Increased Protein Oxidation in Cerebellum, Frontal and Temporal Cortex in Autism

Background: Accumulating evidence suggests that oxidative stress may provide a link between susceptibility genes and pre- and post-natal environmental risk factors 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. Protein oxidation is defined as the covalent modification of a protein induced either directly by reactive oxygen species (ROS) or indirectly by reaction with secondary by-products of oxidative stress. Protein carbonyl derivatives of amino acids (pro, arg, lys, thr) are the most common products of protein oxidation.

Objectives: In this study, the status of protein oxidation was compared in postmortem brain samples from the cerebellum and frontal, temporal, parietal and occipital cortex from autistic subjects with age range of 4 to 39 yrs (N = 7-10 for different brain regions) and age-matched normal subjects (N = 9-10).

Methods: Frozen human brain tissue (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. Protein oxidation was assessed in the brain homogenates by quantitation of protein carbonyls using ELISA kit (Cell Biolab). The method is based on the derivitization of the carbonyl group with dinitrophenylhydrazine (DNPH) to DNP hydrazone, which is probed with anti-DNP antibody.

Results: Protein oxidation, assessed by quantitation of protein carbonyls, was significantly increased in autism by a mean of 123 % in frontal cortex (p<0.001), by 108 % in temporal cortex (p<0.01), and by 100 % in cerebellum (p<0.01) as compared with controls. On the other hand, its levels in parietal and occipital cortex were similar between autism and control groups.  There was no or minimal overlap of protein carbonyl levels in the frontal and temporal cortex between autism and control groups. In cerebellum, 57 % of autism subjects had protein carbonyl levels above the upper cutoff value of range for control group.

Conclusions: These results suggest that oxidative stress differentially affects selective regions of the brain, i.e. cerebellum, frontal and temporal cortex, in autism.