Caption: A CRISPR/cas9 gene editing-based treatment restored production of dystrophin proteins (green) in the diaphragm muscles of dogs with Duchenne muscular dystrophy.
Credit: UT Southwestern
CRISPR and other gene editing tools hold great promise for curing a wide range of devastating conditions caused by misspellings in DNA. Among the many looking to gene editing with hope are kids with Duchenne muscular dystrophy (DMD), an uncommon and tragically fatal genetic disease in which their muscles—including skeletal muscles, the heart, and the main muscle used for breathing—gradually become too weak to function.
Such hopes were recently buoyed by a new study that showed infusion of the CRISPR/Cas9 gene editing system could halt disease progression in a dog model of DMD.
As seen in the micrographs above, NIH-funded researchers were able to use the CRISPR/Cas9 editing system to restore production of a critical protein, called dystrophin, by up to 92 percent in the muscle tissue of affected dogs. While more study is needed before clinical trials could begin in humans, this is very exciting news, especially when one considers that boosting dystrophin levels by as little as 15 percent may be enough to provide significant benefit for kids with DMD.
The CRISPR/Cas9 editing system relies on a guide RNA to direct a scissor-like enzyme (Cas9) to a precise spot in the genome, where it cuts out a recognizable bit of the DNA code. In laboratory studies, the system has shown potential in cutting out the misspellings that underlie many rare genetic conditions.
In the new study, published in the journal Science and led by Eric Olson, University of Texas Southwestern Medical Center, Dallas, DMD presented a special technical challenge in the development of the CRISPR/Cas9 editing system . The dystrophin gene is especially lengthy, and different people with DMD literally have any one of thousands of different DMD-causing mutations!
The mutations disrupt the normal function of dystrophin proteins, which act much like shock absorbers in healthy muscle. Without dystrophins to absorb their daily wear and tear, muscles throughout the body break down and waste away, including those in the heart and the diaphragm, where the muscle is needed for breathing.
Fortunately, there are mutational “hot spots” in the gene. Using Cas9 to cut the dystrophin gene in just the right place, the researchers thought they could skip over a mutated coding area, or exon, to produce an edited version of the gene encoding a slightly shorter, yet still functional, dystrophin protein. Last year, Olson’s team including Leonela Amoasii, lead author of the new study, showed that their single-cut gene editing approach worked in a mouse model of DMD .
But would CRISPR work in a larger mammal? To find out, the researchers teamed up with the Royal Veterinary College, London, to test the treatment in beagle and Cavalier King Charles Spaniel mixed breed dogs that develop the very same DMD symptoms seen in people. Their condition stems from a natural mutation found originally in sick spaniels that corresponds to the most common mutational “hot spot” in people.
In the new study, the researchers used a harmless virus to ferry the CRISPR/Cas9 gene editing system into the dogs’ muscle cells. First, they delivered the treatment through a direct intramuscular injection into the lower legs of two 1-month-old puppies. Six weeks later, the puppies’ treated leg muscles had begun producing dystrophins.
That was encouraging to see, and the next step was to infuse the virus containing Cas9 and the correct guide RNA intravenously into two more young dogs with DMD. Eight weeks later, there were more positive results: the dogs’ skeletal muscles produced dystrophins at levels ranging from 3 to about 90 percent of normal, depending on the muscle type.
Most notably, dystrophin production in the heart muscle reached 92 percent in the dog treated with the highest treatment dose. The gene editing also restored dystrophin production to 58 percent of normal in the diaphragm. That’s key because dogs, like people with DMD, most often die from complications related to heart failure or respiratory collapse.
While the sample size is obviously too small to draw conclusions, Olson noticed the dogs started acting differently after the treatment. They were running and jumping around, as all puppies should do!
Longer-term study in more dogs is clearly needed, but the researchers haven’t seen any ill effects of the treatment so far. Importantly, the dogs didn’t appear to have an immune response to Cas9, nor was there evidence that the enzyme had cut the DNA in other places, which potentially could cause other health problems.
It remains to be seen how long the treatment could last. But, if things continue to look good in the dogs, the next step would be to consider trying this gene editing approach in children and young adults with DMD. In fact, the precise treatment used in the new study could be used to correct the mutation found in 13 percent of DMD patients.
In addition, Olson said they’ve made progress in the lab on approaches to editing mutations in four additional common mutational hot spots, covering about half of all DMD cases. While CRISPR won’t work for everyone—for example, those whose genomes lack large portions of the dystrophin gene—they estimate that the single-cut CRISPR approach ultimately may have potential to treat up to 80 percent of people with DMD.
Meanwhile, researchers continue to explore the possibility of using gene editing for other genetic conditions that affect other parts of the body. Earlier this year, NIH launched a genome editing research program to learn how to advance this groundbreaking research and help more people with rare and even some common diseases live longer and fuller lives in the future.
 Gene editing restores dystrophin expression in a canine model of Duchenne muscular dystrophy. Amoasii L, Hildyard JCW, Li H, Sanchez-Ortiz E, Mireault A, Caballero D, Harron R, Stathopoulou TR, Massey C, Shelton JM, Bassel-Duby R, Piercy RJ, Olson EN. Science. 2018 Aug 30 [Epub ahead of publication].
 Single-cut genome editing restores dystrophin expression in a new mouse model of muscular dystrophy. Amoasii L, Long C, Li H, Mireault AA, Shelton JM, Sanchez-Ortiz E, McAnally JR, Bhattacharyya S, Schmidt F, Grimm D, Hauschka SD, Bassel-Duby R, Olson EN. Sci Transl Med. 2017 Nov 29;9(418).