Medullary thyroid carcinoma exclusively associated with a homozygous RET V778I pathogenic variant: a case report with review of literature.

Medullary thyroid carcinoma (MTC) can occur sporadically or as a hereditary disease. The latter often presents with a multiple endocrine neoplasia type 2 (MEN2) phenotype and is caused by germline-activating pathogenic variants in the RET proto-oncogene, whereas the former may harbor somatic-activating RET pathogenic variants. Here, we report a family with a germline RET V778I pathogenic variant. The proband was a 72-year-old woman with bilateral multifocal MTCs but without other MEN2 features. Germline RET analysis revealed a homozygous V778I pathogenic variant. Postoperative histopathological examination confirmed bilateral multifocal MTC with lymph node metastasis. The patient's parents were cousins. The patient had no family history of MTC or MEN2. Her three middle-aged children were heterozygous for the V778I pathogenic variant, had no symptoms or signs of MTC, and had normal serum calcitonin and CEA levels. The proband died of cardiac and pulmonary diseases at the age of 86, 15 years after surgery, without MTC recurrence. Unlike other dominant RET pathogenic variants, in which a single mutated allele is sufficient for tumor development, V778I may have weak oncogenic activity, requiring homozygosity to develop MTC. Therefore, prophylactic thyroidectomy is not recommended for heterozygous carriers. To the best of our knowledge, this is the second report of a family with MTC exclusively associated with a homozygous RET pathogenic variant. This is also the first report of a germline RET V778I pathogenic variant associated with MTC under homozygous conditions.