Share this article on:

Epigenetic mechanisms and models in the origins of asthma

Karmaus, Wilfried; Ziyab, Ali H.; Everson, Todd; Holloway, John W.

Current Opinion in Allergy and Clinical Immunology: February 2013 - Volume 13 - Issue 1 - p 63–69
doi: 10.1097/ACI.0b013e32835ad0e7
MECHANISMS OF ALLERGY AND ADULT ASTHMA: Edited by Stephen T. Holgate and J. Andrew Grant

Purpose of review Epigenetic mechanisms have the ability to alter the phenotype without changing the genetic code. The science of epigenetics has grown considerably in recent years, and future epigenetically based treatments or prevention strategies are likely. Epigenetic associations with asthma have received growing interest because genetic and environmental factors have been unable to independently explain the cause of asthma.

Recent findings Recent findings suggest that both the environment and underlying genetic sequence variation influence DNA methylation, which in turn seems to modify the risk conferred by genetic variants for various asthma phenotypes. In particular, DNA methylation may act as an archive of a variety of early developmental exposures, which then can modify the risk related to genetic variants.

Summary Current asthma treatments may control the symptoms of asthma but do not modify its natural history. Epigenetic mechanisms and novel explanatory models provide burgeoning approaches to significantly increase our understanding of the initiation and progression of asthma. Due to the inheritance of epigenetics, we anticipate a rapid emergence of critical information that will provide novel treatment strategies for asthma in the current generation and ultimately the prevention of asthma in future generations.

aDepartment of Epidemiology and Biostatistics, University of South Carolina, Columbia, South Carolina, USA

bAcademic Units of Clinical and Experimental Sciences

cHuman Development and Health, Faculty of Medicine, University of Southampton, Southampton, UK

Correspondence to Professor Wilfried Karmaus, Department of Epidemiology and Biostatistics, Norman J. Arnold School of Public Health, University of South Carolina, 800 Sumter St, Columbia, SC 29208, USA. Tel: +1 803 777 9814; fax: +1 803 777 2524; e-mail:

Back to Top | Article Outline


In the 1940s, Conrad Waddington used the term epigenetics to describe how the genotype manifests itself as a phenotype [1]. In 1958, Nanney [2] borrowed the term to describe inherited phenomena that could not be explained by conventional genetics. Recently, epigenetics has been defined concisely by Ptashne in 2007 by three criteria: a change in the activity of a gene that does not involve a mutation, that is initiated by a signal, and that is inherited (mitotically or meiotically) in the absence of the signal that initiated the change [3▪]. Classically, four epigenetic mechanisms have been identified: DNA methylation, histone modification, chromatin remodeling, and small (21–26-nt) and noncoding RNAs.

There is ample evidence that DNA methylation fulfills all three criteria required for it to be considered as an epigenetic mechanism [4–6]. Histone modifications fulfill some of the criteria for being epigenetic mechanisms in that they can result from exogenous signals such as cigarette smoke and that they alter gene activity [7–9]. However, meiotic inheritance has only been demonstrated in Caenorhabditis elegans, a transparent nematode [10]. DNA methylation usually works hand in hand with histone modifications to activate or silence genes by influencing chromatin structure and its accessibility by transcription factors [11]. So, it is possible that DNA methylation constitutes a mechanism of inheritance for some histone modifications. Given the complex and ever-changing structure of chromatin, there is little information on chromatin remodeling regarding initiation, alteration of gene activity, and inheritance [12–14]. MicroRNAs (miRNAs) also have been shown to be caused by exogenous factors and to alter gene activity by either inhibiting translation or degrading messenger RNAs (mRNA) [15,16]. For instance, in humans, miRNAs have been demonstrated to be differentially expressed in current and never smokers and to be related to particulate matter exposure [7,17]. Currently, there is little evidence that miRNAs can be inherited [18]. However, because miRNAs are part of the genetic code, it is possible that DNA methylation affects the activity of miRNAs and thus facilitates inheritance. Hence, in the following, we concentrate on the truly and well established epigenetic mechanism, DNA methylation.

