Abstract 3052: Defining the role of EMT in breast cancer dormancy and metastasis in vivo

Breast cancer patients can develop metastases from dormant tumor cells years after primary tumor resection and treatment. Despite the clinical significance of metastatic latency, the exact cellular and molecular events regulating tumor dormancy remains largely unknown due to challenges in detecting single dormant tumor cells in vivo and characterizing the dormant tumor microenvironment. A developmental program termed Epithelial Mesenchymal Transition (EMT) has been shown to play an important role in tumor cell dissemination, tumor dormancy, and chemoresistance. During EMT, stationary epithelial cells lose their epithelial characteristics, including adherent junctions and apical-basal polarity, and acquire mesenchymal shape and properties. These cells gain the ability to migrate into the circulation, lodge into a distant organ, and form dormant tumor niches. We have previously shown that reversal of EMT is a key driving force in dormant tumor cell proliferation and macrometastasis formation in a skin carcinoma mouse model. Here, we present a novel metastatic breast cancer mouse model in which HER2-driven primary breast tumor cells are labelled with red fluorescent protein with inducible EMT transcription factor TWIST1. We established this mouse model by crossing the TetON-TWIST1 mice with the MMTV-rtTA mice to generate the MMTV-rtTA/TetON-TWIST1 double transgenic mice. To generate HER2-driven breast tumors in these mice, we administered concentrated lentivirus that expresses both the HER2 gene and the tdTomato fluorescent marker under one promoter via intraductal injection. This allows for rapid and synchronized development of primary tumors which recapitulate the formation of sporadic human breast cancer which arises in single cells in otherwise normally developed breast tissue. TWIST1 expression in the luminal layer of the mammary gland was then induced via oral doxycycline. Upon expression of TWIST1, Her2-driven breast tumor cells were found in circulation and in the lungs as micrometastases. We determined that disseminated breast tumor cells mostly remained in dormancy until TWIST expression was switched off by doxycycline withdrawal. We showed that tumor dormancy is partly due to the inability to reverse EMT and regain proliferative capacity. Lastly, using our mouse model to track disseminated breast cancer cells, we will characterize the associated dormant residing niche to further our understanding of tumor dormancy in connection with EMT and immune