A gene therapy for Parkinson's disease seems safe and may help improve movement in people with the disease, a small trial suggests.

Parkinson's is a neurodegenerative disease that affects about one in every 200 people worldwide, leading to muscle tremors, stiff limbs and weakness.

Researchers in the U.S. used a virus to inject a gene directly into the brain cells of 11 men and one woman with Parkinson's with an average age of 58. The participants had severe Parkinson's for at least five years, and current therapies no longer worked for them.

The gene therapy was intended to calm the excessive firing of neurons that occurs in Parkinson's, by adding the glutamic acid decarboxylase or GAD gene. 

All of the people had symptoms on both sides of the body. The procedure was done to one side of the brain, with the untreated side acting as a control.

The results showed the gene therapy can be done safely in the human brain, with no evidence of side-effects after the surgery and for one year after treatment, Prof. Matthew During of Weill Medical College of Cornell University in New York and his colleagues reported in the June 23 issue of the Lancet.

Although the trial was a safety study and was not done to show effectiveness, the researchers said participants showed improvements in movement within three months of the procedure.

Brain scans showed metabolic changes like those seen after surgery for Parkinson's. The benefits were similar to those seen in deep brain stimulation, another therapy for the disease that targets the brain's subthalmic nucleus.

The researchers said the gene therapy approach avoids implants, adjustments and risk of infection that come with deep-brain stimulation.

"Apart from the avoidance of stimulator adjustments and potential hardware problems, what is the real advantage of this approach compared with deep-brain stimulation of the subthalmic nucleus?" asked Dr. Jon Stoessl of the Pacific Parkinson's Research Centre at the University of British Columbia in a commentary accompanying the study.

"During and colleagues have taken a proactive approach to the treatment of neurodegenerative disease, and should be congratulated for their circumspection in the description of the potential implications of their findings.

"They made an important step in showing proof of principle, but much work should be done before neurologists and neuroscientists will regard this therapy as an effective approach."

The study was funded by Neurologix, a company co-founded by two of the study's authors.