September 2020

IN FOCUS

Gene Therapy
Exploring corneal genetics and gene editing


by Ellen Stodola Editorial Co-Director

There have been a number of recent advances in the field of gene editing. Two experts discussed this progress, highlighting the development of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) systems.
Tara Moore, PhD, said that CRISPR/Cas9 is finally progressing into clinical trials, with preliminary results showing evidence of the technique being safe and feasible to treat diseases.1
She added that there is currently a clinical trial in recruitment stage, sponsored and organized by Allergan and Editas, that evaluates safety and efficacy of CRISPR treatment for Leber congenital amaurosis 10, an eye disorder affecting the retina. It is only one of few clinical trials that is being done in vivo, she said.
This treatment was the first one to edit human genes within the body, with the first patients being treated at Oregon Health & Science University.
Due to a number of unique features of the eye, Dr. Moore said that there are several factors that could help genetic treatments for genetic eye disease to be successful. For corneal disease, some of these factors include the exterior element—the location of the eye and cornea—the small size and area to be treated, the ease of viewing pathology and assessing disease regression without invasive technologies, the thinking that there is some immune privilege, and no need for IV treatment or exposure to other organs. More in vivo trials will follow, she added.
Using CRISPR/Cas9 simplifies the basic ways of altering genes and cells, said Albert Jun, MD, PhD. “The process used to be much more involved and to generate even a single method to change genes was much more cumbersome,” he said.
The CRISPR approach simplifies gene therapy design, he said, adding that “it’s theoretically easy to create a genetic treatment for a mutation that’s relatively uncommon.”
“There’s so much excitement because you dial in the treatment,” Dr. Jun said. “The practicalities will come into play when trying to deliver the material, as well as assessing the safety and toxicity of these treatments.”
Dr. Jun added that so far, results in animal testing have been promising.

Personalized allele-specific CRISPR gene editing to treat autosomal dominant disorders

Dr. Jun is working actively on CRISPR treatments for corneal dystrophies. He noted that the two dystrophies he’s “most excited” about applying it to are Fuchs dystrophy and TGFBI disorders.
Fuchs dystrophy is a common disorder and the leading cause of corneal transplants worldwide, Dr. Jun said. There are a lot of patients who have issues and have lost vision but haven’t gotten to the point of needing a transplant, he said, adding that in some parts of the world, there is also limited access to transplant tissue.
While a cornea transplant is a good option, the idea of having a non-surgical treatment would benefit patients, giving them an additional therapeutic option. “Now we wait for patients to develop the disease, even though we could do a genetic diagnosis of Fuchs, but since there’s no medical treatment, we wait until they need surgery.” Using a gene editing approach would be a permanent option, so that a patient might not need a transplant at all.
The other group of diseases that he’s excited about are the TGFBI disorders. “I think this is particularly exciting because there are no good treatments for these conditions,” Dr. Jun said. While a laser treatment or corneal transplant can be performed, the disease will come back, he said.
Dr. Moore said she’s particularly excited about this treatment option for a number of disorders, including TGFBI, toward which her team at Ulster University is working. Additionally, she noted LCA10, which shows promise of being the first in vivo CRISPR treatment. Dr. Moore also mentioned retinitis pigmentosa, inherited retinal dystrophies, and Huntington’s disease and hemophilia disorders as other disorders for which this treatment may be particularly applicable.

How does this treatment work?

According to Dr. Moore, CRISPR’s allele specificity can be exploited in therapeutic treatments where only the disease-causing allele is targeted and disrupted, and healthy allele remains and minimizes adverse effects. To achieve that, Cas9 nuclease together with a guide RNA is introduced into the cornea, she said. “By using this guide, the Cas9 searches for complementary genomic sequence where Cas9 binds and generates double-strand break at the specific location,” Dr. Moore said. “To repair this break, a DNA repair system called non-homologous end joining is activated, which can introduce disruptions at the site of the mutation by introducing indels (insertions and deletions).” The gene is then permanently disrupted, as the translation to mRNA contains premature termination that does not create the protein that contains the mutation. The other allele is not affected, and it produces the healthy wild-type protein, Dr. Moore added.
Dr. Jun detailed how he is currently working on treating tissues but not yet actual patients.
The challenge, he said, is getting the treatment into the cell, and that can be done in a couple of ways. One way is to coat the components with chemicals, and they work their way into cells. Depending on the cells, that approach could be good or not, as some cells don’t let those kinds of chemicals in very easily.
The other more efficient way is with a virus, Dr. Jun said, adding that there are different viruses for delivering CRISPR components. Viruses can get into some cells easily, but they also must be safe to use, he said.
“The way we’re doing it is packaging CRISPR into viruses and putting it onto tissue and checking how effective it is in modifying the genes,” Dr. Jun said. In addition to looking at what percentage of cells are changed, Dr. Jun said it’s important to examine “off-target effects.” He noted that this could occur if there is a slight sequence mismatch to the one that is being targeted. “It’s unlikely there would be that same match somewhere else in the genome,” he said, but if a similar sequence exists, the CRISPR system may still work on it in a more limited fashion, so it’s possible there could be some unintended effects.

Other promising treatments

Dr. Moore added that siRNAs are promising. “The work Avellino and Ulster University are doing with drug delivery company SiSaf is the first time I am seeing promise of delivering gene silencing and gene editing tools to an intact eye and seeing successful cargo delivery not only to the cornea but also to the retina—a real promise for the future of injection-free delivery of drugs for many different major ophthalmology diseases,” Dr. Moore said.

At a glance

• With options for CRISPR gene editing in the works, experts are excited about its potential application to Fuchs dystrophy, TGFBI disorders, retinitis pigmentosa, inherited retinal dystrophies, and Huntington’s disease and hemophilia disorders.
• In order to get this treatment onto the tissue or into a cell, chemicals or a virus may be used.
• CRISPR’s allele specificity can be exploited in therapeutic treatments where only the disease-causing allele is targeted and disrupted and healthy allele remains and minimizes adverse effects. 

About the sources

Albert Jun, MD, PhD

Professor of ophthalmology
Johns Hopkins Hospital
Baltimore, Maryland

Tara Moore, PhD
Professor of personalized medicine 
Ulster University 
Coleraine, U.K.

Reference

1. Stadtmauer EA, et al. CRISPR- engineered T cells in patients with refractory cancer. Science. 2020;367:eaba7365.

Relevant disclosures
Jun: Hunterian Medicine
Moore: Avellino and SiSaf

Contact
Jun: aljun@jhmi.edu
Moore: tara.moore@ulster.ac.uk

Exploring corneal genetics and gene editing Exploring corneal genetics and gene editing
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