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The developmental etiology and pathogenesis of Hirschsprung disease

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The enteric nervous system is the part of the autonomic nervous system that directly controls the gastrointestinal tract. Derived from a multipotent, migratory cell population called the neural crest, a complete enteric nervous system is necessary for proper gut function. Disorders that arise as a consequence of defective neural crest cell development are termed neurocristopathies. One such disorder is Hirschsprung disease (HSCR), also known as congenital megacolon or intestinal aganglionosis. HSCR occurs in 1/5000 live births and typically presents with the inability to pass meconium, along with abdominal distension and discomfort that usually requires surgical resection of the aganglionic bowel. This disorder is characterized by a congenital absence of neurons in a portion of the intestinal tract, usually the distal colon, because of a disruption of normal neural crest cell migration, proliferation, differentiation, survival, and/or apoptosis. The inheritance of HSCR disease is complex, often non-Mendelian, and characterized by variable penetrance. Extensive research has identified a number of key genes that regulate neural crest cell development in the pathogenesis of HSCR including RET, GDNF, GFRα1, NRTN, EDNRB, ET3, ZFHX1B, PHOX2b, SOX10, and SHH. However, mutations in these genes account for only ∼50% of the known cases of HSCR. Thus, other genetic mutations and combinations of genetic mutations and modifiers likely contribute to the etiology and pathogenesis of HSCR. The aims of this review are to summarize the HSCR phenotype, diagnosis, and treatment options; to discuss the major genetic causes and the mechanisms by which they disrupt normal enteric neural crest cell development; and to explore new pathways that may contribute to HSCR pathogenesis.

Section snippets

Gastrointestinal Tract, Enteric Nervous System, and Hirschsprung Disease

The gastrointestinal (GI) tract is an endoderm derived organ system that begins at the mouth and terminates at the anus. The fetal GI tract is divided into 3 segments based on vascular supply. The foregut, supplied by the celiac artery, consists of the esophagus, stomach, part of duodenum, and biliary apparatus. The midgut, supplied by the superior mesenteric artery, comprises the rest of small and large bowel up to the splenic flexure. Lastly, the hindgut consists of the remainder of the large

Clinical Presentation

Histologically, aganglionosis is pathognomonic for HSCR. In 80%-85% of HSCR cases, the aganglionic region is limited to the rectum and sigmoid colon (short segment disease). Long segment disease occurs in up to 20% of cases, and is characterized by aganglionosis extending proximally to the sigmoid colon. Total colonic aganglionosis is rarer, occurring in 3%-8% of patients with HSCR.14 Another rare variant is ultra-short segment disease, affecting only the distal rectum (because this variant has

Diagnosis

The diagnosis of HSCR can be made by a variety of methods. However, the preferred first diagnostic procedure is a contrast enema. This will define the transition zone between normal (dilated) bowel and the narrow aganglionic bowel. This transition zone is seen in 70%-90% of cases.31, 32 The rectosigmoid ratio is used to evaluate the transition zone. A rectosigmoid ratio greater than 1 is normal. A stool-filled proximal bowel will decrease the rectum to sigmoid ratio. Plain radiographs show

Treatment

Currently, the only treatment for HSCR is surgery. Failure to surgically treat HSCR can be fatal due to malnutrition or sepsis following bowel perforation. Although surgery is the routine therapy for HSCR patients, surgical outcomes can vary widely, with a range of long-term consequences, such as constipation, fecal incontinence, and enterocolitis.39, 40 The surgical treatments aim to remove the aganglionic bowel and anastomose the normal bowel to the anus while preserving sphincter function.

Pathogenesis

Proper neural crest cell migration, proliferation, differentiation, survival, and apoptosis all contribute to a functional ENS. Perturbation in any of these processes can lead to a HSCR phenotype. Many genes that play a critical functional role in neural crest cell development have been implicated in HSCR, including the proto-oncogene RET, endothelin signaling genes, and transcription factors.28 Although over a dozen genes have been identified that contribute to the etiology of HSCR, these

Migration

The ENS is derived from migratory neural crest cells that originate at the vagal (somites 1-7) and sacral (caudal to somite 24) regions of the embryonic axis. These subsets contribute to different gut regions.54, 55, 56, 57 Vagal neural crest cells migrate in a rostral to caudal direction and sequentially contribute to the foregut, midgut, and hindgut.2, 55, 58 In contrast, sacral NCCs are thought to contribute to the distal hindgut.59, 60 Migration takes about 6 days in mice, with vagal NCC