Box 1

Box 1

Back to Top | Article Outline


Asthma is the most common chronic disease among children and has a complex etiology including genetic and environmental factors. Human studies have investigated the role of DNA methylation more often than other epigenetic marks due to practical and biological reasons [19]. Table 1 gives a summary of recent population-based studies investigating the association between DNA methylation and asthma [20–22,23▪,24,25▪▪]. Sood et al. [20] investigated the role of DNA methylation of 12 genes selected due to their involvement in oxidative stress pathways in sputum of 695 older adults. They found that the PCDH20 gene coding protocadherin-20, a protein involved in cell adhesion and signal transduction, was statistically more highly methylated in sputum cells from asthma patients. In a subsample of 36 of 637 children, Isidoro-Garcia et al. studied methylation of the D prostanoid receptor (PTGDR) gene. Prostaglandin D2, a metabolite of arachidonic acid, inhibits the apoptosis, prolonging eosinophilic survival, and biases the development of naive T lymphocytes to T helper 2 cells. Isidoro-Garcia et al. [21] showed that genetic variants of the PTGDR gene altered adjacent DNA methylation levels, which was related to hypomethylation of the promoter of the PTGDR gene among asthmatic participants. In gene expression analyses, the authors were able to demonstrate that hypomethylation caused by underlying sequence variants in patients was associated with increased PTGDR expression [21]. A limitation of these two studies is that DNA methylation may not constitute a risk for asthma but may reflect a response due to the disease (reverse causation).

Table 1

Table 1

Among 182 children with asthma, high methylation levels of adrenergic-receptor beta-2 (ADRB2) gene, an important regulator of airway smooth muscle tone, have been associated with severe childhood asthma [22]. Taking environmental exposures into account, an increased risk of severe asthma was associated with the joint effect of indoor NO2 exposure and high levels of ADRB2 methylation, which suggests that DNA methylation can act as an effect modifier for the association between NO2 levels and asthma severity [22]. An environmental study focused on the effects of particulate matter conducted among 940 southern California school children. Salam et al. [23▪] investigated the fraction of exhaled nitric oxide (FeNO) produced by the bronchial epithelium and the NOS2 gene that codes the nitric oxide synthase. The results demonstrated two-way interactions between ‘exposure to particulate matter with an aerodynamic diameter of 2.5 μm or less (PM2.5) × NOS2 genetic variants’ and ‘PM2.5 exposure × NOS2 methylation’ and a three-way interaction between ‘PM2.5 exposure × NOS2 genetic variants × CpG methylation levels’ that jointly influenced FeNO levels [23▪]. In another investigation of this cohort, Breton et al. [24] reported associations between differential DNA methylation of arginase-1 (ARG1) and ARG2 and significantly higher levels of FeNO in children with asthma. The authors suggest that differential methylation of ARG genes may play a role in modifying FeNO production in individuals whose inflammatory and oxidative stress pathways are already upregulated.

Morales et al. [25▪▪] addressed a burgeoning question, namely whether dichlorodiphenyldichloroethylene (DDE), a metabolite of the pesticide DDT, is related to the development of asthma [26,27]. Their results suggest that prenatal DDE exposure and genetic variants were associated with DNA hypomethylation of ALOX12 gene. In turn, this hypomethylation was a risk for persistent wheezing up to 6 years of age [25▪▪].

Back to Top | Article Outline


The epidemiological investigations (Table 1) demonstrate that both environmental and genetic factors may influence DNA methylation levels and could act as effect modifiers for asthma-related phenotypes. Hence, environmental exposures and genetic factors are both essential elements that determine epigenetic state in asthma [28,29]. Multiple past and current exposures have been linked to levels of DNA methylation such as the Dutch famine [30,31], low birth weight and fetal alcohol syndrome [32,33], maternal gestational stress in third trimester [34], gestational folate levels [35–38], early life socio-economic position [39], infections [40–43], and smoking [44–52]. Similarly genetic variants have been shown to affect the susceptibility to DNA methylation, a process named allele-specific or genotype-dependent DNA methylation [53–59]. Such genetic variants have recently been named methylation quantitative trait loci (methQTL) [60,61▪▪].

