| Summary: | Topoisomerase II β-binding protein 1 (TopBP1) is an essential BRCT domain containing protein involved in DNA damage signalling, replication and repair. BRCT domains have the ability to bind single and double stranded DNA as well as phosphopeptides and are found within a large group of proteins involved in the DNA damage response. These domains give the proteins the ability to bind damaged DNA and change their interacting partners following DNA damage, making their role in the DNA damage response essential for efficient signal transduction and the activation of DNA damage checkpoint and repair. TopBP1 binds single and double stranded DNA and co-localises to sites of DNA damage with other sensory proteins including BRCA1, NBS1 and Rad9 and is an essential protein in the activation of the downstream checkpoints and the initiation of cell cycle arrest. No other protein contains as many as eight BRCT domains indicating that TopBP1 plays an essential role in these responses. TopBP1 has been implicated as a possible transcription factor activating transcription of the HPV16 transcription/replication protein E2 and repressing the activation of E2F1 and Miz1. The modification of chromatin is essential for all functions of TopBP1; this thesis has describes the identification of three chromatin modification domains within three distinct BRCT domains of TopBP1. The function of one of these domains is mediated by a direct interaction with the chromatin remodelling enzyme Brgl and the transcriptional regulation by this protein-protein interaction is regulated by DNA damage. The identification of these domains and their regulation by DNA damage provides further insight into the mechanism of DNA damage signalling, repair, transcription and replication by TopBP1 via the modification of chromatin. In addition to the identification of chromatin modification domains, gene targets of TopBP1 were also identified. This was carried out using MCF7 cells with endogenous TopBP1 down-regulated by siRNA treatment. These studies identified several pathways regulated by TopBP1 expression, all of which play a role in breast carcinogenesis. These results provide more evidence supporting a role for TopBP1 in the development of breast cancer; previous studies have shown aberrant expression of TopBP1 in breast cancers as well as polymorphisms pre-disposing towards this disease.
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