Elsevier

Science of The Total Environment

Volume 571, 15 November 2016, Pages 801-808
Science of The Total Environment

GPX1 Pro198Leu polymorphism and GSTM1 deletion do not affect selenium and mercury status in mildly exposed Amazonian women in an urban population

https://doi.org/10.1016/j.scitotenv.2016.07.054Get rights and content

Highlights

  • Study of effects of GPX1 and GSTM polymorphisms on mercury levels in an urban female population in the Amazon

  • Analysis of selenium in erythrocytes and mercury in hair, with oxidative stress markers and genotyping of GPX1 and GSTM

  • High prevalence of selenium deficiency despite ready access to selenium-rich produce

  • Low levels of mercury toxicity despite historically high exposures in this region

  • Polymorphisms to GPX1 and GSTM do not appear to influence mercury levels in mildly-exposed populations

Abstract

Mercury is potent toxicant element, but its toxicity can be reduced by forming a complex with selenium for safe excretion. Considering the impact of mercury exposure in the Amazon region and the possible interaction between these two elements, we aimed to assess the effects of Pro198Leu polymorphism to GPX1 and GSTM1 deletion, on mercury levels in a population from Porto Velho, an urban locality in the Brazilian Amazon region. Two hundred women from the capital city of Rondônia state were recruited for this study with 149 deemed suitable to participate. We assessed dietary intake using 24-hour recall. Selenium levels in plasma and erythrocytes were measured using hydride generation quartz tube atomic absorption spectroscopy and total hair mercury using cold vapor atomic absorption spectrometry. Oxidative stress parameters (GPx activity, oxygen radical absorbency capacity [ORAC] and malondialdehyde [MDA]) were also analyzed. All participants were genotyped for Pro198Leu polymorphism and GSTM1 deletion. We observed that this population presented high prevalence of selenium deficiency, and also low levels of mercury, likely due to food habits that did not include selenium-rich food sources or significant consumption of fish (mercury biomagnifiers) regularly. Univariate statistical analysis showed that Pro198Leu and GSTM1 genotypes did not affect selenium and mercury levels in this population. Pro198Leu polymorphism and GSTM1 deletion had no effect on mercury levels in mildly exposed people, suggesting these genetic variants impact mercury levels only in highly exposed populations.

Introduction

Mercury is a potent toxicant even in low doses, having adverse effects on the renal, immune, cardiovascular, reproductive and central nervous systems (Zahir et al., 2005). In utero transfer of mercury (inorganic or as the more toxic methylmercury species) from mother to fetus is of particular concern, as neurotoxic effects are most marked in the developing brain, and there is concern that current exposure advisories do not sufficiently take into account the potential damage of chronic, low-level exposure during pregnancy (Brown and Austin, 2012, Cardenas et al., 2015, Hui et al., 2016). Selenium is involved in mercury detoxification (Yoneda and Suzuki, 1997); therefore it is important to understand the genetic and biochemical factors that influence interactions between selenium and mercury in at-risk populations, such as women of child-bearing age, as this can result negative health outcomes for the following generation.

Amazonian populations have some of the highest reported mercury exposure levels in the world (Passos and Mergler, 2008). While a small fraction of mercury in the environment occurs through natural geological events (i.e. volcanic activity), the majority of health effects arise from either occupational exposure (e.g. mining) or anthropogenic release (Holmes et al., 2009). Mercury retention in soil in the Amazon region is partially due to geographic factors, but is exacerbated by excessive and poorly-regulated gold-mining, soil cultivation methods and industrial contamination (Comte et al., 2013, Fostier et al., 2015, Miretzky et al., 2005, Pouilly et al., 2013). Biomagnification and the reliance on the aquatic environment for food make local fish intake the most relevant source of methylmercury contamination in this area for subsistence farmers (Barcelos et al., 2013, de Oliveira et al., 2014, Mazzaron Barcelos et al., 2012). Methylmercury exerts the most significant negative health effects through chronic neurotoxicity, particularly during development (Grandjean and Landrigan, 2006, Grandjean and Landrigan, 2014).

Concurrently, Amazonian populations are also exposed to high levels of selenium, also though agriculture, as the soil in this area is the most selenium-rich in Brazil (Cintra and Cozzolino, 1993, Favaro et al., 1997). The most concentrated and widely-consumed selenium food source are Brazil nuts (Bertholletia excelsa, H.B.K), which are widely cultivated in the Amazon basin (Rita Cardoso et al., 2016, Souza and Menezes, 2004). Selenium is an essential nutrient, though it has a therapeutic window that can be exceeded via high dietary intake (MacFarquhar et al., 2010, Martens et al., 2015). Concomitant mercury and selenium exposure in areas such as the Amazon are of significant interest, as selenium is able to partially mitigate mercury toxicity in both animals and humans (Falnoga and Tušek-Žnidarič, 2007). The proposed protective mechanism involves the formation of an inert selenium-mercury complex (Naganuma and Imura, 1983, Yoneda and Suzuki, 1997) that is effective against both inorganic and methylmercury intoxication (Whanger, 1992). However, studies have demonstrated that certain polymorphisms to genes encoding the selenoproteome, a collection of 25 identified selenium-containing proteins (Cardoso et al., 2015) may alter response to dietary intake, thus increasing susceptibility to both mercury intoxication and nutritional selenium deficiencies. In regions like the Amazon, where environmental exposure is practically endemic, assessing these previously identified effects in at-risk populations is an important population health concern.

