By Lola Dong
The retina is a tissue located at the back of the eye that contains rods and cones, which are cells that process light and images and send signals to the brain. Humans with normal color vision typically are born with 3 types of cones that function properly: the L cones (red-sensing cones), the M cones (green-sensing cones), and the S cones (blue-sensing cones). An individual with less than 3 types of functional cones is unable to differentiate between certain colors, thus color blind. The most common type of color blindness is red-green color blindness. An individual with red-green color blindness can not distinguish between green and red colors.
Color blindness can be inherited or acquired, but most individuals with color blindness inherited the disorder from their biological parents. Red-green color blindness is an X-linked recessive inheritance trait, male babies are more likely to inherit red-green color blindness than female babies as both parents contribute an X chromosome to the female babies.
Although there’s no cure for color blindness, most individuals with color blindness can live a normal life with the help of special color-blind glasses. Color-blind glasses have lenses that filter out the wavelengths between 2 colors1. For example, color blind glasses for red-green color blindness filter out specific wavelengths between red color and green color, allowing the individual wearing the glasses to better distinguish between red and green color.
However, color blindness glasses do not treat color blindness as it is only a piece of equipment used to enhance an individual’s ability to see color. Recently, a study done by a group of researchers at University College London found that gene therapy can potentially restore the color vision of an individual with achromatopsia2. Achromatopsia (ACHM) is an autosomal recessively inherited disorder that causes retinal degeneration. Individuals with achromatopsia see only different shades of gray, black, and white because all 3 types of cone photoreceptor cells are affected by the disorder. Most Achromatopsia (ACHM) is caused by mutations in either of the two genes: CNGA3 or CNGB3.3 The CNGA3 gene encodes for the alpha subunit of the cone cyclic nucleotide-gated (CNG) channel, while the CNGB3 gene encodes for the beta subunit of the cone cyclic nucleotide-gated (CNG) channel. The CNG plays an important role in retinal photoreceptor cells because it is involved in the process of sending signals to the brain. In photoreceptors, light trigger a series of biochemical reactions that closes CNG channels and creates a voltage pulse, allowing signals to be passed on to the brain.4 Therefore, an individual with mutated CNGA3 or CNGB3 does not have a functional CNG channel that sends color signals to the brain. The study done by UCL suggests that gene supplementation could potentially become a treatment for those with Achromatopsia. The mechanism of gene supplementation involves transferring a normal copy of the disease-causing gene into cone photoreceptors, and the newly transferred gene would be responsible for encoding the healthy CNG for the patient.5 Currently, this method is being tested in phase I/II clinical trials.
If gene therapy is proven to be successful in helping color-blind individuals to see color, then color-blind individuals will be able to live a more convenient life. Especially for children with color blindness, they will be able to learn like normal children. However, gene therapy will not become prevalent soon because it would require a lot of money and resources to perform. As a result, most families with color blindness still will not have access to this new technology.
References:
Gene therapy partially restores function to color receptors in color blind children. Big Think. (2022, September 25). https://bigthink.com/health/gene-therapy-color-blindness/
Farahbakhsh, M., Anderson, E. J., Rider, A., Greenwood, J. A., Hirji, N., Zaman, S., Jones, P. R., Schwarzkopf, D. S., Rees, G., Michaelides, M., & Dekker, T. M. (2020). A Demonstration of Cone Function Plasticity after Gene Therapy in Achromatopsia. https://doi.org/10.1101/2020.12.16.20246710
Thiadens, A. A. H. J., Roosing, S., Collin, R. W. J., van Moll-Ramirez, N., van Lith-Verhoeven, J. J. C., van Schooneveld, M. J., den Hollander, A. I., van den Born, L. I., Hoyng, C. B., Cremers, F. P. M., & Klaver, C. C. W. (2010). Comprehensive analysis of the achromatopsia genes CNGA3 and CNGB3 in progressive cone dystrophy. Ophthalmology, 117(4). https://doi.org/10.1016/j.ophtha.2009.09.008
Barret, D. C. A., Kaupp, U. B., & Marino, J. (2022a). The structure of cyclic nucleotide-gated channels in rod and cone photoreceptors. Trends in Neurosciences, 45(10), 763–776. https://doi.org/10.1016/j.tins.2022.07.001
Michalakis, S., Gerhardt, M., Rudolph, G., Priglinger, S., & Priglinger, C. (2021). Achromatopsia: Genetics and gene therapy. Molecular Diagnosis & Therapy, 26(1), 51–59. https://doi.org/10.1007/s40291-021-00565-z
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