Metastatic site influences driver gene function in pancreatic cancer [scRNA-seq]
Driver gene mutations can increase the metastatic potential of the primary tumor, but their role in sustaining tumor growth at metastatic sites is poorly understood. A paradigm of such mutations is inactivation of SMAD4 – a transcriptional effector of TGFß signaling – which is a hallmark of multiple gastrointestinal malignancies. SMAD4 inactivation mediates TGFß's remarkable anti- to pro-tumorigenic switch during cancer progression and can thus influence both tumor initiation and metastasis. To determine whether metastatic tumors remain dependent on SMAD4 inactivation, we developed a mouse model of pancreatic ductal adenocarcinoma (PDAC) that enables Smad4 depletion in the pre-malignant pancreas and subsequent Smad4 reactivation in established metastases. As expected, Smad4 inactivation facilitated the formation of primary tumors that eventually colonized the liver and lungs. By contrast, Smad4 reactivation in metastatic disease had strikingly opposite effects depending on the tumor's organ of residence: suppression of liver metastases and promotion of lung metastases. Integrative multiomic analysis and functional perturbations revealed that the divergent effects of Smad4 were underpinned by organ-specific differences in the tumor cells' chromatin state that emerged in the premalignant pancreas and were distinguished by the dominance of KLF (liver-biased) or RUNX (lung-biased) transcription factors. Our results show how epigenetic states favored by the organ of residence can influence the output of driver gene mutations in metastatic tumors. This organ-specific gene–chromatin interplay invites consideration of anatomical site in the interpretation of tumor genetics, with implications for the therapeutic targeting of metastatic disease. Overall design: To explore the origin of liver- and lung metastasis-biased chromatin states, we performed scRNA-seq on premalignant pancreata in the KC-shRen and KC-shSmad4 autochthonous model (at 8 weeks of age) +/– Dox after treatment with PBS or Cerulein (for 2 days). Cerulein treatment induces pancreatitis, which facilitates PDAC development.