The Road to CRISPR Vision

(Posted on Monday, June 19, 2023)

Originally published on Forbes on 6/14/2023

This story is part of a series on the current progression in Regenerative Medicine. This piece is part of a series dedicated to the eye and improvements in restoring vision.

In 1999, I defined regenerative medicine as the collection of interventions that restore to normal function tissues and organs that have been damaged by disease, injured by trauma, or worn by time. I include a full spectrum of chemical, gene, and protein-based medicines, cell-based therapies, and biomechanical interventions that achieve that goal.

Stem cell therapies and gene therapies have been making significant progress in restoring vision loss due to age, disease, or trauma. Here we describe recent advances that use CRISPR to treat retinitis pigmentosa, as seen in a recent study.

Retinitis Pigmentosa is a rare genetic disorder causing severe vision loss that progressively damages the retina. Those impacted feel imprisoned by tunnel vision in an ever-narrowing bubble of sight. Daily tasks become more challenging as the condition progresses, and simple activities feel like insurmountable obstacles. Walking down a street, for instance, becomes intimidating, as you may not be able to see the sidewalk or other people walking nearby, leading to the possibility of tripping or colliding with someone.

Retinitis Pigmentosa in Detail

Retinitis pigmentosa (RP) is an inherited eye disease that affects over one million individuals worldwide. The condition is the leading cause of inherited blindness in developed countries and is estimated to impair the vision of 1 in every 4,000 people.

It is characterized by the progressive and irreversible degeneration of photoreceptor cells, which are the cells lining the back of the eye responsible for converting light into electrical impulses sent to the brain. The disease typically affects both eyes, and most people with retinitis pigmentosa experience symptoms such as night blindness, tunnel vision, and loss of peripheral vision. 

Genetic mutations cause the condition and can be passed down from parents to their children. There are at least 100 genes attributed to the disease. One such gene is phosphodiesterase 6b, also known as PDE6b. It is a protein-coding gene that plays a critical role in the phototransduction pathway. It converts light into an electrical signal the brain can interpret as vision. 

In individuals with retinitis pigmentosa caused by mutations in the PDE6b gene, the gene is either absent or non-functional. As the disease progresses, cone photoreceptor cells, responsible for color vision, are affected. The loss of cone cells contributes to the severe and irreversible deterioration of vision in individuals with retinitis pigmentosa. Traditional gene replacement and CRISPR-Cas gene editing approaches have been used for gene therapy for retinal degenerative diseases. 

What is CRISPR?

CRISPR, or Clustered Regularly Interspaced Short Palindromic Repeats, is a revolutionary gene-editing technology that allows scientists to target and modify specific DNA sequences precisely and efficiently. CRISPR uses specialized cellular machinery to make precise cuts in the DNA strand, opening up the possibility of replacing or removing faulty genes and treating various genetic disorders.

The potential uses of CRISPR are vast and varied, ranging from developing new crops to treating life-threatening genetic diseases. In the medical field, CRISPR has shown great promise in treating degenerative eye diseases.

Researchers in China have developed a new, highly versatile form of CRISPR-based genome editing that has successfully restored the vision of mice with retinitis pigmentosa, a leading cause of blindness in humans. The study will be published in the Journal of Experimental Medicine, focusing on using CRISPR to replace the gene causing retinal degeneration.

The Study Results

In the study, the researchers developed a CRISPR-based genome-editing tool named PESpRY. The system corrected the mutation in the PDE6b gene. The correction restored the gene’s activity in the retinas of mice, prevented the death of rod and cone photoreceptors, and restored their normal electrical responses to light. The gene-edited mice retained their vision even into old age, as shown in behavioral tests, such as finding their way out of a visually guided water maze or head movements in response to visual stimuli. 

In addition, the researchers found that their PESpRY system led to the preservation of photoreceptors in the mice. This resulted in the rescue of visual function in the mice, supported by detailed electroretinograms and behavioral assessments. 

The new system is a breakthrough that offers enormous potential. Its ability to enable practical and versatile genome editing within the body holds great promise for treating inherited retinal disorders, such as retinitis pigmentosa. With further research, the hope is to test these gene therapies in humans to see if we can overcome inherited retinal degeneration. It will be a significant advance in gene therapy.