Clinical importance of beta-adrenoceptor polymorphisms in cardiovascular disease

Role of risk stratification and genetics in sudden cardiac death

Sudden cardiac death (SCD) is a major public health issue due to its increasing incidence in the general population and the difficulty in identifying high-risk individuals. Nearly 300 000 - 350 000 patients in the United States and 4-5 million patients in the world die annually from SCD. Coronary artery disease and advanced heart failure are the main etiology for SCD. Ischemia of any cause precipitates lethal arrhythmias, and ventricular tachycardia and ventricular fibrillation are the most common lethal arrhythmias precipitating SCD. Pulseless electrical activity, bradyarrhythmia, and electromechanical dissociation also result in SCD. Most SCDs occur outside of the hospital setting, so it is difficult to estimate the public burden, which results in overestimating the incidence of SCD. The insufficiency and limited predictive value of various indicators and criteria for SCD result in the increasing incidence. As a result, there is a need to develop better risk stratification criteria and find modifiable variables to decrease the incidence. Primary and secondary prevention and treatment of SCD need further research. This critical review is focused on the etiology, risk factors, prognostic factors, and importance of risk stratification of SCD.

Adrenoreceptors and nitric oxide in the cardiovascular system

Nitric Oxide (NO) is a small molecule that continues to attract much attention from the scientific community. Since its discovery, it has been evident that NO has a crucial role in the modulation of vascular tone. Moreover, NO is involved in multiple signal transduction pathways thus contributing to the regulation of many cellular functions. NO effects can be either dependent or independent on cGMP, and rely also upon several mechanisms such as the amount of NO, the compartmentalization of the enzymes responsible for its biosynthesis (NOS), and the local redox conditions. Several evidences highlighted the correlation among adrenoreceptors activity, vascular redox status and NO bioavailability. It was suggested a possible crosstalk between NO and oxidative stress hallmarks in the endothelium function and adaptation, and in sympathetic vasoconstriction control. Adrenergic vasoconstriction is a balance between a direct vasoconstrictive effect on smooth muscle and an indirect vasorelaxant action caused by α₂- and β-adrenergic endothelial receptor-triggered NO release. An increased oxidative stress and a reduction of NO bioavailability shifts this equilibrium causing the enhanced vascular adrenergic responsiveness observed in hypertension. The activity of NOS contributes to manage the adrenergic pathway, thus supporting the idea that the endothelium might control or facilitate β-adrenergic effects on the vessels and the polymorphic variants in β₂-receptors and NOS isoforms could influence aging, some pathological conditions and individual responses to drugs. This seems to be dependent, almost in part, on differences in the control of vascular tone exerted by NO. Given its involvement in such important mechanisms, the NO pathway is implicated in aging process and in both cardiovascular and non-cardiovascular conditions. Thus, it is essential to pinpoint NO involvement in the regulation of vascular tone for the effective clinical/therapeutic management of cardiovascular diseases (CVD).

Targeting device therapy: genomics of sudden death

This article discusses the development and use of genomic predictors to define the population at risk for sudden cardiac death, which is usually defined as death from cardiac causes within an hour of symptom onset. The identification of genetic predictors of sudden death in heart failure is in its earliest stages. Mutations in ion channels have been shown to cause inherited forms of sudden death; there is, however, little evidence that mutations or rare single nucleotide polymorphisms (SNPs) in those genes are important causes of the common forms of sudden death. Other common variants in ion channels and related genes are associated with sudden death in the setting of acute myocardial infarction or heart failure. Ultimately, we hope to identify a handful of SNPs that modulate the risk of sudden death in heart failure and to develop an algorithm to predict risk based on genotype.

Prevalence of beta-2 adrenergic receptor (beta 2 AR) polymorphisms and its influence on a model used to assess endothelial function using pulse wave analysis (PWA)

BACKGROUND

Pulse wave analysis (PWA) combined with beta(2)-agonist challenge has recently been used to assess endothelial function. beta-2 adrenergic receptor (beta(2)AR) polymorphisms may affect response to beta(2)-agonist. We determined whether beta(2)AR polymorphisms influence endothelial response in our model using PWA and salbutamol.

