Antioxidants in Down syndrome

Biochimica et Biophysica Acta 1822 (2012) 657–663

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Review

Antioxidants in Down syndrome☆ Ira T. Lott ⁎ Department of Pediatrics and Neurology, School of Medicine, University of California Irvine (UCI), Orange, CA 92868, USA

a r t i c l e

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Article history: Received 31 October 2011 Received in revised form 13 December 2011 Accepted 14 December 2011 Available online 21 December 2011 Keywords: Antioxidant Oxidative stress Down syndrome Alzheimer disease Antioxidant clinical trial Mitochondrial disorder

a b s t r a c t Individuals with Down syndrome (DS) have high levels of oxidative stress throughout the lifespan. Mouse models of DS share some structural and functional abnormalities that parallel findings seen in the human phenotype. Several of the mouse models show evidence of cellular oxidative stress and have provided a platform for antioxidant intervention. Genes that are overexpressed on chromosome 21 are associated with oxidative stress and neuronal apoptosis. The lack of balance in the metabolism of free radicals generated during processes related to oxidative stress may have a direct role in producing the neuropathology of DS including the tendency to Alzheimer disease (AD). Mitochondria are often a target for oxidative stress and are considered to be a trigger for the onset of the AD process in DS. Biomarkers for oxidative stress have been described in DS and in AD in the general population. However, intervention trials using standard antioxidant supplements or diets have failed to produce uniform therapeutic effect. This chapter will examine the biological role of oxidative stress in DS and its relationship to abnormalities in both development and aging within the disorder. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Oxidative stress is part of the fundamental biology of Down syndrome (DS). As the most common genetic cause of intellectual disability, DS is due to trisomy of all or part of the chromosome 21. With few exceptions, all of the genes of known function are located on the long arm of chromosome 21. The gene sequence of chromosome 21 has been carried out with 99.7% coverage [1]. Among the more than 500 genes on chromosome 21, functional information exists on about 150 genes. Of these, there are 18 that effect gene expression and over 20 that effect metabolic functioning [2]. Chromosome 21 contains several genes that have been implicated in oxidative stress related to neurodegeneration including Cu/Zn superoxide dismutase (SOD1), amyloid precursor protein (APP), Ets-2 transcription factors, Down Syndrome Critical Region 1 (DSCR1) stress-inducible factor, beta-site APP cleaving enzyme (BACE) and S100 [3–5]. In this chapter, investigations underlying oxidative stress in DS will be discussed with an emphasis on those which have led to therapeutic intervention trials. The scope of the review included a search of PUBMED, MEDLINE AND APAPsychNET Direct (1970–2011). The term “Down Syndrome” was used in combination with the appropriate review target in the individual sections of this report (i.e. mouse models, dementia, clinical trials, etc.). A principle focus was on selected publications over the past 5 years, but older

☆ This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease. ⁎ UC Irvine Medical Center, Department of Pediatrics ZC 4482, 101 The City Dr., Orange, CA 92868, USA. Tel.: + 1 714 456 5333; fax: + 1 714 456 8466. E-mail address: [email protected]. 0925-4439/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.bbadis.2011.12.010

publications were not excluded if considered relevant to the topics. The reference list of articles identified by this search was selected for relevance.

2. Mouse models of DS and oxidative stress Mouse models have been used, in part, to determine how the genes on human chromosome 21 lead to the tissue-wide abnormalities seen in DS [6]. There is 80% synteny between human chromosome 21 (Hsa 21) and mouse chromosome 16 meaning that specific genes are localized on the same chromosome in both the human and mouse species. Approximately 175 genes are highly conserved between Hsa 21 and the mouse genome although the correspondence varies with the different models [7]. The mouse strains labeled Ts65Dn, Ts1Yey, Ts1Cje, and Ts1Rh are trisomic for the segments of mouse chromosome 16 that are homologous to Hsa 21. The relationship of these models to one another and to genes on mouse chromosome 16 is shown in Fig. 1 [8]. The most commonly studied mouse model is Ts65Dn which shares a number of structural and functional neurological similarities with individuals who have DS. These include abnormalities in basal forebrain cholinergic neurons and synapses, reduction of cell density in the hippocampus of aged mice, reduced dendritic branching in layer III pyramidal cells, and abnormalities in the ability to induce long-term potentiation [9–12]. The Ts65Dn mice also show spatial and non-spatial learning disabilities [13]. The advantages of using mouse models for understanding various abnormalities in DS such as oxidative stress are numerous and include the ability to study a sufficient number of animals in a short time, genetic engineering for individual genes or chromosomal regions, control for

