Janaina L S Donadio

Graziela B S Duarte

Patrick Borel

Silvia M F Cozzolino

Marcelo M Rogero

The influence of nutrigenetics on biomarkers of selenium nutritional status

Keywords: single nucleotide polymorphisms, selenoproteins, glutathione peroxidase, selenoprotein P

Selenium (Se) is an essential micronutrient for human biology that executes its functions as the amino acid selenocysteine via selenoproteins, which have important functions, such as, antioxidant, immunomodulatory, thyroid metabolism and human fertility. Se nutritional status is assessed using the quantification of blood Se biomarkers influenced by several factors, including diet, age, gender, smoking status, alcohol consumption, health condition and genetic characteristics of individuals.

Nutrigenetic studies have identified single nucleotide polymorphisms (SNPs) in selenoproteins that might clarify the high variability in values reported for biomarkers of Se nutritional status in different populations and the response of these biomarkers to Se supplementation with either organic or inorganic forms of Se. This review aims to (1) define the basic aspects of Se biology, (2) describe the current most commonly used biomarkers of Se nutritional status and, (3) provide a state of the art of associations observed between functional SNPs in selenoproteins and biomarkers of Se status in healthy populations.

INTRODUCTION

The essentiality of the trace element selenium (Se) to human health was first reported in 1978 when its presence was discovered in the antioxidant erythrocyte enzyme glutathione peroxidase. [START_REF] Forstrom | Identification of the Catalytic Site of Rat Liver Glutathione Peroxidase as Selenocysteine[END_REF] Previous reviews on this topic have described this discovery in detail. [START_REF] Rayman | A Randomized Trial of Selenium Supplementation and Risk of Type-2 Diabetes, as Assessed by Plasma Adiponectin[END_REF][START_REF] Fairweather-Tait | Selenium in Human Health and Disease[END_REF][START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] After recognizing its important functions for human health, several studies have aimed to monitor the Se nutritional status of populations worldwide. [START_REF] Fairweather-Tait | Selenium in Human Health and Disease[END_REF][START_REF] Stoffaneller | A review of dietary selenium intake and selenium status in Europe and the Middle East[END_REF][START_REF] Navarro-Alarcon | Selenium in food and the human body: A review[END_REF] However, the controversial data of epidemiological and intervention studies have demonstrated not only differences in the Se intake of populations from different regions, but also different individual responses. In this context, the influence of genetic variations in genes related to Se metabolism must be considered to understand this scenario. [START_REF] Méplan | Selenium and chronic diseases: A nutritional genomics perspective[END_REF] Following the conclusion of the Human Genome Project (HGP) and the advances in technology, numerous genetic variations in the human DNA sequence have been described. [START_REF] Venter | The sequence of the human genome[END_REF] Concerning Se metabolism, the identification of several functional single nucleotide polymorphisms (SNPs) in selenoproteins has raised the hypothesis that these SNPs could modify the biomarkers of Se status and modulate its beneficial effect on the development of chronic diseases. [START_REF] Méplan | Selenium and chronic diseases: A nutritional genomics perspective[END_REF] In this context, the nutrigenetic research field provides a better understanding of selenoprotein metabolism and its biological function in humans, which could help researchers to understand the data on Se status in different populations and the variable interindividual response to Se dietary interventions. Investigating how the presence of SNPs in selenoprotein genes modifies the biomarkers of Se status is crucial for a better assessment of Se nutritional status and a future revision of Se dietary recommendations according to the genetic characteristics of different populations or individuals.

This review aims to (1) define the basic aspects of Se metabolism, function, food sources, and nutritional recommendations; (2) describe the current most commonly used biomarkers of Se nutritional status; and (3) provide a state of the art of associations observed between functional SNPs in selenoproteins and biomarkers of Se nutritional status in healthy populations.

METHODS

The search for articles was conducted using the PubMed database available at www.ncbi.nlm.nih.gov/pubmed, and the terms "selenium", "nutrigenetics" and the specific SNPs "pro198leu," "rs1050450," "rs713041," "rs3877899," "rs7579," "rs5845," and "rs34713741." These SNPs were selected based on the literature search. The inclusion criteria were as follows: studies conducted with healthy populations, both genders or only males or females, an evaluation of one or more SNPs in selenoprotein genes, and a quantification of biomarkers of Se nutritional status. The studies included were observational, longitudinal, and randomized clinical trials.

Those conducted with a population with chronic diseases such as cancer, cardiovascular disease, and diabetes and without the quantification of biomarkers of Se nutritional status were excluded.

BASIC ASPECTS OF SE BIOLOGY: FUNCTION, FOOD SOURCES, AND DIETARY RECOMMENDATIONS

Se is an essential trace element for human health with its biological role directly related to the functions of selenoproteins and Se metabolites. Unlike other minerals that interact with proteins as cofactors, Se is inserted into mammalian selenoproteins as the 21 st amino acid, selenocysteine (Sec). In mammals, 25 selenoprotein genes have been identified, while only half of them have been functionally characterized. [START_REF] Papp | From selenium to selenoproteins: synthesis, identity, and their role in human health[END_REF][START_REF] Brigelius-Flohé | Selenium and redox signaling[END_REF] Sec is generally located in the active site of the enzymes by using a mechanism that involves the recoding of the stop codon UGA during translation. The incorporation of Sec into proteins occurs in the 3´UTR in the Sec incorporation sequence (SECIS) region. [START_REF] Kryukov | Characterization of mammalian selenoproteomes[END_REF] Most selenoproteins are involved in biological processes that concern the control of the redox state and antioxidant function. [START_REF] Brigelius-Flohé | Selenium and redox signaling[END_REF][START_REF] Fernandes | Selenium at the redox interface of the genome, metabolome and exposome[END_REF] One of the well established functions of selenoproteins is the redox activity mainly attributed to five members of the glutathione peroxidase (GPx) family and three isoforms of thioredoxin reductases (TXNRD) and deiodinases (DIO). They act in the reduction of hydrogen peroxide (H 2 O 2 ) and phospholipid hydroperoxides, decreasing the overall oxidative damage to cell membranes, biomolecules, DNA, and mitochondria. [START_REF] Brigelius-Flohé | Selenium and redox signaling[END_REF] Se is also important for modulating the inflammatory response since it attenuates the activation of the nuclear factor (NF)-kB pathway; [START_REF] Kretz-Remy | Selenium: a key element that controls NFkappa B activation and I kappa B alpha half life[END_REF][START_REF] Duntas | Selenium and inflammation: underlying anti-inflammatory mechanisms[END_REF] protection against toxic heavy metals, such as mercury and methyl mercury; [START_REF] Rayman | Selenium and human health[END_REF] and thyroid function due to the action of deiodinases in the conversion of T 4 into its active form T 3 . [START_REF] Brown | Selenium, selenoproteins and human health: a review[END_REF][START_REF] Beckett | Selenium and endocrine systems[END_REF] Moreover, Se is critical for neurological function as it protects the brain against oxidative damage, [START_REF] Steinbrenner | Selenium homeostasis and antioxidant selenoproteins in brain: Implications for disorders in the central nervous system[END_REF][START_REF] Cardoso | Effects of Brazil nut consumption on selenium status and cognitive performance in older adults with mild cognitive impairment: a randomized controlled pilot trial[END_REF] as well as male fertility and reproduction through the role of GPX4 as an antioxidant in the early phase of spermatogenesis, which guarantees the functional and structural integrity of spermatozoa. [START_REF] Foresta | Male fertility is linked to the selenoprotein phospholipid hydroperoxide glutathione peroxidase[END_REF][START_REF] Brigelius-Flohé | Glutathione peroxidases[END_REF] In the later phase, GPX4 located in the mitochondria is essential to the sperm capsule formation [START_REF] Brigelius-Flohé | Glutathione peroxidases[END_REF] .