METHODS

388 healthy Malay subjects (177 males, 211 females) were genotyped for 5 functionally important single nucleotide polymorphisms (SNPs) of beta(2)AR; 298 subjects proceeded with assessment of endothelial function. The parameter augmentation index (AIx) was recorded non-invasively using SphygmoCor. Recording of AIx at baseline was followed by administration of 500 microg sublingual glyceryl trinitrate (GTN). AIx recordings were repeated at 3, 5, 10, 15 and 20 min post-GTN. Subjects then inhaled 400 microg of salbutamol before AIx recordings at 5 min intervals up to 20 min. Maximum changes in AIx after GTN and salbutamol represented endothelium independent and endothelium dependent vasodilatation (EDV) respectively.

RESULTS

Allele frequencies of mutated Gly16, Glu27, Ile164, -20C and -47C were 47%, 6.8%, 0%, 30% and 9.3% respectively. No significant differences in EDV were noted between genotype groups of each studied SNPs.

CONCLUSIONS

Assessment of endothelial function using PWA and salbutamol was not influenced by beta(2)AR polymorphisms.

Gene Variant of the Bradykinin B2 Receptor Influences Pulmonary Arterial Pressures in Heart Failure Patients

Background

Pulmonary arterial pressure (PAP) varies considerably in heart failure (HF) despite similar degrees of left ventricular (LV) dysfunction. Bradykinin alters vascular tone and common variations in the kinin B2 receptor (BDKRB2) gene exists. We hypothesized that genetic variation in this receptor would influence PAP in HF.

Methods

131 HF patients (>1yr history systolic HF), without COPD, not currently smoking, BMI < 40, without atrial fibrillation completed the study which included a blood draw for genotyping and neurohormones (ACE, A-II, Bradykinin, ANP, BNP, and catecholamines), an echocardiogram for cardiac function and systolic PAP (PAPsys).

Results

Mean LVEF was 29% ∓ 12%, NYHA class 2 ∓ 1, age 56 ∓ 12 yr, BMI 28 ∓ 5 kg/m2. Forty-six patients (35%) were homozygous for the +9 allele, 58 (44%) were heterozygous (+9/-9) and 27 (21%) were homozygous for the -9 allele of the BDKRB2. PAPsys averaged 42 ∓ 13, 38 ∓ 12, and 35 ∓ 11 mmHg for +9/+9, +9/-9 and -9/-9, respectively (p = 0.03). There was a trend towards gene effect for plasma ACE with the highest values in +9/+9 and lowest in -9/-9 patients (9.5 ∓ 10.7, 7.1 ∓ 8.7, and 5.4 ∓ 6.4 U/L, respectively, p = 0.06). There were no differences in plasma bradykinin or A-II, LVEF, or NYHA across genotypes.

Conclusion

These data suggest the +9/+9 polymorphism of the BDKRB2 receptor influences pulmonary vascular tone in stable HF.

GENETIC VARIATION IN THE β(2)-ADRENERGIC RECEPTOR: IMPACT ON INTERMEDIATE CARDIOVASCULAR PHENOTYPES

Genetic variation in drug targets (e.g. receptors) can have pronounced effects on clinical responses to endogenous and exogenous agonists. Polymorphisms in the gene encoding the β(2)-adrenergic receptor (β(2)-AR) have been associated with altered expression, down-regulation, and altered cell signaling in vitro. Because β(2)-ARs play a crucial role in the regulation of the cardiovascular system, the functional importance of genetic variation in the β(2)-AR on cardiovascular responses to physiological or pharmacological stimuli has gained widespread attention. The objective of this review is to characterize these intermediate cardiovascular phenotypes and their influence on cardiovascular disease and adrenergic drug responses.Two common single nucleotide polymorphisms, encoded at codon 46 (Gly(16)Arg) and 79 (Gln(27)Glu) of the β(2)-AR gene, have been studied intensively. They have been shown to be associated with altered vasodilator responses to regional and systemic administration of β(2)-agonists, altered cardiovascular responses to sympathoexcitatory maneuvers, and altered myocardial function. Importantly, these intermediate physiological patterns may influence the development of and the outcomes associated with hypertension and other cardiovascular diseases. As recently reported, β(2)-AR gene variation can risk-stratify patients receiving β-blocker therapy and may predict β-blocker efficacy in patients post acute coronary syndrome or in patients with heart failure.Further studies will advance our understanding of the link between β(2)-AR genotypes, intermediate cardiovascular phenotypes, and clinical phenotypes. In the long term, reassessment of the benefits of β-blocker-therapy within genotype groups should be carried out with the ultimate goal to design the optimal therapeutic regimen for the individual patient.

