It has been hypothesized that carcinoma metastasis is initiated by a subpopulation of circulating tumor cells (CTCs) found in the blood of patients. However, although the presence of CTCs is an indicator of poor prognosis in several carcinoma entities, the existence and phenotype of metastasis-initiating cells (MICs) among CTCs has not been experimentally demonstrated. Here we developed a xenograft assay and used it to show that primary human luminal breast cancer CTCs contain MICs that give rise to bone, lung and liver metastases in mice. These MIC-containing CTC populations expressed EPCAM, CD44, CD47 and MET. In a small cohort of patients with metastases, the number of EPCAM(+)CD44(+)CD47(+)MET(+) CTCs, but not of bulk EPCAM(+) CTCs, correlated with lower overall survival and increased number of metastasic sites. These data describe functional circulating MICs and associated markers, which may aid the design of better tools to diagnose and treat metastatic breast cancer.

The cancer stem cell (CSC) concept, which arose more than a decade ago, proposed that tumor growth is sustained by a subpopulation of highly malignant cancerous cells. These cells, termed CSCs, comprise the top of the tumor cell hierarchy and have been isolated from many leukemias and solid tumors. Recent work has discovered that this hierarchy is embedded within a genetically heterogeneous tumor, in which various related but distinct subclones compete within the tumor mass. Thus, genetically distinct CSCs exist on top of each subclone, revealing a highly complex cellular composition of tumors. The CSC concept has therefore evolved to better model the complex and highly dynamic processes of tumorigenesis, tumor relapse, and metastasis.