Sequential Catalysis of Histone H3K27 Methylation States in the Cell Cycle and Blood Development

Abstract

Polycomb domains form at silenced genes and are marked by histone H3 lysine 27 tri-methylation, but it is unclear how this methylation is maintained following DNA replication in proliferating cells. Here, I use CUT&Tag chromatin profiling to track the dynamics of H3K27 methylation in live human cells. In K562 erythroleukemic cells, I find that tri-methylation in Polycomb domains is maintained by stepwise methylation after DNA replication. Outside of Polycomb domains, thousands of inactive genes gain histone H3K27 di-methylation hours after DNA replication. Acute treatment with small-molecule inhibitors of the Polycomb Repressive Complex 2 (PRC2) histone methyltransferase slows H3K27 methylation during S-phase of the cell cycle and increases acetylation of the residue. By applying multi-factor, single-cell CUT&Tag chromatin profiling to developing blood cells, I observe that Polycomb-silencing at genes in the monocyte and B-lymphoid lineages proceed via a di-methyl intermediate to gain H3K27 tri-methylation. Together, My results explain how H3K27 modification patterns are faithfully duplicated in rapidly proliferating cells and indicate that developmental silencing generally initiates from an inactive chromatin state.