The technique involves sneaking a gene into the retina to correct a faulty stretch of DNA causing a degenerative sight disease called choroideremia. (Agencies)
Six months after the treatment, patients showed improvements in vision in dim light, and two of the six were able to read more lines on the eye chart, researchers reported in The Lancet medical journal.
One patient was stunned to able to see stars in the summer night sky for the first time in years.
"It is still too early to know if the gene therapy treatment will last indefinitely," cautioned Robert MacLaren of the Nuffield Laboratory of Ophthalmology at the University of Oxford, who led the work.
"But we can say that the vision improvements have been maintained for as long as we have been following up the patients, which is two years in one case."
Choroideremia is caused by a mutation in a gene called CHM that leads to degeneration of the layer of tissue between the white of the eye and the retina, which is on the back of the eyeball.
Eventually, the light-capturing retinal cells also die, reducing vision year by year and leading to complete blindness by middle age.
The disease occurs in around one in every 50,000 people and especially strikes males, invariably boys.
Because the disease is slow-moving, it offers a good target if doctors can attack it while the patient is still young, said MacLaren.
The researchers used a disabled cousin to the cold virus as a "Trojan horse," inserting in it a functioning version of the CHM gene.
In an operation similar to cataract surgery, the patient's retina was first detached and the virus with the corrective gene was gently injected underneath, using a very fine needle to limit damage to the precious cells as much as possible.
A total of nine patients have now been treated with the therapy, which is administered in one eye only to allow comparison with the progression of the disease in the other eye.
The results are encouraging news for gene therapy, a frontier of medicine that generated huge excitement in the 1990s but has had many ups and downs since then.
In February 2012, US scientists reported progress in early-stage gene trials among 12 patients with a retinal disease called Leber's congenital amaurosis, or LCA.
The latest experiments "confirm that the virus can deliver its DNA payload without causing significant damage to the
retina," said MacLaren.
The technique involves sneaking a gene into the retina to correct a faulty stretch of DNA causing a degenerative sight disease called choroideremia.