A human yeast artificial chromosome containing the multiple endocrine neoplasia type 2B Ret mutation does not induce medullary thyroid carcinoma but does support the growth of kidneys and partially rescues enteric nervous system development in Ret-deficient mice

We generated a line of transgenic mice using a yeast artificial chromosome containing the Ret mutation responsible for the multiple endocrine neoplasia type 2B syndrome (MEN 2B). The resulting animals did not develop any of the expected neoplasms associated with MEN 2B. Transgenic animals were then bred with animals lacking murine Ret (Ret_M) to further evaluate the function of human mutated Ret (Ret_H^2B) in the murine context. Whereas mice lacking Ret_M exhibit intestinal aganglionosis and the absence of kidneys with other genitourinary anomalies, expression of the Ret_H^2B transgene in Ret_M-deficient mice allowed significant renal development with a partial rescue of the enteric nervous system. These Ret_H^2B-positive/Ret _M-deficient mice exhibit normal Ret expression and survive longer than Ret_M-deficient mice, but still die at 3 to 5 days of age with evidence of enterocolitis. We conclude that the normal expression of a human Ret proto-oncogene with the MEN 2B mutation does not cause any features of MEN 2B in mice. Although the gene is normally expressed in the appropriate target tissues, there is incomplete phenotypic rescue in mice lacking murine Ret. These results suggest important interspecies differences between humans and mice in the function of the Ret oncogene.