Genetic variations associated with interindividual sensitivity in the response to arsenic exposure

Unraveling the genetics of arsenic toxicity with cellular morphology QTL

The health risks that arise from environmental exposures vary widely within and across human populations, and these differences are largely determined by genetic variation and gene-by-environment (gene-environment) interactions. However, risk assessment in laboratory mice typically involves isogenic strains and therefore, does not account for these known genetic effects. In this context, genetically heterogenous cell lines from laboratory mice are promising tools for population-based screening because they provide a way to introduce genetic variation in risk assessment without increasing animal use. Cell lines from genetic reference populations of laboratory mice offer genetic diversity, power for genetic mapping, and potentially, predictive value for in vivo experimentation in genetically matched individuals. To explore this further, we derived a panel of fibroblast lines from a genetic reference population of laboratory mice (the Diversity Outbred, DO). We then used high-content imaging to capture hundreds of cell morphology traits in cells exposed to the oxidative stress-inducing arsenic metabolite monomethylarsonous acid (MMAIII). We employed dose-response modeling to capture latent parameters of response and we then used these parameters to identify several hundred cell morphology quantitative trait loci (cmQTL). Response cmQTL encompass genes with established associations with cellular responses to arsenic exposure, including Abcc4 and Txnrd1, as well as novel gene candidates like Xrcc2. Moreover, baseline trait cmQTL highlight the influence of natural variation on fundamental aspects of nuclear morphology. We show that the natural variants influencing response include both coding and non-coding variation, and that cmQTL haplotypes can be used to predict response in orthogonal cell lines. Our study sheds light on the major molecular initiating events of oxidative stress that are under genetic regulation, including the NRF2-mediated antioxidant response, cellular detoxification pathways, DNA damage repair response, and cell death trajectories.

Cell morphology QTL reveal gene by environment interactions in a genetically diverse cell population

Genetically heterogenous cell lines from laboratory mice are promising tools for population-based screening as they offer power for genetic mapping, and potentially, predictive value for in vivo experimentation in genetically matched individuals. To explore this further, we derived a panel of fibroblast lines from a genetic reference population of laboratory mice (the Diversity Outbred, DO). We then used high-content imaging to capture hundreds of cell morphology traits in cells exposed to the oxidative stress-inducing arsenic metabolite monomethylarsonous acid (MMAIII). We employed dose-response modeling to capture latent parameters of response and we then used these parameters to identify several hundred cell morphology quantitative trait loci (cmQTL). Response cmQTL encompass genes with established associations with cellular responses to arsenic exposure, including Abcc4 and Txnrd1, as well as novel gene candidates like Xrcc2. Moreover, baseline trait cmQTL highlight the influence of natural variation on fundamental aspects of nuclear morphology. We show that the natural variants influencing response include both coding and non-coding variation, and that cmQTL haplotypes can be used to predict response in orthogonal cell lines. Our study sheds light on the major molecular initiating events of oxidative stress that are under genetic regulation, including the NRF2-mediated antioxidant response, cellular detoxification pathways, DNA damage repair response, and cell death trajectories.

Arsenolipids reduce butyrate levels and influence human gut microbiota in a donor-dependent way

Arsenolipids are organic arsenic species with variable toxicity. Accurate assessment of the risks derived from arsenic-contaminated seafood intake requires studying the interplay between arsenolipids and the human gut microbiota. This research used the in vitro mucosal simulator of the human intestinal microbial ecosystem (M-SHIME) to assess the effect of defined chemical standards of arsenolipids (AsFA 362 and AsHC 332) on a simulated healthy human gut microbiota (n = 4). Microbial-derived metabolites were quantified by gas chromatography and microbiota structure was characterized by 16S rRNA gene sequencing. A specific reduction in butyrate production (control=5.28 ± 0.3 mM; AsFAs=4.56 ± 0.4 mM; AsHC 332=4.4 ± 0.6 mM, n = 4 donors), concomitant with a reduction in the abundance of Lachnospiraceae UCG-004 group and the Faecalibacterium genus was observed, albeit in a donor-dependent manner. Furthermore, an increase in Escherichia/Shigella, Proteobacteria and Fusobacterium abundance was observed after arsenolipid treatments, depending on individual microbiota background. These alterations in microbial functionality and microbial community structure suggest a detrimental effect of arsenolipids intake towards the commensal gut microbiome, and consequently, on human health.

The Relationship Between GSTT1, GSTM1, GSTO1, GSTP1 and MTHFR Gene Polymorphisms and DNA Damage of BRCA1 and BRCA2 Genes in Arsenic-Exposed Workers

OBJECTIVE

To investigate the associations between genetic polymorphisms of GSTT1, GSTM1, GSTO1, GSTP1 and MTHFR genes and the DNA damage levels of BRCA1 and BRCA2 genes.

METHODS

Peripheral blood samples were used to measure DNA damage levels and genetic polymorphisms, and urine samples were collected to analyze arsenic metabolites in 79 arsenic-exposed workers and 24 non-arsenic-exposed workers.

RESULTS

The BRCA1 and BRCA2 damage levels in exposure group were significantly higher than that in control group. Significant associations were detected between GSTT1 and GSTO1 polymorphisms and DNA damage levels of BRCA1 and BRCA2 genes in subjects (P < 0.05).

CONCLUSIONS

Our findings suggest that the DNA damage levels of BRCA1 and BRCA2 genes may modulate by genetic variations of GSTT1 and GSTO1 when individuals are exposed to carcinogens, such as arsenic.

GSTM1 and GSTT1 Null Genotype Polymorphisms and Susceptibility to Arsenic Poisoning: a Meta-analysis

The value of the glutathione S-transferase (GST) null genotype in patients with arsenic poisoning has been recognized, but the conclusions of previous studies remain inconsistent. The objective of this study was to evaluate the relationship between GST mu 1 (GSTM1) and GST theta 1 (GSTT1) null genotype polymorphisms and susceptibility to arsenic poisoning. PubMed, Medline, Embase, Web of Science, China National Knowledge Infrastructure (CNKI), WanFang, and WeiPu databases were systematically searched for publications up to March 31, 2020. The quality of the studies was assessed using the Newcastle-Ottawa Quality Assessment Scale. The pooled odds ratios (ORs) and their 95% confidence intervals (CIs) were calculated to estimate the relationship between GSTM1 and GSTT1 null genotype polymorphisms and arsenic poisoning. The meta-analysis was conducted using STATA 14.0 software. Nine articles with 3324 subjects were included in the meta-analysis. A significantly negative correlation was observed between the GSTM1 null genotype and susceptibility to arsenic poisoning (OR = 0.731; 95% CI: 0.536-0.999; P = 0.049; I² = 70.5%). There was no significant correlation between the GSTT1 null genotype (OR = 1.009; 95% CI: 0.856-1.189; P = 0.915, I² = 36.8%) and GSTM1-GSTT1 double null genotype (OR = 1.105; 95% CI: 0.670-1.822; P = 0.695; I² = 64.7%) and the risk of arsenic poisoning. Egger's and Begg's tests indicated no publishing bias. Compared with controls, individuals with the GSTM1 null genotype were less susceptible to arsenic poisoning. The GSTT1 single null genotype and GSTM1-GSTT1 dual-null genotype were not associated with the risk of arsenic poisoning. The GSTM1 single null genotype may have potential as a genotoxic biomarker to identify individuals who are not prone to arsenic poisoning, and as a reference for guiding the prevention of arsenic poisoning.

Early-Life Arsenic Exposure, Nutritional Status, and Adult Diabetes Risk

PURPOSE OF REVIEW

In utero influences, including nutrition and environmental chemicals, may induce long-term metabolic changes and increase diabetes risk in adulthood. This review evaluates the experimental and epidemiological evidence on the association of early-life arsenic exposure on diabetes and diabetes-related outcomes, as well as the influence of maternal nutritional status on arsenic-related metabolic effects.

RECENT FINDINGS

Five studies in rodents have evaluated the role of in utero arsenic exposure with diabetes in the offspring. In four of the studies, elevated post-natal fasting glucose was observed when comparing in utero arsenic exposure with no exposure. Rodent offspring exposed to arsenic in utero also showed elevated insulin resistance in the 4 studies evaluating it as well as microRNA changes related to glycemic control in 2 studies. Birth cohorts of arsenic-exposed pregnant mothers in New Hampshire, Mexico, and Taiwan have shown that increased prenatal arsenic exposure is related to altered cord blood gene expression, microRNA, and DNA methylation profiles in diabetes-related pathways. Thus far, no epidemiologic studies have evaluated early-life arsenic exposure with diabetes risk. Supplementation trials have shown B vitamins can reduce blood arsenic levels in highly exposed, undernourished populations. Animal evidence supports that adequate B vitamin status can rescue early-life arsenic-induced diabetes risk, although human data is lacking. Experimental animal studies and human evidence on the association of in utero arsenic exposure with alterations in gene expression pathways related to diabetes in newborns, support the potential role of early-life arsenic exposure in diabetes development, possibly through increased insulin resistance. Given pervasive arsenic exposure and the challenges to eliminate arsenic from the environment, research is needed to evaluate prevention interventions, including the possibility of low-cost, low-risk nutritional interventions that can modify arsenic-related disease risk.

