The Balance between Myc and Bcl6 Determines Self-Renewal or Differentiation of Pre-B Cells.
Authors: Duy Cihangir , de Alboran Ignacio Moreno , Jumaa Hassan , Muschen Markus 
Published: November 2019

Abstract
Myc and Bcl6 represent classical proto-oncogenes in B-cell malignancies, mainly through translocation into the immunoglobulin (Ig) heavy chain locus in Burkitt’s (MYC) and diffuse large B cell lymphoma (BCL6). While BCL6 was previously established as a factor regulating differentiation of germinal center B cells, the function of MYC and BCL6 in early B-cell development was not previously studied. Investigating requirements for the differentiation of pre-B cells into immature B-cells, we found that both withdrawal of IL7 from murine pre-B-cell cultures and inhibition of BCR-ABL1 in BCR-ABL1-transformed pre-B-cells terminates self-renewal and initiates differentiation into Ig light chain-expressing immature B-cells. Interestingly, IL7 and BCR-ABL1 are exchangeable at this checkpoint: Both IL7 and BCR-ABL1 promote self-renewal and prevent differentiation of pre-B-cells. While inhibition of BCR-ABL1 usually induces apoptosis and partial differentiation, both effects were entirely suppressed by IL7. These findings indicate that IL7 may confer resistance to BCR-ABL1 inhibitors in patients with BCR-ABL1-transformed acute lymphoblastic leukemia. Likewise, inhibition of either IL7 or BCR-ABL1 signaling resulted in complete silencing of Myc expression and strong de novo expression of Bcl6. Because expression of Myc and Bcl6 are mutually exclusive at the pre-B to immature B-cell checkpoint, we tested whether the two proto-oncogenes have distinct functions at this transition. Interestingly, forced expression of Myc rendered BCR-ABL1-transformed pre-B-cells resistant to induction of differentiation upon inhibition of BCR-ABL1. Besides downregulation of Myc, also de novo expression of Bcl6 is critical for the pre-B to immature B-cell differentiation: shmiR-mediated silencing of Bcl6 suppressed B-cell differentiation even if Myc was downregulated. However, forced expression of Bcl6 alone only modestly induced differentiation of pre-B cells if Myc was not downregulated. To test the interplay between Myc and Bcl6 at the pre-B to immature B cell transition more systematically, we analyzed bone marrow pre-B cells from Mycfl/fl mice. Mycfl/fl pre-B cells that also carry MxCre deleted the Myc locus on both alleles upon stimulation with IFNß. As controls, Mycfl/fl pre-B cells without MxCre were used. Pre-B cells were also transduced with a retroviral vector encoding Bcl6/GFP or GFP alone. Upon Myc deletion, more than 80 precent of the Bcl6/GFP transduced pre-B cells underwent differention as compared to 25 percent GFP-transduced pre-B cells. In the absence of Myc deletion, about 15 percent of Bcl6/GFP-transduced pre-B cells initiated differentiation as compared to 5 percent of GFP-transduced pre-B cells. These findings establish that Myc and Bcl6 have critical and antagonistic functions in early B cell development and that both downregulation of Myc together with upregulation Bcl6 are required to initiate differentiation of pre-B cells. The MYC/BCL6 balance may also be a target of leukemic transformation of human pre-B cells: The ratio of MYC/BCL6 mRNA levels in normal human pro- and pre-B cells at 0.52 is dramatically increased in various subtypes of acute lymphoblastic leukemia (6.4 for BCR-ABL1-, 2.6 for E2A-PBX1-, 14.4 for MLL-AF4- and 3.3 for TEL-AML1-transformed acute lymphoblastic leukemia).