Multiple myeloma (MM) is a hematological malignancy of antibody producing plasmablasts/plasma cells. MM is characterized by extensive genetic and clonal heterogeneity, which have hampered the attempts to identify a common underlying mechanism for disease establishment and development of appropriate treatment regimes. This thesis is focused on understanding the role of epigenetic regulation of gene expression mediated by the polycomb repressive complexes 1 and 2 (PRC1 and 2) in MM and their impact on disease biology and therapy. In paper I the genome-wide distribution of two histone methylation marks; H3K27me3 and H3K4me3 were studied in plasma cells isolated from newly diagnosed MM patients or age-matched normal donors. We were able to define targets of H3K27me3, H3K4me3 and bivalent (carry both marks) which are, when compared to normal individuals, unique to MM patients. The presence of H3K27me3 correlated with silencing of MM unique H3K27me3 targets in MM patients at advanced stages of the disease. Notably, the expression pattern of H3K27me3-marked genes correlated with poor patient survival. We also showed that inhibition of the PRC2 enzymatic subunit EZH2 using highly selective inhibitors (GSK343 and UNC1999) demonstrated anti-myeloma activity using relevant in vitro models of MM. These data suggest an important role for gene repression mediated by PRC2 in MM, and highlights the PRC2 component EZH2 as a potential therapeutic target in MM. In paper II we further explored the therapeutic potential of UNC1999, a highly selective inhibitor of EZH2 in MM. We showed that EZH2 inhibition by UNC1999 downregulated important MM oncogenes; IRF-4, XBP-1, BLIMP-1and c-MYC. These oncogenes have been previously shown to be crucial for disease establishment, growth and progression. We found that EZH2 inhibition reactivated the expression of microRNAs genes previously found to be underexpressed in MM and which possess potential tumor suppressor functions. Among the reactivated microRNAs we identified miR-125a-3p and miR-320c as predicted negative regulators of the MM-associated oncogenes. Notably, we defined miR-125a-3p and miR-320c as targets of EZH2 and H3K27me3 in MM cell lines and patients samples.  These findings described for the first time PRC2/EZH2/H3K27me3 as regulators of microRNA with tumor suppressor functions in MM. This further strengthens the oncogenic features of EZH2 and its potential as a therapeutic target in MM. In paper III we evaluated the therapeutic potential of targeting PRC1 in MM using the recently developed chemical PTC-209; an inhibitor targeting the BMI-1 subunit of PRC1. Using MM cell lines and primary cells isolated from newly diagnosed or relapsed MM patients, we found that PTC-209 has a potent anti-MM activity. We showed, for the first time in MM, that PTC-209 anti-MM effects were mediated by on-target effects i.e. downregulation of BMI-1 protein and the associated repressive histone mark H2AK119ub, but that other subunits of the PRC1 complex were not affected. We showed that PTC-209 reduced MM cell viability via significant induction of apoptosis. More importantly, we demonstrated that PTC-209 shows synergistic anti-MM activity with other epigenetic inhibitors targeting EZH2 (UNC1999) and BET-bromodomains (JQ1). This work highlights the potential use of BMI-1 and PRC1 as potential therapeutic targets in MM alone or in combination with other anti-MM agents including epigenetic inhibitors.

Cyclin-Dependent Kinase 9 (CDK9) as part of the PTEFb complex promotes transcriptional elongation by promoting RNAPII pause release. We now report that, paradoxically, CDK9 is also essential for maintaining gene silencing at heterochromatic loci. Through a live cell screen, we discovered that CDK9 inhibition reactivates epigenetically silenced genes in cancer, leading to restored tumor suppressor gene expression and cell differentiation, along with activation of endogenous retrovirus (ERV) genes. CDK9 inhibition results in dephorphorylation of the SWI/SNF protein SMARCA4 and represses HP1α expression, both of which contribute to gene reactivation. Based on gene activation, we developed the highly selective and potent CDK9 inhibitor MC180295 (IC50 =5nM) that has broad anti-cancer activity in-vitro and is effective in in-vivo cancer models. Additionally, CDK9 inhibition sensitizes with the immune checkpoint inhibitor α-PD-1 in vivo, making it an excellent target for epigenetic therapy of cancer. / Molecular Biology and Genetics

Immunotherapy of cancer has been shown to be promising in prolonging patient survival. However, complete elimination of cancer and life-long relapse-free survival remain to be major challenge for anti-cancer therapeutics. We have previously reported that ex vivo reprogramming of tumor-sensitized immune cells by bryostatin 1/ionomycin (B/I) and the gamma-chain (γ-c) cytokines IL-2, IL-7, and IL-15 resulted in the generation of memory T cells as well as CD25+ NKT cells and CD25+ NK cells. Adoptive cellular therapy (ACT) utilizing these reprogrammed immune cells protected FVBN202 mice from tumor challenge, and overcame the suppressive functions of myeloid-derived suppressor cells (MDSCs). We then demonstrated that the presence of CD25+ NKT cells was required for anti-tumor efficacy of T cells as well as their resistance to MDSCs. Similar results were obtained by reprogramming of peripheral blood mononuclear cells (PBMC) from patients with early stage breast cancer, demonstrating that an increased frequency of CD25+ NKT cells in reprogrammed immune cells was associated with modulation of MDSCs to CD11b-HLA-DR+ immune stimulatory cells. Here, we tested the efficacy of immunotherapy in a therapeutic setting against established primary breast cancer (Chapter One), experimental metastatic breast cancer (Chapter Three) as well as against minimal residual disease (MRD) in patients with multiple myeloma (Chapter Two). We evaluated the ability of reprogrammed immune cells, including CD25+ NKT cells, to convert MDSCs to myeloid immune stimulatory cells, in vivo; this resulted in the identification and characterization of a novel antigen presenting cell (APC). These novel immune stimulatory cells differed from conventional APCs, including dendritic cells (DCs) and macrophages. We have also demonstrated that enhancing immunogenicity of mammary tumors by treatment with Decitabine (Dec) along with overcoming MDSCs by utilizing reprogrammed T cells and NKT cells in ACT prolongs survival of animals, but fails to eliminate the tumor. However, targeting cancer during a setting of MDR, when tumor cells are dormant, results in objective responses as evidenced in our multiple myeloma studies. This suggests that targeting breast cancer with immunotherapy following conventional therapies, in a setting of residual disease when tumor cells are dormant, may be effective in eliminating such residual cells or maintaining dormancy and extending time-to-relapse for breast cancer patients.