T-bet polymorphisms are associated with asthma and airway hyperresponsiveness

Abstract

Rationale: T-bet (TBX21 or T-box 21) is a critical regulator of T-helper 1 lineage commitment and IFN-gamma production. Knockout mice lacking T-bet develop airway hyperresponsiveness (AHR) to methacholine, peribronchial eosinophilic and lymphocytic inflammation, and increased type III collagen deposition below the bronchial epithelium basement membrane, reminiscent of both acute and chronic asthma histopathology. Little is known regarding the role of genetic variation surrounding T-bet in the development of human AHR.

Objectives: To assess the relationship between T-bet polymorphisms and asthma-related phenotypes using family-based association.

Methods: Single nucleotide polymorphism discovery was performed by resequencing the T-bet genomic locus in 30 individuals (including 22 patients with asthma). Sixteen variants were genotyped in 580 nuclear families ascertained through offspring with asthma from the Childhood Asthma Management Program clinical trial. Haplotype patterns were determined from this genotype data. Family-based tests of association were performed with asthma, AHR, lung function, total serum immunoglobulin E, and blood eosinophil levels.

Main results: We identified 24 variants. Evidence of association was observed between c.-7947 and asthma in white families using both additive (p = 0.02) or dominant models (p = 0.006). c.-7947 and three other variants were also associated with AHR (log-methacholine PC(20), p = 0.02-0.04). Haplotype analysis suggested that an AHR locus is in linkage disequilibrium with variants in the 3'UTR. Evidence of association of AHR with c.-7947, but not with other 3'UTR SNPs, was replicated in an independent cohort of adult males with AHR.

Conclusions: These data suggest that T-bet variation contributes to airway responsiveness in asthma.

Figures

Figure 1.

T-bet linkage disequilibrium and haplotype block structure in white Childhood Asthma Management Program (CAMP) parents. (a) Pairwise linkage disequilibrium plots (r2) for 16 variants genotyped in white CAMP parents. Arrow denotes relative position of T-bet gene (not to scale). (b) T-bet haplotype block structure in the white CAMP parents was estimated with Haploview using the Gabriel block definition using single nucleotide polymorphisms (SNPs) with minor allele frequencies of at least 0.05 (18). SNPs His33Gln and Pro485Pro have minor allele frequencies of less than 0.05 and are therefore excluded from this analysis. Haplotypes approximating 5% are displayed (frequencies adjacent to blocks). Thin and thick lines connecting blocks denote between-block combinations observed at least at 1% or 10%, respectively. Arrows denote one of several SNP subsets that tag the haplotypes within each block.

Figure 2.

Relationship between T-bet SNPs g.−19698 (a) and c.−7947 (b) and methacholine dose–response slope among 199 methacholine responders in the Normative Aging Study. Dose–response slope is adjusted for age, pack-years of smoking, and current smoking status. Results remained significant after adjustment for baseline FEV1. Minor allele frequencies in this cohort are as follows: g.−19698 = 0.370; c.−7947 = 0.382; His33Gln = 0.041; IVS1+7240 = 0.208; c.2122 = 0.070; c.2314 = 0.424.