Potential Role of Selenoenzymes and Antioxidant Metabolism in relation to Autism Etiology and Pathology

Laura J. Raymond; Richard C. Deth; Nicholas V. C. Ralston

doi:10.1155/2014/164938

Potential Role of Selenoenzymes and Antioxidant Metabolism in relation to Autism Etiology and Pathology

Raymond, Laura J.;Deth, Richard C.;Ralston, Nicholas V. C. 2014-03-05 00:00:00 Hindawi Publishing Corporation Autism Research and Treatment Volume 2014, Article ID 164938, 15 pages http://dx.doi.org/10.1155/2014/164938 Review Article Potential Role of Selenoenzymes and Antioxidant Metabolism in relation to Autism Etiology and Pathology 1 2 1 Laura J. Raymond, Richard C. Deth, and Nicholas V. C. Ralston Energy & Environmental Research Center, University of North Dakota, 15 North 23rd Street, Stop 9018, Grand Forks, ND 58202, USA Department of Pharmaceutical Sciences, Northeastern University, Boston, MA 02115, USA Correspondence should be addressed to Laura J. Raymond; lraymond@undeerc.org Received 11 September 2013; Revised 7 January 2014; Accepted 27 January 2014; Published 5 March 2014 Academic Editor: Klaus-Peter Ossenkopp Copyright © 2014 Laura J. Raymond et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Autism and autism spectrum disorders (ASDs) are behaviorally defined, but the biochemical pathogenesis of the underlying disease process remains uncharacterized. Studies indicate that antioxidant status is diminished in autistic subjects, suggesting its pathology is associated with augmented production of oxidative species and/or compromised antioxidant metabolism. This suggests ASD may result from defects in the metabolism of cellular antioxidants which maintain intracellular redox status by quenching reactive oxygen species (ROS). Selenium-dependent enzymes (selenoenzymes) are important in maintaining intercellular reducing conditions, particularly in the brain. Selenoenzymes are a family of∼25 genetically unique proteins, several of which have roles in preventing and reversing oxidative damage in brain and endocrine tissues. Since the brain’s high rate of oxygen consumption is accompanied by high ROS production, selenoenzyme activities are particularly important in this tissue. Because selenoenzymes can be irreversibly inhibited by many electrophiles, exposure to these organic and inorganic agents can diminish selenoenzyme- dependent antioxidant functions. This can impair brain development, particularly via the adverse influence of oxidative stress on epigenetic regulation. Here we review the physiological roles of selenoproteins in relation to potential biochemical mechanisms of ASD etiology and pathology. 1. Introduction Although ASD was previously thought to be rare, the number of persons receiving treatment for ASD has increased The causes of autism and autism spectrum disorder (col- substantially during the past several decades and continues lectively, ASD) remain unknown, in part because of com- to increase. A recent US government report estimated the plex behavioral phenotypes and the likelihood that multiple prevalenceofASDincreasedby78%from2002to2008[8]. In genetic and environmental factors contribute to its etiology 2011-2012, a prevalence of 20 per 1000 was reported for school [1–3]. In the absence of biochemical tests for ASD, the aged children [9]. However, a portion of ASD’s increasing diagnosis is based solely on clinical assessment of behavioral incidence may reflect changes in diagnostic practice and the criteria that define deficits in social interaction, impairments broadening of diagnostic criteria [10]. Other studies indicate in verbal and nonverbal receptive/expression, speech, and adiagnosticshiftorsubstitutionmay also have contributed hyperfocused repetitive behaviors. eTh pathophysiology of to theriseindiagnosis,whereby theincreaseinautism ASD is primarily expressed in the neurologic, immunologic, diagnoses corresponds with declines in the usage of other and gastrointestinal (GI) systems and aeff cts four times as diagnostic categories [11, 12]. Based on a meta-analysis of many boys as girls [4–6]. Regression, with loss of previ- ASD studies, McDonald and Paul [13] concluded that it does ously acquired skills, can also interrupt apparently normal not seem possible to assess whether or how much of the development. Children with severe autism can exhibit mental observed increases in cumulative incidence are real, although the number of individuals identified as having ASD has retardation, and autistic children have an elevated rate of increased dramatically. seizure disorders [7]. 2 Autism Research and Treatment Table 1: Glutathione and oxidative stress in autism. Authors (Reference) Control𝑛 Autistic𝑛 GSH status Additional related findings 33 20 46%↓ James et al. [14] ↓GSH/GSSG,↓SAM/SAH,↓cysteine James et al. [15] 73 80 32%↓ ↓GSH/GSSG,↓SAM/SAH,↓cysteine Lab-based normal values 10 36%↓ D. A. Geier and M. R. Geier [16] ↓Cysteine 55 43 21%↓ Adams et al. [17] ↓SAM,↓cysteine,↓vitamin E,↑FIGLU 13 15 33%↓ Pa¸c s a et al. [18] ↓Cysteine Pastural et al. [19] 12 15 35%↓ ↓Cysteine 30 30 27%↓ Al-Gadani et al. [20] ↑Lipid peroxides,↓vitamin E,↓SOD,↓GPx ↓GSH/GSSG,↓SAM/SAH,↓cysteine,↓DNA Melnyk et al. [21] 40 40 29%↓ methylation 42 40 28%↓ James et al. [22] ↓GSH/GSSG,↓SAM/SAH,↓cysteine 120 28 24%↓ Geier et al. [23] ↓Cysteine Geier et al. [24] Lab-based normal values 28 24%↓ ↑GSSG,↓cysteine,↓taurine,↓sulfate Until reliably accurate differential diagnoses are achieved, and exogenous antioxidant capacity are commonly reduced it is difficult to attain the goal of defining the biochemical in ASD. Glutathione (GSH) is the primary intracellular and physiological lesions that initiate and/or perpetuate the antioxidant, and the ratio of its reduced (GSH) and oxidized dysfunctions of autism. However, one pathological mecha- (GSSG) forms (GSH/GSSG) provides a useful index of redox nism present in many children with ASD involves defects in status. As shown in Table 1, numerous studies have reported the control of oxidative damage. Distinctions in the nature significantly lower plasma levels of GSH and, in some cases, of these perturbations in redox control may provide insight lower GSH/GSSG levels. Low GSH levels are associated with for identifying biochemically defined patient subgroups that oxidative stress, increased inflammation, impaired immune may respond to specific therapeutic interventions. This response, and a decreased ability to detoxify environmental paper focuses on potential associations between genetic and contaminants. Autistic children have been reported to be acquired defects in control of oxidative damage, particularly increasingly susceptible to recurrent infections, neuroinflam- those that impinge upon selenium- (Se-)dependent enzymes mation, gastroinflammation, and impaired antioxidant and (selenoenzymes). Se physiology is a vital process in the detoxification capacity. Diminished glutathione peroxidase brain and neuroendocrine system, [25, 26]and,because (GPx), superoxide dismutase, and catalase enzyme activities of the high reactivity and low abundance of Se in these have been associated with ASD, as well as low cysteine, Se, tissues, its vulnerability to inhibition by a variety of toxicants zinc (Zn), and Vitamins C, E, and A [32], although these is markedly enhanced. However, its potential role in the associations are not consistently observed. pathophysiology of ASD remains largely unexplored. Accumulation of oxidized glutathione (GS-SG) in plasma is a strong indication of intracellular oxidative stress, as cells export the GS-SG to maintain redox equilibrium. James and 2. Oxidative Stress in ASD coworkers [14] were rfi st to report that plasma levels of cysteine, GSH, and the GSH/GS-SG ratio were significantly Chemically reactive oxygen-derived products like peroxide decreasedinautisticchildren. In that study, totalGSH 2− radicals ( O ), hydrogen peroxide (H O ), superoxide– levels were decreased, and GS-SG was increased, resulting 2 2 − 1 anion (O ), singlet oxygen ( O ), and hydroxyl radicals in a threefold reduction in the redox ratio of GSH/GS-SG. 2 2 ( O H), are products of ongoing aerobic metabolism via Cysteine, the rate-limiting amino acid for GSH synthesis, mitochondrial oxidative phosphorylation [27]. If not inter- was significantly decreased relative to controls in over 65% of the autistic children tested. The finding of lower-plasma cepted and detoxified, reactive oxygen species (ROS) are capable of chemically damaging all forms of cellular macro- GSH has since been replicated by other research groups [17– molecules. To avoid these consequences, numerous ROS- 20, 33], suggesting this is a prevalent feature of ASD. A recent detoxifying reactions enable cells to maintain redox equi- meta-analysis nds fi that children with ASD have decreased librium and metabolic homeostasis. us Th , oxidative stress blood GSH (27%), GPx activity (18%), and methionine (13%) is a condition where the level of ROS production exceeds and increased concentrations of GS-SG (45%) relative to antioxidant capacity. nonautistic children [34]. In addition, levels of NADPH and Increased oxidative stress has been observed in children NADH, which reflect redox status and help maintain GSH in its reduced state, were found to be significantly lower in with ASD [28–30]. Blood collected from autistic children shows low concentrations of membrane polyunsaturated autistic children [32]. Several studies have reported a decrease lipids, higher phospholipase A , and loss of the normal in the level of GSH in postmortem brain samples from ASD asymmetry of membrane lipoproteins, which may indicate subjects, associated with a decrease in the GSH/GS-SG ratio increased oxidative damage [13, 31]. Levels of endogenous and increased levels of oxidative stress biomarkers [35, 36]. Autism Research and Treatment 3 Selenocysteine (—) 2+ HgSe Hg (—) CdSe? PbSe? Adducts with 2+ 2+ Cd , Pb , etc. other electrophiles? (—) Organic electrophiles, Organic electrophile adducts, e.g., NAPQI e.g., NAPQI-Se? Selenoenzyme activities Repair oxidative damage Regulation of redox state (e.g., GPx4, MsrB) (e.g., TRx1, TRx2, ... ) Antioxidant activities (e.g., GPx1, TRx1, ... ) Thyroid hormone regulation (e.g., DIO1, DIO2) Intracellular redox status Methionine DNA and Epigenetic Healthy brain synthase regulation of histone development activity methylation gene expression Figure 1: Selenoprotein synthesis and activities are sensitive to elemental and organic electrophiles. High exposures to soft electrophiles may additively impair redox regulation and thyroid hormone production, disrupting epigenetic regulation and normal brain development. In addition, activities of several GSH-related enzymes, group of autistic children showed dramatic increases. Levels including the selenoenzyme GPx, were lower in cerebellums of urinary 8-hydroxy-2-deoxyguanosine (8-OHdG), a major of ASD subjects [37]. productofDNA oxidation, were also measured butdid eTh folate and Vitamin B -dependent enzyme methio- not reach statistical significance, although they indicated nine synthase (MS) is inhibited under oxidative stress con- a trend toward increasing concentrations in children with ditions, resulting in a decrease in all methylation reactions, autism [30]. No significant correlations were noted between including DNA methylation [38]. The basis for MS inhibi- the levels of these biomarkers and vitamin intake, dietary tion is oxidation of its Vitamin B (cobalamin) cofactor, supplements, medicine, medical disorders, or history which is considered the most readily oxidized biomolecule, of regression. er Th efore, these results suggest that lipid making it an idealsensorofcellularredox status [39, 40]. peroxidation is increased in autistic children and that certain Lower MS activity inhibits methylation by lowering the autistic children have much greater oxidative stress than ratio of the methyl donor S-adenosylmethionine (SAM) to others. the methylation inhibitor S-adenosylhomocysteine (SAH), exerting a global dampening effect on >200 methylation reactions. Arguably, the most important among these are 3. Selenium-Dependent Antioxidant methylation of DNA and histones, which combine to exert Metabolism and ASD epigenetic regulation over gene expression. As noted above, a decrease in SAM/SAH has been documented in plasma Selenoproteins are essential for brain development, redox of ASDsubjectsinassociation with adecreaseinGSH/GS- control, and preventing and reversing oxidative damage in