The beta-adrenergic receptors (ADRBs) are cell surface receptors that play central roles in the sympathetic nervous system. Pharmacological targeting of two of these receptors, ADRB1 and ADRB2, represents a widely used therapeutic approach for common and important diseases including asthma, hypertension and heart failure. Genetic variation in both ADRB1 and ADRB2 has been linked to both in vitro and clinical disease phenotypes. More recently, interest has shifted to studies that explore potential interaction between variation in ADRBs and medications directed at these important receptors. This paper reviews the current state of knowledge and understanding of ADRB genetic variation and explores the likely direction of future studies in this area.
The autonomic nervous system is responsible for regulating the physiological demands of vascular and visceral smooth muscle as well as exocrine and endocrine glands. Through a series of counterbalancing mechanisms, autonomic homeostasis is continually sought as the organism responds to changes in its environment. Endogenous proteins bind to cell surface receptors and transmit signals through the divisions of the autonomic nervous system and coordinate parasympathetic and sympathetic responses to maintain relative constancy of the internal environment of the organism. In the sympathetic nervous system, the adrenergic receptors (both alpha and beta subclasses) are of central importance in mediating the response of cells and organs to sympathetic discharges. In this review, the beta-adrenergic receptors (ADRBs) are discussed in the context of naturally occurring genetic variation in the ADRB genes. Although ADRBs mediate many sympathetic responses, cardiovascular and asthma phenotypes, which have been most thoroughly investigated to date, are primarily discussed here. The described pharmacogenetic studies and the potential relevance of ADRB polymorphisms on clinical phenotypes in humans are highlighted.
Beta-adrenergic receptors are important drug targets for asthma and cardiovascular conditions including hypertension and congestive heart failure. Inhaled beta-receptor agonists (e.g. albuterol) and antagonists (e.g. ‘beta-blockers’) remain among the mostly commonly prescribed medications in adults to treat asthma and cardiovascular disease, respectively. It has long been accepted that abnormal airway function plays a role in the pathogenesis of asthma, and genes, including the ADRBs, are strongly implicated in susceptibility and treatment response to asthma.1, 2, 3, 4 Genetics are also important in the cardiovascular system, which has the tasks of delivering oxygen and nutrients to tissues and organs and must also provide a mechanism for the transport and removal of metabolic by-products for elimination by the body. The cardiovascular system has evolved to function under a wide range of physiologic demands and can rapidly adapt to the changing needs of the host. Beta-adrenergic receptors play an important role in the extrinsic control of cardiac contractility and function.
Beta-adrenergic receptors are G-protein-coupled receptors located on the surface of effector cells. They bind both epinephrine and norepinephrine as well as exogenously administered drugs, including beta-agonists and antagonists (‘beta-blockers’). For the cardiovascular system, ADRBs mediate alterations in myocardial metabolism, heart rate and systolic and diastolic function.5, 6 In general, agonist binding increases cardiac contractile strength, relaxation and heart rate with acute changes observed within seconds of agonist release. Indeed, the prompt responses mediated through activation of ADRBs are critical to regulation of circulatory homeostasis and to the survival of the organism (e.g. providing an ability to maintain adequate blood pressure in the face of acute blood loss). Although the presence of a rapidly responding adrenergic system is critical to survival, prolonged activation of these pathways has deleterious consequences. Chronic stimulation of adrenergic pathways is harmful and inhibits myocyte growth and function.6 Persistent stimulation of ADRBs leads to downregulation of ADRB signaling by decreasing receptor sensitivity to catecholamines and by increasing inhibitory G protein (Gi) activity.7 In chronic heart failure for instance, catecholamines are elevated, leading to tonic adrenergic signaling; this increased level of cardiac adrenergic drive is an important prognostic indicator in heart failure.8
The three beta-adrenergic receptor subtypes
