Activity of the uptake-1 norepinephrine transporter as measured by I-123 MIBG in heart failure patients with a loss-of-function polymorphism of the presynaptic alpha2C-adrenergic receptor

F-18 meta-fluorobenzylguanidine PET imaging of myocardial sympathetic innervation

BACKGROUND

I-123 meta-iodobenzylguanidine (MIBG) imaging has long been employed to noninvasively assess the integrity of human norepinephrine transporter-1 and, hence, myocardial sympathetic innervation. Positron-emitting F-18 meta-fluorobenzylguanidine (MFBG) has recently been developed for potentially superior quantitative characterization. We assessed the feasibility of MFBG imaging of myocardial sympathetic innervation.

METHODS

16 patients were imaged with MFBG PET (30-minute dynamic imaging of chest, followed by 3 whole-body acquisitions between 30 minutes and 4-hour post-injection). Blood kinetics were assessed from multiple samples. Pharmacokinetic modeling with reversible 1- and 2-compartment models was performed. Kinetic rate constants were re-calculated from truncated datasets. All patients underwent concurrent MIBG SPECT.

RESULTS

MFBG myocardial uptake was rapid and sustained; the mean standardized uptake value (SUV (mean ± standard deviation)) was 5.1 ± 2.2 and 3.4 ± 1.9 at 1 hour and 3-4-hour post-injection, respectively. The mean K1 and distribution volume (VT) were 1.1 ± 0.6 mL/min/g and 34 ± 22 mL/cm3, respectively. Both were reproducible when re-calculated from truncated 1-hour datasets (Intraclass Correlation Coefficient of 0.99 and 0.91, respectively). Spearman's ϱ = 0.86 between MFBG SUV and VT and 0.80 between MFBG PET-derived VT and MIBG SPECT-derived heart-to-mediastinum activity concentration ratio.

CONCLUSION

MFBG is a promising PET radiotracer for the assessment of myocardial sympathetic innervation.

Adrenergic-pathway gene variants influence beta-blocker-related outcomes after acute coronary syndrome in a race-specific manner

OBJECTIVES

Overcoming racial differences in acute coronary syndrome (ACS) outcomes is a strategic goal for U.S. health care. Genetic polymorphisms in the adrenergic pathway seem to explain some outcome differences by race in other cardiovascular diseases treated with β-adrenergic receptor blockade (BB). Whether these genetic variants are associated with survival among ACS patients treated with BB, and if this differs by race, is unknown.

BACKGROUND

β-adrenergic receptor blockade after ACS is a measure of quality care, but the effectiveness across racial groups is less clear.

METHODS

A prospective cohort of 2,673 ACS patients (2,072 Caucasian; 601 African-American) discharged on BB from 22 U.S. hospitals were followed for 2 years. Subjects were genotyped for polymorphisms in ADRB1, ADRB2, ADRA2C, and GRK5. We used proportional hazards regression to model the effect of genotype on mortality, stratified by race and adjusted for baseline factors.

RESULTS

The overall 2-year mortality rate was 7.5% for Caucasians and 16.7% for African Americans. The prognosis associated with different genotypes in these BB-treated patients differed by race. In Caucasians, ADRA2C 322-325 deletion carriers had significantly lower mortality as compared with homozygous individuals lacking the deletion (hazard ratio: 0.46; confidence interval [CI]: 0.21 to 0.99; p = 0.047; race × genotype interaction p = 0.053). In African Americans, the ADRB2 16R allele was associated with significantly increased mortality (hazard ratio for RG vs. GG: 2.10; CI: 1.14 to 3.86; RR vs. GG: 2.65; CI: 1.38 to 5.08; p = 0.013; race × genotype interaction p = 0.096).

CONCLUSIONS

Adrenergic pathway polymorphisms are associated with mortality in ACS patients receiving BB in a race-specific manner. Understanding the mechanism by which different genes impact post-ACS mortality differently in Caucasians and African Americans might illuminate opportunities to improve BB therapy in these groups.

Genetic variation, β-blockers, and perioperative myocardial infarction

Perioperative myocardial infarction is a common and potentially fatal complication after noncardiac surgery, particular among patients with cardiovascular risk factors. β-blockers have been considered a mainstay in prevention and treatment of perioperative myocardial infarction, yet recent evidence suggests that β-blockers may have an unfavorable risk profile in this setting, and the use has become controversial. What seems conspicuously absent from the current discussion is the appreciation of how much interindividual genetic variation influences the clinical response to β-blocker therapy. Genetic variation in the adrenergic signaling pathway is common, and has a major impact on adrenergic receptor function and β-blocker efficacy in other cardiovascular diseases, such as heart failure and hypertension. Genetic variation in the cytochrome P450 2D6, or CYP2D6, enzyme, which is responsible for the metabolism of most β-blockers, is also important and can lead to poor metabolizing of β-blockers (potential toxicity) or their ultra-rapid degradation (decreased efficacy). Here, we review the molecular, cellular, and physiologic consequences of polymorphisms in the adrenergic signaling pathway and CYP2D6 gene, and show that these are likely relevant factors influencing efficacy, safety, and toxicity of β-blocker therapy in prevention and treatment of perioperative myocardial infarction.