Differential metabolism of inorganic arsenic in mice from genetically diverse Collaborative Cross strains

Mice have been frequently used to study the adverse effects of inorganic arsenic (iAs) exposure in laboratory settings. Like humans, mice metabolize iAs to monomethyl-As (MAs) and dimethyl-As (DMAs) metabolites. However, mice metabolize iAs more efficiently than humans, which may explain why some of the effects of iAs reported in humans have been difficult to reproduce in mice. In the present study, we searched for mouse strains in which iAs metabolism resembles that in humans. We examined iAs metabolism in male mice from 12 genetically diverse Collaborative Cross (CC) strains that were exposed to arsenite in drinking water (0.1 or 50 ppm) for 2 weeks. Concentrations of iAs and its metabolites were measured in urine and livers. Significant differences in total As concentration and in proportions of total As represented by iAs, MAs, and DMAs were observed between the strains. These differences were more pronounced in livers, particularly in mice exposed to 50 ppm iAs. In livers, large variations among the strains were found in percentage of iAs (15-48%), MAs (11-29%), and DMAs (29-66%). In contrast, DMAs represented 96-99% of total As in urine in all strains regardless of exposure. Notably, the percentages of As species in urine did not correlate with total As concentration in liver, suggesting that the urinary profiles were not representative of the internal exposure. In livers of mice exposed to 50 ppm, but not to 0.1 ppm iAs, As3mt expression correlated with percent of iAs and DMAs. No correlations were found between As3mt expression and the proportions of As species in urine regardless of exposure level. Although we did not find yet a CC strain in which proportions of As species in urine would match those reported in humans (typically 10-30% iAs, 10-20% MAs, 60-70% DMAs), CC strains characterized by low %DMAs in livers after exposure to 50 ppm iAs (suggesting inefficient iAs methylation) could be better models for studies aiming to reproduce effects of iAs described in humans.

Arsenic metabolites; selenium; and AS3MT, MTHFR, AQP4, AQP9, SELENOP, INMT, and MT2A polymorphisms in Croatian-Slovenian population from PHIME-CROME study

The relationships between inorganic arsenic (iAs) metabolism, selenium (Se) status, and genetic polymorphisms of various genes, commonly studied in populations exposed to high levels of iAs from drinking water, were studied in a Croatian-Slovenian population from the wider PHIME-CROME project. Population consisted of 136 pregnant women in the 3^rd trimester and 176 non-pregnant women with their children (n = 176, 8-9 years old). Their exposure to iAs, defined by As (speciation) analyses of biological samples, was low. The sums of biologically active metabolites (arsenite + arsenate + methylated As forms) for pregnant women, non-pregnant women, and children, respectively were: 3.23 (2.84-3.68), 1.83 (1.54-2.16) and 2.18 (1.86-2.54) ng/mL_SG; GM (95 CI). Corresponding plasma Se levels were: 54.8 (52.8-56.9), 82.3 (80.4-84.0) and 65.8 (64.3-67.3) ng/mL; GM (95 CI). As methylation efficiency indexes confirmed the relationship between pregnancy/childhood and better methylation efficiency. Archived blood and/or saliva samples were used for single nucleotide polymorphism (SNP) genotyping of arsenic(3+) methyltransferase - AS3MT (rs7085104, rs3740400, rs3740393, rs3740390, rs11191439, rs10748835, rs1046778 and the corresponding AS3MT haplotype); methylene tetrahydrofolate reductase - MTHFR (rs1801131, rs1801133); aquaporin - AQP 4 and 9 (rs9951307 and rs2414539); selenoprotein P1 - SELENOP (rs7579, rs3877899); indolethylamine N-methyltransferase - INMT (rs6970396); and metallothionein 2A - MT2A (rs28366003). Associations of SNPs with As parameters and urine Se were determined through multiple regression analyses adjusted using appropriate confounders (blood As, plasma Se, ever smoking, etc.). SNPs' influence on As methylation, defined particularly by the secondary methylation index (SMI), confirmed the 'protective' role of minor alleles of six AS3MT SNPs and their haplotype only among non-pregnant women. Among the other investigated genes, the carriers of AQP9 (rs2414539) were associated with more efficient As methylation and higher urine concentration of As and Se among non-pregnant women; poorer methylation was observed for carriers of AQP4 (rs9951307) among pregnant women and SELENOP (rs7579) among non-pregnant women; MT2A (rs28366003) was associated with higher urine concentration of AsIII regardless of the pregnancy status; and INMT (rs6970396) was associated with higher As and Se concentration in non-pregnant women. Among confounders, the strongest influence was observed for plasma Se; it reduced urine AsIII concentration during pregnancy and increased secondary methylation index among non-pregnant women. In the present study of populations with low As exposure, we observed a few new As-gene associations (particularly with AQPs). More reliable interpretations will be possible after their confirmation in larger populations with higher As exposure levels.

Exposure to Trace Elements and Risk of Skin Cancer: A Systematic Review of Epidemiologic Studies

Exposure to environmental trace elements has been studied in relation to many cancers. However, an association between exposure to trace elements and skin cancer remains less understood. Therefore, we conducted a systematic review of published epidemiologic literature examining the association between exposure to trace elements, and risk of melanoma and keratinocyte carcinoma in humans. We identified epidemiologic studies investigating exposure to arsenic, cadmium, chromium, copper, iron, selenium, and zinc and risk of skin cancer in humans. Among the minerals, arsenic, selenium, and zinc had more than five studies available. Exposure to arsenic was associated with increased risk of keratinocyte carcinoma, while too few studies existed on melanoma to draw conclusions. Exposure to selenium was associated with possible increased risk of keratinocyte carcinoma. Studies of zinc and skin cancer were case-control in design and were found to have inconsistent associations. The data on the association between cadmium, chromium, copper, and iron and risk of skin cancer remain too sparse to draw any conclusions. In summary, epidemiologic studies on exposure to trace elements and cutaneous malignancies are limited. Studies with larger sample sizes and prospective designs are warranted to improve our knowledge of trace elements and skin cancer.

Targeted metabolomics to understand the association between arsenic metabolism and diabetes-related outcomes: Preliminary evidence from the Strong Heart Family Study

BACKGROUND

Inorganic arsenic exposure is ubiquitous and both exposure and inter-individual differences in its metabolism have been associated with cardiometabolic risk. A more efficient arsenic metabolism profile (lower MMA%, higher DMA%) has been associated with reduced risk for arsenic-related health outcomes. This profile, however, has also been associated with increased risk for diabetes-related outcomes.

OBJECTIVES

The mechanism behind these conflicting associations is unclear; we hypothesized the one-carbon metabolism (OCM) pathway may play a role.

METHODS

We evaluated the influence of OCM on the relationship between arsenic metabolism and diabetes-related outcomes (HOMA2-IR, waist circumference, fasting plasma glucose) using metabolomic data from an OCM-specific and P180 metabolite panel measured in plasma, arsenic metabolism measured in urine, and HOMA2-IR and FPG measured in fasting plasma. Samples were drawn from baseline visits (2001-2003) in 59 participants from the Strong Heart Family Study, a family-based cohort study of American Indians aged ≥14 years from Arizona, Oklahoma, and North/South Dakota.

RESULTS

In unadjusted analyses, a 5% increase in DMA% was associated with higher HOMA2-IR (geometric mean ratio (GMR)= 1.13 (95% CI: 1.03, 1.25)) and waist circumference (mean difference=3.66 (0.95, 6.38). MMA% was significantly associated with lower HOMA2-IR and waist circumference. After adjustment for OCM-related metabolites (SAM, SAH, cysteine, glutamate, lysophosphatidylcholine 18.2, and three phosphatidlycholines), associations were attenuated and no longer significant.

CONCLUSIONS

These preliminary results indicate that the association of lower MMA% and higher DMA% with diabetes-related outcomes may be influenced by OCM status, either through confounding, reverse causality, or mediation.

Gut microbiome disruption altered the biotransformation and liver toxicity of arsenic in mice

The mammalian gut microbiome (GM) plays a critical role in xenobiotic biotransformation and can profoundly affect the toxic effects of xenobiotics. Previous in vitro studies have demonstrated that gut bacteria have the capability to metabolize arsenic (As); however, the specific roles of the gut microbiota in As metabolism in vivo and the toxic effects of As are largely unknown. Here, we administered sodium arsenite to conventionally raised mice (with normal microbiomes) and GM-disrupted mice with antibiotics to investigate the role of the gut microbiota in As biotransformation and its toxicity. We found that the urinary total As levels of GM-disrupted mice were much higher, but the fecal total As levels were lower, than the levels in the conventionally raised mice. In vitro experiments, in which the GM was incubated with As, also demonstrated that the gut bacteria could adsorb or take up As and thus reduce the free As levels in the culture medium. With the disruption of the gut microbiota, arsenic biotransformation was significantly perturbed. Of note, the urinary monomethylarsonic acid/dimethylarsinic acid ratio, a biomarker of arsenic metabolism and toxicity, was markedly increased. Meanwhile, the expression of genes of one-carbon metabolism, including folr2, bhmt, and mthfr, was downregulated, and the liver S-adenosylmethionine (SAM) levels were significantly decreased in the As-treated GM-disrupted mice only. Moreover, As exposure altered the expression of genes of the p53 signaling pathway, and the expression of multiple genes associated with hepatocellular carcinoma (HCC) was also changed in the As-treated GM-disrupted mice only. Collectively, disruption of the GM enhances the effect of As on one-carbon metabolism, which could in turn affect As biotransformation. GM disruption also increases the toxic effects of As and may increase the risk of As-induced HCC in mice.