SG, reflecting the reciprocal relationship between oxidative the brain and neuroendocrine tissues (Figure 1,Table 2). stress and methylation. In neuronal cells methionine synthase eTh refore, control of intracellular oxidative tone and n fi dings activity is stimulated by growth factors and dopamine but of increased oxidative damage in children with ASD may inhibited by neurodevelopmental toxins, including mercury be indicative of disruptions of selenoenzyme activities. The (Hg) [41]. Methionine synthase mRNA levels are significantly molecular forms of Se most common in foods are the amino decreased in postmortem brains of autistic subjects, indica- acids selenocysteine (Sec) and selenomethionine (SeMet), tive of a deficit in methylation capacity secondary to oxidative although traces of water-soluble inorganic forms (e.g., sele- stress [42]. nate andselenite) canalsobepresent in food anddrinking Elevated urinary levels of 8-isoprostane-F (8-iso- water. For both the organic and inorganic Se forms, bio- 2𝛼 PGF ) and malondialdehyde (MDA), oxidative stress chemical utilization in selenoenzymes is initiated through the 2𝛼 biomarkers, have also been noted in children with autism common intermediate hydrogen selenide (H Se). eTh refore, [28, 30]. A bimodal distribution of 8-iso-PGF was all ingested (and endogenous) forms of Se must be degraded 2𝛼 reported, with the majority of autistic subjects showing to inorganic selenide before Se can be used for synthesis of moderate increases in isoprostane levels, while a smaller Sec, the physiological active form of Se. Although proteins 4 Autism Research and Treatment − Selenocysteine-based − Cysteine-based e e Glucose NADPH redox ednzymes redox proteins (e.g., TRx, TGR, GPx, MsrB) (GSH, T[SH] , Prx) − − GR − − ∙ Reduction of ascorbate and other small molecules. e e GSSG GSH Grx ∙ Reduction of oxidized methionine and phosholipids. ∙ Reduction of glutathionylated proteins. Figure 2: Selenoenzymes are central to providing antioxidant electrons to accomplish reduction of molecules in a number of biochemical processes. NADPH = nicotinamide adenine dinucleotide phosphate; GR = glutathione reductase; T(SH) = reduced thioredoxin; GSH = reduced glutathione; TRx = thioredoxin reductase; GSSG = oxidized glutathione; TGR = thioredoxin-glutathione reductase; GPx = glutathione peroxidase; MsrB = methionine sulfoxide reductase; Prx = peroxiredoxin; Grx = glutaredoxin; e =electron. Levels of plasma GSH, erythrocyte NADH, and NADPH are notably reduced (𝑃<0.001 )inchildrenwithautism[16, 32]. with SeMet contain Se, they are not considered selenoproteins selenoproteins are known, only members of the major sele- because SeMet is nonspecifically incorporated into proteins noenzyme/selenoprotein families with potential relevance to as if it were Met. The nonspecific insertion of SeMet or Met is autism etiology and pathology are discussed below. directed by AUG codons and no significant distinctions in the biochemical functions have been observed. This is in contrast 3.1. Roles of Selenoenzymes in yTh roid Hormone Regulation. to Sec, which is the catalytically active primary amino acid Selenoenzymes regulate thyroid synthesis and metabolic present in all selenoproteins [43] and is responsible for the functions contributing to thyroid hormone biosynthesis, principal functions of these enzymes (Table 2). In contrast to antioxidant defense, redox control of thyrocytes, and thyroid other amino acids, Sec is not recycled for reincorporation into hormone metabolism. yTh roid hormones have important new proteins but is, instead, degraded to release inorganic Se rolesinregulatingmanykeybiochemicalreactions,especially which can be utilized for resynthesis of Sec. protein synthesis and enzymatic activity, accompanied by an While Cys, the analogous sulfur amino acid, is inserted increase in basal metabolic rate. yr Th oid hormone regulates at UGU/UGC codons, the insertion of Sec is in response several processes that are associated with brain differentia- to UGA which is otherwise the “opal” stop codon for other tion, including dendrite and axon growth, synaptogenesis, proteins [44–46]. The selenoprotein mRNAs include a dis- neuronal migration, and myelination [51]. Disruption of tinct Sec insertion sequence (SECIS) stem-loop structure in thyroid hormone production during early child development their untranslated 3 region of the mRNA. This is recognized leads to permanent deficiency in intelligence and sensorimo- by specific SECIS binding proteins which function together tor functions [52], and it is hypothesized that Se deficiency with several transacting factors as well as a unique tRNA may be responsible for the initiation of autoimmune thy- with an anticodon complementary to UGA to initiate de novo roid disorders [53]. Interestingly, thyroid hormone increases Sec synthesis. eTh tRNA is aminoacylated with serine prior plasma Se levels, as well as the levels of selenoenzymes such to biosynthesis of Sec which is inserted into the protein’s as DIO [54]. This relationship is logical, since increased primary structure. In mRNA of most selenoproteins, the metabolic activity places a higher demand on antioxidant UGA of the Sec insertion codon is followed by a second or resources. terminal UGA, which is then read as the stop codon. The central nervous system is very sensitive to thy- Since the discovery of these genetically unique pro- roid hormone supply during growth and development. eTh teins, theirenzymeactivitieshavebecomeincreasinglywell selenoprotein family of DIOs (DIO1, 2, and 3) is involved in den fi ed. Associations between compromised Se and ASD the formation and regulation of the active thyroid hormone, have been reported including low Se levels in red blood cells triiodothyronine (T ). More than 80% of T in the brain 3 3 [47]. Since blood Se is less prone to contamination and more is derived from intracellular deiodination of T by DIO2 indicative of tissue selenoenzyme activities, it is considered [55, 56]. Since circulating T does not readily gain access to to be amorereliableindex than hair.Indeed,hairSeis intracellular nuclear receptors [57], DIO2 provides an impor- variously reported as being increased [48], decreased [49], or tant regulatory function in the brain and central nervous unchanged [50]insubjectswithASD. system (CNS). During early childhood, T bound to nuclear When considering the developmental roles of selenopro- receptors is entirely dependent on its local production from teins, as well as their involvement in redox control and pro- T via this selenoprotein. tection from oxidative stress, the potential for selenoenzyme dysregulation in relation to the pathologies associated with ASD appears worthy of investigation. The three main fam- 3.2. Roles of TRx in Redox Regulation. The three distinct ilies of characterized selenoproteins—iodothyronine deio- forms of TRx—TRx1, TRx2, and TRx3 (collectively, TRx)— dinases (DIO), thioredoxin reductases (TRx), and GPx— are important in controlling redox state within major com- have critical roles in thyroid function, fetal development, partments of the cell. While TRx1 (cytosolic) and TRx2 hormone metabolism, and oxidative stress detoxification, (mitochondrial) restore oxidized thioredoxin (T[S-S]) to its particularly in endocrine and brain tissues. Although∼25 reduced (T[SH] )formand areresponsible forreducinga 2 Autism Research and Treatment 5 Table 2 Selenoprotein References Functions GPx1 [58] Detoxifies peroxides in aqueous compartment of cellular cytosol GPx2 [59] Expressed in cytosol of liver and tissues of the digestive system GPx3 [60] Synthesized primarily by kidney; secreted into plasma for transport to other tissues GPx4 Prevents and reverses oxidative damage to lipids in brain and other tissues [61] Reduces T(SH) , vitamin C, polyphenols, and other substrates to regulate intercellular TRx1 [62–64] redox state TRx2 [63, 64] Located in mitochondria and controls and regulates redox state TRx3 [63, 64] Reduces mitochondrial glutathione disulfide, abundant in testes MsrB1 [64] Restores oxidatively damaged methionine (R-sulfoxides) to native configuration DIO1 Converts T (thyroxine) prohormone into T (active thyroid hormone) [65] 4 3 DIO2 [65] Regulates thyroid hormone status, activating as well as inactivating T DIO3 [65] Activates thyroid hormone in brain, placenta, important in fetal development SPS2 [64] Creates the Se-phosphate precursor required for synthesis of all selenoproteins SelM Notably high expression levels in the brain, possible thiol-disulfide oxidoreductase [64, 66] SelN [64] Interacts with ryanodine receptor, mutations result in congenital muscular dystrophy SelP [64] Transports Se in plasma (10 Sec/molecule) and delivers Se to brain and endocrine tissues SelW [64, 66, 67] Expressed in a variety of tissues and may regulate redox state of 14-3-3 proteins Sel15 Oxidoreductase that may assist in disulfide formation and protein folding [64] variety of other essential antioxidant molecules [63]includ- 3.3. Roles of GPx in Redox Regulation. GPx (vfi e geneti- ing Vitamin C [62]. Thioredoxin is a ubiquitous 12 kDa cally distinct forms, GPx1–4 and GPx6) are selenoenzymes protein that employs vicinal cysteines (CXXC motif) and involved in antioxidant defense and redox regulation and becomes oxidized to intramolecular disulfides T(S-S) during modulation. GPx provide protection against oxidative dam- reduction of other molecules (Figure 2). Itsactionisessential ageand aidinthe maintenanceofmembraneintegrity by for countering oxidative damage in the cytosol of aerobic using GSH as a cofactor to catalyze reduction of hydro- organisms from bacteria to humans [68]. Since T(SH) is gen peroxide, forming oxidized glutathione (GS-SG) in the a central regulator of cellular redox status that is required process. yTh roid hormone synthesis requires a continuous for the redox-regulated function of transcription factors production of high concentrations of H O ,which appears 2 2 and hormonally regulated nuclear receptors, it is critical to be its rate-limiting step [72–74]. eTh refore, since the in DNA production, gene expression, cell survival, and thyrocyte is continually exposed to potentially toxic con- embryogenesis. u Th s TRx maintains T(SH) levels to enable centrations of H O and lipid hydroperoxides, appropriate 2 2 2 basic processes and regulate multiple metabolic events. eTh antioxidant defense systems are essential to control excess antioxidant functions of TRx occur because they directly oxidative stress. Three of the vfi e GPx are expressed in facilitate reduction of oxidized proteins through cysteine- thyrocytes and thyroid tissue [75–77]. Studies indicate a thiol-disulfide exchange, forming an oxidized disulfide T(S- distinct regulation of expression, secretion, and function S) in the process. TRx is also directly involved in prevention of these selenoproteins for controlling thyrocyte growth, and repair of damage caused by H O -based oxidative stress. differentiation, and function [ 76–83]. When Se intake is 2 2 Because intracellular reduction of selenite is required for de adequate, the intracellular GPx and TRx systems protect novo synthesis of Sec selenoproteins, TRx clearly has a central the thyrocyte from peroxides; however, in Se deficiency, the role in all Se physiology. It is assumed that Se’s pivotal role in thyrocyte’s apoptotic response to H O is increased [84]. 