CRISPR Gene Editing: Apollo’s arrow for curing blindness

Magazine
Fig:- CRISPR AAE image
Author: Rupesh Poudel

Since the dawn of human evolution, people have always searched for cause and cure to problems related to health. There was a time where many would die even from simple diseases like diarrhoea and measles. With the industrial revolution and extensive research in science and medicine, technological development accelerated. We made our progress from just observation of symptoms to tracking cells and gene faults for determining the cause of disease.

CRISPR ( Clustered Regularly Interspaced Short Palindromic Repeat) is a DNA sequence found in Prokaryotes like Bacteria which has been infected by a Bacteriophage ( Bacteria affecting virus). When a Bacteria is infected by a virus, it captures Viral DNA and incorporates in its own sequence as a spacer. This captured DNA is surrounded by a clustered repeated sequence of Bacterial genome. So, whenever similar virus attacks again, the spacer will replicate complementary DNA sequences carried by an enzyme called Cas9. Cas9 compares the sequence with viral DNA, if matches; cut and breaks the genome sequence at specific points making viral DNA ineffective. Genome similar to that hybrid clustered sequence in bacteria affected by the virus is called CRISPR fragments.

Such natural phenomenon gives us insight on how we can artificially replicate the process to cut and remove the faulty gene in our DNA sequence and prevent genetic disease. So what we are doing is cut and paste process, cut faulty gene and paste with a good one.

First CRISPR trial on eye:

Clinical researchers at Casey Eye Institute, Oregon Health & Science University did first trial in a patient with Leber’s congenital amaurosis, on March 2020. There are two phases to the trial. With success in 1st phase they will proceed to another one. There are various subtypes of this disease; one of them being LCA10 which is the most common type. Mutation in a gene called CEP290 in LCA10 is what causes the disease.

How would it work?

The goal is to artificially synthesize a sequence of a guide RNA which will be carried by enzyme Cas9 and match with the mutated genome, cut it and replace with a good one. It is done by injecting crisper fragments behind the retina. CRISPR fragments will then be guided toward faulty gene with the help of guide RNA; carried by Cas9 enzyme. This process continues and retinal tissue will begin to recover on its own.

Future of CRISPR in Ophthalmic genome surgery:

Scientists are working on many trial and error studies in animals models for effectively using CRISPR gene technology.

Animal trials of CRISPR:

Wu et al used CRISPR-Cas gene editing in rodless mice. Mice exhibited premature death of rods resembling Retinitis Pigmentosa. It was caused due to mutation in the Pde6b gene, which maintained an internal cGMP level. Using CRISPR technology he was able to restore the phenotype as mice were able to regenerate rods.

CRISPR Gene editing for Preventing glaucoma

The scientist from the University of Lowa did similar research in mice. Certain type of glaucoma has been found to be caused by an alteration to a protein called myocilin. Myocilin is found in Trabecular Meshwork. So, gene editing to prevent the formation of mutant myocilin may prevent open-angle glaucoma.

Gene editing for Treating Retinal Degeneration

Retinal tissue damage and regeneration have been studied by many scientists using Zebra fish as a model. In zebrafish damaged retina is replaced by mullers cell which de-differentiates into a like stem cell to begin a new cell cycle and replace damaged tissue. But in the damaged mammalian retina, mullers cell undergo reactive gliosis and protect the retina by replacing with scar tissue which renders retinal cells functionless. By inducing damage to Zebrafish Retina and following the regeneration process, scientists tend to create similar phenomena in the mammalian retina by editing gene with CRISP technology. TNFα and HB-EGF were the factors that can stimulate Müller glia proliferation in Zebrafish. Hence CRISPR technology can be used if we identify factors and proper sequence for Retinal cells restoration in Zebrafish. CRISPR can mutate human retina to follow a similar sequence.

Conclusion:

Hence we can safely say that we are not very far from getting fruits of successful Genetic engineering. If the current human trial succeeds, we may soon hear about good news where degenerative disease affecting vision may be cured by just simple genetic manipulation.

Author: 
Rupesh Poudel
M.optometry,
Tilganga Institute of Ophthalmology

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