A cure for ‘colorblindness’ may be in sight

A pair of researchers at the Univeristy of Washington have successfully cured colorblindness in two squirrel monkeys.

This may not sound like a big deal to you if you're not a squirrel monkey (or if you have normal vision), but for people with colorblindness, it could be life-altering.

The term colorblindness is actually something of a misnomer. A more accurate term is ‘color deficiency' — and it doesn't mean people see in black and white.

“Ninety-nine percent of all people who suffer from color deficiency have ‘red-green colorblindness,’” says Maureen Neitz, a genetic engineer and one of the researchers who created the cure. “This doesn't mean they can't see red or green; they just experience red or green differently from normal people.”

This small gap in the color spectrum can change the most basic experiences most of us take for granted. Autumn leaves, beautiful sunsets, even telling the difference between a red and a green pepper — these common, shared experiences simply elude people who can’t accurately discern shades of red and green.

(Being a colorblind videographer trying to make a video about color vision is no picnic, either, notes Luke Groskin, Science Friday’s own colorblind video producer.)

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So it’s easy to see why the one-in-12 men and the one-in-200 women affected by the disorder might be interested in a cure. And in 1999, Maureen Neitz and her husband Jay Neitz, a neuroscientist, decided to pursue one.

Easier said than done. Because color deficiency isn't an illness, it's genetic — though in rare cases it can be caused by an unrelated illness.

Here’s what happens: The retina at the back of the human eye holds three types of photoreceptor cells, called cones. One is most sensitive to red, the second to blue and the third to green. Each of these cones and their corresponding pigments are encoded by specific genes.

“Humans and other old-world primates have two genes on the X chromosome that encode visual pigments,” Maureen Neitz explains. “One encodes the red cone pigment and the other encodes the green cone pigment. But if you're colorblind, only one type, red or green, is expressed.”

Finding the faulty gene was relatively simple. Replacing the gene inside the cone cells with a new one that ‘works’ was a bit trickier. “You have to have some way of delivering a gene to the cells that you're trying to treat and not into other cells,” Maureen Neitz says.

Fortunately, nature has crafted a very powerful method of forcing DNA into a very specific cell: a virus. The virus the Neitz’s used is called Adeno Associated Virus, or AAV. Its main advantage is that it does not elicit an immune response in humans.

While figuring out how to load the missing red color pigment gene into the virus, the Neitz’s pursued another line of inquiry. They had discovered that all male squirrel monkeys have red colorblindness. So they designed a test for a pair of colorblind, male squirrel monkeys.

They trained the monkeys to touch the spot on a computer screen where they saw a yellow color blob appear. If the monkeys touched the right spot, they got a treat.

Once the virus was ready and the monkeys had been trained to distinguish the yellow blob from the others on the screen, the animals underwent a fairly elaborate procedure: A vitro-retinal surgeon slipped a needle underneath the retina, then infused a fluid in order to treat the entire back of the eye. Then they waited.

“We didn’t know how long it was going to take for them to change their behavior after the pigment was expressed robustly,” says Maureen Neitz.

Eventually, the two treated monkeys began to pass the colorblindness test, using red blobs instead of yellow. Monkeys who had not received the gene therapy continued to fail the test when it was done with the red blobs.

The Neitzes knew they had nailed it. They knew for a fact that injecting the virus had changed the genes inside the two monkeys' eyes.

“When we look back and see the difference between the animals, it's so dramatic,” Jay Neitz says. “It was an amazing thing.”

While the FDA has yet to approve the procedure for human trials, Maureen Neitz is absolutley confident the gene therapy will work in humans. Indeed, the Neitzes continue their work. They recently developed a one-shot version of the cure. Patients could conceivably see normally by receiving a single shot directly into the eye.

While getting a shot in your eye sounds terrifying to some, it may be a small price to pay for getting to see a sunset in all its glory or experiencing the magic of fall colors — or just knowing you’ve left the house dressed in the right color clothing that day.

A previous version of this story misstated how often patients need to receive a shot.

This article is based on an interview and video on PRI's Science Friday with Ira Flatow.