A Role for Set1/MLL-Related Components in Epigenetic Regulation of the Caenorhabditis elegans Germ Line

The methylation of lysine 4 of Histone H3 (H3K4me) is an important component of epigenetic regulation. H3K4 methylation is a consequence of transcriptional activity, but also has been shown to contribute to “epigenetic memory”; i.e., it can provide a heritable landmark of previous transcriptional activity that may help promote or maintain such activity in subsequent cell descendants or lineages. A number of multi-protein complexes that control the addition of H3K4me have been described in several organisms. These Set1/MLL or COMPASS complexes often share a common subset of conserved proteins, with other components potentially contributing to tissue-specific or developmental regulation of the methyltransferase activity. Here we show that the normal maintenance of H3K4 di- and tri-methylation in the germ line of Caenorhabditis elegans is dependent on homologs of the Set1/MLL complex components WDR-5.1 and RBBP-5. Different methylation states that are each dependent on wdr-5.1 and rbbp-5 require different methyltransferases. In addition, different subsets of conserved Set1/MLL-like complex components appear to be required for H3K4 methylation in germ cells and somatic lineages at different developmental stages. In adult germ cells, mutations in wdr-5.1 or rbbp-5 dramatically affect both germ line stem cell (GSC) population size and proper germ cell development. RNAi knockdown of RNA Polymerase II does not significantly affect the wdr-5.1–dependent maintenance of H3K4 methylation in either early embryos or adult GSCs, suggesting that the mechanism is not obligately coupled to transcription in these cells. A separate, wdr-5.1–independent mode of H3K4 methylation correlates more directly with transcription in the adult germ line and in embryos. Our results indicate that H3K4 methylation in the germline is regulated by a combination of Set1/MLL component-dependent and -independent modes of epigenetic establishment and maintenance.

组蛋白H3（Histone H3）第4位赖氨酸的甲基化（H3K4me）是表观遗传调控的重要组成部分。H3K4甲基化既是转录活性的产物，同时也被证实参与“表观遗传记忆”的构建：即其可作为既往转录活性的可遗传标记，助力后续细胞子代或细胞谱系中相关转录活动的促进与维持。目前已在多种生物中鉴定出多种调控H3K4me添加的多蛋白复合物。这类Set1/MLL复合物或COMPASS复合物（COMPASS）通常共享一套保守蛋白亚基组合，其余组分则可能参与甲基转移酶活性的组织特异性或发育阶段特异性调控。本研究证实，秀丽隐杆线虫（Caenorhabditis elegans）生殖系中H3K4二甲基化与三甲基化的正常维持，依赖于Set1/MLL复合物组分WDR-5.1与RBBP-5的同源蛋白。不同的H3K4甲基化状态分别依赖wdr-5.1与rbbp-5，且各自需要不同的甲基转移酶。此外，在不同发育阶段的生殖细胞与体细胞谱系中，执行H3K4甲基化所需的保守Set1/MLL样复合物亚基子集亦存在差异。在成年生殖细胞中，wdr-5.1或rbbp-5的突变会显著影响生殖系干细胞（GSC）的种群规模与正常生殖细胞发育过程。对RNA聚合酶II（RNA Polymerase II）进行RNA干扰（RNAi）敲降，并不会显著影响早期胚胎或成年生殖系干细胞中依赖wdr-5.1的H3K4甲基化维持，提示该调控机制在这类细胞中并非严格与转录过程偶联。另一种不依赖wdr-5.1的H3K4甲基化模式，则与成年生殖系及胚胎中的转录活动相关性更强。本研究结果表明，生殖系中的H3K4甲基化通过结合Set1/MLL复合物依赖与非依赖的表观遗传建立与维持模式实现调控。