In most patients with chronic myeloid leukemia (CML) clonal cells can be kept under control by BCR::ABL1 tyrosine kinase inhibitors (TKI). However, overt resistance or intolerance against these TKI may occur. We identified the epigenetic reader BRD4 and its downstream-effector MYC as growth regulators and therapeutic targets in CML cells. BRD4 and MYC were found to be expressed in primary CML cells, CD34+ /CD38- leukemic stem cells (LSC), and in the CML cell lines KU812, K562, KCL22, and KCL22T315I . The BRD4-targeting drug JQ1 was found to suppress proliferation in KU812 cells and primary leukemic cells in the majority of patients with chronic phase CML. In the blast phase of CML, JQ1 was less effective. However, the BRD4 degrader dBET6 was found to block proliferation and/or survival of primary CML cells in all patients tested, including blast phase CML and CML cells exhibiting the T315I variant of BCR::ABL1. Moreover, dBET6 was found to block MYC expression and to synergize with BCR::ABL1 TKI in inhibiting the proliferation in the JQ1-resistant cell line K562. Furthermore, BRD4 degradation was found to overcome osteoblast-induced TKI resistance of CML LSC in a co-culture system and to block interferon-gamma-induced upregulation of the checkpoint antigen PD-L1 in LSC. Finally, dBET6 was found to suppress the in vitro survival of CML LSC and their engraftment in NSG mice. Together, targeting of BRD4 and MYC through BET degradation sensitizes CML cells against BCR::ABL1 TKI and is a potent approach to overcome multiple forms of drug resistance in CML LSC.

CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences Vienna Austria

Department for Companion Animals and Horses University Clinic for Small Animals Internal Medicine Small Animals University of Veterinary Medicine Vienna Vienna Austria

Department of Hematology and Oncology Innsbruck Medical University Innsbruck Austria

Department of Hematology Oncology and Rheumatology Center of Integrated Oncology Cologne Bonn University Hospital of Bonn Bonn Germany

Department of Internal Medicine 1 Division of Hematology and Hemostaseology Medical University of Vienna Vienna Austria

Department of Internal Medicine Hematology and Oncology Faculty of Medicine Masaryk University Brno Czech Republic

Department of Internal Medicine Hematology and Oncology University Hospital Brno Brno Czech Republic

Department of Laboratory Medicine Medical University of Vienna Vienna Austria

Department of Pathology Medical University of Vienna Vienna Austria

Institute of Laboratory Animal Science University of Veterinary Medicine Vienna Vienna Austria

Ludwig Boltzmann Institute for Hematology and Oncology Medical University of Vienna Vienna Austria

Medical University of Vienna Vienna BioCenter Vienna Austria

MLL Munich Leukemia Laboratory Munich Germany

Research Institute of Molecular Pathology Vienna Austria

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Myc 9aaTAD activation domain binds to mediator of transcription with superior high affinity