A homozygous missense mutation in the tyrosine E kinase domain of the RET proto-oncogene in an infant with total intestinal aganglionosis

The Polycomb group gene rnf2 is essential for central and enteric neural system development in zebrafish

The development of central nervous system (CNS) and enteric nervous system (ENS) is under precise and strict control in vertebrates. Whether and how the Polycomb repressive complex 1 (PRC1) is involved in it remain unclear. To investigate the role of PRC1 in the nervous system development, using CRISPR/Cas9 technology, we have generated mutant zebrafish lines for the rnf2 gene which encodes Ring1b, the enzymatic component of the PRC1 complex. We show that rnf2 loss of function leads to abnormal migration and differentiation of neural crest and neural precursor cells. rnf2 mutant embryos exhibit aganglionosis, in which the hindgut is devoid of neurons. In particular, the formation of 5-HT serotonin neurons and myelinating glial cells is defective. Furthermore, ectopic expression of ENS marker genes is observed in forebrain of rnf2 mutant embryos. These findings suggest that the rnf2 gene plays an important role in the migration and differentiation of neural precursor cells, and its absence leads to abnormal development of ENS and CNS in zebrafish.

Gut innervation and enteric nervous system development: a spatial, temporal and molecular tour de force

During embryonic development, the gut is innervated by intrinsic (enteric) and extrinsic nerves. Focusing on mammalian ENS development, in this Review we highlight how important the different compartments of this innervation are to assure proper gut function. We specifically address the three-dimensional architecture of the innervation, paying special attention to the differences in development along the longitudinal and circumferential axes of the gut. We review recent information about the formation of both intrinsic innervation, which is fairly well-known, as well as the establishment of the extrinsic innervation, which, despite its importance in gut-brain signaling, has received much less attention. We further discuss how external microbial and nutritional cues or neuroimmune interactions may influence development of gut innervation. Finally, we provide summary tables, describing the location and function of several well-known molecules, along with some newer factors that have more recently been implicated in the development of gut innervation.

Glial Cell-Derived Neurotrophic Factor Induces Enteric Neurogenesis and Improves Colon Structure and Function in Mouse Models of Hirschsprung Disease

BACKGROUND & AIMS

Hirschsprung disease (HSCR) is a life-threatening birth defect in which the distal colon is devoid of enteric neural ganglia. HSCR is treated by surgical removal of aganglionic bowel, but many children continue to have severe problems after surgery. We studied whether administration of glial cell derived neurotrophic factor (GDNF) induces enteric nervous system regeneration in mouse models of HSCR.

METHODS

We performed studies with four mouse models of HSCR: Holstein (Hol^Tg/Tg, a model for trisomy 21-associated HSCR), TashT (TashT^Tg/Tg, a model for male-biased HSCR), Piebald-lethal (Ednrb^s-l//s-l, a model for EDNRB mutation-associated HSCR), and Ret^9/- (with aganglionosis induced by mycophenolate). Mice were given rectal enemas containing GDNF or saline (control) from postnatal days 4 through 8. We measured survival times of mice, and colon tissues were analyzed by histology, immunofluorescence, and immunoblots. Neural ganglia regeneration and structure, bowel motility, epithelial permeability, muscle thickness, and neutrophil infiltration were studied in colon tissues and in mice. Stool samples were collected, and microbiomes were analyzed by 16S rRNA gene sequencing. Time-lapse imaging and genetic cell-lineage tracing were used to identify a source of GDNF-targeted neural progenitors. Human aganglionic colon explants from children with HSCR were cultured with GDNF and evaluated for neurogenesis.

RESULTS

GDNF significantly prolonged mean survival times of Hol^Tg/Tg mice, Ednrb^s-l//s-l mice, and male TashT^Tg/Tg mice, compared with control mice, but not Ret^9/- mice (which had mycophenolate toxicity). Mice given GDNF developed neurons and glia in distal bowel tissues that were aganglionic in control mice, had a significant increase in colon motility, and had significant decreases in epithelial permeability, muscle thickness, and neutrophil density. We observed dysbiosis in fecal samples from Hol^Tg/Tg mice compared with feces from wild-type mice; fecal microbiomes of mice given GDNF were similar to those of wild-type mice except for Bacteroides. Exogenous luminal GDNF penetrated aganglionic colon epithelium of Hol^Tg/Tg mice, inducing production of endogenous GDNF, and new enteric neurons and glia appeared to arise from Schwann cells within extrinsic nerves. GDNF application to cultured explants of human aganglionic bowel induced proliferation of Schwann cells and formation of new neurons.

