Abstract ES10-3: Cell fate plasticity during breast cancer development - where is the translational utility?
Authors: Scheel C 
Published: March 2020

Abstract
To develop more effective, targeted therapies, it is crucial to delineate the dynamics of intra-tumor heterogeneity during disease progression. This is particularly true for patients with metastatic relapse who have limited treatment options. Metastasis is a complex-multistep process starting with local invasiveness and ending with the outgrowth of disseminated cells into actively growing macrometastases. This last step, termed metastatic colonization, marks Stage IV breast cancer and thus, the worst prognostic outcome across all breast cancer subtypes. Understanding the role of cell fate plasticity is arguably most important in the context of metastatic colonization, since earlier steps of metastasis including systemic dissemination are thought to occur early with disseminated cells lying dormant for several years. As a result, the one-time snapshot afforded by molecular analysis of the primary tumor might not provide any information on the impact of cell fate plasticity during metastasis, particularly since cell fate is most likely controlled by dynamic epigenetic cellular programs. Based on these considerations, we will review the latest insights into the role of cell fate changes during metastatic colonization. In this context, new data based on patient-derived samples, such as circulating tumor cells and effusion biopsies or genetically engineered mouse models point towards maintaining an epithelial cell fate. By providing clues how proliferative programs are tied to an epithelial cell fate, these studies may provide the basis for new therapeutic strategies aimed at specifically targeting metastatic outgrowth. In this context, this session will also include an overview of our own current work, where we investigated cellular heterogeneity in patient-derived metastatic serous effusion biopsies based on differential expression of the epithelial cell surface marker EpCAM. Following prospective sorting by FACS, only cells with high EpCAM-expression (EpCAMhi) were able to efficiently form tumors in mice, whereas cells with low EpCAM expression (EpCAMlo) failed to do so. RNA sequencing and GSEA-analysis cells revealed EpCAMhicells to be enriched for gene sets involved in cell cycle regulation and epithelial identity, whereas EpCAMlocells were enriched for mesenchymal genes and those upregulated in response to TGF-beta, suggesting they had undergone an Epithelial-Mesenchymal Transition (EMT). To understand the origin of this epithelial-mesenchymal heterogeneity, we studied EMT-induction in hundreds of single-cell clones derived from in vitroimmortalized breast cancer cells. Thereby, we discovered that some clones underwent EMT whereas others actively resisted mesenchymal reprogramming. ATAC-sequencing revealed that the latter was correlated with widespread chromatin-based repression across many epithelial genes, providing an “epigenetic footprint” that clearly distinguished EMT-resistant and -competent clones. Taken together, our results suggest that epithelial breast cancer cells resist EMT through a novel, chromatin-based mechanism that might be useful for patient stratification. Moreover, our data strongly support the notion that protection of epithelial lineage identity is required for metastatic propagation in breast cancer.
Citation Format: C Scheel. Cell fate plasticity during breast cancer development - where is the translational utility? [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr ES10-3.