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

MicroRNAs (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 developed an automated dual ISH for miRNA-145 coupled with immunohistochemistry for smooth muscle actin, which confirmed the specific distribution of miRNA-145 to smooth muscle cells. These methods may be used routinely for exploratory studies of biomarker development, sample screening and understanding the role of miRNA in the pathophysiology of specific diseases.