Genetic Variation and Obesity in Australian Women: A Prospective Study
Abstract
Objective: A number of candidate genes have been implicated in the pathogenesis of obesity in humans. This study examines associations between longitudinal changes in body mass and composition and the presence of polymorphisms in the β-3 adrenergic receptor, tumor necrosis factor-α, leptin, and leptin receptor (Lepr) in a cohort of Australian women.
Research Methods and Procedures: Healthy white Australian women (n = 335) were randomly selected from the Barwon region of Victoria and underwent baseline anthropometry and double-energy X-ray absorptiometry for assessment of body mass and adiposity. These measurements were repeated again at 2-year follow-up. Genomic DNA was extracted and used for polymerase chain reaction-based genotyping of all polymorphisms.
Results: The Pro1019Pro Lepr polymorphism was associated with longitudinal increases in body weight (p = 0.02), fat mass (p = 0.05), and body mass index (p = 0.01) in this study, and individuals homozygous for the A allele at this locus had a greater propensity to gain body fat over time. The largest effects on body composition seemed to be in individuals already obese at baseline. Changes in body weight, fat mass, percent body fat, and body mass index over a 2-year period were not associated with genetic variation in the β-3 adrenergic receptor (Trp64Arg), tumor necrosis factor-α promoter, or leptin genes in non-obese or obese women.
Discussion: These results suggest that a Lepr polymorphism is involved in the regulation of body mass and adiposity in obese Australian white women, which may have implications for the treatment of obesity in this population.
Introduction
Human obesity is regarded as a multifactorial disease with a large heritable component (1). Family, twin, and adoption studies suggest that the transmission of body fat or obesity in humans may be anywhere from 10% to 80% (2). Furthermore, weight gain and weight loss in humans is itself known to have a heritable component (3) (4) (5). In the Bouchard studies, long-term overfeeding revealed three times more variance in response between twin pairs than within pairs for the gains in body weight, fat mass, and fat free mass (4). Several candidate genes have now been implicated in the regulation of body weight and adiposity in humans and rodents. These candidates include the β3-adrenergic receptor, leptin, leptin receptor, and the tumor necrosis factor (TNF)-α genes.
Lep and Lepr encode leptin and the leptin receptor, respectively. These genes have important roles in the regulation of body weight and fat mass in both humans and rodents, and mutations in these genes result in the development of severe obesity and endocrine abnormalities (6) (7) (8). A highly polymorphic tetranucleotide repeat has been identified in the 3′flanking region of Lep that has been associated with adiposity in a Japanese cohort (9). A number of sequence variants have also been identified throughout the length of Lepr, including an A/G polymorphism in the second position of codon 223 (Gln223Arg) and a G/A variant in the third position of codon 1019 (Pro1019Pro) (10) (11). The Gln223Arg polymorphism has been associated with adiposity and body composition in a number of populations (11) (12) (13), whereas the Pro1019Pro polymorphism has been associated with adiposity in Pima Indians only (11).
The β-3 adrenergic receptor (B3AR) is viewed as a potential regulator of fat metabolism and body weight, with regulation believed to occur through control of thermogenesis (14) (15). In 1995, several groups simultaneously reported the presence of a common polymorphism in this gene, characterized by an amino acid substitution of tryptophan by arginine at position 64, within the first intracellular loop of the receptor (16) (17) (18). This polymorphism has been associated with body weight in several populations, although many conflicting results have been reported (16) (17) (18) (19). Recent studies and one meta-analysis of the B3AR gene have provided evidence that this gene should still be viewed as a candidate for obesity (19) (20), although another meta analysis suggests that there is no association between the Trp64Arg polymorphism and body mass index (BMI) (21).