There are three subtypes of ADRBs (ADRB1, ADRB2 and ADRB3) encoded by three separate genes. ADRB1 and ADRB2 have been well studied and have important effects on pulmonary and cardiac physiology. In the heart, ADRB1s are the predominant subtype expressed and mediate increases in inotropy and chronotropy when stimulated.5 ADRB2s are abundantly expressed in bronchial smooth muscle and activation of them results in bronchodilation, a fact exploited in the use of inhaled beta-agonist medications for asthma. ADRB2s are also expressed on cardiac myocytes (inotropy) and vascular smooth muscle cells (vasodilation). ADRB3 has been least studied to date. Recent data suggest that the ADRB3 subtype might mediate cardiovascular effects to a modest degree and may even act to weaken cardiac contractility.9, 10, 11, 12, 13 ADRB3 is not consistently expressed in the human heart and the importance of its role on cardiovascular disease is not known.14
ADRB1 and ADRB2 are intronless genes encoding 477- and 413-amino-acid proteins, respectively; ADRB3 contains at least one intron15, 16, 17 needed for the 408 amino-acid protein. In spite of moderate differences in amino-acid sequences, the three ADRBs share a common structure with an extracellular amino terminus, seven transmembrane spanning domains and a cytoplasmic carboxy terminus (Figure 1).18 Binding of agonist to the receptor leads to a guanine nucleotide binding protein (Gs protein) coupled response and conversion of adenosine 5′ triphosphate to cyclic adenosine 3′,5′ monophosphate (cAMP) by adenylate cyclase.5, 19 Intracellular rises in cAMP provide an indirect measure of ADRB activity. Increased cAMP stimulates a chain of events that culminates with removal of calcium from contractile protein (faster relaxation in the case of myocytes) and increased activation of contraction through greater calcium cycling; the global effect is improved systolic and diastolic function.5 Stimulation of Gs also directly activates effectors, such as the L-type calcium channel.6 The importance of the ADRB system in cardiovascular physiology has been explored on multiple levels and led to the discovery of multiple polymorphic variants in this system (Figure 1).14, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39
Twenty-three polymorphisms of ADRB1 have been described and 13 of these predict amino-acid changes in the ADRB1 protein.14, 40 As ADRB1 does not have a recognized role in the pathogenesis of asthma, the majority of genetic association studies have focused on cardiovascular phenotypes. Several of the polymorphisms are quite rare and have not been widely studied. The Arg389Gly was the first ADRB1 missense polymorphism reported with the polymorphism localizing to the intracellular cytosplasmic tail of ADRB1 and showing a minor allele (Gly) frequency of 0.26.41 Mason et al.41 have proposed that this tail domain interacts with the Gs protein and influences signal transduction. The NCBI database reports that a valine residue (rs17875445) may also be found at the 389 position, accounting for approximately 3% of the alleles, but this variant has not been studied extensively. Other known ADRB1 missense polymorphisms are Ser49Gly, Ala59Ser, Arg318Ser, Lys324Arg, Ala343Thr, Glu352Asp, Arg399Cys, Arg400Leu, His402Arg, Thr404Ala, Pro418Ala and Asp460Glu.14, 42, 43 The Ser49Gly and Ala59Ser polymorphisms are located in the extracellular amino terminus and are thought to affect ligand binding. Variants in the intracellular carboxy terminus (Arg389Gly, Arg399Cys, Arg400Leu, His402Arg, Thr404Ala, Pro418Ala, Asp460Glu) could modulate Gs protein coupling as is hypothesized for the Arg389Gly variant. The Ser49Gly polymorphism is in linkage disequilibrium with the 389 locus.18
There are 12 polymorphisms reported for the ADRB2 coding region (Figure 1).14, 35, 43 Five of these polymorphisms predict changes to the amino-acid sequence: Gly16Arg, Gln27Glu, Val34Met, Thr164Ile and Ser220Cys. A polymorphism in the untranslated portions of the gene has also been described and may be of biological relevance.44 The separate 19-amino-acid ADRB2 regulatory peptide is encoded in the 5′ upstream region of ADRB2. The last residue of this oligopeptide is polymorphic (Cys-19Arg). Accounting for these 13 polymorphisms of ADRB2 gene region, one would predict a potential for 8192 (213) haplotypes from the theoretical combinations of each variant at each loci. Not surprisingly, there is linkage disequilibrium present and the number of haplotypes encountered in humans is much lower. In one study, all 13 ADRB2 polymorphisms were genotyped in 121 Caucasian asthmatics and 77 controls (396 chromosomes), which resulted in 12 inferred haplotypes; five of these haplotypes appeared to represent 95% of the total haplotype pool.44 Althouth the population size in this study was modest, ethnic differences in haplotype frequency were observed, which may impact the generalizability of future pharmacogenetic studies.
