Hypoxia-induced mitochondrial and nuclear DNA damage in the rat brain

In humans, cerebral hypoxia is a common component of severe brain insults, including trauma, stroke, and perinatal asphyxia. Oxidative stress and free radicals incidental to cerebral hypoxia are implicated in damaging macromolecules, leading to collapse of cellular homeostasis and cell death. Neuronal DNA damage, as a direct measurable event, has not been addressed in cerebral hypoxia. Here, we measured hypoxia-induced damage and repair in nuclear and mitochondrial DNA in rat hippocampus and cortex. Two highly sensitive quantitative polymerase chain reaction (QPCR) assays were used to measure DNA damage. One assay measures the integrity of the entire mitochondrial genome and the other the integrity of nuclear DNA. The latter is a novel assay, developed in our laboratory, which utilizes the high copy number of short interspersed DNA elements (SINEs) residing in introns and untranslated regions of mammalian genes. A unique feature of the SINE- mediated QPCR is its ability to amplify simultaneously long random segments of DNA. Consequently, the SINE assay offers sufficient sensitivity for detecting DNA damage at levels that are compatible with the cellular capacity for DNA repair, and are likely to be consistent with cellular survival and therefore adequate for studying the DNA damage response in the brain. In rats, we found that exposure to an atmosphere of 4% oxygen for 30 min resulted in induction of DNA damage in nuclear and to a greater extent, in mitochondrial DNA. Following a 3-hr recovery period in ambient air, dissimilar repair kinetics for nuclear and mitochondrial DNA were measured.