The Influence of Genetic Polymorphisms on Mercury Toxicokinetics: Evidence from Epidemiological and In Vitro Studies.

Polymorphisms in glutathione pathway and selenoprotein genes are hypothesized to affect Hg accumulation, modify the impact of Hg on blood pressure, and alter enzyme activity of encoded proteins. We utilized a multi-dimensional approach by considering two forms of Hg (methylmercury from fish consumption; elemental Hg from dental amalgams) and 23 polymorphisms in two cohorts- the Michigan Dental Association (MDA, n=511) and Early Life Exposures in Mexico to Environmental Toxicants (ELEMENT, n=353 mothers, 368 children). In the MDA cohort, mean (±SD) urine (1.04±1.18 μg/L) and hair Hg levels (0.49±0.63 μg/g), biomarkers of elemental Hg and methylmercury exposure, respectively, mirrored US population averages, with dentists exhibiting the highest concentrations. While elevated compared to the US population, biomarker Hg levels among ELEMENT mothers (hair) and children (hair, blood, urine) reflected low-level exposures. Simple and multivariate linear regression models investigated the association of polymorphisms with Hg biomarker levels in two cohorts. Statistical modeling in the MDA cohort tested for significant interactions between genotype and exposure sources (fish consumption, dental amalgams) on biomarker (hair, urine) levels. Several polymorphisms were associated with hair Hg (GSTP1: rs1695, rs1138272; GSS: rs3761144; SEPP1: rs7579) and urine Hg (GSTT1 deletion; SEPP1: rs7579). All significant associations (p<0.05) were observed in the MDA cohort, except the rs7579 ‘T’ allele which correlated with lower urine Hg concentrations in both cohorts. We assessed the association of Hg and polymorphisms with systolic and diastolic blood pressure (SBP, DBP) in the MDA cohort. In multivariate models, increasing hair Hg was linked to higher DBP. Contrariwise, urine Hg levels associated with decreasing SBP, a relationship driven by the males. Significant polymorphism-Hg biomarker interactions were not observed in SBP or DBP models. Mechanisms underlying epidemiological associations were explored through in vitro characterization of enzymatic activity and heavy metal inhibition of four GSTP1 variants encoded by two nonsynonymous polymorphisms (rs1695: 105Ile/Val, rs1138272: 114Ala/Val). Kinetic parameters varied significantly (p<0.01) depending on genotype. Allozymes with 105Ile had better catalytic efficiency and greater electrophilic substrate affinity. Inorganic Hg and methylmercury inhibited all GSTP1 variants. Genotype influenced the extent of inhibition with GSTP1 105Val 114Ala the most sensitive to both Hg species.Ph.D.ToxicologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89769/1/gaydojac_1.pd

Methylmercury and elemental mercury differentially associate with blood pressure among dental professionals

Methylmercury-associated effects on the cardiovascular system have been documented though discrepancies exist, and most studied populations experience elevated methylmercury exposures. No paper has investigated the impact of low-level elemental (inorganic) mercury exposure on cardiovascular risk in humans. The purpose of this study was to increase understanding of the association between mercury exposure (methylmercury and elemental mercury) and blood pressure measures in a cohort of dental professionals that experience background exposures to both mercury forms. Dental professionals were recruited during the 2010 Michigan Dental Association Annual Convention. Mercury levels in hair and urine samples were analyzed as biomarkers of methylmercury and elemental mercury exposure, respectively. Blood pressure (systolic, diastolic) was measured using an automated device. Distribution of mercury in hair (mean, range: 0.45, 0.02–5.18 μg/g) and urine (0.94, 0.03–5.54 μg/L) correspond well with the US National Health and Nutrition Examination Survey. Linear regression models revealed significant associations between diastolic blood pressure (adjusted for blood pressure medication use) and hair mercury (n = 262, p = 0.02). Urine mercury results opposed hair mercury in many ways. Notably, elemental mercury exposure was associated with a significant systolic blood pressure decrease (n = 262, p = 0.04) that was driven by the male population. Associations between blood pressure and two forms of mercury were found at exposure levels relevant to the general population, and associations varied according to type of mercury exposure and gender

Relationship of estimated dietary intake of n-3 polyunsaturated fatty acids from fish with peripheral nerve function after adjusting for mercury exposure

