Arsenic Abstracts 2

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Structure of an As(III) S-adenosylmethionine methyltransferase: insights into the mechanism of arsenic biotransformation

         (Ajees, Marapakala et al. 2012) Download

Enzymatic methylation of arsenic is a detoxification process in microorganisms but in humans may activate the metalloid to more carcinogenic species. We describe the first structure of an As(III) S-adenosylmethionine methyltransferase by X-ray crystallography that reveals a novel As(III) binding domain. The structure of the methyltransferase from the thermophilic eukaryotic alga Cyanidioschyzon merolae reveals the relationship between the arsenic and S-adenosylmethionine binding sites to a final resolution of approximately 1.6 A. As(III) binding causes little change in conformation, but binding of SAM reorients helix alpha4 and a loop (residues 49-80) toward the As(III) binding domain, positioning the methyl group for transfer to the metalloid. There is no evidence of a reductase domain. These results are consistent with previous suggestions that arsenic remains trivalent during the catalytic cycle. A homology model of human As(III) S-adenosylmethionine methyltransferase with the location of known polymorphisms was constructed. The structure provides insights into the mechanism of substrate binding and catalysis.

A Toxicological Profile for Arsenic

         (ATSDR 2005) Download

Efficient arsenic metabolism--the AS3MT haplotype is associated with DNA methylation and expression of multiple genes around AS3MT

         (Engstrom, Hossain et al. 2013) Download

Arsenic is a very potent toxicant. One major susceptibility factor for arsenic-related toxicity is the efficiency of arsenic metabolism. The efficiency, in turn, is associated with non-coding single nucleotide polymorphisms (SNPs) in the arsenic methyltransferase AS3MT on chromosome 10q24. However, the mechanism of action for these SNPs is not yet clarified. Here, we assessed the influence of genetic variation in AS3MT on DNA methylation and gene expression within 10q24, in people exposed to arsenic in drinking water. DNA was extracted from peripheral blood from women in the Argentinean Andes (N = 103) and from cord blood from new-borns in Bangladesh (N = 127). AS3MT SNPs were analyzed with Sequenom or Taqman assays. Whole genome epigenetic analysis with Infinium HumanMethylation450 BeadChip was performed on bisulphite-treated DNA. Whole genome gene expression analysis was performed with Illumina DirectHyb HumanHT-12 v4.0 on RNA from peripheral blood. Arsenic exposure was assessed by HPLC-ICPMS. In the Argentinean women, the major AS3MT haplotype, associated with more efficient arsenic metabolism, showed increased methylation of AS3MT (p = 10(-6)) and also differential methylation of several other genes within about 800 kilobasepairs: CNNM2 (p<10(-16)), NT5C2 (p<10(-16)), C10orf26 (p = 10(-8)), USMG5 (p = 10(-5)), TRIM8 (p = 10(-4)), and CALHM2 (p = 0.038) (adjusted for multiple comparisons). Similar, but weaker, associations between AS3MT haplotype and DNA methylation in 10q24 were observed in cord blood (Bangladesh). The haplotype-associated altered CpG methylation was correlated with reduced expression of AS3MT and CNNM2 (r(s) = -0.22 to -0.54), and with increased expression of NT5C2 and USMG5 (r(s) = 0.25 to 0.58). Taking other possibly influential variables into account in multivariable linear models did only to a minor extent alter the strength of the associations. In conclusion, the AS3MT haplotype status strongly predicted DNA methylation and gene expression of AS3MT as well as several genes in 10q24. This raises the possibility that several genes in this region are important for arsenic metabolism.

The two opposite facets of arsenic: Toxin and anticancer drug

(Florea and Büsselberg 2013) Download

Arsenic compounds have been known and used for centuries but their effects in living organisms still represent a large unknown. Arsenic compounds have paradoxical effects: they are threatening to human health, especially upon long-term exposure that can induce the development of cancer; however, they are used as drugs against cancer. This review focuses on the effects shown by clinically and environmentally relevant arsenic compounds in living organisms with a focus on the calcium–apoptosis link.


The enigma of arsenic carcinogenesis: role of metabolism

         (Goering, Aposhian et al. 1999) Download

Inorganic arsenic is considered a high-priority hazard, particularly because of its potential to be a human carcinogen. In exposed human populations, arsenic is associated with tumors of the lung, skin, bladder, and liver. While it is known to be a human carcinogen, carcinogenesis in laboratory animals by this metalloid has never been convincingly demonstrated. Therefore, no animal models exist for studying molecular mechanisms of arsenic carcinogenesis. The apparent human sensitivity, combined with our incomplete understanding about mechanisms of carcinogenic action, create important public health concerns and challenges in risk assessment, which could be met by understanding the role of metabolism in arsenic toxicity and carcinogenesis. This symposium summary covers three critical major areas involving arsenic metabolism: its biodiversity, the role of arsenic metabolism in molecular mechanisms of carcinogenesis, and the impact of arsenic metabolism on human risk assessment. In mammals, arsenic is metabolized to mono- and dimethylated species by methyltransferase enzymes in reactions that require S-adenosyl-methionine (SAM) as the methyl donating cofactor. A remarkable species diversity in arsenic methyltransferase activity may account for the wide variability in sensitivity of humans and animals to arsenic toxicity. Arsenic interferes with DNA methyltransferases, resulting in inactivation of tumor suppressor genes through DNA hypermethylation. Other studies suggest that arsenic-induced malignant transformation is linked to DNA hypomethylation subsequent to depletion of SAM, which results in aberrant gene activation, including oncogenes. Urinary profiles of arsenic metabolites may be a valuable tool for assessing human susceptibility to arsenic carcinogenesis. While controversial, the idea that unique arsenic metabolic properties may explain the apparent non-linear threshold response for arsenic carcinogenesis in humans. In order to address these outstanding issues, further efforts are required to identify an appropriate animal model to elucidate carcinogenic mechanisms of action, and to define dose-response relationships.


