Genetics in Endocrinology: Autosomal dominant osteopetrosis revisited: lessons from recent studies

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  • 1 Section of Specialized Endocrinology, Faculty of Medicine, Nordic Bioscience AS, Department of Endocrinology and Metabolism, University of Southern Denmark, Department of Medical Genetics, Medical Clinic B, Rikshospitalet, Oslo University Hospital, N-0027 Oslo, Norway

(Correspondence should be addressed to J Bollerslev at Section of Specialized Endocrinology, Medical Clinic B, Rikshospitalet, Oslo University Hospital; Email: jens.bollerslev@medisin.uio.no)

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

Abstract

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

Introduction

The concept of osteopetrosis was introduced in the 1920s to describe patients with radio-opaque bones and universal osteosclerosis. The disease corresponded to marble bone disease (1) first described in 1904 by the German radiologist Albers-Schönberg (2). He described a man with multiple fractures and a radiographic osteosclerosis characterized by increased cortical thickness and reduced marrow space. It became clear that osteopetrosis could be seen in families, and based on mode of inheritance, Johnston et al. (1968) (3) suggested two different forms: a benign form designated autosomal dominant osteopetrosis (ADO) and a malignant form seen in childhood and inherited in an autosomal recessive manner, termed autosomal recessive osteopetrosis (ARO). However, other forms were described during the following years, and moreover, it became clear that the clinical presentation of each of the heritable forms varied. Intermediate forms with relatively mild course but a recessive mode of inheritance were identified. In one of these, the patients presented basal ganglia calcifications and renal tubular acidosis, and this syndrome was subsequently found to be caused by carbonic anhydrase deficiency (4, 5, 6, 7). With these observations, malfunctioning bone resorption was introduced as a common pathogenic factor in osteopetrosis, and at least in the intermediate forms, this was demonstrated in the beginning of the 1980s to be due to an acidification defect across the osteoclast cell membrane (5), see section on Genetic studies identifying CLCN7.

Osteopetrosis was described as a heterogeneous group of diseases related to defective bone resorption (3). However, also within ADO, a systematic search for the disease revealed radiographic heterogeneity (8). Based on several families with ADO from the county of Funen, Denmark, in the mid-1980s, we described two distinct radiological forms based on plain radiographs (8). Further studies characterizing ADO at the clinical, biochemical, histological, and biomechanical levels revealed that these types corresponded to two distinct disorders, which we designated as ADO type 1 (ADO1) and ADO2 (9, 10, 11, 12, 13). These early studies were reviewed in 1989 (14). ADO was defined as a condition with diffuse osteosclerosis, primarily involving the axial skeleton, but with symmetrical effects on the long bones and with little or no modeling defects (14). Based on bone biopsies, it was hypothesized that the pathogenesis of these two forms involved defective bone resorption, directly or indirectly.

During the recent 20 years, investigations of osteopetrosis have contributed significantly to our insight into bone biology. After the millennium, it turned out that ADO1 was caused by an activating mutation in LRP5 (15) and thus cannot be recognized as a classical form of osteopetrosis but rather should be regarded as a LRP5-activating bone disease, or high bone mass (HBM) phenotype. Also, the disease most probably described by Albers-Schönberg (2) turned out to be a form of Chloride Channel 7 Deficiency Osteopetrosis (16). However, it should be noted that a mutation in CLCN7 has not been demonstrated in up to 30% of patients presenting with a clinical phenotype of ADO, indicating further heterogeneity (17, 18).

The aim of the present review is to give an overview of clinical lessons learned by studies on large and homogenous populations of patients with ADO and HBM. This review will discuss genetic studies leading to the identification of the mutated genes in the original cohorts, of instrumental importance for separating the syndromes into an intrinsic bone-resorptive defect (Chloride Channel 7 Deficiency Osteopetrosis (ADO)) and a LRP5 activation disease related to the bone formative site. Moreover, we will describe subsequent in vitro investigations on bone-resorptive cells in ADO, increasing the understanding of bone biology and the coupling principle. Also, we will critically review recent data on metabolic consequences of disturbed bone resorption in ADO. Finally, perspectives from these studies regarding treatment of other metabolic bone diseases are outlined. For this purpose, literature was searched through PubMed for the key words autosomal dominant (or benign) osteopetrosis, osteoclasts, bone resorption, HBM, and coupling.

Summary of recent systematic clinical studies

Clinical symptoms, bone mass, and bone structure

Two large cohorts of well-characterized patients with ADO have been described in detail after the millennium (19, 20). Moreover, Frost et al. (21) recently updated the HBMT253I cohort. Bone mass in adults based on dual-energy X-ray absorptiometry (DXA) is almost increased to the same level in patients with the bone-resorptive defect (ADO) as in patients with LRP5 activation bone disease. Thus, ADO patients and our HBMT253I cohort had increased bone mineral density (BMD) at all skeletal sites measured. Z-scores ranged from +2 to +12 at the lumbar spine, which is mostly comprised of trabecular bone, and the whole-body compartment, consisting primarily of cortical bone, Fig. 1, also illustrating studies by high-resolution peripheral quantitative computed tomography (CT) (20, 21, 22).

Figure 1
Figure 1

Bone mass by DXA and high-resolution pQCT in autosomal dominant osteopetrosis and the HBM phenotype. (A) Individual BMD Z-scores in 19 patients with HBMT253I compared with controls. The figure is based on data and reproduced from Frost M, Andersen T, Gossiel F, Hansen S, Bollerslev J, Van Hul W, Eastell R, Kassem M & Brixen K. Levels of serotonin, sclerostin, bone turnover markers as well as bone density and microarchitecture in patients with high bone mass (HBM) phenotype due to a mutation in Lrp5. Journal of Bone and Mineral Research 26 1721–1728, 2011, with permission. (B) Upper panel: high-resolution pQCT of radius and tibia in a case with HBMT253I (left two pictures) and a control subject (right two images); reproduced from Frost M, Andersen T, Gossiel F, Hansen S, Bollerslev J, Van Hul W, Eastell R, Kassem M & Brixen K. Levels of serotonin, sclerostin, bone turnover markers as well as bone density and microarchitecture in patients with HBM phenotype due to a mutation in Lrp5. Journal of Bone and Mineral Research 26 1721–1728, 2011, with permission. Lower panel: high-resolution pQCT of radius and tibia in a case with ADO (left two pictures) and a control subject (right two images). (C) Individual BMD Z-scores from with ADO and obligate carriers. The figure is reproduced from Waguespack SG, Hui SL, Dimeglio LA & Econs MJ. Autosomal dominant osteopetrosis: clinical severity and natural history of 94 subjects with a chloride channel 7 gene mutation. Journal of Clinical Endocrinology and Metabolism 92 771–778, 2007, with permission.

Citation: European Journal of Endocrinology 169, 2; 10.1530/EJE-13-0136

Penetrance is high in ADO; however, several asymptomatic carriers have been described (14, 20). Waguespack et al. (20) described a total of 32 obligate carriers and presented data on BMD in six. In these, BMD Z-scores were modestly elevated with values ranging from 0 to +4 (Fig. 1). The authors had the possibility to reevaluate six patients and one carrier after more than 30 years since the original description (3, 20). In all patients, clinical symptoms progressed, whereas the carrier remained asymptomatic (20). The progression of symptoms is in alignment with progression of the universal osteosclerosis, as indicated by cross-sectional studies using DXA and histomorphometry (11, 12, 20, 21, 23).

Patients with ADO have typical and prevalent symptoms (2, 3, 10, 19, 20, 24). The typical findings are increased fracture frequency, delayed healing, and osteomyelitis, especially in the jaw (2, 10, 19, 20). The latter is of special interest in relation to the current discussion of osteonecrosis of the jaw (ONJ) associated with bisphosphonates and denosumab, i.e. drugs that decrease the number and activity of osteoclasts (25, 26, 27, 28, 29), as will be discussed in more detail below.

With the prevalent and often serious symptoms, the term benign osteopetrosis could be questioned (19, 20). As an example of this, Waguespack et al. (20) found early-onset vision loss in one-fifth of the patients and signs of bone marrow failure in 3%. Thus, their cohort had clearly more serious symptoms than we found in our original studies (14). However, the findings are prone to selection bias, as patients with symptoms are found by clinical work-up, whereas patients with sparse symptoms and carriers are mostly found by family studies (14, 19, 20).

Osteonecrosis in the jaw

ONJ occurs in cancer patients with bone metastasis treated with monthly dosages of i.v. administered bisphosphonates or denosumab (with a frequency of 1.3 and 1.8% respectively) (30) and in rare cases (two cases per 100 000 patient years) of patients with osteoporosis treated with oral bisphosphonates (31). ONJ has also been reported in patients with osteoporosis treated with denosumab every 6 months (32). The risk of ONJ in patients with osteoporosis treated with bisphosphonates increases following tooth extractions and with the duration of therapy. The pathogenesis of ONJ related to bisphosphonates and denosumab is currently unknown. Infection, osteocyte death, soft tissue toxicity, interference with angiogenesis, concomitant therapy, and decreased bone turnover have been suggested as causative factors (33). Occurrence of ONJ following two completely different drugs such as bisphosphonates and denosumab sharing only the common effect of decreasing bone turnover (and number of osteoclasts) as well as case reports indicating that bisphosphonate-related ONJ may respond well to therapy with teriparatide suggests that low bone turnover is essential in the pathogenesis of this condition. In ADO, however, inflammation and not necrosis is the usual feature. Patients with ADO also have other dental abnormalities such as delayed tooth eruption, enamel hypoplasia, and dental caries (34) that may predispose to infection of the jaw. Also, non-jaw osteomyelitis is seen in ADO (19) but not following bisphosphonate therapy. Bisphosphonates are taken up not only by osteoclasts but also by macrophages and enhance apoptosis in these cells. The role of macrophages in ONJ and osteomyelitis in patients with ADO, however, is currently unknown.

Bone remodeling

Histomorphometric analyses of bone biopsies from adult patients with ADO are limited. Early studies at the trabecular (11) and cortical envelope (23) indicated virtually normal bone remodeling and an endosteal resorption defect. However, the studies were limited by few individuals investigated and the immense analytical variability of histomorphometry with the risk of a type 2 error (35). The data were in accordance with syndromes of defective bone resorption and normal, or atleast not increased, bone formation. In normal subjects, administration of triiodothyronine (T3) activates bone remodeling (36). In agreement with the relative benign character of ADO, bone turnover can be stimulated by T3 (37, 38, 39), but the response is blunted for bone resorption as well as for formation markers (38, 39, 40).

ADO is osteoclast rich

Detailed ultrastructural investigations of bone biopsies from ADO patients revealed that osteoclasts were markedly increased in size and number (almost threefold) compared with controls (41) and of the typical multinucleated type. Moreover, biochemical markers of osteoclast number, TRACP and CKBB, were markedly increased in ADO (13, 16, 42, 43, 44, 45). Based on these studies, ADO was described as an osteoclast-rich form, to be discussed in detail below.

Genetic studies identifying CLCN7

Positional cloning was used to identify the disease-causing genes in the original populations. This involved a two-step procedure first aiming to localize the affected gene to a chromosomal region and subsequently perform mutation analysis from the delineated region (46). For the first step, we performed genetic linkage analysis in extended families (47).

Identification of the role of the CLCN7 gene in ADO

In the context of epidemiological and clinical studies in the county of Funen, Denmark, an extended family was ascertained with a large number of family members affected with the classical radiographic appearance of ADO (9). Genome-wide linkage studies revealed a gene localization on chromosome 16p13.3 based on the analysis of six extended families, including the original Danish pedigree (48). By far, the most interesting candidate gene within the delineated region was the CLCN7 gene encoding a chloride channel. Kornak et al. (49) showed that loss of the ClC-7 chloride channel in mice results in severe osteopetrosis. Furthermore, they reported one patient with ARO due to two compound heterozygous mutations in this gene. We identified heterozygous CLCN7 mutations in all ADO families used for the linkage study as well as in six additional ADO patients (50). The ClC-7 chloride channel has 12 transmembrane domains, which turned out to function as a slowly voltage-gated 2Cl(−)/1H(+)-exchanger (51). Currently, 25 different mutations have been documented and found in all regions of the gene but with a cluster around amino acids 313–318 and in the intracellular part before the carboxyterminal end of the protein (Fig. 2). Most mutations are missense mutations causing an amino acid substitution with two exceptions. One results in a deletion of one amino acid (ΔL688), but it can be assumed that this might not disturb the topology of the chloride channel (50). The other is a deletion of two nucleotides (2423delAG) found in two families and causing a frameshift mutation. However, this mutation is located very close to the carboxyterminal part of the protein (50). A possible effect of missense mutations on the functioning of the encoded protein can be complete loss of function. However, for the CLCN7 mutations, this seems to be unlikely. First, severe ARO has been described in patients with loss of function mutations (49), but the parents were clinically and radiographically normal despite having a heterozygous loss of function mutation. Secondly, ClC-7 is known to function in a dimeric state. Together, this indicates that for the ClC-7 protein, there is no effect of haplo-insufficiency in humans while amino acid substitutions might have a dominant negative effect. Several studies have investigated the direct effects of the mutations. Analyses of human osteoclasts showed that the G215R mutation, which is the most common, reduces acid secretion into lysosomes (52, 53). Further studies using over-expression systems indicated that this was related to ER retention of the mutated protein, a finding also observed for other mutations, such as G240R and R526W (51, 54). Another group of mutations was shown to alter the gating kinetics, thereby rendering the transporter inactive (51). However, the full understanding of the relationship between the mutations and osteoclast functionality is generally unclear, and it is further clouded by the presence of unaffected carriers of the mutations, a phenotype that is manifested even in isolated osteoclasts (55).

Figure 2
Figure 2

Topology of the chloride channel 7 (ClC-7) protein. The lysosomal side is indicated with a black line, while the cytosolic side is indicated with a blue line. Known mutations in patients with autosomal dominant osteopetrosis type 2 are indicated. The original Danish pedigree harbored the G215R mutation (50).

