Gene therapy has made color vision to monkeys
Two squirrel monkeys, who could only distinguish yellow and blue, acquired a complete perception of colors through the insertion of a human gene in their retina. This discovery, which is surprising, suggests possible treatments for diseases of vision, even congenital.
Sam and Dalton are two squirrel monkeys, also called sairmi (Saimiri sciureus), hence the name of the first. Two years ago, like all males of their species, they could only distinguish two shades, yellow and blue, but neither red nor green. Hence the name of the second, which is that of John Dalton, the English physician who described in 1798 this hereditary disorder of the vision called since color blindness. Today, Sam and Dalton triumph over all the tests imagined to detect this disease, as evidenced by a publication in the journal Nature.
For the past ten years, these two monkeys have been living with Maureen and Jay Neitz, a couple of ophthalmologists working at the University of Washington ("they're like our kids," they say). Sam and Dalton were extensively trained in color vision screening tests, judging by the video shot by the team.
Meanwhile, William Hauswirth's team (University of Florida) was developing a method of gene therapy to slide a gene into some cells of the retina, the cones, responsible for color vision. In male squirrels, there are no cones perceiving green and red. They are dichromates (like cats of both sexes), while females - and non-colorblind humans - are trichromatic.
The brain has also found the colors
Two years ago, these two monkeys underwent an intervention to receive genes, of human origin, conveyed into the cones by a harmless adenovirus. These genes are used to make a protein of the opsin family, these light-sensitive pigments that function within a molecular complex called rhodopsin.
After the success of this intervention, the question was twofold. Will the cones thus treated develop a real sensitivity to colors? And, above all, will the brains of Sam and Dalton, who have never seen either red or green, know how to analyze these completely new signals?
According to Maureen and Jay Neitz, who live continuously with both animals, the first effects appeared after only five weeks. Twenty weeks after the intervention, Sam and Dalton began to point, during tests, the red and green areas displayed on the screen. Jay Neitz says he himself was surprised.
The team then continued testing for a year and a half to ensure performance and progress. Today, Sam and Dalton visualize the colors as well or almost as the females of the species. Moreover, after two years, no side effects were noted.
A therapeutic hope
"The addition of a third opsine in an adult who does not see either green or red leads to the normal behavior of a trichromatic vision," the authors conclude in the journal Nature. According to Jay Neitz, experience shows that even in these monkeys that do not recognize all the colors, the neural circuits capable of analyzing them are present in the brain. It can also be interpreted as a great plasticity of the central nervous system.
On the medical level, this result is of great interest. This is not the first time that gene therapy improves vision. Success has already been achieved in dogs and humans in the case of Leber's congenital amaurosis.
It shows that it is possible to treat sight disorders, even when the origin is congenital, which was not self-evident. Moreover, as the authors point out, the DNA used for the repair was human, which is one less problem for the transfer of this technique to humans.
Particularly targeted are vision disorders involving cones, such as color blindness but also loss of central vision, such as macular degeneration, due to age or diseases, including diabetes.