Abstract 1288: BCL6 drives an oncogenic transcriptional and epigenetic program in NSCLC whose inhibition results in antineoplastic activity in vivo
Authors: Marullo Rossella , Revuelta Maria V. , Ai Yong , Xue Fengtian , Cerchietti Leandro 
Published: September 2020

Abstract
BCL6 is an oncogenic transcription factor that represses multiple target genes by recruiting three essential co-repressors (SMRT, NCOR, BCOR) via its BTB domain. These, in turn, serve as scaffold to form distinct complexes with various chromatin remodelers. We found that BCL6 is required to sustain oncogenic features in NSCLC such as cell proliferation and colony formation. Furthermore, we demonstrated that BCL6 allows NSCLC cells to survive several stress conditions including exposure to DNA damaging agents. Although these data demonstrated a critical role of BCL6 in NSCLC oncogenesis, the mechanism underlying its oncogenic activity remains unknown. To elucidate the role of BCL6 in NSCLC, we used the small molecule FX1 which specifically disrupts the interaction between the BTB domain and its essential corepressors (Kd 7+/- 3 uM). An in vitro screening demonstrated that FX1 impaired the survival of 6/15 (40%) BCL6-positive NSCLC cell lines that we considered as BCL6-dependent. Furthermore, FX1 (25mg/kg, every other day) significantly decreased the tumor growth of two BCL6-dependent human NSCLC xenograft models (p&lt; 0.001) without toxicity to normal tissues. To gain insight into the transcriptional and epigenetic mechanisms that explain BCL6 oncogenic activity, we exposed two BCL6-dependent cell lines (H1155 and H2122) to FX1 for short-term treatment and performed parallel ATAC-sequencing (chromatin accessibility) and spike-in normalized RNA-sequencing (transcriptional changes) experiments. At the genome-wide level, we found that inhibition of BCL6 broadly affected the epigenome landscape of both cell lines. Notably, 34% (H1155) and 27% (H2122) of the regions for which chromatin accessibility was significantly altered mapped to promoters. Accordingly, we identified 4,825 shared genes that were significantly re-expressed and 1,425 shared genes that were significantly repressed in cells treated with FX1 vs. vehicle. Re-expressed genes were enriched for pathways regulating immune signaling (i.e. CD247, IRF7 and HLAB/C/F), cell-cell interaction (i.e. TNF, IL4R, IL2RB, CXCR4), extracellular matrix organization (i.e. COL1A2, COL2A1, COL5A1, BGN) and neuronal system differentiation (i.e. NEUROD1, SCG3, SCG5). Interestingly, repressed genes included oncogenic transcriptional factors among which MYC, NOTCH1 and NOTCH3. Accordingly, repressed genes were enriched for MYC target genes and genes regulating epithelial to mesenchymal transition (TGFB2, TGFB1, WNT5A, VIM) as well as extracellular matrix organization (COL4A5, COL4A5, COL5A2, MMP10). Overall these findings suggest that BCL6, via its BTB domain, transcriptionally and epigenetically regulates multiple oncogenic pathways in NSCLC and may shape the cellular and extracellular composition of the tumor microenvironment. Furthermore, they provide preliminary evidence of the feasibility of BCL6 BTB inhibition as therapeutic strategy for NSCLC.
Citation Format: Rossella Marullo, Maria V. Revuelta, Yong Ai, Fengtian Xue, Leandro Cerchietti. BCL6 drives an oncogenic transcriptional and epigenetic program in NSCLC whose inhibition results in antineoplastic activity in vivo [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1288.