DNA hypomethylation, ambient particulate matter, and increased blood pressure: findings from controlled human exposure experiments

Biological embedding of early-life exposures and disease risk in humans: a role for DNA methylation

BACKGROUND

Following wider acceptance of 'the thrifty phenotype' hypothesis and the convincing evidence that early-life exposures can influence adult health even decades after the exposure, much interest has been placed on the mechanisms through which early-life exposures become biologically embedded.

MATERIALS AND METHODS

In this review, we summarize the current literature regarding biological embedding of early-life experiences. To this end, we conducted a literature search to identify studies investigating early-life exposures in relation to DNA methylation changes. In addition, we summarize the challenges faced in investigations of epigenetic effects, stemming from the peculiarities of this emergent and complex field. A proper systematic review and meta-analyses were not feasible given the nature of the evidence.

RESULTS

We identified seven studies on early-life socio-economic circumstances, 10 studies on childhood obesity and six studies on early-life nutrition all relating to DNA methylation changes that met the stipulated inclusion criteria. The pool of evidence gathered, albeit small, favours a role of epigenetics and DNA methylation in biological embedding, but replication of findings, multiple comparison corrections, publication bias and causality are concerns remaining to be addressed in future investigations.

CONCLUSIONS

Based on these results, we hypothesize that epigenetics, in particular DNA methylation, is a plausible mechanism through which early-life exposures are biologically embedded. This review describes the current status of the field and acts as a stepping stone for future, better designed investigations on how early-life exposures might become biologically embedded through epigenetic effects.

Carcinogenicity of ambient air pollution: use of biomarkers, lessons learnt and future directions

The association between ambient air pollution (AAP) exposure and lung cancer risk has been investigated in prospective studies and the results are generally consistent, indicating that long-term exposure to air pollution can cause lung cancer. Biomarkers can enhance research on the health effects of air pollution by improving exposure assessment, increasing the understanding of mechanisms, and enabling the investigation of individual susceptibility. In this review, we assess DNA adducts as biomarkers of exposure to AAP and early biological effect, and DNA methylation as biomarker of early biological change and discuss critical issues arising from their incorporation in AAP health impact evaluations, such as confounding, individual susceptibilities, timing, intensity and duration of exposure, and investigated tissue. DNA adducts and DNA methylation are treated as paradigms. However, the lessons, learned from their use in the examination of AAP carcinogenicity, can be applied to investigations of other biomarkers involved in AAP carcinogenicity.

Differential expression and DNA methylation of angiotensin type 1A receptors in vascular tissues during genetic hypertension development

Angiotensin type 1a receptor (AT1aR) is thought to play an important role in the development of hypertension. However, it is unknown how the AT1aR expression in vascular tissue is changed during the development of hypertension or if the degree of methylation in the AT1aR promoter correlates with the expression of AT1aR. To address these questions, we measured AT1aR mRNA, protein expression, and methylation status of the AT1aR promoter in the aorta and mesenteric artery of male spontaneously hypertensive rats (SHRs) and age-matched Wistar-Kyoto (WKY) rats acting as controls at pre-hypertensive (4 weeks), evolving (10 weeks), and established (20 weeks) stages of hypertension. The expression of the AT1aR mRNA and protein was not different between the SHRs and WKY rats at 4 weeks. However, they were significantly greater in SHRs than in WKY rats at 20 weeks. Bisulfite sequencing revealed that the AT1aR promoter from the aorta and mesenteric artery of the SHRs was progressively hypo-methylated with age as compared with their WKY rat counterparts. These results suggest that the heightened AT1aR expression in SHRs is related to the AT1aR promoter hypo-methylation, which might be a consequence of the increased blood pressure and may be important in the maintenance of high blood pressure.

Inducible nitric oxide synthase gene methylation and parkinsonism in manganese-exposed welders

INTRODUCTION

Neurologist-assessed parkinsonism signs are prevalent among workers exposed to manganese (Mn)-containing welding fume. Neuroinflammation may possibly play a role. Inducible nitric oxide synthase, coded by NOS2, is involved in inflammation, and particulate exposure increases the gene's expression through methylation of CpG sites in the 5' region.