ADRB3s are principally localized to adipose tissue, but are also expressed in pancreas, colon, prostate, skeletal muscle and cardiac atrial tissues.45, 46 The adipose tissue expression has generated interest in whether ADRB3s are important in modifying obesity phenotypes. ADRB3 agonists in rodents show an increase in lipolysis and energy expenditure and mice lacking ADRB3s develop obesity.47, 48 Human cardiac tissue and blood vessels contain ADRB3s and upregulation of ADRB3s in failing hearts has been demonstrated.49 ADRB3 may be clinically important in the cardiovascular system as there is evidence for activation by beta-agonists and inhibition by beta-blockers, but the overall significance of cardiac ADRB3s remains uncertain.50, 51
Genetic polymorphisms are present in ADRB3s and the Trp64Arg was the first one to be studied in obesity, diabetes and hypertension.9, 12, 39, 52, 53 In a Pima Indian population, the Arg64 (minor) allele appeared to be a risk factor for earlier onset of diabetes.54 Later studies have reported inconsistent results with respect to the Trp64Arg variant and diabetic and obesity phenotypes.55, 56, 57 More recently, the Trp64Arg polymorphism was suggested to interact with obesity in conferring risk of pre-eclampsia with the rarer Arg64 variant being possibly protective against pre-eclampsia in overweight women.47 As the pharmacogenetic data on ADRB3 polymorphisms are less developed than those for ADRB1 and ADRB2, the remainder of this review will concentrate on the latter two receptors.
Functional studies of ARDB1 polymorphisms
Two ADRB1 polymorphisms (Ser49Gly and Arg389Gly) have been widely studied owing to studies supporting both functional effects on ADRB1 activity and on phenotypes in human studies. The Gly49 allele (as compared to Ser49) showed altered glycosylation and a more pronounced agonist-induced receptor downregulation in a hamster fibroblast model.58, 59 This might translate into resistance to chronic beta-adrenergic stimulation through disease or medication. The Arg389 alleles manifest increased function in vitro as compared to Gly389 receptors showing higher basal levels of adenylyl cyclase activity and heightened sensitivity to stimulation by isoproterenol. Thus, the Arg389 may have inherently superior coupling to Gs (stimulatory G protein) and increased signal transduction, which is fitting with some of the observed population based studies in humans.41
Functional studies of ADRB2 polymorphisms
The Arg16Gly and Gln27Glu polymorphisms show altered downregulation of ADRB2 in a Chinese hamster fibroblasts system.60 In healthy subjects, the Arg16 variant has enhanced isoproterenol-mediated desensitization of the vasculature and enhanced isoproterenol induced-venodilation.61 The Ile164 variant has increased binding affinity for norepinephrine, epinephrine and isoproterenol. The explanation for this is not immediately clear as the 164 position is localized to the fourth transmembrane domain and is unlikely to come into direct contact with ligand.62 Transgenic mice overexpressing Ile164 have lower resting heart rates and lower levels of adenylyl cyclase than mice expressing the Thr164 variant.63 One function of the ADRB2 leader peptide is to modulate ADRB2 expression, and this is influenced by the Cys-19Arg polymorphism.64, 65
ADRB1 association studies
Polymorphisms in ADRB1 have been evaluated against a number of phenotypes, mostly related to the cardiovascular system. The most widely studied variants have been the Ser49Gly and Arg389Gly polymorphisms. In a collection of Asian patients, Ser49 homozygotes had higher mean heart rates than Gly49 homozygotes.66 Later studies did not reproduce this finding in different ethnic populations24, 67 although one study did report a trend towards a more pronounced lowering of mean heart rate after antihypertensive therapy in Gly49 homozygotes with baseline hypertension and left ventricular hypertropy.68 The Arg389 variant was associated with elevated diastolic blood pressure and higher resting heart rate in a study that used both a case–control and a sib pair analysis design.24 The external validity of these findings was strengthened by similar conclusions being made with the Arg389 variant in a different study population.67