Individual susceptibility to arsenic-induced diseases: the role of host genetics, nutritional status, and the gut microbiome

Arsenic (As) contamination in water or food is a global issue affecting hundreds of millions of people. Although As is classified as a group 1 carcinogen and is associated with multiple diseases, the individual susceptibility to As-related diseases is highly variable, such that a proportion of people exposed to As have higher risks of developing related disorders. Many factors have been found to be associated with As susceptibility. One of the main sources of the variability found in As susceptibility is the variation in the host genome, namely, polymorphisms of many genes involved in As transportation, biotransformation, oxidative stress response, and DNA repair affect the susceptibility of an individual to As toxicity and then influence the disease outcomes. In addition, lifestyles and many nutritional factors, such as folate, vitamin C, and fruit, have been found to be associated with individual susceptibility to As-related diseases. Recently, the interactions between As exposure and the gut microbiome have been of particular concern. As exposure has been shown to perturb gut microbiome composition, and the gut microbiota has been shown to also influence As metabolism, which raises the question of whether the highly diverse gut microbiota contributes to As susceptibility. Here, we review the literature and summarize the factors, such as host genetics and nutritional status, that influence As susceptibility, and we also present potential mechanisms of how the gut microbiome may influence As metabolism and its toxic effects on the host to induce variations in As susceptibility. Challenges and future directions are also discussed to emphasize the importance of characterizing the specific role of these factors in interindividual susceptibility to As-related diseases.

Screening for gene-environment (G×E) interaction using omics data from exposed individuals: an application to gene-arsenic interaction

Identifying gene-environment interactions is a central challenge in the quest to understand susceptibility to complex, multi-factorial diseases. Developing an understanding of how inter-individual variability in inherited genetic variation alters the effects of environmental exposures will enhance our knowledge of disease mechanisms and improve our ability to predict disease and target interventions to high-risk sub-populations. Limited progress has been made identifying gene-environment interactions in the epidemiological setting using existing statistical approaches for genome-wide searches for interaction. In this paper, we describe a novel two-step approach using omics data to conduct genome-wide searches for gene-environment interactions. Using existing genome-wide SNP data from a large Bangladeshi cohort study specifically designed to assess the effect of arsenic exposure on health, we evaluated gene-arsenic interactions by first conducting genome-wide searches for SNPs that modify the effect of arsenic on molecular phenotypes (gene expression and DNA methylation features). Using this set of SNPs showing evidence of interaction with arsenic in relation to molecular phenotypes, we then tested SNP-arsenic interactions in relation to skin lesions, a hallmark characteristic of arsenic toxicity. With the emergence of additional omics data in the epidemiologic setting, our approach may have the potential to boost power for genome-wide interaction research, enabling the identification of interactions that will enhance our understanding of disease etiology and our ability to develop interventions targeted at susceptible sub-populations.

Association between arsenic metabolism gene polymorphisms and arsenic-induced skin lesions in individuals exposed to high-dose inorganic arsenic in northwest China

Individuals in a given environment contaminated with arsenic have different susceptibilities to disease, which may be related to arsenic metabolism, age, gender, genetics and other factors. This study recruited 850 subjects, including 331 cases and 519 controls, from populations exposed to high levels of arsenic in drinking water in northwest China. Genotypes were determined using a custom-by-design 48-Plex SNPscan^TM kit. The results indicated that subjects who carried at least one C allele for GSTO1 rs11191979 polymorphism, at least one A allele for GSTO1 rs2164624, at least one A allele for GSTO1 rs4925, the AG genotype for GSTO2 rs156697, the AG genotype or at least one G allele for GSTO2 rs2297235 or the GG genotype or at least one G allele for PNP rs3790064 had an increased risk of arsenic-related skin lesions. In addition, the haplotype CT between rs4925 and rs11191979 appeared to confer a high risk of arsenic-included skin lesions (OR = 1.377, 95% CI = 1.03–1.84), as did the haplotype GCG among rs156697, rs157077 and rs2297235 (OR = 2.197, 95% CI = 1.08–4.44). The results showed that the variants of GSTO1, GSTO2 and PNP render the susceptible toward developing arsenic-induced skin lesions in individuals exposed to high-dose inorganic arsenic in northwest China.

Genetic susceptibility to breast cancer risk associated with inorganic arsenic exposure

OBJECTIVE

To evaluate whether the association between breast cancer (BC) and inorganic arsenic (iAs) exposure is modulated by selected polymorphisms in iAs metabolism.

METHODS

A population based case-control (1016/1028) study was conducted in Northern Mexico. Urinary arsenic metabolites were measured by High Performance Liquid Chromatography. Metabolites percentages and methylation ratios, were estimated. Genotypes of selected polymorphisms were determined by allelic discrimination. The interaction between polymorphisms and iAs metabolites percentages and methylation ratios on BC was assessed with unconditional logistic regression models.

RESULTS

A significant interaction (p=0.002) between MTR c.2756A>G polymorphism and percentage dimethylarsinic acid (DMA) on BC was found; BC risk related with %DMA was lower in AG+GG carriers than in AA carriers. No other significant interactions were found.

CONCLUSION

MTR c.2756A>G polymorphism may confer protection for BC associated with iAs exposure. Further research is warranted to elucidate the potential involvement of other polymorphisms in iAs-related BC.

The Association of Arsenic Metabolism with Cancer, Cardiovascular Disease, and Diabetes: A Systematic Review of the Epidemiological Evidence

BACKGROUND

The available evidence on the role of arsenic metabolism in individual susceptibility to the development of cancer, cardiovascular disease, and diabetes has not been formally and comprehensively reviewed.

OBJECTIVES

Our goal was to systematically investigate the association of arsenic metabolism with cancer, cardiovascular disease, and diabetes-related outcomes in epidemiologic studies. As a secondary objective, we characterized the variation of arsenic metabolism in different populations worldwide.

METHODS

We searched Medline/PubMed and EMBASE from inception to January 2016 and applied predetermined exclusion criteria. Compositional data analysis was used to describe the distribution of arsenic metabolism biomarkers and evaluate the association between arsenic exposure and metabolism.

RESULTS

Twenty-eight studies met the inclusion criteria, 12 on cancer, nine on cardiovascular disease, and seven on diabetes-related outcomes. The median (interquartile range) for mean iAs%, MMA%, and DMA% was 11.2 (7.8-14.9)%, 13.0 (10.4-13.6)%, and 74.9 (69.8-80.0)%, respectively. Findings across studies suggested that higher arsenic exposure levels were associated with higher iAs% and lower DMA% and not associated with MMA%. For cancer, most studies found a pattern of higher MMA% and lower DMA% associated with higher risk of all-site, urothelial, lung, and skin cancers. For cardiovascular disease, higher MMA% was generally associated with higher risk of carotid atherosclerosis and clinical cardiovascular disease but not with hypertension. For diabetes-related outcomes, the pattern of lower MMA% and higher DMA% was associated with higher risk of metabolic syndrome and diabetes.

CONCLUSIONS

Population level of iAs% and DMA%, but not MMA%, were associated with arsenic exposure levels. Overall, study findings suggest that higher MMA% was associated with an increased risk of cancer and cardiovascular disease, while lower MMA% was associated with an increased risk of diabetes and metabolic syndrome. Additional population-based studies and experimental studies are needed to further evaluate and understand the role of arsenic exposure in arsenic metabolism and the role of arsenic metabolism in disease development. https://doi.org/10.1289/EHP577.

Arsenic metabolism and cancer risk: A meta-analysis

OBJECTIVE

To describe the studies that have reported association measures between risk of cancer and the percentage distribution of urinary inorganic arsenic (iAs) metabolites by anatomical site, in non-ecological epidemiological studies.

METHODS

Studies were identified in the PubMed database in the period from 1990 to 2015. Inclusion criteria were: non-ecological epidemiological study, with histologically confirmed cancer cases, reporting the percentage distribution of inorganic arsenic (iAs), monomethylated (MMA) and dimethylated (DMA) metabolites, as well as association measures with confidence intervals (CI) between cancer and %iAs and/or %MMA and/or %DMA. A descriptive meta-analysis was performed by the method of the inverse of the variance for the fixed effects model and the DerSimonian and Laird's method for the random effects model. Heterogeneity was tested using the Q statistic and stratifying for epidemiological design and total As in urine. The possibility of publication bias was assessed through Begg's test.

RESULTS

A total of 13 eligible studies were found, most of them were performed in Taiwan and focused on skin and bladder cancer. The positive association between %MMA and various types of cancer was consistent, in contrast to the negative relationship between %DMA and cancer that was inconsistent. The summary risk of bladder (OR=1.79; 95% CI: 1.42, 2.26, n=4 studies) and lung (OR=2.44; 95% CI: 1.57, 3.80, n=2 studies) cancer increased significantly with increasing %MMA, without statistical heterogeneity. In contrast, lung cancer risk was inversely related to %DMA (OR=0.58; 95% CI: 0.36, 0.93, n=2 studies), also without significant heterogeneity. These results were similar after stratifying by epidemiological design and total As in urine. No evidence of publication bias was found.

