Developing gene therapy to treat blindness caused by Stargardt Disease

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Stargardt disease leads to blindness in young people due to loss of the light sensing cells known as photoreceptors that line the retina at the back of the eye. Stargardt disease is caused by the deficiency of a protein known as ABCA4 which stands for "ATP Binding Cassette subtype A4". This protein is very important for recycling the visual pigments necessary for the light-sensitive photoreceptors. When there is a deficiency of ABCA4, the light-sensitive pigments accumulate in the retina which leads to toxic damage and eventual cell death. Stargardt disease usually presents in childhood and there then follows a period of several years of progressive loss of sight and blindness in many cases. There is therefore a window of opportunity of several years after diagnosis in which to intervene with some form of treatment to prevent sight loss.Gene therapy is a new technique which has been shown to be highly effective at reintroducing defective genes into the retina. Gene therapy was first used in this manner in 2007 in three clinical trials to treat inheriuted blindess caused by defiency of anothe gene known as RPE65. More recently we have started a new gene therapy treatment in Oxford to treat a photoreceptor disease known as choroideraemia and several patients have been treated so far without adverse effects. In these gene therapy trials we are using a small viral particle known as adeno associated virus, or AAV. This viral particle is one of the smallest lifeforms known and it is not associated with any disease in humans. The virus survives by remaining dormant and undetected by the immune system. We have taken advantage of AAV by removing the viral genes and replacing them with the gene that is missing for the particular retinal disease. The ability of the virus to evade the immune system is very helpful because the lack of inflammation means that the virus does not damage the retina when injected into the eye. The AAV particle is however too small to carry many genes that are missing in certain types of retinal degeneration. Unfortunately the ABCA4 gene, which would need to be replaced to cure Stargardt disease, is just too large to fit into AAV.The purpose of this project is to solve this problem by exploring a new property which has recently been identified in relation to AAV. It has recently been discovered that a large gene can be broken into two segments each of which is carried by an AAV particle and these particles are able to recombine the gene back into full length after infecting nerve cells. In this project we aim to perform a series of experiments whereby we optimise the process of recombining two fragments of the ABCA4 gene in order to deliver its successfully into photoreceptors. We will validate the function of the gene in a genetically engineered mouse, which is also deficient of the same gene and has features similar to Stargardt disease on retinal examination. If we can correct or even improve the deficiency of ABCA4 using our new vector then we would have the ideal background information to support a new clinical trial.