Increased excretion of a lipid peroxidation biomarker in autism

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Abstract

It is thought that autism could result from an interaction between genetic and environmental factors with oxidative stress as a potential mechanism linking the two. One genetic factor may be altered oxidative–reductive capacity. This study tested the hypothesis that children with autism have increased oxidative stress. We evaluated children with autism for the presence of two oxidative stress biomarkers. Urinary excretion of 8-hydroxy-2-deoxyguanosine (8-OHdG) and 8-isoprostane-F2α (8-iso-PGF2α) were determined in 33 children with autism and 29 healthy controls. 8-iso-PGF2α levels were significantly higher in children with autism. The isoprostane levels in autistic subjects were variable with a bimodal distribution. The majority of autistic subjects showed a moderate increase in isoprostane levels while a smaller group of autistic children showed dramatic increases in their isoprostane levels. There was a trend of an increase in 8-OHdG levels in children with autism but it did not reach statistical significance. There was no significant correlation between the levels of the biomarkers and vitamin intake, dietary supplements, medicine, medical disorders, or history of regression. These results suggest that the lipid peroxidation biomarker is increased in this cohort of autistic children, especially in the subgroup of autistic children.

Introduction

Oxidative stress has been implicated in the etiology of neurological, neurodevelopmental, and neuropsychiatric disorders including Parkinson's and Alzheimer's diseases, Down's syndrome, and autism. With respect to autism, the notion of enhanced oxidative stress has been derived from several lines of evidence. First, an increased excretion of oxidative stress biomarkers has been reported. Specifically, nitric oxide, a free radical that can block energy production was found to be increased in autism as compared to age- and sex-matched controls [1]. In addition, elevated nitrite concentrations have been detected in autistic individuals [2] along with thiobarbituric acid reactive substances and xanthine oxidase activity in red blood cells [3]. The elevation of these substances indicates excess free radicals in individuals with autism compared to controls. Consistent with the increased oxidative stress biomarkers, children with autism were found to have increased body burdens of environmental toxins that may generate oxidative stress [4], [5].

A second line of evidence that oxidative stress may play a role in autism is suggested by a reduced endogenous antioxidant capacity. Specifically, altered glutathione peroxidase (GPX) [1], [6], [7], superoxide dismutase (SOD) [6], [7] and catalase [3] activities as well as total GSH, and GSH/GSSG and cysteine levels [8] were found in autistic individuals compared to controls. Likewise, levels of exogenous antioxidants were also found to be reduced in autism, including vitamin C, vitamin E, and vitamin A in plasma and zinc and selenium in erythrocytes [8].

A third indicator of altered oxidative stress in autism is derived from evidence of impaired energy metabolism. Magnetic resonance spectroscopic study of the brains of individuals with autism showed reduced synthesis of ATP [9]. In addition, higher lactate [10], [11] and pyruvate [12] levels in autism may suggest mitochondrial dysfunction in autism [13], [14]. One major cause of mitochondria dysfunction is a result of oxidative injury [15].

Finally, improvement in behaviors following antioxidant administration to individuals with autism suggests that oxidative stress may be important in contributing to the etiology of autism. In double-blind, placebo-controlled trials, high-dose vitamin C [16] or carnosine [17] improved the behavior of individuals with autism over baseline observations. Likewise, a 3-week supplement of betaine and folinic acid to 20 children with autism who had lower levels of GSH, GSH/GSSG, cysteine improved their blood levels of the antioxidants [8]. Furthermore, melatonin (an effective NOradical dot and ONOO scavenger [18] with potential for reducing oxidative stress in both brain and gut [19], [20] and increasing expression of GPX [21]) was shown to be effective in the treatment of sleep disorders in autism [22]. Taken together, these lines of evidence support the hypothesis that at least some children with autism exhibit enhanced oxidative stress.

In this study, we determined urinary excretion of 8-isoprostane (8-iso-PGF2α), a lipid peroxidation biomarker, and the biomarker of DNA hydroxylation 8-hydroxy-2-deoxyguanosine (8-OHdG), in children with autism and age-matched controls. 8-OHdG is the product of free radical attack on DNA bound guanosine and hence a marker for oxidative damage to DNA. 8-OHdG is the most abundant oxidative product of cellular DNA oxidation [23], [24] and is also a potent mutagen [23], [24], [25], [26], [27].

Here we report the first observation that the level of one oxidative stress biomarker was significantly increased in a cohort of individuals with autism.

Section snippets

Participants

Thirty-three children with autism were recruited from the UMDNJ Autism Center, and 29 healthy controls were recruited from the Pediatric Ambulatory Center, New Jersey Medical School, UMDNJ, Newark, NJ. Assent and parental consent for these studies was obtained as approved by the Institutional Review Board of the UMDNJ-New Jersey Medical School. The diagnosis of autism was confirmed by Autism Diagnostic Interview-Revised [28], Autism Diagnostic Observation Schedule-Generic [29], or DSM IV [30]

Results

As shown in Fig. 1, 8-iso-PGF2α levels were significantly higher in children with autism (autism group: 32.92±1.98 ng creat−1 M; controls: 5.71±0.98 ng creat−1 M). Log scale was used to transform to the normal distribution and Student's t-test showed P=0.007. Levels of 8-iso-PGF2α among children with autism showed a greater variability than those of controls (autism SE: 1.98, Controls SE: 0.98). Eight of autistic children had isoprostane values greater than 2SD above the mean of the control

Experimental assays

Urine is the preferred body fluid for isoprostane and 8-OHdG analyses [23], [33]. Both isoprostane and 8-OHdG assays in plasma (isoprostane) or blood cells (DNA, e.g. lymphocytes) are technically difficult being subject to auto-oxidation, there are many interfering agents and the sample work-up for analysis often introduces artifacts [34], [35]. If the number of blood samples collected is small and their timing varied, there will be considerable noise in the data. In contrast, urine contains no

Acknowledgement

This work was supported by a Grant from Cure Autism Now Foundation, ES11256, USEPA-R829391, NS43981, ES05022, NJ Governor's Council on Autism, Johnson and Johnson Fund.

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