BACKGROUND: Some clinical studies have suggested that ingestion of n-3 polyunsaturated fatty acids (PUFA) has neuroprotective effects on peripheral nerve function. However, few epidemiological studies have examined the effect of dietary n-3 PUFA intake from fish consumption on peripheral nerve function, and none have controlled for co-occurrence of methylmercury exposure from fish consumption. OBJECTIVES: We evaluated the effect of estimated dietary n-3 PUFA intake on peripheral nerve function after adjusting for biomarkers of methylmercury and elemental mercury in a convenience sample of 515 dental professionals. METHODS: We measured sensory nerve conduction (peak latency and amplitude) of the median, ulnar and sural nerves and total mercury concentrations in hair and urine samples. We estimated daily intake (mg/day) of the total n-3 PUFA, n-3 docosahexaenoic acid (DHA), and n-3 eicosapentaenoic acid (EPA) based on a self-administrated fish consumption frequency questionnaire. We also collected information on mercury exposure, demographics and other covariates. RESULTS: The estimated median intakes of total n-3 PUFA, n-3 EPA, and n-3 DHA were 447, 105, and 179 mg/day, respectively. The mean mercury concentrations in urine (1.05 μg/L) and hair (0.49 μg/g) were not significantly different from the US general population. We found no consistent association between n-3 PUFA intake and sensory nerve conduction after adjusting for mercury concentrations in hair and urine although some positive associations were observed with the sural nerve. CONCLUSIONS: In a convenience sample of dental professionals, we found little evidence suggesting that dietary intake of n-3 PUFAs from fish has any impact on peripheral nerve function after adjustment for methylmercury exposure from fish and elemental mercury exposure from dental amalgam

Glutathione Enzyme and Selenoprotein Polymorphisms Associate with Mercury Biomarker Levels in Michigan Dental Professionals

Mercury is a potent toxicant of concern to both the general public and occupationally exposed workers (e.g., dentists). Recent studies suggest that several genes mediating the toxicokinetics of mercury are polymorphic in humans and may influence inter-individual variability in mercury accumulation. This work hypothesizes that polymorphisms in key glutathione synthesizing enzyme, glutathione S-transferase, and selenoprotein genes underlie inter-individual differences in mercury body burden as assessed by analytical mercury measurement in urine and hair, biomarkers of elemental mercury and methylmercury, respectively. Urine and hair samples were collected from a population of dental professionals (n=515), and total mercury content was measured. Average urine (1.06±1.24 microg/L) and hair mercury levels (0.49±0.63 microg/g) were similar to national U.S. population averages. Taqman assays were used to genotype DNA from buccal swab samples at 15 polymorphic sites in genes implicated in mercury metabolism. Linear regression modeling assessed the ability of polymorphisms to modify the relationship between mercury biomarker levels and exposure sources (e.g., amalgams, fish consumption). Five polymorphisms were significantly associated with urine mercury levels (GSTT1 deletion), hair mercury levels (GSTP1-105, GSTP1-114, GSS 5'), or both (SEPP1 3'UTR). Overall, this study suggests that polymorphisms in selenoproteins and glutathione-related genes may influence elimination of mercury in the urine and hair or mercury retention following exposures to elemental mercury (via dental amalgams) and methylmercury (via fish consumption)

An Investigation of Modifying Effects of Metallothionein Single-Nucleotide Polymorphisms on the Association between Mercury Exposure and Biomarker Levels

Background: Recent studies have suggested that several genes that mediate mercury metabolism are polymorphic in humans

An Investigation of Modifying Effects of Single Nucleotide Polymorphisms in Metabolism-related Genes on the Relationship between Peripheral Nerve Function and Mercury Levels in Urine and Hair

Mercury (Hg) is a potent neurotoxicant. We hypothesized that single nucleotide polymorphisms (SNPs) in genes coding glutathione-related proteins, selenoproteins and metallothioneins may modify the relationship of mercury biomarkers with changes in peripheral nerve function. Dental professionals (n=515) were recruited in 2009 and 2010. Sensory nerve function (onset latency, peak latency and amplitude) of the median, ulnar and sural nerves was recorded. Samples of urine, hair and DNA were collected. Covariates related to demographics, nerve function and elemental and methyl-mercury exposure were also collected. Subjects included 244 dentists (47.4%) and 269 non-dentists (52.2%; mostly dental hygienists and dental assistants). The mean mercury levels in urine (1.06 μg/L) and hair (0.51 μg/g) were not significantly different from the US general population (0.95 μg/L and 0.47 μg/g, respectively). In multivariate linear models predicting nerve function adjusting for covariates, only 3 out of a total of 504 models showed stable and statistically significant interaction of SNPs with mercury biomarkers. Overall, given the possibility of false positives, the results suggested little evidence of effect modification of the SNPs on the relationship between mercury biomarkers with peripheral nerve function at exposure levels that are relevant to the general US population

