Abstract
Triple negative breast cancer (TNBC) lacks estrogen and progesterone receptor expression and HER2 amplification. It is the most aggressive form of breast cancer exhibiting early and higher rates of metastasis and rapid emergence of chemo- and radio-therapy resistance. Resistance to conventional therapies, relapse and metastasis are thought to be linked to the abundance of the cancer stem cells (CSC) or tumor initiating stem cells (TISC) in TNBC. My thesis project investigated the role of an essential histone methyltransferase, DOT1L in maintaining tumor initiating stem cells in TNBC. We found basal, TNBC-enriched breast cancers have elevated expression of DOT1L, and this is associated with poor outcome in patients. We show that the cancer stem cell (CSC) enriched ALDH1+ population exhibits higher levels of DOT1L and cMyc, and higher global DOT1L mediated H3K79 dimethylation than do ALDH1- cells. Global RNA profiling identified that DOT1L promotes expression of genes associated with oxidative phosphorylation, DNA damage response, mitotic cell division, proliferation and WNT pathway activation in ALDH1+ cells isolated from the TNBC model, MDA-MB-468. Our data reveal that DOT1L governs the self-renewing ability of ALDH1+ cells. DOT1L inhibition significantly decreased the proportion of ALDH1+ cells, sphere formation and MYC and SOX2 expression—all properties of CSC. Inhibiting DOT1L catalytic activity using EPZ5676, a specific and potent DOT1L inhibitor, decreased tumor initiating stem cell abundance, extended tumor latency and decreased metastasis from TNBC xenografts arising from ALDH1+ cells in vivo. Our work identifies DOT1L as a key epigenetic regulator of CSC in TNBC and also provides foundational evidence supporting the potential for use of EPZ5676 to target this rare CSC population as a treatment option for triple negative breast cancer.