TNF-α is hypothesized to be a molecular link between obesity and type 2 diabetes (22). Although the mechanism of TNF-α action is unclear, it is believed to involve inhibition of insulin receptor activity (22). Several groups have examined sequence variation in TNF-α and its promoter region, and a number of polymorphisms have been detected (23) (24). One such polymorphism is an A/G base change at position −308, within the promoter of TNF-α, which has been associated with adiposity in humans (24) (25). However, several recent studies examining this polymorphism have reported conflicting results (26) (27).
To date, the majority of studies examining candidate genes for obesity or type 2 diabetes in humans have been cross-sectional in design, and although this type of analysis is useful, it is often limited due to the dynamic nature of obesity-related phenotypes. For this reason, longitudinal studies may provide a more comprehensive insight into the influence of a gene on the regulation of body mass and adiposity. Although there have been a number of studies examining the effects of genotype on longitudinal changes in various phenotypes, there has not been a large amount of work published in relationship to body weight changes (16) (28) (29) (30). The current study examined the presence of polymorphisms in a number of candidate genes (B3AR, Lep, TNF-α, and Lepr) in a population of white Australian women and investigated associations between the selected polymorphisms and changes in body mass and adiposity over a 2-year period.
Research Methods and Procedures
Subjects
In the present study we examined sequence variation in the B3AR, TNF-α, Lep and Lepr genes in a population of white Australian women (n = 335) residing in the Barwon region of Victoria. These healthy women were randomly selected from a larger, population-based Osteoporosis Study covering a range of age (20 to 83 years), body weight (42 to 121 kg), and BMI (18 to 46 kg/m2). At baseline and at 2-year follow-up, subjects underwent basic anthropometry, which was used to calculate BMI, and a DXA scan (Lunar DPX-L; Lunar, Madison, WI), which was used to measure fat mass, lean body mass, and to calculate percent fat mass. Subjects were stratified for obesity using the criteria of the World Health Organization (BMI > 30 kg/m2), with 118 women classified as obese and 217 women classified as non-obese.
Genotyping
Genomic DNA was extracted from lymphocytes using a phenol/chloroform extraction and ethanol precipitation technique and subsequently quantitated by absorbance at 260 nm. Amplification was performed using published oligonucleotide sequences and polymerase chain reaction (PCR) conditions to amplify the regions of interest within the appropriate genes: B3AR (18), Lep (9), Lepr (Gln223Arg and Pro1019Pro) (10), and TNF-α (24). In certain cases for technical reasons, only a subset of the population was genotyped for the polymorphisms.
For the Lep polymorphism, genotypes were classified as genotype 1 if fragment sizes were 121 to 145 base pairs in length, representing fewer tetranucleotide repeats, or as genotype 2 if fragment sizes were 197 to 221 base pairs in length, representing a larger number of tetranucleotide repeats. This assignment of alleles is in accordance with previously published criteria (9).
PCR-restriction fragment length polymorphism (RFLP) was performed to determine genotypes at the B3AR, TNF-α, and Lepr (Gln223Arg and Pro1019Pro) polymorphisms. After amplification, the PCR products were digested with the appropriate restriction enzyme (BstNI for the B3AR polymorphism, Hae III for the Lepr Gln223Arg polymorphism, and Nco I for the Lepr Pro1019Pro and TNF-α promoter polymorphisms) for 2 hours. All PCR-RFLP products were subsequently analyzed by agarose gel electrophoresis and visualized using ethidium bromide, with genotyping performed by sizing of the digested PCR products.
Statistical Analyses
Comparison of mean changes in body mass or composition between the two genotypes for each polymorphism with adjustment for covariates (age and baseline anthropometry as indicated) was performed using ANOVA and multiple linear regression analysis (SPSS, Version 9.0; SPSS, Chicago, IL). First-order interaction between a genotype and a covariate in predicting an outcome (e.g., weight gain) was assessed in a regression model that contained terms for the covariate, the genotype, and the covariate–genotype interaction. The F test was used to determine whether differences were statistically significant. In all cases, p < 0.05 was considered statistically significant.
