GENETIC ABLATION OF 11-CIS RETINAL ACCELERATES RETINAL DEGENERATION IN THE CANINE T4R RHO MODEL OF ADRP BUT PREVENTS LIGHT-INDUCED DAMAGE

Abstract

Purpose: The mechanism by which light exposure modulates the course of retinal degeneration in the T4R RHO dog is currently unknown. Here, we examined whether genetic ablation of 11-cis retinal through the RPE65 null genotype prevents acute light-induced photoreceptor loss in this model of class B1 RHO-Autosomal Dominant RP.

Methods: Littermate dogs with distinct genotypes were used in this study. This included T4R RHO affected (RHO^T4R/+/RPE65^+/-), double affected (RHO^T4R/+/RPE65^-/-), and non-affected (RHO^+/+/RPE65^+/-) dogs. At 7 wks of age, the dogs were anesthetized and had their left eye exposed to a single dose of white light (corneal irradiance of 1 mW/cm² for 1min) with a monocular Ganzfeld. The right eye was shielded and used as control. ONL thickness was measured along the 4 cardinal meridians by non-invasive cSLO/sdOCT imaging before (6 wks of age) and 2 wks after light exposure (LE) (9 wks of age).

Results: Prior to LE (6 wks of age) the ONL in both eyes of double-affected dog was significantly (~ 50%) thinner than that of the T4R RHO affected and normal dogs. Between 6 and 9 wks of age there was a similar decrease (~20%) in ONL thickness in both the exposed and the shielded eyes of the double affected dog. A more severe thinning of the ONL (~ 85% decrease) was seen during that period in the exposed eye of the T4R RHO affected dog. The ONL thickness from the T4R RHO (shielded) and the non-affected (shielded and exposed) retinas decreased by only ~ 7%, and was likely due to age-related thinning of the ONL associated with eye growth.

Conclusions: Our results confirm previously published results (Zhu et al. JBC 2004) that showed that 6.5 month-old double mutant RHO^T4R/+/RPE65^-/- had an earlier and more severe disease phenotype than single mutant T4R RHO dogs. We now show that significant loss of photoreceptors is already present at 6 weeks of age in RHO^T4R/+/RPE65^-/- retina, but that it is not-sensitive to light-damage. These findings further support the claim that 11-cis retinal is critical in stabilizing mutant T4R rod opsin, and that release of the chromophore upon photo-activation may lead to a change in conformation of the apoprotein that disrupts the structural integrity of discal membranes.