Gene therapy in the retina: Exploring neurotrophic treatment and AAV readministration in retinal disease

Inherited retinal degeneration is a devastating illness comprising nearly 200 disease-causing mutations. The difficulty of developing cures for each disease individually makes clear the need for a widely applicable and nonspecific treatment. My first project involved a recently discovered neurotrophic factor, rod-derived cone viability factor (RdCVF), known to protect degenerating cones. My aim was to determine whether achieving constitutive retinal overexpression of RdCVF in mouse models of retinal degeneration could prevent photoreceptor death. I demonstrated that subretinal delivery of an AAV2 vector delivering RdCVF to retinal RPE cells achieves stable expression for at least 9 months. I then delivered the vector to retinal degeneration (Rd10) mice and demonstrated that treated eyes showed significant improvement over control injected eyes. This was seen histologically, with increased outer nuclear layer thickness and improved cone morphology; physiologically, with improved electroretinograph and pupillometry responses; and in behaving animals, with improved optokinetic responses. Gene therapy-mediated expression of RdCVF may therefore be an effective treatment in delaying retinal degeneration. Meanwhile, recent Phase I clinical trials have demonstrated safety and efficacy in restoring vision in Type 2 Leber's Congenital Amaurosis (LCA2), using AAV2-mediated gene replacement of the RPE65 gene. Current clinical trial guidelines do not allow contralateral readministration of vector or treatment of patients with high pre-existing neutralizing antibody (NAb) titers to AAV2, because safety and efficacy are unknown. My second project assessed the safety and feasibility of readministration of the clinical trial-grade vector AAV2-hRPE65 in the contralateral eye of monkeys previously exposed to AAV2 vectors. Results showed that while serum neutralizing antibodies increased following both the first and second subretinal injections, ocular titer only rose following local injection of that eye. Furthermore, all monkeys showed increased RPE65 expression in both injected retinas. One animal developed a CD4+ T-cell response post-first injection, and histology showed minimal inflammatory changes. These results indicate that even in the context of humoral or CD4+ T-cell responses, administration and contralateral readministration of vector are safe and feasible and have high transduction efficiency. It is my hope that these studies will contribute to making retinal gene therapy treatment a reality for more patients.