Patterns of DNA Methylation in Animals: An Ecotoxicological Perspective

Abstract

DNA methylation refers to the addition of a methyl group to nucleotides within DNA. As with other epigenetic endpoints, patterns of DNA methylation are susceptible to alterations due to exposure to environmental stressors, including contaminants. These alterations can persist in the absence of the initial stressor as cells divide, and can even be inherited between generations if they occur in the germ line. Although our knowledge concerning patterns of DNA methylation in animals is increasing, there remains a gap in the literature when it comes to species outside of those typically used for biomedical research. Here, I review the literature relating to DNA methylation in an array of taxa (mammals, fish, birds, amphibians, reptiles, and invertebrates) and discuss these data from an ecotoxicological perspective. The pattern and extent of DNA methylation is well conserved across species of vertebrates; methylation appears mainly on cytosine residues within a CpG context, and much of the genome is methylated, with the notable exception of cytosines within CpG islands in the promoters of genes. Highly methylated genes in vertebrates tend to be transcriptionally repressed. However, large differences occur between classes of vertebrates in terms of the timing and nature of reprogramming and genomic imprinting: epigenetic processes that establish patterns of DNA methylation in the early embryo and which are sensitive to environmental stress. In invertebrates, patterns of DNA methylation are extremely variable and differ significantly from the condition observed in vertebrates. Some invertebrate genomes exhibit no DNA methylation while others are methylated to a level that is comparable to vertebrates. Additionally, DNA methylation may have different functions in invertebrates, e.g., alternative splicing. This variability in basic patterns of DNA methylation among species during sensitive periods of development suggests that responses to epigenetically active environmental contaminants may be similarly variable. For example, the timing of exposure to a contaminant may be a critical factor when considered in the light of variable reprogramming schedules among species. With this in mind, I review data relating to the effects of contaminants on DNA methylation in animals, focusing on non-model organisms and on exposures in natural environments, when possible. An ecotoxicological perspective on patterns of DNA methylation in animals may improve our understanding of the range and diversity of epigenetic phenomena in the natural world.

Regulation ofBDNFchromatin status and promoter accessibility in a neural correlate of associative learning

“…Evidence indicates that epigenetic modifications such as DNA methylation may be affected by environmental conditions including temperature. 29 Whether the procedure using hypothermia to induce torpidity in turtles has any effect on epigenetic marks is unknown. However, the preparation is allowed to warm to room temperature following surgery and the experimental conditioned preparations are compared to na€ ıve, unstimulated preparations that serve as controls.…”

Epigenetic alterations and decreasing insecticide sensitivity of the Asian tiger mosquito Aedes albopictus

“…Even if this epigenetic mechanism is widespread, its function in the different taxa varies drastically (Suzuki and Bird, 2008). Most research on DNA methylation in animals focuses on vertebrates and according to literature, the role of DNA methylation in invertebrates is not yet clear (Head, 2014). In vertebrates, DNA methylation is limited to CpG islands at the gene promotor and it is involved in gene silencing, transposon control, development regulation, parental imprinting, and X chromosome inactivation (Mandrioli, 2007).…”

Ecotoxicological Perspectives of Sex Determination

“…Instead, EDCs are more likely to induce epigenetics changes, perhaps by altering DNA methylation and chromatin remodeling [Crews and McLachlan, 2006] as reported in amphibians and reptiles, where DNA methylation was modified by contaminants and the effect persisted over generations [Head, 2014]. Early-life exposure to EDCs could strongly alter the expression of neuroendocrine genes and DNA methylation, which, together, can cause reproductive dysfunction in mice [Collotta et al, 2013].…”

“…Early-life exposure to EDCs could strongly alter the expression of neuroendocrine genes and DNA methylation, which, together, can cause reproductive dysfunction in mice [Collotta et al, 2013]. Nonetheless, little is known about how exactly contaminants affect DNA methylation in reptiles [Head, 2014].…”