text: CRISPR-Cas9 has, for the first time, been tested by systemic delivery in a large animal—and the results are striking. Working in a dog model of Duchenne muscular dystrophy (DMD), the gene editing not only restored the expression of the protein dystrophin, it also improved muscle histology in the dogs.
  
  Eric Olson, Ph.D., professor and chair of molecular biology at the University of Texas Southwestern Medical Center and lead author tells GEN, “Our technology was developed using human cells and mice to correct the same type of mutation as in these dogs. It was critical for us to test gene editing in a large animal because it harbors a mutation analogous to the most common mutation in DMD patients.” The authors wrote that this is “an essential step toward clinical translation of gene editing as a therapeutic strategy for DMD.”
  
  Indeed, Dame Kay E. Davies, Ph.D., professor of anatomy and director of the MRC Functional Genomics Unit at the University of Oxford and a pioneer in the field of DMD research, echoes this sentiment telling GEN, “This is a very exciting paper as it shows that gene editing can be reasonably affective in a large animal model of DMD.”
  
  The paper, “Gene editing restores dystrophin expression in a canine model of Duchenne muscular dystrophy,” appears in the August 31 issue of Science.
  
  DMD is caused by mutations that inhibit the production of dystrophin, a protein critical for muscle function. The mutation in the dog model, referred to as deltaE50-MD, was first identified as a naturally occurring spontaneous mutation in Cavalier King Charles Spaniels and is now maintained in the beagle model. The mutation leads to loss of exon 50 that can be corrected through the process of exon skipping, the backbone of current techniques for DMD treatments.
  
  Leonela Amosaii, Ph.D., an assistant instructor in Dr. Olson’s lab, and colleagues used adeno-associated virus (AAV) vectors to deliver CRISPR-Cas9 gene-editing components to four one-month-old dogs. The genome-editing approach aimed to correct the mutation by skipping exon 51 in order to reestablish the reading frame of the gene and restore protein expression. Two of the dogs received a systemic delivery through an intravenous injection and the other two dogs were injected intra-muscularly. Dystrophin protein expression was measured 6 weeks after intramuscular delivery and 8 weeks after systemic delivery. An increased production of dystrophin was observed in all of the dogs.
  
  Dr. Olson tells GEN, “the efficiency of gene correction and restoration of dystrophin expression body-wide [was surprising and] it only took 8 weeks.” He adds that “these findings exceeded my most optimistic expectations.”
  
  The actual percentage of increased dystrophin may be less important than the ability to reach a certain threshold. Previous studies have shown that increasing dystrophin expression to 15% of normal levels could provide significant therapeutic benefits for DMD patients.
  
  Dr. Olson says that “These results are extremely exciting because they represent an entirely new therapeutic approach for this disease that goes to the underlying cause: the genetic mutations in the dystrophin gene. Prior therapies for DMD treat the symptoms.” Dr. Davies emphasizes the broad potential for this strategy, adding that it “will be applicable to more patients than single oligonucleotides currently in the clinic or in late-stage trials for exon skipping.”
  
  As exciting as these results are, this is not a cure for DMD. At least, not yet.
  
  Dr. Davies says that “this is some way from the clinic.” Before thinking about the next steps, much more work needs to be done at even the basic research level. The authors point out that “this study, while encouraging, is preliminary and has several limitations.” For example, the paper did not look at CRISPR-Cas9–induced off-target effects. There will be more immunology testing needed, looking particularly at the immunogenicity of Cas9. Only four dogs were used in this study and the time points of 6–8 weeks are short. Lastly, although the dystrophin expression is exciting, there is no meaningful functional data presented in this paper—a necessary piece of the puzzle before moving forward.
  
  Dr. Olson himself states that it is “too early to test this technology in humans. We need to determine safety and durability before trying it in patients” and adds that this will take “several years.” Even if safety is established, there is no guarantee that this will work in humans, just because it worked in dogs.
  
  Single Cut CRISPR strategy restores the dystrophin protein. The images illustrate dystrophin (in green) in a healthy diaphragm muscle (left), absence in a canine model of Duchenne muscular dystrophy (center), and restoration of dystrophin in animals treated with CRISPR/Cas9 (right). [UT Southwestern Medical Center]If this method does jump over the many remaining hurdles that stand in the way to entering the clinic, there will be other barriers. Dr. Davies explains that “since it is AAV therapy, the cost may be very high and it will not be a treatment that can as yet be part of routine clinical practice.”
  
  But, Dr. Olson has a wave of support behind him including an incredibly motivated patient community. The company founded by Dr. Olson just last year, Exonics, which funded some of the work presented in this paper, is supported in large part by the patient advocacy group CureDuchenne.
  
  DMD is a particularly devastating disease, being described by Dr. Olson as the “holy grail of muscle diseases.” It is the most common and severe form of muscular dystrophy, affecting roughly 15,000 children (primarily boys) in the United States and 300,000 worldwide. He adds that “everything has been tried against this disease and so far nothing has shown long-term efficacy. This was the ultimate test for gene editing.”
  
  Up until recently, Labor Day weekend has been synonymous with the Muscular Dystrophy Association Labor Day Telethon, which ran for almost 50 years and was hosted by the late Jerry Lewis for roughly 40 of them. And, although there is still no cure, this latest news on DMD brings us one step closer to finding a cure for “Jerry’s Kids.”