Mutations in the endothelin‐receptor B gene in Hirschsprung disease in Sweden

Hirschsprung disease (HSCR) is a congenital malformation affecting 1 in 5000 live births. Absence of parasympathetic neuronal ganglia in the hindgut results in poor coordination of peristaltic movement and varying degrees of constipation. Familial occurrence with an increased risk for siblings as well as an uneven gender distribution indicate a genetic cause to the disease (1). The first gene to be reported mutated in HSCR was the RET proto-oncogene (2). The second susceptability gene was the endothelin receptor-B gene, encoding a G protein-coupled transmembrane receptor (3). We had previously reported (in Clinical Genetics) our results of SSCP screening of the RET gene in a Swedish population-based study of 69 cases, 7 familial and 62 sporadic (4). We now would like to add our results of single strand conformation polymorphism (SSCP) screening of the EDNRB gene in the same cohort. For further details concerning the patient population, please see (4). Of 69 unrelated HSCR cases, 7, all sporadic cases (10.1% of series, 11.3% of sporadic cases), are heterozygous for three different EDNRB mutations (Table 1). One sporadic case with total colonic and ileal aganglionosis has a de no6o mutation in exon 1, C109R, in the first transmembrane domain. This mutation was recently reported to inhibit the incorporation of the receptor-protein into the cell membrane, and can thus be predicted to cause haploinsufficiency (5). One of the mutations found in two unrelated cases, W226C, is novel and introduces a cysteine in the fourth transmembrane domain of the EDNRB protein (Fig. 1a). Despite identical mutations, one of these patients, a boy, had total colonic and partial ileal involvment and the other, a girl, a short aganglionic segment. Although sporadic cases, the mutations are segregating with a family history of constipation in both cases, and haplotype analysis reveals that this common mutation was not due to a founder effect (Fig. 1b). Four unrelated patients are heterozygous for the previously reported G57S mutation (6). All had sporadic short segment disease, and 3 of them had several cases of constipation in the family branch segregating the mutation. The 4th patient and his family are reported separately as they, in different combinations, also carry the RET R982C mutation (7). The haplotypes of the alleles with the G57S mutation are consistent with at least two different founder alleles (Fig. 1c). All three EDNRB mutations are absent in 100 Swedish control chromosomes. All 7 unrelated patients with EDNRB mutations were the only formally diagnosed HSCR cases in their kindreds, but in 5 of these, there were several relatives with severe constipation on the family branch segregating the mutation. This could indicate that there might be more HSCR cases in these families with ‘subclinical’ disease, and we plan to perform on-going long-term follow-up of these families. In summary, our data, in addition to those from other reports, suggest that some EDNRB mutations can be dosage-sensitive and that the phenotype can be modified by gender or by mutations in other genes, whilst other apparently isolated mutations might be penetrant enough to result in a serious phenotype (3, 5–8). In the combined population-based survey for germline EDNRB and RET mutations, only 11 (15.9%) of all HSCR cases are accounted for by mutations in these two genes (this study, (4)), which in the past were considered the two major susceptibility genes for HSCR. Other susceptibility genes must also be of importance, as shown in cases with combinations of mutations in different genes 1 PCR primers and SSCP conditions are available upon request (par-johan.svensson@cmm.ki.se).