Scientists using a CRISPR–Cas9 gene-editing technique have managed to pump up muscle protein levels in four dogs suffering from the most common form of muscular dystrophy. The advance may hasten clinical trials for similar treatments in humans battling this fatal muscle-wasting illness.

If the approach proves effective in humans, it could fundamentally change the disease trajectory for people with Duchenne muscular dystrophy, a rare genetic disorder that mostly afflicts boys. Genetic mutations cause Duchenne patients’ muscle cells to produce little or no dystrophin, a protein that helps muscles absorb shocks and protects them against degradation over time. The devastating disease, which occurs in about one in 3,500 boys, causes patients’ muscles to progressively break down starting in early childhood. It often confines them to wheelchairs by the time they are teenagers, and usually leads to an early death from heart failure or an inability to breathe. There is no cure.

A research team led by U.T. Southwestern Medical Center has edited muscle cells in young dogs with Duchenne to remove a key barrier to higher protein production—a short, problematic segment of protein-coding DNA that occurs in both canines and human patients. Within about two months of this intervention the dogs were producing greater amounts of dystrophin; levels in muscles connected to the skeleton ranged from 3 to 90 percent of normal, depending on muscle type and dosage used. And in cardiac muscle, a crucial target for treatment, levels climbed to as high as 92 percent of normal. In humans with Duchenne, dystrophin can become almost nonexistent—and the only currently available therapy was approved after researchers showed it could restore dystrophin to less than 1 percent of the normal level. The researchers, who published their findings Thursday in Science, said 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 get this gene-editing technology into 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 cargo. Some of the viruses carried Cas9—molecular “scissors” for cutting out the DNA sequence that hinders dystrophin production in muscle cells. Others carried a guide molecule to help the Cas9 identify where it should make those needed cuts.

Olson’s team had already demonstrated CRISPR could be used to treat Duchenne in rodents and in human cells in the lab, but the new work marks the first success with a large mammal. For this study the team focused on measuring protein-level restoration itself. It has not explored how the intervention changed the dogs’ behavior and day-to-day life.

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 only one dose, but they will need further results to get a clearer idea. If more treatments are needed over time, patients 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 board of Olson’s spinoff company trying to commercialize this Duchenne technology, but was not involved with this study.

The only Duchenne treatment currently approved for the U.S. market—an injected drug that requires continuous delivery—was controversially approved for use in humans after it helped dystrophin levels moderately increase. This approach 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 and requires continual reintroduction to ensure healthy dystrophin production. Sarepta Therapeutics, the company behind the approved therapy, did not immediately respond to a request for comment on the new study. Duchenne researcher Amy Wagers, a professor of stem cell biology and regenerative medicine at Harvard University who is not involved with either therapy, says they could both 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 adds. However, she says, “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 specific type of dystrophin gene mutation that affects about 13 percent of those with the disease. In the U.S. that works out to roughly 1,300 to 1,900 people. “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 this continues to hold up.”