Nucleic acid oxidation: an early feature of Alzheimer's disease

Alzheimer’s disease (AD) is characterized by an insidious onset and progressive cognitive decline (reviewed in (Yaari and Corey-Bloom, 2007)). Altered cellular biomolecules and processes in early and late stages of AD include altered gene expression, increased protein oxidation, altered enzymatic activity, formation of nucleic acid adducts, and lipid peroxidation (reviewed in (Markesbery, 1997; Moreira et al., 2005; Smith et al., 2000)). Although multiple hypotheses have been proposed to explain the progression of AD, no single hypothesis to date can explain both the clinical and pathological features of this multi-faceted disease. One hypothesis of particular interest, the oxidative damage hypothesis, is an extension of the oxidative stress hypothesis of aging proposed by Harman (Harman, 1956). Increased generation and/or prolonged exposure to reactive oxygen species (ROS) including superoxide(O2·−), hydrogen peroxide (H2O2), hydroxyl radicals (OH·), lipid oxyl or peroxyl radicals (ROO·), singlet oxygen and peroxinitrite (NOOO−) may lead to potentially severe cellular consequences. Although it is unclear if elevated levels of ROS are the result of increased generation or reduced efficiency in elimination, or a combination of both, the resulting interaction of ROS with biomacromolecules may lead to disruption of cellular homeostasis and ultimately cell death. The oxidative damage hypothesis is thought to play a role in multiple neurodegenerative disease due to the sensitized nature of the brain resulting from its high energy demand and limited antioxidant defense mechanisms compared to other tissues (Markesbery, 1997). Electron leakage during oxidative phosphorylation resulting in formation of the superoxide radical in mitochondria is thought to be the primary source of endogenous free radical generation (Ernster and Schatz, 1981). Increased oxidative damage of mitochondrial DNA (mtDNA) is attributed to the close proximity to ROS generation, the absence of protective histones, and relatively limited repair mechanisms (Ames et al., 1993; Wallace, 1992). Decreased glucose consumption and depletion of cellular energy, noted in the earliest stages of AD, is thought to be a direct result of ROS mediated damage to mitochondria (Arnaiz et al., 2001; Drzezga et al., 2003; Fukuyama et al., 1994). Additional processes including the iron catalyzed Fenton reaction leading to generation of superoxide (Imam et al., 2006) and the Haber-Weiss reaction leading to generation of hydroxyl radicals (Mello Filho and Meneghini, 1984) may also contribute to radical mediated damage. Multiple lines of research have sought to quantify by-products and identify secondary effects of ROS-mediated damage to biomacromolecules in AD. Collectively these studies suggest that oxidative damage to lipids, proteins, DNA, and RNA may play a role in the pathogenesis of AD. In particular, deoxyribonucleic acids (DNA) including nuclear DNA (nDNA) and mtDNA, are susceptible to hydroxyl radical mediated damage resulting in over 20 potential adducts (Cooke et al., 2003). Oxidation of guanosine at the C8 position results in the formation of 8-hydroxy-2′-deoxyguanosine (8-OHdG), the most commonly studied biomarker of nucleic acid oxidation. Previous studies demonstrate multiple nucleic acid adducts, including increased levels of oxidized guanine in both nDNA and mtDNA in early and late stage AD (de la Monte et al., 2000; Gabbita et al., 1998; Lovell et al., 1999; Lyras et al., 1997; Mecocci et al., 1994; Mecocci et al., 1993; Wang et al., 2006; Wang et al., 2005). Furthermore, protein levels of 8-oxo-guanine glycosylase 1 (OGG1), the primary enzyme responsible for excision of 7, 8-dihydro-8-oxoguanine, were reduced (Iida et al., 2002; Nakabeppu et al., 2004) as were OGG1 activities in AD (Mao et al., 2007; Shao et al., 2008; Weissman et al., 2007). ROS-mediated damage during the earliest clinical phase of AD, mild cognitive impairment (MCI), and in late-stage AD (LAD) has been widely studied. However, there has been limited study of subjects during the earliest pathological phase of disease progression, preclinical AD (PCAD). Clinically, PCAD subjects were undistinguishable from normal control subjects based on neuropsychology evaluations (Schmitt et al., 2000; Schmitt et al., 2012). However, pathological evaluation revealed pronounced AD associated pathology to an extent that these subjects meet the intermediate or high likelihood criteria for the histopathological diagnosis of AD by National Institute on Aging Reagan Institute (NIA-RI) criteria (Schmitt et al., 2000). To fully characterize the extent of nucleic acid oxidation in both nDNA and mtDNA during both the early stages and progression of AD, levels of multiple nucleic acid adducts were quantified from two vulnerable brain regions, the superior and middle temporal gyri (SMTG) and inferior parietal lobule (IPL), and a non-vulnerable brain (cerebellum; CER) of subjects with MCI, PCAD, LAD, and age-matched normal control subjects (NC) by gas chromatography mass spectrometry (GC/MS) operated in selective ion monitoring mode. To determine if oxidized damage is specific to AD levels, DNA oxidation was quantified in the same brain regions from diseased control (DC) subjects with either frontotemporal dementia (FTD) or dementia with Lewy bodies (DLB).