Genome-wide association analysis identifies PDE4D as an asthma-susceptibility gene

Abstract

Asthma, a chronic airway disease with known heritability, affects more than 300 million people around the world. A genome-wide association (GWA) study of asthma with 359 cases from the Childhood Asthma Management Program (CAMP) and 846 genetically matched controls from the Illumina ICONdb public resource was performed. The strongest region of association seen was on chromosome 5q12 in PDE4D. The phosphodiesterase 4D, cAMP-specific (phosphodiesterase E3 dunce homolog, Drosophila) gene (PDE4D) is a regulator of airway smooth-muscle contractility, and PDE4 inhibitors have been developed as medications for asthma. Allelic p values for top SNPs in this region were 4.3 x 10(-07) for rs1588265 and 9.7 x 10(-07) for rs1544791. Replications were investigated in ten independent populations with different ethnicities, study designs, and definitions of asthma. In seven white and Hispanic replication populations, two PDE4D SNPs had significant results with p values less than 0.05, and five had results in the same direction as the original population but had p values greater than 0.05. Combined p values for 18,891 white and Hispanic individuals (4,342 cases) in our replication populations were 4.1 x 10(-04) for rs1588265 and 9.2 x 10(-04) for rs1544791. In three black replication populations, which had different linkage disequilibrium patterns than the other populations, original findings were not replicated. Further study of PDE4D variants might lead to improved understanding of the role of PDE4D in asthma pathophysiology and the efficacy of PDE4 inhibitor medications.

Figure 1

Study Overview (A) 359 CAMP cases and 846 Illumina controls were used in tests for the genome-wide association of individual SNPs to asthma status. Four SNPs with p values less than 10⁻⁶ in a biologically plausible candidate gene and in an LD association peak were analyzed further. (B) Using genome-wide association results of family-based statistical analysis in CAMP probands and their parents, we analyzed the four SNPs that met the criteria described in (A). These four SNPs had p values less than 0.05. (C) Generalizability of the findings was tested through replication attempts in ten independent populations. Numbers under each name are cases/controls for case-control designs; affected/unaffected for population-based designs and GRAAD2; and number of trios for Costa Rica. Data for two SNPs were available in the ten populations. (D) Joint association analysis for the two SNPs in seven populations with similar LD patterns among the top CAMP/Illumina SNPs was performed. Because LD patterns among the most significant SNPs were very different in black populations (red boxes), these were analyzed separately.

Figure 2

EIGENSTRAT-Corrected Association Statistics CAMP/Illumina GWA statistics as adjusted by EIGENSTRAT are highly correlated (r² = 0.98) with unadjusted statistics, demonstrating minimal residual population stratification.

Figure 3

Quantile-Quantile Plot Comparison of CAMP/Illumina allelic p values to those expected for a null distribution. Deviation of measures at the tail may indicate true associations between some SNPs and asthma.

Figure 4

GWA Results for CAMP/Illumina The x axis denotes the position along each chromosome. The y axis denotes −Log₁₀(P), corresponding to allelic p values. Five SNPs have nominal p values less than 1 × 10⁻⁶ (shown in red); four are located on chromosome 5, and one is on chromosome 8.

Figure 5

Region of Association of PDE4D SNPs to Asthma in CAMP/Illumina GWAS (A) The x axis denotes the position along chromosome 5. The y axis denotes –Log₁₀(P), corresponding to allelic p values. LD between the SNP with the lowest p value (rs1544791) to each SNP in the plot is denoted in colors (red, strong LD; orange, moderate LD; and yellow, weak LD). (B) Corresponding location of genomic sequences available in the UCSC genome browser.

Figure 6

Linkage Disequilibrium among Top Chromosome 5 CAMP/Illumina SNPs Panels are for six family-based populations: CAMP, Costa Rica, GRAAD2, HapMap European American (CEU), Chinese and Japanese (CHB+JPT), and Yoruban (YRI). Values in boxes are r² measures on a decimal scale (i.e., 97 represents r² = 0.97). Boxes without numbers have r² = 1. The pattern of LD is very similar in CAMP, Costa Rica, and CEU. There are moderate differences in LD between CHB+JPT and each of the CAMP, Costa Rica, and CEU populations. The patterns in GRAAD2 and YRI are extremely different from those of the other populations. Genotype data for three of the four SNPs were available in Costa Rica. Genotype data for two of the four SNPs were available in GRAAD2.

Figure 7

Summary ORs for Effects of PDE4D SNPs in Seven Populations (A) Summary ORs for rs1588265 are 0.85 (95% CI: 0.77, 0.93) among all populations and 0.88 (95% CI: 0.83, 0.94) among replication populations. (B) Summary ORs for rs1544791 are 0.85 (95% CI: 0.78, 0.94) among all populations and 0.89 (95% CI: 0.84, 0.94) among replication populations. Boxes are at the mean OR for each study, and their size is proportional to study size. Gray bars represent 95% CIs. The summary OR (diamond) was estimated with the DerSimonian-Laird random-effects model.