Breast cancer is one of the world’s leading causes of cancerrelated death among women, characterized by a high degree of heterogeneity in terms of histological, molecular, and clinical features, affecting disease progression and treatment response.

This has led to the classification of breast cancer into several subtypes including classical histological and immunohistochemical definitions of breast cancer types as well as molecularly defined subgroups. The seminal studies identified luminal, HER2-enriched, basal, and normal-breastlike intrinsic breast cancers. At the transcriptomic level, this classification was shown to be mainly driven by estrogen receptor a (ERa), and ERa-related and proliferationrelated genes. ERa-positive (ERa+) and -negative (ERa) breast cancers are well recognized as molecularly and clinically distinct diseases.

Cells of various cancer types have been shown to differ greatly in their tumorigenic, angiogenic, invasive, and metastatic potential. To account for intratumoral heterogeneity the cancer stem cell (CSC) model suggests that tumors are driven by a cellular subpopulation with stem cell properties, giving rise to hierarchically structured tumors.

CSCs of breast tumors are commonly enriched by combinations of several cell-surface antigens, such as CD44/ CD24/EPCAM, or by high ALDH (aldehyde dehydrogenase) activity. However, existing markers lack specificity, also reflective of a substantial proportion of non-CSCs.

Furthermore, the applicability of existing markers is often limited to specific breast cancer subtypes in addition to interindividual intrinsic differences. Previous studies have investigated the CSC content in different breast cancer subtypes; however, thus far it is not exactly known whether distinct subtypes harbor the same or dissimilar CSCs.

The identification of breast cancer cell subpopulations featuring truly malignant stem cell qualities is a challenge due to the complexity of the disease and lack of general markers.

By combining extensive single-cell gene expression profiling with three functional strategies for cancer stem cell enrichment including anchorage-independent culture, hypoxia, and analyses of low-proliferative, label-retaining cells derived from mammospheres, researchers identified distinct stem cell clusters in breast cancer.

Estrogen receptor (ER)α+ tumors featured a clear hierarchical organization with switch-like and gradual transitions between different clusters, illustrating how breast cancer cells transfer between discrete differentiation states in a sequential manner.

ERα− breast cancer showed less prominent clustering but shared a quiescent cancer stem cell pool with ERα+ cancer. The cellular organization model was supported by single-cell data from primary tumors.

The findings allow us to understand the organization of breast cancers at the single-cell level, thereby permitting better identification and targeting of cancer stem cells.

http://www.cell.com/stem-cell-reports/abstract/S2213-6711(15)00371-9