Gene Therapy for Vision: Learning to See
If gene therapy can cure color-blind monkeys, could it someday bring vision to people born without sight?
What was thought to be an essential fact of human development has long hindered efforts to treat blindness. Since the 1960s, experiments have shown that if sight isn’t present at birth, the brain can’t develop the neural circuits required for seeing. But a breakthrough in using gene therapy to counteract color blindness in monkeys suggests there may be exceptions to that rule.
The experiments, involving five-year-old squirrel monkeys, “demonstrate that the nervous system is capable of responding to new sensory input,” says Jay Neitz, professor of ophthalmology at the Eye Institute of the University of Washington. “Somehow the brain seems able to learn that new information coming over the existing neural architecture is different from what came before.”
In squirrel monkeys, as in people, color blindness is caused by a missing gene that ordinarily produces the protein L-opsin, which combines with a derivative of vitamin A to make a photo-pigment molecule that can absorb red or green light in the retina’s photoreceptors (producing not absolute color blindness but difficulty distinguishing light colors). The molecule converts that light energy into electrical signals that travel along the optic nerve to the brain.
Before injecting two monkeys with the missing gene, Neitz and his wife, Maureen (also a vision scientist), taught them to take a test that involved looking for a colored patch hidden among gray dots on a computer screen. On Day 20 after they got the gene, their test performance was dramatically different. “Before, the monkeys would tentatively touch the screen, as if to say, ‘How about here?’” says Jay Neitz. “They were clearly guessing. But when the gene was activated, they touched the red and green spots quickly and accurately. After a week of consistent performance, we knew they could see those colors.”
The Neitzes hope to use gene therapy to treat human color blindness and related disorders, including achromatopsia, in which vision decreases as light levels increase, and cone dystrophies, which result in the loss of color vision and central vision. More important, their work might help disprove the notion that a child born blind will never see.