α2C-adrenoceptor modulators: a patent review

INTRODUCTION

α2-Adrenoceptors (α2-ARs) are membrane proteins belonging to the superfamily of GPCRs. Detailed studies have shown three different subtypes, namely α2A, α2B and α2C. Although numerous α2-AR ligands exist, only a small set of compounds have shown even a degree of selectivity among the three α2-AR subtypes. Moreover, these compounds suffer from binding to receptor sites outside the α2-AR subfamily. Efforts made to understand the biological significance of each α2-AR subtype have greatly been assisted by genetically engineered mice. The main results obtained suggest that α2C-AR stimulation may represent a therapeutic strategy to get an analgesic response with reduced sedative effects and undesirable changes in blood pressure due to α2A-AR activation.

AREAS COVERED

This review summarizes the patent literature about the development of α2C-AR modulators from 2000 to early 2010 and their therapeutic effects evoked by the interaction with this receptor subtype.

EXPERT OPINION

Over 90 patents have been deposited in the last 10 years regarding different methods of α2C-AR modulation (use of agonists or antagonists, nucleic acids and polypeptides) for diagnosis, prognosis and treatment of disorders involving this receptor. Nevertheless, despite the numerous published patents, ligands highly selective for the α2C-AR subtype, which continues to be enigmatic, are lacking.

Bucindolol: a pharmacogenomic perspective on its use in chronic heart failure

Bucindolol is a non-selective β-adrenergic receptor blocker with α-1 blocker properties and mild intrinsic sympatholytic activity. The Beta-Blocker Evaluation of Survival Trial (BEST), which is the largest clinical trial of bucindolol in patients with heart failure, was terminated prematurely and failed to show an overall mortality benefit. However, benefits on cardiac mortality and re-hospitalization rates were observed in the BEST trial. Bucindolol has not shown benefits in African Americans, those with significantly low ejection fraction and those in NYHA class IV heart failure. These observations could be due to the exaggerated sympatholytic response to bucindolol in these sub-groups that may be mediated by genetic polymorphisms or changes in gene regulation due to advanced heart failure. This paper provides a timely clinical update on the use of bucindolol in chronic heart failure.

An α 2C -Adrenergic Receptor Polymorphism Alters the Norepinephrine-Lowering Effects and Therapeutic Response of the β-Blocker Bucindolol in Chronic Heart Failure

Background— Adrenergic activation is an important determinant of outcomes in chronic heart failure. Adrenergic activity is regulated in part by prejunctional α 2C -adrenergic receptors (ARs), which exhibit genetic variation in humans. Bucindolol is a novel β-AR blocking agent that also lowers systemic norepinephrine and thus is also a sympatholytic agent. This study investigated whether α 2C -AR polymorphisms affect sympatholytic effects of bucindolol in patients with heart failure.

Methods and Results— In the β-Blocker Evaluation of Survival Trial, adrenergic activation was estimated by systemic venous norepinephrine measured at baseline, 3 months, and 12 months posttreatment in patients treated with placebo or bucindolol. In the β-Blocker Evaluation of Survival Trial AR polymorphism substudy, DNA was collected from 1040 of the 2708 randomized patients, and α 2C -AR gene polymorphisms (α 2C Del322-325 or the wild-type counterpart) were measured by polymerase chain reaction and gel electrophoresis. Patients who were α 2C Del carriers (heterozygotes or homozygotes) exhibited a much greater sympatholytic response to bucindolol (decrease in norepinephrine at 3 months of 153±57 pg/mL, P =0.012 compared with placebo versus decrease of 50±13 pg/mL in α 2C wild type, P =0.0005 versus placebo; P =0.010 by interaction test). α 2C Del carriers had no evidence of a favorable survival benefit from bucindolol (mortality compared with placebo hazard ratio, 1.09; 95% CI, 0.57 to 2.08; P =0.80), whereas bucindolol-treated subjects who were wild type for the α 2C -AR had a 30% reduction in mortality (hazard ratio, 0.70; 95% CI, 0.51 to 0.96; P =0.025).

