A gene-therapy experiment in monkeys has shown the ability to boost muscle size and strength, giving hope to those looking for treatments for muscular dystrophy and other diseases that cause muscle loss and weakness.

A gene-therapy experiment in monkeys has shown the ability to boost muscle size and strength, giving hope to those looking for treatments for muscular dystrophy and other diseases that cause muscle loss and weakness.

The study was led by Brian Kaspar and neurologist Dr. Jerry Mendell, both researchers at Nationwide Children's Hospital.

Previous studies in mice with muscular dystrophy had shown promise, but tests in an animal more closely related to humans were important before trials in people begin, said Mendell, who heads the Center for Gene Therapy at the hospital's research institute. The researchers published their results last month in the journal Science Translational Medicine.

Based on the lasting and significant improvements - and no documented negative side effects - seen in the study of six macaque monkeys, Mendell said he expects a human study to begin by next fall.

The monkeys each received injections of genes for a protein called follistatin into one of their upper leg muscles. In all cases, the injected legs became stronger and more muscular. The theory behind the therapy is that follistatin stops a protein called myostatin from turning off muscle growth, allowing diseased muscle to become stronger and larger.

One limitation of the experiment is that the animals were healthy, and therefore had healthy muscle. Previous research in mice showed that results were sustainable in animals that were sick.

It's also impossible to know how long the results would have lasted because the monkeys were euthanized, two of them after five months and four of them after 15 months. Researchers examined their bodies for any damage related to the therapy and found none.

"It's a very unequivocal result," said Sharon Hesterlee, the Muscular Dystrophy Association's senior vice president for MDA Venture Philanthropy, the organization's drug-development arm. "This is the first time we have real evidence in an animal that is very similar to humans that you can make muscles bigger and stronger." The only current treatment for muscular dystrophy is steroids, and they provide limited benefit, Hesterlee said.

Approval for routine use in patients with neuromuscular disorders depends on successful human trials and approval from the U.S. Food and Drug Administration. That will take several years, Mendell said.