Next Generation Gene Therapy

Inspired by earlier successes using gene therapy to correct an inherited type of blindness, investigators from the Perelman School of Medicine at the University of Pennsylvania, are poised to extend their approach to other types of blinding disorders. In a previous human trial conducted at the Children's Hospital of Philadelphia and Penn, researchers packaged a normal version of a gene missing in Leber's congenital amaurosis (LCA) inside a genetically engineered vector, called an adeno-associated virus (AAV). The vector delivered the gene to cells in the retina, where the gene produces an enzyme that restores light receptors.

"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," notes co-senior author Jean Bennett, MD, PhD, the F.M. Kirby professor of Ophthalmology. "To broaden treating inherited eye diseases, we will need a larger vector toolkit, and what we have seen of AAV8 gives us hope for creating gene therapies for diseases that attack the retina's photoreceptors. This preclinical study provides the guidance we need to formulate dose level and type of vector to deliver corrective genes to treat blindness caused by the loss of photoreceptors."

In the present study, published in Science Translational Medicine this week, the Penn team compared the safety and efficiency of delivery in an animal model of two different types of AAVs - AAV2, which was used in the human trials for LCA, and AAV8, a second-generation AAV technology initially discovered in the lab of co-senior author James M. Wilson, MD, PhD, professor of Pathology and Laboratory Medicine.

The researchers used both vectors to deliver a green fluorescent protein (GFP) transgene to the retinal pigment epithelial (RPE) cells and photoreceptor cells of nonhuman primates. Photoreceptor cells are the problem area for other retinal diseases such as retinitis pigmentosa (RP) and others, for which there is no treatment. Photoreceptors are specialized nerve cells that convert light into a biological electrical signal and are designated as rods and cones.

"We showed that we can use AAV8 to deliver genes to the photoreceptor of the primate eye at lower doses, both safely and efficiently," says first author Luk H. Vandenberghe, PhD, Senior Investigator, Gene Therapy Program, and currently at Penn's F.M. Kirby Center for Molecular Ophthalmology.

Both AAV2 and AAV8 delivered the gene safely and efficiently to the monkey retinas, but AAV8 was markedly better at targeting photoreceptor cells.

The STM study describes the dose relationship between AAV2 and AAV8 vectors and their target cells and the immune response in the nonhuman primate retina. From this the researchers found dosage thresholds to safely and efficiently target cells in the outer retina such as RPE cells and rod and cone photoreceptors. While AAV2 and AAV8 efficiently delivered the gene to RPE cells at moderate to low doses, expression of the GFP gene in rod and cone photoreceptors was reached only at higher dosages. Substantial delivery to rods was obtained with moderate doses of AAV8, doses similar to those currently used in experimental clinical protocols