In a breakthrough that offers hope to the seriously visually impaired, researchers have used gene therapy to cure color blindness in squirrel monkeys.
The most common genetic disorder in people, color blindness is caused by defective cone cells.
"We've added red sensitivity to cone cells in animals that are born with a condition that is exactly like human color blindness," said William W Hauswirth, a professor of ophthalmic molecular genetics at the University of Florida College of Medicine. "We've shown we can cure a cone disease in a primate, and that it can be done very safely. That's extremely encouraging for the development of therapies for human cone diseases that really are blinding."
Two squirrel monkeys were trained to perform standard color-blindness tests and communicate what colors they were seeing to the researchers via a computer touch screen. When the animals chose correctly, they received a reward of grape juice.
The gene-transfer technique involved using a harmless virus to deliver corrective genes to produce a substance called long-wavelength opsin in the retinas of the monkeys. This is a colorless protein that works in the retina to make pigments that are sensitive to red and green.
"We used human DNAs, so we won't have to switch to human genes as we move toward clinical treatments," said Hauswirth.
About five months after the treatment, the monkeys began to acquire color vision, almost overnight.
"Nothing happened for the first 20 weeks," Neitz said. "But we knew right away when it began to work. It was if they woke up and saw these new colors. The treated animals unquestionably responded to colors that had been invisible to them."
Color-blindness mostly affects men - about eight percent of Caucasians. In addition, about one in 30,000 Americans have a hereditary form of blindness called achromatopsia, which causes nearly complete color blindness and extremely poor central vision. "Those patients would be targets for almost exactly the same treatment," Hauswirth said.
Even in common types of blindness such as age-related macular degeneration and diabetic retinopathy, vision could potentially be rescued by targeting cone cells, he said.
The research is published in Nature.