DISCUSSION
Our study showed that antibiotic use during infancy increased the risk
of AR in childhood, whereas the presence of older siblings had a
protective effect on AR. The risk of AR increased more than twice when
the ERS was high compared to when it was low. Furthermore, although each
SNP, including IL-13 (rs20541), CD14 (rs2569190), TLR4 (rs1927911), and
GSTP1 (rs1695) did not increase the risk of AR individually, a high PRS
was associated with an increased risk of AR. This result suggests that
polygenic effects generated by multiple SNPs may contribute to the
development of AR. Above all, high ERS and PRS were not only
independently associated with the risk of AR but also showed a combined
effect on the risk of AR. These results suggest that the interaction of
susceptibility of multiple genes and hygienic environmental risk factors
might be associated with current AR, and can be applied to early
detection of high-risk groups of school-age AR and prevention of
school-age AR.
Antibiotic use in early childhood and the presence of older siblings are
associated with allergic diseases. The risk of AR is increased in school
children who used antibiotics in infancy and within two years after
birth.25,26The use of antibiotics during early childhood and presence of older
siblings has the potential to affect the distribution of gut bacteria
and induce changes in early immune system
formation.9,27This study showed that antibiotic use had a dose-effect, an important
hygienic environmental factor increased the risk of AR, and that AR risk
was known to decrease as the number of older siblings increased.
Several SNPs have been mentioned as genetic factors affecting the
development of AR, of which SNP rs20541 located in exon 4 of the IL-13
gene was found to be strongly associated with high levels of plasma IgE
and AR
development.1,12,17In our study, SNP rs20541 had a combined effect with ERS, and SNP
rs20541 increased the risk of AR by interaction with mold
exposure.1 These results
suggest that the SNP of the IL-13 gene is involved in the risk of AR by
gene-environment interaction.
The CD14 gene, which is associated with the innate immune response and
located on chromosome 5q31.3, encodes a protein that functions as a
co-receptor for TLR and releases pro-inflammatory
cytokines.28 The effect
of CD14 rs2569190 is conflicting and influenced by lipopolysaccharides-
related factors and interactions with environmental
microorganisms.28 In
our study, the TT genotype has a protective effect on the development of
AR. A previous meta-analysis reported that CD14 SNP rs2569190 did not
affect AR risk in
Asians.12 These results
suggest that the CD14 gene alone does not increase the risk of AR, but
has different effects on the risk of AR through interaction with various
environmental factors.
Toll-like receptor 4 initiates the innate immune system when exposed to
environmental factors, and antagonists of TLR4 have been shown to
aggravate the symptoms of
AR.14 The CC genotype
of TLR4 rs1927911 was associated with a higher risk of
AR.13 Our results are
consistent with the results of previous studies in that the CC genotype
of TLR4 rs1927911 alone did not increase the risk of AR.
Glutathione S-transferase P1 is the most common form of GST found in the
respiratory tract lining fluid, and the GSTP1 genotype is known to be
associated with the severity of airway
dysfunction.15,29Although there have been no previous studies on the association between
GSTP1 genotype and AR risk, several studies showed the association
between GSTP1 and development of
asthma.15,16In our study, the GSTP1 rs1695 SNP showed a weak association with
increase in AR risk, but did not increase the AR risk alone.
Several studies have introduced the concept of ERS to identify the
effects of multiple environmental factors on the development of allergic
diseases in children. In a previous study, ERS was calculated with risk
factors, such as cesarean delivery and antibiotics use during infancy,
which showed that children with higher scores had a higher incidence of
atopic AR at school
age.13 A recent study
in Lebanon showed that children with higher scores based on risk
factors, including environmental factors showed a higher frequency of
allergic diseases, which is consistent with our
study.5
Following studies on ERS, recent studies have been conducted on the
effects of multiple genetic factors on the development of allergic
diseases. A cohort study from Netherlands calculated a weighted PRS
based on 10 SNPs associated with allergies in adults and showed that
high PRS increased parental-reported allergy at 5 years of age and
diagnosis of allergies in childhood by a
physician.18 From two
birth cohorts with 135 SNPs, PRS was associated with an increased risk
of atopic march, but having a weak association with allergic diseases
characterized by the presence of a single
symptom.21 This
suggests that multiple SNPs may influence multiple allergic
comorbidities by unknown interactions. In our study, weighted PRS was
calculated with four SNPs, which were related to the hygiene hypothesis.
Children with high-weighted PRS had an increased risk of developing AR,
suggesting that multiple SNPs may be linked to the development of
allergic disease.
Our study has some limitations. First, information on antibiotic use in
infancy may be biased because it is based on parents’ memories after
several years have passed. However, our study was designed to study the
hygiene hypothesis, and several hygiene related environmental factors
were investigated in detail. Second, AR was defined in the questionnaire
without laboratory tests or skin prick tests. To supplement this, we
used the definition of current AR, which reflects not only the symptoms
but also the history of AR diagnosis by a doctor. Third, target SNP was
selected with only four hygiene related candidate
SNPs.1,13,14However, the selected SNPs have been associated with allergic diseases
in our previous
studies.1,9,13,14Further studies using genome-wide association study or prospective birth
cohort will be needed. Lastly, this study had a relatively limited
number of children in Korea; however, it is a general population-based
study.
Our study has the following strengths. First, our study is meaningful in
that it is a general population-based study. Finally, the ISAAC
questionnaire was verified in many previous studies in Korea, and the
response rate to the questionnaire was high at over 95% in this study.
In conclusion, polygenic
susceptibility and exposure to multiple hygienic environmental risk
factors during infancy increase the risk of AR at school age, which
suggests gene-environment interaction. Therefore, it is necessary to
decrease exposure to unhygienic environmental risk factors in infancy to
prevent AR in school children, especially in susceptible children.
Further studies are needed to elucidate the mechanism for this
interaction between PRS and ERS that contributes to the development of
AR