Circulating microRNAs: emerging biomarkers for diagnosis and prognosis in patients with gastrointestinal cancers

To identify novel non-invasive biomarkers for improved detection, risk assessment and prognostic evaluation of cancer, expression profiles of circulating microRNAs are currently under evaluation. Circulating microRNAs are highly promising candidates in this context, as they present some key characteristics for cancer biomarkers: they are tissue-specific with reproducible expression and consistency among individuals from the same species, they are potentially derived directly from the tumour and therefore might correlate with tumour progression and recurrence, and they are bound to proteins or contained in subcellular particles, such as microvesicles or exosomes, making them highly stable and resistant to degradation. The present review highlights the origin of circulating microRNAs, their stability in blood samples, and techniques to isolate exosomal microRNAs, and then addresses the current evidence supporting potential clinical applications of circulating miRNAs for diagnostic and prognostic purposes. Abbreviations: Ago, Argonaute; AUC, area under the curve; CA 19-9, carbohydrate antigen 19-9; ESCC, oesophageal squamous cell carcinoma; EV, extracellular vesicle; 5-FU, 5-fluorouracil; HBV, hepatitis B virus; HCC, hepatocellular carcinoma; HDL, high-density lipoprotein; LDL, low-density lipoprotein; qRT–PCR, quantitative reverse transcription–PCR; TNM, tumour node metastasis