Association Between Uncoupling Protein 2 Gene Polymorphisms And Obesity In Japanese

Body weight in humans appears to be a trait with strong genetic determinants and for adult body weight heritability is estimated to be between 50% and 80%. As it well known, obesity is presumed to develop when there is an imbalance between energy intake and energy expenditure, candidate genes for body weight regulation include those that might be important for the regulation of energy expenditure. Uncoupling proteins(UCPs) which can translocate protons into the mitochondrial matrix resulting in heat generation without ATP synthesis, have been examined for association with body weight.UCP2 was discovered in 1997 and widely expressed in human tissues, including white fat, brown fat, the pancreas, and skeletal muscle. UCP2 is upregulated in white fat in response to fat feeding in the obesity-resistant A/J mouse, showing a role of UCP2 in the metabolic adaptation to fasting. UCP2 was also shown to negatively regulate insulin secretion in UCP2 deficient mice exhibiting higher islet ATP levels and increased glucose stimulated insulin secretion; however, mice lacking UCP2 followingtargeted gene disruption are not obese.Several UCP2 gene variants have been reported in human studies: a G/A polymorphism in the promoter region (-866G/A), a valine-for-alanine substitution at amino acid 55 in exon 4 (Ala55Val), and a 45 base-pair insertion/deletion in the 3'-untranslated region. All of these variants have been investigated for possible association with obesity, but up to now, no conclusive results have been obtained .Esterbauer et al reported that the A allele of UCP2 -866G/A polymorphism is associated with decreased risk of obesity; Qiuhe Ji et al reported that this polymorphism is associated with a higher frequency of hypertension,but not with obesity; D'Adamo M's report showed that the A allele of UCP2 -866G/A polymorphism is associated with a higher frequency of insulin resistance and type 2 diabetes.As for the Ala/Val substitution in exon 4 of UCP2, Yiman Zheng et al reported that Ala55Val variant in the UCP2 gene is associated not only with femoral subcutaneous adipose tissue area but also with BMI; Hua Wang et al found that Ala55Val variant is associated with an index of beta-cell compensation for insulin sensitivity. These data suggest the possibility that the variation of -866G/A in the promoter and Ala55/Val in the exon 4 are associated with hypertension, obesity, or insulin resistance.Contradidtory findings on the role of UCP2 in the metabolism prompted us to investigate the presence of the -866G/A in the promoter and Ala55/Val in the exon 4 in a large cohort of Japanese subjects and evaluate whether these two polymorphisms are associated with hypertension , insulin resistance and obesity.The subjects (n=l,084) were recruited from the inhabitants of the towns of Tanno and Sobetsu, Hokkaido, Japan. Systolic blood pressure(SBP), diastolic blood pressure(DBP), immunoreactive insulin (IRI), fasting plasma glucose(FPG). cholesterol (TC), triglyceride (TG),high density lipoprotein cholesterol(HDL), body mass index (BMI), waist circumference (WC) were measured in all subjects.Insulin resistance (IR) was assessed by homeostasis model assessment (HOMA) .According to BMI and blood pressure, We divided the subjects into two groups respectively: obese and non-obese group; hypertension and normotensive group. DNA was extracted from buffy coat using blood samples. The genotypes of the -866 G/A polymorphism in the promoter of UCP2 were determined by polymerase chain reaction (PCR)-restriction fragment length polymorphism(RFLP).The genotypes of the Ala55Val polymorphism in the exon-4 of UCP2 were determined using the TaqMan PCR method. Statistical analysis was conducted using StatView 4.5J (Abacus Concepts) and JMP 5.0.1 (SAS Inc). Genotype frequencies and Hardy-Weinberg equilibrium were examined by y2 analysis. Compansion of clinical parameters among groups was performed using one-way ANOVA. A multiple logistic regression analysis was performed to assess the quantitative effects of covariates (sex, age, BMI, etc).The results showed that the genotype distribution of UCP2 polymorphisms was in agreement with Hardy-Weinberg equilibrium. The two variants , the -866G/A in the promoter and Ala55val in the exon 4 of UCP2 gene were in strong linkage disequilibrium (PO.001). The genotype frequency of AA, AG, and GG of -866G/A polymorphism was 22.7%, 51.1%, 26.2%, respectively, and the frequency of the A allele (48.2%) in this study was higher than that previously shown in the Caucasian population (37.1%, /><0.001) .The genotype distribution of Ala55Val in the exon 4 polymorphisms (Ala:51.8%versusVal:48.2%) of the UCP2 in Japanese subjects of this study was similar to that of Kubota's report(Ala:52.7%versusVal:47.3%),but was significantly different from thedata in Caucasians (Ala:79.5%versusVal:20.5%) (P<0.001).The frequencies of both genotypes were not different between the obese and non-obese group or between hypertension and normotensive group. In the total studied subjects, no association was found between UCP2 -866G/A and Ala55Val polymorphisms and clinical parameters related to insulin secretion and obesity, including BMI, WC, SBP, DBP, FPG, IRI, HOMA-R and frequency of hypertension. But for the obese subjects, AA type of UCP2 -866G/A polymorphism had significantly higher BMI and WC than AG+GG type (PO.05) .This association was significantly positive only when the population was divided into AA and AG+GG genotypes. Multiple logistic regression analysis was conducted to exclude the possible effects of confounding factors, such as sex, age, frequency of HT and BMI on WC. The result of the logistic analysis showed that the AA genotype of UCP2 -866G/A polymorphism remained an independment factor associating with WC.According to the results above, some conclusions might be achieved: 1. The AA type of UCP2 -866G/A polymorphism was associated with higher BMI and WC only in the obese Japanese subjects. The result was contrary to that in Caucasian population, in which GG types had higher levels of BMI in the obese subjects. This contradiction may be caused by the difference in genetic background. Different genes or different single nucleitide polymorphism may contribute to controlling body weight and the level of BMI in that Caucasian is much higher than Japanese.2. In the total population studied, no association was found between UCP2 -866G/A or Ala55Val polymorphism and insulin or hypertension.We investigated the association between insulin levels and UCP2 -866G/A and Ala55Val polymorphisms in the obese and non-obese groups. Although not statistically significant, IRI, HOMA-IR and FPG levels were higher in...