The effect of the beta(2) adrenoceptor gene Thr164Ile polymorphism on human adipose tissue lipolytic function

Genetic Variants of Adrenoceptors

Adrenoceptors are class A G-protein-coupled receptors grouped into three families (α1-, α2-, and β-adrenoceptors), each one including three members. All nine corresponding adrenoceptor genes display genetic variation in their coding and adjacent non-coding genomic region. Coding variants, i.e., nucleotide exchanges within the transcribed and translated receptor sequence, may result in a difference in amino acid sequence thus altering receptor function and signaling. Such variants have been intensely studied in vitro in overexpression systems and addressed in candidate-gene studies for distinct clinical parameters. In recent years, large cohorts were analyzed in genome-wide association studies (GWAS), where variants are detected as significant in context with specific traits. These studies identified two of the in-depth characterized 18 coding variants in adrenoceptors as repeatedly statistically significant genetic risk factors - p.Arg389Gly in the β1- and p.Thr164Ile in the β2-adrenoceptor, along with 56 variants in the non-coding regions adjacent to the adrenoceptor gene loci, the functional role of which is largely unknown at present. This chapter summarizes current knowledge on the two coding variants in adrenoceptors that have been consistently validated in GWAS and provides a prospective overview on the numerous non-coding variants more recently attributed to adrenoceptor gene loci.

Loss of a water-mediated network results in reduced agonist affinity in a β2-adrenergic receptor clinical variant

The β₂-adrenergic receptor (β₂AR) is a member of the G protein-coupled receptor (GPCR) family that is an important drug target for asthma and COPD. Clinical studies coupled with biochemical data have identified a critical receptor variant, Thr164Ile, to have a reduced response to agonist-based therapy, although the molecular mechanism underlying this seemingly "non-deleterious" substitution is not clear. Here, we couple molecular dynamics simulations with network analysis and free-energy calculations to identify the molecular determinants underlying the differential drug response. We are able to identify hydration sites in the transmembrane domain that are essential to maintain the integrity of the binding site but are absent in the variant. The loss of these hydration sites in the variant correlates with perturbations in the intra-protein interaction network and rearrangements in the orthosteric ligand binding site. In conjunction, we observe an altered binding and reduced free energy of a series of agonists, in line with experimental trends. Our work identifies a functional allosteric pathway connected by specific hydration sites in β₂AR that has not been reported before and provides insight into water-mediated networks in GPCRs in general. Overall, the work is one of the first step towards developing variant-specific potent and selective agonists.

Multicomponent Training Changes Blood Pressure and Redox Status in Older Women: Influence of β2 Adrenergic Receptor Haplotypes

The β2 adrenergic receptor (β2-AR) plays an important role in vascular smooth muscle. However, the interaction between Arg16Gly and Gln27Glu β2-AR polymorphisms, and exercise training has not yet been established. This study evaluated the influence of these β2-AR polymorphisms on body composition, aerobic capacity, blood pressure, lipid profile, nitric oxide, and redox status at baseline and in response to an exercise program in women aged 50-79 years. Genotype and haplotypes were analyzed in association with the previously mentioned variables before and after the multicomponent training (12 weeks, 2 sessions/week, 90 min/session, and exercise intensity between 13 and 15 on the Borg scale). Individuals who carry β2-AR Arg16Arg/Gln27Gln genotypes presented more improvements in blood pressure, body composition, aerobic capacity, and redox status in response to a multicomponent training program compared with individuals who do not carry this haplotype. In some years, the genetic profile may be used to predict which exercise program can induce more health benefits for each person.

