Tobacco Carcinogen Induces Persistent DNA Damage and Homologous Recombination Dysregulation in Pancreatic Ductal Carcinogenesis

Cigarette smoking is a well-established risk factor for the development of pancreatic ductal adenocarcinoma (PDAC), yet the mechanisms by which tobacco carcinogens, such as Nicotine-Derived Nitrosamine Ketone (NNK), drive pancreatic carcinogenesis remain incompletely understood. We investigated the impact of NNK on pancreatic epithelium by treating human pancreatic ductal epithelial cells (HPDE6-E6E7; H6c7) with NNK for 5 or 50 days followed by genome-wide RNA sequencing, qRT-PCR, immunofluorescence, comet assay, and γH2AX ELISA. We assessed γH2AX by immunohistochemistry in human PDAC tissue microarrays and in TCGA-PAAD datasets to identify clinical correlates. Acute NNK exposure (5 days) induced DNA double-strand breaks and activated ATM-γH2AX signaling, accompanied by increased RAD51 and BRCA2 recruitment, consistent with activation of homologous recombination (HR)-mediated DNA repair. In contrast, chronic NNK exposure (50 days) resulted in persistent DNA damage signaling and transcriptional upregulation of HR-associated genes, including RAD51, BRCA1, BRCA2, FANCD2, and PLK1. Despite sustained accumulation of RAD51- and BRCA2-positive repair foci, chronic NNK exposure was associated with altered RAD51 localization, persistent DNA damage, and chromosomal instability, suggesting dysregulation of HR repair fidelity during prolonged genotoxic stress. γH2AX expression was significantly elevated in NNK-treated cells, KRAS-driven premalignant pancreatic lesions, and pancreatic tissues from smokers. Increased γH2AX expression correlated with tumor progression and poor overall survival, while elevated RAD51 expression was associated with adverse outcomes in patients with PDAC. Collectively, these findings reveal a biphasic DNA damage response to NNK characterized by early activation of ATM-γH2AX-dependent DNA repair followed by persistent DNA damage signaling, dysregulated HR repair, and genomic instability during chronic exposure. Tobacco-induced disruption of DNA repair homeostasis may represent a key mechanism driving pancreatic carcinogenesis and supports γH2AX as a clinically relevant prognostic biomarker in tobacco-associated PDAC.