Results
Table 1 shows the allele and genotype frequencies of each of the polymorphisms investigated in this population. The variant alleles were classified as designated in previously published studies: B3AR (18), Lep (9), Lepr (Gln223Arg and Pro1019Pro) (10), TNF-α (24). The frequency of the least frequent allele was 0.44, 0.42, 0.35, 0.08, and 0.20 for the Lep tetranucleotide repeat, Lepr Gln223Arg, Lepr Pro1019Pro, B3AR Trp64Arg, and TNF-α promoter polymorphisms, respectively. All genotype frequencies were found to be in Hardy–Weinberg equilibrium in this population.
| Allele frequency | Genotype frequency | ||||
|---|---|---|---|---|---|
| Lep | I: 0.44 | II: 0.56 | I/I: 0.23 | I/II: 0.42 | II/II: 0.35 |
| Gln223Arg | A: 0.58 | G: 0.42 | A/A: 0.32 | A/G: 0.52 | G/G: 0.16 |
| Pro1019Pro | G: 0.65 | A: 0.35 | G/G: 0.43 | G/A: 0.44 | A/A: 0.13 |
| B3AR | T: 0.92 | C: 0.08 | T/T: 0.846 | T/C: 0.145 | C/C: 0.009 |
| TNF-α | A: 0.80 | G: 0.20 | A/A: 0.69 | A/G: 0.21 | G/G: 0.10 |
- I, fewer tetranucleotide repeats; II, larger number of tetranucleotide repeats (5).
To examine the effect of the variant allele in each polymorphism on various phenotypic measures, the heterozygous and homozygous variant genotypes were combined (genotype 2). This grouping was performed to increase the numbers in the group with the variant allelle to allow a meaningful analysis of the data. The results from this analysis do not differ significantly from those obtained when the three genotypes were examined without grouping. Interaction effects were also tested between baseline body weight and all five polymorphisms and baseline BMI and all five polymorphisms in the prediction of change in body weight or composition over time. No significant interactions were detected.
Table 2 shows baseline and longitudinal variables for the whole population and obese and non-obese women separately.
| Variable | All (n = 335) | Non-obese (n = 217) | Obese (n = 118) |
|---|---|---|---|
| Baseline age | 42.16 ± 0.69 | 41.23 ± 0.88 | 49.69 ± 1.23 |
| Baseline weight | 71.65 ± 0.86 | 62.04 ± 0.50 | 89.30 ± 1.04 |
| Baseline fat (kg) | 28.22 ± 0.62 | 21.19 ± 0.39 | 41.14 ± 0.64 |
| Baseline fat (%) | 38.0 ± 0.43 | 33.70 ± 0.42 | 45.90 ± 0.32 |
| Baseline fat free mass (kg) | 43.43 ± 0.31 | 40.85 ± 0.26 | 48.17 ± 0.53 |
| Baseline BMI | 27.28 ± 0.33 | 23.36 ± 0.16 | 34.47 ± 0.34 |
| Δ weight (kg) | 0.90 ± 0.25 | 1.36 ± 0.24 | 0.04 ± 0.57 |
| Δ fat (kg) | 1.42 ± 0.24 | 1.71 ± 0.27 | 0.85 ± 0.50 |
| Δ fat (%) | 1.65 ± 0.21 | 2.07 ± 0.27 | 0.82 ± 0.29 |
| Δ fat free mass (kg) | −0.57 ± 0.14 | −0.41 ± 0.14 | −0.86 ± 0.31 |
| Δ BMI | 0.33 ± 0.10 | 0.51 ± 0.09 | 0.003 ± 0.22 |
Table 3 shows associations between longitudinal changes in body weight and body fat and the five polymorphisms typed for the whole population. No statistically significant differences were found between change in body weight, fat mass, or percent body fat and the B3AR, Lep, or TNF-α polymorphisms. There was a tendency for subjects with genotype 2 for the Gln223Arg polymorphism (A/G + G/G) to have a larger increase in body weight and BMI over the 2-year period, compared with subjects with genotype 1 for this polymorphism (A/A), although this did not reach statistical significance.