In foods, Se can be presented as either organic (Sec and selenomethionine

[SeMet], Se-methyl-selenocysteine [SeMCys], and glutamyl-Se-methylselenocysteine) or inorganic forms (selenate). The inorganic form selenite is found in supplements. In a normal physiological condition with an adequate intake, most organic and inorganic forms are well absorbed (~70 -90%), except for selenite, which has an absorption no greater than 60%. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] Se concentration in foods varies worldwide according to Se content and bioavailability in the soil and the capacity of plants to accumulate Se. [START_REF] Rayman | Food-chain selenium and human health: emphasis on intake[END_REF][START_REF] Lemire | Elevated levels of selenium in the typical diet of Amazonian riverside populations[END_REF] This geographical pattern of variation has a direct impact on the Se nutritional status of global populations, with the lowest values found in Eastern Europe, and the highest in Venezuela, the United States and Canada.

China has regions of deficiency and toxicity. [START_REF] Rayman | Selenium and human health[END_REF][START_REF] Combs | Biomarkers of Selenium Status[END_REF] While seafood, meat, grains, eggs, and cereals are excellent sources of Se, Brazil nuts have the highest concentration. [START_REF] Navarro-Alarcon | Selenium in food and the human body: A review[END_REF] The Brazil nut tree (Bertholletia excelsa), a native species from South America, is found mainly in the Brazilian Amazon region, which has a Se-rich soil that influences the high Se content in the nuts and the Se status of the population. [START_REF] Lemire | Elevated levels of selenium in the typical diet of Amazonian riverside populations[END_REF][START_REF] Cardoso | Brazil nuts: Nutritional composition, health benefits and safety aspects[END_REF] Nevertheless, for this population, the higher intake that reflects plasma Se concentrations that could reach 900 µg/L, with a median of 135 µg/L, is not toxic. [START_REF] Lemire | No evidence of selenosis from a selenium-rich diet in the Brazilian Amazon[END_REF] Some factors that could explain the lack of selenosis in this population are as follows: (1) the population might have adapted to a higher than average Se status through metabolic mechanisms; (2) the high Se content might be protective against Hg exposure since plasma selenoprotein P can bind Hg and reduce its bioavailability to target proteins 26 ; (3) this population might have a specific gut microbiota profile due to their exposure to high Se that helps excrete the excess Se more rapidly 27 ; (4) they might have polymorphisms in genes that regulate Se metabolism and excretion. [START_REF] Rayman | Selenium intake, status, and health: a complex relationship[END_REF] The Brazil nut tree is considered a secondary accumulator tree that can accumulate about 100 to 1,000 mg/Se/g -1 of dry weight in their seeds. The organic forms of Se (C-Se-C species) are predominantly present in this nut, which includes SeMet (the main Se compound), SeMCys, and Se-lanthionine. [START_REF] Lima | Selenium accumulation, speciation and localization in Brazil nuts (Bertholletia excelsa H.B.K.)[END_REF] However, the Se content in nuts varies according to the region of the Amazon rainforest where the Brazil nut tree was planted. [START_REF] Junior | Natural variation of selenium in Brazil nuts and soils from the Amazon region[END_REF] Studies have shown the Se concentration of one Brazil nut from the Brazilian Amazon rainforest can vary from 290 to 1,261 g. [START_REF] Cardoso | Effects of Brazil nut consumption on selenium status and cognitive performance in older adults with mild cognitive impairment: a randomized controlled pilot trial[END_REF][START_REF] Cominetti | Brazilian nut consumption improves selenium status and glutathione peroxidase activity and reduces atherogenic risk in obese women[END_REF][START_REF] Stockler-Pinto | ) improves oxidative stress and inflammation biomarkers in hemodialysis patients[END_REF][START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF][START_REF] Duarte | Consumption of Brazil nuts with high selenium levels increased inflammation biomarkers in obese women: a randomized controlled trial[END_REF] The dietary intake recommendation values proposed by the Food and Nutrition Board of the Institute of Medicine (IOM) were based on the intake needed to maximize plasma GPX3 activity. [START_REF]Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids[END_REF] The values of the estimated average requirement (EAR), the recommended dietary allowance (RDA), tolerable upper intake level (UL), no observed adverse effect level (NOAEL), and lowest observed adverse effect level (LOAEL) for children, adults, pregnancy, and lactation are summarized in Table 1.

BIOMARKERS OF SE NUTRITIONAL STATUS

The evaluation of Se nutritional status remains a matter of debate since the distribution of this mineral into selenoproteins depends on several factors, such as the hierarchy system of incorporation, bioavailability from food sources, dietary intake, health state, and the presence of genetic polymorphisms in selenoproteins. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] Overall, Se status involves Se dietary intake, Se content in tissues, Se function, and excretion. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] The current most commonly used biomarkers are 1) total Se concentration in whole blood, plasma, serum, erythrocytes, or urine; 2) GPx activity in plasma (GPx3), erythrocytes (GPx1), and whole blood (total GPx); and 3) plasma selenoprotein P (SELENOP) levels. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] A reliable marker of Se intake must to be sensitive to changes in Se status and reflect the current intake through food. It should be noted that plasma Se levels can be used to predict Se intake if SeMet is mostly consumed by using a linear regression equation (Se in, μg/kg0.75/day = 0.44 + 0.03 × Se plasma, ng/mL ). [START_REF] Combs | Biomarkers of Selenium Status[END_REF] Food questionnaires can also measure Se intake; nevertheless, the available food composition tables do not address the huge geographical variation of Se content in soil.

