The following article by Laura Spinney appears in BioMednews Reports (6- 25-99,#57)
Glycogen storage disease type II causes enlargement of certain muscles, particularly the heart and tongue, due to their inability to break down glycogen. A team of American scientists is developing a method of delivering the enzyme required for that breakdown to all the body’s muscles with a single intravenous injection of adenovirus. They presented their findings at the Second Annual Meeting of the American Society of Gene Therapy in Washington, D.C., held June 9-13, 1999.
Glycogen storage disease type II (GSD-II) is an autosomal recessive disorder in which the enzyme acid alpha-glucosidase (GAA), which normally clears glycogen from the muscles, is missing or reduced. The disease occurs in neonatal, juvenile, and adult forms with high mortality, and babies born with it usually die by the age of 18 months. In the lethal neonatal form, GSD-II first shows itself as a general weakness and susceptibility to colds and pneumonia. But the hallmark is a massively enlarged heart, and occasionally tongue, caused by glycogen buildup. There is currently no cure. GSD-II, along with Duchenne muscular dystrophy, is a perfect example of the kind of disease that would be amenable to virus-mediated gene therapy. Any potential treatment would have to reach all the muscles of the body, including the notoriously inaccessible cardiac muscle, to be effective. That means either multiple injections of the enzyme into different muscle types, or else some kind of systemic delivery. At Duke University Medical Center in Durham, North Carolina, Dr. Andrea Amalfitano and his colleagues have been testing both strategies. Clinical trials for the direct enzyme therapy got underway last week. But the approach that may hold out most promise in the long run involves systemic delivery of GAA via a modified adenovirus vector that Dr. Amalfitano’s team has been studying in a mouse model of GSD-II.
The idea is simple, and requires the organs of the affected individual to do most of the work. An adenovirus that has been rendered harmless by the removal of two genes (called E1 and E2b) that it needs to replicate, but that is still capable of infecting cells and transforming their DNA, is loaded with the gene that encodes for GAA and then injected into a vein. Once in the bloodstream, the virus infects cells, migrating naturally to the liver where the transgene is expressed and the enzyme product is secreted back into the blood. Thereafter it travels to all the muscles in the body. “We transform the liver into an enzyme factory,” says Amalfitano. And because the adenovirus is so infectious, a single injection is capable of transducing all the cells of a particular organ, in this case the liver.
Within twelve days of the injection, the researchers measured GAA activity in the livers of the treated mice and found that it had increased 100-fold. Levels of a precursor of the enzyme circulating in the blood had also risen. The muscles of the leg showed a 10-fold increase in GAA expression while in the diaphragm the increase was 100-fold. Even the heart muscle showed quadrupled GAA activity. But most importantly staining for glycogen revealed that it had been reduced to a normal, healthy level. “There’s nothing out there that can deliver genetic information as efficiently as adenovirus,” says Amalfitano.
But he says that although the findings are promising, there is a lot more work to be done. For instance, his team found some evidence of an immune response to either the vector, the transgene, or both that could herald possible problems to explore the treatment’s long term consequences, including the virus’s ability to survive in vivo, the duration of the GAA enzyme’s activity once it enters the muscle, and the speed with which glycogen builds up again after the end of the treatment.