The central theme of epigenetics is that DNA-independent phenotypic changes or gene expression alterations are inheritable through cell divisions. However, how epigenetic marks are transmitted from parental cells to daughter cells remains poorly understood. Increasing evidence indicates that PRC2-mediated H3K27me3 is important for cells to ‘memorize’ gene expression patterns during embryogenesis and development.16
It is believed that maintenance of the H3K27me3 mark during cell division cycles is essential for preservation of cell identity.32,33
Results from a recent study suggest that the PRC2 complex binds to the preexisting H3K27me3 mark at the sites of DNA replication and catalyzes K27 trimethylation on newly synthesized H3 incorporated in the daughter strands of chromatin.34
According to this model, H3K27me3 taking place on de novo synthesized H3 in nascent nucleosomes must occur after S (DNA replication) but before M (cell division) phases. Thus, there seems to be a demand for an active EZH2 during the S-G2
phases of the cell cycle. Interestingly, CDK1 and CDK2 are highly activated at these two stages, and we and others demonstrate recently that activation of CDK1 and CDK2 results in EZH2 phosphorylation at T350, which augments EZH2-mediated H3K27me3 in cells.25,26
Based upon these findings, we envision a model that in addition to their roles in driving DNA replication and mitosis, activation of CDK1 and CDK2 in S and G2
phases also promotes EZH2 T350 phosphorylation. The phosphorylation of EZH2 triggers the binding of PRC2 to recruiters such as HOTAIR and XIST ncRNAs and facilitates H3K27 trimethylation on newly synthesized H3, thereby maintaining H3K27me3 marks in daughter cells (). It is worth noting that Hox gene clusters, the inactive X-chromosome and paternally imprinted loci are extensively studied models for H3K27me3-mediated epigenetic silencing. Intriguingly, they invariably produce ncRNAs and accumulate methylated H3K27.15
Therefore, phosphorylation and activation of EZH2 by CDK1 and CDK2 may play a key role in the maintenance of the H3K27me3 marks in the inactive X-chromosome, paternally imprinted promoters and other PRC2 target loci through cell divisions.
A hypothetical model deciphering the potential role of CDK phosphorylation of EZH2 at T350 in maintenance of the H3K27me3 mark through a cell division cycle. Cell cycle phases G1, S, G2 and M are indicated.
EZH1 is another SET domain-containing protein. It is 65% identical to EZH2.15
EZH1 and EZH2 are partnered with the same core subunits of the PRC2 complex. As demonstrated by in vitro HMTase assays, they exhibit comparable histone methyltransferase activity.12
While knockdown of EZH2 affects global H3K27me3 levels in cells, knockdown of EZH1 is ineffectual on global H3K27me3.11
Also, EZH1 only partially complements EZH2 in executing pluripotency during embryonic stem cell differentiation.12
Notably, EZH1 lacks all the threonine residues on EZH2 that are phosphorylated by CDKs and p38 (). These residues are important for the binding of EZH2 to PRC2 recruiters either ncRNAs or YY1.26,31
It would be interesting to examine whether EZH2 phosphorylation mediated by CDKs and p38 contributes to the functional differences between EZH1 and EZH2 in cells. In line with the finding that the effect of EZH2 on global gene silencing largely relies on CDK1 and CDK2-mediated phosphorylation,25
EZH2 is mainly expressed in proliferating cells whereas EZH1 predominantly expressed in adult tissues and non-dividing cells.11,12,21
Thus, the functional difference between EZH1 and EZH2 may due to the distinct mechanisms that regulate their expression and activity.
Figure 2 EZH1 lacks the threonine residues in EZH2 that are phosphorylated by CDK1, CDK2 and p38. Middle: domain structure of human EZH2. Top and bottom: human EZH2 and EZH1 protein sequence alignment in the areas that contain CDK phosphorylation sites T350 and (more ...)
While CDK phosphorylation of EZH2 at T350 augments EZH2-mediated H3K27me3 in proliferating cells,25,26
EZH2 is also functional in resting cells.31
Activation of p38α in differentiating muscle satellite cells promotes cell cycle arrest and inhibits proliferation.31
Concordance with the essential role of CDKs in activation of EZH2 in cells,25
many muscle genes that are PRC2 targets are expectedly derepressed in non-proliferating myotubes.31
However, activation of p38 signaling seems to specifically direct PRC2 to repress genes that are typically downregulated during muscle differentiation, such as Pax7.31
This effect of p38 is mediated by p38 phosphorylation of EZH2 at T372 and by T372 phosphorylationenhanced interaction of PRC2 and YY1.31
While activation of most PRC2 target genes inhibits cell proliferation, expression of Pax7 is important for satellite cell proliferation,31
suggesting that Pax7 is a ‘noncanonical’ PRC2 target gene. In line with this observation, YY1, which is required for p38-mediated repression of Pax7,31
is not a general recruiter of PRC2 because there is only limited overlap of YY1 and PRC2 in mouse embryonic stem cell chromatin.35
Based on these findings, we hypothesize that in proliferating cells, CDK1 and CDK2 phosphorylates EZH2, which facilitates the interaction of PRC2 with ‘common’ recruiters such as ncRNAs and promotes H3K27me3 in the promoters of canonical PRC2 target loci. Upon cell cycle exit at certain stages of development such as in terminal differentiated muscles, CDK-mediated phosphorylation of EZH2 at T350 declines, which may trigger the disassociation of PRC2 from ncRNAs and thereby dismiss PRC2 from their canonical target loci. In contrast, environment cues from adult tissues produce tissue-specific signaling, such as activation of p38 in muscle cells. Phosphorylation of EZH2 at T372 due to p38 activation triggers the binding of PRC2 to the gene-specific recruiter YY1, thereby promoting context-dependent gene silencing in non-dividing cells.
In summary, recent findings from us and others demonstrate that EZH2 is phosphorylated at the T350 residue by CDK1 and CDK2. T350 phosphorylation is required for the effective binding of EZH2 to PRC2 target loci in cells. Accordingly, EZH2 is phosphorylated at T350 during S and G2/M phases of the cell cycle where CDK1 and CDK2 are highly activated. Thus, CDK phosphorylation of EZH2 at T350 represents a mechanism that augments EZH2 function during S and G2 phases of the cell cycle. This mechanism may play a role in preservation of cell identity by maintaining H3K27me3 marks through cell divisions.