These monkeys were unable to make a distinction between red and green, a condition that affects mostly males because the genes for red and green receptors are located on the X-chromosome. Females have two X-chromosomes so if they have a bad gene its damaging effects can be neutralized by a good gene on the other X-chromosome. Not so for males. However, scientists from the University of Washington used a gene therapy based treatment to cure these squirrel monkeys of their congenital color blindness condition and restored full color vision to the animals.
What is particularly spectacular about this research is that it has smashed a once preconceived notion about the plasticity of brains. Previously it had been thought that it was not possible to manipulate an adult brain in this way, and that a gene-based therapy would only work if intervention came at an early stage of development when the brain is more malleable - "plastic". But this research has demonstrated that no other brain "re-wiring" was needed; the introduction of the gene was sufficient to restore full color vision.
click to enlarge
The University of Washington researchers were able to introduce therapeutic
genes into the light-sensing cells at the back of the eye by incorporating them into a virus that had its virulence factors silenced. Then they injected the viral load into an area just behind the monkey retina. The gene was the human form of the red-detecting opsin gene. The virus offloaded the genes which were taken up by monkey cells. The result was that the light-sensing cells were able to distinguish between green and red. This was tested by having the monkeys recognise a colored target that was surrounded by grey dots (this is similar to the tests humans have to check for color blindness). Before the experiment the monkeys would guess, but several months after the gene therapy procedure they were picking out the correct target immediately.
