MDA supports the most comprehensive research program in Duchenne dystrophy in the United States, allocating some $3 million a year to this endeavor.

Gene therapy for DMD has been high on MDA's priority list since the early 1990s and continues to command a large portion of MDA's resources. It's hoped that clinical trials of gene therapy involving boys with Duchenne will begin by 1999. Such trials will likely test limited aspects of gene therapy, such as its safety.

The concept is a broad one, however, and the obstacles to gene therapy of muscle are currently not well understood.

GENE THERAPY BY INSERTING THE DYSTROPHIN GENE

The original goal of gene therapy for DMD or any genetic disorder was straightforward (not the same as "simple"): It was to insert a new gene that would compensate for the existing, flawed gene.

In DMD, the gene in question is, of course, the one for the muscle protein dystrophin. MDA hopes a gene that can produce dystrophin would rescue any muscle tissue that hasn't degenerated too far. It's not clear whether gene therapy would merely halt the progress of DMD or whether it could actually restore any degree of lost function.

Delivering this gene poses some formidable obstacles as numerous studies in animals have shown over the last several years. For one thing, the dystrophin gene is among the largest genes ever identified. Its size makes it hard to work with and hard to insert inside "transport vehicles," such as viruses, that gene therapists use to get genes to targeted tissues.

This problem has nearly been solved, in two ways. First, researchers have created dystrophin "minigenes," which carry instructions for a slightly smaller version of dystrophin, but which can fit inside a virus nicely. The proteins these minigenes can make would probably improve, although not cure, DMD, if they could be effectively delivered to enough muscle tissue and if the genes survived to produce the shortened dystrophin forms.

Second, researchers have also created the so-called gutted virus, a virus that's had its own genes removed so that it's carrying only the dystrophin gene. This "gutting" of the virus lets it take on the full-length dystrophin gene. Experiments indicate this hollow virus can probably function well enough to deliver dystrophin genes.

But the size of the dystrophin gene isn't the only problem with gene therapy of muscle in Duchenne. The problem is also that the muscle tissue itself is so large and relatively impenetrable.

Getting new genes to a large enough area of the body, including the heart and respiratory muscles, which pose some of their own problems, is a challenge that doesn't concern researchers who only need to get a gene into the bloodstream or skin cells.

Many animal experiments to test the effectiveness of gene therapy have used intramuscular injections. But covering the area of a mouse's leg is different from doing so in the area of a human leg, and intramuscular injections don't seem a practical solution for gene delivery in boys with DMD.

Researchers are now working on ways to deliver the genes systemically -- through the bloodstream. The genes will have to be carefully targeted so they get into muscle and, at least for the most part, don't get into other body tissues.

As if that weren't enough, there is still the immunologic problem that plagues researchers trying to transfer muscle cells from one animal to another. Genes by themselves don't normally provoke the immune system; but genes by themselves don't go very far either. At this time in the development of gene therapy techniques, genes have to be inserted into viruses in order to travel to muscle and enter muscle fibers when they get there. (There are researchers working on non-viral methods of delivery, however.) Most viruses provoke the immune system and can cause it to kill muscle fibers that have taken up the new genes. (If the immune system didn't kill viruses, we'd all suffer from serious or even fatal viral infections.)

And some researchers wonder what will happen in the immune systems of boys who have never had any dystrophin if they suddenly start making some. Will the immune system see the dystrophin as "foreign," and try to destroy it and maybe the cell around it? (That's what the immune system normally does with proteins it doesn't recognize as "self.")

This question hasn't yet been fully answered, but it's something that has to be taken into consideration in careful studies to test the safety of gene therapy.

Immunosuppression using drugs like cyclosporine (Sandimmune) and a newer drug known as FK506, or tacrolimus (Prograf), is possible, but will it be effective? And, more important, will it be safe? These questions also need to be answered.

Next... Other Kinds of Gene Therapy