Discussion
For the first time, we identified a key locus that many confer susceptibility to adult asthma in the Korean population. The SNP rs1770 on chromosome 6, which is located near HLA-DQB1, demonstrated a strong association with genome-wide significance. Regarding the predicted mRNA expression levels in lung tissue and whole blood, HLA genes were found to have a significant association with mRNA expression. Additionally, we compared our GWAS results with significant SNPs and genes associated with asthma identified by GWAS using UK Biobank data. A total of 8 SNPs were significant with 6 SNPs, IL1RL1, TSLP, GATA3, MUC5AC, CLEC16A and C5orf56, having the same odds ratio direction. HLA-DQA1 and C11orf30 showed opposing directions for their odds ratios.
The first GWAS identified variants near the ORMDL3/GSDML loci on chromosome 17q21, which were related to asthma in three populations with European ancestry23. A meta-analysis of 23 GWASs involving Europeans was conducted by the GABRIEL study3, which verified the strongest genome-wide significant associations between asthma and SNPs was rs9273349 in the HLA-DQ region of chromosome 6p21.3. The EVE consortium, which includes GWAS datasets of 5,416 asthmatics in the US, reported several associated regions including loci on 17q21, near IL1RL1, TSLP and IL33, and PYHIN1 which was new asthma susceptibility locus in populations of African descent.24 Recently, a meta-analysis of 23,948 asthmatics from the Trans-National Asthma Genetic Consortium (TAGC) reported 18 loci with genome-wide significance.25 To date, the majority of GWAS have involved European populations and the little was known about GWAS for Asian, African and mixed populations. In Asia, a study identified the 5 loci that have susceptibility in Japanese asthmatics, and the peak of association at 6p21 was located nearby rs2070600, which was related to FEV1/FVC of lung function.26 A next generation sequencing studies were observed differences in allele frequencies and haplotype structures at the 17q12-21 asthma-related locus between Chinese and other populations.27 These result supported that it is important to study populations of various ethnic backgrounds of identifying shared and unique genetic predictors of asthma.
A GWAS in Korea focused on patients with AERD.9,28-30A study of 117 Korean asthmatics with AERD reported that HLA-DPB1 rs1042151 showed the most significant association with susceptibility of AERD (p = 5.11 × 10-7; OR = 2.40)9 and confirmed the importance of HLA-DPB1 in the genetic etiology of AERD. Other study performed the replication to validate the previously identified genetic variants in 237 Korean asthmatics.8 They found that 6 SNPs were significantly associated with at least one allergy-related phenotype, which included KZF2, SHROOM3, IL13, SLC22A5, WDR36, and TSLP. Limited comprehensive GWAS have been conducted for adult asthmatics in the Korean population and we present here the largest Korean asthma GWAS examining 1,386 Korean asthmatics.
In the present study, GWAS demonstrated that rs1770, which is located near HLA-DQB1, had the most significant genome-wide association (p =4.5×10-10; OR=1.41). In addition, the expressed HLA genes on chromosome 6 with imputed mRNA expression levels for lung tissue and whole blood were significant. These findings indicated that there may be substantial genetic variants within the HLA-DQ region, which can influence asthma susceptibility. The HLA region on chromosome 6 has been reported to be highly polymorphic and play important roles in the regulation of the immune system, which included over 224 genes associated with autoimmune and infectious diseases31. Studies of HLA genes in the susceptibility to asthma and related phenotypes have involved various countries and populations, e.g., USA, Chinese, UK, and Caucasian32. Recently, a study showed that the HLA-DR/DQ region is closely associated with both interleukin 4 and tumor necrosis factor (TNF) alpha and emphasized the association of HLA with type2 inflammatory responses33. These results further suggest that HLA-DQ typing could be a predictive gene marker in the Korean population, as well as other populations, for asthma and be used to instigate primary prevention against asthma progression.
