Persistent organic pollutants : aberrant DNA methylation underlying potential health effects

Abstract

Wild caught fish, especially marine fish, can contain high levels of persistent organic pollutants (POPs). In the Netherlands, especially eel from the main rivers have high POP levels. This led to a ban in 2011 on eel fishing due to health concerns. Many of the marine POPs have been related to adverse health effects such as endocrine disruption, neurodevelopmental problems, immune suppression and cancer. Although some mechanisms of action of POPs are clear, like dioxins binding to the aryl hydrocarbon receptor and OH-PCBs binding to thyroid transport proteins, not all adverse health effects can be explained by these mechanisms of action. Epigenetic phenomena, such as DNA methylation, have been proposed as a possible molecular mechanism underlying adverse health effects. DNA methylation is a heritable modification, which refers to the addition of a methyl group to cytosine in a CpG dinucleotide. Observational studies have indeed shown that POPs can affect global DNA methylation, although results are inconsistent. Some animal studies as well as in vitro experiments suggest that POPs can affect gene-specific DNA methylation, however, the biological significance and relevance for humans is not clear. Therefore, this thesis aimed to 1) study the accumulation of POPs in men consuming eel from high-polluted areas 2) elucidate whether seafood-related POPs can induce aberrant DNA methylation and 3) to determine whether DNA methylation is related to functional endpoints and gene expression in vitro. For this purpose eight POPs that are abundantly present in seafood were chosen, namely 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorobiphenyl (PCB) 126 and 153, perfluorooctanesulfonic acid (PFOS), hexabromocyclododecane (HBCD), 2,2′,4,4′- tetrabromodiphenyl ether (BDE-47), tributyltin (TBT), and methylmercury (MeHg). Chapter 2 describes the in vitro effects of these POPs and mixtures thereof in H295R adrenocortical carcinoma cells. Relative responses for 13 steroid hormones and 7 genes involved in the steroidogenic pathway, and CYP1A1, were analysed. PFOS induced the most pronounced effects on steroid hormone levels by significantly affecting 9 out of 13 hormone levels measured, with the largest increases found for 17β-estradiol, corticosterone, and cortisol. Furthermore, TCDD, both PCBs, and TBT significantly altered steroidogenesis. Increased steroid hormone levels were accompanied by related increased gene expression levels. The differently expressed genes were MC2R, CYP11B1, CYP11B2, and CYP19A1 and changes in gene expression levels were more sensitive than changes in hormone levels. The POP mixtures tested showed mostly additive effects, especially for DHEA and 17β-estradiol levels. This study shows that some seafood POPs are capable of altering steroidogenesis in H295R cells at concentrations that mixtures might reach in human blood, suggesting that adverse health effects cannot be excluded. DNA methylation was not measured in this study due to the short exposure time, which was expected not to be sufficient for long-term epigenetic marks. Therefore, in chapters 3A and 3B a differentiation experiment was performed enabling long-term exposure to POPs. Human mesenchymal stem cells (hMSCs) were differentiated into mature adipocytes over a time-course of 10 days. The transcriptional regulatory cascade involved in adipocyte differentiation has been extensively studied, however the mechanisms driving the transcription are poorly understood. In chapter 3A we therefore first explored the involvement of DNA methylation in transcriptional regulation during adipocyte differentiation. Genome-wide changes in DNA methylation were measured as well as the expression of adipogenic genes. The majority of these genes showed significant expression changes during the differentiation process. There were, however, only a couple of these differentially expressed genes that were differentially methylated. Genome-wide DNA methylation changes were most often located in intergenic regions, and underrepresented close to the transcription start site. This suggested that changes in DNA methylation are not the underlying mechanism regulating gene expression during adipocyte differentiation. Nevertheless, we explored DNA methylation differences after continuous exposure to POPs to investigate whether this could be an underlying mechanism by which POPs affect adipocyte differentiation. TCDD and PFOS decreased lipid accumulation, while TBT increased lipid accumulation. TCDD and TBT induced opposite gene expression profiles, whereas after PFOS exposure gene expression remained relatively stable. Genome-wide DNA methylation analysis showed that all three POPs affected DNA methylation patterns in adipogenic and other genes, but without concomitant gene expression changes. Differential methylation was again predominantly detected in intergenic regions, where the biological relevance of alterations in DNA methylation is unclear. This study demonstrated that POPs, at environmentally relevant levels, are able to induce differential DNA methylation in differentiating adipocytes. However, the biological relevance of this aberrant DNA methylation remains unclear. The in vitro results showed a proof of principle that POPs could be capable of altering DNA methylation. To this date, no human studies were performed investigating the relationship between POP levels and genome-wide DNA methylation. In order to investigate this, we first measured POP levels in eel consumers from the high-polluted areas (areas with a ban on eel fishing) and compared these levels to men consuming eel from low-polluted areas or aquaculture (chapter 4). We aimed to investigate the accumulation of these POPs and determine whether the predictions made in an earlier risk assessment were valid. This was indeed the case as levels of dioxins and dioxin-like compounds were on average 2.5 times higher in men consuming eel from high-polluted areas. Furthermore, PCBs with their hydroxylated metabolites, and perfluoroalkyl substances (PFASs) were, up to ten times, higher in these consumers. Especially the high levels of dioxins and dioxin-like compounds as well as the OH-PCBs are expected to be of health concern. We continued this research in chapter 5 by associating all the measured POPs to clinical parameters related to e.g. thyroid hormones and liver enzymes, but found no relationship. Subsequently, we investigated the association between dioxins and dioxin-like compounds, the sum of seven indicator PCBs, and PFOS with genome-wide DNA methylation. We detected a number of differentially methylated regions (DMRs) related to genes involved in carcinogenesis (e.g. BRCA1, MAGEE2, HOXA5), the immune system (e.g. RNF39, HLA-DQB1), in retinol homeostasis (DHRS4L2), or in metabolism (CYP1A1). In contrast to the in vitro data, most significant effects were detected in CpG islands and were annotated close to the promoter region. This suggests that the differential methylation might be related to differential expression and possibly induce adverse health effects. The hypermethylation of some of these gene related to cancer could be an explanation of the carcinogenic effects that are observed with POP exposure. Based on the results of this thesis we can conclude that the consumption of eel from high-polluted areas lead to accumulation of POPs above safe levels and that POP levels are associated with gene-specific DNA methylation in vitro as well as in environmentally exposed men. More research, however, is needed to fully elucidate the biological implications of this aberrant DNA methylation. A first step can be to measure histone modifications, as these two epigenetic marks together are likely better in predicting gene expression. The second step can be to investigate the potential health effects related to these epigenetic marks and to determine whether there is a causal relationship. Although at this point there is a lack of knowledge with regard to health effects caused by DNA methylation, the consumption of eel from these high-polluted areas is ill- advised, because adverse health effects cannot be excluded based on our results and can even be expected based on literature.