Targeted Inhibition of Polycomb Repressive Complexes in Multiple Myeloma Implications for Biology and Therapy

Sammanfattning: 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.