Molecular Mechanisms of Chemotherapy Resistance in Oestrogen Receptor Positive Breast Cancer

Resistance to chemotherapy is a major obstacle to the successful treatment of breast cancer. In order to optimise treatment regimens and identify novel drug targets, it is important first to understand the molecular mechanisms that can lead to chemoresistance. In this thesis, I have investigated mRNA and miRNA expression profiles of tumour samples taken from patients with oestrogen receptor positive primary breast cancers treated with neoadjuvant chemotherapy (NAC) who displayed only a partial response. Samples were analysed before and after chemotherapy treatment. Gene expression profiles post-NAC suggested that the MAPK and PI3K-AKT pathways were activated. MiRNA expression profiles demonstrated three miRNAs that were consistently deregulated post-NAC. Further in vitro studies revealed that the increased expression of miR-26b and miR-195 contributed to significant increases in chemoresistance (p<0.05). Pulldown assays using mimics of miR-26b and miR-195 as bait, together with RNA-Seq, led to the identification of possible mRNA targets of these two miRNAs. Further in vitro studies confirmed REEP4 and SEMA6D as targets of miR-26b and miR-195 respectively. As targets of these miRNAs, decreased expression of these mRNAs would be expected to contribute to chemoresistance. Chemosensitivity assays suggested a consistent but not significant increase in resistance when REEP4 was silenced, and a significant increase in resistance when SEMA6D was silenced (p<0.05). Investigations were performed to determine whether the expression of either of the corresponding proteins had any prognostic value. Results suggested that REEP4 expression was significantly related to disease free survival, although the precise relationship was unclear. The effect of increased expression of the xenobiotic drug pump BCRP induced by endocrine therapy on chemoresistance was also investigated. Results suggested that increased BCRP expression led to significant increases in chemoresistance (p<0.05), thus suggesting that a treatment regimen of endocrine therapy followed by chemotherapy may not be beneficial. I have identified in this thesis several molecular changes that are induced by chemotherapy or endocrine therapy that contribute to chemoresistance, including changes in mRNA, miRNA and xenobiotic drug pump expression.