Breast cancer will affect one in nine women. ∼10% of breast cancer cases are hereditary, often caused by mutations in tumor suppressors (e.g., BRCA1 and 2). However, the majority of breast cancers are sporadic. Mechanisms of oncogenesis in sporadic breast cancers remain largely unknown, in part due to their cytogenetic complexity. Thus, model systems are needed to define key events leading to their formation. To address this, we previously generated a novel mouse model of human breast cancer based on a spontaneously acquired chromosomal translocation [t(12;15) ETV6-NTRK3 (EN) translocation] uniquely found in human secretory breast carcinoma, a type of basal-like breast cancer. The model is designed to initiate breast cancer from Wap+ (whey acidic protein promoter) cells in virgin mammary glands (MGs) by conditionally expressing the EN fusion oncoprotein from the endogenous Etv6 locus, using the Cre-lox approach. A unique feature of this model is that by including a conditional Cre-reporter (Rosa26-Stop-YFP), we can genetically mark affected cells by YFP when we turn on EN to initiate tumorigenesis. This allows us to track progression of the YFP-marked cells from their normal state to malignancy. We characterized these YFP+ cells at different stages by FACS analysis and by microarray expression profiling (of sorted YFP+ cells), followed by a cross-species comparison with published expression signatures of different subpopulations of cells sorted from normal human breast. We found that most normal Wap+ YFP+ cells in virgin MGs (i.e., normal target cells, <1% of total mammary cells) are CD24hi CD29med Sca-1- CD61+/− cells, whereas most YFP+ premalignant cells are CD24hi CD29med Sca-1low CD61+ cells. Furthermore, we found that at the molecular level, normal Wap+ YFP+ cells resemble human CD49f+ EpCAM− basal breast cells, whereas YFP+ premalignant cells resemble human CD49f+ EpCAM+ luminal progenitors. Thus, during cancer progression, there appears to be a phenotypic switch from basal-like Wap+ normal cells to luminal progenitors-like premalignant cells, and eventually to basal-like breast cancer cells. The YFP+ premalignant cells exhibit relaxed cell cycle control, upregulate genes related to DNA double-strand break repair (DSBR), and downregulate cell adhesion molecules. In sorted EN basal-like tumor cells, key DSBR genes are downregulated, suggesting this may represent a rate-limiting step in tumor progression. In addition, we found multiple stem cell-related pathways are initially active in normal Wap+ cells, while they are downregulated in YFP+ premalignant cells. Intriguingly, some of these pathways (e.g., hedgehog signaling) appear to be reactivated in (or acquired by) EN tumor cells. We hypothesize that such pathways may represent important self-renewal pathways required by cancer cells in order to progress from the premalignant stage.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-35. doi:10.1158/1538-7445.AM2011-LB-35