New Horizons In Asthma: Importance Of
b₂ Adrenergic Receptor Polymorphisms

John J. Lima, Pharm D.
John J. Lima, Pharm D., is Director, Center for Clinical Pediatric Pharmacology, Nemours Children's Clinic.

Asthma affects 14 million to 15 million persons in the United States and is the most common disease of childhood, affecting 4.8 million children. Pharmacological intervention is aimed at the prevention and control of asthma symptoms, reducing the frequency and severity of exacerbations, and reversing airflow obstruction. Inhaled short-acting b₂ agonists like albuterol and terbutaline are recommended for the relief of acute symptoms, while long-acting agents like salmeterol are used in combination with corticosterioids for long-term asthma control.¹ Two main beneficial effects of inhaled b₂ agonists in asthma are bronchodilation and inhibition bronchoconstriction induced by exercise and other provocative stimuli. The main beneficial effects of inhaled corticosteroids in asthma are anti-inflammatory and reversing b₂ agonist-promoted tolerance.

Despite advances in our understanding and treatment of asthma, morbidity and mortality continue to rise. The reasons for this paradox are not clear. The development of tolerance, (also known as tachyphylaxis or subsensitivity) to the bronchodilating and bronchoprotective effects following chronic b₂ agonist use has been associated with, and is thought to contribute to functional deterioration and increases in asthma death.^2-3 The development of tolerance is thought to be due to agonist-induced desensitization of b₂ receptor-stimulated adenylyl cyclase. Adenylyl cyclase catalyzes the conversion of ATP to cAMP, the second messenger, which in turn activates several cytosolic kinases to induce relaxation of airway smooth muscles and bronchodilaltion. b₂ agonists activate the b₂ receptor to couple with G_s-binding protein, which is responsible for transducing the signal produced by the b₂ agonist to adenylyl cyclase. Two mechanisms of desensitization have been identified: short-term desensitization, in which the b₂ receptor is phosphorylated and uncoupled from G_s, and long-term desensitization, which involves receptor down-regulation.⁴

b₂ adrenergic receptors are expressed in all cell types comprising the airway including smooth muscle, epithelial cells, blood vessels and cholinergic nerves; they are also expressed in inflammatory cells, and in cells and vessels of the peripheral lung and in circulating lymphocytes. Recently, nine different missense mutations (exchange of one nucleic acid base for another resulting in an amino acid substitution) within the coding block of the b₂ adrenergic receptor gene have been identified (see Figure 1).⁵ Five of these mutations are degenerate in that they encode the same gene product, while four mutations at positions 16, 27, 34 and 164 encode different gene products (Figure 1). However, only the polymorphisms at positions 16 and 27 are present in high allelic frequencies. Table 1 lists the frequencies of b₂ adrenergic receptor genotypes at positions 16 and 27 in the human population.

Figure 1. Primary amino acid sequence human b2 adrenergic receptor polymorphisms. Codons with nucleic acid deviations from wild-type are indicated in parentheses; those that result in amino acid changes are indicated. (Reprinted from Reference 5, with permission).

The functional significance of polymorphisms at loci 16 and 27 have been studied in cell lines which express the homozygous forms of each mutation. Substitutions of the amino acid glycine (Gly) for arginine at position 16 (Arg 16®Gly), and glutamic acid (Glu) for glutamine (Gln) and position 27 (Gln 27®Glu) (Figure 1) resulted in b₂ adrenergic receptor polymorphisms that displayed similar agonist binding and functional coupling to G_s. However, when exposed to continuous isoproterenol stimulation, the individual polymorphisms showed different patterns of receptor down-regulation. Compared to the Arg 16 form, the Gly 16 variant of the b₂ adrenergic receptor was more down-regulated by isoproterenol, while the Glu 27 variant was completely resistant to agonist-promoted down-regulation compared to the Gln 27 form.^6-7 These studies led to the idea that polymorphisms of the b₂ adrenergic receptor may play an important role in asthma phenotypes, and in regulating receptor-mediated response to drugs.

The relation between b₂ adrenergic receptor polymorphisms and asthma phenotypes is summarized in Table 2. No difference in the frequency of receptor polymorphisms between asthmatics and non-asthmatics was found,⁸ indicating that mutations of the b₂ adrenergic receptor are not a primary cause of asthma. However, the Gly 16 and Gln 27 variants have been associated with nocturnal asthma,⁹ airway hyperreactivity^10,11 and increased levels of IgE.¹² Although no association was found between b₂ adrenergic receptor polymorphism or haploytpe and the risk of sudden death, the Gly 16/Gln 27 haplotype was more prevalent in moderate than mild asthmatics.¹³ Taken together these data indicate that genetic polymorphisms of the b₂ adrenergic receptor are important risk factors in certain asthma phenotypes.

