Improving the Delivery of Gene Therapy
August 16, 2011 - People living with inherited retinal degenerative diseases such as retinitis pigmentosa (RP), Leber congenital amaurosis (LCA) or Stargardt disease were born with genetic changes that undermine the functioning of the light-sensing cells in their retinas (called photoreceptors).
The essential light-sensing and signalling functions of the photoreceptors are controlled by hundreds of proteins, and an error in any one of the genes that encode those proteins can damage the photoreceptors and impair a person’s vision. Over the last twenty years, scientists in Canada and elsewhere have discovered many of the specific genetic changes associated with retinal degenerative conditions. This has allowed the development of potential new therapies.
In many cases, the mutation responsible for damage to photoreceptors means that a necessary protein is not produced or doesn’t work correctly. This is called a “loss-of-function” mutation. One strategy to treat such mutations is to replace the damaged genes in the photoreceptors with healthy copies of the gene. This treatment would allow any surviving photoreceptors to function again and prevent further loss of healthy ones.
This idea has been used to develop a gene therapy for one type of LCA, and early-phase clinical trials of gene-replacement therapy in small numbers of human LCA patients have already produced encouraging results. Canadian Dale Turner was the recipient of gene therapy in one of these trials. In those trials, a modified virus called adeno-associated virus type 2 (AAV2) was used to insert the new genes into the cell.
The AAV2 virus has been engineered to carry the therapeutic replacement gene and not to reproduce itself. This “viral vector” does not cause disease, but only fools the therapeutically targeted cells – photoreceptors and retinal pigment epithelial (RPE) cells – into taking in the virus, along with the replacement gene that it carries (think of the viral vector as a sort of Trojan horse). However, the viruses that have been used to date are limited in a number of important ways. For one thing, AAVs can carry only fairly small genes, and many genes that have been implicated in retinal degeneration are too large to be introduced in this way. For another, a relatively high dose of the virus (large number of virus particles) is needed to assure that sufficient numbers of photoreceptors will take up the replacement gene, and the higher the dose, the more likely it is that a person’s immune system will try to reject the treatment.
"The results from three Phase I clinical trials for LCA showed the potential for gene therapy based on adeno-associated viruses delivering corrective genes to the retina," says Dr. Jean Bennett of the University of Pennsylvania, one of the world leaders in developing gene therapy for inherited retinal diseases. "To broaden treating inherited eye diseases, we will need a larger vector toolkit.”
The forms of LCA that were treated successfully in the Phase I trials are due to mutations in genes that are expressed only in the RPE, which is a relatively easy target for AAV vectors. However, most cases of RP and similar diseases are due to mutations in genes that are peculiar to photoreceptor cells, which have proven to be relatively resistant to AAV2. Dr. Bennett and her colleagues, Dr. James Wilson and Dr. Luk Vandenberghe have been studying ways to deal with this limitation. Recently, they compared the ability of AAV2 and a related virus, AAV8, to carry genes into the photoreceptors. Their findings were just published in the journal, Science Translational Medicine.
"We showed that we can use AAV8 to deliver genes to the photoreceptor of the primate eye at lower doses [than AAV2], both safely and efficiently," says Dr. Vandenberghe. They also showed that AAV8 was markedly better at targeting photoreceptor cells than AAV2.
"To address patients with other retinal diseases, we need a renaissance of technology – new and better vectors to safely and effectively deliver corrective genes to a range of diseases," says Dr. Wilson, who originally discovered AAV8 and who continues to study different viral vectors.
Scientists all over the world are working to solve these challenges. In Canada, a team led by Dr. Robert Molday at the University of British Columbia and funded by the FFB and the Canadian Institutes of Health Research is working to find ways of carrying large genes, such as the gene associated with Stargardt disease, into the photoreceptors.
“Stay tuned!” says Dr. Bill Stell, Director of Research Programs, FFB. “Advances in therapy for inherited retinal degeneration are coming thick and fast. There has never been a better time to invest your dollars in research to combat blindness!”