Objective: To determine the types of mutations and the clinical significance of a specific genotype in familial medullary thyroid carcinoma (MTC) syndromes. Design: We retrospectively and prospectively studied patients with MTC at a tertiary referral center. Material and Methods: The study cohort consisted of 348 affected patients and at-risk family members of MTC kindreds, including 33 multiple endocrine neoplasia type IIA (MEN IIA) kindreds with 165 members, 13 familial MTC alone (FMTC) kindreds (at least 4 affected members with MTC per kindred, without evidence of pheochromocytoma and hyperparathyroidism) with 108 members, 15 'other hereditary MTC' kindreds (2 or 3 affected members) with 42 members, and 33 individuals with sporadic MTC. An additional 53 subjects from the aforementioned MEN IIA kindreds who were clinically affected but not genetically tested were also included in an analysis of the relationship between genotype and phenotype. The presence of germline mutations in the RET protooncogene was studied by DNA sequence analysis of exons 10, 11, and 13. Results: Germline RET mutations in exons 10 and 11 were identified in 32 of 33 MEN IIA kindreds (97%), 10 of 13 FMTC kindreds (77%), and 10 of 15 'other hereditary MTC' kindreds (67%). No mutations were identified in exon 13. No patient with sporadic MTC had a germline mutation. In MEN IIA, codon 634 was affected in 73% of the kindreds, whereas in FMTC, the main affected codon was codon 618 (54%). In MEN IIA, patients with codon 634 mutations had a higher risk of having C-cell disease, pheochromocytoma, and hyperparathyroidism than did those with other mutations (P<0.05, P<0.001, and P<0.01, respectively). Conclusion: RET analysis is a reliable, practical, and cost-effective test in the screening of at-risk family members of MEN IIA and FMTC kindreds. In addition, RET analysis may be helpful in the follow-up of gene carriers and for the early detection of pheochromocytoma and hyperparathyroidism in patients with codon 634 mutations.
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