Genomics: Insight

Introduction

Loss of vision can be devastating for the patient and their family. Those gifted with the ability of sight might brush over just how fortunate we are in our ability to perceive the world visually. Vision is something that we have to use every day: whether it be to perform day-to-day tasks such as driving and reading consistently or to simply avoid colliding with objects around us. Unfortunately, like many human functions, vision is susceptible to being impaired. 2.2 billion people globally have some form of vision deficiency and at least 1 billion of those people could have been prevented or at least addressed the issue with the advanced technology we have today¹. This essentially means that a quarter of the world is subjected to struggle as they try to adapt in the absence of sight. However, scientists have been working tirelessly in order to refine the quality of life as they strive to fully cure those with vision deficiencies. Specifically, retinitis pigmentosa (RP), an eye disease in which the retina is damaged, is one of the vision deficiencies scientists are working to cure.¹

What is Retinitis Pigmentosa?

Retinitis pigmentosa (RP) is a group of rare eye diseases that affect the retina, which is the light-sensitive layer of tissue in the back of the eye. Some symptoms of the disease are loss of night and peripheral vision along with the loss of vision entirely. As RP causes cells in the retina to break down slowly over time, treatments such as modern vision aids and rehabilitation act only as methods of delaying vision loss instead of curing it.²

Without any cure for RP, those diagnosed with the disease could only wait for the untimely loss of their ability to see until March 4th, 2020, when doctors first utilized CRISPR, a gene-editing tool, to treat a type of blindness known as a form of Leber congenital amaurosis.³ Out of six people who received this new form of therapy, two have gained a better sense of light while another can now navigate a maze in dim light, both large improvements from being totally blind. Showing promising results, some researchers have begun experiments to find cures for RP.⁴ To that end, recent research has used CRISPR-based gene-editing techniques in mouse models of RP to correct mutations associated with RP symptoms.

What is CRISPR?

First discovered as a defense system against viruses in Escherichia coli, CRISPR (clustered regularly interspaced short palindromic repeat) has become an increasingly promising gene-editing tool since it can very precisely edit genes at a much more efficient rate compared to past gene-editing technology. Therefore, CRISPR shows great potential to make a positive impact on health and medicine and any industry related to biological sciences (food, drug, and agriculture) as it allows scientists to edit disease-associated DNA with precision and ease. Assisted by a guide RNA that detects the right place to genetically modify a DNA, CRISPR technology can correct mutations related to inherited diseases, even those that scientists have yet to find a cure for, such as RP.⁵

Analysis & Data

Retinitis pigmentosa (RP) is an inherited blindness disorder that affects 1/3500 people globally. PRPF31⁶ is a gene that, when mutated, is known to cause retinitis pigmentosa, specifically accounting for 6-11.1%⁷ of all RP. RP is a genetically heterogeneous group of retinal dystrophies that progressively degenerate photoreceptors—proteins that are activated by and help process light, eventually resulting in severe visual impairment. Using mouse models, the injection of the PRPF31-Knockout vectors— CRISPR-mediated deletion of the PRPF31 gene immediately resulted in rapid structural and functional degeneration inside the photoreceptors.⁸ The PRPF31-Knockout mice (PRPF31-KO) is engineered to inactivate the specific PRPF31 gene, in order to create a model system that mimics the molecular defects seen in RP in terms of electroretinogram (ERG), a measure of the functional response of the retina to light; and apoptosis—cell death, and gliosis—production of glial cells, that are both characteristics of RP.

When the PRPF31-KO mice were treated with a CRISPR rescue vector that inserts the wild-type PRPF31 gene—to rescue the PRPF31-KO, it resulted in the restoration of ERG recordings, indicating restoration of retinal function, and reduction of retinal thickness and loss of photoreceptors, indicating improvement of structural defects relative to the control sample. Scotopic vision allows one to see in low-light conditions by utilizing rod cells, while photopic vision uses cone cells to see in brighter conditions. To investigate these visual processes, scotopic and photopic waves were employed to measure and analyze the response of the retina's photoreceptors, bipolar cells, and RPE in mice to a light stimulus. 5 weeks in, ERG recordings showed significantly reduced scotopic and photopic wave responses in PRPF31-KO mice eyes across a range of stimulus intensities, indicating photoreceptor dysfunction. When the PRPF31 KO mice were treated with the PRPF31 rescue vector, many of the RP-like defects of the KO mice like retinal thickness, loss of photoreceptors, and impaired RPE function were improved relative to controls.⁹

In another mouse model of RP that involves the mutation of the rhodopsin gene (P23H¹⁰ ) in a heterozygous condition (only one of the alleles was mutated), scientists used a CRISPR-mediated gene-editing strategy to correct the rhodopsin P23H mutation and reduce RP-like symptoms. When these mice were treated with a CRISPR- mediated vector that corrects the mutant P23H allele to the wild-type (normal) allele, the RP-like symptoms such as increased photoreceptor degeneration were alleviated relative to controls. Specifically, in the CRISPR-treated regions, photoreceptors began to work again as the progression of photoreceptor cell degeneration in the outer nuclear layer was significantly delayed in treated regions of the Rho-P23H retinas at 5 weeks of age. These results suggest that CRISPR-mediated editing of the P23H mutant Rho allele resulted in photoreceptor cell preservation in the treated retinas.¹¹

“[CRISPR]’s success in reducing RP symptoms in mice models suggest that it could be a promising candidate to cure RP.”

Conclusion

Even though CRISPR has not yet been used on a human with RP, the experiments described above using mouse models indicate that while we are yet to achieve a complete cure, promising improvements in vision by correcting mutations using CRISPR technology can be achieved. Given the adverse effects of having RP or knowing someone who has RP, it is imperative that there is a continued effort to find a cure. While CRISPR’s novelty as a gene-editing technology may elicit skepticism amongst those wary of its potential, its success in reducing RP symptoms in mice models suggests that it could be a promising candidate to cure RP.

References

1. World Health Organization. “Vision Impairment and Blindness.” World Health Organization, World Health Organization, https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment.
2. “Retinitis Pigmentosa.” National Eye Institute, U.S. Department of Health and Human Services, https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/retinitis-pigmentosa#:~:text=Retinitis%20pigmentosa%20(RP)%20is%20a,over%20time%2C%20causing%20vision%20loss.
3. “CRISPR Helps a Blind Woman See, but Doesn’t Help All Patients.” AAAS Articles DO Group, 2021, https://doi.org/10.1126/science.acx9258.
4. Mahmoudian-sani, Mohammad-Reza, et al. “CRISPR Genome Editing and Its Medical Applications.” Biotechnology & Biotechnological Equipment, vol. 32, no. 2, 2017, pp. 286–292., https://doi.org/10.1080/13102818.2017.1406823.
5. Xi, Zhouhuan, et al. “Gene Augmentation Prevents Retinal Degeneration in a CRISPR/cas9-Based Mouse Model of PRPF31 Retinitis Pigmentosa.” Nature News, Nature Publishing Group, 13 Dec. 2022, https://www.nature.com/articles/s41467-022-35361-8.
6. Li, Pingjuan, et al. “Allele-Specific CRISPR-Cas9 Genome Editing of the Single-Base P23H Mutation for Rhodopsin-Associated Dominant Retinitis Pigmentosa.” The CRISPR Journal, vol. 1, no. 1, 2018, pp. 55–64., https://doi.org/10.1089/crispr.2017.0009.