Comparative Biochemistry and Physiology Part C: Toxicology & Pharmacology
Effects of methylmercury on epigenetic markers in three model species: Mink, chicken and yellow perch
Introduction
Methylmercury (MeHg) is a ubiquitous toxicant that biomagnifies through aquatic food webs (Scheuhammer et al., 2007). Once inside the body, MeHg is effectively absorbed from the bloodstream and can readily enter sensitive tissues such as the brain (Clarkson and Magos, 2006). Numerous studies have documented that under real-world exposure scenarios, MeHg causes adverse effects to the health of individuals and populations of various taxa, including fish (Depew et al., 2012a), birds (Depew et al., 2012b), and mammals (Basu and Head, 2010).
The toxic actions of MeHg are diverse owing to its high affinity for thiol groups (Clarkson and Magos, 2006). A myriad of effects have been described at a molecular and cellular level including changes in neurotransmission, oxidative stress, and gene expression. There is now preliminary evidence that MeHg, like other toxic metals, may cause epigenetic changes (Pilsner et al., 2010). Epigenetics refers to heritable factors affecting gene expression that occur outside of modifications to the DNA sequence itself (Robertson and Wolffe, 2000, Head et al., 2012). These include DNA methylation and histone modification. Epigenetic marks are susceptible to environmental influences such as exposure to chemicals, and these changes may be inherited as cells divide mitotically, or even between generations. Epigenetic mechanisms may even help explain the observation of temporal disconnects between exposure and effect, as early-life exposure may leave epigenetic marks that result in adverse health outcomes later in life. This is noteworthy as MeHg is known to have a long latency of effect in many organisms (Clarkson and Magos, 2006, Basu and Head, 2010).
In the biomedical sciences, MeHg-associated changes in epigenetic markers are starting to be found in several experimental model systems. Developmental exposure of mice to MeHg resulted in a range of epigenetic changes in brain-derived neurotrophic factor (BDNF) in male offspring, including increased DNA methylation and altered methylation and acetylation of histones (Onishchenko et al., 2008). These epigenetic changes were associated with depressive symptoms. In female offspring of rats exposed to MeHg throughout pregnancy, reduced hepatic expression of DNA methyltransferases (DNMT1, DNMT3a, DNMT3b) was reported (Desaulniers et al., 2009). DNA methyltransferases are a family of enzymes that establish and maintain patterns of DNA methylation in the genome. An in vitro study using mouse embryonic cells found that acute exposure to Hg2 + impairs histone production and reduces H3-K27 methylation (Gadhia et al., 2012). Such epigenetic findings have been extended to humans as increased methylation of GSTM1/5 promoter in blood was associated with MeHg exposure in women undergoing in vitro fertilization (Hanna et al., 2012), and hypomethylation of SEPP1 was associated with hair Hg levels among a cohort of male dentists (Goodrich et al., in press). Collectively these aforementioned studies provide founding evidence that mercury compounds can cause epigenetic changes.
Of all organisms, fish-eating wildlife are among those with the greatest exposures to MeHg (Scheuhammer et al., 2007, Basu and Head, 2010). However, little is known about MeHg-associated epigenetic effects in these organisms, (Vandegehuchte and Janssen, 2011, Head et al., 2012). We previously observed reduced global DNA methylation in association with MeHg exposure in brain stem tissues of male polar bears (Pilsner et al., 2010). While the results of this polar bear study were informative, the DNA methylation results were highly variable and likely influenced by a range of known (e.g., other toxicants, health status, age, gender, tissue quality) and unknown factors that could not be well-controlled when studying tissues from wild-caught animals. Given the need to generate data from well-controlled laboratory experiments, the current study was performed to increase understanding of possible MeHg-associated epigenetic changes using a mammalian, avian, and fish model species. All three classes contain species with well-documented sensitivities to MeHg (Scheuhammer et al., 2007). For each class we chose to study a model ecotoxicological test organism, namely mink (Neovison vison), chicken (Gallus gallus domesticus), and yellow perch (Perca flavescens). As part of other research projects, animals of each species were previously exposed in the laboratory to levels of MeHg ranging from ecologically relevant to high. Here we opportunistically used tissues preserved from those studies. The brain tissue (focus: cerebrum) was investigated in each model since this organ is the principal target of MeHg (Clarkson and Magos, 2006, Basu and Head, 2010). We examined global genomic DNA methylation using the LUMA assay and the enzymatic activity of DNMT via a commercially available ELISA kit. The guiding hypothesis of this study was that brain global genomic DNA methylation and DNMT enzyme activity would be reduced in association with methylmercury exposure in each of the three model species.
