A genome-wide search for linkage to asthma. German Asthma Genetics Group

Complement and IL-12: yin and yang

Interleukin 12 (IL-12) is central to the orchestration of cell-mediated immune responses in the innate as well as the adaptive immune system. Recent studies of the pathogenesis of diseases as disparate as measles and asthma have suggested that the complement system, itself at the interface of innate and adaptive immunity, is a biologically relevant regulator of IL-12 production. These data are reviewed here.

Testing for gene-gene interaction controlling total IgE in families from Barbados: evidence of sensitivity regarding linkage heterogeneity among families

Genetic heterogeneity has been proposed as a hallmark feature of allergic disease. To test the hypothesis that total IgE levels are jointly influenced by a locus on chromosome 12q21.1-q21.31 and a locus on 17q11.2-q21.2, we conducted multipoint allele-sharing analyses using nonparametric linkage (NPL) methods on Afro-Caribbean families from Barbados to test for evidence of gene-gene interactions. Significant correlations were observed between NPL scores at D12S1052 and both D17S1293 and D17S1299 for a dichotomized phenotype of total IgE. An analysis of family-specific NPL scores revealed that evidence for interaction was being driven largely by one multiplex pedigree (NPL = 12.01, 12.23, and 12.16 at D12S1052, D17S1293, and D17S1299, respectively). Using the programs SIMWALK (v2.0) and GOLD, a different set of haplotypes in this influential family was observed around D12S1052 and the 17q loci compared to the other Barbados pedigrees. Our findings are a classic example of founder effect, provide evidence for sensitivity of this type of linkage analysis to unusual pedigrees, and highlight an element of genetic heterogeneity that has been given little attention in the study of complex traits.

Tree-based linkage and association analyses of asthma

Tree-based linkage analyses and association studies are introduced and applied for two data sets on asthma from Genetic Analysis Workshop 12. Consistent and strong evidence of linkage and association to markers on chromosomes 1 and 11 is revealed. Linkage to chromosome 16 in one data set and association with D6S276 in the other data set are also detected.

Homogeneity of asthma genome scan results

Three different samples of families with asthmatic patients, the German data set and the CSGA data set subdivided into Caucasian and African American groups, were analyzed using the maximum lod score statistic. Although different scores were obtained in each sample, the Wald's likelihood homogeneity test did not reveal any significant genetic heterogeneity. This may be due to the very large variance of the model-free linkage statistics.

Using single nucleotide polymorphisms as a means to understanding the pathophysiology of asthma

Asthma is the most common chronic childhood disease in the developed nations, and is a complex disease that has high social and economic costs. Studies of the genetic etiology of asthma offer a way of improving our understanding of its pathogenesis, with the goal of improving preventive strategies, diagnostic tools, and therapies. Considerable effort and expense have been expended in attempts to detect specific polymorphisms in genetic loci contributing to asthma susceptibility. Concomitantly, the technology for detecting single nucleotide polymorphisms (SNPs) has undergone rapid development, extensive catalogues of SNPs across the genome have been constructed, and SNPs have been increasingly used as a method of investigating the genetic etiology of complex human diseases. This paper reviews both current and potential future contributions of SNPs to our understanding of asthma pathophysiology.

A high-density genetic map of the chromosome 13q14 atopy locus

Atopy describes a syndrome of immunoglobulin E (IgE)-mediated allergy that underlies asthma and infantile eczema. We have previously identified a locus on chromosome 13q14 that is linked to atopy and to the total serum immunoglobulin A concentration. We have therefore made a saturation genetic map of the region by typing 59 polymorphic microsatellite loci on chromosome 13q. Multipoint linkage analysis identified a 1-LOD support unit for the location of the atopy locus with a 7.5-cM region flanked by the loci D13S328 and D13S1269. The peak of linkage was at locus D13S161 with a nonparametric -log of P score of approximately 4.5. Parent of origin effects were present, with linkage primarily observed to paternally derived alleles. The genetic map of this region provides a basis for the effective identification of the chromosome 13 atopy gene.

