By Margo Schneidman
Medical Center Public Relations
A team of researchers from the Department of Human Genetics has used gene therapy to cure Duchenne muscular dystrophy (DMD) in mice. DMD is the most common childhood form of this fatal disease.
As reported in the Aug. 19 issue of Nature, the researchers inserted a corrected copy of the dystrophin gene into the embryos of mice with muscular dystrophy. Instead of developing the disease as they normally would, the mice remained disease-free.
“We have clearly shown that if you can get the gene into the muscle, you can cure the disease,” said Jeffrey Chamberlain, assistant professor of human genetics, who led the team. “This has never been done before.”
According to Chamberlain, the path-breaking study results prove that gene therapy offers the best hope for halting the progression of DMD and Becker muscular dystrophy, a milder form of the muscle disorder.
“This demonstrates for the first time that it is possible to cure the disease,” Chamberlain said. “If you can find an effective way to get the gene into humans, and you can control it properly, then you may have a cure. In this study, we found a way to control the gene once it’s in the muscle; now we need to find a way to deliver it.”
Chamberlain cautions against premature optimism.
“The way we did the experiment with mice cannot be done with humans because of the damage to the embryo that can result,” he said. Chamberlain speculates that the best way to deliver the gene safely and effectively into humans may be by inserting it into an adenovirus similar to one that causes the common cold.
“Duchenne muscular dystrophy is among the most common genetic diseases, occurring in one out of every 3,500 males. While females can be carriers of the disease, they rarely develop its symptoms. DMD accounts for almost one-half of all muscular dystrophy cases. Symptoms first appear around age 3 and include progressive muscle weakness, joint stiffening and spinal curvature. Children are often in wheelchairs by age 11. By their mid-20s, weakened heart and breathing muscles result in heart or respiratory failure, which causes death.
DMD is caused by mutations in the gene that produces dystrophin, a protein important in the function of muscle cells. The severity of symptoms correlates with the degree to which the expression of dystrophin is impaired.
The dystrophin gene, which is on the X-chromosome, is the largest known gene, some 100 times larger than a typical gene. As a result of its size, Chamberlain said, it has a large target area for damage, which makes it highly prone to mutations. This, he explained, is why one-third of DMD cases are not inherited.
This large size also presented an obstacle for Chamberlain and colleague Greg-ory Cox, a researcher in the Department of Human Genetics and lead author of the study.
Researchers had to construct a mini version of the dystrophin gene before they could inject it into the mice. Once they made the artificial gene, the challenge remained to find a way to regulate it so it expressed the right amount of protein, and in the right cells.
To do this, they took the muscle promoter—a genetic on/off switch—from another gene and connected it to the mini-gene, creating a hybrid that would express protein only in muscle cells. Finding the right promoter was, according to Chamberlain, a key to the success of the study.
“We’ve found a good combination of mini-gene and an on/off switch that works in the right tissues and also makes enough of the protein to eliminate symptoms of the disease,” Chamberlain said.
By studying the physiologic responses of the mice to the therapy, John Faulkner, professor of physiology and director of biological research at the Institute of Gerontology, was able to show that the diaphragm muscles in mice with the transferred dystrophic gene functioned normally, while those in mice with the defective gene were severely damaged and impaired. Although the injected mice still retained their mutant gene, the corrected copy functioned to compensate for the defective one. The researchers also found that the transferred gene released up to 50 times the normal amount of protein, but that this excess produced no ill effects.
Chamberlain’s lab will now focus on finding ways to insert the mini gene and switches into adenoviruses. In addition, he will be developing an optimized miniature version of the dystrophin gene, perhaps even smaller than the mini-gene injected into mice.
He is confident that the findings of his current research will have implications for the treatment of other types of muscle disease.
“I think the techniques that will come out of this, particularly what we learn about how to get this into humans, will be applicable to any muscular dystrophy,” he said.
The other collaborators in the study were Neil Cole, a U-M doctoral fellow in physiology; Stephen Hauschka, professor of biochemistry, University of Washington, who specializes in the study of gene promoters; and Kevin Campbell, professor of physiology and biophysics, University of Iowa, who studies the interaction of dystrophin with other proteins in muscle cells.
The study was funded by grants from the National Institutes of Health, the Muscular Dystrophy Association and the March of Dimes.
