Review Hopeful that Muscular Dystrophies Will Be Treated Using Gene Editing in Not-Too-Distant Future

We are approaching an age in which gene-editing techniques are entering the therapeutic portfolio of many diseases. Clinical trials, using a variety of approaches, report successful results. For muscular dystrophies, however, clinical trials are still a future hope.

Gene editing is made possible by the targeted introduction of breaks in the double-stranded DNA molecule. Today, four methods are being used by researchers to introduce breaks in the DNA at specific sites: Zink finger nucleases, transcription activator-like effector nucleases (TALENs), meganucleases, and, most recently, the CRISPR/Cas system. Once breaks are introduced, genes, or gene fragments, can be inserted, deleted, inactivated, or corrected.

One of the main challenges in gene editing is the delivery of the genome-editing tools. Many approaches have been tried with varying degrees of success. Viral vectors are an optimal option for many applications, and are associated with low toxicity. When researchers manipulate cells inside the body, additional challenges arise and particular strategies need to be used, compared to editing done in isolated cells in the laboratory.

Among muscular dystrophies, genome editing has advanced most in Duchenne muscular dystrophy (DMD). Approaches being developed for DMD are also informing research on other types of dystrophies.

DMD is particularly challenging for genome editing because of the exceptionally large size of dystrophin’s coding region. Dystrophin, the causative gene of DMD, is one of the largest human genes known, making it difficult to package into a small viral delivery system.

Researchers have, therefore, experimented with the delivery of smaller gene segments that offer a partial improvement in functionality. Studies aiming to correct mutations by inserting mutated exons or restoring the function of the gene by introducing a so-called frame-shift — allowing for the correct ‘reading’ of the gene sequence — are limited by their lack of generalizability. Since the variability of mutations in the dystrophin gene is so rich, each individual correction addresses only a small proportion of patients.

Current studies are focusing on deleting a larger part of the gene, encompassing several exons. As recently reported by Muscular Dystrophy News, one such strategy has resulted in a successful deletion of exons 45-55, restoring the expression of dystrophin. If this method reaches the clinic, it would target 62 percent of all DMD patients.

Optimization of such a system for clinical use employs the CRISPR/Cas9 system, using adeno-associated virus (AVV) delivery that specifically targets skeletal and cardiac muscle. In a mouse model of DMD, intramuscular or intravenous injections restored dystrophin expression and improved muscle pathology and strength. One study successfully used this method to edit the gene in muscle progenitor cells in mice, opening the possibility of targeting cells that can act as a renewable source of muscle cells with a functional dystrophin gene.

Researchers hope that, in the future, this knowledge might be used to develop methods where patients’ own cells can act as a source of muscle stem cells — possibly by backward reprogramming of adult cells to stem cells. Isolation of patient-derived cells would allow for production of patient-specific stem cells, in which genes can be manipulated before the cells are expanded and engrafted back into the patient.

Before such therapies can be realized, several challenges need to be resolved. These particularly regard patient safety, such as a better understanding of potential off-target effects. Researchers are also still in the dark about how the immune system will respond to genetically altered cells or the presence of gene-editing tools.

The rapid development of new technological approaches, however, are expected to produce increasingly specific and sensitive tools. The authors conclude on an optimistic note for the not-too-distant future introduction of genome-editing techniques as a clinical therapeutic option for muscular dystrophies.