Genetic factors

HSCRisanon-Mendelian,sex-linkedmultigenic,partiallypenetrantand genetically heterogeneous disease with variable severity. For instance, a single deleterious mutation can cause HSCR in 20% of cases, whereas multiple genes are implicated in 80% of cases22,81. Syndromic HSCR and familial forms of HSCR can follow complex patterns of inheritance. An increased knowledge of common variants involved in HSCR have revealed its heterogeneity, and advances in genetic sequencing technologies have highlighted the genetic complexity of HSCR. Currently,

aMigration from neural crest to gut

rare coding variants in >24 genes related to HSCR have been identified, although some require further functional confirmation22,38.

With the detection of more common variants in the population, advances in genetics may soon be able to explain more isolated cases of HSCR22,82. Genome-wide association studies, epigenome-wide association studies and next-generation sequencing have expanded the list of known genes involved in HSCR pathogenesis. These genes include, but are not limited to, RET, GDNF, GFRA1, NRTN, PHOX2B, NKX2.1, SOX10, EDNRB, EDN3, ECE1, KIAA1279, ZFXH1B, NRRK3, L1CAM, TCF4, HOXB5,

bCranio-caudal migration through the gut (4–7 weeks of gestation)

Fig.3|PathophysiologyandsignallingpathwaysinHirschsprungdisease.

The normal development of the enteric nervous system (ENS) requires the successful migration of enteric neural crest cells (NCCs) in the correct direction, and their proliferation, survival and differentiation into neurons and glia. Neural crest cells (NCCs) originate from the neural tube and migrate through the somites where they are exposed to retinoic acid and begin to express RET prior to entering the foregut mesenchyme (part a). Vagal NCCs migrate from the oesophagus to the anal canal between the fourth and the seventh weeks of gestation, whereas sacral NCCs contribute to the pelvic plexus (part b). Undifferentiated NCCs have high expression of several genes including SOX10, PHOX2B, EDNRB and RET227. The RET

signalling pathway plays a critical role in promoting enteric NCC proliferation and migration. RET is expressed on the surface of enteric NCCs, and its transcription is regulated by SOX10 and PHOX2B. Upon activation by its ligand, GDNF, RET triggers multiple downstream pathways. EDNRB is the second critical pathway for ENS development228. Activation of the EDNRB pathway leads to enteric NCC proliferation45, whereas loss of this pathway results in premature differentiation and decreased proliferation. The extracellular matrix also plays a critical role, with several proteins that promote enteric NCC migration, whereas others have an inhibitory effect74,229. Part a adapted with permission from ref. 45, Elsevier. Part b adapted with permission from ref. 230, JCI.