'This trial is the first application of gene therapy to neurons in the retina', said Robert MacLaren, professor of ophthalmology at the University of Oxford, who led the study. 'Our findings hold great promise for gene therapy to prevent loss of sight in other retinal diseases'.
Choroideremia is a condition where the retina degenerates and vision gets progressively worse, and it affects one in 50,000 people. It is caused by mutations in a single gene, CHM, which is present on the X chromosome, meaning the condition affects mostly men.
In an operation that took less than an hour, surgeons carefully detached the retina from the back of the eye before injecting the virus underneath it.
The first person to undergo the procedure, Jonathan Wyatt, 65, had lived with worsening vision for more than 40 years. 'My eyesight has improved enormously after the operation', he said.
'Before the operation, I had to work hard to read 17 letters on the Snellen [eyesight] chart. Six months afterwards, I could manage 42 letters', Wyatt said.
Toby Stroh, 56, received treatment in February 2012. 'Immediately after the operation, I could see another couple of lines on the sight chart', he said.
'For the last 30 years, I'd been living under the awful, insidious inevitability of going blind, and now as a result of this work, there's a very real prospect that I will continue to be able to see', Stroh added.
In people with choroideremia, the retina gradually gets smaller as cells die off. Light-sensing cells in the retina cannot divide, so new cells cannot be made. However, this could be an advantage in gene therapy, as genes from the one-off injection will remain within their target cells, meaning effects should be long-lasting.
'It is still too early to know if the gene therapy treatment will last indefinitely, but we can say that the vision improvements have been maintained for as long as we have been following up the patients, which is two years in one case', said Professor MacLaren.
The researchers are now testing a higher dose of the gene-carrying virus in a further six patients.
There are 195 known genes linked to retinal degeneration, and around one-third of eye disorders are genetic in nature. The researchers hope that similar therapies can be developed for more complex eye disorders such as age-related macular degeneration and retinitis pigmentosa.
'This is an exciting scientific development and a step in the right direction for treating retinal diseases with gene therapy; however, it is early days yet', Clara Eaglen, eye health campaigns manager at the Royal National Institute for the Blind told BioNews.
'The findings are positive and do support the case for conducting more research in this area but extensive trials would be needed to establish efficacy and safety', she added.