WASHINGTON (Reuters) – A patient with a rare genetic form of anaemia is getting by without blood transfusions after experimental gene therapy, French and US researchers reported last week.

The case, reported in the journal Nature, is a rare success for the troubled field of gene therapy, although the researchers and other experts said it still needs fine-tuning.

The patient has beta-thalassemia, a group of conditions caused by genetic defects in the production of hemoglobin. Researchers used gene therapy to fix the faulty gene responsible for the condition in some of the patient's own bone marrow stem cells, and re-infused them.

Nearly three years later, the patient has been healthy without the need for the usual blood transfusions, the team at Brigham & Women's Hospital and Harvard Medical School in Boston and University of Paris reported.

"For the first time, a patient with severe beta-thalassemia is living without the need for transfusions over a sustained period of time," Dr. Marina Cavazzana-Calvo of the University of Paris and the Necker Hospital said in a statement.

Gene therapy is a promising but difficult approach to medicine, based on the idea that faulty genes that underlie many diseases can be corrected.

Researchers have struggled to find good ways to get the repaired genes back into the body safely. Viruses are usually used to carry the genes into the body and they can themselves be dangerous.

One experiment cured two French boys with a rare immune disorder but gave them leukemia in 2002, and an Arizona teenager died in a 1999 gene therapy experiment, evidently from an immune system overreaction to the virus that was used.

But researchers at Children's Hospital of Philadelphia have used gene therapy to help improve worsening eyesight in 12 patients with a rare blinding disease called Leber congenital amaurosis, or LCA.

For the beta-thalassemia study, the researchers used a modified AIDS virus to repair the DNA in the patient's bone marrow stem cells. A small, Massachusetts-based biotech company called Bluebird Bio is developing the approach as a product called LentiGlobin.

In an unusual step, they first destroyed the patient's bone marrow – a method used before giving patients bone marrow transplants for cancer and other conditions.

The patient got the first transfusion in 2007, when he was 18, and again in 2008. "At present, approximately three years post-transplantation, the biological and clinical evolution is remarkable, and the patient's quality of life is good," the researchers wrote in Nature.

Dr. Derek Persons of St. Jude Children's Research Hospital in Memphis, Tennessee, said destroying the patient's bone marrow appears to have been crucial to making the treatment work.

"Although, three years on, he remains mildly anemic and shows signs of compensatory expansion of red-blood cell precursors in his bone marrow, absence of the need for blood transfusions means that this case can be viewed as a clinical success," Parsons, who was not involved in the research, wrote in a commentary.

Beta-thalassemia is diagnosed in about 60,000 children globally each year.

It is caused by defects in a gene involved in making hemoglobin, the component that carries iron in red blood cells, and patients generally need monthly blood transfusions to treat severe anaemia and then special therapy called chelation to clear out the extra iron buildup caused by the transfusions.