CONCLUSION

These findings provide additional support that methylation needs to be taken into account when assessing the potential iAs carcinogenicity risk.

Cellular arsenic transport pathways in mammals

Natural contamination of drinking water with arsenic results in the exposure of millions of people world-wide to unacceptable levels of this metalloid. This is a serious global health problem because arsenic is a Group 1 (proven) human carcinogen and chronic exposure is known to cause skin, lung, and bladder tumors. Furthermore, arsenic exposure can result in a myriad of other adverse health effects including diseases of the cardiovascular, respiratory, neurological, reproductive, and endocrine systems. In addition to chronic environmental exposure to arsenic, arsenic trioxide is approved for the clinical treatment of acute promyelocytic leukemia, and is in clinical trials for other hematological malignancies as well as solid tumors. Considerable inter-individual variability in susceptibility to arsenic-induced disease and toxicity exists, and the reasons for such differences are incompletely understood. Transport pathways that influence the cellular uptake and export of arsenic contribute to regulating its cellular, tissue, and ultimately body levels. In the current review, membrane proteins (including phosphate transporters, aquaglyceroporin channels, solute carrier proteins, and ATP-binding cassette transporters) shown experimentally to contribute to the passage of inorganic, methylated, and/or glutathionylated arsenic species across cellular membranes are discussed. Furthermore, what is known about arsenic transporters in organs involved in absorption, distribution, and metabolism and how transport pathways contribute to arsenic elimination are described.

Expression profile of hydrogen sulfide and its synthases correlates with tumor stage and grade in urothelial cell carcinoma of bladder

BACKGROUND

Hydrogen sulfide (H2S) is a newly discovered gas transmitter. It is synthesized by cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (MPST). Endogenous hydrogen sulfide has never been studied in bladder cancer.

PURPOSE

We evaluated H2S production and its synthases expression levels in transitional cell carcinoma (urothelial cell carcinoma of bladder [UCB]) of human bladder tissue and cell lines.

MATERIALS AND METHODS

Immunostaining was performed in urothelial cell lines and bladder specimens from 94 patients with UCB of different stages/grades. The expression levels/activities of CBS, CSE, and MPST of specimens and cell lines were analyzed by image semiquantity assay, western blot, and a sulfur-sensitive electrode. We tried to find the correlation between hydrogen sulfide and its synthases with tumor stage in UCB. All experiments were repeated at least 3 times.

RESULTS

Immunoreactivity for CBS, CSE, and MPST was detected in malignant uroepithelium and muscular layer of all tissues examined and cultured cells. The expression levels of CBS, CSE, and MPST were associated with UCB stage/grade. Muscle-invasive bladder cancer samples showed the highest production of H2S (52.6±2.91 nmol/[mg·min]) among all tested samples and EJ cells (transitional cell carcinoma, grade IIIshowed the highest production of H2S among all tested cell lines (53.3±7.02nmol/[mg·min]).

CONCLUSIONS

Protein levels and catalytic activities of CBS, CSE, and MPST increased with the increase of malignant degrees in human bladder tissues and human UCB cell lines. Our findings may promote the application of these novel enzymes to UCB diagnosis or treatment.

Determinants and Consequences of Arsenic Metabolism Efficiency among 4,794 Individuals: Demographics, Lifestyle, Genetics, and Toxicity

BACKGROUND

Exposure to inorganic arsenic (iAs), a class I carcinogen, affects several hundred million people worldwide. Once absorbed, iAs is converted to monomethylated (MMA) and then dimethylated forms (DMA), with methylation facilitating urinary excretion. The abundance of each species in urine relative to their sum (iAs%, MMA%, and DMA%) varies across individuals, reflecting differences in arsenic metabolism capacity.

METHODS

The association of arsenic metabolism phenotypes with participant characteristics and arsenical skin lesions was characterized among 4,794 participants in the Health Effects of Arsenic Longitudinal Study (Araihazar, Bangladesh). Metabolism phenotypes include those obtained from principal component (PC) analysis of arsenic species.

RESULTS

Two independent PCs were identified: PC1 appears to represent capacity to produce DMA (second methylation step), and PC2 appears to represent capacity to convert iAs to MMA (first methylation step). PC1 was positively associated (P <0.05) with age, female sex, and BMI, while negatively associated with smoking, arsenic exposure, education, and land ownership. PC2 was positively associated with age and education but negatively associated with female sex and BMI. PC2 was positively associated with skin lesion status, while PC1 was not. 10q24.32/AS3MT region polymorphisms were strongly associated with PC1, but not PC2. Patterns of association for most variables were similar for PC1 and DMA%, and for PC2 and MMA% with the exception of arsenic exposure and SNP associations.

CONCLUSIONS

Two distinct arsenic metabolism phenotypes show unique associations with age, sex, BMI, 10q24.32 polymorphisms, and skin lesions.

IMPACT

This work enhances our understanding of arsenic metabolism kinetics and toxicity risk profiles.

Effect of heme oxygenase-1 gene promoter polymorphism on cancer risk by histological subtype: A prospective study in arseniasis-endemic areas in Taiwan

Heme oxygenase (HO)-1 is upregulated by many stressful stimuli, including arsenic. A GT-repeat ((GT)n) polymorphism in the HO-1 gene promoter inversely modulates the levels of HO-1 induction. Previous HO-1 (GT)n polymorphism studies in relation to cancer risk have shown disparate results. We prospectively investigated the associations between HO-1 (GT)n polymorphism and cancer risk related to arsenic from drinking water. Totally, 1,013 participants from community-based cohorts of arseniasis-endemic areas in Taiwan were followed for 13 years. Allelic polymorphisms were classified into long (L, ≥ 27 (GT)n) and short (S, <27 (GT)n). Newly developed cases were identified through linkage with National Cancer Registry of Taiwan. Multivariate Cox proportional hazard methods were used to evaluate effects of the HO-1 polymorphism alone or combined with arsenic exposure. Results showed that participants with the S/S genotype had an increased risk of Bowen's disease (HR = 10.49; 95% CI: 2.77-39.7), invasive skin cancer (HR = 2.99; 95% CI: 1.13-7.87), and lung squamous cell carcinoma (HR = 3.39; 95% CI: 1.15-9.95) versus those with L/S or L/L genotype. The S/S genotype combined with high arsenic exposure (>300 μg/L) had a greater risk of skin cancer compared to the genotype alone. Consistent with previous findings, participants with the S-allele had a reduced risk of lung adenocarcinoma (HR = 0.21; 95% CI: 0.03-0.68) versus those with L/L genotype. There were no significant differences in risk of urothelial carcinoma among the three genotypes. Associations of HO-1 (GT)n polymorphism with cancer risk differs by histological subtype and the polymorphism should be considered a modifier in the risk assessment of arsenic exposure.

The Genetic Architecture of Arsenic Metabolism Efficiency:A SNP-Based Heritability Study of Bangladeshi Adults

BACKGROUND

Consumption of arsenic-contaminated drinking water adversely affects health. There is interindividual variation in arsenic metabolism efficiency, partially due to genetic variation in the arsenic methyltransferase (AS3MT) gene region.

OBJECTIVES

The goal of this study was to assess the overall contribution of genetic factors to variation in arsenic metabolism efficiency, as measured by the relative concentration of dimethylarsinic acid (DMA%) in urine.

METHODS

Using data on genome-wide single nucleotide polymorphisms (SNPs) and urinary DMA% for 2,053 arsenic-exposed Bangladeshi individuals, we employed various SNP-based approaches for heritability estimation and polygenic modeling.

RESULTS

Using data on all participants, the percent variance explained (PVE) for DMA% by all measured and imputed SNPs was 16% (p = 0.08), which was reduced to 5% (p = 0.34) after adjusting for AS3MT SNPs. Using information on close relatives only, the PVE was 63% (p = 0.0002), but decreased to 41% (p = 0.01) after adjusting for AS3MT SNPs. Regional heritability analysis confirmed 10q24.32 (AS3MT) as a major arsenic metabolism locus (PVE = 7%, p = 4.4 × 10(-10)), but revealed no additional regions. We observed a moderate association between a polygenic score reflecting elevated DMA% (composed of thousands of non-AS3MT SNPs) and reduced skin lesion risk in an independent sample (p < 0.05). We observed no associations for SNPs reported in prior candidate gene studies of arsenic metabolism.

CONCLUSIONS

Our results suggest that there are common variants outside of the AS3MT region that influence arsenic metabolism in Bangladeshi individuals, but the effects of these variants are very weak compared with variants near AS3MT. The high heritability estimates observed using family-based heritability approaches suggest substantial effects for rare variants and/or unmeasured environmental factors.

