Age-Dependent Disease Expression Determines Remodeling of the Retinal Mosaic in Carriers ofRPGRExon ORF15 Mutations

Abstract

Purpose To characterize the retinal histopathology in carriers of X-linked progressive retinal atrophy (XLPRA1 & XLPRA2), two canine models of X-linked retinitis pigmentosa caused, respectively, by a stop and a frameshift mutation in RPGRORF15. Methods Retinas of XLPRA2 and XLPRA1 carriers of different ages were processed for morphologic evaluation, TUNEL assay, and immunohistochemistry. Cell-specific markers were used to examine retinal remodeling events. Results A mosaic pattern composed of patches of diseased and normal retina was first detected in XLPRA2 carriers at 4.9 weeks of age. A peak of photoreceptor cell death led to focal rod loss; however, in these patches an increased density of cones was found to persist over time. Patches of disease gradually disappeared such that by 39 weeks of age the overall retinal morphology, albeit thinner, had improved lamination. In older XLPRA2 carriers (≥8.8 years), extended regions of severe degeneration occurred in the peripheral/mid-peripheral retina. In XLPRA1 carriers, opsin mislocalization and rare events of rod death were detected by TUNEL assay at 20 weeks of age, however patchy degeneration was only seen by 1.4 years, and was still apparent at 7.8 years. Conclusion The time of onset and the progression of the disease differed between the two models. In the early onset form (XLPRA2) the morphologic appearance of the retinal mosaic changed as a function of age, suggesting that structural plasticity persists in the early postnatal canine retina as mutant photoreceptors die. In the late onset form (XLPRA1), patches of disease persisted until later ages.

Genetic rescue models refute nonautonomous rod cell death in retinitis pigmentosa

“…However, data from both of our mutants strongly suggest that surviving photoreceptors undergo active or passive rearrangement in response to the dramatic physical changes to the photoreceptor layer landscape-essentially filling in the void left by degenerating rods to yield an ONL that is either uniform in thickness (Pde6b H620Q /Pde6b STOP , Pde6g::CreERT2 mutant) or graded (Pde6b H620Q /Pde6b STOP , Pax6α::Cre mutant). In fact, remodeling of photoreceptor cells has been observed in a canine model of X-linked RP (XLPRA2); these diseased retinas exhibit patches of diseased and normal retina, which gradually disappear over time (24). In addition, retinas from chimeric mice (produced by combining morulae from rds −/− and wild-type mice) showed regions with normal, intermediate, and thin ONL thickness (25).…”

Gene therapy rescues photoreceptor blindness in dogs and paves the way for treating human X-linked retinitis pigmentosa

“…At least for the stages of disease studied, this therapeutic vector was highly effective and warrants further studies for translational applications. In both models, treatment with the hIRPB-hRPGR therapeutic vector prevented (XLPRA1) or reversed (XLPRA2) rod and R/G cone opsin mislocalization, a feature of the disease in human (34), mouse (17), and dog (22,35) and a putative early marker of photoreceptor cell death (58,59).…”

“…In XLPRA, as in other primary photoreceptor diseases, OPL and inner retinal abnormalities are common secondary effects (22,(35)(36)(37). In untreated regions, narrowing of the OPL was associated with compressed photoreceptor synaptic terminals (Fig.…”

“…S4) and their lateral processes was normal and unchanged between treated and untreated regions. These last-mentioned hallmarks, however, are of late-stage retinal remodeling in XLPRA (22,35) and were not expected to be present at the age when dogs were terminated.…”

“…S3). Finally, the mislocalization of rod and cone opsins, a feature of the disease in human (34), mouse (17), and dog (22,35), was reversed ( Fig. 3, panels 9, 10, 12, and 13; Fig.…”