Hence, we do not only need to understand the mechanisms by which alterations in the epigenome alter phenotype but also to test different models of interplay between genetic variants, environmental factors, and DNA methylation in the etiology of asthma. A common idea is that the epigenome is an integrator of multiple signals in the pathway to diseases. Although different steps seem to be involved in structuring the DNA methylation profiles, the integrative role often remains a black box (Fig. 1, model A) [62,63]. Here, we propose a two-stage model (Fig. 1, model B), allowing these stages to develop in different phases of life. In stage 1, specific exposures and methQTLs interact within one gene and change the DNA methylation status of specific genetic elements (either promoter or intragenic). Once a methylation change close to a methQTL has been established, for instance, at the promoter site, the gene may be differentially regulated. The response to additional exposures that interact with other genetic variants of the same gene depends on whether, for example, the promoter is silenced or activated. To contrast these other genetic variants, whose response may be modified as a consequence of prior DNA methylation, from methQTLs, we call these modifiable genetic variants (modGV). The three-way interaction in the study by Salam et al. [23▪] in children in southern California showed that NOS2 genetic variants were modifiable (Table 1). Similarly, the study of asthma severity by Fu et al. [22] demonstrates that DNA methylation is an effect modifier for the association between indoor exposure to NO2 and severe asthma. Recently for eczema, Ziyab et al. [64▪] demonstrated that the haploinsufficiency of the filaggrin gene can be modified by DNA methylation within the intragenic region that worsens the insufficiency. Experimentally, in lymphoblastoid cell lines, similar models have been identified by Berlivet et al. [65▪] for the asthma-associated locus 17q12–q21.



Comparable to the studies described above that have focused on gene promoter methylation, these models also apply to intragenic methylation. DNA methylation is more frequent within gene bodies (intragenic) than in promoters. Whereas hypermethylation of promoter sites has been associated with transcriptional silencing, intragenic methylation has been observed to have a positive or a bell-shaped correlation with gene expression [66]. Recently, for the CD45 transcript, it has been demonstrated that intragenic DNA methylation is related to alternative premRNA splicing [67]. In particular, it has been suggested that a specific binding factor was involved in splicing regulation [67]. We speculate that methQTLs and exposure may also affect intragenic DNA methylation (stage 1) and then modify premRNA splicing (stage 2).

DNA methylation is likely to have contributed to discrepancies found among genome-wide association studies. Both methQTL and modGV are part of the set of genetic variants (e.g. SNP: single nucleotide polymorphism, haplotypes) that are the focus of genome-wide association studies. The detection of associations between such genetic variants and phenotypes may therefore depend on other modifiers of DNA methylation levels such as environmental exposure. For instance, a SNP may facilitate DNA methylation in an exposed study group but not in the unexposed group. As stage 1 changes may penetrate through stage 2 to some extent, a methQTL in the exposed group may be associated with increased risk of the disease. However, in another study group, a different genetic sequence (nonrisk genotype) in the same methQTL may not be favoring DNA methylation, and thus not establishing a risk for a disease. Also, modGV with the same genetic code may be masked by DNA methylation in one study group but unmasked in another study. Such settings lead to disagreements between genetic studies and reduce the chance to replicate candidate genes [68]. Hence, a methQTL cannot be assessed without knowing the exposure and modGV cannot be assessed without taking the methylation of other SNPs/haplotypes into account that may influence gene regulation or splicing.

Back to Top | Article Outline


As exemplified by the study by Morales et al. [25▪▪], it is important to consider the timing of exposure, measurement of DNA methylation, and phenotypic outcome assessment with reference to the life course. In the study by Morales et al. [25▪▪], first, change in methylation related to prenatal DDE in DNA obtained at 4 years of age was assessed (replication study: cord blood), and second, the altered DNA methylation was linked to wheezing. This approach avoids the problem of reverse causation that can result if DNA methylation may either result as a response to the disease or may be considered as a risk factor. The concept of the ‘developmental programming’ has been well accepted [69–71] and there is increasing awareness of its importance in asthma [72]. Environmental pollutants may influence crucial cellular functions during critical periods of fetal development and permanently alter the structure or function of specific organ systems.

