Abstract PR01: Downregulation of PTF1A is a crucial and rate-limiting step in pancreatic cancer initiation

The goal of this study is to understand the endogenous mechanisms limiting the ability of oncogenic KRAS to initiate pancreatic tumorigenesis. Recently, our lab and others discovered that although pancreatic ductal adenocarcinoma (PDAC) shares phenotypic characteristics with normal pancreatic duct cells, it actually originates from mature exocrine acinar cells. In analyzing the initiating events of PDAC, we have identified the acinar cell transcription factor, PTF1A, as a critical factor inhibiting KRAS from reprogramming acinar cells to ductal tumor precursors. Binding sites for PTF1A are found upstream of essentially all acinar differentiation products and, importantly, PTF1A maintains its own expression through a positive autoregulatory loop. Given the central role of PTF1A in regulating acinar-specific gene expression, we wanted to determine if loss of this transcription factor plays a role in pancreatic intraepithelial neoplasia (PanIN) formation. Importantly, we find that PTF1A is downregulated in early (stage 1) PanINs of humans and mouse models, suggesting that loss of PTF1A expression could be a rate-limiting step in pancreatic cancer initiation. To test whether PTF1A loss is a functionally important step in PanIN development, we deleted the Ptf1a gene in mice using an acinar-specific inducible Cre (Ptf1a cKO). These studies confirm that PTF1A downregulation is an essential step in PanIN formation, as Cre-mediated deletion of acinar cell Ptf1a in the presence of oncogenic KRAS (KrasG12D) acutely and dramatically accelerates PanIN formation. Widespread PanINs are observed within two weeks of combined KRAS activation/Ptf1a deletion, by which time KRAS alone has induced few or no PanINs. At six weeks post-recombination, the combination of KRAS/Ptf1a cKO induces >15-fold more PanINs than KRAS alone. Additionally, we have found that Ptf1a heterozygosity sensitizes pancreatic cells to KRAS-mediated PanIN formation, indicating that transformation requires reduction of PTF1A activity below a critical autoregulatory threshold needed to maintain acinar identity. Based on these current studies, we hypothesize the existence of mutual antagonism between acinar differentiation, which is maintained through PTF1A, and transformation driven by KRAS. KRAS normally transforms acinar cells only weakly, but both endogenous and mutant KRAS activity can be increased by inflammation, such as that stimulated by caerulein-induced pancreatitis. As KRAS is potentiated by Ptf1a deletion, we asked whether inflammation would be sufficient to transform Ptf1a-deficient acinar cells even without mutant KRAS. In the absence of pancreatitis, Ptf1a deletion alone has relatively little short-term effect: Ptf1a-deficient acinar cells eventually lose their differentiated phenotype and express ductal markers, but they do not undergo hyperplasia or dysplasia. After subjecting Ptf1a cKO mice to caerulein-induced pancreatitis, however, we observed widespread acinar-to-ductal metaplasia, histological structures that resemble PanINs and stain positively with Alcian blue, and loss of amylase staining throughout the pancreas. Our current data indicate that PTF1A has a critical role in guarding against genetic (oncogenic KRAS) and environmental (pancreatitis) insults, and demonstrate that loss of Ptf1a expression is the critical event of acinar cell transformation. Going forward, we will use gene expression profiling and ChIP-seq analysis to identify PTF1A target genes responsible for inhibiting KRAS, and to characterize the epigenetic processes by which KRAS and inflammation reprogram acinar cells to a PanIN phenotype. This abstract is also presented as Poster A3. Citation Format: Nathan Michael Krah, Mary P. Bronner, Christopher V. Wright, L. Charles Murtaugh. Downregulation of PTF1A is a crucial and rate-limiting step in pancreatic cancer initiation. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr PR01.