Pre-B Cell Receptor-Mediated Activation of BCL6 Induces Pre-B Cell Quiescence Through Transcriptional Repression of MYC

Abstract

Abstract 1406 Background:Pre-B cell receptor signaling is critical to induce initial expansion of the pool of differentiating B cell precursors but also mediates subsequent cell cycle exit and quiescence. Initial cell surface expression of the pre-B cell receptor induces a series of tyrosine phosphorylation events resulting in MYC and CCND2-mediated proliferation. After 2–5 divisions, however, large pre-BII (Fraction C’) cells exit cell cycle to become resting, small pre-BII cells (Fraction D). While the signaling pathway resulting in pre-B cell receptor-induced proliferation is well understood, the mechanism by which pre-BII cells exit cell cycle, however, is currently unclear. Results:This checkpoint and cell cycle exit at the Fraction C'-D transition is critical for immunoglobulin light chain gene recombination and to prevent malignant transformation into acute lymphoblastic leukemia. Here we demonstrate that inducible activation of pre-B cell receptor signaling recapitulates the initial proliferative burst followed by cell cycle exit, which is characterized by strong upregulation of BCL6 and subsequent loss of MYC and CCND2 expression. ChIP-on-chip analysis revealed that the transcriptional repressor BCL6 is directly recruited to the MYC promoter. Inducible activation of pre-B cell receptor signaling in pre-B cells from BCL6-deficient mice failed to induce cell cycle exit and quiescence. We conclude that activation of BCL6 downstream of the pre-B cell receptor is crucial for the induction of quiescence at the Fraction C'-D checkpoint. As expected, inducible activation of BCL6 downstream of the pre-B cell receptor results in transcriptional repression of MYC and CCND2. Overexpression of MYC prevented pre-B cell receptor/BCL6-induced cell cycle exit. Conclusion:Hence, pre-B cell receptor signaling induces cellular quiescence of Fraction C' pre-B cells through activation of BCL6 and BCL6-mediated transcriptional repression of MYC and CCND2. Disclosures:No relevant conflicts of interest to declare.