Diagnostic Relevance of miR-185, miR-141, and miR-21 in Colon Carcinoma: Insights into Tumor Sidedness and Reference Gene Selection

<b>Background/Objectives:</b> MicroRNAs (miRNAs) regulate gene expression and are proposed as biomarkers in colorectal cancer (CRC). This study evaluated miR-185-5p, miR-141-5p, and miR-21-5p expression in CRC tissues; their association with tumor location, histopathology, and clinical outcomes; and the suitability of miR-16-5p and miR-151a-3p as housekeeping controls. Previous reports suggest tumor-suppressive roles for miR-185 and miR-141 and an oncogenic function for miR-21, though findings remain inconsistent. <b>Methods:</b> Paired tumor and adjacent normal tissues from 70 CRC patients were analyzed. RNA was extracted from FFPE samples, and miRNA expression quantified by RT-qPCR. Relative expression values were normalized to miR-151a-3p. Tumor-normal differences, localization effects, and associations with clinicopathological and outcome variables were assessed using repeated-measures ANOVA and non-parametric tests. <b>Results:</b> miR-185-5p and miR-141-5p were significantly reduced in tumors compared with normal mucosa while miR-21-5p was upregulated. miR-16-5p showed higher expression in normal tissue, indicating its instability and unsuitability as a housekeeping control. A modest but significant localization effect was observed for miR-185, while other miRNAs were minimally influenced by location. Baseline asymmetry between non-tumor samples, observed for miR-185-5p, further indicated sidedness effects. None of the miRNAs were associated with stage, histological type, grade, invasion, immune infiltration, progression, or five-year survival. <b>Conclusions:</b> miR-185-5p, miR-141-5p, and miR-21-5p show robust tumor-normal differences, supporting their diagnostic potential, while miR-16-5p is unsuitable as a housekeeper. Modest but significant localization effect was observed for miR-185 in right-sided tumors. None showed prognostic value in stage I-III CRC. Larger, location-stratified studies are warranted.