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.