Vision Science Program and School of Optometry, University of California, Berkeley, Berkeley, California, United States.
PURPOSE. To identify a new mouse mutation developing early-onset dominant retinal degeneration, to determine the causative gene mutation and investigate the underlying mechanism. METHODS. Retinal phenotype was examined by an indirect ophthalmoscope, histology, transmission electron microscopy, immunohistochemistry, Western blots and electroretinograms. Causative gene mutation was determined by a genome-wide linkage analysis and DNA sequencing. Structural modeling was used to predict the impact of the mutation on protein structure. RESULTS. An ENU-mutagenized mouse line (R3), displaying attenuated retinal vessels and pigmented patches, was identified by fundus examination. Homozygous R3/R3 mice lost photoreceptors rapidly, leaving only a single row of photoreceptor nuclei at postnatal day 18. A- and b-waves of ERG were flat in R3/R3 mice while heterozygous R3/+ mice showed reduced a- and b-waves. The R3/+ mice had a slower rate of photoreceptor cell loss than compound heterozygous R3/- mice with a null mutant allele. The R3 mutation was mapped and verified to be a rhodopsin point mutation, a c.553T>C for a p.C185R substitution. The side chain of Arg185 impacted on the extracellular loop of the protein. Mutant rhodopsin-C185R protein accumulated in the photoreceptor inner segments and/or cellular bodies. CONCLUSIONS. Rhodopsin C185R mutation leads to severe retinal degeneration in R3 mutant mice. A dosage dependent accumulation of misfolded mutant proteins likely triggers and/or stimulates the death of rod photoreceptors. The presence of a wild-type rhodopsin allele can delay the loss of photoreceptor cells in R3/+ mice.