When the owl swooped, the “blind” mice ran away. This was thanks to a new type of gene therapy to reprogramme cells deep in the eye to sense light.
After treatment, the mice ran for cover when played a video of an approaching owl, just like mice with normal vision. “You could say they were trying to escape, but we don’t know for sure,” says Rob Lucas of the University of Manchester, UK, co-leader of the team that developed and tested the treatment. “What we can say is that they react to the owl in the same way as sighted mice, whereas the untreated mice didn’t do anything.”
This is the team’s best evidence yet that injecting the gene for a pigment that detects light into the eyes of blind mice can help them see real objects again.
This approach aims to treat all types of blindness caused by damaged or missing rods and cones, the eye’s light receptor cells. Most gene therapies for blindness so far have concentrated on replacing faulty genes in rarer, specific forms of inherited blindness, such as Leber congenital amaurosis.
The new treatment works by enabling other cells that lie deeper within the retina to capture light. While rod and cone cells normally detect light and convert this into an electrical signal, the ganglion and bipolar cells behind them are responsible for processing these signals and sending them to the brain. By giving these cells the ability to produce their own light-detecting pigment, they can to some extent compensate for the lost receptors, or so it seems.
The team used the human gene for rhodopsin, the pigment used by rod cells to detect brightness. They hooked this up to a genetic “switch” that would only turn on the gene inside ganglion and bipolar cells, and then inserted this DNA, via a virus, into the eyes of mice whose rods and cones had been destroyed.
After treatment, Lucas and his colleagues found that the mice could tell objects apart by their size quite well, although not as well as sighted mice. “The treated mice could discriminate black and white bars, but only ones that were 10 times thicker than what sighted mice could see,” says Lucas.
Just ordinary light
In earlier attempts, mice could only tell objects apart under extremely bright light, so the new finding is crucial. “Our mice could respond in ordinary light, the equivalent of looking at a computer monitor under ordinary office lighting,” says Lucas.
This is also the first time a human gene has been tested this way. The virus they used to deliver the gene therapy is already approved for use in humans, and Lucas says he hopes to begin trials of a human treatment in about five years.
“This is the most effective example yet of the use of genetic therapy to treat advanced retinal degeneration,” says Robin Ali, whose team at University College London has given gene therapy to people with Leber congenital amaurosis.
But Robert Lanza, chief medical officer at Ocata Therapeutics in Marlborough, Massachusetts, warns that we don’t yet know how long the beneficial effects of the new treatment might last. There is some evidence that sight has begun to wane in people with Leber congenital amaurosis who were treated with gene therapy between one and three years ago, according to New Scientist.