Some studies [73,74] suggest that intrauterine and early life exposures to a farming environment are associated with decreased risk of allergic disorders, including asthma. This protective effect is believed to be associated with epigenetic mechanisms that are induced during early developmental stages. Recently, Slaats et al. [75] demonstrated that profiles of promoter DNA methylation of CD14 gene measured in placentas were different among mothers living on a farm compared with mothers not living on a farm. However, this finding needs to be replicated in a larger sample and the biological pathway underlying the protective effect needs further elucidation.

Another example of prenatal exposure is maternal smoking. Using the Norwegian Mother and Child Cohort Study (cord blood), Joubert et al. [76▪▪] reported that DNA methylation in cord blood derived DNA of genes including the cytochrome P450 aryl-hydrocarbon-hydroxylase CYP1A1 gene and the aryl hydrocarbon receptor repressor gene (AHRR) are differentially methylated after gestational exposure to cigarette smoking. The CYP1A1 gene codes an enzyme that catalyzes the conversion of chemical species into reactive intermediates such as quinones; AHRR competes with AHR for binding at xenobiotic response elements and is related to active smoking. In addition, Karmaus et al. (unpublished observation) have demonstrated that these genes were also differentially methylated in individuals exposed to in-utero cigarette smoke in blood DNA samples at 18 years of age in the Isle of Wight Birth cohort. Given that early life DNA methylation leads to a cell memory [77,78], children may be programmed to metabolize xenobiotics differently, which can increase their disease risk due to smoke exposure later in life. Hence, DNA methylation builds gene-activation memories during key periods of development (e.g. in utero and adolescence) producing aberrant activation patterns later in life, which may elevate disease risk.

Back to Top | Article Outline


To date, only a few studies have reported associations between epigenetic marks and the diverse asthma-related phenotypes. Although most studies have focused on different candidate genes, they have shown similar models of interactions between genetic variants (methQTLs and modGV), environmental exposures, and DNA methylation. There is a need to improve our knowledge about the black box of epigenetics with regard to exposures and diseases. The biological mechanisms that lead to specific changes in gene regulation in response to specific exposures are not known. We need to determine whether epigenetics should be considered as a major integrator of multiple signals, or, alternatively, whether DNA methylation acts differently at various developmental stages depending on genetic variants and exposures, such as in the proposed two-stage model. In addition, as there is a lack of critical knowledge on which genes are programmed or re-programmed at what time during gestation and in which developmental phase, birth cohort studies need to trace DNA methylation over time, and ideally over generations. This will provide critical information about which phases in the course of life are most suitable to prevent deviant DNA methylation (preventive epigenomics) or intervene to normalize DNA methylation to prevent disease (pharmacoepigenomics) [79]. Given that patterns of DNA methylation can be inherited through meiosis, future research will provide a unique chance to not only prevent and treat asthma in the current generation but also prevent it in subsequent generations.

Back to Top | Article Outline


The authors’ work was funded and supported by the National Institute of Allergy and Infectious Diseases under award number R01 AI091905. Research reported in this review was supported by the National Institute of Allergy and Infectious Diseases under Award Number R01 AI091905-01 (PI: W.K.).

Back to Top | Article Outline

Conflicts of interest

The authors have no conflicts of interest to declare in connection with this work.

Back to Top | Article Outline


Papers of particular interest, published within the annual period of review, have been highlighted as:

  • ▪ of special interest
  • ▪▪ of outstanding interest

Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 122).

Back to Top | Article Outline


1. Ledford H. Language: disputed definitions. Nature 2008; 455:1023–1028.
2. Nanney DL. Epigenetic control systems. Proc Natl Acad Sci U S A 1958; 44:712–717.
3▪. Ptashne M. On the use of the word ‘epigenetic’. Curr Biol 2007; 17:R233–R236.

