Researchers Put Dystrophin Minigenes to Ultimate Test
Highlights from the American Society of Gene Therapy meeting
Seattle, May 30-June 3, 2001
"Minigene" therapy for Duchenne muscular dystrophy (DMD) has been put through one of the most rigorous tests yet, MDA-supported scientists announced today at the American Society of Gene Therapy meeting in Seattle.
DMD is caused by mutations in dystrophin -- a large gene that challenges scientists seeking to devise a way to deliver a replacement copy of it via gene therapy. Mini-dystrophins - dystrophin genes that have been whittled down to their most essential parts - were designed to solve that problem.
But would the resulting mini-dystrophin protein work as well as the natural version? Normally, the dystrophin protein spans from the inside of a muscle cell to just beneath the muscle cell surface, creating a scaffold that's essential for muscle integrity. When muscle cells lack dystrophin -- as in DMD -- they fall apart, causing progressive muscle weakness and wasting, and eventual death from respiratory failure.
MDA grantee Jeffrey Chamberlain and his team at the University of Washington in Seattle, who've made several mini-dystrophins, set out to test whether the shortened dystrophin proteins could preserve muscle integrity in a mouse model of DMD. To do so, rather than using a gene therapy method to deliver the mini-dystrophins, the researchers genetically engineered the DMD mice to produce their own mini-dystrophins.
Production of the mini-dystrophins, the team found, protected the mice from the devastating effects of DMD, including the wasting of respiratory muscles. The mini-dystrophins tested were about one-third the size of full-length dystrophin (so small that Chamberlain actually prefers to call them micro-dystrophins).
In independent studies, Xiao Xiao of the University of Pittsburgh recently showed that mini-dystrophins delivered via gene therapy also have a protective effect in DMD mice.
Both Chamberlain and Xiao presented their findings at the gene therapy meeting Thursday.
Packaging dystrophin into an adeno-associated virus (AAV) and then injecting the virus into the body is considered one of the best strategies for DMD gene therapy, since the virus has a natural attraction for muscle and evokes little response from the immune system. But the virus is too small to carry full-length dystrophin, and can only accommodate compact versions of the gene, like the mini-dystrophins designed by Chamberlain and Xiao.
With current technology, AAV doesn't travel far beyond the site of injection, so it has to be injected into individual muscles to achieve efficient gene delivery. Xiao's study showed that intramuscular injection of AAV carrying mini-dystrophin has a therapeutic effect on individual leg muscles in DMD mice.
With the expectation that better gene therapy techniques will soon enable systemic (whole-body) AAV delivery of mini-dystrophin, Chamberlain and his group have essentially mimicked that situation by making DMD mice that produce mini-dystrophin in all of their muscles. The results show that mini-dystrophins can rescue all of the muscles in DMD mice: The mini-dystrophins reduced wasting of the skeletal muscles (muscles that control voluntary movement) and diaphragm (the muscle that controls breathing) in the mice. Not only that, the mini-dystrophins helped prevent contraction-induced injury of the skeletal muscles and improved motor performance in the mice.
Soon, Chamberlain and Xiao plan to test AAV-mediated delivery of mini-dystrophin in clinical trials for DMD.
Read more highlights from the American Society of Gene Therapy meeting:
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