Outer retinal transduction by AAV2-7m8 following intravitreal injection in a sheep model of CNGA3 achromatopsia

Sheep carrying a mutated CNGA3 gene exhibit diminished cone function and provide a naturally occurring large animal model of achromatopsia. Subretinal injection of a vector carrying the CNGA3 transgene resulted in long-term recovery of cone function and photopic vision in these sheep. Research is underway to develop efficacious vectors that would enable safer transgene delivery, while avoiding potential drawbacks of subretinal injections. The current study evaluated two modified vectors, adeno-associated virus 2-7m8 (AAV2-7m8) and AAV9-7m8. Intravitreal injection of AAV2-7m8 carrying enhanced green fluorescent protein under a cone-specific promoter resulted in moderate photoreceptor transduction in wild-type sheep, whereas peripheral subretinal delivery of AAV9-7m8 resulted in the radial spread of the vector beyond the point of deposition. Intravitreal injection of AAV2-7m8 carrying human CNGA3 in mutant sheep resulted in mild photoreceptor transduction, but did not lead to the clinical rescue of photopic vision, while day-blind sheep treated with a subretinal injection exhibited functional recovery of photopic vision. Transgene messenger RNA levels in retinas of intravitreally treated eyes amounted to 4–23% of the endogenous CNGA3 levels, indicating that expression levels >23% are needed to achieve clinical rescue. Overall, our results indicate intravitreal injections of AAV2.7m8 transduce ovine photoreceptors, but not with sufficient efficacy to achieve clinical rescue in CNGA3 mutant sheep.