Defining the mechanism and specificity of the 53BP1-Rev7 non-homologous end joining pathway in immunity and oncogenesis

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The major aim of this research project is to refine our understanding of the 53BP1 pathway: a poorly understood sub-pathway of the vertebrate non-homologous end joining (NHEJ) DNA repair system that plays a vital role in the human adaptive immune response, yet also drives the initiation and progression of breast, ovarian and other tumours in patients harbouring a faulty copy of the BRCA1 tumour suppressor gene. A key objective of the project is to challenge a widely accepted paradigm for the molecular function of this pathway. We will set out to test a hypothesis that proposes an alternative explanation for this pathway's activity. Namely, that the single-stranded DNA enrichments detected at DNA double-strand break (DSB) sites in cells deficient for the 53BP1 pathway, do not represent product of a failure to protect DNA ends from resection as is widely perceived, but instead represents a structured DNA repair intermediate/substrate that cannot be overcome when the pathway fails. If correct, the genetically modified mice we plan to engineer will be immune-deficicient as a result of an inability to efficiently generate antibody diversity via 53BP1-dependent NHEJ. Nevertheless, cells from these animals will be able to survive and divide relatively healthily in the absence of the activity of BRCA1, a protein normally essential for sustaining normal cell proliferation and the maintenance of genome stability. Both scenarios will provide us with valuable biological tools with which we will attribute mechanism to the 53BP1 DNA repair system, in biological contexts directly related to human health and disease. Other distinct, yet complementary experiments will examine secondary predictions of our model, including one that may explain the context specificity of the 53BP1-dependent DNA repair system, thereby linking its immune system functions to its potent oncogenic activities in cancer.