CtBP1 Is a Novel and Essential Corepressor for BCL6 Autoregulation.
Authors: Mendez Lourdes M. , Polo Jose , Krupski Melissa , Yu Jessica , Melnick Ari M. , Ye B.H. 
Published: April 2020

Abstract
BCL6 is POZ/BTB transcription repressor that is required for the germinal center (GC)- stage of B cell development and its deregulated expression underlies the development of many GC-derived B cell lymphomas such as diffuse large B cell lymphoma (DLBCL). BCL6 carries out its biological function by repressing target genes involved in various aspects of B cell biology such as DNA damage response, cell-cycle regulation and plasma cell differentiation. Recent publications indicate that BCL6 differentially utilizes its corepressor partners to silence target genes involved in different biological processes. Negative autoregulation of BCL6 is likely to play an important role in B-cell differentiation, and is frequently disrupted in DLBCL due to translocation or point mutation of the BCL6 promoter. However, from a mechanistic standpoint, it is not known how BCL6 mediates negative autoregulation. BCL6 is reported to repress its target genes through binding of the SMRT, NCoR and BCoR corepressors to its N-terminal POZ domain and through binding of the MTA3 and HDAC2 corepressors to its second repression domain. However, a BCL6 mutant unable to bind these corepressors retained near wild-type repression activity on the BCL6 promoter. The expression of endogenous BCL6 was unchanged in DLBCL cell lines treated with BCL6 Peptide Inhibitor, which selectively disrupts the association between BCL6 and its POZ domain corepressors, or with MTA3 siRNA. This led us to consider the possibility that BCL6 autoregulation proceeds through a novel corepressor. Several POZ transcription factors can interact with CtBP as their corepressor. We found BCL6 and CtBP can interact in both the ectopically expressed and endogenous settings in DLBCL cells. Moreover, our ChIP experiments demonstrate that CtBP is present in the 5′UTR of BCL6 at sites that were previously shown by us and others to mediate BCL6 negative autoregulation. Nearly half of DLBCL patients are estimated to carry translocations and “activating” point mutations in the 5′UTR of BCL6 which allow negative autoregulation to be bypassed. In DLBCL cell lines carrying BCL6 promoter mutations or translocations, CtBP was preferentially bound to the wild-type BCL6 allele. Moreover, CtBP siRNA specifically derepressed the wild-type allele sparing the translocated BCL6 allele driven by heterologous promoters. This allelic analysis of BCL6 is consistent with a model in which BCL6 recruits CtBP to carry out negative autoregulation. Tiling ChIP-on-chip of BCL6 target genes showed colocalization of CtBP in a BCL6 repression complex at only a subset of target genes, including BCL6. However, the BCL6 locus was the only target dependent exclusively on CtBP for repression. In an effort to address the corepressor requirements of BCL6 autoregulation, we have uncovered a novel BCL6 corepressor, CtBP. Our results substantiate the growing body of evidence that BCL6’s mechanism of repression is dynamic, selectively calling upon corepressors to silence different cohorts of target genes perhaps reflecting segregation of biological functions. Our study provides new insight into normal BCL6-driven biology and also informs BCL6-targeted lymphoma therapies.