Conclusions— In the β-Blocker Evaluation of Survival Trial AR polymorphism substudy, the norepinephrine lowering and clinical therapeutic responses to bucindolol were strongly influenced by α 2C receptor genotype.

Mechanisms of pharmacogenomic effects of genetic variation within the cardiac adrenergic network in heart failure

One of the goals of pharmacogenomics is the use of genetic variants to predict an individual's response to treatment. Although numerous candidate and genome-wide associations have been made for cardiovascular response-outcomes, little is known about how a given polymorphism imposes the phenotype. Such mechanisms are important, because they tie the observed human response to specific signaling alterations and thus provide cause-and-effect relationships, aid in the design of hypothesis-based clinical studies, can help to devise workaround drugs, and can reveal new aspects of the pathophysiology of the disease. Here we discuss polymorphisms within the adrenergic receptor network in the context of heart failure and beta-adrenergic receptor blocker therapy, where multiple approaches to understand the mechanism have been undertaken. We propose a comprehensive series of studies, ranging from transfected cells, transgenic mice, and ex vivo and in vitro human studies as a model approach to explore mechanisms of action of pharmacogenomic effects and extend the field beyond observational associations.

Alpha-adrenoceptor gene variants and autonomic nervous system function in a young healthy Japanese population

alpha(1A)-adrenergic receptor (alpha(1A)-AR) regulates the cardiac and peripheral vascular system through sympathetic activation, and alpha(2A)-AR and alpha(2C)-AR subtypes are essential for presynaptic feedback regulation of catecholamine release from the central and peripheral sympathetic nerve. Genetic variations in each human alpha-AR subtype gene have been identified and have been implicated in hypertension and cardiovascular disease. It is not yet clear whether these genetic variations actually have an effect on sympatho-vagal modulation. The aim of the present study was to evaluate the relation between the five representative genetic polymorphisms of alpha-AR subtypes (Arg347Cys of alpha(1A)-AR; C-1291G, Asn251Lys, and DraI RFLP of alpha(2A)-AR; and Del322-325 of alpha(2C)-AR) and autonomic nervous system (ANS) function in young and healthy Japanese males. One hundred forty-nine subjects were genotyped for each alpha-AR polymorphism, and underwent evaluation of ANS function by power spectral analysis of heart rate variability (HRV) during supine rest and in a standing position. In a supine position, the alpha(1A)-AR 347Cys allele was significantly associated with lower HRV sympathetic index (normalized low frequency power [LF(%)] and LF:HF ratio) and higher HRV parasympathetic index [HF(%)]. Meanwhile, subjects with the alpha(2C)-AR Del322-325 allele had markedly higher LF(%) and LF:HF ratio and lower HF(%) than noncarriers. Thus, the alpha(1A)-AR and alpha(2C)-AR genetic variations influence sympatho-vagal balance even in young and healthy normotensive states, which could be postulated to constitute an intermediate phenotype for future pathological episodes of various ANS dysfunction-related diseases.

Role of β1- and α2c-adrenergic receptor polymorphisms and their combination in heart failure: A case-control study

BACKGROUND

Adrenergic activation has a central role in the development of HF. The function of the beta1- and the alpha2C-adrenergic receptors is influenced by gene polymorphisms: the beta1Arg389 variant is associated with increased beta1-receptor sensitivity and the alpha2C-receptor Del322-325 variant is associated with decreased alpha2C receptor function and increased norepinephrine release. We hypothesised that these polymorphisms could influence the prevalence of heart failure.

METHODS

The role of the beta1- and alpha2C-adrenergic receptor gene polymorphisms as risk factors for heart failure (HF) was assessed in an Italian white Caucasian population using a case-control study design. Genomic DNA was analysed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RLFP).

RESULTS

We compared 260 Caucasian patients with HF and 230 normal subjects. The beta1Arg389 allele was frequent both in the patients with HF (69%) and in the normal subjects (73%). The alpha2CDel322-325 variant was rare in both groups (9% and 8%, respectively). Patients homozygotes for either the beta1Arg389 or the alpha(2C)Del322-325 alleles had no increased risk of HF (odds ratio [OR], 0.8; 95%CI: 0.5-1.2 and OR, 0.8; 95% CI: 0.4-1.8, respectively). Patients homozygotes for both the beta1Arg389 and the alpha(2C)Del322-325 alleles had no increased risk of HF as well (OR: 0.6; 95% CI: 0.2-2.1).

CONCLUSIONS

Beta1-ARs and alpha2C-ARs polymorphisms are not associated with an increased risk of HF in an Italian white Caucasian population.

