OBJECTIVE: Gene alterations leading to activation of the MAPK pathway are of interest for targeted therapy in patients with advanced radioactive iodine-refractory (RAI-R) thyroid carcinoma. Due to technical reasons gene fusion analysis in RNA isolated from formalin-fixed tumor tissues has till now been limited. The objective of the present study was to identify targetable gene rearrangements in RNA isolated from formalin-fixed RAI-R thyroid carcinomas.
DESIGN: Retrospective study in 132 patients with RAI-R thyroid carcinoma (59 papillary-, 24 follicular-, 35 Hürthle cell-, and 14 anaplastic thyroid carcinoma).
METHODS: Total nucleic acid (undivided DNA and RNA) was isolated from formalin-fixed tissue. Extensive gene fusion analysis was performed in all samples that tested negative for pathogenic BRAF, NRAS, HRAS and KRAS variants.
RESULTS: Seven targetable gene fusions were identified in the remaining 60 samples without known DNA variants. This includes frequently reported gene fusions such as CCDC6/RET [PTC1], PRKAR1A/RET [PTC2] and ETV6/NTRK3 (n=2), and gene fusions that are less common in thyroid cancer (TPM3/NTRK1, EML4/ALK and EML4/NTRK3). Of note, most gene fusions were detected in papillary thyroid carcinoma and MAPK-associated alterations in Hürthle cell carcinomas are rare (2/35).
CONCLUSION: Targetable gene fusions were found in 12% of RAI-R thyroid carcinoma without DNA variants, and can be effectively identified in formalin-fixed tissue. These gene fusionsmight provide a preclinical rationale to include specific kinase inhibitors in the treatment regimen for these patients. The latter intends to restore iodine transport and/or take advantage of the direct effect on tumor cell vitality once progressive disease is seen.