Restoration of Vision and Retinal Responses After Adeno-Associated Virus-Mediated Optogenetic Therapy in Blind Dogs.

Purpose: Optogenetic gene therapy to render remaining retinal cells light-sensitive in end-stage retinal degeneration is a promising strategy for treatment of individuals blind because of a variety of different inherited retinal degenerations. The clinical trials currently in progress focus on delivery of optogenetic genes to ganglion cells. Delivery of optogenetic molecules to cells in the outer neural retina is predicted to be even more advantageous because it harnesses more of the retinal circuitry. However, this approach has not yet been tested in large animal models. For this reason, we evaluated the safety and efficacy of optogenetic therapy targeting remaining diseased cone photoreceptors in the Rcd1 dog model of retinitis pigmentosa. Methods: Imaging and measures of retinal function and functional vision were carried out, as well as terminal studies evaluating multi-electrode array recordings and histology. Results: Animals remained healthy and active throughout the study and showed improved retinal and visual function as assessed by electroretinography and visual-evoked potentials, improved navigational vision, and improved function of cone photoreceptors and the downstream retinal circuitry. Conclusions: The findings demonstrate that an optogenetic approach targeting the outer retina in a blind large animal model can partially restore vision. Translational Relevance: This work has translational relevance because the approach could potentially be extrapolated to treat humans who are totally blind because of retinal degenerative disease.

Protocols for Visually Guided Navigation Assessment of Efficacy of Retina-Directed Cell or Gene Therapy in Canines.

There has been marked progress in recent years in developing gene delivery approaches for the treatment of inherited blinding diseases. Many of the proof-of-concept studies have utilized rodent models of retinal degeneration. In those models, tests of visual function include a modified water maze swim test, optokinetic nystagmus, and light-dark activity assays. Test paradigms used in rodents can be difficult to replicate in large animals due to their size and awareness of non-visual aspects of the test system. Two types of visual behavior assays have been utilized in canines: an obstacle avoidance course and a forced choice Y maze. Given the progress in developing cell and gene therapies in large animals, such tests will become more and more valuable. This study provides guidelines for carrying out such tests and assesses the challenges and benefits associated with each test.

Evaluation of Dose and Safety of AAV7m8 and AAV8BP2 in the Non-Human Primate Retina.

Within the next decade, we will see many gene therapy clinical trials for eye diseases, which may lead to treatments for thousands of visually impaired people around the world. To target retinal diseases that affect specific cell types, several recombinant adeno-associated virus (AAV) serotypes have been generated and used successfully in preclinical mouse studies. Because there are numerous anatomic and physiologic differences between the eyes of mice and "men" and because surgical delivery approaches and immunologic responses also differ between these species, this study evaluated the transduction characteristics of two promising new serotypes, AAV7m8 and AAV8BP2, in the retinas of animals that are most similar to those of humans: non-human primates (NHPs). We report that while AAV7m8 efficiently targets a variety of cell types by subretinal injection in NHPs, transduction after intravitreal delivery was mostly restricted to the inner retina at lower doses that did not induce an immune response. AAV8BP2 targets the cone photoreceptors efficiently but bipolar cells inefficiently by subretinal injection. Additionally, transduction by both serotypes in the anterior chamber of the eye and the optic pathway of the brain was observed post-intravitreal delivery. Finally, we assessed immunogenicity, keeping in mind that these AAV capsids may be used in future clinical trials. We found that AAV8BP2 had a better safety profile compared with AAV7m8, even at the highest doses administered. These studies underscore the differences in AAV transduction between mice and primates, highlighting the importance of careful evaluation of therapeutic vectors in NHPs prior to moving to clinical trials.

Exploiting metabolic and antioxidant pathways to maintain vision in blinding disease.

The use of gene therapy for blinding disease shows growing promise; however, due to an ever-expanding list of disease-causing genes and mutations, the identification of a generic gene-based treatment is urgently needed. In many forms of degenerative retinal disease, there may be a window of opportunity to preserve daylight vision, as the cone photoreceptors degenerate more slowly than do the rods. In this issue of the JCI, Venkatesh et al. and Xiong et al. exploit two different pathways to promote cone cell survival and preserve vision in murine retinal degeneration models. These studies provide hope for developing a universal reagent to treat many different blinding disorders.

Nonallele specific silencing of ataxin-7 improves disease phenotypes in a mouse model of SCA7.

Spinocerebellar ataxia type 7 (SCA7) is a late-onset neurodegenerative disease characterized by ataxia and vision loss with no effective treatments in the clinic. The most striking feature is the degeneration of Purkinje neurons of the cerebellum caused by the presence of polyglutamine-expanded ataxin-7. Ataxin-7 is part of a transcriptional complex, and, in the setting of mutant ataxin-7, there is misregulation of target genes. Here, we designed RNAi sequences to reduce the expression of both wildtype and mutant ataxin-7 to test if reducing ataxin-7 in Purkinje cells is both tolerated and beneficial in an animal model of SCA7. We observed sustained reduction of both wildtype and mutant ataxin-7 as well as a significant improvement of ataxia phenotypes. Furthermore, we observed a reduction in cerebellar molecular layer thinning and nuclear inclusions, a hallmark of SCA7. In addition, we observed recovery of cerebellar transcripts whose expression is disrupted in the presence of mutant ataxin-7. These data demonstrate that reduction of both wildtype and mutant ataxin-7 by RNAi is well tolerated, and contrary to what may be expected from reducing a component of the Spt-Taf9-Gcn5 acetyltransferase complex, is efficacious in the SCA7 mouse.

RNA interference-based therapy for spinocerebellar ataxia type 7 retinal degeneration.

Spinocerebellar ataxia type 7 (SCA7) is an autosomal dominant neurodegenerative disease characterized by loss of motor coordination and retinal degeneration with no current therapies in the clinic. The causative mutation is an expanded CAG repeat in the ataxin-7 gene whose mutant protein product causes cerebellar and brainstem degeneration and retinal cone-rod dystrophy. Here, we reduced the expression of both mutant and wildtype ataxin-7 in the SCA7 mouse retina by RNA interference and evaluated retinal function 23 weeks post injection. We observed a preservation of normal retinal function and no adverse toxicity with ≥50% reduction of mutant and wildtype ataxin-7 alleles. These studies address an important safety concern regarding non-allele specific silencing of ataxin-7 for SCA7 retinal therapy.

Recent advances in RNA interference therapeutics for CNS diseases.

Over the last decade, RNA interference technology has shown therapeutic promise in rodent models of dominantly inherited brain diseases, including those caused by polyglutamine repeat expansions in the coding region of the affected gene. For some of these diseases, proof-of concept studies in model organisms have transitioned to safety testing in larger animal models, such as the nonhuman primate. Here, we review recent progress on RNA interference-based therapies in various model systems. We also highlight outstanding questions or concerns that have emerged as a result of an improved (and ever advancing) understanding of the technologies employed.