DICER1 dysregulation triggers miRNA reprogramming, promotes mesenchymal cell fates and slows TNBC tumorigenic phenotypes

DICER1 is a key processing enzyme required for the generation of mature and active miRNAs. Mutations that diminish DICER1 function result in widespread changes in miRNA levels, resulting in changes in the transcriptome and cellular phenotypes. Previously, we have found that mutations within the 3’UTR of the Dicer1 mRNA greatly diminish DICER1 protein levels and miRNA production. Triple negative breast cancer cells that contain these mutations have slower growth and migration and diminished tumorigenic potential. By comparing the transcriptome and miRNA profile of these cells, we find that miR30d-5p are significantly reduced in DICER1 mutant cells. This reduction in miR30d-5p results in increased mRNA stability and protein levels of the miR30d-5p target, SNAIL, a transcription factor that promotes the transcription of genes required for mesenchymal cell fates. We show that elevated SNAIL protein levels drive an epithelial- mesenchymal transition in these cells that can be partially rescued by the addition of miR30d-5p.Our results highlight the important role for DICER1 and miRNA levels in modulating cell fate in breast cancer.