Gene therapy is going to be very influential in medicine of all types in the next few years, and this is due to the advent of CRISPR, a cheap and reliable genetic editing technology. However, while it is reliable in embryos, since researchers only have to ensure coverage of a small number of cells to create a change that will later be present throughout the whole of the adult body, obtaining that same coverage in a therapy delivered to adults has been a challenge. If a gene therapy fails to change a large enough percentage of cells in an adult individual, then it will have no useful effect. Hence we should be watching for progress on this front.
The research results linked here focus on an inherited disorder with no relevance to aging, but the importance lies in the delivery mechanism and its demonstration, not the therapeutic goals. It is an example of a methodology for adult gene therapy with CRISPR that is (a) easy to carry out for existing labs and (b) generates good tissue coverage in adults. This is significant: it means that all of the gene therapies we might like to carry out as treatments to compensate for age-related damage and decline, such as myostatin deletion to boost muscle growth, adding extra lysosomal receptors to better clear out damage in old tissues, or moving mitochondrial genes to the cell nucleus, are now much more technically feasible. Progress is presently very rapid in this space.
Researchers had previously used CRISPR to correct genetic mutations in cultured cells from Duchenne muscular dystrophy patients, and other labs had corrected genes in single-cell embryos in a laboratory environment. But the latter approach is currently unethical to attempt in humans, and the former faces many obstacles in delivering treated cells back to muscle tissues. Another approach, which involves taking CRISPR directly to the affected tissues through gene therapy techniques, also faces challenges, particularly with delivery. In the new study, researchers overcame several of these obstacles by using a non-pathogenic carrier called adeno-associated virus, or AAV, to deliver the gene-editing system.
To use viruses as delivery vehicles for gene therapy, researchers take all the harmful and replicative genes out of the virus and put in the therapeutic genes they want to deliver. While early virus types didn't work well for various reasons, such as integrating into the genome and causing problems or triggering immune responses, AAV thus far has proven special. It's a virus that many people are exposed to anyway and is non-pathogenic, but still exceptionally effective at getting into cells. 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. There are also different versions of AAV that can preferentially go to different tissues, such as skeletal and cardiac muscle, so researchers can deliver them systemically.
But there's always a catch. "AAV is a really small virus and CRISPR is relatively large. It simply doesn't fit well, so we had a packaging problem." The solution came from a CRISPR system in a different bacterium than the one commonly used. In the natural bacterial immune system, CRISPR is the mug shot that helps identify the target DNA, and Cas9 is the blade that slices the strands. The large Cas9 protein typically used by researchers comes from the bacterial species Streptococcus pyogenes. After scouring the bacterial kingdom, researchers discovered the much smaller Cas9 protein of Staphylococcus aureus - small enough to fit comfortably inside of AAV.
In the study, researchers worked with a mouse model that has a debilitating mutation on one of the exons of the dystrophin gene. They programmed the new CRISPR/Cas9 system to snip out the dysfunctional exon, leaving the body's natural repair system to stitch the remaining gene back together to create a shortened - but functional - version of the gene. Researchers 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. They then injected the CRISPR/AAV combination into a mouse's bloodstream to reach every muscle. The results showed some correction of muscles throughout the body, including in the heart - a major victory because heart failure is often the cause of death for Duchenne patients.