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M Lorentzon, R Lorentzon, UH Lerner and P Nordstrom

OBJECTIVE: Bone mineral density (BMD) in adolescence is under strong genetic control. The calcium sensing receptor (CASR) is involved in the regulation of calcium homeostasis and bone resorption. The A986S polymorphism of the CASR has recently been associated with serum calcium levels, in one hitherto unconfirmed report. We investigated whether this polymorphism was related to BMD, circulating calcium and parathyroid hormone (PTH) concentrations in girls. DESIGN: BMD, plasma calcium and serum PTH were measured in adolescent girls and compared with regard to CASR genotype. METHODS: In 97 healthy Caucasian girls (mean age 16.9+/-1.2 years (mean+/-s.d.)), the A and S alleles were determined using PCR with a mismatched primer and the restriction enzyme BsaHI. BMD (g/cm) of the total body, humerus, femoral neck and lumbar spine was measured using dual energy X-ray absorptiometry. RESULTS: The genotype frequencies were 71% AA, 26% AS and 3% SS. The genotypes were divided into presence (29%) or absence of S allele (71%). Subjects with the S allele had higher levels of plasma calcium, corrected for albumin (2.17+/-0.06>2.14+/-0.06; P < 0.05, using independent samples t-test), lower BMD at the lumbar spine (P=0.02) and total body (P=0.04), and were significantly less physically active (2.9+/-2.6 vs 4.3+/-2.6 h/week; P=0.01) than the subjects lacking the S allele. PTH levels were not significantly different between the two allelic groups. A multiple regression analysis, including age, height, weight and physical activity, revealed that the CASR allelic variants were not independent predictors of BMD at any site measured (beta=-0.03-0.09; P>0.05). Physical activity was an independent predictor of BMD, was significantly different between the CASR genotypes, and could therefore have a role in explaining the difference in BMD between the CASR genotypes. CONCLUSIONS: The CASR alleles are related to BMD, but it cannot be definitely concluded whether the CASR polymorphism has a direct influence on BMD, or whether the differences in BMD were mediated via an influence of the amount of physical activity.

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P Wennberg, P Nordstrom, R Lorentzon, UH Lerner and M Lorentzon

OBJECTIVE: The cytokine tumor necrosis factor alpha (TNF-alpha) is an important regulator of bone metabolism. Polymorphisms in the promoter region of the TNF-alpha gene at positions -308 and -863 have been identified. We investigated whether these polymorphisms and circulating TNF-alpha levels were related to bone mineral density and bone area in adolescent girls. DESIGN: Bone mineral density (BMD), bone area (BA), anthropometric characteristics and biochemical analyses were measured in adolescent girls and compared with regard to TNF-alpha genotype. METHODS: Allelic variants of the TNF-alpha gene in 97 girls, aged 16.9+/-1.2 years (mean+/-S.D.), were identified using polymerase chain reaction and the restriction endonucleases NcoI and TaiI. Bone mineral density and bone area of the femoral neck, lumbar spine and total body were measured using dual energy X-ray absorptiometry. RESULTS: Carriers of the rare -863 A allele (n=25) had higher body weight (P=0.03), lumbar spine BMD (P=0.02), and larger total BA (P=0.03), femoral neck area (P<0.05), and lumbar spine area (P=0.01). The independent predictors of BMD and BA were investigated using multiple regression. The TNF-alpha-863 genotypes (beta=0.18, P=0.03) and the TNF-alpha plasma levels (beta=0.19, P=0.04) independently predicted BA of the lumbar spine but not BA or BMD of any other measured sites. No statistically significant differences in body constitution parameters, biochemical parameters, bone density, or bone area at the measured skeletal sites were found when comparing the groups defined by the allelic variants at position -308 (P=0.17-0.84). CONCLUSIONS: We found the TNF-alpha-863 polymorphism and the TNF-alpha plasma levels to be independent predictors of lumbar spine area in healthy Caucasian adolescent females.