Mice given 'human sight' in colour blindness study

Scientists have created a genetically modified mouse that has the same full-colour vision as humans as part of a study that may open new avenues to correcting colour blindness.

Mice see only in blends of two colours, but an experiment has shown that it is possible to modify their genes so they can see in the three-colour vision of humans and other primates. Men are at greater risk than women of being colour blind because the genes for colour vision are on the X-chromosome and women have two, whereas men have just one.

The scientists created the mouse by inserting a human gene into the genome responsible for generating a particular protein in the retina of the animal's eye that was sensitive to light at the red end of the visible spectrum. Like most mammals, mice can only see light in the blue and green regions of the spectrum, but the GM mice demonstrated that they were also able to learn how to sense red light - just like monkeys, apes and people.

Jeremy Nathans, of the Johns Hopkins Medical School in Baltimore, Maryland, said that the study emulates the important evolutionary transition in mammals from two-colour, dichromatic vision to full trichromatic vision, which is estimated to have occurred in the ancestors of primates about 40 million years ago.

The retinas in the eyes of primates have three types of protein "photoreceptor" that are each responsive to light wavelengths in either the short (blue), medium (green), or long (red) part of the visible spectrum. It is thought that this is an adaptation that allowed primates to see ripened fruit in a forest canopy.

Professor Nathans and his colleagues inserted the gene for the red photoreceptor into mice and trained them to see if they could distinguish red-based colours from other colours. Normal mice failed to discriminate yellow versus red light when the light intensities were set to give equal activation of their middle-wavelength receptor. Mice with both the human long-wavelength receptor and the mouse middle-wavelength receptors learnt to tell the difference, although it took more than 10,000 trials for them to make the distinction, according to the study in the journal Science.

"Each photoreceptor absorbs a range of wavelengths, but the efficiency changes with wavelength," Professor Nathans said. "One photoreceptor might absorb green light only half as efficiently as red light. If an animal had only this type ofphotoreceptor, then a green light that was twice as bright as a red light would look identical to the red one.

"But if the animal adds a second photoreceptor with different absorption properties, then by comparing both receptors the red and green lights could always be distinguished," he added.