ADRB1 polymorphisms have been studied in the context of heart failure, where beta-blocking medications are widely used therapeutic agents. The Ser49Gly polymorphism does not appear to be an independent risk factor for heart failure in case–control studies.22, 25, 69 One study did report Gly49 as a risk factor for idiopathic dilated cardiomyopathy, but the modest population size studied (37 cases and 40 controls) and difference in allele frequency as compared to other studies limit a firm conclusion that Gly49 carriers are at elevated risk.21, 42 This polymorphism, however, may have a role in progression of heart failure as the Ser49 variant was associated with a reduced 5-year survival (risk ratio 2.34; P=0.003). The other major ADRB1 polymorphism, Arg389Gly, is not a clear risk factor for heart failure. In a study of 224 patients with heart failure treated with the beta-blocker carvedilol, Arg389 homozygotes experienced a greater improvement in the ejection fraction (8.7 vs 0.93%; P<0.05) as compared to Gly389 homozgyotes.70
Interaction of ADRB1 Arg389Gly and alpha-2-C adrenergic polymorphism
The ADRB1 Arg389Gly polymorphism appears to interact with another polymorphism in the alpha-2-C adrenergic receptor (ADRA2C). A functional 4-amino-acid deletion of the ADRA2C (del322–325) modulates presynaptic release of norepinephrine. In a case–control study of heart failure (189 affected; 159 controls), del322–325 homozygotes, who tonically release more norepinephrine, had an increased odds ratio of 5.65 (95% CI 2.57–11.95) of heart failure. When the Arg389 homozygotes, who experience enhanced ADRB1 signaling, were added into this model, the odds of heart failure was increased to 10.11 (95% CI 2.11–44.53) for carriers of both rare variants. These data were only true in Black subjects, as the number of double homozygotes in Caucasians was too small for analysis.27
ADRB2 association studies
Polymorphisms in ADRB2 have been more extensively studied because the potential phenotypes of interest extend beyond cardiovascular traits to asthma, obesity and diabetes. Although environmental triggers are believed to be necessary for the development of asthma, a major focus of current research is on the genetic factors, including ADRB2s, which interact with environmental exposures.1, 71 A recent meta-analysis concluded that the Gly16 allele predisposes to nocturnal asthma and asthma severity.72 Another study reported reduced forced expiratory volume and a greater decline in lung function in cystic fibrosis patients carrying the Gly16 allele.30 Gly16 has also been linked to differential response to beta-agonist therapy (albuterol) in asthmatics.73, 74
In hypertensive populations, ADRB2 genetic association studies have yielded mixed results. Hypertension has been associated with the Arg16 and Gly16 alleles in different studies.23, 75 These contradictory findings could be explained by differences in the populations. A subsequent combined analysis of over 1800 hypertensive cases and controls found no association with the hypertensive phenotype at the Arg16Gly locus.31 In an analysis of siblings highly discordant for blood pressure (427 subjects from 55 families), the Gly16 was associated with elevated resting blood pressure. This work was extended to nearly 1300 subjects and the association findings remained significant.76 The investigators then added 291 additional families from a differing geographic region and the Gly16 allele no longer associated with elevated blood pressure; interestingly, the Gln27 allele was found to be predictive of systolic and mean arterial blood pressure in the final population (odds ratio 1.80; P=0.023).
In healthy individuals, ADRB1s are far more abundant than ADRB2s in the human heart; this situation changes in heart failure where the proportion of ADRB2s is increased to approximately 40%.77, 78, 79 These data could be interpreted to mean that ADRB2 polymorphisms might have greater effects in heart failure.14 In a study of 259 heart failure patients, the Ile164 variant was predictive of a combined end point of death or cardiac transplantation (relative risk 4.81; P<0.001). The Arg16Gly and Gln27Glu loci were also studied and did not yield significant findings.37 In another heart failure study, the Ile164, Gly16 and the combination of Gly16 and Gln27 variants predicted lower cardiopulmonary exercise capacity.80 More recently, the Arg16 allele has been linked to higher heart rate and elevated cardiac adrenergic drive (measured by norepinephrine spillover) and the Gln27 allele predicted a more favorable response to carvedilol in heart failure subjects.81, 82
Thus, polymorphisms in ADRB1 and ADRB2 have support for biological relevance in both in vitro and clinical studies. Not all results have been consistent, however, and few studies have successfully replicated findings in larger populations. In most clinical studies, surrogate markers of clinically important end points have been studied and the importance of variation at these polymorphisms on major indicators of morbidity and mortality is still not understood. Furthermore, the utilization of ADRB polymorphic data in the clinical care of patients affected with cardiovascular or pulmonary diseases remains largely theoretical.
