After a genomewide screen in the Hutterites was completed, the IL4RA gene was examined as the 16p-linked susceptibility locus for asthma and atopy. Seven known variants and one novel variant, representing all nonsynonymous substitutions in the mature protein, were examined in the Hutterites; on the basis of studies in the Hutterites, outbred white, black, and Hispanic families were genotyped for selected markers. All population samples showed evidence of association to atopy or to asthma (P values.039-.0044 for atopy and. 029-.0000061 for asthma), but the alleles or haplotypes showing the strongest evidence differed between the groups. Overall, these data suggest that the IL4RA gene is an atopy- and asthma-susceptibility locus but that variation outside the coding region of the gene influences susceptibility.

The genomewide screen to search for asthma-susceptibility loci, in the Collaborative Study on the Genetics of Asthma (CSGA), has been conducted in two stages and includes 266 families (199 nuclear and 67 extended pedigrees) from three U.S. populations: African American, European American, and Hispanic. Evidence for linkage with the asthma phenotype was observed for multiple chromosomal regions, through use of several analytical approaches that facilitated the identification of multiple disease loci. Ethnicity-specific analyses, which allowed for different frequencies of asthma-susceptibility genes in each ethnic population, provided the strongest evidence for linkage at 6p21 in the European American population, at 11q21 in the African American population, and at 1p32 in the Hispanic population. Both the conditional analysis and the affected-sib-pair two-locus analysis provided further evidence for linkage, at 5q31, 8p23, 12q22, and 15q13. Several of these regions have been observed in other genomewide screens and linkage or association studies, for asthma and related phenotypes. These results were used to develop a conceptual model to delineate asthma-susceptibility loci and their genetic interactions, which provides a promising basis for initiation of fine-mapping studies and, ultimately, for gene identification.

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Asthma is an inflammatory airways disease associated with intermittent respiratory symptoms, bronchial hyper-responsiveness (BHR) and reversible airflow obstruction and is phenotypically heterogeneous. Patterns of clustering and segregation analyses in asthma families have suggested a genetic component to asthma. Previous studies reported linkage of BHR and atopy to chromosomes 5q (refs 7-9), 6p (refs 10-12), 11q (refs 13-15), 14q (ref. 16), and 12q (ref. 17) using candidate gene approaches. However, the relative roles of these genes in the pathogenesis of asthma or atopy are difficult to assess outside of the context of a genome-wide search. One genome-wide search in atopic sib pairs has been reported, however, only 12% of their subjects had asthma. We conducted a genome-wide search in 140 families with > or = 2 asthmatic sibs, from three racial groups and report evidence for linkage to six novel regions: 5p15 (P = 0.0008) and 17p11.1-q11.2 (P = 0.0015) in African Americans; 11p15 (P = 0.0089) and 19q13 (P = 0.0013) in Caucasians; 2q33 (P = 0.0005) and 21q21 (P = 0.0040) in Hispanics. Evidence for linkage was also detected in five regions previously reported to be linked to asthma-associated phenotypes: 5q23-31 (P = 0.0187), 6p21.3-23 (P = 0.0129), 12q14-24.2 (P = 0.0042), 13q21.3-qter (P = 0.0014), and 14q11.2-13 (P = 0.0062) in Caucasians and 12q14-24.2 (P = 0.0260) in Hispanics.