The Prognostic Value of Small Noncoding microRNA-21 Expression in the Survival of Cancer Patients: A Meta-Analysis

microRNA-21 (miR-21) is a small noncoding RNA that regulates gene expression in different types of human malignancies. The potential prognostic value of miR-21 in cancer progression is controversial. This meta-analysis includes 76 studies of 10,213 cancer patients to test miR-21 prognostic value in various human cancers. We obtain hazard ratios (HRs) and corresponding 95% confidence intervals (CIs) for overall survival (OS) to assess association strength. In the pooled analysis, high miR-21 expression is associated with poor OS, with a combined HR of 1.59 (95% CI, 1.49-1.70; p < 0.001; random-effects model). Furthermore, subgroup analysis demonstrates that high miR-21 expression is related to shorter OS in patients with digestive system cancers (HR = 1.02; 95% CI, 1.002 to 1.04; p = 0.026), respiratory system cancers (HR = 1.93; 95% CI, 1.48 to 2.51; p < 0.001), and breast cancer (HR = 2.20; 95% CI, 1.78 to 2.73; p = 0.001). These results indicate that miR-21 may be a clinically useful prognostic biomarker for cancer progression.