Section snippets
Animal exposure studies
Mink have been exposed to a range of contaminants in the laboratory largely for the purposes of ecological risk assessment, and in the field they have been shown to accumulate potentially toxic concentrations of MeHg (Basu et al., 2007a). Here, captive juvenile male mink (n = 12 per treatment) were exposed daily to MeHgCl (0, 0.1, 0.5, 1, and 2 mg/kg nominal concentrations in diet) as previously described (Basu et al., 2006, Basu et al., 2007b). Following 3 months of exposure, animals were
Mink studies
In the occipital cortex of control (unexposed) captive mink, the mean percent DNA methylation for individual samples as determined via the LUMA assay was 70.1 ± 1.9% (Table 1), and ranged between 67.2% and 73.0% for individual control animals. The mean percent DNA methylation was highest in the control group and lowest in the 1 ppm MeHg dietary group (68.2 ± 1.6%). This difference between the control group and 1 ppm MeHg dietary group was the only pairwise comparison that was of statistical
Discussion
We looked for evidence that MeHg disrupts global DNA methylation in brain tissue of representative species from three different classes of animals; mammals, birds and fish. This was undertaken by studying a convenience sample of brain tissues from previous MeHg laboratory exposures studies. The data presented here provide evidence that MeHg has the potential to cause epigenetic changes in a mammalian species; DNA hypomethylation and reduced DNMT activity were observed in mink exposed to dietary
Acknowledgments
This study was funded by a Collaborative Mercury Research Network (COMERN) grant to A.S. and H.M.C., Natural Science and Engineering Research Council of Canada (NSERC) grants to H.M.C. and N.B., a National Oceanic and Atmospheric Administration grant to J.H., and University of Michigan School of Public Health funding to N.B. We are thankful to the Canadian Centre for Fur Animal Research (especially Merridy Rankin, Rena Currie, Sarah Gatti-Yorke, Tanya Morse, Cindy Crossman, Jody Muise, and
References (27)
- et al.
Mammalian wildlife as complementary models in environmental neurotoxicology
Neurotoxicol. Teratol.
(2010) - et al.
Mink as a sentinel in environmental health
Environ. Res.
(2007) - et al.
Decreased N-methyl-d-aspartic acid (NMDA) receptor levels are associated with mercury exposure in wild and captive mink
Neurotoxicology
(2007) - et al.
Trace metals alter DNA repair and histone modification pathways concurrently in mouse embryonic stem cells
Toxicol. Lett.
(2012) - et al.
Evolutionary changes in CpG and methylation levels in the genome of vertebrates
Gene
(1997) - et al.
Toxicological significance of mercury in yellow perch in the Laurentian Great Lakes region
Environ. Pollut.
(2012) - et al.
DNA hypomethylation and human diseases
Biochim. Biophys. Acta
(2007) - et al.
Methylmercury impairs components of the cholinergic system in captive mink (Mustela vison)
Toxicol. Sci.
(2006) - et al.
Is dietary mercury of neurotoxicological concern to polar bears (Ursus maritimus)?
Environ. Toxicol. Chem.
(2009) - et al.
Inherited effects of low-dose exposure to methylmercury in neural stem cells
Toxicol. Sci.
(2012)
The toxicology of mercury and its chemical compounds
Crit. Rev. Toxicol.
Toxicity of dietary methylmercury to fish: derivation of ecologically meaningful threshold concentrations
Environ. Toxicol. Chem.
Derivation of screening benchmarks for dietary methylmercury (MeHg) exposure for the common loon (Gavia immer): rationale for use in ecological risk assessment
Environ. Toxicol. Chem.
Cited by (37)
Neurotoxic effects of heavy metal pollutants in the environment: Focusing on epigenetic mechanisms
2024, Environmental PollutionEffects of blood mercury accumulation on DNA methylation levels in the Khorat snail-eating turtle (Malayemys khoratensis)
2024, Ecotoxicology and Environmental SafetyEpigenetics of methylmercury
2023, NeuroToxicologyGestational blood levels of toxic metal and essential element mixtures and associations with global DNA methylation in pregnant women and their infants
2021, Science of the Total EnvironmentEpigenetic influence of environmentally neurotoxic metals
2020, NeuroToxicologyCitation Excerpt :Furthermore, the rate of proliferation of neural progenitor cells in the sub granule region of the dentate gyrus decreases due to exposure to MeHg (Bose et al., 2012). In another study, Basu and colleagues reported reduction in global DNA methylation and DNMT activity in minks exposed to MeHg (Basu et al., 2013). A recent study by Carvan et al. (2017) shows MeHg is capable of eliciting a transgenerational effect on unexposed offspring in zebrafish.
Methylmercury induces transgenerationally transmissible epigenetic changes influencing zebrafish behavior
2020, Behavioral and Neural Genetics of Zebrafish