Application of benchmark dose modeling to protein expression data in the development and analysis of mode of action/adverse outcome pathways for testicular toxicity.

Reliable quantification of gene and protein expression has potential to contribute significantly to the characterization of hypothesized modes of action (MOA) or adverse outcome pathways for critical effects of toxicants. Quantitative analysis of gene expression by benchmark dose (BMD) modeling has been facilitated by the development of effective software tools. In contrast, protein expression is still generally quantified by a less robust effect level (no or lowest [adverse] effect levels) approach, which minimizes its potential utility in the consideration of dose-response and temporal concordance for key events in hypothesized MOAs. BMD modeling is applied here to toxicological data on testicular toxicity to investigate its potential utility in analyzing protein expression relevant to the proposed MOA to inform human health risk assessment. The results illustrate how the BMD analysis of protein expression in animal tissues in response to toxicant exposure: (1) complements other toxicity data, and (2) contributes to consideration of the empirical concordance of dose-response relationships, as part of the weight of evidence for hypothesized MOAs to facilitate consideration and application in regulatory risk assessment. Lack of BMD analysis in proteomics has likely limited its use for these purposes. This paper illustrates the added value of BMD modeling to support and strengthen hypothetical MOAs as a basis to facilitate the translation and uptake of the results of proteomic research into risk assessment.

A 50-gene biomarker identifies estrogen receptor-modulating chemicals in a microarray compendium.

High throughput transcriptomics (HTTr) profiling has the potential to rapidly and comprehensively identify molecular targets of environmental chemicals that can be linked to adverse outcomes. We describe here the construction and characterization of a 50-gene expression biomarker designed to identify estrogen receptor (ER) active chemicals in HTTr datasets. Using microarray comparisons, the genes in the biomarker were identified as those that exhibited consistent directional changes when ER was activated (4 ER agonists; 4 ESR1 gene constitutively active mutants) and opposite directional changes when ER was suppressed (4 antagonist treatments; 4 ESR1 knockdown experiments). The biomarker was evaluated as a predictive tool using the Running Fisher algorithm by comparison to annotated gene expression microarray datasets including those evaluating the transcriptional effects of hormones and chemicals in MCF-7 cells. Depending on the reference dataset used, the biomarker had a predictive accuracy for activation of up to 96%. To demonstrate applicability for HTTr data analysis, the biomarker was used to identify ER activators in a set of 15 chemicals that are considered potential bisphenol A (BPA) alternatives examined at up to 10 concentrations in MCF-7 cells and analyzed by full-genome TempO-Seq. Using benchmark dose (BMD) modeling, the biomarker genes stratified the ER potency of BPA alternatives consistent with previous studies. These results demonstrate that the ER biomarker can be used to accurately identify ER activators in transcript profile data derived from MCF-7 cells.

Harnessing genomics to identify environmental determinants of heritable disease.

Next-generation sequencing technologies can now be used to directly measure heritable de novo DNA sequence mutations in humans. However, these techniques have not been used to examine environmental factors that induce such mutations and their associated diseases. To address this issue, a working group on environmentally induced germline mutation analysis (ENIGMA) met in October 2011 to propose the necessary foundational studies, which include sequencing of parent-offspring trios from highly exposed human populations, and controlled dose-response experiments in animals. These studies will establish background levels of variability in germline mutation rates and identify environmental agents that influence these rates and heritable disease. Guidance for the types of exposures to examine come from rodent studies that have identified agents such as cancer chemotherapeutic drugs, ionizing radiation, cigarette smoke, and air pollution as germ-cell mutagens. Research is urgently needed to establish the health consequences of parental exposures on subsequent generations.

Lack of change in microRNA expression in adult mouse liver following treatment with benzo(a)pyrene despite robust mRNA transcriptional response.

Benzo(a)pyrene (BaP) is a mutagenic and carcinogenic environmental contaminant. Metabolic activation of BaP is required for it to exert its mutagenic effects. Metabolism occurs via BaP interaction with the aryl hydrocarbon receptor (AHR) resulting in induction of phase 1 enzymes. Exposure to BaP is expected to cause differential regulation of AHR-responsive genes as well as pathways responding to DNA damage induced by its metabolites. MicroRNAs (miRNAs) are short non-coding molecules that control mRNA levels and protein translation. MiRNA regulation may also be affected by chemical insult. Here we investigate the correlation between hepatic mRNA and miRNA response to BaP in vivo. Mature male mice were orally exposed to 3 daily doses of 150mg/kg BaP. DNA microarrays were used to profile gene and miRNA expression in the liver 4 and 24h following the final dose. Despite widespread changes in gene expression (>400 genes) in pathways consistent with the known effects of BaP, we found no changes in miRNA. This was confirmed on two microarray platforms and by qRT-PCR. Analysis of positive controls (2 distinct reference pools) indicated that the Agilent technology could identify differences in miRNA. The effects of sample storage at -80°C were also compared. We found little effect of prolonged freezing on the technical correlation between samples or on differential expression. Our results are consistent with the lack of response of miRNA in rodent liver to dioxin, another potent AHR-agonist. We conclude that hepatic miRNA in vivo is not directly responsive to BaP exposure.

Pulmonary gene and microRNA expression changes in mice exposed to benzo(a)pyrene by oral gavage.

Exposure to the environmental mutagen benzo(a)pyrene (BaP) alters the expression of AHR-responsive genes as well as genes involved in other pathways. We recently reported that exposure of adult mice to BaP resulted in a robust transcriptome response in the liver, but this was accompanied by a complete lack of change in microRNA (miRNA) expression. Since BaP exposure does not result in hepatocarcinogenicity, but does cause lung cancer, in the present study we examine the pulmonary mRNA and miRNA responses to BaP in the same mice. Adult male B6C3F1 mice were exposed to 150 and 300 mg/kg BaP by oral gavage for three consecutive days and sacrificed 4h after the last exposure. Serum clinical chemistry was performed for both the doses to assess the general toxicity of BaP; a modest decrease in serum inorganic phosphorous was observed at both the doses. A small decrease in serum glucose following 150 mg/kg and alkaline phosphatase following 300 mg/kg BaP was observed. BaP-DNA adduct levels in whole lung and liver tissues were assessed by (32)P post labelling and similar dose dependent increases were observed for lung and liver. Using DNA microarrays, pulmonary mRNA and miRNA expressions were analysed. Over 1000 genes were statistically differentially expressed (p<0.05). The perturbed pathways included oxidative stress, xenobiotic metabolism, cell proliferation, cell cycle, B and T-cell receptor signalling and primary immunodeficiency signalling pathways. Analysis of miRNA profiles revealed downregulation of miR-150, miR-142-5p, miR-122 and upregulation of miR-34c, miR-34b-5p and miR-29b. These miRNAs are involved in the biological processes, immune response, cell proliferation and cell cycle, which are the main pathways affected at the mRNA level. Thus, miRNAs are more responsive to BaP in lungs than in liver, and are likely to be involved in the regulation of the pulmonary responses to BaP exposure.