Linkage Analysis of Urine Arsenic Species Patterns in the Strong Heart Family Study

Arsenic toxicokinetics are important for disease risks in exposed populations, but genetic determinants are not fully understood. We examined urine arsenic species patterns measured by HPLC-ICPMS among 2189 Strong Heart Study participants 18 years of age and older with data on ~400 genome-wide microsatellite markers spaced ~10 cM and arsenic speciation (683 participants from Arizona, 684 from Oklahoma, and 822 from North and South Dakota). We logit-transformed % arsenic species (% inorganic arsenic, %MMA, and %DMA) and also conducted principal component analyses of the logit % arsenic species. We used inverse-normalized residuals from multivariable-adjusted polygenic heritability analysis for multipoint variance components linkage analysis. We also examined the contribution of polymorphisms in the arsenic metabolism gene AS3MT via conditional linkage analysis. We localized a quantitative trait locus (QTL) on chromosome 10 (LOD 4.12 for %MMA, 4.65 for %DMA, and 4.84 for the first principal component of logit % arsenic species). This peak was partially but not fully explained by measured AS3MT variants. We also localized a QTL for the second principal component of logit % arsenic species on chromosome 5 (LOD 4.21) that was not evident from considering % arsenic species individually. Some other loci were suggestive or significant for 1 geographical area but not overall across all areas, indicating possible locus heterogeneity. This genome-wide linkage scan suggests genetic determinants of arsenic toxicokinetics to be identified by future fine-mapping, and illustrates the utility of principal component analysis as a novel approach that considers % arsenic species jointly.

A distinct and replicable variant of the squamous cell carcinoma gene inositol polyphosphate-5-phosphatase modifies the susceptibility of arsenic-associated skin lesions in Bangladesh

BACKGROUND

Single-nucleotide polymorphisms (SNPs) in inflammation, one-carbon metabolism, and skin cancer genes might influence susceptibility to arsenic-induced skin lesions.

METHODS

A case-control study was conducted in Pabna, Bangladesh (2001-2003), and the drinking-water arsenic concentration was measured for each participant. A panel of 25 candidate SNPs was analyzed in 540 cases and 400 controls. Logistic regression was used to estimate the association between each SNP and the potential for gene-environment interactions in the skin lesion risk, with adjustments for relevant covariates. Replication testing was conducted in an independent Bangladesh population with 488 cases and 2,794 controls.

RESULTS

In the discovery population, genetic variants in the one-carbon metabolism genes phosphatidylethanolamine N-methyltransferase (rs2278952, P for interaction  = .004; rs897453, P for interaction = .05) and dihydrofolate reductase (rs1650697, P for interaction = .02), the inflammation gene interleukin 10 (rs3024496, P for interaction =.04), and the skin cancer genes inositol polyphosphate-5-phosphatase (INPP5A; rs1133400, P for interaction = .03) and xeroderma pigmentosum complementation group C (rs2228000, P for interaction = .01) significantly modified the association between arsenic and skin lesions after adjustments for multiple comparisons. The significant gene-environment interaction between a SNP in the INPP5A gene (rs1133400) and water arsenic with respect to the skin lesion risk was successfully replicated in an independent population (P for interaction = .03).

CONCLUSIONS

Minor allele carriers of the skin cancer gene INPP5A modified the odds of arsenic-induced skin lesions in both main and replicative populations. Genetic variation in INPP5A appears to have a role in susceptibility to arsenic toxicity.

Drinking Water Arsenic Contamination, Skin Lesions, and Malignancies: A Systematic Review of the Global Evidence

Skin lesions and cancer are known manifestations of chronic exposure to arsenic contaminated drinking water. Epidemiologic data primarily comes from regions with exposures 1-2 orders of magnitude above the current World Health Organization (WHO) guidelines of 10 μg/L. Emerging evidence indicates that more common exposures may also be related to both noncancerous and cancerous changes to the skin. In this review, we focus on the body of epidemiologic literature that encompasses exposures within the WHO guidelines, excluding studies that lacked individual exposure estimates and case reports. For skin lesions and skin cancers, 15 and 10 studies were identified that met our criteria, respectively. For skin lesions, a consistent dose-response relationship with water arsenic has been observed, with increased risk evident at low- to moderate-dose exposure. Of the larger studies of specific histologic types of skin cancers, although with differing exposure definitions, there was evidence of dose-related relationships with both basal cell carcinomas and squamous cell carcinomas. The effect of arsenic exposure on skin lesion risk is likely modified by genetic variants that influence arsenic metabolism. Accumulating evidence suggests that arsenic may increase risk of skin lesions and skin cancers at levels not previously considered harmful, and that genetic factors may influence risk.

Arsenic thiolation and the role of sulfate-reducing bacteria from the human intestinal tract

BACKGROUND

Arsenic (As) toxicity is primarily based on its chemical speciation. Although inorganic and methylated As species are well characterized in terms of metabolism and formation in the human body, the origin of thiolated methylarsenicals is still unclear.

OBJECTIVES

We sought to determine whether sulfate-reducing bacteria (SRB) from the human gut are actively involved in the thiolation of monomethylarsonic acid (MMAV).

METHODS

We incubated human fecal and colon microbiota in a batch incubator and in a dynamic gut simulator with a dose of 0.5 mg MMAV in the absence or presence of sodium molybdate, an SRB inhibitor. We monitored the conversion of MMAV into monomethyl monothioarsonate (MMMTAV) and other As species by high-performance liquid chromatography coupled with inductively coupled plasma mass spectrometry analysis. We monitored the sulfate-reducing activity of the SRB by measuring hydrogen sulfide (H2S) production. We used molecular analysis to determine the dominant species of SRB responsible for As thiolation.

RESULTS

In the absence of sodium molybdate, the SRB activity-primarily derived from Desulfovibrio desulfuricans (piger)-was specifically and proportionally correlated (p < 0.01) to MMAV conversion into MMMTAV. Inactivating the SRB with molybdate did not result in MMAV thiolation; however, we observed that the microbiota from a dynamic gut simulator were capable of demethylating 4% of the incubated MMAV into arsenous acid (iAsIII), the trivalent and more toxic form of arsenic acid (iAsV).

CONCLUSION

We found that SRB of human gastrointestinal origin, through their ability to produce H2S, were necessary and sufficient to induce As thiolation. The toxicological consequences of this microbial As speciation change are not yet clear. However, given the efficient epithelial absorption of thiolated methylarsenicals, we conclude that the gut microbiome-and SRB activity in particular-should be incorporated into toxicokinetic analysis carried out after As exposure.

Arsenic in drinking water and urinary tract cancers: a systematic review of 30 years of epidemiological evidence

BACKGROUND

Arsenic in drinking water is a public health issue affecting hundreds of millions of people worldwide. This review summarizes 30 years of epidemiological studies on arsenic exposure in drinking water and the risk of bladder or kidney cancer, quantifying these risks using a meta-analytical framework.

METHODS

Forty studies met the selection criteria. Seventeen provided point estimates of arsenic concentrations in drinking water and were used in a meta-analysis of bladder cancer incidence (7 studies) and mortality (10 studies) and kidney cancer mortality (2 studies). Risk estimates for incidence and mortality were analyzed separately using Generalized Linear Models. Predicted risks for bladder cancer incidence were estimated at 10, 50 and 150 μg/L arsenic in drinking water. Bootstrap randomizations were used to assess robustness of effect size.

RESULTS

Twenty-eight studies observed an association between arsenic in drinking water and bladder cancer. Ten studies showed an association with kidney cancer, although of lower magnitude than that for bladder cancer. The meta-analyses showed the predicted risks for bladder cancer incidence were 2.7 [1.2-4.1]; 4.2 [2.1-6.3] and; 5.8 [2.9-8.7] for drinking water arsenic levels of 10, 50, and 150 μg/L, respectively. Bootstrapped randomizations confirmed this increased risk, but, lowering the effect size to 1.4 [0.35-4.0], 2.3 [0.59-6.4], and 3.1 [0.80-8.9]. The latter suggests that with exposures to 50 μg/L, there was an 83% probability for elevated incidence of bladder cancer; and a 74% probability for elevated mortality. For both bladder and kidney cancers, mortality rates at 150 ug/L were about 30% greater than those at 10 μg/L.

CONCLUSION

Arsenic in drinking water is associated with an increased risk of bladder and kidney cancers, although at lower levels (<150 μg/L), there is uncertainty due to the increased likelihood of exposure misclassification at the lower end of the exposure curve. Meta-analyses suggest exposure to 10 μg/L of arsenic in drinking water may double the risk of bladder cancer, or at the very least, increase it by about 40%. With the large number of people exposed to these arsenic concentrations worldwide the public health consequences of arsenic in drinking water are substantial.

Body composition and arsenic metabolism: a cross-sectional analysis in the Strong Heart Study

OBJECTIVE

The objective of this study was to evaluate the association between measures of body composition and patterns of urine arsenic metabolites in the 1989-1991 baseline visit of the Strong Heart Study, a cardiovascular disease cohort of adults recruited from rural communities in Arizona, Oklahoma, North Dakota and South Dakota.

METHODS

We evaluated 3,663 Strong Heart Study participants with urine arsenic species above the limit of detection and no missing data on body mass index, % body fat and fat free mass measured by bioelectrical impedance, waist circumference and other variables. We summarized urine arsenic species patterns as the relative contribution of inorganic (iAs), methylarsonate (MMA) and dimethylarsinate (DMA) species to their sum. We modeled the associations of % arsenic species biomarkers with body mass index, % body fat, fat free mass, and waist circumference categories in unadjusted regression models and in models including all measures of body composition. We also considered adjustment for arsenic exposure and demographics.

