Naturally occurring rhodopsin mutation in the dog causes retinal dysfunction and degeneration mimicking human dominant retinitis pigmentosa

Abstract

Rhodopsin is the G protein-coupled receptor that is activated by light and initiates the transduction cascade leading to night (rod) vision. Naturally occurring pathogenic rhodopsin (RHO) mutations have been previously identified only in humans and are a common cause of dominantly inherited blindness from retinal degeneration. We identified English Mastiff dogs with a naturally occurring dominant retinal degeneration and determined the cause to be a point mutation in the RHO gene (Thr4Arg). Dogs with this mutant allele manifest a retinal phenotype that closely mimics that in humans with RHO mutations. The phenotypic features shared by dog and man include a dramatically slowed time course of recovery of rod photoreceptor function after light exposure and a distinctive topographic pattern to the retinal degeneration. The canine disease offers opportunities to explore the basis of prolonged photoreceptor recovery after light in RHO mutations and determine whether there are links between the dysfunction and apoptotic retinal cell death. The RHO mutant dog also becomes the large animal needed for preclinical trials of therapies for a major subset of human retinopathies. R hodopsin, the visual pigment of rod photoreceptors, is one of many G protein-coupled receptors involved in human disease (1-3). Numerous rhodopsin gene (RHO) mutations (mostly point mutations) cause retinitis pigmentosa (RP), a blinding human retinal degeneration (see RetNet, http:͞͞ www.sph.uth.tmc.edu͞Retnet͞; refs. 4 and 5). RP caused by RHO mutations exhibits two different phenotypes (4, 5). One is an early-onset disorder with rapid loss of rods uniformly across the retina. The second has a protracted natural history of vision loss and puzzling features: rod vision can be normal early in life, and degeneration slowly spreads from a disease focus in one retinal region. Furthermore, and independent of disease stage, there is abnormally slow recovery of rod vision after bright light exposure. Variation in severity of this second phenotype may indicate that epigenetic factors play an important role in its progression (5-7) and suggests that it may be particularly amenable to preventive or ameliorative therapies.Other than in humans, naturally occurring disease-causing RHO mutations have not been identified previously in mammals. Genetically engineered animals and mutagenized flies have been the mainstay for in vivo research and treatment attempts in the past decade (reviewed in ref. 8).Naturally occurring hereditary retinal degenerations in dogs, termed progressive retinal atrophies (PRAs), are widespread and have provided several models of autosomal recessive and X-linked RP (9-13). We have now identified an autosomal dominant form of PRA, one that closely resembles the second human phenotype described above. This model should advance understanding of the pathophysiology of the disease and therapies for this major subset of dominant RP. Materials and MethodsRhodopsin Gene Sequence Analysis. Mutation analysis by comparative sequencing of the five c...

A Naturally Occurring Mutation of the Opsin Gene (T4R) in Dogs Affects Glycosylation and Stability of the G Protein-coupled Receptor

“…2, A1-A3). It should be noted that older animals (Ͼ10 months) begin to exhibit severe retinal degeneration, initially in distinct topographic regions that later become more generalized (22). Fundus photomontages of the left eye of a 6.5-month-old RHO T4R/ϩ RPE65 Ϫ/Ϫ dog, which could not produce the chromophore because of mutation in the RPE65 gene (24,25,44), displayed hyperreflectivity of the tapetal fundus (Fig.…”

“…RHO T4R/ϩ and RHO T4R/T4R dogs derive their mutant RHO allele from English mastiff dogs affected with autosomal dominant progressive retinal atrophy as described previously (22,23). RPE65 Ϫ/Ϫ and RPE65 ϩ/Ϫ dogs derive their mutant allele, a 4-bp deletion in the canine RPE65 (retinal pigment epithelium protein of 65 kDa) gene, from a briard dog affected with the canine homolog of Leber's congenital amaurosis (24,25).…”

“…RPE65 Ϫ/Ϫ and RPE65 ϩ/Ϫ dogs derive their mutant allele, a 4-bp deletion in the canine RPE65 (retinal pigment epithelium protein of 65 kDa) gene, from a briard dog affected with the canine homolog of Leber's congenital amaurosis (24,25). All dogs used in this study were tested to determine their genotype at either or both the RHO (22,23) and RPE65 (25) loci as described previously. To produce RHO T4R/ϩ RPE65 Ϫ/Ϫ double mutant dogs, an initial crossbreeding was undertaken between an affected female from the RPE65 Ϫ/Ϫ strain (genotype RHO ϩ/ϩ RPE65 Ϫ/Ϫ ) and a heterozygous affected male from the RHO T4R/ϩ strain (genotype RHO T4R/ϩ RPE65 ϩ/ϩ ).…”

“…The affected dogs display a dramatically slowed time course for recovery of rod photoreceptor function after light exposure and a distinctive topographic pattern of retinal degeneration (22). Here, we present the properties of T4R Rho isolated from the scarce native tissue.…”

“…An opportunity to study the mechanism of retinitis pigmen-tosa (RP) 1 pathogenesis was presented when the T4R mutation was discovered in the English mastiff dog (22,23). The affected dogs display a dramatically slowed time course for recovery of rod photoreceptor function after light exposure and a distinctive topographic pattern of retinal degeneration (22).…”