Leber congenital amaurosis (LCA) is the name of a group of hereditary diseases that cause blindness in infants and children. Changes in any one of a number of different genes can cause the blindness, which usually comes on quite soon after birth. About 20% of children with LCA have the most common type called LCA1 which is caused by defects in a gene called retinal guanylate cyclase (GUCY2D). Defects in the gene leave the light sensitive photoreceptors in the eye unable to respond to light, and so cause blindness. Loss of function of these receptor cells leads to their degeneration and eventually breakdown of the retina –the light sensitive tissue at the back of the eye.
There is an animal model of LCA1, a strain of chickens with a mutation in the chicken GUCY2D gene. These chickens develop a disease very similar to that of humans. Gene therapy works by replacing a defective gene with a normal functional one, usually by packaging the normal gene into a harmless virus and injecting it into the affected area, in this case the eye.
The researchers injected a virus containing the normal gene into chicken embryos that had the abnormal gene, and allowed the chicks to hatch normally. Of seven treated chicks, six appeared to develop sight, in contrast to untreated animals. The improvement in the chicks' sight occurred despite the fact that only a relatively small percentage of the receptor cells were infected by the virus.
This study shows that in theory it is possible to treat this form of childhood blindness by gene therapy. However, before clinical trials can be considered for humans, researchers would need to demonstrate that the virus is safe in humans, and additional studies would need to be done to improve the virus and determine the dose required to achieve long term restoration of sight. In addition, it will be important to examine existing patients to determine when therapeutic intervention would be most appropriate.
Citation: Williams ML, Coleman JE, Haire SE, Aleman TS, Cideciyan AV, et al. (2006) Lentiviral expression of retinal guanylate cyclase-1 (RetGC1) restores vision in an avian model of childhood blindness. PLoS Med 3(7): e201.
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