Abstract 1271: The transcription factor BCL6 is a rational target in non-small cell lung cancer (NSCLC)
Authors: Marullo Rossella , Ahn Haelee , Cardenas Mariano , Melnick Ari , Xue Fengtian , Cerchietti Leandro 
Published: July 2016

Abstract
BCL6 is a transcription factor that promotes lymphomagenesis by repressing target genes involved in DNA damage sensing and response. In NSCLC patients, BCL6 is amplified in 40% of Squamous Cell Carcinomas (197/501, TCGA) and in 2.2% of Adenocarcinomas (5/230, TCGA), suggesting a potential oncogenic role for BCL6 in lung cancer.
To determine whether BCL6 plays a role in sustaining NSCLC, we first measured the expression of BCL6 in a panel of 15 NSCLC cell lines and found BCL6 expression in 15/15 cell lines. Furthermore, in 7/15 cells lines (46%) we found BCL6 gene amplification. To gain insight into BCL6 function, we engineered two NSCLC cell lines with BCL6 amplification (H1299 and H838) to stably express BCL6-shRNA. BCL6 silencing resulted in 3-5 fold mRNA increase of key DNA damage response genes such as ATR, Chek1, p21 and p53. BCL6-mediated repression of DNA damage response genes has functional effects as BCL6 silencing resulted into i) G1 cell-cycle arrest as determine by flow cytometry analysis, ii) 1.5-2 fold increase in cell doubling time, iii) 40-60% reduction in colony formation. These results suggest that a major role of BCL6 in lung cancer could be to enable proliferation under genotoxic stress, a function that BCL6 exerts physiologically in B-cells during antibody generation. Under these premises, BCL6 inhibition should preferentially affect the proliferation of cells with elevated genomic instability. Thus, we measured the amount of chromosomal aberration harbored by the proliferating fractions of BCL6-shRNA cells compared to the isogenic parental cells using micronuclei assay. BCL6 silencing resulted in 30-40% reduction of micronuclei amount in the proliferating fraction of cells, supporting our hypothesis. Moreover, exposing NSCLC cells to DNA damaging agents leads to 2-3 fold increase in BCL6 expression regardless the presence of BCL6 amplification.
In lymphomas, BCL6 represses genes by recruiting co-repressors to form “repressosome” complexes in gene promoters and enhancers. In particular, repression of DNA damage response genes is achieved by the recruitment of three co-repressors (SMRT, N-CoR, and BCOR) to the BTB-domain of BCL6. To determine whether the BTB-domain of BCL6 is mediating the repression of DNA damage genes in NSCLC, we employed FX1085, a small molecule that specifically interacts with the BCL6 BTB domain and prevents the formation of the repressosome complex. Exposure to FX1085 resulted in 3-10 fold increase in ATR, Chek1, p21 and p53 mRNA levels and prevented the proliferation of NSCLC cells with elevated genomic instability. Moreover, FX1085 affected the survival of 6/15 NSCLC cell lines (40%) at GI50 doses comparable to those effective in lymphoma cells, indicating that BCL6 maybe a suitable therapeutic target in NSCLC.
Overall, our data indicate a role for BCL6 in sustaining NSCLC genomic instability and suggest that this protein may be a potential therapeutic target in this disease.
Citation Format: Rossella Marullo, Haelee Ahn, Mariano Cardenas, Ari Melnick, Fengtian Xue, Leandro Cerchietti. The transcription factor BCL6 is a rational target in non-small cell lung cancer (NSCLC). [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1271.