Sympathoneural and adrenomedullary functional effects of ??2C-adrenoreceptor gene polymorphism in healthy humans

OBJECTIVES

alpha2-Adrenoreceptors restrain sympathetic nervous outflows and inhibit release of noradrenaline from sympathetic nerves. In-frame deletion of the alpha2C-adrenoreceptor subtype (alpha2CDel322-325) increases the risk of congestive heart failure. Increased delivery of catecholamines to cardiovascular receptors might explain this increased risk.

METHODS

Twenty-nine healthy African-Americans genotyped for alpha2-adrenoreceptor subtype polymorphisms underwent 3H-noradrenaline and 3H-adrenaline intravenous infusion and arterial blood sampling for measurements of rates of entry of endogenous noradrenaline and adrenaline into arterial plasma (total body spillovers) by the tracer dilution technique. Eleven subjects were homozygotes for the alpha2CDel322-325 polymorphism, nine heterozygotes, and nine non-carriers. Subjects were studied during supine rest and during and after i.v. infusion of the alpha2-adrenoreceptor antagonist, yohimbine.

RESULTS

At rest, homozygotes for the alpha2CDel322-325 polymorphism had higher total body noradrenaline spillover than did heterozygotes (t=2.90, df=18, P=0.023) or non-carriers (t=3.22, df=18, P=0.010). Adrenaline spillover was higher in homozygotes than non-carriers (t=2.61, df=18, P=0.045). Administration of yohimbine produced larger, more sustained increments in noradrenaline spillover, heart rate, and anxiety in homozygotes than in the other groups.

CONCLUSION

In healthy people, alpha2CDel322-325 polymorphism is associated with increased sympathetic nervous and adrenomedullary hormonal activities, both during supine rest and during pharmacologically evoked catecholamine release. Polymorphisms of the alpha2C-adrenoreceptor may help explain individual differences in predisposition to a variety of disorders of catecholaminergic function, such as cardiovascular disorders, depression or anxiety disorders.

The human gene map for performance and health-related fitness phenotypes: the 2003 update

This review presents the 2003 update of the human gene map for physical performance and health-related fitness phenotypes. It is based on peer-reviewed papers published by the end of 2003 and includes association studies with candidate genes, genome-wide scans with polymorphic markers, and single-gene defects causing exercise intolerance to variable degrees. The genes and markers with evidence of association or linkage with a performance or fitness phenotype in sedentary or active people, in adaptation to acute exercise, or for training-induced changes are positioned on the genetic map of all autosomes and the X chromosome. Negative studies are reviewed but a gene or locus must be supported by at least one positive study before being inserted on the map. By the end of 2000, 29 loci were depicted on the first edition of the map. In contrast, the 2003 human gene map for physical performance and health-related phenotypes includes 109 autosomal gene entries and QTL, plus two on the X chromosome. Moreover, there are 15 mitochondrial genes in which sequence variants have been shown to influence relevant fitness and performance phenotypes.

Polymorphisms of cardiac presynaptic alpha2C adrenergic receptors: Diverse intragenic variability with haplotype-specific functional effects

The presynaptic alpha2C adrenergic receptors (AR) act to inhibit norepinephrine release in cardiac and other presynaptic nerves. We have recently shown that a genetic variant in the alpha2CAR coding region (Del322-325), which renders the receptor partially uncoupled from Gi, is a risk factor for heart failure. However, variability of heart failure phenotypes and a dominance of Del322-325 in those of African descent led us to hypothesize that other regions of this gene have functional polymorphisms. In a multiethnic population, we found 20 polymorphisms within 4,625 bp of contiguous sequence of this intronless gene encompassing the promoter, 5' UTR, coding, and 3' UTR. These polymorphisms occur in 24 distinct haplotypes with complex organizations, including multiple 5'-upstream polymorphisms in regions known to direct expression, a 3' UTR substitution polymorphism within an insertion/deletion sequence, and the radical coding polymorphism that deletes four amino acids. Relatively low linkage disequilibrium between many polymorphisms, few cosmopolitan haplotypes, prevalent ethnic-specific haplotypes, and substantial genetic divergence among haplotypes was noted. The dysfunctional Del322-325 allele was partitioned into multiple haplotypes, with frequencies of 48% to 2%. The functional implications of the haplotypes were ascertained by whole-gene transfections of human neuronal cells, where haplotype was significantly related (P < 0.001) to expression levels of receptor transcript and protein. Expression varied by as much as approximately 50% by haplotype, and such studies enabled haplotype clustering by phenotypic, rather than genotypic, similarities. Thus, depending on phenotype, expression-specific haplotypes may amplify, attenuate, or dominate the cardiomyopathic effect attributed to the alpha2CDel322-325 marker.