Muscle defects corrected with CRISPR gene editing

New, body-wide gene repair technique successful in mice.

Duchenne muscular dystrophy is a disease that causes a progressive loss of muscle mass, eventually leading to a loss of the ability to walk and ultimately respiratory or heart failure. The disease is caused by a gene coding defect for the protein dystrophin. In a new Nature Communications study, Jeffrey Chamberlain and colleagues used the CRISPR/Cas9 repertoire to restore dystrophin to levels that improved muscle function in mice. The results point to CRISPR as a possible treatment for a wide range of muscular dystrophies and disorders of other organs, including liver and blood cells.

We spoke with him about the work.

ResearchGate: What is Duchenne muscular dystrophy?

Jeffrey Chamberlain: DMD is an inherited disorder caused by mutations in a gene that is critical for maintaining normal muscle cell structure. Patients display a progressive loss of muscle mass, with increasing weakness throughout life. The disorder leads to a loss of the ability to walk and ultimately patients die from respiratory or heart failure. There is currently no cure and only modest therapies that serve to slow loss of walking ability.

RG: What motivated this study?

Chamberlain: My lab has been exploring genetic therapies for DMD for more than 25 years. Our major focus has been on a therapy to replace the defective gene (called the dystrophin gene) in the muscle by delivering a synthetic replacement. While that approach is showing considerable promise, and is moving into human clinical trials, it is somewhat limited by the need to deliver small “micro-dystrophin” versions of the gene. The natural gene is the largest known gene, so it is too large to deliver. We became interested in gene editing as a potential alternative to gene replacement. With gene editing one can potentially generate larger proteins that might be more functional for therapy.

RG: Can you give us a brief insight into your results?

Chamberlain: We showed about 12 years ago, that gene delivery shuttles, or vectors, derived from AAV (adeno-associated virus) can be used to deliver new genes to muscles body-wide. This formed the basis for our gene replacement methods using micro-dystrophins. However, since gene editing methods have the potential to permanently modify a gene, we sought to determine if similar vectors could achieve a body-wide gene repair in muscle tissues. We found that delivery of the CRISPR/Cas9 components using AAV did indeed lead to gene editing in muscles body-wide, although to date the efficiency is significantly lower than our previous gene replacement approach.

We also tested four different ways to edit the dystrophin gene. All worked, but with varying efficiencies. This is important, as DMD is a “new mutation” syndrome, meaning every patient has a unique mutation. Thus, normal strategies for gene repair would only work for a subset of patients. Our studies show the methods can work body-wide and can be adapted for different types of mutations.

RG: Does this have applications for other disorders?

Chamberlain: Yes, many disorders. Certainly, it would be applicable to any of the muscular dystrophies, of which there are more than 50 types (DMD is the most common). Other labs are applying similar approaches to treat disorders of other organs, including liver and blood cells.

RG: What’s next for your research?

Chamberlain: We are continuing to advance the gene replacement methods into human trials, but with gene editing we are working to increase the efficiency and address the overall safety and specificity of the approach. My feeling is that several improvements need to be made before the editing methods can be tested in the clinic, but the field is moving rapidly.

RG: When do you hope to start clinical trials?

Chamberlain: That’s still unclear; it would be several years from now and will depend on the success of the AAV-gene replacement trials and technological enhancements to the efficiency of the editing methods.

RG: Could this offer a permanent fix?

Chamberlain: At present the AAV approach for gene editing does not work in muscle stem cells, an important target for a “permanent” fix to muscles. However, targeting stem cells is an active goal, and even without hitting the stem cells, the method could be highly useful for treatment options. This technology is still in early stages of development, but I feel it has important, long-term potential.

Image courtesy of Stefano.