Haplotype approaches to ADRB1 and ADRB2
The results of previous genetic association studies described above have been intriguing and several support hypotheses that functional polymorphisms in ARDB1 and ADRB2 influence the differential expression of clinically important phenotypes. The enthusiasm for these data are tempered by problems generally common to genetic association studies: limited sample sizes, suboptimal matching of cases and controls, subgroup analyses and focuses on secondary outcomes, lack of corrections for multiple comparisons and few successful attempts to replicate results in independent populations.83, 84 Some have suggested that haplotype approaches may address some of these problems and the density of haplotype data generated by the HapMap project (http://www.hapmap.org/) makes these types of studies increasingly feasible.84 Currently, only a few haplotype-based ADRB studies have been published and their focus has largely been on those ADRB polymorphisms that have been previously reported to associate with clinical phenotypes (Figure 2). By transfecting ADRB1 genes with different combinations of the Ser49Gly and Arg389Gly polymorphisms, Sandilands et al.85 noted differences in the production of cAMP after isoprenaline stimulation. In a study of 862 subjects undergoing bicycle exercise training, ADRB1 Haplotype 1 (Figure 2) predicted greater aerobic power (measured by peak oxygen uptake) as compared to Haplotype 2 or 3.86
Four polymorphisms have been included in studies of ADRB2 haplotypes: Cys-19Arg, Gly16Arg, Gln27Glu and Thr163Ile. In one such analysis of terbutaline-mediated venodilation of the dorsal hand vein in 35 subjects, Haplotype 1 (Figure 2) predicted greater response to terbutaline than Haplotype 2 or 3 (the Cys-19Arg polymorphism was not genotyped in this work).87 Oostendorp et al.88 used human lymphocytes and measured desensitization to isoproterenol-induced cAMP accumulation; ADRB2 Haplotype 2 showed significantly greater desensitization as compared to Haplotypes 1 or 3 (the Thr164Ile polymorphism was not genotyped in this work). The authors suggest that the differential haplotype effect may be important for the regulation of type 2 helper cells in asthma inflammation. In a larger study of 642 overweight or obese subjects, ADRB2 Haplotype 1 predicted increased levels of triglycerides and LDL-cholesterol (the Thr164Ile polymorphism was not genotyped in this work).89 The largest ADRB2 haplotype study to date involved genotyping of 523 patients who had myocardial infarction from a prospectively followed cohort of 14 916 initially healthy American men. Four controls (n=2092) were matched for each case and Haplotype 4 and the combined group of Haplotypes 1 and 3 had significantly altered risk of myocardial infarction with odds ratios of 0.178 (95% CI: 0.043–0.737; P=0.017) and 1.235 (95% CI: 1.031–1.480; P=0.022) respectively (the Cys-19Arg polymorphism was not genotyped in this work).90
Pharmacogenetic studies of ADRBs
Accumulating evidence suggests that ADRBs are important in the pathogenesis and severity of human disease states, including cardiovascular, pulmonary and obesity disorders. That ADRBs are major targets of pharmacological therapy (beta receptor agonists and antagonists) has not escaped notice and efforts are now underway to account for polymorphic variation in ADRBs to develop more rationale therapeutic approaches in humans.83, 91, 92 For ADRB1, four recent studies have reported exciting finding of interactions between adrenergic directed agents and cardiac phenotypes. In a study of 30 healthy males, ADRB1 Arg389 homozygotes showed greater increases in fractional shortening upon exposure to the non-selective beta-agonist dobutamine as compared to subjects carrying Gly389 alleles (61.9 vs 52.8; P=0.001). This translated to a 19 mmHg difference in systolic blood pressure at the highest dobutamine dose (P=0.005).93 In 61 patients with heart failure who had not previously received beta-blocking therapy, the ADRB1 Arg389 homozygotes had significant improvements in ejection fraction in response to an ADRB1 blocking agent (metoprolol) and showed overall improvements as compared to Gly389 carriers. Carriers