Scientists have improved the vision of a small number of patients suffering from a rare and incurable eye disease by replacing a defective gene with a healthy one-a boost for a technique known as gene therapy.

The patients have choroideremia, a degenerative disease caused by defects in a single gene that leads to blindness and affects 1 in 50,000 people. In an early-stage trial published Wednesday in the Lancet, the researchers used a deactivated virus to safely ferry billions of healthy, lab-made versions of the gene into the retina. That appeared to restore the function of light-sensitive cells, which the disease impairs.

'We were surprised by the magnitude of vision improvement' in the patients, said Robert MacLaren, a professor of ophthalmology at the University of Oxford in England and leader of the clinical trial.

The experiment marks one of the first times that gene therapy has targeted the main light-sensing cells in the retina. It thus offers a possible route for treating far more common causes of blindness that affect the same cells, such as retinitis pigmentosa and age-related macular degeneration.

Gene therapy involves the use of DNA, rather than a protein or drug, to treat an ailment. The idea is that if the DNA gets properly embedded in the target cells, it can potentially remain there indefinitely and deliver benefits for a long time.

Gene therapy fell out of favor after a handful of early studies led to cancer and death. That's a big reason why it hasn't yet led to a single authorized treatment in the U.S.

Now, the technique is making a comeback. For example, in three early-stage clinical trials done a few years ago, the technique was used against a retinal disease called Leber congenital amaurosis. And in November 2012, European regulators approved a gene therapy treatment for a rare condition that leaves patients unable to properlydigest fats-the first such approval in the Western world.

The ailment treated in the Lancet study, choroideremia, is caused by defects in a single gene on the X chromosome and mainly affects boys. Many start losing night vision by age 10 and become legally blind in their 40s.

Because of the defective gene, light-sensitive cells in the retina slowly stop working and then die. Prof. MacLaren's team decided to make healthy versions of the gene in the lab, load each onto a small virus (one that doesn't cause disease in people) and inject the mix under the retina.

The therapy is given in one eye so it's easy to compare the progression of the disease with the untreated eye. 'Every injection has 10 billion viral particles, each carrying one copy of the gene,' said Prof. MacLaren. 'We have to target millions of cells.'

The trial began with six patients. Two still had excellent visual acuity-clearness of vision-which is measured by reading lines of letters on a sight chart. Two other patients had good acuity and two had reduced acuity.

Six months after the operation, the two patients with reduced acuity showed improved vision, being able to read two and four more lines on the sight chart. The others could see better in dim light. The gains were sustained over several months of follow-up.

A 65-year-old in the trial said that when he now watches a soccer game on TV the 'green of the pitch is brighter and the numbers on the shirt much clearer.' Another who went through the procedure says he can now see stars in the night sky, which he hadn't seen for a long time.

Altering a patient's DNA is risky because it can trigger dangerous side-effects. In this case, there was no sign of an immune reaction in the first six months of the follow-up, according to the Lancet study.

The scientists hope to treat patients before their sight falters. 'We want to preserve the vision they've got,' said Prof. MacLaren. He now plans to test the technique on a larger group of about 30 patients.