CrxRdy Cat: An Excellent Large Animal Model For Severe Dominant Retinopathies Associated With CRX Mutations Based On Its Functional And Structural Characterization

Abstract

Purpose: : CRX is a photoreceptor transcription factor essential for normal photoreceptor development and survival. The Crx^Rdy cat model has a spontaneous mutation in Crx resulting in dysfunction and degeneration of photoreceptors. The purpose of this study was to further investigate the phenotype of the Crx^Rdy cat at the cellular and molecular levels.

Methods: : Heterozygous Crx^Rdy cats were investigated by ophthalmic examination, fundus photography, electroretinography (ERG) and optical coherence tomography (OCT). Retinal gene expression changes were assessed by immunohistochemistry (IHC) and QRT-PCR.

Results: : Heterozygous Crx^Rdy kittens showed severe vision deficits, developed ophthalmoscopically detectable changes in the area centralis by 7 weeks and showed generalized retinal thinning and vascular attenuation by 12 weeks of age. ERGs of the affected kittens were very reduced in amplitude and increased in latency, and could not be recorded before 6 weeks of age and were extinguished by about 20 weeks of age. OCT showed a progressive thinning of the outer nuclear layer (ONL) and combined inner/outer segment layer (IS/OS). This was more pronounced in the area centralis with an approximately 50% decrease in ONL and IS/OS thickness compared to normal by 7 weeks of age. The inner segment/outer segment junction was indistinct on OCT in the affected kittens. IHC at 6 weeks of age showed that Crx^Rdy kittens had thinning of the ONL and stunting of the photoreceptor inner and outer segments. QRT-PCR in 6-week and 20-week-old Crx^Rdy kittens revealed significantly decreased expression of cone and rod opsins with a more severe and earlier decrease in cone opsins. Interestingly, the expression of Crx itself appeared to be elevated, similar to that observed in Crx^DNH mouse model (see abstract by Tran and Chen), suggesting a possible mechanism for pathogenesis.

Conclusions: : Crx^Rdy kittens have a severe cone-rod dystrophy characterized by failure to develop normal photoreceptor structure and function followed by a rapid photoreceptor degeneration. The Crx^Rdy cat represents a valuable model for the severe dominant forms of human CRX retinal dystrophies.

Keywords: transcription factors • gene/expression • degenerations/dystrophies