Citation: European Journal of Endocrinology 169, 2; 10.1530/EJE-13-0136

Genotype–phenotype correlation

Many of the mutations identified are private to one family but some have been reported in several families. So far, no clear-cut genotype–phenotype correlation has been established (56). This is not unexpected taking into account the high intra-familiar clinical variability ranging from asymptomatic to severely affected. This also implies a reduced disease penetrance, which has been estimated to be between 66 and 94% (3, 19, 20, 57, 58). A clear explanation for the reduced penetrance and the intra-familial variability is not available. In theory, single-nucleotide polymorphisms in the promoter region might influence the ratio between the mutated and the wild-type copy of the protein. Interestingly, a coding polymorphism, V418M, has been reported to be associated with the severity of the ADO presentation (59, 60). Furthermore, a 50 bp variable number of tandem repeat (VNTR) polymorphism with a copy number between 2 and 9 is present in intron 8 of the human CLCN7 gene. This repeat is associated with BMD in the general population, but also with the severity of ADO (61). The mechanism by which this repeat influences the function of ClC-7 is unclear, but an effect on exon–intron splicing cannot be excluded. The V418M and the VNTR polymorphism are to some degree in linkage disequilibrium (62). Therefore, the real causal variant remains to be demonstrated.

In addition to this, it is clear that ADO is genetically heterogeneous as no CLCN7 mutation could be found in up to 30% of patients presenting with a clinical phenotype of ADO (17, 18). No clinical or radiological difference between those with or without a CLCN7 mutation has been reported.

Analysis of osteoclasts from ADO patients

Osteoclast-rich forms of osteopetrosis and in particular ADO have been studied in detail in vitro. Histomorphometric analyses of ADO and osteoclast-rich ARO patients showed increased numbers of very large osteoclasts in vivo (17, 41). In vitro analyses showed that osteoclastogenesis was normal, both with respect to time frame and numbers of osteoclasts, and with respect to morphology (17, 52, 53, 63, 64). The underlying reason for the lack of bone resorption by these osteoclasts was shown to be reduced acid secretion into the resorption lacunae (52, 53, 63, 64, 65), an effect also seen in ClC-7-deficient mice (49).

Further studies of the ADO osteoclasts indicated that the reason for the increased number of osteoclasts in vivo is related to increased survival of the osteoclasts. The osteoclasts have an attenuated capacity to resorb the calcified bone matrix. This mechanism, at least partially, seems to have an auto-regulatory effect of calcium directly on osteoclast survival (63, 66). However, other studies have also shown that release of transforming growth factor β from the bone matrix caused induction of osteoclast apoptosis (67) and thus also participates in this auto-regulatory control of osteoclast activity. In addition to increased survival due to decreased release of pro-apoptotic factors from the mineral, increases in parathyroid hormone could contribute to the increased number of osteoclasts (68). However, this increase in survival is also seen in vitro in pure cultures of osteoclasts where acid secretion is reduced due to mutations or blocked pharmacologically hence showing at least some PTH-independent osteoclast survival effects (63, 66, 69).

In addition to shedding light on the regulation of the life span of osteoclasts, another highly interesting aspect of bone remodeling has been studied extensively based on the pathophysiology of patients with ADO, namely the coupling of bone resorption to bone formation. In the ADO patients, as well as other osteoclast-rich forms of osteopetrosis, it has been shown that bone formation is ongoing, despite the absence of bone resorption (11, 17, 41, 42), a phenomenon that is also found in rodent models of osteoclast-rich osteopetrosis (70, 71), and in animal models treated with inhibitors of osteoclastic acid secretion (63, 72, 73, 74).

In accordance with these studies, a series of in vitro studies of osteoclasts have shown that independent of their resorptive activity, osteoclasts produce anabolic signals for osteoblasts (69, 75, 76, 77), thus explaining the origin of the ongoing bone formation in the ADO/osteoclast-rich osteopetrosis patients, see Fig. 3. Furthermore, these studies showed that only mature osteoclasts produce the anabolic signal (76) and hence illustrated the specificity of this phenomenon, while also providing some evidence as to why bone formation appears very low or even missing in the absence of osteoclasts, as seen in the RANK-deficient patients (78). However, controversies still exist, as demonstrated recently where inhibition of bone resorption in vitro with the V-ATPase inhibitor bafilomycin blunted release of anabolic factors from the bone matrix (79). Thus, there are still discussions related to the origin of the anabolic molecules initiating and driving bone formation as a consequence of bone resorption by osteoclasts.

Figure 3
Figure 3

Schematic illustration of the interplay between bone lining cells, osteoclasts, and osteoblasts under normal circumstances (upper panel) and with autosomal dominant osteopetrosis (ADO; lower panel). Upper panel: bone resorption releases apoptotic factors ensuring termination of resorption and the osteoclasts secrete anabolic molecules for the osteoblasts ensuring that bone formation follows bone resorption and thereby completing the bone remodeling cycle. Lower panel: in ADO, resorption per osteoclast is greatly reduced, and this has been shown to lower the release of apoptotic factors from the bone matrix, resulting in larger numbers of large and multinuclear osteoclasts. These osteoclasts are still anabolically active, and they secrete factors to the osteoblasts ensuring rather normal bone formation, which when combined with defective bone formation is the reason for the increased bone mass in these patients. An interesting phenomenon, namely deposition of TRACP residue along the resorbed surface, has been shown in these patients and corresponding animal models; however, the functional consequence of this has not been shown yet. The figure was inspired by Segovia-Silvestre et al. (213).

Citation: European Journal of Endocrinology 169, 2; 10.1530/EJE-13-0136

With respect to molecular candidates for the bone anabolic factors derived from the osteoclasts, there are several. They include well-known bone-stimulating molecules, such as IGF1, BMP6, and Wnt10b (76, 79, 80). Furthermore, there are also molecules that are still being explored in detail, such as the ephrinB2–ephB4 interaction and sphingosine-1-phosphate (76, 77). However, what remains to be demonstrated is clear-cut in vivo proof that these are indeed coupling factors. Conversely, showing this is immensely complex when considering the ubiquitous nature of the molecules, and the very likely possibility that it is a combination of molecules that serves this function in vivo.

In addition to osteoclast-produced and resorption-derived coupling factors for osteoblasts, there is also some evidence that the bone surface in the resorption pits is of utmost importance for bone formation (81). In this context, it is of interest that the resorbed surface area in the ADO patients is very high, likely as a consequence of the increased numbers of osteoclasts, which initiate, but fail, to perform a complete process of resorption (41), thus leaving the bone surface ‘scratched’ as opposed to completely resorbed. Along the same line, data from in vitro experiments show that allowing osteoclasts to resorb bone facilitates bone formation by osteoblasts in the resorbed areas (82, 83). Furthermore, studies indicate that osteoclasts deposit TRACP on the bone surface leading to recruitment of osteoblasts (84), a finding that correlates with the high levels of TRACP found on the resorbed bone surfaces in ADO patients and other acid secretion-deficient systems (41, 85, 86) (Fig. 3).

In summary, in vitro studies of ADO osteoclasts have highlighted relevant and important aspects not only related to bone resorption but also for the coupling principle and thereby for bone formation. Thus, it has been clearly demonstrated that the chloride–proton antiporter ClC-7 is a highly relevant potential pharmacological target for treatment of osteoporosis.

Treatment of and potential therapies identified from studies of ADO

Treatment of ADO

The pivotal studies by Walker (87) in the early seventies for the first time demonstrated that osteoclast-rich forms of osteopetrosis could be cured by parabiosis, also demonstrating that osteoclasts derive from hematopoietic stem cell. Thereby, the scientific basis was established for experimental treatment of the severe clinical (malignant) forms of ARO with bone marrow transplantation in children (88, 89) in the early 1980s. This is now an established treatment modality (90, 91). However, no specific pharmacological therapy for ADO is currently available. Treatment of osteopetrosis including ADO is mainly supportive. Multidisciplinary management is required in order to manage skeletal related complications such as osteoarthritis, fractures, and osteomyelitis, as well as compression of cranial nerves including most notably the optic nerve (14, 19, 20, 24, 45, 47). Osteopetrosis may cause bone marrow failure and seizures due to hypocalcaemia, but these complications are infrequent in late-onset ADO.

Insight into the pathophysiology of the different forms of osteopetrosis may be important for the design of therapy for these diseases. A number of drugs have been shown to increase osteoclast activity including calcitriol, which ameliorates ARO (92). Furthermore, treatment of ARO with interferon γ1b increases bone resorption, causing a reduction in trabecular bone area and an increase in bone marrow space (93). While treatment of ARO with RANKL recently was shown to improve the bone phenotype in mice and may prove effective in humans (94), specific pharmaceutical treatment of ADO has not been investigated systematically.

Patients with ADO have a single-allele dominant negative mutation of CLCN7. Consequently, it has been suggested that siRNA therapy could transform the phenotype by silencing the affected allele. Indeed, preliminary results have shown that siRNA rescued the phenotype of human osteoclasts transfected with mutant ClC-7 constructs (95). Another possible approach is suggested by the demonstration that osteoclasts cultured from unaffected gene carriers function normally while osteoclasts from affected patients resorbed much less bone in vitro (55, 60). These studies strongly suggest that modifying pathways might exist. Potentially, these pathways could be drug targets. Age, sex, and the specific CLCN7 mutation do not seem to be important in this respect and osteoclastic response to RANKL did not differ between carriers and affected patients either. Finally, it is unknown whether reduction in osteoclast number by treatment with, e.g. bisphosphonates or denosumab, could be beneficial.

Insight into the pathophysiology of ADO is also important for the design of anti-osteoporosis therapy. A number of limitations in this respect should be recognized. First, ADO exists throughout life and thus still show effects that are also related to bone development (modeling) as opposed to bone remodeling (70). The studies have provided proof-of-concept that the chloride–proton antiporter ClC-7 is an attractive target as well.

ClC-7 inhibitors

Studies of ADO patients have demonstrated the potential mode of action of a ClC-7 inhibitor, and the most intriguing finding is the apparent uncoupling between bone resorption and bone formation, a finding that clearly indicates that bone resorption can be inhibited without detrimental effects on bone formation (81, 96, 97).

Studies in the aged ovariectomized rat model using these inhibitors have shown that bone resorption is lowered, while osteoclast numbers and bone formation are maintained (63, 73), thereby mimicking the phenotype of the ADO patients and ClC-7-deficient mice (11, 41, 85, 86). Treatment with these inhibitors also resulted in increased BMD and bone strength (63, 73), thereby underlining the potential of these molecules. The contrast to the fracture pattern in ADO patients (10) might be related to the difference between an inherited condition with early effects during development and bone modeling. As osteopetrosis patients, including ADO patients, show poor bone quality and increased numbers of fractures (14, 18, 98), it should be considered whether molecules mimicking these phenotypes would also result in bone fragility. However, a recent study in which osteopetrosis was induced in adult mice (>3 months of age) showed that the bone brittleness observed in osteopetrosis is a developmental phenomenon during growth and bone modeling caused by the presence of calcified cartilage (70) and thus not a phenomenon likely to occur in adult osteoporotic patients with normal childhood bone development.

In addition, the patients and mice with complete loss of function or expression of ClC-7 are known to show primary neurological issues (49, 91, 99); however, analyses of the ADO patients, which, based on gene doses, have 25% residual ClC-7 activity remaining, have not shown any primary neurological phenomena (14, 20, 91), indicating that the neurological problems only arise when ClC-7 is completely inactive or absent, and this level of inhibition of ClC-7 is not expected with small-molecule inhibitors.

In relation to the specificity of the target, the A3 subunit of the osteoclastic proton pump, which is functionally close to identical to ClC-7, in addition to only having a function in the osteoclasts, has also been studied in detail (100). However, as the A3 subunit is a structural component docked in the membrane and as the enzymatic subunit of the V-ATPase complex has a broad tissue distribution, this has complicated development of small-molecule inhibitors of this target significantly (100).

Hence, the acid secretion process in the osteoclasts is a highly attractive target from a pharmacological point-of-view. However, it is also a difficult target, and more research is clearly needed before a final conclusion can be reached, although the phenotype observed in ADO with decreased resorption and normal or increased bone formation in adults is very promising.

Interestingly, inhibition of cathepsin K, the acid-activated protease mediating type I collagen cleavage in the resorption lacunae, has been explored extensively as a pharmacological target, including publication of data from phase II trials (101, 102, 103, 104). Initially, inhibition of cathepsin K was thought to lead to inhibition of resorption, increases in osteoclast numbers, and no secondary inhibition of formation, as seen in the ADO patients, due to the very close relationship between these processes and the bone phenotype observed in cathepsin K-deficient mice (105). However, studies on monkeys, the clinical trials, and the phenotype observed in the cathepsin K-deficient pycnodysostosis patients have demonstrated that inhibition/ablation of cathepsin K in humans is more complex, as bone formation is reduced secondary to the inhibition of resorption. Hence, the final data from the phase III study of the cathepsin inhibitor odanacatib is awaited with interest. In summary, inhibitors of ClC-7 are attractive as candidates for treatment of postmenopausal osteoporosis, inflammatory osteolysis, and other bone metabolic disorders.

Identification of the role of the LRP5 gene in bone

Two Danish families with HBM were instrumental in localizing the disease-causing gene by linkage analysis on chromosome 11q12–13 (106). Within the delineated region, two genes of interest were localized: TCIRG1 encoding for the a3 subunit of the proton pump V-ATPase and LRP5 encoding for the LDL receptor-related protein 5. Mutations in TCIRG1 were found to cause ARO (107) but could not be demonstrated in our cohort. On the other hand, loss-of-function mutations in LRP5 were identified to be causative for autosomal recessive osteoporosis-pseudoglioma syndrome (OPPG), characterized by congenital blindness and severe juvenile-onset osteoporosis (108). Heterozygous carriers of these mutations showed an increased incidence of osteoporotic fractures, indicating a dominant negative effect on bone mass (108). At the same time, Little et al. (110) and subsequently Boyden et al. (109) reported the identical missense mutation, G171V, in two kindred diagnosed with the so-called HBM phenotype. Patients from the first family were asymptomatic but showed radiographic very dense bones, especially involving the cortices of the long bones, as well as an increased thickness of the skull. In the second family, some additional features including a wide and deep mandible and a torus palatinus were described (55). In both families, the patients seem to be protected against fractures. Analysis of the LRP5 gene in patients from the two Danish families indicated a heterozygous missense (T253I) mutation in both (15). In addition, a more extended set of patients was screened for LRP5 mutations. All these patients, despite different diagnoses such as endosteal hyperostosis, Van Buchem disease, or autosomal dominant osteosclerosis, showed a similar radiographic and clinical picture (111, 112). In several of these families, we found a missense mutation in LRP5 (15). As shown in Fig. 4, all these mutations cluster within the first propeller domain of the LRP5 protein. This has also been the case for all mutations reported later on (113, 114, 115). The clinical variability observed in patients with these LRP5 mutations cannot be correlated with the specific mutation identified. Moreover, intra-familial variability supports the idea that other genetic modifiers as well as environmental factors influence the expression and severity of the phenotype.

