Colorblindness Cured

Old Monkeys See New Colors
October 1, 2009 Using gene therapy, scientists recently cured two colorblind monkeys. These adult monkeys were introduced to a Technicolor world for the first time in their lives. Human and monkey colorblindness have the same cause, which means there may be hope on the horizon for colorblind people too. Monkeys and humans can sense the color red because of their L-opsin gene. Without a working copy of this gene, red looks the same as green to some unlucky monkeys -- they are red-green colorblind.

Courtesy of Marieke
Scientists cured
colorblindness in
squirrel monkeys
like this one.
Scientists cured these monkeys of colorblindness by injecting working copies of the L-opsin gene into these monkeys' eyes. The monkeys were then able to pass a color test that they couldn't before the treatment. The discovery is surprising because previous studies showed that what happens early in life is critical for a working visual system. But this study shows that that's not always the case: a vision deficiency can be cured in an adult animal. The study also suggests that gene therapy treatments may be used in the future to treat colorblindness and maybe even other eye disorders too. The future is a little brighter for the colorblind.
Making Monkeys See Red, Literally
Monkeys (and people) can't tell red from green when their L-opsin gene isn't working well. So an obvious solution is to simply add a working copy of the gene to their eyes. Of course, it sounds simple, but it took a little ingenuity and a little luck to pull off. The first hard part was communicating with the monkeys. Unlike King Louie of the Jungle Book, real monkeys don't speak human. They can't say what they see However, they like juice. So, scientists had already designed a test based on that. They took a board with multiple gray dots and displayed colors in one of three areas. If the monkey touched the correct patch with color, they got some juice. However, monkeys with a broken L-opsin gene remained thirsty when red and green were shown. They couldn't distinguish between the two colors, and that confirmed that they were colorblind.

Viruses can inject genes
into cells.
The next step was getting a working copy of L-opsin into these colorblind monkeys. That meant getting genes into the cells of the monkey's eyes. To do this, scientists used two common tools: a syringe and a virus. The syringe was used by a surgeon to inject the virus into the eyes of anesthetized monkeys. The virus was used to inject the L-opsin gene into the cells of the monkey's eye. While the syringe is familiar to anybody who has received a shot, the use of a virus may not make much sense. How and why did scientists use a virus to cure colorblindness? The why mostly has to do with getting past the protective membrane that surrounds a cell. The membrane is kind of like the skin of the cell and keeps unwanted stuff out. But scientists wanted to get the L-opsin gene in. So they used something like a needle that has evolved to do just this -- a virus. Scientists have used viruses for getting genes into cells for quite awhile, so the technique wasn't new. Normally, these viruses work by attaching to cells and injecting their own genes. But scientists didn't want to inject virus genes; that would probably make things worse. So, they substituted some of the virus's genes with what they wanted: the working L-opsin gene. To recap, surgeons injected viruses into the eyes. The viruses then injected the genes into the cells. Amazingly, the monkeys with the new gene passed the color test. They got a taste of juice when red or green appeared -- a pretty and sweet result. This told scientists that the monkeys were cured of their colorblindness. Scientists used some not-so-new tools to address a new problem. But science isn't just about being knowledgeable or clever. It often requires a little bit of luck, too. Most scientists did not think that curing adult monkeys of colorblindness would be possible. In the 1960s, scientists saw that eyes needed to be stimulated early in life to develop properly. This led to the belief that things have to go right at an early age for an eye to work properly. Most scientists would have thought that it was impossible to add a gene to an adult monkey to cure a genetic defect related to sight. It just goes to show that sometimes believing leads to seeing.

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A Glimpse into Color Vision Genetics

Colorblind people can still
see colors. The world
just looks slightly different.
Colorblindness isn't as black and white as it looks. Most people who are "colorblind" can actually see colors. They just find it hard to distinguish between some colors. Before addressing this, it's good to be clear on how color vision works. At the back of the eyeball is the retina, which is a layer of nerve cells. There are two types of cells in the retina: rods and cones. Rods are very sensitive and used in low light conditions. However, they are not sensitive to color. Cones, on the other hand, are sensitive to colors. The three different cones are each sensitive to a different color -- red, green, or blue. The color depends on which opsin gene is turned on in a cone. If an opsin gene is defective, a person can't sense certain hues. The most common defects happen in the red or green pigment genes making it difficult to distinguish red, green, and yellow. The protein responsible for seeing red is L-opsin. "L" stands for "long" because red light has a long wavelength. The protein responsible for green is M-opsin, and "M" stands for "middle," which also refers to wavelength. Both of the genes for these proteins are on the X-chromosome which is bleak news for men. Women have two X-chromosomes (XX). That means if a woman has a broken copy, she probably has a second working copy. And she only needs one working copy to avoid colorblindness. On the other hand, men have only one X-chromosome (XY). If they get a bum copy, they are guaranteed to be colorblind. This explains why 7% (about 1 out 14) of American men are red-green colorblind. And why only 0.4% (1 out of 250) of American women have the same difficulties. There is another type of colorblindness where a person has difficulty telling blue from yellow. The gene for blue, S-opsin ("S" for "short"), is not on the X chromosome, which means that everybody has a back up copy. Just as red-green colorblind women are rare, so are people with blue colorblindness. Andrew Hellman