"There is a compelling need to explore gene therapy in the eye," according to Paul A. Sieving, MD, PhD.
His current work is in X-linked retinoschisis (XLRS) with the goal of understanding it and to advance gene therapy. During delivery of the Gertrude Pyron Award Lecture, he described this work and provided a rationale for pursuing gene therapy for XLRS.
“We now know that XLRS is a retinal synaptic disease that can readily be treated in adult mice,” said Dr. Sieving, director, National Eye Institute, Bethesda, MD. “The question remains about whether treatment will work in humans.”
The eye is a good place to explore gene therapy because it is a small, closed compartment. The vector would carry a low risk of systemic toxicity, he noted.
“It also is worth noting that the cell types in the eye are evolutionarily conserved from mouse to human,” he said, citing the study of RPE65 gene therapy that began with gene discovery in 1993 and resulted in gene therapy in humans for Leber’s congenital amaurosis over 15 years.
“We now understand that RPE65 is the most critical factor in retinoid cycling in the retinal pigment epithelium,” he said.
Dr. Sieving focused on two components of XLRS, i.e., structure and function. Regarding the former, optical coherence tomography shows cavitation throughout the retinal layers, particularly in the deeper retina and the plexiform layer. Another facet is the electronegative response, which is key to what happens in the disease.
At the outset of Dr. Sieving’s interest in XLRS, he found a pedigree of 119 family members, but his cloning of the gene was pre-empted by a German investigator, Bernard Weber.
The next step was to determine the gene function by disabling it in a mouse model, which, in turn, showed that the B-wave was missing and normally there was copious protein in the rod inner retinal segments and in the synaptic zone. In both regions in the knockout mice there was disruption in the outer plexiform layer (OPL).
Dr. Sieving showed the presence of retinoschisin protein on the outer membrane of the inner segment, not the outer segment. He showed when two inner segments were side by side, in the cleft between them there is a large amount of retinoschisin protein, the function of which is as yet unclear.
When the investigators examined the protein itself using cryoelectronmicroscopy, they found collections of the retinoschisin protein that were lined up like a frisbee, with two such configurations coupled back to back.