CRISPR-Cas9 System to Treat Age Related Macular Degeneration

Scientists have developed a gene surgery in the tissue layer supporting the retina which promises to be a potential form of therapy for patients with age related macular degeneration. Among people over the age of 65 years, one in 10 people show signs of this condition, with its prevalence increasing with age. Age related macular degeneration (AMD) occurs commonly among Caucasians, resulting in blind spots and vision distortion. The study published in the journal Genome Research is based on laboratory studies as well as animal studies.

Retinopathy

Retinopathy is a condition in which there is blood vessel damage in the retina. The retina offers a view of the health of the circulatory system; it provides a glimpse of the blood vessels of the body and can thus be used to detect early signs of diabetes or high blood pressure complications. It is the inner layer of the eye where the light received is changed into visual messages that are received by the brain.

Retinopathy can be

• 'Retinopathy of prematurity' – This is a potentially blinding disease that occurs due to the abnormal retinal blood vessel development among premature infants. There is a high risk for premature babies to have abnormal blood vessel development. The condition may be resolved without damage being caused to the retina but in severe cases, it could result in the retina being pulled away and lead to permanent blindness.
• ‘Hypertensive retinopathy’- This type of retinopathy is caused due to high blood pressure. Normally, this does not impair vision; however, blockage of retinal veins and arteries could result in the loss of vision.
• ‘Diabetic retinopathy’ – This type of retinopathy occurs due to the deterioration of the blood vessels in the retina and can affect both eyes. People who have diabetes for longer than 20 years are normally associated with retinal damage; diabetic retinopathy is the major cause of blindness in North America.
• ‘Age-related macular degeneration’ - This type of retinopathy affects older adults and is caused by the deterioration of the central portion of the retina called the macula.

In the different types of retinopathy, there are abnormal levels of the vascular endothelial growth factor (VEGF) which are secreted. In age related macular degeneration, VEGF aids in the formation of new blood vessels which causes the leakage of blood as well as fluid into the eye, resulting in damage to the macula.

Injections with Anti-VEGF Drugs

The most widely used treatment for age related macular degeneration is injections with anti-VEGF drugs. The main drawback of this method of treatment is that since vascular endothelial growth factors (VEGF) are over expressed by the retinal pigment epithelial cells, at least 7 injections may be required every year. Dr. Kim Jin-Soo who is the Director of the Center of Engineering at the Institute of Basic Science (IBS) said that these anti-VEGF injections do not address the main problem of the disease but only its effects.

To avoid such invasive procedures the research team from The Institute of Basic Science decided to use the CRISPR-Cas9 system to provide a third generation gene therapy solution. Dr. Kim further detailed that editing the VEGF gene would provide a long term cure.

CRISPR-Cas9 for Gene Editing

CRISPR-Cas9 gene editing tool can be used to precisely cut and correct desired strands of DNA in the genome. This gene editing tool cuts inside the VEGF gene and earlier studies conducted by the research team showed that

• Cas9 ribonucleoprotein (RNP) which is a pre-assembled variant of CRISPR-Cas9, could be used to deliver stem cells to change genes that are being targeted.
• The assembled variant works fast and then degrades quickly before an immune response is mounted by the host body.

Though there are many advantages in using this procedure, the hurdles involved in the delivery of these pre-assembled complexes have limited its use in therapeutics.

Injection of the CRISPR-Cas9 into the Eyes

The research team injected the CRISPR-Cas9 into the eyes of mice with wet age related macular degeneration which resulted in the modification of the VEGF gene. The findings of the study were

• The pre-assembled CRISPR-Cas9 complex’s delivery is more efficient that of the gene editing tool complex in a plasmid form.
• In a period of 72 hours, the complex dissolved.
• When the entire genome was assessed by the research team, the CRISPR-Cas9 complex was found to have modified only the VEGF gene but not any other gene.
• Choroidal neovascularization (CNV) (the creation of new blood vessels) was monitored to understand the progression of the disease. There was reduction in CNV by 58%.
• Cone dysfunction is a possible side effect of age related macular degeneration which usually occurs within 3 days in these mice; in this treatment method it did not occur even a week later.

The CRISPR–Cas9 system has always been used in the treatment of hereditary diseases. However, the current study showed that even degenerative diseases that were non-hereditary could be corrected without the risk of affecting other genes.

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The first human trial of the CRISPR-Cas9 system was conducted on patients with lung cancer last year with a potential for use among people with sickle cell anemia and other forms of hereditary diseases. The current study offers hope for treatment of diseases that occur due to increased age, which could increase the quality of life of older people.

References:

1. Retinopathy of Prematurity - (https://www.aapos.org/terms/conditions/94)
2. Diabetic Retinopathy - (http://care.diabetesjournals.org/content/26/suppl_1/s99)

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Source-Medindia