Aside from helping those with red–green blindness perhaps become a pilot or just select a better tie, the researchers also hope to eventually treat those who are blind to all colors. Called achromatopsia, the hereditary disease affects about one in 30,000 people in the U.S. Afflicted individuals can only see with 5 percent of their photoreceptors and thus have a very low level of vision.
The success with monkeys also gives hope for other retinal disorders. "The principal is related to almost everything," Neitz explains.
Ongoing phase I clinical trials have reported safe gene therapy treatment for the heritable Leber's congenital amaurosis (LCA), a rare degenerative disease which can cause near-blindness by young adulthood. The results have been welcomed in the gene-therapy field, whose successes have occasionally been overshadowed by negative health impacts and even death.
Stout notes that in order to make it to the next steps, color-blindness research will have to demonstrate that the treatment consistently targets the proper photoreceptor cells and that both the virus and the genetic material are safe. "It's hard to imagine that the expression of a cone [a type of photoreceptor] would cause a problem, but in someone who has never had that, you could have the potential issue of rejection," Stout says.
Gene therapy also has the potential to cure other forms of vision impairment that are not inherited genetic mutations. Stout and his team are working on genetic therapy treatments for vision problems "where there's not a gene that's caused a problem, but it's age and a combination of proteins that are [or are not] being expressed," such as age-related macular degeneration. He also sees promise in the advancement of stem cell research to help nurse defective retinal cells back into health. The application of such nascent findings, however, is still years away.
In the meantime, Jay and Maureen Neitz, along with their team, will continue to test Dalton and Sam's vision daily and watch for side effects. Two years in, however, the lucky monkeys haven't showed any signs of fading to gray.