Epigenetics, oxidative stress, and Alzheimer disease

Abstract

Alzheimer disease (AD) is a progressive neurodegenerative disorder whose clinical manifestations appear in old age. The sporadic nature of 90% of AD cases, the differential susceptibility to and course of the illness, as well as the late age onset of the disease suggest that epigenetic and environmental components play a role in the etiology of late-onset AD. Animal exposure studies demonstrated that AD may begin early in life and may involve an interplay between the environment, epigenetics, and oxidative stress. Early life exposure of rodents and primates to the xenobiotic metal lead (Pb) enhanced the expression of genes associated with AD, repressed the expression of others, and increased the burden of oxidative DNA damage in the aged brain. Epigenetic mechanisms that control gene expression and promote the accumulation of oxidative DNA damage are mediated through alterations in the methylation or oxidation of CpG dinucleotides. We found that environmental influences occurring during brain development inhibit DNA-methyltransferases, thus hypomethylating promoters of genes associated with AD such as the beta-amyloid precursor protein (APP). This early life imprint was sustained and triggered later in life to increase the levels of APP and amyloid-beta (Abeta). Increased Abeta levels promoted the production of reactive oxygen species, which damage DNA and accelerate neurodegenerative events. Whereas AD-associated genes were overexpressed late in life, others were repressed, suggesting that these early life perturbations result in hypomethylation as well as hypermethylation of genes. The hypermethylated genes are rendered susceptible to Abeta-enhanced oxidative DNA damage because methylcytosines restrict repair of adjacent hydroxyguanosines. Although the conditions leading to early life hypo- or hypermethylation of specific genes are not known, these changes can have an impact on gene expression and imprint susceptibility to oxidative DNA damage in the aged brain.

Figure 1. Structural modifications in CpG dinucleotides and their impact on Sp1-binding and DNA Repair

Sp1 binding and DNA repair glycosylase (Ogg1) activities were evaluated in oligonucleotides containing a 5-methyl cytosine and/or an adjacent 8-oxo-dG. An unmodified oligonucleotide was used as control. Top panels illustrate results for assessments of Sp1 binding (A) or Ogg1 activity (B); bottom panels illustrate the biological consequences of these modifications. A) Sp1 binding was reduced by methylation or by presence of an oxidized G. However, the inhibition was larger when both base modifications were present in the same sequence. B) Similarly, presence of a methylated cytosine next to an oxidized G reduced the activity of Ogg1, preventing repair of oxidative damage to DNA. The solid line represents time-associated repair when only 8-oxo-dG is present; while, the dashed line represents repair when 5-mC is adjacent to the oxidized G. Methods for Sp1 binding and Ogg1 activity have previously been published [60, 66]. The values presented were derived from 3–4 experiments.

Figure 2. DNA methylation of the APP promoter across the lifespan

Pyrosequencing was used in order to quantify the methylation levels at different CpG sites of the APP promoter. The methylation levels of the CpG dinucleotides located in the illustrated region were quantified after bisulfite conversion. The graph shows changes in methylcytosine content in these three different sites at different ages in the frontal cortex of monkeys. Three monkeys per age group were used for this sequencing.

Figure 3. Epigenetic modifications during development and their impact on gene expression, DNA damage and neurodegeneration in the aging brain

It is presumed that DNA methylation during development sets the level of responsiveness of a gene for life. The higher the methylation burden, the more silenced a gene is. Exposure to Pb (or other perturbations) during development may inhibit DNA methylation of target genes such as APP. The inhibition of DNA methylation patterns resets the responsiveness of the APP promoter and the expression of the APP gene to a higher ceiling. The reset gene is over-expressed when challenged by an aging trigger. This leads to an increase in the production of APP and its amyloidogenic Aβ cleavage. Aβ forms aggregates and generates free radicals which attack macromolecules such as DNA. Exposure to Pb (or other perturbation) early in life may also enhance the methylation of some genes. The epigenetically modified genes may be more susceptible to oxidative stress later in life. Epigenetic modulations of 5-methylcytosine residues impair the capacity to repair adjacent oxidized guanine bases thereby rendering neurons more susceptible to damage. The increase in the levels of Aβ promotes aggregation, free radical formation, and DNA damage. These events enhance neurodegeneration and the formation of senile plaques in the aging brain.