We compared our GWAS results with results using UK Biobank data consisting mainly of Europeans. Interestingly, our results demonstrated that the genetic signals of type2 inflammation tend to be similar with the same odds ratio direction and included IL1RL1, a receptor for key cytokine interlukin-33; TSLP, which leads type2 inflammation; and GATA3, an important transcription factor in Th2 cells6. TSLP has been associated with various atopic traits including asthma in Japanese26, African Americans24, and European Americans24. Tezepelumab, which is a biologic targeting TSLP, was developed, and it resulted in a significant reduction in annual asthma exacerbation rates34. GATA3 has been associated with the concentrations of blood eosinophils and effector cells in allergic diseases such as asthma and allergic rhinitis6. Several studies suggested that multiple genetic signals within the GATA3 locus might contribute to asthma progression5,25,26. Taken together, the selection of patients with genes related to type2 inflammation may be essential for the prediction of asthma progression and transition to severe asthma, which would be linked to personalized therapies based on genotypic profiling.
The MUC5AC and CLEC16A genes also had the same odds ratio direction. CLEC16A is known as a regulator of autophagy involved in the homeostasis of airway cells.35 MUC5AC has been linked to mucus plugging and airway hyperresponsiveness, and it is a major cause of morbidity in asthma36. Defective mucociliary clearance is observed in both mild and severe asthma, and this decreases further during acute exacerbation37. The upregulation of MUC5AC has been verified in animal models of T2-low inflammation and obesity-related asthma38. Given the interest in the effect of MUC5AC on asthma progression, mucin-targeting treatment based on genomic profiling might be an effective alternative therapy for severe asthma, especially for patients with T2- low inflammation.
Although HLA-DQA1 and C11orf30 have been associated with a risk of allergic disease39,40, our analysis results showed an opposite odds ratio direction compared with GWAS results using UK Biobank data. We speculated that there may be discrepancies in the genetic architecture between European and Korean populations. Particularly, the inconsistency of HLA-DQA1 may be attributed to (1) differences in LD and opposite effect of the nearby causal variant of HLA-DQA1 SNPs or (2) the reversed effect of HLA-DQA1 SNPs between different ethnicities. Therefore, further GWAS of multi-ethnic populations are needed to provide valuable information on genetic diversity according to racial and ethnic differences.
Our study has several limitations. First, although controls were selected to exclude subjects with asthma and other respiratory diseases using a standardized questionnaire, the choice of controls was potentially vulnerable to misclassification bias due to the inclusion of undiagnosed cases. Second, this study did not conduct a replication analysis; however, we showed significant similarities in the GWAS results between Korean and UK asthmatics. Third, genetic variation according to phenotypes, such as allergen sensitization, onset age, and severity, was not analyzed in the present study. Some recent studies have performed GWAS to detect asthma phenotypes, age at asthma onset (childhood vs. adult onset)3, and disease severity (moderate-to-severe asthma)6. Further GWAS related to clinical phenotypes and endotypes are needed to identify potential individuals with asthma risk. Fourth, asthma is a heterogeneous disease with gene-environmental interactions, and we have not fully assessed environmental exposure. In future studies, we may investigate gene-environmental associations by performing epigenetic analysis of blood DNA methylation status in Korean asthmatics and link this to exposure to environmental pollution.
In conclusion, this is the first GWAS for the Korean population. We demonstrated that the SNP rs1770 of HLA-DQB1 was the most significant SNP in adult asthmatic patients. In the comparison of genetic susceptibility between Korean asthmatics and UK asthmatics from UK Biobank, genetic variants associated with type2 inflammation (IL1RL1, TSLP, and GATA3) and mucus plugging in the airway (MUC5AC) were significant with a similar odds ratio direction. A discrepancy was observed for HLA-DQA1 with opposite odds ratio direction. Our findings of the GWAS of Korean asthmatics should be replicated and compared with those of GWAS of other ethnicities. Moreover, further GWAS would focus on asthma phenotypes and clinical usefulness including asthma treatment strategies beyond genetic profiling.