EpCAM-Independent Enrichment of Circulating Tumor Cells in Metastatic Breast Cancer

Circulating tumor cells (CTCs) are the potential precursors of metastatic disease. Most assays established for the enumeration of CTCs so far–including the gold standard CellSearch—rely on the expression of the cell surface marker epithelial cell adhesion molecule (EpCAM). But, these approaches may not detect CTCs that express no/low levels of EpCAM, e.g. by undergoing epithelial-to-mesenchymal transition (EMT). Here we present an enrichment strategy combining different antibodies specific for surface proteins and extracellular matrix (ECM) components to capture an EpCAM(low/neg) cell line and EpCAM(neg) CTCs from blood samples of breast cancer patients depleted for EpCAM-positive cells. The expression of respective proteins (Trop2, CD49f, c-Met, CK8, CD44, ADAM8, CD146, TEM8, CD47) was verified by immunofluorescence on EpCAM(pos) (e.g. MCF7, SKBR3) and EpCAM(low/neg) (MDA-MB-231) breast cancer cell lines. To test antibodies and ECM proteins (e.g. hyaluronic acid (HA), collagen I, laminin) for capturing EpCAM(neg) cells, the capture molecules were first spotted in a single- and multi-array format onto aldehyde-coated glass slides. Tumor cell adhesion of EpCAM(pos/neg) cell lines was then determined and visualized by Coomassie/MitoTracker staining. In consequence, marginal binding of EpCAM(low/neg) MDA-MB-231 cells to EpCAM-antibodies could be observed. However, efficient adhesion/capturing of EpCAM(low/neg) cells could be achieved via HA and immobilized antibodies against CD49f and Trop2. Optimal capture conditions were then applied to immunomagnetic beads to detect EpCAM(neg) CTCs from clinical samples. Captured CTCs were verified/quantified by immunofluorescence staining for anti-pan-Cytokeratin (CK)-FITC/anti-CD45 AF647/DAPI. In total, in 20 out of 29 EpCAM-depleted fractions (69%) from 25 metastatic breast cancer patients additional EpCAM(neg) CTCs could be identified [range of 1–24 CTCs per sample] applying Trop2, CD49f, c-Met, CK8 and/or HA magnetic enrichment. EpCAM(neg) dual-positive (CK(pos)/CD45(pos)) cells could be traced in 28 out of 29 samples [range 1–480]. By single-cell array-based comparative genomic hybridization we were able to demonstrate the malignant nature of one EpCAM(neg) subpopulation. In conclusion, we established a novel enhanced CTC enrichment strategy to capture EpCAM(neg) CTCs from clinical blood samples by targeting various cell surface antigens with antibody mixtures and ECM components.