Researchers Identify Better Virus for Retinal Gene Delivery

June 12, 2013
While human studies of vision-restoring gene therapies have been hailed as a major breakthrough, Foundation-funded scientists have not rested on their laurels; they continue their intensive research to make them even better.

The latest case in point: Scientists from the University of California, Berkeley and the University of Rochester identified a new adeno-associated virus, known as a 7m8 AAV, for more safely and effectively delivering therapeutic genes to the retinas of people with a variety of retinal degenerations. In contrast to existing AAVs, which are often administered through an injection underneath the retina, the new AAV can penetrate most cells in the retina through a less-invasive injection into the vitreous, the gel-like substance in the middle of the eye. The investigators reported the results of their AAV research in the journal Science Translational Medicine.

“This is a critical next step in the development of retinal gene therapies,” says Dr. Stephen Rose, Ph.D., chief research officer, Foundation Fighting Blindness. “The enhanced AAV holds potential for treating more of the retina and doing so more safely. Incremental advancements like this are essential to getting the best treatments out to the patients.”

The investigators showed efficacy for the 7m8 AAV in a large animal as well as mouse models of retinoschisis and Leber congenital amaurosis, or LCA (RPE65 mutations). In the mouse studies, the virus was able to penetrate the retina and deliver a corrective gene to enable the retina to function normally.

While the large animal did not have a retinal disease, the virus transduced many regions of its retina. Ultimately, in both types of animals, the AAV was able to deliver genetic cargo to a variety of retinal cells, including: photoreceptors, the cells that provide vision; the retinal pigment epithelium, a layer of cells providing nutrients and waste disposal; and ganglion cells, which are a target for emerging, vision-restoring optogenetic therapies.

Most notably, the intravitreally administered AAV was able to penetrate the fovea, a small pit in the center of the retina rich in cones, which provides the vision most critical to daily living, but is often made fragile by degenerative diseases. Researchers have been concerned that injections underneath the fovea could cause permanent damage and vision loss in patients with advanced degeneration in their central retina.

AAVs are currently used for gene delivery in several retinal gene therapy clinical trials, including those that have restored vision in children and young adults with LCA (RPE65). AAVs are attractive for gene delivery because of their natural ability to penetrate a variety of cells. In addition, humans are exposed to the virus in nature and, therefore, tolerate it well.

To identify the optimal AAV for intravitreal gene delivery, the scientists used a process called “directed evolution” to randomly create millions of variations of the virus. The variants were then screened in mice to identify the top candidates for gene delivery to the retina. In addition to looking for an AAV that could penetrate retinal cells well, the researchers searched for a variant that could pass through a formidable barrier in the eye known as the inner limiting membrane, or ILM, which separates the vitreous from the retina.

The scientists from UC Berkeley plan to perform additional toxicology and efficacy studies to ready the 7m8 AAV for study in humans.

The study’s investigators were: Drs. Deniz Dalkara, Leah C. Byrne, Ryan R. Klimczak, Meike Visel, Lu Yin, William H. Merigan, John G. Flannery, and David V. Schaffer.