Figure 4
Figure 4

Gain-of-function mutations in the gene encoding the LDL receptor-related protein 5 (LRP5). All identified mutations in the high bone mass phenotypes cluster in the first β-propeller of the extracellular domain, thereby affecting the binding of inhibitory proteins as DKK1 and sclerostin. The consequence of these mutations is an increased signaling of the canonical Wnt pathway. The original Danish pedigrees harbor the T253I mutation (15).

Citation: European Journal of Endocrinology 169, 2; 10.1530/EJE-13-0136

Minor changes in the LRP5 gene, i.e. polymorphisms in LRP5, have been associated with slightly lower bone mass and an increased risk of fracture (116, 117). This suggests that the pathway may be a drug target to achieve a regulated response on bone homeostasis.

As discussed in detail below, the LRP5 protein plays a role as a co-receptor for extracellular Wnt proteins to induce the canonical Wnt signaling and this signaling pathway is regulated by inhibitors including DKK1 and sclerostin (118). Structural analysis of the LRP5 protein revealed that all amino acids involved in any of the HBM are clustered at an open binding pocket near the surface of the first β-propeller of LRP5 (119). The mutations do not have any effect on the functioning of the protein but rather disrupt the ligand binding of the extracellular inhibitors DKK1 and sclerostin (120, 121, 122), in which a short binding motif was found by structure analysis (123). The latter protein was identified by positional cloning for two other sclerosing bone disorders, Van Buchem disease and sclerosteosis (124, 125, 126, 127). The radiographic picture in these patients is very reminiscent to HBM. However, the phenotype is more severe (128, 129), with pronounced enlargement of the mandible and extreme thickening of the skull causing cranial nerve encroachments resulting in facial nerve palsy, optic atrophy, and hearing loss. Thus, the similarities between the dominant phenotypes (HBM) and the recessive phenotypes (Sclerosteosis and Van Buchem disease) can be explained by a shared increased Wnt signaling, but by different mechanisms. In the latter conditions, the patients are lacking an inhibitor (sclerostin) while in the former ones the inhibitor can no longer bind its mutated receptor. However, a recent study indicated that this might be a somewhat simplified model. Niziolek et al. (130) made knock-in mouse models for two HBM mutations and compared them to a SOST knockout model. All models showed an increased thickness of the skull, but clear differences were noticed at the appendicular skeleton. The A214V and SOST mutants were identical with increased bone formation periosteally, while in the G171V model, bone was added preferentially at the endocortical envelope (130), as suggested (14) and demonstrated by histomorphometry in the HBMT253I cohort (11, 23). This indicates, at least for the G171V mutant, that additional mechanisms not associated with sclerostin are involved.

The involvement of the canonical Wnt signaling pathway in bone metabolism

The most important implication of the LRP5 mutations revealed by positional cloning efforts in skeletal disorders is that they linked, for the first time, the canonical Wnt/β-catenin signaling pathway to bone biology. This is the best-known pathway triggered by extracellular Wnt molecules. In the absence of Wnts, intracellular β-catenin is bound to a protein complex including axin1/2, APC, casein kinase 1, glycogen synthase kinase 3β (GSK3β), as well as WTX (AMER1) and subsequently degraded in the proteasome (131). However, upon the presence of Wnt molecules, these will bind to the extracellular frizzled receptor and a co-receptor (Lrp5/Lrp6). This results in destabilization of the destruction complex and release of β-catenin, leading to intracellular accumulation of β-catenin that can translocate to the nucleus and, after binding to Tcf/Lef transcription factors, induce the expression of target genes (131).

Over the last years, plenty of evidence became available supporting a role of canonical Wnt signaling in bone homeostasis from molecular genetic studies on skeletal diseases. In addition to the already mentioned mutations in the co-receptor LRP5 and the Wnt inhibitor sclerostin, mutations were found in WTX, a member of the destruction complex of β-catenin, causing osteopathia striata (132, 133, 134). Furthermore, a loss-of-function mutation in LRP6 results in a condition with osteoporosis, coronary artery diseases, and metabolic syndrome (135). Finally, patients diagnosed with Williams–Beuren syndrome have a low BMD, and this condition is associated with a deletion of FZD9, another Wnt co-receptor (136). Both in vitro and in vivo studies have been performed gaining insight into the underlying mechanisms involving both the processes of bone formation and bone resorption.

Evidence for a role of canonical Wnt signaling in bone formation

Canonical Wnt signaling is involved in the bone formation process at different levels. This pathway is a key regulator of the differentiation of mesenchymal stem cells toward chondrocytes, osteoblasts, or adipocytes. The Wnt/β-catenin signaling pathway has been shown to inhibit the adipogenic differentiation potential, thus altering the fate of cells from adipocytes to osteoblasts (137, 138, 139, 140). This is induced by suppressing the expression of the adipogenic transcription factors peroxisome proliferator-activated receptor γ (PPARγ (PPARG)) and CCAAT/enhancer-binding protein α (C/EBPα (CEBPA)) (141). Next, an extended set of mouse models was generated to evaluate the role of LRP5 and β-catenin in osteoblastogenesis and bone formation. Lrp5 knockout mice showed a decreased bone formation rate mainly due to reduced osteoblast proliferation (142). In order to model the human HBM phenotype, Babij et al. (143) generated a Lrp5 G171V transgenic mouse using the 3.6 Col1A1 promoter and were able to also show a decrease in the apoptosis of osteoblasts and osteocytes, which might also contribute to the phenotype. Mouse models in which β-catenin was conditionally deleted indicated an essential role in osteoblast differentiation as absence of β-catenin in embryonic mesenchymal progenitors abolished the generation of mature osteoblasts (144) and had an effect on the balance between osteo- and chondroblastogenesis (145).

In different studies, the role of Wnt/β-catenin signaling in bone formation was linked to the responsiveness to mechanical loading on bone, which seemed to be increased in the LRP5 (G171V) mutant models and reduced in the LRP5 knockout models (146). Furthermore, deletion of one copy of the β-catenin gene in osteocytes was linked to reduced new bone formation upon mechanical loading (147). Thus, a model has been suggested combining a direct β-catenin-dependent but LRP5-independent osteocytic effect with an LRP5-dependent feedback loop to explain the responsiveness to mechanical loading (148).

Evidence for a role of canonical Wnt signaling in bone resorption

Mouse studies have provided strong indications that at least β-catenin influences the process of bone resorption. The generation of conditional knockout mouse models for β-catenin supports that regulation of bone resorption is mediated by osteoblasts and osteocytes. Deletion of β-catenin in a later stage of osteoblast differentiation results in a severe bone loss (149). An increased number of osteoclasts were present, and subsequently, it was demonstrated that canonical Wnt signaling within differentiated osteoblasts induces the expression and secretion of osteoprotegerin (OPG), an important inhibitor of osteoclastogenesis (137). Thus, by stabilizing β-catenin, the OPG:RANKL ratio increased followed by decreased osteoclastogenesis and defective tooth eruption (137, 150), which are classical signs of osteopetrosis in murine models (151, 152, 153, 154). Along the same lines, deletion of β-catenin in osteocytes resulted in decreased BMD due to an increased number and activity of osteoclasts (155). This effect was associated with a decreased level of OPG, thus increasing the RANKL:OPG ratio and consequently stimulating osteoclastogenesis.

Because β-catenin is also involved in many Wnt-independent processes, this might suggest, but does not confirm, a direct role for Wnts in osteoclast differentiation. However, Ruan et al. (156) recently reported an increased osteoclast differentiation of osteoclast precursors lacking functional LRP5 and LRP6, indicating that canonical Wnt signaling indeed suppresses osteoclast differentiation. Furthermore, the administration of anti-sclerostin antibody, which is assumed to increase canonical Wnt signaling, also results in a decreased osteoclastogenesis and reduction in bone resorption in rats (157).

Early based investigations of our HBMT253I cohort identified low bone resorption biochemically (68) reflecting reduced osteoclast profiles by immunohistochemistry and electron microscopy (41). Although we confirmed the biochemical findings recently (21, 40), these findings have never been established in other HBM cohorts, as systematic bone metabolic studies have not been published.

Treatment of and potential therapies identified from studies of HBM

Treatment of HBM

Owing to the benign nature of the HBM disorders, treatment has so far been mainly supportive, as for ADO. As the pathogenic defect is on the formative side of bone remodeling, bone marrow transplantation is not an option (151, 158, 159). Two considerations are of importance in relation to treatment of HBM: i) serious side effects are unacceptable given the relatively benign nature of the disease and ii) bone formation is increased and accompanied with some alteration in bone resorption (118, 160).

With respect to reversing the phenotype, there appears to be some possibilities as both glucocorticoids and glitazones are characterized by reducing bone formation and increasing bone resorption (161, 162). However, these drugs are associated with unacceptable side effects with high dosages and long-term treatment. Moreover, it would be more appealing to treat directly by targeting the regulation of the Wnt signaling.

Sclerostin and stimulators of sclerostin secretion such as calcitonin (163) could potentially be used for the treatment of HBM, but calcitonin is also a potent inhibitor of bone resorption. Mechanical unloading (i.e. bed rest) is followed by increased resorption and decreased formation, controlled at least to some extent by increased sclerostin levels and thereby inhibition of the overactive Wnt cascade (164). It is at present unknown whether manipulating sclerostin levels will have unintended effects on the neuromuscular response as seen in humans in relation to unloading (165). Moreover, some of the HBM mutations appear to prevent sclerostin by itself from binding to LRP5 (122), possibly explaining the increased level of sclerostin observed in patients with HBM (21).

Alternatively, application of continuous PTH or RANKL could lead to aggressive induction of bone resorption and thereby potential removal of the excess bone (166, 167). However, this would need careful long-term studies and could potentially be complicated by antibody production and difficulties in controlling the RANKL dose.

Taken together, there are several hypothetical approaches to treat HBM. However, so far, they are all limited by side effects and lack clinical validation. For future studies, side effects and long-term consequences of treatment should be balanced against the, in general, benign nature of the disorder.

The Wnt/LRP5 system

With the identification of the highly interesting bone phenotype of increased bone mass and strength in the HBM patients, who have loss-of-inhibition mutations in LRP5 (15, 109, 140, 160, 168), an intensive search for therapies targeting this molecule was initiated. As the Wnt ligands, as well as the downstream signaling molecules GSK3β and β-catenin, are rather ubiquitous and have been implicated in cancer progression, these are less attractive targets despite their obvious anabolic potential (169, 170). On the other hand, the identification of the soluble inhibitors of Wnt signaling, such as sclerostin, DKK1-4, WIF, and sFRPs, identified a series of interesting targets for antibody and small-molecule inhibitor therapy (169), and especially sclerostin that appears rather bone specific and DKK1 have been explored extensively (171, 172, 173, 174).

Anti-sclerostin antibodies, which prevent the binding of sclerostin to LRP5 and the co-receptor (175), have been tested in a host of animal models of osteoporosis, and in all these models that resulted in a large increase in BMD, due to a powerful acceleration of modeling-based bone formation, i.e. activation of bone formation at otherwise quiescent surfaces (176). Also, a reduction of bone resorption was demonstrated, and the resulting increase in bone strength at various sites was substantial (173, 174). AMG-785, a sclerostin MAB, is currently tested in an extensive clinical program. In a phase-1 trial, a large increase in the bone formation marker P1NP and a decrease in the bone resorption marker CTX-I, led to significant increases in BMD at both the lumbar spine and the hip, confirming the pre-clinical results (177).

Anti-DKK antibodies have also been explored, although not to the level of the anti-sclerostin antibodies. Recent studies indicated that these antibodies induce bone formation, reduce bone resorption, and thereby lead to an increase in bone volume in rodent and monkey models (171, 172, 177). Thus, anti-DKK may have a potential as treatment of osteoporosis.

While there is no doubt about the anabolic potential of these molecules, a question is the potential effect of inducing bone formation at surfaces that are normally quiescent or slowly remodeled. A site in question is the subchondral plate in the joints, which has been highlighted by a series of studies indicating that loss of the Wnt inhibitors is a key player in the development of osteoarthritis (178, 179); however, whether this will occur with the therapeutic strategies and thereby become a potential serious side effect remains to be elucidated. In addition, nerve compression is frequently observed in osteopetrotic/osteosclerotic phenotypes (14, 18, 24), and in the case of the mutations related to the Wnt/LRP5 system, this is a consequence of the high bone volume.

In summary, the antibodies against soluble Wnt inhibitors are highly promising in terms of the bone response, but they will need to be carefully monitored in clinical development for osteoporosis due to the potential consequences of inducing bone formation systemically.

Metabolic aspects in relation to a homeostatic model involving bone

Interaction between bone and glucose metabolism

Recent investigations have linked bone metabolism and whole-body glucose homeostasis, and studies indicate that the ADO and HBM phenotypes may include changes in glucose metabolism.

Lee et al. (180) showed that bone cells interact with glucose metabolism through the osteoblast-specific protein, uncarboxylated osteocalcin. Mice expressing lower levels of uncarboxylated osteocalcin were hyperglycemic, hypoinsulinemic, had reduced insulin sensitivity in peripheral tissue, lower-cell mass, and increased fat mass (180). Subsequently, Ferron et al. (65) demonstrated that insulin signaling in osteoblasts increased osteoclast activity and thereby the level of uncarboxylated osteocalcin, thus affecting glucose homeostasis. Fulzele et al. reported that insulin promotes osteoblast development and osteocalcin expression and that bone formation and the number of osteoblasts were reduced in mice without insulin receptors in bone (181, 182). With age, these mice developed adiposity and insulin resistance (181).

