Epigenetics provides stability and diversity to the cellular phenotype through chromatin marks that affect local transcriptional activity. Epigenetic marks are preserved or regenerated during cell division and embryonic development. The complex time/space patterns of gene expression necessary for normal development are likely to require multiple epigenetic signals (Bonasio et al., 2010).
We have demonstrated H3K27me3 and H3K4me3, the bivalent domain, were localized on Hoxa2 gene promoter and affect the transcriptional level of Hoxa2 gene. As H3K4me3 is associated with RNA Pol II, it is necessary to further investigate how H3K27me3 cooperates with H3K4me3 to regulate the initiation of gene transcription on Hox gene clusters. How are bivalent domains timely patterned on the gene promoter, and associated with RNA Pol II to exert the effect on gene expression during embryonic development?
H3K9 is a substrate for both acetylation and methylation. Acetylation of H3K9 correlates with activation of gene expression (Agalioti et al., 2002). In contrast, methylation of H3K9 can lead to the recruitment of silencing proteins (Bannister et al., 2001; Lachner et al., 2001) and is directly linked to DNA methylation in a number of human cell lines (Esteve et al., 2006). Since the acetylation of H3K9 precludes methylation, it would be interesting to investigate the effect of VPA on modification status of H3K9 associated with bivalent domains. In this case, the action of TrxG protein MLL1 in the Hox loci may generate and stabilize a pattern of chromatin modifications (H3K4me3) required for Hox gene expression.
Progress has been made in understanding the versatile role of PcG proteins, especially PRC2 protein. PRC is not only involved in the regulation of a broad array of biological processes, but it also establishes regulatory cues that are stable and propagated throughout embryonic development (Margueron and Reinberg, 2011). Previous studies have demonstrated PRC2 must be targeted to chromatin by a coordinated and intricate process to maintain the repression of different sets of genes dependent on cell types and developmental stages. These steps may require specific DNA sequences, non-coding RNA (nc RNA) and the chromatin structure proteins associated with its target genes (Margueron and Reinberg, 2011). Answering the questions such as: how ncRNA can recognize defined genomic locations? What is the exact
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mechanism by which histone methyltransferase Ezh2 and histone demethylase JARID2 proteins contribute to PRC2 recruitment, will provide a future direction for pinpointing the aspects of PRC2-mediated processes on Hoxa2 gene promoter.
Studies developed during the last two decades have also shown the importance of GSH during the process of mammalian chromatin sperm decondensation (Maeda et al., 1998; Perreault et al., 1984; Romanato et al., 2005). The alteration of nuclear redox conditions modulates chromatin conformation and stability (Markovic et al., 2010). Oxidants induce degradation of higher ordered chromatin structures (Bai and Konat, 2003). Oxidation of chromatin affects interaction between nucleosomes and the levels of reducing agents in the nucleus (Tas and Walford, 1982). However, the overall redox mechanisms that may regulate all these events remain unknown, although it has been speculated that ROS generation induced by a shift in the redox status of cells could affect gene expression by altering chromatin conformation (Hitchler and Domann, 2007).
Glutathione is closely connected with epigenetic mechanisms. Indeed, synthesis of S- adenosylmethionine (SAM) by SAM synthetases is a redox regulated process that depends on the GSH/GSSG ratio. When cells are depleted of glutathione by chemical means, methyl donors become deficient, leading to genome-wide DNA hypomethylation (Lertratanangkoon et al., 1996; Lertratanangkoon et al., 1997). However, we found VPA induced oxidative stress and also increased DNA methylation at Hoxa2 gene. There could be other mechanisms related to VPA- induced hypermethylation at Hoxa2 gene. Therefore, it will provide valuable insight to showing chromatin remodeling events occurring or directly linking histone modification with GSH depletion induced by oxidative stress.
Telomere attrition is modulated by oxidant-antioxidant balance (Saretzki, 2009). GSH is the
most abundant non-protein thiol in most cells. The thiol moiety of GSH is important in antioxidant defense metabolism (Ketterer 1982; Meister 1983; Ziegler 1985). The response of a cell to an oxidative stress often involves changes in thiol content, and is then replaced through either enzymatic reduction of a disulfide, or by de novo synthesis. These changes in thiol content and metabolism can have effects on signaling pathways (Arrigo 1999; Dickinson and Forman 2002). We have shown the changes in telomerase activity by VPA associated with VPA-induced decreases in glutathione and thiol status (Fig.4.1.2, Fig. 4.1.5, Fig. 4.1.6, Fig. 4.1.8). Further
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studies are needed to determine whether VPA-induced dysfunction in telomerase activity modulates apoptosis, and what is the enzyme machinery?
Telomerase regulation is complex and has prominent consequences not just for telomere length, but also for the development of many diseases (Koziel et al., 2011). Obtaining further insight into the role of telomerase gene regulation and therefore telomerase activity in disease development will assist in the generation of clinical therapies (Koziel et al., 2011). More recently, regulation of telomerase activity has been found not only to be controlled at the TERT transcriptional level, but also determined at the post-transcriptional level (Liu et al., 1999). Phosphorylation and nuclear translocation both play an important role in the post-transcriptional regulation of telomerase (Liu et al., 2001; Zhu et al., 2011). Therefore, understanding how telomerase is regulated at the epigenetic level will lead to improved targeting of telomerase as a therapy for genetic diseases and cancers.
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