Gene therapy for muscular dystrophy shows promise in trial with mice

A new gene therapy technique that has shown promise in treating skin disease and hemophilia might one day be useful as a remedy for muscular dystrophy, according to new medical school research.

In the study, published online Jan. 2 in the Proceedings of the National Academy of Sciences, the researchers used gene therapy to introduce a healthy copy of the gene dystrophin into mice with a condition that mimics muscular dystrophy. When the dystrophin gene is mutated, it produces a defective protein, evident in the roughly 20,000 people in the United States with the most common form of the disease.

Using gene therapy to treat muscular dystrophy isn't a new idea. Thomas Rando, MD, PhD, associate professor of neurology and neurological sciences, said that researchers have tried several different techniques with variable success. One hurdle is getting genes into muscle cells all over the body. Another is convincing those cells to permanently produce the therapeutic protein made by those genes.

The gene therapy technique Rando and postdoctoral fellow Carmen Bertoni, PhD, used was developed by Michele Calos, PhD, associate professor of genetics. One of the main advantages of this method is that it could potentially provide a long-term fix for a variety of genetic diseases, including muscular dystrophy.

In muscular dystrophy, the muscle cells break down and are slowly replaced by fat. Eventually people with the disease are confined to a wheelchair and usually die in their 20s. There is currently no effective treatment for the disease, which explains why gene therapy remains a hope despite the significant hurdles.

Rando said the PNAS paper highlights an additional requirement for any gene therapy to be successful: The introduced gene must produce healthy dystrophin protein in large quantities to repair the entire muscle cell. Previous muscular dystrophy gene therapy studies did not look at whether the introduced dystrophin spread along the entire length of the muscle cell, which can be many millimeters long in mice or inches long in humans.

In this study, the researchers used a standard gene therapy method to introduce two genes—dystrophin and a gene that makes a glowing protein—into mice with a mouse version of muscular dystrophy. They found that in mice producing insufficient dystrophin, they could see the glowing

protein leak out of the cell. This leakiness showed that the cell had not healed.

But when the researchers used Calos' gene therapy technique to introduce the genes, the muscle cell contained high levels of dystrophin distributed along the length of the cell and the glowing protein stayed within the cell, suggesting that the abundant dystrophin repaired the ailing muscle.

"If you have a single cell that's a foot long and you only correct a few inches, you've done very little," Rando said. "If you correct it from end to end, you truly cure the disease in that cell."

Rando and Calos noted that the road to a gene therapy cure for muscular dystrophy remains a long one. However, Calos is confident that her technique will be a part of the journey toward a cure for this and other diseases such as hemophilia and epidermolysis bullosa. Early trials using her approach have looked promising in animal models of both of these diseases.

"Our approach has a lot of potential to overcome issues that have slowed the field of gene therapy," Calos said. She cited two advantages to her approach: One is that in her method the gene gets inserted directly into the cell's own DNA, which is why the correction is permanent. In some other methods the gene stays outside the DNA and slowly breaks down.

The second advantage is that her method doesn't rely on a virus to disperse the DNA and therefore avoids some of the issues, including cancer and an immune reaction, that have turned up in viral gene therapy trials. Instead this approach uses naked DNA that travels through the bloodstream to the cells of the body.

For his part, Rando said that no matter how well gene therapy works in an isolated muscle, researchers still must figure out how to get that gene to muscles throughout the body.