Investigating mechanisms of acquired drug resistance in Triple Negative Breast Cancer

Triple negative breast cancer (TNBC) is an aggressive, heterogenous, metastatic disease characterised by lack of oestrogen, progesterone and human epidermal growth factor 2 receptors. Due to the lack of druggable targets, chemotherapy remains the mainstay of treatment. Whilst patients initially respond to this therapy, they often relapse due to acquired drug resistance. Given the poor outlook, it is evident that an appropriate second line therapy is required following chemoresistance, as well as clinically relevant biomarkers to identify when this change of therapy is required.To address this, a panel of six chemotherapeutic agents and 16 inhibitors of the DNA damage response and repair (DDR) pathways were evaluated as potential second line therapy options against a panel of three chemo-naive and 15 chemo-resistant TNBC cell lines. This showed that the PARP inhibitors, olaparib and rucaparib, and the chemotherapeutic agent, doxorubicin, may be ineffective as second treatment strategies for chemo-refractory TNBC, whilst inhibitors targeting CHK2, RAD51 recombinase and PLK1 may be effective. Analysis of the TNBC cell lines exome sequencing data, in combination with data extracted from The Cancer Genome Atlas, identified 70 genes as candidate biomarkers of chemoresistance. This included a loss of function frameshift variant in TOP2A within the doxorubicin resistant cell line, HCC1806rDOX12.5, as a candidate biomarker of doxorubicin resistance. SiRNA mediated knockdown of TOP2A in the chemo-naive cell line, HCC1806, confirmed a doxorubicin resistance phenotype.In conclusion, this thesis provides a novel insight into the use of chemotherapeutic agents and DDR inhibitors as a potential second line therapy options after the emergence of chemoresistance in TNBC. It identified 70 clinically relevant candidate biomarkers of chemoresistance, and provides new avenues of research for further exploration of these findings. The work presented in this thesis has the potential to advance understanding of chemoresistance in the clinic and improve the outcome of patients with chemo-refractory TNBC.