After absorption, Se can be found in tissues, such as whole blood, erythrocyte, and plasma. In humans, whole blood Se is not the best biomarker option because it does not reflect its biological function, executed through selenoproteins activity. The evaluation of specific selenoproteins can thus provide a more precise diagnostic of Se status. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF][START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF] Erythrocyte Se represents a long-term evaluation of Se status since the half-life of these cells is about 120 days. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] Several factors, such as geographical location, gender, age, race, BMI, smoking status and alcohol consumption, affect plasma Se concentrations. Studies have found a reduction in plasma Se concentration with age, [START_REF] Lloyd | Effect of smoking, alcohol, and other factors on the selenium status of a healthy population[END_REF] as well as in protein malnutrition, [START_REF] Mathias | Selenium deficiency in kwashiorkor[END_REF] chronic inflammation, [START_REF] Maehira | Alterations of serum selenium concentrations in the acute phase of pathological conditions[END_REF] in smokers, [START_REF] Arnaud | Serum selenium determinants in French adults: the SU.VI.M.AX study[END_REF][START_REF] Karunasinghe | Serum selenium and singlenucleotide polymorphisms in genes for selenoproteins: Relationship to markers of oxidative stress in men from Auckland, New Zealand[END_REF] in obese individuals, [START_REF] Arnaud | Serum selenium determinants in French adults: the SU.VI.M.AX study[END_REF][START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF] in African Americans, [START_REF] Hargreaves | Plasma selenium biomarkers in low income black and white americans from the southeastern United States[END_REF] and in daily alcohol consumers. [START_REF] Lloyd | Effect of smoking, alcohol, and other factors on the selenium status of a healthy population[END_REF] Red wine consumers had higher than average plasma Se levels in the SU.VI.MAX study conducted in France. [START_REF] Arnaud | Serum selenium determinants in French adults: the SU.VI.M.AX study[END_REF] The measurement of selenoproteins activity, involved mostly with redox control, is the best option when evaluating Se status because this biomarker reflects Se function. It is important to note that Se have biological effects that do not occur through the selenoproteins. [START_REF] Christensen | Selenium and prostate cancer prevention: What next -If anything?[END_REF] In a xenograft model of prostate cancer, Semethylselenocysteine and methylseleninic acid inhibited tumor growth while selenite and SeMet did not. [START_REF] Li | Superior in vivo inhibitory efficacy of methylseleninic acid against human prostate cancer over selenomethionine or selenite[END_REF] The direct biomarkers are GPx1 and GPx3 activities and plasma SELENOP concentration. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] GPx1 activity is a suitable biomarker in cases of low Se status (plasma concentration < 60 µg/L) since the activity of this enzyme cannot reach its maximal level in that range of Se concentration and can be assessed in erythrocytes using spectrophotometry. [START_REF] Paglia | Studies on the quantitative and qualitative characterization of erythrocyte glutathione peroxidase[END_REF] This biomarker is also effective and sensitive for evaluating Se supplementation due to its rapid response observed in a period of one to two weeks, depending on the baseline Se status and the chemical form of Se administrated. [START_REF] Combs | Biomarkers of Selenium Status[END_REF][START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF] The response of the biomarkers to different chemical forms of Se has been reviewed extensively elsewhere and is not addressed in this article. [START_REF] Meltzer | The form of selenium determines the response to supplementation in a selenium replete population[END_REF][START_REF] Persson-Moschos | Plasma selenoprotein P levels of healthy males in different selenium status after oral supplementation with different forms of selenium[END_REF][START_REF] Burk | Effects of chemical form of selenium on plasma biomarkers in a high-dose human supplementation trial[END_REF][START_REF] Xia | Effectiveness of selenium supplements in a low selenium area of China[END_REF][START_REF] Ravn-Haren | A short-term intervention trial with selenate, selenium-enriched yeast and selenium-enriched milk: effects on oxidative defence regulation[END_REF][START_REF] Rayman | Food-chain selenium and human health: spotlight on speciation[END_REF][START_REF] Hurst | Establishing optimal selenium status : results of a randomized , double-blind, placebo-controlled trial[END_REF] GPx1 activity has been found to vary with gender and age, being higher in females than males [START_REF] Rush | Plasma glutathione peroxidase in healthy young adults: influence of gender and physical activity[END_REF][START_REF] Donadio | Influence of gender and SNPs in GPX1 gene on biomarkers of selenium status in healthy Brazilians[END_REF][START_REF] Massafra | Gender related differences in erythrocyte glutathione peroxidase activity in healthy subjects[END_REF][START_REF] Malling | Sex determines the influence of smoking and gene polymorphism on glutathione peroxidase activity in erythrocytes[END_REF][START_REF] Habif | Age and gender dependent alterations in the activities of glutathione related enzymes in healthy subjects[END_REF] and females under 55 years than males and females over 55 years. [START_REF] Blankenberg | Glutathione peroxidase 1 activity and cardiovascular events in patients with coronary artery disease[END_REF] GPx1 activity was reduced in smokers, [START_REF] Malling | Sex determines the influence of smoking and gene polymorphism on glutathione peroxidase activity in erythrocytes[END_REF][START_REF] Blankenberg | Glutathione peroxidase 1 activity and cardiovascular events in patients with coronary artery disease[END_REF][START_REF] Ravn-Haren | Associations between GPX1 Pro198Leu polymorphism, erythrocyte GPX activity, alcohol consumption and breast cancer risk in a prospective cohort study[END_REF] in patients with cardiovascular events [START_REF] Blankenberg | Glutathione peroxidase 1 activity and cardiovascular events in patients with coronary artery disease[END_REF] and in patients with asthma. [START_REF] Nadeem | Increased oxidative stress and altered levels of antioxidants in asthma[END_REF] GPx3 is an extracellular isoform that the kidney produces by and is responsible for around 10 -25% of total plasma Se. The measurements of plasma GPx3 activity and/or concentration can be both used as Se biomarkers and they usually correlate with total plasma Se concentration. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF][START_REF] Combs | Biomarkers of Selenium Status[END_REF] Regarding the two GPx activities, it is essential to highlight that they reach a maximal activity at different plasma Se concentrations: GPx1 around 80-120 µg Se/L 61 and GPx3 around 70-90 µg Se/L. [START_REF] Hurst | EURRECA-Estimating selenium requirements for deriving dietary reference values[END_REF] In this context, both activities are useful biomarkers of Se status to assess a nutritional deficiency. GPx3 activity varies with age and gender, being lower in adults between the ages of 40 and 49 years and obese females compared to adults of other ages and obese males. [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF] SELENOP acts as the primary Se transporter to peripheral tissues, being the only selenoprotein that can contain as many as 10 residues of Sec and comprises 40-60% of total plasma Se. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] While the measurement of plasma SELENOP concentration is considered the most conclusive biomarker of Se status, no reference values for it exist. [START_REF] Combs | Biomarkers of Selenium Status[END_REF][START_REF] Burk | Selenoprotein P-Expression, functions, and roles in mammals[END_REF][START_REF] Xia | Optimization of selenoprotein P and other plasma selenium biomarkers for the assessment of the selenium nutritional requirement: A placebo-controlled, double-blind study of selenomethionine supplementation in selenium-deficient Chinese subjects[END_REF] SELENOP reaches a plateau when plasma Se concentration is around 120 µg Se/L. [START_REF] Hurst | Establishing optimal selenium status : results of a randomized , double-blind, placebo-controlled trial[END_REF][START_REF] Kipp | Revised reference values for selenium intake[END_REF] Factors such as ethnicity and BMI affected SELENOP concentrations, for example, SELENOP is reduced in African-Americans [START_REF] Hargreaves | Plasma selenium biomarkers in low income black and white americans from the southeastern United States[END_REF] and obese adults. [START_REF] Combs | Determinants of selenium status in healthy adults[END_REF] Urinary Se is a marker of Se excretion associated with plasma Se concentration and Se intake. Results from balance studies confirm that urinary Se represents 50-60% of the total amount excreted; Se intake can therefore be estimated as twice the urinary Se. [START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF] Hair and toenails are tissues that accumulate Se, being markers of Se retention, and are thus more useful as markers of longer exposure over the past six to 12 months. [START_REF] Navarro-Alarcon | Selenium in food and the human body: A review[END_REF][START_REF] Hurst | EURRECA-Estimating selenium requirements for deriving dietary reference values[END_REF][START_REF] Filippini | Toenail selenium as an indicator of environmental exposure: A cross-sectional study[END_REF] Nevertheless, hair samples must be analyzed carefully due to possible contamination from by chemical products, such as Se-containing shampoos. [START_REF] Navarro-Alarcon | Selenium in food and the human body: A review[END_REF][START_REF] Combs | Biomarkers of Selenium Status[END_REF]

