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Integrative analysis of transcriptional and DNA methylation changes associated with PCB126 exposure in adult zebrafish (Danio rerio)

ABSTRACT: The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor well known for mediating the toxicity of environmental chemicals such as polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs). There is extensive knowledge on the range of target genes regulated by AHR ligands. However, there is limited information on the effect of AHR ligands on DNA methylation. The objective of this study is to investigate genome-wide changes in DNA methylation and gene expression patterns in response to PCB126 exposure. Adult zebrafish were exposed to 10 nM PCB126 for 24 hours (waterborne exposure) and were reared in clean water for 7 days before tissue sampling. DNA methylation and transcriptional changes in the liver and brain tissues were quantified by Reduced Representation Bisulfite Sequencing (RRBS) and RNAseq, respectively. RRBS analysis revealed DNA hypomethylation in response to PCB exposure in both liver and brain tissues. We observed 482 and 476 differentially methylated regions (DMRs) in the liver and brain tissues respectively. Most of the DMRs are located more than 20 kilobases upstream of the transcriptional start sites. RNAseq results from the liver revealed differential expression of genes related to xenobiotic metabolism, oxidative stress and carbohydrate metabolism in response to PCB exposure. In the brain, PCB exposure altered the expression of genes involved in myelination and glutamate signaling. Our results suggest that there is very little correlation between DNA methylation and gene expression patterns among the differentially expressed genes (DEGs). We are currently investigating the relationship between DMRs and DEGs.

ORGANISM(S): Danio rerio

PROVIDER: GSE104221 | GEO | 2017/09/30

SECONDARY ACCESSION(S): PRJNA412050

REPOSITORIES: GEO

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Project description:Chemical modifications of proteins, DNA and RNA moieties play critical roles in regulating gene expression. Emerging evidence suggests these RNA modifications (epitranscriptomics) have substantive roles in basic biological processes. One of the most common modifications in mRNA and noncoding RNAs is N6-methyladenosine (m6A). In a subset of mammalian mRNAs, m6A sites are preferentially enriched near stop codons, in 3′ UTRs, and within exons, suggesting an important role in the regulation of mRNA processing and function including alternative splicing and gene expression. Very little is known about the effect of environmental chemical exposure on m6A modifications. As many of the commonly occurring environmental contaminants alter gene expression profiles and have detrimental effects on physiological processes, it is important to understand the effects of exposure on this important layer of gene regulation. Hence, the objective of this study was to characterize the acute effects of developmental exposure to PCB126, an environmentally relevant dioxin-like PCB, on m6A methylation patterns. We exposed zebrafish embryos to PCB126 for 6 hours starting from 72 hours post-fertilization and profiled m6A RNA using methylated RNA immunoprecipitation followed by sequencing (MeRIP-seq), as well as assessing changes in mRNA splicing. We observed several hundreds of peaks that are differentially expressed in response to PCB126 exposure (FDR 5%). The majority of the peaks are preferentially located around the 3’UTR and stop codons. Pathway analysis of m6A marked transcripts induced by PCB126 exposure revealed that these transcripts are associated with important developmental pathways (MAPK, Hedgehog, Notch and Wnt). These results suggest that PCB126 could affect developmental gene expression patterns by altering m6A levels. Gene expression analysis revealed upregulation of classical aryl hydrocarbon receptor (AHR) target genes such as cytochrome P450s in response to PCB126 exposure. Interestingly, none of the AHR target genes overlapped with the m6A altered transcripts, suggesting that xenobiotic metabolism may not be under m6A regulation. Further studies are necessary to understand the functional consequences of exposure-associated alterations in m6A levels. This work is supported by NIEHS ES024915.

Project description:There is growing evidence from epidemiological and experimental studies suggesting that early life exposure to environmental chemicals can have long-term consequences that are seen in adults and not apparent early in life. We recently demonstrated that developmental exposure of zebrafish embryos to low, non-embryotoxic levels of PCB126 did not affect larval behavior but caused changes in adult behavior (Glazer et al., 2016, NeuroToxicology 52:134-143). Zebrafish embryos were exposed to either vehicle (DMSO) or low concentrations of PCB126 (0.3, 0.6, 1.2 nM) for 20 h (4–24 h post fertilization), and then reared to adulthood in clean water. Locomotor activity of the larvae at 7 and 14 days post fertilization (dpf) was not affected by PCB126. In contrast, adult fish (4 months old) tested in novel tank and shoaling assays showed impaired habituation to a novel environment. In order to investigate the underlying molecular basis of these phenotypes, we determined the transcriptional profiles in whole embryos (48 hpf), larvae (5 dpf) and adult brain (4 mo) using strand-specific RNA-sequencing. Our results show that 0.3 nM PCB126 exposure induced cyp1a transcript levels 12.5-fold in 48-hpf embryos but there was no induction in 5-dpf larvae, suggesting transient activation of aryl hydrocarbon receptor during early development. No significant induction of cyp1a was observed in the brains of adults exposed as embryos to PCB126. However, we observed significant changes in gene expression profiles in the adult brain samples. A total of 2209 and 1628 genes were differentially expressed in 0.3 nM and 1.2 nM PCB126-exposed groups, respectively. KEGG pathway analysis of differentially expressed genes in the brain suggest enrichment of genes involved in oxidative phosphorylation, neurodegenerative diseases, circadian rhythm and calcium signaling pathways. We are currently investigating the role of these genes in altered behavior observed in the adults. Overall, our results suggest that PCB exposure during sensitive periods of early development alters normal development of the brain by reprogramming gene expression patterns. [Funded by NIH P01ES021923 and NSF OCE-1314642].

Project description:To further our understanding of the role of DNA methylation in development, Methylated DNA Immunoprecipitation (MeDIP) was used in conjunction with a NimbleGen promoter plus CpG island array to identify Tissue and Developmental Stage specific Differentially Methylated DNA Regions (T-DMRs and DS-DMRs) on a genome-wide basis. Four tissues (brain, heart, liver, and testis) from C57BL/6J mice were analyzed at three developmental stages (15 day embryo, E15; new born, NB; 12 week adult, AD). Almost 5,000 adult T-DMRs and 10,000 DS-DMRs were identified. Surprisingly, almost all DS-DMRs were tissue specific (i.e., methylated and ummenthylated in one or more non-overlapping tissues), indicating that the vast majority of unique sequence DNA methylation has tissue specificity. Also, many DS-DMRs were methylated at early development stages (E15 and NB) but unmethylated in adult, indicating “demethylation” has a prominent role in tissue differentiation. The pattern of DNA methylation in adult testis was dramatically different from somatic tissues in many aspects, mostly notably with a very strong bias of methylation in non-CpGi (CpG island) promoter regions (94%). Although the majority of T-DMRs and DS-DMRs tended to be in non-CpGi promoter regions, a relatively large number were also located in CpGi in promoter, intra-genic and inter-genic regions (>15% of all CpGi). Gene Ontology analysis of genes with methylation in non-CpGi promoters indicates enrichment of genes related to membrane proteins and G-protein coupled receptors. Our data also suggest regulatory roles of DNA methylation outside of promoter regions and in alternative promoter selection. Overall, our studies indicate that change in DNA methylation during development is a dynamic, widespread and tissue-specific process involving both DNA methylation and demethylation. Comparison of DNA methylation across 3 developmental stages (15 day embryo, newborn, and adult) for four tissues (brain, heart, liver and testis)

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Integrative analysis of transcriptional and DNA methylation changes associated with PCB126 exposure in adult zebrafish (Danio rerio)

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