CONCLUSIONS

GDNF prolonged survival, induced enteric neurogenesis, and improved colon structure and function in 3 mouse models of HSCR. Application of GDNF to cultured explants of aganglionic bowel from children with HSCR induced proliferation of Schwann cells and formation of new neurons. GDNF might be developed for treatment of HSCR.

Gene-environment interactions and the enteric nervous system: Neural plasticity and Hirschsprung disease prevention

Intestinal function is primarily controlled by an intrinsic nervous system of the bowel called the enteric nervous system (ENS). The cells of the ENS are neural crest derivatives that migrate into and through the bowel during early stages of organogenesis before differentiating into a wide variety of neurons and glia. Although genetic factors critically underlie ENS development, it is now clear that many non-genetic factors may influence the number of enteric neurons, types of enteric neurons, and ratio of neurons to glia. These non-genetic influences include dietary nutrients and medicines that may impact ENS structure and function before or after birth. This review summarizes current data about gene-environment interactions that affect ENS development and suggests that these factors may contribute to human intestinal motility disorders like Hirschsprung disease or irritable bowel syndrome.

RET gene is a major risk factor for Hirschsprung's disease: a meta-analysis

PURPOSE

During the past two decades several genes have been identified that control morphogenesis and differentiation of the enteric neuron system (ENS). These genes, when mutated or deleted, interfere with ENS development. RET gene is the major gene causing Hirschsprung's disease (HD). Mutations in RET gene are responsible for 50% of familial HD cases and 15-20% of sporadic cases. The aim of this meta-analysis was to determine the incidence of RET gene mutations in patients with HD and to correlate RET mutations with the extent of aganglionosis.

METHODS

A systematic literature-based search for relevant cohorts was performed using the terms "Hirschsprung's disease AND RET Proto-oncogene", "Hirschsprung's disease AND genetic polymorphism" and "RET Gene". The relevant cohorts of HD were systematically searched for reported mutations in the RET gene (RET+). Data on mutation site, phenotype, and familial or sporadic cases were extracted. Combined odds ratio (OR) with 95% CI was calculated to estimate the strength of the different associations.

RESULTS

In total, 23 studies concerning RET with 1270 individuals affected with HD were included in this study. 228 (18%) of these HDs were RET+. Of these 228, 96 (42%) presented as rectosigmoid, 81 (36%) long segment, 18 (8%) as TCA, 16 (7%) as total intestinal aganglionosis and 17 (7%) individuals were RET+ but no extent of aganglionosis was not reported. In the rectosigmoid group, no significant association between phenotype and RET mutation could be shown (P = 0.006), whereas a clear association could be shown between long-segment disease, total colonic- and total intestinal aganglionosis and RET mutations (P = 0.0002). Mutations most often occurred in Exon 13 (24) and showed significant association with rectosigmoid disease (P = 0.004). No significance could be shown between RET+ and sporadic cases (P = 0.53), albeit a trend towards RET+ and Familial cases could be observed (P = 0.38).

CONCLUSIONS

The association with the RET gene and HD is well recognized. This study showed a clear association between RET+ mutations and the long-segment, total colonic- and total intestinal aganglionosis. Exon 13 appears to be a mutational "hot spot" in rectosigmoid disease.

Building a second brain in the bowel

The enteric nervous system (ENS) is sometimes called the "second brain" because of the diversity of neuronal cell types and complex, integrated circuits that permit the ENS to autonomously regulate many processes in the bowel. Mechanisms supporting ENS development are intricate, with numerous proteins, small molecules, and nutrients that affect ENS morphogenesis and mature function. Damage to the ENS or developmental defects cause vomiting, abdominal pain, constipation, growth failure, and early death. Here, we review molecular mechanisms and cellular processes that govern ENS development, identify areas in which more investigation is needed, and discuss the clinical implications of new basic research.

Advances in Hirschsprung disease genetics and treatment strategies: an update for the primary care pediatrician

Hirschsprung disease (HSCR) is a multigenic condition with variable presentation. Most commonly, it presents in the neonatal period as a functional intestinal obstruction secondary to failure of caudal migration of the enteric nervous system. Classically, this manifests as dilated proximal bowel and constricted distal bowel with absent ganglia and hypertrophic nerve trunks. When recognized early, medical and surgical therapies can be instituted to minimize associated morbidity and mortality. This article reviews current understanding of the etiology of HSCR, its multigenic associations, the historical evolution of HSCR diagnosis and treatment, and current HSCR therapies.

