Background

IL-4 is a Th2 cytokine that plays an essential role in IgE regulation. It triggers isotype switching from IgM to IgE, induces differentiation to Th2 phenotype on T cells and plays a critical role in the induction and maintenance of allergy.IL4 gene has been mapped to chromosome 5q31 where asthma and atopy have also been linked [1–3]. Evidence that IL4 polymorphisms are associated with total IgE levels and potentially with asthma and other allergy related phenotypes has been provided, although ethnical differences have been reported [4]. Specifically, the promoter region of IL4 has been associated with asthma phenotype [5] and a -33C>T polymorphism has been reported in this region [6]. An association between this polymorphism and asthma or atopy has been found, although this relation is still controversial [4, 7–10]. IL-4 acts through the IL-4 receptor (IL-4R) that consists of two subunits, the α chain (IL-4Rα) and the γ chain (γc) [11, 12]. IL-4Rα is a component of both the IL-4 and the IL-13 receptor complexes [13]. The IL4RA gene is located on chromosome 16p (16p12.1) [14], a region reported in linkage with atopy in different populations [15, 16]. Several single-nucleotide polymorphisms (SNPs) have been identified in the coding region of the IL4RA gene, many of them resulting in aminoacid substitutions [17, 18]. One of these polymorphisms, 576Q>R, consists of an A-to-G transition at nucleotide 1902, causing a change from glutamine to arginine at codon 576 (Q576R) in the cytoplasmic domain of the IL-4Rα. It has been reported that B-lymphocytes isolated from allergic patients bearing the 576Q>R mutation have an enhanced CD23 induction in response to IL-4 [19]. However, this result has not been confirmed by other authors [20]. Association of the 576Q>R polymorphism with the atopic phenotype has been described, but this relationship is still controversial [19, 21–31]. Due to the central role of the IL-4/IL-4RA pathway in atopy and the scarce information about combinations of both genes in South European populations, we have analysed the -33C>T polymorphism of IL4 gene and the 576Q>R polymorphism of IL4RA gene in a Spanish population of patients with a well-characterized phenotype of asthma.

Methods

Subjects

We studied 212 unrelated Caucasian individuals, 133 patients and 79 controls, recruited from the outpatient Allergy Department of the University Hospital of Salamanca. The study was performed following the recommendations of the Ethical Committee of the University Hospital of Salamanca and informed written consent was obtained from each patient. Individuals who met all the following criteria were selected as controls: (i) no symptoms or history of asthma or other pulmonary diseases; (ii) no symptoms or history of atopy; (iii) negative skin prick tests to a battery of common aeroallergens (<1 mm wheal greater than saline) and (iv) absence of first-degree relatives with a history of asthma or atopy. Asthmatic patients were recruited if they had specialist physician-diagnosed asthma with the following characteristics: (i) at least two symptoms consistent with asthma (cough, wheeze and dyspnoea); (ii) either a positive bronchial hyperresponsiveness or a positive bronchodilator test defined as a ≥ 15% increase in baseline FEV1 after bronchodilator use; (iii) absence of other pulmonary disorders. Lung function was measured by spirometry according to ATS (American Thoracic Society) standards and severity of asthma was classified following GINA (Global Initiative for Asthma) guidelines. Asthma patients were grouped into intermittent and persistent by the clinical severity and into allergic and non allergic asthma by the clinical etiology.

Skin prick tests were performed according to EAACI recommendations with a battery of common aeroallergens that included D pteronisynuss, D farinae, L destructor, T putrescentiae, A siro, G domesticus, E maynei, mix of grasses, mix of trees, P judaica, C album, A vulgaris, P lanceolata, O europaea, A alternata, C herbarum, P notatum, A fumigatus, dog, cat, hamster, horse and rabbit dander and cockroach (ALK-Abelló, Madrid, Spain). Saline was used as negative control and histamine 10 mg/ml was used as positive control. Antihistamines were discontinued before skin testing according to published guidelines. Skin tests were considerer positive if at least one allergen elicited a wheal reaction of more than 3 mm of diameter after subtraction of the negative control. Patients were considered atopic if at least they had one positive skin test result. Total serum IgE was measured by a fluoroenzymeimmunoassay (Pharmacia Cap System®; Pharmacia, Uppsala, Sweden), according to the manufacturer's instructions.