RESULTS

Increasing body mass index was associated with higher mean % DMA and lower mean % MMA before and after adjustment for sociodemographic variables, arsenic exposure, and for other measures of body composition. In unadjusted linear regression models, % DMA was 2.4 (2.1, 2.6) % higher per increase in body mass index category (< 25, ≥25 & <30, ≥30 & <35, ≥35 kg/m2), and % MMA was 1.6 (1.4, 1.7) % lower. Similar patterns were observed for % body fat, fat free mass, and waist circumference measures in unadjusted models and in models adjusted for potential confounders, but the associations were largely attenuated or disappeared when adjusted for body mass index.

CONCLUSION

Measures of body size, especially body mass index, are associated with arsenic metabolism biomarkers. The association may be related to adiposity, fat free mass or body size. Future epidemiologic studies of arsenic should consider body mass index as a potential modifier for arsenic-related health effects.

Gut microbiome perturbations induced by bacterial infection affect arsenic biotransformation

Exposure to arsenic affects large human populations worldwide and has been associated with a long list of human diseases, including skin, bladder, lung, and liver cancers, diabetes, and cardiovascular disorders. In addition, there are large individual differences in susceptibility to arsenic-induced diseases, which are frequently associated with different patterns of arsenic metabolism. Several underlying mechanisms, such as genetic polymorphisms and epigenetics, have been proposed, as these factors closely impact the individuals' capacity to metabolize arsenic. In this context, the role of the gut microbiome in directly metabolizing arsenic and triggering systemic responses in diverse organs raises the possibility that perturbations of the gut microbial communities affect the spectrum of metabolized arsenic species and subsequent toxicological effects. In this study, we used an animal model with an altered gut microbiome induced by bacterial infection, 16S rRNA gene sequencing, and inductively coupled plasma mass spectrometry-based arsenic speciation to examine the effect of gut microbiome perturbations on the biotransformation of arsenic. Metagenomics sequencing revealed that bacterial infection significantly perturbed the gut microbiome composition in C57BL/6 mice, which in turn resulted in altered spectra of arsenic metabolites in urine, with inorganic arsenic species and methylated and thiolated arsenic being perturbed. These data clearly illustrated that gut microbiome phenotypes significantly affected arsenic metabolic reactions, including reduction, methylation, and thiolation. These findings improve our understanding of how infectious diseases and environmental exposure interact and may also provide novel insight regarding the gut microbiome composition as a new risk factor of individual susceptibility to environmental chemicals.

Ogg1 genetic background determines the genotoxic potential of environmentally relevant arsenic exposures

Inorganic arsenic (i-As) is a well-established human carcinogen to which millions of people are exposed worldwide. It is generally accepted that the genotoxic effects of i-As after an acute exposure are partially linked to the i-As-induced production of reactive oxygen species, but it is necessary to better determine whether chronic sub-toxic i-As doses are able to induce biologically significant levels of oxidative DNA damage (ODD). To fill in this gap, we have tested the genotoxic and oxidative effects of environmentally relevant arsenic exposures using mouse embryonic fibroblast MEF mutant Ogg1 cells and their wild-type counterparts. Effects were examined by using the comet assay complemented with the use of FPG enzyme. Our findings indicate that MEF Ogg1-/- cells are more sensitive to arsenite-induced acute toxicity, genotoxicity and ODD. Long-term exposure to sub-toxic doses of arsenite generates a detectable increase in ODD and genotoxic DNA damage only in MEF Ogg1-deficient cells. Altogether, the data presented here point out the relevance of ODD and Ogg1 genetic background on the genotoxic risk of i-As at environmentally plausible doses. The persistent accumulation of DNA 8-OH-dG lesions in Ogg1-/- cells during the complete course of the exposure suggests a relevant role in arsenic-associated carcinogenic risk in turn.

Nuclear factor erythroid 2-related factor gene variants and susceptibility of arsenic-related skin lesions

Inorganic arsenic (iAs) is an important pollutant associated with various chronic-degenerative diseases. The cytoprotective protein nuclear factor erythroid 2-related factor (NRF2) has been proposed as an important responsive mechanism against iAs exposure. The aim of this study was to determine whether the risk of skin lesions in people exposed to iAs-contaminated water could be modified by the presence of single nucleotide polymorphisms in the NRF2 coding gene. We studied 117 individuals with long-term iAs exposure and 120 nonexposed individuals. Total As was determined in water, meanwhile iAs and its metabolites were measured in urine. The iAs-induced skin lesion status was evaluated by expert dermatologists. We sequenced the promoter region of NRF2 in a sample of 120 healthy donors. We found four polymorphisms previously reported and one novel polymorphism in the 5′ regulatory region of the NRF2. In this study, we did not find allelic and genotype association of NRF2 polymorphisms with iAs-related skin lesion. However, the analysis of haplotypes composed by –653GA, and –617CA NRF2 single nucleotide polymorphisms showed a significant association with protection against skin lesions in the low-As exposure group. This is the first report studying the association between NRF2 polymorphisms and susceptibility of As-related skin lesions. Increasing the sample size will allow us to confirm this data.

SLCO1B1 variants and urine arsenic metabolites in the Strong Heart Family Study

Arsenic species patterns in urine are associated with risk for cancer and cardiovascular diseases. The organic anion transporter coded by the gene SLCO1B1 may transport arsenic species, but its association with arsenic metabolites in human urine has not yet been studied. The objective of this study is to evaluate associations of urine arsenic metabolites with variants in the candidate gene SLCO1B1 in adults from the Strong Heart Family Study. We estimated associations between % arsenic species biomarker traits and 5 single-nucleotide polymorphisms (SNPs) in the SLCO1B1 gene in 157 participants, assuming additive genetics. Linear regression models for each SNP accounted for kinships and were adjusted for sex, body mass index, and study center. The minor allele of rs1564370 was associated with lower %MMA (p = .0003) and higher %DMA (p = .0002), accounting for 8% of the variance for %MMA and 9% for %DMA. The rs1564370 minor allele homozygote frequency was 17% and the heterozygote frequency was 43%. The minor allele of rs2291075 was associated with lower %MMA (p = .0006) and higher %DMA (p = .0014), accounting for 7% of the variance for %MMA and 5% for %DMA. The frequency of rs2291075 minor allele homozygotes was 1% and of heterozygotes was 15%. Common variants in SLCO1B1 were associated with differences in arsenic metabolites in a preliminary candidate gene study. Replication of this finding in other populations and analyses with respect to disease outcomes are needed to determine whether this novel candidate gene is important for arsenic-associated disease risks.

Association of urinary monomethylated arsenic concentration and risk of hypertension: a cross-sectional study from arsenic contaminated areas in northwestern China

BACKGROUND

Although some studies mainly from Taiwan, Bangladesh and the United States, have suggested a consistent dose-response increase in the prevalence of hypertension with increasing arsenic exposure, the association between chronic environmental arsenic exposure and the risk of hypertension is still inconclusive. Most of the studies discussed the association from the point of view of arsenic concentration in drinking water or cumulative arsenic exposure (CAE), few involved arsenic speciation into the discussion. In this cross-sectional study, we evaluated the potential association between environmental arsenic exposure through drinking water and the prevalence of hypertension by analyzing not only CAE but also urinary arsenic speciation, and provided data on arsenic exposure and hypertension from mainland of China.

METHODS

A cross-sectional study was conducted in one of the arsenic contaminated areas in the northwest of China. Among a total of 1005 residents who voluntarily participated in the study, 604 of eligible subjects were confirmed and interviewed door to door. Standing height, body weight, and blood pressure were measured. First void urine was collected and measured for the concentration of urinary arsenic speciation. CAE was calculated in a subpopulation of 360 subjects with detailed water consumption history. The association between urinary arsenic speciation, CAE and the risk of hypertension were analyzed by multiple logistic regressions.

RESULTS

We found that the levels of urinary arsenic species of inorganic arsenic (iAs), monomethylated arsenic (MMA), dimethylated arsenic (DMA) and total arsenic (tAs) were significantly correlated with systolic or pulse blood pressure. A positive relationship was found between the highest tertile of CAE and hypertension in a dose-dependent manner. Subjects with higher concentration of urinary MMA or lower percentage of DMA tended to be liable to suffer from hypertension. A significant increasing trend of the risk of hypertension with increasing tertiles of MMA concentration was also observed in the logistic regression models both before and after adjustment for confounders.

CONCLUSIONS

Our findings suggested that arsenic exposure, especially high level of CAE, was positively associated with the prevalence of hypertension, and that higher concentration of urinary MMA might be related to the increased susceptibility to hypertension.

Heritability and preliminary genome-wide linkage analysis of arsenic metabolites in urine

BACKGROUND

Arsenic (III) methyltransferase (AS3MT) has been related to urine arsenic metabolites in association studies. Other genes might also play roles in arsenic metabolism and excretion.

OBJECTIVE

We evaluated genetic determinants of urine arsenic metabolites in American Indian adults from the Strong Heart Study (SHS).

METHODS

We evaluated heritability of urine arsenic metabolites [percent inorganic arsenic (%iAs), percent monomethylarsonate (%MMA), and percent dimethylarsinate (%DMA)] in 2,907 SHS participants with urine arsenic measurements and at least one relative within the cohort. We conducted a preliminary linkage analysis in a subset of 487 participants with available genotypes on approximately 400 short tandem repeat markers using a general pedigree variance component approach for localizing quantitative trait loci (QTL).