N-6-adenine-specific DNA methyltransferase 1 (N6AMT1) polymorphisms and arsenic methylation in Andean women

         (Harari, Engstrom et al. 2013) Download

BACKGROUND: In humans, inorganic arsenic is metabolized to methylated metabolites mainly by arsenic (+3 oxidation state) methyltransferase (AS3MT). AS3MT polymorphisms are associated with arsenic metabolism efficiency. Recently, a putative N-6-adenine-specific DNA methyltransferase 1 (N6AMT1) was found to methylate arsenic in vitro. OBJECTIVE: We evaluated the role of N6AMT1 polymorphisms in arsenic methylation efficiency in humans.Methods: We assessed arsenic methylation efficiency in 188 women exposed to arsenic via drinking water (~ 200 microg/L) in the Argentinean Andes by measuring the relative concentrations of arsenic metabolites in urine [inorganic arsenic, methylarsonic acid (MMA), and dimethylarsinic acid] by high-performance liquid chromatography coupled with hydride generation and inductively coupled plasma mass spectrometry. We performed genotyping for N6AMT1 and AS3MT polymorphisms by Taqman assays, and gene expression (in blood; n = 63) with Illumina HumanHT-12 v4.0. RESULTS: Five N6AMT1 single nucleotide polymorphisms (SNPs; rs1997605, rs2205449, rs2705671, rs16983411, and rs1048546) and two N6AMT1 haplotypes were significantly associated with the percentage of MMA (%MMA) in urine, even after adjusting for AS3MT haplotype. %MMA increased monotonically according to the number of alleles for each SNP (e.g., for rs1048546, mean %MMA was 7.5% for GG, 8.8% for GT, and 9.7% for TT carriers). Three SNPs were in linkage disequilibrium (R2 > 0.8). Estimated associations for joint effects of N6AMT1 (haplotype 1) and AS3MT (haplotype 2) were generally consistent with expectations for additive effects of each haplotype on %MMA. Carriers of N6AMT1 genotypes associated with lower %MMA showed the lowest N6AMT1 expression, but associations were monotonic according to copy number for only one genotype and one haplotype. CONCLUSIONS: N6AMT1 polymorphisms were associated with arsenic methylation in Andean women, independent of AS3MT. N6AMT1 polymorphisms may be susceptibility markers for arsenic-related toxic effects.


Arsenic biotransformation as a cancer promoting factor by inducing DNA damage and disruption of repair mechanisms

         (Martinez, Vucic et al. 2011) Download

Chronic exposure to arsenic in drinking water poses a major global health concern. Populations exposed to high concentrations of arsenic-contaminated drinking water suffer serious health consequences, including alarming cancer incidence and death rates. Arsenic is biotransformed through sequential addition of methyl groups, acquired from s-adenosylmethionine (SAM). Metabolism of arsenic generates a variety of genotoxic and cytotoxic species, damaging DNA directly and indirectly, through the generation of reactive oxidative species and induction of DNA adducts, strand breaks and cross links, and inhibition of the DNA repair process itself. Since SAM is the methyl group donor used by DNA methyltransferases to maintain normal epigenetic patterns in all human cells, arsenic is also postulated to affect maintenance of normal DNA methylation patterns, chromatin structure, and genomic stability. The biological processes underlying the cancer promoting factors of arsenic metabolism, related to DNA damage and repair, will be discussed here.

Estimation of Safe and Adequate Daily Intake for Arsenic

         (Uthos 1994) Download


References

Ajees, A. A., K. Marapakala, et al. (2012). "Structure of an As(III) S-adenosylmethionine methyltransferase: insights into the mechanism of arsenic biotransformation." Biochemistry 51(27): 5476-85. [PMID: 22712827]

ATSDR (2005). A Toxicological Profile for Arsenic. P. H. S. U.S. Department of Health and Human Services. Atlanta, GA.

Engstrom, K. S., M. B. Hossain, et al. (2013). "Efficient arsenic metabolism--the AS3MT haplotype is associated with DNA methylation and expression of multiple genes around AS3MT." PLoS One 8(1): e53732. [PMID: 23341986]

Florea, A.-M. and D. Büsselberg (2013). "The two opposite facets of arsenic: Toxin and anticancer drug." J Local Global Health Science 1. [PMID:

Goering, P. L., H. V. Aposhian, et al. (1999). "The enigma of arsenic carcinogenesis: role of metabolism." Toxicol Sci 49(1): 5-14. [PMID: 10367337]

Harari, F., K. Engstrom, et al. (2013). "N-6-adenine-specific DNA methyltransferase 1 (N6AMT1) polymorphisms and arsenic methylation in Andean women." Environ Health Perspect 121(7): 797-803. [PMID: 23665909]

Martinez, V. D., E. A. Vucic, et al. (2011). "Arsenic biotransformation as a cancer promoting factor by inducing DNA damage and disruption of repair mechanisms." Mol Biol Int 2011: 718974. [PMID: 22091411]

Uthos, E. (1994). Estimation of Safe and Adequate Daily Intake for Arsenic. Risk Assessment of Essential Elements. W. Mertz. Washington, DC, ILSI Press.