There are currently no treatment options for choroideremia, a rare form of progressive blindness that affects 1 in every 50,000 people. However, an operation that involves injecting a gene into the eye has been shown to improve eyesight as well as stall degeneration.
Choroiderema is caused by a mutation in the CHM gene on the X chromosome, resulting in the degeneration of the choroid, retinal pigment epithelium and retina. This ultimately leads to the death of the photoreceptor cells in the retina, causing complete blindness by the time the patient reaches middle age.
"It's like looking down through a telescope at a small central island of vision," said lead author Robert MacLaren, a professor at the University of Oxford. "And by the time they're in their 40s and 50s, they lose vision completely."
The new gene therapy involves injecting patients with a vital gene that is either missing or defective in their genetic code. The researchers genetically altered an adeno-associated virus (AAV), so that it carried a corrective copy of the CHM gene.
The researchers then injected the genetially engineered virus into the retinas of six patients aged 35 to 63, all afflicted with choroiderema. After six months, the participants were examined, and the findings were published in the Lancet, which says, "Despite undergoing retinal detachment, which normally reduces vision, two patients with advanced choroideremia who had low baseline best corrected visual acuity gained 21 letters and 11 letters (more than two and four lines of vision). Four other patients with near normal best corrected visual acuity at baseline recovered to within one to three letters. Mean gain in visual acuity overall was 3·8 letters."
Prior to this therapy, some suffering from degenerative eye diseases that cause loss of vision were able to try out the Argus II, a bionic eye with a small mounted camera that is under trial in Europe. The Argus II is specially appropriate for those with retinitis pigmentosa, a genetic disorder where the rods and cones of the eye cease from functioning properly.
But with this gene therapy, patients suffering from choroiderema will not be limited to having an implant and wearing a camera over the eye.
MacLaren said that the success of this study could open doors for gene therapy on other conditions that cause loss of vision, as long as they are caused by mutations in only a single gene. "This has huge implications for anyone with a genetic retinal disease such as age-related macular degeneration or retinitis pigmentosa, because it has for the first time shown that gene therapy can be applied safely before the onset of vision loss," MacLaren explained.
