Evidence of Oxidative Damage and Inflammation Associated with Low Glutathione Redox Status in the Autism Brain

Background: Despite increasing evidence of oxidative stress in the pathophysiology of autism, the source and functional consequences of oxidative stress are relatively understudied.  Oxidative stress and damage occurs when antioxidant defense mechanisms fail to effectively counter endogenous or exogenous sources of reactive oxygen species (ROS). Glutathione is the primary antioxidant responsible for maintaining the reducing intracellular microenvironment that is essential for normal cellular function and viability. The ratio of reduced to oxidized glutathione (GSH/GSSG) is an established indicator of cellular redox status.  A chronic reduction in GSH/GSSG reflects a decrease in antioxidant/detoxification capacity and increased vulnerability to oxidative damage. We recently reported that an increase in oxidative protein and DNA damage was associated with the decrease in intracellular and plasma GSH/GSSG in children with autism suggesting that the reduced antioxidant defense capacity in these children may have functional consequence in terms of overt oxidative damage.

Objectives: The aim of this investigation was to determine whether systemic indicators of oxidative stress are also present in two brain regions reported to be abnormal in autism, cerebellum and BA22.  We also sought to determine whether these oxidative stress biomarkers are associated with protein and DNA damage and markers of inflammation and mitochondrial superoxide production.

Methods: Frozen samples of cerebellum and temporal cortex (BA22) from individuals with autism and unaffected controls (n=15 and n=12 per group, respectively) were matched for sex, age, post mortem interval (PMI), mode of death and race. Biomarkers of oxidative stress including reduced glutathione (GSH), oxidized glutathione (GSSG) and glutathione redox/antioxidant capacity (GSH/GSSG) were measured.  Biomarkers of oxidative protein damage (3-nitrotyrosine; 3-NT) and oxidative DNA damage (8-oxo-deoxyguanosine; 8-oxo-dG) were also assessed.  Functional indicators of oxidative stress included relative levels of 3-chlorotyrosine (3-CT), an established biomarker of inflammation, and reduced aconitase activity, a biomarker of excessive mitochondrial superoxide production. The biomarkers GSH/GSSG, 3-NT and 3-CT were measured by HPLC elution and electrochemical detection while 8-oxo-dG was measured by HPLC/MS. Aconitase activity was measured spectrophotometrically.

Results: Consistent with previous studies on plasma and immune cells, GSH/GSSG was significantly decreased in both autism cerebellum (p<0.001) and BA22 (p<0.001). Both 3-NT and 3-CT were inversely correlated with GSH/GSSG in both brain regions (p=0.04 and p=0.02, respectively).  Further, 8-oxo-dG was inversely correlated with GSH/GSSG in the cerebellum (p<0.0001). Aconitase activity was significantly decreased in autism cerebellum (p<0.01) and positively correlated with GSH/GSSG (p=0.01).

Conclusions: Overall, the findings of this study support a role for glutathione redox imbalance and oxidative stress in the neuropathology of autism and provide new evidence suggesting that a neuroinflammatory process and excessive mitochondrial superoxide production may promote oxidative damage in the affected brain regions in autism.