RPE65 gene therapy slows cone loss in Rpe65-deficient dogs

Abstract

Recent clinical trials of retinal pigment epithelium gene (RPE65) supplementation therapy in Leber congenital amaurosis type 2 patients have demonstrated improvements in rod and cone function, but it may be some years before the effects of therapy on photoreceptor survival become apparent. The Rpe65-deficient dog is a very useful pre-clinical model in which to test efficacy of therapies, because the dog has a retina with a high degree of similarity to that of humans. In this study, we evaluated the effect of RPE65 gene therapy on photoreceptor survival in order to predict the potential benefit and limitations of therapy in patients. We examined the retinas of Rpe65-deficient dogs after RPE65 gene therapy to evaluate the preservation of rods and cone photoreceptor subtypes. We found that gene therapy preserves both rods and cones. While the moderate loss of rods in the Rpe65-deficient dog retina is slowed by gene therapy, S-cones are lost extensively and gene therapy can prevent that loss, although only within the treated area. Although LM-cones are not lost extensively, cone opsin mislocalization indicates that they are stressed, and this can be partially reversed by gene therapy. Our results suggest that gene therapy may be able to slow cone degeneration in patients if intervention is sufficiently early and also that it is probably important to treat the macula in order to preserve central function.

Human retinal gene therapy for Leber congenital amaurosis shows advancing retinal degeneration despite enduring visual improvement

“…Noninvasive OCT studies localized the region of preservation, and histological studies showed intact ONL and preservation of inner retinal layers. Our studies confirm and extend previous studies (14,17,35), and they support the hypothesis that, in RPE65-mutant dogs, early treatment of the primary RPE defect prevents the later onset of photoreceptor degeneration. Similar experiments in mutant mice treated early with gene therapy have shown survival of cone (12,13,15,16) and rod photoreceptors (49).…”

“…To explain both findings, we postulate that there exists a critical limit in terms of accumulating molecular changes that are detrimental to photoreceptor survival under this condition of severe RPE65 dysfunction. There is evidence for molecular changes occurring in rod and cone photoreceptors in the predegenerative phase of RPE65 disease (17,18,32,54,58), and a specific translocation of one of the proapoptotic Bax isoforms from the cytosol to mitochondria has been observed to correlate with the initiation of photoreceptor cell loss (54,55). To date, there has been a paucity of murine studies of gene therapy in older animals after the onset of retina-wide rod degeneration (6,59).…”

“…Next, we asked whether the posttreatment results of advancing retinal degeneration despite enduring visual improvement were unique to human RPE65-LCA or also occurred in an animal model of the disease. RPE65-mutant dogs are the only large animal model for RPE65-LCA and are known to show congenital visual dysfunction followed later by retinal degeneration (5,17,(28)(29)(30)(31)(32). First, we defined the natural history of the canine disease.…”

“…It is hypothesized that correction of the underlying cellular dysfunction in a human recessive retinal degeneration will promote greater neuronal survival by slowing the natural rate of photoreceptor loss. Gene therapy experiments that have evaluated photoreceptor degeneration have been performed mainly in mouse and dog models treated at young ages (12)(13)(14)(15)(16)(17)(18) and have implied that at least a subset of photoreceptors could be rescued from degeneration. However, accumulating evidence has suggested that human RPE65-LCA, which can show degeneration prenatally or within the first years of life (19,20), is better modeled at the more advanced disease stages in older animals (3,21).…”

Halting progressive neurodegeneration in advanced retinitis pigmentosa

“…This same logic could be used to explain the failure to halt degeneration in some previous animal studies (13,35). In support of this idea are gene therapy studies demonstrating a delay in photoreceptor degeneration that was limited to the transduced area (36)(37)(38).…”