Implications of Different Cancer Stem Cell Phenotypes in Breast Cancer

The attempts to identify, isolate and characterize cancer stem cell populations are mostly dependent on cell-surface markers. In breast cancer, several putative breast cancer stem cell (BCSC) markers have already been reported, but the agreement on their phenotypic characterization is still absent. In fact, it became unfeasible to obtain a universal combination of markers that could specifically identify BCSCs in all cases of breast cancer. Breast cancer heterogeneity as reflected by various histological subtypes, with variable clinical presentations and diverse molecular signatures also contributes to major drawbacks. Indeed, intra-tumor heterogeneity leads to a single tumor to contain, at any given time, tumor cell populations displaying different molecular profiles and biological properties. As a consequence, several BCSC phenotypes were described, with some being associated with aggressive forms of breast cancer. Although the validation of the CSC model remains an ongoing task, it is important to define which BCSC phenotypes have high tumorigenic potential and ability to resist therapeutic agents. For this reason, a concise review is presented here regarding the implications of the most studied BCSC markers and phenotypes in breast cancer progression and treatment.