RPE65 knockout Xenopus laevis have a compromised but detectable electroretinogram and altered visual responses, without retinal degeneration or altered melanophore dispersion

The isomerohydrolase RPE65 is a critical element of the visual cycle, the series of enzymatic reactions by which the chromophore of the visual pigments is regenerated following light exposure. In humans, mutations in the RPE65 gene cause a severe form of blindness called Leber's congenital amaurosis. Studies of Rpe65^-/- mice have shown dramatic depletion of 11-cis-retinal in the retina, resulting in a slow retinal degeneration. However, a number of studies suggest that RPE65 may not be necessary for the regeneration of photopigment in all photoreceptor types. Using CRISPR/Cas9 technology, we previously generated RPE65 knockout Xenopus laevis in order to test the involvement of rhodopsin chromophore in the cell death mechanisms associated with rhodopsin mutations and rhodopsin quality control. Here we further characterize the effects of RPE65 knockout in these animals, and show their rod photoreceptors have shortened outer segments that lack detectable rhodopsin photopigment. However, there is no progressive degeneration of rods or cones. Via electroretinography we found greatly reduced but significant responses to light under scotopic and photopic conditions. We also found reduced behavioral sensitivity to light, while light-induced melanophore dispersion was unaffected. RPE65 knockout X. laevis may be a useful system for examining RPE65-independent photosensation mechanisms in vertebrates.