MDA RESEARCHERS PIONEER VASCULAR GENE THERAPY APPROACH
Single Injection Delivers Gene to Muscles of Entire Limb
TUCSON, Ariz., March 29, 1999 -- Researchers sponsored by the Muscular Dystrophy Association (MDA) have strengthened prospects for gene therapy as a treatment for muscular dystrophy. For the first time, they've delivered corrective genetic material throughout the entire limb of a naturally occurring hamster model of limb-girdle muscular dystrophy. They've also confirmed earlier MDA-funded research showing that genes for the sarcoglycan muscle protein can, when injected directly into leg muscles, correct the defects causing the disease.
"These exciting achievements confirm that we're on the right track," said Donald S. Wood, MDA director of Science Technology. "We're more committed than ever to begin clinical trials of gene therapy for muscular dystrophy."
Because muscle tissue represents a large percentage of total body mass and is highly compartmentalized, delivering a therapeutic gene to all or even most of the muscles to cure diseases like muscular dystrophy has presented a formidable challenge.
Now MDA grantees Hansell Stedman, James Wilson and Rita Balice-Gordon at the University of Pennsylvania in Philadelphia have pioneered an innovative method to efficiently deliver therapeutic genes to skeletal muscle using the circulatory system. Previously, viruses carrying therapeutic genes were too large to pass through the walls of blood vessels. The study, being published as the cover article in the April issue of Nature Medicine, builds on prior MDA-sponsored research.
"We've demonstrated proof of concept that the methodology can work for both skeletal and cardiac muscle," said Stedman, assistant professor of surgery at Penn's Institute for Human Gene Therapy, "so the next step will be to establish its safety and effectiveness in patients with this form of muscular dystrophy." Stedman cautioned that clinical trials need to address safety issues one step at a time before significant therapeutic benefit can be approached.
Dr. Leon Charash, chairman of MDA's Medical Advisory Committee, said the new gene therapy approach has the potential for broad application. "The sarcoglycan protein complex is implicated in several neuromuscular diseases, and it can be delivered virally without immunosuppressive drugs and still be well tolerated by the immune system," Charash added. "The Penn team's new vascular delivery approach may ultimately be harnessed to treat several forms of muscular dystrophy, metabolic diseases of the muscle and cardiomyopathy (heart failure)."
To improve transfer of the viruses carrying therapeutic genes, also known as "vectors," from the bloodstream to the skeletal muscles, the researchers developed a method that temporarily enlarges openings in the blood vessels, allowing passage of the virus. This procedure relies on the injection of two substances -- histamine, which makes blood vessel walls permeable to various substances, and papaverine, which keeps the blood vessels dilated during histamine administration. Stedman described the procedure as "creating a sieve that allows the virus to leave the bloodstream and be taken up by the muscle cells."
Using this technique, the researchers showed that a single injection into the circulation of the hind limb of a hamster with a form of muscular dystrophy enabled the adeno-associated virus vector to deliver the sarcoglycan gene needed to correct the problem in all muscles of the hind limb. The sarcoglycan complex is a critical muscle structure composed of four membrane-spanning proteins. Flaws in the genes for any of these four sarcoglycans compromise the integrity of the muscle membrane, fostering progressive muscle weakness.
"The success of these experiments in a small-animal model represents an important step toward solving the problem of large-scale gene delivery to muscle," said Wilson, who directs the Institute for Human Gene Therapy at the University of Pennsylvania.
"The route of administration seems to be the key element in getting a vector carrying therapeutic genes into enough muscle cells to be effective," Stedman said. He and his colleagues also showed that an adeno-associated virus (AAV) vector, which triggers almost no response from the immune system, is able to efficiently infect skeletal muscle cells and direct long-term production of a missing protein, if given sufficient access to the muscle.
The researchers emphasized that this new technique isn't yet ready for human trials. "One huge challenge we are faced with," Stedman said, "is that histamine cannot be used systemically (injected into the entire bloodstream) unless additional measures are taken to prevent shock. Our current success relies on isolating blood supply to the limb that is being treated to protect the rest of the body from the negative effects of fluid loss through the permeable blood vessels."
The eventual goal of this research is to deliver the gene therapy vector with a single injection to all of the muscles of the body, including the heart, that are affected by muscular dystrophy. Stedman, a surgeon, pointed out that, during open heart surgery, the heart is stopped and bypassed temporarily. He speculated that with the appropriate supportive measures, it may be possible to cope with the histamine-induced effects of anaphylactic shock long enough to deliver gene therapy systemically.
Wilson and Stedman are coordinating a group of MDA-funded researchers planning to do human trials of gene therapy for four kinds of muscular dystrophy that result from defects in genes and the muscle proteins that are produced from those genes. This trial would be used mainly to evaluate the safety of the procedure, and would involve injections into a single foot muscle.
MDA is a voluntary health agency working to defeat 40 neuromuscular diseases through programs of worldwide research, comprehensive patient and community services, and far-reaching professional and public health education. Recognized by the American Medical Association with a Lifetime Achievement Award "for significant and lasting contributions to the health and welfare of humanity," MDA maintains 230 hospital-affiliated clinics that offer families the best in care for progressive neuromuscular diseases.
MDA annually funds some 400 scientific teams worldwide. These investigators have made significant advances toward cures for several muscle-wasting diseases. They've pioneered breakthroughs that may well lead to therapies for heart disease, cancer, AIDS, Alzheimer's, Parkinson's, Huntington's, and cystic fibrosis. For information about MDA research progress and/or referrals to MDA clinics, call 1-800-572-1717, or visit MDA's Web site at www.mdausa.org. MDA programs are funded almost entirely by individual private contributors.