THE CHOICE OF SE BIOMARKERS DEPENDS ON THE SE STATUS OF THE

POPULATION

The choice of a specific biomarker to evaluate Se status must consider several factors, such as the health status of the individual, the specific biological function of selenoproteins, and the genetic polymorphisms. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] The use of different biomarkers is an adequate approach because it provides information related to Se nutritional status and the functionality of specific selenoproteins. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] An ideal measurement of Se status must reflect the amount available for the functional activity of selenoproteins. [START_REF] Thomson | Assessment of requirements for selenium and adequacy of selenium status: a review[END_REF] For instance, in populations with low plasma Se concentrations (< 60 µg/L), more options of biomarkers are available because selenoproteins do not reach their maximal activity. In this case, while plasma Se is an adequate option, so is erythrocyte GPx1 activity, plasma GPx3 activity, and plasma SELENOP concentration. [START_REF] Elsom | Functional markers of selenium status: UK Food Standards Agency workshop report[END_REF] In individuals with adequate Se status and intake, the increase in Se biomarkers does not depend on chemical forms of Se since almost all inorganic and organic forms, including selenite in the presence of reduced glutathione (GSH), are rapidly absorved. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF] SeMet appears to be more bioavailable because it is being incorporated inespecifically into plasma proteins such as albumin. The raise in plasma Se due to the increase in SeMet is directly correlated with the increase in Se in albumin. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] However, SeMet can raise plasma Se and SELENOP concentrations even in those individuals with adequate Se status. [START_REF] Combs | Biomarkers of Selenium Status[END_REF][START_REF] Burk | Effects of chemical form of selenium on plasma biomarkers in a high-dose human supplementation trial[END_REF][START_REF] Hurst | Establishing optimal selenium status : results of a randomized , double-blind, placebo-controlled trial[END_REF][START_REF] Combs | Differential responses to selenomethionine supplementation by sex and genotype in healthy adults[END_REF] The increase in those two biomarkers is possible since its absorption occurs through a transcellular pathway that transporters mediate and the elimination of inorganic species is increased, while SeMet is retained and incorporated in an unspecific way into other proteins, such as serum albumin and hemoglobin. SeMCys can also be absorbed through the same process that SeMet contributes to the increased plasma Se. [START_REF] Roman | Selenium biochemistry and its role for human health[END_REF][START_REF] Müller | Functional Biomarkers for the Selenium Status in a Human Nutritional Intervention Study[END_REF] The consumption of Se as SeMet in individuals with adequate Se status can therefore be estimated using plasma Se concentrations as a biomarker. [START_REF] Combs | Biomarkers of Selenium Status[END_REF][START_REF] Combs | Differential responses to selenomethionine supplementation by sex and genotype in healthy adults[END_REF] As to populations with high Se exposure, the indicated biomarker is plasma Se concentration because all selenoproteins reach their plateau with a plasma Se concentration of 150 µg/L. The supplementation with 1,600 µg and 3,200 µg of Se yeast of individuals with baseline plasma Se concentration of 135 and 129 µg/L, respectively, was able to increase this biomarker. [START_REF] Reid | A report of high-dose selenium supplementation: Response and toxicities[END_REF]

FUNCTIONAL SINGLE NUCLEOTIDE POLYMORPHISMS AFFECTING BIOMARKERS OF SE NUTRITIONAL STATUS

Evidence of the functional consequences of some single nucleotide polymorphisms in selenoproteins

After the conclusion of the HGP, several genetic variations were discovered.

