Using Mouse Models to Investigate the Initiation and Progression of Pancreatic Cancer

Abstract

Pancreatic cancer is one of the deadliest human malignancies. Standard chemotherapy approaches have proven ineffective , highlighting the need to understand the basic biology of the disease in order to identify future therapeutic targets. In humans, pancreatic tumors almost universally display activating mutations in the oncogene Kras, which are considered to be the initiating factor in carcinogenesis. Pancreatic cancer is also associated with later mutations in tumor suppressors, commonly point mutations in the gene p53. This information from human tumors is used to model pancreatic cancer in mice, where expression of oncogenic Kras recapitulates the precursor lesions seen in humans. Addition of tumor suppressor mutations leads to mice that reliably mimic all the stages of human disease. The overarching goal of this work is to use existing mouse models, as well as develop new ones, in order to further our understanding of the initiation and progression of pancreatic cancer. In the first part of this dissertation, I focus on the initiation of pancreatic cancer and the role of the epigenetic regulator Bmi1. Using a mouse model of early pancreatic carcinogenesis, I show that pancreatic expression of Bmi1 is required for the development of precancerous lesions, and therefore the initiation of pancreatic cancer. I showed that Bmi1 knockdown in pancreatic cancer cell lines increased levels of reactive oxygen species (ROS), indicating that the requirement of Bmi1 expression during pancreatic cancer initiation may be due to its control of cellular ROS levels. Next, I explore the mechanism for Bmi1 requirement in pancreatic cancer initiation. I find that when Bmi1 is knocked down in pancreatic cancer cells, HIF1a is also down, suggesting that Bmi1 is regulating HIF1a. In vivo, pancreatic HIF1a stabilization recovers the lack of precancerous lesion phenotype seen in animals lacking Bmi1 expression, indicating that the reason for Bmi1 requirement in pancreatic cancer initiation is through regulation of HIF1a levels. In the next sections of this dissertation work, I develop new mouse models that will help in the understanding of the basic biology of pancreatic cancer. First, I analyze HIF2a stabilization in the murine pancreas. Pancreatic HIF2a stabilization results in a phenotype that resembles human chronic pancreatitis, including inflammatory infiltrates and extensive fibrosis. In the context of oncogenic Kras expression, HIF2a stabilization leads to the development of large cystic lesions that resemble human mucinous cystic neoplasm (MCN), a less common precancerous lesion of pancreatic cancer. This work provides new mouse models of chronic pancreatitis and MCN, which can be of use in the future to study these conditions. Next, I create a new mouse model to study the role of tumor suppressor mutations in pancreatic cancer. In this work I create a new model that uses the second most common point mutation in p53, as well as models the sequence of events that occur in human tumors. I use these mice to determine that mutant p53 expression promotes formation of precancerous lesions, but it is not necessary for the growth of established tumors. Taken together, this dissertation work utilizes both existing and newly developed mouse models in order to provide new insights into the initiation and progression of pancreatic cancer. Overall, the use of mouse models provides an important scientific basis for experimentation that will eventually lead to new therapeutic options for pancreatic cancer, a truly devastating disease.