Detection of microRNAs in circulating tumor cells
Tremendous efforts have been made to develop reliable methods for detection and characterization of circulating tumor cells (CTCs). Hence, currently available approaches enable enumeration of carcinoma-derived CTCs with high sensitivity and specificity as prerequisite for estimation of prognosis and therapeutic monitoring of cancer patients. Moreover, phenotypic characterization of CTCs regarding the expression of selected therapeutic targets has already been included in clinical decision making and first assay formats for genomic as well as transcriptomic and epigenetic analyses of CTCs up to the single cell level were established to complement quantification of CTCs. Comparably less efforts have been directed towards uncovering the expression of microRNAs (miRNAs) in CTCs. Increasing numbers of miRNAs which are small non-coding RNAs have been described to regulate gene expression by interfering with mRNAs at posttranscriptional level. Acting on a multitude of different transcripts, miRNAs are capable to influence various processes associated with tumor development and progression. While miRNA expression was comprehensively studied in tumor tissues and in plasma and serum of cancer patients by now, the current review summarizes results of several proof-of-concept studies aimed to analyze miRNA expression in CTCs. Investigation of CTCs is of utmost interest especially in the metastatic setting where tumor tissues are only rarely available. Although not yet ready as additional biomarker to characterize CTCs, CTC miRNA analysis holds great promise to improve knowledge of regulatory mechanisms involved in tumor cell dissemination and might provide insights into the cellular origin of circulating miRNAs.