A gene therapy that increases the size and strength of muscle tissue could soon be used to treat neuromuscular disorders such as muscular dystrophy, according to scientists.

In a study on monkeys, the treatment, which modifies the body’s natural regulation of muscle growth, was found to have long-lasting effects on muscle mass and tone.

The treatment produced no obvious negative side-effects and clinical trials are expected to start next year.

If similar results are seen in human trials, the technique could transform treatments for a range of muscle-wasting diseases.

“If we can improve the strength of muscles we can make a difference to the lives of these patients,” said Professor Jerry Mendell, a specialist in muscle disease at Ohio State University and a co-author on the study.

The technique works by manipulating the action of a natural protein called myostatin, which regulates muscle growth.

In infancy and childhood, when muscles are continuously developing, myostatin is almost absent, but as a stable muscle mass is reached in adulthood, myostatin begins inhibiting muscle growth.

Scientists discovered that a second protein called follistatin can bind to myostatin and prevent it carrying out its role as a muscle growth inhibitor. By artificially introducing a follistatin-producing gene into the body, muscle growth can be encouraged.

In the study, published in the journal Science Translational Medicine, six macaque monkeys received the treatment in their thigh muscles. A common cold virus was used to carry the follistatin gene into muscle cells, where it slotted into the muscle cells’ DNA.

Once in place, the gene produced follistatin protein, which paved the way for increased muscle growth.

After the treatment, the monkeys’ leg muscles grew steadily, and were 15 per cent bigger in circumference on average after eight weeks.

Using electrical muscle stimulation, the scientists showed that the treated legs were also significantly stronger than the untreated legs. In one monkey, the treated leg was 78 per cent stronger.

The enhanced size and power were retained 15 months after the treatment and caused no apparent health problems.

While this therapy would not offer a cure for hereditary conditions such as Duchennes Muscular Dystrophy (DMD), it could limit the impact of disease and allow patients to retain the ability to walk and use their arms for much longer.

The researchers have now applied to test the therapy in patients with muscle disorders.

Professor Dominic Wells, a DMD specialist at Imperial College London, described the findings as very promising but said that one drawback was the need for patients to take immunosuppressant drugs alongside the therapy.

This can be a particular problem for diseases such as DMD, which affects the lungs and leaves patients prone to respiratory infections.

It will also have to be established that increasing muscle mass is effective in treating diseases such as DMD, which affect muscle quality.

“If you don’t address the underlying genetic defect then you could just get a larger amount of weak muscle,” said Professor Wells.

He said it was likely that the full benefit of the technique would only be seen in combination with other treatments.

Marita Pohlschmidt, director of research at the Muscular Dystrophy Campaign, said: “If proven to be safe and efficient, reducing the activity of myostatin could potentially be used to increase muscle strength for people with a broad range of neuromuscular conditions.

“These results, shown in healthy monkeys, are encouraging and represent another step forward in taking this therapeutic approach to clinical trial.”

Although the monkeys appeared to suffer no serious side-effects, Professor Wells said that the treatment would be unlikely to be attractive to athletes wishing to enhance their performance, because high levels of follistatin would be easily detected.

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