Scientists in the USA have used gene therapy to restore full vision to two male squirrel monkeys with red-green colour blindness, raising hopes that the technique might one day be used to treat humans. The researchers injected both monkeys with the human form of a gene which enables detection of the colour red. Five months after the treatment, the monkeys were able to successfully identify a red pattern on a background of grey dots.
'We've added red sensitivity to cone cells in animals that are born with a condition exactly like human colour blindness,' said William Hauswirth, professor of ophthalmic molecular genetics at the University of Florida, and co-author of the study. 'We've shown we can cure a cone disease in a primate, and that it can be done very safely, that's extremely encouraging for the development of therapies for human cone [diseases] that are really blinding,' he added.
Colour blindness is the most common form of genetic disorder in humans, affecting one in 12 men and approximately one in 30,000 people. The condition is caused by a lack of either the red- or green- sensitive photoreceptor protein usually present in the cones (colour-sensing cells) of the retina.
The US researchers taught the monkeys to undertake colour vision tests, similar to tests given to school children who are asked to identify a specific pattern of coloured dots among a field of dots varying in size, colour and intensity. The monkeys used a touch-screen computer to select images containing red, and on choosing correctly, were rewarded with grape juice.
Before treatment, the animals simply guessed which screen was coloured, at a success rate of around 33 per cent. Fifteen weeks after the gene injection, the monkeys' success rate was increased to 95 per cent.
'We used human DNA so we won't have to switch to human genes as we move toward clinical treatments,' explained Hauswirth. However, Jay Neitz, a professor of ophthalmology at the University of Washington who trained the monkeys with his wife Maureen and co-wrote the study, warned that 'the biggest issue is that people who are colour blind have very good vision, so before people are going to want to treat colour blindness, you're going to want to ensure that this is completely safe, and that's going to take some work.'