A comprehensive screen of the USH2A gene in 185 patients with autosomal recessive retinal disease

Abstract

Purpose: Mutations in the USH2A gene are the commonest cause of both Usher syndrome and autosomal recessive retinitis pigmentosa (RP). The USH2A gene has multiple transcripts; the longest transcript (72 exons) encodes usherin, a protein of 5,202 amino acids. The aim of this study is to provide insights into the clinical and genetic characteristics of nonsyndromic USH2A-related retinal disease. Of particular interest was the identification of “retina-specific” alleles as these may point to protein domains essential for photoreceptor function.

Methods: The USH2A gene was screened in 185 probands with autosomal recessive retinal dystrophy without documented sensorineural hearing loss. Publicly available datasets (NHLBI GO Exome Sequencing Project [ESP], 1000 genomes) were used to distinguish disease-causing alleles from the background of nonpathogenic variation. Clinical investigations of patients with two likely pathogenic mutations were performed including OCT, fundus autofluorescence imaging (FAF) and audiological assessment.

Results: A total of 68 coding or splice site variants with a minor allele frequency of 0.20% or less in the ESP dataset were identified (5 nonsense or frameshifting indels, 4 splice site, 47 nonsynonymous); 24/185 (13%) probands were found to harbour at least two such variants. Interestingly, only in one individual were both variants previously associated with syndromic disease. Three alleles that are likely to be specific to those with nonsyndromic disease were found in more than one proband: p.Cys759Phe, p.Cys3358Tyr and c.12295-3T>A. All 24 patients presented with visual symptoms (night blindness and visual field loss; median age 29 years, range 19-43) and none reported early-onset hearing loss. The retinal phenotype was consistent with RP in all cases and a ring of high density on FAF was observed in 19/24 patients. Audiology testing revealed a phenotype consistent with Usher syndrome type 2 in 1/17 patients.

Conclusions: USH2A retinopathy is a common cause of nonsyndromic autosomal recessive retinal disease. Identifying disease-causing alleles remains challenging but the extensive catalogues of human genetic variation yielded by exome sequencing projects provide significant insights. Three likely “retina-specific” variants in USH2A were identified. The results of this study are expected to increase the sensitivity of molecular testing in this highly heterogeneous condition.

Keywords: 696 retinal degenerations: hereditary • 537 gene screening • 688 retina