Genotyping analysis

After purification from peripheral blood leukocytes, DNA was amplified by polymerase chain reaction (PCR). Genotyping of -33C>TIL4 SNP was performed using a TaqMan assay in the ABI 7700 sequence detector and the allelic discrimination software Sequence Detector v1.7 according to the manufacturer's recommendations (Applied Biosystems). Primers and probes were obtained by means of the Assays-by-Demand SNP genotyping service of Applied Biosystems, Assay ID: C 16176215. Genotyping of the 576Q>RIL4RA polymorphism was performed according to a previously published assay [28]. Two oligonucleotides were used to amplify the polymorphic region of IL4RA: 5'-CCCCCACCACCAGTGGCTACC-3' and 5'-CCAGGAATGAGGTCTTGGAA-3' [24]. PCR reactions were carried out in a total volume of 25 μL, containing 50 ng of DNA and 12.5 μL of PCR Master Mix 2 × (Promega, Madison, Wisconsin). Amplification was performed with an initial denaturation step at 94°C for 5 min followed by 30 cycles of denaturation at 94°C for 1 min, primer annealing at 55°C for 1 min, and extension at 72° for 1 min. A final extension was carried out at 72° for 10 min. A blank amplification tube was always run to check for the presence of contamination. Strict rules were taken to avoid contamination. PCR reactions were prepared on a laminar flow hood and PCR products were examined in a different room. PCR products were digested for 4 h at 37°C with 1 U of MspI (New England Biolab, Boston, Massachusetts) restriction enzyme. After enzymatic digestion of the amplified fragments, the samples were analyzed by electrophoresis in 3% nusieve agarose gel. Control and patients were not genotyped in separated batched and the analysis was performed blindly with respect to case-control status.

Statistical analysis

For case-control studies, the allele and genotype frequencies in patients with asthma were compared to a control non-asthmatic population. All the groups were tested for Hardy-Weinberg equilibrium using χ2 analyses. The dichotomous variables were analysed using χ2, Fisher exact test, Monte Carlo simulation test (after 104 iterations) and odds ratio test. IgE levels were transformed to log10 values to produce a normal distribution for statistical analysis and analysed by ANOVA. To model the effects of multiple covariates on the dichotomous and continuous variables, logistic regression was used. In multivariate analysis, sex and age were included as potential covariates. A p-value less than 0.05 was considered statistically significant. Bonferroni correction was applied when appropriate. Case-control studies were also undertaken using combination of polymorphisms. Frequencies of combinations were estimated individually in controls and in samples to give the results of both single combinations and global data. For management of data, SHEsis software platform [32] and SPSS version 11 (SPSS Inc, Chicago, IL, USA) were used.

Results

-33C>TIL4 SNP

Characteristics of patients and controls are shown in Table 1. Genotype and allele frequencies are shown in Table 2. The -33TIL4 allele was found at a frequency of 0.15 in patients with asthma versus 0.09 in controls. No statistically significant differences between patients with asthma and controls were found. However, we observed an increase of the -33T IL4 allele in patients with persistent asthma compared to controls [Fisher's p value = 0.014, Monte Carlo p value (after 104 simulations) = 0.019]. Multivariate analysis of the genotypes adjusted for age and sex confirmed a trend of association of -33C>TIL4 polymorphism with an increased risk of persistent asthma (OR: 2.77, 95% CI: 1.18–6.49; p = 0.019). No differences were found in the group of subjects suffering allergic asthma compared to controls. Analysis of the total IgE levels failed to reveal any significant difference (p = 0.22), even when separate analysis for each gender was performed (data not shown).

Table 1 Demographic characteristics of patients
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Table 2 Genotype and allele frequencies of -33C>TIL4 and 576Q>RIL4RA SNPs
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576Q>RIL4RA SNP

576RIL4RA arginine allele (G) was found at a frequency of 0.16 in patients with asthma versus 0.21 in healthy subjects (Table 2). We did not observe differences between patients and controls in allele or genotype frequencies. No association was detected with asthma phenotype or with asthma severity (Table 2). 576Q>RIL4RA polymorphism was not related to total serum IgE levels in our population (p = 0.35).

Gene-gene interaction analysis

We did not detect differences in the global distribution of -33C>TIL4 / 576Q>RIL4RA combinations between the group of patients with asthma and the group of controls (Monte Carlo p value = 0.074) (Table 3). However, patients who were carriers of both the T allele of -33C>TIL4 and the A allele of IL4RA had an increased risk of asthma [Fisher's p value = 0.017, Monte Carlo p value (after 104 simulations = 0.012, odds ratio, 2.58; 95 % CI, 1.18–5.66].

Table 3 Distribution of combinations of -33C>TIL4 and 576Q>RIL4RA SNPs
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Slight differences in the global distribution of genetic variants were detected between the group of patients with atopy and the group of controls [Fisher's p value = 0.05, Monte Carlo p value (after 104 simulations) = 0.045]. Patients who were carriers of both the T allele of -33C>TIL4 and the A allele of IL4RA had an increased risk of atopy [Fisher's p value = 0.013, Monte Carlo p value (after 104 simulations = 0.016, odds ratio, 2.64; 95 % CI, 1.93–5.87] (Table 3).

