Increased RET activity coupled with a reduction in the RET gene dosage causes intestinal aganglionosis in mice

Abstract

Mutations of the gene encoding the RET tyrosine kinase causes Hirschsprung disease (HSCR) and medullary thyroid carcinoma (MTC). Current consensus holds that HSCR and MTC are induced by inactivating and activating RET mutations, respectively. However, it remains unknown whether activating mutations in the RET gene have adverse effects on ENS development in vivo. We addressed this issue by examining mice engineered to express RET51(C618F), an activating mutation identified in MTC patients. Although Ret^{51(C618F)/51(C618F)}mice displayed hyperganglionosis of the ENS, Ret^51(C618F)/- mice exhibited severe intestinal aganglionosis due to premature neuronal differentiation. Reduced levels of GDNF, a RET-activating neurotrophic factor, ameliorated the ENS phenotype of Ret^51(C618F)/- mice, demonstrating that GDNF-mediated activation of RET51(C618F) is responsible for severe aganglionic phenotype. The RET51(C618F) allele showed genetic interaction with Ednrb gene, one of modifier genes for HSCR. These data reveal that proliferation and differentiation of ENS precursors are exquisitely controlled by both the activation levels and total dose of RET. Increased RET activity coupled with a decreased gene dosage can cause intestinal aganglionosis, a finding that provides novel insight into HSCR pathogenesis.

SIGNIFICANCE STATEMENT

Mutations of the RET gene have been identified in Hirschsprung disease (HSCR) and neuroendocrine tumors (NET). It has been thought that HSCR and NET are caused by inactivating and activating mutations of the RET gene, respectively. However, little is known about whether enhanced RET activity exerts any roles in the pathogenesis of HSCR. We show that mice carrying an activating mutation in the Ret gene display intestinal aganglionosis when the Ret gene dosage is halved. The aganglionosis phenotype is caused by premature neuronal differentiation and impaired migration of ENS precursors These findings raise the possibility that RET-activating mutations can cause HSCR when associated with a reduction in the dosage or expression of the RET gene.