| Genotype | Baseline body weight (kg)* | Δ Body weight (kg)† | Δ Fat mass (kg)‡ | Δ Percentage of body fat§ | Δ BMI¶ |
|---|---|---|---|---|---|
| Lep | |||||
| 1 (n = 68) | 74.9 ± 1.8 | 0.28 ± 0.67 | 1.87 ± 0.59 | 1.60 ± 0.47 | 0.12 ± 0.25 |
| 2 (n = 221) | 75.2 ± 1.0 | 0.56 ± 0.37 | 1.18 ± 0.33 | 1.10 ± 0.26 | 0.20 ± 0.14 |
| Gln223Arg | |||||
| 1 (n = 116) | 72.5 ± 1.5 | 0.62 ± 0.43 | 1.12 ± 0.41 | 1.43 ± 0.34 | 0.23 ± 0.16 |
| 2 (n = 219) | 71.2 ± 1.1 | 1.05 ± 0.31 | 1.58 ± 0.30 | 1.77 ± 0.25 | 0.39 ± 0.12 |
| Pro1019Pro | |||||
| 1 (n = 140) | 71.2 ± 1.3 | 0.26 ± 0.39 | 0.97 ± 0.37 | 1.53 ± 0.31 | 0.07 ± 0.15 |
| 2 (n = 186) | 71.4 ± 1.1 | 1.45 ± 0.34** | 1.97 ± 0.34†† | 1.99 ± 0.28 | 0.56 ± 0.13‡‡ |
| B3AR | |||||
| 1 (n = 247) | 74.3 ± 1.0 | 0.30 ± 0.36 | 1.31 ± 0.32 | 1.21 ± 0.26 | 0.10 ± 0.14 |
| 2 (n = 42) | 78.6 ± 2.3 | 1.42 ± 0.86 | 1.75 ± 0.77 | 1.54 ± 0.62 | 0.53 ± 0.33 |
| TNF-α | |||||
| 1 (n = 199) | 73.9 ± 1.1 | 0.93 ± 0.39 | 1.61 ± 0.34 | 1.42 ± 0.28 | 0.36 ± 0.15 |
| 2 (n = 80) | 73.5 ± 1.7 | 0.11 ± 0.61 | 1.26 ± 0.54 | 1.54 ± 0.45 | −0.06 ± 0.23 |
- Genotype 1, homozygous for wild-type allele; genotype 2,heterozygous and homozygous for the variant allele.
- * Adjusted for age.
- † Adjusted for age and baseline body weight.
- ‡ Adjusted for age and baseline fat mass.
- § Adjusted for age and baseline percent body fat.
- ¶ Adjusted for age and baseline BMI.
- ** Significantly different from genotype 1, p =0.02.
- †† Significantly different from genotype 1, p =0.05.
- ‡‡ Significantly different from genotype 1, p = 0.01.
Subjects with genotype 2 for the Pro1019Pro polymorphism (G/A + A/A) were found to have a significantly greater increase in body weight (p = 0.02), fat mass (p = 0.05), and BMI (p = 0.01) over the 2-year period compared with subjects with genotype 1. This difference was independent of age and baseline anthropometry. The percent variance accounted for by the gene (adjusted R2 × 100) was <3.5%, suggesting that although the results are significant, the Pro1019Pro polymorphism accounts for only a small degree of the variance in the changes seen. There was also a tendency for subjects with genotype 2 to have a larger increase in percent body fat during the follow-up period, although this did not reach statistical significance (Table 3).
