Color blindness that runs in families, rather than being caused by medication or an underlying condition, has had no treatment
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A gene therapy has enabled people with a rare form of total color blindness to see faint red. In a small trial, those who could previously only detect shades of gray set out to distinguish a red object from its dark background.
Ayelet McKyton at the Hebrew University of Jerusalem, Israel, and her colleagues studied four people with a rare condition called achromatopsia. In about 1 in 30,000 to 40,000 peopleachromatopsia disrupts the cone cells of the eyes, which are otherwise responsible for seeing colors.
The participants, three of whom were adults and one was 7 years old, all had some version of the condition caused by a single genetic mutation. The researchers therefore hoped that inserting working copies of the defective gene into cone cells would provide some measure of color vision.
To test the idea, they injected a virus carrying the correct gene into the subretinal region, home to cone cells, in one eye of each participant. “The virus then enters the cells with the faulty gene and corrects it,” says Mckyton.
In the hours following the procedure, there were no major changes in the participants’ vision, but in the months that followed, some reported seeing shades of gray that “glow” differently than before the injection, Mckyton says.
After running a series of tests, the researchers found that the participants could see red objects against a dark background in their treated eye, when they couldn’t see the color at all beforehand.
A previous study that gave gene therapy to sheep used to model human achromatopsia found that the animals developed full color vision. In people with the condition, their so-called rod cells, which are highly sensitive to light and provide night vision, are active in light, preventing them from going blind during the day. These cells are inactive during the day in sheep with achromatopsia or those without the condition.
In the trial, the active rod cells may have interfered with the signal from the treated cone cells, preventing the participants from seeing in color, Mckyton says. However, they may have been able to see red because rod cells are particularly insensitive to its wavelength, she says. Thus, the rod cells remained inactive when exposed to red and the signals from the cone cells were not disrupted.
Mckyton isn’t sure if treatment can be modified to more effectively treat achromatopsia. “We don’t know how to silence the bars,” she says. “But I think it’s generally a good thing that these people have active rods, otherwise they would have been blind.”
Gene therapies probably won’t work for other types of color blindness, because they’re generally not caused by a single mutation that can be corrected, Mckyton says.
“This is an intriguing study that highlights the complexity of developing a therapy for regaining color vision,” says Abigail Hackam at the University of Miami, Florida. The participants’ brain circuitry for color vision may be relatively dormant and not activated enough to regain color vision after the injection, hence the therapy’s limited effectiveness, she says.
The participants will be followed for several years, and the injection can then be repeated on their other, untreated eye, says Mckyton.
Subjects: