The powerful gene editing technology CRISPR is a small step closer to treating a human disease.
In a recent work by Science researchers working for Eric Olson, Professor and Chair of Molecular Biology at the University of Washington UT Southwestern Medical Center, reported that he and his team successfully used the CRISPR to correct genetic defect for Duchenne muscular dystrophy in four beagles bred with the disease-causing gene. It is the first application of CRISPR for the treatment of muscular dystrophy in a large animal. (Previous studies had tested the technology on rodents.) To varying degrees, gene therapy stopped the muscle breakdown associated with the disease.
Duchenne is caused by mutations in the dystrophin gene, which encodes a protein essential for normal muscle function. People born with the disease often end up in wheelchairs as their muscles get weaker, and in the later stages, many rely on ventilators to breathe when their diaphragmatic muscles stop working. Finally, they develop heart and respiratory failure.
Olson and his team "fixed" the mutant dystrophin gene in four dogs by splicing a disruptive portion of the gene with CRISPR. The gene editing technology that was discovered in 201
Olson has tried two different methods of injecting CRISPR molecular scissors. With two dogs, he directly injected the CRISPR technology into the muscle, while in two other dogs he injected the same CRISPR technology into the bloodstream, allowing him to access more parts of the body and exert a broader effect on different muscle types from the limbs could go to the heart and to the diaphragm. Because Duchenne prefers to affect the muscles of the heart and respiratory system, he has loaded the CRISPR cut complex onto a molecular vehicle, a cold virus that has been modified to detect and splice DNA, especially in these muscle cells.
"Obviously, I was effusive of the results, "says Olson. "It was breathtaking."
In the dogs who received the systemic injections, he found that muscle cells in different parts of their body, including the heart and diaphragm, produced healthy dystrophin protein from 3% to 3%, 90% of normal levels for eight weeks after the injection. Olson says that experts in muscular dystrophy believe that if the dystrophin level in affected people were raised to 15% of normal, this would mean a dramatic difference in their lives and functioning. The dogs that received the CRISPR injections directly into their muscle also showed higher levels of dystrophin production, but especially in those muscles. Because Duchenne affects deep-seated organs such as the heart and respiratory system, Olson believes it is better to continue spreading CRISPR without repeated and multiple injections.
The idea is that CRISPR essentially clears the mutation in muscle cells, and bring the affected dogs back to almost normal condition. So far, the animals continue to make higher levels of dystrophin after eight weeks.
The study is the latest in an encouraging series of results in the use of CRISPR for the treatment of human disease. Researchers have also successfully used gene-editing technology to eliminate HIV from both infected human cells in the laboratory and in live mice and rats, and are nearing the beginning of studies on blood disorders such as beta-thalassemia and sickle cell anemia. Scientists have even corrected a genetic heart defect in laboratory embryos that could not be further developed or implanted for ethical reasons. While serious questions remain regarding the safety of CRISPR-based therapies – some studies revealed more than expected side effects of over-diligent DNA cutting – both academic and commercial researchers are pursuing CRISPR as a new tool in the treatment of genetic diseases such as Duchenne
Olson is encouraged by the results, even if they come from just a few dogs. They give some assurance that Duchenne, which is often diagnosed before people begin to notice symptoms of muscle weakness, can be stopped in its orbits before major skeletal, cardiac, and respiratory muscles become damaged beyond saving. A CRISPR-based therapy, he says, could be most effective in treating young people who have recently been diagnosed with the disease to prevent them from ever experiencing the symptoms of Duchenne.
Even people with advanced disease could benefit from this, Olson thinks as long as there is still some muscle to maintain a certain amount of function, be it moving the legs and arms or functioning the heart. "I am firmly convinced that at any stage we can intervene with this therapy, we can stop or slow the progression of the disease," he says.
First, however, more studies must be done on larger animals such as dogs. Olson is planning a longer-term study to see how long the CRISPR cells remain in the dogs and how safe the therapy is.
The hope is that if these animal studies and human studies prove this technique to be safe and effective, CRISPR could lead to a cure for Duchenne, says Olson. "We're going for a cure, not a treatment," he says. "All other therapies for Duchenne muscular dystrophy have treated the symptoms and consequences of the disease, which is the cause of the genetic mutation."