CRISPR Used to Modify Vision Loss Caused by Genetic Mutation

Retinitis pigmentosa is a degenerative disease that leads to vision loss in humans. Though there is currently no cure for this condition, a novel study has identified that silencing the Nrl gene could prevent the loss of cells that occurs due to retinal degenerative diseases. The study was published in the journal Nature Communications and was conducted by scientists from the National Eye Institute, which is associated with The National Institute of Health.

Retina

The retina is a tissue that is present at the back of the eye and it consists of two types of cells, rod photoreceptors and cone photoreceptors. These cells help in converting the light that enters the eye into electrical signals which reaches the brain.

• Rod Photoreceptors – These cells help in vision in dim light
• Cone Photoreceptors- These cells help in vision during well-lit conditions and are used for color vision.

Genetic Mutations

The Retinal Information Network (RetNet) states that there are over 242 genetic mutations with over 202 genes that are associated with the development of various vision associated genetic disorders. Retinitis pigmentosa, is one such genetic disorder that leads to a predictable, yet inevitable, course of disease. This disease is characterized by

• Rod dysfunction that is characterized by night blindness or a loss in vision during dim light
• Rods in the peripheral retina die leading to a loss in peripheral vision
• Complete blindness during the late stages of the disease

There is currently no available cure for the disease and therapy is mainly used to delay the progression of the disease. The rods provide nutritional support and structure for cones, so their dysfunction leads to the death of the cones as well, resulting in total blindness.

Chief of NEI's Neurodegeneration and Repair Laboratory, Dr. Anand Swaroop, and his team wanted to save vision by saving the rods from dysfunction.

The Gene Nrl

In humans, the Nrl gene codes for neural retina specific leucine zipper protein, which is essential for the development of the photoreceptors.

In an earlier study conducted by Dr. Swaroop and colleagues, mice that were grown without the Nrl gene developed retinas that had only cones. Nrl gene expression is knocked out in mice that are older, according to other studies.

Potential Strategy to Override the Mutations

Dr. Swaroop stated that knocking out the Nrl gene could be a potential strategy to veto the genetic mutations that lead to rod degeneration. The rods could be guided to become cone like after the Nrl gene is knocked out. The cones that are present adjacent to these cone like rods would remain functional.

CRISPR

The team of researchers utilized a technique called CRISPR that acts like molecular scissors to snip specific DNA sequences with precision. Dr. Wenhan Yu, who is the study's first author and an NEI postdoctoral research fellow, designed a method to use CRISPR in photoreceptors.

The steps designed by Dr. Yu were

• The use of an adeno-associated virus (AAV) as a vector or carrier that could be used to introduce the CRISPR into the retinal cells.
• The Nrl gene was removed using CRISPR in wild-type mice, and also in three different mouse models which suffered from retinal degeneration.
• The retinal cells were examined along with the gene expression. It was found that rods developed into cone like cells. These cone like cells were not capable of detecting light but they helped in the survival of neighboring cells.

There was prevention of rod degeneration in mouse models, though the benefit was not considerable when the therapy was carried out on older animals. Dr. Zhijian Wu, who was the Head of NEI Ocular Gene Therapy and also the senior author of the study, said that this method of replacing the defective gene was different from the conventional gene therapy.

There are further research studies that need to be carried before this procedure reaches the stage of a clinical trial. Studies are yet to prove the safety of CRISPR and more information is required to prove that there are no adverse effects. The current study, though, provides considerable support that CRISPR-could be used for genetic disorders associated with vision loss.

The study is of particular importance as there is a high prevalence of retinitis pigmentosa in South India which is evident from a study titled “Prevalence of retinitis pigmentosa in South Indian population aged above 40 years.” Dr. Sen P and colleagues carried out an ophthalmic examination of 9576 people, both from rural as well as urban Tamilnadu.
It was found that

• 1 in 930 in the urban population had retinitis pigmentosa
• 1 in 372 in the rural population had the condition
• 1 in 4000prevalence in the Western population

The alarmingly high level of retinitis pigmentosa highlights the need to find therapeutic measures which will help in preventing vision loss and maintaining the patient’s quality of life.

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References:

1. NRL neural retina leucine zipper - (https://www.ncbi.nlm.nih.gov/gene/4901)
2. Genetics of Blindness: Inherited Retinal Diseases - (http://massgenomics.org/2013/07/genetics-of-blindness.html)
3. Prevalence of retinitis pigmentosa in South Indian population aged above 40 years - (https://www.ncbi.nlm.nih.gov/pubmed/18780262)

Source-Medindia