Circumstantial evidence of interaction between bone and whole-body metabolism in humans has recently become available. Thus, osteocalcin was inversely associated with plasma glucose, insulin levels, and serum triglycerides in elderly men and women as well as elderly men with high cardiovascular risk (183, 184, 185, 186, 187, 188). Also, osteocalcin was lower in patients with T2DM (189, 190, 191) and gestational diabetes (192). Moreover, in active acromegaly, a condition with profound insulin resistance, osteocalcin levels were the major determinant of insulin resistance and β-cell function, both in vivo and in vitro (193). In animals, the potent inhibitor of bone resorption, alendronate, changed the levels of both uncarboxylated osteocalcin and glucose metabolism. In accordance with these results, treatment of humans with alendronate reduced whereas PTH 1–84 (a bone-forming agent) increased circulating levels of uncarboxylated osteocalcin. These changes were associated with fat mass and other markers of metabolism including adiponectin (194). In contrast to these findings, teriparatide (PTH (1–34)) had no impact on glucose homeostasis (195). Furthermore, it has recently been reported that treatment with antiresorptive drugs including alendronate, zoledronic acid, and denosumab has no clinically important effect on fasting glucose levels (196). However, these findings need to be studied further in trials designed to fully clarify to what extent bone resorption has an effect on glucose homeostasis.

Glucose and fat metabolism in relation to ADO and HBM

LRP5 is expressed in several tissues including bone and pancreas (197, 198). Mutations in LRP5 have been shown to influence the differentiation of human mesenchymal stem cells into osteoblasts or adipocytes (140). Moreover, polymorphisms in LRP5 have been associated with osteoporosis and osteoporotic fractures, as well as obesity and metabolic syndrome (116, 117, 199, 200, 201). Glucose-induced insulin secretion is impaired in mice deficient in Lrp5 (202). Recently, Palsgaard et al. (203) reported that LRP5 promotes insulin signaling in pre-adipocytes, suggesting that modulation of LRP5 could promote insulin sensitivity in type 2 diabetes. Furthermore, patients with OPPG due to a loss of function mutation in LRP5 are more frequently glucose intolerant or diabetic, possibly due to β-cell dysfunction (204). However, glucose homeostasis remains to be investigated in detail in patients with HBM.

Compared with age- and sex-matched controls, fat mass and BMI were higher in our patients with HBMT253I (21). Additionally, patients with clinical, not genetically verified, HBM had higher BMI compared with their relatives (205). Thus, these observations suggest that the clinical phenotype of HBM and increased bone mass may also involve fat mass and glucose metabolism.

Individuals with impaired bone resorption due to ADO appeared to have lower levels of uncarboxylated osteocalcin and decreased levels of insulin after food intake, strengthening the notion that bone metabolism at least in part regulates glucose homeostasis (65). However, as the study comprised few individuals with ADO, further studies on glucose metabolism in ADO are needed for confirmation, in as much as ADO seems not to entail a higher risk of diabetes, based on the literature.

Taken together, these studies suggest that bone metabolism and whole-body glucose and fat homeostasis are integrated. Monogenetic bone disorders characterized by abnormal bone formation or resorption may prove useful as a platform for further investigations.

Serotonin in relation to HBM

Rather than acting directly on osteoblast differentiation, LRP5 may regulate bone formation through changes in the expression of tryptophan hydroxylase I, an enzyme that influences the serotonin synthesis in the gut (206), and pharmacological inhibition of gut-derived serotonin may increase bone mass (207). In accordance with the animal study, a subsequent study demonstrated that the level of serotonin was lower in patients with HBM (207). We reported lower levels of serotonin in platelet-poor plasma as well as serum serotonin measured in samples collected at two different time points in our patients with HBMT253I (21, 208), but the level of serotonin was not associated with bone mass or structure. Furthermore, a recent study did not find any effect of gut-derived serotonin on bone metabolism (209). Instead, the study indicated that LRP5 regulates bone mass through osteocytes and, possibly, late-stage osteoblasts. Taken together, the LRP5–HBM has provided support of a potential association between serotonin levels and bone mass; however, further studies including other genotypes are clearly needed for confirmation.

Conclusions and perspectives

Updated definition of clinical osteopetrosis

Previously, we have described ADO in two forms (ADO1 and ADO2) (14). However, it turned out that the former was caused by an activating mutation in LRP5 (LRP5 activation bone disease) defined as an HBM phenotype (15), leaving ADO to the disease related to chloride channel 7 deficiency. An updated definition taking into account recent advantages in pathophysiological understanding should describe osteoclast-rich and osteoclast-poor forms (17, 18). Thus, in a modern sense, osteopetrosis is an inherited group of generalized bone disorders characterized by increased bone mass in all compartments due to osteoclast failure and impaired bone resorption. This definition focuses on the resorptive side of bone remodeling, however, and recognizes direct (for example, ADO due to ClC-7 defects within the osteoclast) and indirect regulatory defects. Examples of the latter are defects in the receptor for RANKL at the resorptive, osteoclastic site (RANK) (78), and mutations in RANKL by itself on the osteoblastic site (210). This definition acknowledges regulatory (autocrine/paracrine/endocrine) pathways within bone remodeling and also points to the fact that bone formation is ongoing in osteopetrosis. For ADO, the defective osteoclasts seem to regulate bone formation (75, 81, 96). This updated definition is also in alignment with the original description of the naturally occurring murine forms of osteopetrosis, depicting osteoclast-poor (ex the tl-rat, (151, 158, 159)) and -rich forms (ex the ia-rat, (211, 212)).

Potential therapeutic aspects

From a therapeutic perspective, the studies of ADO have highlighted several new targets, all of which employ novel modes of action and could thereby provide benefits to the field of osteoporosis. The possibility to have a pure anabolic response or an anti-resorptive response without the secondary reduction in bone formation associated with presently available treatments is enticing (97).

Based on the studies of the HBM phenotype, an array of new drugs for treatment of osteoporosis is in development and several of these target the canonical Wnt signaling pathway. The antibodies against soluble Wnt inhibitors are promising in terms of the bone response, but they will need to be carefully monitored in clinical development due to the potential serious side effects.

Whole-body energy homeostasis

The role of bone as an integrated part of whole-body energy homeostasis is controversial. Both ADO and HBM have contributed directly to this discussion. Detailed metabolic studies hypothesized a positive feed-forward loop integrating bone remodeling in glucose and insulin homeostasis, exemplified with ADO as the human counterpart (65). Insulin via the insulin receptor on osteoblasts stimulates bone resorption and thereby the release of uncarboxylated osteocalcin from the bone matrix, thus in turn stimulating insulin secretion. Indeed, ADO patients had decreased uncarboxylated osteocalcin (reduced bone resorption) and reduced insulin levels (65). The extensive, however, diverging studies performed in various models have clearly challenged previous concepts in this field. However, only very few patient samples have been presented and the clinical importance remains unclear.

Declaration of interest

K Henriksen has economical interest in development of ClC-7 inhibitors. The other authors have nothing to declare.

Funding

This review did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.

References

  • 1

    Karshner RG. Osteopetrosis. American Journal of Roentgenology 1926 16 405.

  • 2

    Albers-Schonberg H. Rontgenbilder einer seltenen Knochenerkrankung. Münchener Medizinische Wochenschrift 1904 51 365.

  • 3

    Johnston CC Jr, Lavy N, Lord T, Vellios F, Merritt AD, Deiss WP Jr. Osteopetrosis. A clinical, genetic, metabolic, and morphologic study of the dominantly inherited, benign form. Medicine 1968 47 149167. (doi:10.1097/00005792-196803000-00004).

    • Search Google Scholar
    • Export Citation
  • 4

    Sato S, Zhu XL, Sly WS. Carbonic anhydrase isozymes IV and II in urinary membranes from carbonic anhydrase II-deficient patients. PNAS 1990 87 60736076. (doi:10.1073/pnas.87.16.6073).

    • Search Google Scholar
    • Export Citation
  • 5

    Sly WS, Hewett-Emmett D, Whyte MP, Yu YS, Tashian RE. Carbonic anhydrase II deficiency identified as the primary defect in the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. PNAS 1983 80 27522756. (doi:10.1073/pnas.80.9.2752).

    • Search Google Scholar
    • Export Citation
  • 6

    Sly WS, Whyte MP, Sundaram V, Tashian RE, Hewett-Emmett D, Guibaud P, Vainsel M, Baluarte HJ, Gruskin A, Al-Mosawi M. Carbonic anhydrase II deficiency in 12 families with the autosomal recessive syndrome of osteopetrosis with renal tubular acidosis and cerebral calcification. New England Journal of Medicine 1985 313 139145. (doi:10.1056/NEJM198507183130302).

    • Search Google Scholar
    • Export Citation
  • 7

    Whyte MP, Murphy WA, Fallon MD, Sly WS, Teitelbaum SL, McAlister WH, Avioli LV. Osteopetrosis, renal tubular acidosis and basal ganglia calcification in three sisters. American Journal of Medicine 1980 69 6474. (doi:10.1016/0002-9343(80)90501-X).

    • Search Google Scholar
    • Export Citation
  • 8

    Andersen PE Jr, Bollerslev J. Heterogeneity of autosomal dominant osteopetrosis. Radiology 1987 164 223225.

  • 9

    Bollerslev J, Andersen PE Jr. Radiological, biochemical and hereditary evidence of two types of autosomal dominant osteopetrosis. Bone 1988 9 713. (doi:10.1016/8756-3282(88)90021-X).

    • Search Google Scholar
    • Export Citation
  • 10

    Bollerslev J, Andersen PE Jr. Fracture patterns in two types of autosomal-dominant osteopetrosis. Acta Orthopaedica Scandinavica 1989 60 110112. (doi:10.3109/17453678909150106).

    • Search Google Scholar
    • Export Citation
  • 11

    Bollerslev J, Steiniche T, Melsen F, Mosekilde L. Structural and histomorphometric studies of iliac crest trabecular and cortical bone in autosomal dominant osteopetrosis: a study of two radiological types. Bone 1989 10 1924. (doi:10.1016/8756-3282(89)90142-7).

    • Search Google Scholar
    • Export Citation
  • 12

    Bollerslev J, Mosekilde L, Nielsen HK, Mosekilde L. Biomechanical competence of iliac crest trabecular bone in autosomal dominant osteopetrosis type I. Bone 1989 10 159164. (doi:10.1016/8756-3282(89)90048-3).

    • Search Google Scholar
    • Export Citation
  • 13

    Gram J, Antonsen S, Horder M, Bollerslev J. Elevated serum levels of creatine kinase BB in autosomal dominant osteopetrosis type II. Calcified Tissue International 1991 48 438439. (doi:10.1007/BF02556458).

    • Search Google Scholar
    • Export Citation
  • 14

    Bollerslev J. Autosomal dominant osteopetrosis: bone metabolism and epidemiological, clinical, and hormonal aspects. Endocrine Reviews 1989 10 4567. (doi:10.1210/edrv-10-1-45).

    • Search Google Scholar
    • Export Citation
  • 15

    Van Wesenbeeck L, Cleiren E, Gram J, Beals RK, Benichou O, Scopelliti D, Key L, Renton T, Bartels C, Gong Y et al. . Six novel missense mutations in the LDL receptor-related protein 5 (LRP5) gene in different conditions with an increased bone density. American Journal of Human Genetics 2003 72 763771. (doi:10.1086/368277).

    • Search Google Scholar
    • Export Citation
  • 16

    Whyte MP, Kempa LG, McAlister WH, Zhang F, Mumm S, Wenkert D. Elevated serum lactate dehydrogenase isoenzymes and aspartate transaminase distinguish Albers-Schonberg disease (Chloride Channel 7 Deficiency Osteopetrosis) among the sclerosing bone disorders. Journal of Bone and Mineral Research 2010 25 25152526. (doi:10.1002/jbmr.130).

    • Search Google Scholar
    • Export Citation
  • 17

    Del Fattore A, Peruzzi B, Rucci N, Recchia I, Cappariello A, Longo M, Fortunati D, Ballanti P, Iacobini M, Luciani M et al. . Clinical, genetic, and cellular analysis of 49 osteopetrotic patients: implications for diagnosis and treatment. Journal of Medical Genetics 2006 43 315325. (doi:10.1136/jmg.2005.036673).

    • Search Google Scholar
    • Export Citation
  • 18

    Del Fattore A, Cappariello A, Teti A. Genetics, pathogenesis and complications of osteopetrosis. Bone 2008 42 1929. (doi:10.1016/j.bone.2007.08.029).

    • Search Google Scholar
    • Export Citation
  • 19

    Benichou OD, Laredo JD, de Vernejoul MC. Type II autosomal dominant osteopetrosis (Albers-Schonberg disease): clinical and radiological manifestations in 42 patients. Bone 2000 26 8793. (doi:10.1016/S8756-3282(99)00244-6).

    • Search Google Scholar
    • Export Citation
  • 20

    Waguespack SG, Hui SL, Dimeglio LA, Econs MJ. Autosomal dominant osteopetrosis: clinical severity and natural history of 94 subjects with a chloride channel 7 gene mutation. Journal of Clinical Endocrinology and Metabolism 2007 92 771778. (doi:10.1210/jc.2006-1986).

    • Search Google Scholar
    • Export Citation
  • 21

    Frost M, Andersen T, Gossiel F, Hansen S, Bollerslev J, Van Hul W, Eastell R, Kassem M, Brixen K. Levels of serotonin, sclerostin, bone turnover markers as well as bone density and microarchitecture in patients with high-bone-mass phenotype due to a mutation in Lrp5. Journal of Bone and Mineral Research 2011 26 17211728. (doi:10.1002/jbmr.376).

    • Search Google Scholar
    • Export Citation
  • 22

    Grodum E, Gram J, Brixen K, Bollerslev J. Autosomal dominant osteopetrosis: bone mineral measurements of the entire skeleton of adults in two different subtypes. Bone 1995 16 431434.

    • Search Google Scholar
    • Export Citation
  • 23

    Brockstedt H, Bollerslev J, Melsen F, Mosekilde L. Cortical bone remodeling in autosomal dominant osteopetrosis: a study of two different phenotypes. Bone 1996 18 6772. (doi:10.1016/8756-3282(95)00424-6).

    • Search Google Scholar
    • Export Citation
  • 24

    Bollerslev J, Grontved A, Andersen PE Jr. Autosomal dominant osteopetrosis: an otoneurological investigation of the two radiological types. Laryngoscope 1988 98 411413. (doi:10.1288/00005537-198804000-00011).