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Mouse syntenic regions

MMU16

MMU17 MMU10

Fig. 1. Schematic representation of mouse models of trisomy 21. The first bar on the left represents mouse chromosomal regions (MMU10, MMU16, and MMU17) that are syntenic to human chromosome 21 (Hsa21). This bar is proportional to the number of genes carried by the fragment. The approximate mapping of the genes is shown on the left of the bar. All, except one, models are syntenic to MMU16. The estimated number of syntenic genes is indicated on the right of each bar for each mouse model. Reprinted from American Journal of Medical Genetics Part C (Seminars in Medical Genetics), 154C, Roubertoux P and Carlier M, Mouse Models of Cognitive Disabilities in Trisomy 21 (Down Syndrome), 400–416, 2010, with permission from John Wiley and Sons.

genetic background, low cost of screening, ability to study both developing and mature subjects, modeling therapeutic interventions and the lack of postmortem delays [6,14]. Oxidative stress represents an imbalance between the production of reactive oxygen species (ROS) and the ability of a biological system to detoxify the reactive intermediates or to repair the resulting damage. Toxic effects may be caused through the production of lipid peroxidation products and these compounds are increased in the brains of two mouse models of DS called Ts1Cje and Ts2Cje [15]. The TsiCje model excludes APP and SOD1 yet shows evidence of oxidative stress as characterized by decreased mitochondrial membrane potential and other biomarkers [16]. In Ts65Dn mice, supplementation of the antioxidant α-tocopherol from the embryonic stage onward showed improvement of spatial learning and alertness in the treated mice but no amelioration of background hyperactivity [17]. There was also improvement in hypocellularity in the dentate gyrus in the treated mice. In another study, markers of oxidative stress were found to be elevated in Ts65Dn brain and treatment with αtocopherol was associated with improved performance on spatial working memory tasks and an attenuation in cholinergic neuronal pathology in forebrain [18]. A nootropic drug with antioxidant properties showed mixed results in cognition and behavior of Ts65Dn mice [19]. Thus, several treatment approaches to oxidative stress have shown some benefit in mouse models of DS. 3. Biological foundation of oxidative stress in individuals with DS It is the area of Alzheimer disease (AD) that oxidative stress in DS seems to have the most significance. By 40 years of age, virtually all individuals with DS show the characteristic neuropathological changes of AD including beta-amyloid (Aβ) plaques and neurofibrillary tangles [20]. The accumulation of Aβ plaques in brain is seen across the lifespan in DS with appearance by 8 years of age or before [21]. In most individuals with DS, the rate of Aβ deposition increases markedly between 35 and 45 years and becomes associated with neurofibrillary tangles [22].

