A Hybrid Mechanism of Action for BCL6 in B Cells Defined by Formation of Functionally Distinct Complexes at Enhancers and Promoters

Abstract

SUMMARY The BCL6 transcriptional repressor is required for development of germinal center (GC) B-cells and diffuse large B-cell lymphomas (DLBCL). Although BCL6 can recruit multiple corepressors, its transcriptional repression mechanism of action in normal and malignant B-cells is unknown. We find that in B-cells, BCL6 mostly functions through two independent mechanisms that are collectively essential to GC formation and DLBCL, both mediated through its N-terminal BTB domain. These are: i) formation of a unique ternary BCOR-SMRT complex at promoters with each corepressor binding to symmetrical sites on BCL6 homodimers, linked to specific epigenetic chromatin features, and ii) the “toggling” of active enhancers to a poised but not erased conformation through SMRT-dependent H3K27 de-acetylation, which is mediated by HDAC3 and opposed by p300 histone acetyltransferase. Dynamic toggling of enhancers provides a basis for B-cells to undergo rapid transcriptional and phenotypic changes in response to signaling or environmental cues.

Bcl6 middle domain repressor function is required for T follicular helper cell differentiation and utilizes the corepressor MTA3

“…Furthermore, its repressive capability, at least in B cells, is dependent on association with a variety of corepressors (7). One key function of Bcl6 in T and B cells is the repression of Prdm1, the gene encoding Blimp1.…”

Mechanisms of action of BCL6 during germinal center B cell development

“…This is evident by this fact that BCl6 binding sites are significantly coincident with the cistrome of these proteins including MYB (v-myb myeloblastosis viral oncogene homology), IRF8 (interferon-regulatory factor 8) and BACH2 [41,44,45]. BCL6 seems to antagonize or synergize these transcription factors in programing the GC phenotype.…”

“…The BBDs of NCOR and SMRT are identical, whereas the BCOR BBD is completely different, yet all three bind to the BCL6 BTB lateral groove in perfectly overlapping configurations [37,39]. BCL6 appears to recruit two distinct repression complexes at promoters and enhancers via its BTB domain [41]. At promoters, BCL6 dimers can recruit a unique ternary BCOR-SMRT/NCOR complex to effectively repress transcription in a repressed chromatin environment.…”

“…Recently, wholegenome chromatin immunoprecipitation plus sequencing (ChIP-seq) analysis identified genome-wide BCL6 binding sites [33,41,44]. BCL6 binding sites were shown to be significantly enriched in the BCL6 consensus binding motif TTCCT(A/C)GAA that was initially identified in vitro by examining the ability of recombinant BCL6 protein to bind to synthetic oligonucleotides.…”

“…Although the BCL6 target genes are well defined [33,41,44], the precise functions of separate BCL6 repression domain is less clear. BCL6 appears to recruit distinct co-repressor complex to different subsets of targets (Figure 1) [47,48], suggesting that transcriptional programming by BCL6 may be finely compartmentalized through distinct domains.…”