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First Gene Therapy Results for Choroideremia Suggest Cautious Optimism

Jan 16, 2014 - The first human trial of a gene therapy for choroideremia has announced preliminary results on six people; two of these patients have experienced significant improvements in vision. The findings were published in the British journal, The Lancet.

Dr. Robert MacLaren performing the surgery to inject choroideremia gene therapy.The novel gene treatment was developed by Dr. Robert MacLaren, at Oxford University, in collaboration with Dr. Miguel Seabra of Imperial College London. Dr. MacLaren is shown here performing the surgery to insert the gene therapy. This therapy is designed to provide the gene whose function is missing in people with choroideremia and intended to stop the deterioration that gradually leads to blindness. A trial of the same therapy has been funded at the University of Alberta under the direction of Dr. Ian MacDonald. Potential participants are currently being screened at the Canadian site, and a letter from Health Canada allowing the trial to go forward was received in December 2013 - although some conditions remain to be met. The first Canadian patients will  be treated this year.

Dr. MacLaren and colleagues reported on the effect of gene therapy on retinal and visual function in six patients aged 35–63 years with different stages of choroideremia. Choroideremia is a condition that develops slowly with the most severe effects occurring late in life. It is caused by mutations in the gene REP1 and the loss of the protein that gene would normally make. Four of the men treated in the study had near normal visual acuity, while two had more severe vision loss.

The treatment procedure requires a temporary detachment of the retina and an associated loss of vision. However the treatment caused no lasting harm and all six patients treated in this initial group recovered their visual acuity from before the procedure. In one patient, some complications were encountered during the surgery, so that the dose of viral vector and the way it was delivered needed to be modified.

Six months after delivery of the gene, the two most visually impaired patients both experienced significant improvements in how well they could see. Both could read more letters from an eye chart, one was able to read three additional lines.

For the four patients with relatively good vision before the trial, benefits were present, but less obvious. Technical measurements showed that all the patients have some increase in retinal light sensitivity, and all the treated eyes are producing the proteins missing in the eyes of people with choroideremia.

In the conclusion to their paper, the scientists note that this is the first report on "the effects of retinal gene therapy in patients with good visual acuity of 6/6 or better and before the onset of clinically significant retinal thinning are described. These findings support the development and use of gene therapy to prevent loss of sight in other retinal diseases.”

However researchers caution that these preliminary findings do not yet show that this treatment will halt, or even slow, vision loss in these patients. In fact, small but measurable degenerations of the retina have been measured in these patients over the two years since the treatment began. Since the development of choroideremia is so slow, it is difficult to determine the degree to which this therapy may be protecting vision.

A commentary by the respected vision scientists Dr. Hendrik Scholl from John Hopkins University, and Dr. José Sahel from the Institut de la Vision, Paris, France, says, "The ultimate goal of gene therapy in choroideremia would be to save the central retina from further degeneration. The short follow-up of the new study prevents any conclusion about preventing degeneration in the long term; indeed, results from the morphological assessment suggested that degeneration is rather continuous…It remains to be determined if gene therapy targeting REP1 will have an effect on the progression of photoreceptor degeneration. Even if the effect turns out to be only slight, this might have important positive implications."

In their research paper, Drs. MacLaren and Seabra also discuss how the dosage and placement of the therapeutic injection may influence the effect of the therapy. Six additional patients are in the early stages of treatment in London, and the Canadian patients will be starting treatment soon. Dr. MacDonald confirms that the Canadian and British sites are in close contact, sharing information so that the Canadian study can build on the knowledge being gained by the British team.

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