Background: The HLA (human leukocyte antigen) class II genes DQB1 and DRB1 and the Tumor Necrosis Factor alpha gene (TNFA) within the HLA complex (chromosome 6p21) have been associated with asthma and allergy. Due to the strong linkage disequilibrium characterizing this complex and the multiple asthma/allergy expressions, we aimed to determine which of these genes were primarily involved in specific asthma/allergy traits.
Methods: The DRB1-DQB1 alleles and TNFA-308 polymorphism were genotyped in 959 children from the Environment and Childhood Asthma study and analyzed for possible associations with allergic and non-allergic asthma (with/without at least one positive skin prick test for allergens) and specific allergic sensitization, as well as bronchial hyperresponsiveness and total IgE, using both allele and extended haplotype analyses.
Results: Different genes within the HLA complex were associated with separate asthma and allergy traits. Nonallergic asthma was associated with both the TNFA-308A allele [Odds ratio (OR) 1.7 (1.3-2.3)] and DRB1 03 allele [OR 1.6(1-2.6)], but extended DRB1 03-TNFA-308 haplotype analysis suggested that the DRB1-DQB1 association was secondary to linkage disequilibrium with the TNFA-308 polymorphism. Allergies were associated with HLA class II alleles only; birch sensitization with DQB1 0603-DRB1 13 [OR 2.3 (1.4-4.0)] and mugwort sensitization with DQB1 0609-DRB1 13 [OR 7.1 (1.9-27.0)] and DQB1 0501-DRB1 01 [OR 2.0 (1.0-4.0)].
Conclusions: Our data suggests that asthma is not associated with DRB1 or DQB1 but rather TNFA or a gene(s) in linkage disequilibrium, while sensitization to specific allergens is associated with particular alleles at the DQ and/or DR loci. A novel association between DQB1 0603-DRB1 13 and birch allergy is identified.