Prognostic and Predictive Significance of Breast Cancer Stem Cells

The existence of tumorigenic (stem) cells has been first described in acute myeloid leukemia (AML), and afterwards in some solid tumors. Recently, they were identified also in breast cancer showing high levels of CD44 surface marker and low level or lacking of CD24. Latest development revealed also significance of presence of ALDH1+ in this certain subpopulation of cells. These cells - CD44+/CD24- with ALDH1 have additional capability to induce tumors in NOD/SCID mice, have been extensively investigated in human tumors, and are claimed by some investigators to be responsible for cancer growth, relapse, resistance to chemotherapy, endocrine and radiation treatment, as well as widespread metastasis in breast cancer patients. However, the hope to solve the problem of breast cancer treatment focusing all attention to breast cancer stem cells (BCSC) has not yet been entirely successful and rather speculative. Investigations in animal models and human breast cancer cells grown as single cell suspensions or mammospheres in vitro have enlighted some of the latter problems, but the results until now are scarce and contradictory. In our review we have focused only on problems of vital interest for clinical practice, trying to elucidate the significance of breast cancer stem cells (or broadly speaking tumorigenic breast cancer cells) for prognostic and predictive significance in breast cancer treatment and prognosis. As a rule, BCSC are immunohistochemically detectable, having CD44+/CD24-/low or ALDH1+ as a surface markers, they as a rule present a subpopulation of cells with intrinsic resistance to chemotherapy, some of them (CD44+/CD24-/low) to radiotherapy; as to endocrine treatment, no satisfactory data are available until now. The BCSC problem remains unsolved, in vitro, in vivo or using them as indicators for treatment strategy. More data are to be needed for definite answers to many questions put forward by investigators and clinicians.