The lack of balance in the metabolism of ROS appears to have a direct role in producing the neuropathology of DS [23–25]. Oxidative damage has been observed in brain tissue from individuals with DS [26,27]. In the brain of fetuses with DS, two forms of glycation end products indicative of cellular oxidative stress were increased [28]. Cortical neurons from fetuses with DS show intracellular accumulation of ROS that lead to neuronal apoptosis [29]. These findings suggest that glycoxidation occurs very early in the life of individuals with DS. Early deposition of oxidized Aβ has also been seen within plaques from a 31 year old individual with DS, a finding which suggests that oxidative modification of Aβ may be an early event in the pathogenesis of AD [30]. Oxidative stress is also manifested in the brain of patients with AD in the general population [31,32]. Cultured human cortical neurons have provided a model in which oxidative stress may be treated in DS. Peptides released from naturally occurring glial proteins have been shown to possess potent antioxidative effects in cultured DS cortical neurons [33]. 4. Genes encoded on chromosome 21 and their relationship to oxidative stress SOD1 has been shown to be a part of many physiological processes such as transcription regulation, protein activation, bioenergy output, and cell proliferation. Simultaneous disorders of SOD1 and the uncoupling of proteins related to mitochondrial dysfunction have been implicated in the pathogenesis of AD and may precede Aβ deposition [34,35]. SOD1 gene is located just above the DS minimal critical region and more than 99% are trisomic for this gene [36]. The Down syndrome critical region is located on the long arm (q) of chromosome 21 and is considered to be responsible for some, if not all, of the features of the DS phenotype. This cytosolic enzyme catalyzes the dismutation of superoxide radicals that are formed during oxidative metabolism to hydrogen peroxide and oxygen. Under normal circumstances, the hydrogen peroxide is removed by glutathione peroxidase or catalase. However, brain levels of both of these enzymes maybe low in mammalian species causing an imbalance between the first and second step in superoxide catabolism which then results in an increase in hydroxyl radicals [37]. Hydroxyl radicals are highly reactive compounds that can damage neuronal membranes, cellular structures and organelles [38]. In DS, the exact role played by SOD1 overexpression is not clear. For example, congenital heart disease which is seen in up to 50% of children with DS does not appear to be associated with abnormalities in redox metabolism [39]. However, circulating endothelial progenitor cells involved in angiogenesis in DS have an increased susceptibility to oxidative stress which is thought to be related to the immune disorder associated with this syndrome [40]. Proliferation defects and gene expression dysregulation seen in neural progenitor cells in a mouse model of DS (Ts1Cje) required neither overexpression of SOD1 or APP [41]. In double APP/SOD1 transgenic mice, SOD1 overexpression prevented toxicity from APP and significantly increased the lifespan of the animals [42]. The relationship of genes encoded on chromosome 21 to oxidative stress has been previously reviewed [25]. Ets-2 is a protein that is encoded by Ets-2 gene and functions as a transcription factor that is overexpressed on chromosome 21. Ets-2 has been involved in multiple cellular processes including differentiation, maturation and signaling cascades [43,44]. It appears to be markedly upregulated by oxidative stress. In fetal cortical cells Ets-2 induces a stereotyped pattern of apoptotic changes leading to neuronal cell death [4]. In AD, increased expression of Ets-2 is associated with pathological markers suggesting that oxidative stress-driven Ets-2 upregulation contributes to neuronal death in both DS and AD. In Ts65Dn mouse, the Ets-2 tumor repressor gene has been related to a reduced incidence of solid tumors, one of the multiple complex genetic mechanisms that underlie cancer pathogenesis in DS [45,46].

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APP is involved in the development of AD and DS [47,48]. Aβ plasma levels are increased in DS and soluble forms of Aβ have been seen down to 21 weeks of gestational age [49,50]. Oxidized Aβ is seen in a high percentage of diffuse and core plaques within the brains of individuals with DS and oxidative modification of Aβ may be an early event in plaque biogenesis. Animal exposure studies to Aβ demonstrate that the neurobiological factors that foster AD may have their origins in early life and result from interplay between the environment, epigenetic factors and oxidative stress [51]. Environmental influences occurring during brain development inhibit DNA methyltransferases, thus hypomethylating promoters of genes associated with AD such as APP. This early life imprinting may act later in life to increase the levels of APP and Aβ. Increased Aβ levels may promote the production of ROS, damage DNA, and accelerate neurodegenerative events. However, the resulting picture is complex since other AD-associated genes may be repressed rather than overexpressed. Thus, hypo- or hypermethylation of AD-specific genes and their subsequent effect on Aβ production may be one of the factors accounting for the variability of the incidence of AD-type dementia in individuals with DS. DSCR1 is a stress-inducible factor located on chromosome 21 that is modulated by oxidative stress [52]. DSCRI is also known as Adapt78 and RCAN (regulator of calcineurin). Transient overexpression of DSCR1 appears to be protective but chronic upregulation is deleterious to cells likely through the expression of SOD1 [53]. DSCR1 is also upregulated in AD [54]. DSCR1 has been found to be a regulator of the calcineurin signaling pathway related to the reduced risk of certain cancers in individuals with DS [55]. DYRK1A is a gene that is overexpressed in individuals with trisomy 21 as well as mouse models for DS. This gene may be responsible for processes affecting synaptic plasticity and memory consolidation. Overepxression of DYRK1A in the brain of individuals with DS causes neurofibrillary degeneration via hyperphosphorylation of tau [56]. In aged mice overexpressing DYRK1A, marked motor abnormalities were associated with cell loss in the CA1–CA3 areas of the hippocampus and in the dentate gyrus [57]. Epigallocatechin gallate — a member of a natural polyphenols family, is found in great amount in green tea leaves. It appears to be a specific and safe DYRK1A inhibitor which rescues the brain defects in transgenic mouse that overexpresses the gene [58]. The relationship of this effect to oxidative stress has not been elucidated. In summary, overexpression of several chromosome 21 genes leads to a genetic imbalance that creates a “vicious cycle” by which oxidative stress is exacerbated resulting in further genetic imbalance. By virtue of their role in oxidative metabolism, mitochondria may serve as one of the pathways for this genetic imbalance.

