Color blindness and gene therapy
What we see is less than meets the eye, especially for those who are color-blind. About ten percent of men are to some degree red-green color-blind. Roughly 1.5 percent of men cannot distinguish reds from greens because they lack either the red- or green-sensitive cone pigments, but for the most part, color blindness results when one of the three types of visual pigments doesn’t work normally. A more apt term for this condition might be color deficiency: men who are affected—the condition is sex-linked—see altered or weak colors, rather than no colors at all.
To someone with red-green color deficiency, including the Irish painter Paul Henry, reds and greens are difficult to distinguish. Although Henry used red for the men’s trousers in Launching the Curragh, he could not differentiate it from yellow or orange. He may well have had help from friends when choosing the tubes of paint to use.
“Artists don’t live in a vacuum,” says Michael Marmor ’66, a professor of ophthalmology at Stanford University School of Medicine. “Friends tell them, ‘Grass is green, so use green paint.’ There are labels on tubes.”
People who are color-blind, however, may one day have an opportunity to experience the full spectrum of color vision, according to Jason Comander ’06, an instructor in ophthalmology at Massachusetts Eye and Ear. Researchers at the University of Washington have developed gene therapy that restores the gene that codes for the missing or faulty light-sensitive pigment, allowing cone cells to detect colors that they could not detect previously.
The therapy has been tested in animals that have cone cells sensitive to wavelengths of blue and green light but insensitive to those for red light. A few months after therapy, treated animals could distinguish an image formed from red dots embedded within a field of dots of varying colors, while untreated animals remained blind to the embedded image.
It isn’t clear yet whether the therapy works beyond restoring red sensitivity to the cone cells. Does it, for example, also affect the complex wiring inside the retina and the brain that contributes to the processing of color vision within the visual cortex?
Comander finds it amazing that a visual system sensitive to only two colors can gain a third color and that the brain somehow figures out how to recognize the new color. “For any other part of the visual system,” he adds, “if you haven’t exercised it since childhood, it won’t work.”
People with normal three-color vision are trichromats, says Comander, while those with garden-variety color blindness are anomalous trichromats with one pigment being partially defective. Those rare individuals with just two types of cone cells are called dichromats. In contrast, certain females have a four-color visual system; this is a sex-linked trait. Known as tetrachromats, these women have a fourth type of cone cell that is sensitive to a slightly different range of light wavelengths. Tetrachromats may see billions of colors, although among the handful of such women that researchers have studied, only one person was able to distinguish color differences beyond those distinguished by trichromats.
Color blindness can limit career choices. It can, for example, bar individuals from serving as police officers and from engaging in certain roles in the military. Because the condition is so physiologically benign for most affected people, few may be eager to try gene therapy. “There’s a cost-benefit to consider,” says Marmor.
Testing of a similar gene therapy for achromatopsia, a more serious condition, is slated to begin in 2016, notes Comander. This condition affects cone cells and robs patients of both color and high-acuity vision. It can arise because of missing pigments in the cone cells or because of malfunctions in the cone cells’ machinery that transmits light signals to neurons downstream. This novel treatment requires genes that create functioning versions of the faulty machinery to be inserted into cone cells.
“Gene therapy is starting to work and is changing this field,” says Comander. “There’s a real need for new therapies for the people I see who are losing most or all of their vision due to inherited retinal diseases.”
If modern gene therapy does catch on, treatments for color blindness may eventually be approved. That, in turn, could open the door to color vision enhancement.
Elizabeth Dougherty is a science writer based in Massachusetts.
Image: John Soares