Beta-2 adrenergic receptor dysfunction and polymorphism in asthma

INTRODUCTION — While the powerful bronchodilatory properties of beta-adrenergic agonists have been appreciated for many years, understanding of the molecular basis of their activity at the beta-2 adrenergic receptor did not begin until the late 1980s. The mechanisms of signal transduction from this receptor and the potential role of beta-2 adrenergic receptor dysfunction in the pathogenesis of asthma and its response to therapy will be reviewed here. The clinical use of beta agonists in the treatment of asthma is discussed separately. (See "Beta agonists in asthma: Acute administration and prophylactic use".)

NORMAL RECEPTOR REGULATION — The gene encoding the beta-2 adrenergic receptor is situated on chromosome 5q31 [1]. It encodes a protein that is a member of the large family of G-protein coupled, seven transmembrane-spanning domain receptors. The receptor is expressed on a variety of cell types in the lung, including airway smooth muscle and epithelial cells, vascular endothelium and smooth muscle cells, and inflammatory cells such as mast cells, eosinophils, and lymphocytes.

Stimulation of the beta-2 receptor results in activation of the associated G-protein, Gs, which dissociates to release a protein subunit, free Gs-alpha. Gs-alpha in turn activates adenylyl cyclase, resulting in a rise in intracellular cyclic adenosine monophosphate (AMP) levels. Most of the intracellular effects of beta-2 adrenergic receptor stimulation are due to the elevation in cyclic AMP and consequent activation of protein kinase A (PKA); there may also be direct, cyclic AMP-independent effects of Gs-alpha, for example, on the calcium-activated potassium channel [2,3]. (See "Peptide hormone signal transduction and regulation", section on 'G proteins'.)

The expression of beta-2 adrenergic receptors and their coupling with intracellular signaling pathways are dynamically regulated, providing a negative feedback loop that reduces cell responsiveness to long-term occupation of the receptor by an agonist. Phosphorylation of the receptor, either by PKA-dependent pathways or by activation of one of a family of G-protein receptor kinases termed beta-adrenergic receptor kinase (beta-ARKs), leads to reduced coupling with the intracellular signaling pathway following agonist stimulation. Different tissues vary in the degree of uncoupling seen with prolonged agonist exposure, probably due to differences in the amount and activity of beta-ARK and/or PKA in different cell types.

The number of receptors also is actively regulated.

●Beta-2 receptor number is reduced after exposure to agonist, typically for more than two hours

●Beta-2 adrenoceptor expression in airway cells is downregulated after exposure to viruses or to pro-inflammatory cytokines, such as interleukin 1-beta [4,5]

●Beta-2 receptor number is increased by in vitro exposure to glucocorticoids [6,7]

RECEPTOR DYSFUNCTION IN ASTHMA — The possibility that medication-induced changes in beta receptor density or signal transduction impact detrimentally upon asthma control has been repeatedly suggested in the literature, although supporting evidence has been scant. Beta-2 adrenergic receptor dysfunction has been documented in in vitro studies of tissue obtained from patients with severe asthma [8-10]; however, clinically significant tachyphylaxis to the bronchodilator effects of beta-2 agonists has been difficult to demonstrate in patients with mild asthma [11]. (See "Beta agonists in asthma: Acute administration and prophylactic use", section on 'Tolerance'.)

Responses to beta-adrenergic medications may also be mediated by genes other than the beta-2 receptor gene itself. A genome wide association study (GWAS) identified a region on chromosome 2 that is associated with a decreased bronchodilator response to inhaled beta-2 agonist [12].

A large number of studies have examined the effect of scheduled treatment with short-acting beta-2 agonists on asthma control [13-21]. The majority of these reports found no significant deterioration in symptoms, lung function, or the need for rescue medications. Several studies demonstrated a small deterioration in symptom score, FEV₁, or response to bronchoprovocation when beta agonists were used regularly [2,13,21], and rebound hyperresponsiveness has been noted following beta agonist withdrawal [22]. While more intense beta agonist use does not have a large detrimental effect, regular treatment with a short-acting beta-agonist appears to confer no advantage over "as required" use [18,19]. (See "An overview of asthma management".)

RECEPTOR POLYMORPHISMS — A number of polymorphic forms of the beta-2 adrenergic receptor were described in 1993 [23]. These polymorphisms are in linkage disequilibrium in populations studied to date, but the frequency of the different variants varies between ethnic groups. Each individual has two copies of the gene for the beta-2 adrenergic receptor and hence can be heterozygous or homozygous for each polymorphism.