The Pro198Leu polymorphism (rs1050450) to the selenoprotein glutathione peroxidase 1 gene (GPX1) is characterized by a variant T-allele, encoding leucine (Leu) instead of proline (Pro) at codon 198 (Forsberg et al., 1999, Ravn-Haren et al., 2006). This polymorphism has been associated with changes to selenium status and the selenoprotein hierarchy (Combs et al., 2011, Jablonska et al., 2009, Karunasinghe et al., 2012, Xiong et al., 2010), although its effects in response to selenium supplementation remain unclear (Jablonska et al., 2015, Miller et al., 2012, Soares et al., 2016).

Additionally, glutathione-S-transferases (GSTs) are a family of phase II biotransformation enzymes that are also involved in mercury detoxification and elimination by mediating conjugation of mercury and glutathione (GSH), which is then excreted in bile and by the kidneys (Clarkson and Magos, 2006). The human GST family is subdivided into eight separate classes: alpha, mu, kappa, omega, pi, sigma, theta and zeta; which are encoded by the GSTA, GSTM, GSTK, GSTO, GSTP, GSTS, GSTT, and GSTZ genes, respectively (Hayes and Strange, 2000, Josephy, 2010). These genes are highly polymorphic, and deletion of GSTM (GSTM1) and GSTT (GSTT1) are significant as they result in a complete loss of activity of the enzymes, and studies have associated this deficiency with increased cancer risk and mercury retention (Barcelos et al., 2013, Barcelos et al., 2015, de Oliveira et al., 2014, Goodrich et al., 2011, Jain et al., 2006, Klautau-Guimarães et al., 2005), as well as increased maternal blood mercury levels (Lee et al., 2010). However, other reports have failed to find a link between GSTM1 concentration or activity and an outcome-biomarker of mercury retention (Wang et al., 2012), and thus further studies are required to determine the precise role of this deletion polymorphism on mercury detoxification.

Therefore, considering the importance of mercury exposure in the Amazon region and the interaction between this toxic metal and selenium, this pilot study aimed to evaluate the effect of Pro198Leu polymorphism and GSTM1 deletion on mercury levels, taking into account the previously reported role these mutations play in generalized selenium metabolism. In the context of the Amazonian region, where isolated populations are fast becoming juxtaposed with densely populated urban regions, it is valuable to understand the interaction between exposure to this environmental toxicant and individual genetic susceptibility; a necessity highlighted at the National Institutes of Health Gene-Environment Interplay Workshop (Bookman et al., 2011). Because of the established toxic effect on fetus in addition to the mother, we conducted this study in a female cohort from Porto Velho, representing a diverse urban population in Amazon region.

Section snippets

Study participants

This study was conducted according to the Declaration of Helsinki guidelines and approved by Ethics Committee of the Faculty of Pharmaceutical Sciences at the University of São Paulo (protocol number 574). Written informed consent was obtained from all participants. Sample size calculation, based on prevalence of the variant allele of Pro198Leu (32%) estimated that c.a. 200 participants would be required to test the effects of GSTM1 deletion and Pro198Leu variants of GPX1 on biological selenium

General information

The mean age of participants was 26.6 ± 7.8 years. According to BMI classification (World Health Organisation, 2000), 80.5% were eutrophic, 15.4% were overweight and 4% were underweight. Waist circumference was adequate in 88% of the population indicating low risk for cardiovascular diseases. Full population demographics are shown in Table 1.

Dietary assessment suggested the observed selenium deficiency was due to inadequate intake in 42% of the participants. Dietary recalls revealed that the main

Discussion

Environmental exposure to mercury represents one of the major public health concerns related to chemical pollution and because this metal is especially toxic in utero (Hui et al., 2016, Karagas et al., 2012) it is important to evaluate women in a reproductive age from an area of known high mercury exposure (the Amazon basin). Furthermore, studies have demonstrated the role of gene-environment interaction in regard to susceptibility to mercury toxicity (Julvez and Grandjean, 2013, Llop et al.,

Conclusions

For the first time, our study showed that Pro198Leu does not influence mercury levels in an established biomarker of exposure within a mildly exposed population. We also observed that GSTM1 deletion had no effect on mercury levels in mildly exposed people, suggesting that the isolate deletion of GSTM gene has impact only in highly exposed populations.

Acknowledgments

We are extremely grateful to the Alphaclin Lab which provided the locale for sample collection.

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