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A few years ago, scientists created a genetically modified mouse with huge muscles. It became known as Mighty Mouse. Now, researchers have used a similar approach to produce muscle-bound monkeys.

As NPR's Jon Hamilton explains, this brings scientists a step closer to new treatments for people with diseases such as muscular dystrophy.

JON HAMILTON: All of this muscle research has to do with a substance called myostatin. Normally, myostatin in the body prevents muscles from getting too big. Take it away and muscles can become huge. So, researchers have been looking for a way to block the effects of myostatin in people with muscle-wasting diseases. You could do that with regular injections of a drug. Another way is to ramp up the body's own ability to block myostatin.

Brian Kaspar of Ohio State University is part of a team that's done that by putting an extra dose of genes right where they're needed.

Dr. BRIAN KASPAR (Professor of Pediatrics, Ohio State University): Our approach has been to develop a one-time gene delivery, given it's a simple shot into the muscle.

HAMILTON: And it works great in mice. But of course, lots of things that work in mice don't work in people.

Jerry Mendell is another member of the gene team. He directs the Center for Gene Therapy at Nationwide Children's Hospital in Columbus.

Dr. JERRY MENDELL (Director, Center for Gene Therapy, Nationwide Childrens Hospital): So, we decided that the best way to test our gene therapy model was to take it to the monkey. And if it worked, it had a better chance of working in patients.

HAMILTON: The team injected the right thigh muscles of six macaque monkeys. Kaspar says researchers didn't have to wait long.

Dr. KASPAR: We found that muscles started to get larger in circumference. And when we measured the strength generated out of those muscles, indeed the treated muscles were significantly higher than untreated muscles.

HAMILTON: The success means the team is ready to move on to people. Mendell says they want to begin with patients who have a disease called inclusion body myositis. It leaves thigh muscles so weak, people can't stand up.

Dr. MENDELL: If everything works out, we should be in a clinical trial by next summer.

HAMILTON: That would be a big step. But even cautious scientists have been impressed by the results in monkeys.

Dr. SE-JIN LEE (Professor of Molecular Biology and Genetics, Johns Hopkins University): Certainly very intriguing, very interesting, potentially very exciting.

HAMILTON: That's Se-Jin Lee of Johns Hopkins University. He's the scientist who discovered myostatin. Lee says many different groups have now found ways to manipulate the myostatin pathway and produce muscle growth in a wide range of animals. He says that bodes well for treating people.

Dr. LEE: Tampering with this pathway will probably work in terms of increasing muscle growth. You know, that still is hypothesis. It definitely has not been proven, but there are more and more signs that that probably will work.

HAMILTON: And not just on sick people. Athletes and bodybuilders have been following the research closely. Some of them are looking for a new kind of doping. Lee says it would be pretty hard for these would-be dopers to make drugs that block myostatin.

Dr. LEE: The scary part is that that's just not true in the case of gene therapy. It's a fairly easy thing for a, you know, a small group to set up and produce, and you only need to give essentially a single injection, presumably, or a limited number of injections and it's there forever.

HAMILTON: The World Anti-Doping Agency is already working on a test for this sort of doping. The new research appears in the journal Science Translational Medicine.

Jon Hamilton, NPR News.

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