Tim Keith

Association of the ADAM33 gene with asthma and bronchial hyperresponsiveness

Asthma is a common respiratory disorder characterized by recurrent episodes of coughing, wheezing and breathlessness. Although environmental factors such as allergen exposure are risk factors in the development of asthma, both twin and family studies point to a strong genetic component. To date, linkage studies have identified more than a dozen genomic regions linked to asthma. In this study, we performed a genome-wide scan on 460 Caucasian families and identified a locus on chromosome 20p13 that was linked to asthma (log10 of the likelihood ratio (LOD), 2.94) and bronchial hyperresponsiveness (LOD, 3.93). A survey of 135 polymorphisms in 23 genes identified the ADAM33 gene as being significantly associated with asthma using case-control, transmission disequilibrium and haplotype analyses (P = 0.04–0.000003). ADAM proteins are membrane-anchored metalloproteases with diverse functions, which include the shedding of cell-surface proteins such as cytokines and cytokine receptors. The identification and characterization of ADAM33, a putative asthma susceptibility gene identified by positional cloning in an outbred population, should provide insights into the pathogenesis and natural history of this common disease.

A genetic linkage map of the human genome

We report the construction of a linkage map of the human genome, based on the pattern of inheritance of 403 polymorphic loci, including 393 RFLPs, in a panel of DNAs from 21 three-generation families. By a combination of mathematical linkage analysis and physical localization of selected clones, it was possible to arrange these loci into linkage groups representing 23 human chromosomes. We estimate that the linkage map is detectably linked to at least 95% of the DNA in the human genome.

Genome-wide association study for Crohn's disease in the Quebec Founder Population identifies multiple validated disease loci

Genome-wide association (GWA) studies offer a powerful unbiased method for the identification of multiple susceptibility genes for complex diseases. Here we report the results of a GWA study for Crohns disease (CD) using family trios from the Quebec Founder Population (QFP). Haplotype-based association analyses identified multiple regions associated with the disease that met the criteria for genome-wide significance, with many containing a gene whose function appears relevant to CD. A proportion of these were replicated in two independent German Caucasian samples, including the established CD loci NOD2 and IBD5. The recently described IL23R locus was also identified and replicated. For this region, multiple individuals with all major haplotypes in the QFP were sequenced and extensive fine mapping performed to identify risk and protective alleles. Several additional loci, including a region on 3p21 containing several plausible candidate genes, a region near JAKMIP1 on 4p16.1, and two larger regions on chromosome 17 were replicated. Together with previously published loci, the spectrum of CD genes identified to date involves biochemical networks that affect epithelial defense mechanisms, innate and adaptive immune response, and the repair or remodeling of tissue.

TOLL-LIKE RECEPTOR 4 POLYMORPHISM AND SEVERITY OF ATOPY IN ASTHMATICS

Endotoxin exposure may have a protective effect against asthma and atopy. An Asp299Gly polymorphism in the Toll-like receptor 4 (TLR4) gene reduces responsiveness to endotoxin. This study determined the effect of TLR4 polymorphism on the risk and severity of asthma and atopy. In all, 336 UK Caucasian families with ≥2 affected sibs (physicians diagnosis of asthma and current medication use) and 179 Caucasians without asthma or a family history of asthma were genotyped using ARMS-PCR. No association of the TLR4 polymorphism was found with the risk of developing asthma, either in parent-affected sibling trios, or in case–control analyses (P>0.05). In the first affected asthmatic siblings, the atopy severity score (based on size and number of positive skin-prick tests and specific IgE) was higher in those with the Asp/Gly or Gly/Gly genotypes (mean 1.8, s.d. 1.1, n=39) compared to those with the Asp/Asp genotype (mean 1.2, s.d. 1.0, n=279) (P=0.003, t-test). No associations were found with total IgE, FEV1 % predicted, slope of FEV1 response to methacholine or asthma severity score (P>0.05). This study confirms the previously observed lack of association of TLR4 polymorphisms with asthma. In contrast, the findings suggest that genetically determined hyporesponsiveness to endotoxin may increase atopy severity.

Polymorphisms in the interleukin-4 and interleukin-4 receptor alpha chain genes confer susceptibility to asthma and atopy in a Caucasian population

Background  IL‐4 by binding to its receptor (IL‐4R) is essential for the development of airway inflammation present in asthma, through the induction of IgE synthesis in B cells and differentiation of T cells to a Th2 phenotype.

