Collaboration between scientists from the University of Pennsylvania and the University of Florida, Gainesville has hit pay dirt when it comes to treating an inherited eye disease. This study used gene therapy to treat the disease and the results of this research project make a definitive contribution to the development of gene therapies for people with the blinding eye disorders for which there is presently no cure.
The disease in question is called retinitis pigmentosa, which is a group of rare, genetic disorders characterized by the degradation and subsequent loss of photoreceptors in the retina. People who suffer from retinitis pigmentosa have difficulty seeing at night and experience a loss of peripheral vision.
As mentioned, retinitis pigmentosa is an inherited disorder that results from mutations in any one of more than 50 different genes. These genes encode proteins that are required for retinal photoreceptors, and mutations in these genes compromises photoreceptor survival and function.
In human patients, retinitis pigmentosa is the most common inherited disease that results in degeneration of the photoreceptors of the retina. Approximately 1 in 4,000 people are affected with retinitis pigmentosa and 10 to 20 percent have a particularly severe form called X-linked retinitis pigmentosa. This disease predominately affects males, who experience night blindness by age 10 and progressive loss of the visual field by age 45. 70 percent of people with the X-linked retinitis pigmentosa harbor loss-of-function mutations in the retinitis pigmentosa GTPase Regulator (RPGR) gene. RPGR encodes a protein that maintains the health and survival of retinal photoreceptors. There are two types of photoreceptors; rods that give us the ability to see in dim light, and cones that allow us to see fine detail and color in bright light. Loss of the RPGR protein damages both types of photoreceptors.
Because there are no treatments for retinitis pigmentosa, gene therapy might be the best option to treat this disease. Fortunately, some varieties of dogs have a naturally occurring, late-stage retinitis pigmentosa that closely resembles the human disease. In previous experiments, gene therapies were used in diseased dogs, but such studies showed that benefits from gene therapy were only observed when it was used in the earliest stages of the disease.
“The study shows that a corrective gene can stop the loss of photoreceptors in the retina, and provides good proof of concept for gene therapy at the intermediate stage of the disease, thus widening the therapeutic window,” said Neeraj Agarwal, Ph.D., a program director at National Eye Institute, a part of the National Institutes of Health, who funded this research.
The dogs used in this study all suffered from a naturally occurring canine form of RPGR X-linked retinitis pigmentosa that is observed in some mixed breeds. These animals provided an excellent model system for their gene therapy tests, since affected dogs with early to late stages of the disease could be treated with the experimental therapy in one eye while the other untreated eye could be evaluated in parallel as a control.
To treat these blind dogs, the team utilized adeno-associated virus (AAV). They engineered AAV particles that possessed the entire RPGR gene. Then they devised a way to deliver these viruses to the retinal cells so that the viruses could infect the retinal cells and produce normal copies of the RPGR protein.
When the eyes treated with the AAV vectors were subjected to detailed imaging, it was clear that the gene therapy protocol arrested the thinning of the retinal layer. This shows that the treatment halted the degeneration of the photoreceptors in the affected dogs. When the treated eyes were compared with the untreated eye, the structure of the rod and cone photoreceptors was obviously improved and better preserved in the treated eye in comparison to the untreated eye. When the neural physiology of the retinas from the treated and untreated eyes was compared, once again, the retinas from the eyes treated with the gene therapy were more normal than the untreated eyes. In fact, the gene therapy halted the photoreceptor cell death associated with retinitis pigmentosa for two and a half years, which was the length of the study.
The team also treated dogs who suffered from later-stage disease in the hope that the gene therapy could not only improve the condition of dogs in the early stages of the disease, but also those with later stages of the disease. Interestingly, the gene therapy also froze the loss of retinal thickness and preserved the structure of surviving photoreceptors, but the retinas in the untreated eyes continued to thin and their photoreceptor function deteriorated as well. When the dogs were sent through an obstacle course and a maze under dim light, the animals did significantly better when they used their eye that had been treated with the gene therapy compared with their performance when they used the untreated eye. This shows that this gene therapy also works in dogs suffering from the late-stages of retinitis pigmentosa.
Can such a therapy be used in people in human clinical trials? Not yet. More safety testing must be done in order to properly determine if it is safe over long periods of time, according to this study’s co-leaders, Gustavo Aguirre, V.M.D., Ph.D., and William Beltran, D.V.M., Ph.D., of the University of Pennsylvania. Other collaborators, University of Pennsylvania scientists Artur Cideciyan, Ph.D., and Samuel Jacobson, M.D., Ph.D. are presently screening potential patients who have RPGR mutations as a prolegomena for a future clinical trial.
Their results are published in Proceedings of the National Academy of Sciences.