This article provides a concise definition of epigenetics.

4. Daxinger L, Whitelaw E. Understanding transgenerational epigenetic inheritance via the gametes in mammals. Nat Rev Genet 2012; 13:153–162.
5. Alegria-Torres JA, Baccarelli A, Bollati V. Epigenetics and lifestyle. Epigenomics 2011; 3:267–277.
6. Jones PA. Functions of DNA methylation: islands, start sites, gene bodies and beyond. Nat Rev Genet 2012; 13:484–492.
7. Lovinsky-Desir S, Miller RL. Epigenetics, asthma, and allergic diseases: a review of the latest advancements. Curr Allergy Asthma Rep 2012; 12:211–220.
8. Clifford RL, John AE, Brightling CE, Knox AJ. Abnormal histone methylation is responsible for increased vascular endothelial growth factor 165a secretion from airway smooth muscle cells in asthma. J Immunol 2012; 189:819–831.
9. Royce SG, Karagiannis TC. Histone deacetylases and their role in asthma. J Asthma 2012; 49:121–128.
10. Greer EL, Maures TJ, Ucar D, et al. Transgenerational epigenetic inheritance of longevity in Caenorhabditis elegans. Nature 2011; 479:365–371.
11. Callinan PA, Feinberg AP. The emerging science of epigenomics. Hum Mol Genet 2006; 15 Spec. No. 1:R95–R101.
12. Travers AA, Vaillant C, Arneodo A, Muskhelishvili G. DNA structure, nucleosome placement and chromatin remodelling: a perspective. Biochem Soc Trans 2012; 40:335–340.
13. Berr A, Menard R, Heitz T, Shen WH. Chromatin modification and remodelling: a regulatory landscape for the control of Arabidopsis defence responses upon pathogen attack. Cell Microbiol 2012; 14:829–839.
14. Grigoryev SA, Woodcock CL. Chromatin organization: the 30 nm fiber. Exp Cell Res 2012; 318:1448–1455.
15. Angulo M, Lecuona E, Sznajder JI. Role of microRNAs in lung disease. Arch Bronconeumol 2012; 48:325–330.
16. Su WY, Xiong H, Fang JY. Natural antisense transcripts regulate gene expression in an epigenetic manner. Biochem Biophys Res Commun 2010; 396:177–181.
17. Yang IV, Schwartz DA. Epigenetic control of gene expression in the lung. Am J Respir Crit Care Med 2011; 183:1295–1301.
18. Buckley BA, Burkhart KB, Gu SG, et al. A nuclear Argonaute promotes multigenerational epigenetic inheritance and germline immortality. Nature 2012; 489:447–451. doi: 10.1038/nature11352.
19. Talens RP, Boomsma DI, Tobi EW, et al. Variation, patterns, and temporal stability of DNA methylation: considerations for epigenetic epidemiology. FASEB J 2010; 24:3135–3144.
20. Sood A, Petersen H, Blanchette CM, et al. Methylated genes in sputum among older smokers with asthma. Chest 2012; 142:425–431.
21. Isidoro-Garcia M, Sanz C, Garcia-Solaesa V, et al. PTGDR gene in asthma: a functional, genetic, and epigenetic study. Allergy 2011; 66:1553–1562.
22. Fu A, Leaderer BP, Gent JF, et al. An environmental epigenetic study of ADRB2 5’-UTR methylation and childhood asthma severity. Clin Exp Allergy 2012. doi: 10.1111/j.1365-2222.2012.04055.x. [Epub ahead of print]
23▪. Salam MT, Byun HM, Lurmann F, et al. Genetic and epigenetic variations in inducible nitric oxide synthase promoter, particulate pollution, and exhaled nitric oxide levels in children. J Allergy Clin Immunol 2012; 129:232–239.e1–e7.

This is the first article that investigates the interaction between genetic variants, exposures, and DNA methylation.

