January 12, 2016

Gene editing improves muscle in mice with muscular dystrophy

At a Glance

  • Three teams independently used the CRISPR/Cas9 gene-editing system to restore expression of the gene responsible for Duchenne muscular dystrophy in mouse models.
  • With further development, the approach might be used to correct mutations responsible for muscular dystrophy and other genetic disorders.
Illustration of scissors cutting DNA. 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.

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References: 

In vivo genome editing improves muscle function in a mouse model of Duchenne muscular dystrophy. Nelson CE, Hakim CH, Ousterout DG, Thakore PI, Moreb EA, Rivera RM, Madhavan S, Pan X, Ran FA, Yan WX, Asokan A, Zhang F, Duan D, Gersbach CA. Science. 2015 Dec 31. pii: aad5143. [Epub ahead of print]. PMID: 26721684.

In vivo gene editing in dystrophic mouse muscle and muscle stem cells. Tabebordbar M, Zhu K, Cheng JK, Chew WL, Widrick JJ, Yan WX, Maesner C, Wu EY, Xiao R, Ran FA, Cong L, Zhang F, Vandenberghe LH, Church GM, Wagers AJ. Science. 2015 Dec 31. pii: aad5177. [Epub ahead of print]. PMID: 26721686.

Postnatal genome editing partially restores dystrophin expression in a mouse model of muscular dystrophy. Long C, Amoasii L, Mireault AA, McAnally JR, Li H, Sanchez-Ortiz E, Bhattacharyya S, Shelton JM, Bassel-Duby R, Olson EN. Science. 2015 Dec 31. pii: aad5725. [Epub ahead of print]. PMID: 26721683.

Funding: NIH’s National Heart, Lung, and Blood Institute (NHLBI), National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), National Eye Institute (NEI), National Human Genome Research Institute (NHGRI), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institute of General Medical Sciences (NIGMS), National Institute of Mental Health (NIMH), National Institute of Neurological Disorders and Stroke (NINDS), NIH Director’s New Innovator Award, and NIH Director’s Pioneer Award; Howard Hughes Medical Institute; Muscular Dystrophy Association; Duke-Coulter Translational Partnership Grant; Hartwell Foundation; March of Dimes Foundation; National Science Foundation; Keck Foundation; Damon Runyon Foundation; Searle Scholars Foundation; Merkin Family Foundation; Vallee Foundation; Simons Foundation; Paul G. Allen Foundation; New York Stem Cell Foundation; Bob Metcalfe; Hope for Javier Foundation; American Heart Association; Paul and Daisy Soros Fellowship; Agency for Science, Technology, and Research, Singapore; Fondation Leducq; and Robert A. Welch Foundation.