Within the five human DNA samples used for the entire HGP, more than 3 million SNPs with an average density of 1/1,250 bp were discovered. [START_REF] Venter | The sequence of the human genome[END_REF] Other initiatives to map the entire diversity of the human genetic variation includes the International Hapmap Project [START_REF]The International HapMap Project[END_REF] and the 1000 Genomes Project Consortium. [START_REF] Auton | A global reference for human genetic variation[END_REF] The latter observed more than 85 million SNPs in a sample of 2,504 subjects with different ethnic backgrounds. [START_REF] Auton | A global reference for human genetic variation[END_REF] Different types of polymorphisms occur the human genome, including SNPs, short insertions/deletions (indels), and structural variants. By definition, a polymorphic locus in a chromosome is one in which the most common variant occurs in less than 99% of the population; in terms of the variant allele, the rarer allele must occur in more than 1% of the population. [START_REF] Schork | Single nucleotide polymorphisms and the future of genetic epidemiology[END_REF][START_REF] Kauwell | Emerging concepts in nutrigenomics: a preview of what is to come[END_REF] The substitution of a single nucleotide in the DNA sequence is the simplest and most common type of polymorphism, the SNP. In the nutritional genomics field, the most studied type of genetic variation is the SNP. Polymorphisms located in intron were thought not to affect the structure of the final protein; however, this concept has changed because if the variation is located in an intron close to the splicing junction, it will change the entire open reading frame and, as a consequence, the entire sequence of aminoacids of the protein after the SNP. [START_REF] Corella | Basic Concepts in Molecular Biology Related to Genetics and Epigenetics[END_REF] It is worth noting that most SNPs have no biological effect for several reasons: they could be in exons and do not change the amino acid because the genetic code is degenerated, or even if they change the amino acid, this modification is not essential for the structure, stability, and function of the protein. [START_REF] Corella | Basic Concepts in Molecular Biology Related to Genetics and Epigenetics[END_REF][START_REF] Hesketh | Nutrigenomics and selenium: gene expression patterns, physiological targets, and genetics[END_REF] Concerning genes that encode selenoproteins, the great challenge is therefore to discover SNPs that, alone or interacting with other SNPs, affect the expression or the activity of these proteins that modulate Se nutritional status. [START_REF] Hesketh | Nutrigenomics and selenium: gene expression patterns, physiological targets, and genetics[END_REF] Single nucleotide polymorphisms in at least 10 selenoprotein genes are believed to be associated with the variation of several biomarkers of Se status. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] One of the most studied SNPs in selenoprotein genes is the Pro198Leu in the GPX1 gene (rs1050450). It is a C > T substitution located in the coding region of the gene that changes the amino acid proline to leucine in position 198 of the protein. [START_REF] Moscow | Loss of heterozygosity of the human cytosolic glutathione peroxidase I gene in lung cancer[END_REF] Previous studies have demonstrated that the presence of the rare allele T was associated with a reduction in GPx1 activity [START_REF] Ravn-Haren | Associations between GPX1 Pro198Leu polymorphism, erythrocyte GPX activity, alcohol consumption and breast cancer risk in a prospective cohort study[END_REF][START_REF] Ratnasinghe | Glutathione peroxidase codon 198 polymorphism variant increases lung cancer risk[END_REF][START_REF] Jablonska | Lipid peroxidation and glutathione peroxidase activity relationship in breast cancer depends on functional polymorphism of GPX1[END_REF][START_REF] Hansen | GPX1 Pro198Leu polymorphism, erythrocyte GPX activity, interaction with alcohol consumption and smoking, and risk of colorectal cancer[END_REF][START_REF] Cardoso | Pro198Leu polymorphism affects the selenium status and GPx activity in response to Brazil nut intake[END_REF][START_REF] Bastaki | Genotype-activity relationship for Mn-superoxide dismutase, glutathione peroxidase 1 and catalase in humans[END_REF][START_REF] Cominetti | Associations between glutathione peroxidase-1 Pro198Leu polymorphism, selenium status, and DNA damage levels in obese women after consumption of Brazil nuts[END_REF] and with a reduction in GPX1 mRNA expression. [START_REF] Cardoso | Pro198Leu polymorphism affects the selenium status and GPx activity in response to Brazil nut intake[END_REF][START_REF] Hamanishi | Functional variants in the glutathione peroxidase-1 (GPx-1) gene are associated with increased intima-media thickness of carotid arteries and risk of macrovascular diseases in Japanese type 2 diabetic patients[END_REF][START_REF] Donadio | Influence of genetic variations in selenoprotein genes on the pattern of gene expression after supplementation with Brazil nuts[END_REF] It is hypothesized that the change of the amino acid proline to leucine alters the secondary and tertiary structures of the protein, affecting its activity and stability. [START_REF] Ratnasinghe | Glutathione peroxidase codon 198 polymorphism variant increases lung cancer risk[END_REF] Indeed, in vitro studies have confirmed a reduction in the thermostability of the enzyme-containing the leu variant. [START_REF] Zhuo | Molecular consequences of genetic variations in the glutathione peroxidase 1 selenoenzyme[END_REF] GPx4 is the only GPx isoform that can reduce phospholipid hydroperoxides in the cell membrane. [START_REF] Bellinger | Regulation and function of selenoproteins in human disease[END_REF] A substitution C > T is located in the 3'UTR (rs713041), the mRNA region important for Sec insertion. Experimental evidence, both in vitro and in vivo, has demonstrated that this SNP has functional consequences. Levels of lymphocyte lipoxygenase products were higher in individuals with the CC genotype than in those with the TT genotype, and the C allele was associated with a higher risk for colorectal cancer compared to the T allele. [START_REF] Villette | A novel single nucleotide polymorphism in the 3' untranslated region of human glutathione peroxidase 4 influences lipoxygenase metabolism[END_REF] In vitro studies have revealed that GPX4 transcripts that contain the C allele indcate a better ability to bind proteins during translation, competing strongly with not only the transcripts that contain the T allele but also the GPX1 transcripts. Under a limitation of Se supply, this better ability to bind proteins means that GPX4 with the C allele have a preference for protein synthesis. [START_REF] Méplan | Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex[END_REF] In human endothelial cells, the T allele was associated with higher than average levels of lipid hydroperoxides and was also more susceptible to oxidative stress. [START_REF] Crosley | The single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene influences endothelial cell function: Interaction with selenium and fatty acids[END_REF] In addition, the rare T allele was associated with a higher risk for breast cancer [START_REF] Udler | Common germline genetic variation in antioxidant defense genes and survival after diagnosis of breast cancer[END_REF] and colorectal cancer compare to the C allele. [START_REF] Méplan | Genetic variants in selenoprotein genes increase risk of colorectal cancer[END_REF] Two SNPs with functional consequences were described in the SELENOP gene that encodes the plasma SELENOP, the main plasma Se transporter for tissues and the brain. Both SNPs are G > A substitutions: one located in the coding region, predicted to change the amino acid alanine to threonine at position 235 (rs3877899), and the other located in the 3´ UTR region, important for Sec insertion (rs7579). [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF] It is hypothesized that rs3877899 regulates SELENOP stability and uptake through the cells, and the rs7579 affects the synthesis of this selenoprotein. [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF] Plasma SELENOP has two isoforms, the 50 kDa and the 60 kDa, with the second one containing the Sec-rich domain. [START_REF] Mostert | A novel method for the purification of selenoprotein P from human plasma[END_REF] The two SNPs in the SELENOP gene affect both isoforms. At baseline, individuals with the GA genotype for rs3877899 had a lower proportion of the SELENOP 60 kDa, with the Sec-rich domain. After Se supplementation with 200 µg of selenite for eight weeks, individuals with the AA genotype for rs7579 presented a lower proportion of SELENOP 60 kDa compared to individuals with the GG genotype. [START_REF] Méplan | Relative abundance of selenoprotein P isoforms in human plasma depends on genotype, se intake, and cancer status[END_REF] In addition, rs7579 was associated with a higher than average risk of developing prostate cancer. [START_REF] Steinbrecher | Effects of selenium status and polymorphisms in selenoprotein genes on prostate cancer risk in a prospective study of European men[END_REF] . Moreover, the presence of the rare allele A for rs7579 was associated with an increase in SELENOP mRNA expression [START_REF] Cardoso | Pro198Leu polymorphism affects the selenium status and GPx activity in response to Brazil nut intake[END_REF][START_REF] Donadio | Influence of genetic variations in selenoprotein genes on the pattern of gene expression after supplementation with Brazil nuts[END_REF] and higher than average cholesterol levels after supplementation with Brazil nuts [START_REF] Donadio | SEPP1 polymorphisms modulate serum glucose and lipid response to Brazil nut supplementation[END_REF] , while the presence of the rare allele A for rs3877899 was associated with an increase in SELENOP mRNA expression [START_REF] Cardoso | Pro198Leu polymorphism affects the selenium status and GPx activity in response to Brazil nut intake[END_REF] and with lower levels of cholesterol after supplementation with Brazil nuts. [START_REF] Donadio | SEPP1 polymorphisms modulate serum glucose and lipid response to Brazil nut supplementation[END_REF] The selenoprotein F (SELENOF), previously known as 15 kDa selenoprotein (SEP15), has two functional polymorphisms located in the 3' UTR region (rs5845 and rs5859) that are in linkage disequilibrium within the same haplotype, which means that the presence of the rare allele for one SPN implicates the presence of the other one. The first SNP is a G > A substitution at position 1,125 in the apical loop of the SECIS element, and the second one is a C > T substitution at position 811. [START_REF] Gladyshev | A New Human Seleniumcontaining Protein. Purification, characterization and cDNA sequence[END_REF] The variation in the apical loop (rs5845) might influence the efficiency of Sec incorporation into proteins. [START_REF] Hu | Distribution and functional consequences of nucleotide polymorphisms in the 3'-untranslated region of the human Sep15 gene[END_REF] In vitro studies have verified that the TA haplotype was less responsive to sodium selenite supplementation in NIH 3T3 mouse fibroblasts [START_REF] Hu | Distribution and functional consequences of nucleotide polymorphisms in the 3'-untranslated region of the human Sep15 gene[END_REF] and mesothelioma cells than the GC haplotype. 100 In addition, the polymorphism rs5845 was associated with a higher risk of lung cancer in plasma Se concentrations below 60 µg/L, 101 an increased risk of colorectal cancer, 102 lower scores for verbal learning memory 103 and a higher histological tumor grade in patients who have undergone radical prostatectomy compared to individuals with the normal genotype. 104 Selenoprotein S (SELENOS), located in the endoplasmatic reticulum (ER), is associated with the protection of the ER from the stress that by misfolded proteins cause 105 and with the control of the inflammatory response. 106,107 A substitution C > T occurs in the promoter region of the SELENOS gene (rs34713741) that has been associated with an increased risk of rectal, 102 colorectal, [START_REF] Méplan | Genetic variants in selenoprotein genes increase risk of colorectal cancer[END_REF] and gastric cancer 108 as well as ischemic stroke. 109 Another relevant polymorphism is the rs28665122, located in the promoter region of the SELENOS gene, also known as -105G/A. This SNP has been correlated with plasma IL-1, IL-6, and TNF-α levels 106 , an increased susceptibility to Hashimoto´s Thyroiditis 110 , preeclampsia in Norwegian women 107 and an increased risk of gastric cancer. 111