METHODS

We assessed DNA methylation at three CpG sites in the NOS2 exon 1 from blood from 201 welders. All were non-Hispanic Caucasian men 25-65 years old who were examined by a neurologist specializing in movement disorders. We categorized the workers according to their Unified Parkinson Disease Rating Scale motor subsection 3 (UPDRS3) scores as parkinsonism cases (UPDRS3 ≥ 15; n = 49), controls (UPDRS3 < 6; n = 103), or intermediate (UPDRS3 ≥ 6 to < 15; n = 49).

RESULTS

While accounting for age, examiner and experimental plate, parkinsonism cases had lower mean NOS2 methylation than controls (p-value for trend = 0.04), specifically at CpG site 8329 located in an exonic splicing enhancer of NOS2 (p-value for trend = 0.07). These associations were not observed for the intermediate UPDRS3 group (both p-value for trend ≥ 0.59).

CONCLUSIONS

Inflammation mediated by inducible nitric oxide synthase may possibly contribute to the association between welding fume and parkinsonism, but requires verification in a longitudinal study.

Basic mechanisms for adverse cardiovascular events associated with air pollution

Air pollution is a significant cause of cardiovascular morbidity and mortality worldwide. Although the epidemiologic association between air pollution exposures and exacerbation of cardiovascular disease (CVD) is well established, the mechanisms by which these exposures promote CVD are incompletely understood. This review provides an overview of the components of air pollution, an overview of the cardiovascular effects of air pollution exposure, and a review of the basic mechanisms that are activated by exposure to promote CVD.

Heart rate variability and DNA methylation levels are altered after short-term metal fume exposure among occupational welders: a repeated-measures panel study

Background

In occupational settings, boilermakers are exposed to high levels of metallic fine particulate matter (PM_2.5) generated during the welding process. The effect of welding PM_2.5 on heart rate variability (HRV) has been described, but the relationship between PM_2.5, DNA methylation, and HRV is not known.

Methods

In this repeated-measures panel study, we recorded resting HRV and measured DNA methylation levels in transposable elements Alu and long interspersed nuclear element-1 (LINE-1) in peripheral blood leukocytes under ambient conditions (pre-shift) and right after a welding task (post-shift) among 66 welders. We also monitored personal PM_2.5 level in the ambient environment and during the welding procedure.

Results

The concentration of welding PM_2.5 was significantly higher than background levels in the union hall (0.43 mg/m³ vs. 0.11 mg/m³, p < 0.0001). The natural log of transformed power in the high frequency range (ln HF) had a significantly negative association with PM_2.5 exposure (β = -0.76, p = 0.035). pNN10 and pNN20 also had a negative association with PM_2.5 exposure (β = -0.16%, p = 0.006 and β = -0.13%, p = 0.030, respectively). PM_2.5 was positively associated with LINE-1 methylation [β = 0.79%, 5-methylcytosince (%mC), p = 0.013]; adjusted for covariates. LINE-1 methylation did not show an independent association with HRV.

Conclusions

Acute decline of HRV was observed following exposure to welding PM_2.5 and evidence for an epigenetic response of transposable elements to short-term exposure to high-level metal-rich particulates was reported.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2458-14-1279) contains supplementary material, which is available to authorized users.

Short-term diesel exhaust inhalation in a controlled human crossover study is associated with changes in DNA methylation of circulating mononuclear cells in asthmatics

Background

Changes in DNA methylation have been associated with traffic-related air pollution in observational studies, but the specific mechanisms and temporal dynamics therein have not been explored in a controlled study of asthmatics. In this study, we investigate short-term effects of diesel exhaust inhalation on DNA methylation levels at CpG sites across the genome in circulating blood in asthmatics.

Methods

A double-blind crossover study of filtered air and diesel exhaust exposures was performed on sixteen non-smoking asthmatic subjects. Blood samples were collected pre-exposure, and then 6 and 30 hours post-exposure. Peripheral blood mononuclear cell DNA methylation was interrogated using the Illumina Infinium HumanMethylation450 Array. Exposure-related changes in DNA methylation were identified. In addition, CpG sites overlapping with Alu or LINE1 repetitive elements and candidate microRNA loci were also analyzed.

