The common PPARgamma Pro12Ala polymorphism is associated with decreased risk of type 2 diabetes.

Genetic association studies are viewed as problematic and plagued by irreproducibility. Many associations have been reported for type 2 diabetes, but none have been confirmed in multiple samples and with comprehensive controls. We evaluated 16 published genetic associations to type 2 diabetes and related sub-phenotypes using a family-based design to control for population stratification, and replication samples to increase power. We were able to confirm only one association, that of the common Pro12Ala polymorphism in peroxisome proliferator-activated receptor-gamma(PPARgamma) with type 2 diabetes. By analysing over 3,000 individuals, we found a modest (1.25-fold) but significant (P=0.002) increase in diabetes risk associated with the more common proline allele (85% frequency). Moreover, our results resolve a controversy about common variation in PPARgamma. An initial study found a threefold effect, but four of five subsequent publications failed to confirm the association. All six studies are consistent with the odds ratio we describe. The data implicate inherited variation in PPARgamma in the pathogenesis of type 2 diabetes. Because the risk allele occurs at such high frequency, its modest effect translates into a large population attributable risk-influencing as much as 25% of type 2 diabetes in the general population.

No relationship between identified variants in the uncoupling protein 2 gene and energy expenditure.

OBJECTIVE: The uncoupling protein 2 (UCP2) uncouples respiration from the oxidative phosphorylation in most cell types, predominantly in white fat and skeletal muscle. Since a decreased basal metabolic rate (BMR) would increase the susceptibility to weight gain, genetic alterations in the UCP2 gene could contribute to the pathogenesis of obesity and the metabolic syndrome (MSDR). DESIGN AND METHODS: To test this hypothesis, we PCR amplified the introns of the UCP2 gene and sequenced the exon/intron boundaries. This information was used to construct intronic primers and to screen obese patients with low BMR for mutations in the coding regions of the UCP2 gene, using the single-strand conformational polymorphism technique. Furthermore, we examined whether there is an association between a biallelic marker in the UCP2 gene and BMR or MSDR. RESULTS: The UCP2 gene is composed of six coding exons, covering 5 kb of chromosome 11q13. One polymorphism, but no mutations, were identified in the coding regions of the UCP2 gene. There were no significant differences in the allele or genotype frequencies of the Ala55Val polymorphism between 55 patients with MSDR and 46 healthy controls. No association was found between the UCP2 gene and BMR in patients with MSDR or in healthy controls. CONCLUSIONS: Mutation screening and association studies suggest that mutations in the coding regions of the UCP2 gene do not affect BMR and do not contribute to increased susceptibility to obesity or MSDR. The results cannot, however, exclude the possibility that variants in regulatory elements of the gene could contribute to the development of obesity or MSDR.

Interaction between the Asn291Ser variant of the LPL gene and insulin resistance on dyslipidaemia in high risk individuals for Type 2 diabetes mellitus.

AIMS: Lipoprotein lipase (LPL) is a major regulator of triglyceride clearance. A genetic variant of the LPL gene on chromosome 8p22, Asn291Ser, has previously been associated with dyslipidaemia and an increased frequency of cardiovascular disease as well as familial disorders of lipoprotein metabolism. The aim of this study was to test whether the phenotypic expression of the LPL Asn291Ser variant is dependent upon glucose tolerance and insulin resistance. Therefore, the Asn291Ser variant was examined in 192 patients with Type 2 diabetes, 278 subjects with normal glucose tolerance who are first degree relatives of patients with Type 2 diabetes and 226 healthy control spouses without family history of diabetes. METHODS: The subjects were genotyped with an allele-specific mini-sequencing method. Insulin resistance was estimated using the homeostasis model assessment (HOMA) index. RESULTS: The frequency of the Asn/Ser genotype was significantly increased in normoglycaemic subjects with hypertriglyceridaemia (> 1.7 mmol/1), and was associated with dyslipidaemia and increased systolic blood pressure. There was a significant interaction between Asn291Ser and insulin resistance in normoglycaemic subjects, indicating that dyslipidaemia is more severe in Asn/ Ser carriers with reduced insulin sensitivity. The frequency of the Asn/Ser genotype was not increased in diabetic subjects with hypertriglyceridaemia, but was associated with increased systolic blood pressure. CONCLUSIONS: The Asn/Ser genotype of the LPL gene is associated with dyslipidaemia in normoglycaemic subjects, and the dyslipidaemic phenotype is more severe in insulin-resistant subjects. This association is not seen in diabetic subjects.