Plasma Se

Plasma Se is a short-term biomarker that reflects the current nutritional status of this mineral. [START_REF] Combs | Biomarkers of Selenium Status[END_REF]112 Since this biomarker is widely used to evaluate Se status and responds to dietary intake, genetic variations that affect plasma Se should be investigated as they can influence the assessment of Se nutritional status.

Lower plasma Se concentrations were associated with four SNPs in different selenoproteins (rs1050450 [GPX1], rs3877899 and rs7579 [SELENOP] and rs3471374 [SELENOS) and higher plasma Se with one SNP (rs7579 [SELENOP]). In a study conducted with 261 adults from the United States with a mean age of 50 years, lower plasma Se concentrations were observed in subjects with the rare TT genotype for the SNP in the antioxidant enzyme GPX1 Pro198Leu (rs1050450). [START_REF] Combs | Determinants of selenium status in healthy adults[END_REF] In a Polish supplementation trial that included 95 adults with a mean age of 35 years who were administred 200 µg of SeMet a day for six months, the coding SNP in the SELENOP gene (rs3877899) was associated with lower plasma Se only at baseline in individuals with the rare genotype AA. 113 In the SU.BRA.NUT trial, the association of SNPs in selenoproteins with biomarkers of Se status was evaluated in 130 adults after the supplementation with one unit of Brazil nut (~300-400 g/Se) for eight weeks following a washout period of eight weeks. Two SNPs were associated with lower plasma Se after only four weeks of Brazil nut intake: SELENOP gene (rs7579) and SELENOS gene (rs34713741). [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF] In the SELGEN trial, the association of SNPs in the SELENOP gene with biomarkers of Se status before and after supplementation was investigated in 75 healthy adults from England with a mean age of 40 years, who were supplemented with 200 µg of sodium selenite for six weeks following six weeks of washout. Overall, individuals with the rare genotype AA for the 3´ UTR on SELENOP rs7579 had higher plasma Se concentrations after the intervention period and two weeks of washout compared to individuals with the GG genotype. [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF]

Erythrocyte Se

Erythrocyte Se concentration is a biomarker of Se status that reflects longterm Se intake. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] Although it is a useful biomarker, it is not commonly used in studies that investigate the association of SNPs with biomarkers of Se status in healthy adults. Specifically for patients with systemic inflammatory response, erythrocyte Se is a better biomarker than plasma Se since the acute phase response does not affect it. 114 Two studies have found this association. In the observational study, there was an association between the coding SNP rs8179169, a substitution G > C that changes the amino acid arginine to proline in position five of the protein (Arg5Pro) in the GPX1 gene, and erythrocyte Se concentrations: lower concentrations were observed in individuals with the GC genotype. [START_REF] Donadio | Influence of gender and SNPs in GPX1 gene on biomarkers of selenium status in healthy Brazilians[END_REF] In the SU.BRA.NUT trial, erythrocyte Se concentration was associated with two SNPs: rs713041 (GPX4) and rs34713741 (SELENOS). The 3´UTR SNP in GPX4 gene rs713041 was associated with lower erythrocyte Se in T-carriers at baseline. Moreover, an increase in the variation (∆1 [8 weeks nutsbaseline] ) of erythrocyte Se concentration in T-carriers for rs34713741 occurred in the SELENOS gene, which means that in these individuals, erythrocyte Se concentration was higher compared to the CC genotype after Se supplementation with Brazil nuts; the T-carriers were more responsive to Se supplementation. [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF]

