Mutational screening of the USH2A gene in Spanish USH patients reveals 23 novel pathogenic mutations

Abstract

Background: Usher Syndrome type II (USH2) is an autosomal recessive disorder, characterized by moderate to severe hearing impairment and retinitis pigmentosa (RP). Among the three genes implicated, mutations in the USH2A gene account for 74-90% of the USH2 cases.

Methods: To identify the genetic cause of the disease and determine the frequency of USH2A mutations in a cohort of 88 unrelated USH Spanish patients, we carried out a mutation screening of the 72 coding exons of this gene by direct sequencing. Moreover, we performed functional minigene studies for those changes that were predicted to affect splicing.

Results: As a result, a total of 144 DNA sequence variants were identified. Based upon previous studies, allele frequencies, segregation analysis, bioinformatics' predictions and in vitro experiments, 37 variants (23 of them novel) were classified as pathogenic mutations.

Conclusions: This report provide a wide spectrum of USH2A mutations and clinical features, including atypical Usher syndrome phenotypes resembling Usher syndrome type I. Considering only the patients clearly diagnosed with Usher syndrome type II, and results obtained in this and previous studies, we can state that mutations in USH2A are responsible for 76.1% of USH2 disease in patients of Spanish origin.

Figure 1

In vitro splicing assays for p.E2496E and p.V382M mutations. Gel electrophoresis shows the different splicing processes for WT minigene and mutants constructions. COS-7 cells transfection experiments were performed in duplicate. Numbers at the bottom of gels indicate the proportion (%) of misspliced transcripts compared to the full-length transcript. For the p.E2496E mutation, an evident increase of band b (corresponding to the aberrant transcript that only contains 37pb of the exon 40) can be observed with regard to the WT minigene expression product. For the p.V382M variant a small increase of the exon 7 skipping in the mutant minigene expression is observed (band d).

Figure 2

Schematic illustration of the pathogenic and possibly pathogenic (UV3) mutations identified in this study along the USH2A protein domains.