Coordinated regulation of cellular identity-associated H3K4me3 breadth by the COMPASS family
Histone H3 lysine 4 trimethylation (H3K4me3) facilitates recruitment of transcription factors and epigenetic effectors to promote transcriptional activation and ensure cellular identity. This conserved histone mark is implemented by the COMPASS (COMplex of Proteins ASsociated with Set1) family of H3K4 methyltransferases. COMPASS members Set1A and Set1B have been accredited as primary depositors of global H3K4me3 in mammalian cells. Our previous studies in mouse embryonic stem cells (ESCs) demonstrated that deleting the enzymatic SET domain of Set1A does not perturb bulk H3K4me3, indicating possible compensatory roles played by other COMPASS methyltransferases. Here, we generated a series of ESC lines harboring compounding mutations of the COMPASS enzymes. We find that Set1B is functionally redundant to Set1A in implementing H3K4me3 at highly expressed genes, while Mll2 deposits H3K4me3 at less transcriptionally active promoters. Furthermore, Set1A and Set1B are responsible for broad H3K4me3 peaks, whereas Mll2 establishes H3K4me3 with narrow breadth. Most importantly, Mll2 helps preserve global H3K4me3 levels and peak breadth. Our results illustrate the biological flexibility of such enzymes in regulating transcription in a context-dependent manner to maintain stem cell identity, which could assist our understanding of their disease liability. Overall design: Elucidate roles of various COMPASS methyltransferases on H3K4me3 levels and breadth by investigating a series of generated ESC lines with compounding mutations of the COMPASS enzymes