To further examine the influence of these polymorphisms on changes in body mass and adiposity, subjects were divided into those that were obese (BMI ≥ 30 kg/m2) and non-obese (BMI < 30 kg/m2) at the initial assessment (baseline). Table 4 shows that there were no statistically significant differences in the phenotypic measures examined between the two genotypes for the Lep, Lepr Gln223Arg, B3AR, or TNF-α promoter polymorphisms in non-obese women after adjustment for age and baseline anthropometry (Table 4).
| Genotype | Baseline body weight (kg)* | Δ Body weight (kg)† | Δ Fat mass (kg)‡ | Δ Percentage of body fat§ | Δ BMI¶ |
|---|---|---|---|---|---|
| Lep | |||||
| 1 (n = 37) | 63.7 ± 1.2 | 0.66 ± 0.97 | 2.22 ± 0.89 | 2.36 ± 0.90 | 0.30 ± 0.38 |
| 2 (n = 130) | 65.6 ± 0.7 | 0.88 ± 0.41 | 1.38 ± 0.39 | 1.17 ± 0.35 | 0.31 ± 0.15 |
| Gln223Arg | |||||
| 1 (n = 71) | 62.2 ± 0.9 | 1.19 ± 0.41 | 1.41 ± 0.48 | 1.80 ± 0.47 | 0.45 ± 0.15 |
| 2 (n = 146) | 62.0 ± 0.6 | 1.45 ± 0.29 | 1.86 ± 0.32 | 2.20 ± 0.33 | 0.54 ± 0.11 |
| Pro1019Pro | |||||
| 1 (n = 96) | 63.0 ± 0.7 | 1.19 ± 0.36 | 1.59 ± 0.39 | 0.22 ± 0.39 | 0.44 ± 0.14 |
| 2 (n = 117) | 61.0 ± 0.7 | 1.54 ± 0.33 | 1.94 ± 0.36 | 0.25 ± 0.37 | 0.58 ± 0.12 |
| B3AR | |||||
| 1 (n = 146) | 64.7 ± 0.7 | 0.85 ± 0.42 | 1.61 ± 0.41 | 1.50 ± 0.38 | 0.32 ± 0.16 |
| 2 (n = 20) | 65.4 ± 1.9 | 1.56 ± 0.87 | 1.81 ± 1.01 | 1.66 ± 1.18 | 0.61 ± 0.32 |
| TNF-α | |||||
| 1 (n = 126) | 64.7 ± 0.69 | 1.20 ± 0.43 | 1.84 ± 0.42 | 1.69 ± 0.39 | 0.45 ± 0.17 |
| 2 (n = 47) | 63.9 ± 1.2 | 0.44 ± 0.67 | 2.07 ± 0.57 | 2.44 ± 0.71 | 0.18 ± 0.24 |
- Genotype 1, homozygous for wild-type allele; genotype 2,heterozygous and homozygous for the variant allele.
- * Adjusted for age.
- † Adjusted for age and baseline body weight.
- ‡ Adjusted for age and baseline fat mass.
- § Adjusted for age and baseline percent body fat.
- ¶ Adjusted for age and baseline BMI.
Table 5 shows that in women who were obese at baseline, polymorphisms in Lep, B3AR gene, and TNF-α promoter were not associated with changes in body mass or adiposity during follow-up after adjustment for age and baseline anthropometry. A similar result was seen for the Gln223Arg polymorphism in the Lepr. However, the A allele at the Pro1019Pro polymorphism in Lepr was associated with an increase in body weight (p = 0.01), fat mass (p = 0.01), percent body fat (p = 0.04), and BMI (p = 0.01) over the 2-year period. The percent variance accounted for by the polymorphism against each of these variables was <6%, again suggesting that only a small degree of the variability seen in the phenotypic changes is accounted for by this gene. These differences were independent of age and baseline anthropometry.
