*3.2. RPE65*

Large animal models with visual cycle gene mutations include the *Rpe65*−/− dog. This is a model for LCA2. This model was crucial in the development of translational gene augmentation therapy which led to the first FDA-approved gene therapy product. The first animal injected with a therapeutic vector for LCA2 was an *Rpe65*−/− dog. Therapies were developed by three independent groups and consisted in each instance of recombinant adeno-associated virus vectors packaged with *RPE65* cDNA. The precise details of promoters and other features such as polyadenylation signals and enhancers differed between the groups. Four groups with colonies of *Rpe65*−/− dogs reported successful restoration of rod and cone function [16,57–59]. Loss of rod photoreceptors in the *Rpe65*−/− dog was slow and gene therapy showed ERG rescue even in middle-age [60]. Studies showed that S-cones were sensitive to the lack of normal 11-cis-retinal supply and s-cone opsin immunoreactivity was lost at an early age [61]. There were some phenotypic differences between the *Rpe65*−/− dog colonies, with one showing early photoreceptor degeneration in the area centralis (canine equivalent of the human macula) [62]. Trials in human subjects have not resulted in the same restoration of function shown by the dramatic improvement in ERG responses seen in dog and mouse models. A possible explanation for this species difference in therapy efficacy was provided by a comparison of the Rpe65 function of primates and dogs. This suggested that primates require a higher level of Rpe65 than dogs for the function of the visual cycle and that the current therapy might not result in adequate levels of enzymatic function in humans [63].
