In asthma genetics, the association of highly replicated susceptibility genes lacks consistency across populations. To identify genuine associations, we investigated the reproducibility of the 23 most promising asthma and asthma-related candidate genes in a moderately sized sample from the Japanese population. We compared the frequency of 33 polymorphisms in unrelated cases and controls and tested for their association with asthma, atopy and serum total IgE levels using allele frequency, codominant, dominant and recessive genotype models. On the basis of the consistency of our findings with previous metaanalyses and large replication studies, IL13, TNF, ADAM33, IL4RA and TBXA2R might represent common major asthma and asthma-related trait genes. Individual gene assessment was extended to the interactions between two polymorphisms using our original method. Interactions between TBXA2R and ADAM33, and IL4RA and C3 were suggested to increase the risk for childhood and all asthma (adult and childhood asthma combined). The confirmation of previously reported associations between gene polymorphisms and phenotypes was problematic when as few as several hundred samples per group were used. Stratification of the subjects by environmental factors or other confounding factors may be necessary to improve the sensitivity and reliability of association results. Keywords: association; asthma; atopy; polymorphism; replication
INTRODUCTIONAsthma is a heritable trait 1 and investigations to determine the genetic components underlying asthma using linkage mapping and the candidate gene approach have been carried out. By 2006, more than 100 genes were associated with asthma and asthma-related phenotypes; 2 25 of these genes have been replicated in more than six independent association studies. In 2008, this list was complemented with an additional three genes, FLG, NAT2 and CCL15. 3 However, no single polymorphic marker or gene locus has been unanimously labeled as a strong and independent genetic determinant of asthma, and the results for the highly replicated genes have been inconsistent across the tested populations. 3 To identify true associations, it is of critical importance to comprehensively replicate the initial finding. 4 To this aim, we investigated whether the 23 most replicated genes for asthma and asthma-related phenotypes were positively associated with extrinsic childhood and adult asthma, atopy and total serum IgE levels in a moderately sized sample drawn from the Japanese population.We also tested eight genes that were significantly associated with asthma in our subjects: IL13, 5 TBXA2R, 6 GSTP1, 7 ADAM33, 8 MMP9, 9 IL12B, 10 C3 11 and SOCS1. 12 The re-evaluation of these associations is