Increased DNA oxidation and decreased levels of repair products in Alzheimer's disease ventricular CSF

One of the leading etiologic hypotheses regarding Alzheimer's disease (AD) is the involvement of free radical-mediated oxidative stress in neuronal de generation. Although several recent studies show an increase in levels of b rain DNA oxidation in both aging and AD, there have been no studies of leve ls of markers of DNA oxidation in ventricular CSF. This is a study of level s of 8-hydroxy-2'-deoxyguanosine (8-OHdG), the predominant marker of oxidat ive DNA damage, in intact DNA and as the "free" repair product that results from repair mechanisms. Free 8-OHdG was isolated from CSF from nine AD and five age-matched control subjects using solid-phase extraction columns and measured using gas chromatography/mass spectrometry with selective ion mon itoring. Intact DNA was isolated from the same samples and the levels of 8- OHdG determined in the intact structures. Quantification of results was car ried out using stable isotope-labeled 8-OHdG. By using this sensitive metho dology, statistically significant elevations (p < 0.05) of 8-OHdG were obse rved in intact DNA in AD subjects compared with age-matched control subject s. In contrast, levels of free 8-OHdG, removed via repair mechanisms, were depleted significantly in AD samples (p < 0.05). Our results demonstrate an increase in unrepaired oxygen radical-mediated damage in AD DNA as evidenc ed by the increased presence of 8-OHdG in intact DNA and decreased concentr ations of the free repair product. These data suggest that the brain in AD may be subject to the double insult of increased oxidative stress, as well as deficiencies in repair mechanisms responsible for removal of oxidized ba ses.