Downregulation of miR-9 correlates with poor prognosis in colorectal cancer

microRNAs (miRNAs) are frequently dysregulated in many human cancers including colorectal cancer (CRC) and are useful candidate biomarkers in liquid biopsy of cancer for their stability in the blood.We compared the expression of microRNA-9 (miR-9) in tissues (n = 357) and sera (n = 109) of CRC patients to determine whether miR-9 in serum reflects that in the cancer tissue in parallel. Also, we examined the miR-9 role in CRC by in vitro functional studies in four CRC cell lines.On multivariate analysis of colorectal cancer tissues and sera, miR-9 low expressions were significantly associated pN stage (tissues; p 0.01, serum; p = 0.013), and clinical stage (tissues; p 0.01, serum; p = 0.031). Moreover, patients with low miR-9 expression had shorter survival than those with high miR-9 expression (log-rank test, tissue; p = 0.021, serum; p = 0.011). miR-9 level in serum reflects that in the tumor. The CRC cells with low miR-9 expression was significantly increased cell proliferation, migration, invasion and colony formation than cells with high miR-9 expression.Serum miR-9 is an useful early detection marker in liquid biopsy of CRC and overexpression of miR-9 in CRC may be a novel prognostic marker as well.