He says, however, that results would take a while to see, since the virus takes time to fully insert the replacement genes into the cells of the retina. "That process takes something on the order of several weeks to several months."
But he says that if the procedure turns out to be effective, is should be performed in patients "as soon as you can identify the people."
"It's a very exciting time for the science of genetic diseases," says Sieving. "The issue is to get the science converted to practice so it can benefit you, my children, everybody."
He adds that what is happening now will also help determine the safety of putting a gene and viral vector into the eye.
"I am presuming that this is safe and usable for other genes," he said. "This opens the door to treatment of a whole repertoire of eye diseases."
Several ophthalmology experts refused to comment on the procedure, some saying it was too early to tell whether the approach would show success in a human patient.
And Bensinger says the procedure comes attached with considerations that could make it difficult to achieve the desired outcome.
"The difficulty in gene therapy is the lack of ability to insert the normal gene into the existing DNA of the recipient," Bensinger says, adding that the treatment normally causes the death of cells in the retina.
"The retinal cells do not reproduce after infancy, which makes gene transfer more difficult, as a virus-infected cell is likely to die -- not the desired result since it cannot replace itself," he says. "The bottom line is that more details are needed to analyze this. It is probably a 'first,' but that does not mean it is likely to succeed."
There is also the question of safety. While some patients have benefited from gene therapy, the 1999 death of an 18-year-old patient receiving gene therapy for a metabolic disorder caused many to reconsider the approach.
Johnson's procedure, however, differs from some other gene-therapy procedures performed in the past, as it deals with a very small area of the body -- namely, a spot of nerves on the back of the eye. This may make it safer than some gene therapy procedures that affect the entire body.
Despite the uncertainties, Sieving says the treatment could have implications for a number of other genetic diseases if successful.
And many of these treatments could start in the eye. According to the National Eye Institute, nearly 500 genes that contribute to inherited eye diseases have been identified over the past 15 years. These genes are associated with conditions ranging from glaucoma and cataracts to retinal degeneration.
Sieving believes that this most recent procedure will just be the first of several that will come out over the next few years. He says a comparable trial for retinal treatment is slated soon in the United States at the University of Pennsylvania, and he adds that gene therapy solutions for other conditions may be on the way.
"Whatever transpires in Moorfields will likely be replicated and vetted elsewhere," he says. "I think that we should celebrate a success story here. We, of course, have to make sure that this is a success, but even to get here, to this point, is a remarkable story."