Founder USH2A Mutations in French–Canadian Families Affected by Usher Type II Syndrome

Abstract

Abstract: : Purpose: Usher syndrome is an autosomal recessive (AR) disorder characterized by retinitis pigmentosa and hearing loss. Mutations in the USH2A gene, which encodes usherin at 1q41, cause Usher type II, a moderate to severe form of the syndrome. USH2A generates 2 isoforms: A (21 exons) and B (A+51 exons). To decipher the causes of Usher in French–Canadian type II families, we screened USH2A for mutations in 5 pedigrees. Genotyping was performed to identify founder effects. Methods: Five (5) families (12 affected, 40 asymptomatic) were investigated. The largest PV pedigree included 7 affected and 15 unaffected. Usher type II was confirmed by ophthalmological and medical examinations. Mutations screening was performed by direct sequencing of USH2A isoforms A and B using primers specific for the 72 exons and their intron–exon boundaries. Variations were confirmed by second strand sequencing. 142 unrelated asymptomatic individuals were used as controls. Haplotypes were constructed by genotyping 4 Genethon markers in conjonction with 10 SNPs within the USH2A locus. Results: AR inheritance was confirmed in the 5 pedigrees. One branch of the PV pedigree displayed pseudo–dominance. Six disease haplotypes were detected in the 5 families. Haplotype analysis in the PV family showed homozygosity for 5 of the 7 patients. Sequencing USH2A isoform A in these 5 homozygotes revealed a CT deletion, C1447FS, in exon 21. The other 2 PV patients carried this first haplotype plus one additional disease haplotype. They were compound carriers for the CT deletion and a novel C3575Y mutation in exon 54. RT–PCR confirmed that exons 53 to 55 were expressed in retinal RNA. Interestingly, 1 patient in the RR family also harbored the C3575Y mutation in an homozygous fashion. The RP family was homozygous for the CT deletion. The last 2 families displayed 4 different haplotypes. The 2 FN patients carried a compound R626X stop codon in exon 11 and a GA insertion, I2422FS, in exon 38 while the TD patient presented 2 distinct haplotypes. Conclusions: Founder effects were detected for 2 haplotypes/mutations conveyed by 9 patients in 3 different families. In the PV family, we further observed 2 distinct combinations of 2 haplotypes explaining pseudo–dominance in one of its branches. One novel homozygous USH2A mutation, C3575Y, was also characterized in isoform B. Genetic screening for USH2A should be offered to Usher type II families in the Province of Quebec.

Keywords: retinal degenerations: hereditary • genetics • retinitis