2 2 TRx explains why targeted disruption of the TRx1 [69]and Furthermore, in iodine deficiency, where hyperstimulation T(SH) [70] genes are embryonically lethal. of the thyroid-stimulating hormone (TSH) receptor signals Additionally, investigations have shown that TRx1 syn- increased H O production, GPx production is also stim- 2 2 thesized without its penultimate Sec is an apoptosis initiator ulated, thus upregulating antioxidant protection. By virtue (GRIM-12) [71]. The only difference between the truncated of its ability to increase basal metabolism, thyroid hormone GRIM-12 and full-length TRx1 is the absence of the na fi l increases oxygen utilization, thereby increasing the demand two amino acids, Sec and glycine. Truncation occurs when for antioxidant. the codon UGA is interpreted as a stop codon instead of a GPx4 reduces hydroperoxides of membrane phospho- signal for Sec insertion during times of Se deficiency or when lipidfatty acidsand hasparticularrelevance forautism. its Sec is selectively derivatized. eTh Sec-deficient form of Along with TRx1, TRx2, DIO2, DIO3, and selenoprotein P TRx1, GRIM-12, is a notably powerful apoptosis initiator that (SelP), GPx4 is considered an essential selenoprotein whose rapidly induces cell death [71]. levels are preserved in brain and endocrine tissues during Se 6 Autism Research and Treatment deficiency. Suppression of neuronal GPx4 expression resulted that their redox regulation eect ff s are important in these in a selective loss of parvalbumin-expressing GABAergic tissues. eTh low GSH level of neurons ( ∼0.2 mM) provides a interneurons [85] that are essential for dopamine-dependent unique opportunity for redox signaling as a mechanism for regulation during attention [86]and hasbeenlinkedto epigenetic control. Neurotrophic factors stimulate cysteine attention deficit hyperactivity disorder (ADHD) [ 87, 88]. uptake and increase both GSH/GS-SG and SAM/SAH in Earlier studies also showed that expression of these interneu- association with a significant increase in DNA methylation rons was inhibited when glutathione synthesis was impaired, [102]. However, Se-dependent redox regulation is more indicating a critical role for redox status in establishing the vulnerable to soft electrophiles, positively charged chemical capacity for attention [89, 90]. species which bind to selenoproteins with exceptionally high affinity, as discussed below. 3.4. Selenoenzyme Metabolism and Physiology. In addition to the TRx and GPx selenoenzyme families, further selenopro- 4. Genetic Influences and Metabolic teinshaverecentlybeenimplicatedinprocesses knownto Disturbances in ASD be involved in neurodegenerative diseases, including protein folding, degradation of misfolded membrane proteins, and Observations from family studies suggest that ASD has a control of cellular calcium homoeostasis [91]. Cerebral Se strong genetic component [103–105], although the failure to deficiency is associated with neurological disorders such as identify genetic factors aeff cting more than a small propor- seizures and ataxia [92, 93], consistent with a restriction tion of ASD cases suggests that multiple etiologies may be in the development of inhibitory interneurons. Knockout responsible for the pathologies and neurobehavioral features of selenoprotein synthesis in neurons specifically interfered of the disorder [106–117]. Moreover, genetic factors may with development of parvalbumin-expressing GABAergic increasetheprobabilityofoxidativedamageanddiminishthe interneurons, and knockout of GPx4 produced a similar body’s ability to detoxify ROS and free radicals. Interactions deficit, indicating that these neurons have a particular between genetic and environmental factors may potentiate requirement for Se [85]. Thus impaired selenoprotein syn- increased oxidative stress in autistic children. thesis or loss of their activities could contribute to the Genetic risk of autism may be related to a differential neurocognitive dysfunction and seizure activity in ASD. sensitivity to environmental factors. Using a strict definition The cerebral cortex, hippocampus, cerebellum, and olfac- of autism, a recent study found a 58% concordance rate tory bulb express the highest numbers of selenoproteins for monozygotic male twins and 60% for females and 21% [94]. The brain and endocrine tissues are preferentially sup- and 27% for male and female dizygotic twins, respectively plied with Se, predominantly through directed distribution [118]. Using the broader definition of ASD, monozygotic and cellular uptake of SelP. Although other selenoproteins concordance increased to 77% and 50% for males and uniformly incorporate only a single Sec per molecule, SelP females, while dizygotic concordance was 31% and 36%, uniquely contains 10 Sec per molecule. Studies with SelP- respectively. eTh se rates are substantially lower than earlier deficient mice indicate that moderate reductions of brain Se estimates, and the authors concluded that environmental content will impair brain function [95, 96]. factors are more important than genetic factors, although NotonlyisSelPimportant forSec transport, butitalso genetic factors clearly play an important role. Moreover, appearstohaveavitalroleinneurogenesis. Astudy of its no individual genetic cause of autism has been identiefi d brain distribution found a remarkably higher concentration to account for more than 1%-2% of cases and, with the in ependymal cells, which are found at the ventricle surface exception of Rett syndrome, there is no current evidence that [97]. Ependymal cells are a source of neural stem cells ASD is linked to any specific genetic or nongenetic disorder. which are produced upon asymmetric cell division and give However, there is evidence suggesting that epigenetic factors rise to neuronal, astrocyte and oligodendrocyte lineages in and exposures to environmental modifiers may contribute the subependymal region [98]. Neurotrophic growth factors to variable expression of autism-related traits [42]. Variants stimulatemitosis of theseprecursor cells[99]and provide of major eeff ct genes and numerous common variants with an important source of postnatal neurons. SelP is taken up smaller eeff ct genes have been identified in individuals by neurons via apolipoprotein E receptor 2 (ApoER2), which with ASD and related conditions. These genetic variances is localized to synapses, and ApoER2 knockout mice show are providing insights to common pathways and metabolic a decrease in synapse density as well as a decrease in the disturbances affected in ASD, particularly genes involved in number of dendritic spines [100, 101]. oxidative stress and detoxification pathways [ 15, 119, 120]. All brain selenoenzymes are aeff cted by loss of Se in the Polymorphisms of genes involved in glutathione absence of SelP, but the DIOs, TRxs, and GPxs are the seleno- metabolism, including genes for GPx and glutathione S- proteins considered to have the most critical roles in the transferase (GST), have been reportedly associated with ASD. brain and endocrine system. Therefore, loss or compromise GPx1 is the predominant and most abundant isoenzyme of of their functions [96] would have dramatic effects on matu- GPx and plays an integral role in reducing oxidative stress ration of the neuroendocrine system. The unusual capacity by catalyzing the reduction of potentially harmful peroxides. of the brain and endocrine tissues (e.g., pituitary, testes) Ming et al. [121] found significant disequilibrium in the to retain their selenoenzyme activities during prolonged overall transmission of a sequence polymorphism of GPx1 in or even multigenerational deficiency states [ 43]indicates ASD. Williams et al. [122] showed that the GSTP1-313A allele Autism Research and Treatment 7 may be acting as a teratogenic allele, contributing to the in global DNA methylation, while inhibition of MS activity by phenotype of the affected child. GST proteins conjugate and ethanolisassociatedwithalargedecrease[41]. u Th s, xenobi- detoxify products of oxidative stress and conjugate toxins that otics aeff cting redox status can exert an epigenetic influence. produce oxidative stress. By assessing genotypes of mothers Beyond its direct epigenetic regulation of gene tran- and maternal grandparents, it was shown that the GSTP1A scription, DNA methylation also regulates the activity of haplotype was signicfi antly more frequently transmitted to repetitive transposable elements dispersed throughout the mothers of individuals with ASD, suggesting that it may human genome. Transposable elements comprise about 45% be acting in mothers during pregnancy to contribute to the of the genome, and their earlier description as “junk DNA” phenotype of autism during fetal development [122]. has recently been revised in recognition of their ability James et al. [15] examined the frequency of several single to modulate gene transcription, mRNA splicing, micro- nucleotide polymorphisms (SNPs) capable of aeff cting redox RNA formation, and other processes [129]. Reflecting their and methylation pathways in autistic subjects. eTh y found viral origin, retrotransposons such as the LINE-1 (long significant differences in allele frequencies for the reduced interspersed nuclear element-1) family are suppressed by folate carrier (RFC 80G>A),transcobalaminII(TCN2 776 methylation but can replicate and transpose to new locations, G>C), methylenetetrahydrofolate reductase (MTHFR) 677 especially during early development and especially when C>Tand 1298 (A>C), catechol-O-methyltransferase (COMT methylation is suppressed. Based upon its impressive quanti- 472 G>A), and GST M1 between autistic and control cohorts. tative contribution to the genome, methylation of LINE-1 has These differences were associated with abnormal metabolite been used as a surrogate for global DNA methylation [130], levels, suggesting that individuals with genetic vulnerability and factors regulating MS activity aeff ct LINE-1 methylation affecting redox and methylation capacity may be linked [131]. LINE-1 retrotransposition is reported to occur at a to a higher risk for autism. Any decfi it in the function of higher rate in brain than to other tissues [132], and a selenoproteins could synergize with these genetic risk factors. higher rate was observed in Rett syndrome subjects carrying DNA copy number variants (CNVs) represent a major mutations in the methylated DNA binding protein MeCP2 category of genetic risk for ASD and are implicated in approx- [130]. Although more studies are needed to clarify their imately 10% of cases [123, 124]. Several of the genes likely specific contribution, transposable elements such as LINE- affected by homozygous deletions are regulated by neuronal 1 are poised to provide a global genomic influence during activity, and the expression of these genes can change in development, so agents aeff cting their methylation state are response to neuronal stimulation. Synapses mature partially likely to disrupt this process. as a function of experience-dependent neuronal activity, Oxidative stress and decreased methylation capacity are so disruption of those genes by mutation or copy number common in autism and abnormal epigenetic regulation may variationmay alterthe processofsynapticdevelopment. link the metabolic abnormalities to disruptions in brain DNA methylation status is associated with the occurrence of development. Other comorbid features of autism, such as CNVs [125], raising the possibility that impaired methylation autoimmunity and gastrointestinal (GI) dysfunction [133, capacity could contribute to increased CNVs in ASD. 134], may reflect similar manifestations of abnormal epi- genetic regulation. A genome-wide comparison of DNA methylation in monozygotic twins discordant for autism found numerous differentially methylated regions associated 5. Epigenetic Disturbances in ASD with ASD, and the extent of these differences were correlated with severity of autistic trait scores [135]. Epigenetic regulation utilizes covalent modifications such as DNA methylation and the addition/removal of various From conception to maturation, human development is chemical moieties to histone tails (collectively known as epi- a highly orchestrated expression of epigenetic regulation, so genetic marks) to provide stable, transgenerational changes it is not surprising that genetic and environmental factors in gene expression without alteration of the underlying adversely affecting oxidative tone and methylation status nucleotide sequence [126, 127]. Epigenetic marks are dynamic can contribute to developmental disorders. The exceptionally and highly sensitive to cellular changes [128]. u Th s, normal dynamic redox-dependent epigenetic regulation in the brain physiologic changes in the cellular environment, such as increases its vulnerability to neurodevelopmental disorders. Autism is a prominent feature of Rett, Angelman, Prader- levels of growth factors, hormones, and neurotransmitters, as well as xenobiotic exposure, can translate into modifications Willi, and Fragile-X syndromes, each of which has been in gene expression mediated by epigenetic regulation. Xeno- linked to interruption of methylation-dependent regula- biotic exposures aeff cting epigenetic status can, therefore, tion [136–138]. Therefore, environmental exposures aeff cting not only produce lifelong consequences, but their eeff cts redox and methylation status could reasonably result in can be transmitted through germline cells to aeff ct multiple neurodevelopment disorders such as ASD. succeeding generations [126, 