Sudden Cardiac Death: Epidemiology, Mechanisms, and Therapy

Sudden cardiac death is a major public health problem affecting 500,000 patients annually in the United States alone. The major risk factor for sudden cardiac death is the presence of coronary artery disease, usually in the setting of reduced ejection fraction. Globally, the incidence is expected to rise sharply as the prevalence of coronary artery disease and heart failure continue to increase. However, sudden cardiac death is a heterogeneous condition and may be caused by acute ischemia, structural defects, myocardial scar, and/or genetic mutations. Sudden death may occur even in a grossly normal heart. Beta-blockers can reduce the risk of sudden cardiac death, while implantable cardioverter defibrillators are effective at terminating malignant arrhythmias. Ejection fraction remains the major criterion to stratify patients for defibrillator implantation but this strategy alone is insensitive and nonspecific. Novel clinical, electrophysiologic, and genetic markers have been identified that may increase precision in patient selection for primary prevention therapy. This review discusses the epidemiology, mechanisms, etiologies, therapies, treatment guidelines, and future directions in the management of sudden cardiac death.

Heart Failure Management in African Americans: Meeting the Challenge

Heart failure affects 3% of African Americans. The etiology of disease and prognosis for these patients differs substantially from those for non-African Americans. A history of hypertension is associated with development of heart failure more often in African Americans than in non-African Americans and it also appears that target organ involvement is more severe in African Americans with hypertension than in other patient subgroups. Reviewing the results from large-scale clinical end point studies suggests that optimal treatment for heart failure in African Americans may differ from that of their non-African American counterparts. More importantly, concomitant use of beta blockers and angiotensin-converting enzyme inhibitors may be as effective in African Americans as in non-African Americans. Utilizing angiotensin-converting enzyme inhibitors alone may not represent ideal therapy. Of the drugs studied, especially among the beta blockers, carvedilol may be the most effective to use for this population.

Equine sweating and anhidrosis Part 2: anhidrosis

The condition of anhidrosis is described in this review, and the latest theories on the causal factors are explored. The evidence supports the hypothesis that anhidrosis is an inappropriate response to prolonged climatic stress (generally combined heat and high humidity), which can be evoked in a small (approximately 10 +/- 5%) proportion of the equine population. It is caused by gradual failure of the glandular secretory cell processes, initiated by desensitization and subsequent down-regulation of the cell receptors as a result of continued adrenaline-driven hyperactivity. It progresses through secretory failure and culminates in gradual, probably irreversible, glandular dedifferentiation and ultimate degeneration. There is a need for considerably more research on the secretory and transcriptional processes to document the changes arising within the glandular secretory mechanism as a prelude to development of a corrective treatment.

The catecholamine-beta-adrenoreceptor-cAMP system and prediction of cardiovascular events in hypertension