A major susceptibility locus for atopic dermatitis maps to chromosome 3q21

Atopic dermatitis (eczema) is a chronic inflammatory skin disease with onset mainly in early childhood It is commonly the initial clinical manifestation of allergic disease, often preceding the onset of respiratory allergies. Along with asthma and allergic rhinitis, atopic dermatitis is an important manifestation of atopy that is characterized by the formation of allergy antibodies (IgE) to environmental allergens. In the developed countries, the prevalence of atopic dermatitis is approximately 15%, with a steady increase over the past decades. Genetic and environmental factors interact to determine disease susceptibility and expression, and twin studies indicate that the genetic contribution is substantial. To identify susceptibility loci for atopic dermatitis, we ascertained 199 families with at least two affected siblings based on established diagnostic criteria. A genome-wide linkage study revealed highly significant evidence for linkage on chromosome 3q21 (Zall=4.31, P= 8.42 10(-6)). Moreover, this locus provided significant evidence for linkage of allergic sensitization under the assumption of paternal imprinting (hlod=3.71,alpha=44%), further supporting the presence of an atopy gene in this region. Our findings indicate that distinct genetic factors contribute to susceptibility to atopic dermatitis and that the study of this disease opens new avenues to dissect the genetics of atopy.

Genome screen for asthma and related phenotypes in the French EGEA study

A genome-wide search was conducted in 107 nuclear families with at least two siblings with asthma, as part of the French EGEA study. A two-stage analysis strategy was applied to the 107 families divided into two independent subsets of 46 and 61 families, where all regions detected in the first set of families were tested for replication in the second set. In addition, all regions reported by published genome scans in different populations were examined in the total sample. A total of 254 markers were typed in the first set of families and 70% of them in the second set. Linkage was investigated by model-free methods for asthma and four asthma-related phenotypes: bronchial responsiveness (BR), skin test response, total immunoglobulin E (IgE) levels, and eosinophil count. The two-stage analysis led to the detection of three regions: 11p13 for IgE, 12q24 for eosinophils, and 17q12-21 for asthma and skin tests. Among the regions reported by published genome screens, seven were found in the 107 French EGEA families: three being already detected by the two-stage analysis, 11p13 (p = 0.005), 12q24 (p = 0.0008), and 17q12-21 (p = 0.001), and four additional ones, 1p31 (p = 0.005) for asthma, 11q13 (p = 0.006) for IgE, 13q31 (p = 0.001) for eosinophils, and 19q13 (p = 0.02) for BR.

Major genes regulating total serum immunoglobulin E levels in families with asthma

Immunoglobulin E (IgE) has a major role in the pathogenesis of allergic disorders and asthma. Previous data from 92 families, each identified through a proband with asthma, showed evidence for two major genes regulating total serum IgE levels. One of these genes mapped to 5q31-33. In the current study, the segregation analysis was extended by the addition of 108 probands and their families, ascertained in the same manner. A mixed recessive model (i.e., major recessive gene and residual genetic effect) was the best-fitting and most-parsimonious one-locus model of the segregation analysis. A mixed two-major-gene model (i.e., two major genes and residual genetic effect) fit the data significantly better than did the mixed recessive one-major-gene model. The second gene modified the effect of the first recessive gene. Individuals with the genotype aaBB (homozygous high-risk allele at the first gene and homozygous low-risk allele at the second locus) had normal IgE levels (mean 23 IU/ml), and only individuals with genotypes aaBb and aabb had high IgE levels (mean 282 IU/ml). A genomewide screening was performed using variance-component analysis. Significant evidence for linkage was found for a novel locus at 7q, with a multipoint LOD score of 3. 36 (P=.00004). A LOD score of 3.65 (P=.00002) was obtained after genotyping additional markers in this region. Evidence for linkage was also found for two previously reported regions, 5q and 12q, with LOD scores of 2.73 (P=.0002) and 2.46 (P=.0004), respectively. These results suggest that several major genes, plus residual genetic effects, regulate total serum IgE levels.

A second-generation genomewide screen for asthma-susceptibility alleles in a founder population