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5. Mitochondria and oxidative stress in DS Mitochondria are subcellular organelles that contain enzymes for ATP production, calcium homeostasis, ROS, and apoptotic signaling [59]. As such, they are a target for oxidative stress. Dysfunction of mitochondria has surfaced as one of the most discussed hypotheses for a trigger in AD. Within the general population and to some extent in DS, the strongest genetic association with dementia is the ε-4 allele of the apolipoprotein gene (Apoε4) [60,61]. Cleavage of Apoε4 yields peptides that increase oxidative stress within the mitochondria [62]. Mitochondrial dysfunction has been observed in individuals with AD as manifested by defects in oxidative phosphorylation specifically in the respiratory complex IV [63,64]. The Aβ peptide itself can elevate oxidative stress by entering the mitochondria and producing ROS [65,66]. Mitochondrial dysfunction is one of the earliest events in the brains of mutant mice overexpressing APP as well as tau and presenilin1 [65,67]. Somatic mitochondrial DNA mutations may be elevated 15-fold in AD brain [68]. In DS, mitochondrial dysfunction and elevated levels of oxidative stress have been reported in neuronal cell cultures, brain, fibroblasts, erythrocytes and urine [69]. While somatic mitochondrial DNA control region mutations accumulate with age in post-mitotic tissues including the brain, the level of these mutations is markedly elevated in the brains of individuals with DS and AD in the general population [70]. This increased mutation rate is also seen in peripheral blood and in lymphoblastoid cell lines (Fig. 2). Mitochondrial dysfunction in DS is not only seen in the aging-AD process but earlier in life as well. In human neural progenitor cells from fetal cortex, those genes overexpressed on chromosome 21 (such as S100B) disturb the redox state within mitochondria and ultimately impair neurogenesis [71]. Human skin fibroblasts from fetuses with DS show that a complex I deficit may be responsible for the oxidative defect in mitochondria [72]. The mitochondria from DS fibroblasts are particularly sensitive to okadaic acid, a toxin that may induce malformations and interfere with the growth of cell lines [73]. In another study evaluating the cause of mitochondrial dysfunction in DS, impairment of the methyl cycle was shown to effect mitochondrial methyl availability and glutathione concentrations [74]. Mitochondrial abnormalities in mouse models for DS have been discussed in the introductory section to these models. Taken collectively, the findings of mitochondrial DNA mutations in DS and AD may be responsible for dementia in this disorder. Mitochondrial dysfunction also occurs at early age epochs in DS and could

Fig. 2. mtDNA Regulatory Control Region (RCR) mutation frequency in the peripheral tissue samples from control and dementia patients. (A) Analysis of mtDNA RCR mutation frequency in the frontal cortex and serum DNA (CF-DNA) from the same AD and control cases (n = 6, between the ages of 81 to 95 years). (B) Analysis of lymphoblastoid cell lines (LCLs) from DS, DS and AD, AD, and controls (n = 5–7, between the ages of 45 to 62 years). Reprinted from Journal of Alzheimer’s Disease, 20, Coskun P, Wyrembaka J, Derbereva O, Melkonian G, Doran E, Lott IT, Head E, Cotman CW, and Wallace D, Systemic Mitochondrial Dysfunction and the Etiology of Alzheimer’s Disease and Down Syndrome Dementia, S293-S310, 2010, with permission from IOS press.

