Targeting Ataxia telangiectasia mutated (ATM) and DNA dependent protein kinase catalytic subunit (DNA-PKcs) for synthetic lethality application in breast cancer

BRCA1 germ-line mutations predispose to hereditary breast and ovarian cancers. Cells lacking functional BRCA1 protein are deficient in the homologous recombination DNA repair pathway. Base excision repair (BER) is essential for processing base damage induced by endogenous and exogenous sources. Recently, BRCA1 was shown to transcriptionally regulate expression of genes involved in BER. The primary aim of the work described in this thesis was to investigate whether targeting the double-strand break pathway in BRCA1-BER deficient cells using ATM or DNA-PKcs inhibitors would be synthetically lethal. DNA repair gene and protein expression in BRCA1 deficient and proficient cells were investigated. Initially 84 DNA repair genes were investigated. Data demonstrated down-regulation of several DNA repair mRNAs in BRCA1 mutant/knockdown cell lines as compared to proficient cell lines. RT-qPCR was performed for selected DNA repair genes and confirmed statistically significant down-regulation of these genes. Protein expression of the selected group was assessed and showed down-regulation. These results suggest that BRCA1 deficiency may be associated with a global defect in the BER pathway. BRCA1 deficient cells were targeted by ATM/DNA-PKcs inhibitors. BRCA1-BER deficient cells were sensitive to ATM and DNA-PKcs inhibitor treatment either alone or in combination with cisplatin and synthetic lethality was evidenced by DNA double strand breaks accumulation, cell cycle arrest and apoptosis. This in vitro study suggests that a potential synthetic lethality relationship exists between BRCA1 deficiency and ATM/DNA-PKcs inhibition. Moreover, results support the hypothesis that cisplatin increases the efficacy of ATM and DNA-PKcs inhibition in BRCA1 deficient cells. Taken together, this study provides the pre-clinical evidence that ATM and DNA-PKcs could be alternative synthetic lethality targets in BRCA1 deficient breast cancer