Drug library screening highlights synthetic lethal interactions between proteasome inhibition and epigenetic regulators in multiple myeloma cell lines.

Multiple myeloma is a blood cancer of monoclonal plasma cells, the second most common hematological malignancy in high-income countries. MM cells vastly rely on proper proteasome function, and proteasome inhibitors (PI) are currently the backbone of all current therapies. Despite enormous progress in the past years, multiple myeloma still remains incurable, largely because of its intrinsic heterogeneity that often leads MM patients to experience drug refractoriness and relapses. Drug combination is the most effective approach to overcome these limitations, allowing to reach synergistic interactions that strongly reduce the capability of cancer cells to adapt to therapeutic pressure. Here, we exploited a drug library screening assembled with 320 small-molecule inhibitors covering more than 100 targets to unbiasedly detect compounds that could boost Carfilzomib (CFZ)-mediated cell death in PI-resistant cell lines. Among top scoring candidates, UNC0642, an EHMT2 inhibitor, showed synergistic interactions in both PI-resistant and sensitive MM cell lines. EHMT2, also called G9a, is a histone methyltransferase mainly catalyzing mono- and di-methylation of histone 3 on lysine 9. CFZ and UNC0642 combinatorial treatment triggered synthetic lethal interactions in several multiple myeloma cell lines, and exhibited a favorable cytotoxicity profile toward peripheral blood mononuclear cells and bone marrow-derived stromal cells. To exclude off-target effects, we demonstrated that UNC0642 treatment reduces EHMT2-related molecular markers such as H3K9 methylation levels and MYC transcript levels. Finally, we demonstrated perturbation of the autophagy pathway and the DNA damage repair pathway after the combinatorial treatment, suggesting a multi-layered mechanism of action. In conclusion, the present study demonstrates that EHMT2 inhibition sensitizes MM cells to proteasome inhibitor treatment, thus providing new strategies to enhance sensitivity and overcome resistance in MM patients.