128]. As noted above, MS exerts powerful control over all methylation reactions via its influence over the SAM/SAH 6. ASD in relation to Exposures to ratio, and MS inhibition by oxidative stress will cause both a Potentially Neurotoxic Agents decrease in SAM and an increase in SAH, while reducing con- ditions will have the opposite effect. A twofold increase in MS Certainsoft electrophilesare knowntobeneurotoxicat activity induced by IGF-1 is associated with a twofold increase high exposures, presumably due to their eeff cts on sulfur 8 Autism Research and Treatment metabolism. Although these electrophilic species are highly metals [143, 144]. Thisisconsistentwiththe nfi ding that interactive with cellular nucleophiles such as thiols, the Se of lower Hg concentrations are present in the hair of young Sec is by far the strongest intracellular nucleophile. er Th efore, (<6 y) children with ASD [144–146]althoughMajewskaet selenoproteins are very vulnerable to enzyme inhibition by al. [146] found older ASD children have higher hair Hg. binding to neurotoxic electrophiles. eir Th toxic concentra- Although serum was studied instead of RBC’s or whole blood, tions are generally miniscule in relation to sulfur, toxic levels alarge studyofHginrelationtoautism[147]reported generally equal or exceed the normal tissue concentrations of finding no significant differences between nonautistic and Se. er Th efore, because of their high reactivity and low molar ASDchildren. Thisissupported by thefindingthatno abundance, selenoenzymes are highly vulnerable to selective distinctions in expression levels of four genes that are known inhibition by high concentrations of electrophiles such as Hg. to respond to metal exposures were noted between ASD and Soft electrophiles such as Hg have larger ionic radii and a typical children [148]. However, Stamova et al. [149]found more dispersed surface charge, making them more reactive a distinctive correlation between gene expression and blood with soft nucleophiles such as the Se of Sec at the active sites Hg levels in boys with autism suggesting it is associated of enzymes. with a different pattern of gene transcription in response Only certain electrophiles are notably neurotoxic. o Th se to Hg exposure. The ability of both Hg and Se to exert that bind and potentially sequester Se would cooperatively epigenetic effects was recently demonstrated in embryonic diminish the biological availability of Se for performance stem cells [150]. Several studies have reported potent toxic of its necessary physiological roles (Figure 1). For example, effects of methylmercury on neural stem cell differentiation a multitude of electrophilic agents are naturally present in and survival [151, 152], indicating its potential capacity for food in small amounts and the Se-sequestering eeff cts of each altering gene expression during development. would usually be minor. However, their additive effects on To prevent bacterial contamination in multiple-dose selenoenzyme synthesis and function could be detrimental in vials, thimerosal, the ethylmercury derivative of thiosalicylic individuals with compromised Se status or metabolism. acid, has been used as a preservative in various medical Likewise, additional exposures to electrophiles are products, including vaccines. As autism rates increased, encountered in the form of environmental contaminants, Bernard et al. [153] suggested that vaccine-derived Hg might such as toxic metals, pesticides, herbicides, and others. be a contributing cause, a highly controversial proposal In addition, soft electrophiles are present as the active [154–156]. As a result, thimerosal was removed from all ingredients in many pharmaceuticals and food preservatives, pediatric vaccines, except for some influenza vaccines, in whilestill others areproducedduringthe degradation of the United States starting in 2001, but the incidence of these products. eTh refore, instead of examining relationships autism continued to rise [157], furthering the doubts that between ASD incidence and exposures to individual agents, vaccine-derived Hg exposures contributes to autism inci- it may be more informative to examine ASD incidence in dence. While a number of epidemiological studies do not relation to aggregate exposures to these soft electrophiles indicate an association between thimerosal exposure and and/or their effect on selenoenzyme activity in vulnerable ASD [158, 159], possible associations between developmental individuals. disorders with Hg-containing vaccines [157]and delayedor even transgenerational influence of epigenetic changes have been suggested [160]. Such genetic or epigenetic defects of 6.1. iTh o-/Selenoreactive Elements. High exposures to soft the antioxidant enzyme system could cooperatively inter- act with other environmental electrophiles and make vul- electrophiles have the potential to incapacitate various sulfur- and Se-dependent metabolic processes, thus disrupting many nerable individuals more sensitive to exposure levels that redox regulatory mechanisms that are required for healthy would otherwise be harmless. Since deficits in selenoen- cell growth and function, particularly in brain and endocrine zyme synthesis or function can increase the potential for tissues. Increased selenium status is known to counteract oxidative stress and epigenetic dysregulation, sensitivity to neurotoxins, such as Hg and similar soft electrophiles, may the adverse effects of elevated exposures to neurotoxic electrophiles such as Hg, cadmium (Cd), lead (Pb), and differ among individuals. In this regard, there have been vanadium (V) [139]. These electrophilic elements may all be several reports of decreased GPx activity in autism [161, 162] and selective transmission of GPx-1 allelic variants [121]. capable of selective, irreversible inhibition of selenoenzyme activities similar to the mechanism of Hg toxicity [140, 141]. Interestingly, Pa¸c s a et al. [120] found an inverse correlation However, relationships between Se status and the neurotoxic between homocysteine and GPx activity in autistic subjects, effects of these other elements have not been adequately indicating an association between low GPx activity and examined, and additional mechanisms of toxicity have been impaired methylation. Therefore, Casta neda ˜ et al. suggested recognized forsomeofthese elements [142]. Because it is rich that the removal of thimerosal from vaccines might not in cysteine, hair has oen ft been used to provide a reflection be immediately reflected as a reversal of epigenetic effects, of circulating amounts of thio-reactive electrophiles present especially if they involved effects on germline cells [ 163]. Notably, lower levels of DNA methylation increase novel in exposed individuals. The concentrations of Hg, As, Cd, or Pb in hair do not indicate consistent relationships with insertions of transposable elements and increase the fre- ASD incidence; however, some studies report unusually low quency of CNVs in germline cells [164–166], which are also elevated in autism. eTh refore, the eeff cts of elemental concentrations in hair samples and suggest that individu- als with ASD have diminished abilities to eliminate toxic electrophiles on selenometabolism may be a contributing Autism Research and Treatment 9 Predisposing factors Epigenetic factors Genetic factors Poor dietary Se intakes Potential etiologic egents Defective Se–enzyme synthesis Inordinate exposures to soft Diminished toxin excretion Diminished toxin degradation electrophiles, e.g., Hg, Pb, Cd, and/or potentially hazardous organic molecular species that Biochemical outcomes interact with selenoenzymes. Increased oxidative damage Decreased reversal of oxidative damage Endocrine disruption Epigenetic disruption Impaired immune response Pathological effects Degradation of cell functions Diminished neuroplasticity Impaired neurodevelopment Neuronal dysfunction Neuronal cell death Autism/ASD Figure 3: Depiction of potential etiologic contributors to disruptions of selenoenzyme physiology that may lead to disruptions of redox control and pathological consequences of autism and ASD. eTh factors and agents depicted are not all necessarily involved, but increases in predisposing factors along with additive contributions of increased exposures to thio- and selenoreactive electrophiles would be expected to increase the likelihood of progression to pathology. factor to ASD etiology and/or progression in vulnerable inactivate N-acetyl-p-benzoquinone imine (NAPQI), a individuals. metabolic product of acetaminophen breakdown that is thoughttobethe proximal causeofhepatotoxicityfollowing acetaminophen overdoses. When excessive quantities of 6.2. iTh o-/Selenoreactive Organic Electrophiles. Organic mol- NAPQI are formed, the primary metabolic (glucuronide ecules such as acrylamide, acrolein, and diethyldithio- and sulfate conjugation) pathways apparently become carbamate are structurally diverse, but these electrophilic saturated. N-acetylcysteine is thought to counteract toxicity species all share the potential to chemically react with by either reducing NAPQI to the parent compound or strong nucleophiles [167]. Just as for inorganic electrophiles, providing sulyfh dryl for conjugation of this metabolite [ 176]. the most likely target will be the most nucleophilic moi- If supplementation with a sulfhydryl-containing compound eties of enzymes, such as Sec or thiols of Cys residues. such as N-acetylcysteine can directly inactivate NAPQI, Acetaminophen, the most commonly used analgesic and supplementation with Se to restore selenoenzyme status antipyretic drug in much of the world, is associated with may be an important adjunct therapy to restore healthy toxic eeff cts at high exposures. Excessive intake leads to redox status in the affected tissues. eTh signicfi ance of the impaired sulfur metabolism and life-threatening hepatotox- liver in providing Se for delivery to the brain suggests that icity, involving depletion of GSH [168]. A survey of parents compromised Se availability in the liver temporarily induced reported a higher frequency of acetaminophen use aer ft the by exposures to NAPQI and/or other electrophiles could MMR (measles-mumps-rubella) vaccine for autistic children diminish theamountofSethe brainreceives. Exposures than for unaffected children [ 169], leading to the suggestion to agents which could cause either prolonged or excessive that use of acetaminophen might be causally linked to an diminishments in the supply of Se to neuroendocrine tissues increase in autism rates [170]. may therefore be important factors to consider in relation to Interestingly, the major acetaminophen-binding protein ASD. in the liver is Se binding protein-2 (SeBP2) [171, 172]. Both SeBP1 and SeBP2 bind Se, but not in the Sec form charac- teristic of the genetically encoded selenoproteins. Increased 7. Summary and Conclusions expression of SeBP2 is associated with increased suscepti- bility to acetaminophen cytotoxicity [173]. In view of the The nosology of ASD is complicated by the difficulties to male predominance of autism, it is interesting to note that differentiate the syndrome into subsets with similar symp- SeBP2 levels are higher in males [174] and their vulnerability toms and distinct etiologies. Children that share the diagnosis to acetaminophen hepatotoxicity is also greater. Males also of ASD represent more than one distinct pathophysiological display a decreased capacity to restore their GSH levels to condition. Recognizing and distinguishing between groups normal [175] following high acetaminophen exposures. with separate etiologies require identification of objective N-acetylcysteine protects against acetaminophen- laboratory indices that clinicians can use for diagnosis and induced hepatotoxicity by maintaining or restoring hepatic to monitor progression and treatment eeff cts. eTh objective of concentrations of GSH [176]. Glutathione is required to this review is to discuss metabolic defects that may contribute 10 Autism Research and Treatment to theonset andpathology of ASD, particularly in relation to [5] I. N. Pessah, R. F. Seegal, P. J. Lein et al., “Immunologic and neurodevelopmental susceptibilities of autism,” NeuroToxicol- Se physiology. ogy,vol.29, no.3,pp. 532–545, 2008. Existing evidence indicates children with ASD have disruptions in GSH metabolism, and that impaired selenoen- [6] M. L. 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Publisher: Hindawi Publishing Corporation
