Abstract PR-005: Chromatin dynamics in vivo define coordinate functions of inflammation and mutant Kras in pancreatic tumorigenesis

• Published in: Cancer research (Chicago, Ill.) Ill.), 2021-11, Vol.81 (22_Supplement), p.PR-005-PR-005
• Main Authors: Falvo, David, Pitarresi, Jason, Osterhoudt, Alexa, Grimont, Adrien, Stanger, Ben, Leach, Steven D., Rustgi, Anil K., Chandwani, Rohit
• Format: Article
• Language: English
• ISSN: 0008-5472; 1538-7445
• DOI: 10.1158/1538-7445.PANCA21-PR-005

Pancreatic cancer initiation features abundant rewiring of the normal acinar cell,. but little is known about the chromatin specification of pancreatic cell types and the epigenetic dysregulation of normal acinar cells in tumor initiation. To address these questions, we employed a lineage-traced autochthonous mouse model to examine systematically perturbed acinar cells. We coupled the spatiotemporal control of inflammation (via caerulein injections) with inducible oncogene activation (mutant Kras) in the adult mouse along with bulk RNA-seq and ATAC-seq to sorted acinar and acinar-derived cells. In addition, we generated Ptf1a-TdTomato mice to reliably sort pancreatic progenitors at e10.5 and e15.5. We observe that Kras activation alone does not disturb acinar cell chromatin nor the histologic appearance of the pancreas. By contrast, caerulein alters chromatin significantly in metaplasia and even in regeneration, with putative enhancers derepressed despite normal histology. In the context of Kras activation and caerulein administration, we find a broad and stable reorganization of chromatin, reflecting cooperativity between oncogenic stress and an inflammatory insult. We also find that in PanIN, the chromatin state bears few, if any, ductal, progenitor, or islet features and instead reflects a largely novel cell fate. To understand the dependencies of these findings on an inflammatory insult and mutant Kras, we leveraged temporal resolution of pancreatitis and the iKras system to define the reversibility of this epigenetic rewiring. Notably, neither resolution of inflammation nor withdrawal of mutant Kras expression are sufficient to revert an acinar cell to its initial chromatin state. Analysis of the acinar-derived cells undergoing the transition to PanIN reveals the induction of specific proliferative and progenitor master transcription factors and activation of associated transcriptional programs. In these data we also observe a specific unveiling of the AP-1 isoform Fra-1 (Fosl1) transcript, protein, and binding sites in chromatin. To address if Fra-1-associated alterations to chromatin are bona fide requirements for tumorigenesis, we coupled conditional Fra-1 knockout alleles with the iKras system, finding nearly complete ablation of PanIN in the absence of Fra-1. Together, our findings suggest that (1) loss of acinar cell identity is resistant to oncogenic stress and is susceptible to inflammation; (2) the acquired acinar cell fate reflects neither ‘