Circulating tumor cell markers for early detection and drug resistance assessment through liquid biopsy

Abstract

Circulating tumor cells (CTCs) are cancerous cells that extravasate from the primary tumor or metastatic foci and travel through the bloodstream to distant organs. CTCs provide crucial insights into cancer metastasis, the evolution of tumor genotypes during treatment, and the development of chemo- and/or radio-resistance during disease progression. The process of Epithelial-to-mesenchymal transition (EMT) plays a key role in CTCs formation, as this process enhances cell's migration properties and is often associated with increased invasiveness thereby leading to chemotherapy resistance. During the EMT process, tumor cells lose epithelial markers like EpCAM and acquire mesenchymal markers such as vimentin driven by transcription factors like Snail and Twist. CTCs are typically identified using specific cell surface markers, which vary depending on the cancer type. Common markers include EpCAM, used for epithelial cancers; CD44 and CD24, which are associated with cancer stem cells; and cytokeratins, such as CK8 and CK18. Other markers like HER2/neu and vimentin can also be used to target CTCs in specific cancer types and stages. Commonly, immune-based isolation techniques are being implemented for the isolation and enrichment of CTCs. This review emphasizes the clinical relevance of CTCs, particularly in understanding drug resistance mechanisms, and underscores the importance of EMT-derived CTCs in multidrug resistance (MDR). Moreover, the review also discusses CTCs-specific surface markers that are crucial for their isolation and enrichment. Ultimately, the EMT-specific markers found in CTCs could provide significant information to halt the disease progression and enable personalized therapies.

Keywords: circulating tumor cells; early detection; liquid biopsies; multidrug resistance; nanotechnology; tumor plasticity.

Figure 1

Biological mechanisms responsible for the MDR phenotype in CTCs and their inter-relationship analysis via bioinformatic studies. (a) Different mechanisms for the development of MDR phenotype in CTCs: The CTCs achieve stemness phenotypes characterized by upregulation of EMT factors (TWIST, cytokeratins, Snail, etc.) and generation of ROS via activation of oxidative phosphorylation. Elevated ROS is also responsible for genetic mutation, and modulation of redox-sensitive kinases that lead to tumor progression. The CTCs also attain the MDR phenotype via the upregulation of drug-resistance transporters that enhance the intracellular drug efflux from the CTCs. The upregulation of DNA repair activation networks such as the ECRR1 gene helps CTCs to survive and attain chemoresistance phenotype. The process involved in CTCs to attain MDR phenotypes: 1) The epigenetic mutations are responsible for the MDR phenotype; 2) Upregulation of the MDR gene which results in overexpression of ABC transporters; 3) DNA repair activation also facilitates the cells to attain MDR phenotypes; 4) Elevated ROS due also triggers the cells to achieve and develop stemness characteristic; 5) Different transcription factors (TFs) are involved to achieve EMT plasticity; 6) Overexpressed MDR drug transporters are involved in the efflux of an intracellular drug outside from the cells and CTCs become chemoresistant; (b) the Venn diagram illustrates the link between the genes involved in the process of EMT, CTCs generation, and MDR development; (c) the illustration highlighting the protein-protein interaction among the common top 424 gene involved in EMT, CTCs and MDR mechanisms. The protein-protein linkages were constructed using the STRING database.

Figure 2

(a) Comprehensive representation of dynamic equilibrium between CTCs, drug resistance, and tumor relapse. The EMT plasticity, Cancer heterogeneity, and MDR phenotypes play key roles in the development of chemoresistant CTCs. The chemoresistant CTCs act as seeds for the development of tumors at secondary locations. (b) Schematic illustration of CTCs enrichment, analysis, and clinical significance. The isolated CTCs from the cancer patient are used for the single-cell analysis, testing of drug susceptibility, and biological properties of individual tumors. Genetic and molecular profiling facilitates tailoring novel diagnostic and prognostic markers for early cancer detection.