| Genotype | Baseline body weight (kg)* | Δ Body weight (kg)† | Δ Fat mass (kg)‡ | Δ Percentage of body fat§ | Δ BMI¶ |
|---|---|---|---|---|---|
| Lep | |||||
| 1 (n = 31) | 87.3 ± 1.9 | −0.17 ± 1.14 | 1.57 ± 1.03 | 1.00 ± 0.61 | 0.09 ± 0.44 |
| 2 (n = 91) | 89.3 ± 1.1 | 0.12 ± 0.66 | 0.84 ± 0.58 | 0.86 ± 0.35 | 0.04 ± 0.25 |
| Gln223Arg | |||||
| 1 (n = 45) | 89.4 ± 1.5 | −0.32 ± 0.93 | 0.60 ± 0.84 | 0.61 ± 0.49 | −0.17 ± 0.35 |
| 2 (n = 73) | 89.3 ± 1.2 | 0.25 ± 0.72 | 1.00 ± 0.63 | 0.94 ± 0.37 | 0.11 ± 0.27 |
| Pro1019Pro | |||||
| 1 (n = 44) | 87.8 ± 1.5 | −1.7 ± 0.9 | −0.60 ± 0.82 | 0.13 ± 0.48 | −0.70 ± 0.35 |
| 2 (n = 69) | 90.0 ± 1.2 | 1.3 ± 0.7** | 2.14 ± 0.73** | 1.44 ± 0.42†† | 0.50 ± 0.28** |
| B3AR | |||||
| 1 (n = 101) | 87.9 ± 1.0 | −0.54 ± 0.65 | 0.82 ± 0.56 | 0.75 ± 0.33 | −0.22 ± 0.25 |
| 2 (n = 22) | 91.7 ± 2.9 | 1.46 ± 1.39 | 1.94 ± 1.25 | 1.57 ± 0.74 | 0.53 ± 0.53 |
| TNF-α | |||||
| 1 (n = 73) | 89.7 ± 1.4 | 0.41 ± 0.76 | 1.10 ± 0.64 | 0.82 ± 0.39 | 0.16 ± 0.29 |
| 2 (n = 33) | 87.3 ± 1.9 | −0.73 ± 1.13 | 0.35 ± 0.97 | 0.58 ± 0.59 | −0.33 ± 0.43 |
- Genotype 1, homozygous for wild-type allele; genotype 2,heterozygous and homozygous for the variant allele.
- * Adjusted for age.
- † Adjusted for age and baseline body weight.
- ‡ Adjusted for age and baseline fat mass.
- § Adjusted for age and baseline percent body fat.
- ¶ Adjusted for age and baseline BMI.
- ** Significantly different from genotype 1, p =0.01.
- †† Significantly different from genotype 1, p =0.04.
Discussion
This study provides an examination of longitudinal changes in body mass and adiposity in Australian white women and demonstrates an association between increases in body weight, fat mass, percent body fat, and BMI in women who were obese at baseline and the A allele of the Pro1019Pro polymorphism of Lepr. We hypothesize, therefore, that genetic variation in the leptin receptor may contribute to excessive accumulation of body fat over time, which leads to the development of obesity.
We formally tested for different effects of the A allele of the Pro1019Pro polymorphism of Lepr by baseline body weight and BMI through the insertion of interaction terms between each polymorphism and these baseline measures. None was significant at p < 0.05, indicating that the effects of this polymorphism on changes in body mass and composition did not formally differ by these baseline measures. There was, however, some support for an interaction between this polymorphism and baseline body weight and BMI, because p values of 0.051 and 0.08 were obtained, respectively. Although the results were not significant, they suggest that we cannot dismiss completely the idea that there is an interaction effect between this polymorphism and body weight and BMI. Furthermore, when we subdivided our analysis of change in body mass and composition over time, the effects of this polymorphism seemed mainly confined to obese women, again supporting the idea that there may be some interaction. Further follow-up and analysis in this cohort may reveal the presence of a statistically significant interaction between the gene and the phenotypes examined.
The frequencies of the polymorphic alleles examined in these Australian women are similar to those reported in other white populations. The TNF-α promoter polymorphism has been reported to occur at a frequency of ∼0.20 in a Spanish population (24), whereas the variant alleles of the Lepr Gln223Arg and Pro1019Pro polymorphisms were reported to occur at a frequency of 0.44 and 0.36, respectively, in a population of white British men (10). The arginine allele of the Trp64Arg polymorphism has been found at a frequency of 0.08 in elderly Australians (31), and in a population of Japanese subjects, allele II of the Lep tetranucleotide repeat polymorphism was found at a frequency of 0.30 (9).
