MiR-21 expression in the tumor cell compartment holds unfavorable prognostic value in gliomas.

High-grade gliomas are some of the most lethal forms of human cancer, and new clinical biomarkers and therapeutic targets are highly needed. MicroRNAs (miRNAs), a group of short noncoding RNAs, hold great potential as new biomarkers and targets as they are commonly deregulated in a variety of diseases including gliomas. MicroRNA-21 (miR-21) is the most consistently overexpressed miRNA in several cancers including gliomas and is therefore very promising as a useful clinical biomarker and therapeutic target. To better understand the role of miR-21 in gliomas, paraffin-embedded glioma tissue samples from 193 patients with grade I, II, III, and IV tumors were analyzed by in situ hybridization (ISH) using LNA-DNA chimeric probes. We found miR-21 expression in tumor cells and tumor-associated blood vessels, whereas no expression was seen in adjacent normal brain parenchyma. Using advanced image analysis we obtained quantitative estimates reflecting the miR-21 expression levels in each of these compartments. The miR-21 levels correlated significantly with grade [p = 0.027, r (s) = 0.161, 95 % confidence interval (CI), 0.015-0.301] with the highest levels measured in glioblastomas. Only tumor cell miR-21 was associated with poor prognosis when adjusting for known clinical parameters (age, grade, and sex) in a multivariate analysis [p = 0.049, hazard ratio (HR) = 1.545, 95 % CI, 1.002-2.381]. In conclusion, we have shown that miR-21 is located in both tumor cells and tumor blood vessels and that its level in the tumor cell compartment holds unfavorable prognostic value in gliomas.