Two adults with a rare disease that causes gradual loss of eyesight had their vision improved after being treated with a new gene therapy, according to preliminary results from a new study.

The study involved six patients ages 35 to 63 with choroideremia, an inherited condition with no cure that causes vision problems early in life, and eventually leads to blindness. Patients have a mutation in a gene called CHM, which causes light-sensitive cells in the eye to slowly stop working.

The goal behind the new gene therapy is to use a safe virus to deliver a working copy of the gene to the right part of the eye to prevent the cells from degenerating. [7 Diseases You Can Learn About From a Genetic Test]

The new study was an early test of the therapy in which the researchers aimed to carry out the treatment without causing damage to the eye. (Patients must have an eye surgery so that the virus can be injected under the retina with a fine needle).

The result showed that the treatment did not cause harm, and in fact, improved vision in a few of the patients.

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Six months after the treatment, four patients recovered the visual acuity (clearness or acuteness of vision) that they had before the surgery, and developed increased sensitivity to light. And two patients had improvements in vision: They were able to read two to four more lines on a sight chart.

"We did not expect to see such dramatic improvements in visual acuity," study researcher Robert MacLaren, of the Nuffield Laboratory of Ophthalmology at the University of Oxford in the U.K., said in a statement. It is still too early to know if the improvements will last, but they have so far been maintained for as long as two years, MacLaren said.

The study is the first to test gene therapy in patients before they'd experienced significant thinning of the retinal cells, MacLaren said.

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Gene therapy improves vision for some with rare disease