Mutations of the Opsin Gene (Y102H and I307N) Lead to Light-induced Degeneration of Photoreceptors and Constitutive Activation of Phototransduction in Mice

Ewa Budzynski,Alecia K Gross,Suzanne D Mcalear,Neal S Peachey,Meera Shukla,Feng He,Malia Edwards,Jungyeon Won,Wanda L Hicks,Theodore G Wensel,Jurgen K Naggert,Patsy M Nishina

doi:10.1074/jbc.m110.112409

Abstract

Mutations in the Rhodopsin (Rho) gene can lead to autosomal dominant retinitis pigmentosa (RP) in humans. Transgenic mouse models with mutations in Rho have been developed to study the disease. However, it is difficult to know the source of the photoreceptor (PR) degeneration in these transgenic models because overexpression of wild type (WT) Rho alone can lead to PR degeneration. Here, we report two chemically mutagenized mouse models carrying point mutations in Rho (Tvrm1 with an Y102H mutation and Tvrm4 with an I307N mutation). Both mutants express normal levels of rhodopsin that localize to the PR outer segments and do not exhibit PR degeneration when raised in ambient mouse room lighting; however, severe PR degeneration is observed after short exposures to bright light. Both mutations also cause a delay in recovery following bleaching. This defect might be due to a slower rate of chromophore binding by the mutant opsins compared with the WT form, and an increased rate of transducin activation by the unbound mutant opsins, which leads to a constitutive activation of the phototransduction cascade as revealed by in vitro biochemical assays. The mutant-free opsins produced by the respective mutant Rho genes appear to be more toxic to PRs, as Tvrm1 and Tvrm4 mutants lacking the 11-cis chromophore degenerate faster than mice expressing WT opsin that also lack the chromophore. Because of their phenotypic similarity to humans with B1 Rho mutations, these mutants will be important tools in examining mechanisms underlying Rho-induced RP and for testing therapeutic strategies.

Highlights

From the ‡The Jackson Laboratory, Bar Harbor, Maine 04609, the §Department of Vision Sciences, University of Alabama, Birmingham, Alabama 35294, the ¶Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio 44195, the ʈResearch Service, Cleveland Veterans Affairs Medical Center, Cleveland, Ohio 44106, the **Department of Ophthalmology, Lerner College of Medicine, Case Western Reserve University, Cleveland, Ohio 44195, and the Departments of ‡‡Biochemistry and Molecular Biology and §§Neuroscience and Ophthalmology, Baylor College of Medicine, Houston, Texas 77030
Both mutants express normal levels of rhodopsin that localize to the PR outer segments and do not exhibit PR degeneration when raised in ambient mouse room lighting; severe PR degeneration is observed after short exposures to bright light. Both mutations cause a delay in recovery following bleaching. This defect might be due to a slower rate of chromophore binding by the mutant opsins compared with the wild type (WT) form, and an increased rate of transducin activation by the unbound mutant opsins, which leads to a constitutive activation of the phototransduction cascade as revealed by in vitro biochemical assays
We report two new mouse models identified through screening of cohorts of chemically mutagenized mice by indirect ophthalmoscopy in the Translational Vision Research Models (TVRM) program sited at The Jackson Laboratory

Summary

Introduction

From the ‡The Jackson Laboratory, Bar Harbor, Maine 04609, the §Department of Vision Sciences, University of Alabama, Birmingham, Alabama 35294, the ¶Cole Eye Institute, Cleveland Clinic Foundation, Cleveland, Ohio 44195, the ʈResearch Service, Cleveland Veterans Affairs Medical Center, Cleveland, Ohio 44106, the **Department of Ophthalmology, Lerner College of Medicine, Case Western Reserve University, Cleveland, Ohio 44195, and the Departments of ‡‡Biochemistry and Molecular Biology and §§Neuroscience and Ophthalmology, Baylor College of Medicine, Houston, Texas 77030. We report two chemically mutagenized mouse models carrying point mutations in Rho (Tvrm with an Y102H mutation and Tvrm with an I307N mutation) Both mutants express normal levels of rhodopsin that localize to the PR outer segments and do not exhibit PR degeneration when raised in ambient mouse room lighting; severe PR degeneration is observed after short exposures to bright light. Both mutations cause a delay in recovery following bleaching. These observations have spurred the development of transgenic lines expressing comparable rhodopsin point mutations (9 –13) These models are a valuable resource, rod degeneration is observed in mice overexpressing wild type (WT) rhodopsin [14]. Difficult to know with certainty whether rod degeneration observed in transgenic lines reflects the effects of the introduction of rhodopsin point mutations or the overexpression of rhodopsin

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Conclusion