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Gene therapy for Leber congenital amaurosis: promising results in phase 1


Three young adults with an inherited form of blindness showed evidence of improved day and night vision following a specialized gene transfer procedure in a phase 1 clinical trial funded by the National Eye Institute ( NEI ), part of the National Institutes of Health.
In addition no adverse effects from the therapy were reported.

These findings are published in the Proceedings of the National Academy of Sciences ( PNAS ) and in the Human Gene Therapy. These new reports extend the findings of two other papers published earlier this year in the New England Journal of Medicine.

Patients in the study had one genetic form of Leber congenital amaurosis ( LCA ) caused by mutations in the RPE65 gene. In this form of LCA, retinal neurons called photoreceptor cells do not respond to light because the defective RPE65 proteins cannot produce sufficient vitamin A molecules necessary for healthy vision. Vision loss is severe from retinal degeneration in all forms of LCA. However, unlike many other forms of LCA, the RPE65 disease retains some relatively intact retina. Knowledge of the exact retinal locations of these non-functioning photoreceptor cells provides the opportunity to target the therapy and overcome the RPE65 gene defects.

Patients in the study received a subretinal injection to replace the nonfunctioning gene in pre-selected regions of the retina with less degeneration of photoreceptor cells. Each patient had visual impairment that had been present since birth due to the defective RPE65 gene. Over the 90-day period of the study, gene therapy was associated with improvement of visual function. This research was conducted at the University of Pennsylvania, Philadelphia, and the University of Florida, Gainesville.

This study is the first to show that gene therapy can improve both day and night vision in patients with LCA. Day vision was improved by 50-fold and night vision by 63,000-fold compared to pre-treatment levels. Restored vision was localized to the area of treatment in the treated eye.

Researchers in this study were first to examine the enzymatic cycle of vision targeted by the treatment by measuring the speed with which the patients’ vision adjusted from bright to dim environments. They learned that, while the speed of day vision was near normal, that of night vision took more than eight hours to adjust to darkness as compared to one hour in normal eyes. " We did not suspect this from pre-clinical studies. The first clues came while interviewing patients about their visual experiences after treatment, and we immediately altered our testing strategies appropriately " says Artur V. Cideciyan, at the University of Pennsylvania. " In future studies, we will seek ways to make the restored vision even more useful to the daily lives of patients."

" The converging results from three independent and contemporaneous clinical trials are remarkably encouraging for patients and for researchers and clinicians who have worked tirelessly for decades in the field of retinal degenerative disease " said Samuel G. Jacobson of the University of Pennsylvania’s Scheie Eye Institute, principal investigator of the trial.

Given the positive results, the study will now be expanded to include more patients, confirm the safety and effectiveness of the therapy and advance the gene transfer techniques.

Source: National Institutes of Health, 2008

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