Gene therapy restores useful vision to a model of the GUCY2D-LCA1 patient fovea, the all-cone Nrl^-/-GC1^-/- mouse

Abstract

Purpose: Mutations in GUCY2D are a leading cause of the most severe form of early-onset retinal dystrophy, Leber congenital amaurosis (LCA1). GUCY2D encodes retinal guanylate cyclase-1 (retGC1), a protein expressed exclusively in the outer segments of photoreceptors that plays a role in the recovery phase of phototransduction. Recent clinical data shows that, despite a high degree of visual disturbance stemming predominantly from a loss of cone function, LCA1 patients retain normal photoreceptor laminar architecture, except for foveal cone outer segment abnormalities and, in some patients, foveal cone loss. These results point to the cone-rich central retina as a target for GUCY2D replacement. Relevant proof of concept studies have thus far been conducted in non-foveated, rod-dominant models. Thus, the question of how a cone-dominant mammal would respond to GUCY2D gene replacement remained unanswered. The purpose of our study was to examine the effects of gene replacement in a model that mimics the retGC1-deficient LCA1 patient fovea, the cone-only Nrl^-/- GC1^-/- mouse.

Methods: AAV8(Y733F)-hGRK1-Gucy2e was delivered subretinally to one eye of Nrl^-/-GC1^-/- at various treatment ages. Electroretinogram, visually guided behavior tests, optical coherence tomography, guanylate cyclase activity assays, immunohistochemistry and immunoblot were used in treated mice and age-matched Nrl^-/- controls to assess therapeutic outcome.

Results: We show that AAV-retGC1 treatment fully restores cone function, cone-mediated visual behavior and retGC1 enzyme activity and preserves cones in treated Nrl^-/- GC1^-/- mice over the long term. The finding that retinal function could be restored to levels seen in age-matched Nrl^-/- controls contrasts results obtained in other models of retGC1 deficiency. We attribute this finding to the underlying preservation of retinal structure in Nrl^-/- GC1^-/- mice relative to Nrl^-/- controls.

Conclusions: The Nrl^-/- GC1^-/- mouse possesses an increased dynamic range relative to other models for testing candidate clinical vectors. Our finding that AAV-mediated retGC1 expression is therapeutic over the long term in an all-cone model of LCA1 further supports clinical application of a gene therapy targeted to the cone-rich central retina of LCA1 patients.