CRISPR gene editing has stopped the progression of Duchenne muscular dystrophy (DMD) in dogs, according to a study by UT Southwestern that provides a strong indication that a lifesaving treatment may be in the pipeline.
unprecedented improvement in the muscle fibers of dogs with DMD – the most common fatal genetic disease in children, caused by a mutation that inhibits the production of dystrophin, a protein critical for muscle function.
Researchers used a single-cut gene-editing technique to restore dystrophin in muscle and heart tissue by up to 92 percent of normal levels. Scientists have estimated a 15 percent threshold is needed to significantly help patients.
“Children with DMD often die either because their heart loses the strength to pump, or their diaphragm becomes too weak to breathe,” said Dr. Eric Olson, Director of UT Southwestern’s Hamon Center for Regenerative Science and Medicine. “This encouraging level of dystrophin expression would hopefully prevent that from happening.”
DMD, which affects one in 5,000 boys, leads to muscle and heart failure, and premature death by the early 30s. Patients are forced into wheelchairs as their muscles degenerate and eventually onto respirators as their diaphragms weaken. No effective treatment exists, though scientists have known for decades that a defect in the dystrophin gene causes the condition.
The Science study establishes the proof-of-concept for single-cut gene editing in dystrophic muscle and represents a major step toward a clinical trial. Already Dr. Olson’s team has corrected DMD mutations in mice and human cells by making single cuts at strategic points of the mutated DNA.
The latest research applied the same technique in four dogs that shared the type of mutation most commonly seen in DMD patients. Scientists used a harmless virus called adeno-associated virus (AAV) to deliver CRISPR gene-editing components to exon 51, one of the 79 exons that comprise the dystrophin gene.
CRISPR edited the exon, and within several weeks the missing protein was restored in muscle tissue throughout the body, including 92% correction in the heart and 58% in the diaphragm, the main muscle needed for breathing.
“Our strategy is different from other therapeutic approaches for DMD because it edits the mutation that causes the disease and restores normal expression of the repaired dystrophin,” said Dr. Leonela Amoasii, lead author of the study and Assistant Instructor of Molecular Biology in Dr. Olson’s lab. “But we have more to do before we can use this clinically.”
Mutations in the gene encoding dystrophin, a protein that maintains muscle integrity and function, cause Duchenne muscular dystrophy (DMD). The deltaE50-MD dog model of DMD harbors a mutation corresponding to a mutational “hot spot” in the human DMD gene. We used adeno-associated viruses to deliver CRISPR gene editing components to four dogs and examined dystrophin protein expression 6 weeks after intramuscular delivery or 8 weeks after systemic delivery. After systemic delivery in skeletal muscle, dystrophin was restored to levels ranging from 3 to 90% of normal, depending on muscle type. In cardiac muscle, dystrophin levels in the dog receiving the highest dose reached 92% of normal. The treated dogs also showed improved muscle histology. These large animal data support the concept that, with further development, gene editing approaches may prove clinically useful for the treatment of DMD.