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provide a platform for early effects of oxidative stress. Measures of mitochondrial overproliferation and deletion have been proposed as a marker for AD [75]. Mitochondrial deficits have been noted in non-neural peripheral tissues and there are a number of studies that have tried to capture peripheral effects of oxidative stress. 6. Biomarkers of oxidative stress in individuals with DS In a study aimed at defining the relationship between peripheral markers of oxidative stress and cognitive ability in adults with DS, low ratios of SOD1 to glutathione peroxidase were associated with worse memory ability, a finding which held after adjustment for gender, age, and the use of nutritional supplements. However, there was no relationship of the above findings to measures of Fisoprostanes, a proxy for lipid peroxidation [76]. The authors concluded that relationship between antioxidant capacity and cognitive phenotype in DS is complex and could be affected by the interaction of several genes rather than a formulation of a simple association. Individuals with DS often have high uric acid concentration, a metabolite that may have antioxidant properties. For example, children with DS had higher level of uric acid and total antioxidant capacity than the control group whereas no differences were found in adults [77]. However, in another study of children with DS, increased uric acid concentrations were associated with an increase in oxidative stress markers as measured by elevated allantoin concentrations in comparison to controls [78]. The mediator of this increase in reactive oxygen species may be xanthine oxidase. Allantoin and F-isoprostanes appear to be related to the oxidative assault that may occur as the result of chemotherapy in women with breast cancer in the general population [79]. The role of uric acid and oxidative stress in DS will require elucidation from additional studies. The antioxidant properties of melatonin as well as the neurotoxic factors measured by biomarkers in the kynurenine pathway were studied in a small group of children with DS [80]. Plasma melatonin and urinary kynurenine concentrations were lower than in agematched controls but levels of kynurenic and anthranilic acids were elevated. The authors concluded that this pattern of biomarkers constituted an added oxidative risk to children with DS. Markers of homocysteine and folic acid were examined in 42 pregnant mothers with a previous history of having delivered a child with DS in comparison to 48 control pregnant women. The pattern of metabolites suggested to the authors that mothers who delivered a child with DS had an abnormal oxidant/antioxidant ratio [81]. Gelsolin is actin-binding protein that binds to Aβ and inhibits fibrillogenesis, a pathogenetic step in AD. The active metabolite of gelsolin is a proteolytic product which contains a carboxy-terminal fragment (CTF). Gelsolin-CTF is decreased in the frontal cortex of adults with DS (43–63 years) but not in younger individuals (0.5– 23 years). There was a correlation in brain between the gelsolin fragment and both soluble and total concentrations of Aβ40 and Aβ42. This suggests a potential role for gelsolin in increasing of Aβ levels in DS brain. Oxidative pathways may be involved since gelsolin is cleaved under apoptotic conditions involving oxidative stress [82]. Nitric oxide is a regulatory molecule that may play a role in the pathogenesis of AD [83]. Inflammation appears to be a mediator of nitric oxide effect through glial cells that produce pro-inflammatory and neurotoxic factors [84]. Neopterin is an indicator for systemic inflammation as well as immune activation. This compound increases with age in DS and appears related to the development of dementia [85]. In a large population of individuals with DS (401), there was a strong correlation between neopterin levels in plasma and the production of nitric oxide [86]. These findings are similar to those found in the plasma of AD patients in the general population [87]. Neopterin measures may reflect oxidative stress in DS and dementia. Biomarkers of oxidative and inflammatory stress in blood and urine have the advantage of easy accessibility but their ability to

