S. Rorke

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.

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.

Evolving concepts on the value of adenosine hyperresponsiveness in asthma and chronic obstructive pulmonary disease.

Adenosine is a purine nucleoside which mediates a variety of cellular responses relevant to asthma and COPD through interaction with specific receptors. Administration of adenosine by inhalation to patients with asthma and COPD is known to cause concentration related bronchoconstriction. Responses elicited by this purine derivative in asthma and COPD should not be considered as a mere reflection of non-specific airways hyperresponsiveness. Evaluation of airways responsiveness by adenosine induced bronchoconstriction may be valuable in differentiating asthma from COPD, monitoring of anti-inflammatory treatment in asthma, surveying disease progression, and assessing disease activity in relation to allergic airways inflammation.

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.

Role of cysteinyl leukotrienes in adenosine 5`-monophosphate induced bronchoconstriction in asthma

Background: Adenosine induced bronchoconstriction in patients with asthma is thought to be mediated by the synthesis and release of autacoids from airway mast cells. In vitro, adenosine induced constriction of asthmatic bronchi is blocked by a combination of specific histamine and cysteinyl leukotriene receptor antagonists, but the relative contribution of these mediators in vivo is unclear. We hypothesised that adenosine induced bronchoconstriction in asthmatic patients may be blocked by pretreatment with the orally active selective cysteinyl leukotriene-1 (CysLT1) receptor antagonist, montelukast. Methods: In a randomised, double blind, crossover study, oral montelukast (10 mg) or placebo was administered once daily on two consecutive days to 18 patients with mild to moderate persistent atopic asthma. Incremental doses of adenosine 5`-monophosphate (AMP) from 0.39 to 400 mg/ml were inhaled by dosimeter and the dose producing a 20% fall in FEV1 (PC20AMP) after AMP inhalation was recorded. Leukotriene E4 (LTE4) urinary concentrations were measured by enzyme immunoassay 4 hours after AMP challenge. Results: Montelukast pretreatment provided highly significant protection against adenosine induced bronchoconstriction, with geometric mean PC20AMP values of 52.6 mg/ml (95% CI 35.2 to 78.7) after placebo and 123.9 mg/ml (95% CI 83.0 to 185.0) after montelukast (p=0.006). The geometric mean of the montelukast/placebo PC20AMP ratio was 2.4 (95% CI 1.3 to 4.2). Montelukast had no significant effect on 4 hour urinary excretion of LTE4 compared with placebo. Conclusions: Selective CysLT1 receptor antagonism with montelukast provides highly significant protection against AMP induced bronchoconstriction in patients with atopic asthma, implying that cysteinyl leukotrienes are generated from airway mast cells through preferential activation of their A2B receptors.

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.

Targeting adenosine receptors: novel therapeutic targets in asthma and chronic obstructive pulmonary disease.

Adenosine, an endogenous signaling nucleoside that modulates many physiological processes has been implicated in playing an ever increasingly important role in the pathogenesis of asthma and chronic obstructive pulmonary disease (COPD). All cells contain adenosine and adenine nucleotides and the cellular production of adenosine is greatly enhanced under conditions of local hypoxia as may occur in inflammatory conditions such as asthma and COPD. In 1983, it was first reported that inhaled adenosine causes dose-related bronchoconstriction in patients with both allergic and non-allergic asthma but not in healthy volunteers. This hyperresponsiveness was also reported in patients with COPD, with those patients who smoked exhibiting a significantly greater response. This bronchoconstrictor effect of adenosine is orchestrated through the stimulation of specific cell membrane receptors and involves an important inflammatory cell, the mast cell.There is substantial evidence which suggests that mast cell activation is central to this unique response to adenosine. Mast cell mediator release makes a significant contribution towards airflow obstruction and the consequent symptoms in patients with asthma. Over the last two decades, researchers have investigated the effect of mast cell inhibitors as well as mast cell mediator receptor antagonists and their role in attenuating the bronchoconstrictor response to inhaled adenosine 5′-monophosphate (AMP). Promising results have been shown using mast cell stabilizers, histamine H1 receptor antagonists, selective cysteinyl leukotriene-1 receptor antagonists and inhibitors of 5-lipoxygenase and cyclo-oxygenase. Through these findings, the mast cell has been recognized as being a critical inflammatory cell in the adenosine-induced response in patients with asthma and COPD.To date, four subtypes (A1, A2A, A2B, A3) of adenosine receptors have been cloned each with a unique pattern of tissue distribution and signal transduction. Activation of these receptors has pro- and anti-inflammatory consequences making the development of agonists and/or antagonists at these receptor sites a novel approach in the treatment of patients with asthma and COPD.This review highlights the importance of adenosine in the pathophysiology of asthma and COPD, the critical role of the mast cell and the potential to target the adenosine receptor subtype in patients with asthma and COPD. The complete characterization of these adenosine receptor subtypes in terms of their distribution in humans and the development of selective agonists and antagonists, holds the key to our complete understanding of the role of this important mediator in asthma and COPD.

