Characterization of the RET protooncogene transmembrane domain mutation S649L associated with nonaggressive medullary thyroid carcinoma.

ContextFor rare and novelRETmutations associated with hereditary medullary thyroid carcinoma (MTC), clinical and functional studies are needed to classify theRETmutation into one of the three clinical risk groups.ObjectiveWe analyzed proliferative properties and clinical implications associated with theRETprotooncogene transmembrane domain mutation S649L.DesignThe transforming potential and mitogenic properties of S649L mutation were investigated clinically and by evaluating kinase activity, cell proliferation, and colony formation.PatientsFifteen individuals from five kindreds were identified as carriers of aRETprotooncogene mutation in exon 11 codon 649 (TCGSer→TTGLeu). In two out of five index patients, a secondRETmutation (C634W or V804L) was detected.ResultsEight gene carriers were operated on. Histology revealed MTC and C-cell hyperplasia in three index and three screening patients respectively. In all other gene carriers (aged 41–64 years), calcitonin levels were in the normal range, and pentagastrin-stimulated calcitonin levels were <100 pg/ml. Therefore, thyroidectomy had not yet been performed. In one index patient carrying the S649L mutation, hyperparathyroidism was confirmed histologically.RETS649L-expressing NIH3T3 cells exhibited a clear increase of phosphotyrosine and proliferation rate when compared with parental NIH3T3 cells but a significantly lower kinase activity and cell growth rate when compared withRETC634R-expressing cells. When compared withRETC634R, the S649L mutant showed moderate transforming potential with small-sized colonies.ConclusionsOur clinical and in vitro findings indicate that the transmembraneRETS649L mutation is associated with late-onset non-aggressive disease. Recommendations for prophylactic thyroidectomy should be individualized depending on stimulated calcitonin levels.