The presence of EpCAM-positive circulating tumor cells (CTCs) in the peripheral blood is associated with poor clinical outcomes in breast, colorectal and prostate cancer, as well as the prognosis of other tumor types. In addition, recent studies have suggested that the presence of CTCs undergoing epithelial-to-mesenchymal transition and, as such, may exhibit reduced or no expression of epithelial proteins e.g. EpCAM, might be related to disease progression in metastatic breast cancer (MBC) patients. Analyzing the neoplastic nature of this EpCAM-low/negative (EpCAM-neg) subpopulation remains an open issue as the current standard detection methods for CTCs are not efficient at identifying this subpopulation of cells. The possible association of EpCAM-neg CTCs with EpCAM-positive (EpCAM-pos) CTCs and role in the clinicopathological features and prognosis of MBC patients has still to be demonstrated. Several technologies have been developed and are currently being tested for the identification and the downstream analyses of EpCAM-pos CTCs. These technologies can be adapted and implemented into workflows to isolate and investigate EpCAM-neg cells to understand their biology and clinical relevance. This chapter will endeavour to explain the rationale behind the identification and analyses of all CTC subgroups, as well as to review the current strategies employed to enrich, isolate and characterize EpCAM-negative CTCs. Finally, the latest findings in the field will briefly be discussed with regard to their clinical relevance.
Keywords: EPCAM-negative; Molecular analysis; Single cell isolation.