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

Context: For rare and novel RET mutations associated with hereditary medullary thyroid carcinoma (MTC), clinical and functional studies are needed to classify the RET mutation into one of the three clinical risk groups. Objective: We analyzed proliferative properties and clinical implications associated with the RET protooncogene transmembrane domain mutation S649L. Design: The transforming potential and mitogenic properties of S649L mutation were investigated clinically and by evaluating kinase activity, cell proliferation, and colony formation. Patients: Fifteen individuals from five kindreds were identified as carriers of a RET protooncogene mutation in exon 11 codon 649 (TCG(Ser) --> TTG(Leu)). In two out of five index patients, a second RET mutation (C634W or V804L) was detected. Results: Eight 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-stimutated 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 RET S649L-expressing NIH3T3 cells exhibited a clear increase of phosphotyrosine and proliferation rate when compared with parental NFH3T3 cells but a significantly lower kinase activity and cell growth rate when compared with RET C634R-expressing cells. When compared with RET C634R, the S649L mutant showed moderate transforming potential with small-sized colonies. Conclusions: Our clinical and in vitro findings indicate that the transmembrane RET S649L mutation is associated with late-onset non-aggressive disease. Recommendations for prophylactic thyroidectomy should be individualized depending on stimulated calcitonin levels.