AAV-mediated gene replacement, either alone or in combination with physical and pharmacological agents, results in partial and transient protection from photoreceptor degeneration associated with betaPDE deficiency

Abstract

Purpose: Mutations in the PDE6B gene cause recessive, severe retinitis pigmentosa (RP). PDE6B encodes the β subunit of the rod-specific phosphodiesterase (βPDE), which, when absent, results in toxic levels of intracellular Ca(2+) and photoreceptor cell death. Ca(2+) blockers, such as nilvadipine, as well as light restriction, slow photoreceptor degeneration in animal models of βPDE deficiencies. The goal of the study was to evaluate the efficacy of AAV2/5- or AAV2/8-mediated gene replacement in combination with nilvadipine and/or with light restriction in the rd10 mouse bearing homozygous pde6b mutations.

Methods: AAV vectors encoding either βPDE or EGFP were subretinally administered at postnatal day (P)2. Nilvadipine was administered from P7 to P28. For light restriction, pregnant rd10 mice were kept in a dark environment until their pups were 28 days old. All functional and histologic analyses were performed at P35.

Results: Significant morphologic photoreceptor protection was observed after subretinal administration of AAV vectors encoding EGFP. This protection further increased after administration of AAV2/8 or -2/5 encoding for βPDE and was not associated with significant functional improvement. Photoreceptor protection was higher after AAV2/8- than after AAV2/5-mediated delivery and was not significantly augmented by additional drug therapy and/or light restriction. The protective effect was lost after P35.

Conclusions: In conclusion, more efficient gene transfer tools than those used in this study, as well as a better understanding of the disease pathogenesis, should be explored to increase the effect of gene replacement and to design gene-based strategies that block the apoptotic pathways activated by βPDE deficiency.