RESULTS

The medians (interquartile ranges) for %iAs, %MMA, and %DMA were 7.7% (5.4-10.7%), 13.6% (10.5-17.1%), and 78.4% (72.5-83.1%), respectively. The estimated heritability was 53% for %iAs, 50% for %MMA, and 59% for %DMA. After adjustment for sex, age, smoking, body mass index, alcohol consumption, region, and total urine arsenic concentrations, LOD [logarithm (to the base of 10) of the odds] scores indicated suggestive evidence for genetic linkage with QTLs influencing urine arsenic metabolites on chromosomes 5 (LOD = 2.03 for %iAs), 9 (LOD = 2.05 for %iAs and 2.10 for %MMA), and 11 (LOD = 1.94 for %iAs). A peak for %DMA on chromosome 10 within 2 Mb of AS3MT had an LOD of 1.80.

CONCLUSIONS

This population-based family study in American Indian communities supports a genetic contribution to variation in the distribution of arsenic metabolites in urine and, potentially, the involvement of genes other than AS3MT.

Arsenic metabolism and thioarsenicals

Arsenic has received considerable attention in the world, since it can lead to a multitude of toxic effects and has been recognized as a human carcinogen causing cancers. Here, we focus on the current state of knowledge regarding the proposed mechanisms of arsenic biotransformation, with a little about cellular uptake, toxicity and clinical utilization of arsenicals. Since pentavalent methylated metabolites were found in animal urine after exposure to iAs(III), methylation was considered to be a detoxification process, but the discovery of methylated trivalent intermediates and thioarsenicals in urine has diverted the view and gained much interest regarding arsenic biotransformation. To further investigate the partially understood phenomena relating to arsenic toxicity and the uses of arsenic as a drug, it is important to elucidate the exact pathways involved in metabolism of this metalloid, as the toxicity and the clinical uses of arsenic can be best recognized in context of its biotransformation. Thereby, in this perspective, we have focused on arsenic metabolic pathways including three proposed mechanisms: a classic pathway by Challenger in 1945, followed by a new metabolic pathway proposed by Hayakawa in 2005 involving arsenic-glutathione complexes, while the third is a new reductive methylation pathway that is proposed by our group involving As-protein complexes. According to previous and present in vivo and in vitro experiments, we conclude that the methylation reaction takes place with simultaneous reductive rather than stepwise oxidative methylation. In addition, production of pentavalent methylated arsenic metabolites are suggested to be as the end product of metabolism, rather than intermediates.

Methylation of arsenic by recombinant human wild-type arsenic (+3 oxidation state) methyltransferase and its methionine 287 threonine (M287T) polymorph: Role of glutathione

Arsenic (+3 oxidation state) methyltransferase (AS3MT) is the key enzyme in the pathway for methylation of arsenicals. A common polymorphism in the AS3MT gene that replaces a threonyl residue in position 287 with a methionyl residue (AS3MT/M287T) occurs at a frequency of about 10% among populations worldwide. Here, we compared catalytic properties of recombinant human wild-type (wt) AS3MT and AS3MT/M287T in reaction mixtures containing S-adenosylmethionine, arsenite (iAs(III)) or methylarsonous acid (MAs(III)) as substrates and endogenous or synthetic reductants, including glutathione (GSH), a thioredoxin reductase (TR)/thioredoxin (Trx)/NADPH reducing system, or tris (2-carboxyethyl) phosphine hydrochloride (TCEP). With either TR/Trx/NADPH or TCEP, wtAS3MT or AS3MT/M287T catalyzed conversion of iAs(III) to MAs(III), methylarsonic acid (MAs(V)), dimethylarsinous acid (DMAs(III)), and dimethylarsinic acid (DMAs(V)); MAs(III) was converted to DMAs(III) and DMAs(V). Although neither enzyme required GSH to support methylation of iAs(III) or MAs(III), addition of 1mM GSH decreased K(m) and increased V(max) estimates for either substrate in reaction mixtures containing TR/Trx/NADPH. Without GSH, V(max) and K(m) values were significantly lower for AS3MT/M287T than for wtAS3MT. In the presence of 1mM GSH, significantly more DMAs(III) was produced from iAs(III) in reactions catalyzed by the M287T variant than in wtAS3MT-catalyzed reactions. Thus, 1mM GSH modulates AS3MT activity, increasing both methylation rates and yield of DMAs(III). AS3MT genotype exemplified by differences in regulation of wtAS3MT and AS3MT/M287T-catalyzed reactions by GSH may contribute to differences in the phenotype for arsenic methylation and, ultimately, to differences in the disease susceptibility in individuals chronically exposed to inorganic arsenic.

Genetic variation in glutathione S-transferase omega-1, arsenic methyltransferase and methylene-tetrahydrofolate reductase, arsenic exposure and bladder cancer: a case-control study

BACKGROUND

Ingestion of groundwater with high concentrations of inorganic arsenic has been linked to adverse health outcomes, including bladder cancer, however studies have not consistently observed any elevation in risk at lower concentrations. Genetic variability in the metabolism and clearance of arsenic is an important consideration in any investigation of its potential health risks. Therefore, we examined the association between genes thought to play a role in the metabolism of arsenic and bladder cancer.

METHODS

Single nucleotide polymorphisms (SNPs) in GSTO-1, As3MT and MTHFR were genotyped using DNA from 219 bladder cancer cases and 273 controls participating in a case-control study in Southeastern Michigan and exposed to low to moderate (<50 μg/L) levels of arsenic in their drinking water. A time-weighted measure of arsenic exposure was constructed using measures from household water samples combined with past residential history, geocoded and merged with archived arsenic data predicted from multiple resources.

RESULTS

While no single SNP in As3MT was significantly associated with bladder cancer overall, several SNPs were associated with bladder cancer among those exposed to higher arsenic levels. Individuals with one or more copies of the C allele in rs11191439 (the Met287Thr polymorphism) had an elevated risk of bladder cancer (OR = 1.17; 95% CI = 1.04-1.32 per 1 μg/L increase in average exposure). However, no association was observed between average arsenic exposure and bladder cancer among TT homozygotes in the same SNP. Bladder cancer cases were also 60% less likely to be homozygotes for the A allele in rs1476413 in MTHFR compared to controls (OR = 0.40; 95% CI = 0.18-0.88).

CONCLUSIONS

Variation in As3MT and MTHFR is associated with bladder cancer among those exposed to relatively low concentrations of inorganic arsenic. Further investigation is warranted to confirm these findings.

Genetic variation in glutathione S-transferase omega-1, arsenic methyltransferase and methylene-tetrahydrofolate reductase, arsenic exposure and bladder cancer: a case-control study

BACKGROUND

Ingestion of groundwater with high concentrations of inorganic arsenic has been linked to adverse health outcomes, including bladder cancer, however studies have not consistently observed any elevation in risk at lower concentrations. Genetic variability in the metabolism and clearance of arsenic is an important consideration in any investigation of its potential health risks. Therefore, we examined the association between genes thought to play a role in the metabolism of arsenic and bladder cancer.

METHODS

Single nucleotide polymorphisms (SNPs) in GSTO-1, As3MT and MTHFR were genotyped using DNA from 219 bladder cancer cases and 273 controls participating in a case-control study in Southeastern Michigan and exposed to low to moderate (<50 μg/L) levels of arsenic in their drinking water. A time-weighted measure of arsenic exposure was constructed using measures from household water samples combined with past residential history, geocoded and merged with archived arsenic data predicted from multiple resources.

RESULTS

While no single SNP in As3MT was significantly associated with bladder cancer overall, several SNPs were associated with bladder cancer among those exposed to higher arsenic levels. Individuals with one or more copies of the C allele in rs11191439 (the Met287Thr polymorphism) had an elevated risk of bladder cancer (OR = 1.17; 95% CI = 1.04-1.32 per 1 μg/L increase in average exposure). However, no association was observed between average arsenic exposure and bladder cancer among TT homozygotes in the same SNP. Bladder cancer cases were also 60% less likely to be homozygotes for the A allele in rs1476413 in MTHFR compared to controls (OR = 0.40; 95% CI = 0.18-0.88).

CONCLUSIONS

Variation in As3MT and MTHFR is associated with bladder cancer among those exposed to relatively low concentrations of inorganic arsenic. Further investigation is warranted to confirm these findings.

