Left Ventricular Mass and +276 G/G Single Nucleotide Polymorphism of the Adiponectin Gene in Uncomplicated Obesity

Authors


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McMaster University, Hamilton General Hospital, Cardiovascular Obesity Research & Management, 237 Barton Street East, Hamilton, Ontario, L8L 2X2 Canada. E-mail: gianluca@ccc.mcmaster.ca

Abstract

The single nucleotide polymorphism at position 276 in the adiponectin gene (APM1/ACDC +276 G>T) and left ventricular mass (LVM) have been associated with increased cardiovascular risk. We sought to evaluate whether +276 G>T variants in the adiponectin gene are correlated with LVM in uncomplicated obese subjects. APM1/ACDC +276 G>T single nucleotide polymorphism, echocardiographic indexed LVM (LVM/body surface area and LVM/height2.7), insulin sensitivity by euglycemic clamp, and plasma adiponectin levels were analyzed in 62 obese subjects without complications (51 women and 11 men; mean age, 34.2 ± 10.2 years; BMI, 38.6 ± 9.1 kg/m2). Forty lean subjects formed the control group for LVM evaluation. We found 23 (37%) uncomplicated obese subjects with the APM1/ACDC +276 G/G genotype, 25 (40%) with the G/T genotype, and 14 (23%) with the T/T genotype. G/G uncomplicated obese subjects showed significant higher LVM/body surface area and LVM/height2.7 (within the normal range in the majority of them) than uncomplicated obese subjects carrying the G/T and T/T genotypes (p < 0.01 and p < 0.05, respectively). This study showed that LVM is significantly higher in uncomplicated obese subjects carrying the G/G genotype at position 276 of the human adiponectin gene.

The single nucleotide polymorphism (SNP)1 at positions 276 of the adiponectin gene (APM1/ACDC +276 G>T) has been correlated with low circulating adiponectin levels, type 2 diabetes, insulin resistance, and increased risk for coronary artery disease (CAD) (1, 2, 3, 4, 5, 6). Low plasma adiponectin levels have been reported to be associated with CAD (7, 8, 9) and, recently, with increased left ventricular mass (LVM) (10), although the results in this area are still controversial (11, 12). LVM is a well-known independent cardiovascular risk factor, and its relationship with insulin resistance in obese subjects has been previously described (13). The interactions between insulin resistance, obesity, and left ventricular morphology could suggest an association between the adiponectin gene, especially APM1/ACDC +276 G>T, and LVM.

Therefore, in this study, we sought to evaluate whether +276 G>T variants in the adiponectin gene are correlated with LVM in uncomplicated obese subjects. Uncomplicated obesity is a well-defined clinical entity, as we recently reported (14), that allows us to study these parameters without the confounding effect of obesity-related comorbidities.

Clinical characteristics of uncomplicated obese and lean control subjects are summarized in Table 1. No differences in BMI, age, glucose, and lipid pattern among the three obese groups were observed. Subjects carrying G/G and G/T genotypes had blood pressure values higher than subjects with T/T genotype but within the normal range (Table 1).

Table 1. . Anthropometric and metabolic characteristics of uncomplicated obese subjects according to the APM1/ACDC +276 genotype
 G/G (n = 23)G/T (n = 25)T/T (n = 14)p
  1. p values were adjusted for sex. Data are expressed as means ± standard deviation. ANOVA with Newman-Keuls post hoc test. OGTT, oral glucose tolerance test; M index, whole body glucose use; HDL-C, high-density lipoprotein cholesterol; SBP, systolic blood pressure; DBP, diastolic blood pressure; NS, not significant.

Age (years)36.3 ± 9.535.8 ± 10.131.5 ± 10.10.33
Height (m)1.63 ± 91.65 ± 101.63 ± 70.69
BMI (kg/m2)41.7 ± 10.338.0 ± 3.740.5 ± 8.40.36
Women (n)191913 
Smokers (n)221 
Fasting glucose (mg/dL)86.1 ± 7.984.5 ± 7.585 ± 7.80.75
2-h OGTT (mg/dL)107.2 ± 20.2108.3 ± 21.4105.5 ± 24.70.91
Fasting insulin (μU/mL)21.6 ± 15.717.7 ± 9.220.8 ± 110.50
M index (mg/FFMkg/min)7.7 ± 2.48.4 ± 3.49.5 ± 3.80.11
Adiponectin (mg/liter)25.6 ± 14.830.6 ± 11.731 ± 10.5NS
Total cholesterol (mg/dL)189.2 ± 22.4181.1 ± 23.1182.2 ± 21.10.42
HDL-C (mg/dL)52.7 ± 13.451.6 ± 10.550.9 ± 10.10.94
Triglycerides (mg/dL)83.2 ± 24.388.9 ± 22.586.9 ± 29.50.76
SBP (mm Hg)125 ± 10126 ± 10115 ± 120.006
DBP (mm Hg)82 ± 680 ± 774.5 ± 80.007

Genotype Distribution

We found 23 (37%) uncomplicated obese subjects with the APM1/ACDC +276 G/G genotype, 25 (40%) with the G/T genotype, and 14 (23%) with the T/T genotype. The genotype frequencies were under Hardy-Weinberg equilibrium (calculated χ2 with N − 1 degrees of freedom was 1.92).