    • Search Google Scholar
    • Export Citation
  • 25

    Bilezikian JP. Osteonecrosis of the jaw – do bisphosphonates pose a risk? New England Journal of Medicine 2006 355 22782281. (doi:10.1056/NEJMp068157).

    • Search Google Scholar
    • Export Citation
  • 26

    Marx RE, Sawatari Y, Fortin M, Broumand V. Bisphosphonate-induced exposed bone (osteonecrosis/osteopetrosis) of the jaws: risk factors, recognition, prevention, and treatment. Journal of Oral and Maxillofacial Surgery 2005 63 15671575. (doi:10.1016/j.joms.2005.07.010).

    • Search Google Scholar
    • Export Citation
  • 27

    Pazianas M, Abrahamsen B. Safety of bisphosphonates. Bone 2011 49 103110. (doi:10.1016/j.bone.2011.01.003).

  • 28

    Shane E, Goldring S, Christakos S, Drezner M, Eisman J, Silverman S, Pendrys D. Osteonecrosis of the jaw: more research needed. Journal of Bone and Mineral Research 2006 21 15031505. (doi:10.1359/jbmr.060712).

    • Search Google Scholar
    • Export Citation
  • 29

    Smith MR, Saad F, Coleman R, Shore N, Fizazi K, Tombal B, Miller K, Sieber P, Karsh L, Damiao R et al. . Denosumab and bone-metastasis-free survival in men with castration-resistant prostate cancer: results of a phase 3, randomised, placebo-controlled trial. Lancet 2012 379 3946. (doi:10.1016/S0140-6736(11)61226-9).

    • Search Google Scholar
    • Export Citation
  • 30

    Saad F, Brown JE, Van Poznak C, Ibrahim T, Stemmer SM, Stopeck AT, Diel IJ, Takahashi S, Shore N, Henry DH et al. . Incidence, risk factors, and outcomes of osteonecrosis of the jaw: integrated analysis from three blinded active-controlled phase III trials in cancer patients with bone metastases. Annals of Oncology 2012 23 13411347. (doi:10.1093/annonc/mdr435).

    • Search Google Scholar
    • Export Citation
  • 31

    Khosla S, Burr D, Cauley J, Dempster DW, Ebeling PR, Felsenberg D, Gagel RF, Gilsanz V, Guise T, Koka S et al. . Bisphosphonate-associated osteonecrosis of the jaw: report of a task force of the American Society for Bone and Mineral Research. Journal of Bone and Mineral Research 2007 22 14791491. (doi:10.1359/jbmr.0707onj).

    • Search Google Scholar
    • Export Citation
  • 32

    Papapoulos S, Chapurlat R, Libanati C, Brandi ML, Brown JP, Czerwinski E, Krieg MA, Man Z, Mellstrom D, Radominski SC et al. . Five years of denosumab exposure in women with postmenopausal osteoporosis: results from the first two years of the FREEDOM extension. Journal of Bone and Mineral Research 2012 27 694701. (doi:10.1002/jbmr.1479).

    • Search Google Scholar
    • Export Citation
  • 33

    Reid IR, Cornish J. Epidemiology and pathogenesis of osteonecrosis of the jaw. Nature Reviews. Rheumatology 2012 8 9096.

  • 34

    Bedi RS, Goel P, Pasricha N, Sachin , Goel A. Osteopetrosis – a rare entity with osteomyelitis. Annals of Maxillofacial Surgery 2011 1 155159. (doi:10.4103/2231-0746.92783).

    • Search Google Scholar
    • Export Citation
  • 35

    Podenphant J. Methodological problems in bone histomorphometry and its application in postmenopausal osteoporosis. Danish Medical Bulletin 1990 37 424433.

    • Search Google Scholar
    • Export Citation
  • 36

    Hasling C, Eriksen EF, Charles P, Mosekilde L. Exogenous triiodothyronine activates bone remodeling. Bone 1987 8 6569. (doi:10.1016/8756-3282(87)90072-X).

    • Search Google Scholar
    • Export Citation
  • 37

    Bollerslev J, Thomas S, Grodum E, Brixen K, Djoseland O. Collagen metabolism in two types of autosomal dominant osteopetrosis during stimulation with thyroid hormones. European Journal of Endocrinology 1995 133 557563. (doi:10.1530/eje.0.1330557).

    • Search Google Scholar
    • Export Citation
  • 38

    Bollerslev J, Ueland T, Grodum E, Haug E, Brixen K, Djoseland O. Biochemical markers of bone metabolism in benign human osteopetrosis: a study of two types at baseline and during stimulation with triiodothyronine. European Journal of Endocrinology 1998 139 2935. (doi:10.1530/eje.0.1390029).

    • Search Google Scholar
    • Export Citation
  • 39

    Bollerslev J, Ueland T, Odgren PR. Serum levels of TGF-β and fibronectin in autosomal dominant osteopetrosis in relation to underlying mutations and well-described murine counterparts. Critical Reviews in Eukaryotic Gene Expression 2003 13 163171. (doi:10.1615/CritRevEukaryotGeneExpr.v13.i24.90).

    • Search Google Scholar
    • Export Citation
  • 40

    Henriksen K, Gram J, Hoegh-Andersen P, Jemtland R, Ueland T, Dziegiel MH, Schaller S, Bollerslev J, Karsdal MA. Osteoclasts from patients with autosomal dominant osteopetrosis type I caused by a T253I mutation in low-density lipoprotein receptor-related protein 5 are normal in vitro, but have decreased resorption capacity in vivo. American Journal of Pathology 2005 167 13411348. (doi:10.1016/S0002-9440(10)61221-7).

    • Search Google Scholar
    • Export Citation
  • 41

    Bollerslev J, Marks SC Jr, Pockwinse S, Kassem M, Brixen K, Steiniche T, Mosekilde L. Ultrastructural investigations of bone resorptive cells in two types of autosomal dominant osteopetrosis. Bone 1993 14 865869. (doi:10.1016/8756-3282(93)90316-3).

    • Search Google Scholar
    • Export Citation
  • 42

    Alatalo SL, Ivaska KK, Waguespack SG, Econs MJ, Vaananen HK, Halleen JM. Osteoclast-derived serum tartrate-resistant acid phosphatase 5b in Albers-Schonberg disease (type II autosomal dominant osteopetrosis). Clinical Chemistry 2004 50 883890. (doi:10.1373/clinchem.2003.029355).

    • Search Google Scholar
    • Export Citation
  • 43

    Bollerslev J, Ueland T, Landaas S, Marks SC Jr. Serum creatine kinase isoenzyme BB in mammalian osteopetrosis. Clinical Orthopaedics and Related Research 2000 377 241247. (doi:10.1097/00003086-200008000-00032).

    • Search Google Scholar
    • Export Citation
  • 44

    Waguespack SG, Hui SL, White KE, Buckwalter KA, Econs MJ. Measurement of tartrate-resistant acid phosphatase and the brain isoenzyme of creatine kinase accurately diagnoses type II autosomal dominant osteopetrosis but does not identify gene carriers. Journal of Clinical Endocrinology and Metabolism 2002 87 22122217. (doi:10.1210/jc.87.5.2212).

    • Search Google Scholar
    • Export Citation
  • 45

    Chalhoub N, Benachenhou N, Rajapurohitam V, Pata M, Ferron M, Frattini A, Villa A, Vacher J. Grey-lethal mutation induces severe malignant autosomal recessive osteopetrosis in mouse and human. Nature Medicine 2003 9 399406. (doi:10.1038/nm842).

    • Search Google Scholar
    • Export Citation
  • 46

    Wicking C, Williamson B. From linked marker to gene. Trends in Genetics 1991 7 288293.

  • 47

    Balemans W, Van Wesenbeeck L, Van Hul W. A clinical and molecular overview of the human osteopetroses. Calcified Tissue International 2005 77 263274. (doi:10.1007/s00223-005-0027-6).

    • Search Google Scholar
    • Export Citation
  • 48

    Benichou O, Cleiren E, Gram J, Bollerslev J, de Vernejoul MC, Van Hul W. Mapping of autosomal dominant osteopetrosis type II (Albers-Schonberg disease) to chromosome 16p13.3. American Journal of Human Genetics 2001 69 647654. (doi:10.1086/323132).

    • Search Google Scholar
    • Export Citation
  • 49

    Kornak U, Kasper D, Bosl MR, Kaiser E, Schweizer M, Schulz A, Friedrich W, Delling G, Jentsch TJ. Loss of the ClC-7 chloride channel leads to osteopetrosis in mice and man. Cell 2001 104 205215. (doi:10.1016/S0092-8674(01)00206-9).

    • Search Google Scholar
    • Export Citation
  • 50

    Cleiren E, Benichou O, Van Hul E, Gram J, Bollerslev J, Singer FR, Beaverson K, Aledo A, Whyte MP, Yoneyama T et al. . Albers-Schonberg disease (autosomal dominant osteopetrosis, type II) results from mutations in the ClCN7 chloride channel gene. Human Molecular Genetics 2001 10 28612867. (doi:10.1093/hmg/10.25.2861).

    • Search Google Scholar
    • Export Citation
  • 51

    Leisle L, Ludwig CF, Wagner FA, Jentsch TJ, Stauber T. ClC-7 is a slowly voltage-gated 2Cl(−)/1H(+)-exchanger and requires Ostm1 for transport activity. EMBO Journal 2011 30 21402152. (doi:10.1038/emboj.2011.137).

    • Search Google Scholar
    • Export Citation
  • 52

    Henriksen K, Gram J, Schaller S, Dahl BH, Dziegiel MH, Bollerslev J, Karsdal MA. Characterization of osteoclasts from patients harboring a G215R mutation in ClC-7 causing autosomal dominant osteopetrosis type II. American Journal of Pathology 2004 164 15371545. (doi:10.1016/S0002-9440(10)63712-1).

    • Search Google Scholar
    • Export Citation
  • 53

    Henriksen K, Gram J, Neutzsky-Wulff AV, Jensen VK, Dziegiel MH, Bollerslev J, Karsdal MA. Characterization of acid flux in osteoclasts from patients harboring a G215R mutation in ClC-7. Biochemical and Biophysical Research Communications 2009 378 804809. (doi:10.1016/j.bbrc.2008.11.145).

    • Search Google Scholar
    • Export Citation
  • 54

    Schulz P, Werner J, Stauber T, Henriksen K, Fendler K. The G215R mutation in the Cl−/H+-antiporter ClC-7 found in ADO II osteopetrosis does not abolish function but causes a severe trafficking defect. PLoS ONE 2010 5 e12585. (doi:10.1371/journal.pone.0012585).

    • Search Google Scholar
    • Export Citation
  • 55

    Chu K, Snyder R, Econs MJ. Disease status in autosomal dominant osteopetrosis type 2 is determined by osteoclastic properties. Journal of Bone and Mineral Research 2006 21 10891097. (doi:10.1359/jbmr.060409).

    • Search Google Scholar
    • Export Citation
  • 56

    Frattini A, Pangrazio A, Susani L, Sobacchi C, Mirolo M, Abinun M, Andolina M, Flanagan A, Horwitz EM, Mihci E et al. . Chloride channel ClCN7 mutations are responsible for severe recessive, dominant, and intermediate osteopetrosis. Journal of Bone and Mineral Research 2003 18 17401747. (doi:10.1359/jbmr.2003.18.10.1740).

    • Search Google Scholar
    • Export Citation
  • 57

    Bollerslev J. Osteopetrosis. A genetic and epidemiological study. Clinical Genetics 1987 31 8690. (doi:10.1111/j.1399-0004.1987.tb02774.x).

    • Search Google Scholar
    • Export Citation
  • 58

    Waguespack SG, Koller DL, White KE, Fishburn T, Carn G, Buckwalter KA, Johnson M, Kocisko M, Evans WE, Foroud T et al. . Chloride channel 7 (ClCN7) gene mutations and autosomal dominant osteopetrosis, type II. Journal of Bone and Mineral Research 2003 18 15131518. (doi:10.1359/jbmr.2003.18.8.1513).

    • Search Google Scholar
    • Export Citation
  • 59

    Campos-Xavier AB, Casanova JL, Doumaz Y, Feingold J, Munnich A, Cormier-Daire V. Intrafamilial phenotypic variability of osteopetrosis due to chloride channel 7 (CLCN7) mutations. American Journal of Medical Genetics. Part A 133A 2005 216218. (doi:10.1002/ajmg.a.30490).

    • Search Google Scholar
    • Export Citation
  • 60

    Chu K, Koller DL, Snyder R, Fishburn T, Lai D, Waguespack SG, Foroud T, Econs MJ. Analysis of variation in expression of autosomal dominant osteopetrosis type 2: searching for modifier genes. Bone 2005 37 655661. (doi:10.1016/j.bone.2005.06.003).

    • Search Google Scholar
    • Export Citation
  • 61

    Kornak U, Ostertag A, Branger S, Benichou O, de Vernejoul MC. Polymorphisms in the CLCN7 gene modulate bone density in postmenopausal women and in patients with autosomal dominant osteopetrosis type II. Journal of Clinical Endocrinology and Metabolism 2006 91 9951000. (doi:10.1210/jc.2005-2017).

    • Search Google Scholar
    • Export Citation
  • 62

    Pettersson U, Albagha OM, Mirolo M, Taranta A, Frattini A, McGuigan FE, Vezzoni P, Teti A, Van Hul W, Reid DM et al. . Polymorphisms of the CLCN7 gene are associated with BMD in women. Journal of Bone and Mineral Research 2005 20 19601967. (doi:10.1359/JBMR.050717).

    • Search Google Scholar
    • Export Citation
  • 63

    Karsdal MA, Henriksen K, Sorensen MG, Gram J, Schaller S, Dziegiel MH, Heegaard AM, Christophersen P, Martin TJ, Christiansen C et al. . Acidification of the osteoclastic resorption compartment provides insight into the coupling of bone formation to bone resorption. American Journal of Pathology 2005 166 467476. (doi:10.1016/S0002-9440(10)62269-9).

    • Search Google Scholar
    • Export Citation
  • 64

    Taranta A, Migliaccio S, Recchia I, Caniglia M, Luciani M, De Rossi G, Dionisi-Vici C, Pinto RM, Francalanci P, Boldrini R et al. . Genotype–phenotype relationship in human ATP6i-dependent autosomal recessive osteopetrosis. American Journal of Pathology 2003 162 5768. (doi:10.1016/S0002-9440(10)63798-4).

