Clinical features of a polymorphism of the beta3-adrenergic receptor gene in patients with type 2 diabetes mellitus--a study using a pin-point sequencing method

Meta-Analysis of the Association of the Trp64Arg Polymorphism in the β3 Adrenergic Receptor with Insulin Resistance

OBJECTIVE

To clarify the possible association between the Trp64Arg polymorphism and insulin resistance (IR).

RESEARCH METHODS AND PROCEDURES

Articles evaluating the effect of the Trp64Arg polymorphism on IR were identified on the MEDLINE and PubMed databases from 1995 to February, 2004. After extraction of relevant data, main and subgroup meta-analyses were performed to assess the differences in IR indices between Trp/Trp and Trp/Arg genotypes.

RESULTS

Forty eligible papers containing 56 subgroups were included in this meta-analysis. Among a total of 12,805 subjects, 21.9% had Trp64Arg mutation: 20.8%, heterozygotes and 1.1%, homozygotes. Significant associations were found between this mutation and some indices of IR. The weighted mean difference in fasting insulin, 120-minute insulin level after oral glucose tolerance test, and homeostasis model assessment between Arg64 and Trp64 was 0.23 [95% confidence interval (CI), 0.05 to 0.42] pM, 0.89 (95% CI, 0.30 to 1.48) pM, and 0.55 (95% CI, 0.14 to 0.96), respectively. Subgroup analysis further indicated that this significant association existed only in the Asian population (p < 0.01) and in the obese (p = 0.02) and diabetes subgroups (p = 0.03).

DISCUSSION

Numerous studies have been conducted to examine the relationship between the beta3-adrenergic receptor Trp64Arg polymorphism and components of IR syndrome. However, the results have been inconsistent and have led to controversy about whether this polymorphism is associated with these clinical features. The current meta-analysis demonstrated the moderate effects of the Trp64Arg polymorphism on IR in the Asian population and in obese and diabetic subgroups.

LEPR, ADBR3, IRS-1 and 5-HTT genes polymorphisms do not associate with obesity

Obesity is a growing problem and is associated with numerous medical conditions. In several genes coding for molecules involved in the regulation of body weight (fat mass) and thermogenesis, polymorphisms have been reported which possibly modify human obesity risk. The aim of this study was to determine the incidence of the following polymorphisms in the following genes in 262 obese (BMI > or = 30) and 138 control (BMI < or = 25) subjects: leptin receptor (LEPR)-Gln223Arg, B3-adrenergic receptor (B3-AR)-Trp64Arg, serotonin transporter (5-HTT)--a 44-base pair insertion/deletion functional polymorphism in the 5-HTTLPR and insulin receptor substrate-1 (IRS-1)-Gly972Arg. Our hypothesis was that these polymorphisms would occur more frequently in the obese population. The polymorphisms were determined by polymerase chain reaction (PCR) and restriction genotyping in study population. In our results, no strong associations were observed between BMI status and these polymorphisms. Weak, though significant, association coefficients obtained with HTT and LEPR loci indicate that the genotype numbers at these loci may depend on BMI status to some extent.

Beta3-adrenoceptors in urinary bladder

The beta-adrenoceptor (AR) is currently classified into beta(1), beta(2), and beta(3) subtypes. A third subtype, beta(3)-AR, was first identified in adipose tissue, but has also been identified in smooth muscle tissue, particularly in the gastrointestinal tract and urinary bladder smooth muscle. There is a predominant expression of beta(3)-AR messenger RNA (mRNA) in human bladder, with 97% of total beta-AR mRNA being represented by the beta(3)-AR subtype and only 1.5 and 1.4% by the beta(1)-AR and beta (2)-AR subtypes, respectively. Moreover, the presence of beta(1)-, beta(2)-, and beta(3)-AR mRNAs in the urothelium of human bladder has been identified. The distribution of beta-AR subtypes mediating detrusor muscle relaxation is species dependent, the predominant subtype being the beta(3)-AR in humans. Recent studies have suggested that cAMP-dependent routes are not exclusive mechanisms triggering the beta-AR-mediated relaxation of smooth muscle. It has been demonstrated in rats detrusor muscle that cAMP plays a greater role in beta-adrenergic relaxation against basal tone than against KCl-induced tone and that conversely calcium-activated K(+) channels (BKca channels) play a greater role under the latter circumstances. In rat models, beta(3)-AR agonists increase bladder capacity without influencing bladder contraction and have only weak cardiovascular side effects. Although this evidence points toward the clinical utility of beta(3)-AR agonists as therapy for overactive bladder (OAB), pharmacological differences exist between rat and human beta(3)-ARs. Development of compounds with high selectivity for the human beta(3)-AR, identified by screening techniques using cell lines transfected with the human beta(1)-, beta(2)-, and beta(3)-AR genes, may mitigate against such problems. The association between the tryptophan 64 arginine polymorphism in the beta(3)-AR gene and idiopathic OAB is discussed.

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.

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.