Gene therapy has partly restored colour detection in two children who were born totally colour blind.
There are two types of light-detecting cells in the retina: cones responsible for colour vision, and rods which can detect much lower light levels but cannot differentiate between colours. Achromatopsia (ACHM) is an inherited retinal disorder that disrupts the function of cone cells which results in patients being born completely colour blind. The commonest forms of ACHM are caused by mutations in genes that code for proteins crucial to cone receptor signalling pathways, meaning that these patients still have intact cone cells, but their messages are not getting to the brain.
'Our study is the first to directly confirm widespread speculation that gene therapy offered to children and adolescents can successfully activate the dormant cone photoreceptor pathways and evoke visual signals never previously experienced by these patients' said lead author Dr Tessa Dekker from the University College London Institute of Ophthalmology.
In this clinical trial four children aged ten to 15 years whose ACHM is caused by mutations in the CNGA3 or CNGB3 genes were treated with subretinal gene therapy using adeno-associated virus vectors that expressed CNGA3 or CNGB3.
The researchers employed a novel functional magnetic resonance imaging mapping approach to distinguish post-treatment cone signalling from existing rod signals in the patients. They used a 'silent substitution' technique which involves selectively stimulating rods or cones using a pair of lights. The results obtained from the treated patients were compared to measures from nine untreated patients 28 normal-sighted controls.
Patients received treatment in one eye which enabled doctors to compare the effectiveness of the treatment with the untreated eye.
The findings were published in Brain and the researchers reported that there was strong evidence for cone-mediated signalling in the treated eyes for two out of the four children, six to 14 months after treatment. There was no evidence of cone function prior to treatment, but after treatment the measures obtained from these two patients were comparable to the normal-sighted controls.
'We are still analysing the results from our two clinical trials, to see whether this gene therapy can effectively improve everyday vision for people with achromatopsia. We hope that with positive results, and with further clinical trials, we could greatly improve the sight of people with inherited retinal diseases,' said co-lead author Dr Michel Michaelides from Moorfields Eye Hospital in London.
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