Researchers at the National Eye Institute have delivered a CRISPR/Cas9–based therapy directly to the eye to prevent retinal degeneration in a mouse model of retinitis pigmentosa.
Their work, published last week in Nature Communications, provides proof that the CRISPR gene-editing technique can be delivered through a viral vector to correct genetic defects underlying degenerative retinal diseases.
CRISPR/Cas9 is the genome editing tool that is creating a buzz in the science world. The technique can effectively repair or knockout a target stretch of DNA without reproduction for each new target, making it simpler and more cost effective than previous gene editing methods.
This combination of CRISPR technology with an adeno-associated virus vector, a system with a proven track record to deliver genetic information to the retina, may represent the first step in a global treatment approach for rod-mediated degenerative disease.
Anand Swaroop, PhD, chief of NEI’s Neurodegeneration and Repair Laboratory, and colleagues designed a CRISPR single guide RNA (sgRNA) to target a retinal transcription factor, Neural retina leucine zipper (Nrl), which specifies rod cell fate during retinal development and maintains rod cells within the mature retina. The idea was to coax rod cells into becoming more like cone cells, preventing their deaths as well as the secondary cone cell death seen in retinitis pigmentosa.
“The evidence suggested to us that coaxing rods into becoming more cone-like by knocking out Nrl was a potential strategy for overriding mutations that would otherwise lead to rod degeneration,” said Swaroop. “Consequently, the neighboring cones would remain functional and viable.”
Dr. Swaroop and his team tested the therapy in wild-type mice and 3 different mouse models of retinal degeneration. They confirmed that rods became more cone-like. Although these cone-like rods could not detect light, they survived and improved survival of their neighboring cones.
In all 3 different 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 three 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,” explained lead author Zhijian Wu, PhD, head of the NEI Ocular Gene Therapy Core.
Several questions must be answered before this approach can be tested in humans, including the safety of long-term expression of Cas9 in the retina, and whether ablation of Nrl will have the same effect in humans.