Is the red-green color blindness gene therapy cure ready for humans?

Researchers at the Eye Institute of the University of Washington have successfully used gene therapy to cure color blindness in adult monkeys. In the photo on the left (photo credit), a squirrel monkey, who was treated for red-green color blindness, is seen enjoying a feast of colored fruits and vegetables. The image was digitally altered to simulate what the scene would look like to a person (or monkey) with red-green color blindness. 

Red-green color blindness, which results from loss of either the red- or green-sensitive visual pigment in the eye, is the most common genetic disorder; 1 in 12 men and 1 in 230 women are affected, while 1 in 6 women is a carrier.  

The work is a culmination of years of research performed in collaboration with laboratories at the University of Florida and the Medical College of Wisconsin.

The researchers used a computerized test for human color blindness that was similar to the well-known testing books in which colored numbers or symbols are concealed in a pattern of dots.  Prior to treatment, the monkeys were trained to touch the location of a colored patch hidden among gray dots.  Similar to color-blind humans, the monkeys could not distinguish red or green, but following treatment that added the missing visual pigment gene, the monkeys passed the test easily for all colors.

A popular belief has been that “critical periods” for the development of many capacities end prior to adolescence, implying that treatments involving the adult visual system would be impossible.  To the contrary, the results of Mancuso and colleagues reported in Nature indicate that in the case of color vision, the nervous system is capable of responding to newly-added sensory input, allowing adult monkeys to respond to colors that they could not see previously. 

The success in treating color blindness complements ongoing gene therapy trials for a blinding disorder, Leber’s congenital amaurosis (LCA), a progressive disease that causes cell death and retinal degeneration.  The initial phases of the LCA trials are safety studies in which a therapeutic gene was administered to patients with advanced disease.  Early results indicate that the treatment is safe; however, the effectiveness of the therapy for LCA will not be known until younger patients with healthier retinas are treated.  In contrast to LCA, retinal degeneration is not associated with color blindness and its successful treatment demonstrates the full potential of gene therapy to restore a visual capacity.  As most vision disorders have a genetic component involving the retina, the encouraging results of both studies open the way to treatments for a broad range of eye diseases.

People with severe forms of color vision deficiency struggle with many everyday tasks that most people take for granted, such as detecting a sunburn or determining when tomatoes are ripe.  Color blindness also excludes people from numerous professions.  Some obvious examples include employment as police officers, fire fighters, commercial/public transit drivers, and pilots.  In other professions, the requirement is not as obvious, and some people spend years training for careers as designers, geologists, chemists, or ophthalmologists before being excluded by their color blindness.  Color-coding is used extensively in education and children can be mistakenly labeled as learning-disabled before it is discovered that they have a color vision deficiency. 

The prospect of ameliorating the problems caused by color blindness makes it an attractive future target for human gene therapy.  Because the monkey visual system is similar to that of humans, and a human gene was used to replace the missing visual pigment of the monkeys, the scientists are optimistic about the possibility of gene therapy to cure colorblindness in humans.  In the same way that few would settle for a black-and-white television or monochrome computer-monitor, it is easy to imagine that many color-blind people would want the cure if there was no risk to their vision or health.  While no adverse side-effects were observed in the monkeys, the most important barrier in moving the treatment forward will be insuring its safety for use in humans. 

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