Scientists for the first time have used gene therapy to dramatically improve sight in people with a rare form of blindness, a development experts called a major advance for the experimental technique.

Some vision was restored in four of the six young people who got the treatment, teams of researchers in the United States and Britain reported Sunday. Two of the volunteers who could only see hand motions were able to read a few lines of an eye chart within weeks.

"It's a phenomenal breakthrough," said Stephen Rose, chief research officer of the Foundation Fighting Blindness, which helped pay for one study done at Children's Hospital of Philadelphia.

If successful in larger numbers, experts said, the technique has the potential to reverse blindness from other kinds of inherited eye diseases.

"I think this is incredibly exciting," said Dr. Jean Bennett, a professor of ophthalmology at the University of Pennsylvania and a leader of the Philadelphia study. "It's the beginning of a whole new phase of studies."

The research was published online Sunday by the New England Journal of Medicine in conjunction with presentations at a medical meeting in Florida.

The two teams of scientists, working separately, each tested gene replacement therapy in three patients with a form of a rare hereditary eye disease called Leber's congenital amaurosis. There's no treatment for the disease, which appears early in infancy and causes severe vision loss, especially at night.

An estimated 2,000 Americans have the form of the disease they targeted, Bennett said.

Gene therapy _ replacing faulty genes with a normal version _ has been studied in humans for over 15 years with limited success. The field suffered a setback with the 1999 death of Jesse Gelsinger, 18, in an experiment for a liver disorder at Penn. And some children treated for an immune disorder called the "bubble boy disease" later developed leukemia.

The early results of the eye experiments should give the field a boost, some experts said.

"I think it's really a big shot in the arm for gene therapy and for medicine in general," said Dr. Ronald Crystal, head of genetic medicine at Weill Cornell Medical College in New York.

Each of the study participants had mutations in a gene that makes a protein needed by the retina, which senses light and sends images to the brain. Those without the gene gradually lose sight until they are blind in early adulthood.

The retina itself stays in relatively good shape for a while, making it a good candidate for gene therapy, said Robin Ali, a professor at University College London, who led the British team. He likened the defective gene to a missing spark plug in a car engine.

"The whole engine can be absolutely fine, but if it doesn't have a spark plug, the car's not going to work," said Ali.

For the experiment, the scientists injected millions of copies of a working gene beneath the retina in the back of the eye. Only one eye was treated _ the worst one _ in case anything went wrong; the untreated eye was used for comparison. After the treatment, their eyesight and light sensitivity were measured periodically; mobility was tested in a maze or an obstacle course.

All three of those treated in Philadelphia showed significant improvement in their vision, the researchers said. The volunteers _ two women, 19 and 26, and a man, 26 _ were from Italy, where they had been screened by researchers there. The longest follow-up was six months.

Besides reading lines on an eye chart, they could see better in dim lit, Bennett said.

"We were not expecting to restore their vision to 20/20," she said.

In the British group, the treatment only worked in 18-year-old Steven Howarth, whose disease was less advanced than the other two _ a girl, 17; and a man, 23, who was followed for a year.

Howarth said he used to rush home from school because he was worried about getting around in the dark, according to remarks issued by the university.

"Now, my sight when it's getting dark or it's badly lit is definitely better. It's a small change _ but it makes a big difference to me," said Howarth, who lives in Bolton, near Manchester.

After the injection last July, Howarth said his eye felt like sandpaper. It was better after a week, and his eyesight gradually improved. He was able to negotiate a dimly lit maze in 14 seconds without bumping into any obstacles; before it took him 77 seconds with eight errors.

There were no serious side effects reported in either group. One of the patients in Philadelphia developed a hole in his retina which didn't affect his eyesight. The researchers think the hole was related to the surgery and not the injected gene.

The researchers said there was no evidence that the altered virus used to ferry the gene into the retina's cells had traveled outside the eye to other areas of the body.

The groups have each treated a fourth patient, including a preteen in England. The researchers hope to see better results with higher doses and in younger patients with less eye damage.

The National Eye Institute is funding a third similar study at the University of Florida.

The research in Philadelphia and London was paid for by a variety of government agencies and private foundations. An employee of Targeted Genetics Corp., which made the altered virus used in London, is a co-author of their report. Four of the Philadelphia researchers, including Bennett, have either applied for or have patents related to gene therapy. Ali and another British researcher have also applied for a patent for the procedure.

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AP Science Writer Malcolm Ritter contributed to this report.

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On the Net:

New England Journal of Medicine: http://www.nejm.org

National Eye Institute: http://www.nei.nih.gov