BackgroundMicroRNAs (miRNAs) are short, single-stranded RNAs that function as post-transcriptional regulators of gene expression. Although circulating miRNAs have been linked to carcinogenesis, they have not yet been systematically investigated in relation to risk of colorectal cancer (CRC).MethodsWe used Mendelian randomization (MR) and colocalization analyses to investigate the association of genetically predicted plasma miRNA concentrations (2083 miRNAs in 710 individuals) with risk of CRC (58,221 cases and 67,694 controls). For miRNAs associated with CRC risk, we also investigated their association with circulating plasma proteins (4907 proteins in 35,559 participants), bidirectionally, using MR. We performed pathway enrichment analysis (PEA) to explore downstream molecular pathways.ResultsAssociations of five miRNAs with CRC were found in MR and supported in colocalization analyses. Specifically, miR-146a-5p, miR-21-5p, and miR-4707-3p were positively, and miR-1908-5p and miR-6810-3p were inversely associated with CRC risk. Several protein associations were found for these miRNAs (range of proteins with P < 0.05: 78-796; 211 with FDR < 5%), and 11 pathways were identified in PEA, including regulation of Erb-B2 receptor tyrosine kinase 4 (miR-6810-3p) and insulin-like growth factor pathways (miR-1908-5p).ConclusionsOur results support a potential implication of miR-146a-5p, miR-21-5p, miR-4707-3p, miR-1908-5p, and miR-6810-3p to CRC risk. However, their downstream effects should be elucidated before they can be utilized as preventive targets.