Although the importance of elevated circulating plasma catecholamines on cardiac structural and functional remodelling of hypertension is well documented, it is unclear whether the catecholamine-beta-adrenoreceptor (beta AR)-cAMP system can predict different cardiovascular events. 2. A total of 601 identified hypertensive patients with baseline and follow-up plasma levels of noradrenaline (NA) and adrenaline (Adr), lymphocyte beta AR density (B(max)) and intra-lymphocyte cAMP levels in peripheral blood (last examination 60+/-26 months apart) were followed up for an additional 24+/-12 months. 3. After the last follow up, a composite end-point of cardiovascular death, non-fatal myocardial infarction (MI) and stroke occurred in 139 patients (23.1%). In Cox analyses, adjusting for other standard factors as well as treatment effect, NA (hazard ratio 1.22; 95% confidence interval (CI) 1.17-1.28; P=0.0008), Adr (hazard ratio 1.53; 95% CI 1.18-2.00; P=0.002), beta AR (hazard ratio 1.12; 95% CI 1.06-1.17; P=0.007) and cAMP (hazard ratio 1.15; 95% CI 1.09-1.21; P=0.005) separately predicted cardiovascular mortality. Noradrenaline, Adr, beta AR and intra-lymphocyte cAMP separately predicted fatal/non-fatal MI; NA and Adr predicted fatal/non-fatal stroke, whereas B(max) and intra-lymphocyte cAMP levels were not a significant predictor of fatal/non-fatal stroke. When stratifying the study population by NA or Adr (median 4 nmol/L), B(max) (median 600 fmol/10(7) cells) and cAMP (median 5.0 pmol/mg protein) above and below the median values in both parameters categories, patients above the median had composite cardiovascular end-point (all P<0.001) and high cardiovascular death (all P<0.01, log-rank test). 4. These results suggest that plasma NA and Adr are significant predictors of cardiovascular mortality, MI and stroke. The B(max) and intra-lymphocyte cAMP levels are significant predictors of cardiovascular mortality and MI, but not stroke.

Genetic variations related to hypertension: a review

Hypertension is a complex multifactorial disorder with genetic, environmental and demographic factors contributing to its prevalence. The genetic element contribution to blood pressure variation ranges from 30 to 50%. Therefore, identifying hypertension susceptibility genes will help understanding the pathophysiology of the disease. In addition to the potential impact of genomic information in selecting antihypertensive drug therapy, it may also help in recognizing those at risk of developing the disease, which may lead to new preventive approaches. Several strategies and methods have been used to identify hypertension susceptibility genes. Currently, genetic analysis of such data produced complex results, which makes it difficult to draw final conclusion on the use of genomic data in management of hypertension. This review attempts to summarize present known genetic variations that may be implicated in the pathogenesis of hypertension and to discuss various research strategies used to identify them. It also highlights some of the opportunities and challenges, which may be encountered in interpreting the value of these genetic variations to improve management of hypertension.

Pharmacogenomics in cardiovascular clinical trials

Genomics - having quickly emerged as the central discipline in basic science and biomedical research - is poised to take the center stage in clinical medicine as well over the next few decades. Although there is no specific regulatory guideline on the application of pharmacogenetics to drug development, some recommendations are already included in several published guidelines on drug development. The patients more likely to provide the most valuable information on the specific contribution of a given gene or its variant are those who fail to respond to a drug ('therapeutic failures') and those who develop toxicity to the drug. However, before drawing definite conclusions on subgroups following pharmacogenomic analyses, one must be aware of disease classification, data collection, and how much is known about the disease process. It seems reasonable to collect genomic DNA from all patients enrolled in clinical drug trials (along with appropriate consent to permit pharmacogenetic studies) for the purpose of post hoc analyses. One exception to post hoc genomic analysis is when patients with a specific genotype are excluded from randomization into a clinical trial. Physicians will need to understand the concept of genetic variability, its interactions with the environment (e.g. drug-drug or drug-disease interactions), and its implication for patient care.

β2-adrenoceptor polymorphisms: Relation between in vitro and in vivo phenotypes

Beta2-adrenoceptors are expressed in many cell types throughout the body and play a pivotal role in the regulation of the cardiac, pulmonary, vascular, endocrine and central nervous system. Recent studies have discovered that Beta2-adrenoceptor are polymorphic. Three common polymorphisms appear to influence receptor function: Arg16Gly, Gln27Glu, and Thr164Ile. In vitro studies of agonist-stimulation have shown that the Gly16 Beta2-adrenoceptors demonstrate enhanced agonist-promoted down-regulation, while Glu27 variants seem to be resistant. The Ile164 variant, on the other hand, demonstrates decreased responsiveness to agonist stimulation in vitro. However, the functional relevance and phenotypic consequence of such Beta2-adrenoceptor variants in vivo is still unclear. The aim of this review is therefore to provide an overview about the somewhat controversy in vitro, ex vivo and in vivo studies.