A genomewide screen for asthma- and atopy-susceptibility loci was conducted, using 563 markers, in 693 Hutterites who are members of a single 15-generation pedigree, nearly doubling the sample size from the authors' earlier studies. The resulting increase in power led to the identification of 23 loci in 18 chromosomal regions showing evidence for linkage that is, in general, 10-fold more significant (P<.001 vs. P<.01) than the linkages reported previously in this population. Moreover, linkages to loci in 11 chromosomal regions were identified for the first time in the Hutterites in this report, including five regions (5p, 5q, 8p, 14q, and 16q) showing evidence both of linkage, by the likelihood ratio (LR) chi(2), and of disequilibrium, by the transmission/disequilibrium test. A region on chromosome 19 continues to show evidence for linkage, by both tests, in this study. Studies of 17 candidate genes provide evidence for association with variation in the IL4RA gene (16p12), the HLA class II genes (6p21), and the interferon-alpha gene cluster (9p22), but the lack of evidence for linkage in these regions by the LR chi(2) test suggests that these are minor susceptibility loci. A polymorphism in the CD14 gene is in linkage disequilibrium with an as yet unidentified susceptibility allele in the 5q cytokine cluster, a region showing evidence for linkage among the Hutterites. Finally, 10 of the regions showing evidence for linkage in the Hutterites have shown evidence of linkage to related phenotypes in other genome screens, suggesting that these regions may contain common alleles that have relatively large effects on asthma and atopy phenotypes in diverse populations.

A European study on the genetics of mite sensitization

BACKGROUND

Sensitization to mite allergens represents a prominent feature of atopy and an important predictor of bronchial asthma.

OBJECTIVE

It was the intention of this study to define genetic loci linked to mite sensitization because these could represent the genetic basis of the important atopic component of asthma.

METHODS

We studied a multiethnic white population of 99 families, including 224 sib pairs sensitized to Dermatophagoides pteronyssinus. A genome-wide candidate-region search was performed that covered potential asthma and atopy regions.

RESULTS

As for nonparametric linkage (NPL) analysis, 14 of the candidate regions showed evidence for linkage (NPL > 2.0), and 4 of them showed prominent linkage (NPL > 3.0). However, there were substantial ethnic differences. Maximizing the LOD score analysis identified candidate regions with suspected dominant and recessive mode of inheritance. Furthermore, genetic imprinting models provided significant evidence for linkage in the 8p23 region and revealed potential maternal imprinting. The regions found are distinct to those in asthma searches that have been found to be linked to asthma, as well to other inflammatory diseases. In addition, we could not find linkage to the HLA region. By different cutoff points of the phenotype definition, the IL cluster showed evidence of being linked to the degree of sensitization rather than to sensitization per se.

CONCLUSION

The results indicate that the genetic basis of the atopic component of asthma is different from that of the inflammatory component. Furthermore, it seems reasonable to assume that specific sensitizations are influenced by distinct genetic variants leading to their initiation versus those leading to their enhancement.

Evidence for common genetic elements in allergic disease

Genetic research in allergic disease has focused primarily on asthma and its associated phenotypes (eg, total IgE), with very little attention given to the presence or absence of concomitant allergic diseases, especially allergic rhinitis and atopic dermatitis. Because asthma, allergic rhinitis, and atopic dermatitis share common systemic characteristics, it is reasonable to propose that a number of susceptibility genes could contribute to the allergic process regardless of the specific clinical phenotype. Consequently, the many genetic linkages previously reported for asthma may not be specific for asthma per se but rather may reflect an overall predisposition for allergic disease. Finally, epidemiologic data suggest that asthma and allergic rhinitis represent a continuum of disease, whereby those individuals with less severe disease will express rhinitis without asthma and those individuals with more severe disease express more than 1 phenotype. Alternatively, it is plausible that, in addition to the "allergic disease genes," there are "phenotype-specific genes" or possibly certain combinations of susceptibility genes (eg, gene-gene interactions) that contribute to the expression of asthma, allergic rhinitis, or atopic dermatitis.

Application of an algorithm for the diagnosis of asthma in Chinese families: limitations and alternatives for the phenotypic assessment of asthma in family-based genetic studies

