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By RANDOLPH E. SCHMID, AP Science Writer

WASHINGTON -- Scientists have taken a new step in the quest for gene therapy for muscular dystrophy. Three years after researchers experimenting with mice discovered that gene therapy might one day work, another team has found a way to deliver such therapy throughout the body.

University of Pittsburgh researchers report they were able to deliver a miniature gene, similar to the defective gene involved in muscular dystrophy, throughout the bodies of mice, showing that systemic therapy is possible.

Their findings are reported in the online edition of Proceedings of the National Academy of Science.

In 2002, researchers at the University of Washington reported successful treatment of muscular dystrophy by a virus containing a gene that produces a protein lacking in people with the disease. However, they found that virus had to be injected directly into each muscle.

While the new study by the Pittsburgh team shows systemwide gene treatment can be done, the scientists weren't able simply to replace the defective gene with a good version because the gene itself is so large it couldn't fit inside the disabled virus used to deliver it.

Instead, the researchers used a miniature gene, similar in function to the defective one, and showed it could be delivered to muscles throughout the mouse, and the animals exhibited some improvement.

"While we have much farther to go until we can say gene therapy will work in children, we have shown here a glimmer of hope by presenting the first evidence of a successful gene therapy approach that improved both the general health and longevity in mice with congenital muscular dystrophy," said Dr. Xiao Xiao, associate professor of molecular genetics and biochemistry.

Co-author Dr. Chungping Qiao said in a statement that it's probably unrealistic to expect complete success using the mini-gene even though it is similar to the defective one.

Future research will seek a way to deliver correct versions of the defective gene throughout the body.

The National Institutes of Health describes muscular dystrophy as a group of genetic diseases characterized by progressive weakness and degeneration of muscles that control movement.

The Mayo Clinic reports that the most common muscular dystrophies appear to be due to a genetic deficiency of the muscle protein dystrophin. The various types of the disease affect more than 50,000 Americans. There is no cure, but medication and therapy can slow the disease.

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On the Net:

Proceedings of the National Academy of Sciences: http://www.pnas.org

National Institute of Neurological Disorders and Stroke: http://www.ninds.nih.gov