Results

DNA methylation at 2827 CpG sites were affected by exposure to diesel exhaust but not filtered air; these sites enriched for genes involved in protein kinase and NFkB pathways. CpG sites with significant changes in response to diesel exhaust exposure primarily became less methylated, with a site residing within GSTP1 being among the significant hits. Diesel exhaust-associated change was also found for CpG sites overlapping with Alu and LINE1 elements as well as for a site within miR-21.

Conclusion

Short-term exposure to diesel exhaust resulted in DNA methylation changes at CpG sites residing in genes involved in inflammation and oxidative stress response, repetitive elements, and microRNA. This provides plausibility for the role of DNA methylation in pathways by which airborne particulate matter impacts gene expression and offers support for including DNA methylation analysis in future efforts to understand the interactions between environmental exposures and biological systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-014-0071-3) contains supplementary material, which is available to authorized users.

Arterial blood pressure responses to short-term exposure to low and high traffic-related air pollution with and without moderate physical activity

BACKGROUND

Short-term exposure to traffic-related air pollution (TRAP) has been associated with adverse cardiovascular outcomes. Physical activity (PA) in polluted air may increase pollutant uptake and increase these effects.

METHODS

Crossover real-world exposure study in 28 healthy participants comparing systolic (SBP) and diastolic blood pressure (DBP) responses to four different exposure scenarios: 2 h exposure in high or low-TRAP environment, each at rest and combined with intermittent moderate PA consisting of 15 min intervals alternating rest and cycling on a stationary bicycle. Data was analyzed using mixed effect models for repeated measures.

RESULTS

Exposure to high TRAP was associated with higher DBP (1.1 mm/Hg, p = 0.002) post-exposure, irrespective of exercise status. Ultrafine particles (UFP) increased DBP post-exposure (0.9 mm/Hg, p = 0.004). Interquartile increases in black carbon (BC), fine particulate matter (PM10 and PMcoarse), UFP, and nitric oxides (NOx) were associated with statistically significantly higher SBP post-exposure (1.2, 1.0, 1.1, and 1.1 mm/Hg, respectively). Intermittent PA compared with rest was associated with lower SBP post-exposure (-2.4 mm/Hg, p < 0.001). PA lowered SBP more after exposure to the low-TRAP site (-2.3 mm/Hg) compared with the high-TRAP site (-1.6 mm/Hg). We only found evidence of an interaction between PA and both PM10 and PMcoarse, increasing SBP.

CONCLUSION

Both SBP and DBP increase after exposure to TRAP. Intermittent PA attenuates the TRAP-related increases in SBP, with the exception of PM10 and PMcoarse, which potentiate these increases. We showed that in low-TRAP environments intermittent PA has stronger beneficial effects on SBP than in high-TRAP environments.

Ambient Coarse Particulate Matter and Human Health: A Systematic Review and Meta-Analysis

Airborne particles have been linked to increased mortality and morbidity. As most research has focused on fine particles (PM2.5), the health implications of coarse particles (PM10-2.5) are not well understood. We conducted a systematic review and meta-analysis of associations for short- and long-term PM10-2.5 concentrations with mortality and hospital admissions. Using 23 mortality and 10 hospital admissions studies, we documented suggestive evidence of increased morbidity and mortality in relation to higher short-term PM10-2.5 concentrations, with stronger relationships for respiratory than cardiovascular endpoints. Reported associations were highly heterogeneous, however, especially by geographic region and average PM10-2.5 concentrations. Adjustment for PM2.5 and publication bias resulted in weaker and less precise effect estimates, although positive associations remained for short-term PM10-2.5 concentrations. Inconsistent relationships between effect estimates for PM10-2.5 and correlations between PM10-2.5 and PM2.5 concentrations, however, indicate that PM10-2.5 associations cannot be solely explained by co-exposure to PM2.5. While suggestive evidence was found of increased mortality with long-term PM10-2.5 concentrations, these associations were not robust to control for PM2.5. Additional research is required to better understand sources of heterogeneity of associations between PM10-2.5 and adverse health outcomes.

