Triple negative breast cancers are a subtype of breast cancers affecting a mostly a younger population. They are highly aggressive and metastasize to visceral organs and brain resulting in high mortality. There are no targeted therapies available and chemotherapy is the only form of therapy with limited success. Recently poly (ADP-Ribose) polymerase inhibitors (PARPi) have been approved for this indication, but they are only effective in patients with BRCA mutation or other DNA repair enzyme defects. In addition, most patient still do not respond and survival benefit is yet to be established. Cancer stem cells (CSC) are crucial cells that display unique characteristics and are responsible for Tumourigenesis and metastasis. These cells are resistant to most standard anti-cancer therapies. Previous experiments in the Rao lab have revealed epigenetic modification via lysine-specific demethylase 1 (LSD1) inhibition may regulate epithelial mesenchymal transition (EMT) related genes and transcription. This project explored the interaction between PARP1 and LSD1.This project generated several novel findings: firstly, PARP1 regulated EMT and CSC maintenance, where PARP1 inhibition caused upregulation of EMT and had no effect on CSC. Secondly, LSD1 and PARP1 showed strong colocalization and co-binding on chromatin with unique histone modifications. Thirdly, phenelzine, an LSD1 inhibitor in combination with olaparib (PARP inhibitor) induced unique global differential gene expression including EMT/CSC related genes, MAPK, RAS, PI3K/AKT pathway genes and transcription factors and reduced early tumour growth in the BRCA proficient tumour model. This also unravelled novel targets including TCF3 or BMPR1A/B. Phenelzine inhibited c-MET and HSP90 which render tumours resistant to PARP inhibitors. Finally, LSD1 inhibition with phenelzine reversed the immune inhibitory signal (PD-L1) induced by PARP1 inhibition which will render the tumours susceptible to T-cell mediated killing. This thesis identified HER2+ circulating tumour cells (CTCs) as a potential subset responsible for metastasis, especially brain metastasis where nuclear DUSP6, a MAP phosphatase was identified as a biomarker for resistance and brain metastasis. This outcome may be crucial in future drug developmental therapeutics, especially in early breast cancer. Overall, this project demonstrates for the first time the interplay between and LSD1 and PARP1 and establishes the synergism of PARP1/LSD1 inhibition by unravelling the differential gene expression, histone modifications and immune-related changes. Intra-nuclear LSD1, PARP1 and DUSP6 are potential targets for drug therapeutics with the capability to reduce tumour growth and metastasis, especially brain metastasis in breast cancer.
|Date of Award
|Sudha Rao (Supervisor), David Pyne (Supervisor), Kris Hardy (Supervisor) & Fan Wu (Supervisor)