Three young adults who received gene therapy for a blinding eye condition remained healthy and maintained previous visual gains one year later, according to an August online report in Human Gene Therapy. One patient also noticed a visual improvement that helped her perform daily tasks, which scientists describe in an August 13 letter to the editor in the New England Journal of Medicine.

These findings have emerged from a phase I clinical trial supported by the National Eye Institute (NEI) at the National Institutes of Health, and conducted by researchers at the University of Pennsylvania, Philadelphia, and the University of Florida, Gainesville. This is the first study that reports the one-year safety and effectiveness of successful gene therapy for a form of Leber congenital amaurosis (LCA), a currently untreatable hereditary condition that causes severe vision loss and blindness in infants and children.

"These results are very significant because they represent one of the first steps toward the clinical use of gene therapy for an inherited form of blindness," said NEI director Paul A Sieving. "I anticipate that it is only a matter of time before similar techniques will be applied to other genetic diseases affecting vision."

The three patients in the study-aged 22, 24 and 25-have been legally blind since birth due to a specific form of LCA caused by mutations in the RPE65 gene. The protein made by this gene is a crucial component of the visual cycle. The RPE65 protein is necessary for the production of a retina-specific form of vitamin A that is required for the light-sensitive photoreceptor cells to function. Mutations in the RPE65 gene prevent this production, which halts the visual cycle and blocks vision.

The RPE65 disease offers an opportunity for treatment in that it leaves some photoreceptors intact. In this study, researchers pinpointed an area of intact photoreceptors in the retina of each patient. They injected healthy copies of the RPE65 gene under the retina in this area in an attempt to repair the visual cycle.

One year after the procedure, the therapy had not provoked an immune response in the eye or in the body. Though the patients' visual acuity, or ability to read letters on an eye chart, remained unchanged, all three patients could detect very dim lights that they were unable to see prior to treatment. This visual benefit provides evidence that the newly introduced RPE65 gene is functional and is increasing the light sensitivity of the retina.

"These new reports extend our previous findings from three months after the procedure. At one year, we have now found that the RPE65 gene therapy appears to be safe and leads to a stable visual improvement in the patients studied. We are cautiously optimistic about these results and look forward to additional reports that address the key issues of safety and effectiveness," said Artur V Cideciyan, research associate professor of ophthalmology at the University of Pennsylvania and lead author of the publications.

At 12 months, one patient also noticed that while riding in a car, she could read an illuminated clock on the dashboard for the first time in her life. When researchers performed additional visual testing, they found that this patient focused on images with a different part of the retina than they expected.

The fovea is the area of the retina where the sharpest central vision normally occurs. However, instead of focusing on images with the fovea, this patient had gradually begun to use the area of the retina that had been treated with gene therapy. The area had already become more light sensitive than her fovea at one month after treatment, but it took 12 months for her to read dim numerals-such as the illuminated clock-that she was previously unable to read.

"This interesting finding shows that over time, a person visually adapted to gene therapy in a meaningful way," said Samuel G. Jacobson, professor of ophthalmology at the University of Pennsylvania's Scheie Eye Institute and principal investigator of the clinical trial. "As we continue our studies, we will look more closely at whether these slow visual gains could be accelerated with visual training."

Researchers will continue to follow these patients over the next several years to monitor safety and to learn whether the visual benefits remain. This ongoing phase I trial also includes additional groups of LCA patients-children as well as adults -- who are receiving different doses of the RPE65 gene therapy.