Expression of asthma susceptibility genes in bronchial epithelial cells and bronchial alveolar lavage in the Severe Asthma Research Program (SARP) cohort

<p><i>Objective</i>: Genome-wide association studies (GWASs) have identified genes associated with asthma, however expression of these genes in asthma-relevant tissues has not been studied. This study tested expression and correlation between GWAS-identified asthma genes and asthma or asthma severity. <i>Methods</i>: Correlation analyses of expression levels of GWAS-identified asthma genes and asthma-related biomarkers were performed in cells from human bronchial epithelial biopsy (BEC, <i>n</i> = 107) and bronchial alveolar lavage (BAL, <i>n</i> = 94). <i>Results</i>: Expression levels of asthma genes between BEC and BAL and with asthma or asthma severity were weakly correlated. The expression levels of <i>IL18R1</i> were consistently higher in asthma than controls or in severe asthma than mild/moderate asthma in BEC and BAL (<i>p</i> < 0.05). In <i>RAD50-IL13</i> region, the expression levels of <i>RAD50</i>, not <i>IL4, IL5</i>, or <i>IL13</i>, were positively correlated between BEC and BAL (ρ = 0.53, <i>P</i> = 4.5 × 10⁻⁶). The expression levels of <i>IL13</i> were positively correlated with <i>IL5</i> in BEC (ρ = 0.35, <i>P</i> = 1.9 × 10⁻⁴) and <i>IL4</i> in BAL (ρ = 0.42, <i>P</i> = 2.5 × 10⁻⁵), respectively. rs3798134 in <i>RAD50</i>, a GWAS-identified SNP, was correlated with <i>IL13</i> expression and the expression levels of <i>IL13</i> were correlated with asthma (<i>P</i> = 0.03). rs17772583 in <i>RAD50</i> was significantly correlated with <i>RAD50</i> expression in BAL and BEC (<i>P</i> = 7.4 × 10⁻⁷ and 0.04) but was not associated with asthma. <i>Conclusions</i>: This is the first report studying the expression of GWAS-identified asthma genes in BEC and BAL. <i>IL13</i>, rather than <i>RAD50, IL4</i>, or <i>IL5</i>, is more likely to be the asthma susceptibility gene. Our study illustrates tissue-specific expression of asthma-related genes. Therefore, whenever possible, disease-relevant tissues should be used for transcription analysis.</p

Probability of severe asthma as a function of age, and asthma duration.

<p><b>[A]</b> The probability of severe asthma increases as a function of age until the age of 45 years, at which time the probability of severe asthma plateaus. The fitted final logistic regression model, which included variables associated with severe asthma and adjusted for asthma duration, a history of Gastro-esophageal Reflux Disease, and nasal corticosteroids use, was applied to calculate the probability of severe asthma. <b>[B]</b> Individuals were stratified by age groups into older or young adult asthma in order to evaluate the effect of asthma duration on risk of severe asthma in those older or younger than 45 years. Asthma duration has a lesser effect than age on the probability of severe asthma in younger adult asthmatics. In older asthmatics (age 45 years and older), asthma duration has no significant effect on risk of severe asthma.</p

Logistic regression model of risk factors for severe asthma.^*

<p>* OR denotes odds ratio, CI confidence interval.</p><p>^† Older (age 45 years and older) vs. young adult asthma (18–45 year old).</p><p>^‡ OR per each year increase in asthma duration.</p><p>Logistic regression model of risk factors for severe asthma.^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133490#t002fn001" target="_blank">*</a>}</p

The histogram of age distribution by asthma severity.

<p>This histogram shows the difference in age distribution among severe and nonsevere asthmatics. The distribution of nonsevere asthmatics was skewed to the left reflecting younger age, and severe asthmatics were shifted to older age.</p

The probability of severe asthma as a function of age and stratified by gender.

<p><b>[A]</b> The association between age and the probability of severe asthma by applying a LOWESS (Locally Weighted Scatterplot Smoother) smoother in the overall population. The relationship between age and probability of severe asthma resembles a spline with an inflection point at the age of 45 years. <b>[B]</b> The stratification by gender shows the probability of asthma severity is higher in men than women after age 45.</p

Medication requirement and health care utilization with PS matching and subsequent multivariate adjustment for asthma duration and GERD.^*

<p>* OR denotes odds ratio, ICS inhaled corticosteroids, LABAs long acting beta agonists, ER emergency room, UC urgent care, and OCS oral corticosteroids.</p><p>^† History of previous health care use related to asthma.</p><p>^‡ Health care utilization was self-reported.</p><p>Comparing older asthma vs. young adult asthma groups.</p

Characteristics of patients with asthma, according to age group.^*

<p>* Plus—minus values are means ±SD. BMI denotes body mass index, GERD gastroesophageal reflux disease, FEV1 forced expiratory volume in 1 second, FVC forced vital capacity, PC20 for methacholine the concentration of inhaled methacholine causing a 20% reduction in FEV1, FENO fraction of exhaled nitric oxide, and BAL broncho-alveolar lavage.</p><p>^† P values are for the comparison of older asthma with the young adult asthma group and were calculated with the use t-test for approximately normally distributed clinical characteristics, Pearson's chi-squared test for differences in proportions, and Wilcoxon signed rank test for all other variables.</p><p>Characteristics of patients with asthma, according to age group.^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0133490#t001fn001" target="_blank">*</a>}</p