Project abstract Analysis of tumor-stroma signaling that mediates HER2-therapy resistance in breast cancer Drug resistance remains a great concern in HER2+ breast cancer, and a significant portion of patients (40-75%) will ultimately not respond to therapy. A better understanding of the causes of HER2-therapy resistance is therefore needed, in order to improve patient outcomes. To date, investigations on mechanisms contributing to HER2-therapy resistance have focused on tumor cell intrinsic factors that promote cell growth and survival. The role of extrinsic signals stemming from the tumor microenvironment (i.e. stroma), however, remains unclear. The goal of this proposal is to investigate the role of tumor-stroma interactions in HER2-therapy resistance and develop new strategies to re-sensitize tumor cells to therapy by (1) evaluating whether sensitivity to lapatinib correlates with stroma-rich niches in a panel of HER2+ breast cancer models ex vivo and in vivo (Aim 1), (2) examining tumor cell apoptotic priming as a tumor cell survival mechanism in HER2+ breast cancer patient-derived xenografts and patient samples from the clinical trial TRIO-US-B07 (Aim 2), (3) investigating additional tumor cell survival mechanisms by an unbiased mass-spectrometry analysis and developing and optimizing novel combination-therapies using computational modeling and in vivo studies with patient-derived xenograft models (Aim 3). The results from the proposed studies will lead to the identification of microenvironmental niches and associated tumor cell signaling pathways that could serve as biomarkers for patient stratification and provide important information for the design of rational combination therapies that will re-sensitize tumors to treatment. This proposal draws on my bioengineering background and postdoctoral training in cancer biology, however in order to ensure completion of the goals and successful transition to independence, I have also devised a detailed training plan, under the guidance of my mentor Dr. Brugge and my advisory team Dr. Letai, Dr. Michor and Dr. Sorger. During the mentored phase I plan to acquire new skills in (1) systems analysis and modeling of high-dimensional proteomic datasets to elucidate tumor-stroma signaling, (2) development of predictive mathematical models for designing effective drug combinations, (3) multiplexed immunofluorescence to profile in situ molecular markers in tumor and stroma cells and (4) analysis of predictive biomarkers in samples from clinical trials. The environment at Harvard Medical School presents an excellent opportunity for training in quantitative cancer biology and maximizing the translational potential of my research. I will also mentor students and further develop my grantsmanship and lab management skills. The proposed research plan combined with the career development training will significantly aid me to become an independent investigator in the field of tumor microenvironment bioengineering.