The results from these two cohorts confirm data from other studies, which have reported that common polymorphisms in IL4Rα
are associated with asthma outcomes, such as FEV1
. However, this study expands the understanding of the importance of the variation in this gene by also identifying a relationship between SNPs coding for amino acid substitutions in the intracellular signaling portion of the receptor and severe exacerbations of asthma. This study further identified a possible inflammatory mechanism for this effect, specifically an increase in mast cells and their IgE binding in airway tissue. Finally, there are distinct differences in allele frequencies at all coding SNPs in exon 12 between African Americans and whites that have the potential to contribute to disparities in asthma severity/health care utilization in these two racial groups (18
The primary and replicating databases used in this study contain large numbers of the most extensively phenotyped subjects with severe and milder asthma studied to date (1
). This phenotyping enabled us to associate relatively small numbers of subjects with asthma with specific asthma outcomes, such as health care utilization, lung function, and inflammatory processes. More general asthma phenotypes, such as distinctions by level of severity and total serum IgE levels, were not associated with the IL4Rα
SNPs that were studied. Because severe asthma, in particular, is recognized to consist of those individuals with both frequent/severe exacerbations and those with more persistent but severe airway obstruction (often without severe exacerbations), a specific association of genotype with one particular phenotype could be predicted. In contrast, those with milder asthma may be a slightly more homogeneous population, less confounded by treatment modalities, comorbid conditions, and phenotypes. This may explain why, in the SARP cohort, in whom the majority had mild or moderate asthma, a stronger association of IL4Rα
SNPs to %predicted FEV1
was present for both E375A and Q551R than was observed in the full cohort. This relationship was even stronger when analyzing whites separately. Interestingly, even in this milder subgroup, there remained a marginal relationship to higher intensity health care utilization (p = 0.08 for hospitalizations/ED visits in the nonsevere white subgroup for the C allele at E375A).
Th2 inflammatory pathways have long been associated with asthma, including severe asthma. IL-4Rα is likely the most important receptor involved in these pathways. IL4Rα
is known to be a highly polymorphic gene, yet only a few SNPs in this gene have been evaluated to date (8
). Of these, the SNPs in exon 12, which encode for amino acid changes in the intracellular signaling portion of the receptor, have received the most attention. These SNPs are of interest both from their persistent significance in genetic association studies of asthma and allergy and because of their potential to alter receptor function. Published studies have suggested the less common alleles at E375A, S478P, or Q551R are associated with lower lung function, higher IgE levels, and an in vitro
gain of function of the receptor (5
). In contrast to our study, where the evaluation of haplotypes consisting of the Ile50 (in the extracellular portion of the receptor) and exon 12 SNPs did not add to the significance of our findings, others have suggested that change in function of the receptor requires the presence of two specific amino acid changes in these two regions of the gene (20
). Interestingly, one study has suggested that these IL4Rα
polymorphisms contribute to a loss of function of the receptor, perhaps through a lower level of activation of STAT-6 (9
). Certainly, interactions between other genes in the Th2 pathway (including IL-4
, and others) and IL4Rα
could also contribute to clinical, physiologic, and immunologic phenotypes (4
). Future studies in larger numbers of subjects will need to address gene-by-gene interactions in these cohorts.
