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Gene Therapy Cures Color Blindness in Monkeys

By Eliza Strickland
Sep 16, 2009 11:25 PMOct 10, 2019 2:51 PM


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For two squirrel monkeys nicknamed Dalton and Sam, life has gotten a lot more colorful. Researchers used gene therapy to correct the color blindness of the two adult monkeys, giving them the ability to distinguish between red and green for the first time. The fascinating accomplishment suggests that scientists may someday be able to cure other kinds of blindness in humans. And because the treated monkeys were “middle aged”, it challenges the assumption that gene therapies cannot work in adults because their brain connections are too set in their ways to change beneficially [New Scientist].

The field of gene therapy, in which a malfunctioning gene in a patient’s body is replaced with a functional one, fell into disarray one decade ago following the death of an 18-year-old in a clinical trial. But since then scientists have regrouped, using animal studies to probe the technique’s safety. Last year, researchers progressed to the point of safety trials in humans for the treatment of one rare eye condition called Leber congenital amaurosis, and were able to dramatically improve the patients’ sight. Those results were stunning, but they were also achieved in children, whose still-growing brains can rewire themselves on the fly in response to new sources of visual stimuli [Wired.com].

In the new study, published in Nature, the researchers used a type of squirrel monkey in which the males lack a visual pigment called L-opsin. Its absence renders the monkeys color-blind, unable to distinguish reds and green. Most of the females, on the other hand, see in full color. So the scientists got to wondering: what would happen if they gave a boy squirrel monkey the same opsin that girls have [Scientific American]. They used a harmless virus to ferry in the gene that makes opsin, injecting the virus behind the monkeys’ retinas.

The results were dramatic. The monkeys had already been trained to recognize patterns of blue and yellow dots against a gray screen in order to get a grape juice reward. About 20 weeks after the procedure, they began to also recognize the patterns of green and red dots that they had previously ignored. Two years on, the monkeys can still see the colors and have experienced no obvious side effects.

The researchers say that the work’s most interesting aspect is that the monkeys gained full-color vision even though their adult brains couldn’t grow new neural circuits. Instead the existing neural circuitry and visual pathways apparently adapted to make use of the new opsin pigment that was suddenly produced in the eye. This realization will open the way for new research on treating a wide range of eye disorders in adults.

Study coauthor Jay Neitz says the findings could, theoretically, lead to work that goes beyond curing eye diseases, and forges ahead into eye enhancements. One very speculative and futuristic possibility is that of using gene therapy to equip humans to see ranges of light invisible to them at present, such as ultraviolet or infrared light. Some birds and reptiles can sense UV light, for example, and some fish can sense wavelengths approaching those in the infrared. “I wouldn’t rule it out completely, but it’s very futuristic,” says Neitz [New Scientist].

But enough of the medical implications of the research. Here’s a pressing question: What must it have been like for the monkeys to suddenly see the world in full color? While we can’t get an answer from Dalton and Sam, Neitz is willing to speculate. “You go out and look at a rainbow, or the fall leaves, or sunset over the ocean, and it’s not something where you just say, ‘I can see colors.’ It has a deep effect on us,” he said. “These emotions are something we inherited from our evolutionary past. I think monkeys have that, too. I think these animals must have the real experience of, “Oh! Wow!” [Wired.com]

Image: Neitz Laboratory

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