Regulation of adipocyte lipolysis

In adipocytes the hydrolysis of TAG to produce fatty acids and glycerol under fasting conditions or times of elevated energy demands is tightly regulated by neuroendocrine signals, resulting in the activation of lipolytic enzymes. Among the classic regulators of lipolysis, adrenergic stimulation and the insulin-mediated control of lipid mobilisation are the best known. Initially, hormone-sensitive lipase (HSL) was thought to be the rate-limiting enzyme of the first lipolytic step, while we now know that adipocyte TAG lipase is the key enzyme for lipolysis initiation. Pivotal, previously unsuspected components have also been identified at the protective interface of the lipid droplet surface and in the signalling pathways that control lipolysis. Perilipin, comparative gene identification-58 (CGI-58) and other proteins of the lipid droplet surface are currently known to be key regulators of the lipolytic machinery, protecting or exposing the TAG core of the droplet to lipases. The neuroendocrine control of lipolysis is prototypically exerted by catecholaminergic stimulation and insulin-induced suppression, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and perilipin. Interestingly, in recent decades adipose tissue has been shown to secrete a large number of adipokines, which exert direct effects on lipolysis, while adipocytes reportedly express a wide range of receptors for signals involved in lipid mobilisation. Recently recognised mediators of lipolysis include some adipokines, structural membrane proteins, atrial natriuretic peptides, AMP-activated protein kinase and mitogen-activated protein kinase. Lipolysis needs to be reanalysed from the broader perspective of its specific physiological or pathological context since basal or stimulated lipolytic rates occur under diverse conditions and by different mechanisms.

The Human Obesity Gene Map: The 2003 Update

This is the tenth update of the human obesity gene map, incorporating published results up to the end of October 2003 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. Transgenic and knockout murine models relevant to obesity are also incorporated (N = 55). As of October 2003, 41 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. QTLs reported from animal models currently number 183. There are 208 human QTLs for obesity phenotypes from genome-wide scans and candidate regions in targeted studies. A total of 35 genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 272 studies reporting positive associations with 90 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, more than 430 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.

The Human Obesity Gene Map: The 2004 Update

This paper presents the eleventh update of the human obesity gene map, which incorporates published results up to the end of October 2004. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTLs) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2004, 173 human obesity cases due to single-gene mutations in 10 different genes have been reported, and 49 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 166 genes which, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 221. The number of human obesity QTLs derived from genome scans continues to grow, and we have now 204 QTLs for obesity-related phenotypes from 50 genome-wide scans. A total of 38 genomic regions harbor QTLs replicated among two to four studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably with 358 findings of positive associations with 113 candidate genes. Among them, 18 genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. Overall, >600 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful publications and genomic and other relevant sites can be found at http://obesitygene.pbrc.edu.

Pharmacogenomics of β-adrenergic receptor physiology and response to β-blockade

Myocardial β-adrenergic receptors (βARs) are important in altering heart rate, inotropic state, and myocardial relaxation (lusitropy). The β1AR and β2AR stimulation increases cyclic adenosine monophosphate concentration with the net result of myocyte contraction, whereas β3AR stimulation results in decreased inotropy. Downregulation of β1ARs in heart failure, as well as an increased β3AR activity and density, lead to decreased cyclic adenosine monophosphate production and reduced inotropy. The βAR antagonists are commonly used in patients with coronary artery disease and heart failure; however, perioperative use of βAR antagonists is controversial. Individual patient's response to beta-blocker therapy is an area of intensive research, and apart from pharmacokinetics, pharmacodynamics, and ethnic differences, genetic alterations have become more important in the last 20 years. The most common genetic variants in humans are single nucleotide polymorphisms (SNPs). There are 2 clinically relevant SNPs for the β1AR (Ser49Gly, Arg389Gly), 3 for the β2AR (Arg16Gly, Gln27Glu, Thr164Ile), and 1 for the β3AR (Trp64Arg). Although results are somewhat controversial, generally large datasets have the potential to show a relationship between βAR SNPs and outcomes such as development and progression of heart failure, coronary artery disease, vascular reactivity, hypertension, asthma, obesity, and diabetes. Although βAR SNPs may not directly cause disease, they appear to be risk factors for, and modifiers of, disease and the response to stress and drugs. In the perioperative setting, this has specifically been demonstrated for the Arg389Gly β1AR polymorphism with which patients with the Gly variant had a higher incidence of adverse perioperative events. Knowing that genetic variants play an important role, perioperative medicine will likely change from simple therapeutic intervention to a more personalized way of adrenergic receptor modulation.

