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Epigenetics investigates heritable changes in gene expression that occur without changes in DNA sequence. Several epigenetic mechanisms, including DNA methylation and histone modifications, can change genome function under exogenous influence. We review current evidence indicating that epigenetic alterations mediate effects caused by exposure to environmental toxicants. Results obtained from animal models indicate that in utero or early-life environmental exposures produce effects that can be inherited transgenerationally and are accompanied by epigenetic alterations. The search for human equivalents of the epigenetic mechanisms identified in animal models is under way. Recent investigations have identified a number of environmental toxicants that cause altered methylation of human repetitive elements or genes. Some exposures can alter epigenetic states and the same and/or similar epigenetic alterations can be found in patients with the disease of concern. On the basis of current evidence, we propose possible models for the interplay between environmental exposures and the human epigenome. Several investigations have examined the relationship between exposure to environmental chemicals and epigenetics, and have identified toxicants that modify epigenetic states. Whether environmental exposures have transgenerational epigenetic effects in humans remains to be elucidated. In spite of the current limitations, available evidence supports the concept that epigenetics holds substantial potential for furthering our understanding of the molecular mechanisms of environmental toxicants, as well as for predicting health-related risks due to conditions of environmental exposure and individual susceptibility. [ABSTRACT FROM AUTHOR]

An important feature of cancer is dysregulation of gene activity and gene expression, which is driven by a combination of acquired genetic and epigenetic alterations. Here, we will highlight how insights into the epigenetic processes underpinning tumor biology have led to the emerging field of cancer pharmaco-epigenomics. First, we will discuss how interference with the epigenetic machinery in cancer is leading to novel promising therapies, with several DNA methyltransferase and histone deacetylase inhibitors being approved for cancer treatment. Second, we will discuss how epigenetic markers in cancer may increasingly be used as complementary diagnostic tools, prognostic markers of disease progression, and predictive markers of treatment response. Although the anti-tumoral activities of epigenetic therapies have thus far been attributed to reactivation of silenced tumor-suppressor and/or apoptotic genes, they may also influence the tumor environment by directly affecting stromal cells. As an example, we will discuss how tumor-endothelial cells are regulated at the epigenetic level and are affected by methyltransferase and histone deacetylase inhibitors. [ABSTRACT FROM AUTHOR]

The dentate gyrus of the hippocampus is an exception to a ‘neurogenesis-unfriendly’ environment of the adult brain. New functional neurons generated in this region contribute to learning and mood regulation, and thus represent a unique form of neural plasticity. The rate of hippocampal neurogenesis significantly changes on physiological or pathological influences, such as physical activity, environmental enrichment, stress, and aging. We suggest that epigenetic mechanisms could be sensors of environmental changes and fine modulators of adult hippocampal neurogenesis. Here, we examine the role of DNA methylation and methylation of core histones mediated by the Polycomb and Trithorax complexes in the regulation of adult neurogenesis. Given the recent surprising discovery of dynamic and reversible DNA methylation in the hippocampus, we speculate regarding its regulation and its role in adult neurogenesis. [ABSTRACT FROM AUTHOR]

Epigenetic mechanisms contribute to the establishment and maintenance of cell-type-specific gene expression patterns. In this review, we focus on the functions of histone lysine methylation in the context of epigenetic gene regulation during developmental transitions. Over the past few years, analysis of histone lysine methylation in active and repressive nuclear compartments and, more recently, genome-wide profiling of histone lysine methylation in different cell types have revealed correlations between particular modifications and the transcriptional status of genes. Identification of histone methyltransferases (HMTases) and specific binding factors for most methylated lysine positions has provided a novel insight into the mechanisms of epigenetic gene regulation. In addition, analyses of HMTase knockout mice show that histone lysine methylation has important functions for normal development. In this study, we review mechanisms of gene activation and repression by histone lysine methylation and discuss them in the context of the developmental roles of HMTases. [ABSTRACT FROM AUTHOR]

DNA methylation is an epigenetic mark that has key roles in the control of genome activity in plants and mammals. It is critical for the stable silencing of repeat elements and is also involved in the epigenetic regulation of some genes. Despite similarities in the controlling functions of DNA methylation, its dynamics and deposition patterns differ in several respects between plants and mammals. One of the most striking differences is that plants tend to propagate pre-existing DNA methylation states across generations, whereas mammals re-establish them genome wide at every generation. Here, we review our current understanding of DNA methylation in the flowering plant Arabidopsis. We discuss in particular the role of RNAi in the incremental methylation and silencing of repeat elements over successive generations. We argue that paramutation, an epigenetic phenomenon first described in maize, is an extreme manifestation of this RNAi-dependent pathway. [ABSTRACT FROM AUTHOR]

Transcription, translation and subsequent protein modification represent the transfer of genetic information from the archival copy of DNA to the short-lived messenger RNA, usually with subsequent production of protein. Although all cells in an organism contain essentially the same DNA, cell types and functions differ because of qualitative and quantitative differences in their gene expression. Thus, control of gene expression is at the heart of differentiation and development. Epigenetic processes, including DNA methylation, histone modification and various RNA-mediated processes, are thought to influence gene expression chiefly at the level of transcription; however, other steps in the process (for example, translation) may also be regulated epigenetically. The following paper will outline the role epigenetics is believed to have in influencing gene expression. [ABSTRACT FROM AUTHOR]

Accumulating evidence suggests that hybrid genetic dysfunctions accrue not only because of sequence divergence of incompatible alleles but also result from a broad variety of mechanisms related to the maintenance of chromatin integrity. For example, it has been observed that hybridization in plants and mammals disrupts patterns of DNA methylation and imprinting. These epigenetic changes can be associated with transcriptional activation and mobilization of transposable elements in hybrids. It raises a question of how these alterations are matched by small regulatory RNAs, such as piwi-interacting RNAs, which play a potent role in both suppressing transposable elements and epigenetic control. The review offers a handful of glimpses into these complex dynamics.Heredity (2009) 102, 45–50; doi:10.1038/hdy.2008.48; published online 11 June 2008 [ABSTRACT FROM AUTHOR]