Germline mutations of the RET ligand GDNF are not sufficient to cause Hirschsprung disease

Abstract

Hirschsprung disease (HSCR, aganglionic megacolon) is a common congenital malformation leading to bowel obstruction, with an incidence of 1/5,000 live births. It is characterized by the absence of intrinsic ganglion cells in the myenteric and submucosal plexuses along variable lengths of the gastrointestinal tract¹. As enteric neurons are derived from the vagal neural crest, HSCR is regarded as a neurocristopathy². On the basis of a skewed sex-ratio ( M/F = 4/1) and a risk to relatives much higher than the incidence in the general population, HSCR has long been regarded as a sex-modified multifactorial disorder^3,4. Accordingly, segregation analysis suggested an incompletely penetrant dominant inheritance in HSCR families with agan-glionosis extending beyond the sigmoid colon³. We and others have mapped a dominant gene for HSCR to chromosome 10q11.2 and have ascribed the disease to mutations in the RET protoonco-gene^5–8. However, the lack of genotype-phenotype correlation, the low penetrance and the sex-dependent effect of RET mutations supported the existence of one or more modifier gene(s) in familial HSCR9. In addition, thus far, RET mutations only accounted for 50% and 15–20% of familial and sporadic HSCR patients, respectively^9–11. RET encodes a tyrosine kinase receptor whose ligand was unknown¹². Recently, the Glial cell line- derived neurotrophic factor (GDNF) has been identified to be a ligand for RET^13,14. Moreover, Gdnf−/− knockout mutant mice display congenital intestinal agan-glionosis and renal agenesis^15–17, a phenotype very similar to the Ret −/− mouse¹⁸. These data prompted us to hypothesize that mutations of the gene encoding GDNF could either cause or modulate the HSCR phenotype in some cases.