Gene Editing Restores Vision in Mouse RP Models

The CRISPR/Cas9 gene-editing tool was used to reprogram defective optical rod photoreceptor cells and transform them into functioning cone photoreceptors, which restored impaired vision in two mouse models of retinitis pigmentosa.

Retinitis pigmentosa (RP) is one of the most common forms of inherited retinal degeneration. This disorder is characterized by the progressive loss of photoreceptor cells and may eventually lead to blindness. Mutations in a number of different genes have been found to cause the retinitis pigmentosa phenotype.

Investigators at the University of California, San Diego applied the CRISPR/Cas9 approach to the RP problem. CRISPRs (clustered regularly interspaced short palindromic repeats) are segments of prokaryotic DNA containing short repetitions of base sequences. Each repetition is followed by short segments of "spacer DNA" from previous exposures to a bacterial virus or plasmid. CRISPRs are found in approximately 40% of sequenced bacteria genomes and 90% of sequenced archaea. CRISPRs are often associated with cas genes that code for proteins related to CRISPRs. Since 2013, the CRISPR/Cas system has been used in research for gene editing (adding, disrupting, or changing the sequence of specific genes) and gene regulation. By delivering the Cas9 enzyme and appropriate guide RNAs into a cell, the organism's genome can be cut at any desired location. The conventional CRISPR/Cas9 system is composed of two parts: the Cas9 enzyme, which cleaves the DNA molecule and specific RNA guides (CRISPRs) that shepherd the Cas9 protein to the target gene on a DNA strand.

The investigators reported in the April 21, 2017, online edition of the journal Cell Research that they had used adeno-associated virus (AAV) to deliver CRISPR/Cas9 to retinal cells. The effect of the gene-editing tool was to deactivate a master switch gene in the retina called Nrl (Neural retina-specific leucine zipper protein) and a downstream transcription factor called Nr2e3 (photoreceptor cell-specific nuclear receptor). Inactivation of these two proteins reprogrammed rod cells into cone-like photoreceptors, which rescued retinal rod and cone degeneration and restored visual function in two different mouse RP models.

"Cone cells are less vulnerable to the genetic mutations that cause RP," said senior author Dr. Kang Zhang, professor of ophthalmology at the University of California, San Diego. "Our strategy was to use gene therapy to make the underlying mutations irrelevant, resulting in the preservation of tissue and vision. Human clinical trials could be planned soon after completion of preclinical study. There is no treatment for RP so the need is great and pressing. In addition, our approach of reprogramming mutation-sensitive cells to mutation-resistant cells may have broader application to other human diseases, including cancer."