Nine point mutations have been studied within the beta-2 adrenergic receptor gene, although only four result in amino acid substitutions because of redundancy in the amino acid translation code.

●The arginine to glycine 16 and glutamine to glutamate 27 substitutions are common in the White population; allele frequencies at each locus are between 0.3 and 0.7 [24]. In an in vitro study, homozygosity of Gly 16 resulted in greater downregulation (96 percent) and homozygosity of Glu 27 in less downregulation (29 percent) of the beta-2 receptor in response to agonists than occurred with the wild type (78 percent), although agonist binding kinetics were not affected [25]. Both the codon 16 and codon 27 polymorphisms are in Hardy-Weinberg equilibrium in the general population. However, there is linkage disequilibrium between the two loci, with individuals who have glycine 16 being more likely (in the White population) to have glutamate 27 [26,27].

●The threonine to isoleucine 164 mutation is rare (allelic frequency 3 percent), but produces marked alterations in the in vitro behavior of the receptor. Cells transfected with this form of the receptor display reduced agonist binding to catechol ligands and have altered receptor trafficking [28]. This polymorphism is also close to the putative salmeterol exosite. Data from a large population based study, the Copenhagen City Heart Study, found an association between this polymorphism and the presence of airflow limitation [29]. However, few individuals homozygous for this polymorphism have been identified, and therefore its physiologic effects on airway responses are uncertain.

●The valine to methionine 34 substitution is rare and does not appear to alter receptor function.

●The regulatory region (promoter) for the beta-2 adrenergic receptor gene also contains functionally relevant polymorphisms; the clinical importance of this remains to be fully determined [30,31]. The possibility that combinations of these polymorphisms are deleterious is also worthy of further study, although functional data are currently limited on haplotype driven effects [32,33].

Clinical significance — The possibility that beta-2 adrenergic receptor polymorphism may alter the response to treatment or influence susceptibility to asthma has been investigated in a number of studies [26,27,34-45].

Treatment response to regular SABA — Although not consistent across all studies [37], several studies have found that frequent or regularly scheduled short-acting beta-agonists (eg, albuterol) impaired asthma control in patients who were homozygous for arginine at the 16th amino acid position (ie, Arg16Arg) [35,42,43,46]. This was best illustrated by a prospective, blinded study of 78 patients with mild asthma who were treated with regularly scheduled albuterol or placebo for eight weeks, then crossed over to the other treatment for eight weeks [35]. Patients with the Arg16Arg genotype experienced a decrease in morning peak expiratory flow rate (PEFR) (-10 L/m compared to placebo), while patients with the Gly16Gly genotype experienced a significant increase in morning PEFR (+14 L/m).

Treatment response to LABA — Two small retrospective cohort studies found that patients with the Arg16Arg genotype had a worse response to the long-acting beta-agonist (LABA) salmeterol compared with patients with the Gly16Gly genotype [47,48]; however, most larger prospective studies have not confirmed these results [49-52]. As examples:

●In a double blind study of 2250 patients with more severe asthma (833 Gly16Gly, 1028 Gly16Arg, 361 Arg16Arg) on regular inhaled glucocorticoids and either salmeterol or formoterol, no difference between genotypes was found in the frequency of exacerbations or measures of asthma control [50]. As all of the patients were taking inhaled glucocorticoids and used limited doses of short acting beta agonists (SABA), this study does not resolve whether beta-adrenergic receptor polymorphism affects outcomes in patients on LABA monotherapy or frequent dosing of SABA.

●To evaluate whether beta-adrenergic receptor polymorphisms affect outcomes in patients on LABA monotherapy, 544 subjects, whose asthma was poorly controlled on SABA alone, were randomly assigned based on Arg16Gly genotype to salmeterol alone or salmeterol with fluticasone [52]. The primary efficacy measure was morning peak expiratory flow rate. No difference was found for modulation of the treatment response due to beta-adrenergic receptor genotype variation.

In contrast, an observational study of 1182 young asthmatics suggested that exacerbations were more common in those individuals carrying the Arg16 allele who were exposed to daily beta-2 agonists, regardless of whether the exposure was to SABA or LABA [46], and a meta-analysis of data on 4226 children with asthma from five different populations showed an association between LABA use and exacerbation risk in those carrying the Arg16 allele [53]. An additional meta-analysis of nearly 6000 children and young adults from 10 studies also demonstrated an increase in risk for asthma exacerbation in patients with the Arg16Gln27 haplotype who were treated with LABA and ICS [54].

