Modification of additive effect between vitamins and ETS on childhood asthma risk according to GSTP1 polymorphism : a cross -sectional study

Asthma is a chronic inflammatory disease of the respiratory tract, and the bronchial airways are particularly susceptible to oxidation-induced tissue damage [ 1]. Environmental tobacco smoke (ETS) worsens asthma symptoms and leads to poor asthma control in adults and children [ 2, 3]. Current guidelines for asthma treatment recommend avoiding exposure to tobacco smoke, including both active smoking and ETS [ 4]. Children who are exposed to ETS in their homes have lower peak lung function than those who are not exposed [ 5].

Oxidative stress results in inflammation and tissue damage in the respiratory system and, subsequently, immune system damage. Indeed, individuals with lower cellular reducing capacity have an increased risk of developing asthma [ 6]. Tobacco smoke is a key source of free radicals related to oxidative stress and therefore increases the risk for asthma [ 7, 8].

Antioxidants provide protection by preventing oxidative DNA damage [ 9]. A deficiency in dietary antioxidants is associated with increased asthma risk by increasing susceptibility to oxidative stress [ 10, 11]. The combination of smoking and low antioxidant levels increase free radicals and therefore lead to antioxidant depletion and oxidative stress [ 12– 14]. Smoking is strongly associated with reduced blood concentration of vitamin C, as well as α- and β-carotene [ 15]. Smoking has also been shown to deplete endogenous antioxidants such as vitamins C and E, β-carotene, ubiquinol, glutathione, and α-lipoic acid [ 16]. Antioxidant depletion increases individual vulnerability to free radicals and other oxidant species produced by tobacco smoking, and thus increases morbidity, the rate of aging, and risk of death.

Dietary supplementation with antioxidants may reduce the overall oxidative burden that is increased by cigarette smoking [ 16]. Previous studies indicate that consumption of carotenoids, a group of dietary antioxidants, may reduce the smoking-induced increase in colorectal cancer risk [ 17]. Another study shows that the intake of nutrients with antioxidant properties may reduce lung function decline in older adults exposed to cigarette smoke [ 18]. Enhancing antioxidant defenses may reduce the cumulative effects of oxidative damage and perhaps the risk of developing childhood asthma. To the best of our knowledge, no research has evaluated the impact of ETS and dietary antioxidants on the risk of childhood asthma.

Genetic factors, such as gene polymorphisms that alter the antioxidant response, may also contribute to the relationship between asthma and ETS or deficiency of dietary antioxidants. Glutathione S-transferases (GSTs) are a group of enzymes that are reported to neutralize the effects of tobacco smoke and reduce oxidative stress [ 19]. Most studies investigating genetically associated susceptibility to respiratory abnormalities have focused on GSPT1, the most abundant GST isozyme in the lungs, as well as GSTM1 and GSTT1. The functional sequence variant in GSTP1 at codon 105 (Ile105Val -rs1695) has been associated with asthma in some [ 20, 21] but not all studies [ 22]. This variant has been shown to protect against and increase the risk of asthma. Several studies report that GSTP1 genotypes modulate the effect of environment-induced respiratory symptoms and asthma in children [ 23, 24]. However, the effect of gene polymorphisms on the response to dietary antioxidants and ETS, and subsequently asthma risk, is poorly understood.

This study showed an additive effect of low dietary intake of vitamin A and ETS exposure for increasing risk for asthma symptoms. Additionally, the AA GSTP1 polymorphism was associated with an increased risk for asthma in children who were exposed to ETS and had a low dietary intake of vitamin A and carotene. Children who were exposed to ETS were significantly more likely to report wheeze within the previous 12 months and have an asthma diagnosis. Although overall antioxidant intake was not associated with presence of asthma symptoms, children who were exposed to ETS and had a low vitamin A intake were more likely to report asthma symptoms. This trend was particularly notable in children carrying the GSTP1 genotype AA, which has been associated with an increased risk for asthma. Our study suggested that low vitamin A intake increased susceptibility to development of ETS-associated childhood asthma by decreasing antioxidant capacity, and the oxidative stress-related GSTP1 gene further modified this association.

Oxidative stress occurs when the generation of oxidant molecules (i.e., free radicals) exceeds the available antioxidant defenses [ 30]. Inflammatory disorders such as asthma and allergic rhinitis may be mediated by oxidative stress [ 25], which occurs as a result of endogenous inflammation and following environmental exposure to toxic substances such as cigarette smoke and air pollutants in allergic airway diseases [ 6]. Exposure to ETS is a major environmental factor that influences the development and aggravation of asthma and impaired lung function in childhood [ 7].

