Background: A novel Cys-Ser Ret germline point mutation in a 58-year-old woman with bilateral medullary thyroid carcinoma (MTC) prompted us to perform genetic analysis of the family and evaluate the biological consequences of such a mutation. Methods: Ret analysis by direct sequencing was performed in five family members. The biological activity and biochemical properties of the Ret- Cys515Ser mutant were analyzed in NIH-3T3 cells. Results: The proband's son, age 35, had the Ret- Cys515Ser mutation and the L769 CTT/CTG exon 13 polymorphic variant, which was also found in his father. Clinical evaluation of the son also revealed bilateral multifocal microscopic MTC and papillary thyroid carcinoma (PTC). In vitro and in vivo analysis indicated ligand-independent activation of the Ret-Cys515Ser mutant due to aberrant disulfide homodimerization, increased mitogenic activity, and ability to induce anchorage-independent growth in NIH-3T3 cells in comparison to wild-type Ret, suggesting a possible role of Cys515Ser in tumor development. Conclusions: The Cys515Ser mutation adds to cysteine substitution groups that have been described in association with MTC. Our data also highlight the importance of performing a complete genetic analysis in patients who present with MTC.
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