Retinitis pigmentosa (RP), a major cause of vision loss in inherited retinal degenerations affects more than 150,000 people in the USA alone. There are diverse genetic causes of RP including different mutations with dominant, recessive and sex- linked inheritance patterns. The identification of disease-causing genes has paved the way for development of gene therapies. The successful example is RPE 65 gene therapy for Leber congenital amaurosis type 2 (LCA2). LCA2, an autosomal recessive (loss of function) RP, is caused by mutations in RPE65, an essential gene in the retinal visual cycle pathway. However, a recent report from one clinical trial showed that the rate of retinal degeneration in the vector-treated eye was similar to that in the contralateral untreated eye, raising concerns that once started, the degeneration processes may not be entirely stopped just by supplying a correct copy of the gene. Moreover, majority of the inherited retinal disease patients are affected by dominant form diseases (gain of function), in these cases, reducing the toxic product is required rather than supplying a correct copy of the gene. There is no treatment available. The advent of customized DNA-binding modules, including TALE and CRISPR/cas9 provides new tools to specifically ablate mutant genes. As proof of principle, we demonstrate the feasibility of using CRISPR/Cas9 in vivo as gene therapy by selectively ablating the dominant rhodopsin S334ter transgene (rhoS334) in rats that model severe autosomal dominant retinitis pigmentosa (adRP). Subretinal injection of guide RNA (gRNA)/Cas9 plasmid generated allele-specific disruption of rhoS334, prevented retinal dystrophy, and improved visual function (Mol Ther. 2016 Mar; 24 (3): 556-63). The feasibility of using CRISPR/Cas9 as in vivo gene therapy to repair dominant-inherited diseases for patients is not yet clear, but development of personalized medicines for previously untreatable diseases can be realized through investigation into new technologies, of which CRISPR is the most novel and promising so far. We propose to investigate the long-term safety and efficacy of CRISPR/Cas9 mediated specific ablation on the dominant mutation in S334ter-3 rats by translational delivery. In addition, we incorporated a second adRP rodent model (P23H-1) that recapitulates the most frequent adRP human mutation and contrasts the fast degeneration in S334ter-3 rats. These two models for adRP will allow us to investigate the efficacy and long-term safety after CRISPR/CAS9 delivered into the photoreceptors at early, mid and late stages of retinal degeneration and to characterize the relationship of efficacy to time-of-intervention.
Inherited retinal degenerative diseases are major cause of vision loss affecting million people worldwide. So far treatment is very limited. Gene therapy, especially the development of CRISPR/Cas9 mediated specific gene editing holds real promise to treat these debilitating diseases.