Oxidatively modified nucleic acids in preclinical Alzheimer's disease (PCAD) brain

Abstract

Previous studies show increased oxidative DNA and RNA damage and diminished 8-oxoguanine glycosylase (OGG1) mediated base excision repair in vulnerable brain regions of mild cognitive impairment and late-stage Alzheimer's disease (LAD) subjects compared to normal control (NC) subjects. Recently, a preclinical stage of AD (PCAD) has been described in which subjects show no overt clinical manifestations of AD but demonstrate significant AD pathology at autopsy. To determine if DNA or RNA oxidation are significantly elevated in PCAD brain we quantified 8-hydroxyguanine (8-OHG) in sections of hippocampus/parahippocamapal gyri in PCAD and NC subjects using immunohistochemistry and confocal microscopy and in superior and middle temporal gyri (SMTG) using gas chromatography/mass spectrometry. To determine if increased DNA oxidation is associated with altered repair capacity, levels of OGG1 protein in HPG were measured by immunohistochemistry and levels of OGG1 mRNA were measured in SMTG using quantitative PCR. Results show significantly increased (p<0.05) 8-OHG immunostaining in DNA and RNA of PCAD HPG and significantly increased 8-OHG in PCAD SMTG. Quantification of OGG1 showed significantly elevated mRNA in PCAD SMTG and a trend toward elevated protein immunostaining in PCAD HPG. Overall, the data suggest oxidative damage to nucleic acids and a compensatory increase in OGG1 expression occur early in the pathogenesis of AD.

Figure 1

Representative confocal micrographs showing RNA associated 8-OHG staining (A) is associated with Tuj-1 positive neurons (C) but not GFAP-positive astrocytes (B). The last panel shows a merged image.

Figure 2

Representative confocal micrographs showing total, RNA-associated, and DNA associated 8-OHG in NC (A) and PCAD (B) hippocampus/parahippocampal gyri. Note elevated immunofluorescence associated with DNA and RNA associated 8-OHG.

Figure 3

Quantification of 8-OHG and OGG1 immunostaining. 8-OHG was significantly (p < 0.05) elevated in RNA and DNA in PCAD subjects compared to age-matched NC subjects. Levels of OGG1 were elevated in PCAD, although the difference was not statistically significant.

Figure 4

Representative confocal micrographs of sections of NC (A) and PCAD (B) hippocampus/parahippocampal gyri immunostained for OGG-1. Note elevated OGG1 immunostaining in PCAD.