Erythrocyte GPx1 activity

Erythrocyte GPx1 activity is a widely used biomarker of Se status and intake, especially in cases of suspected deficiency. [START_REF] Combs | Biomarkers of Selenium Status[END_REF] While several studies have evaluated the influence of SNPs in selenoprotein genes that can affect GPx1 activity, the most studied is the rs1050450 in the GPX1 gene (Pro198Leu). [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF][START_REF] Karunasinghe | Serum selenium and singlenucleotide polymorphisms in genes for selenoproteins: Relationship to markers of oxidative stress in men from Auckland, New Zealand[END_REF][START_REF] Donadio | Influence of gender and SNPs in GPX1 gene on biomarkers of selenium status in healthy Brazilians[END_REF][START_REF] Malling | Sex determines the influence of smoking and gene polymorphism on glutathione peroxidase activity in erythrocytes[END_REF]113,115,116 However, SNPs in other selenoproteins have also been associated with alterations in GPx1 activity in healthy volunteers. These include rs713041 (GPX4), [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF][START_REF] Donadio | Influence of gender and SNPs in GPX1 gene on biomarkers of selenium status in healthy Brazilians[END_REF][START_REF] Méplan | Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex[END_REF] rs3877899, rs7579 (SELENOP) [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF][START_REF] Karunasinghe | Serum selenium and singlenucleotide polymorphisms in genes for selenoproteins: Relationship to markers of oxidative stress in men from Auckland, New Zealand[END_REF] and rs5845 (SELENOF) [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF] . Overall, the presence of the rare allele T or genotype TT for rs1050450 has been associated with lower GPx1 activity in seven studies (see Table 2). [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF][START_REF] Karunasinghe | Serum selenium and singlenucleotide polymorphisms in genes for selenoproteins: Relationship to markers of oxidative stress in men from Auckland, New Zealand[END_REF][START_REF] Malling | Sex determines the influence of smoking and gene polymorphism on glutathione peroxidase activity in erythrocytes[END_REF][START_REF] Bastaki | Genotype-activity relationship for Mn-superoxide dismutase, glutathione peroxidase 1 and catalase in humans[END_REF]113,115,116 In three of these studies, a gender-SNP interaction was modulating GPx1 activity. [START_REF] Donadio | Influence of gender and SNPs in GPX1 gene on biomarkers of selenium status in healthy Brazilians[END_REF][START_REF] Malling | Sex determines the influence of smoking and gene polymorphism on glutathione peroxidase activity in erythrocytes[END_REF][START_REF] Bastaki | Genotype-activity relationship for Mn-superoxide dismutase, glutathione peroxidase 1 and catalase in humans[END_REF] Moreover, T-carriers for rs1050450 in the GPX1 gene had a reduction in GPX1 mRNA expression after supplementation with Brazil nuts [START_REF] Donadio | Influence of genetic variations in selenoprotein genes on the pattern of gene expression after supplementation with Brazil nuts[END_REF] . A reduction in GPx1 activity was also associated with rs5845 in the SELENOF gene in the SU.BRA.NUT trial during the washout period [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF] and with rs3877899 in the SELENOP gene in adults from New Zealand. [START_REF] Karunasinghe | Serum selenium and singlenucleotide polymorphisms in genes for selenoproteins: Relationship to markers of oxidative stress in men from Auckland, New Zealand[END_REF] The presence of the rare allele T for rs713041 (GPX4) was associated with both higher [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF][START_REF] Karunasinghe | Serum selenium and singlenucleotide polymorphisms in genes for selenoproteins: Relationship to markers of oxidative stress in men from Auckland, New Zealand[END_REF] and lower than average GPx1 activity. [START_REF] Méplan | Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex[END_REF] It is initially surprising that SNP in genes other than GPX1 can affect GPx1 enzyme activity; nevertheless, the hierarchy of selenoprotein expression could explain this result. Indeed, selenoproteins compete with one another against components of the selenoprotein synthesis machinery and for available Se, as the amino acid Sec, during the translation process. [START_REF] Bellinger | Regulation and function of selenoproteins in human disease[END_REF]117 The differences in the 3´ UTR region, where the Sec insertion occurs, are the main factor that drives this competition. Single nucleotide polymorphisms in this specific region of Sec insertion can therefore change the hierarchical order of selenoprotein expression. [START_REF] Crosley | The single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene influences endothelial cell function: Interaction with selenium and fatty acids[END_REF] RNA-protein binding assays in vitro demonstrated that the C-variant for rs713041 in GPX4 transcripts bind to protein more strongly than the T-variant and GPX1 transcripts. [START_REF] Méplan | Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex[END_REF] In addition, GPX4 protein is ranked high in the hierarchy of selenoprotein expression, which indicates that it has a preference for protein synthesis when Se supply is scarce. 117 Such evidence suggests that in individuals with the C allele for rs713041 in the GPX4 gene, the synthesis of GPX1 protein is expected to be lower because, during translation, the C transcript for GPX4 can compete more strongly against the GPX1 transcript for Se incorporation into GPX4 protein, which means that Sec will be incorporated preferentially into GPX4 protein rather than GPX1. By contrast, in individuals with the T allele for the GPX4 SNP the opposite scenario occurs, changing the hierarchy: Sec will be directed to GPX1 synthesis because the T-transcript is not strong enough to compete against GPX1 during translation. [START_REF] Méplan | Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex[END_REF][START_REF] Crosley | The single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene influences endothelial cell function: Interaction with selenium and fatty acids[END_REF] In the SU.BRA.NUT trial, an increase in the variation (∆1 [8 weeks nuts -baseline] ) of erythrocyte Se concentration in A-carriers for rs7579 occurred in the SELENOP gene, which suggests that in these individuals, erythrocyte Se concentration was higher compared to the GG genotype after Se supplementation with Brazil nuts; A-carriers were more responsive to Se supplementation. [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF]

Plasma GPx3 activity

Studies conducted with healthy adults found an influence of genetic polymorphisms in selenoprotein genes on GPx3 activity, and overall, the presence of the rare allele for rs7579 (SELENOP), rs713041 (GPX4), and rs5845 (SELENOF) was associated with a reduction in its activity. In the SELGEN trial, rs713041 (GPX4) and rs7579 (SELENOP) were associated with lower GPx3 activity after the supplementation and washout period. [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF][START_REF] Méplan | Functional effects of a common single-nucleotide polymorphism (GPX4c718t) in the glutathione peroxidase 4 gene: interaction with sex[END_REF] The same rationale for the influence of the SNP in rs713041 (GPX4) on GPx1 activity could be applied to explain the influence of SNPs in SELENOP and SELENOF on GPx3 activity. In the SU.BRA.NUT trial, rs5845 (SELENOF) was associated with lower variation in GPx3 activity after the intervention period (∆1 [8 weeks nuts -baseline] ), which means that in T-carriers, the GPx3 activity was less responsive to Brazil nut supplementation compared to the CC genotype. [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF] In a study conducted with healthy pregnant women in the United Kingdom, A-carriers for rs3877899 (SELENOP) increased GPx3 activity after supplementation with Se-yeast; in the placebo group, the A-carriers were able to maintain their Se status measured by whole blood Se. 118

