CRISPR is a gene editing technique with incredible potential. But the traditional CRISPR-Cas9 approach involves cutting the double helix of DNA in cells, which can have many unintended and dangerous consequences. There are some alternatives to cutting the double-stranded genome that could be much safer, and still help us treat or even cure genetic diseases. To successfully perform gene editing, the cells that carry mutated DNA also have to be accessible so they can be treated. It can be very challenging to access the right cells when they are deep within the body. But cells in the eye are far easier to reach, making it possible to treat some forms of blindness that are caused by genetic mutations.
Scientists have now used a modified version of CRISPR gene editing to restore vision in a mouse model of retinitis pigmentosa, which is a major cause of blindness in humans. The work has been reported in the Journal of Experimental Medicine.
Retinitis pigmentosa is thought to impair vision in one of every 4,000 people, and it can be caused by errors in more than 100 different genes. Retinitis pigmentosa affects rod cells first, disrupting their function. The disease moves onto cone cells next, and causes irreversible and severe loss of vision over time.
Various gene editing methods have been used to repair vision loss in mice that model various genetic diseases that cause blindness, like Leber congenital amaurosis. In this disorder, the retinal pigment epithelium, a layer of cells that supports other cells that sense light, is disrupted. But most forms of blindness that are genetic, such as retinitis pigmentosa affect the neural photoreceptors directly. Any gene therapy used to treat these disorderx also have to target the affected photreceptor cells directly.
In this work, the researchers used a mouse model that carried mutations in a gene called PDE6β, a portion of a protein complex found in rod cells. When PDE6β is disrupted, it causes retinitis pigmentosa. A variation of CRISPR was developed called PESpRY, which can be applied to any part of the genome. CRISPR-Cas9 can only target certain sequences, when they are near other sequences called protospacer adjacent motifs or PAMs. But not every target sequence is close to a PAM, which can hamper gene editing in certain places. PESpRY overcomes that limitation, and hence, it's called a PAM-less SpCas9 variant (SpRY).
In the mouse model that was treated with PESpRY, the function of the complex was restored and the mice became responsive to light again.
The researchers also followed these mice and continued to observe them, which showed hat the animals retained their vision even as they became elderly.
The PESpRY method is still not ready for use in humans, however.
This research has shown that this editing method can be used in vivo, and it has "... potential in diverse research and therapeutic contexts, in particular for inherited retinal diseases such as retinitis pigmentosa," said study co-author Kai Yao, a professor at the Wuhan University of Science and Technology.
Sources: Rockefeller University Press, Journal of Experimental Medicine
