Abstract BL1: Deciphering stem and progenitor cells to understand breast cancer
Authors: Visvader JE , Lindeman GJ 
Published: March 2020

Abstract
Breast cancer is a highly heterogeneous disease at the molecular and pathological levels. To understand heterogeneity and potential ‘cells of origin’ of breast cancer, it is important to dissect the normal mammary epithelial differentiation hierarchy as well as the cellular composition of breast tumors. Discrete populations of mouse and human mammary epithelial cells have been isolated on the basis of cell surface marker expression, yielding three primary populations including a stem cell-enriched population and two luminal populations. Their responsiveness to female steroid hormones was investigated, revealing the importance of paracrine regulation of ER/PR– stem/progenitor cells by ER/PR+ luminal ‘hormone sensor’ cells. RANKL was found to be a key paracrine effector of progesterone signaling, underscoring the central role that female hormones play in normal mammary gland homeostasis and cancer. Consistent with this observation, a RANK+ luminal progenitor cell was found to be the likely target cell that gives rise to breast cancer in BRCA1 mutation carriers. Pre-clinical studies further indicate that it may be possible to target this ‘culprit’ cell with the RANKL inhibitor denosumab. This finding is now being tested in a randomized phase III prevention study, ‘BRCA-P’.
In further studies, a quiescent stem cell that remains hormone responsive has been identified. Lineage tracing, combined with high resolution 3D imaging (visualizing large regions of intact tissue), was carried out to track stem and progenitor cells in situ. These studies have revealed the presence of long-lived stem cells that replenish mammary epithelium, while unipotent progenitors appear to play an essential role in sustaining the independent lineages on a day-to-day basis. In order to generate a comprehensive single cell expression resource and explore molecular heterogeneity in the mouse mammary gland, single cell profiling at key developmental stages was performed. This has revealed novel intermediates and unexpected changes in the transcriptional landscape that could inform a role for distinct cell types during mammary oncogenesis.
To gain insight into cancer heterogeneity and clonal dynamics at cellular resolution, 3D imaging was combined with lineage tracing to track specific cells during cancer development. Through the application of a novel pipeline, the epithelial-to-mesenchymal transition (EMT) was found to be a frequent event, thus highlighting the inherent plasticity of mammary tumors. These findings are currently being extended to human tumors to further understand inter- and intra-tumoral heterogeneity.
In parallel, our group has established a bank of patient-derived xenograft (PDX) and patient derived organoid (PDO) models to facilitate the transfer of laboratory findings to the clinic. These serve as useful pre-clinical models to test promising agents. Initial efforts have focused on BH3 mimetics, a new class of drug that targets the BCL-2 family of pro-survival proteins. This work is leading to early phase clinical trials, currently focused on patients with metastatic ER+ breast cancer.
Citation Format: JE Visvader, GJ Lindeman. Deciphering stem and progenitor cells to understand breast cancer [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 BL1.