While gene therapies for retinal degenerative diseases are making groundbreaking strides in both human and laboratory studies, the most widely and successfully used human-engineered virus for delivering replacement genes to retinal cells — the adeno-associated virus, or AAV — has one significant limitation. It can’t deliver relatively large genes, namely those larger than about 4.5 or 5 kilobases (kb). (Bases are the building blocks of a gene, and its size is expressed in kilobases.)
This capacity limitation has been a challenge for researchers trying to develop gene therapies for a number of retinal conditions, including those caused by mutations in the gene CEP290, which happens to be a little more than 8 kilobases.
CEP290 mutations are the leading cause of Leber congenital amauorsis (LCA), a retinal degenerative disease that often leads to severe vision loss in young children. CEP290 defects can also cause syndromic conditions — e.g., Bardet-Biedl, Joubert and Senor-Loken syndromes — which result in a constellation of symptoms, including LCA.
At the annual meeting of the Association for Research in Vision and Ophthalmology (ARVO), it is great to see reports from FFB-funded scientists who are developing a number of creative approaches to overcoming the delivery challenge for CEP290.
Take, for example, Erin Burnight, Ph.D., at the University of Iowa, who is using a lentivirus, which has the cargo capacity of about 9 kilobases, to deliver copies of CEP290 to stem cells derived from the skin of an LCA (CEP290 mutations) patient. Dr. Burnight demonstrated that the treatment effectively corrected the genetic defect in the cells. An advantage of the lentivirus is that it’s already being used in gene-therapy human studies for age-related macular degeneration, Stargardt disease and Usher syndrome type 1B.
Another approach discussed at ARVO involves delivering CEP290 in two packages. Renee Ryalls, at the University of Florida, is developing a dual-AAV gene therapy to deliver CEP290 to human cells in two packages. She and her team continue to fine-tune the approach to get the gene to re-assemble once delivered and function normally.
Alejandro Garanto, Ph.D., at the Radboud University Medical Centre in the Netherlands, is developing what are known as antisense oligonucleotides, or AOs, to correct the defective messaging that occurs with certain types of CEP290 mutations. At ARVO, he discussed how his therapy worked successfully in LCA patient cells.
Two important points about all three of these studies: 1) They are important steps toward clinical trials of emerging therapies; and 2) the approaches have the potential to work for treating retinal diseases caused by mutations in other large genes, including USH2A (Usher syndrome) and EYS (retinitis pigmentosa).
A final note: These aren’t the only alternatives for delivering large genes. Check out articles on the work of Muna Naash, Ph.D., David Schaffer, Ph.D., and Luk Vandenberghe, Ph.D., to learn more about the other options.
Pictured, above: Renee Ryalls at ARVO, explaining the dual-AAV gene therapy she’s developing.