of ADRB1 Gly49 had improved left-ventricular end-diastolic diameters.94 In a related study, patients with Gly389 treated with metoprolol required more increases in their heart failure medications in a 10-week study as compared to Arg389 subjects; interestingly, the same held true for carriers of Gly49 alleles.95 Somewhat at odds with these findings are data from a subgroup analysis of 375 idiopathic dilated cardiomyopathy patients (375 cases and 492 controls) where Ser49 carriers required higher doses of beta-blocking medications. In patients receiving 50% of their targeted full dose of beta-blockade, 5-year mortality was lower in the Gly49 carriers, an effect that was not observed in those patients taking higher doses.69
Two studies looked at ADRB2 genotypes in reactive airway disease. In a case–control study of 543 Caucasian males, ADRB2 Arg16 showed a trend towards bronchial hyper-responsiveness to methacholine (cholinergic agonist) challenge. The effect became statistically significant when only non-smokers were considered, where Arg16/Gln27 (Figure 2, ADRB2 Haplotype 1) subjects had greater hyper-responsiveness.96 In the first prospective trial to stratify treatment arms by ARDB2 genotypes in asthma, Arg16 and Gly16 homozygotes (n=37 and 41, respectively) were enrolled and randomized to placebo or inhaled albuterol in a crossover design. In the treatment phase, albuterol-treated Gly16 homozygotes had improvements in morning peak expiratory flow rates as compared to placebo (14 l/min improvement; 95% CI 3–25; P=0.0175). The opposite was observed for the Arg16 homozygotes, who actually showed an average of 10 l/min drop in their expiratory flow rates (95% CI: −19 to −2; P=0.0209). The unfavorable response to albuterol therapy in the Arg16 homozygotes prompted the authors to suggest that caution may need to be exercised in this group of subjects.73
Implications for pharmacogenomics
The bulk of the data indicates that variation in ADRBs does impact clinically relevant phenotypes. A recent shift towards studies that evaluate the response to medications as determined by ADRB genotype adds momentum to the concept that individualized therapy for heart failure, hypertension and/or asthma may soon be realized. For ADRB1, the Gly49 and Arg389 variants seem, on balance, to be the variants that perform more favorably in heart failure patients given beta-blockade. In the case of ADRB2, the data reported by Israel et al.73 suggest an intriguing negative effect of Arg16 alleles on short-term beta agonist therapy. Whether this finding holds true for long-term exposure to beta-agonist therapy remains to be clarified. Haplotypes of ADRB2 may also be important in myocardial infarction risk as suggested by at least one study.90 Much remains to be learned and studies that integrate haplotype data and also account for interactions between ADRB1 and ADRB2 loci may ultimately prove more fruitful studies of individual polymorphisms. The example of the interaction between ADRB1 Arg389Gly and the ADRA2C del 322–325 polymorphism highlights the importance of other genes in the adrenergic pathway.
The road ahead remains a steep one for translating these sorts of data into clinical practice. A foundation for this journey will likely be developed from extending the studies of ADRB polymorphisms to large DNA banks, ideally where prospective data have been collected. Such an effort may require the partnership of several research groups, and data sharing and replication in multiple data sets is recommended. Convincing data from this method of approach can then form the basis for prospective clinical trials where stratification by haplotype (or genotype) is performed. The shift from the current approach in trials of ‘mere’ randomization of clinically homogenous subjects (who are actually genetically heterogenous) to a genetic-based paradigm will be essential if the benefits of ADRB haplotype-directed therapies are to be realized.
Ms Sharon Bobel is acknowledged for her assistance with the preparation of the figures.
About this article
Beta 1, Beta 2 and Beta 3 Adrenergic Receptor Gene Polymorphisms in a Southeastern European Population
Frontiers in Genetics (2018)