{beta}-Adrenoceptor gene variation and intermediate physiological traits: prediction of distant phenotype

Intermediate physiological phenotype is the genetic and environmental influence on functional physiological characteristics with direct prognostic relevance to distant, more complex phenotypes, such as cardiovascular and metabolic disease. Increasingly available and affordable genotyping techniques have created an explosion of information on candidate gene variation and its relationship to intermediate physiological traits. Variation in beta-adrenoceptor genes is an intense focus of investigation because beta-adrenoceptors are: (1) ubiquitous in organ system distribution; (2) integral to a multitude of physiological processes; (3) well described in cardiovascular and metabolic disease; and (4) major pharmacological treatment targets. Furthermore, knowledge of functional gene variants in these receptors predates the description of the human genome. This review highlights the influence of common gene variation in the three beta-adrenoceptor subtypes on intermediate physiological phenotype predictive of cardiovascular disease and obesity. Although further information is needed to replicate this information across populations, this review condenses and summarizes growing trends in specific pleiotropic effects of beta-adrenoceptor polymorphisms and suggests which variants may be predictive of distant phenotype.

Beta-1 and beta-2 adrenoceptor polymorphisms: functional importance, impact on cardiovascular diseases and drug responses

Beta-1 and beta-2 adrenoceptors (AR) play a pivotal role in regulation of the activity of the sympathetic nervous system and agonists and antagonists at both beta AR subtypes are frequently used in treatment of cardiovascular diseases. Both beta-1 and beta-2 AR genes have several polymorphisms that encode different amino acids. This review summarizes new insights into the functional importance of these polymorphisms, as well as their relationship to cardiovascular diseases and their impact on responses to adrenergic drug treatment. At present, it seems that, for cardiovascular diseases, beta-1 and beta-2 AR polymorphisms do not play a role as disease-causing genes; they might, however, be associated with disease-related phenotypes. In addition they could influence adrenergic drug responses. Thus, the Arg389Gly beta-1 AR polymorphism might predict responsiveness to beta-1 AR agonist and blocker treatment: patients homozygous for the Arg389 beta-1 AR polymorphism should be good responders, while patients homozygous for the Gly389 beta-1 AR polymorphism should be poor or nonresponders. Furthermore, the Arg16Gln27 beta-2 AR seems to have strong impact on long-term agonist-induced beta-2 AR desensitization. Thus, patients carrying this haplotype appear to suffer from rapid loss of therapeutic efficacy of chronic agonist treatment, as has been demonstrated in asthma patients. Moreover, the Arg16Gln27 beta-2 AR haplotype might have some predictive value for poor outcome of heart failure. Future large prospective studies have to replicate these findings in order to reach the final goal of pharmacogenomic research: to optimize and individualize drug therapy based on the patient's genetic determinants of drug efficacy.

Association of a beta-2 adrenoceptor (ADRB2) gene variant with a blunted in vivo lipolysis and fat oxidation

BACKGROUND AND AIMS

Obesity is associated with a blunted beta-adrenoceptor-mediated lipolysis and fat oxidation. We investigated whether polymorphisms in codon 16, 27 and 164 of the beta (2)-adrenoceptor gene (ADRB2) and exon 10 of the G protein beta (3)-subunit gene (GNB3) are associated with alterations in in vivo lipolysis and fat oxidation.

DESIGN AND METHODS

Sixty-five male and 43 female overweight and obese subjects (body mass index (BMI) range: 26.1-48.4 kg/m(2)) were included. Energy expenditure (EE), respiratory quotient (RQ), circulating free fatty acid (FFA) and glycerol levels were determined after stepwise infusion of increasing doses of the non-selective beta-agonist isoprenaline (ISO).

RESULTS

In women, the Arg16 allele of the ADRB2 gene was associated with a blunted increase in circulating FFA, glycerol and a decreased fat oxidation during ISO stimulation. In men, the Arg16 allele was significantly associated with a blunted increase in FFA but not in glycerol or fat oxidation.

CONCLUSION

These results suggest that genetic variation in the ADRB2 gene is associated with disturbances in in vivo beta-adrenoceptor-mediated lipolysis and fat oxidation during beta-adrenergic stimulation in overweight and obese subjects; these effects are influenced by gene-gender interactions.

