Potential therapy for progressive vision loss due to PCDH15-associated Usher syndrome developed in an orthologous Usher mouse

Usher syndrome type I (USH1) is characterized by congenital deafness, vestibular areflexia, and progressive retinal degeneration with age. The protein-truncating p.Arg245* founder variant of PCDH15 has an ~2% carrier frequency among Ashkenazi Jews, accounting for nearly 60% of their USH1 cases. Here, longitudinal ocular phenotyping in thirteen USH1F individuals harboring the p.Arg245* variant revealed progressive retinal degeneration, leading to severe loss of vision with macular atrophy by the sixth decade. Half of the affected individuals met either the visual acuity or visual field loss definition for legal blindness by the middle of their fifth decade of life. Mice homozygous for p.Arg250* (Pcdh15R250X; equivalent to human p.Arg245*) also have early visual deficits evaluated using electroretinography. Light-dependent translocation of phototransduction cascade proteins, arrestin and transducin, was found to be impaired in Pcdh15R250X mice. Retinal pigment epithelium-(RPE) specific visual retinoid cycle proteins, RPE65 which converts all-trans retinoids to 11-cis retinoids and CRALBP that transports retinoids, and key retinoid levels were also reduced in Pcdh15R250X mice, suggesting a dual role for protocadherin-15 in photoreceptors and RPE. Administration of exogenous 9-cis retinal, an analog of the naturally occurring 11-cis retinal, improved ERG amplitudes in these mutant mice, suggesting a basis for a clinical trial of exogenous FDA approved retinoids to preserve vision in USH1F patients. SummaryIn a preclinical setting studying exogenous retinoids using a novel Usher syndrome mouse model, we describe a potential therapy to treat PCDH15-mediated visual dysfunction.