LVM

No significant difference in LVM/body surface area (BSA), and LVM/height2.7 (h) between overall groups of uncomplicated obese and lean control subjects was found (90 ± 10 vs. 86.5 ± 10 g/m2, p = 0.08; and 45.2 ± 8 vs. 43 ± 6 g/m2.7, p = 0.12, respectively). G/G uncomplicated obese subjects showed significant higher LVM/BSA (p = 0.004; G/G vs. G/T, p < 0.01; G/G vs. T/T, p < 0.05) and LVMh2.7 (p = 0.03) than uncomplicated obese subjects carrying the G/T and T/T genotypes (Figure 1).

Figure 1.

Indexed LVM in the three different genotypes at locus 276 in the adiponectin gene. G/G uncomplicated obese subjects showed significant higher LVM/BSA (p = 0.004; G/G vs. G/T, p < 0.01; G/G vs. T/T, p < 0.05) and LVMh2.7 (p = 0.03) than uncomplicated obese subjects carrying the other two genotypes.

The G/T and T/T obese subjects (n = 39) grouped together showed lower LVM/BSA and LVMh2.7 (p < 0.01 for both) than obese subjects with the G/G genotype.

Univariate linear regression analysis showed that fasting insulin, whole body glucose use (M index), and BMI correlated with LVMh2.7 (r = 0.48, p = 0.03; r = 0.39, p = 0.03; and r = 0.32, p = 0.05; respectively) and fasting insulin and M index with LVM/BSA (r = 0.45, p = 0.03; and r = 0.38, p = 0.03; respectively). No statistically significant correlation between LVM/BSA and LVMh2.7 and adiponectin and blood pressure was observed, even after adjustment for sex.

A number of adiponectin gene variations have been studied (1, 2, 3, 4, 5, 6, 7). Among them, APM1/ACDC +276 SNP was more frequently associated with CAD risk (3, 4, 5, 6) and with early-onset CAD (7) than other SNPs of the adiponectin gene. Therefore, in this study, we chose to evaluate the potential correlation between APM1/ACDC +276 SNP and LVM, which is a well-established risk factor for CAD (15).

Our data showed that uncomplicated obese subjects carrying the +276 G/G genotype of the adiponectin gene have increased indexed LVM. The uncomplicated obese subjects, despite a large fat amount, showed no left ventricular hypertrophy or pathological increase of LVM, as we previously reported (16, 17, 18, 19). Nevertheless, uncomplicated obese subjects who are homozygotes for G/G had significantly higher LVM than subjects carrying the G/T and T/T genotypes and lean controls, although LVM values were within the normal range in the majority of subjects, and only 10% of G/G obese subjects showed eccentric LVH.

Subjects carrying the G/G and G/T genotypes presented with higher systolic and diastolic blood pressure, albeit within the normal range, than subjects with the T/T genotype. In fact, all of the uncomplicated obese subjects included in this study were normotensive, according to our restricted inclusion criteria. Although arterial blood pressure is a well-known determinant of LVM, no conclusions about cause-and-effect mechanisms of increased LVM can be drawn from our observational study. Moreover, obese subjects with the G/G and G/T genotypes had similar blood pressure values but significantly different LVM. Taken together, these findings highlight the unfavorable effect of the G/G genotype. The potential role of blood pressure and metabolic features in increasing LVM among obese subjects carrying APM1/ACDC +276 G/G is still not completely clear.

In conclusion, the novel finding of this study is that LVM is significantly higher in uncomplicated obese subjects carrying G/G genotype at position 276 in the human adiponectin gene. Nevertheless, the limited sample evaluated, justified by the accurate characterization of the uncomplicated obesity that represents the strength and originality of this study, could suggest further evaluations in a larger population.

