January 27, 2012 – A Foundation-funded research team led by the University of Pennsylvania recently used gene therapy to prevent and reverse vision loss in two canine models of X-linked retinitis pigmentosa (XLRP). The results bode well for future approval of a human study of the treatment approach.
Drs. William Beltran and Gus Aguirre, study investigators at the University of Pennsylvania, say they are very pleased with the treatment’s ability to restore cells in the degenerating retinas to a normal state. The visual function in both canines, they add, improved dramatically.
Demonstrating safety and effectiveness in the XLRP large animal model are crucial to gaining FDA approval to launch a clinical trial. “Canine studies performed by Dr. Aguirre at the University of Pennsylvania were a pivotal step in getting the FDA to OK all gene therapy studies for Leber congenital amaurosis,” Dr. Stephen Rose, the Foundation’s chief research officer, says, referring to LCA studies underway at the Universities of Pennsylvania and Florida and four other clinical trial centers. “And he and Dr. Beltran are now positioning researchers well to move XLRP gene therapy into the clinic.”
The XLRP treatment involves delivery of healthy copies of the RPGR gene to rods and cones, the cells that provide vision in the retina. Because 75 percent of all cases of XLRP are caused by defects in RPGR, the gene therapy has the potential to help most people affected, including some with late-stage disease.
The gene therapy uses a manmade adeno-associated virus, or AAV, to penetrate rods and cones with healthy copies of the RPGR gene. Researchers believe a single treatment, which is contained in a tiny drop of liquid injected underneath the retina, will last years. Dr. William Hauswirth of the University of Florida developed the AAV, which is similar to the virus used in all five LCA trials now underway.
The other members of the XLRP gene therapy team are Drs. Artur Cideciyan and Samuel Jacobson, of the University of Pennsylvania, and Dr. Al Lewin, of the University of Florida. All of the team’s members are investigators for the LCA gene therapy effort as well.
Drs. Beltran and Aguirre are performing additional studies of XLRP gene therapy in canines to confirm which promoter — the switch in all gene therapies that ensures the therapeutic gene is activated in the right cells — will work best in humans. They are also evaluating the treatment in older canines and determining which areas of the retina are most amenable to the therapy.
XLRP mainly affects males. Usually, boys with XLRP are diagnosed by the age of 10 and are legally blind by 40. Like other forms of RP, XLRP initially causes loss of night and peripheral vision, followed by diminished visual acuity.
Women are usually unaffected carriers of XLRP, and they have a 50 percent chance of passing the condition down to their sons. However, some women do experience vision loss, sometimes severe, from XLRP.
“XLRP is a tough, aggressive, and complex disease,” says Dr. Rose. “There’s still a lot we don’t know about it. But we are on the right track to overcoming it. The success of this canine study is a big step forward.” Approximately 7,000 to 11,000 people in the U.S. are affected by XLRP.
Results of the study were published online January 23, 2012, in the journal Proceedings of the National Academy of Sciences.