For the first time Duchenne muscular dystrophy (DMD) progression was halted in a mammal as large as a dog using the CRISPR gene editor. DMD is a genetic disorder which leads to severe muscle weakness and degradation. Mutations in the dystrophin gene, which is involved in the production of dystrophin protein, is the known cause for the disorder. Abnormal dystrophin production prevents muscle cells from being kept intact and muscle atrophy occurs. The dystrophin gene is X-linked recessive, therefore the main population affected is males. DMD is debilitating and affects approximately 300,000 males globally. Prior to modern healthcare advancements in respiratory and cardiac care; DMD patients were not expected to survive past their teenage years.
The recent gene editing of dogs provides hope for human DMD treatment. Researchers at the University of Texas Southwestern Medical Center are working with CRISPR technology and the deltaE50-MD canine model of DMD in an attempt to fix the DMD gene mutation. How does CRISPR work exactly? The gene editing technology uses an RNA strand to guide Cas9 (an enzyme) to cut a specified potion of DNA. The logic behind using a canine model, specifically beagles, is that they exhibit many of the pathological features (muscle degeneration, weakness, and fibrosis) of human DMD . Positive results of the study include CRISPR induced restoration of dystrophin protein in major body muscles (i.e. the heart). The study monitored four dogs over the course of less than two months. It is expected that the small duration and cohort of the study left some scientists unimpressed. However the lead researcher, Eric Olson, replied that the fast pace of the study was to establish proof of the ability to restore dystrophin for future research. In response to criticism regarding the amount of animals used Olson replied “We’re very mindful of ethical concerns and have done our best to keep our use of dogs to an absolute minimum”.
A total of 75 exons are contained in the dystrophin gene, the largest gene in humans. Due to its size, there are several opportunities for mutations on this but only one functional copy is required for normal dystrophin production. One mutation that causes DMD occurs between exon 45 and exon 50 which in turn leads to an “out of frame” exon 51; preventing dystrophin from being produced. The CRISPR molecular scissors used in the study were designed to make a cut at exon 51 in the beagles with DMD. The expectation was that when attempting to repair the splice, the cell will induce errors to exon 51 that would lead dystrophin protein production mechanism to skip the exon entirely. As a result of exon-skipping there was production of a shorter (normal dystrophin protein is 3500 amino acids in length) but still functional dystrophin that was partially restored in the beagles. To assist in altering the billions of muscle cells in the dog models, the team used an adeno-associated virus (AAV) carrying the CRISPR components, to infect skeletal muscle and heart tissue. The intravenous route of treatment occurred after intramuscular injections of CRISPR-carrying AAV proved successful in restoring production of dystrophin. Two of the 1-month-old canines were given intravenous infusion of the CRISPR-carrying AAV’s which resulted in dystrophin levels “reaching 58% of normal in the diaphragm and 92% in the heart”.
Certainly there are still several questions regarding this advancement. Since muscle damage is irreversible, treatment will only be effective in early life? Also will the CRISPR treatment induce cancer-causing mutations? Is there a way for the treatment to reach stem cells? Will normal muscle function result with shortened dystrophin? Regardless, the further exploration of CRISPR as a medical tool is essential to treating genetic illness to improve the quality of life and overall life expectancy of afflicted patients.
Fajar Alam is a contributor for The Daily Campus. She can be reached via email at firstname.lastname@example.org.