Targeting Usher syndrome with CRISPR mediated base editing

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Usher syndrome is an hereditary deaf-blindness syndrome inherited in a recessive manner. There are different types of Usher syndrome displaying a range in the severity of symptoms: Type II is the most common and patients demonstrate progressive retinal degeneration occurring after the onset of deafness. Usher syndrome is commonly caused by point mutations in the USH2A gene that result in a defective version of a protein called usherin. It is thought that the usherin protein is involved in long term maintenance of photoreceptors, the cells involved in light detection in the retina.The aim of this DPhil is to correct individual point mutations in the USH2A gene using a technique called base editing. A guide RNA directs the base editing apparatus to a specific section of DNA adjacent to the point mutation so that an enzyme can change the mutated nucleotide to its original form, thus restoring visual acuity. The base editing system will be delivered to patients inside a viral vector via an injection underneath the retina. This method of delivery has already had success in human clinical trials involving other genetic retinal diseases. After the base editing system has been designed, I will begin by testing its efficacy in vitro and will then progress to testing it on a mouse model of Usher syndrome created by the Harwell Institute. Experiments can then be performed to investigate whether any physiological rescue has occurred. It will be important to optimise the base editing system to reduce any unwanted side effects and to maximise its efficiency. The work involved in this DPhil has the potential to make a significant impact on the lives of patients with Usher syndrome. Additionally, if the study is successful it will provide an excellent basis for future base editing experiments directed at reversing harmful genetic mutations both inside and outside the retina.