Phenotype assessment is a crucial issue in gene mapping studies of asthma. Recently, Panhuysen and coworkers proposed an algorithm to define the asthma phenotype in gene mapping family-based studies. We classified members of 2,756 Chinese families ascertained on the basis of the presence of two or more siblings and no more than one parent with asthma using a slightly modified version of the aforementioned algorithm. Among 4,097 Chinese parents, 404 (9.9%) were classified as having "definite asthma," 284 (6.9%) as "probable asthma," 1,193 (29.1%) as "unclassifiable obstructive airway disease, " 626 (15.3%) as "COPD," and 1,590 (38.8%) as "unaffected" (no obstructive airway disease). Among 6,424 Chinese offspring, 1,065 (16.6%) were classified as having "definite asthma," 820 (12.8%) as "probable asthma," 1,996 (31.1%) as "unclassifiable obstructive airway disease," 228 (3.5%) as "COPD," and 2,315 (36%) as "unaffected." The use of the algorithm proposed by Panhuysen and coworkers in a Chinese population with a high prevalence of smoking would result in the exclusion of subjects with asthma who smoke or who have severe airflow obstruction from linkage analysis, as well as in an inability to explore any potential interactions between genetic factors and cigarette smoking in the pathogenesis of asthma. In the absence of a "gold standard," definitions of asthma that incorporate a combination of respiratory symptoms, increased airway responsiveness or bronchodilator response, and a physician's diagnosis of asthma are reasonable. The choice of a particular diagnostic algorithm for family-based genetic studies of asthma should be made according to factors such as the prevalence of smoking in the study population. Genetic studies of intermediate phenotypes related to asthma, which are objectively defined and may be influenced by a smaller number of genes, continue to be of great importance.

Linkage analysis of chromosome 12 markers in Italian families with atopic asthmatic children

We investigated 116 Italian atopic families (560 individuals) for linkage with 13 DNA markers on chromosome 12. All the subjects were phenotyped for asthma, total serum IgE, bronchial hyperresponsiveness, skin-prick positivity to common aeroallergens, and atopy. A relative location map of the markers was prepared from Centre d'Etude du Polymorphisme Humain families. Affected sib pair multipoint linkage methods were used to perform the statistical analyses. We report suggestive linkage for asthma with markers on chromosome 12. The region of interest centers around marker D12S390 (maximum logarithm of odds [mlod] = 2.81; p = 0.003). These results provide additional support that asthma susceptibility factors are located on chromosome 12q.

Quantitative trait loci controlling allergen-induced airway hyperresponsiveness in inbred mice

Identification of the genetic loci underlying asthma in humans has been hampered by variability in clinical phenotype, uncontrolled environmental influences, and genetic heterogeneity. To circumvent these complications, the genetic regulation of asthma-associated phenotypes was studied in a murine model. We characterized the strain distribution patterns for the asthma-related phenotypes airway hyperresponsiveness (AHR), lung eosinophils, and ovalbumin (OVA)-specific serum immunoglobulin (Ig) E induced by allergen exposure protocols in A/J, AKR/J, BALB/cJ, C3H/HeJ, and C57BL/6J inbred strains and in (C3H/HeJ x A/J)F1 mice. Expression of AHR differed between strains and was sometimes discordant with lung eosinophils or serum IgE. Furthermore, we identified two distinct quantitative trait loci (QTL) for susceptibility to allergen-induced AHR, Abhr1 (allergen-induced bronchial hyperresponsiveness) (lod = 4. 2) and Abhr2 (lod = 3.7), on chromosome 2 in backcross progeny from A/J and C3H/HeJ mice. In addition, a QTL on chromosome 7 was suggestive of linkage to this trait. These QTL differ from those we have previously found to control noninflammatory AHR in the same crosses. Elucidation of the genes underlying these QTL will facilitate the identification of biochemical pathways regulating AHR in animal models of asthma and may provide insights into the pathogenesis of human disease.

A first trial of retrospective collaboration for positional cloning in complex inheritance: assay of the cytokine region on chromosome 5 by the consortium on asthma genetics (COAG)

The central problem of complex inheritance is to map oligogenes for disease susceptibility, integrating linkage and association over samples that differ in several ways. Combination of evidence over multiple samples with 1,037 families supports loci contributing to asthma susceptibility in the cytokine region on 5q [maximum logarithm of odds (lod) = 2.61 near IL-4], but no evidence for atopy. The principal problems with retrospective collaboration on linkage appear to have been solved, providing far more information than a single study. A multipoint lod table evaluated at commonly agreed reference loci is required for both collaboration and metaanalysis, but variations in ascertainment, pedigree structure, phenotype definition, and marker selection are tolerated. These methods are invariant with statistical methods that increase the power of lods and are applicable to all diseases, motivating collaboration rather than competition. In contrast to linkage, positional cloning by allelic association has yet to be extended to multiple samples, a prerequisite for efficient combination with linkage and the greatest current challenge to genetic epidemiology.

