m6A RNA methylation orchestrates transcriptional dormancy during paused pluripotency [dCas_RNAseq]
Development is often assumed to be a sequential unfolding of genetic programs towards increased complexity and occurring with a very stereotypical timing. However, embryos across all lineages of metazoans can enter reversible states of developmental pausing in response to adverse environmental conditions. In mammals, pausing manifests as a delayed implantation of the blastocyst, the source of embryonic stem cells (ESCs). Paused pluripotency can be induced in mouse blastocysts and ESCs by inhibition of mTOR, a conserved growth-promoting kinase, and is characterized by a drastic global decrease in biosynthetic activity, including gene transcription. The molecular mechanisms by which this remarkable dormant cellular state is achieved remain largely unknown. Here we show that m6A RNA methylation by Mettl3 is essential for transcriptional dormancy and maintenance of the paused pluripotent state in vitro and in vivo. Mass spectrometry and transcriptome-wide sequencing revealed an increase in m6A in paused ESCs. Knockout of the RNA methyltransferase Mettl3 suppresses transcriptional and proliferation dormancy of ESC specifically in the paused state, and leads to loss of paused blastocysts. Integration of several datasets in vitro and in vivo identified the transcriptional amplifier and oncogene Mycn as a key “anti-pausing factor” regulated by m6A mRNA methylation. Mett3-mediated methylation at a specific site of the Mycn mRNA is essential for its destabilization, which in turns mediates suppression of nascent transcription and proliferation in paused conditions. Our results highlight an intricate feedback between signaling, regulation of RNA stability and nascent transcription during pausing, with Mettl3 as an essential integrator and Mycn as a key downstream anti-pausing factor. These findings shed light on the mechanisms that underlie tuning of global transcriptional output during mammalian developmental pausing, and that may be redeployed in adult stem cells or during cancer dormancy. Overall design: ESCs expressing dCas_Alkbh5 cells were grown on gelatin in FBS/LIF Cells were transfected with 1µg lenti-sgRNA-BSD (coding for guide RNAs) and selected with 8µg/mL blasticidin for 3 days. Cells were then paused with 200 nM INK128 as mTor inhibitor for 5 days. RNA was extracted from ESCs.