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The X-Link Factor

Consortium Makes Major Strides in the Development of Treatments for X-Linked RP

After three years of intensive research on multiple fronts, a consortium funded by the Foundation Fighting Blindness is reporting significant progress in the development of vision-saving  treatments for X-linked retinitis pigmentosa (XLRP).
“X-linked RP is a particularly aggressive retinal degenerative disease, and at the same time, is relatively prevalent. So the need to take action is strong,” says Dr. Stephen Rose, chief research officer, Foundation Fighting Blindness. “By establishing the consortium, we are able to collaborate as a team to identify treatment opportunities and then apply a full court press in developing them. The strategy has paid off well.”

The XLRP consortium is focusing its efforts on multiple gene therapy approaches, as well as a protein-based treatment that inhibits retinal cell death. The team is also performing natural history studies to better understand the progression of XLRP and how well potential treatments are working. Gene therapy for XLRP is furthest along in development and could move into clinical trials within the next two to three years.

Dr. Rose says that consortium investigators have been able to leverage the research successes for other retinal conditions, including gene therapy clinical trials that are restoring vision in children and young adults with Leber congenital amaurosis (LCA).

Dr. Jean Bennett, the lead investigator for the LCA gene therapy clinical trial at The Children’s Hospital of Philadelphia, is achieving excellent results in preclinical studies of XLRP gene therapy, which uses the same gene delivery technology — adeno-associated viruses (AAVs) — used in the LCA trial.

Gene therapy is an attractive approach to treating XLRP, because 70-80 percent of XLRP cases are caused by a single gene called RPGR. In other words, an RPGR gene therapy could potentially help 70-80 percent of all people with XLRP.

One of the challenges in delivering the RPGR gene to the retina is its size. As genes go, it is rather large. To overcome that limitation, Dr. William Hauswirth of the University of Florida is developing an XLRP gene therapy that delivers a region of RPGR known as ORF-15 to the retina. ORF-15 is where disease-causing variations in RPGR commonly occur. He, too, has obtained excellent results in preclinical studies.

On another research front, Drs. David Zack and Debra Thompson at the University of Michigan are conducting preclinical experiments of a protein called X-linked Inhibitor of Apoptosis Protein (XIAP) to slow or halt vision loss from XLRP. The investigators have shown that XIAP slows retinal cell death and loss of vision in preclinical testing.

Functional and structural evaluation of the retina is critical to the development of these various XLRP treatments. Dr. John Heckenlively at the University of Michigan, a leading retinal physician, is an expert in conducting both retinal examinations and tests that measure retinal activity. He will help investigators understand how well emerging treatments are working in both lab and clinical studies.

Dr. Rose notes that while the consortium is aiming at XLRP, the knowledge coming into and out of their studies is relevant to a wide range of retinal conditions. “The consortium has benefited greatly from the knowledge gained in the LCA clinical trials and other cross-cutting studies, and I know other research groups are watching the progress being made in XLRP very closely,” he says.


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