Involvement of N-6 Adenine-Specific DNA Methyltransferase 1 (N6AMT1) in Arsenic Biomethylation and Its Role in Arsenic-Induced Toxicity

Genes and Environment Laboratory, Division of Environmental Health Sciences, School of Public Health, University of California-Berkeley, Berkeley, California 94720, USA.
Environmental Health Perspectives (Impact Factor: 7.98). 12/2010; 119(6):771-7. DOI: 10.1289/ehp.1002733
Source: PubMed


In humans, inorganic arsenic (iAs) is metabolized to methylated arsenical species in a multistep process mainly mediated by arsenic (+3 oxidation state) methyltransferase (AS3MT). Among these metabolites is monomethylarsonous acid (MMAIII), the most toxic arsenic species. A recent study in As3mt-knockout mice suggests that unidentified methyltransferases could be involved in alternative iAs methylation pathways. We found that yeast deletion mutants lacking MTQ2 were highly resistant to iAs exposure. The human ortholog of the yeast MTQ2 is N-6 adenine-specific DNA methyltransferase 1 (N6AMT1), encoding a putative methyltransferase.
We investigated the potential role of N6AMT1 in arsenic-induced toxicity.
We measured and compared the cytotoxicity induced by arsenicals and their metabolic profiles using inductively coupled plasma-mass spectrometry in UROtsa human urothelial cells with enhanced N6AMT1 expression and UROtsa vector control cells treated with different concentrations of either iAsIII or MMAIII.
N6AMT1 was able to convert MMAIII to the less toxic dimethylarsonic acid (DMA) when overexpressed in UROtsa cells. The enhanced expression of N6AMT1 in UROtsa cells decreased cytotoxicity of both iAsIII and MMAIII. Moreover, N6AMT1 is expressed in many human tissues at variable levels, although at levels lower than those of AS3MT, supporting a potential participation in arsenic metabolism in vivo.
Considering that MMAIII is the most toxic arsenical, our data suggest that N6AMT1 has a significant role in determining susceptibility to arsenic toxicity and carcinogenicity because of its specific activity in methylating MMAIII to DMA and other unknown mechanisms.

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Available from: Martyn Smith, Sep 23, 2014
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    • "Human epidemiology studies have suggested that variability in arsenic biomethylation and the resulting changes of arsenic metabolite profiles, in particular the elevated ratio between mono-and dimethylated metabolites in urine and blood, is associated with an increased susceptibility to skin lesions (Valenzuela et al., 2009) and several types of cancer (Ahsan et al., 2007; Chung et al., 2010; De Chaudhuri et al., 2008; Lin et al., 2006). We recently reported that N-6 adenine-specific DNA methyltransferase 1 (N6AMT1), a putative methyltransferase, was one of the promising methyltransferases involved in arsenic methylation (Ren et al., 2011). We demonstrated that in contrast to As3MT, which methylates iAs to the mono-and dimethylated arsenicals, N6AMT1 appears to be specifically involved in the conversion of MMA III to the less toxic DMA V . "
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    ABSTRACT: In humans, arsenic is primarily metabolized by arsenic (+3 oxidation state) methyltransferase (As3MT) to yield both trivalent and pentavalent methylated metabolites. We recently reported that the putative N-6 adenine specific DNA methyltransferase 1 (N6AMT1) can biotransform monomethylarsonous acid (MMA(III)) to dimethylarsinic acid (DMA(V)), conferring resistance of human cells to arsenic exposure. To further decipher the role of N6AMT1 and its interaction with As3MT in arsenic biomethylation, we examined the relative contribution of N6AMT1 and As3MT in metabolizing arsenic using several newly modified UROtsa human urothelial cells, i.e., UROtsa cells with either a constant level of N6AMT1 or AS3MT in combination with an inducible level of As3MT or N6AMT1, respectively. Our analysis confirmed the involvement of N6AMT1 in MMA(III) biomethylation but not for inorganic arsenic. In a comparable level of N6AMT1 and As3MT, the effect of N6AMT1 mediated MMA(III) biomethylation was obscured by the action of As3MT. Furthermore, we showed that the levels of N6AMT1 and As3MT proteins were varied among and within human normal and cancerous tissues. Overall, the data showed that N6AMT1 has a role in MMA(III) biomethylation, but its effect is relatively minor and limited compared to As3MT. In addition, the varied levels and distributions of N6AMT1 and As3MT among human tissues may potentially contribute to the tissue specificity and susceptibility to arsenic toxicity and carcinogenicity. © The Author 2015. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For permissions, please email:
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    • "Arsenic N6AMT1 (N-6 adenine-specific DNA methyltransferase 1, putative) and MYST1 (MYST histone acetyltransferase 1) are genes that play an important role in arsenic methylation and histone acetylation, respectively. Results from studies in yeast [Jo et al., 2009a,b] and validation in mammalian cells [Ren et al., 2011] suggested that N6AMT1 and MYST1 are candidate human susceptibility genes involved in arsenic toxicity. Studies are undergoing for investigating the biochemical roles of these two proteins in in vitro studies and examining the role of N6AMT1 polymorphisms in susceptibility to lung cancer associated with arsenic exposure in DNA samples from a case control study conducted in Chile. "
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    • "Likewise, a functional screen in yeast identified mitochondrial translation inhibition as the lethality mechanism of the antimicrobial and antileukemic compound tigecycline, and this activity was confirmed in leukemic cells (Skrtić et al., 2011). Finally, Jo et al. (2009a) used yeast to show that a S-adenosylmethionine dependent methyltransferase conferred resistance to various arsenic species, while Ren et al. (2011) showed the corresponding gene in humans (N6AMT1) could metabolize arsenic in human urothelial cells. Harari et al. (2013) expanded upon these studies by demonstrating that N6AMT1 polymorphisms were associated with arsenic methylation in Andean women, and posited that the polymorphisms could be used as susceptibility markers for arsenic toxicity. "
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