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Genetic engineering prevents retinal cell loss in mice

The upper panel shows normal Nrl expression in the mouse retina. The bottom panel shows reduced Nrl expression when the gene is knocked down with CRISPR/Cas9. NEI

Retinitis pigmentosa is a group of fairly rare genetic disorders that involve a breakdown and loss of cells in the retina—the light sensitive tissue that lines the back of the eye. Initial symptoms include difficulty seeing at night and a loss of side (peripheral) vision. In later stages, people also lose their central vision, which is associated with reading, driving, and other daily activities. Eventually, people with retinitis pigmentosa lose most of their sight.

There are 2 types of light-sensing cells in the retina: rods and cones. Rod photoreceptors enable vision in dim light, while cone photoreceptors enable color vision and the ability to see fine detail in well-lit conditions. Mutations in almost 100 genes have been linked to inherited retinal degeneration. The majority of these mutations affect rods at first, leading to night blindness such as that seen with retinitis pigmentosa. Because rods provide vital structural and nutritional support to cones, rod dysfunction or death can also lead to cone degeneration and vision loss.

Previous studies have investigated the use of gene therapy or genome editing to correct the individual genetic issues that lead to retinal diseases. However, because there are so many underlying genetic causes, scientists have been exploring approaches that target common disease pathways.

Deleting the gene for Nrl—the key transcription factor that determines whether a photoreceptor becomes a rod or a cone during development—has been shown to improve cone cell survival in mouse models of retinal degeneration. Loss of Nrl can also make mature rod photoreceptors more resistant to disease-causing gene mutations and prevent cone cell death. To explore a practical strategy to treat retinal degeneration, a team led by Drs. Anand Swaroop and Zhijian Wu at NIH’s National Eye Institute carried out a series of studies in mice. Results appeared online on March 14, 2017, in Nature Communications.

The scientists developed a method for deleting Nrl in rod photoreceptors using a gene snipping tool called CRISPR/Cas9. The strategy used a harmless virus as a carrier, or vector, to introduce CRISPR/Cas9 into retinal cells. They tested this genome editing tool to remove the Nrl gene in wild-type mice and 3 different mouse models of retinal degeneration. By measuring gene expression and examining the retinal photoreceptor cells, the researchers confirmed that rods became more cone-like, as predicted. Although these cone-like rods could not detect light, they survived and improved survival of their neighboring cones.

In all 3 mouse models, rod degeneration was prevented or slowed, although less benefit was achieved when the therapy was introduced in older animals. Importantly, the benefit was evident in all 3 models, regardless of the specific gene defect in the mouse.

“Unlike conventional gene therapy, in which a normal gene is introduced to replace the defective gene, this approach could treat retinal degeneration caused by a variety of mutant genes,” Wu explains.

More research is needed before this therapy is ready for testing in a clinical trial. The safety of CRISPR/Cas9 has yet to be established, and information is needed about its possible adverse effects. Nevertheless, these findings provide proof-of-concept for therapies to treat degenerative retinal diseases.