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In Three Studies, CRISPR Effective in Improving Symptoms of Mice With Muscular Dystrophy

NEW YORK (GenomeWeb) – Several independent researchers last week shared promising results of their work using the genome-editing tool CRISPR/Cas9 to advance treatments for Duchenne muscular dystrophy.

In a group of three studies published in the Jan. 1 issue of Science, researchers found that after viral delivery into muscle tissue or blood, CRISPR/Cas9 was able to excise disease-associated exons from the dystrophin gene, and improve the symptoms of mice with a rodent version of the disease.

The past year has been a breakout one for CRISPR, and this is not the first promising data from researchers using the tool in the search for a treatment for Duchenne, or DMD, an X chromosome-linked disorder characterized by deficiency of the protein dystrophin, an important component of muscle tissue.

In December, a group from Toronto's Hospital for Sick Children published a study in which they demonstrated several ways CRISPR/Cas9 could be used to treat hereditary diseases. Among the applications demonstrated was the in vitro removal of exon duplications in cells isolated from a patient with the disease that appeared to lead to the production of normal dystrophin.

In that study, researchers were also able to use a Cas9 protein modified to promote gene expression to increase the amount of utrophin, a DMD-ameliorating protein.

In the new findings published in Science, three other research groups — led by Eric Olson of the University of Texas Southwestern Medical Center, Charles Gersbach of Duke University, and Amy Wagers of Harvard University — shared data from their own uses of CRISPR/Cas9 in mouse models of the disease.

In all three studies, researchers used a virus to transport CRISPR/Cas9 into muscle cells in mice with an alteration in an exon of the dystrophin gene, directing the tool to snip out the deleterious mutation in question.

Olsen's UT Southwestern team had tried a similar approach a year earlier, editing out the damaged exon in the fertilized egg of a mouse, but the newer study is the first evidence for the ability of the approach in the tissue of a living adult animal.

In their new study, the researchers reported that after they injected these viruses into the muscles or blood of their DMD model mice, the animals' muscle cells began to make a truncated, or shortened form of dystrophin, skipping the excised exons. Along with this shift, the rodents appeared to perform better on tests of muscle strength than untreated DMD mice, indicating that the shorter protein was functional.

The improvement was not a complete cure however. While the mice's muscles performed better, they were not up to the abilities of completely unaffected healthy mice. However, the authors wrote that even a mediated improvement in expression of the dystrophin protein could provide significant benefit to humans with DMD.

"It has been estimated that even low-level expression of dystrophin … can partially ameliorate cardiomyopathy and protect against eccentric contraction-induced injury in skeletal muscle. The efficiency of restoration of dystrophin expression observed [in the current study] is therefore within the range expected to provide therapeutic benefit," the investigators wrote.

The other two independent teams, led by Gersbach at Duke and Wagers at Harvard — both collaborating with CRISPR pioneer Feng Zhang of Harvard and the Broad Institute — reported similar results in two other studies published in the journal.

Gersbach’s group has published previously on CRISPR/Cas9 editing to remove exons of the dystrophin gene from Duchenne patient cells grown in vitro. In their new study, the team showed that viral delivery of the CRISPR/Cas9 system into DMD mice to remove exon 23 of the dystrophin gene resulted in expression of a modified dystrophin protein, improvement of muscle biochemistry, and significant enhancement of muscle force.

Wagers and colleagues demonstrated similarly in their study that programmable CRISPR complexes could be delivered locally and systemically to skeletal muscle fibers and cardiomyocytes, as well as muscle satellite cells, in neonatal and adult mice.

As in the other team's experiments, resulting exon excision restored dystrophin expression and partially repaired the functional deficiencies of dystrophic muscle in these mice.

None of the teams saw any serious off-target effects in their studies, but Olsen and colleagues wrote that because unexpected mutations may occur at sites beyond those predicted in silico, a comprehensive and unbiased analysis, such as whole genome sequencing, would be an "essential component of future efforts to establish the safety of this approach."

All three teams reported in their Science publications that they have filed for patents on aspects of gene editing in muscle disease.  

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