A Novel Constitutively Active Rhodopsin Mutation Causes Autosomal Dominant CSNB

Abstract

Purpose: : Mutations in RHO, PDE6B and GNAT1 can lead to autosomal dominant congenital stationary night blindness (adCSNB). Constitutive activation of the phototransduction cascade has been suggested to be the pathogenic mechanism underlying this disease. Here we aimed to identify the genetic defect in a large Swiss family affected with adCSNB and to investigate the pathogenic mechanism of the mutation.

Methods: : Two affected cousins of a large Swiss family were clinically examined with standard clinical examinations, funduscopy, EOG, ERG and dark adaptometry. For genetic diagnosis, the coding exons and flanking regions of RHO were PCR amplified and sequenced in the DNA of 7 affected and 5 unaffected members of this family. The ability of mutant rhodopsin to constitutively activate transducin was monitored by measuring the catalytic exchange of bound GDP for radiolabeled [³⁵S]GTPγS in transducin.

Results: : We identified a novel mutation in RHO (c.884C>T,p.Ala295Val) in patients with adCSNB. Two affected cousins were examined clinically. They had full vision under photopic conditions, showed no fundus abnormalities, revealed EOG results in the normal range but presented night blindness. Only the scotopic ERG of both patients was altered. One patient revealed an electronegative ERG due to a severely reduced b-wave, while the affected cousin had no electronegative ERG but also a reduced b-wave. In both, the oscillatory potentials were severely reduced. In vitro studies in the presence of 11-cis retinal showed that the mutant rhodopsin is inactive, similar to wildtype, responding only when exposed to light. However, in the absence of 11-cis-retinal, unlike wildtype opsin, the mutant opsin constitutively activates transducin.

Conclusions: : Our study adds a fourth rhodopsin mutation associated with CSNB. Although the phenotype of autosomal dominant CSNB may vary slightly in patients showing mutations in RHO, PDE6B or GNAT1, the disease course seems to be stationary with only scotopic vision being affected. Our data indicate that the mutant opsin is able to constitutively activate transducin, which is a consistent and common feature of all four CSNB-associated rhodopsin mutations reported to date.

Keywords: mutations • proteins encoded by disease genes • photoreceptors: visual performance