Copyright: Copyright © 2014 Laura J. Raymond et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ISSN: 2090-1925
eISSN: 2090-1933
DOI: 10.1155/2014/164938
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Abstract

Hindawi Publishing Corporation Autism Research and Treatment Volume 2014, Article ID 164938, 15 pages http://dx.doi.org/10.1155/2014/164938 Review Article Potential Role of Selenoenzymes and Antioxidant Metabolism in relation to Autism Etiology and Pathology 1 2 1 Laura J. Raymond, Richard C. Deth, and Nicholas V. C. Ralston Energy & Environmental Research Center, University of North Dakota, 15 North 23rd Street, Stop 9018, Grand Forks, ND 58202, USA Department of Pharmaceutical Sciences, Northeastern University, Boston, MA 02115, USA Correspondence should be addressed to Laura J. Raymond; lraymond@undeerc.org Received 11 September 2013; Revised 7 January 2014; Accepted 27 January 2014; Published 5 March 2014 Academic Editor: Klaus-Peter Ossenkopp Copyright © 2014 Laura J. Raymond et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Autism and autism spectrum disorders (ASDs) are behaviorally defined, but the biochemical pathogenesis of the underlying disease process remains uncharacterized. Studies indicate that antioxidant status is diminished in autistic subjects, suggesting its pathology is associated with augmented production of oxidative species and/or compromised antioxidant metabolism. This suggests ASD may result from defects in the metabolism of cellular antioxidants which maintain intracellular redox status by quenching reactive oxygen species (ROS). Selenium-dependent enzymes (selenoenzymes) are important in maintaining intercellular reducing conditions, particularly in the brain. Selenoenzymes are a family of∼25 genetically unique proteins, several of which have roles in preventing and reversing oxidative damage in brain and endocrine tissues. Since the brain’s high rate of oxygen consumption is accompanied by high ROS production, selenoenzyme activities are particularly important in this tissue. Because selenoenzymes can be irreversibly inhibited by many electrophiles, exposure to these organic and inorganic agents can diminish selenoenzyme- dependent antioxidant functions. This can impair brain development, particularly via the adverse influence of oxidative stress on epigenetic regulation. Here we review the physiological roles of selenoproteins in relation to potential biochemical mechanisms of ASD etiology and pathology. 1. Introduction Although ASD was previously thought to be rare, the number of persons receiving treatment for ASD has increased The causes of autism and autism spectrum disorder (col- substantially during the past several decades and continues lectively, ASD) remain unknown, in part because of com- to increase. A recent US government report estimated the plex behavioral phenotypes and the likelihood that multiple prevalenceofASDincreasedby78%from2002to2008[8]. In genetic and environmental factors contribute to its etiology 2011-2012, a prevalence of 20 per 1000 was reported for school [1–3]. In the absence of biochemical tests for ASD, the aged children [9]. However, a portion of ASD’s increasing diagnosis is based solely on clinical assessment of behavioral incidence may reflect changes in diagnostic practice and the criteria that define deficits in social interaction, impairments broadening of diagnostic criteria [10]. Other studies indicate in verbal and nonverbal receptive/expression, speech, and adiagnosticshiftorsubstitutionmay also have contributed hyperfocused repetitive behaviors. eTh pathophysiology of to theriseindiagnosis,whereby theincreaseinautism ASD is primarily expressed in the neurologic, immunologic, diagnoses corresponds with declines in the usage of other and gastrointestinal (GI) systems and aeff cts four times as diagnostic categories [11, 12]. Based on a meta-analysis of many boys as girls [4–6]. Regression, with loss of previ- ASD studies, McDonald and Paul [13] concluded that it does ously acquired skills, can also interrupt apparently normal not seem possible to assess whether or how much of the development. Children with severe autism can exhibit mental observed increases in cumulative incidence are real, although the number of individuals identified as having ASD has retardation, and autistic children have an elevated rate of increased dramatically. seizure disorders [7]. 2 Autism Research and Treatment Table 1: Glutathione and oxidative stress in autism. Authors (Reference) Control𝑛 Autistic𝑛 GSH status Additional related findings 33 20 46%↓ James et al. [14] ↓GSH/GSSG,↓SAM/SAH,↓cysteine James et al. [15] 73 80 32%↓ ↓GSH/GSSG,↓SAM/SAH,↓cysteine Lab-based normal values 10 36%↓ D. A. Geier and M. R. Geier [16] ↓Cysteine 55 43 21%↓ Adams et al. [17] ↓SAM,↓cysteine,↓vitamin E,↑FIGLU 13 15 33%↓ Pa¸c s a et al. [18] ↓Cysteine Pastural et al. [19] 12 15 35%↓ ↓Cysteine 30 30 27%↓ Al-Gadani et al. [20] ↑Lipid peroxides,↓vitamin E,↓SOD,↓GPx ↓GSH/GSSG,↓SAM/SAH,↓cysteine,↓DNA Melnyk et al. [21] 40 40 29%↓ methylation 42 40 28%↓ James et al. [22] ↓GSH/GSSG,↓SAM/SAH,↓cysteine 120 28 24%↓ Geier et al. [23] ↓Cysteine Geier et al. [24] Lab-based normal values 28 24%↓ ↑GSSG,↓cysteine,↓taurine,↓sulfate Until reliably accurate differential diagnoses are achieved, and exogenous antioxidant capacity are commonly reduced it is difficult to attain the goal of defining the biochemical in ASD. Glutathione (GSH) is the primary intracellular and physiological lesions that initiate and/or perpetuate the antioxidant, and the ratio of its reduced (GSH) and oxidized dysfunctions of autism. However, one pathological mecha- (GSSG) forms (GSH/GSSG) provides a useful index of redox nism present in many children with ASD involves defects in status. As shown in Table 1, numerous studies have reported the control of oxidative damage. Distinctions in the nature significantly lower plasma levels of GSH and, in some cases, of these perturbations in redox control may provide insight lower GSH/GSSG levels. Low GSH levels are associated with for identifying biochemically defined patient subgroups that oxidative stress, increased inflammation, impaired immune may respond to specific therapeutic interventions. This response, and a decreased ability to detoxify environmental paper focuses on potential associations between genetic and contaminants. Autistic children have been reported to be acquired defects in control of oxidative damage, particularly increasingly susceptible to recurrent infections, neuroinflam- those that impinge upon selenium- (Se-)dependent enzymes mation, gastroinflammation, and impaired antioxidant and (selenoenzymes). Se physiology is a vital process in the detoxification capacity. Diminished glutathione peroxidase brain and neuroendocrine system, [25, 26]and,because (GPx), superoxide dismutase, and catalase enzyme activities of the high reactivity and low abundance of Se in these have been associated with ASD, as well as low cysteine, Se, tissues, its vulnerability to inhibition by a variety of toxicants zinc (Zn), and Vitamins C, E, and A [32], although these is markedly enhanced. However, its potential role in the associations are not consistently observed. pathophysiology of ASD remains largely unexplored. Accumulation of oxidized glutathione (GS-SG) in plasma is a strong indication of intracellular oxidative stress, as cells export the GS-SG to maintain redox equilibrium. James and 2. Oxidative Stress in ASD coworkers [14] were rfi st to report that plasma levels of cysteine, GSH, and the GSH/GS-SG ratio were significantly Chemically reactive oxygen-derived products like peroxide decreasedinautisticchildren. In that study, totalGSH 2− radicals ( O ), hydrogen peroxide (H O ), superoxide– levels were decreased, and GS-SG was increased, resulting 2 2 − 1 anion (O ), singlet oxygen ( O ), and hydroxyl radicals in a threefold reduction in the redox ratio of GSH/GS-SG. 2 2 ( O H), are products of ongoing aerobic metabolism via Cysteine, the rate-limiting amino acid for GSH synthesis, mitochondrial oxidative phosphorylation [27]. If not inter- was significantly decreased relative to controls in over 65% of the autistic children tested. The finding of lower-plasma cepted and detoxified, reactive oxygen species (ROS) are capable of chemically damaging all forms of cellular macro- GSH has since been replicated by other research groups [17– molecules. To avoid these consequences, numerous ROS- 20, 33], suggesting this is a prevalent feature of ASD. A recent detoxifying reactions enable cells to maintain redox equi- meta-analysis nds fi that children with ASD have decreased librium and metabolic homeostasis. us Th , oxidative stress blood GSH (27%), GPx activity (18%), and methionine (13%) is a condition where the level of ROS production exceeds and increased concentrations of GS-SG (45%) relative to antioxidant capacity. nonautistic children [34]. In addition, levels of NADPH and Increased oxidative stress has been observed in children NADH, which reflect redox status and help maintain GSH in its reduced state, were found to be significantly lower in with ASD [28–30]. Blood collected from autistic children shows low concentrations of membrane polyunsaturated autistic children [32]. Several studies have reported a decrease lipids, higher phospholipase A , and loss of the normal in the level of GSH in postmortem brain samples from ASD asymmetry of membrane lipoproteins, which may indicate subjects, associated with a decrease in the GSH/GS-SG ratio increased oxidative damage [13, 31]. Levels of endogenous and increased levels of oxidative stress biomarkers [35, 36]. Autism Research and Treatment 3 Selenocysteine (—) 2+ HgSe Hg (—) CdSe? PbSe? Adducts with 2+ 2+ Cd , Pb , etc. other electrophiles? (—) Organic electrophiles, Organic electrophile adducts, e.g., NAPQI e.g., NAPQI-Se? Selenoenzyme activities Repair oxidative damage Regulation of redox state (e.g., GPx4, MsrB) (e.g., TRx1, TRx2, ... ) Antioxidant activities (e.g., GPx1, TRx1, ... ) Thyroid hormone regulation (e.g., DIO1, DIO2) Intracellular redox status Methionine DNA and Epigenetic Healthy brain synthase regulation of histone development activity methylation gene expression Figure 1: Selenoprotein synthesis and activities are sensitive to elemental and organic electrophiles. High exposures to soft electrophiles may additively impair redox regulation and thyroid hormone production, disrupting epigenetic regulation and normal brain development. In addition, activities of several GSH-related enzymes, group of autistic children showed dramatic increases. Levels including the selenoenzyme GPx, were lower in cerebellums of urinary 8-hydroxy-2-deoxyguanosine (8-OHdG), a major of ASD subjects [37]. productofDNA oxidation, were also measured butdid eTh folate and Vitamin B -dependent enzyme methio- not reach statistical significance, although they indicated nine synthase (MS) is inhibited under oxidative stress con- a trend toward increasing concentrations in children with ditions, resulting in a decrease in all methylation reactions, autism [30]. No significant correlations were noted between including DNA methylation [38]. The basis for MS inhibi- the levels of these biomarkers and vitamin intake, dietary tion is oxidation of its Vitamin B (cobalamin) cofactor, supplements, medicine, medical disorders, or history which is considered the most readily oxidized biomolecule, of regression. er Th efore, these results suggest that lipid making it an idealsensorofcellularredox status [39, 40]. peroxidation is increased in autistic children and that certain Lower MS activity inhibits methylation by lowering the autistic children have much greater oxidative stress than ratio of the methyl donor S-adenosylmethionine (SAM) to others. the methylation inhibitor S-adenosylhomocysteine (SAH), exerting a global dampening effect on >200 methylation reactions. Arguably, the most important among these are 3. Selenium-Dependent Antioxidant methylation of DNA and histones, which combine to exert Metabolism and ASD epigenetic regulation over gene expression. As noted above, a decrease in SAM/SAH has been documented in plasma Selenoproteins are essential for brain development, redox of ASDsubjectsinassociation with adecreaseinGSH/GS- control, and preventing and reversing oxidative damage in SG, reflecting the reciprocal relationship between oxidative the brain and neuroendocrine tissues (Figure 1,Table 2). stress and methylation. In