BCL6 Transcriptionally Reprograms the DLBCL Genome by Forming Distinct Biochemically Active Complexes on Genes Corresponding to Different Pathways.

Aberrant gene expression is a hallmark of cancer, and so it is not surprising that the most common category of oncogenes and tumor suppressors involved in hematologic malignancies are transcription factors. These factors mediate their effects by nucleating biochemically active cofactor complexes to modify the chromatin structure of their respective target genes. BCL6 is a transcriptional repressor and the most commonly involved oncogene in diffuse large B-cell lymphomas. BCL6 represses genes by recruiting several corepressor complexes including SMRT, N-CoR, BCoR; all of which bind to BCL6 through its BTB domain. Each of these complexes has different biochemical functions (e.g. BCoR forms a polycomb complex vs. SMRT which forms an HDAC3 complex). Moreover, our preliminary data suggested that BCL6 uses different sets of corepressors to mediate distinct biological effects, possibly by using different biochemical mechanisms at specific sets of target genes. Therefore, we hypothesized that BCL6 regulates its target genes using different biochemical tools, allowing it to exquisitely fine tune gene expression and provide specific control mechanisms for different biological functions. In order to test this hypothesis we first identified the direct target genes of BCL6 SMRT, N-CoR and BCoR by ChIP-on-chip in DLBCL cells (Ly1 cells) in multiple replicates, and examined whether the overlapping sets of genes corresponded to different gene pathways. We used a 24,000 promoter microarray representing 1.5 KB of sequence for each gene. The results show reproducible binding of BCL6 at 940 promoters, While BCoR bound to 770, SMRT to 545 and N-CoR to 487 promoters respectively. BCL6 and BCoR overlapped at 400 genes, preferentially involved in involve in cell cycle, cell death chromatin structure, ubiquitin dependent process and chemotaxis. BCL6 and SMRT overlapped on 376 genes, involved in immune response, cell motility and also as BCOR cell death, while N-CoR and BCL6 overlapped on 100 genes including transcriptional control and cell death pathways. The overlap between BCoR and SMRT was at 200 genes, BCoR and N-CoR at 60 genes and SMRT and N-CoR at 85 genes. All three overlapped at 50 genes. We also examined whether these corepressors were associated with specific combinations of histone modifications including H3K9 acetylation, H3K9 methylation, H3K4 methylation, H3K27 methylation, H4K16 acetylation and H3K36 acetylation. Taken together, the data indicate that specific subsets of BCL6 target genes are dependent on distinct biochemical mechanisms, suggesting that additional layers of biochemical complexity govern formation of gene repression complexes in DLBCL cells and providing opportunities for highly specific therapeutic targeting of specific gene programs.