Evaluation of flap endonuclease-1 (FEN1) as a prognostic, predictive, and therapeutic target in breast (BC) and ovarian cancer (OVC).

1016 Background: FEN1 is a multifunctional protein with essential roles in long patch base excision repair (LP-BER), Okazaki fragment maturation during replication, resolution of tri-nucleotide repeat sequence-derived secondary structures, rescue of stalled replication forks, maintenance of telomere stability and apoptotic fragmentation of DNA. In the current study we have evaluated FEN1 as a prognostic, predictive and therapeutic target in BC and OVC. Methods: Clinico-pathological significance of FEN1 mRNA expression was evaluated in a BC training cohort [n=128], test cohort (n=249) and validated in a large cohort of 1,980 BC (METABRIC). Neural network analysis (NNA) was conducted to identify FEN1 interaction genes. FEN1 protein expression was investigated in three consecutive series of BC and OVC: 568 ER negative (ER- BC), 894 ER positive (ER+ BC) and 195 OVC cohorts. Pre-clinically, FEN1 deficient and proficient HeLa cell lines were investigated for chemotherapy sensitization. A high throughput screening (HTS) strategy was developed to identify FEN1inhibitors for therapeutic application. Results: FEN1mRNA overexpression is associated with adverse clinicopathological features such as high grade, high mitotic index, and triple negative (ps<0.0001). High FEN1 mRNA expression was highly significantly associated with PAM50 Her2, PAM50 Basal and PAM50 LumB (ps<0.0001) BC. FEN1 mRNA overexpression is associated with resistance to chemotherapy (p=0.019), endocrine therapy (p<0.0001) and independently with poor survival (p<0.0001). NNA revealed novel interaction genes with predominant roles in proliferation, cell growth, DNA repair, differentiation, invasion, migration, metabolism and apoptosis. FEN1 protein overexpression is significantly associated with aggressive clinico-pathological features and independently with poor survivals in ER+ BC, ER- BC and in OVC (ps<0.001). HTS assay has identified novel FEN1 inhibitors for therapeutic development. Conclusions: This is the first study to investigate FEN1 in BC and OVC. We provide confirmatory evidence that FEN1 is a promising biomarker as well as a therapeutic anti-cancer drug target for clinical application.