The human obesity gene map: the 2005 update

This paper presents the 12th update of the human obesity gene map, which incorporates published results up to the end of October 2005. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, transgenic and knockout murine models relevant to obesity, quantitative trait loci (QTL) from animal cross-breeding experiments, association studies with candidate genes, and linkages from genome scans is reviewed. As of October 2005, 176 human obesity cases due to single-gene mutations in 11 different genes have been reported, 50 loci related to Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and causal genes or strong candidates have been identified for most of these syndromes. There are 244 genes that, when mutated or expressed as transgenes in the mouse, result in phenotypes that affect body weight and adiposity. The number of QTLs reported from animal models currently reaches 408. The number of human obesity QTLs derived from genome scans continues to grow, and we now have 253 QTLs for obesity-related phenotypes from 61 genome-wide scans. A total of 52 genomic regions harbor QTLs supported by two or more studies. The number of studies reporting associations between DNA sequence variation in specific genes and obesity phenotypes has also increased considerably, with 426 findings of positive associations with 127 candidate genes. A promising observation is that 22 genes are each supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. The electronic version of the map with links to useful publications and relevant sites can be found at http://obesitygene.pbrc.edu.

Beta2-adrenoceptor gene polymorphisms

Beta2-adrenoceptors (AR) play an important role in regulation of vascular and bronchial smooth muscle tone; functional beta2-AR, however, also exist in human heart where they can mediate positive inotropic and chronotropic effects. Recent studies have discovered that beta2-AR are polymorphic. The most common single nucleotide polymorphisms (SNPs) are: Arg16Gly, Gln27Glu, Thr164Ile in the coding region, and Arg-19Cys in the 5' upstream peptide. These SNPs affect receptor function in vitro; however, conflicting data exist on their functional relevance in vivo. This might be due to the fact that the four SNPs in the 5' upstream peptide and in the coding region, respectively, are linked and form certain haplotypes. This review gives an overview on the contribution of beta2-AR polymorphisms to cardiovascular diseases or altered drug responses. In addition, the relevance of SNPs vs. haplotypes for beta2-AR functional responsiveness is discussed.

Influence of sex and beta2 adrenergic receptor haplotype on resting and terbutaline-stimulated whole body lipolysis

beta 2 adrenergic receptors ( beta 2 ARs) are important mediators of lipolysis. The beta 2 AR gene is highly polymorphic. To determine the contribution of beta 2 AR polymorphisms to variability in whole body lipolysis, we compared basal and terbutaline-stimulated lipolytic rates (Ra) using tracer techniques in 14 healthy, non-obese males (n=7) and females (n=7) who were homozygous for Cys-19/Arg16/Gln27 or Arg-19/Gly16/Glu27 haplotypes. Fasting (overnight) Ra values were higher in females compared to males. Mean+/-SD Ra, Ra/body weight, Ra/fat free mass, Ra/fat, and Ra/energy expenditure rates in males and females were 155+/-46 vs 311+/-111 micromol/min (P=.007); 2.0+/-0.61 vs 5.2+/-2.3 micromol/(min kg) (P=.006); 2.5+/-0.75 vs 7.8+/-3.4 micromol/(min kg) (P=.003); 10+/-3.7 vs 17+/-7.4 micromol/(min kg) (P=.09); and 144+/-45.5 vs 392+/-111 micromol/d (P=.0001), respectively. Mean+/-SD basal glycerol concentrations were higher in females compared to males: 62+/-5.6 vs 36+/-17 micromol/L (P=.003). Basal glycerol concentrations and Ra values were similar by beta2 AR haplotype. Basal glucose and insulin concentrations tended to be higher in males compared to females and were similar by haplotype. Terbutaline-stimulated changes in glycerol concentrations were variable and are not related to either sex or haplotype. We conclude that compared to haplotype, sex is a more important determinant of basal lipolysis after a 12-hour fast in healthy, non-obese individuals.

Haplotype analysis of the beta2 adrenergic receptor gene and risk of myocardial infarction in humans

Polymorphisms in the beta2 adrenergic receptor (ADRB2), in particular G16R, Q27E, and T164I, have been implicated in the pathogenesis of cardiovascular and metabolic phenotypes. However, no prospective, genetic-epidemiological data are available on the risk of cardiovascular disease associated with these variants. Using DNA samples collected at baseline in a prospective cohort of 14,916 initially healthy American men, we evaluated the G16R, Q27E, and T164I polymorphisms among 523 individuals who subsequently developed myocardial infarction and among 2092 individuals who remained free of reported cardiovascular events during follow-up. The haplotype frequency distribution was significantly different among cases and controls (chi(2)(7d.f.) = 20.92, P = 0.0039). Haplotype-based logistic regression, adjusting for age, smoking, and randomized treatment group, indicated that G16-Q27-I164 (odds ratio 0.178, 95% C.I. 0.043-0.737, P = 0.017) and (non-G16-Q27)-T164 (odds ratio 1.235, 95% C.I. 1.031-1.480, P = 0.022) haplotypes were significantly associated with altered risk of myocardial infarction. These findings remained after further adjustment for BMI, history of hypertension, and presence or absence of diabetes. In conclusion, variation in haplotype frequencies for the beta2 adrenergic receptor gene was found to be associated with risk of myocardial infarction.

