New Gene Therapy Vector May Lead to Treatment for Muscular Dystrophy

For release: Monday, April 18, 2005

One of the biggest challenges in developing useful gene therapy is finding a way to get the beneficial gene into enough cells of the body to effectively treat the disease.  Now, researchers have shown in rodents that a virus called adeno-associated virus 8 (AAV8) can effectively deliver a gene to all the skeletal muscles of the body.  If it works the same way in humans, this virus-based approach may allow the first effective gene therapy for muscular dystrophy (MD) and similar diseases. 

The study was funded in part by the National Institute of Neurological Disorders and Stroke (NINDS).

The most common approach to gene therapy uses viruses as vehicles, or vectors, that transport a beneficial gene into cells.  Viruses normally insert their genes into cells and then use the cells' machinery to replicate themselves.  Scientists have learned to manipulate the viral DNA to remove disease-causing genes and add beneficial ones.  Different viruses target different types of cells and tissues, so researchers carefully test each potential vector to learn how it works. 

Previous research in animal models of MD has successfully shown long-term expression of beneficial genes with intramuscular injections, but these methods have only affected the muscles near the injection site.  Investigators have tried using intravenous (vein) injections to get more widespread gene delivery, but the tiny blood vessels in muscles keep viruses from entering the muscle tissue unless researchers manipulate the situation using high pressure or a drug that makes the blood vessels leaky.  These approaches increase the risk of side effects.

In the new study, published in the March 2005 issue of Nature Biotechnology,[1] Xiao Xiao, Ph.D., and colleagues at the University of Pittsburgh School of Medicine tested several types of AAV, including two recently identified varieties called AAV7 and AAV8.  By introducing the gene for a marker called green fluorescent protein (GFP), they showed that injecting several types of AAV into the abdomen or the blood vessels of mice led to gene delivery in muscles far from the injection site.  AAV8 was the most efficient vector tested; it led to GFP production throughout the body, including the forelimb, face, and heart muscles.  The researchers found similar results with intravenous injections in hamsters. 

The treatment worked without using high pressure or drug therapy to move the AAV out of the blood vessels and into the muscle tissue.  In newborn mice, it also affected muscles without any lasting effects on other organs such as the brain.  In older animals, however, the gene became active in organs such as the liver, pancreas, and gonads as well as the muscle tissue.

The researchers also tested AAV8 to see if it could deliver a beneficial gene, delta-sarcoglycan, in a hamster model for one type of limb-girdle MD.  In young adult hamsters, intravenous injections of AAV8 carrying the delta-sarcoglycan gene led to strong expression of the delta-sarcoglycan gene in the heart and leg muscles.   The therapy also corrected the degeneration and other problems normally seen in muscles affected by this disease. 

Limb-girdle MD is one of nine major types of muscular dystrophy.  It causes weakness and wasting of muscles, starting with those around the shoulders and hips, and sometimes leads to heart problems.  While the investigators chose to study a gene for limb-girdle MD in their initial tests, gene therapy using the AAV8 virus may also be beneficial for other types of MD.  "For muscular dystrophy, there currently is no treatment that can halt the disease," says Dr. Xiao.  Because the vector also targets heart muscle, it might be useful for treating a heart problem called cardiomyopathy. 

"This is an important advance for gene therapy in muscular dystrophy—a new AAV vector that provides efficient gene targeting to skeletal and cardiac muscle," says John Porter, Ph.D. the NINDS program director for Dr. Xiao's grant.  Dr. Xiao’s studies are among the first in the muscular dystrophy field to receive support through a novel NINDS translational research mechanism designed to accelerate the development of new therapies.

As with all therapies, gene therapy using AAV to treat MD has potential dangers, Dr. Xiao says.  Since the amount of the vector needed is relatively high, researchers can't predict what immune response the body may launch against the virus.  Also, there is a small possibility that some part of the vector could integrate into the chromosomes and cause new gene mutations.  However, these risks are small, and clinical trials of AAV vectors so far have shown no obvious problems, he adds.

Dr. Xiao and his colleagues now plan studies to optimize the AAV8 vector by modifying its gene sequence.  They hope to improve its ability to deliver genes to muscle tissue while minimizing or abolishing its infectivity in the liver and other organs, he says.  They also plan to study AAV8 in a dog model for Duchenne MD to learn if it can effectively treat disease in large animals.  If those experiments are successful, researchers might eventually be able to test AAV8-based gene therapies in human clinical trials.   

The NINDS is a component of the National Institutes of Health within the Department of Health and Human Services and is the nation’s primary supporter of biomedical research on the brain and nervous system.

-by Natalie Frazin

Date Last Modified: Wednesday, January 31, 2007

---