Gene therapy is a therapeutic strategy where, as opposed to using traditional drugs, doctors manipulate specific genes in patients suffering from inherited genetic diseases, such as cystic fibrosis and severe combined immunodeficiency (SCID). Although conceptualized in the 1970s, gene therapy has had little clinical success and has suffered several setbacks as an approach to treating disease. Much of the difficulty in applying gene therapy lies in the technical difficulty of targeting and disrupting faulty genes. However, recent advances in gene editing technology, including the development of the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system, are providing new options and excitement for realizing the potential of gene therapy.

As a proof-of-principle study, Benjamin Bakondi and colleagues at Cedars-Sinai Medical Center (Los Angeles, CA) have now successfully applied CRISPR/Cas9 to a rat model of retinitis pigmentosa, an inherited degenerative eye disease that causes severe loss of photoreceptors leading to visual impairment and blindness (Mol. Ther. doi: 10.1038/mt.2015.220; published online 15 December 2015). Transgenic S334ter rats suffer from an autosomal-dominant mutation in an allele for the rhodopsin gene (Rho), and display similar visual phenotypes to humans suffering from retinitis pigmentosa caused by Rho mutations. Using CRISPR/Cas9, Bakondi et al. were able to disrupt the allele-specific RhoS334 and rescue the effects of retinitis pigmentosa.

Bakondi et al. injected plasmids containing targeting-guide RNA and the Cas9 enzyme directly into the eyes of S334ter rat pups at age P0, along with a fluorescent dye for histological verification of the spread of the injection. Using fluorescent confocal microscopy, they confirmed that the plasmids were successfully taken up by photoreceptors in the injected retinas. To test how well the CRISPR/Cas9 strategy functionally rescued retinal degeneration, the researchers performed histology on injected retinal tissue at different periods throughout development of the S334ter rats and into adulthood. Bakondi et al. found that injected retinas had significantly higher levels of photoreceptors compared with retinas in non-injected eyes. Further, they showed immunohistochemical evidence that synaptic connectivity was maintained between photoreceptors and downstream neurons. Most importantly, they found that intact eyes with the injections had enhanced visual acuity (as tested by the optokinetic reflex response) compared with eyes that were not injected, demonstrating that this CRISPR/Cas9 strategy functionally rescued the deleterious effects of the RhoS334 mutation. Although only a proof-of-principle study, this work provides an important advancement for developing gene therapy as a treatment option for patients suffering from genetic disorders.