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.

Autonomic Nervous System Pharmacogenomics: A Progress Report

Pharmacogenetics, the inherited basis for interindividual differences in drug response, has rapidly expanded with the advent of new molecular tools and the sequencing of the human genome, yielding pharmacogenomics. We review here recent ideas and findings regarding pharmacogenomics of components of the autonomic nervous system, in particular, neuronal nicotinic acetylcholine receptors, postsynaptic receptors with which the parasympathetic and sympathetic neurotransmitters, acetylcholine (ACh) and norepinephrine, respectively, interact. The receptor subtypes that mediate these responses, M(1-3) muscarinic cholinergic receptors (mAChRs), and alpha(1A,B,D)-, alpha(2A,B,C)-, and beta(1,2,3)-adrenergic receptors (AR), show highly variable expression of genetic variants; variants of mAChRs and alpha(1)-ARs are relatively rare, whereas alpha(2)-AR and beta-AR subtype variants are quite common. The largest amount of data is available regarding variants of the latter ARs and represents efforts to associate certain receptor genotypes, most commonly, single nucleotide polymorphisms, with particular phenotypes (e.g., cardiovascular and metabolic responses). In vitro and in vivo studies have yielded inconsistent results; definitive conclusions are limited. We identify several conceptual and methodological problems with available data: sample size, ethnicity, tissue differences, coding versus noncoding variants, limited studies of haplotypes, and interaction among variants. Thus, although progress has been made in identifying genetic variation that influences drug response fo autonomic nervous system components, we are still at the early stages of defining the most critical genetic determinants and their role in human physiology and pharmacology.

Beta-adrenoceptor polymorphisms

There can be no doubt that beta(1)-, beta(2)- and beta(3)-adrenoceptor genes have genetic polymorphisms. Two single nucleotide polymorphisms have been described for the beta(1)- (Ser49Gly; Gly389Arg), three for the beta(2)- (Arg16Gly; Gln27Glu; Thr164Ile) and one for the beta(3)-adrenoceptor subtype (Trp64Arg) that might be of functional importance. The possibility that changes in expression or properties of the beta-adrenoceptors due to single nucleotide polymorphisms might have phenotypic consequences influencing their cardiovascular or metabolic function or may contribute to the pathophysiology of several disorders like hypertension, congestive heart failure, asthma or obesity is an idea that has attracted much interest during the last 10 years. At present, it appears that these beta-adrenoceptor polymorphisms are very likely not disease-causing genes, but might be risk factors, might modify disease and/or might influence progression of disease. The aim of this review is to provide an overview of the functional consequences of such beta-adrenoceptor polymorphisms in vitro, ex vivo and in vivo.

Relationship between β-2 adrenoceptor gene haplotypes and adipocyte lipolysis in women

BACKGROUND AND AIMS

The beta(2)-adrenergic receptors are important for adipocyte lipolysis regulation by catecholamines in humans. The beta(2)-adrenoceptor gene is highly polymorphic. The role of these genetic variations for adipocyte lipolysis was investigated.

DESIGN AND METHODS

Six single-nucleotide polymorphisms (SNPs) in the promotor region and four SNPs in the coding region (leading to amino-acid substitution) of the beta(2)-adrenoceptor gene were determined in 141 overweight or obese, but otherwise healthy women. Lipolysis experiments were performed on isolated subcutaneous adipocytes.

RESULTS

Three homozygous haplotypes (6/6, 4/4 and 2/2) were found that differed about 500-fold in noradrenaline sensitivity or beta(2)-adrenoceptor sensitivity (6/6>2/2>4/4, P=0.01). The haplotypes also differed by 100% in maximum noradrenaline-induced lipolysis rates (6/6>2/2>4/4). However, there was no influence on beta(1)-, beta(3)- or alpha(2)A-adrenoceptor sensitivity. Heterozygosity at one or several SNPs in the haplotypes influenced the beta(2)-adrenoceptor sensitivity significantly.

