Double heterozygosity for a RET substitution interfering with splicing and an EDNRB missense mutation in Hirschsprung disease

Hirschsprung disease in an infant with a contiguous gene syndrome of chromosome 13

Hirschsprung disease is a developmental disorder resulting from the arrest of the craniocaudal migration of enteric neurons from the neural crest along gastrointestinal segments of variable length; see Behrman [Nelson textbook of pediatrics, 1992:954-956]. It is a heterogeneous disorder in which familial cases map to at least three loci whose function is necessary for normal neural crest-derived cell development. Homozygous mutations in the endothelin-B receptor gene (EDNRB) on 13q22 have been identified in humans and mice with Hirschsprung disease type 2 (HSCR2). The auditory pigmentary disorder, Waardenburg-Shah syndrome, comprises Waardenburg syndrome and Hirschsprung disease and has also been mapped to the EDNRB locus. Hirschsprung disease, malrotation, isochromia, a profound sensorineural hearing loss, and several other anomalies were found in an infant with an interstitial deletion of 13q, suggesting the existence of a contiguous gene syndrome involving developmental genes necessary for the normal growth of the neural crest derivatives of the eye, inner ear, and colon. We report on an additional patient with a deletion in 13q and Hirschsprung disease. Congenital anomalies associated with deletions of the distal long arm of chromosome 13 are sufficiently consistent to suggest a clinical syndrome.

Genetics of Hirschsprung disease

Hirschsprung disease (HSCR), or congenital intestinal aganglionosis, is a relatively common disorder of neural crest migration. It has a strong genetic basis, although simple Mendelian inheritance is rarely observed. Hirschsprung disease is associated with several other anomalies and syndromes, and animal models for these conditions exist. Mutations in the RET gene are responsible for approximately half of familial cases and a smaller fraction of sporadic cases. Mutations in genes that encode RET ligands (GDNF and NTN); components of another signaling pathway (EDNRB, EDN3, ECE-1); and the transcription factor, SOX10, have been identified in HSCR patients. A subset of these mutations is associated with anomalies of pigmentation and/or hearing loss. For almost every HSCR gene, incomplete penetrance of the HSCR phenotype has been observed, probably due to genetic modifier loci. Thus, HSCR has become a model of a complex polygenic disorder in which the interplay of different genes is currently being elucidated.

Multiple endocrine neoplasia type 2 and RET: from neoplasia to neurogenesis

Multiple endocrine neoplasia type 2 (MEN 2) is an inherited cancer syndrome characterised by medullary thyroid carcinoma (MTC), with or without phaeochromocytoma and hyperparathyroidism. MEN 2 is unusual among cancer syndromes as it is caused by activation of a cellular oncogene, RET. Germline mutations in the gene encoding the RET receptor tyrosine kinase are found in the vast majority of MEN 2 patients and somatic RET mutations are found in a subset of sporadic MTC. Further, there are strong associations of RET mutation genotype and disease phenotype in MEN 2 which have led to predictions of tissue specific requirements and sensitivities to RET activity. Our ability to identify genetically, with high accuracy, subjects with MEN 2 has revolutionised our ability to diagnose, predict, and manage this disease. In the past few years, studies of RET and its normal ligand and downstream interactions and the signalling pathways it activates have clarified our understanding of the roles played by RET in normal cell survival, proliferation, and differentiation, as well as in disease. Here, we review the current knowledge of the normal functions of RET and the effects of mutations of this gene in tumorigenesis and in normal development.

Characterisation of the human GFRalpha-3 locus and investigation of the gene in Hirschsprung disease

BACKGROUND

The GDNF family receptor alpha (GFRalpha) proteins are extracellular cell surface bound molecules that act as adapters in binding of the GDNF family of soluble neurotrophic factors to the RET receptor. These molecules are essential for development of many neural crest derived cell types and the kidney. Mutations in RET and in two members of the GDNF ligand family are associated with Hirschsprung disease (HSCR), a congenital absence of the enteric ganglia. Members of the GFRalpha family are also candidates for HSCR mutations. One such gene is GFRalpha-3, which is expressed in the peripheral nervous system and developing nerves.

OBJECTIVE

We have characterised the structure of the human GFRalpha-3 locus and investigated the gene for sequence variants in a panel of HSCR patients.

METHODS

Long range PCR or subcloning of PAC clones was used to investigate GFRalpha-3 intron-exon boundaries. A combination of single strand conformation polymorphism (SSCP) analysis and direct sequencing was used to investigate GFRalpha-3 sequence variants.

RESULTS

GFRalpha-3 spans eight coding exons and has a gene structure and organisation similar to that of GFRalpha-1. We identified three polymorphic variants in GFRalpha-3 in a normal control population, a subset of which also occurred in HSCR patients. We did not detect any sequence variants within the coding sequence of GFRalpha-3. We found a base substitution in the 5' UTR of GFRalpha-3, 15 base pairs upstream of the translation start site. A second substitution was identified in intron 4 (IVS4-30G>A) between the splice branch site and the splice acceptor site. The final variant was a 2 base pair insertion within the splice donor consensus sequence of exon 7 (IVS7+4ins GG).

CONCLUSIONS

We did not detect any correlation between variants of GFRalpha-3 and the HSCR phenotype. Our data suggest that mutations of this gene are not a cause of HSCR.

Incidence of RET mutations in patients with Hirschsprung's disease

BACKGROUND

RET mutations have been reported variously to affect 7% to 41% of Hirschsprung's disease (HSCR) patients depending on familial or sporadic occurrence of the disease, length of aganglionosis and possible association with other disease phenotypes. The authors report a study of the incidence of RET mutations in unselected HSCR patients from two regional centers and correlate their genotypes and phenotypes.

METHODS

The records of HSCR patients treated in 2 regional centers with a combined population of 5 million were reviewed, and blood samples were obtained from 57 patients. During the same period, 39 patients with similar demographic data refused or provided insufficient blood for study. DNA was extracted, and the 21 exons of the RET protooncogene were screened for mutations using denaturing gradient gel electrophoresis (DGGE).

RESULTS

Of 57 patients, 48 were sporadic, and 9 were familial. Lengths of aganglionosis were total colonic, 4; long, 11; short, 39; ultrashort, 1; unclassified, 2. Associated anomalies were present in 20. Causative mutations were identified in 4 (7%): missense or "silent" in 3 (exons 5, 11, 13) and deletion in 1. The silent mutation of exon 11 recently has been shown to have effects on correct RET mRNA splicing. One mutation occurred in total colonic aganglionosis (25%), 1 in long segment dysganglionosis (9%), and 2 in short segment aganglionosis (5%). Surprisingly, all these mutations occurred in sporadic cases (10%). Five patients (9%) had rare polymorphic alleles at exon 14 (n = 1) and exon 18 (n = 4). Fifty patients (88%) showed common polymorphic alleles (sequence variants) in 1 or more exons (> 4, n = 5).

CONCLUSIONS

RET mutation as a primary cause for Hirschsprung's disease in the general surgical population is less frequent than previously thought. This observation is compatible with the hypothesis that HSCR could be a polygenic disease caused by additive subclinical effects of more than one gene, including RET.