(Representative image) by Dr. Indumathi MariappanResearch Scientist, LV Prasad Eye Institute, Hyderabad

Retinal Blindness

Millions of people the world over suffer visual disability as a result of retinal dystrophy that involves the death of retinal cells that are important for the light sensing function of the eye. Enormous progress has been made in other blinding conditions involving the cornea, lens, among others. However, the retinal dystrophies and optic nerve atrophies do not have any proven therapy till date. The major forms of retinal dystrophies such as Age-related macular degeneration (AMD), retinitis pigmentosa (RP), Lebers congenital amaurosis (LCA), Stargardts disease etc. are either inherited disorders or developed with aging. In most cases, the retinal cells are present at birth, but undergo gradual death during the later stages of life. It is typically characterized by initial symptoms of low vision and night blindness during early childhood, which progresses to severe visual impairment and total blindness at different stages of adulthood. Inherited defects in many genes involved in retina-specific functions and vitamin A metabolism are linked to various forms of retinal dystrophies. These genetic defects affect the normal cellular functions of the retina, leading to gradual cell death and ultimately the patient becomes legally blind.

Recent Technologies and Novel Treatment Options

The current modalities for the treatment of such patients mainly include dietary supplements, visual aids and rehabilitation support. However, a radical approach is required either to preserve or to restore visual function in these patients. Some of them include the replacement of either the lost retinal cells or the defective genes within the surviving, but non-functional retinal cells. This has been the principle behind the massive efforts involved in the development of cell and gene-based therapies. They are currently at different stages of product development and clinical trial evaluation. In cell therapy, normal retinal cells are prepared from specialized stem cells and are injected into the eye to replace the lost cells and to restore retinal functions. Clinical safety trials using cell therapy are ongoing in many countries such as USA, Japan, UK and others (Weblinks 1-4). In gene therapy, the prime strategy is to introduce a normal copy of the affected gene into the surviving retinal cells of the patient, to restore normal cellular functions and improvements in vision. This is achieved by engineering safe viral vectors to carry a normal copy of the desired gene as their cargo. When injected into the eye, the viruses can infect the retinal cells once and deliver the normal gene to restore cellular functions (Weblinks 5-7). A step further is an advanced method of DNA microsurgery, wherein, the defective part of the retinal cell DNA is precisely edited to correct the genetic defect and to restore cellular functions. This could be achieved using the latest gene editing tools such as ZFNs, TALENs, CRISPR/Cas systems etc. These are naturally occurring molecular scissors, employed as host defense mechanism and immune memory to combat viral infections in different species of bacteria. These systems are now engineered to enable DNA and RNA editing in almost any living cells. Such tools are now combined with either cell therapy or gene therapy to develop novel drugs for the treatment of various inherited genetic diseases (Weblink 8).

Gene therapy products approved for clinical use:

LUXTURNATM (Weblink 5)

This is the first commercial gene therapy drug approved by the US-FDA and European Commission for the treatment of an early childhood retinal dystrophic condition called the Leber Congenital Amaurosis 2 (LCA2). This disease is caused due to genetic defects in the gene called RPE65. LUXTURNA (AAV2-hRPE65v2 or Voretigene neparovec-rzyl) is an engineered adeno-associated virus 2 (AAV2) vector carrying a normal copy of the human RPE65 gene. This product was developed and marketed by Spark Therapeutics, a US-based startup now owned by Roche, a Swiss pharma company.

This drug has been tested on 20 patients, aged 3 years or older, in a randomized, controlled, open label, phase 3 interventional clinical trial at two sites in the US from June 2015. All treated individuals showed significantly improved functional vision, with no product-related serious adverse events or deleterious immune responses. The treated patient will be followed for further 15 years until March 2030 to assess the long-term retinal gene expression and stable maintenance of functional vision. It is administered as a onetime injection behind the retina of an eye of patients genetically diagnosed to carry mutations in RPE65 gene and also have sufficient viable retinal cells. It is priced at $850,000 for two eyes in the US and UK, which translates to about 6.5 crores in Indian rupees.

Many such gene therapy vectors are currently under clinical trial evaluation for the delivery of other retinal gene such as REP1, PDE6B, RPGR, OAT (Ornithine aminotransferase), MERTK, sFLT1etc.

EDIT101 (Weblink 8)

This is the first gene editing based drug approved by US-FDA, for the treatment of another early childhood retinal dystrophic condition called LCA10, caused by defects in the CEP290 gene. Here, it is important to understand that a gene editing approach is different from a gene therapy. In gene therapy, a normal copy of entire gene is delivered to the retina to complement the defective gene. In CRISPR/Cas9 based gene editing, only the mutated region of the gene is edited/corrected in situ inside the target cells. This is an attractive approach for correcting a variety of gene mutations, especially those in large genes which exceed the cargo capacity of the commonly used AAV-based gene therapy vectors.

EDIT101 (AGN-151587) is an engineered adeno-associated virus 5 (AAV5) vector carrying a CRISPR/Cas9 based DNA editing machinery to locate and remove a specific mutation hotspot within the intron 26 of human CEP290 gene. When injected behind the retina, the virus will infect the surviving photoreceptor cells and deliver the CRISPRs to enable mutation editing. Successful DNA edits in photoreceptor cells would inactivate a spurious splice site created by the mutation and restore normal protein expression and retinal function.

Preclinical testing in mice and monkey eyes has proved significant edit efficiency of up to 28%, which was above the expected 10% threshold required for clinical efficacy in human trials. This drug was developed by the gene editing company, Editas Medicine, Inc. and is being tested in 18 participants in a Phase 1/2 clinical trial sponsored by Allergan, at four sites in the US from March 2019 and the outcomes are awaited.

Similar gene editing strategy is being explored at different centers for mutation correction in other retinal genes such as KCNJ13, RP1, USH2A, MYO7A, RDH12 etc.

Who can benefit?

Both gene therapy and gene editing approaches have opened up newer hopes for the treatment of various genetic condition affecting different cell types of the body. However, only a small subset of patients can benefit from such therapies at the moment. Such treatment considerations require a thorough genetic screening/genotyping to confirm the identity of the gene affected in a specific patient. Further, the patients should retain some viable cells in the retina for the treatment to be clinically effective.

Research efforts in India

Many labs in the country are developing gene therapies and gene editing based therapeutics for the treatment of various diseases affecting the blood, retina, liveretc. Researchers at the CMC, Vellore, CSIR-IGIB, Delhi, CSIR-CCMB, Hyderabad are developing gene therapeutics for the treatment of different forms of blood disorders. Narayana Nethralaya, Bangalore is engaged in developing AAV-based gene therapies for various retinal dystrophies. Our lab at the LV Prasad Eye Institute is collaborating with the research teams at IIT-Kanpur and CSIR-IGIB, Delhi to develop modified gene therapy vectors for retinal gene delivery and cell-based therapies using CRISPR edited stem cells and retinal cells respectively.

The way forward

As of May 2020, the RetNet database lists about 271 genes to be associated with different forms of retinal dystrophies. This requires a larger library of gene delivery vectors to be developed and made available at affordable costs for the treatment of a large number of patients. This mandates the need for developing indigenous and cost-effective therapeutics and ICMR has set up a dedicated task force on gene therapy research, to identify and support promising research ideas in this newly emerging area of biomedical research. A national guideline for gene therapy product development and clinical trials has been jointly formulated and released by the DBT and ICMR in 2019. It is hoped that the streamlined regulatory framework would fast track our basic and translational research efforts into developing novel and cost-effective treatment options in the near future.

Read more here:
Gene Therapy and Editing : Novel options for inherited retinal blindness - ETHealthworld.com