Both BC, primarily from Diesel exhaust in Boston, and Sulfate particles, primarily from coal burning powerplants, were associated with changes in the methylation of genes in the asthma pathway. This finding provides a potential mechanism for the reported association of traffic pollution, particularly Diesel exhaust, and especially the particles, and the exacerbation of asthma. Diesel particles have been found to be an adjuvant for atopic responses,[11
] and heavily traffic roads, particularly with high bus or truck traffic, have been linked to respiratory symptoms and asthma.[20
] The genes modified by BC exposure include IgE signaling and eosinophil control, which fits in well with those observations. Hence altered epigenetic control of the immunoregulatory cells may be in the causal pathway between black carbon and asthmic responses. The specific genes identified suggest target proteins for toxicologic studies of Diesel particles.
Equally interesting, we found that sulfate particles, which are primarily from coal burning power plants, are also associated with altered methylation in the asthma pathway. While most attention has focused on traffic exposure and asthma in recent years, it is important to remember that a number panel and ER studies [36
] reported sulfate associated with asthma exacerbation, and there are reported drops in pulmonary function and increased symptoms in asthmatic children following exposure to episodes of high ozone and sulfate particles.[39
] While the genes whose methylation is associated with sulfate exposure were mostly different from those influenced by BC, they also included IgE receptors, cytokines, and eosinophil control. Hence reduced asthma severity may be an underappreciated benefit of control of sulfate precursors.
Another study linked exposure to air pollution and methylation in a gene related to asthma. [40
] and found that increased exposure to AAP is associated with hypermethylation of the Foxp3 locus, a regulatory T cell that with impaired function in asthmatics. Note, however, that this gene is not a member of the asthma pathway that we used.
A major limitation of this study is the cells that were used, which were peripheral leukocytes. Target tissue cells are difficult to obtain in human studies, but differential methylation by cell type can occur, raising questions about their relevance. In this case, white cells are key players in asthma, and particularly acute exacerbations, and the methylation targets identified, eosinophils, mast cells, cytokines, IgE, are all central to asthma. While BAL cells would perhaps have greater relevance, peripheral cells circulate through the lung, and are exposed there to many of the same air pollution derived signals that resident cells are. Support for this comes also from the association of air pollution with increased markers of inflammation in peripheral blood, such as C reactive protein, fibrinogen, sICAM and sVCAM, IL6, etc. Hence this limitation seems less of a concern in the case of asthma and air pollution.
Another limitation is the population being studied—elderly men. The differential diagnosis of asthma in the elderly is difficult, and many immunologic responses decline with age. Hence the same pattern of methylation changes may not be seen in younger subjects. Further, it is possible that the observed association does not hold in the same manner in females, as our investigation was conducted in a male-only cohort. Nevertheless, the finding that DNA methylation in the asthmatic pathway is modified by exposure to black carbon and sulfates is unlikely to be a response only seen in the male elderly, although the magnitude of response, and specific genes affected, may differ with age and gender. In addition, many of the participants take medications, mainly to treat high blood pressure, and it is possible that the treated condition is affected by air pollution, making drugs possible confounders in the association between pollution and methylation. However, since asthmatic subjects are excluded, it is unlikely that the drugs used are associated with methylation in the asthma pathway. Finally, a limitation is the small sample size (n = 132 for BC, and n = 85 for sulfate association analysis). These results should be validated in a separate study.