The role of environmental exposures and the epigenome in health and disease

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152782/1/em22311_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/152782/2/em22311.pd

Time-Course Effects of Acute Aflatoxin B1 Exposure on Hepatic Mitochondrial Lipids and Oxidative Stress in Rats

Aflatoxins are secondary metabolites of certain Aspergillus species, that contaminate staple foods, particularly in developing countries. Aflatoxin B1 (AFB1) is the most toxic and common of the major types of aflatoxins. AFB1 is hepatotoxic and has been implicated in increasing the risk of hepatocellular carcinoma (HCC). We have previously shown that subacute exposure to AFB1 for 7 days disrupts hepatic lipids; therefore, this study determined the timecourse effects of acute aflatoxin exposure on hepatic mitochondrial lipids and oxidative stress. To achieve this, thirty male albino rats were randomly assigned to six groups. The groups received an oral dose of 1 mg/kg body weight AFB1 or vehicle only (controls) for one, four, or seven days, respectively. Twenty-four hours after the last dose, the animals were sacrificed and liver excised. Mitochondria and cytosolic fractions were obtained from the liver after which lipids (cholesterol, triacylglycerols) were determined in the mitochondria while biomarkers of oxidative stress (glutathione, glutathione transferase (GST), glutathione peroxidase (GPx), glutathione reductase, nitric oxide (NO), malonaldehyde (MDA), thioredoxin reductase (TR), and superoxide dismutase (SOD) were determined spectrophotometrically in the mitochondria and cytosolic fractions. The expression of genes (Nrf2, Acc, Nqo1, and HmgCoa) were determined using quantitative RT-PCR Results showed that AFB1 significantly increased mitochondrial cholesterol at day seven (treatment vs. control, p = 0.016). It also increased the concentrations of NO and MDA at day one and day seven while the activity of GPx and concentration of GSH were increased at day seven (p = 0.030) and day one (p = 0.025) alone, respectively, compared to control. The activities of cytosolic GR (p = 0.014), TR (p = 0.046) and GST (p = 0.044) were increased at day seven. AFB1 significantly increased the expression of Nrf2 (p = 0.029) and decreased the expression of Acc (p = 0.005) at day one. This study revealed that AFB1 disrupts hepatic mitochondrial lipids and antioxidant capacity. These changes were dependent on the timing of exposure and did not follow a linear time-course trend. These alterations could be part of the hepatic mitochondria response mechanism to acute AFB1 toxicity

Hepatic Lipid Accumulation and Nrf2 Expression following Perinatal and Peripubertal Exposure to Bisphenol A in a Mouse Model of Nonalcoholic Liver Disease

Background: Exposure to chemicals during critical windows of development may re-program liver for increased risk of nonalcoholic fatty liver disease (NAFLD). Bisphenol A (BPA), a plastics component, has been described to impart adverse effects during gestational and lactational exposure. Our work has pointed to nuclear factor E2-related factor 2 (Nrf2) being a modulator of hepatic lipid accumulation in models of NAFLD. Objectives: To determine if chemical exposure can prime liver for steatosis via modulation of NRF2 and epigenetic mechanisms. Methods: Utilizing BPA as a model exposure, pregnant CD-1 mice were administered 25μg/kg/day role= presentation \u3e25μg/kg/day BPA via osmotic minipumps from gestational day 8 through postnatal day (PND)16. The offspring were weaned on PND21 and exposed to same dose of BPA via their drinking water through PND35. Tissues were collected from pups at week 5 (W5), and their littermates at week 39 (W39). Results: BPA increased hepatic lipid content concomitant with increased Nrf2 and pro-lipogenic enzyme expression at W5 and W39 in female offspring. BPA exposure increased Nrf2 binding to a putative antioxidant response element consensus sequence in the sterol regulatory-element binding protein-1c (Srebp-1c) promoter. Known Nrf2 activators increased SREBP-1C promoter reporter activity in HepG2 cells. Methylated DNA immunoprecipitation-PCR and pyrosequencing revealed that developmental BPA exposure induced hypomethylation of the Nrf2 and Srebp-1c promoters in livers of W5 mice, which was more prominent in W39 mice than in others. Conclusion: Exposure to a xenobiotic during early development induced persistent fat accumulation via hypomethylation of lipogenic genes. Moreover, increased Nrf2 recruitment to the Srebp-1c promoter in livers of BPA-exposed mice was observed. Overall, the underlying mechanisms described a broader impact beyond BPA exposure and can be applied to understand other models of NAFLD