Two men with progressive blindness have partially regained their vision after taking part in the first clinical trial of a certain gene therapy. The men were among six patients to have experimental treatment for a rare, inherited, disorder called choroideremia, which steadily destroys eyesight and leaves people blind in middle age.

After therapy to correct a faulty gene, the men could read two to four more lines on an optician’s sight chart, a dramatic improvement that has held since the doctors treated them.

Writing in The Lancet, doctors maintain that further trials are as effective, the team could apply for approval for the therapy in the next five years. Some other forms of blindness could be treated in a similar way.

Choroideremia is caused by a faulty gene, called CHM, on the X chromosome. The disease mostly affects men because they have only one copy of the X chromosome. Women have two copies of the X chromosome, so a healthy version of the gene on one chromosome can largely make up for any defects on the other.

The therapy uses a genetically modified virus to smuggle healthy copies of the CHM gene into light-sensitive cells in the retina and supporting tissue called retinal pigment epithelium. Surgeons injected modified virus particles behind the retinas of the patients in an operation that could be completed in an hour under general anaesthetic.

The therapy only works on cells that have not been destroyed by the disease. It cannot replace cells that have died off. — © Guardian Newspapers Limited, 2014

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