neuronal cells methionine synthase eTh refore, control of intracellular oxidative tone and n fi dings activity is stimulated by growth factors and dopamine but of increased oxidative damage in children with ASD may inhibited by neurodevelopmental toxins, including mercury be indicative of disruptions of selenoenzyme activities. The (Hg) [41]. Methionine synthase mRNA levels are significantly molecular forms of Se most common in foods are the amino decreased in postmortem brains of autistic subjects, indica- acids selenocysteine (Sec) and selenomethionine (SeMet), tive of a deficit in methylation capacity secondary to oxidative although traces of water-soluble inorganic forms (e.g., sele- stress [42]. nate andselenite) canalsobepresent in food anddrinking Elevated urinary levels of 8-isoprostane-F (8-iso- water. For both the organic and inorganic Se forms, bio- 2𝛼 PGF ) and malondialdehyde (MDA), oxidative stress chemical utilization in selenoenzymes is initiated through the 2𝛼 biomarkers, have also been noted in children with autism common intermediate hydrogen selenide (H Se). eTh refore, [28, 30]. A bimodal distribution of 8-iso-PGF was all ingested (and endogenous) forms of Se must be degraded 2𝛼 reported, with the majority of autistic subjects showing to inorganic selenide before Se can be used for synthesis of moderate increases in isoprostane levels, while a smaller Sec, the physiological active form of Se. Although proteins 4 Autism Research and Treatment − Selenocysteine-based − Cysteine-based e e Glucose NADPH redox ednzymes redox proteins (e.g., TRx, TGR, GPx, MsrB) (GSH, T[SH] , Prx) − − GR − − ∙ Reduction of ascorbate and other small molecules. e e GSSG GSH Grx ∙ Reduction of oxidized methionine and phosholipids. ∙ Reduction of glutathionylated proteins. Figure 2: Selenoenzymes are central to providing antioxidant electrons to accomplish reduction of molecules in a number of biochemical processes. NADPH = nicotinamide adenine dinucleotide phosphate; GR = glutathione reductase; T(SH) = reduced thioredoxin; GSH = reduced glutathione; TRx = thioredoxin reductase; GSSG = oxidized glutathione; TGR = thioredoxin-glutathione reductase; GPx = glutathione peroxidase; MsrB = methionine sulfoxide reductase; Prx = peroxiredoxin; Grx = glutaredoxin; e =electron. Levels of plasma GSH, erythrocyte NADH, and NADPH are notably reduced (𝑃<0.001 )inchildrenwithautism[16, 32]. with SeMet contain Se, they are not considered selenoproteins selenoproteins are known, only members of the major sele- because SeMet is nonspecifically incorporated into proteins noenzyme/selenoprotein families with potential relevance to as if it were Met. The nonspecific insertion of SeMet or Met is autism etiology and pathology are discussed below. directed by AUG codons and no significant distinctions in the biochemical functions have been observed. This is in contrast 3.1. Roles of Selenoenzymes in yTh roid Hormone Regulation. to Sec, which is the catalytically active primary amino acid Selenoenzymes regulate thyroid synthesis and metabolic present in all selenoproteins [43] and is responsible for the functions contributing to thyroid hormone biosynthesis, principal functions of these enzymes (Table 2). In contrast to antioxidant defense, redox control of thyrocytes, and thyroid other amino acids, Sec is not recycled for reincorporation into hormone metabolism. yTh roid hormones have important new proteins but is, instead, degraded to release inorganic Se rolesinregulatingmanykeybiochemicalreactions,especially which can be utilized for resynthesis of Sec. protein synthesis and enzymatic activity, accompanied by an While Cys, the analogous sulfur amino acid, is inserted increase in basal metabolic rate. yr Th oid hormone regulates at UGU/UGC codons, the insertion of Sec is in response several processes that are associated with brain differentia- to UGA which is otherwise the “opal” stop codon for other tion, including dendrite and axon growth, synaptogenesis, proteins [44–46]. The selenoprotein mRNAs include a dis- neuronal migration, and myelination [51]. Disruption of tinct Sec insertion sequence (SECIS) stem-loop structure in thyroid hormone production during early child development their untranslated 3 region of the mRNA. This is recognized leads to permanent deficiency in intelligence and sensorimo- by specific SECIS binding proteins which function together tor functions [52], and it is hypothesized that Se deficiency with several transacting factors as well as a unique tRNA may be responsible for the initiation of autoimmune thy- with an anticodon complementary to UGA to initiate de novo roid disorders [53]. Interestingly, thyroid hormone increases Sec synthesis. eTh tRNA is aminoacylated with serine prior plasma Se levels, as well as the levels of selenoenzymes such to biosynthesis of Sec which is inserted into the protein’s as DIO [54]. This relationship is logical, since increased primary structure. In mRNA of most selenoproteins, the metabolic activity places a higher demand on antioxidant UGA of the Sec insertion codon is followed by a second or resources. terminal UGA, which is then read as the stop codon. The central nervous system is very sensitive to thy- Since the discovery of these genetically unique pro- roid hormone supply during growth and development. eTh teins, theirenzymeactivitieshavebecomeincreasinglywell selenoprotein family of DIOs (DIO1, 2, and 3) is involved in den fi ed. Associations between compromised Se and ASD the formation and regulation of the active thyroid hormone, have been reported including low Se levels in red blood cells triiodothyronine (T ). More than 80% of T in the brain 3 3 [47]. Since blood Se is less prone to contamination and more is derived from intracellular deiodination of T by DIO2 indicative of tissue selenoenzyme activities, it is considered [55, 56]. Since circulating T does not readily gain access to to be amorereliableindex than hair.Indeed,hairSeis intracellular nuclear receptors [57], DIO2 provides an impor- variously reported as being increased [48], decreased [49], or tant regulatory function in the brain and central nervous unchanged [50]insubjectswithASD. system (CNS). During early childhood, T bound to nuclear When considering the developmental roles of selenopro- receptors is entirely dependent on its local production from teins, as well as their involvement in redox control and pro- T via this selenoprotein. tection from oxidative stress, the potential for selenoenzyme dysregulation in relation to the pathologies associated with ASD appears worthy of investigation. The three main fam- 3.2. Roles of TRx in Redox Regulation. The three distinct ilies of characterized selenoproteins—iodothyronine deio- forms of TRx—TRx1, TRx2, and TRx3 (collectively, TRx)— dinases (DIO), thioredoxin reductases (TRx), and GPx— are important in controlling redox state within major com- have critical roles in thyroid function, fetal development, partments of the cell. While TRx1 (cytosolic) and TRx2 hormone metabolism, and oxidative stress detoxification, (mitochondrial) restore oxidized thioredoxin (T[S-S]) to its particularly in endocrine and brain tissues. Although∼25 reduced (T[SH] )formand areresponsible forreducinga 2 Autism Research and Treatment 5 Table 2 Selenoprotein References Functions GPx1 [58] Detoxifies peroxides in aqueous compartment of cellular cytosol GPx2 [59] Expressed in cytosol of liver and tissues of the digestive system GPx3 [60] Synthesized primarily by kidney; secreted into plasma for transport to other tissues GPx4 Prevents and reverses oxidative damage to lipids in brain and other tissues [61] Reduces T(SH) , vitamin C, polyphenols, and other substrates to regulate intercellular TRx1 [62–64] redox state TRx2 [63, 64] Located in mitochondria and controls and regulates redox state TRx3 [63, 64] Reduces mitochondrial glutathione disulfide, abundant in testes MsrB1 [64] Restores oxidatively damaged methionine (R-sulfoxides) to native configuration DIO1 Converts T (thyroxine) prohormone into T (active thyroid hormone) [65] 4 3 DIO2 [65] Regulates thyroid hormone status, activating as well as inactivating T DIO3 [65] Activates thyroid hormone in brain, placenta, important in fetal development SPS2 [64] Creates the Se-phosphate precursor required for synthesis of all selenoproteins SelM Notably high expression levels in the brain, possible thiol-disulfide oxidoreductase [64, 66] SelN [64] Interacts with ryanodine receptor, mutations result in congenital muscular dystrophy SelP [64] Transports Se in plasma (10 Sec/molecule) and delivers Se to brain and endocrine tissues SelW [64, 66, 67] Expressed in a variety of tissues and may regulate redox state of 14-3-3 proteins Sel15 Oxidoreductase that may assist in disulfide formation and protein folding [64] variety of other essential antioxidant molecules [63]includ- 3.3. Roles of GPx in Redox Regulation. GPx (vfi e geneti- ing Vitamin C [62]. Thioredoxin is a ubiquitous 12 kDa cally distinct forms, GPx1–4 and GPx6) are selenoenzymes protein that employs vicinal cysteines (CXXC motif) and involved in antioxidant defense and redox regulation and becomes oxidized to intramolecular disulfides T(S-S) during modulation. GPx provide protection against oxidative dam- reduction of other molecules (Figure 2). Itsactionisessential ageand aidinthe maintenanceofmembraneintegrity by for countering oxidative damage in the cytosol of aerobic using GSH as a cofactor to catalyze reduction of hydro- organisms from bacteria to humans [68]. Since T(SH) is gen peroxide, forming oxidized glutathione (GS-SG) in the a central regulator of cellular redox status that is required process. yTh roid hormone synthesis requires a continuous for the redox-regulated function of transcription factors production of high concentrations of H O ,which appears 2 2 and hormonally regulated nuclear receptors, it is critical to be its rate-limiting step [72–74]. eTh refore, since the in DNA production, gene expression, cell survival, and thyrocyte is continually exposed to potentially toxic con- embryogenesis. u Th s TRx maintains T(SH) levels to enable centrations of H O and lipid hydroperoxides, appropriate 2 2 2 basic processes and regulate multiple metabolic events. eTh antioxidant defense systems are essential to control excess antioxidant functions of TRx occur because they directly oxidative stress. Three of the vfi e GPx are expressed in facilitate reduction of oxidized proteins through cysteine- thyrocytes and thyroid tissue [75–77]. Studies indicate a thiol-disulfide exchange, forming an oxidized disulfide T(S- distinct regulation of expression, secretion, and function S) in the process. TRx is also directly involved in prevention of these selenoproteins for controlling thyrocyte growth, and repair of damage caused by H O -based oxidative stress. differentiation, and function [ 76–83]. When Se intake is 2 2 Because intracellular reduction of selenite is required for de adequate, the intracellular GPx and TRx systems protect novo synthesis of Sec selenoproteins, TRx clearly has a central the thyrocyte from peroxides; however, in Se deficiency, the role in all Se physiology. It is assumed that Se’s pivotal role in thyrocyte’s apoptotic response to H O is increased [84]. 2 2 TRx explains why targeted disruption of the TRx1 [69]and Furthermore, in iodine deficiency, where hyperstimulation T(SH) [70] genes are embryonically lethal. of the thyroid-stimulating hormone (TSH) receptor signals Additionally, investigations have shown that TRx1 syn- increased H O production, GPx production is also stim- 2 2 thesized without its penultimate Sec is an apoptosis initiator ulated, thus upregulating antioxidant protection. By virtue (GRIM-12) [71]. The only difference between the truncated of its ability to increase basal metabolism, thyroid hormone GRIM-12 and full-length TRx1 is the absence of the na fi l increases oxygen utilization, thereby increasing the demand two amino acids, Sec and glycine. Truncation occurs when for antioxidant. the codon UGA is interpreted as a stop codon instead of a GPx4 reduces hydroperoxides of membrane phospho- signal for Sec insertion during times of Se deficiency or when lipidfatty acidsand hasparticularrelevance forautism. its Sec is selectively derivatized. eTh Sec-deficient form of Along with TRx1, TRx2, DIO2, DIO3, and selenoprotein P TRx1, GRIM-12, is a notably powerful apoptosis initiator that (SelP), GPx4 is considered an essential selenoprotein whose rapidly induces cell death [71]. levels are preserved in brain and endocrine tissues during Se 6 Autism Research and Treatment deficiency. Suppression of neuronal GPx4 expression resulted that their redox regulation eect ff s are important in these in a selective loss of parvalbumin-expressing GABAergic tissues. eTh low GSH level of neurons ( ∼0.2 mM) provides a interneurons [85] that are essential for dopamine-dependent unique opportunity for redox signaling as a mechanism for regulation during attention [86]and hasbeenlinkedto epigenetic control. Neurotrophic factors stimulate cysteine attention deficit hyperactivity disorder (ADHD) [ 87, 88]. uptake and increase both GSH/GS-SG and SAM/SAH in Earlier studies also showed that expression of these interneu- association with a significant increase in DNA methylation rons was inhibited when glutathione synthesis was impaired, [102]. However, Se-dependent redox regulation is more indicating a critical role for redox status in establishing the vulnerable to soft electrophiles, positively charged chemical capacity for attention [89, 90]. species which bind to selenoproteins with exceptionally high affinity, as discussed below. 