Demonstrating the possibility that some permanent forms of blindness can be reversed, scientists have restored sight to three dogs suffering from a rare genetic eye disease.

"We hope we have the same biology in humans," said Albert Maguire, an ophthalmic surgeon at the University of Pennsylvania's Scheie Eye Institute, one of the principal authors of the study. Maguire said that in the best of circumstances it would likely take three to five years before researchers could begin human trials.

Scientists used a form of gene therapy, injecting one eye in each of three dogs with a healthy form of the gene that was defective. The researchers used a modified virus to deliver the engineered gene, essentially infecting cells with the normal version.

The dogs had a canine version of Leber Congenital Amaurosis (LCA), which has been found in purebreds that have been subject to too much inbreeding. The Penn team used a breed called Swedish Briards.

LCA also is a rare disease in humans, affecting several thousand people in the United States. But the research might have further applications.

"This is specifically for one type of retinal degeneration, but the implications are serious for any genetic eye disease," Santa Tumminia, a specialist in diseases of the retina at the Foundation Fighting Blindness in Hunt Valley, Md., said yesterday. The class of hereditary diseases for which the procedure might be investigated is known as retinitis pigmentosa, which affects 100,000 to 200,000 Americans.

The researchers, led by Scheie's Jean Bennett, have published their findings in today's issue of Nature Genetics.

The faulty gene causes LCA by preventing the body from forming a critical protein, one that in healthy people transports a form of Vitamin A to the retina. This causes a loss of vision at birth.

A few experiments had attempted to inject the protein directly into the eye, Maguire said, but these met with marginal success. A better strategy seemed to be to administer the gene that would allow the dogs to make the protein themselves.

Maguire injected the gene into cells behind the retina six months ago. A week after the surgery, he said, they were able to see perhaps half or two-thirds as well as a normal dog.

The dogs, blind since birth, could now see well enough to navigate around objects. Now, more than six months since the treatment, they have maintained that ability and seem happy, Maguire said.