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Department of Hypertension and Diabetology (K.N.), Medical University of Gdansk, 80-952 Gdansk, Poland; Department of Statistics (C.J.W.), University of Idaho, Moscow, Idaho 83844; and Department of Clinical and Experimental Medicine (M.C., R.D.T., E.A., G.P.R.), Clinica Medica 4, University Hospital, University of Padova Medical School, 35128 Padova, Italy
Address all correspondence and requests for reprints to: Gian Paolo Rossi, M.D., F.A.C.C., F.A.H.A., Department of Clinical and Experimental Medicine, Clinica Medica 4 University Hospital, via Giustiniani, 2, 35126 Padova, Italy. E-mail: gianpaolo.rossi{at}unipd.it.
| Abstract |
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Objective: The aim of the study was to investigate whether ADPN and body mass index (BMI) are genetically determined.
Design, Setting, Participants, and Main Outcome Measures: In a series of 60 pairs of healthy twins, we estimated genetic variance and heritability of ADPN and BMI using both ANOVA and path analysis methods. Twins were genotyped at two biallelic single nucleotide polymorphisms (SNPs) at the gene encoding adiponectin: the +45 T/G (on exon 2) and the –11377 G/C (on the promoter).
Results: A total of 30 pairs of twins were Monozygotic (MZ), and 30 were dizygotic (DZ). The mean ADPN (±SD) was 10.6 ± 5.7 in MZ and 11.1 ± 4.5 in DZ twins (nonsignificant). Three tests of heritability (within pair = 1.13, P < 0.0001; among components = 1.62, P = 0.005; and intraclass correlation 1.34, P < 0.0001) consistently showed ADPN heritability. The preferred model of a likelihood-based analysis included an additive genetic influence and an individually unique environmental influence for ADPN, accounting for 88% and 12% of ADPN variance, respectively. We found a significantly higher within-pair difference of ADPN in DZ than in MZ pairs, and in +45 T/G SNP discordant compared with concordant DZ twins, indicating a significant effect of this adiponectin gene SNP on ADPN.
Conclusions: ADPN shows significant genetic variance and heritability, which is independent of BMI and partly accounted for by the +45 T/G, but not the –11377 G/C adiponectin gene SNP.
| Introduction |
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A genome-wide scan (19, 20) revealed a susceptibility locus for type 2 diabetes and the metabolic syndrome (21) in chromosome 3q27, in which the gene encoding adiponectin (ACDC) sits. Several single nucleotide polymorphisms (SNPs) have been identified in the promoter, and coding and noncoding regions of the ACDC gene (22, 23, 24). Although some might be functional and could influence ADPN, available data are conflicting (23, 24, 25, 26). Therefore, the relative importance of the genetic and environmental factors in influencing ADPN remains unclear. Numerous SNPs (available at http://hgvbase.cgb.ki.se) have been identified, and a strong linkage disequilibrium across the ACDC gene has been reported (23, 27). Two of the most studied entail the –11377 G/C SNP in the promoter, which is in linkage disequilibrium with other SNPs in the promoter (28), and the +45 T/G in exon 2, which is in linkage disequilibrium not only with the promoter SNPs, but also with other SNPs, as the +276 (G/T) in intron 2 that has been associated with plasma ADPN in diabetic men (29).
Studies of monozygotic (MZ) and dizygotic (DZ) twins offer a powerful method of partitioning genetic and environmental sources of covariance of quantitative traits, potentially relevant for the development of phenotypes as the metabolic syndrome and/or diabetes mellitus type 2. Therefore, the comparison of the values of ADPN, between fraternal (DZ) and identical (MZ) twins, represents a streamlined approach to the question of whether this variable is under genetic control. Thus, in identical and fraternal healthy normotensive twins, we investigated whether and to what extent ADPN is genetically determined. Moreover, based on the hypothesis that SNPs in the promoter and the coding sequence could have functional relevance, we elected to genotype our twins at the 11377 G/C (in the promoter) and the +45 T/G SNP (in exon 2) to gather information on their potential role as a determinant of ADPN.
