Epigenetic Biomarkers of Diesel Exhaust Exposure and Pediatric Respiratory Health

Background: Global, interferon-gamma ( IFN-γ) and forkhead box transcription factor 3 (FOXP3) methylation are associated with air pollution exposures and have shown to be modified by common glutathione S-transferase (GST) variants. It has yet to be established if chronic diesel exhaust exposure during childhood is associated with global methylation levels or alterations in the promoters of IFN-γ and FOXP3 and whether the epigenetic changes are associated with the respiratory health effects, such as wheezing and asthma development, of diesel exhaust particles (DEP) exposure.;Objectives: The objectives of this dissertation were threefold; 1) determine if chronic childhood DEP exposure is associated with global methylation and gene-specific methylation of IFN-γ and FOXP3, 2) examine the modifying effects of the GST Pi-1 ( GSTP1) isoluecine105valine polymorphism, and 3) evaluate the relationship between global and gene-specific methylation patterns and wheezing and asthma development during childhood.;Methods: Children were randomly selected from the Cincinnati Childhood Allergy and Air Pollution birth cohort (n=92). DEP exposure from birth through age seven was estimated from a land-use regression model using a complete address history. Children with consistent DEP exposure throughout childhood and no exposure to tobacco smoke were eligible for inclusion. Global methylation was assessed as total methylated cytosine, IFN-γ promoter methylation was determined by methylation-specific polymerase chain reaction (MS-PCR), and FOXP3 promoter methylation was determined by pyrosequencing. GSTP1 genotype was dichotomized as carriers and noncarriers of the Val105 allele. The association between DNA methylation and DEP exposure was analyzed using generalized linear regression (adjusted for an interquartile range increase in DEP of 0.14 µg/m 3) and logistic regression (adjusted for a 1 standard deviation increase in methylation) was used for respiratory outcomes (longitudinal wheezing phenotypes and asthma).;Results: Chronic exposure to high levels of DEP resulted in significantly lower global methylation (p<0.01); this effect was over two-times greater among children carrying the GSTP1-Val105 allele (β, -0.48; 95%CI: -0.88, -0.08) compared to non carriers (β, -0.21; 95%CI: -0.46, 0.03). Significant hypermethylation of the IFN-γ promoter was observed as a result of increased DEP exposure (p<0.01) but was not modified by GSTP1 genotype. Similar to IFN-γ, increases in estimated DEP exposure were associated with increases in FOXP3 methylation (P<0.05), although this effect was not modified by GSTP1 genotype. In addition, children with increased FOXP3 methylation were also at greater risk for developing persistent wheezing (OR=3.05; 95% CI, 1.54-6.05), early transient wheezing (OR=1.57; 95% CI, 1.01-2.41), and asthma (OR=2.1; 95%CI, 1.14-3.85). Neither global nor IFN-γ methylation was associated with wheezing or asthma development.;Conclusions: Global methylation was significantly lower among children carrying the valine allele suggesting an indirect role of DEP exposure through oxidative stress mechanisms. Increasing DEP exposure was associated with increasing IFN-γ and FOXP3 methylation but these associations were not modified by GSTP1 genotype, suggesting the changes in methylation may result directly from exposure to DEP rather than indirectly through oxidative stress. FOXP3 methylation may serve as a clinically relevant mediating predictor for wheezing and/or asthma risk in children exposed to traffic-related air pollution.