Early reports of the functional importance of b₂ adrenergic receptor polymorphisms in determining response to drugs were encouraging. For example, asthmatics with the homozygous Arg 16 genotype were more likely to respond to an inhaled dose of albuterol compared to Gly 16 homozygotes or heterozygotes,¹⁴ and a greater degree of tolerance was observed in Gly 16 homozygotes compared to Arg 16 homozygotes.¹⁵ However, more recent studies do not support a role for b₂ adrenergic receptor polymorphisms in determining bronchodilator response and tolerance to inhaled b₂ agonists.^16-17

Recently we completed a study of the impact of b₂ adrenergic receptor polymorphism on FEV₁ evoked by albuterol.¹⁸ Stable, moderate asthmatics ranging in age from 18 to 50 years volunteered to take a single oral solution containing 8 mg of albuterol, following a 12- and 24-hour washout from inhaled b₂ agonists and corticosteroids, respectively. b₂ Receptor genotype, plasma concentrations and FEV₁ values were determined prior to and for 12 hours following administration of the albuterol oral dose in 16 patients. The results of our study are summarized in Figure 2. Compared to asthmatics who carried the Gly 16 variant (Gly 16 homozygotes and heterozygotes), Arg 16 homozygotes responded more rapidly and to a greater extent at equivalent plasma albuterol concentrations. Furthermore, 7 asthmatics failed to respond, or had FEV₁ values less than at baseline. All were carriers of the Gly 16 variant. Of the nine patients who did respond in a predictable way to albuterol, 56% were genotyped as Arg 16 homozygotes, and 44% were carriers of the Gly 16 variant. This indicates that not all carriers of the Gly 16 variant are poor responders to b₂ agonists, however those who are poor responders, carry the Gly 16 variant.

The results of this study are important for several reasons. First, they clearly establish that b₂ receptor polymorphisms at loci 16 are major determinants of agonist-evoked bronchodilation. Second, b₂ receptor polymorphisms are responsible for contributing in a major way to variability in FEV₁ response to b₂ agonists. Third, our study provides a solid foundation to conduct future studies designed to examine the impact of b₂ receptor polymorphisms on bronchodilator response to long-term agonist treatment, the development of tolerance and to corticosteroid reversal of tolerance. It is possible that in the near future we will design drug therapy for asthmatics based on their b₂ receptor genotype.

REFERENCES

1. National Asthma Education and Prevention Program. Expert Panel Report 2: Guidelines for the diagnosis and management of asthma. April 1997; 97.
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4. Lohse MJ. Molecular mechanisms of membrane receptor desensitization. Biochim Biophys Acta. 1993; 1179:171-188.
5. Liggett SB. Polymorphisms of the beta2-adrenergic receptor and asthma. Am J Respir Crit Care Med. 1997;156:S156-S162
6. Green SA, Turki J, Innis M, Liggett SB. Amino-terminal polymorphisms of the human beta 2-adrenergic receptor impart distinct agonist-promoted regulatory properties [published erratum appears in Biochemistry. 1994 29;33(47):14368]. Biochemistry. 1994; 33:9414-9419.
7. Green SA, Turki J, Bejarano P, Hall IP, Liggett SB. Influence of beta 2-adrenergic receptor genotypes on signal transduction in human airway smooth muscle cells. Am J Respir Cell Mol Biol. 1995;13:25-33.
8. Reihsaus E, Innis M, MacIntyre N, Liggett SB. Mutations in the gene encoding for the beta 2-adrenergic receptor in normal and asthmatic subjects. Am J Respir Cell Mol Biol. 1993;8:334-339.
9. Turki J, Pak J, Green SA, Martin RJ, Liggett SB. Genetic polymorphisms of the beta 2-adrenergic receptor in nocturnal and nonnocturnal asthma. Evidence that Gly16 correlates with the nocturnal phenotype. J Clin Invest. 1995; 95:1635-1641.
10. Hall IP, Wheatley A, Wilding P, Liggett SB. Association of Glu 27 beta 2-adrenoceptor polymorphism with lower airway reactivity in asthmatic subjects. Lancet. 1995;345:1213-1214.
11. Holroyd KJ, Levitt RC, Dragwa C, Amelung PJ, Panhuysen CM, Meyers DA. Evidence for b-adrenergic receptor (ADR?) polymorphism at amino acid 16 as a risk factor for bronchial hyperresponsiveness (BHR). Am J Respir Crit Care Med. 1995;151 (suppl).
12. Dewar JC, Wilkinson J, Wheatley A, et al. The glutamine 27 beta2-adrenoceptor polymorphism is associated with elevated IgE levels in asthmatic families. J Allergy Clin Immunol. 1997; 100:261-265.
13. Weir TD, Mallek N, Sandford AJ, et al. beta2-Adrenergic receptor haplotypes in mild, moderate and fatal/near fatal asthma. Am J Respir Crit Care Med. 1998;158:787-791.
14. Martinez FD, Graves PE, Baldini M, Solomon S, Erickson R. Association between genetic polymorphisms of the beta2-adrenoceptor and response to albuterol in children with and without a history of wheezing. J Clin Invest. 1997;100:3184-3188.
15. Tan S, Hall IP, Dewar J, Dow E, Lipworth B. Association between beta 2-adrenoceptor polymorphism and susceptibility to bronchodilator desensitisation in moderately severe stable asthmatics [see comments]. Lancet. 1997; 350:995-999.
16. Hancox RJ, Sears MR, Taylor DR. Polymorphism of the beta2-adrenoceptor and the response to long-term beta2-agonist therapy in asthma. Eur Respir J. 1998;11:589-593.
17. Aziz I, Hall IP, McFarlane LC, Lipworth BJ. Beta2-adrenoceptor regulation and bronchodilator sensitivity after regular treatment with formoterol in subjects with stable asthma. J Allergy Clin Immunol. 1998; 101:337-341.
18. Lima JJ, Thomason D, Mohamad M, Eberle LV, Self TH, Johnson JA. Impact of genetic polymorphisms of the b₂-adrenergic receptor on albuterol bronchodilator pharmacodynamics. Clin Pharmacol Ther. 1999;(In Press).

Jacksonville Medicine / November, 1999

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