| Subjects and Methods |
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The twins were enrolled among the students and employees of the high school and University of Gdansk, Poland, and from an Italian association of twins based in Padua, Italy, as already detailed (30, 31). Monozygosity and dizygosity were assessed by the analysis of highly discriminating variable number of tandem repeats microsatellites and minisatellites systems. Three amplified fragment length polymorphisms (ApoB, D1S80, and YNZ22) and four short tandem repeats (HUMACTPB2, HUMTH01, HUMFES/FPS, and HUMMVWA31) were assessed by separate PCR amplification of genomic DNA, followed by PAGE and silver staining. Anthropometric data and information on cohabitation were gathered by a predefined form. Smokers were asked to refrain from smoking for at least 12 h before blood sampling. All subjects were healthy, nondiabetic, normotensive, with normal diet and mostly normal body mass index (BMI) (30, 31). They gave informed written consent, and the Institutional Human Subjects Review Committees of both universities approved the study protocol.
Blood sampling and ADPN assessment
Blood was drawn at about 0900 h after an overnight fast and 15-min. supine rest from an antecubital vein without stasis. Five milliliters of whole blood with 100 µl 6% Na2EDTA were immediately put on ice. After centrifugation at 3000 x g (at 4 C for 10 min), separated aliquots of plasma and buffy coat were stored at –40 C. ADPN concentration was evaluated by an ELISA method (Quantikine Human Adiponectin; R&D Systems, Inc., Minneapolis, MN), in which the monoclonal antibodies recognize the globular domain of adiponectin (essentially the C terminus of the protein) as well as full-length adiponectin. Interarray and intraarray coefficients of variation were 6.9% and 4.7%, respectively.
Genotyping
Genotyping at the –11377 C/G (on the promoter) and +45 T/G (on exon 2) SNPs of the ACDC gene was performed by real-time PCR, followed by melting curve analysis using fluorescence resonance energy transfer probes. Primer and probe sequences are available from the corresponding author (G.P.R.) upon request. Genotyping at both SNPs was confirmed by sequencing.
Statistical analysis
Results are expressed as mean and range, as appropriate (32). Because subjects in each twin pair cannot be regarded as independent unrelated individuals, either twin 1 or twin 2 of each pair was considered for the purpose of statistical comparison. Comparison of MZ and DZ twins was performed with a Mann-Whitney U test or (for the variables normally distributed) with Students t test for independent samples. Analysis was performed with the SPSS for Windows statistical package (version 14.0; SPSS, Inc., Chicago, IL). A P value < 0.05 was considered statistically significant.
Analyses of twin data were performed with TWINAN90, a program specifically developed for this purpose (33). The consistency of ADPN with the normal distribution assumption and with the hypothesis of equal variance between zygosity was verified beforehand. Because we previously found that age, gender, and BMI predict ADPN, we used for this genetic both raw and age-, gender-, and BMI-adjusted ADPN values (34). Estimates of genetic variance, including the within pair (WP) and among components (ACs), were thereafter attained, and a test for genetic variance based on the average absolute difference between twins (33), which is standardized to yield an approximate t test for the null hypothesis of no genetic variance, was performed. Three estimates of heritability were also calculated; the first two are derived from the WP and AC genetic variance test statistics, respectively, according to the following equations:
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The third estimate of heritability (h2) is based on intrapair correlation coefficients calculated from the MZ and DZ twins as follows: h2 = 2(rMZ – rDZ). A maximum-likelihood-based approach yields estimates of the proportion of variance of ADPN accounted for by additive genetic influences (A), nonadditive genetic influences (D), environmental influences shared by cotwins within a family (S), and unique to individuals (E) (35). Finally, the results of the likelihood-based analyses were obtained for the following models that allow combinations of effects: the ADE model, the ASE model, the AE model, the SE model, and the E model. A similar analysis was performed for BMI.