24. Breton CV, Byun HM, Wang X, et al. DNA methylation in the arginase–nitric oxide synthase pathway is associated with exhaled nitric oxide in children with asthma. Am J Respir Crit Care Med 2011; 184:191–197.
25▪▪. Morales E, Bustamante M, Vilahur N, et al. DNA hypomethylation at ALOX12 is associated with persistent wheezing in childhood. Am J Respir Crit Care Med 2012; 185:937–943.

This is article provides an excellent example of how to analyze DNA methylation data, addressing two steps: (1) from exposure to DNA methylation and (2) from DNA methylation to the phenotype.

26. Karmaus W. Infections and atopic disorders in childhood and organochlorine exposure. Arch Environ Health 2001; 56:485–492.
27. Sunyer J, Torrent M, Munoz-Ortiz L, et al. Prenatal dichlorodiphenyldichloroethylene (DDE) and asthma in children. Environ Health Perspect 2005; 113:1787–1790.
28. Koppelman GH, Nawijn MC. Recent advances in the epigenetics and genomics of asthma. Curr Opin Allergy Clin Immunol 2011; 11:414–419.
29. Blumenthal MN. Genetic, epigenetic, and environmental factors in asthma and allergy. Ann Allergy Asthma Immunol 2012; 108:69–73.
30. Heijmans BT, Tobi EW, Stein AD, et al. Persistent epigenetic differences associated with prenatal exposure to famine in humans. Proc Natl Acad Sci U S A 2008; 105:17046–17049.
31. Tobi EW, Lumey LH, Talens RP, et al. DNA methylation differences after exposure to prenatal famine are common and timing- and sex-specific. Hum Mol Genet 2009; 18:4046–4053.
32. Tobi EW, Heijmans BT, Kremer D, et al. DNA methylation of IGF2, GNASAS, INSIGF and LEP and being born small for gestational age. Epigenetics 2011; 6:171–176.
33. Haycock PC. Fetal alcohol spectrum disorders: the epigenetic perspective. Biol Reprod 2009; 81:607–617.
34. Oberlander TF, Weinberg J, Papsdorf M, et al. Prenatal exposure to maternal depression, neonatal methylation of human glucocorticoid receptor gene (NR3C1) and infant cortisol stress responses. Epigenetics 2008; 3:97–106.
35. Boeke CE, Baccarelli A, Kleinman KP, et al. Gestational intake of methyl donors and global LINE-1 DNA methylation in maternal and cord blood: Prospective results from a folate-replete population. Epigenetics 2012; 7:253–260.
36. Dominguez-Salas P, Cox SE, Prentice AM, et al. Maternal nutritional status, C(1) metabolism and offspring DNA methylation: a review of current evidence in human subjects. Proc Nutr Soc 2012; 71:154–165.
37. Hoyo C, Murtha AP, Schildkraut JM, et al. Methylation variation at IGF2 differentially methylated regions and maternal folic acid use before and during pregnancy. Epigenetics 2011; 6:928–936.
38. Chowdhury S, Cleves MA, MacLeod SL, et al. Maternal DNA hypomethylation and congenital heart defects. Birth Defects Res A Clin Mol Teratol 2011; 91:69–76.
39. Borghol N, Suderman M, McArdle W, et al. Associations with early-life socio-economic position in adult DNA methylation. Int J Epidemiol 2012; 41:62–74.
40. Maekita T, Nakazawa K, Mihara M, et al. High levels of aberrant DNA methylation in Helicobacter pylori-infected gastric mucosae and its possible association with gastric cancer risk. Clin Cancer Res 2006; 12 (3 Pt 1):989–995.
41. Yanagawa N, Osakabe M, Hayashi M, et al. Detection of HPV-DNA, p53 alterations, and methylation in penile squamous cell carcinoma in Japanese men. Pathol Int 2008; 58:477–482.
42. Bonvicini F, Manaresi E, Di Furio F, et al. Parvovirus b19 DNA CpG dinucleotide methylation and epigenetic regulation of viral expression. PLoS One 2012; 7:e33316.
