Background: microRNAs (miRNAs) are small noncoding RNAs that guide degradation of mRNA and regulate protein expression. miRNA based diagnostic biomarkers for ulcerative colitis (UC) and Crohn's disease (CD) are emerging but information about the cellular localization of many miRNAs is limited and more detailed histologic evaluation of miRNA expression patterns is needed to understand their immunobiological function.
Methods: Formalin-fixed paraffin-embedded colon biopsies from 10 patients with UC and 8 patients with CD together with 9 controls were examined by RT-qPCR and quantitative in situ hybridization (ISH). The cellular expression of miR-21 positive cells was further characterized using immunohistochemical cellular markers.
Results: Increased levels of miR-21 and miR-126 were found in UC compared with controls and increased levels of miR-21 were observed in UC compared with CD by both RT-qPCR and quantitative in situ hybridization. miR-126 was localized to endothelial cells and miR-21 to cells in the lamina propria. Multiplex immunohistochemical staining showed miR-21 expression in subsets of CD68 macrophages and CD3 T cells in UC, however, far the majority of the miR-21 positive cells could not be categorized among CD68, CD3, and CD19 cells.
Conclusions: This study shows that miR-126 levels are increased in UC and expressed in endothelial cells. miR-21 is expressed in subsets of monocytes/macrophages and T cells and may work as a potential biomarker to distinguish UC from CD. Quantitative in situ hybridization may be a powerful tool for such analysis as it combines overall expression with validation of cellular origin. Studies in larger cohorts may confirm this for clinical diagnostics.