Colour blind monkeys 'cured'

Thursday September 17 2009

“Scientists have cured colour blindness in monkeys,” said The Daily Telegraph . It said this signals hope for millions of people with the condition and other more serious defects such as age-related macular degeneration.

Full-colour vision was restored using gene therapy in two adult monkeys that were born without the ability to distinguish between red and green. The newspaper said this type of colour blindness mostly affects men and affects about one in 30,000 people.

The squirrel monkeys used in this experiment could not distinguish the red and green parts of the colour spectrum but could see yellow and blue, so that objects mostly appear grey.

The monkeys’ vision was tested by training them to touch a coloured target on a screen with their heads and rewarding them with fruit juice if they touched the target correctly. After the treatment, the monkeys had a greatly improved accuracy in identifying the colours.

Although more studies are needed, this advance paves the way for research into genetic treatments for a range of genetic eye disorders in humans. Experts say this technique may work for humans who are colour blind but trials in humans are still some distance in the future.

Where did the story come from?

This research was carried out by Dr Katherine Mancuso and colleagues from the departments of ophthalmology at the University of Washington, the University of Florida and the Medical College of Wisconsin. The study was supported by grants from several US organisations including the National Institutes of Health and charitable foundations. It was published in the peer-reviewed journal Nature .

What kind of scientific study was this?

This animal study explored the possibility of curing colour blindness using gene therapy in experiments on two adult monkeys that had been colour blind since birth.

The researchers explain that in red–green colour blindness, substances called photopigments at the back of the eye, which are sensitive to either long or middle wavelengths of light, are missing. In humans, this condition is the most common genetic disorder caused by a defect in a single gene, the L-opsin gene, found on the X chromosome.

A similar condition affects all male squirrel monkeys (Saimiri sciureus) , which naturally see the world in just two colours: blue and yellow. Two versions of the L-opsin gene are needed for full-colour vision. One codes for a red-detecting photoreceptor, the other for a green-detecting photoreceptor. As male squirrel monkeys have only one X chromosome, they carry only one version of the gene, making them red–green colour blind.

Fewer female squirrel monkeys have the condition as they have two X chromosomes, and often carry both versions of the L-opsin gene.

The researchers say that because nerve connections are usually established early during development, it was previously believed that the treatment of inherited vision disorders would be ineffective unless administered to the very young.

The researchers used a virus (a vehicle used by scientists in gene therapy for delivery of specific material into the host) to introduce a human form of the red-detecting L-opsin gene into the monkeys. They injected the virus behind the retina and then assessed the monkeys' ability to find coloured patches of dots on a background of grey dots. They did this by training the monkeys to touch coloured patches on a screen with their heads, and rewarded them with grape juice if they touched the correct patch.

They showed the monkeys a coloured patch of dots surrounded by different coloured dots including grey ones. The intensity of the red target was increased, so the point at which the monkeys could accurately discriminate the red from the grey could be determined.

When the monkeys touched the coloured target, a positive tone sounded and they were rewarded with juice, and the next trial started immediately. If the wrong position was chosen, a negative tone sounded, and a two- to three-second penalty time occurred before the next trial began.

Earlier experiments in three monkeys used a different form of the gene but that gene did not work and the monkeys’ vision did not improve. This latest experiment tested a different form of the gene in two monkeys and used more virus to introduce the gene.

What were the results of the study?

After 20 weeks, the monkeys' colour skills improved and the monkeys could see in three colours or shades. They retained this skill for more than two years with no apparent side effects.

What interpretations did the researchers draw from these results?

The researchers point out that adding the missing gene was sufficient to restore full-colour vision without further rewiring of the brain, despite the fact that the monkeys had been colour blind since birth.

This is an important finding as it implies that the additional cone L-opsin pigment can be added at older ages than previously thought possible.

The researchers say: “This provides a positive outlook for the potential of gene therapy to cure adult vision disorders.”

What does the NHS Knowledge Service make of this study?

The importance of this small animal study lies in its implications for human disease, but this has yet to be tested. Although colour blindness is not a life-threatening disease it can be disabling. The fact that the researchers have shown a cure for a similar condition in monkeys brings the day closer when other cone (the colour sensitive cells) diseases in humans could be treated in this way.

Overall, this was a well-conducted study with some limitations noted by the authors. One issue is that people who are colour blind usually have good vision and so the safety of injections of viruses and genes into the retina needs to be ensured before the treatment can be tested more thoroughly.

Gene therapy trials in humans for more serious forms of blindness known as retinal degeneration have begun, and these use the same principles of viral introduction of genes to the retina. The researchers say that their model of how to do this in monkeys, where all photoreceptors are intact and healthy, will make it possible to assess the full potential of gene therapy to restore eyesight.

Analysis by Bazian
Edited by NHS Choices