CONCLUSION

Multiple SNPs in the beta(2)-adrenoceptor gene form several haplotypes that markedly influence beta(2)-receptor function- and catecholamine-induced lipolysis in fat cells. These haplotypes may be important genetic factors behind impaired lipolysis in obesity.

Thr164Ile polymorphism of the human beta2-adrenoceptor exhibits blunted desensitization of cardiac functional responses in vivo

In subjects heterozygous for Thr164Ile beta2-adrenoceptor (beta2AR) polymorphism, cardiac responses to beta2AR agonist stimulation are blunted. In this study, we investigated agonist-induced desensitization of Thr164Ile beta2ARs. For this purpose, we assessed in six subjects with heterozygous Thr164Ile beta2ARs and in 10 subjects with homozygous wild-type (WT) beta2ARs the effects of 2-wk oral treatment with 3 x 5 mg/day terbutaline on terbutaline infusion-induced increases in heart rate (HR) and contractility [measured as shortening of HR-corrected duration of electromechanical systole (QS2c)]. Compared with WT beta2AR subjects, Thr164Ile subjects exhibited a blunted terbutaline-induced maximum increase in HR (WT 32 +/- 4 beats/min, Thr164Ile 19 +/- 3 beats/min, P < 0.05) and contractility (WT -54 +/- 2 ms, Thr164Ile -37 +/- 6 ms, P < 0.05). Two-week oral terbutaline treatment desensitized cardiac beta2AR responses to terbutaline infusion (increase in HR: WT 10 +/- 2 beats/min, Thr164Ile 8 +/- 4 beats/min; increase in contractility: WT -22 +/- 5 ms Thr164Ile: -17 +/- 6 ms); however, the extent of desensitization was larger in WT than Thr164Ile beta2AR subjects. Thus, after 2-wk oral terbutaline treatment cardiac, beta2AR responses did not differ anymore between WT and Thr164Ile beta2AR subjects. We conclude that agonist-induced desensitization of cardiac beta2ARs is more pronounced in WT than Thr164Ile subjects. Thus cardiac Thr164Ile subjects appear to be somewhat protected against agonist-induced desensitization.

Beta2 adrenoceptor functional gene variants, obesity, and blood pressure level interactions in the general population

We investigated the association of beta2 adrenoceptor functional gene variants (Arg16Gly, Gln27Glu, and Thr164Ile polymorphisms), obesity phenotypes, and blood pressure levels in a large, ethnically mixed urban population. The individuals (n=1576) were randomly selected for a cross-sectional study of cardiovascular risk factors in Vitória, Brazil. Statistically significant associations among systolic blood pressure and the Arg16Gly and Thr164Ile variants were identified in univariate analysis. The Gly16/Gly16 genotype was still associated with systolic blood pressure (SBP) in multivariate analysis adjusting for age, gender, ethnicity, total cholesterol, diabetes, and body mass index (BMI) (P=0.01). The Arg16 allele was the only genotypic variable associated with BMI, and, in a dominant model, it remained associated with an increased BMI even after adjustment for age, gender, ethnicity, triglycerides, HDL cholesterol, LDL cholesterol, diabetes, and hypertension status (P=0.02). Although the different polymorphisms did not interact in the determination of SBP, a significant interaction with BMI (P=0.02), not through linkage disequilibrium, was identified between the Gln27Glu and the Thr164Ile variants. Furthermore, a significant interaction among the Arg16Gly polymorphism and BMI (P=0.036) and waist-hip ratio (P=0.003) in determining SBP was disclosed by ANOVA factorial modeling, with SBP used as the dependent variable. An interaction between the Thr164Ile polymorphism and waist-hip ratio was also identified (P=0.018). Finally, multiple logistic regression models showed a 1.48-fold increase in the risk of hypertension in individuals harboring the Gly16/Gly16 genotype and a 1.31-fold (P=0.01) and a 1.49-fold (P=0.003) increased risk of obesity in individuals harboring the Gln27/Gln27 genotype or the presence of the Arg16 allele, respectively. Taken together, these data provide evidence for a strong but complex relation between beta-adrenoceptor gene variants, hypertension, and obesity.

