RET S409Y Germline Mutation and Associated Medullary Thyroid Carcinoma.

BACKGROUND:Inherited medullary thyroid cancinoma (MTC) is primarily caused by the RET mutation, commonly localised in exons 5, 8, 10, 11 and 13-16. We report herein four Chinese pedigrees with MTC harbouring a germline S409Y variant within exon 6 of RET proto-oncogene. METHODS:Using targeted sequencing, four apparently sporadic MTC index cases were diagnosed with the germline RET S409Y (c.1226 C>A) variant. Subsequently, their 27 relatives underwent clinical, genetic assessments and/or thyroid surgery. Furthermore, in silico analysis and in vitro assays were performed to predict or verify the potential oncogenic ability and protein structure of the S409Y variant. RESULTS:Overall, 15 of 31 participants were diagnosed with the RET S409Y variant; of these, 6 presented isolated MTC (mean age: 50.2 years; range: 41-75 years) with 3 presenting neck lymph node metastases, including 2 presenting distant liver or lung metastases. Among the remaining 9 carriers, 3 (mean age, 56 years; range, 41-76 years) reported elevated serum calcium-stimulated calcitonin (sCtn) or a concurrent marginally elevated serum calcitonin (Ctn) level; the remaining 6 (mean age, 37.5 years; range, 14-52 years) exhibited no abnormal Ctn/sCtn level (P < 0.05). None of all the 15 carriers presented clinical evidence for pheochromocytom or hyperparathyroidism in these four families. The in silico analysis revealed a 'possibly damaging' mutation for S409Y, resulting that could affect the organisation of the RET protein structural inter-domain interface; the in vitro assay revealed that phosphorylation level of the tyrosine 905 was a relatively stronger in the S409Y RET mutant, indicating ligand-independent autophosphorylation. Moreover, transfection with the S409Y induced an enhance effect of the transforming activity of PI3K/AKT, MAPK/ERK pathways and a greater aggressiveness ability of migration and invasion compared with wild-type RET, but all to a lesser degree than the C618Y and C634Y mutations. CONCLUSIONS:This study reveals that the novel germline RET S409Y variant is probably pathogenic and associated with lower penetrance of MTC. Nonetheless, individuals with S409Y should be managed using a personalized approach, and 'at-risk' additional families should be confirmed; the correlation between the S409Y mutation and MEN2-specific tumours should be further elucidated.