A new treatment for Duchenne muscular dystrophy (DMD) using the CRISPR-Cas9-based method is being developed by scientists at the University of Texas Southwestern. The researchers employed a novel gene therapy type to successfully treat animals with DMD. They believe that the procedure used could possibly lead to a DMD treatment in humans.
What is Duchenne Muscular Dystrophy?
Duchenne muscular dystrophy (DMD) is a type of genetic disorder that causes progressive degeneration and weakness of the muscles due to the mutation of dystrophin -- a protein keeping the muscle cells unimpaired.
Symptoms of DMD appear in early childhood, normally between 2-3 years. This genetic disorder predominantly affects boys, but girls can also get affected in rare instances. In North America and Europe, DMD’s rampancy is about 6 per 100,000 individuals.
Available Clinical Therapies for Duchenne Muscular Dystrophy Treatment
Currently, there are two available clinical therapies for treating DMD: morpholino antisense oligomer injection and steroid supplementation.
In morpholino antisense oligomer injections, the mutant DMD exons are being skipped. However, only less than 1% of dystrophin can be restored using this method. On the other hand, with the supplementation of corticosteroid therapy, the DMD pathological phenotypes are being alleviated but the dystrophin levels cannot be restored.
There are also a few clinical trials presently assessing the therapeutic benefits of shortened dystrophin versions carried by adeno-associated virus (AAV). However, these kinds of gene replacement treatments do not have the ability to repair endogenous dystrophin expression. Gene replacement treatment also depends on the exogenous promoter expression pattern within the AAV and the continuance of AAV expression. With this said, generating a method to efficiently and permanently correct the alteration of the endogenous DMD gene may bring forth a treatment for this fatal genetic disorder.
Application of CRISPR/Cas9 to Correct DMD Gene Mutations
The emergence of CRISPR/Cas9 gene-editing method pointed to the likelihood of correcting DMD gene mutations and possibly refining permanently the pathological concerns of the genetic disorder.
Dr. Eric Olson, Director of University of Texas Southwestern's Hamon Center for Regenerative Science and Medicine, along with his team utilized CRISPR/Cas9 to stop DMD progression in animals and human cells. The gene-editing tool has the ability to correct gene mutations that cause diseases and has shown positive results in DMD mice models.
Dr. Olson and his colleagues also tested this method in one of their small, short-term studies using DMD dog models exhibiting a lot of human disease features. CRISPR/Cas9’s intramuscular delivery yields significantly increased levels of dystrophin both in the heart and skeletal muscles. Additionally, muscle histology has significantly improved. The researchers discovered an 80% improvement in heart function in 8 weeks.
At present, Dr. Olson and his team also use CRISPR/Cas9 to alter the dystrophin gene in cells from patients with Duchenne muscular dystrophy. The team’s next objective is to conduct a clinical trial that will develop a treatment or cure for DMD patients.
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Possible Challenges for Duchenne Muscular Dystrophy Treatment Through CRISPR
While the studies about using CRISPR/Cas9 show notable results for correcting DMD mutations in animals, there are various possible challenges that need to be addressed. Below are some of the challenges for DMD treatment through gene editing:
The duration of sustaining gene editing benefits in DMD
In mdx mice models, dystrophin expression and sustained genome editing lasted for 12 to 18 months after AAV expression. Additionally, the dystrophin expression in skeletal muscles showed maintenance for more than a year after a single-cut gene editing.
Skeletal muscles are long-lived tissues, however, the possibility that myoedited nuclei might disappear in a period of time remains to be known.
One common gene-editing concern is off-target mutagenesis. In Dr. Olson’s study, the researchers observed only less than 1% off-targeted gene editing. However, this needs to be assessed more in the future.
Clinical-grade AAV manufacturing in high quantities for a large number of patients
For all gene therapy types, the manufacturing in high quantities of clinical-grade AAV for large-scale treatment of patients remains a challenge. A range of 1 × 1014 vg/kg is the required dosing regimen for both animal models and patients to have an effective muscle tissue expression. To make the doses lower, there is a need for novel techniques through AAV production optimization, expression, infectivity, and tissue-specific tropism.
Ideal age to treat DMD patients with myoediting
When it comes to DMD, it is better to intervene earlier. A young patient will have more preserved muscle, unlike an older patient where their muscle has been replaced already with fat and fibrous tissue. In the case of Dr. Olson’s study, the animal models were able to preserve muscles during myoediting intervention but cannot build new muscles. Additionally, if a patient is younger, the AAV dose needed would be lower. With this, the risk of having liver toxicity is minimized.
The Future of Duchenne Muscular Dystrophy Treatment Through CRISPR
Further research is needed in using gene editing to treat or cure DMD. The researchers still need to measure the stability of dystrophin levels and to make sure there are no harmful side effects with the gene edits. The University of Texas Southwestern researchers are optimistic to carry out clinical trials on DMD in the coming years.
Additionally, there are various gene-editing techniques being developed such as base editing. Some of the companies working on CRISPR/Cas9 method for DMD treatment include CRISPR Therapeutics, Editas Medicine, Sarepta Therapeutics, and Vertex Pharmaceuticals. All studies by these companies are at their preclinical stage.
In the long run, studies involving DMD treatment through CRISPR should be applicable to various genetic disorders and not just for dystrophinopathies.
While further studies are still needed for the development of Duchenne muscular dystrophy cure, the promising results using CRISPR method give everyone a reason to be optimistic.