UBC Theses and Dissertations

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UBC Theses and Dissertations

Identification of synthetic cytotoxic interactions to improve the efficacy of DNA damaging therapeutic agents Moshgabadi, Noushin


Cancer therapies ideally selectively kill tumour cells and have little effect on normal cells. However, many current therapies are toxic to both tumour and normal cells. Differential killing can be achieved by exploiting the differences between tumour cells and normal cells. Tumour specific somatic mutations can be leveraged to affect selective killing by targeting secondary proteins required for the viability of mutant cancer cells. This rationale is called synthetic lethality (SL). Genome instability (GIN) genes, which are frequently mutated in tumours, are good candidates for finding SL interactions in combination with DNA repair enzyme inhibitors. If a somatically mutated GIN gene is involved in a DNA repair pathway, then inhibiting a second, parallel DNA repair pathway enzyme might trigger SL due to an inability to repair endogenous DNA damage. The combination of a GIN mutation and a DNA repair enzyme inhibitor would not be expected to result in SL in all cases. However, it may be possible that the combination of a GIN mutation and inhibition of a DNA repair enzyme could sensitize cells to low doses of a DNA damaging agent (DDA). We termed this phenomenon “synthetic cytotoxicity” (SC). The aim of my research was to discover and characterize SC relationships between GIN mutations found in tumours and loss of function of DNA repair enzymes, using Saccharomyces cerevisiae as a model experimental organism. I tested a matrix of 27 GIN mutations, three DNA repair enzyme mutations, and six different DDAs to test single and double mutants in the presence and absence of low concentrations of DDAs and discovered 6 SL interactions, 21 SC interactions, and surprisingly 19 cases of phenotypic suppression (PS). I also characterized the SC interaction between tpp1⊿ and mre11⊿ (homologous recombination defective) in response to bleomycin and revealed a crucial role for Tpp1 in the non homologous endjoining DNA repair pathway. I propose a model in which the deletion of both Mre11 and Tpp1 inhibits the resection of bleomycin-induced double strand DNA breaks, resulting in unrepaired DNA damage and synthetic cytotoxicity.

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