Inhibition of c-Myc Expression and Function in Hematologic Malignancies

Abstract 1409

MYC is among the most prolific oncogenes in cancer, yet pharmacologic strategies to modulate the function of the c-Myc oncoprotein do not exist. Toward this objective, we have undertaken to target c-Myc transcription by interfering with chromatin-dependent signal transduction to RNA polymerase, specifically by inhibiting the acetyl-lysine recognition domains (bromodomains) of putative co-activator proteins implicated in transcriptional initiation and elongation. Using a selective small-molecule bromodomain inhibitor, JQ1, we identify BET bromodomain-containing proteins as transcriptional regulatory factors for c-Myc. BET inhibition with JQ1 rapidly downregulates c-Myc transcription, followed by depletion of chromatin-bound c-Myc and genome-wide downregulation of Myc-dependent target genes. In translational model systems of multiple myeloma and Burkitt's lymphoma, both canonical MYC -dependent hematologic malignancies, JQ1 treatment produces a potent antiproliferative effect associated with cell cycle arrest. In multiple myeloma, adhesion to bone marrow stroma is associated with upregulation of BRD4, a BET bromomdomain coactivator protein. Inhibition of Myc function with JQ1 leads to impaired adhesion to stroma and cellular senescence, a classical Myc-specific phenotype. Mechanistically, JQ1 treatment depletes BRD4 from IgH enhancers, leading to prompt and robust downregulation of MYC transcription. In vivo efficacy of JQ1 in two disseminated models of multiple myeloma and in a Burkitt's lymphoma human xenograft establishes the therapeutic rationale for BET bromodomain inhibition in these diseases. Together, these studies identify a mechanistic rationale for targeting c-Myc in human cancer, and potentially other undruggable oncogenes driven by immunoglobulin rearrangement.

Note: G.C.I. and J.E.D. have made equal contributions to this research; C.S.M. and J.E.B. are jointly senior authors