Researchers have tinkered with CRISPR in treating an adult mouse with a type of muscular dystrophy.
A team from Duke University has used the gene editing tool — hailed as Science magazine's 2015 Breakthrough of the Year — for the first time to treat the genetic disorder Duchenne muscular dystrophy inside a living mammal, to be potentially replicated in humans.
They also overcame issues using adeno-associated virus (AAV) to deliver the gene-editing system.
The researchers published their findings in the journal Science.
Duchenne muscular dystrophy, which strikes in 1 out of 5,000 newborn boys, results from a genetic deficiency in dystrophin, a protein involved in repairing muscle fibers affected by daily movement and activities such as exercise. The lack of dystrophin leads to shredding and slowly deteriorating muscles.
The disease makes most patients wheelchair-bound by age 10 and risk for death by their early 30s.
Restored Muscle Function
The team led by biomedical engineering professor Charles A. Gersbach used a mouse model suffering from a mutated exon of the dystrophin gene, programming CRISPR/CAS9 — a bacterial-protein derived process of cutting and pasting DNA portions — to snip out the defective exon.
They then left the natural repair system of the body to stitch the remaining gene back together, creating a shortened yet functional gene version.
The therapy delivered directly to a leg muscle restored dystrophin and increased muscle strength in the adult mouse, after which the researchers injected a combination of CRISPR and AAV, the most popular virus today for delivering genes, into the bloodstream to reach every muscle.
Some muscles, including the heart, were corrected by the therapy, and this is considered a major success because heart failure surfaces as the common cause of death among Duchenne sufferers.
Gersbach said in a press release that while there is substantial work ahead in seeing if their findings will work and are safe in humans, the results aren't any less exciting.
"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," he explained.
Delivery Hurdles
Gersbach, who has been exploring Duchenne treatments since he began his Duke University lab in 2009, lately focused on CRISPR. But there were hurdles along the way.
"[There's] delivery. 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," said Chris Nelson, a fellow in Gersbach's lab who led the work, adding that viruses, which evolved for billions of years, are the best way to go right now.
This led the team to package the gene editing tool into AAV, used in late-phase clinical trials in the U.S. and also already approved in a gene therapy drug in the European Union.
To use viruses as delivery vehicles for the gene therapy, the researchers took all the harmful genes out of the virus and put in the beneficial genes in. AAV is said to be special in that it is non-pathogenic, many people are exposed to it, and it is "exceptionally effective" at penetrating cells.
Size was another challenge, as AAV is a small virus and CRISPR is relatively huge.
To address the packaging issue, Feng Zhang of MIT and Harvard scoured bacteria and discovered the much smaller Cas9 protein of the Staph bacteria — small enough to comfortably fit inside AAV.
Ethical Aspects
Duke University researchers previously attempted to use CRISPR to solve genetic mutations in Duchenne patients' cultured cells, while other labs had fixed genes in single-cell embryos. The former faced delivery problems, while the latter remains unethical to be performed in humans.
Gersbach acknowledge ethical concerns in using CRISPR for addressing genetic mutations in human embryos, but argued that CRISPR's role in correcting mutations via sick individuals' tissues "is not under debate."
Their study, according to him, shows the way to the elusive cure.