Receptor Tyrosine Kinase and Glial Cell Line-Derived Neurotrophic Factor

Many genes contribute to normal enteric neural crest cell migration and the formation of a functional enteric nervous system, and mutations in any of these genes may cause an HSCR phenotype (Table 1). Two of the major contributing gene families responsible for HSCR cases are receptor tyrosine kinase (RET) and glial cell line-derived neurotrophic factor (GDNF). Mutations in the RET pathway account for 15%-35% of patients with sporadic HSCR (HSCR in a single family member) and 50% of familial

Endothelin Pathway

Endothelin signaling is also necessary for normal ENCC migration and may help maintain a permissive NCC environment. Endothelin 3 (EDN3 or ET3) is a secreted peptide expressed by gut mesenchyme97 that binds to the G-protein coupled receptor ENDRB on migrating ENCC. Endothelin converting enzyme 1 (ECE1) post-translationally modifies the immature form of EDN3 into the active form.98, 99 The EDN3-EDNRB signaling pathway is involved in regulating the normal migration of ENCCs and maintains enteric

Additional Genes Regulating Neural Crest Cell Migration

The zinc finger homeobox 1b/SMAL interacting protein (ZFHX1B/SIP1/SMADIP1), is expressed in premigratory and migratory vagal neural crest cells. This transcription factor is involved in neural specification and the epithelial to mesenchymal transition during early NCC development. Zfxh1b-/- mice exhibit complete absence of vagal NCC precursors and die around E9.5 because of cardiovascular and neural defects.118 A human mutation in ZFHX1B is associated with Mowat-Wilson syndrome of which HSCR is

Proliferation, Survival, and Differentiation

Proliferation rates in the developing GI system are equivalent throughout the ENS, with active ENCC proliferation at the wavefront necessary to colonize the gut, and behind the wavefront to fully populate the expanding intestine131 and generate the millions of enteric neurons and glia present in the adult intestine.17 The balance between proliferation and differentiation is necessary to maintain a sufficient progenitor pool of cells necessary to ensure complete ENS colonization.1, 18, 71, 113,

SOX10

Sox10 is expressed in vagal NCCs as they emigrate from the neural tube. Sox10 is used as a marker of ENS progenitors as it is expressed during NCC migration and maintains the ENS progenitor state.100, 145 This transcription factor regulates key genes required for ENS, melanocyte, and glial development,146 such as Ednrb and Ret in ENCCs.147, 148, 149

Mouse and zebrafish Sox10 homozygous mutants display abnormal ENS and melanocyte phenotypes. Sox10Dom mice have a single base pair insertion leading

Vitamin A Metabolism

Of all the human genes and mouse models identified, the RET pathway and its interacting components are the most commonly disrupted genes contributing to the HSCR phenotype. However, the mutations discussed above account for only about 50% of the documented cases of HSCR. Thus, there must be other pathways involved in the pathogenesis of Hirschsprung disease. Recently, a role for retinoid signaling in ENS development and the pathogenesis of colonic aganglionosis has been suggested.160, 161

Retinoid Deficient Mouse Models

Currently, there are a 2 mouse models of depleted retinoid signaling that may phenotypically display some evidence for colonic aganglionosis, Rbp4-/- and Raldh2-/-.160, 161 Rbp4-/- mice are unable to store retinol in their livers, and when subjected to an RA deficient diet during embryogenesis via maternal restriction from E7.5, these mice become fully depleted of Vitamin A and its active form, RA. Subsequently, these Rbp4-/- mice exhibit mild retinoid deficiency, with distal bowel

Discussion

Although more than one-half of the cases associated with HSCR have associated genetic mutations, there is still much more to be learned about this disease. A continued effort is needed to identify responsible genes in experimental models but also to identify relevant mutations in humans. Whole exome and whole genome sequencing of patient samples are an ideal way to identify these genes and mutations. With new genetic models of HSCR and new tools to examine them, new modes of pathogenesis may be

Naomi E. Butler Tjaden is currently pursuing an MD/PhD degree through the University of Kansas Medical Center. Her article is based on a presentation given at the Combined Annual Meeting of the Central Society for Clinical and Translational Research and Midwestern Section American Federation for Medical Research held in Chicago, Ill, on April 2012.

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    Naomi E. Butler Tjaden is currently pursuing an MD/PhD degree through the University of Kansas Medical Center. Her article is based on a presentation given at the Combined Annual Meeting of the Central Society for Clinical and Translational Research and Midwestern Section American Federation for Medical Research held in Chicago, Ill, on April 2012.

    Conflict of interest: None.

    Research in the Trainor laboratory is supported by the Stowers Institute for Medical Research, the National Institute for Dental and Craniofacial Research (DE 016082), and the March of Dimes FY08-265.

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