Genome-wide association study identifies chromosome 10q24.32 variants associated with arsenic metabolism and toxicity phenotypes in Bangladesh

Arsenic contamination of drinking water is a major public health issue in many countries, increasing risk for a wide array of diseases, including cancer. There is inter-individual variation in arsenic metabolism efficiency and susceptibility to arsenic toxicity; however, the basis of this variation is not well understood. Here, we have performed the first genome-wide association study (GWAS) of arsenic-related metabolism and toxicity phenotypes to improve our understanding of the mechanisms by which arsenic affects health. Using data on urinary arsenic metabolite concentrations and approximately 300,000 genome-wide single nucleotide polymorphisms (SNPs) for 1,313 arsenic-exposed Bangladeshi individuals, we identified genome-wide significant association signals (P<5×10⁻⁸) for percentages of both monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) near the AS3MT gene (arsenite methyltransferase; 10q24.32), with five genetic variants showing independent associations. In a follow-up analysis of 1,085 individuals with arsenic-induced premalignant skin lesions (the classical sign of arsenic toxicity) and 1,794 controls, we show that one of these five variants (rs9527) is also associated with skin lesion risk (P = 0.0005). Using a subset of individuals with prospectively measured arsenic (n = 769), we show that rs9527 interacts with arsenic to influence incident skin lesion risk (P = 0.01). Expression quantitative trait locus (eQTL) analyses of genome-wide expression data from 950 individual's lymphocyte RNA suggest that several of our lead SNPs represent cis-eQTLs for AS3MT (P = 10⁻¹²) and neighboring gene C10orf32 (P = 10⁻⁴⁴), which are involved in C10orf32-AS3MT read-through transcription. This is the largest and most comprehensive genomic investigation of arsenic metabolism and toxicity to date, the only GWAS of any arsenic-related trait, and the first study to implicate 10q24.32 variants in both arsenic metabolism and arsenical skin lesion risk. The observed patterns of associations suggest that MMA% and DMA% have distinct genetic determinants and support the hypothesis that DMA is the less toxic of these two methylated arsenic species. These results have potential translational implications for the prevention and treatment of arsenic-associated toxicities worldwide.

Exposure to arsenic through drinking water is a serious public health issue in many countries, including Bangladesh and the United States. Although there is substantial inter-individual variation in arsenic metabolism and toxicity, the biological basis of this variation is not well understood. Here, we have conducted the first genome-wide association study of arsenic-related traits within a unique population cohort of arsenic-exposed Bangladeshi individuals. Using data on 1,313 well-characterized individuals, we identify multiple association signals for urinary arsenic metabolite concentrations in the 10q24.32 regions, near the AS3MT (arsenite methyltransferase) gene. In a subsequent analysis of >2,000 individuals, we show for the first time that variants that influence arsenic metabolism can also influence risk for arsenical skin lesions (the classical sign of arsenic toxicity) through interaction with arsenic exposure. Using array-based genome-wide gene expression data, we show that several of our lead genetic variants are associated with expression of AS3MT and neighboring gene C10orf32, providing a potential mechanism by which 10q24.32 variants influence arsenic metabolism and toxicity. Knowledge of variation in this region and associated biological processes could be used to develop intervention and pharmacological strategies aimed at preventing large numbers of arsenic-related deaths in arsenic-exposed populations.

Arsenic-glutathione conjugate transport by the human multidrug resistance proteins (MRPs/ABCCs)

Millions of people world-wide are chronically exposed to inorganic forms of the environmental toxicant arsenic in drinking water. This has led to a public health crisis because arsenic is a human carcinogen, and causes a myriad of other adverse health effects. In order to prevent and treat arsenic-induced toxicity it is critical to understand the cellular handling of this metalloid. A large body of literature describes the importance of the cellular tripeptide glutathione (γ-Glu-Cys-Gly,GSH/GS) in the excretion of arsenic. The triglutathione conjugate of arsenite [As(III)(GS)(3)] and the diglutathione conjugate of monomethylarsonous acid [MMA(III)(GS)(2)] have been isolated from rat bile and mouse urine, and account for the majority of excreted arsenic, suggesting these are important transportable forms. The ATP-binding cassette (ABC) transporter proteins, multidrug resistance protein 1 (MRP1/ABCC1) and the related protein MRP2 (ABCC2), are thought to play an important role in arsenic detoxification through the cellular efflux of arsenic-GSH conjugates. Current knowledge on the cellular handling of arsenic with a special emphasis on the transport pathways of the arsenic-GSH conjugates As(III)(GS)(3), MMA(III)(GS)(2), and dimethylarsenic glutathione DMA(III)(GS), as well as, the seleno-bis(S-glutathionyl) arsinium ion [(GS)(2)AsSe](-) are reviewed.

Arsenic toxicology: translating between experimental models and human pathology

BACKGROUND

Chronic arsenic exposure is a worldwide health problem. How arsenic exposure promotes a variety of diseases is poorly understood, and specific relationships between experimental and human exposures are not established. We propose phenotypic anchoring as a means to unify experimental observations and disease outcomes.

OBJECTIVES

We examined the use of phenotypic anchors to translate experimental data to human pathology and investigated research needs for which phenotypic anchors need to be developed.

METHODS

During a workshop, we discussed experimental systems investigating arsenic dose/exposure and phenotypic expression relationships and human disease responses to chronic arsenic exposure and identified knowledge gaps. In a literature review, we identified areas where data exist to support phenotypic anchoring of experimental results to pathologies from specific human exposures.

DISCUSSION

Disease outcome is likely dependent on cell-type-specific responses and interaction with individual genetics, other toxicants, and infectious agents. Potential phenotypic anchors include target tissue dosimetry, gene expression and epigenetic profiles, and tissue biomarkers.

CONCLUSIONS

Translation to human populations requires more extensive profiling of human samples along with high-quality dosimetry. Anchoring results by gene expression and epigenetic profiling has great promise for data unification. Genetic predisposition of individuals affects disease outcome. Interactions with infectious agents, particularly viruses, may explain some species-specific differences between human pathologies and experimental animal pathologies. Invertebrate systems amenable to genetic manipulation offer potential for elaborating impacts of specific biochemical pathways. Anchoring experimental results to specific human exposures will accelerate understanding of mechanisms of arsenic-induced human disease.

Monomethylarsenic diglutathione transport by the human multidrug resistance protein 1 (MRP1/ABCC1)

The ATP-binding cassette (ABC) transporter protein multidrug resistance protein 1 (MRP1; ABCC1) plays an important role in the cellular efflux of the high-priority environmental carcinogen arsenic as a triglutathione conjugate [As(GS)(3)]. Most mammalian cells can methylate arsenic to monomethylarsonous acid (MMA(III)), monomethylarsonic acid (MMA(V)), dimethylarsinous acid (DMA(III)), and dimethylarsinic acid (DMA(V)). The trivalent forms MMA(III) and DMA(III) are more reactive and toxic than their inorganic precursors, arsenite (As(III)) and arsenate (As(V)). The ability of MRP1 to transport methylated arsenicals is unknown and was the focus of the current study. HeLa cells expressing MRP1 (HeLa-MRP1) were found to confer a 2.6-fold higher level of resistance to MMA(III) than empty vector control (HeLa-vector) cells, and this resistance was dependent on GSH. In contrast, MRP1 did not confer resistance to DMA(III), MMA(V), or DMA(V). HeLa-MRP1 cells accumulated 4.5-fold less MMA(III) than HeLa-vector cells. Experiments using MRP1-enriched membrane vesicles showed that transport of MMA(III) was GSH-dependent but not supported by the nonreducing GSH analog, ophthalmic acid, suggesting that MMA(III)(GS)(2) was the transported form. MMA(III)(GS)(2) was a high-affinity, high-capacity substrate for MRP1 with apparent K(m) and V(max) values of 11 μM and 11 nmol mg(-1)min(-1), respectively. MMA(III)(GS)(2) transport was osmotically sensitive and inhibited by several MRP1 substrates, including 17β-estradiol 17-(β-D-glucuronide) (E(2)17βG). MMA(III)(GS)(2) competitively inhibited the transport of E(2)17βG with a K(i) value of 16 μM, indicating that these two substrates have overlapping binding sites. These results suggest that MRP1 is an important cellular protective pathway for the highly toxic MMA(III) and have implications for environmental and clinical exposure to arsenic.

Significantly increased risk of carotid atherosclerosis with arsenic exposure and polymorphisms in arsenic metabolism genes

Individual susceptibility to arsenic-induced carotid atherosclerosis might be associated with genetic variations in arsenic metabolism. The purpose of this study is to explore the interaction effect on risk of carotid atherosclerosis between arsenic exposure and risk genotypes of purine nucleoside phosphorylase (PNP), arsenic (+3) methyltransferase (As3MT), and glutathione S-transferase omega 1 (GSTO1) and omega 2 (GSTO2). A community-based case-control study was conducted in northeastern Taiwan to investigate the arsenic metabolic-related genetic susceptibility to carotid atherosclerosis. In total, 863 subjects, who had been genotyped and for whom the severity of carotid atherosclerosis had been determined, were included in the present study. Individual well water was collected and arsenic concentration determined using hydride generation combined with flame atomic absorption spectrometry. The result showed that a significant dose-response trend (P=0.04) of carotid atherosclerosis risk associated with increasing arsenic concentration. Non-significant association between genetic polymorphisms of PNP Gly51Ser, Pro57Pro, As3MT Met287Thr, GSTO1 Ala140Asp, and GSTO2 A-183G and the risk for development of carotid atherosclerosis were observed. However, the significant interaction effect on carotid atherosclerosis risk was found for arsenic exposure (>50μg/l) and the haplotypes of PNP (p=0.0115). A marked elevated risk of carotid atherosclerosis was observed in subjects with arsenic exposure of >50μg/l in drinking water and those who carried the PNP A-T haplotype and at least either of the As3MT risk polymorphism or GSTO risk haplotypes (OR, 6.43; 95% CI, 1.79-23.19). In conclusion, arsenic metabolic genes, PNP, As3MT, and GSTO, may exacerbate the formation of atherosclerosis in individuals with high levels of arsenic concentration in well water (>50μg/l).