Enteric nervous system development: migration, differentiation, and disease

The enteric nervous system (ENS) provides the intrinsic innervation of the bowel and is the most neurochemically diverse branch of the peripheral nervous system, consisting of two layers of ganglia and fibers encircling the gastrointestinal tract. The ENS is vital for life and is capable of autonomous regulation of motility and secretion. Developmental studies in model organisms and genetic studies of the most common congenital disease of the ENS, Hirschsprung disease, have provided a detailed understanding of ENS development. The ENS originates in the neural crest, mostly from the vagal levels of the neuraxis, which invades, proliferates, and migrates within the intestinal wall until the entire bowel is colonized with enteric neural crest-derived cells (ENCDCs). After initial migration, the ENS develops further by responding to guidance factors and morphogens that pattern the bowel concentrically, differentiating into glia and neuronal subtypes and wiring together to form a functional nervous system. Molecules controlling this process, including glial cell line-derived neurotrophic factor and its receptor RET, endothelin (ET)-3 and its receptor endothelin receptor type B, and transcription factors such as SOX10 and PHOX2B, are required for ENS development in humans. Important areas of active investigation include mechanisms that guide ENCDC migration, the role and signals downstream of endothelin receptor type B, and control of differentiation, neurochemical coding, and axonal targeting. Recent work also focuses on disease treatment by exploring the natural role of ENS stem cells and investigating potential therapeutic uses. Disease prevention may also be possible by modifying the fetal microenvironment to reduce the penetrance of Hirschsprung disease-causing mutations.

Development of the enteric nervous system and its role in intestinal motility during fetal and early postnatal stages

Motility patterns in the mature intestine require the coordinated interaction of enteric neurons, gastrointestinal smooth muscle, and interstitial cells of Cajal. In Hirschsprung's disease, the aganglionic segment causes functional obstruction, and thus the enteric nervous system (ENS) is essential for gastrointestinal motility after birth. Here we review the development of the ENS. We then focus on motility patterns in the small intestine and colon of fetal mice and larval zebrafish, where recent studies have shown that the first intestinal motility patterns are not neurally mediated. Finally, we review the development of gastrointestinal motility in humans.

Allele dosage-dependent penetrance of RET proto-oncogene in an Israeli-Arab inbred family segregating Hirschsprung disease

Hirschsprung disease (HSCR) is characterised by intestinal obstruction resulting from an absence of ganglion cells in the intestinal tract. The mutations in the major gene, RET, associated with isolated HSCR, are dominant loss-of-function mutations with incomplete penetrance and variable expressivity. We have ascertained a large inbred Israeli-Arab family segregating HSCR. Sequencing of the RET gene showed a splicing mutation, IVS6+5G- >A, in the homozygous state in all the females with severe forms of HSCR and in the heterozygous state in the male patient with short-segment HSCR. The recently described hypomorphic-RET predisposing allele, rs2435357, was transmitted in the heterozygous state to the male patient, but was not transmitted to the three affected females. Although the heterozygous IVS6+5G- >A is of low-penetrance for short-segment HSCR disease, the homozygous state is fully penetrant for total aganglionosis or long-segment HSCR. As in other inbred populations segregating a weakly penetrant RET allele (Mennonite), our findings support the hypothesis that the penetrance of RET gene mutations for the HSCR phenotype depends on: (i) the nature of the mutation, (ii) the allele dosage and (iii) modifier-loci.

Molecular pathogenesis of MEN2-associated tumors

Although the gene responsible for multiple endocrine neoplasia type 2 (MEN2) was discovered many years ago, the exact mechanisms of tumor development in patients affected with RET germline mutations remain unknown. In vitro studies have certain pitfalls, one of which is the use of cell culture systems such as the NIH3T3 cells, in which RET usually is not expressed in contrast to the in vivo situation. Recent data suggest that an overrepresentation of mutant RET as a 'second hit' event might trigger tumorigenesis. However, alterations in other genes might contribute to this overrepresentation of RET or impact on MEN 2-related tumor development through completely different mechanisms and pathways. The final goal of further elucidating the natural history and pathogenesis of MEN2-related tumors should be the chance to offer patients with RET germline mutations an optimal cancer prevention (e.g. codon specific recommendations for prophylactic thyroidectomy) and treatment program, especially for metastatic medullary thyroid carcinoma for which presently no effective therapy other than surgery exists.