Imbalance production between interleukin-1beta (IL-1beta) and IL-1 receptor antagonist (IL-1Ra) in bronchial asthma

Genes of the IL-1 family encode three different peptides, IL-1alpha, IL-1beta, and IL-1Ra, respectively. IL-1 operates through IL-1RI, and is involved in airway inflammation in asthmatic subjects, whereas IL-1Ra appears to be a specific competitive inhibitor of IL-1. All genes are on chromosome 2q12-21 where genomewide searches have identified linkage for asthma. To test whether variants of IL-1 relate to asthma, we conducted a genetic association study in a Japanese population. We show that the A2 allele of IL1RN (encoding IL-1Ra) associates with nonatopic asthma [OR = 5.71, 95% CI: 1.63-19. 8, Pc = 0.007]. Both atopic and nonatopic asthmatics with the A2 allele had significantly lower serum IL-1Ra levels in both types of asthmatics. Peripheral blood cells from asthmatics with A2 alleles, however, produced as much IL-1 as did those with A1 homozygotes. Since Th1 and Th2 cytokines differentially regulate the ratio between IL-1beta and IL-1Ra, these findings suggest that dysregulation of IL-1beta/IL-1Ra, probably due to interaction between epithelium and immuno-competent cells in the airway, is important in asthma inflammation.

Genomic approaches to understanding asthma

Asthma is the most common chronic childhood disease in developed nations, and it is a complex disease that has high social and economic costs. Asthma and its associated intermediate phenotypes are under a substantial degree of genetic control. The genetic aetiology of asthma offers a means of better understanding its pathogenesis and, thus, improving preventive strategies, diagnostic tools, and therapies. Considerable effort and expense have been expended in attempts to detect genetic loci contributing to asthma susceptibility, and extensive candidate gene studies and a number of whole-genome screens have been undertaken. This article reviews the current state of knowledge of the genetics of asthma, with a focus on genomic approaches to understanding allergic diseases.

Identification of complement factor 5 as a susceptibility locus for experimental allergic asthma

The prevalence and severity of allergic asthma continue to rise, lending urgency to the search for environmental triggers and genetic substrates. Using microarray analysis of pulmonary gene expression and single nucleotide polymorphism-based genotyping, combined with quantitative trait locus analysis, we identified the gene encoding complement factor 5 (C5) as a susceptibility locus for allergen-induced airway hyperresponsiveness in a murine model of asthma. A deletion in the coding sequence of C5 leads to C5-deficiency and susceptibility. Interleukin 12 (IL-12) is able to prevent or reverse experimental allergic asthma. Blockade of the C5a receptor rendered human monocytes unable to produce IL-12, mimicking blunted IL-12 production by macrophages from C5-deficient mice and providing a mechanism for the regulation of susceptibility to asthma by C5. The role of complement in modulating susceptibility to asthma highlights the importance of immunoregulatory events at the interface of innate and adaptive immunity in disease pathogenesis.

Contributing factors to the pathobiology. The genetics of asthma

Markers in 19 chromosomal regions have shown some evidence of linkage to asthma, atopy, or related phenotypes in multiple independent genome-wide searches. Linkages to five of these regions (5q, 6p, 11q, 12q, and 13q) have also been reported in non-genome-wide screens. In addition, at least two independent studies have reported linkages to markers on 16p. Numerous candidate genes in these regions have shown varying levels of association to asthma or atopic phenotypes, potentially implicating them as disease susceptibility loci. These include the IL4, CD14, and B2ADR genes on 5q, the HLA-DRB1 and TNF genes on 6p, the FCERB1 and CC16 genes on 11q, and the IL4RA gene on 16p. It still remains to be determined whether polymorphisms in these genes account for the reported linkages in these regions. Studies are underway in laboratories around the world to identify the disease-causing variations in these genes that account for the linkages just discussed. Identifying specific genetic polymorphisms that influence asthma and atopic phenotypes will shed light on the molecular pathways involved in these complex disorders and provide a better understanding of the pathophysiology of asthma and atopy.

