Hirschsprung disease (HSCR, aganglionic megacolon) is a complex genetic disorder 
of the enteric nervous system (ENS) characterized by the absence of enteric 
neurons along a variable length of the intestine. While rare variants (RVs) in 
the coding sequence (CDS) of several genes involved in ENS development lead to 
disease, the association of common variants (CVs) with HSCR has only been 
reported for RET (the major HSCR gene) and NRG1. Importantly, RVs in the CDS of 
these two genes are also associated with the disorder. To assess independent and 
joint effects between the different types of RET and NRG1 variants identified in 
HSCR patients, we used 254 Chinese sporadic HSCR patients and 143 ethnically 
matched controls for whom the RET and/or NRG1 variants genotypes (rare and 
common) were available. Four genetic risk factors were defined and interaction 
effects were modeled using conditional logistic regression analyses and 
pair-wise Kendall correlations. Our analysis revealed a joint effect of RET CVs 
with RET RVs, NRG1 CVs or NRG1 RVs. To assess whether the genetic interaction 
translated into functional interaction, mouse neural crest cells (NCCs; enteric 
neuron precursors) isolated from embryonic guts were treated with NRG1 (ErbB2 
ligand) or/and GDNF (Ret ligand) and monitored during the subsequent neural 
differentiation process. Nrg1 inhibited the Gdnf-induced neuronal 
differentiation and Gdnf negatively regulated Nrg1-signaling by down-regulating 
the expression of its receptor, ErbB2. This preliminary data suggest that the 
balance neurogenesis/gliogenesis is critical for ENS development.