A squirrel monkey named Dalton takes a color vision test in which he has to touch the area on the screen showing a patch of colored dots.

Monkeys once color-blind can now see the world in full color thanks to gene therapy. The results demonstrate the potential for such methods to eventually cure human vision disorders, from color blindness to possibly other conditions leading to full blindness.

The primate patients, named Dalton and Sam, are two adult, male squirrel monkeys that were red-green color-blind since birth — a condition that similarly affects human males more than females. Five months after researchers injected human genes into the monkeys' eyes, the duo could see red as if they had always had this ability.

Since human genes were used and the monkeys' eyes and brains are similar to ours, at least in terms of color vision, the researchers hope the same procedure could work in humans.

"People who are color-blind feel that they are missing out," said study researcher Jay Neitz, a professor of ophthalmology at the University of Washington, Seattle. "If we could find a way to do this with complete safety in human eyes, as we did with monkeys, I think there would be a lot of people who would want it."

The findings are detailed in the Sept. 17 issue of the journal Nature.

Color-coded

The researchers chose squirrel monkeys partly because all males of the species show some form of red-green color blindness, which is the most common form of color blindness in humans and certain monkeys. About 8 percent of Caucasian men in the United States are color-blind.

The blindness primarily afflicts males because the genes encoding red and green receptors are located on the X-chromosome, of which men only have one. Women have two X-chromosomes, and a normal gene can often balance out a defective one.

Like humans, monkeys' eyes contain cone and rod cells. Each cone contains different photopigments that can detect specific wavelengths of light. The monkeys, Dalton and Sam, had cones that were unable to detect red light.

Monkey, see …

To try to correct their vision, Neitz and his colleagues put a needle into the monkeys' eyes, just behind the retina, and injected a virus whose disease-causing genes had been replaced with human genes for red photopigments. Viruses dump their genes into host cells, where the viral DNA can replicate. In this case, the virus was used to insert photopigment genes.

Throughout the study, the monkeys were tested daily. They had to distinguish patches of colored dots that vary in size and brightness from surrounding grey dots on a screen. When the animals touch the colored target with their hands or nose, a positive tone sounds and the monkey garners a juice reward. When wrong, a negative tone sounds and a two- to three-second pause, considered a penalty, ensues before the next test.

Before the genetic injection, "Occasionally he'll guess right and if he guesses right, right away he'll try that same spot, like 'Oh, maybe this is the spot,'" Neitz said, referring to the male monkeys.

About five months after the injection, the two monkeys showed no hesitation in the colored-dot tests, getting them all correct. The monkeys could pick out colored patches even when just a hint of red was added to the target patch of dots.

And now, some two years later, the monkeys show no signs of their color senses waning and no adverse effects.

{{ video="LS_090916_colorblind-monkey" title="Monkey Gets Color Vision" caption="After successful gene therapy, this male squirrel monkey that was once colorblind can now pick out red dots from the grey background. When the monkey correctly noses the red patch, a positive tone sounds and the monkey gets a drop of juice. Credit: Neitz Laboratory." }}

What's going on?

The study suggests more than meets the eye, however. Merely giving the monkeys red-sensing photopigment receptors would not necessarily give them the ability to perceive red, the researchers knew. Some new ability must have been triggered in the monkeys' brains, since it's the noggin that ultimately analyzes the information from the eyes, Neitz said.

"People thought in order to add some new information to the brain you'd have to add some kind of new neural circuits. And once you get to be an adult all of your neural circuits are in place," Neitz told LiveScience. So scientists had thought that adding new sensory information to the brain would be possible only early in life.

Rather than brewing up new neurons or rewiring itself, the monkeys' brains probably harnessed the abilities of existing circuitry, according to Neitz.

"Amazingly the animals behave almost exactly as if they had this capacity all along from birth," Neitz said.

Next up: humans

Before such gene therapy could help humans, Neitz said that he and others would need to perfect it, and ensure its complete safety. For instance, the genetic insertion could have some secondary effects in humans not yet seen in the monkey subjects.

"That's something we have to think about before this ever happens in a human — how to make it perfectly safe," Neitz said. "Given that, we put a human gene in the monkeys and their eyes and brain are like ours, at least that part of their brain. I have to assume that if we did this exact same thing in a human being today, the human would respond exactly as the monkeys did."

He added, "I get calls from people every day who say they wish they weren't color-blind, but nobody wants to risk their vision in order to get color vision."

In addition to color blindness, most of the major blinding diseases involve the retina and the inability of certain cells to sense light, Neitz said. "This could be a first step to curing a huge number of problems that cause people to be blind," Neitz said.

He hopes that within 10 years his monkey research will at least be moving in the direction of human clinical trials.