Abstract 969: Characterization of murine mammary epithelial populations throughout development and in premalignant tissue via mass cytometry
Authors: Gray Gary K. , Brugge Joan S. 
Published: August 2019

Abstract
Single-cell technologies provide a unique opportunity to develop a comprehensive understanding of the dynamic transitions of mammary epithelial cells during normal development, pregnancy, and tumorigenesis. To provide a higher resolution map of epithelial cell dynamics, we developed a mass cytometry panel composed of 39 antibodies targeting cell surface antigens of distinct murine mammary populations. We first utilized this panel to evaluate mammary epithelial composition through puberty and virgin adulthood. This analysis identified subpopulations of the three major mammary epithelial lineages (basal, alveolar progenitor [AP], and hormone-sensing cells [HS]). Interestingly, no clusters expressed markers consistent with their being intermediate between basal and luminal populations or the two luminal populations. We also identified puberty-restricted populations in each of the three lineages. Intriguingly, the luminal pubertal populations expressed basal markers such as CD104 and CD73. We also examined epithelial dynamics during a pregnancy cycle, including mid-pregnancy, lactation, involution, and post-involution. Three hitherto undescribed transit-amplifying clusters were enriched during pregnancy and persisted through involution. These clusters were highly proliferative (as measured by Idu incorporation) and expressed several basal markers, whose expression persisted among AP cells during involution and among a subset of post-involution AP cells. Finally, we analyzed alterations in mammary tissue in the context of loss of tumor suppressors (Apcmut/+, Atm+/-, Cdkn1a-/-, Arf-/-, Trp53+/-, or Trp53-/-) or expression of oncogenes (MMTV-Errb2, -neu, or -PyMT). Myriad alterations were observed across the conditions, the most striking of which was restriction of alterations in MMTV models to cells in the AP lineage. The expression of PyMT increased the frequency and basal-like features of AP cells. Analysis of a PyMT-induced tumor also showed that tumor cells were overwhelming of an AP-like phenotype. In conclusion, our data paint a high-resolution portrait of mammary epithelial dynamics throughout development and in the context of pro-tumorigenic genetic alterations. We report numerous novel populations during puberty and the pregnancy cycle. A particularly intriguing observation is that luminal populations appear to acquire basal-like differentiation features during puberty, pregnancy, and pre-tumorigenesis, suggesting the oft-reported mixed luminal/basal differentiation status of breast cancer could be a coopting of normal developmental programs. This result may provide clues for uncovering and blocking the molecular mechanisms of breast carcinogenesis.
Citation Format: Gary K. Gray, Joan S. Brugge. Characterization of murine mammary epithelial populations throughout development and in premalignant tissue via mass cytometry [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 969.