Significant evidence for linkage of mite-sensitive childhood asthma to chromosome 5q31-q33 near the interleukin 12 B locus by a genome-wide search in Japanese families

Childhood-onset asthma is frequently found in association with atopy. Although asthmatic children may develop IgE antibodies against variety of allergens, asthma is associated primarily with allergy to house-dust mites, molds, or other allergens. In this study, we conducted a genome-wide linkage search in 47 Japanese families (197 members) with more than two mite-sensitive atopic asthmatics (65 affected sib-pairs) using 398 markers. Multipoint linkage analysis was carried out for atopic asthma as a qualitative trait using the MAPMAKER/SIB program. We observed significant evidence for linkage with maximum lod scores (MLS) of 4.8 near the interleukin 12 B gene locus on chromosome 5q31-q33. In addition, suggestive evidence on 4q35 with MLS = 2.7 and on 13q11 with MLS = 2.4 was obtained. The other possible linkage regions included 6p22-p21.3 (MLS = 2.1), 12q21-q23 (MLS = 1.9), and 13q14.1-q14.3 (MLS = 2.0). Many of the linkage loci suggested in this study were at or close to those suggested by genome-wide studies for asthma in Caucasian populations. The present study suggests the contribution of the interleukin 12 B gene or nearby gene(s) to mite-sensitive atopic asthma and a considerable number of genetic variants common across Caucasians and Japanese populations contributing to asthma, although the relative importance of various variants may differ between the groups.

The Ile198Thr and Ala379Val variants of plasmatic PAF-acetylhydrolase impair catalytical activities and are associated with atopy and asthma

The platelet-activating factor (PAF) represents a phospholipid with complex biological functions, including involvement in inflammatory processes. The degrading enzyme PAF acetylhydrolase (PAFAH) represents a candidate for asthma and other atopic diseases. Two loss-of-function mutations of PAFAH are associated with severe asthma in Japanese individuals. Our aim was to look for further PAFAH variants in white populations, their possible association with atopic and asthmatic phenotypes, and their functional importance. We picked up three common variants in the PAFAH gene: Arg92His (exon 4), Ile198Thr (exon 7), and Ala379Val (exon 11). The known loss-of-function mutations were not seen. The variant allele Thr198 was found to be highly associated with total IgE concentrations in an atopic population (P=.009) and with "atopic asthma" in an asthmatic population (P=.008). The variant allele Val379 was found to be highly associated with "specific sensitization" in the atopic population (P=.002) and with "asthma" in the asthmatic population (P=.003). By use of recombinant PAFAH enzymes, the variant Val379 showed increased (14 microM) and Thr198 markedly increased (42 microM) KM values compared to the wild type (7 microM); furthermore, Vmax of Val379 was highly increased (132%). Thr198 and Val379 influence plasmatic PAFAH toward lower substrate affinities and therefore are very likely to prolong the activities of PAF. At the same time, they are associated with an increased risk to develop asthma and atopy. Thus, two PAFAH variants seem to play a key role in atopic and asthmatic processes in Caucasian populations.

A two-stage variable-stringency semiparametric method for mapping quantitative-trait loci with the use of genomewide-scan data on sib pairs

Genomewide scans for mapping loci have proved to be extremely powerful and popular. We present a semiparametric method of mapping a quantitative-trait locus (QTL) or QTLs with the use of sib-pair data generated from a two-stage genomic scan. In a two-stage genomic scan, either the entire genome or a large portion of the genome is saturated with low-density markers at the first stage. At the second stage, the intervals that are identified as probable locations of the trait loci, by means of analysis of data from the first stage, are then saturated with higher-density markers. These data are then analyzed for fine mapping of the loci. Our statistical strategy for analysis of data from the first stage is a low-stringency method based on the rank correlation of squared trait-difference values of the sib pairs and the estimated identity-by-descent scores at the marker loci. We suggest the use of a low-stringency method at the first stage, to save on computational time and to avoid missing any marker interval that may contain the trait loci. For analysis of data from the second stage, we have developed a high-stringency nonparametric-regression approach, using the kernel-smoothing technique. Through extensive simulations, we show that this approach is more powerful than is a currently used method for mapping QTLs by use of sib pairs, particularly in the presence of dominance and epistatic effects at the trait loci.

The gene encoding interleukin-13: a susceptibility locus for asthma and related traits

Asthma is a complex inflammatory disorder controlled by both genetic and environmental influences. Multiple genetic analyses have identified the T helper type 2 (Th2) cytokine gene cluster on chromosome 5q as a susceptibility locus for asthma. Recently, the Th2 cytokine interleukin-13 has been shown to be a critical mediator of the asthma phenotype in murine models. In this commentary we discuss several recent studies that have identified polymorphisms in the gene encoding interleukin-13. The consistent genetic associations of interleukin-13 with asthma and related traits across diverse ethnic populations in these studies provides strong support for the candidacy of this cytokine as a susceptibility locus for asthma and atopy on chromosome 5q31.