43. Niwa T, Tsukamoto T, Toyoda T, et al. Inflammatory processes triggered by Helicobacter pylori infection cause aberrant DNA methylation in gastric epithelial cells. Cancer Res 2010; 70:1430–1440.
44. Ally MS, Al-Ghnaniem R, Pufulete M. The relationship between gene-specific DNA methylation in leukocytes and normal colorectal mucosa in subjects with and without colorectal tumors. Cancer Epidemiol Biomarkers Prev 2009; 18:922–928.
45. Breton CV, Byun HM, Wenten M, et al. Prenatal tobacco smoke exposure affects global and gene-specific DNA methylation. Am J Respir Crit Care Med 2009; 180:462–467.
46. Breitling LP, Yang R, Korn B, et al. Tobacco-smoking-related differential DNA methylation: 27K discovery and replication. Am J Hum Genet 2011; 88:450–457.
47. Ehrlich S, Walton E, Roffman JL, et al. Smoking, but not malnutrition, influences promoter-specific DNA methylation of the proopiomelanocortin gene in patients with and without anorexia nervosa. Can J Psychiatry 2012; 57:168–176.
48. Flom JD, Ferris JS, Liao Y, et al. Prenatal smoke exposure and genomic DNA methylation in a multiethnic birth cohort. Cancer Epidemiol Biomarkers Prev 2011; 20:2518–2523.
49. Suter M, Ma J, Harris AS, et al. Maternal tobacco use modestly alters correlated epigenome-wide placental DNA methylation and gene expression. Epigenetics 2011; 6:1284–1294.
50. Philibert RA, Beach SR, Gunter TD, et al. The effect of smoking on MAOA promoter methylation in DNA prepared from lymphoblasts and whole blood. Am J Med Genet B Neuropsychiatr Genet 2010; 153B:619–628.
51. Terry MB, Ferris JS, Pilsner R, et al. Genomic DNA methylation among women in a multiethnic New York City birth cohort. Cancer Epidemiol Biomarkers Prev 2008; 17:2306–2310.
52. Hillemacher T, Frieling H, Moskau S, et al. Global DNA methylation is influenced by smoking behaviour. Eur Neuropsychopharmacol 2008; 18:295–298.
53. Gertz J, Varley KE, Reddy TE, et al. Analysis of DNA methylation in a three-generation family reveals widespread genetic influence on epigenetic regulation. PLoS Genet 2011; 7:e1002228.
54. Bergman Y, Cedar H. Epigenetic control of recombination in the immune system. Semin Immunol 2010; 22:323–329.
55. Bosviel R, Garcia S, Lavediaux G, et al. BRCA1 promoter methylation in peripheral blood DNA was identified in sporadic breast cancer and controls. Cancer Epidemiol 2012; 36:e177–e182.
56. Naghibalhossaini F, Zamani M, Mokarram P, et al. Epigenetic and genetic analysis of WNT signaling pathway in sporadic colorectal cancer patients from Iran. Mol Biol Rep 2012; 39:6171–6178.
57. Tycko B. Allele-specific DNA methylation: beyond imprinting. Hum Mol Genet 2010; 19 (R2):R210–R220.
58. Bock C, Paulsen M, Tierling S, et al. CpG island methylation in human lymphocytes is highly correlated with DNA sequence, repeats, and predicted DNA structure. PLoS Genet 2006; 2:e26.
59. Shoemaker R, Deng J, Wang W, Zhang K. Allele-specific methylation is prevalent and is contributed by CpG-SNPs in the human genome. Genome Res 2010; 20:883–889.
60. Zhang D, Cheng L, Badner JA, et al. Genetic control of individual differences in gene-specific methylation in human brain. Am J Hum Genet 2010; 86:411–419.
61▪▪. Rakyan VK, Down TA, Balding DJ, Beck S. Epigenome-wide association studies for common human diseases. Nat Rev Genet 2011; 12:529–541.

This article provides an excellent overview of epigenetic association studies.

