Abstract 6203: CTC1-STN1-TEN1 (CST) complex-independent roles for STN1 in DNA double-strand break repair in pancreatic ductal adenocarcinoma

Abstract KRAS mutation occurs in 90-95% of pancreatic ductal adenocarcinoma (PDAC) and is a driver of PDAC; however, the underlying molecular mechanisms by which KRAS mutation maintains survival and promotes progression of PDAC need further refinement. We herein report that KRAS mutations increase STN1 (also named OBFC1) expression leading to enhanced repair of DNA double strand breaks (DSBs) in PDAC. Analysis of The Cancer Genome Atlas databases reveal that STN1 is significantly upregulated in PDAC tissue and, is associated with KRAS mutation and poor survival. Silencing KRAS or pharmacologic inhibition of KRAS signaling decreased STN1 expression in PDAC cells. Canonically, STN1 forms a CST (CTC1-STN1-TEN1) complex, which is important for telomere duplication and maintenance. Intriguingly, siRNA-mediated depletion of STN1 (but not CTC1 or TEN1) sensitized PDAC cells to ionizing radiation and chemotherapy as assessed by radiation clonogenic assay and alamarBlue assay, respectively. Consistently, immunoblotting and immunofluorescence analyses of γH2AX (marker of DNA damage) demonstrated that depletion of STN1 (but not CTC1 or TEN1) resulted in DNA damage. To explore the underlying molecular mechanisms, we performed proteomic analysis and found that STN1 physically interacts with many proteins important for DNA repair, replication and cell cycle progression. Of note, we revealed unreported interactions of STN1 with ATM, ATR and DNA-PKcs, key components of DNA damage response signaling. Furthermore, homologous recombination and non-homologous end joining repair reporter assays demonstrated that STN1 silencing reduced both homologous recombination and non-homologous end joining repair of DSBs in U2OS and PDAC cells. Moreover, flow cytometry and immunofluorescence analyses showed that knockdown of STN1 by siRNA impaired cell cycle arrest at S and G2/M phases which was accompanied with mitotic catastrophe in response to ionizing radiation. Our findings have revealed novel CST complex-independent roles for STN1 in DSB repair and suggest STN1 is a novel biomarker for PDAC as well as potentially other cancers. Citation Format: Changxian Shen, Linlin Yang, Sergio Corrales-Guerrero, Sindhu Nair, Terence Williams. CTC1-STN1-TEN1 (CST) complex-independent roles for STN1 in DNA double-strand break repair in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6203.