(Circulation. 1995;92:1089-1093.)
© 1995 American Heart Association, Inc.
Articles |
From the Departments of Medicine (R.A.H., P.W.C.), Clinical Biochemistry (R.A.H., P.W.C.), and Biochemistry (P.W.C.), St Michael's Hospital, University of Toronto, Ontario; and School of Nursing (J.H.B.), University of Victoria, British Columbia, Canada.
Correspondence to Robert A. Hegele, MD, DNA Research Laboratory, St Michael's Hospital, 30 Bond St, Toronto, Ontario, Canada M5B 1W8.
| Abstract |
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Methods and Results In 316 male Hutterites, we tested for phenotype-genotype association of two DNA polymorphisms on chromosome 1q and the ratio of waist-to-hip circumference. We included control loci on 10 other chromosomes in the multivariate model. We observed that DNA variation on chromosome 1q was significantly associated with variation in the ratio of waist-to-hip circumference in men (P=.0029).
Conclusions The association of DNA variation chromosome 1q with the ratio of waist-to-hip circumference in male Hutterites suggests that there are important structural elements in this genomic region that have a functional impact on body fat distribution.
Key Words: genetics obesity
| Introduction |
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| Methods |
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Hutterite society has a static intergenerational and intragenerational lifestyle.13 Colonies are effective surrogates for extended families; women marry between colonies, but men tend to remain within a colony.19 The prevalence of atherosclerosis risk factors is comparable to that found in other populations.20 Smoking is forbidden, but alcohol is not.21 Major meals are taken communally, and the diet is high in animal fat.21 Mechanized farming techniques have reduced the amount of aerobic work-related exercise.20
Subjects from 21 colonies of the Alberta Dariusleut and Lerherleut sects took part in the Canadian Heart Health Survey screening for coronary heart disease risk factors.22 23 Physical examination included determination of circumferences of waist and hip, body mass index (BMI) defined as ratio of weight to height2 (kg/m2), and four separate blood pressure determinations. Plasma samples from 846 Hutterites were obtained with informed consent. Exclusion criteria included an inadequate blood sample available for all biochemical and/or genetic determinations. The study was approved by the ethics review panels of the Universities of Alberta and Toronto.
Genetic Analyses
Sufficient DNA and phenotypic information
were obtained for most
analyses from 793 Hutterites. Established methods were used to
obtain genotypes for AGT codons 174 and
235,15 APOB codons 3611 and
4154,24 PON codon 192,25
LPL intron 6,26 VLDLR
trinucleotide repeat in the 5'-untranslated
region,27 APOC3 Sac I site polymorphism in
the 3'-untranslated region,28 LRP
tetranucleotide repeat in the 5'-untranslated region
(5'-tetra),29 clotting factor VII (F7) codon
353 protein polymorphism,30 HL codon
202,31 ACE (angiotensin-converting
enzyme) insertion-deletion polymorphism,32
LDLR exon 12 HincII site
polymorphism,33 and APOE.34
The chromosomal localization of these genes is included in Table
1
.
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Statistical Analysis
SAS (Version 6) was used for
all statistical
comparisons.35 Each quantitative variable was
transformed and subjected to analysis of normality. Because
indexes of obesity differ between men and women, the sexes were
analyzed separately. Logarithmically transformed WHR, BMI, and
systolic blood pressure had distributions that did not differ from
normal (data not shown). The transformed variables were used for
statistical analyses, but the nontransformed mean and standard
deviation values are presented in Table 2
.
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ANOVA was performed using the General Linear Models procedure to determine the sources of variation of WHR. F tests were computed from the type III sums of squares35 ; this form of the sums of squares applies to unbalanced study designs and reports the effect of an independent variable after adjustment for all other variables included in the model. The analysis was performed separately for men and women. The dependent variable was the natural logarithm of WHR. The independent variables were age, colony of origin (which was included to correct for variation that was related to other shared genetic and environmental factors), logarithm of height (which was included because it was postulated to be related to the dependent variable), and logarithm of systolic blood pressure (which was included because it was previously shown to be associated with variation in AGT, the locus of interest). Also included as independent variables were genotypes of AGT, APOB, PON, LPL, VLDLR, APOC3, LRP, F7, HL, ACE, LDLR, and APOE.
When a significant association was
identified within the entire group,
baseline traits among individuals classified by genotype were
subsequently compared with the use of either a t test for
least-squares mean values35 or a nonparametric
test for significant differences between groups (Kruskal-Wallis test,
2 approximation, NPAR1WAY
routine35 ).
| Results |
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Determinants of Variation in WHR
The ANOVA for men is shown
in Table 2
. At a nominal
P=.05, F values in the analysis of the natural
logarithm of WHR as the dependent variable and the 14 genetic
markers as independent class variables showed that only the
genotype of AGT codon 174 contributed significantly
to phenotypic variation. Age, colony of origin, and natural logarithm
of systolic blood pressure were also found to be significantly
associated with variation in the natural logarithm of WHR. The ANOVA
for women showed similar associations with age, colony of origin, and
systolic blood pressure but no significant association with any genetic
variable (data not shown).
Between-Genotype Differences
Least-squares mean values for
the significant
phenotype-genotype association were compared with the
use of a t test (Table 3
). The AGT
codon 174 genotype system had three genotype classes:
T/T (218 subjects), T/M (98 subjects), and M/M (3 subjects). We
excluded the 3 M/M subjects, because this group was too small to carry
out meaningful pairwise comparisons. Significant pairwise differences
between the least-squares mean values of the T/T and T/M classes were
found for the natural logarithms of WHR and systolic blood pressure
(P=.0011) but not of BMI. Remarkably, the mean systolic
blood pressure was higher in T/M subjects than in T/T subjects, but WHR
was lower in T/M subjects than in T/T subjects
(P=.0066).