predict oxidative changes in the brain has not yet been established conclusively. Peripheral markers carry a promise to assist in the evaluation of oxidative risk throughout the lifespan with potential intervention through antioxidant trials. In evaluating the reliability of peripheral markers, of particular importance is the validation of assays, determination of the steady state concentrations of the biomarkers, utilization of multianalyte panels, and correlation with in vivo imaging findings [88]. The relationship of peripheral marker to cerebrospinal fluid (CSF) concentrations has been rarely studied, a point for consideration in future studies since CSF markers may more closely reflect brain function than plasma. However, even with CSF markers, lower levels of cytokines may reflect influx from plasma rather than intrathecal synthesis [89]. 7. Antioxidant trials in the general AD population In the general population, studies assessing whether the consumption of antioxidants from both dietary and supplemental sources in preventing AD have shown mixed results [90]. In the Chicago Health and Aging Project, increased vitamin E intake from dietary sources reduced the risk for dementia in individuals who were positive for Apoε4 allele. However, actual supplements of vitamin E were not associated with benefits. Vitamin E administration did not influence the conversion of mild cognitive impairment to dementia [91]. Dietary intake of beta-carotene or vitamin C did not influence the course of dementia. [92]. In the Rotterdam study, no effect of the ApoE genotype was found but high vitamin C and vitamin E intake was associated with reduced risk for AD [93]. No association between disease risk and intake of beta-carotene, vitamin E, and vitamin C was found in the Washington Heights-Inwood Columbia Aging Project, the Honolulu-Aging Study, and the Cache County Study [94–96]. In another study, supplements of vitamins C and E together reduced the risk for AD [97]. 8. The canine model for the study and treatment of oxidative stress The often contradictory results in human trials have been one of the factors leading to the investigation of a canine animal model of aging which has had implications for antioxidant trials in DS. Beagle dogs have a median lifespan of 13.5 years and show many of the changes of AD including evidence of oxidative stress [98]. Fig. 3 shows the relationship of oxidized Aβ in the brains of individuals with DS, AD as compared to the canine model. Their neuropathology parallels many of the changes of AD including cortical atrophy, neuronal loss, Aβ plaques, and amyloid angiopathy and oxidative damage. With aging, this particular breed of beagle dog exhibits decline in executive functioning, learning, and memory. However, the model is somewhat incomplete as the dogs do not develop neurofibrillary changes. Nonetheless, many of the neuropathological features in the canines correlate with the extent of cognitive decline. Topographic concentrations of Aβ in the dogs exhibit patterns that parallel those in human brain [99]. The cloning, sequencing and expression of APP isoforms and the enzymes related to their processing also parallel the findings in human brain [100]. The dogs also show cholinergic hypofunction similar to that seen in human AD brain, suggesting that this model could be used for therapeutic screening as well as understanding the links between cholinergic functioning, Aβ pathology, and cognitive decline [101]. Of particular interest to potential clinical trials in DS was an antioxidant supplementation study in which cellular and mitochondrial antioxidants were given to the beagle dogs [102]. A broad based diet in the treated group of dogs contained dl-alpha-tocopherol acetate, l-carnitine, dl-alpha-lipoic acid, ascorbic acid and other dietary antioxidants. After one and six months of treatment, the animals were tested for spatial attention ability [103,104]. The treated aged animals performed much better than controls and better than the

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Fig. 3. Characterization of plaques containing oxidized Aβ in the brain tissues. Diffuse plaques in the prefrontal cortex contain oxidized Aβ in the brain of: (A) an aged nondemented 86 year old individual; (B) a 31 year old non-demented individual with DS; and (C) an aged canine. Scale = 50 μm.

younger animals. The beneficial effects of the antioxidant diet were enhanced further by environmental stimulation.

be weighed against potential risk and disease progression. Targets that attempt to treat several dysfunctions simultaneously may hold promise for future studies of alternative therapies.

