Dicer loss induces an oncogenic epigenetic switch in mesenchymal stem cells

Abstract

MicroRNAs (miRNAs) are post-transcriptional regulators that tune gene expression. Despite the modest 2-fold repression that miRNA activity generally confers on a target, miRNAs are critical for many biological processes including development and differentiation. Due to this mild repression directly conferred by miRNA activity, miRNAs coordinate with other regulators such as transcription factors to shape the gene expression landscape and phenotypes of a cell. To understand the function of global miRNA activity in regulating the specification of the somatic state, we deleted Dicer in a murine mesenchymal stem cell model. Upon exploring the consequences of Dicer deletion, we identify a specific let-7 regulated mid-embryonic program within the global de-repression of miRNA targets accompanying Dicer loss. We further observe within the landscape of let-7 regulated targets, an activation of greater than 50-fold of known oncofetal (Igf2bp1/2/3) genes, an effect much greater than that typically reported for miRNA-mRNA interactions. This suggests a requirement of let-7 for the continual suppression of mid-embryonic programs in adult cells. To investigate the regulation of these oncofetal genes, we restored miRNAs through re-expression of Dicer. Despite complete reconstitution of the posttranscriptional activity of miRNAs, the activated oncofetal genes are incompletely suppressed. Igf2bp1-3 are components of a larger set of irreversible oncogenes whose chromatin signature indicate that they are transcriptionally activated upon Dicer deletion. This transcriptional activation is maintained, despite miRNA restoration in Dicer rescued cells. Consistent with this expression pattern, Dicer rescued cells are able to form tumors in mice, a phenotype absent in the parental wild-type and Dicer knockout cells. Moreover, the irreversible gene set is amplified in human cancers and is predictive of patient survival indicating that our observations are relevant to human disease. Finally, we develop a computational method to decipher the indirect, transcription factor mediated effects of miRNAs on gene expression. Through comprehensive analysis of ChIP-Seq, CLIP-Seq and RNA-Seq datasets, we quantitatively assess the relative contributions of direct posttranscriptional miRNA activity and transcriptional activity on gene expression changes resulting from Dicer deletion. We find that transcriptional changes contribute significantly to perturbations in gene expression resulting from global miRNA loss upon Dicer deletion. In summary, our work expands the current knowledge of fundamental roles for miRNAs in differentiated mammalian cells. As further work builds on our observations, the increased understanding of miRNA-mediated regulation will inform therapeutic strategies for human disease.