102). The above examples have enabled important progress in single-cell RNA profiling, particularly in transcriptome sequencing. primary tumor, are considered to be responsible for the establishment of distant metastasis for a given cancer.1C3 Numerous recent studies have indicated that CTCs may act as a real-time biomarker to better understand disease progression and IRAK inhibitor 4 therapy assessment, complementary to traditional biopsy sampling.4,5 As a new type of liquid biopsy, CTC analysis offers the possibility to avoid invasive tissue biopsy with practical implications for cancer diagnostics. Despite this great potential, until now CTC analysis has barely entered the clinical arena, largely because of the daunting technical challenge in isolating these rare IRAK inhibitor 4 cancer cells with ultrahigh sensitivity and selectivity. The main technical challenge lies in the fact that CTCs are very rare in the bloodstream, with concentrations generally estimated to be several CTCs among billions of red blood cells (RBCs) and millions of leukocytes per milliliter of whole blood.6 IRAK inhibitor 4 Thus, highly efficient and selective capture is the first critical step in CTC-based analysis. Another challenge is the heterogeneity observed extensively in cancer cells.7 For example, individual blood cancers exhibit significant intra-clonal heterogeneity, indicating the need for single cell analysis.8 Like most tumor tissues, CTCs show distinct morphological and phenotypic features, including potential morphological, genetic, metabolomic, proteomic Rabbit polyclonal to KIAA0494 and metastatic variations (Fig. 1).9,10 For instance, to intrude blood vessels, some of the cancer cells may undergo an epithelial to mesenchymal transition (EMT), resulting in progressive loss of the expression of epithelial markers.11 This heterogeneity poses the greatest challenge for enrichment, as there is no unique common biomarker for identification. More importantly, heterogeneity of CTCs highlights the importance of analyzing CTCs at the single-cell level, because bulk analysis may lose important information on individual CTCs.12,13 Thus, beyond CTC enumeration, the profiling of the phenotype and genotype of single CTCs may provide deeper insights into CTCs, which are important for the identification, origin, evolution and elucidation of cancer metastasis. Open in a separate window Fig. 1 Schematic of current methods for enrichment and single-cell analysis of CTCs. Modified with permission (adapted from ref. 21, 27, 51 and 70, and modified from ref. 25). This perspective provides a broad picture of CTC analysis, including advanced techniques for enrichment and single-cell analysis of CTCs, as well as current developmental trends and promising research directions (Fig. 1). To date, various techniques have led to exciting opportunities for CTC study. We first summarize the significant progress in CTC enrichment with satisfactory efficiency and purity, with IRAK inhibitor 4 special attention to emerging methods based on microfluidic technologies. We also discuss a number of key platforms for single-cell CTC characterization at the molecular level, including genomic, proteomic and phenotypic profiling and drug screening, which will lead to a comprehensive understanding of CTCs. Finally potential promising research directions regarding CTCs are also discussed. 2.?Methods for CTC enrichment The key technical challenge in CTC research is isolation and detection. Detecting a few CTCs among the vast numbers of other cells and differentiating the CTCs from epithelial non-tumor cells and leukocytes represent daunting technical challenges. Many CTC detection platforms utilize physical and morphological features of cancer cells, such as size, deformability, electrical charge or density.14,15 In addition, numerous strategies based on specific biological properties, for instance, tumor specific markers, have also been developed for CTC isolation.13,16 2.1. Traditional methods Immunomagnetic separation has been the most widely used approach to isolate CTCs from patient samples. Among all the platforms, the CellSearch? system is the IRAK inhibitor 4 1st and only clinically validated and FDA (US Food and Drug Administration) cleared test for taking and enumerating CTCs of metastatic breast, prostate or colorectal cancer. CellSearch? is intended for the enumeration of CTCs of epithelial source and uses anti-EpCAM-coated magnetic beads. After capture, the enriched cells are treated having a nuclear stain (DAPI+), fluorescent antibody conjugates against epithelial markers (EpCAM+) and cytokeratins (CK+), and a leukocyte marker (CD45C) for distinguishing CTCs from white blood cells. CellSearch? has been widely used mainly because an objective indication of CTC counting for evaluating additional new approaches. However, the standard immunomagnetic separation methods based on the acknowledgement of epithelial markers are not usually able to isolate the subpopulation of CTCs undergoing the EMT process, because of their lower or absent epithelial marker manifestation.17 Consequently,.