ADAM 33 and its association with airway remodeling and hyperresponsiveness in asthma.

Asthma is known to be a Th2 inflammatory syndrome that leads to intermittent airway obstruction. However, the mechanisms involved in development of the clinical features remain enigmatic, although genetic elements clearly are involved. Recently, based on a large genome-wide screen involving families in the United Kingdom and the United States with at least two siblings with asthma, a locus was identified that encoded for a family of proteases. This group of proteins is now known as the ADAM superfamily. In this review, we discuss the ADAM superfamily and, in particular ADAM 33, a member of a family of genes which encode a subgroup of zinc dependent metalloproteinase (metzincin). The potential for therapeutic intervention with ADAM 33 is extremely attractive and further work will not only focus on the specific domains of ADAM 33, but also the mechanisms by which they lead to bronchial hyperreactivity.

Combination therapy with anti‐mediator drugs in allergic disease

Inflammatory mediators are local hormones that act via specific cell-surface receptors in an autocrine or paracrine manner to modulate the behaviour of inflammatory leucocytes and structural cells in inflamed tissues. The classical inflammatory mediators are the autacoids such as histamine, cysteinyl-leukotrienes (cys-LT), LTB4, prostanoids, bradykinin, and platelet-activating factor (PAF). Potent, selective, safe and long-acting compounds that inhibit the synthesis or block the receptors of individual autacoids have been developed over the last 20 years. The leukotriene (LT) modifiers (montelukast, pranlukast, zafirlukast, zileuton) have entered clinical practice for asthma, and the non-sedating H1 histamine antagonists (cetirizine, loratadine, terfenadine) are front-line therapies for allergic rhinitis and allergic skin disease. Potent inhibitors of prostanoid synthesis (NSAIDs) have been available for 100 years, but the new cycloxygenase-2-selective inhibitors (celecoxib, rofecoxib) may combat inflammation without the gastric and other adverse effects associated with traditional NSAIDs. Experimental antagonists of LTB4, PAF and the mast cell protease, tryptase, have also been investigated in clinical trials in asthma and other diseases, with mixed results. LT modifiers and other anti-mediator therapies have been shown to provide excellent clinical benefit in significant subgroups of patients. Many have advantages with respect to patient compliance and favourable adverse effect profiles compared to anti-inflammatory corticosteroids. The perception emerging from clinical trials is that anti-mediator monotherapies may be less efficacious than corticosteroids against the broad spectrum of inflammatory processes in allergic disease, arousing growing interest in the concept of combination therapy with two or more antimediator drugs, not only in asthma but also in inflammatory diseases of the nasal airway and skin. The paper from Professor Lipworths group at Dundee University published in this issue of Clinical & Experimental Allergy [1] provides important evidence that combination therapy with two anti-mediator drugs, specifically a LT modifier and an H1 histamine antagonist, can provide additive and complementary anti-inflammatory outcomes that match those afforded by topical corticosteroids. This review will focus on clinical trials of anti-mediator cotherapy, particularly with combinations of LT modifiers and antihistamines, in the treatment of allergic asthma, exercise-induced bronchoconstriction, allergic skin disease (eczema, urticaria), and allergic rhinitis.