This is the first genetic study of subjects with asthma to evaluate the association of genotypes with tissue inflammatory findings. In subjects with the higher risk IL4Rα
polymorphisms, there was an increase in tissue mast cell numbers and a more specific increase in IgE+
mast cells in the tissue biopsies. These results support the concept that the amino acid switch from glutamic acid to alanine at position 375 contributes to a gain of function of the receptor. This could lead to an increase in serum IgE (as has previously been reported) but could also increase the binding of IgE to mast cells in the lung tissue where they may play a causal role in the development of severe asthma exacerbations (11
). Interestingly, the degree of binding of IgE to mast cells in cell culture has been suggested to prolong the survival of mast cells, outlining a mechanism by which these SNPs could increase total mast cell numbers (25
This is also the second study to address the frequency of these SNP alleles in African Americans (7
). The current study confirms the previous differences in Q551R frequencies but suggests substantially larger differences by racial origin in E375A than those previously reported (7
). Additional studies of SNPs in IL4Rα
revealed greater overall diversity of allelic frequency in IL4Rα
in African Americans as compared with that in whites, particularly in exon 12, where the SNPs of interest in this study are located (8
). The authors of that study proposed that the dense distribution of coding SNPs in this single exon, associated with broad racial diversity, supported a strong environmental influence on these mutations. Allelic substitutions in these coding SNPs, which alter signaling through IL-4Rα, could almost certainly impact parasite host defense, a response of greater importance to equatorial populations as compared with populations in colder climate regions. These coding and functional SNPs likely evolved to be adaptive and advantageous traits in host responses to parasites. What is not yet clear is whether these evolutionarily advantageous parasite responses may, in fact, have negative implications for the development or progression of allergic diseases in industrialized societies.
In the replication cohort, in particular, most of the severe exacerbation events were seen in the African-American population. Although nearly 30% of the SARP cohort subjects are African Americans, their overall numbers are still relatively small. This smaller sample size, the low incidence of homozygotes for the allele common to whites, as well as the generally much higher severe asthma exacerbation rate in the African Americans studied likely diluted the ability to discern a specific difference in this subgroup by IL4Rα
genotype. In contrast, the larger white sample sizes in both cohorts allowed analysis of the relevance of these SNPs to that population subgroup. In the substantially more severe NJ cohort, the removal of the African-American subjects only marginally diminished the significance of the associations for whites alone with exacerbations and tissue mast cells, especially for the E375A SNP. In the overall milder SARP cohort, the significance for the IL4Rα
SNPs in relationship to the white population remained marginally present for ED exacerbations (p = 0.08), whereas the associations with lung function became stronger, consistent with previous reports (11
). Although these findings continue to support the relevance of these polymorphisms to the white population, it cannot be determined from these studies whether these polymorphisms contribute to the higher morbidity and mortality rates observed in African Americans, above and beyond the contribution from the recognized disparities in health care (18
). Larger cohort studies of African Americans will be required to investigate these relationships.
These studies are limited by the relatively small sample sizes, heterogeneity of allele frequencies in the racial subgroups, and heterogeneity of the disease itself. Although we analyzed small numbers of SNPs in IL4Rα (four of which were in exon 12), these SNPs were selected based on previously described associations with asthma and allergy. It is conceivable that a more important risk-associated SNP exists in exon 12 or elsewhere in this gene that is in LD with the currently genotyped SNPs. This may explain the disconnect between E375A and Q551R SNPs, with the allele substitutions at E375A associated more closely to exacerbations and IgE/mast cell processes and Q551R changes more associated with lung function abnormalities. In addition, tissue studies are limited by the small pieces of tissue obtained and the single point in time of these investigational bronchoscopic studies, such that these results in relatively small numbers are important. Questionnaire data may also be limited by recall bias and different clinical models for urgent health care utilization, especially given the mixed geographic/health systems (one site in the United Kingdom and multiple sites in the United States). However, the marked consistency of the direction of the associations supports the validity of the results and indicates that a more comprehensive analysis of the IL4Rα gene should be pursued.
In conclusion, these results confirm prior population studies, which report associations of allele substitutions in IL4Rα with asthma and decreased lung function. In addition, these observations expand our understanding by demonstrating an association with asthma exacerbations and suggest a mechanism by which variations in IL4Rα may modify asthma severity through increases in mast cells and associated local IgE expression. Finally, these studies demonstrate large racial differences in IL4Rα SNPs between racial groups, which may be related to positive evolutionary adaptations to local environments, but which may now have negative effects in industrialized countries. Further studies to determine the functional changes in IL-4Rα signaling as they relate to asthma and parasite host defense are needed.