Gene therapy of achromatopsia
Gene therapy partially restored color vision in colorblind people
Alexandra Medvedeva, Naked Science
Monochromasia is a complete lack of color perception, absolutely normal for some animal species, such as cetaceans and some pinnipeds. In humans, this is a pathological condition called achromatopsia, color blindness or complete color blindness. It is caused by a mutation of one or more genes encoding cone proteins — one of two types of retinal photoreceptors. Since cones are responsible for color vision, people with achromatopsia perceive the color of any wave as gray and distinguish only its brightness. In addition, they have poor eyesight, they suffer from photophobia and nystagmus — involuntary oscillatory movements of the eyeballs.
In achromatopsia, cones do not send signals to the brain, but may be present on the retina. Scientists from University College London (UK) have found a way to activate these dormant cells in order to partially restore color perception to patients with complete color blindness. Preliminary results of the study are published in the journal Brain (Farahbakhsh et al., A demonstration of cone function plasticity after gene therapy in achromatopsia).
Four adolescents with achromatopsia aged 10 to 15 years were selected for the experiment. A child's brain is more neuroplasticity than an adult, because at an early age neural connections develop more actively. Gene therapy included the introduction of adeno-associated viral vectors expressing CNGA3 or CNGB3 genes associated with color blindness. It was in these genes that mutations were detected in four patients. The treatment was carried out only on one eye so that scientists could fully evaluate the result.
The researchers used functional magnetic resonance imaging (fMRI) to separate new brain signals that appeared after treatment from pre-existing signals. This made it possible to accurately record all changes in visual function. The experiments involved pairs of light sources for selective stimulation of cones or rods. The results were compared with tests in which patients with achromatopsia and healthy volunteers took part.
6-14 months after treatment, two out of four children recorded signals that the cones of the treated eye sent to the brain. Their fMRI readings now resembled those of study participants with normal vision. The subjects also underwent a psychophysical test that assessed their ability to distinguish contrast levels. The same two children had a noticeable improvement in vision.
So far, the researchers cannot confirm that the treatment of the other two participants was ineffective — perhaps the result will manifest itself later. In addition, the study demonstrated unprecedented plasticity of neurons in the children's brain: this gives hope that gene therapy can activate visual functions and neural pathways that have been inactive for years.
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