Polymerase chain reaction-single strand conformational polymorphism analysis of rearranged during transfection proto-oncogene in Chinese familial hirschsprung’s disease

AIM: To investigate the relationship between mutations of rearranged during transfection (RET) proto-oncogene and Chinese patients with Hirschsprung’s disease (HD), and to elucidate the genetic mechanism of familial HD patient at the molecular level.

METHODS: Genomic DNA was extracted from venous blood of probands and their relatives in two genealogies. Polymerase chain reaction (PCR) products, which were amplified using specific primers (RET, exons 11, 13, 15 and 17), were electrophoresed to analyze the single-strand conformational polymorphism (SSCP) patterns. The positive amplified products were sequenced. Forty-eight sporadic HD patients and 30 normal children were screened for mutations of RET proto-oncogene simultaneously.

RESULTS: Three cases with HD in one family were found to have a G heterozygous insertion at nucleotide 18974 in exon 13 of RET cDNA (18974insG), which resulted in a frameshift mutation. In another family, a heterozygosity for T to G transition at nucleotide 18888 in the same exon which resulted in a synonymous mutation of Leu at codon 745 was detected in the proband and his father. Eight RET mutations were confirmed in 48 sporadic HD patients.

CONCLUSION: Mutations of RET proto-oncogene may play an important role in the pathogenesis of Chinese patients with HD. Detection of mutated RET proto-oncogene carriers may be used for genetic counseling of potential risk for HD in the affected families.

Mice expressing a dominant-negative Ret mutation phenocopy human Hirschsprung disease and delineate a direct role of Ret in spermatogenesis

The Ret receptor tyrosine kinase mediates physiological signals of glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) and is essential for postnatal survival in mice. It is implicated in a number of human diseases and developmental abnormalities. Here, we describe our analyses of mice expressing a Ret mutant (RetDN) with diminished kinase activity that inhibits wild-type Ret activity, including its activation of AKT. All RetDN/+ mice died by 1 month of age and had distal intestinal aganglionosis reminiscent of Hirschsprung disease (HSCR) in humans. The RetDN/+ proximal small intestine also had severe hypoganglionosis and reduction in nerve fiber density, suggesting a potential mechanism for the continued gastric dysmotility in postsurgical HSCR patients. Unlike Ret-null mice, which have abnormalities in the parasympathetic and sympathetic nervous systems, the RetDN/+ mice only had defects in the parasympathetic nervous system. A small proportion of RetDN/+ mice had renal agenesis, and the remainder had hypoplastic kidneys and developed tubulocystic abnormalities postnatally. Postnatal analyses of the testes revealed a decreased number of germ cells, degenerating seminiferous tubules,maturation arrest and apoptosis, indicating a crucial role for Ret in early spermatogenesis.

Clinical relationship between EDN-3 gene, EDNRB gene and Hirschsprung’s disease

AIM: To investigate the mutation of EDNRB gene and EDN-3 gene in sporadic Hirschsprung’s disease (HD) in Chinese population.

METHODS: Genomic DNA was extracted from bowel tissues of 34 unrelated HD patients which were removed by surgery. Exon 3, 4, 6 of EDNRB gene and Exon 1, 2 of EDN-3 gene were amplified by polymerase chain reaction (PCR) and analyzed by single strand conformation polymorphism (SSCP).

RESULTS: EDNRB mutations were detected in 2 of the 13 short-segment HD. One mutant was in the exon 3, the other was in the exon 6. EDN-3 mutation was detected in one of the 13 short-segment HD and in the exon 2. Both EDNRB and EDN-3 mutations were detected in one short-segment HD. No mutations were detected in the ordinary or long-segment HD.

CONCLUSION: The mutations of EDNRB gene and EDN-3 gene are found in the short-segment HD of sporadic Hirschsprung’s disease in Chinese population, which suggests that the EDNRB gene and EDN-3 gene play important roles in the pathogenesis of HD.

Mutation of RET gene in Chinese patients with Hirschsprung’s disease

AIM: To investigate the pathogenic mechanism of Hirschsprung’s disease (HD) at the molecular level and to elucidate the relationship between RET oncogene and Chinese patients with HD.

