BCL6 Programs Lymphoma Cells for Survival and Differentiation through Distinct Biochemical Mechanisms, Both of Which Can Be Therapeutically Targeted.

The BCL6 transcriptional repressor is the most commonly involved oncogene in diffuse large B-cell lymphomas (DLBCL). Constitutive expression of BCL6 has been proposed to mediate lymphomagenesis through several mechanisms, including evasion of cell death, proliferation and differentiation blockade. We show here that BCL6 mediates these effects through distinct mechanisms. First, we show that blocking the association of the SMRT corepressor with BCL6 using our specific peptide inhibitor (BPI) abrogates only the survival effects of BCL6 but has no effect on differentiation. Accordingly, BPI upregulates survival genes such as ATR and p53, but not genes associated with differentiation such as Blimp1, XBP and Syndecan. In contrast, BCL6 shRNA upregulates both survival and differentiation genes and induces both cell death and differentiation. We and others have shown that BCL6 can also directly bind to the MTA3 corepressor, which is implicated in differentiation of Burkitt lymphoma cells. We found that BCL6 and MTA3 are co-expressed in DLBCL cells and primary human centroblasts (the precursor cell for most DLBCLs). The endogenous BCL6 and MTA3 proteins interacted in DLBCLs cells in co-immunoprecipitation experiments. In contrast to SMRT blockade with BPI, siRNA depletion of MTA3 induced expression of the Blimp1, XBP and Syndecan but not p53 and ATR. MTA3 depletion induced plasmacytic differentiation within 72 hours as shown in functional assays and by surface markers. We performed ChIP on chip using custom arrays densely tiling with oligonucleotides covering the entire genomic loci of 20 BCL6 target genes. Interestingly, BCL6 formed different types of repression complexes at differentiation genes (Complex with MTA3/NuRD) vs. survival genes (complex with SMRT and N-CoR). BCL6-mediated repression of genes involved in survival and differentiation thus depend on distinct biochemical mechanisms. The relevance of these findings for human disease was underscored by the fact that we found a statistically significant positive correlation between MTA3 and BCL6 gene expression in a database of 176 human DLBCLs (p<0.00001). Likewise, protein expression of BCL6 and MTA3 was also highly correlated (p<0.00001) (74 cases examined) and staining for Blimp1 revealed mutually exclusive expression from MTA3. Taken together these results illustrate the basic mechanisms through which BCL6 mediates DLBCL lymphomagenesis and provide the basis for powerful targeted therapy regimens that could be translated to the clinical setting.