The dystrophin gene, discovered in the late 1980s, is carried on the X chromosome. Boys born to mothers who carry a dystrophin gene mutation on one of their X chromosomes have a 50-50 chance of inheriting the gene and having muscular dystrophy. Daughters also may inherit the gene and become MD carriers. Unlike other genetic diseases that can be prevented by prenatal diagnosis and genetic counseling, however, one out of three Duchenne muscular dystrophy patients comes from de novo mutation; that is to say, there is no family history of MD and the parents' genes are normal.

When a child cannot produce sufficient dystrophin, his muscle contractions put damaging stress on the connections between muscle fibers and membranes. Membranes become leaky, fibers are destroyed and, eventually, muscle cells die.

The dystrophin gene happens to be one of the largest in the human genome, something that poses real challenges for Duan and other MD researchers. That's because the virus Duan uses to deliver healthy genes, adeno-associated virus or AAV, is one of the smallest vectors available for gene therapy. Even after its contents have been removed to create what Duan calls a "gutless virus," AAV is still far too small to hold the entire dystrophin gene.

Still, despite the difficulties, AAV holds much advantage for gene therapy. "It's almost the perfect tool," Duan says. For one thing, the virus is exceptionally safe; it doesn't cause any known human disease. Some viruses used as vectors can cause inflammation; as the body tries to rid itself of the virus, it eliminates the therapeutic genes as well. And AAV hasn't presented the kind of risks that were encountered by the French researchers.

The French used a retrovirus as their vector. That virus allowed the new gene to be incorporated into the patient's genome. In one case, however, the gene located in the wrong place and activated other genes that caused the leukemia-like illness.

Genes delivered by AAV behave differently. They take up residence in cells but remain separate from a cell's genome. These genes can persist in human cells for many years, researchers have found.

Duan and other researchers, such as his collaborator, Jeffrey Chamberlain of the University of Washington, have developed some ingenious strategies for packaging the dystrophin gene into AAV. Chamberlain, for example, has created a "micro" dystrophin gene, an abridged edition of the gene that's been stripped down to its essential information.

Duan and his research team used AAV to deliver Chamberlain's microgene, injecting the virus directly into the heart cavities of newborn mouse pups with DMD. That was a significant technical challenge, Duan said. Mouse hearts are smaller than the nail on a pinky finger and they beat as fast as 700 times a minute, 10 times the speed of a human heart.

Published by the Office of Research.

©2006 Curators of the University of Missouri. Click here to contact the editor.