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.

Les cancers du sein triple-négatifs (CSTN) présentent un taux élevé de récidive dû à la résistance aux chimiothérapies. Les adaptations du métabolisme cellulaire dans les CSTN contribuent à la résistance thérapeutique. Des changements du métabolisme de la glycolyse ou des acides aminés, peuvent permettre aux cellules de CSTN de s’adaptent rapidement en situation de stress. Toutefois, de nouvelles vulnérabilités peuvent être exposées chez les cellules du CSTN au cours de ces adaptations métaboliques. La méthyltransférase Enhancer of Zest Homologe 2 (EZH2) est connue pour son rôle dans l’identité cellulaire et la régulation de l’expression génique. Récemment, il a été montré qu’EZH2 joue un rôle dans la reprogrammation cellulaire des CSTN et contribue au développement de la résistance à la chimiothérapie. Mais les implications de cette reprogrammation sur le métabolisme des cellules CSTN, ne sont pas encore clairement identifiées. Ce projet a pour but d’étudier si les modifications épigénétiques résultant de l’activité d’EZH2 contribuent à la régulation du métabolisme cellulaire et permet les adaptations métaboliques dans les CSTN. Dans cette étude, nous utilisons les molécules UNC1999 et EPZ-4638, deux inhibiteurs sélectifs de l’activité enzymatique d’EZH2. Par des approches génomiques, transcriptomiques et métabolomiques, nous montrons que l’inhibition pharmacologique d’EZH2 induit des changements métaboliques caractérisés par une perturbation de la glycolyse et une résistance accrue à la privation de glucose. Nous observons aussi une augmentation de la dépendance à la glutamine et une utilisation accrue de la glutamine intracellulaire lors de l’inhibition d’EZH2. Ces vulnérabilités constituent des cibles potentielles pour un traitement concomitant avec le UNC1999. Finalement, nous proposons un mécanisme impliquant le facteur de transcription Estrogen Related Receptor-alpha (ERR) comme médiateur contribuant à l’effet métabolique résultant de l’inhibition d’EZH2 dans les CSTN. Les données préliminaires présentées dans ce mémoire proposent pour la première fois que les inhibiteurs d’EZH2 pourraient être utilisés pour induire des vulnérabilités métaboliques qui pourraient potentiellement être exploitées dans les CSTN. / Triple-negative breast cancers (TNBC) have a high rate of recurrence due to resistance to chemotherapy. Adaptations of cellular metabolism in TNBCs contribute to therapeutic resistance. Changes in glycolysis or amino acid metabolism may allow TNBC cells to adapt rapidly under stress. However, new and potentially targetable vulnerabilities may be exposed in TNBC cells during these metabolic adaptations. Methyltransferase Enhancer of Zest Homologue 2 (EZH2) is known for its role in cell identity and regulation of gene expression. Recently, EZH2 was shown to play a role in the cellular reprogramming of TNBCs and to contribute to the development of resistance to chemotherapy. But the implications of this reprogramming on the TNBC metabolism are not yet clearly identified. This project aims to investigate whether epigenetic modifications resulting from EZH2 activity contribute to the regulation of cellular metabolism and enable metabolic adaptations in TNBCs. In this study, we use the molecules UNC1999 and EPZ-4638, two selective inhibitors of the enzymatic activity of EZH2. Using genomic, transcriptomic and metabolomic approaches, we show that pharmacological inhibition of EZH2 induces metabolic changes characterized by disruption of glycolysis and increased resistance to glucose starvation. We also observe an increase in glutamine dependence and increased use of intracellular glutamine upon inhibition of EZH2. We show that these vulnerabilities are potential targets for concurrent treatment with UNC1999. Finally, we propose a mechanism proposing that the transcription factor Estrogen Related Receptor-alpha (ERR) contributes to the metabolic effect resulting from EZH2 inhibition in TNBCs. The preliminary data presented in this thesis propose for the first time that EZH2 inhibitors could be used to induce metabolic vulnerabilities that may potentially be exploited in TNBC.