Ingenuity Fuels Advance of Gene Therapy for LCA and RP

February 27, 2015

The gene CRB1 is a key target in the fight against inherited retinal diseases such as retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) because mutations in it lead to significant, often devastating vision loss. Approximately 4,000 people in the United States are affected by the mutation, as many as 80,000 worldwide.

CRB1 leads to the production of a protein that plays a critical role in the development and function of photoreceptors, the cells in the retina that convert light into electrical signals that are sent to the brain and formed into images. CRB1 is also essential for maintenance of the retina’s layered network of cells, which process and route the signals.

However, for two reasons, gene therapy development for CRB1 has been a tough nut to crack for scientists. First, the gene is too large for common delivery systems—namely human-engineered adeno-associated viruses, or AAVs, which are used widely and successfully in human studies of retinal-disease gene therapies. Standard AAVs don’t have the capacity to deliver the therapeutic CRB1 cargo to the cells that need it. Second, in mouse models, the locations for expression of CRB1 and the closely related gene CRB2 are the reverse of those in humans.

Foundation-funded researchers at the Netherlands Institute for Neuroscience creatively overcame these hurdles by developing a gene therapy that improves retinal function and structure in mice. Instead of replacing the defective CRB1 copies with normal CRB1 copies—the strategy typically used in gene-replacement therapy—they replaced them with CRB2 copies. CRB2 is small enough to fit into an AAV, and the researchers’ hypothesis that CRB2 was similar enough to CRB1 to do the job was correct. Results of the team’s efforts were recently published in Human Molecular Genetics.

“The functions of the proteins expressed by CRB2 and CRB1 are very similar in the mouse and human retina, and, in some species, these functions are executed by CRB2 proteins only,” explains Jan Wijnholds, Ph.D., the study’s lead investigator. “Before moving CRB2 gene therapy into a human study, we will perform additional experiments to examine if long-term increased expression of CRB2 is safe to the retina. If so, we will evaluate it in human studies as a potential treatment for RP and LCA caused by mutations in the CRB1 gene.”

In humans, CRB1 is normally expressed in both rods and cones, the two types of photoreceptors, as well as Müller glial cells, which extend through the retina like spokes of a wheel, providing structural support and guiding light through the inner retina. CRB2 is expressed at significant levels in Müller cells only and at lower levels in photoreceptors in humans. The emerging treatment involves delivery of CRB2 to rods, cones and Müller cells as a replacement for the mutated CRB1.

“I salute the ingenuity of the Netherlands team,” says Stephen Rose, Ph.D., chief research officer, Foundation Fighting Blindness. “They had to think way outside of the box to come up with their approach to gene replacement for CRB1 mutations. While more work remains, we’re excited about the potential their innovative treatment holds for moving into a clinical trial.”