Glutathione Redox Imbalance and Increased DNA Oxidation In Specific Brain Regions In Autism

Background:  Accumulating evidence suggests that oxidative stress may provide a link between susceptibility genes and pre- and post-natal environmental stressors in the pathophysiology of autism. Brain tissue is highly heterogeneous with different functions localized in specific areas, and it is highly vulnerable to oxidative stress due to its limited antioxidant capacity and higher energy requirement.  DNA is a major target for free radical-induced damage, and oxidative DNA damage refers to the functional or structural alterations of DNA resulting from the insults of free radicals, i.e. reactive oxygen species (ROS). Hydroxyl radical is a potent inducer of DNA damage, and 8-hydroxy-2^’-deoxyguanosine (8-OH-dG) is formed during oxidative DNA damage through the oxidation of guanosine bases in DNA. Glutathione (GSH) is the most important endogenous antioxidant in human tissues, which neutralizes ROS, and participates in detoxification and elimination of environmental toxins. Glutathione in its reduced state (GSH) and oxidised disulfide form (GSSG) are the primary determinants of redox status in all human cells. A decrease in GSH-to-GSSG redox ratio is a marker of oxidative stress.

Objectives:  To compare DNA oxidation and glutathione redox status in postmortem brain samples from the cerebellum and frontal, temporal, parietal and occipital cortex from autistic subjects and age-matched normal subjects.

Methods: Frozen human brain tissues (cerebellum, frontal cerebral cortex, temporal cortex, parietal cortex and occipital cortex) of autistic and age-matched control subjects were obtained from the NICHD Brain and Tissue Bank for Developmental Disorders at the University of Maryland. DNA oxidation was assessed by quantitation of 8-OH-dG. The glutathione redox status was determined by measuring the levels of GSH and GSSG.

 Results:  DNA oxidation was significantly increased by two-fold in frontal cortex, temporal cortex, and cerebellum in individuals with autism as compared with control subjects. On the other hand, its levels in parietal and occipital cortex were similar between autism and control groups. The levels of reduced glutathione GSH were significantly reduced and the levels of oxidized glutathione GSSG were significantly increased in the cerebellum and temporal cortex in autism group compared with control group. On the other hand, similar levels of GSH and GSSG were observed in frontal, parietal and occipital cortices between autism and control groups. The ratio of GSH/GSSG, an indicator of redox status was also significantly reduced in the cerebellum and temporal cortex in autism compared with control subjects, but it was similar in other brain regions.

Conclusions:  A decrease in reduced glutathione (GSH), an increase in its oxidized form (GSSG) and a decrease in redox ratio of GSH/GSSG in cerebellum and temporal cortex in autism subjects, but not in other brain regions suggest brain region-specific glutathione redox imbalance in autism. Increase in DNA oxidation in frontal cortex, temporal cortex and cerebellum in autism but not in parietal and occipital cortex further confirms that oxidative stress differentially affects selective brain regions in autism. These results indicate increased oxidative damage coupled with reduced antioxidant status in the brain of individuals with autism.