We demonstrate evidence for association of the GSTM1 null variant with asthma affection, age of asthma symptom onset, and pulmonary function among children with a history of passive smoke exposure. Though our results are compelling because of their consistency in both population- and family-based analysis (the latter rules out cryptic population stratification as a biasing confounder), our findings are best understood in the context of previously published studies. summarizes all prior studies that assessed asthma affection status. Though statistically significant evidence of increased asthma risk among GSTM1-null carriers was observed in only 7 of 20 populations previously studied, trends of similar association (i.e., OR >1) were observed in 12 of the 14 publications in which an OR was either reported or could be calculated from the published data. Given the large sample sizes typically required to demonstrate statistically significant modest genetic effects, the consistency in the direction of association across most of these studies suggests that the effects observed across populations are likely meaningful.
Literature review: Effects of GSTM1-null on asthma affection status
Though asthma is known to be highly heritable, with relative risk to siblings of an asthmatic of ~ 3, the role of environmental exposures in asthma pathogenesis is critical and is often underappreciated in genetic association studies. We note that, though many potential asthma genes are emerging from candidate gene and genome-wide association studies,27, 28
the vast majority of even the most promising asthma candidate genes have failed to show association in multiple other populations. One explanation for such disparate results in different populations is a failure to account for gene-by-environment interaction. Our data and others suggest that defining the importance of GSTM1 in asthma requires careful consideration of such interactions. It is certainly plausible that the cohorts that showed GSTM1 effects in all subjects simply had higher levels of exposure to environmental tobacco smoke, ozone, or other sources of increased oxidative stress.
Our cohort, with 511 asthmatics and 489 complete trios, is among the largest asthmatic cohorts tested for GSTM1 association (see ). The study was thus powered to appreciate clinically meaningful differences in the entire cohort (with 80% power to find a change in FEV1 of 4%, or onset of asthma that was 7 months earlier for example); adequate power makes our findings of no association between GSTM1 and various asthma phenotypes in the entire cohort more meaningful. In contrast to the findings in all subjects, we found strong evidence of association with asthma among subjects exposed to IUS (with 23:10 Transmitted: Untransmitted ratio of the GSTM1-null allele). Similarly, GSTM1-null individuals who were exposed to IUS had a younger age of asthma onset and a lower FEV1/FVC ratio than those with at least one copy of GSTM1; GSTM1 status did not influence these phenotypes in subjects without IUS exposure. We also note that four of the five studies that assessed GSTM1 in the context of high oxidative stress found significant association (); similarly, all studies that evaluated GSTM1 copy number and environmental effects on lung function were significant (). Thus, the literature strongly supports a role for environmental modulation of GSTM1 effects.
GSTM1 effects on lung function in asthma
Our results are most similar to those of Gilliland et al., who found that in a population-based cohort of 2590 children, associations of the GSTM1-null variant with active asthma, medication use, and earlier age of asthma onset were restricted entirely to children with a history of in utero smoke exposure.9
We note that early onset of asthma was assessed only in Gilliland’s work and ours, and both studies identified significant GSTM1 effects in the context of IUS exposure. Given the putative mechanism of GSTM1 as a defense against oxidative stress, exposure to IUS or ETS early in life could logically result in an earlier age of asthma onset among those who lack GSTM1, suggesting that this phenotype may be of particular interest for studying genetic interactions with early-life exposures.
GSTM1-null status was associated with a lower FEV1/FVC and higher FVC in IUS-exposed subjects in our cohort. These differences were present at baseline and sustained over the 4-year trial; in contrast, the slope of FEV1/FVC and FVC did not change with GSTM1 status. These lung function findings are most consistent with dysanapsis, in which airway size is relatively smaller than lung parenchyma.29
Nicotine exposure has been shown to stimulate lung branching morphogenesis and increase lung size in embryonic murine explants,30
while IUS exposure is associated with increased airway collagen deposition in rhesus monkeys31
and with increased airway thickness in human infants.32
While our results are consistent with the hypothesis that the anti-oxidant effects of GSTM1 prevent such changes in lung vs. airway growth in utero, this has not been previously examined.
A strength of this study is our use of quantitative PCR to genotype GSTM1 for 0, 1, or 2 copies. Most previous publications genotyped only GSTM1 presence or absence, and therefore are unable to distinguish whether the associations observed are driven by GSTM1 hemizyogtes or homozygotes. Our results demonstrate that a single copy of GSTM1 is sufficient to confer protective effects. Due to the relatively small number of 2-copy individuals in the IUS-exposed cohort (N=4), we are underpowered to make firm inferences regarding additional effects of a second copy of GSTM1 in asthma.
We note that our findings were significant in the IUS-exposed subjects (n=50) but not significant in the much larger subset exposed to ETS (n=175). Yet 46 of the 50 subjects exposed to IUS were also exposed to ETS later in life, and tended to have more ongoing smoke exposure throughout the 4-year follow-up period. Due to this very high degree of correlation between IUS and ETS, we are unable to discern whether the observed associations with IUS reflect specific gene-by-environment interactions that are established primarily in utero, or whether IUS is simply a marker for more extensive and sustained ETS exposure later in life. We acknowledge that our findings were significant only in a small subset of subjects, with correspondingly marginal p values; though this subgroup analysis was planned given the previous GSTM1 literature, confirmation of the importance of the interactions of GSTM1 and smoke exposure in additional asthma cohorts is necessary.
In summary, we found that lack of GSTM1 is associated with asthma, an early onset of asthma, and low FEV1/FVC among asthmatic children exposed to IUS. One copy of GSTM1 appears to be sufficient to confer protective effects. Our findings support the growing literature on the importance of GSTM1 in asthmatics exposed to tobacco smoke.