| Results |
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Heritability of ADPN
The relationship between ADPN in each pair of identical and fraternal twins, after log transformation and adjustment for the predictors of ADPN (age, gender, and BMI), is shown as a scatter plot (Fig. 1
, top). The correlation coefficient of covariate-adjusted ADPN in MZ (r = 0.904; P < 0.0001) was almost 2-fold higher than in DZ [r = 0.233; nonsignificant (NS)] twins. The null hypothesis that the mean of the MZ and DZ twins absolute difference did not differ from zero was accepted (t = 1.36, with 52.3 df; P = 0.18; NS), and the equal variance hypothesis was not rejected (F = 0.78; P = 0.41; NS). Estimates of genetic variance and intraclass correlation coefficients for both MZ and DZ twins were obtained, and a test for genetic variance was performed (Table 2
). Both the preferred tests of genetic variance (WP and average absolute difference test) and also the AC estimate were statistically significant. The three estimates of heritability (WP, AC, and intraclass correlations) (Table 2
) also showed significant results, thus leading to accept the hypothesis that ADPN is heritable (33).
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To explore further the impact of shared environment and lifestyle on ADPN, we performed separate analyses for cohabitant and noncohabitant twins. Results showed that the WP correlation coefficient of cohabitant twins was 0.979 (P < 0.001) in MZ and 0.390 (P = 0.031) in DZ; in noncohabitant twins, corresponding values were 0.165 and 0.074 (both NS).
We also calculated the WP difference in ADPN in MZ and DZ twins. Moreover, we could compare plasma ADPN between DZ twins who were concordant or discordant for the exon 2 + 45 T/G and the promoter –11377 G/C SNP in the ACDC gene. We found a significantly higher WP difference in DZ than in MZ. Although there was no effect of the –11377 G/C SNP, a significantly higher WP difference was observed in DZ twins who were discordant than in those who were concordant for the +45 T/G SNP (Fig. 2
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The vast majority of our twins had a normal BMI, and only a few had a BMI in the overweight-obesity range (Table 1
and Fig. 1
, bottom). Because BMI is a predictor of ADPN, we also examined the relationship between BMI in each pair of identical and fraternal twins. This analysis, shown in Fig. 1
(bottom), also highlighted a higher correlation coefficient in MZ (r = 0.843; P < 0.0001) than in DZ (r = 0.471; P = 0.003) twins (Table 3
), thus suggesting strong genetic variance of BMI. The null hypothesis that the mean of the MZ and DZ twins absolute difference did not differ from zero was thereafter accepted (t = –1.54, with 72.8 df; P = 0.13; NS), and the equal variance hypothesis was not rejected (F = 0.76; P = 0.33; NS). Both the tests of genetic variance WP and average absolute difference tests and also the AC estimate were statistically significant (Table 3
). The three estimates of heritability (WP, AC, and intraclass correlations) were significant, thereby indicating that BMI levels are heritable. The results of the likelihood-based analyses for different models (ADE, ASE, and the AE, SE, and E) identified the AE as the best fitting model (Table 3
). The heritability from the AE model was 0.814, and the proportion of variance of BMI explained by E was about 18%, thus indicating that BMI is highly heritable. Because BMI derives from two components, e.g. body weight and height, we examined the heritability of each of these components. We found evidence that both height [WP correlation coefficient 0.955 (P < 0.001) in MZ, and 0.406 (P = 0.011) in DZ] and weight [WP correlation coefficient 0.880 (P < 0.001) in MZ, and 0.324 (P = 0.036) in DZ] were highly heritable.
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| Discussion |
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We separately analyzed cohabitant and noncohabitant twins to explore further the impact of shared environment and lifestyle on ADPN. This analysis does not seem to suggest strong heritability because the significant WP correlation disappeared in noncohabitant twins. In DZ twins, cohabitants showed significant relationship (r = 0.390; P = 0.031), but cohabitants (r = 0.074; NS) did not. Cohabitant MZ twins also exhibited a much higher WP correlation (r = 0.979; P < 0.001) than DZ twins (r = 0.390; P = 0.031), suggesting that more similar lifestyle and/or more similar hereditary personality contributes to ADPN. The loss of statistical power might explain the loss of statistical significance in the less numerous noncohabitant twins. However, regardless of the cohabitation status, the WP resemblance of covariate-adjusted ADPN was much higher in the MZ than the DZ twins, suggesting again a genetic component to ADPN.
