Dissecting the Heterogeneity of Circulating Tumor Cells in Metastatic Breast Cancer: Going Far Beyond the Needle in the Haystack

Abstract

Although the enumeration of circulating tumor cells (CTC) defined as expressing both epithelial cell adhesion molecule and cytokeratins (EpCAM+/CK+) can predict prognosis and response to therapy in metastatic breast, colon and prostate cancer, its clinical utility (i.e., the ability to improve patient outcome by guiding therapy) has not yet been proven in clinical trials. Therefore, scientists are now focusing on the molecular characterization of CTC as a way to explore its possible use as a “surrogate” of tumor tissues to non-invasively assess the genomic landscape of the cancer and its evolution during treatment. Additionally, evidences confirm the existence of CTC in epithelial-to-mesenchymal transition (EMT) characterized by a variable loss of epithelial markers. Since the EMT process can originate cells with enhanced invasiveness, stemness and drug-resistance, the enumeration and characterization of this population, perhaps the one truly responsible of tumor recurrence and progression, could be more clinically useful. For these reasons, several devices able to capture CTC independently from the expression of epithelial markers have been developed. In this review, we will describe the types of heterogeneity so far identified and the key role played by the epithelial-to-mesenchymal transition in driving CTC heterogeneity. The clinical relevance of detecting CTC-heterogeneity will be discussed as well.

Role of Circulating Tumor Cells and Cell-Free Tumor Deoxyribonucleic Acid Fragments as Liquid Biopsy Materials in Breast Oncology

“…[10,11] The other frequently used methods for CTC detection from LB of BC patients are AdnaTest, which is another immunomagnetic enrichment based method, EPISPOT-assay that detects CTCs via specific proteins such as CK19, fluorescence in situ hybridization (FISH) technique that detects chromosome aberrations in the CTCs, micro RNA (miRNA)-profiling that detects CTCs by profiling altered miRNA expression, 'CTC-chips' that detect EpCAM-expressing cells and mRNA-based PCR detection methods can be used. [12] Clinical utilization of CTCs in breast oncology Breast tumor cells encountered at secondary homing sites, such as bone marrow (BM) and PB, are currently seen as surrogate markers and precursors of distant metastasis. [13] At the late 90's, several studies investigated the role of BM disseminated tumor cells (DTC) in the micrometastatic process of BC.…”

“…However, most of the current CTC detection methods are able to isolate and detect only epithelial type CTCs by targeting EpCAM-positive epithelial CTCs; yet it is known that mesenchymal-type of CTCs have also been observed as a result of epithelial-to-mesenchymal transition (EMT) in human BC patients. [12] For example, EpCAM-negative breast CTCs, which metastasize to brain have recently been identified. [50] Identification of these CTCs suggests that, EpCAM-negative CTC sub-populations may be present in LB samples of BC patients that need to be identified.…”

Recent Advances in Microfluidic Platforms Applied in Cancer Metastasis: Circulating Tumor Cells' (CTCs) Isolation and Tumor‐On‐A‐Chip

“…Label‐free‐based strategies, refer to CTC capture by biophysical properties other than tumor specific markers, have attracted much attention in recent years. The affinity‐based strategies usually employ expensive antibodies for capture, and the specificity of targeted protein must be known in advance . Moreover, expression of targeted protein is not a comprehensive method for identification of CTCs' heterogeneity since some CTC subpopulations exhibit no or little expression of tumor‐specific markers (EpCAM, HER2, and EGFR) .…”

Epithelial–mesenchymal plasticity and circulating tumor cells: Travel companions to metastases

“…Although CTC isolation techniques have been extensively reviewed elsewhere and will not be detailed here, it is important to perceive that the potential contribution of EMT to CTC biology largely complicates their purification and characterization (Bednarz-Knoll et al, 2012;Alix-Panabières and Pantel, 2014;Joosse et al, 2015;Pantel and Speicher, 2015;Hyun et al, 2016;Alix-Panabieres et al, 2017). This has stimulated the development of technologies to include the detection of EMT-shifted CTCs and their more systematic examination (Bednarz-Knoll et al, 2012;Barriere et al, 2014;Bulfoni et al, 2016b). It has thus been recognized that certain mesenchymally-shifted CTCs could be excluded from different CTC assays.…”