Differences in genetic variants distribution were also observed in the group of patients with allergic asthma compared to controls, although these differences did not reach statistical signification globally considered (Fisher p value = 0.053, Monte Carlo p value = 0.053). Again the combination of T and A alleles showed a trend of association [Fisher's p value = 0.016, Monte Carlo p value = 0.017, odds ratio, 2.62; 95 % CI, 1.17–5.90] (Table 3).

When we compared patients with persistent asthma to controls, significant differences in the global combination distribution were observed (Fisher's p value = 0.018, Monte Carlo p value = 0.017). Patients who carried both the T allele of -33C>TIL4 and the A allele of IL4RA had an increased risk of persistent asthma [Fisher's p value = 0.0021, Monte Carlo p value = 0.0016, odds ratio, 3.39; 95 % CI, 1.50–7.66] (Table 3).

Discussion

-33C>TIL4 SNP

We studied -33C>TIL4 and 576Q>RIL4RA polymorphisms in a well-characterized Spanish population of patients with asthma and in a healthy control population. Controls were older than patients allowing a longer period for asthma diagnosis to be made. When we analysed the -33C>T polymorphism independently, we did not detect significant differences in allele or genotype frequencies between the group of patients and the group of controls. Nevertheless, we observed a higher incidence of the T allele of -33IL4 in the group of patients with asthma (Table 2).

An association between -33C>TIL4 polymorphism and asthma or atopy has been previously reported, although this association is controversial [4, 7–9]. In previous studies, no statistically significant association between -33C>TIL4 polymorphism and atopic dermatitis, bronchial hyperresponsiveness, atopic rhinitis and skin prick test reactivity was found [7, 9]. However, a significant trend for an association between serum IgE levels and this SNP has been detected in children with positive skin prick tests, independent of asthma status [7].

As shown in table 2, we detected that the allele -33TIL4 is more frequent in patients with allergic asthma, although, we did not detect an association between this polymorphism and IgE levels.

It has been reported that polymorphisms within the promoter region of IL4 gene seems to correlate with enhanced IL4 activity [5, 33], secondary to modification of IL4 gene transcription [34]. In this sense, it has been hypothesized that the T allele may be associated with severity of asthma [23, 34, 35]. In our study, a trend of association of -33TIL4 allele with persistent asthma was observed.

576Q>RIL4RA SNP

Analysis of 576Q>RIL4RA polymorphism did not reveal any association with asthma phenotype. It has been reported a relationship of the 576Q>RIL4RA SNP with the atopic phenotype, however this relationship is still controversial [19, 21–31, 36–38]. In a previous study, we did not find any association of this polymorphism with atopy or IgE levels, except in a specific of group of patients with family history of atopy [28].

Kruse et al [21] associated the R allele with lower total IgE values, but Hersey et al [19]found an association with high levels of total IgE. We did not detect any association with IgE levels. Genetic association studies are often difficult to interpret due to poor reproducibility in different populations [39, 40]. This may well result, among other reasons, from the fact that these studies are focused only on one SNP [18, 26, 27] and that penetrance of the alleles may be influence by other factors [19].

Gene-gene interaction analysis

When we analysed both polymorphisms simultaneously, differences between the group of patients with asthma and controls were detected. Particularly, patients who were carriers of both the T allele of -33C>TIL4 and the A allele of 576Q>RIL4RA had an increased risk of asthma.

Significant differences were observed in the group of patients with allergic asthma compared to controls. In addition, patients who carried both the T allele of -33C>TIL4 and the A allele of 576Q>RIL4RA had an increased risk of persistent asthma, in our population.

It has been previously suggested that both SNPs may modify the susceptibility to atopy or atopic asthma, independently. The importance of analysis of genetic variants has been previously illustrated, because the functional significance of a given polymorphism may only be evident in a specific setting of additional SNPs in the same or different genes [40]. It has also been pointed out that genetic association studies need careful classification of phenotypes, application of quality control in the performance of laboratory procedures and very stringent significant levels to assure reproducibility [20, 39]., although it also may indicate true heterogeneity in gene-disease associations. We describe for the first time a specific genetic combination of IL4/IL4RA polymorphisms that shows a trend of association to persistent asthma in a South European population. As long as this study was performed in a small population, further studies in other populations are needed to confirm these results.

Conclusion

We show a trend of association between -33C>TIL4 and 576Q>RIL4RA polymorphisms and asthma phenotype in a Spanish population. Patients who carried both the T allele of -33C>TIL4 and the A allele of 576Q>RIL4RA showed an increased risk of allergic asthma. In the population included in our study this combination was observed more frequently in patients with persistent asthma.