The first Phase I clinical trial of a gene therapy for an inherited cause of progressive blindness called choroideremia — an X-linked recessive disease — has shown promising initial results.

The objective of the study was to get the gene into the cells in the retina of the eye using an AAV vector without causing damage. After six months, however, the patients actually showed improvements in their vision in dim light; two of the six patients enrolled for the study were able to read more lines on the eye chart. The authors caution that the results of the study “should not be overinterpreted.” The paper is published today (January 16) in The Lancet. Robert MacLaren, the first author of the study, is from the University of Oxford.

In RPE65-associated retinal degeneration, “the disease starts with loss of night vision in the first decade of life and then progresses with a gradual loss of peripheral vision and legal blindness by the fifth decade,” notes the paper.

Vision loss results from both dysfunction and degeneration of photoreceptors. Since the loss of vision is due to dysfunction, gene therapy is able to restore vision. However, the photoreceptors that have already undergone cell death cannot be saved. The results hold promise for other disorders like age-related macular degeneration and rod-cone dystrophies.

Two patients who had the most advanced disease had “substantial gains.” On the whole, the group of six showed an improvement in maximal retinal sensitivity in the eye that was treated. Five of the six patients who received the full dose of AAV vector had “improvements in mean retinal sensitivity in their detached retinas six months after surgery.”

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