This is the first report of a genomewide, sex-stratified linkage analysis of serum allergen–specific IgEs in families of children with asthma. Although sex had no strong influence in our findings for IgE to Ascaris
and IgE to dust mite, we found distinct genetic architectures for cockroach-specific IgE in male and female subjects and identified significant linkage to IgE to cockroach on chromosome 5 (peak LOD, 4.14 at 127 cM) in female subjects. The T allele of a SNP (rs2289276) in a candidate gene near this linkage peak (TSLP
) was inversely associated with cockroach-specific IgE in Costa Rican girls. In addition, this allele was inversely associated with total IgE in girls in each of two family-based study samples (in Costa Rica and in CAMP). To our knowledge, this is the first sex-specific association with allergy-related phenotypes to be replicated in independent populations (45
Among members of the eight large families of Costa Rican children with asthma, the estimated heritability of IgE to Ascaris, IgE to dust mite, and IgE to cockroach was similar in all subjects (h2N range, 0.33–0.39) and in male subjects only (h2N range, 0.43–0.49). In contrast, the estimated heritability of IgE to cockroach was higher (h2N, 0.52) than that of Ascaris-specific IgE (h2N, 0.34) or dust mite–specific IgE (h2N, 0.28) among female subjects, suggesting that genetic factors play a more significant role in the production of IgE to cockroach than in the production of IgE to Ascaris or dust mite in female Costa Ricans.
We report the first genomewide linkage analysis of IgE to Ascaris
(a phenotype associated with asthma severity in Costa Rican children) (3
). Among all members of large Costa Rican families, there was suggestive evidence of linkage to Ascaris
-specific IgE on chromosome 7q, a genomic region previously linked to obesity (a trait often associated with asthma) (46
). Previous genomewide studies in families of subjects with asthma revealed two chromosomal regions (19p13 [LOD, 3.51] and 20q13 [LOD, 4.93]), with significant evidence of linkage to dust mite allergy (assessed by skin test positivity and not by continuous measurement of specific IgE) (48
). Although “usual” genomewide linkage analyses of allergen-specific IgE measurements have not revealed loci meeting criteria for genomewide significance, an analysis that took maternal imprinting into account showed significant evidence of linkage to dust mite–specific IgE on chromosome 8p22 (maximized LOD score with respect to disease-model parameters, 4.76) (50
). There was little to no overlap in our findings for each allergen-specific IgE, suggesting that these phenotypes are influenced by different genomic regions.
The 1.5 LOD-unit support interval for the observed linkage peak to cockroach-specific IgE in female subjects is broad (chromosome 5q21-q32, encompassing ~87 known genes) and includes a region previously shown to be significantly linked to mite-sensitive asthma in Japanese subjects (17
). Although genes within 5q21-q32 (e.g., IL4
, and CD14
) have been previously examined, this is the first association study of TSLP
for asthma- and allergy-related phenotypes.
Gender-specific analyses of genetic association can lead to false-positive results due to multiple testing (45
). Studies that claim to identify sex-specific effects are often limited by lack of replication in independent populations, absence of replication of the findings with regard to the direction of the original association, and no formal testing for sex-by-genotype interaction. We have tested for sex-by-genotype interaction and replicated our female-specific association between TSLP
and total IgE in two independent populations for the same SNP, in a consistent direction of association.
, a gene with two splice variants, can prime dendritic cells and stimulate production of Th2 cytokines by naive T cells (54
), and TSLP
knockout mice are protected from allergic airway responsiveness (55
). Increased expression of TSLP
in the airways of human subjects with asthma is associated with increased levels of Th2 cytokines and airway responsiveness (56
). Of relevance to our findings, overexpression of TSLP
in transgenic mice leads to prominent pulmonary pathology, including perivascular leukocytic infiltration and prominent eosinophilia, with more severe effects in female mice than in male mice (23
Our study has several limitations. First, we had strong motivation to perform sex-stratified, genomewide linkage analyses of allergen-specific IgEs (9
) and found evidence for an inverse association between the T allele of SNP rs2289276 in TSLP
and IgE to cockroach in Costa Rican girls, but replication of this finding is needed. However, we are encouraged by our finding that this allele was also inversely associated with total IgE in Costa Rican girls and girls in CAMP. Second, evidence of a sex-by-SNP (rs2289276) interaction for total IgE in Costa Rica and CAMP may implicate that TSLP
is involved more generally in the sex-dependent regulation of total and allergen-specific IgEs. Third, the functional effects of TSLP
polymorphisms are not known. However, SNP rs2289276 is predicted to disrupt an exonic splicing enhancer site (57
). Finally, data from the International HapMap consortium suggest that SNP rs2289276 is within a large block of linkage disequilibrium that includes WDR36
(WD repeat-containing protein 36), a gene involved in T-cell activation. We cannot rule out the possibility that other SNPs within WDR36
alternately or additionally explain our findings of association. Although our findings provide evidence to support a role of TSLP
in human allergy (in a sex-specific fashion), a fine-mapping association study of chromosome 5q21-q32 is needed to firmly identify the gene(s) responsible for the observed female-specific linkage peak to IgE to cockroach.
In summary, we have identified a female-specific locus for cockroach-specific IgE on chromosome 5q21-q32 in Costa Ricans and a female-specific association between a variant in a candidate gene in this region (TSLP
) and IgE to cockroach in Costa Rica and total serum IgE in Costa Rica and CAMP. The strong influence of gender on age-associated asthma prevalence (58
) and expression of various atopic phenotypes has been well documented but largely unexplored. Our results suggest an important role of sex-specific genetic effects on allergic phenotypes.