Effects of short-term exposure to inhalable particulate matter on DNA methylation of tandem repeats

There is compelling evidence that particulate matter (PM) increases lung cancer risk by triggering systemic inflammation, and leukocyte DNA hypomethylation. However, previous investigations focused on repeated element sequences from LINE-1 and Alu families. Tandem repeats, which display a greater propensity to mutate, and are often hypomethylated in cancer patients, have never been investigated in individuals exposed to PM. We measured methylation of three tandem repeats (SATα, NBL2, and D4Z4) by polymerase chain reaction-pyrosequencing on blood samples from truck drivers and office workers (60 per group) in Beijing, China. We used lightweight monitors to measure personal PM2.5 (PM with aerodynamic diameter ≤2.5 µm) and elemental carbon (a tracer of PM from vehicular traffic). Ambient PM10 data were obtained from air quality measuring stations. Overall, an interquartile increase in personal PM2.5 and ambient PM10 levels was associated with a significant covariate-adjusted decrease in SATα methylation (-1.35% 5-methyl cytosine [5mC], P = 0.01; and -1.33%5mC; P = 0.01, respectively). Effects from personal PM2.5 and ambient PM10 on SATα methylation were stronger in truck drivers (-2.34%5mC, P = 0.02; -1.44%5mC, P = 0.06) than office workers (-0.95%5mC, P = 0.26; -1.25%5mC, P = 0.12, respectively). Ambient PM10 was negatively correlated with NBL2 methylation in truck drivers (-1.38%5mC, P = 0.03) but not in office workers (1.04%5mC, P = 0.13). Our result suggests that PM exposure is associated with hypomethylation of selected tandem repeats. Measuring tandem-repeat hypomethylation in easy-to-obtain blood specimens might identify individuals with biological effects and potential cancer risk from PM exposure.

Dietary flavanols modulate the transcription of genes associated with cardiovascular pathology without changes in their DNA methylation state

Background

In a recent intervention study, the daily supplementation with 200 mg monomeric and oligomeric flavanols (MOF) from grape seeds for 8 weeks revealed a vascular health benefit in male smokers. The objective of the present study was to determine the impact of MOF consumption on the gene expression profile of leukocytes and to assess changes in DNA methylation.

Methodology/Principal Findings

Gene expression profiles were determined using whole genome microarrays (Agilent) and DNA methylation was assessed using HumanMethylation450 BeadChips (Illumina). MOF significantly modulated the expression of 864 genes. The majority of the affected genes are involved in chemotaxis, cell adhesion, cell infiltration or cytoskeleton organisation, suggesting lower immune cell adhesion to endothelial cells. This was corroborated by in vitro experiments showing that MOF exposure of monocytes attenuates their adhesion to TNF-α-stimulated endothelial cells. Nuclear factor kappa B (NF-κB) reporter gene assays confirmed that MOF decrease the activity of NF-κB. Strong inter-individual variability in the leukocytes' DNA methylation was observed. As a consequence, on group level, changes due to MOF supplementation could not be found.

Conclusion

Our study revealed that an 8 week daily supplementation with 200 mg MOF modulates the expression of genes associated with cardiovascular disease pathways without major changes of their DNA methylation state. However, strong inter-individual variation in leukocyte DNA methylation may obscure the subtle epigenetic response to dietary flavanols. Despite the lack of significant changes in DNA methylation, the modulation of gene expression appears to contribute to the observed vascular health effect of MOF in humans.

Controlled exposure of humans with metabolic syndrome to concentrated ultrafine ambient particulate matter causes cardiovascular effects

