Genetically correcting a muscle disorder
Three independent groups of researchers provide preliminary evidence that CRISPR can treat genetic disorders by editing a gene involved in muscle functioning, restoring some muscle function in mice with a specific type of muscular dystrophy. While much controversy surrounds the editing of germline (reproductive) cells to correct genetic disorders, these results demonstrate the potential to correct some genetic disorders post-birth. Duchenne muscular dystrophy (DMD) is a debilitating genetic disease that occurs in about 1 in 3,500 males, causing muscle degeneration, loss of mobility, and premature death. DMD mutations most often involve deletion of one or more exons in the dystrophin gene, causing a "shift" in DNA coding that leads to a complete loss of dystrophin, a protein essential for muscle functioning. 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. In this instance, they used adenovirus-associated virus 8 (AAV8) to deliver the gene editing system into muscle cells of mice. This resulted in deletion of exon 23 in roughly 2% of all muscle cells in areas that received an injection and restoration of dystrophin protein levels to roughly 8% of the normal level (previous reports suggest that as little as 4% is sufficient to achieve adequate muscle function in DMD). Additional tests using six-week-old mice found improvements in muscles that are responsible for cardiac and pulmonary health, which are severely weakened in DMD patients, and associated with premature death.
In a second study, Chengzu Long and colleagues used adeno-associated virus-9 (AAV9), which displays a high affinity for muscle, to deliver the CRISPR-Cas9 editing components to the same mouse model of DMD. They first made sure their gene editing strategy worked in egg and sperm cells of mice, and found that it was highly efficient: 80% of baby mice born following germline modification showed a deletion of exon 23, which in turn boosted dystrophin protein expression in these animals. The team then applied their strategy to the more clinically relevant setting of gene editing in somatic cells. They used AAV9 to deliver the editing system to mice a few days after birth via injections into the abdomen, into muscles, or into the backs of eyes. While each delivery method had its unique benefits and improved muscle function, they found that dystrophin protein levels were highest when the treatment was injected directly into muscles.
A third study by Mohammadsharif Tabebordbar et al. also used CRISPR and AAV9 to edit out exon 23, finding similar beneficial restoration of muscle functioning. Using fluorescent markers, the team analyzed the effects of the gene editing system on satellite cells, located away from injection sites. They found that some satellite myotubes, a "young" stage of muscle cells, had restored expression of dystrophin. Therefore, the authors suggest that AAV-CRISPR may provide ongoing genetic repair in vivo.