3.4. Selenoenzyme Metabolism and Physiology. In addition to the TRx and GPx selenoenzyme families, further selenopro- 4. Genetic Influences and Metabolic teinshaverecentlybeenimplicatedinprocesses knownto Disturbances in ASD be involved in neurodegenerative diseases, including protein folding, degradation of misfolded membrane proteins, and Observations from family studies suggest that ASD has a control of cellular calcium homoeostasis [91]. Cerebral Se strong genetic component [103–105], although the failure to deficiency is associated with neurological disorders such as identify genetic factors aeff cting more than a small propor- seizures and ataxia [92, 93], consistent with a restriction tion of ASD cases suggests that multiple etiologies may be in the development of inhibitory interneurons. Knockout responsible for the pathologies and neurobehavioral features of selenoprotein synthesis in neurons specifically interfered of the disorder [106–117]. Moreover, genetic factors may with development of parvalbumin-expressing GABAergic increasetheprobabilityofoxidativedamageanddiminishthe interneurons, and knockout of GPx4 produced a similar body’s ability to detoxify ROS and free radicals. Interactions deficit, indicating that these neurons have a particular between genetic and environmental factors may potentiate requirement for Se [85]. Thus impaired selenoprotein syn- increased oxidative stress in autistic children. thesis or loss of their activities could contribute to the Genetic risk of autism may be related to a differential neurocognitive dysfunction and seizure activity in ASD. sensitivity to environmental factors. Using a strict definition The cerebral cortex, hippocampus, cerebellum, and olfac- of autism, a recent study found a 58% concordance rate tory bulb express the highest numbers of selenoproteins for monozygotic male twins and 60% for females and 21% [94]. The brain and endocrine tissues are preferentially sup- and 27% for male and female dizygotic twins, respectively plied with Se, predominantly through directed distribution [118]. Using the broader definition of ASD, monozygotic and cellular uptake of SelP. Although other selenoproteins concordance increased to 77% and 50% for males and uniformly incorporate only a single Sec per molecule, SelP females, while dizygotic concordance was 31% and 36%, uniquely contains 10 Sec per molecule. Studies with SelP- respectively. eTh se rates are substantially lower than earlier deficient mice indicate that moderate reductions of brain Se estimates, and the authors concluded that environmental content will impair brain function [95, 96]. factors are more important than genetic factors, although NotonlyisSelPimportant forSec transport, butitalso genetic factors clearly play an important role. Moreover, appearstohaveavitalroleinneurogenesis. Astudy of its no individual genetic cause of autism has been identiefi d brain distribution found a remarkably higher concentration to account for more than 1%-2% of cases and, with the in ependymal cells, which are found at the ventricle surface exception of Rett syndrome, there is no current evidence that [97]. Ependymal cells are a source of neural stem cells ASD is linked to any specific genetic or nongenetic disorder. which are produced upon asymmetric cell division and give However, there is evidence suggesting that epigenetic factors rise to neuronal, astrocyte and oligodendrocyte lineages in and exposures to environmental modifiers may contribute the subependymal region [98]. Neurotrophic growth factors to variable expression of autism-related traits [42]. Variants stimulatemitosis of theseprecursor cells[99]and provide of major eeff ct genes and numerous common variants with an important source of postnatal neurons. SelP is taken up smaller eeff ct genes have been identified in individuals by neurons via apolipoprotein E receptor 2 (ApoER2), which with ASD and related conditions. These genetic variances is localized to synapses, and ApoER2 knockout mice show are providing insights to common pathways and metabolic a decrease in synapse density as well as a decrease in the disturbances affected in ASD, particularly genes involved in number of dendritic spines [100, 101]. oxidative stress and detoxification pathways [ 15, 119, 120]. All brain selenoenzymes are aeff cted by loss of Se in the Polymorphisms of genes involved in glutathione absence of SelP, but the DIOs, TRxs, and GPxs are the seleno- metabolism, including genes for GPx and glutathione S- proteins considered to have the most critical roles in the transferase (GST), have been reportedly associated with ASD. brain and endocrine system. Therefore, loss or compromise GPx1 is the predominant and most abundant isoenzyme of of their functions [96] would have dramatic effects on matu- GPx and plays an integral role in reducing oxidative stress ration of the neuroendocrine system. The unusual capacity by catalyzing the reduction of potentially harmful peroxides. of the brain and endocrine tissues (e.g., pituitary, testes) Ming et al. [121] found significant disequilibrium in the to retain their selenoenzyme activities during prolonged overall transmission of a sequence polymorphism of GPx1 in or even multigenerational deficiency states [ 43]indicates ASD. Williams et al. [122] showed that the GSTP1-313A allele Autism Research and Treatment 7 may be acting as a teratogenic allele, contributing to the in global DNA methylation, while inhibition of MS activity by phenotype of the affected child. GST proteins conjugate and ethanolisassociatedwithalargedecrease[41]. u Th s, xenobi- detoxify products of oxidative stress and conjugate toxins that otics aeff cting redox status can exert an epigenetic influence. produce oxidative stress. By assessing genotypes of mothers Beyond its direct epigenetic regulation of gene tran- and maternal grandparents, it was shown that the GSTP1A scription, DNA methylation also regulates the activity of haplotype was signicfi antly more frequently transmitted to repetitive transposable elements dispersed throughout the mothers of individuals with ASD, suggesting that it may human genome. Transposable elements comprise about 45% be acting in mothers during pregnancy to contribute to the of the genome, and their earlier description as “junk DNA” phenotype of autism during fetal development [122]. has recently been revised in recognition of their ability James et al. [15] examined the frequency of several single to modulate gene transcription, mRNA splicing, micro- nucleotide polymorphisms (SNPs) capable of aeff cting redox RNA formation, and other processes [129]. Reflecting their and methylation pathways in autistic subjects. eTh y found viral origin, retrotransposons such as the LINE-1 (long significant differences in allele frequencies for the reduced interspersed nuclear element-1) family are suppressed by folate carrier (RFC 80G>A),transcobalaminII(TCN2 776 methylation but can replicate and transpose to new locations, G>C), methylenetetrahydrofolate reductase (MTHFR) 677 especially during early development and especially when C>Tand 1298 (A>C), catechol-O-methyltransferase (COMT methylation is suppressed. Based upon its impressive quanti- 472 G>A), and GST M1 between autistic and control cohorts. tative contribution to the genome, methylation of LINE-1 has These differences were associated with abnormal metabolite been used as a surrogate for global DNA methylation [130], levels, suggesting that individuals with genetic vulnerability and factors regulating MS activity aeff ct LINE-1 methylation affecting redox and methylation capacity may be linked [131]. LINE-1 retrotransposition is reported to occur at a to a higher risk for autism. Any decfi it in the function of higher rate in brain than to other tissues [132], and a selenoproteins could synergize with these genetic risk factors. higher rate was observed in Rett syndrome subjects carrying DNA copy number variants (CNVs) represent a major mutations in the methylated DNA binding protein MeCP2 category of genetic risk for ASD and are implicated in approx- [130]. Although more studies are needed to clarify their imately 10% of cases [123, 124]. Several of the genes likely specific contribution, transposable elements such as LINE- affected by homozygous deletions are regulated by neuronal 1 are poised to provide a global genomic influence during activity, and the expression of these genes can change in development, so agents aeff cting their methylation state are response to neuronal stimulation. Synapses mature partially likely to disrupt this process. as a function of experience-dependent neuronal activity, Oxidative stress and decreased methylation capacity are so disruption of those genes by mutation or copy number common in autism and abnormal epigenetic regulation may variationmay alterthe processofsynapticdevelopment. link the metabolic abnormalities to disruptions in brain DNA methylation status is associated with the occurrence of development. Other comorbid features of autism, such as CNVs [125], raising the possibility that impaired methylation autoimmunity and gastrointestinal (GI) dysfunction [133, capacity could contribute to increased CNVs in ASD. 134], may reflect similar manifestations of abnormal epi- genetic regulation. A genome-wide comparison of DNA methylation in monozygotic twins discordant for autism found numerous differentially methylated regions associated 5. Epigenetic Disturbances in ASD with ASD, and the extent of these differences were correlated with severity of autistic trait scores [135]. Epigenetic regulation utilizes covalent modifications such as DNA methylation and the addition/removal of various From conception to maturation, human development is chemical moieties to histone tails (collectively known as epi- a highly orchestrated expression of epigenetic regulation, so genetic marks) to provide stable, transgenerational changes it is not surprising that genetic and environmental factors in gene expression without alteration of the underlying adversely affecting oxidative tone and methylation status nucleotide sequence [126, 127]. Epigenetic marks are dynamic can contribute to developmental disorders. The exceptionally and highly sensitive to cellular changes [128]. u Th s, normal dynamic redox-dependent epigenetic regulation in the brain physiologic changes in the cellular environment, such as increases its vulnerability to neurodevelopmental disorders. Autism is a prominent feature of Rett, Angelman, Prader- levels of growth factors, hormones, and neurotransmitters, as well as xenobiotic exposure, can translate into modifications Willi, and Fragile-X syndromes, each of which has been in gene expression mediated by epigenetic regulation. Xeno- linked to interruption of methylation-dependent regula- biotic exposures aeff cting epigenetic status can, therefore, tion [136–138]. Therefore, environmental exposures aeff cting not only produce lifelong consequences, but their eeff cts redox and methylation status could reasonably result in can be transmitted through germline cells to aeff ct multiple neurodevelopment disorders such as ASD. succeeding generations [126, 128]. As noted above, MS exerts powerful control over all methylation reactions via its influence over the SAM/SAH 6. ASD in relation