Abstract P011: STN1 (OBFC1) promotes DNA double-strand break repair in a potentially CTC1-STN1-TEN1 (CST) complex-independent role in pancreatic cancer
Authors: Cui Tiantian , Shen Changxian , Yang Linlin , Gui Ling , Corrales-Guerrero Sergio , Nair Sindhu , Karasinska Joanna M. , Topham James T. , Ping Xiaoli , Stark Jeremy M. , Williams Terence M. 
Published: January 2025

Abstract
Purpose: Pancreatic cancer (PC) is an aggressive lethal tumor with an unmet need for novel therapeutic approaches. KRAS activating mutations occur in 90-95% of PC and contribute to tumor progression and resistance to therapy, including radiation. The mechanisms by which oncogenic KRAS promotes radiation resistance are critical to understand in order to identify novel therapies. Methods: We first analyzed the expression levels of DNA damage response and repair genes using Affymetrix RNA expression microarray in isogenic HCT116 and SW48 cells with KRAS wide-type and KRASG13D activating mutations to identify novel targets by which KRAS mutations may confer radiation resistance. We analyzed the expression of STN1 in pancreas normal and cancer tissues and assessed the correlation with PC clinical outcomes using TCGA dataset. Human tumor xenografts were generated to explore the role of STN1 on tumor growth in vivo. Radiation response was assessed through clonogenicity and gH2AX foci assays. Homologous recombination (HR) and non-homologous end joining (NHEJ) repair reporter assays, chromatin spreading assay, cell cycle analysis, mitotic catastrophe, Annexin-V assays were performed to investigate the mechanisms of radiation-induced cell death. Mass spectrometry analysis was performed to identify STN1 interacting proteins important in DNA damage response and further validated by immunoprecipitation and immunoblotting. Results: We find that KRAS activation increases STN1 expression to enhance DNA double strand break repair capacity in PC. STN1 is a component of the CST complex normally important for telomere duplication and maintenance. We find that STN1 is significantly upregulated in PC, especially in aggressive subtypes of PC, associates with KRAS oncogenic mutations, and correlates with poor patient clinical outcomes. Genetic silencing or pharmacologic inhibition of KRAS signaling decreases STN1 expression in PC cells, suggesting KRAS signaling positively regulates STN1 expression. Interestingly, STN1 depletion reduces tumor growth in a heterotopic model of KRAS mutant PC. Mechanistically, depletion of STN1 potentiates DNA damage, replication stress, and sensitizes PC cells to ionizing radiation independent of CTC1 and TEN1. In support of this finding, STN1 silencing reduces both HR and NHEJ repair of DSBs. Furthermore, knockdown of STN1 impairs cell cycle arrest at the G2/M phase in response to ionizing radiation, which is accompanied with increased mitotic catastrophe, radiation-induced apoptosis. Proteomic analysis reveals that STN1 physically interacts with many proteins important for DNA repair, replication and cell cycle progression, including ATM, DICER1, CEP164, and CEP250. Conclusion: Our findings have revealed a novel, potentially CST complex-independent role of STN1 in DSB repair after radiation. STN1 may function at one of the apical nodes in the DNA damage response pathway by interacting with ATM. Our findings suggest STN1 may be a promising target for improving genotoxic therapies in KRAS mutant cancers, including PC.
Citation Format: Tiantian Cui, Changxian Shen, Linlin Yang, Ling Gui, Sergio Corrales-Guerrero, Sindhu Nair, Joanna M. Karasinska, James T. Topham, Xiaoli Ping, Jeremy M. Stark, Terence M. Williams.STN1 (OBFC1) promotes DNA double-strand break repair in a potentially CTC1-STN1-TEN1 (CST) complex-independent role in pancreatic cancer.[abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Translating Targeted Therapies in Combination with Radiotherapy; 2025 Jan 26-29; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(2_Suppl):Abstract nr P011