9. Antioxidant trials in DS 10. Conclusions Since individuals with DS share many of the patterns of oxidative stress seen in the canine model, a parallel clinical trial was of interest. We carried out a randomized, double-blind, placebo-controlled trial to assess whether daily antioxidant supplementation (900 IU of αtocopherol, 200 mg of ascorbic acid and 600 mg of alpha-lipoic acid) was effective, safe, and tolerable for 53 individuals with DS and ADtype dementia [105]. The outcome measures comprised a battery of neuropsychological assessments administered at baseline and every 6 months throughout two years of study. Compared to the placebo group, those individuals receiving the antioxidant supplement showed neither an improvement in cognitive functioning nor a stabilization of cognitive decline. Mean plasma levels of α-tocopherol increased ~2-fold in the treatment group and were consistently higher than the placebo group over the treatment period. Pill counts indicated good compliance with the regimen. No serious adverse events attributed to the treatment were noted. We conclude that antioxidant supplementation is safe, though ineffective as a treatment for dementia in individuals with DS and AD-type dementia. A similar lack of antioxidant efficacy was noted in a study of 156 infants with DS b7 months of age. Supplementation of the diet with antioxidants, and folinic acid did not show efficacy in a randomized controlled trial [106]. Another approach to treating oxidative stress in AD has been through environmental modification, particularly exercise. There is evidence from experimental models that exercise has time-dependant benefits to learning and memory [107]. While definitive clinical studies are lacking, there is evidence among older people with AD in the general population that weight-bearing exercise reduces the decline in activities of daily living [108]. Neurogenesis appears to be enhanced by environmental factors such as exercise, environmental enrichment and dietary energy restriction [109]. In this context, exercise was tested in a small study of adults with DS and lipid peroxidation, a marker of oxidative stress, was reported to be reduced in saliva [110]. Plasma oxidative stress markers were reduced in adolescents with DS who underwent a 12 week aerobic training program [111]. The role of exercise in lowering oxidative stress in DS merits further investigation. Many individuals with DS receive complementary and alternative therapies, some of which are aimed at oxidative stress. The pros and cons of pharmaceutical, nutritional, botanical and stimulatory therapies for AD have been reviewed [112]. The author concludes that potential risks of both approved and non-approved therapies should

Oxidative stress is strongly associated with both development and aging in DS. The mouse models of DS that provide homology to chromosome 21 have afforded a fruitful approach for the experimental manipulation of oxidative stress through strategic interventions. Several genes on chromosome 21 have been associated with oxidative stress and their overexpression may contribute to a genetic imbalance in DS which propagates the development of ROS. Mitochondrial dysfunction is impacted by and also creates oxidative stress in DS. Mitochondria may be a common pathway by which oxidative stress affects basic mechanisms underlying DS and AD. However, despite strong animal evidence for oxidative stress and its amelioration, clinical trials in both DS and AD have been disappointing to date. Intervention, once dementia has begun in DS, is unlikely to be therapeutic, in part because of the underlying intellectual disability and factors related to decreased synpatogenesis in the disorder. Prophylactic trials may hold more encouragement in DS and it is possible that early intervention with strategically developed antioxidants may positively impact intellectual functioning decades before dementia sets in. But which antioxidants? The generation of newer classes of antioxidants and new approaches to intervention will be needed to combat oxidative stress in both AD and DS. Acknowledgements This research was supported by NIH grants AG-21912, HD-65160, AG-16573 Alzheimer's Disease Research Center. Nina Movsesyan, PhD, assisted in the preparation of this manuscript. References [1] M. Hattori, A. Fujiyama, T.D. Taylor, H. Watanabe, T. Yada, H.S. Park, A. Toyoda, K. Ishii, Y. Totoki, D.K. Choi, Y. Groner, E. Soeda, M. Ohki, T. Takagi, Y. Sakaki, S. Taudien, K. Blechschmidt, A. Polley, U. Menzel, J. Delabar, K. Kumpf, R. Lehmann, D. Patterson, K. Reichwald, A. Rump, M. Schillhabel, A. Schudy, W. Zimmermann, A. Rosenthal, J. Kudoh, K. Schibuya, K. Kawasaki, S. Asakawa, A. Shintani, T. Sasaki, K. Nagamine, S. Mitsuyama, S.E. Antonarakis, S. Minoshima, N. Shimizu, G. Nordsiek, K. Hornischer, P. Brant, M. Scharfe, O. Schon, A. Desario, J. Reichelt, G. Kauer, H. Blocker, J. Ramser, A. Beck, S. Klages, S. Hennig, L. Riesselmann, E. Dagand, T. Haaf, S. Wehrmeyer, K. Borzym, K. Gardiner, D. Nizetic, F. Francis, H. Lehrach, R. Reinhardt, M.L. Yaspo, The DNA sequence of human chromosome 21, Nature 405 (2000) 311–319. [2] X. Sturgeon, K.J. Gardiner, Transcript catalogs of human chromosome 21 and orthologous chimpanzee and mouse regions, Mamm. Genome, 22, 2011, pp. 261–271.

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