to Exposures to ratio, and MS inhibition by oxidative stress will cause both a Potentially Neurotoxic Agents decrease in SAM and an increase in SAH, while reducing con- ditions will have the opposite effect. A twofold increase in MS Certainsoft electrophilesare knowntobeneurotoxicat activity induced by IGF-1 is associated with a twofold increase high exposures, presumably due to their eeff cts on sulfur 8 Autism Research and Treatment metabolism. Although these electrophilic species are highly metals [143, 144]. Thisisconsistentwiththe nfi ding that interactive with cellular nucleophiles such as thiols, the Se of lower Hg concentrations are present in the hair of young Sec is by far the strongest intracellular nucleophile. er Th efore, (<6 y) children with ASD [144–146]althoughMajewskaet selenoproteins are very vulnerable to enzyme inhibition by al. [146] found older ASD children have higher hair Hg. binding to neurotoxic electrophiles. eir Th toxic concentra- Although serum was studied instead of RBC’s or whole blood, tions are generally miniscule in relation to sulfur, toxic levels alarge studyofHginrelationtoautism[147]reported generally equal or exceed the normal tissue concentrations of finding no significant differences between nonautistic and Se. er Th efore, because of their high reactivity and low molar ASDchildren. Thisissupported by thefindingthatno abundance, selenoenzymes are highly vulnerable to selective distinctions in expression levels of four genes that are known inhibition by high concentrations of electrophiles such as Hg. to respond to metal exposures were noted between ASD and Soft electrophiles such as Hg have larger ionic radii and a typical children [148]. However, Stamova et al. [149]found more dispersed surface charge, making them more reactive a distinctive correlation between gene expression and blood with soft nucleophiles such as the Se of Sec at the active sites Hg levels in boys with autism suggesting it is associated of enzymes. with a different pattern of gene transcription in response Only certain electrophiles are notably neurotoxic. o Th se to Hg exposure. The ability of both Hg and Se to exert that bind and potentially sequester Se would cooperatively epigenetic effects was recently demonstrated in embryonic diminish the biological availability of Se for performance stem cells [150]. Several studies have reported potent toxic of its necessary physiological roles (Figure 1). For example, effects of methylmercury on neural stem cell differentiation a multitude of electrophilic agents are naturally present in and survival [151, 152], indicating its potential capacity for food in small amounts and the Se-sequestering eeff cts of each altering gene expression during development. would usually be minor. However, their additive effects on To prevent bacterial contamination in multiple-dose selenoenzyme synthesis and function could be detrimental in vials, thimerosal, the ethylmercury derivative of thiosalicylic individuals with compromised Se status or metabolism. acid, has been used as a preservative in various medical Likewise, additional exposures to electrophiles are products, including vaccines. As autism rates increased, encountered in the form of environmental contaminants, Bernard et al. [153] suggested that vaccine-derived Hg might such as toxic metals, pesticides, herbicides, and others. be a contributing cause, a highly controversial proposal In addition, soft electrophiles are present as the active [154–156]. As a result, thimerosal was removed from all ingredients in many pharmaceuticals and food preservatives, pediatric vaccines, except for some influenza vaccines, in whilestill others areproducedduringthe degradation of the United States starting in 2001, but the incidence of these products. eTh refore, instead of examining relationships autism continued to rise [157], furthering the doubts that between ASD incidence and exposures to individual agents, vaccine-derived Hg exposures contributes to autism inci- it may be more informative to examine ASD incidence in dence. While a number of epidemiological studies do not relation to aggregate exposures to these soft electrophiles indicate an association between thimerosal exposure and and/or their effect on selenoenzyme activity in vulnerable ASD [158, 159], possible associations between developmental individuals. disorders with Hg-containing vaccines [157]and delayedor even transgenerational influence of epigenetic changes have been suggested [160]. Such genetic or epigenetic defects of 6.1. iTh o-/Selenoreactive Elements. High exposures to soft the antioxidant enzyme system could cooperatively inter- act with other environmental electrophiles and make vul- electrophiles have the potential to incapacitate various sulfur- and Se-dependent metabolic processes, thus disrupting many nerable individuals more sensitive to exposure levels that redox regulatory mechanisms that are required for healthy would otherwise be harmless. Since deficits in selenoen- cell growth and function, particularly in brain and endocrine zyme synthesis or function can increase the potential for tissues. Increased selenium status is known to counteract oxidative stress and epigenetic dysregulation, sensitivity to neurotoxins, such as Hg and similar soft electrophiles, may the adverse effects of elevated exposures to neurotoxic electrophiles such as Hg, cadmium (Cd), lead (Pb), and differ among individuals. In this regard, there have been vanadium (V) [139]. These electrophilic elements may all be several reports of decreased GPx activity in autism [161, 162] and selective transmission of GPx-1 allelic variants [121]. capable of selective, irreversible inhibition of selenoenzyme activities similar to the mechanism of Hg toxicity [140, 141]. Interestingly, Pa¸c s a et al. [120] found an inverse correlation However, relationships between Se status and the neurotoxic between homocysteine and GPx activity in autistic subjects, effects of these other elements have not been adequately indicating an association between low GPx activity and examined, and additional mechanisms of toxicity have been impaired methylation. Therefore, Casta neda ˜ et al. suggested recognized forsomeofthese elements [142]. Because it is rich that the removal of thimerosal from vaccines might not in cysteine, hair has oen ft been used to provide a reflection be immediately reflected as a reversal of epigenetic effects, of circulating amounts of thio-reactive electrophiles present especially if they involved effects on germline cells [ 163]. Notably, lower levels of DNA methylation increase novel in exposed individuals. The concentrations of Hg, As, Cd, or Pb in hair do not indicate consistent relationships with insertions of transposable elements and increase the fre- ASD incidence; however, some studies report unusually low quency of CNVs in germline cells [164–166], which are also elevated in autism. eTh refore, the eeff cts of elemental concentrations in hair samples and suggest that individu- als with ASD have diminished abilities to eliminate toxic electrophiles on selenometabolism may be a contributing Autism Research and Treatment 9 Predisposing factors Epigenetic factors Genetic factors Poor dietary Se intakes Potential etiologic egents Defective Se–enzyme synthesis Inordinate exposures to soft Diminished toxin excretion Diminished toxin degradation electrophiles, e.g., Hg, Pb, Cd, and/or potentially hazardous organic molecular species that Biochemical outcomes interact with selenoenzymes. Increased oxidative damage Decreased reversal of oxidative damage Endocrine disruption Epigenetic disruption Impaired immune response Pathological effects Degradation of cell functions Diminished neuroplasticity Impaired neurodevelopment Neuronal dysfunction Neuronal cell death Autism/ASD Figure 3: Depiction of potential etiologic contributors to disruptions of selenoenzyme physiology that may lead to disruptions of redox control and pathological consequences of autism and ASD. eTh factors and agents depicted are not all necessarily involved, but increases in predisposing factors along with additive contributions of increased exposures to thio- and selenoreactive electrophiles would be expected to increase the likelihood of progression to pathology. factor to ASD etiology and/or progression in vulnerable inactivate N-acetyl-p-benzoquinone imine (NAPQI), a individuals. metabolic product of acetaminophen breakdown that is thoughttobethe proximal causeofhepatotoxicityfollowing acetaminophen overdoses. When excessive quantities of 6.2. iTh o-/Selenoreactive Organic Electrophiles. Organic mol- NAPQI are formed, the primary metabolic (glucuronide ecules such as acrylamide, acrolein, and diethyldithio- and sulfate conjugation) pathways apparently become carbamate are structurally diverse, but these electrophilic saturated. N-acetylcysteine is thought to counteract toxicity species all share the potential to chemically react with by either reducing NAPQI to the parent compound or strong nucleophiles [167]. Just as for inorganic electrophiles, providing sulyfh dryl for conjugation of this metabolite [ 176]. the most likely target will be the most nucleophilic moi- If supplementation with a sulfhydryl-containing compound eties of enzymes, such as Sec or thiols of Cys residues. such as N-acetylcysteine can directly inactivate NAPQI, Acetaminophen, the most commonly used analgesic and supplementation with Se to restore selenoenzyme status antipyretic drug in much of the world, is associated with may be an important adjunct therapy to restore healthy toxic eeff cts at high exposures. Excessive intake leads to redox status in the affected tissues. eTh signicfi ance of the impaired sulfur metabolism and life-threatening hepatotox- liver in providing Se for delivery to the brain suggests that icity, involving depletion of GSH [168]. A survey of parents compromised Se availability in the liver temporarily induced reported a higher frequency of acetaminophen use aer ft the by exposures to NAPQI and/or other electrophiles could MMR (measles-mumps-rubella) vaccine for autistic children diminish theamountofSethe brainreceives. Exposures than for unaffected children [ 169], leading to the suggestion to agents which could cause either prolonged or excessive that use of acetaminophen might be causally linked to an diminishments in the supply of Se to neuroendocrine tissues increase in autism rates [170]. may therefore be important factors to consider in relation to Interestingly, the major acetaminophen-binding protein ASD. in the liver is Se binding protein-2 (SeBP2) [171, 172]. Both SeBP1 and SeBP2 bind Se, but not in the Sec form charac- teristic of the genetically encoded selenoproteins. Increased 7. Summary and Conclusions expression of SeBP2 is associated with increased suscepti- bility to acetaminophen cytotoxicity [173]. In view of the The nosology of ASD is complicated by the difficulties to male predominance of autism, it is interesting to note that differentiate the syndrome into subsets with similar symp- SeBP2 levels are higher in males [174] and their vulnerability toms and distinct etiologies. Children that share the diagnosis to acetaminophen hepatotoxicity is also greater. Males also of ASD represent more than one distinct pathophysiological display a decreased capacity to restore their GSH levels to condition. Recognizing and distinguishing between groups normal [175] following high acetaminophen exposures. with separate etiologies require identification of objective N-acetylcysteine protects against acetaminophen- laboratory indices that clinicians can use for diagnosis and induced hepatotoxicity by maintaining or restoring hepatic to monitor progression and treatment eeff cts. eTh objective of concentrations of GSH [176]. Glutathione is required to this review is to discuss metabolic defects that may contribute 10 Autism Research and Treatment to theonset andpathology of ASD, particularly in relation to [5] I. N. Pessah, R. F. Seegal, P. J. Lein et al., “Immunologic and neurodevelopmental susceptibilities of autism,” NeuroToxicol- Se physiology. ogy,vol.29, no.3,pp. 532–545, 2008. Existing evidence indicates children with ASD have disruptions in GSH metabolism, and that impaired selenoen- [6] M. L. 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Potential Role of Selenoenzymes and Antioxidant Metabolism in relation to Autism Etiology and Pathology

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