Because BMI is a determinant of ADPN, one might argue that the heritability of ADNP was only, or predominantly due to that of BMI. However, at least two considerations should be made. First, while a significant WP correlation exists for BMI in both MZ and DZ twins, for age-, gender-, and BMI-adjusted ADPN, such significant correlation was observed only in MZ, and not in DZ (Fig. 1
). Second, our twin analysis yielded even stronger estimates of heritability when age-, gender-, and BMI-adjusted ADPN rather than raw ADPNs were examined. Thus, this contention does not seem to be supported by our results.
Moreover, the observation that the WP difference of ADPN was lower in MZ than in DZ twins (Fig. 2
) also supports the conclusion that ADPN is under strong genetic control. To gather insight into the genetic factors underlying this association, we genotyped our twins at two widely investigated SNPs, the +45 T/G (on exon 2) and the –11377 G/C (on the promoter) in the ACDC gene, whose influence on ADPN is conflicting. Overall, studies in non-Caucasian populations of nondiabetics and nonobese subjects were negative (25, 36), while positive results for both SNPs were described in Caucasians (23, 37). We calculated the WP difference of ADPN in the MZ and DZ twins who were concordant or discordant for each genotype. We found a significant effect of the +45 T/G SNP, but no evidence for an effect of the –11377 G/C SNP. Although some caution is advised because of the small number of our twins who had the rarer alleles, this result favors the contention that the +45 T/G but not the –11377 G/C SNP affects ADPN. Noteworthy the finding that in the STOP-NIDDM Study the +45 T/G SNP was associated with the risk of conversion from impaired glucose tolerance to type 2 diabetes (38) accords with our suggestion. However, other SNPs exist in the ACDC gene that were not determined in this study but might be in linkage disequilibrium with those investigated here. Therefore, our finding of strong heritability of ADPN warrants further investigation of other variants in the ACDC gene.
The relatively small sample size might also suggest some caution; however, we would like to point out that this sample size was adequate to show heritability for other variables, as plasminogen activator inhibitor type 1, homocysteine, and angiotensin-converting enzyme (30, 31), which are known to exhibit a higher biological and within-assay variability than ADPN. Moreover, it could be that the shared family environment of most of our twins might have led to some inflation of the estimates of heritability. Therefore, to explore the impact of shared environment and lifestyle on ADPN, we performed a subanalysis of cohabitant and noncohabitant twins. Despite the obvious limitations of post hoc analyses and the inherent loss of statistical power, this disclosed a much higher WP correlation coefficient of ADPN in MZ than in DZ twins, regardless of the cohabitation status, thus suggesting the predominant effect of genetic identity over cohabitation in determining ADPN.
In conclusion, our twins data provide novel information on the relative effects of environmental and genetic factors on ADPN and BMI, two quantitative traits that are relevant for IR, the metabolic syndrome, and diabetes mellitus type 2. They indicate that both ADPN and BMI are genetically determined, and that minor influences of environmental factors unique to individuals also exist. Moreover, they suggest that the +45 T/G SNP, but not the –11377 G/C SNP, has an effect on plasma levels of adiponectin. Whether other SNPs in the ACDC gene, which might be in linkage disequilibrium with the +45 T/G but were not assessed in this study, contribute to ADPN remains to be investigated.
| Footnotes |
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Disclosure Statement: The authors have nothing to declare.
First Published Online May 29, 2007
Abbreviations: A, Additive genetic influence; AC, among component; ADPN, plasma adiponectin levels; BMI, body mass index; D, nonadditive genetic influence; DZ, dizygotic; E, environmental influence unique to individuals; IR, insulin resistance; MZ, monozygotic; NS, nonsignificant; S, environmental influences shared by cotwins within a family; SNP, single nucleotide polymorphism; WP, within pair.
Received February 22, 2007.
Accepted May 18, 2007.
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