DICER1 hotspot mutation induces 3p microRNA gain of function via Argonaute strand switch

Dicer has RNase activity and is an essential enzyme in microRNA (miRNA) biogenesis. Mutations in the DICER1 gene have been linked to various cancers, notably DICER1 syndrome. To investigate the impact of pathogenic hotspot mutations in DICER1-associated tumors, we introduced a hotspot mutation into the endogenous Dicer1 locus of a mouse embryonic carcinoma cell line using CRISPR. Our findings not only confirm the loss of 5p-miRNAs, as previously reported, but also demonstrate unexpected upregulation of specific 3p-miRNAs. These upregulated 3p-miRNAs, which are usually considered to be passenger strands in wild-type cells, are selectively loaded into the Argonaute protein in mutant cells based on their 5'-end characteristics, resulting in a 'strand-switch' phenomenon. Functional assays and transcriptome analyses demonstrate the activity of the passenger 3p-miRNAs. These results suggest that the Dicer hotspot mutation is not merely a loss-of-function mutation for 5p-miRNAs but also a gain-of-function mutation for passenger 3p-miRNAs, potentially contributing to DICER1-associated tumorigenesis.