The polycomb repressive complex 2 (PRC2) methylates lysine 27 of histone H3 (H3K27me) via its catalytic subunit Ezh2. While H3K27me3 and H3K27me1 have been associated with gene repression and activation respectively, the exact role of H3K27me2 remains under-explored.
However, recent works in temperature-sensitive drosophila and in PRC2 knock out embryonic stem cells (ESCs) suggest a role of H3K27me2 in preventing inappropriate enhancer activation. Armed with the knowledge that variable degrees of H3K27 methylation are of vital significance for stem cells, Juan et al. decided to probe further the exact role of different H3K27 methylation ratios on ESC fate.
To accomplish their task, the team used Y641F mutation in one of the two Ezh2 alleles of ESCs via transcription activator-like effector nuclease (TALEN) mediated genome editing and captured the specific and dynamic genome-wide distribution of H3K27me2 and H3K27me3 in undifferentiated and differentiating ESCs, using chromatin immunoprecipitation followed by sequencing (ChIP-seq).
To disquiet H3K27 methylation, Juan and colleagues generated isogenic ESCs via TALEN-mediated genome editing of the Ezh2 coding region, causing the replacement of Tyr641 with phenylalanine (Y641F). This mutation results in Ezh2Y641F-containing PRC2 complex formation with increased catalytic activity on H3K27me2 substrates, thereby shifting the steady state of H3K27 to favor in vivo trimethylation.
By altering the ratios of H3K27 di/tri-methylation in ESCs, the team discovered that:
- H3K27me2/H3K27me3 are redistributed during ESC differentiation.
- H3K27 methylation recapitulates changes during differentiation in Ezh2-edited ESCs.
- In pluripotent culture conditions, Ezh2-edited ESCs activate neurogenic programing.
- Ezh2-edited and wild-type ESCs are phenotypically indistinguishable, and resistant to a medium containing MEK and GSK3 inhibitors induced “ground state”.
The researchers conclude that their findings indicate that modifying the ratio of H3K27me2 and H3K27me3 is sufficient for the acquisition and repression of defined cell lineage transcriptional programs and phenotypes. This is because the presence or absence of certain PRC2 subunits determine its methylation efficiency, and thus consequently partakes in regulating ESC biology.
To learn more about this important piece of work, ration ahead to Cell Reports, October 2016.