This study investigated the combined effect of exposure to tobacco smoke and haplotype pairs of the IL13
gene on wheeze phenotypes in the first decade of life using the data from the Isle of Wight birth cohort. Maternal smoking during pregnancy was associated with early-transient
and early-onset persistent
wheeze. No independent effect for the IL13
gene was detected. However, common variant of IL13
gene polymorphisms were observed to increase the adverse effect of maternal smoking during pregnancy on early-onset persistent
wheeze, the phenotype with the highest morbidity [23
]. A similar association was observed for the persistent asthma phenotype. Tobacco smoke exposure after pregnancy did not modify the association of IL13
and wheezing nor asthma.
In this study, the information was available from a subset of children who were followed up at age 10 and who agreed to provide blood for genotyping. Although these children appeared to have slightly (~3%) more wheeze and less exposure to tobacco smoke in comparison to all children who were followed up at age 10, these differences were not significant. The presence of a selection bias could result in a violation of the Hardy-Weinberg equilibrium and different allele frequencies than other Caucasian populations [29
]. The latter scenario was not present, hence, a selection bias is unlikely.
The wheeze phenotypes were specified at age 10 based on longitudinal records from ages 1, 2, 4 and 10. This strengthens the study as we used longitudinal wheeze phenotypes instead of an outcome measured at a single cross section. To avoid recall bias, the analysis was restricted to children who were seen prospectively with information at all study visits [23
]. Previously, we have shown that a preceding diagnosis of asthma was less likely to produce biased reports in later follow-ups [24
Information on individual SNPs was used and the most likely pairs of haplotypes were estimated from genotype data. We demonstrated a significant association between exposure to maternal smoking during pregnancy and early-onset persistent wheeze when children had the common variant for each individual SNP as well as haplotype pairs.
One motivation for using haplotype pairs was the consistent pattern observed for the combined effect of ETS-2 and all markers (individual SNPs and haplotype pairs) on early-onset persistent
wheeze (Figure ). Additionally, it has been suggested, specifically for IL13
], that haplotype analysis could confer more information than individual marker analysis [39
]. Haplotype pair analysis may misclassify genotypes when parents' genetic information is not available (ambiguous phase). However, the probability of having CCG/CCG, the major haplotype pair, was 1.00 in 429 out of 435 children with this genotype (the probability for the other 6 children was 0.89). The distribution of the data, with respect to minor haplotypes, did not allow testing for their interactions with tobacco smoke exposure. However, when children homozygous for minor haplotypes (frequency = 0.071, table ) were removed from the analysis, the results did not change substantially.
Previous studies have shown an increased risk of asthma in children who were exposed to tobacco during pregnancy [3
]. In a prior examination of this cohort, a possible association between exposures during pregnancy and early-onset persistent
wheeze was suggested [37
]. Additionally, several studies have suggested a gene-environment interaction for the effect of tobacco smoke exposure and asthma phenotypes [13
]. Specifically, in two genome-wide screens, Colilla et al and Meyers et al reported that exposure to tobacco modifies the linkage between 5q, the region containing IL13
, and asthma phenotypes [13
]. The current study demonstrates a scenario in which a gene modifies the effect of tobacco smoke exposure during pregnancy but not after, thus, time of the exposure may be of critical importance in gene-environment interaction studies. The finding of an interaction between IL13
and ETS in the present study suggests that negative reports for the effect of a candidate gene for IL13
and asthma [20
], could be explained by a failure to take into account environmental exposures. It is therefore of utmost importance, for genetic studies, to describe environmental exposures in the target population.
Most of the previous studies, including a report on the Isle of Wight cohort [41
], suggest that the minor alleles of IL13
SNPs rs20541 (R130Q) and rs1800925 (-1112C/T) are associated with increased adverse effects [10
]. In the current study we observed that children who had the common genotype for IL13
polymorphisms (haplotype pairs or SNP rs20541) have increased risk of early onset persistent wheeze
and persistent childhood asthma in relation to maternal smoking during pregnancy.
Our finding for common IL13
variants to be a risk factor when mothers smoked during pregnancy is unexpected. Nevertheless, the observed association is statistically significant and cannot be explained by chance (p = 0.014). Additionally, we see a constant pattern for the effect of smoking across the SNPs and their haplotypes. This latter observation also suggests that it is less likely that the findings are due to a random effect. An unexpected finding for the effect of SNP rs20541 alleles on persistent wheeze is not scientifically untenable as the interplay between genes and environment is complex. Similar discrepancy has been shown for CD14, which turned out to be an interesting gene-environment interaction [43
It has been suggested that tobacco smoke increases IL-13 and there are some reports on the combined effects of IL13
polymorphisms or other genetic variants in the IL13
region on asthma-related outcomes [11
Liu et al
. reported a synergistic effect of smoking and IL13
promoter polymorphism on the level of serum IgE [44
], and genome-wide analyses have suggested a gene-environment interaction for the effect of tobacco exposure on asthma [13
]. Noakes and colleagues demonstrated that cord blood cells produce significantly higher levels of IL-13 in response to both house dust mite and ovalbumin when newborns are exposed to tobacco smoke during pregnancy [11
]. This suggests that prenatal exposure to tobacco elicits immunological effects. In our analysis, the interaction between tobacco exposure and IL13
polymorphisms was present in those who were exposed during and after pregnancy, but was not evident for the group with the same polymorphisms who were exposed only after pregnancy. As there was no group of children that was exposed only during pregnancy, it is not possible to distinguish definitively the effect of tobacco exposure before and after pregnancy. However, considering both the relatively short time of pregnancy and the large difference of risk between the two exposed groups, we suggest that exposure to tobacco during pregnancy has more influence on asthma than tobacco exposure after pregnancy.
In summary, in a sub-sample of the Isle of Wight cohort, the combined effect of exposure to tobacco smoke during pregnancy and the common haplotype pair of the IL13 gene resulted in an increased relative risk of early-onset persistent wheeze and asthma. For tobacco smoke exposure later in childhood we did not observe this association. The IL13 gene did not pose a risk in its own right. These results demonstrate that the association between exposures to environmental risk factors, like tobacco smoke, can be modified by gene polymorphisms. Given that there are various patterns and prevalences of exposure to tobacco smoke in different populations, this study suggests that negative reports of genetic association studies may be due to differences in environmental exposures. We propose that the next step in the investigation of the interaction between IL13 and exposure to tobacco smoke is to examine the influence of ETS exposure on IL13 expression.