The human obesity gene map: the 2002 update

This is the ninth update of the human obesity gene map, incorporating published results through October 2002 and continuing the previous format. Evidence from single-gene mutation obesity cases, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) from human genome-wide scans and various animal crossbreeding experiments, and association and linkage studies with candidate genes and other markers is reviewed. For the first time, transgenic and knockout murine models exhibiting obesity as a phenotype are incorporated (N = 38). As of October 2002, 33 Mendelian syndromes relevant to human obesity have been mapped to a genomic region, and the causal genes or strong candidates have been identified for 23 of these syndromes. QTLs reported from animal models currently number 168; there are 68 human QTLs for obesity phenotypes from genome-wide scans. Additionally, significant linkage peaks with candidate genes have been identified in targeted studies. Seven genomic regions harbor QTLs replicated among two to five studies. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 222 studies reporting positive associations with 71 candidate genes. Fifteen such candidate genes are supported by at least five positive studies. The obesity gene map shows putative loci on all chromosomes except Y. More than 300 genes, markers, and chromosomal regions have been associated or linked with human obesity phenotypes. The electronic version of the map with links to useful sites can be found at http://obesitygene.pbrc.edu.

The human obesity gene map: the 2001 update

This report constitutes the eighth update of the human obesity gene map, incorporating published results up to the end of October 2001. Evidence from the rodent and human obesity cases caused by single-gene mutations, Mendelian disorders exhibiting obesity as a clinical feature, quantitative trait loci (QTLs) uncovered in human genome-wide scans and in crossbreeding experiments in various animal models, association and linkage studies with candidate genes and other markers is reviewed. The human cases of obesity related in some way to single-gene mutations in six different genes are incorporated. Twenty-five Mendelian disorders exhibiting obesity as one of their clinical manifestations have now been mapped. The number of different QTLs reported from animal models currently reaches 165. Attempts to relate DNA sequence variation in specific genes to obesity phenotypes continue to grow, with 174 studies reporting positive associations with 58 candidate genes. Finally, 59 loci have been linked to obesity indicators in genomic scans and other linkage study designs. The obesity gene map depicted in Figure 1 reveals that putative loci affecting obesity-related phenotypes can be found on all chromosomes except chromosome Y. A total of 54 new loci have been added to the map in the past 12 months, and the number of genes, markers, and chromosomal regions that have been associated or linked with human obesity phenotypes is now above 250. Likewise, the number of negative studies, which are only partially reviewed here, is also on the rise.

Genetic variance and lipolysis regulation: implications for obesity

Catecholamines are the major lipolytic hormones in human fat cells, and lipolytic catecholamine resistance is described in obesity. Studies on twins and in rare genetic disorders suggest a strong heredity component of catecholamine-induced lipolysis. Polymorphisms in catecholamine receptor signalling pathways have been described, several of which associate with obesity. Many polymorphisms in adrenoceptor genes are functional in transfected cell lines. The importance of polymorphisms in catecholamine signalling pathways for lipolysis regulation is discussed. A Trp64Arg polymorphism in the beta3-receptor, which associates with obesity, is accompanied by changes in lipolytic sensitivity of the receptor in human fat cells. Similarly, a Gln16Glu and an Arg164Ile variation in the beta2-adrenoceptor cause marked variations in the lipolytic sensitivity of this receptor in human adipocytes. Furthermore, beta2-adrenoceptor gene polymorphisms associate with obesity. A dinucleotide (CA) intron repeat in hormone-sensitive lipase gene is linked to obesity and markedly decreases the ability of catecholamines to activate the lipase and thereby lipolysis in human fat cells. However, an Arg389Gly polymorphism in the beta1-adrenoceptor, which alters receptor function in transfected cell lines, has no effect on lipolysis in human fat cells and is not associated with obesity. Thus, polymorphism in human genes that are involved in catecholamine signal transduction have effects on fat cell lipolysis and also relate to obesity. The lipolysis effects of these polymorphisms cannot always be predicted from gene transfer experiments on artificial cell lines. It is possible that genetic variance in catecholamine signalling pathways, through alterations in adipocyte lipolysis, may promote obesity.