BCL6, p53 and Molecular Targeted Therapy in B-Cell Lymphomas.
Authors: Cerchietti Leandro C.A. , Polo Jose M. , Da Silva Gustavo F. , Dowdy Steve M. , Giorgio Catoretti M. , Rita Shaknovich M. , Melnick Ari M. 
Published: April 2020

Abstract
The BCL6 transcriptional repressor is an oncogene often constitutively expressed in diffuse large B-cell lymphomas (DLBCL). The oncogenic mechanism of action of BCL6 presumably involves repression of its direct target genes. We recently developed a targeted therapy agent (called BPI - BCL6 peptide inhibitor) that specifically blocks transcriptional repression by BCL6, and which causes apoptosis in lymphoma cells in vitro and in vivo. We present here potent and stable derivatives of BPI able to specifically eradicate lymphoma cells after a single dose in vitro. Expression array studies of BCL6 target genes reactivated by BPI revealed that one such gene is the p53 tumor suppressor. p53 was also recently shown to be BCL6 target gene by Phan et. al., Nature 2004. We find that BCL6 represses p53 in DLBCL cells through recruitment of the SMRT and N-CoR corepressors, which explains how BPI, which blocks recruitment of these corepressors, reactivates p53. We next wished to determine the contribution of BCL6-mediated repression of p53 to lymphomagenesis, and how p53 modulation might affect BCL6 targeted therapy strategies for DLBCL. We found that BPI could induce p53 target gene expression in DLBCL cells with wild-type p53 and that small molecules or peptides that block p53 rescue apoptosis induced by BPI. In contrast, although BPI also induces p53 in DLCBL cells with mutant p53, there was no activation of p53 target genes and no rescue by p53 blocking molecules. However BPI causes apoptosis of DLBCL cells regardless of p53 status indicating the BCL6 mediates its oncogenic actions through both p53 dependent and independent pathways. p53 is usually wild-type in DLBCL and our analysis of &gt;100 patients show that p53 protein is, surprisingly, still expressed in these tumors. These data suggest that p53 is not fully active in DLBCL cells, consistent with the fact that we found that BCL6 also directly represses upstream activators of p53 such as Chk1 and ATR. BCL6 blockade thus can fully restore activity of p53, both by increasing its expression levels and by enhancing its activation by upstream mediators. Accordingly, sequential administration of p53 activating molecules that enhance p53 activity, potently synergizes with BPI in killing lymphoma cells. BPI also synergizes with chemotherapy drugs that act in part through p53, such as doxorubicin. From these studies we conclude that i) BCL6 mediates lymphomagenesis by direct repression of p53 and upstream target gene pathways; ii) BCL6 positive lymphomas are dependent on BCL6 for their survival regardless of whether p53 is wild type or mutated; iii) Sequential targeting of BCL6 and p53 with BPI and a p53 activating molecule or doxorubicin is likely to be a highly effective therapeutic regimen for patients with DLBCL, especially for the majority who have wild-type p53; iv) The new BPI derivatives are sufficiently potent and stable to be tested in the clinical setting.