In this study, we aimed to evaluate the contribution of SNPs in the TSLP gene towards AR susceptibility in Han Chinese subjects by employing a population-based case–control association analysis.Our study demonstrated that the allele frequencies for none of the nine tag SNPsselected for AR risk assessment were significantly different between patients with AR and control subjects. Similarly, no differences were found between the AR patients and control subjects for either the genotype distributions of the selected SNPs or the haplotype frequencies. Overall, these findings suggest that although the TSLP gene presents itself as a good candidate involved in the development of allergy, this gene is unlikely to be associated with increased susceptibility to AR in Han Chinese subjects.
The TSLP gene is located on human chromosome 5q22, near the gene cluster encoding T helper (Th) 2 cytokines [29
], and plays a critical role in Th2 cell differentiation [31
]. Association studies of allergy related phenotypes using genetic polymorphisms have been performed in different populations [32
]; with some recent studies showing possible roles of human genetic polymorphisms of the TSLP gene in allergic diseases. One study by Harada and colleagues [33
] demonstrated that the SNP rs3806933 in the promoter region of TSLP created a binding site for the transcription factor activating protein (AP)-1, and in vitro enhanced AP-1 binding to the regulatory element. In a more recent study these authors demonstrated that TSLP gene promoter polymorphisms (rs3806933 and rs2289276) were significantly associated with disease susceptibility in both childhood atopic and adult asthma [33
]. Similarly, Liu and colleagues [34
] demonstrated that one particular variant of TSLP (rs1898671) contributed to asthma susceptibility in admixed urban populations and that the risk of asthma was significantly increased in ex-smokers; suggesting gene and environment interaction. In contrast, a large study from Canada demonstrated that variant rs1837253, which is 5.7
kb upstream of the transcription start site of the TSLP gene, was associated with protection from asthma, atopic asthma, and airway hyperresponsiveness [35
]. More recently, a genome-wide association study in a Japanese population-based cohort of adult asthma utilizing fine mapping analysis of the region on chromosome 5q22 using 13 tag SNPs showed that rs1837253 represented an associated LD block spanning 88
kb that included two genes, TSLP and WDR36. The authors further concluded that TSLP was the most plausible susceptibility gene in this locus [36
]. Similarly, a large consortium-based genome-wide study has demonstrated significant association between asthma and several other SNPs [37
]. However, with respect to the association between TSLP polymorphisms and AR, the study by Bunyavanich and colleagues [16
] indicated that TSLPSNP rs1837253 was associated with reduced odds for AR in boys with asthma. In the present study, we selected the representative tag SNPs in and near the TSLP gene region and the SNPs confirmed to be the candidate locus associated with allergy in previous studies. In the present study, rs1898671 and rs3806933 were substituted for rs10455025 and rs12110124 in terms of the r2
value (>0.8), but despite the inclusion of these main described TSLP variants (rs1837253, rs1898671 and rs3806933) in the adult Han Chinese population, we were not able to demonstrate any significant association between these variants and AR susceptibility, similar to that shown by Bunyavanich and colleagues [16
]. However, unlike the study of Bunyavanich and colleagues [16
] which investigated individuals with comorbid AR and asthma, a major strength of our study is that subjects with AR and asthma were excluded and all subjects in this study only had AR. This is of particular relevance because up to 80% of asthmatics have AR and as noted from several studies described above, TSLP is a well replicated asthma susceptibility gene. Thus, it is possible that the lack of an association between the TSLP gene and AR susceptibility noted in our study is an accurate reflection of the real-life situation, because the TSLP gene is associated with asthma but not AR.
It is of interest that there is evidence which suggests that gender might modify the role of TSLP in asthma. Transgenic expression of TSLP in mice leads to perivascular leukocytic infiltration with prominent eosinophilia, with increased severity noted in female mice compared to malemice [38
]. Hunninghake and colleagues [39
] reported a sex-specific association between a polymorphism, rs2289276, and serum total IgE in girls in two independent populations. More recently, these authors demonstrated that TSLP polymorphisms were also associated with asthma in a sex-specific fashion [40
]. The T allele of rs1837253 was significantly associated with a reduced risk of asthma in males only, whereas the T allele of rs2289276 was significantly associated with a reduced risk of asthma in females only [40
]. However, in stratified analyses of AR associations with gender, no SNPs showed significant associated effects among either male nor female groups in present study.
Despite the possibility that the TSLP gene is generally not associated with risk of AR, as discussed above, it is also possible that the discordance noted between the findings of the present study and previous studies for any associations between TSLP polymorphisms and risk of AR [14
], may be a consequence of difference in study protocols as well as some limitations of the present study. Firstly, we cannot completely exclude the possibility that some of our findings are false negative due to the relative small sample size.It could also be that SNPs in the TSLP gene region confer a relatively small risk of developing AR, which we could not detect. Future efforts to identify TSLPSNPs carrying a smaller TSLP risk will thus require a larger sample size than has been used here. A second consideration is the ethnic variability in SNP frequency known for the gene. It is clear that in this study, we have replicated SNPs associated in other asthmatic populations and the representative tag SNPs in terms of the Hapmap CHB population data, but have not performed extensive fine mapping studies of the gene which might identify other SNPs with increased risk. Moreover, as with many other complex disorders, AR is thought to be the result of a complicated network of numerous susceptibility loci, many of which exert additive or synergistic effects, but have only a small role when considered in isolation [41
]. Further studies including genes involved in the Th2 pathway are needed in larger cohorts of adult Chinese subjects with AR. However, it is important to note that based on the current study, a particular TSLP variant alone does not show a significant association with the development of AR. Although estimation of TSLP protein in the nasal mucosa of the subjects would undoubtedly have provided further direct information in the elucidation of any association between a TSLP gene variant and susceptibility/development of AR in this cohort, it was unfortunately not possible to accomplish this because of the difficulty in convincing both the AR and control subjects to provide nasal mucosal samples. Moreover, it was also not possible to perform this aspect of the study from the ethical aspects.