Duchenne muscular dystrophy is a life-threatening muscle-wasting illness. Occurring mostly in males, it is the most common type of muscular dystrophy, striking about one in 3,500 boys and causing their muscles to start breaking down in early childhood. It often confines patients to wheelchairs by the time they are teenagers and usually leads to an early death from heart or respiratory failure. There is no cure—but a genetic fix tested in dogs may offer new hope.
The disease is caused by gene mutations that make patients' muscle cells unable to produce enough dystrophin, a protein that helps muscles absorb shocks and protects them against degradation over time. In a recent study, scientists used a gene-editing technique called CRISPR/Cas9 to pump up muscle protein levels in four dogs suffering from Duchenne. The advance may hasten clinical trials for similar treatments in humans.
The research team, led by the University of Texas Southwestern Medical Center, worked with young beagles bred to have Duchenne. The scientists edited the dogs' muscle cells to remove a key barrier to higher protein production—a short, problematic segment of protein-coding DNA that occurs in both canines and humans with the illness. Within about two months the dogs were producing greater amounts of dystrophin; levels in skeletal muscle ranged up to 90 percent of normal, depending on the muscle type and dosage used. (Some dogs produced significantly less.) In cardiac muscle, a crucial target for treatment, levels climbed to as high as 92 percent of normal. The U.T. Southwestern researchers, who published their findings in August in Science, report that they did not detect any unintended changes to other regions of the genome—a common concern with gene-editing technology—and there was no evidence the technique made the dogs ill.
To deliver this technology to the dogs' muscles, senior author Eric Olson, a molecular biologist at U.T. Southwestern, and his colleagues engineered viruses to act as delivery trucks, stripping out some of the viruses' own DNA to make room for gene-editing machinery. A number of the viruses were then loaded up with the Cas9 enzyme, which acts like molecular “scissors”; this was used to cut out the DNA sequence that hinders dystrophin production in muscle cells. Other viruses carried a guide molecule to help the Cas9 identify where it should make the needed cuts.
Olson's team had previously demonstrated that CRISPR could be used to treat Duchenne in rodents and in human cells in the laboratory. The new work marks the first success in a large mammal. For this study, the team focused only on measuring protein-level restoration. It has not explored how the intervention might have changed the dogs' behavior or day-to-day lives.
Exactly how long one injection with CRISPR gene-editing machinery might last in human Duchenne patients remains unknown. Olson and his colleagues hope the intervention might be durable enough with a single dose, but they need further results to get a clearer idea. If patients require more treatments over time, they might not be able to use the same viral vehicle, says Elizabeth McNally, a geneticist and cardiologist who directs the Center for Genetic Medicine at Northwestern University. “The body may develop neutralizing antibodies, so there are a lot of questions about the viral delivery piece of that,” says McNally, who is also on the scientific advisory board of Olson's spin-off company trying to commercialize this Duchenne technology but was not involved with this study.
The sole Duchenne treatment currently approved for the U.S. market—an injectable drug made by Sarepta Therapeutics that requires continuous delivery—increases dystrophin levels by less than 1 percent. This approach, which has yet to show a clinical benefit, differs from Olson's in that it works on RNA (the molecule into which DNA is eventually transcribed) but leaves the abnormal DNA sequence unchanged.
Duchenne researcher Amy Wagers, a professor of stem cell biology and regenerative medicine at Harvard University, who is not involved with developing either therapy, says these two approaches could potentially be used in tandem to help boost dystrophin. “I think it's really exciting to see this new work in mice now translated to a large animal model,” she says, adding that “the authors very appropriately note that this is a preliminary study with a small number of animals and a short follow-up time.”
Both Sarepta's approved technology and Olson's experimental one target a subset of the Duchenne population: patients with a particular dystrophin gene mutation that affects about 13 percent of those with the disease. There are at least 1,000 such cases in the U.S. “We need to do long-term safety and efficacy studies in dogs,” Olson says. “It will be a few years before we're ready to test this in humans if it continues to hold up.”