Taken together, these studies suggest that any clinical effects driven by genetic variation are likely to be minimized by prescribing LABAs in combination with inhaled glucocorticoids in adults. In children with asthma, there may be a small increased risk of exacerbations in those carrying the Arg16 polymorphism. In accordance with current treatment guidelines, we do not recommend LABA monotherapy or regular dosing of SABA in patients with asthma.

Response to ultra-LABAs — The development of very long acting once daily beta-2 agonists such as indacaterol and vilanterol has increased the number of beta-2 agonists available for use in patients with COPD. Whether clinical responses to ultra-LABAs are affected by beta-2 receptor polymorphism has not been determined, although in vitro responses show similar profiles to existing LABAs [55]. (See "Stable COPD: Initial pharmacologic management", section on 'Long-acting beta-agonists'.)

Acute treatment response to SABA — The Arg16Arg genotype has been associated with an increased physiological response to a single dose of short-acting beta-agonist [38,41]. This was best demonstrated by a study of 269 children with asthma who underwent genotyping, then had spirometry measured before and after the administration of albuterol [38]. Patients with the Arg16Arg genotype were 5.3 times more likely to have an increase in FEV₁ of more than 15 percent, compared to patients with the Gly16Gly genotype.

Disease risk — There have been a number of studies examining possible relationships between beta-2 receptor polymorphisms and disease risk with conflicting results:

●In a report of 60 families with multiple asthmatic family members, no association was found between asthma severity and either the glycine or glutamate polymorphisms at the 16th and 27th positions of the beta-2 adrenergic receptor (Gly16/Gln27), respectively [44]. Similarly, in a random sample of 332 individuals, neither the glycine nor the glutamate polymorphism was associated with the presence of clinician-diagnosed asthma [27].

●In contrast to the negative results of the studies described above, other studies have reported an association between disease and beta-2 receptor polymorphisms [39,40]. In one study of 167 patients with asthma and 84 control subjects, the Gly16/Gln27 genotype was more prevalent in patients with moderate asthma than in patients with mild asthma [40]. In another study, the Gly16 polymorphism correlated with nocturnal asthma [39].

●A study of the UK 1958 birth cohort examined relationships between beta-2 adrenergic receptor polymorphism and asthma risk [56]. In this study, which included 8018 subjects, no overall association was seen between the codon 16, 27, or 164 polymorphisms and asthma risk either in children or adults, although the Arg16Gln27 haplotype was associated weakly with progression of wheezing illness from childhood into adult life. This study shows that at least in the UK population, these polymorphisms are not important determinants of asthma risk.

A number of GWAS studies have examined asthma risk, but have not identified genome wide associations in the ADRB2 locus.

Cross talk with other signaling cascades — Studies on transgenic mice either lacking or overexpressing the beta-2 adrenergic receptor have provided additional insight into the regulation of airway responses by the beta-2 adrenergic receptor pathway. Although mice with very high levels of beta-2 adrenergic receptor expression have dilated airways, mice with lower levels of overexpression actually have increased airway reactivity, while mice lacking the beta-2 adrenergic receptor demonstrate decreased airway responsiveness [57]. These counterintuitive findings are probably due to increased phospholipase C-beta1 expression in mice overexpressing the beta-2 adrenergic receptor.

SUMMARY

●Normal receptor regulation – The expression and coupling of beta-2 adrenergic receptors are dynamically controlled in airway cells by a variety of mechanisms that may modify the effect of receptor stimulation. Tachyphylaxis following chronic beta agonist stimulation is demonstrable in vitro, but the clinical impact of this phenomenon is less well defined. (See 'Normal receptor regulation' above and 'Receptor dysfunction in asthma' above.)

●Receptor polymorphisms – Common polymorphisms of the beta-2 adrenergic receptor are found at amino acid positions 16 and 27. Studies examining the influence of these polymorphisms are conflicting; the weight of the evidence is that they do not increase the risk of developing asthma or the severity of asthma but may be associated with a greater likelihood of asthma persisting from childhood to adulthood. Further study may clarify the precise role of these and other beta receptor polymorphisms in the genesis and course of asthma. (See 'Receptor polymorphisms' above.)

●Clinical significance of the Arg16 polymorphism – The Arg/Arg polymorphism at position 16 of the beta-2 adrenergic receptor may be associated with adverse outcomes related to regular use of short acting beta agonists (SABA). However, at least within a clinical trial setting, this polymorphism does not appear to affect the safety or efficacy of currently available long-acting beta agonists (LABAs) in adults when used in combination with inhaled glucocorticoids. (See 'Clinical significance' above.)