Two patients with a rare and fatal brain disease have been successfully treated using an experimental gene therapy, French scientists say.

Researchers used a disabled form of HIV to transfer working genes into two boys with X-linked adrenoleukodystrophy or ALD — a rare, hereditary condition featured in the 1992 movie Lorenzo's Oil.

The disease is caused by a deficiency of a protein needed to degrade fatty acids. People with ALD gradually lose the myelin sheath, a protective layer coating the brain's nerve fibers. The disease leads to blindness, deafness, progressive dementia, seizures and loss of muscle control.

The two-year study appears in Friday's issue of the journal Science.

The two boys showed the same degree of improvement they would have with a bone marrow transplant, the usual way the disease is treated, said lead researcher Dr. Patrick Aubourg of the University Paris-Descartes.

Bone marrow transplants help people with ALD by allowing new myelin-forming stem cells to grow, but it is difficult to find a match and the transplant procedure is risky.

Auburg's team used gene therapy to eliminate the ALD mutation in the bone marrow stem cells of the seven-year-old boys.

A healthy version of the gene was attached to a disabled version of HIV, which delivered the gene after harmlessly infecting cells.

'Only the beginning'

The healthy ALD protein was produced in about 15 per cent of the cells. It was enough to slow ALD, but that may not be sufficient for other diseases, Aubourg cautioned.

"There is a lot of work to be done to make this gene therapy vector more powerful, less complicated and less expensive. This is only the beginning," he said in a release.

Gene therapy research suffered a setback 10 years ago when an American teenager died in one experiment, and two boys got leukemia after treatment for severe combined immunodeficiency (SCID) or "bubble boy disease" in 2002.

The research was funded by the National Institute of Health and Research Medical, Assistance Publique des Hôpitaux de Paris, PHRC programs, the Deutsche Forchungsgemeinschaft and the German Ministry of Education and Research, the European Leukodystrophy Association, the Association Française contre les Myopathies, the Stop ALD Foundation and University Paris-Descartes.