Development of medullary thyroid carcinoma in transgenic mice expressing the RET protooncogene altered by a multiple endocrine neoplasia type 2A mutation.

Multiple endocrine neoplasia type 2 (MEN 2) is a dominantly inherited cancer syndrome that comprises three clinical subtypes: MEN type 2A (MEN-2A), MEN type 2B (MEN-2B), and familial medullary thyroid carcinoma (FMTC). Medullary thyroid carcinoma (MTC), a malignant tumor arising from calcitonin-secreting thyroid C cells, is the cardinal disease feature of this syndrome, and mortality in affected MEN-2 patients is mainly caused by this malignancy. Germ-line mutations of the RET protooncogene, which encodes a receptor tyrosine kinase, are responsible for these three neoplastic-prone disorders. MEN2 mutations convert the RET protooncogene in a dominantly acting oncogene as a consequence of the ligand-independent activation of the tyrosine kinase. The majority of MEN2A and FMTC mutations are located in the extracellular domain and cause the replacement of one of five juxtamembrane cysteines by a different amino acid. To examine whether expression of a MEN2A allele of RET results in transformation of C cells, we have used the transgenic approach. Expression of the RET gene altered by a MEN2A mutation was targeted in C cells by placing the transgene under the control of the calcitonin gene-related peptide/calcitonin promoter. Animals of three independent transgenic mouse lines, which expressed the transgene in the thyroid, displayed overt bilateral C cell hyperplasia as early as 3 weeks of age and subsequently developed multifocal and bilateral MTC. Moreover, these tumors were morphologically and biologically similar to human MTC which afflicts MEN2 individuals. These findings provide evidence that the MEN2A mutant form of RET is oncogenic in parafollicular C cells and suggest that these transgenic mice should prove a valuable animal model for hereditary MTC.