The use of circulating microRNAs as diagnostic biomarkers in colorectal cancer

BACKGROUND: Abnormal levels of microRNAs (miRNAs) have been found in the blood or its components in a number of different cancers including colorectal cancer. In addition to being abundant in circulation, miRNAs show remarkable stability in both plasma and serum making miRNAs ideal markers for early detection in colorectal cancer. Several miRNAs have been identified as potential circulating biomarkers although none have been incorporated into clinical practice. OBJECTIVE: To identify the most consistently dysregulated circulating miRNAs in colorectal cancer patients according to current literature and postulate reasons for heterogeneity in results. METHODS: A literature review was performed using the electronic databases PubMed, Embase and the Cochrane Library. RESULTS: The 6 circulating miRNAs most frequently found to be dysregulated in colorectal cancer are miR-18a-5p, miR21-5p, miR-29a-5p, miR-92a-5p, miR-143-5p and miR-378-5p. There are, however, multiple studies with conflicting findings. Studies vary significantly in ethnicity of populations, use of endogenous controls, source of miRNAs (whole blood, serum and plasma) and methods of detection. CONCLUSIONS: Circulating miRNAs are promising diagnostic biomarkers in colorectal cancer. Further studies identifying the source of tumour derived miRNAs in circulation, including identification of exosomal miRNA content, are required. Identifying pre-profiling factors affecting miRNA expression and determining stable endogenous controls will expedite the incorporation of miRNAs into clinical practice.