The biological and molecular heterogeneity of breast cancers correlate with their cancer stem cell content

Pathways that govern normal stem cell (SC) function are often subverted in cancer. Here, we report the isolation to near purity of human normal mammary SC (hNMSCs), from cultured mammospheres, based on their ability to retain the lipophilic dye PKH26 as a consequence of their quiescent nature. We demonstrated that PKH26-positive cells possess all the characteristics of hNMSCs. The transcriptional profile of PKH26-positive cells (hNMSC signature) was able to predict biological and molecular features of breast cancers. By using markers of the hNMSC signature, we could prospectively isolate SCs from the normal gland and from breast tumors. Poorly-differentiated aggressive (G3) cancers displayed higher content of prospectively isolated cancer SCs, than well-differentiated less aggressive (G1) cancers. By comparing G3 and G1 tumors in xenotransplantation experiments, we directly demonstrated that G3s are enriched in cancer SCs. Our data support the notion that the heterogeneous phenotypical and molecular traits of human breast cancers are a function of their SC content.