Duchenne muscular dystrophy is caused by problems with the body's ability to produce dystrophin, a long protein chain that binds the interior of a muscle fiber to its surrounding support structure.
In the study published today in Science, UTSW researchers used a gene-editing approach to permanently correct the DMD mutation that causes the disease in young mice.
Now, the research team is working to apply this gene-editing technique to cells from DMD patients and in larger preclinical animal models. In a recent publication, scientists used this same virus to treat DMD in dogs, albeit using a gene therapy approach. To restore expression of dystrophin proteins, Christopher Nelson et al. used the CRISPR-Cas9 gene editing system to delete exon 23, causing an additional shift in the genetic coding that allows dystrophin proteins to be expressed. Duchenne muscular dystrophy causes progressive weakness because of genetic mutations that interfere with the production of dystrophin, a protein needed to form healthy muscle.
Only shown to work in human cells three years ago, the system has such enormous potential it was hailed as the 2015 "breakthrough of the year" by the prestigious journal Science. However, those trials faced problems with delivering cells back to muscle tissues and have been now deemed unethical for human trial. Since germ line editing is not feasible in humans, strategies would need to be developed to deliver gene-editing components to postnatal tissues. "We know what genes need to be fixed for certain diseases, but getting the gene editing tools where they need to go is a huge challenge", noted lead author Chris Nelson, Ph.D., postdoctoral fellow in Dr. Gersbach's laboratory. "The best way we have to do it right now is to take advantage of viruses, because they have spent billions of years evolving to figure out how to get their own viral genes into cells". It's a virus that many people are exposed to anyway and is non-pathogenic, but still exceptionally effective at getting into cells. Moreover, AAV is in use in many late-stage clinical trials in the United States and has already been approved for use in one gene therapy drug in the European Union.
"AAV is a really small virus and CRISPR is relatively large", said Gersbach.
The gene-editing tool, known as CRISPR-Cas9, harnesses a defence mechanism bacteria used against viruses to home in on targeted sections of DNA which are then snipped away with an enzyme that acts like molecular "scissors".
Histopathology of gastrocnemius muscle from patient who died of pseudohypertrophic muscular dystrophy, Duchenne type.
Gersbach and his team first delivered the therapy directly to a leg muscle in an adult mouse, resulting in the restoration of functional dystrophin and an increase in muscle strength. Subsequently, they injected the CRISPR/AAV combination into a mouse's bloodstream to reach every muscle.
"There is still a significant amount of work to do to translate this to a human therapy and demonstrate safety", said Charles Gersbach, associate professor of biomedical engineering at Duke University in a statement. "From here, we'll be optimizing the delivery system, evaluating the approach in more severe models of DMD, and assessing efficiency and safety in larger animals with the eventual goal of getting into clinical trials".