62. Relton CL, Davey Smith G. Is epidemiology ready for epigenetics? Int J Epidemiol 2012; 41:5–9.
63. Bjornsson HT, Fallin MD, Feinberg AP. An integrated epigenetic and genetic approach to common human disease. Trends Genet 2004; 20:350–358.
64▪. Ziyab AH, Karmaus W, Holloway JW, et al. DNA methylation of the filaggrin gene adds to the risk of eczema associated with loss-of-function variants. J Eur Acad Dermatol Venereol 2012. doi: 10.1111/jdv.12000. [Epub ahead of print]

This is the first epidemiologic study that demonstrates the genetic variants (haploinsufficiency) can be modified by DNA methylation.

65▪. Berlivet S, Moussette S, Ouimet M, et al. Interaction between genetic and epigenetic variation defines gene expression patterns at the asthma-associated locus 17q12-q21 in lymphoblastoid cell lines. Hum Genet 2012; 131:1161–1171.

Experimentally, this article examines different models of the interaction of genetic and epigenetic variants.

66. Jjingo D, Conley AB, Yi SV, et al. On the presence and role of human gene-body DNA methylation. Oncotarget 2012; 3:462–474.
67. Shukla S, Kavak E, Gregory M, et al. CTCF-promoted RNA polymerase II pausing links DNA methylation to splicing. Nature 2011; 479:74–79.
68. Genuneit J, Cantelmo JL, Weinmayr G, et al. A multicentre study of candidate genes for wheeze and allergy: the International Study of Asthma and Allergies in Childhood Phase 2. Clin Exp Allergy 2009; 39:1875–1888.
69. Chavatte-Palmer P, Tarrade A, Levy R. Developmental origins of health and disease in adults: Role of maternal environment. Gynecol Obstet Fertil 2012; 40:517–519.
70. Gluckman PD, Hanson MA, Bateson P, et al. Towards a new developmental synthesis: adaptive developmental plasticity and human disease. Lancet 2009; 373:1654–1657.
71. Hanson M, Gluckman P. Developmental origins of noncommunicable disease: population and public health implications. Am J Clin Nutr 2011; 94 (6 Suppl):1754S–1758S.
72. Krauss-Etschmann S, Bush A, Bellusci S, et al. Of flies, mice and men: a systematic approach to understanding the early life origins of chronic lung disease. Thorax 2012. doi:10.1136/thoraxjnl-2012-201902 [Epub ahead of print]
73. Lampi J, Canoy D, Jarvis D, et al. Farming environment and prevalence of atopy at age 31: prospective birth cohort study in Finland. Clin Exp Allergy 2011; 41:987–993.
74. Ege MJ, Herzum I, Buchele G, et al. Prenatal exposure to a farm environment modifies atopic sensitization at birth. J Allergy Clin Immunol 2008; 122:407–412.12 e1–12 e4.
75. Slaats GG, Reinius LE, Alm J, et al. DNA methylation levels within the CD14 promoter region are lower in placentas of mothers living on a farm. Allergy 2012; 67:895–903.
76▪▪. Joubert BR, Haberg SE, Nilsen RM, et al. 450K epigenome-wide scan identifies differential DNA methylation in newborns related to maternal smoking during pregnancy. Environ Health Perspect 2012; 120:1425–1431.

This is the first study that analyzes the effects of maternal smoking during pregnancy on DNA methylation of specific genes using an epigenome-wide approach.

77. Kabesch M, Adcock IM. Epigenetics in asthma and COPD. Biochimie 2012; 94:2231–2241.
78. Tammen SA, Friso S, Choi SW. Epigenetics: the link between nature and nurture. Mol Aspects Med 2012. doi: 10.1016/j.mam.2012.07.018 [Epub ahead of print]
79. Ingelman-Sundberg M, Gomez A. The past, present and future of pharmacoepigenomics. Pharmacogenomics 2010; 11:625–627.

asthma; DNA methylation; epigenetics; methylation quantitative trait loci; modifiable genetic variants

Copyright © 2013 Wolters Kluwer Health, Inc. All rights reserved.