Date of Award
Doctor of Philosophy (PhD)
There are currently no prevention strategies for Hirschsprung disease (HSCR), a potentially lethal birth defect caused by the failure of the enteric nervous system (ENS) to develop completely, resulting in an aganglionic segment of intestine. HSCR can result from one or a combination of partially-penetrant mutations in genes affecting the enteric neural crest-derived cells (ENCDCs) that colonize the intestine to form the ENS. While the genetics of HSCR and ENS development has been extensively studied, little is known about environmental risk factors for HSCR. To investigate maternal medication exposure as a possible HSCR risk factor, we conducted a chemical screen in zebrafish embryos for drugs that inhibit ENS development. Among several identified inhibitors of ENS development, we selected the antimetabolite immunosuppressant medication mycophenolic acid (MPA) and its prodrug mycophenolate mofetil (MMF) for further study. Here we demonstrate that maternal MPA or MMF exposure impairs ENS development in mice through MPA-mediated inhibition of inosine-5'-monophosphate dehydrogenase (IMPDH), which catalyzes the rate-limiting step of de novo guanosine monophosphate (GMP) synthesis. This inhibition results in reduced ENCDC proliferation that then impairs colonization of the intestine. We also show that MMF exposure is sufficient to cause permanent ENS defects and greatly enhances the penetrance and severity of aganglionosis in the Sox10 and Ret mouse models of HSCR. Prompted by this interaction between IMPDH inhibition and ENS development and by the proximity of an essential IMPDH gene, IMPDH2, to a mapped HSCR susceptibility region, we sequenced IMPDH2 in a cohort of HSCR patients. We detected a single individual heterozygous for a nonsynonymous variant, P123R, and show that it produces an enzyme with slightly reduced activity. Furthermore, we deleted Impdh2 in neural crest derivatives in mice, resulting in major developmental defects including drastically reduced ENCDC colonization of the intestine. However, heterozygosity for a null allele of Impdh2 did not affect the penetrance of either Sox10 or Ret mutations. These studies reinforce the importance of ENCDC proliferation to ENS development, demonstrate a dramatic gene-drug interaction, and provide the first evidence that maternal medications may increase HSCR risk.
Chair and Committee
Robert O Heuckeroth
Thomas Baranski, Aaron DiAntonio, Gregory Longmore, Jason Mills
Lake, Jonathan Ian, "Guanine Nucleotide Synthesis and The Enteric Nervous System" (2015). Arts & Sciences Electronic Theses and Dissertations. 433.