18 September 2009
Red-green colour blindness is the most common single gene disorder in humans, affecting approximately 1 in 12 men. It is not usually considered to be a serious or debilitating condition, but those affected can be barred entry into certain occupations that depend upon the ability to reliably discriminate between different hues.
The study used male squirrel monkeys, all of whom are born red-green colour blind: Full colour vision requires two versions of the opsin gene located on the X chromosome; since male monkeys have only one X chromosome they only have one version of the gene, and so are unable to distinguish between red and green. A similar mechanism accounts for the same form of colour blindness in men.
The researchers injected human opsin genes contained within a viral vector into the subretinal tissue of the monkeys’ eyes. The genes were accompanied by regulatory elements selected to induce the expression of the gene only in colour-sensitive cone cells. The monkeys were repeatedly tested before and after treatment by being required to touch coloured patches on a screen in order to receive a treat. 20 weeks following treatment there was a ‘dramatic’ improvement in the monkeys’ ability to distinguish colours, and this enhanced vision has remained stable for the past two years without adverse side effects.
Despite the monkeys having been colour blind since birth, adding the missing gene was enough to give them full colour vision, indicating that the formation of new neural connections was not required. This is in contrast to developmental conditions such as monocular deprivation, whereby if one eye is covered during early development sight cannot subsequently be restored in that eye – despite no tissue being damaged – because the neurons no longer respond to input from that eye.
Comment: The results of this study suggest the possibility that future gene therapies could enable function to be added or restored to the eye. Since colour blindness is not considered a serious condition, extensive further research will be required to categorically establish the safety of this technique before it is even considered for clinical trials. It is plausible however that this method could be employed to treat more debilitating visual problems with a genetic basis – trials are currently underway using gene therapy to treat Leber’s congenital amaurosis – perhaps ultimately leading to gene therapies that are able to restore sight in some blind people.