Cigarette smoke inhalation increases exposure to reactive oxygen species [ 31] to a level that may overwhelm endogenous antioxidant defenses in asthmatic patients who already have exacerbated levels of oxidative stress [ 6]. A controlled human exposure model has shown that glutathione levels are reduced in the bronchial and nasal airways following exposure to air pollutants [ 32]. The body produces numerous antioxidants endogenously, but the quantity is often insufficient to prevent oxidative stress. Exogenous antioxidants, such as dietary nutrients, can supplement the endogenous system to help defend against free radicals. Smoking is associated with reduced circulating concentration of antioxidants in the blood [ 33]. This may be because dietary and supplemental antioxidant intake tends to be lower in smokers than in non-smokers and because smoke-induced oxidative stress increases the degradation or transformation of circulating antioxidant micronutrients into biologically inactive components [ 34] or even into pro-oxidants [ 35]. Therefore, dietary antioxidant intake may influence the relationship between exposure to ETS and the risk of asthma symptoms.

Several studies suggest that low serum levels or dietary intake of antioxidants may be risk factors for asthma [ 9, 10]. Dietary vitamin A intake and serum vitamin A concentrations are significantly lower in patients with asthma than in healthy control subjects and are lower in patients with severe asthma than in those with mild asthma [ 36, 37]. One study shows that low vitamin A status increases susceptibility to cigarette smoke-induced lung emphysema in a mouse model [ 38]. However, other studies suggest that dietary supplementation with vitamins, such as vitamin A and ascorbate, does not improve lung function or asthma symptoms [ 39– 41]. We found no relationship between overall dietary antioxidant intake and asthma diagnosis or wheeze in the previous 12 months, although children who were exposed to ETS and had a low dietary vitamin A intake were more likely to report symptoms of asthma. Dietary antioxidants may ameliorate the effects of smoking on asthma symptoms, although future human studies that assess the benefits of antioxidant intake should focus on selecting an appropriate exposure to oxidative stress.

Recent studies suggest that genetic factors may also contribute to an individual’s susceptibility to respiratory disorders induced by ETS exposure [ 7]. GSTP1 encodes for an enzyme that belongs to a large family of GST enzymes, which are important for detoxification of potentially harmful compounds from tobacco smoke, such as the polyaromatic hydrocarbon molecules benzopyrene and chrysene [ 42, 43]. GSTP1 is widely expressed in human airways, predominantly in alveolar macrophages and epithelial cells [ 44]. Polymorphisms in the GST genes, such as GSPT1 (rs1695), affect the ability to respond to excessive oxidative stress by altering activity of the GST enzymes [ 45]. Based on the hypothesis that dietary intake of antioxidants and endogenous antioxidant capacity contribute to the susceptibility to oxidative stress in asthmatic children, researchers investigated the effects of antioxidant supplementation on ozone-related decreases in lung function according to GSTM1 genotype [ 46, 47]. Children in the placebo group that lacked the GSTM1 gene had a significant reduction in FEF _25–75% after ozone exposure, whereas the GSTM1-positive children in the placebo group did not. Therefore, asthmatic children with compromised antioxidant defense systems caused by genetic susceptibility and deficiencies in antioxidant intake may be at increased risk for oxidative stress induced by ozone or ETS. However, a recent meta-analysis indicates that the GSTP1 single nucleotide polymorphism rs1695 did not affect the prevalence of asthma, suggesting that presence of GST variants contribute to airway diseases through interactions with the environment [ 48].

Active GSTP1 variant proteins produced by the GSTP1 gene play a role in xenobiotic metabolism and influence susceptibility to asthma and other diseases [ 49]. Some studies show that GSTP1 encodes for an important enzyme in the anti-oxidative pathway that buffers the harmful effects of air pollution [ 21, 50]. The interaction between GSTP1 and different types of air pollutants has a higher information gain than other gene-air pollutant combinations [ 21]. Therefore, we investigated the influence of interactions between ETS, dietary antioxidant intake, and the GSTP1 gene on risk for childhood asthma. We also investigated the relationship between MTHFR (rs1801133) and NQO1 (rs1800566) genes and asthma symptoms, but we did not present these data because we did not find an association.

To the best of our knowledge, few publications have investigated the overall effects of GST variants and ETS exposure on asthma symptoms [ 51– 55], and whether such effects could be modulated by dietary antioxidant intake has not yet been explored. This was the first study to assess how ETS, low dietary vitamin A intake, and GSTP1 genotype affect asthma symptoms in children. However, our study had some limitations. First, this was a cross-sectional study, and therefore we could not determine causal relationships among the factors studied. Second, we focused on only one well-known candidate gene involved in oxidative stress, and other genes likely also regulate the influence of ETS and dietary antioxidants. Third, our study may also have recall bias because our dietary data were based on the semi-quantitative FFQ completed by parents or guardians, who may have underreported unhealthy foods and overreported healthy foods. Fourth, the number of children in the asthma group was smaller than the number in the control group, a discrepancy that is common in community-based studies. In addition, we did not record the use of other supplements, such as multivitamins, and we could not confirm the association between dietary intake and serum levels of antioxidants because we did not measure serum levels. Nonetheless, the clinical implications of these findings are important because exposure to ETS is common in children. Further prospective, long-term follow-up studies are needed to confirm and extend these findings.