Genetics of asthma and inflammation: the status

Genome-wide screens are consistently finding linkage between asthma-associated traits and specific chromosomal loci. Several loci coincide with linkages to other inflammatory diseases, suggesting the presence of common pathways in their pathogenesis. Candidate-gene studies have found an association between a CD14 polymorphism and IgE levels, suggesting a mechanism for the increased prevalence of allergic disease. A polymorphism in Fc epsilon RI-beta shows parent-of-origin effects when associated with severe infantile eczema, further illustrating the complexity of gene-environment effects on the developing immune system.

Mapping autoimmunity genes

Rat and mouse models for the major human autoimmune/inflammatory diseases are under intense genetic scrutiny. Genome-wide linkage studies reveal that each model is regulated by multiple genetic loci. Many of these loci colocalize to homologous genomic regions associated with several different autoimmune diseases of mice, rats and humans. Candidate genes are being identified. Polymorphic alleles associated with these chromosomal segments may represent predisposing genetic elements common to a number of human diseases with very different clinical presentations.

Genetic and environmental interaction in allergy and asthma

Asthma is an inflammatory disorder of the airways involving coordinate up-regulation of T(H)2-type cytokines encoded in a cluster on chromosome 5q(31-33) on T cells and inflammatory cells. There is also a requirement for local airway susceptibility factors that, together with T(H)2 polarization, results in hyperresponsiveness, variable airflow obstruction, and, over time, remodeling of the airway wall. Asthma has strong genetic and environmental components that interact both in the induction and subsequent expression of the disease phenotypes. Multiple genes are involved and probably interact. Whole genome screens are beginning to identify gene-rich regions of special relevance to asthma and atopy, although a novel disease-related gene has yet to be discovered from these. By contrast, there are a plethora of candidate genes whose function in relation to disease pathophysiologic mechanisms and response to treatment are known. Two examples are polymorphisms involving IL-4 receptors and the enzymes controlling cysteinyl leukotriene production. Abnormal signaling between the epithelium, which is in contact with the environment, and the underlying (myo)fibroblasts and dendritic cells indicating reactivation of the epithelial mesenchymal trophic unit, which is involved in fetal lung development and branching, provide a basis for asthma that encapsulates both T(H)2 polarization and airway wall remodeling.

The alliance of genes and environment in asthma and allergy

The diseases of asthma, eczema and hay fever are typified by reactions to common allergens, which are mediated by immunoglobulin E. These allergic diseases are increasing in prevalence, and are now a major source of disability throughout the developed world. They are the result of complex interactions between largely unknown genetic and environmental mechanisms. The identification of the environmental factors offers the real possibility of prevention of disease, and unravelling the genetics of allergic illnesses is likely to change their classification and treatment. Early life seems particularly important, when the initiation of allergic disease may result from genetic and environmental modification of the immune interaction between mother and child.

Use of genetic models in respiratory neurobiology and sleep

In this review, two issues are highlighted: 1) the difficulties that can form major hurdles in trying to understand a disease or a fundamental biologic process at the genetic and molecular level and 2) the potential opportunities that genetic models such as the Drosophila or c-elegans can provide in answering clinically or biologically relevant questions. This review also lists in some detail the areas in which these models have been helpful and successes have been scored. For such models to be used, however, requires the "dissection" of a biologic or a disease process into a tractable phenotype that can be assayed in a genetic model and have relevant and interpretable conclusions. The hope is that questions pertaining to sleep, arousal, respiratory neurobiology, and their disorders can be formulated in such a way to be addressed in models that can lend themselves to very exciting discoveries.

Chemokines and allergic disease

Understanding the chemokine network has become one of the great challenges for researchers interested in inflammatory mechanisms and inflammation-based diseases. The complexity and diversity of the system provide not only a daunting task for its comprehension but also numerous opportunities for development of new, targeted therapies. It is now certain that chemokines are involved as important mediators of allergic inflammation; the fine details and scope of their roles are now under investigation. Presumably, because of distinct pressures on the immune systems of people living in different geographic regions, genetic variation of ligands, receptors, and regulatory regions in the network have emerged. Establishing the roles of these polymorphisms in determining disease susceptibility or progression among individuals and in distinct ethnic groups will provide a basis for improved understanding and treatment of allergic diseases.