General principles and methods for routine automated microRNA in situ hybridization and double labeling with immunohistochemistry

AbstractMicroRNAs (miRNAs) are small non-coding RNAs that modulate gene expression by binding to complementary sequences on target messenger RNA transcripts. Changes in the expression levels of specific miRNAs have been associated with a variety of disease conditions. We developed a reliable and high throughput in situ hybridization (ISH) method and optimized tissue fixation conditions for formalin fixed, paraffin embedded (FFPE) tissues. ISH methods were automated to visualize four miRNAs: miRNA-145 (smooth muscle cells), miRNA-126 (endothelial cells), miRNA-21 (neoplastic cells) and U6 small nuclear RNA (nuclear marker) using locked nucleic acid (LNA) probes and the Discovery Ultra Ventana™ platform. The FFPE tissue sections were pretreated with protease 3, hybridized with probe concentrations of ≤ 25 nM; signal was detected using an enhanced, polymer-based detection method. The ISH signal was stronger and more uniform for tissue samples fixed for ≥ 48 h. To investigate the specificity of the method, we...