Circulating cell-free microRNAs as biomarkers for colorectal cancer

MicroRNAs (miRNAs) are a class of small, endogenous, non-coding, single-stranded RNAs that act as post-transcriptional regulators. Their discovery has provided new avenues for the diagnosis and treatment of cancer. The expression of both oncogenic and tumor suppressor miRNAs can be aberrantly either up- or down-regulated in cancer cells. These miRNAs target mRNAs of genes that either promote or inhibit tumor growth, and are one of several epigenetic factors that control the initiation and progression of colorectal cancer (CRC) and other cancers. Investigations of miRNAs as CRC biomarkers have employed the expression profiling of traditional tissue samples and, more recently, non-invasive samples, such as feces and body fluids, have been analyzed. MiRNAs may also be able to predict responses to chemo- and radiotherapy, and may be manipulated to modify CRC characteristics. We herein discuss the use of circulating miRNAs as possible non-invasive biomarkers of early CRC onset, relapse, or response to treatment. We also discuss the obstacles that currently limit the routine use of epigenetic biomarkers in clinical settings.