Abstract C56: Bmi1 is widely expressed in acini and regulates Kras-driven transcription factor networks in early pancreatic neoplasia
Authors: Thompson Joyce K. , Wu Emily , Alkhalili Osama , Crawford Howard C. , Magliano Marina Pasca di , Bednar Filip 
Published: March 2020

Abstract
Rare acinar cells expressing Bmi1, a component of the Polycomb Repressor Complex 1 (PRC1), represent a reservoir of cells contributing to pancreatic repair during injury and stress. This is marked by acinar-ductal metaplasia (ADM), a transition of acinar cells to a more duct-like phenotype, which is seen in pancreatitis and early stages of Kras-driven pancreatic neoplasia. The mechanism behind Kras-driven ADM at the genomic level is not completely understood. Developmental transcription factor (TF) networks regulate and maintain the fate of cells and rely on appropriate chromatin context for their activity. Deletion of Bmi1 represses Kras-driven ADM and neoplasia in genetically engineered mouse models (GEMMs) of PDA. Thus, we hypothesized that Kras reprograms the acinar fate TF network to promote PDA in a Bmi1-dependent epigenetic context. We used GEMMs to study the expression of Bmi1 in the acinar compartment and its role in early pancreatic neoplasia. We combined an acinar cell-specific tamoxifen-inducible Cre-recombinase under the elastase promoter (Ela-CreER) with the Kras G12D oncogenic allele and the conditional Bmi1 knockout allele and the Rosa26 tdTomato reporter to generate Ela-CreER, KrasLSL-G12D/+, Bmi1, R26 tdTomato mice and their littermate controls. We also generated Ela-CreER, Bmi1GFP, R26tdTomato and Bmi1-CreER, R26 tdTomato mice to quantify the fraction of acinar cells expressing Bmi1. We activated the Cre-recombinase in 6- to 8-week-old Ela-CreER mice with 5 tamoxifen gavages (4mg/day) prior to induction of acute pancreatitis by intraperitoneal caerulein administration (8 hourly injections x 2 days) one week after the first tamoxifen dose. We FACS sorted tdTomato+ cells from untreated mice (0 hr) and one week (168 hr) after pancreatitis induction and analyzed RNA levels of 8 TFs, reported to play a role in pancreatic development, homeostasis, and neoplasia. Expression levels of two lineage markers (amylase and elastase) were also measured by RT-qPCR. Lineage tracing in Bmi1-CreER and Ela-CreER, Bmi1GFP mice revealed expression of Bmi1 in vast majority of acinar cells. RT-qPCR of tdTomato+ cells in the Ela-CreER models indicated a loss of acinar-specific TFs in mice expressing mutant Kras one week after induction of pancreatitis consistent with cells undergoing ADM. Genetic ablation of Bmi1 in the presence of mutant Kras restored elastase and amylase mRNA levels. At the morphologic level, loss of Bmi1 also led to recovery of acinar cells from ADM and correlated with an increase in RNA expression of three key acinar-specific fate TFs—Mist1, Hnf1a, and Nr5a2. The mRNA levels of other key acinar TFs—Pdx1 and Ptf1a/p48—did not recover with the loss of Bmi1 in the presence of KrasG12D. Together, our data suggest that Bmi1 is expressed in the majority of acinar cells and regulates oncogenic Kras-driven ADM by altering master TF gene regulatory networks. Its deletion leads to partial reprogramming of these networks to allow acinar cells to resist Kras-driven oncogenesis.
Citation Format: Joyce K. Thompson, Emily Wu, Osama Alkhalili, Howard C. Crawford, Marina Pasca di Magliano, Filip Bednar. Bmi1 is widely expressed in acini and regulates Kras-driven transcription factor networks in early pancreatic neoplasia [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr C56.