METHODS: Exon 13 of RET oncogene from 20 unrelated HD patients was analyzed with polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP). The positive amplifying products were then sequenced. According to the results of SSCP and DNA sequence, SSCP was done as well for the samples from the family other members of some cases with mutated RET gene.

RESULTS: SSCP analysis indicated that mobility abnormality existed in 4 unrelated HD patients. Direct DNA sequence analysis identified a missense mutation, T to G at the nucleotide 18888 and a frameshift mutation at the nucleotide 18926 insG. In a HD family, the sicked child and his father were the same heterozygous missense mutation (T to G at nucleotide 18888).

CONCLUSION: Among Chinese HD patients, RET gene mutations may exist in considerable proportion with different patterns. These new discoveries indicate that RET mutations may play an important role in the pathogenesis of unrelated HD in the Chinese population. PCR-SSCP combined with DNA sequence can be used as a tool in the genetic diagnosis of HD.

In ovo transplantation of enteric nervous system precursors from vagal to sacral neural crest results in extensive hindgut colonisation

The enteric nervous system (ENS) is derived from vagal and sacral neural crest cells (NCC). Within the embryonic avian gut, vagal NCC migrate in a rostrocaudal direction to form the majority of neurons and glia along the entire length of the gastrointestinal tract, whereas sacral NCC migrate in an opposing caudorostral direction, initially forming the nerve of Remak, and contribute a smaller number of ENS cells primarily to the distal hindgut. In this study, we have investigated the ability of vagal NCC, transplanted to the sacral region of the neuraxis, to colonise the chick hindgut and form the ENS in an experimentally generated hypoganglionic hindgut in ovo model. Results showed that when the vagal NC was transplanted into the sacral region of the neuraxis, vagal-derived ENS precursors immediately migrated away from the neural tube along characteristic pathways, with numerous cells colonising the gut mesenchyme by embryonic day (E) 4. By E7, the colorectum was extensively colonised by transplanted vagal NCC and the migration front had advanced caudorostrally to the level of the umbilicus. By E10, the stage at which sacral NCC begin to colonise the hindgut in large numbers, myenteric and submucosal plexuses in the hindgut almost entirely composed of transplanted vagal NCC, while the migration front had progressed into the pre-umbilical intestine, midway between the stomach and umbilicus. Immunohistochemical staining with the pan-neuronal marker, ANNA-1, revealed that the transplanted vagal NCC differentiated into enteric neurons, and whole-mount staining with NADPH-diaphorase showed that myenteric and submucosal ganglia formed interconnecting plexuses, similar to control animals. Furthermore, using an anti-RET antibody, widespread immunostaining was observed throughout the ENS, within a subpopulation of sacral NC-derived ENS precursors, and in the majority of transplanted vagal-to-sacral NCC. Our results demonstrate that: (1) a cell autonomous difference exists between the migration/signalling mechanisms used by sacral and vagal NCC, as transplanted vagal cells migrated along pathways normally followed by sacral cells, but did so in much larger numbers, earlier in development; (2) vagal NCC transplanted into the sacral neuraxis extensively colonised the hindgut, migrated in a caudorostral direction, differentiated into neuronal phenotypes, and formed enteric plexuses; (3) RET immunostaining occurred in vagal crest-derived ENS cells, the nerve of Remak and a subpopulation of sacral NCC within hindgut enteric ganglia.

The pathogenesis of Hirschsprung disease

Hirschsprung disease is the most common congenital malformation of the enteric nervous system. Phenotypic expression is variable because of incomplete penetrance, and the pathogenesis is multifactorial. Although mutations of the RET tyrosine kinase gene remain the most commonly identified cause, there are now eight separate human gene loci identified whose mutations result in this disease. Analysis of these gene products in experimental animal models and cell systems has led to an increasing elucidation of the signaling pathways that are in operation during specific embryonic time stages and that direct the spatial arrangements and differentiation of enteric neuroblasts. Mutation analysis through in vitro cell expression studies has led to detailed descriptions of the affected microdomains of signal pathway receptors and the cellular pathogenesis of abnormal signaling that leads to apoptosis of developing neurons before the completion of enteric nervous system development. The full description of the pathogenesis of this disorder awaits the definition of new genetic loci, multiple gene interactions, and the acknowledgment of random events that may lead to aganglionosis of the distal bowel.