Gene editing improves muscle in mice with muscular dystrophy

Advances in gene editing show the potential of the technique to correct mutations responsible for certain genetic disorders. Wildpixel/ iStock/Thinkstock

Muscular dystrophy is a group of more than 30 genetic conditions that cause progressive weakness and degeneration of the muscles that control body movement and heart contraction. Duchenne muscular dystrophy (DMD) is the most common type in children and affects boys beginning at about 2-4 years. Progressive weakness and wasting of muscles leads to a loss of the ability to walk as teenagers. People with DMD are now surviving into their 30s and beyond due to advances in the management of breathing and heart complications. However, no specific treatment can stop or reverse the progression of any form of muscular dystrophy.

DMD arises from mutations in the gene for dystrophin, a protein crucial for muscle cell structure. Since DMD results from errors in a single gene, scientists have tried using gene therapy to treat the disease. Among the many challenges is that the dystrophin gene is one of the largest known. Delivering an entire working version to muscles throughout the body isn’t possible. In one experimental approach, scientists developed a miniature version of the dystrophin gene and achieved successful gene therapy in dystrophic mice and dogs.

Three NIH-funded teams—led by Dr. Charles A. Gersbach at Duke, Dr. Amy J. Wagers at Harvard, and Dr. Eric N. Olson at the University of Texas Southwestern Medical Center—have been pursuing a different approach. An estimated 4 of every 5 DMD patients have mutations in the dystrophin gene that disrupt expression of the protein but are in regions that aren’t essential for function. If these regions were excised, they could be bypassed, removing the need to precisely correct each disease-causing mutation.

The CRISPR/Cas9 gene-editing system uses short “guide RNAs” (gRNAs) to identify specific target sequences to cleave. When 2 nearby sites are cut, the cell’s machinery can repair the breaks by joining the broken DNA ends. The teams used the CRISPR/Cas9 system to remove a nonessential region of the dystrophin gene called exon 23 in a mouse model of DMD. They published their results online in separate papers in Science on December 31, 2015.

The researchers delivered the necessary Cas9 gene and gRNAs into the animals’ bodies using adeno-associated virus vectors. Skeletal and cardiac muscle showed partial recovery of functional dystrophin protein, and muscle biochemistry had measurable improvements. The technique also repaired the dystrophin gene in muscle stem cells, which are needed for new muscle to form.

Both the structure and function of muscle, including contractile function and forelimb grip strength, improved in the treated mice. Different timing and injection methods restored dystrophin protein expression in cardiac and skeletal muscle to varying degrees from 3 to 12 weeks after injection. Together, these studies support further research into the potential for CRISPR/Cas9 genome editing to treat DMD and possibly other genetic diseases.

“There is still a significant amount of work to do to translate this to a human therapy and demonstrate safety,” Gersbach says. “From here, we'll be optimizing the delivery system, evaluating the approach in more severe models of DMD, and assessing efficiency and safety in larger animals with the eventual goal of getting into clinical trials.”

—by Harrison Wein, Ph.D.