We have analysed 118 families with inherited medullary thyroid carcinoma (MTC) for mutations of the RET proto–oncogene. These included cases of multiple endocrine neoplasia types 2A (MEN 2A) and 2B (MEN 2B) and familial MTC (FMTC). Mutations at one of 5 cysteines in the extracellular domain were found in 97% of patients with MEN 2A and 86% with FMTC but not in MEN 2B patients or normal controls. 84% of the MEN2A mutations affected codon 634. MEN 2A patients with a Cys634 to Arg substitution had a greater risk of developing parathyroid disease than those with other codon 634 mutations. Our data show a strong correlation between disease phenotype and the nature and position of the RET mutation, suggesting that a simple, constitutive activation of the RET tyrosine kinase is unlikely to explain the events leading to MEN 2A and FMTC.
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