U.S, Japanese Scientists Test Treatmentfor Muscular Dystrophy: Pilot StudyShows "Exon Skipping" DNA Therapy Effective in Dogs

Exon Skipping 

To produce a protein, disparate portions of DNA are stitched together, or spliced, to create a contiguous template for protein production. Exons are the portions of DNA that are spliced together.

In Duchenne muscular dystrophy, mutations tell the cellular machinery to prematurely stop making the protein. With the exon-skipping strategy, researchers use synthetic DNA to trick cells into skipping over the exon that contains the mutation. The cell will make a shorter version of the dystrophin protein, but, as in Becker muscular dystrophy, researchers hope the short version is enough to restore some function to muscle.

One complication is that multiple exons must be skipped in order to avoid the mutations and produce enough of the protein. Each exon requires a separate synthetic DNA sequence. The current study is the first to show that a cocktail of different DNA sequences can work together to treat muscular dystrophy.

"Systemic treatment of the majority of Duchenne dystrophy will require multiple sequences to be delivered in the blood, and this study also is the first proof-of-principle of multiple exon skipping in any organism," said Dr. Shin'ichi Takeda, a researcher at the National Center of Neurology and Psychiatry in Tokyo and a senior author of the study.

Hoffman, Takeda and their colleagues were able to restore dystrophin to 26 percent of normal levels, on average. The treatment dramatically improved dogs’ ability to walk and run.

Scientists administered the synthetic DNA cocktail intravenously to expose all muscles to the treatment, which does not appear to be toxic.

Despite the recovery of walking and running ability, the treatment’s effectiveness varied in muscles throughout the body. Dystrophin protein levels were not restored to similar levels in every muscle. Different levels were apparent in the same muscle on different sides of the body. In the heart, dystrophin was concentrated in small patches of muscle and present in “far lower levels” than in skeletal muscle, scientists found.

Future studies will test how much muscle function can be restored long term, will treat a greater number of animals and will experiment with ways to improve the treatment’s access to the heart, the authors wrote in the paper.

"In order to realize that promise in human trials, it also will be important to re-evaluate current measures of toxicity, efficacy, and marketing that ensure both safety for the patient, as well as rapid development and distribution of life-saving drugs,” Takeda said.

Hoffman said that these results are a proof-of-concept that therapy targeting specific pieces of DNA is effective in a large animal “for Duchenne muscular dystrophy or any disease."

“Of course, this success has also introduced even more avenues for investigation, but these findings finally overcome a significant hurdle to our progress,” Yokota said. “We've solved the riddle of an effective systemwide delivery to muscle tissue, and seen promising results.

" Videos by the researchers of an untreated dog and a treated dog are available on the YouTube Web site.