Allelic association and functional studies of promoter polymorphism in the leukotriene C4 synthase gene (LTC4S) in asthma

Background: LTC4 synthase is essential for the production of cysteinyl leukotrienes (Cys-LT), critical mediators in asthma. We have identified a novel promoter polymorphism at position −1072 (G/A) and a −444 (A/C) polymorphism has previously been reported. The role of these polymorphisms in the genetic susceptibility to asthma was examined. Methods: To test for genetic association with asthma phenotypes, 341 white families (two asthmatic siblings) and 184 non-asthmatic control subjects were genotyped. Genetic association was assessed using case control and transmission disequilibrium test (TDT) analyses. LTC4S promoter luciferase constructs and transiently transfected human HeLa and KU812F cells were generated to determine the functional role of these polymorphisms on basal transcription. Results: No associations were observed in case control analyses (–1072 A, q=0.09; −444 C, q=0.29); the TDT identified a borderline association between the −444 C allele and bronchial responsiveness to methacholine (p=0.065). Asthmatic children with the −444 C allele had a lower mean basal forced expiratory volume in 1 second (97.4 v 92.7% predicted, p=0.005). LTC4S promoter luciferase analyses provided no evidence for a functional role of either polymorphism in determining basal transcription. Conclusion: This study does not support a role for these polymorphisms in genetic susceptibility to asthma but provides evidence to suggest a role in determining lung function parameters.

Promoter polymorphism in the 5-lipoxygenase (ALOX5) and 5-lipoxygenase-activating protein (ALOX5AP) genes and asthma susceptibility in a Caucasian population

Background 5‐Lipoxygenase (5‐LO) and 5‐lipoxygenase‐activating protein (FLAP) are essential for cysteinyl‐leukotriene (cys‐LT) production, critical mediators in asthma.

Polymorphism of the mast cell chymase gene (CMA1) promoter region: lack of association with asthma but association with serum total immunoglobulin E levels in adult atopic dermatitis

Background Mast cell chymase has the potential to be an important mediator of inflammation and remodelling in the asthmatic lung. Previous studies have examined association between promoter polymorphism of the chymase gene (CMA1) and allergic phenotypes but the significance of this polymorphism is unclear. We have examined association of a CMA1 variant in relation to asthma in a large UK Caucasian family cohort.

The role of ADAM33 in the pathogenesis of asthma

While asthma is a disorder of the conducting airways characterised by Th2-directed inflammation, a second set of mechanisms is being increasingly recognised as fundamental to disease chronicity and severity, for which the term “remodelling” has been used. The cellular and mediator responses underpinning airway remodelling involve aberrant communication between the airway epithelium and underlying mesenchyme, involving the generation of growth factors that lead to proliferation of fibroblasts and smooth muscle and the deposition of matrix proteins to cause airway wall thickening linked to bronchial hyperresponsiveness and fixed airflow obstruction. The identification of ADAM33 on chromosome 20p13 from positional cloning as a novel candidate gene involved in the pathogenesis of these structural and functional changes has opened the way to further insight into these processes that contribute to corticosteroid refractoriness. The preferential expression of ADAM33 in mesenchymal cells and its multiple molecular actions provide ample opportunity for incriminating this molecule in chronic asthma. Its association with progressive asthma and in predicting reduced lung function in young children suggest that ADAM33 has an important role in the natural history and possibly the origins of asthma, a disease unique to humans.

The discovery and role of ADAM33, a new candidate gene for asthma

Asthma is a complex disorder in which major genetic and environmental factors interact to initiate the disease and propagate it as a chronic relapsing disorder. Until recently, genetic factors implicated in the disease pathogenesis have been restricted to variants in known molecules involved in the inflammatory or remodelling pathways. This review discusses evidence for a new susceptibility gene for asthma, ADAM33, which was identified by positional cloning and shown to be selectively expressed in mesenchymal but not immune or inflammatory cells. ADAM33 belongs to a family of membrane-anchored metalloproteinases that also have fusagenic, adhesion and intracellular signalling properties. ADAM33 might play a key role in predisposing to the reduced lung function characteristic of asthma, possibly by influencing airway wall remodelling.