Abstract MIP-044: FOXM1 INDUCED REPLICATION STRESS IS MITIGATED BY ITS BIDIRECTIONAL GENE PARTNER, RHNO1, IN HIGH GRADE SEROUS OVARIAN CANCER
Authors: Barger Carter J , Chee Linda , Drapkin Ronny , Odunsi Kunle , Karpf Adam R. 
Published: June 2017

Abstract
PURPOSE: FOXM1 is located at 12p13.33, a significantly amplified locus in high-grade serous ovarian cancer (HGSC). The 12p13.33 amplicon contains ~33 genes, which may cooperate with FOXM1 to exert oncogenic function. Although FOXM1 expression is associated with cancer genomic instability, it is unclear whether FOXM1 causes this phenotype and, if so, by what mechanism. We hypothesized that FOXM1 induces replication stress, which is the slowing or stalling of replication forks. We further noted that RHNO1, a component of the 9-1-1 complex required for ATR-CHK1 signaling, is contained within the 12p13.33 amplicon, and arranged in a head-to-head orientation with FOXM1. Here, we characterized the FOXM1-RHNO1 bidirectional promoter, FOXM1 and RHNO1 expression patterns in HGSC, and determined the relationship between FOXM1 and RHNO1 in replication stress.
METHODS: We used 5' RACE and a luciferase reporter construct to characterize the FOXM1-RHNO1 bidirectional promoter. We analyzed copy number, mRNA and protein expression datasets in TCGA HGSC and additional TCGA cancers. We measured FOXM1 and RHNO1 expression in HGSC cell lines and primary HGSC tissues. We overexpressed FOXM1 in human immortalized fallopian tube epithelial (FTE) cells to determine its contribution to replication stress. We used RHNO1 knockdown to dissect its functional contribution to ATR-CHK1 signaling in HGSC cells.
RESULTS: FOXM1 and RHNO1 were co-amplified in ~12% of HGSC and their mRNA expressions were highly correlated. The FOXM1-RHNO1 bidirectional promoter showed similar activity in each direction in HGSC cells, and correlated with mRNA expression. Analysis of TCGA HGSC data revealed that FOXM1 associates with markers of DNA replication and CHK1-Ser345 phosphorylation, a canonical marker of replication stress. FOXM1 overexpression in immortalized FTE cells induced CHK1-Ser345 phosphorylation. RHNO1 knockdown attenuated CHK1-Ser345 phosphorylation in response to replication stress, and, importantly, reduced clonogenic growth in FOXM1 overexpressing HGSC cells.
CONCLUSIONS: Our data reveal that FOXM1 and RHNO1 share a bidirectional promoter resulting in their frequent co-expression. FOXM1 induces replication stress whereas RHNO1 is necessary for efficient ATR-CHK1 signaling. We hypothesize that balanced FOXM1 and RHNO1 expression promotes HGSC development and progression. Ongoing studies are characterizing the impact of FOXM1 and RHNO1 on replication stress and genomic instability. Finally, our findings suggest ATR-CHK1 signaling as a potential therapeutic vulnerability in FOXM1 activated HGSC.
Citation Format: Carter J Barger, Linda Chee, Ronny Drapkin, Kunle Odunsi, Adam R. Karpf. FOXM1 INDUCED REPLICATION STRESS IS MITIGATED BY ITS BIDIRECTIONAL GENE PARTNER, RHNO1, IN HIGH GRADE SEROUS OVARIAN CANCER [abstract]. In: Proceedings of the 11th Biennial Ovarian Cancer Research Symposium; Sep 12-13, 2016; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(11 Suppl):Abstract nr MIP-044.