Research Methods and Procedures

Sixty-two uncomplicated obese subjects (BMI >30 kg/m2; 51 women and 11 men; mean age, 34.2 ± 10.2 years; BMI, 38.6 ± 9.1 kg/m2), with a history of excess fat for at least 10 years, were selected from 200 consecutive obese individuals who were all screened from our Day Hospital. Uncomplicated obesity was defined according to the following parameters: normal fasting plasma glucose (<110 mg/dL), normal glucose tolerance (glucose after a 2-hour oral glucose tolerance test <140 mg/dL), normal resting arterial blood pressure (systolic <140 mm Hg; diastolic <90 mm Hg for at least three measurements), normal serum level of total cholesterol (<220 mg/dL) and triglycerides (<150 mg/dL), high-density lipoprotein-cholesterol >40 mg/dL for men and >50 mg/dL for women, and normal thyroid hormones. None had any evidence of cardiovascular, hepatic, renal, respiratory, or other metabolic diseases by routine history and physical examination, and no subject was taking any medication. Forty lean control volunteers (30 women and 10 men; mean age, 33 ± 10 years; BMI, 23.5 ± 1.5 kg/m2) without history of hypertension and cardiac diseases formed the control group for LVM comparison.

This study was conducted in accordance with the guidelines proposed in the Declaration of Helsinki and has been approved by a review committee of La Sapienza University. All subjects gave informed consent before the study began.

Echocardiographic Study

Each subject underwent a transthoracic two-dimensional guided one-dimensional echocardiogram. Echocardiograms were performed with a Toshiba instrument (Toshiba American Medical Systems, Tustin, CA) using standard techniques, with subjects in the left lateral decubitus position. LVM was estimated by the anatomically validated formula of Devereux et al. (20). The indices used to adjust LVM were obtained as follows: LVM/BSA and LVM/h2.7. Left ventricular hypertrophy (LVH) was defined as LVM/BSA >134 g/m2 for men and >110 g/m2 for women (21) and as LVMh2.7 >51 g/m2.7 in both sexes (22). Four left ventricular geometric patterns (normal, concentric remodeling, concentric hypertrophy, and eccentric hypertrophy) were determined using values of relative wall thickness (RWT) and LVM as follows: normal (no LVH and RWT <0.44), concentric remodeling (no LVH and RWT >0.44), concentric hypertrophy (LVH and RWT >0.44), and eccentric hypertrophy (LVH and RWT <0.44) (23).

Clamp Study

We performed euglycemic hyperinsulinemic clamp in all obese subjects after 10 to 12 hours of overnight fasting, following previously described methods (24). Insulin was continuously infused at the rate of 4.0 mU/kg per minute for 5 minutes, 2.0 mU/kg per minute for 5 minutes, and 1.0 mU/kg per minute for 110 minutes. The steady state of the test was considered the interval between 60 and 120 minutes. M index was calculated from the infusion rate of exogenous glucose during the second hour of the insulin clamp period after correction for changes in glucose levels in a distribution volume of 250 mL/kg. The M index was adjusted by kilograms of fat-free mass calculated using a bioelectrical impedance analyzer (BIA-103; Akern, Florence, Italy).

Genotype Analysis

SNP +276 of the APM1/ACDC gene was genotyped using the fluorogenic 5′ nuclease assay application of the ABI PRISM 7900 HT Sequence Detection System (ABI, Foster City, CA). The conditions for Taqman reaction were as follows: 95 °C for 10 minutes and 40 cycles of 95 °C for 15 seconds and 60 °C for 1 minute.

Analytical Procedures

Plasma adiponectin concentrations were measured by radioimmunoassay (Linco Research, Inc., St. Charles, MO; intra-assay CV, 3.5 ± 0.3%; inter-assay CV, 4.7 ± 0.4%). Samples were diluted 500 times before assay.

Statistical Analysis

Data in the text and in the tables are expressed as means ± SD. One-way ANOVA with Newman-Keuls post hoc multiple comparison test was applied to evaluate the differences between continuous variables among groups. Unpaired Student's t test was used to calculate the difference on indexed LVM between the G/G group and the G/T and T/T groups together. Conformity to Hardy-Weinberg equilibrium to the expected genotype distribution was calculated by the χ2 test. Power analysis was carried out and showed that the sample size (n = 62) of this study had a statistical power of 0.81 (α= 0.05, two-tailed) to detect significant differences among examined variables and association between variables. Univariate linear regression analysis was performed to identify correlates of indexed LVM. Two-tailed p < 0.05 indicates statistical significance. Analysis was done using Stata 5.0 (Stata Corp., College Station, TX).

Acknowledgement

There was no funding/outside support for this study.

Footnotes

  • 1

    Nonstandard abbreviations: SNP, single nucleotide polymorphism; CAD, coronary artery disease; LVM, left ventricular mass; BSA, body surface area; h, height; M index, whole body glucose use; LVH, left ventricular hypertrophy; RWT, relative wall thickness.

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