Many studies have reported associations between air pollution particles with an aerodynamic diameter <2.5 μm (fine particulate matter (PM)) and adverse cardiovascular effects. However, there is an increased concern that so-called ultrafine PM which comprises the smallest fraction of fine PM (aerodynamic diameter <0.1 μm) may be disproportionately toxic relative to the 0.1-2.5 μm fraction. Ultrafine PM is not routinely measured in state monitoring networks and is not homogenously dispersed throughout an airshed but rather located in hot spots such as near combustion sources (e.g., roads), making it difficult for epidemiology studies to associate exposure to ultrafine PM with adverse health effects. Thirty four middle-aged individuals with metabolic syndrome were exposed for 2 h while at rest in a randomized crossover design to clean air and concentrated ambient ultrafine particles (UCAPS) for 2 h. To further define potential risk, study individuals carrying the null allele for GSTM1 (a prominent antioxidant gene) were identified by genotyping. Blood was obtained immediately prior to exposure, and at 1 and 20 h afterward. Continuous Holter monitoring began immediately prior to exposure and continued for 24 h. Based on changes we observed in previous CAPS studies, we hypothesized that ultrafine CAPS would cause changes in markers of blood inflammation and fibrinolysis as well as changes in heart rate variability and cardiac repolarization. GSTM1 null individuals had altered cardiac repolarization as seen by a change in QRS complexity following exposure to UCAPS and both the entire study population as well as GSTM1 null individuals had increased QT duration. Blood plasminogen and thrombomodulin were decreased in the whole population following UCAPS exposure, whereas C-reactive protein (CRP) and SAA were increased. This controlled human exposure study is the first to show that ambient ultrafine particles can cause cardiovascular changes in people with metabolic syndrome, which affects nearly a quarter of the U.S. adult population.

Genetics and epigenetics of arrhythmia and heart failure

Heart failure (HF) is the end stage of several pathological cardiac conditions including myocardial infarction, cardiac hypertrophy and hypertension. Various molecular and cellular mechanisms are involved in the development of HF. At the molecular level, the onset of HF is associated with reprogramming of gene expression, including downregulation of the alpha-myosin heavy chain (α-MHC) gene and sarcoplasmic reticulum Ca ²⁺ ATPase genes and reactivation of specific fetal cardiac genes such as atrial natriuretic factor and brain natriuretic peptide. These deviations in gene expression result in structural and electrophysiological changes, which eventually progress to HF. Cardiac arrhythmia is caused by altered conduction properties of the heart, which may arise in response to ischemia, inflammation, fibrosis, aging or from genetic factors. Because changes in the gene transcription program may have crucial consequences as deteriorated cardiac function, understanding the molecular mechanisms involved in the process has become a priority in the field. In this context, various studies besides having identified different DNA methylation patterns in HF patients, have also focused on specific disease processes and their underlying mechanisms, also introducing new concepts such as epigenomics. This review highlights specific genetic mutations associated with the onset and progression of HF, also providing an introduction to epigenetic mechanisms such as histone modifications, DNA methylation and RNA-based modification, and highlights the relation between epigenetics, arrhythmogenesis and HF.

Endotoxin in concentrated coarse and fine ambient particles induces acute systemic inflammation in controlled human exposures

BACKGROUND

Knowledge of the inhalable particulate matter components responsible for health effects is important for developing targeted regulation.

OBJECTIVES

In a double-blind randomised cross-over trial of controlled human exposures to concentrated ambient particles (CAPs) and their endotoxin and (1→3)-β-D-glucan components, we evaluated acute inflammatory responses.

METHODS

35 healthy adults were exposed to five 130-min exposures at rest: (1) fine CAPs (~250 µg/m(3)); (2) coarse CAPs (200 µg/m(3)); (3) second coarse CAPs (~200 µg/m(3)); (4) filtered air; and (5) medical air. Induced sputum cell counts were measured at screening and 24 h postexposure. Venous blood total leucocytes, neutrophils, interleukin-6 and high-sensitivity C reactive protein (CRP) were measured pre-exposure, 3 and 24 h postexposure.

RESULTS

Relative to filtered air, an increase in blood leucocytes 24 h (but not 3 h) postexposure was significantly associated with coarse (estimate=0.44×10(9) cells/L (95% CI 0.01 to 0.88); n=132) and fine CAPs (0.68×10(9) cells /L (95% CI 0.19 to 1.17); n=132), but not medical air. Similar associations were found with neutrophil responses. An interquartile increase in endotoxin (5.4 ng/m(3)) was significantly associated with increased blood leucocytes 3 h postexposure (0.27×10(9) cells/L (95% CI 0.03 to 0.51); n=98) and 24 h postexposure (0.37×10(9) cells/L (95% CI 0.12 to 0.63); n=98). This endotoxin effect did not differ by particle size. There were no associations with glucan concentrations or interleukin-6, CRP or sputum responses.

CONCLUSIONS

In healthy adults, controlled coarse and fine ambient particle exposures independently induced acute systemic inflammatory responses. Endotoxin contributes to the inflammatory role of particle air pollution.