    • Search Google Scholar
    • Export Citation
  • 65

    Ferron M, Wei J, Yoshizawa T, Del Fattore A, DePinho RA, Teti A, Ducy P, Karsenty G. Insulin signaling in osteoblasts integrates bone remodeling and energy metabolism. Cell 2010 142 296308. (doi:10.1016/j.cell.2010.06.003).

    • Search Google Scholar
    • Export Citation
  • 66

    Nielsen RH, Karsdal MA, Sorensen MG, Dziegiel MH, Henriksen K. Dissolution of the inorganic phase of bone leading to release of calcium regulates osteoclast survival. Biochemical and Biophysical Research Communications 2007 360 834839. (doi:10.1016/j.bbrc.2007.06.145).

    • Search Google Scholar
    • Export Citation
  • 67

    Houde N, Chamoux E, Bisson M, Roux S. Transforming growth factor-β1 (TGF-β1) induces human osteoclast apoptosis by up-regulating Bim. Journal of Biological Chemistry 2009 284 2339723404. (doi:10.1074/jbc.M109.019372).

    • Search Google Scholar
    • Export Citation
  • 68

    Bollerslev J, Nielsen HK, Larsen HF, Mosekilde L. Biochemical evidence of disturbed bone metabolism and calcium homeostasis in two types of autosomal dominant osteopetrosis. Acta Medica Scandinavica 1988 224 479483. (doi:10.1111/j.0954-6820.1988.tb19614.x).

    • Search Google Scholar
    • Export Citation
  • 69

    Henriksen K, Andreassen KV, Thudium CS, Gudmann KN, Moscatelli I, Cruger-Hansen CE, Schulz AS, Dziegiel MH, Richter J, Karsdal MA et al. . A specific subtype of osteoclasts secretes factors inducing nodule formation by osteoblasts. Bone 2012 51 353361. (doi:10.1016/j.bone.2012.06.007).

    • Search Google Scholar
    • Export Citation
  • 70

    Henriksen K, Flores C, Thomsen JS, Bruel AM, Thudium CS, Neutzsky-Wulff AV, Langenbach GE, Sims N, Askmyr M, Martin TJ et al. . Dissociation of bone resorption and bone formation in adult mice with a non-functional V-ATPase in osteoclasts leads to increased bone strength. PLoS ONE 2011 6 e27482. (doi:10.1371/journal.pone.0027482).

    • Search Google Scholar
    • Export Citation
  • 71

    Marzia M, Sims NA, Voit S, Migliaccio S, Taranta A, Bernardini S, Faraggiana T, Yoneda T, Mundy GR, Boyce BF et al. . Decreased c-Src expression enhances osteoblast differentiation and bone formation. Journal of Cell Biology 2000 151 311320. (doi:10.1083/jcb.151.2.311).

    • Search Google Scholar
    • Export Citation
  • 72

    Rzeszutek K, Sarraf F, Davies JE. Proton pump inhibitors control osteoclastic resorption of calcium phosphate implants and stimulate increased local reparative bone growth. Journal of Craniofacial Surgery 2003 14 301307. (doi:10.1097/00001665-200305000-00007).

    • Search Google Scholar
    • Export Citation
  • 73

    Schaller S, Henriksen K, Sveigaard C, Heegaard AM, Helix N, Stahlhut M, Ovejero MC, Johansen JV, Solberg H, Andersen TL et al. . The chloride channel inhibitor NS3736 [corrected] prevents bone resorption in ovariectomized rats without changing bone formation. Journal of Bone and Mineral Research 2004 19 11441153. (doi:10.1359/JBMR.040302).

    • Search Google Scholar
    • Export Citation
  • 74

    Visentin L, Dodds RA, Valente M, Misiano P, Bradbeer JN, Oneta S, Liang X, Gowen M, Farina C. A selective inhibitor of the osteoclastic V-H(+)-ATPase prevents bone loss in both thyroparathyroidectomized and ovariectomized rats. Journal of Clinical Investigation 2000 106 309318. (doi:10.1172/JCI6145).

    • Search Google Scholar
    • Export Citation
  • 75

    Karsdal MA, Neutzsky-Wulff AV, Dziegiel MH, Christiansen C, Henriksen K. Osteoclasts secrete non-bone derived signals that induce bone formation. Biochemical and Biophysical Research Communications 2008 366 483488. (doi:10.1016/j.bbrc.2007.11.168).

    • Search Google Scholar
    • Export Citation
  • 76

    Pederson L, Ruan M, Westendorf JJ, Khosla S, Oursler MJ. Regulation of bone formation by osteoclasts involves Wnt/BMP signaling and the chemokine sphingosine-1-phosphate. PNAS 2008 105 2076420769. (doi:10.1073/pnas.0805133106).

    • Search Google Scholar
    • Export Citation
  • 77

    Zhao C, Irie N, Takada Y, Shimoda K, Miyamoto T, Nishiwaki T, Suda T, Matsuo K. Bidirectional ephrinB2–EphB4 signaling controls bone homeostasis. Cell Metabolism 2006 4 111121. (doi:10.1016/j.cmet.2006.05.012).

    • Search Google Scholar
    • Export Citation
  • 78

    Guerrini MM, Sobacchi C, Cassani B, Abinun M, Kilic SS, Pangrazio A, Moratto D, Mazzolari E, Clayton-Smith J, Orchard P et al. . Human osteoclast-poor osteopetrosis with hypogammaglobulinemia due to TNFRSF11A (RANK) mutations. American Journal of Human Genetics 2008 83 6476. (doi:10.1016/j.ajhg.2008.06.015).

    • Search Google Scholar
    • Export Citation
  • 79

    Fuller K, Lawrence KM, Ross JL, Grabowska UB, Shiroo M, Samuelsson B, Chambers TJ. Cathepsin K inhibitors prevent matrix-derived growth factor degradation by human osteoclasts. Bone 2008 42 200211. (doi:10.1016/j.bone.2007.09.044).

    • Search Google Scholar
    • Export Citation
  • 80

    Hayden JM, Mohan S, Baylink DJ. The insulin-like growth factor system and the coupling of formation to resorption. Bone 1995 17 93S98S. (doi:10.1016/8756-3282(95)00186-H).

    • Search Google Scholar
    • Export Citation
  • 81

    Karsdal MA, Martin TJ, Bollerslev J, Christiansen C, Henriksen K. Are nonresorbing osteoclasts sources of bone anabolic activity? Journal of Bone and Mineral Research 2007 22 487494. (doi:10.1359/jbmr.070109).

    • Search Google Scholar
    • Export Citation
  • 82

    Jones SJ, Gray C, Boyde A. Simulation of bone resorption–repair coupling in vitro. Anatomy and Embryology 1994 190 339349. (doi:10.1007/BF00187292).

    • Search Google Scholar
    • Export Citation
  • 83

    Schwartz Z, Lohmann CH, Wieland M, Cochran DL, Dean DD, Textor M, Bonewald LF, Boyan BD. Osteoblast proliferation and differentiation on dentin slices are modulated by pretreatment of the surface with tetracycline or osteoclasts. Journal of Periodontology 2000 71 586597. (doi:10.1902/jop.2000.71.4.586).

    • Search Google Scholar
    • Export Citation
  • 84

    Sheu TJ, Schwarz EM, O'Keefe RJ, Rosier RN, Puzas JE. Use of a phage display technique to identify potential osteoblast binding sites within osteoclast lacunae. Journal of Bone and Mineral Research 2002 17 915922. (doi:10.1359/jbmr.2002.17.5.915).

    • Search Google Scholar
    • Export Citation
  • 85

    Neutzsky-Wulff AV, Karsdal MA, Henriksen K. Characterization of the bone phenotype in ClC-7-deficient mice. Calcified Tissue International 2008 83 425437. (doi:10.1007/s00223-008-9185-7).

    • Search Google Scholar
    • Export Citation
  • 86

    Neutzsky-Wulff AV, Sims NA, Supanchart C, Kornak U, Felsenberg D, Poulton IJ, Martin TJ, Karsdal MA, Henriksen K. Severe developmental bone phenotype in ClC-7 deficient mice. Developmental Biology 2010 344 10011010. (doi:10.1016/j.ydbio.2010.06.018).

    • Search Google Scholar
    • Export Citation
  • 87

    Walker DG. Osteopetrosis cured by temporary parabiosis. Science 1973 180 875. (doi:10.1126/science.180.4088.875).

  • 88

    Coccia PF, Krivit W, Cervenka J, Clawson C, Kersey JH, Kim TH, Nesbit ME, Ramsay NK, Warkentin PI, Teitelbaum SL et al. . Successful bone-marrow transplantation for infantile malignant osteopetrosis. New England Journal of Medicine 1980 302 701708. (doi:10.1056/NEJM198003273021301).

    • Search Google Scholar
    • Export Citation
  • 89

    Sieff CA, Chessells JM, Levinsky RJ, Pritchard J, Rogers DW, Casey A, Muller K, Hall CM. Allogeneic bone-marrow transplantation in infantile malignant osteopetrosis. Lancet 1983 1 437441. (doi:10.1016/S0140-6736(83)91438-1).

    • Search Google Scholar
    • Export Citation
  • 90

    Steward CG. Hematopoietic stem cell transplantation for osteopetrosis. Pediatric Clinics of North America 2010 57 171180. (doi:10.1016/j.pcl.2009.11.006).

    • Search Google Scholar
    • Export Citation
  • 91

    Pangrazio A, Pusch M, Caldana E, Frattini A, Lanino E, Tamhankar PM, Phadke S, Lopez AG, Orchard P, Mihci E et al. . Molecular and clinical heterogeneity in CLCN7-dependent osteopetrosis: report of 20 novel mutations. Human Mutation 2010 31 E1071E1080. (doi:10.1002/humu.21167).

    • Search Google Scholar
    • Export Citation
  • 92

    Key L, Carnes D, Cole S, Holtrop M, Bar-Shavit Z, Shapiro F, Arceci R, Steinberg J, Gundberg C, Kahn A. Treatment of congenital osteopetrosis with high-dose calcitriol. New England Journal of Medicine 1984 310 409415. (doi:10.1056/NEJM198402163100701).

    • Search Google Scholar
    • Export Citation
  • 93

    Key LL Jr, Rodriguiz RM, Willi SM, Wright NM, Hatcher HC, Eyre DR, Cure JK, Griffin PP, Ries WL. Long-term treatment of osteopetrosis with recombinant human interferon γ. New England Journal of Medicine 1995 332 15941599. (doi:10.1056/NEJM199506153322402).

    • Search Google Scholar
    • Export Citation
  • 94

    Lo IN, Blair HC, Poliani PL, Marrella V, Ficara F, Cassani B, Facchetti F, Fontana E, Guerrini MM, Traggiai E et al. . Osteopetrosis rescue upon RANKL administration to Rankl(−/−) mice: a new therapy for human RANKL-dependent ARO. Journal of Bone and Mineral Research 2012 27 25012510. (doi:10.1002/jbmr.1712).

    • Search Google Scholar
    • Export Citation
  • 95

    Del Fattore A, Capannalo M, Rucci N, Teti A. New siRNA-based therapy for autosomal dominant osteopetrosis. Journal of Bone and Mineral Research 2010 25 (suppl 1) abstract # 150 doi:10.1002/jbmr565025130325).

    • Search Google Scholar
    • Export Citation
  • 96

    Henriksen K, Neutzsky-Wulff AV, Bonewald LF, Karsdal MA. Local communication on and within bone controls bone remodeling. Bone 2009 44 10261033. (doi:10.1016/j.bone.2009.03.671).

    • Search Google Scholar
    • Export Citation
  • 97

    Henriksen K, Bollerslev J, Everts V, Karsdal MA. Osteoclast activity and subtypes as a function of physiology and pathology – implications for future treatments of osteoporosis. Endocrine Reviews 2011 32 3163. (doi:10.1210/er.2010-0006).

    • Search Google Scholar
    • Export Citation
  • 98

    Tolar J, Teitelbaum SL, Orchard PJ. Osteopetrosis. New England Journal of Medicine 2004 351 28392849. (doi:10.1056/NEJMra040952).

  • 99

    Kasper D, Planells-Cases R, Fuhrmann JC, Scheel O, Zeitz O, Ruether K, Schmitt A, Poet M, Steinfeld R, Schweizer M et al. . Loss of the chloride channel ClC-7 leads to lysosomal storage disease and neurodegeneration. EMBO Journal 2005 24 10791091. (doi:10.1038/sj.emboj.7600576).

    • Search Google Scholar
    • Export Citation
  • 100

    Thudium CS, Jensen VK, Karsdal MA, Henriksen K. Disruption of the V-ATPase functionality as a way to uncouple bone formation and resorption – a novel target for treatment of osteoporosis. Current Protein & Peptide Science 2012 13 141151. (doi:10.2174/138920312800493133).

    • Search Google Scholar
    • Export Citation
  • 101

    Bone HG, McClung MR, Roux C, Recker RR, Eisman JA, Verbruggen N, Hustad CM, DaSilva C, Santora AC, Ince BA. Odanacatib, a cathepsin-K inhibitor for osteoporosis: a two-year study in postmenopausal women with low bone density. Journal of Bone and Mineral Research 2010 25 937947.

    • Search Google Scholar
    • Export Citation
  • 102

    Brixen K, Chapurlat R, Cheung AM, Keaveny TM, Fuerst T, Engelke K, Recker R, Dardzinski B, Verbruggen N, Ather S et al. . Bone density, turnover, and estimated strength in postmenopausal women treated with odanacatib: a randomized trial. Journal of Clinical Endocrinology and Metabolism 2013 98 571580. (doi:10.1210/jc.2012-2972).

    • Search Google Scholar
    • Export Citation
  • 103

    Eastell R, Walsh JS, Watts NB, Siris E. Bisphosphonates for postmenopausal osteoporosis. Bone 2011 49 8288. (doi:10.1016/j.bone.2011.02.011).

  • 104

    Eisman JA, Bone HG, Hosking DJ, McClung MR, Reid IR, Rizzoli R, Resch H, Verbruggen N, Hustad CM, DaSilva C et al. . Odanacatib in the treatment of postmenopausal women with low bone mineral density: three-year continued therapy and resolution of effect. Journal of Bone and Mineral Research 2011 26 242251. (doi:10.1002/jbmr.212).

