RNA methylation maintains hematopoietic stem cell identity and symmetric commitment

Stem cells balance cellular fates through asymmetric and symmetric divisions in order to self-renew or to generate downstream progenitors. Symmetric commitment divisions in stem cells are required for rapid regeneration during tissue damage and stress. The control of symmetric commitment remains poorly defined. N6-methyladenosine (m6A), the most abundant posttranscriptional mRNA modification controls cellular states and its abundance is dysregulated in cancer. Here we show that mRNA methylation controls symmetric commitment and cell identity of hematopoietic stem cells (HSCs). Using single-cell RNA sequencing (scRNA-seq) in combination with transcriptomic profiling of HSPCs (hematopoietic stem and progenitor cells) from control and m6A methyltransferase Mettl3 conditional knockout mice, we found that m6A-deficient HSC fail to symmetrically differentiate. Dividing HSCs are expanded and are blocked in an intermediate state that molecularly and functionally resembles multipotent progenitors. Mechanistically, RNA methylation controls Myc mRNA abundance in differentiating HSCs. Importantly, we identified MYC as a new marker for HSC asymmetric and symmetric commitment. Furthermore, forced expression of MYC rescued m6A’s requirement for engraftment indicating its importance in the early stage of HSC cellular fate. Overall our results indicate that RNA methylation is critical for normal blood homeostasis and may provide a general mechanism for how stem cells regulate differentiation fate choice. For single cell RNA sequencing, 3,000 cells were targeted for each replicate using the 10X Genomics Chromium Single Cell RNA solution, and sequenced to a depth of 50,000 reads per cell. Bulk RNA sequencing was performed in 3 replicates, for sorted hematopoietic stem cells (HSC), and the following multi potent progenitors (MPP) populations: MPP1, MPP2 and MPP4