Functionally, Polycomb repressive complex 2 (PRC2) mediates transcriptional repression of differentiation genes critical for mouse embryonic stem cell (mESC) maintenance. Culturing mESCs in 2i serum-free medium inhibits FGF/ERK signaling and activates the Wnt/Îø-catenin pathway, which induces a naive cell state characterized by a reduced expression of lineage-specific genes. Interestingly, in naive mESCs, both PRC2 chromatin occupancy and the repressive histone3 lysine 27 trimethylation (H3K27me3) modification they mediate are largely depleted. To explore the molecular mechanism attributing to the transcriptional changes in naive cells, we performed RNA-sequencing for mESCs cultured in serum versus 2i medium. Gene expression analysis revealed reduced Jarid2 levels in naive mESCs. Reactivation of FGF/ERK signaling caused elevated Jarid2 expression, whereas ERK1/2 deletion decreased its expression levels. Ectopic expression of ERK depleted cells restored Jarid2 expression, showing that Jarid2 expression dependent on ERK signaling. Using ChIP-seq analysis, we observed reduced occupancy for Jarid2 and PRC2 and decreased H3K27me3 levels in both naive and Erk1/2 depleted mESCs. Expression of Jarid2 in Erk1/2 depleted cells reestablished PRC2 occupancy and H3K27me3 modifications. Taken together, these results reveal the molecular mechanism associated with FGF/ERK signaling and PRC2 recruitment in mESCs. The TrxG (Trithorax) group member ASH1L serves as a regulator of cell development. However, its functional role in MLL-rearranged leukemia initiation and maintenance is not well understood. Using an Ash1L conditional knockout mouse model, we demonstrated that ASH1L in hematopoietic progenitor cells (HPCs) impaired initiation of MLL-AF9-induced leukemic transformation in vitro. Ablation of ASH1L in the MLL-AF9-transformed leukemia cells impeded maintenance in vitro and leukemia progression in vivo. Furthermore, Ash1L depleted cells expressing wild-type ASH1L rescued MLL-AF9-induced leukemia transformation, while the transformation of cells expressing enzymatically inactive ASH1L inhibited their maintenance. Implementation of RNA-sequencing analysis revealed that ASH1L controls the expression of MLL-AF9 target genes by occupying their promoters and depositing H3K36me2 marks at these sites. Altogether, these results demonstrate that the enzymatic activity of ASH1L is crucial for MLL-AF9-induced leukemic transformation and maintenance. In addition, our study identifies a potential therapeutic target in MLL-AF9-induced leukemias. Histone post-translational modifications are vital for epigenetic mediated gene regulation. While past studies have characterized the functional role of many histone H3 lysine residues modifications, the post-translational modification of histone H3 lysine 37 and the factors contributing to these modifications remain undefined in mammals. Using in vitro methyltransferase assays, we found that SMYD family member 5 (SMYD5) catalyzes mono-methylation of H3 lysine 36 and 37 (H3K36/K37me1). Mutation of the conserved histidine within the catalytic SET domain abolished SMYD5 methyltransferase activity in vitro. Additionally, loss of Smyd5 in mESCs reduces the global histone H3K37me1 level in cells. Thus, our data functionally identifies SMYD5 as an H3 specific methyltransferase that mediates H3K36me/H3K37me1 in vitro. It also reveals that SMYD5 serves as one of the histone methyltransferases catalyzing histone H3K37me1 in vivo.
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