Cell lineage tracing links ERα loss in Erbb2-positive breast cancers to the arising of a highly aggressive breast cancer subtype.

HER2-positive (HER2) breast cancers (BrCs) contain approximately equal numbers of ERαHER2 and ERαHER2 cases. An enduring obstacle is the unclear cell lineage-related characteristics of these BrCs. Although ERαHER2 BrCs could lose ERα to become ERαHER2 BrCs, direct evidence is missing. To investigate ERα dependencies and their implications during BrC growth and metastasis, we generated ERαRFP-T mice that produce an RFP-marked ERα mammary gland epithelial cell (MGEC) lineage. RCAS virus-mediated expression of Erbb2, a rodent Her2 homolog, first produced comparable numbers of ERαRFPErbb2 and ERαRFPErbb2 MGECs. Early hyperplasia developed mostly from ERαRFPErbb2 cells and ERαRFPErbb2 cells in these lesions were rare. The subsequently developed ductal carcinomas in situ had 64% slow-proliferating ERαRFPErbb2 cells, 15% fast-proliferating ERαRFPErbb2 cells derived from ERαRFPErbb2 cells, and 20% fast-proliferating ERαRFPErbb2 cells. The advanced tumors had mostly ERαRFPErbb2 and ERαRFPErbb2 cells and only a very small population of ERαRFPErbb2 cells. In ERαRFPErbb2 cells, GATA3 and FoxA1 decreased expression and ERα promoter regions became methylated, consistent with the loss of ERα expression. Lung metastases consisted of mostly ERαRFPErbb2 cells, a few ERαRFPErbb2 cells, and no ERαRFPErbb2 cells. The high metastatic capacity of ERαRFPErbb2 cells was associated with ERK1/2 activation. These results show that the slow-proliferating, nonmetastatic ERαRFPErbb2 cells progressively lose ERα during tumorigenesis to become fast-proliferating, highly metastatic ERαRFPErbb2 cells. The ERαErbb2 BrCs with an ERα origin are more aggressive than those ERαErbb2 BrCs with an ERα origin, and thus, they should be distinguished and treated differently in the future.