Lepr is involved in body weight regulation, and mutations in this gene result in the development of a morbidly obese phenotype in both humans and rodents (6) (7) (8). In the present study, obese women with the A allele for Pro1019Pro polymorphism in Lepr had excessive increases in body weight, fat mass, percent body fat, and BMI over the 2-year period, compared with subjects homozygous for the G allele. Although this polymorphism does not encode an amino acid change, these findings suggest that this polymorphism may be involved in body weight regulation in obese white women. Recent studies examining the pathophysiology of type 2 diabetes in humans revealed the importance of an intronic single nucleotide polymorphism in determining the function and activity of the calpain 10 gene (32). Furthermore, this intronic single nucleotide polymorphism was found to be important for the development of insulin resistance and, thereby, played a role in the progression toward the diabetic state (32) (33). Because the leptin receptor is undoubtedly involved in a metabolic pathway important for the regulation of body weight, the possibility that a polymorphism in the leptin receptor not encoding an amino acid change could alter body mass or composition is plausible. This study does not rule out the possibility that the Pro1019Pro polymorphism may be a marker for another polymorphism within the leptin receptor gene that does alter the amino acid sequence of the protein or alters protein function or stability. Our findings, however, do imply some sort of synergism between baseline obesity and the leptin receptor gene.
Previous studies have indicated that the genes examined may be involved in the regulation of body mass or adiposity in humans. However, in this population of white Australian women, the Lep, Lepr Gln223Arg, B3AR, or TNF-α promoter polymorphisms were not associated with longitudinal changes in body weight, fat mass, percentage body fat, or BMI over a 2-year period. There was no association in the population as a whole or when the subjects were separated into obese and non-obese groups.
A number of studies have examined the effects of genotype on longitudinal changes in various phenotypes, although there has not been a large amount of work published in relationship to body weight changes. Despite this, longitudinal changes in body weight have been associated with the apolipoprotein E gene (28), the uncoupling protein 1 gene (29) (30), and the β3-adrenergic receptor gene (16) polymorphisms. In this latter study, French white patients with morbid obesity who were heterozygous for the Trp64Arg mutation showed an increased capacity to gain weight over 25 years. In our population of Australian women, there was a trend toward increased weight gain in those heterozygous and homozygous for the Arg allele at 2-year follow-up, although this did not reach significance. It is possible that a longer follow-up period may reveal an association between weight gain and this B3AR gene polymorphism in Australian women, similar to the French white population previously studied.
Obesity, like type 2 diabetes, is a polygenic disease and susceptibility to obesity is influenced by a number of genes, each believed to have a small effect on disease development. The leptin receptor has been associated with adiposity in a number of human studies, and here we provide evidence that this gene is associated with the obese phenotype and increasing adiposity over time in white women. The leptin receptor may only exert a small effect on body mass or composition at any given time; however, over a two-year period, the effects of genetic variation in this gene on adiposity become apparent. This work, therefore, highlights the importance of examining longitudinal changes in body composition when assessing dynamic obesity-related phenotypes and further supports the idea that obesity can develop through key genes exerting minor effects to influence adiposity.
In conclusion, this study examined five polymorphisms in candidate genes previously shown to be associated with body weight regulation. The Lepr Pro1019Pro polymorphism was associated with increased gain in body mass and adiposity over a 2-year period in already obese women. These results suggest that a Lepr polymorphism is involved in the regulation of body mass and adiposity in obese Australian white women, which may have implications for the treatment of obesity in this population.
Acknowledgments
This work was funded by the Autogen Ltd., Australia, the Victorian Health Promotion Foundation, and the Geelong Region Medical Research Foundation.
