Gene therapy cures red-green colour blindness in adult monkeys

Published in The Hindu on September 17, 2009

The number on the demo plate (left) can be identified even by the colour-blind. The number on the test plate can be read only by a normal person.

There is hope for millions of people who are unable to see or have trouble seeing red and green colours. Red-green colour blindness is the most common colour-vision disorder. It mostly affects males as it is a gene defect that is carried in the X chromosome.

In India about 13 million people suffer from red-green colour blindness. It is 16 million in the case of China.

While normal people can perceive hues of blue, yellow, green and red, those with red-green colour deficiencies can see only hues of blue and yellow.

Scientists from the University of Washington, University of Florida and the Medical College of Wisconsin have been able to cure red-green colour blindness in adult monkeys that are born blind to these two colours by resorting to gene therapy.

“Treated monkeys unquestionably respond to colours that were previously invisible to them,” they write. Their experiments were on squirrel monkeys (Saimiri sciureus). Their work is published online in the journal Nature.

Correction possible on adults

What makes the study particularly important is the fact that researchers used adult monkeys for the experiment. According to the authors, their work has disproved the long held notion that colour blindness cannot be corrected in adults.

They have also debunked the notion that neural connections are established during development when a child is very young, and any corrections done later will be of little use as the neural connections are not present.

Ability to seen red-green “can arise from single gene addition of a third cone class and it does not require an early development process,” they note. “This proves a positive outlook for the potential of gene therapy to cure adult vision disorders.”

It took nearly five months for the treated monkeys’ threshold to perceive blue-green and red-violet improved.

Based on the improvement, the authors stress that “some form of inherent plasticity” in the monkeys’ visual system was present which enabled the adult ones to acquire the new colour vision.

Taking advantage of pre-existing neural circuitry

“These experiments demonstrate that a new colour-vision capacity, as defined by new discrimination abilities, can be added by taking advantage of pre-existing neural circuitry,” they note.

The experiment shows that restoring full vision could have happened in the absence of any other change in the visual system except the addition of a third cone type using gene therapy.

It may be possible in the future to cure red-green colour blindness in people using gene therapy provided all photoreceptors are intact and healthy. The age when such a correction is done may not be an issue, as this experiment shows.

According to them, the new colour vision acquisition does not necessarily mean that any rewiring of the neural circuitry has taken place. Though the monkeys certainly responded to colours that were previously invisible to them, the researchers are not sure if the monkeys experienced any new internal sensations to red and green.