    • Search Google Scholar
    • Export Citation
  • 105

    Pennypacker B, Shea M, Liu Q, Masarachia P, Saftig P, Rodan S, Rodan G, Kimmel D. Bone density, strength, and formation in adult cathepsin K (−/−) mice. Bone 2009 44 199207. (doi:10.1016/j.bone.2008.08.130).

    • Search Google Scholar
    • Export Citation
  • 106

    Van Hul E, Gram J, Bollerslev J, Van Wesenbeeck L, Mathysen D, Andersen PE, Vanhoenacker F, Van Hul W. Localization of the gene causing autosomal dominant osteopetrosis type I to chromosome 11q12–13. Journal of Bone and Mineral Research 2002 17 11111117. (doi:10.1359/jbmr.2002.17.6.1111).

    • Search Google Scholar
    • Export Citation
  • 107

    Frattini A, Orchard PJ, Sobacchi C, Giliani S, Abinun M, Mattsson JP, Keeling DJ, Andersson AK, Wallbrandt P, Zecca L et al. . Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a subset of human autosomal recessive osteopetrosis. Nature Genetics 2000 25 343346. (doi:10.1038/77131).

    • Search Google Scholar
    • Export Citation
  • 108

    Gong Y, Slee RB, Fukai N, Rawadi G, Roman-Roman S, Reginato AM, Wang H, Cundy T, Glorieux FH, Lev D et al. . LDL receptor-related protein 5 (LRP5) affects bone accrual and eye development. Cell 2001 107 513523. (doi:10.1016/S0092-8674(01)00571-2).

    • Search Google Scholar
    • Export Citation
  • 109

    Boyden LM, Mao J, Belsky J, Mitzner L, Farhi A, Mitnick MA, Wu D, Insogna K, Lifton RP. High bone density due to a mutation in LDL-receptor-related protein 5. New England Journal of Medicine 2002 346 15131521. (doi:10.1056/NEJMoa013444).

    • Search Google Scholar
    • Export Citation
  • 110

    Little RD, Carulli JP, Del Mastro RG, Dupuis J, Osborne M, Folz C, Manning SP, Swain PM, Zhao SC, Eustace B et al. . A mutation in the LDL receptor-related protein 5 gene results in the autosomal dominant high-bone-mass trait. American Journal of Human Genetics 2002 70 1119. (doi:10.1086/338450).

    • Search Google Scholar
    • Export Citation
  • 111

    Johnson ML, Harnish K, Nusse R, Van Hul W. LRP5 and Wnt signaling: a union made for bone. Journal of Bone and Mineral Research 2004 19 17491757. (doi:10.1359/JBMR.040816).

    • Search Google Scholar
    • Export Citation
  • 112

    Piters E, Boudin E, Van Hul W. Wnt signaling: a win for bone. Archives of Biochemistry and Biophysics 2008 473 112116. (doi:10.1016/j.abb.2008.03.006).

    • Search Google Scholar
    • Export Citation
  • 113

    Balemans W, Devogelaer JP, Cleiren E, Piters E, Caussin E, Van Hul W. Novel LRP5 missense mutation in a patient with a high bone mass phenotype results in decreased DKK1-mediated inhibition of Wnt signaling. Journal of Bone and Mineral Research 2007 22 708716. (doi:10.1359/jbmr.070211).

    • Search Google Scholar
    • Export Citation
  • 114

    Pangrazio A, Boudin E, Piters E, Damante G, Lo Iacono N, D'Elia AV, Vezzoni P, Van Hul W, Villa A, Sobacchi C. Identification of the first deletion in the LRP5 gene in a patient with autosomal dominant osteopetrosis type I. Bone 2011 49 568571. (doi:10.1016/j.bone.2011.05.006).

    • Search Google Scholar
    • Export Citation
  • 115

    Whyte MP, Reinus WH, Mumm S. High-bone-mass disease and LRP5. New England Journal of Medicine 2004 350 20962099. (doi:10.1056/NEJM200405133502017).

    • Search Google Scholar
    • Export Citation
  • 116

    Bollerslev J, Wilson SG, Dick IM, Islam FM, Ueland T, Palmer L, Devine A, Prince RL. LRP5 gene polymorphisms predict bone mass and incident fractures in elderly Australian women. Bone 2005 36 599606. (doi:10.1016/j.bone.2005.01.006).

    • Search Google Scholar
    • Export Citation
  • 117

    van Meurs JB, Trikalinos TA, Ralston SH, Balcells S, Brandi ML, Brixen K, Kiel DP, Langdahl BL, Lips P, Ljunggren O et al. . Large-scale analysis of association between LRP5 and LRP6 variants and osteoporosis. Journal of the American Medical Association 2008 299 12771290. (doi:10.1001/jama.299.11.1277).

    • Search Google Scholar
    • Export Citation
  • 118

    Monroe DG, McGee-Lawrence ME, Oursler MJ, Westendorf JJ. Update on Wnt signaling in bone cell biology and bone disease. Gene 2012 492 118. (doi:10.1016/j.gene.2011.10.044).

    • Search Google Scholar
    • Export Citation
  • 119

    Bhat BM, Allen KM, Liu W, Graham J, Morales A, Anisowicz A, Lam HS, McCauley C, Coleburn V, Cain M et al. . Structure-based mutation analysis shows the importance of LRP5β-propeller 1 in modulating Dkk1-mediated inhibition of Wnt signaling. Gene 2007 391 103112. (doi:10.1016/j.gene.2006.12.014).

    • Search Google Scholar
    • Export Citation
  • 120

    Ai M, Holmen SL, Van Hul W, Williams BO, Warman ML. Reduced affinity to and inhibition by DKK1 form a common mechanism by which high bone mass-associated missense mutations in LRP5 affect canonical Wnt signaling. Molecular and Cellular Biology 2005 25 49464955. (doi:10.1128/MCB.25.12.4946-4955.2005).

    • Search Google Scholar
    • Export Citation
  • 121

    Balemans W, Piters E, Cleiren E, Ai M, Van Wesenbeeck L, Warman ML, Van Hul W. The binding between sclerostin and LRP5 is altered by DKK1 and by high-bone mass LRP5 mutations. Calcified Tissue International 2008 82 445453. (doi:10.1007/s00223-008-9130-9).

    • Search Google Scholar
    • Export Citation
  • 122

    Semenov MV, He X. LRP5 mutations linked to high bone mass diseases cause reduced LRP5 binding and inhibition by SOST. Journal of Biological Chemistry 2006 281 3827638284. (doi:10.1074/jbc.M609509200).

    • Search Google Scholar
    • Export Citation
  • 123

    Bourhis E, Wang W, Tam C, Hwang J, Zhang Y, Spittler D, Huang OW, Gong Y, Estevez A, Zilberleyb I et al. . Wnt antagonists bind through a short peptide to the first β-propeller domain of LRP5/6. Structure 2011 19 14331442. (doi:10.1016/j.str.2011.07.005).

    • Search Google Scholar
    • Export Citation
  • 124

    Balemans W, Van Den Ende J, Freire Paes-Alves A, Dikkers FG, Willems PJ, Vanhoenacker F, de Almeida-Melo N, Alves CF, Stratakis CA, Hill SC et al. . Localization of the gene for sclerosteosis to the van Buchem disease-gene region on chromosome 17q12–q21. American Journal of Human Genetics 1999 64 16611669. (doi:10.1086/302416).

    • Search Google Scholar
    • Export Citation
  • 125

    Balemans W, Ebeling M, Patel N, Van Hul E, Olson P, Dioszegi M, Lacza C, Wuyts W, Van Den Ende J, Willems P et al. . Increased bone density in sclerosteosis is due to the deficiency of a novel secreted protein (SOST). Human Molecular Genetics 2001 10 537543. (doi:10.1093/hmg/10.5.537).

    • Search Google Scholar
    • Export Citation
  • 126

    Brunkow ME, Gardner JC, Van Ness J, Paeper BW, Kovacevich BR, Proll S, Skonier JE, Zhao L, Sabo PJ, Fu Y et al. . Bone dysplasia sclerosteosis results from loss of the SOST gene product, a novel cystine knot-containing protein. American Journal of Human Genetics 2001 68 577589. (doi:10.1086/318811).

    • Search Google Scholar
    • Export Citation
  • 127

    Van Hul W, Balemans W, Van Hul E, Dikkers FG, Obee H, Stokroos RJ, Hildering P, Vanhoenacker F, Van Camp G, Willems PJ. Van Buchem disease (hyperostosis corticalis generalisata) maps to chromosome 17q12–q21. American Journal of Human Genetics 1998 62 391399. (doi:10.1086/301721).

    • Search Google Scholar
    • Export Citation
  • 128

    Beighton P, Davidson J, Durr L, Hamersma H. Sclerosteosis – an autosomal recessive disorder. Clinical Genetics 1977 11 17. (doi:10.1111/j.1399-0004.1977.tb01269.x).

    • Search Google Scholar
    • Export Citation
  • 129

    Van Buchem FS, Hadders HN, Ubbens R. An uncommon familial systemic disease of the skeleton: hyperostosis corticalis generalisata familiaris. Acta Radiologica 1955 44 109120. (doi:10.3109/00016925509170789).

    • Search Google Scholar
    • Export Citation
  • 130

    Niziolek PJ, Farmer TL, Cui Y, Turner CH, Warman ML, Robling AG. High-bone-mass-producing mutations in the Wnt signaling pathway result in distinct skeletal phenotypes. Bone 2011 49 10101019. (doi:10.1016/j.bone.2011.07.034).

    • Search Google Scholar
    • Export Citation
  • 131

    Westendorf JJ, Kahler RA, Schroeder TM. Wnt signaling in osteoblasts and bone diseases. Gene 2004 341 1939. (doi:10.1016/j.gene.2004.06.044).

  • 132

    Jenkins ZA, van Kogelenberg M, Morgan T, Jeffs A, Fukuzawa R, Pearl E, Thaller C, Hing AV, Porteous ME, Garcia-Minaur S et al. . Germline mutations in WTX cause a sclerosing skeletal dysplasia but do not predispose to tumorigenesis. Nature Genetics 2009 41 95100. (doi:10.1038/ng.270).

    • Search Google Scholar
    • Export Citation
  • 133

    Perdu B, Lakeman P, Mortier G, Koenig R, Lachmeijer A, Van Hul W. Two novel WTX mutations underscore the unpredictability of male survival in osteopathia striata with cranial sclerosis. Clinical Genetics 2011 80 383388. (doi:10.1111/j.1399-0004.2010.01553.x).

    • Search Google Scholar
    • Export Citation
  • 134

    Perdu B, de Freitas F, Frints SG, Schouten M, Schrander-Stumpel C, Barbosa M, Pinto-Basto J, Reis-Lima M, de Vernejoul MC, Becker K et al. . Osteopathia striata with cranial sclerosis owing to WTX gene defect. Journal of Bone and Mineral Research 2010 25 8290. (doi:10.1359/jbmr.090707).

    • Search Google Scholar
    • Export Citation
  • 135

    Mani A, Radhakrishnan J, Wang H, Mani A, Mani MA, Nelson-Williams C, Carew KS, Mane S, Najmabadi H, Wu D et al. . LRP6 mutation in a family with early coronary disease and metabolic risk factors. Science 2007 315 12781282. (doi:10.1126/science.1136370).

    • Search Google Scholar
    • Export Citation
  • 136

    Wang YK, Sporle R, Paperna T, Schughart K, Francke U. Characterization and expression pattern of the frizzled gene Fzd9, the mouse homolog of FZD9 which is deleted in Williams–Beuren syndrome. Genomics 1999 57 235248. (doi:10.1006/geno.1999.5773).

    • Search Google Scholar
    • Export Citation
  • 137

    Glass DA, Bialek P, Ahn JD, Starbuck M, Patel MS, Clevers H, Taketo MM, Long F, McMahon AP, Lang RA et al. . Canonical Wnt signaling in differentiated osteoblasts controls osteoclast differentiation. Developmental Cell 2005 8 751764. (doi:10.1016/j.devcel.2005.02.017).

    • Search Google Scholar
    • Export Citation
  • 138

    Kubota T, Michigami T, Ozono K. Wnt signaling in bone metabolism. Journal of Bone and Mineral Metabolism 2009 27 265271. (doi:10.1007/s00774-009-0064-8).

    • Search Google Scholar
    • Export Citation
  • 139

    Li HX, Luo X, Liu RX, Yang YJ, Yang GS. Roles of Wnt/β-catenin signaling in adipogenic differentiation potential of adipose-derived mesenchymal stem cells. Molecular and Cellular Endocrinology 2008 291 116124. (doi:10.1016/j.mce.2008.05.005).

    • Search Google Scholar
    • Export Citation
  • 140

    Qiu W, Andersen TE, Bollerslev J, Mandrup S, Abdallah BM, Kassem M. Patients with high bone mass phenotype exhibit enhanced osteoblast differentiation and inhibition of adipogenesis of human mesenchymal stem cells. Journal of Bone and Mineral Research 2007 22 17201731. (doi:10.1359/jbmr.070721).

    • Search Google Scholar
    • Export Citation
  • 141

    Ross SE, Hemati N, Longo KA, Bennett CN, Lucas PC, Erickson RL, Macdougald OA. Inhibition of adipogenesis by Wnt signaling. Science 2000 289 950953. (doi:10.1126/science.289.5481.950).

    • Search Google Scholar
    • Export Citation
  • 142

    Kato M, Patel MS, Levasseur R, Lobov I, Chang BH, Glass DA, Hartmann C, Li L, Hwang TH, Brayton CF et al. . Cbfa1-independent decrease in osteoblast proliferation, osteopenia, and persistent embryonic eye vascularization in mice deficient in Lrp5, a Wnt coreceptor. Journal of Cell Biology 2002 157 303314. (doi:10.1083/jcb.200201089).

    • Search Google Scholar
    • Export Citation
  • 143

    Babij P, Zhao W, Small C, Kharode Y, Yaworsky PJ, Bouxsein ML, Reddy PS, Bodine PV, Robinson JA, Bhat B et al. . High bone mass in mice expressing a mutant LRP5 gene. Journal of Bone and Mineral Research 2003 18 960974. (doi:10.1359/jbmr.2003.18.6.960).

