EZH2 Variants Differentially Regulate Polycomb Repressive Complex 2 in Histone Methylation and Cell Differentiation

Background: Polycomb repressive complex 2 (PRC2) is responsible for establishing and maintaining histone H3K27 methylation during cell differentiation and proliferation. H3K27 can be mono-, di-, or tri-methylated, resulting in differential gene regulation. However, it remains unknown how PRC2 specifies the degree and biological effects of H3K27 methylation within a given cellular context. One way to determine PRC2 specificity may be through alternative splicing of Ezh2, PRC2’s catalytic subunit, during cell differentiation and tissue maturation. Results: We fully characterized the alternative splicing of Ezh2 in somatic cells and male germ cells and found that Ezh’s exon 14 was differentially regulated during mitosis and meiosis. The Ezh2 isoform containing exon 14 (ex14-Ezh2) is upregulated during cell cycle progression, consistent with a role in maintaining H3K27 methylation during chromatin replication. In contrast, the isoform lacking exon 14 (ex14D-Ezh2) was almost exclusively present in spermatocytes when new H3K27me2 is established during meiotic differentiation. Moreover, Ezh2’s transcript is normally controlled by E2F transcription activators, but in spermatocytes, Ezh2’s transcription is controlled by the meiotic regulator MYBL1. Compared to ex14-EZH2, ex14D-EZH2 has a diminished efficiency for catalyzing H3K27me3 and promotes embryonic stem cell differentiation. Conclusions: Ezh2’s expression is regulated at transcriptional and post-transcriptional levels in a cellular context-dependent manner. EZH2 variants determine functional specificity of PRC2 in histone methylation during cell proliferation and differentiation.

背景：多梳抑制复合体2（Polycomb repressive complex 2, PRC2）负责在细胞分化与增殖过程中建立并维持组蛋白H3K27甲基化。H3K27可发生单甲基化、双甲基化或三甲基化，进而介导差异化的基因调控。但目前仍不清楚PRC2如何在特定细胞环境中调控H3K27甲基化的程度及其生物学效应。解析PRC2特异性的潜在途径之一，是在细胞分化与组织成熟过程中对PRC2的催化亚基Ezh2进行可变剪接。
研究结果：我们全面表征了体细胞与雄性生殖细胞中Ezh2的可变剪接模式，发现Ezh2的第14号外显子在有丝分裂与减数分裂过程中受到差异化调控。包含第14号外显子的Ezh2剪接异构体（ex14-Ezh2）在细胞周期进程中表达上调，这与其在染色质复制过程中维持H3K27甲基化的功能相符。与之相反，缺失第14号外显子的Ezh2剪接异构体（ex14D-Ezh2）几乎仅存在于精母细胞中，而该阶段正是减数分裂分化过程中新的H3K27me2被建立的时期。此外，Ezh2的转录本通常受E2F转录激活因子调控，但在精母细胞中，Ezh2的转录转而由减数分裂调控因子MYBL1所控制。与ex14-EZH2相比，ex14D-EZH2催化H3K27me3的效率显著降低，并可促进胚胎干细胞的分化。
研究结论：Ezh2的表达在不同细胞环境中分别受到转录水平与转录后水平的调控。EZH2变体决定了PRC2在细胞增殖与分化过程中介导组蛋白甲基化的功能特异性。