Reduced repressive epigenetic marks, increased DNA damage and Alzheimer's disease hallmarks in the brain of humans and mice exposed to particulate urban air pollution

Highlights

• Pollution-exposed urbanites and mice have reduced frontal repressive epigenetic marks.

• Hyperphosphorylated tau is a common denominator in pollution-exposed young humans and mice.

• Combustion-derived nanoparticles are present in brain cell nuclei of pollution-exposed urbanites.

Abstract

Exposure to air pollutants is associated with an increased risk of developing Alzheimer's disease (AD). AD pathological hallmarks and cognitive deficits are documented in children and young adults in polluted cities (e.g. Metropolitan Mexico City, MMC). Iron-rich combustion- and friction-derived nanoparticles (CFDNPs) that are abundantly present in airborne particulate matter pollution have been detected in abundance in the brains of young urbanites. Epigenetic gene regulation has emerged as a candidate mechanism linking exposure to air pollution and brain diseases. A global decrease of the repressive histone post-translational modifications (HPTMs) H3K9me2 and H3K9me3 (H3K9me2/me3) has been described both in AD patients and animal models. Here, we evaluated nuclear levels of H3K9me2/me3 and the DNA double-strand-break marker γ-H2AX by immunostaining in post-mortem prefrontal white matter samples from 23 young adults (age 29 ± 6 years) who resided in MMC (n = 13) versus low-pollution areas (n = 10). Lower H3K9me2/me3 and higher γ-H2A.X staining were present in MMC urbanites, who also displayed the presence of hyperphosphorylated tau and amyloid-β (Aβ) plaques. Transmission electron microscopy revealed abundant CFDNPs in neuronal, glial and endothelial nuclei in MMC residents' frontal samples. In addition, mice exposed to particulate air pollution (for 7 months) in urban Santiago (Chile) displayed similar brain impacts; reduced H3K9me2/me3 and increased γ-H2A.X staining, together with increased levels of AD-related tau phosphorylation. Together, these findings suggest that particulate air pollution, including metal-rich CFDNPs, impairs brain chromatin silencing and reduces DNA integrity, increasing the risk of developing AD in young individuals exposed to high levels of particulate air pollution.

Introduction

Urban air pollution is a worldwide environmental health problem affecting millions of people. Epidemiological studies in developed and developing nations have linked exposure to air pollutants, particularly ultrafine (PM_0.1) and fine particulate matter (PM_2.5), to increased morbidity and mortality (Cohen et al., 2017) and to diverse neuropathological and neurological abnormalities, including brain cancer (Weichenthal et al., 2019) and impaired cognitive abilities (Calderón-Garcidueñas et al., 2008a, 2019; Chen et al., 2017a; Forns et al., 2017; Harris et al., 2015; Perera et al., 2006; Suglia et al., 2008; Zhang et al., 2018). Moreover, traffic-derived air pollutants, residency close to heavily trafficked roads and exposure to high concentrations of PM_2.5 have been associated with increased risk of dementia and Alzheimer's disease (AD) (Calderón-Garcidueñas et al., 2004; Chen et al., 2017b; Jung et al., 2015; Zhang et al., 2018). Children and young adults from Metropolitan Mexico City (MMC) exposed to moderate PM_2.5 levels exhibit neuropathological hallmarks of AD, including hyperphosphorylated tau and amyloid-β (Aβ) plaques (Calderón-Garcidueñas et al., 2008b, 2012, 2018). Abundant magnetite and other co-associated metal-rich combustion- and friction-derived nanoparticles (CFDNPs) have been detected in brains of children and young adult MMC residents (Maher et al., 2016), showing that air pollution particles can translocate to the human brain.

Recently, epigenetic gene regulation has emerged as a potential mechanism linking pollution and disease outcomes including heart disease, cancer and asthma (Li et al., 2018; Vecoli et al., 2016; Yang et al., 2017). Epigenetic mechanisms organize chromatin structure to regulate changes in gene activity. Epigenetic gene regulation can be mediated through DNA methylation, histone post-translational modifications (HPTMs), exchange of histone variants, and nucleosome remodeling. HPTMs are key components for defining chromatin status as they can either promote or inhibit transcription, depending on the modification and on the modified histone residue. It is notable that deregulation of epigenetic control is a common feature of a number of diseases, including brain disorders (Berson et al., 2018; Frost et al., 2014; Graff and Tsai, 2013; Klein et al., 2019). Specifically, recent analysis of tissue derived from AD patients and tau animal models indicate that a widespread loss of the repressive HPTMs, H3K9me2 and H3K9me3 (H3K9me2/me3) may underlie DNA damage and altered gene expression in AD (Frost et al., 2014; Mansuroglu et al., 2016).

Whether from fission yeast to humans, H3K9me2/me3 deposits are required to transcriptionally silence tissue-specific and developmental stage-specific coding genes in euchromatic and facultative heterochromatic (fHC) regions, typically to ensure lineage specificity (Allis and Jenuwein, 2016; Bannister and Kouzarides, 2011; Bustos et al., 2017; Saksouk et al., 2015; Trojer and Reinberg, 2007; Zeller and Gasser, 2017). Additionally, H3K9me3 is enriched at constitutive HC (cHC) regions which comprise diverse repeat element classes, including tandem repeats and transposable elements (Dumbovic et al., 2017; Harr et al., 2016; Zeller and Gasser, 2017). H3K9me2 is also found at cHC regions; however, this mark is particularly enriched at minor satellites of centromeric regions (Allis and Jenuwein, 2016; Peters et al., 2001; Saksouk et al., 2015; Trojer and Reinberg, 2007; Zheng et al., 2019).

Here we analyzed the repressive H3K9me2/me3 marks in post-mortem frontal white matter samples of young adults from MMC that exhibit AD hallmarks and abundant CFDNPs in the brain. Frontal white matter samples from low-pollution areas were used as controls. In addition, frontal cortex samples of mice chronically exposed to urban air pollution or HEPA-filtered air were also examined.