Genetic study of a fami segregating Waardenburg-Shah syndrome

Abstract

Waardenburg-Shah syndrome (WS4, MIM_277580) is a congenital developmental disorder characterized by pigmentary abnormalities of the skin, eyes and hair, sensorineural deafness and intestinal aganglionosis (HSCR; Hirschsprung disease). Mutations in EDN3, EDNRB, or SOX10 account for approximately 65-85% of the WS4 patients. The disorder is clinically and genetically heterogeneous with either autosomal recessive or autosomal dominant inheritance with incomplete penetrance. EDN3, EDNRB, and SOX10 were sequenced in a three-generation family (14 individuals; 3 members affected with HSCR and 1 with WS4). A novel heterozygous missense mutation in the EDNRB (chromosome 13) was identified in four affected and three unaffected family members. The mutation changes the translation initiation codon Met1 of the EDNRB receptor isoforms 1 and 2 (M1V; predicted damaging) or the residue Met91 of the EDNRB isoform 3 (M91V; predicted benign). Importantly, the three EDNRB isoforms are expressed in the gut of newborns. Functional analyses coupled with confocal microscopy revealed the presence of a shorter EDNRB isoform 1 (M1V) as a result of alternative translation initiation site usage (Met46). Small quantities of this shorter protein were observed in the cellular membrane. Both EDNRB isoform 3 wild-type and the mutated (M91V) counterpart were only found in the cytosol, apparently not being implicated in the receptor-ligand relationship necessary for the differentiation of the neural crest cells from where both enteric neurons and melanocytes derived. Reduced amounts of a shorter form of the EDNRB receptor can indeed account for the disease phenotype. Yet, given the reduced penetrance of M1V, additional loci contributing to this disorder may exist in this family. To identify those, family members were genotyped using Illumina Human Omni2.5-quad BeadChip. After quality control and SNP pruning the Merlin software was used for parametric and non-parametric linkage. A susceptibility locus on chromosome 4q13.3-q24 (28.35 cM) was identified by both nonparametric and parametric linkage analysis. Haplotype analysis refined the linkage region to a 27.76 cM interval (between rs13146209 and rs17033669) encompassing two biologically plausible genes: MAPK10 and PPP3CA which are currently being sequenced in these individuals.