Inhibitors of histone methylation

Two classes of inhibitors of histone methyltransferase I from calf thymus are reported. High concentrations ($\text{≧ 10}\text{m}\text{M}$) of various alkyl or aralkyl amines and polyamines were inhibitory to the enzyme. Spermine and spermidine were among the most potent compounds in this group. The best monoamine inhibitor was 2-phenylethylamine, which gave 47% inhibition at 10 mM.The substituted phenanthridinium compound ethidium bromide was also an inhibitor of the enzyme. A number of analogs of ethidium bromide were tested, and the most potent compound (17) gave 50% inhibition at 0.125 mM. $\text{S-}\text{Adenosyl}\text{-}\text{l}\text{-}\text{ethionine}$ (SAM) showed competitive inhibition of the enzyme as determined from a Lineweaver-Burke plot, while ethidium bromide was noncompetitive.     

In vivo HP1 targeting causes large-scale chromatin condensation and enhanced histone lysine methylation

Changes in chromatin structure are a key aspect in the epigenetic regulation of gene expression. We have used a lac operator array system to visualize by light microscopy the effect of heterochromatin protein 1 (HP1) alpha (HP1alpha) and HP1beta on large-scale chromatin structure in living mammalian cells. The structure of HP1, containing a chromodomain, a chromoshadow domain, and a hinge domain, allows it to bind to a variety of proteins. In vivo targeting of an enhanced green fluorescent protein-tagged HP1-lac repressor fusion to a lac operator-containing, gene-amplified chromosome region causes local condensation of the higher-order chromatin structure, recruitment of the histone methyltransferase SETDB1, and enhanced trimethylation of histone H3 lysine 9. Polycomb group proteins of both the HPC/HPH and the EED/EZH2 complexes, which are involved in the heritable repression of gene activity, are not recruited to the amplified chromosome region by HP1alpha and HP1beta in vivo targeting. HP1alpha targeting causes the recruitment of endogenous HP1beta to the chromatin region and vice versa, indicating a direct interaction between the two HP1 homologous proteins. Our findings indicate that HP1alpha and HP1beta targeting is sufficient to induce heterochromatin formation.     

Selective interactions between vertebrate polycomb homologs and the SUV39H1 histone lysine methyltransferase suggest that histone H3-K9 methylation contributes to chromosomal targeting of Polycomb group proteins

Polycomb group (PcG) proteins form multimeric chromatin-associated protein complexes that are involved in heritable repression of gene activity. Two distinct human PcG complexes have been characterized. The EED/EZH2 PcG complex utilizes histone deacetylation to repress gene activity. The HPC/HPH PcG complex contains the HPH, RING1, BMI1, and HPC proteins. Here we show that vertebrate Polycomb homologs HPC2 and XPc2, but not M33/MPc1, interact with the histone lysine methyltransferase (HMTase) SUV39H1 both in vitro and in vivo. We further find that overexpression of SUV39H1 induces selective nuclear relocalization of HPC/HPH PcG proteins but not of the EED/EZH2 PcG proteins. This SUV39H1-dependent relocalization concentrates the HPC/HPH PcG proteins to the large pericentromeric heterochromatin domains (1q12) on human chromosome 1. Within these PcG domains we observe increased H3-K9 methylation. Finally, we show that H3-K9 HMTase activity is associated with endogenous HPC2. Our findings suggest a role for the SUV39H1 HMTase and histone H3-K9 methylation in the targeting of human HPC/HPH PcG proteins to modified chromatin structures.     

Epigenetic regulation by histone methylation and histone variants

Epigenetics is the study of heritable changes in gene expression that are not mediated at the DNA sequence level. Molecular mechanisms that mediate epigenetic regulation include DNA methylation and chromatin/histone modifications. With the identification of key histone-modifying enzymes, the biological functions of many histone posttranslational modifications are now beginning to be elucidated. Histone methylation, in particular, plays critical roles in many epigenetic phenomena. In this review, we provide an overview of recent findings that shape the current paradigms regarding the roles of histone methylation and histone variants in heterochromatin assembly and the maintenance of the boundaries between heterochromatin and euchromatin. We also highlight some of the enzymes that mediate histone methylation and discuss the stability and inheritance of this modification.     

DNA and histone methylation in plants

Heritable patterns of gene activity and gene silencing arise by the formation and the propagation of specific chromatin states that restrict or permit gene expression. In mammals and in plants, restrictive heterochromatin is associated with the hypermethylation of DNA at CG sites and with the specific modification of histones, such as the methylation of histone H3 at lysine 9 (H3K9(Me)). In addition to CG methylation, plant nuclear DNA packaged in restrictive chromatin is also usually methylated in cytosines outside a CG sequence context. The functional relationship between an unexpectedly complex plant DNA-methylation system and histone modifications that lead to chromatin compaction and gene silencing is under intense scrutiny. The results of recent studies indicate intriguing links between chromatin remodeling, histone methylation, DNA methylation and RNA interference.     

A crack in histone lysine methylation

Histone lysine methylation is regarded as a very stable modification with important functions in epigenetic gene control and for organizing chromatin domains. While more robust modifications of the chromatin template are essential to stabilize epigenetic information, there is now the first evidence for a histone lysine demethylase that reverts an activating methyl mark to the unmodified state (Shi et al., 2004 [this issue of Cell]).     

The epigenetic magic of histone lysine methylation

Epigenetic mechanisms control eukaryotic development beyond DNA-stored information. There are several pathways, including histone tail modifications, histone variant incorporation, nucleosome remodelling, DNA methylation and noncoding RNAs that together all contribute to the dynamic 'make-up' of chromatin under distinct developmental options. The histone tail modifications are most variable and over 50 marks have by now been mapped. While the majority of these modifications are transient, histone lysine methylation and, in particular, a histone lysine tri-methyl state has been regarded as a more robust signal, consistent with proposed roles to impart long-term epigenetic memory. Based on the paradigm of SET-domain histone lysine methyltransferases (HMTases) and chromo-domain adaptor proteins, and in conjunction with the Sir Hans Krebs Medal 2005, I describe here my personal view on the discovery of the first HMTase in 2000, and the subsequent advances on the biology of histone lysine methylation. This discovery has changed my scientific career and significantly contributed to a better understanding of epigenetic control, with important implications for heterochromatin formation, X inactivation, Polycomb group silencing and novel insights into stem cell research, nuclear reprogramming and cancer.     

组蛋白甲基化修饰效应分子的研究进展

作为一种重要的表观遗传学调控机制,组蛋白甲基化修饰在多种生命过程中发挥了重要的作用。细胞内有多种组蛋白甲基化酶和去甲基化酶共同调节组蛋白的修饰状态,在组蛋白甲基化状态确定后,多种效应分子特异的读取修饰信息,从而参与基因转录调控过程。文章从组蛋白甲基化效应分子的作用机制方面综述了这一领域的研究进展。