Abstract
BACKGROUND: Tobacco smoke is worldwide one of the main preventable lifestyle inhalative pollutants causing severe adverse health effects. Epidemiological studies revealed association of tobacco smoking with epigenetic changes at single CpGs in blood. However, the biological relevance of the often only marginal methylation changes remains unclear. RESULTS: Comparing genome-wide changes in CpG methylation of three recently reported epidemiological datasets, two obtained on whole blood and one on peripheral blood mononuclear cells (PBMCs), it becomes evident that the majority of methylation changes (86.7 and 93.3 %) in whole blood account for changes in granulocytes. Analyzing, in more detail, seven highly significant reported smoking-induced methylation changes at single CpGs in different blood cell types of healthy volunteers (n = 32), we confirmatively found a strong cell-type specificity. Two CpGs in GFI1 and F2RL3 were significantly hypomethylated in granulocytes (-11.3 %, p = 0.001; -8.7 %, p = 0.001, respectively) but not in PBMCs of smokers while two CpGs in CPOX and GPR15 were found to be hypomethylated in PBMC (-4.3 %, p = 0.003; -4.2 %, P = 0.009, respectively) and their subtypes of GPR15 non-expressing (-3.2 %, p = 0.027; -2.5 %, p = 0.032, respectively) and smoking-evoked GPR15 expressing T cells (-15.8 %, p < 0.001; -13.8 %, p = 0.018, respectively) but not in granulocytes. In contrast, cg05575921 within AHRR was hypomethylated in every analyzed cell type of smokers, but with a different degree. Both, hypomethylation at cg05575921 in granulocytes (-55.2 % methylation change in smokers, p < 0.001) and the frequency of GPR15+ T cells (9.8-37.1 % in smokers), possessing a specific hypomethylation at cg19859270, were strongly associated with smoking behavior at individual level and could therefore serve as valuable biomarkers indicating a disturbed homeostasis in smokers. In contrast to the reported long-term persistent methylation changes in adult smokers after cessation, the hypomethylation at cg05575921 in prenatally tobacco smoke-exposed children (n = 13) from our LINA cohort was less stable and disappeared already within 2 years after birth. CONCLUSIONS: Studying cell type-specific methylation changes provides helpful information regarding the biological relevance of epigenetic modifications. Here, we could show that smoking differently affects both cells of the innate and adaptive immune systems.