Rare Daily Staff

Researchers reported that a novel gene therapy that halts vision loss in a canine model of autosomal dominant retinitis pigmentosa (adRP) could provide a means of combatting the development of blindness in people with adRP, a group of rare genetic disorders that damage the light-sensing cells in the retina.

The work, funded by the National Eye Institute, demonstrates a strategy that could one day be used to slow or prevent vision loss in people with the disease.

“We’ve developed and shown proof-of-concept for a gene therapy for one of the most common forms of retinitis pigmentosa,” said William Beltran of the University of Pennsylvania School of Veterinary Medicine, Philadelphia, a lead author of the study available online in the Proceedings of the National Academy of Sciences.

Rod photoreceptor cells enable vision in low light and require a protein called rhodopsin for their light-sensing ability. People with adRP caused by mutations in the rhodopsin gene usually have one good copy of the gene and a second, mutated copy that codes for an abnormal rhodopsin protein. The abnormal rhodopsin is often toxic, slowly killing the rod cells. As the photoreceptors die, vision deteriorates over years or decades. There are more than 150 rhodopsin mutations known to cause adRP, which has made the development of effective therapies challenging.

Beltran, in collaboration with researchers at the University of Pennsylvania Scheie Eye Institute, University of Pennsylvania School of Veterinary Medicine, and the University of Florida at Gainesville, generated a gene therapy construct that knocks down the rod cells’ ability to produce rhodopsin using a technology known as shRNA (short-hairpin RNA) interference.

The researchers tested their gene therapy construct on dogs with a rhodopsin mutation. As in people with adRP, these dogs slowly lose their rod photoreceptors. In the dogs’ retinas, the construct knocked down about 98-99 percent of rhodopsin (both mutated and normal). But because normal rhodopsin is required for the rods to detect light, the researchers added a “hardened” shRNA-resistant rhodopsin gene to the same vector.

Initially, the researchers tried using two separate vectors to deliver the shRNA and rhodopsin replacement but getting the right amount of each into all the cells without overloading the retina with viral vector was challenging. The solution the researchers hit upon was delivering a single vector that carried both the shRNA and the replacement gene to ensure balance.

When treated with the combined vector, the dogs maintained healthy, functional photoreceptors, and because the vector was designed to produce human rhodopsin, it could potentially work in humans as well.

“This all-in-one, remove-and-replace approach needed a lot of fine-tuning,” Beltran said. “The naturally-occurring dog model of adRP was critical. Only in a model where you have natural levels of both the mutant and wild-type copies can you actually test that fine-tuning and see whether you’re knocking down sufficiently; whether you’re replacing sufficiently.”

The team hopes to start pre-clinical safety studies within the next year or two, with the goal of eventual human trials. Because the treatment prevents photoreceptor loss rather than regenerating photoreceptors, it is expected the approach could be helpful for patients who have slowly progressing forms of adRP, and whose retinas include areas with living rod cells.

August 21, 2018
Photo:  William Beltran of the University of Pennsylvania School of Veterinary Medicine, Philadelphia