New findings from a team led by University of Pennsylvania vision scientists, who have in the past taken gene therapies into clinical trials, are proving successful, this time treating a form of retinitis pigmentosa, a disease that progressively robs people of their night and peripheral vision before blindness develops

The researchers, from Penn's School of Veterinary Medicine and Perelman School of Medicine, in collaboration with University of Florida scientists, developed a therapy that effectively eliminates the abnormal copy of rhodopsin, a light-sensing molecule, and then restores it with a healthy copy of the protein.

This knockdown and replacement approach preserved the retina's light-sensing photoreceptor cells in affected dogs, which can develop a very similar disease to affected humans.

What's more, they accomplished this using a single viral vector to co-deliver the genetic material needed to achieve both the knockdown and replacement. Though more than 150 different mutations in rhodopsin have been identified to cause retinitis pigmentosa, this approach is intended to work regardless of the mutation or the mechanism by which rod photoreceptor cells, those responsible for vision in dim light, die.

That means that a large percentage of patients with rhodopsin autosomal dominant retinitis pigmentosa could benefit if the therapy is found to be safe and effective in people.

"The treatment targets a region of the rhodopsin gene that is homologous in humans and dogs and is separate from where the mutations are located. That gives us great hope about making this a translational treatment."

Retinitis pigmentosa refers to a set of progressive hereditary retinal disorders. Three decades ago, researchers identified mutations in the gene encoding rhodopsin as the first known genetic cause of the condition. Other genes have since been implicated as well, but rhodopsin mutations remain a major contributor, accounting for up to 30 percent of autosomal dominant retinitis pigmentosa.

The vast majority of rhodopsin mutations are passed through families in a dominant fashion, meaning that a parent need only pass one mutated copy on for their child to be affected.

"In our investigations, we have seen people in the 1990s with this genetic type of retinitis pigmentosa, and now we're seeing their grandchildren also affected," Jacobson says. "It's a multi-generational disease, and it's a serious disease."

Gene mutations

Several rhodopsin mutations that lead to retinitis pigmentosa result in what's known as a toxic gain-of-function thought to produce a protein that is harmful to the photoreceptor cells. Thus, to address the problems that arise in these patients, researchers have determined that the best strategy is to eliminate the mutant protein.

"We have developed in the past gene therapies for other conditions where the mutation causes a loss of function," says Aguirre, "so in these cases we just needed to add back the normal copy of the gene for the photoreceptors to regain their normal structure and function. When you have a dominant disease like this one, where the gene product is damaging to the cell, you have to get rid of it."

In the current work, the researchers employed this strategy, which relied on both in vitro assays and, significantly, on a canine animal model with a naturally occurring mutation in the rhodopsin gene that faithfully recapitulates the form of human retinitis pigmentosa caused by rhodopsin class B mutations.

While patients with class A mutations in rhodopsin lose functioning rod photoreceptor cells early in life, those with class B mutations may retain their rod cells for decades, making them candidates for vision-preserving gene therapy.

Because mutant rhodopsin dogs are very sensitive to ambient light, a short exposure equivalent to midday luminance was used to accelerate the degeneration of photoreceptors. This sensitivity allowed the researchers to control which areas of the retina were most affected and when, and thus evaluate treatment success in a matter of weeks rather than the years it would take to develop degeneration without this trigger.

The Penn researchers were able to confirm the beneficial effects on both structure and function of rod cells (as well as cones, responsible for color vision) by using specialized imaging techniques that can be used in human patients along with electroretinography, which provides a measure of rod and cone function.

Thus far, tracking the treatment effect more than eight months after delivery of the gene therapy, the effect seems stable and lasting. The research team is currently working to move the findings into clinical trials.

"The current work has strong implications for the treatment of patients with autosomal dominant retinitis pigmentosa due to Class B rhodopsin mutations," Cideciyan says.