Direct Transcriptional Repression of BCL2 by BCL6 in Germinal Centre B Cells and Its Disruption in B Cell Lymphomas with BCL2 Locus Alterations

The human proto-oncogene BCL6 encodes a BTB/POZ-zinc finger transcriptional repressor that is required for germinal centre (GC) development and is expressed in the majority of normal GC B cells and in the majority of B cell lymphoma (B-NHL), including follicular lymphoma (FL) and a subset of diffuse large B cell lymphomas (DLBCLs). Deregulation of BCL6, by chromosomal translocation or somatic hypermutation, is implicated in the pathogenesis of B-NHL. The precise function of BCL6 in GC development and lymphomagenesis is still unclear also due to the very few direct BCL6 target genes that have been identified. In order to identify physiologically relevant BCL6 direct target genes, we used a novel approach combining high-throughput biochemical (ChIP-on-chip), bioinformatics (ARACNe) (Basso et al., Nature Genetics, 2005) and gene expression profile analysis (Klein et al., Proc. Natl. Acad. Sci., 2004) of normal GC B cells. The results (see abstract by Saito et al.) have identified a core of bona fide target genes whose promoter is bound by BCL6, whose transcription is dynamically linked to BCL6, and whose expression is down-regulated in GC B cells. Among the genes meeting these stringent criteria, we found BCL2, encoding the anti-apoptotic molecule whose expression is deregulated by chromosomal translocations or gene amplification in the majority of FL and in a subset of DLBCL. Further investigations showed that BCL6 represses BCL2 transcription by binding specific DNA sites within the BCL2 promoter region. Suppression of BCL6 expression via specific siRNAs leads to increased levels of BCL2, and, conversely, constitutive BCL6 expression prevents B cell receptor (BCR)-induced upregulation of BCL2 in B cells. Consistent with a physiological role for BCL6- mediated BCL2 suppression, immunohistochemical analysis shows that BCL2 expression is absent in GC B cells where BCL6 is highly expressed. Notably, the comparative analysis of BCL6 and BCL2 expression in B-NHL cell lines showed a conserved inverse correlation between BCL6 and BCL2 levels in cell lines carrying a normal BCL2 locus. However, cell lines carrying chromosomal translocations or amplifications affecting the BCL2 gene displayed the pathologic co-expression of both proteins, suggesting that the alterations affecting the BCL2 locus prevent BCL6-mediated suppression. These results indicate that one critical role of BCL6 is the modulation of antiapoptotic function in GC B cells via BCL2 transcriptional repression, suggesting a mechanism by which B cells may die in the GC if not rescued by BCR and CD40 engagement, both of which downregulate BCL6 while inducing BCL2 expression. Alterations of the BCL2 locus may contribute to lymphomagenesis by making the gene resistant to suppression by BCL6, and therefore allowing B cells to avoid apoptosis in the absence of antigen- and T-cell-mediated rescue.