Crx-L253X Mutation Produces Dominant Photoreceptor Defects in TVRM65 Mice

The cone-rod homeobox (CRX) transcription factor is essential for photoreceptor gene expression, differentiation, and survival. Human CRX mutations can cause dominant retinopathies of varying onset and phenotype severity. In animal models, dominant frameshift Crx mutations introduce a premature termination codon (PTC), producing inactive truncated proteins that interfere with normal CRX function. Previously, a mutant mouse, TVRM65, was reported to carry a recessive late PTC mutation, Crx-L253X. More detailed phenotype analysis of the pathogenicity of Crx-L253X sheds new light on the variability of CRX-linked diseases. Homozygous (L253X/X); heterozygous (L253X/+); Crx-/- and control C57BL/6J (WT) mice were analyzed at various ages for changes in retinal function (ERG), morphology (histology) and photoreceptor gene expression (qRT-PCR). At 1 month, L253X/X mice lack visual function, show greater reductions in retinal thickness, and distinct gene expression changes relative to Crx-/-, suggesting that the phenotype of L253X/X is more severe than Crx-/-. L253X/+ mice have reduced rod/cone function, but normal retinal morphology at all ages tested. qRT-PCR assays described a complex phenotype in which both developing and mature photoreceptors are unable to maintain proper gene expression. L253X mRNA/protein is overexpressed relative to normal Crx, suggesting a pathogenic mechanism similar to early PTC mutations. However, the overexpression is less pronounced, correlating with a relatively mild dominant phenotype. The L253X mouse provides a valuable model for CRX-associated retinopathy. The pathogenicity of CRX frameshift mutations depends on the position of the PTC, which in turn determines the degree of mutant mRNA/protein overproduction.