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| Discussion |
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Because of its association with higher systolic blood pressure, the AGT M174 isoform might have been expected to have been associated with an increased WHR. Systolic blood pressure and indexes of body fat are closely correlated within families,8 but blood pressure and body fat might share relatively few genetic determinants.37 In the Hutterites, it is unlikely that the association between the AGT M174 isoform and a lower WHR was due to a direct physiological role of angiotensinogen in determining the distribution of body fat. Instead, we hypothesize that in this sample of male Hutterites, who are closely related and share a restricted gene pool, the AGT M174 isoform acted as a genetic marker for a closely linked gene on the long arm of chromosome 1 that was the actual determinant of the difference in body fat distribution. The loci corresponding on porcine chromosome 4 that determined abdominal and back fatness were syntenic with a region that is more centromeric on chromosome 1q than AGT. Thus, although AGT was a helpful surrogate marker in the Hutterites, the actual genetic determinant of variation of body fat distribution likely lies in close proximity to, but is not identical with, AGT. An approach analogous to quantitative trait locus mapping in inbred animals38 39 40 will be required using closely spaced markers in this region to specifically identify the causative gene on human chromosome 1.
The associations of this locus with variations in both blood pressure and WHR were detected only in men. There are several possible explanations for the sex-specificity of these associations, particularly with respect to WHR. For example, WHR differs significantly between the sexes.6 7 This could reflect fundamental sex-related differences in physiological determinants of WHR, including differences in the genetic determinants and differences in the gene-environment interactions that produce the measured phenotype. It is also possible that the variation in WHR among women is determined more by environmental factors than is WHR in men. Also, there may be tissue-specific hormonal modulation of the association of the locus with the phenotype, adding another level of complexity to any genotype-phenotype association. The sex-specificity of the association in the Hutterites could also relate to the cultural practice of women marrying between colonies but men remaining within a colony over time, although the mechanistic basis for such a relation is unclear.
We previously showed that there was strong, but not complete, linkage disequilibrium between alleles at AGT codons 174 and 235.15 This was due mainly to a higher prevalence of subjects homozygous for both 235M and 174T than would have been expected had there been no linkage disequilibrium.15 The absence of association between alleles of codon 235 and variation in WHR has several possible explanations. For example, alleles at codons 174 and 235 were not independent. Thus, our statistical model would not have detected associations with both if the association was primarily with alleles at one site. Also, linkage disequilibrium between alleles of codon 235 and the actual causative locus might have been dissipated, as has been observed for genes located near telomeres, like AGT.41 However, the actual basis for the association with WHR observed only for alleles of AGT codon 174 and not for alleles of codon 235 is unclear.
Others have reported associations between obesity and some of the candidate genes that we have tested. For example, variation at APOB was found to be associated with variation in percent body fat42 and in BMI.43 Also, variation at LDLR was found to be associated with variation in BMI in hypertensive subjects.44 However, variation in these two genes was not found to be associated with either WHR or BMI in the Hutterites. These discrepancies may reflect differences between the study samples related to ethnicity, genetic heterogeneity, environmental differences, ascertainment, or a combination.
Positive phenotype-genotype associations have been found with other candidate genes in unrelated populations. For example, variation at DRD2 (dopamine receptor; chromosome 11q22-23) was found to be associated with obesity45 and variation in both weight and height.46 Interestingly, one patient with a major deletion affecting chromosome 11p14-p12 had morbid obesity.47 Also, variation at INS (insulin; chromosome 11p15.5) was found to be associated with variation in body fat distribution in women.48 However, in the Hutterites, variation at APOC3 on chromosome 11q23 was not associated with WHR or BMI in either sex. Although the discrepancies might reflect differences between the study samples related to genetic heterogeneity, it is also possible that APOC3 is not in linkage disequilibrium with a functional locus on chromosome 11 that might actually contribute to variation in obesity phenotypes. More loci will need to be examined as a part of a genomic screen.
Recently, Zhang and colleagues49 reported the structure of a mutation of the obesity gene ob on mouse chromosome 6. Homozygosity for this mutation results in the obese phenotype in the mouse. The human homologue has been mapped on chromosome 7q31.17 This is close to the PON locus on chromosome 7q21-22, which was included in our analysis of the Hutterites. We found no association between variation in either BMI or WHR and PON genotypes in the Hutterites. Although this could be attributed to a loss of linkage disequilibrium between PON and the human homologue of ob in the Hutterites, it might also reflect the heterogeneity of genetic determinants of obesity-related phenotypes and the relative importance of different genes in various populations.
There was a significant phenotype-genotype association between WHR and AGT genotype in the study population, but the absolute difference between genotypic classes was small. This suggests that although genetic variation on chromosome 1q was a statistically significant determinant of WHR, it explains only a small proportion of the total variation in this trait. Thus, environmental factors, such as diet and exercise, and additional genetic loci certainly also contribute to variation in WHR. Analysis of genetic determinants of obesity in the general population will be further obscured by the unique landscape of environmental factors for each person. Genomic screening in related samples could help identify new genes that determine obesity. This might help both to identify individuals who are candidates for intervention and to identify metabolic pathways that may be important targets for new interventional strategies.
| Acknowledgments |
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Received January 23, 1995; revision received March 3, 1995; accepted March 19, 1995.
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