Plasma Selenoprotein P

Plasma SELENOP concentration is considered one of the best biomarkers of short-term Se status and intake. [START_REF] Hurst | EURRECA-Estimating selenium requirements for deriving dietary reference values[END_REF] Three SNPs have been associated with a reduction in plasma SELENOP concentration: two on its gene (rs3877899 and rs7579) and one in the SELENOF gene (rs5845). In the SELGEN trial, heterozygote females for rs3877899 had lower plasma SELENOP concentrations than GA males. [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF] In healthy Americans, GA adults for rs7579 had lower plasma SELENOP than GG and GA genotypes. [START_REF] Combs | Determinants of selenium status in healthy adults[END_REF] In the SU.BRA.NUT trial, T-carriers for rs5845 (SELENOF) had a lower variation of plasma SELENOP concentration during the washout period (∆2 [washout -nuts] ), which indicates that this biomarker dropped after Se withdrawal, being more responsive to a reduction in Se availability. [START_REF] Donadio | Genetic variants in selenoprotein genes modulate biomarkers of selenium status in response to Brazil nut supplementation (the SU.BRA.NUT study)[END_REF] By contrast, the same two SNPs on the SELENOP gene were associated with an increase in plasma SELENOP concentrations after Se supplementation. An SNP-gender interaction occurred after the supplementation: higher than average plasma SELENOP concentration was associated with the GG genotype for rs3877899 in women [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF]116 and with the GA genotype for rs7579 in men. [START_REF] Méplan | Genetic polymorphisms in the human selenoprotein P gene determine the response of selenoprotein markers to selenium supplementation in a gender-specific manner (the SELGEN study)[END_REF] In American adults, plasma SELENOP concentrations were lower in GA individuals compared to GG individuals for rs3877899. [START_REF] Combs | Determinants of selenium status in healthy adults[END_REF] Moreover, in A-carriers for rs7579, SELENOP mRNA expression was higher than in GG individuals before and after Brazil nut intake. [START_REF] Donadio | Influence of genetic variations in selenoprotein genes on the pattern of gene expression after supplementation with Brazil nuts[END_REF]

Conclusions

This review provided an overview of the effects of functional SNPs in selenoprotein genes on the most common biomarkers of Se nutritional status.

Evidence demonstrates a significant reduction of GPX1 activity in the presence of the rare allele T for the coding SNP rs1050450 (GPX1). Concerning the other functional SNPs, insufficient evidence prevents stating that some genotypes can modulate one or more biomarkers of Se status.

As to the association between the reduction on GPX1 activity and the SNP rs1050450 in the GPX1 gene, non-RCT and RCT interventions and observational studies with a baseline plasma Se below 100µg/L have confirmed this association; however, in populations with high Se status, such as that of the United States, this association has not been observed. One explanation is the saturation of selenoproteins activity or concentration with plasma Se above 120 µg/L. Even with a supplementation, populations with high Se status would not increase selenoprotein activity because they are already at their maximal and the presence of the SNP would not change this response.

Nutrigenetics studies have helped the great advances in Se biology of the past two decades years. The ultimate goal of a more personalized nutritional recommendation is closer to being realized than ever before. Nevertheless, more research in this area must be encouraged and be integrated with other omics tools (metabolomics, epigenomic, proteomic and transcriptomic) to better understand all the data provided so far. When strong evidence on the effects of some genotypes on biomarkers of Se status becomes available, it will likely be used to update the current dietary recommendation based on not only the regional particularities but also on the genetic characteristics of the population or even of individuals (personalized nutrition).

Table 1 Summary of Selenium dietary reference intake values

 1

	EAR = estimated average requirement; RDA = recommended dietary allowance, UL =
	tolerable upper intake level; NOAEL = no observed adverse effect level; LOAEL = lowest
	observed adverse effect level. 34

Table 2 Observational studies evaluating the effects of functional SNPs on biomarkers of Se status in healthy subjects. 528 529

 2 no association, ↑ increased, ↓ decreased. *: statistically different at p< 0.05. BMI: body mass index (kg/m 2 ), F: females, GPx: Glutathione Peroxidase enzyme, GPX: Glutathione Peroxidase gene, M: males, MAF: minor allele frequency, nd: not determined, n.i: not informed, Se: selenium, SELENOP: Plasma Selenoprotein P, SELENOF: Selenoprotein F (former Selenoprotein 15kDa), SELENOP: Selenoprotein P, SELENOS: Selenoprotein S, suppl: supplementation, wks: weeks. Studies are ordered according to baseline plasma Se concentration.

			Plasma		Genotype %
	Country	Population	Se	SNP	and MAF	Main results	Reference
			(µg/L)		(allele)
	Sweden	214 adults, age and BMI	n.i	GPX1	53/ 40/ 7	↔ GPx1 activity with genotypes	Forsberg et
		n.i		rs1050450	MAF (T):		al. (2000) 119
				(C > T)	0.27
	USA	115 Asians, 63	n.i	GPX1	A: 86/ 11/ 3	No effect of ethnicity on GPx1 activity	Bastaki et al.
		Caucasians, 20 Hispanics,		rs1050450	C: 46/ 46/ 8	↓ GPx activity in males with TT genotype	(2006) 83
		19 Others,		(C > T)	H: 60/ 35/ 5	All: 13.16 U/g Hb. Males/females: 2.49/ 2.62 a , 2.60/
		adults, 17-21 y, BMI n.i,			O: 58/ 21/	2.65 a , 2.20/ 2.63* a (CC, CT, TT)
		M= 102, F= 129			21
	Brazil (North	149 women, mean age 26	49.7	GPX1	58/ 35/ 7	↔ Se biomarkers with genotypes	Rocha et al.
	Region)	y, BMI= 22.9		rs1050450	MAF (T):		(2016) 120
				(C > T)	0.25
	Brazil (Southeast	116 adults, mean age 28	53.2	GPX1	48/ 48/ 4	↑ GPx1 activity in females with CC genotype	Donadio et
	Region)	y, BMI= 23.1,		rs1050450	MAF (T):		al. (2016) 54
		M= 44, F= 72		(C > T)	0.28
				GPX1	29/ 70/ 0	↓ erythrocyte Se in GC genotype
				rs8179169	MAF (C):	↑ GPx1 activity in females with GC genotype
				(G > C)	0.35
				GPX4	39/ 45/ 16	↑ GPx1 activity in females with CC genotype
				rs713041	MAF (T):
				(C > T)	0.15
	Brazil (Northeast	343 adults, mean age	54.0	GPX1	52/ 39/ 9	↓ GPx1 activity in T-carriers	Almondes et
	Region)	24y, BMI= 22.8, M= 145,		rs1050450	MAF (T):	↑ ORAC levels in T-carriers	al. (2018) 115
		F= 198,		(C > T)	0.28
	Poland	405 adults, mean age 57	54.4	GPX1	48/ 42/ 10	↓ correlation of GPx1 activity and Plasma Se in TT	Jablonksa et
		y, BMI n.i, M= 282, F=		rs1050450	MAF (T):	genotype	al.

↔

Acknowledgments

Author contributions. J.L.S.D. and G.B.S.D. wrote the manuscript with the supervision of M.M.R., P.B. and S.M.F.C. All authors read and approved the final manuscript.

Funding and support. This manuscript was funded by Brazilian grants from Sao Paulo Research Foundation to J.L.S.D. (Fundação de Amparo à Pesquisa do Estado de São Paulo -FAPESP process: 2011/17720-0, 2013/ 03224-0 and 2015/10146-8).

Conflict of interest. The authors declare no conflict of interest.