    • Search Google Scholar
    • Export Citation
  • 144

    Rodda SJ, McMahon AP. Distinct roles for Hedgehog and canonical Wnt signaling in specification, differentiation and maintenance of osteoblast progenitors. Development 2006 133 32313244. (doi:10.1242/dev.02480).

    • Search Google Scholar
    • Export Citation
  • 145

    Day TF, Guo X, Garrett-Beal L, Yang Y. Wnt/β-catenin signaling in mesenchymal progenitors controls osteoblast and chondrocyte differentiation during vertebrate skeletogenesis. Developmental Cell 2005 8 739750. (doi:10.1016/j.devcel.2005.03.016).

    • Search Google Scholar
    • Export Citation
  • 146

    Sawakami K, Robling AG, Ai M, Pitner ND, Liu D, Warden SJ, Li J, Maye P, Rowe DW, Duncan RL et al. . The Wnt co-receptor LRP5 is essential for skeletal mechanotransduction but not for the anabolic bone response to parathyroid hormone treatment. Journal of Biological Chemistry 2006 281 2369823711. (doi:10.1074/jbc.M601000200).

    • Search Google Scholar
    • Export Citation
  • 147

    Jahaveri B, Dallas M, Zhao H, Bonewald L, Johnsom M. β-Catenin haploinsufficiency in osteocytes abolishes the osteogenic effect of mechanical loading. abstract Journal of Bone and Mineral Research 2011 26 (suppl 1) abstract # 1068 doi:10.1002/jbmr584).

    • Search Google Scholar
    • Export Citation
  • 148

    Bonewald LF, Johnson ML. Osteocytes, mechanosensing and Wnt signaling. Bone 2008 42 606615. (doi:10.1016/j.bone.2007.12.224).

  • 149

    Holmen SL, Zylstra CR, Mukherjee A, Sigler RE, Faugere MC, Bouxsein ML, Deng L, Clemens TL, Williams BO. Essential role of β-catenin in postnatal bone acquisition. Journal of Biological Chemistry 2005 280 2116221168. (doi:10.1074/jbc.M501900200).

    • Search Google Scholar
    • Export Citation
  • 150

    Glass DA, Karsenty G. Canonical Wnt signaling in osteoblasts is required for osteoclast differentiation. Annals of the New York Academy of Sciences 2006 1068 117130. (doi:10.1196/annals.1346.015).

    • Search Google Scholar
    • Export Citation
  • 151

    Marks SC Jr. Osteopetrosis in the toothless (t1) rat: presence of osteoclasts but failure to respond to parathyroid extract or to be cured by infusion of spleen or bone marrow cells from normal littermates. American Journal of Anatomy 1977 149 289297. (doi:10.1002/aja.1001490212).

    • Search Google Scholar
    • Export Citation
  • 152

    Marks SC Jr. The basic and applied biology of tooth eruption. Connective Tissue Research 1995 32 149157. (doi:10.3109/03008209509013718).

  • 153

    Marks SC Jr, Schroeder HE. Tooth eruption: theories and facts. Anatomical Record 1996 245 374393. (doi:10.1002/(SICI)1097-0185(199606)245:2<374::AID-AR18>3.0.CO;2-M).

    • Search Google Scholar
    • Export Citation
  • 154

    Popoff SN, Marks SC Jr. The heterogeneity of the osteopetroses reflects the diversity of cellular influences during skeletal development. Bone 1995 17 437445. (doi:10.1016/8756-3282(95)00347-4).

    • Search Google Scholar
    • Export Citation
  • 155

    Kramer I, Halleux C, Keller H, Pegurri M, Gooi JH, Weber PB, Feng JQ, Bonewald LF, Kneissel M. Osteocyte Wnt/β-catenin signaling is required for normal bone homeostasis. Molecular and Cellular Biology 2010 30 30713085. (doi:10.1128/MCB.01428-09).

    • Search Google Scholar
    • Export Citation
  • 156

    Ruan M, Pederson L, Hachfeld C, Thomson M, Prakash YS, Howe A, Williams B, Davey R, Khosla S, Westendorf J et al. . Deletion of Wnt Receptors Lrp5 and Lrp6 or β-catenin in late osteoclast precursors differentially suppress osteoclast differentiation and bone metabolism. Journal of Bone and Mineral Research 2012 27 (Suppl 1) abstract 1047.

    • Search Google Scholar
    • Export Citation
  • 157

    Liu M, Kurimoto P, Qing-Tian N, Warmington KS, Xiaodong L, Scott Simonet W, Zhu Ke H. Decreased osteoclastogenesis in bone marrow cells derived from ovariectomized rats treated with sclerostin antibody. Journal of Bone and Mineral Research 2012 27 (Suppl 1) abstract SA0404.

    • Search Google Scholar
    • Export Citation
  • 158

    Osier LK, Popoff SN, Marks SC Jr. Osteopetrosis in the toothless rat: failure of osteoclast differentiation and function. Bone and Mineral 1987 3 3545.

    • Search Google Scholar
    • Export Citation
  • 159

    Van Wesenbeeck L, Odgren PR, MacKay CA, D'Angelo M, Safadi FF, Popoff SN, Van Hul W, Marks SC Jr. The osteopetrotic mutation toothless (tl) is a loss-of-function frameshift mutation in the rat Csf1 gene: evidence of a crucial role for CSF-1 in osteoclastogenesis and endochondral ossification. PNAS 2002 99 1430314308. (doi:10.1073/pnas.202332999).

    • Search Google Scholar
    • Export Citation
  • 160

    Balemans W, Van Hul W. The genetics of low-density lipoprotein receptor-related protein 5 in bone: a story of extremes. Endocrinology 2007 148 26222629. (doi:10.1210/en.2006-1352).

    • Search Google Scholar
    • Export Citation
  • 161

    Dormandy J, Bhattacharya M, van Troostenburg de Bruyn AR. Safety and tolerability of pioglitazone in high-risk patients with type 2 diabetes: an overview of data from PROactive. Drug Safety 2009 32 187202. (doi:10.2165/00002018-200932030-00002).

    • Search Google Scholar
    • Export Citation
  • 162

    Lane NE, Yao W. Glucocorticoid-induced bone fragility. Annals of the New York Academy of Sciences 2010 1192 8183. (doi:10.1111/j.1749-6632.2009.05228.x).

    • Search Google Scholar
    • Export Citation
  • 163

    Gooi JH, Pompolo S, Karsdal MA, Kulkarni NH, Kalajzic I, McAhren SH, Han B, Onyia JE, Ho PW, Gillespie MT et al. . Calcitonin impairs the anabolic effect of PTH in young rats and stimulates expression of sclerostin by osteocytes. Bone 2010 46 14861497. (doi:10.1016/j.bone.2010.02.018).

    • Search Google Scholar
    • Export Citation
  • 164

    Gaudio A, Pennisi P, Bratengeier C, Torrisi V, Lindner B, Mangiafico RA, Pulvirenti I, Hawa G, Tringali G, Fiore CE. Increased sclerostin serum levels associated with bone formation and resorption markers in patients with immobilization-induced bone loss. Journal of Clinical Endocrinology and Metabolism 2010 95 22482253. (doi:10.1210/jc.2010-0067).

    • Search Google Scholar
    • Export Citation
  • 165

    Suetta C, Hvid LG, Justesen L, Christensen U, Neergaard K, Simonsen L, Ortenblad N, Magnusson SP, Kjaer M, Aagaard P. Effects of aging on human skeletal muscle after immobilization and retraining. Journal of Applied Physiology 2009 107 11721180. (doi:10.1152/japplphysiol.00290.2009).

    • Search Google Scholar
    • Export Citation
  • 166

    Boyle WJ, Simonet WS, Lacey DL. Osteoclast differentiation and activation. Nature 2003 423 337342. (doi:10.1038/nature01658).

  • 167

    Ma YL, Cain RL, Halladay DL, Yang X, Zeng Q, Miles RR, Chandrasekhar S, Martin TJ, Onyia JE. Catabolic effects of continuous human PTH (1–38) in vivo is associated with sustained stimulation of RANKL and inhibition of osteoprotegerin and gene-associated bone formation. Endocrinology 2001 142 40474054. (doi:10.1210/en.142.9.4047).

    • Search Google Scholar
    • Export Citation
  • 168

    Mason JJ, Williams BO. SOST and DKK: antagonists of LRP family signaling as targets for treating bone disease. Journal of Osteoporosis 2010 2010 460120. (doi:10.4061/2010/460120).

    • Search Google Scholar
    • Export Citation
  • 169

    Krishnan V, Bryant HU, Macdougald OA. Regulation of bone mass by Wnt signaling. Journal of Clinical Investigation 2006 116 12021209. (doi:10.1172/JCI28551).

    • Search Google Scholar
    • Export Citation
  • 170

    Moon RT, Kohn AD, De Ferrari GV, Kaykas A. WNT and β-catenin signalling: diseases and therapies. Nature Reviews. Genetics 2004 5 691701. (doi:10.1038/nrg1427).

    • Search Google Scholar
    • Export Citation
  • 171

    Glantschnig H, Hampton RA, Lu P, Zhao JZ, Vitelli S, Huang L, Haytko P, Cusick T, Ireland C, Jarantow SW et al. . Generation and selection of novel fully human monoclonal antibodies that neutralize Dickkopf-1 (DKK1) inhibitory function in vitro and increase bone mass in vivo. Journal of Biological Chemistry 2010 285 4013540147. (doi:10.1074/jbc.M110.166892).

    • Search Google Scholar
    • Export Citation
  • 172

    Glantschnig H, Scott K, Hampton R, Wei N, McCracken P, Nantermet P, Zhao JZ, Vitelli S, Huang L, Haytko P et al. . A rate-limiting role for Dickkopf-1 in bone formation and the remediation of bone loss in mouse and primate models of postmenopausal osteoporosis by an experimental therapeutic antibody. Journal of Pharmacology and Experimental Therapeutics 2011 338 568578. (doi:10.1124/jpet.111.181404).

    • Search Google Scholar
    • Export Citation
  • 173

    Li X, Ominsky MS, Warmington KS, Morony S, Gong J, Cao J, Gao Y, Shalhoub V, Tipton B, Haldankar R et al. . Sclerostin antibody treatment increases bone formation, bone mass, and bone strength in a rat model of postmenopausal osteoporosis. Journal of Bone and Mineral Research 2009 24 578588. (doi:10.1359/jbmr.081206).

    • Search Google Scholar
    • Export Citation
  • 174

    Li X, Warmington KS, Niu QT, Asuncion FJ, Barrero M, Grisanti M, Dwyer D, Stouch B, Thway TM, Stolina M et al. . Inhibition of sclerostin by monoclonal antibody increases bone formation, bone mass, and bone strength in aged male rats. Journal of Bone and Mineral Research 2010 25 26472656. (doi:10.1002/jbmr.182).

    • Search Google Scholar
    • Export Citation
  • 175

    Lewiecki EM. Sclerostin monoclonal antibody therapy with AMG 785: a potential treatment for osteoporosis. Expert Opinion on Biological Therapy 2011 11 117127. (doi:10.1517/14712598.2011.540565).

    • Search Google Scholar
    • Export Citation
  • 176

    Ominsky M, Niu Q-T, Kurimoto P, Ke H. Tissue level mechanism of increased bone formation by sclerostin antibody in male Cynomolgus monkeys. Journal of Bone and Mineral Research 2010 25 1174. (doi:10.1002/jbmr.14).

    • Search Google Scholar
    • Export Citation
  • 177

    Padhi D, Jang G, Stouch B, Fang L, Posvar E. Single-dose, placebo-controlled, randomized study of AMG 785, a sclerostin monoclonal antibody. Journal of Bone and Mineral Research 2011 26 1926. (doi:10.1002/jbmr.173).

    • Search Google Scholar
    • Export Citation
  • 178

    Chan BY, Fuller ES, Russell AK, Smith SM, Smith MM, Jackson MT, Cake MA, Read RA, Bateman JF, Sambrook PN et al. . Increased chondrocyte sclerostin may protect against cartilage degradation in osteoarthritis. Osteoarthritis and Cartilage 2011 19 874885. (doi:10.1016/j.joca.2011.04.014).

    • Search Google Scholar
    • Export Citation
  • 179

    Power J, Poole KE, van Bezooijen R, Doube M, Caballero-Alias AM, Lowik C, Papapoulos S, Reeve J, Loveridge N. Sclerostin and the regulation of bone formation: effects in hip osteoarthritis and femoral neck fracture. Journal of Bone and Mineral Research 2010 25 18671876. (doi:10.1002/jbmr.70).

    • Search Google Scholar
    • Export Citation
  • 180

    Lee NK, Sowa H, Hinoi E, Ferron M, Ahn JD, Confavreux C, Dacquin R, Mee PJ, McKee MD, Jung DY et al. . Endocrine regulation of energy metabolism by the skeleton. Cell 2007 130 456469. (doi:10.1016/j.cell.2007.05.047).

    • Search Google Scholar
    • Export Citation
  • 181

    Fulzele K, Riddle RC, DiGirolamo DJ, Cao X, Wan C, Chen D, Faugere MC, Aja S, Hussain MA, Bruning JC et al. . Insulin receptor signaling in osteoblasts regulates postnatal bone acquisition and body composition. Cell 2010 142 309319. (doi:10.1016/j.cell.2010.06.002).

    • Search Google Scholar
    • Export Citation
  • 182

    Nakamura T, Toita H, Yoshimoto A, Nishimura D, Takagi T, Ogawa T, Takeya T, Ishida-Kitagawa N. Potential involvement of Twist2 and Erk in the regulation of osteoblastogenesis by HB–EGF–EGFR signaling. Cell Structure and Function 2010 35 5361. (doi:10.1247/csf.10001).

    • Search Google Scholar
    • Export Citation
  • 183

    Fernandez-Real JM, Izquierdo M, Ortega F, Gorostiaga E, Gomez-Ambrosi J, Moreno-Navarrete JM, Fruhbeck G, Martinez C, Idoate F, Salvador J et al. . The relationship of serum osteocalcin concentration to insulin secretion, sensitivity, and disposal with hypocaloric diet and resistance training. Journal of Clinical Endocrinology and Metabolism 2009 94 237245. (doi:10.1210/jc.2008-0270).

    • Search Google Scholar
    • Export Citation
  • 184

    Iglesias P, Arrieta F, Pinera M, Botella-Ca