The researchers, led by Susan Semple-Rowland of the University of Florida, carried out their work on a type of Rhode Island Red chicken which carries a genetic mutation making it blind at hatching. This organism provides an experimental model which closely resembles a human disorder called Leber's congenital amaurosis (LCA) type 1.
LCA is a group of retinal disorders that are a common cause of inherited blindness in infants and children. Several types of LCA are caused by mutations in the GUCY2D gene, which encodes retinal guanylate cyclase-1, an enzyme involved in phototransduction (the conversion of light energy into electrical impulses).
Using fine glass needles, Semple-Rowland and her team injected a lentivirus-based vector carrying the GUCY2D gene through the eggshell into the developing nervous system of early-stage Rhode Island Red chicken embryos.
"The process sounds straightforward but it really isn't," said Semple-Rowland. "It took quite a long time to build the vector, develop the injection procedure and figure out how to hatch the eggs. By doing the injection early during development, we actually treat the cells before they become photoreceptors." Subsequent behavioural tests showed that vision had been restored in six out of seven of the birds, as a result of transfer of the GUCY2D gene, and electrophysiological analyses showed that retinal cells from the chicks were responsive to light.
"The vision capabilities of the treated animals far exceeded our expectations," says Semple-Rowland, who is an associate professor of neuroscience at the University of Florida's McKnight Brain Institute.
The work shows that, at least in principle, a similar treatment may be developed for humans. Children with LCA are not born blind but experience gradual degeneration of the retina. A future treatment would involve an injection of vectors directly into the eyeball during the first few years of life.
Semple-Rowland is optimistic about the development of such a treatment, saying that "the first treatments for some of these genetic eye diseases [will be available] soon."