Decreased expression and function of adipocyte hormone-sensitive lipase in subcutaneous fat cells of obese subjects.

Decreased lipolytic effect of catecholamines in adipose tissue has repeatedly been demonstrated in obesity and may be a cause of excess accumulation of body fat. However, the mechanisms behind this lipolysis defect are unclear. The role of hormone-sensitive lipase was examined using abdominal subcutaneous adipocytes from 34 obese drug-free and otherwise healthy males or females and 14 non-obese control subjects. The enzyme catalyzes the rate-limiting step of the lipolysis pathway. The maximum lipolytic capacity of fat cells was significantly decreased in obesity when measured using either a non-selective beta-adrenergic receptor agonist (isoprenaline) or a phosphodiesterase resistant cyclic AMP analogue (dibutyryl cyclic AMP). Likewise, enzyme activity, protein expression, and mRNA of hormone-sensitive lipase were significantly decreased in adipocytes of obese subjects. The findings were not influenced by age or gender. The data suggest that a decreased expression of hormone-sensitive lipase in subcutaneous fat cells, which in turn causes decreased enzyme function and impaired lipolytic capacity of adipocytes, is present in obesity. Impaired expression of the hormone-sensitive lipase gene might at least in part explain the enzyme defect.

The putative role of the hormone-sensitive lipase gene in the pathogenesis of Type II diabetes mellitus and abdominal obesity.

Impaired lipolysis has been proposed as a pathogenic factor contributing to clustering of abdominal obesity and dyslipidaemia in Type II (non-insulin-dependent) diabetes mellitus--that is, the metabolic syndrome (MSDR). As this syndrome clusters in families, alterations in the hormone-sensitive lipase (HSL) gene could contribute to the genetic predisposition to MSDR. To test this hypothesis we carried out population and intrafamily association studies in individuals with MSDR, using a polymorphic marker (LIPE) in the HSL gene. There was a significant difference in allele frequency distribution between 235 Type II diabetic patients and 146 control subjects (p = 0.002), particularly between 78 abdominally obese Type II diabetic patients with MSDR and the control group (p = 0.010). An extended transmission disequilibrium test (TDT) showed transmission disequilibrium of 66 alleles to 42 nondiabetic, abdominally obese offspring in families with Type II diabetes (p < 0.05). A slight difference in allele frequency distribution was seen between 71 individuals from the lowest and 71 from the highest tertile of isoprenaline-induced lipolysis in fat tissue (p = 0.07). No missense mutations were found with single-strand conformational polymorphism (SSCP) in 20 abdominally obese subjects with MSDR. In conclusion, our population and intrafamily association studies suggest that the LIPE marker in the HSL gene is in linkage disequilibrium with an allele and/or gene which increases susceptibility to abdominal obesity and thereby possibly to Type II diabetes.

Pro-opiomelanocortin gene is associated with serum leptin levels in lean but not in obese individuals.

OBJECTIVE: Mutations in the pro-opiomelanocortin and melanocortin 4 receptor genes (POMC and MC4R) cause monogenic obesity, and the POMC locus (2p21) has been linked to leptin levels and body mass index (BMI). We searched for monogenic obesity due to mutations in POMC and MC4R among morbidly obese Swedes and studied the association of POMC variants with BMI and serum leptin levels. DESIGN: MC4R and POMC were screened for mutations in 102 obese Swedish subjects (40+/-11 y, 41.3+/-5.0 kg/m(2)) using the single-strand conformation polymorphism technique. The detected polymorphisms were genotyped in 118 lean control subjects (56+/-11 y, 22.6+/-1.3 kg/m(2)) and studied for association with BMI and serum leptin levels. RESULTS: No cases of monogenic obesity due to mutations in POMC or MC4R were identified and none of the four common POMC polymorphisms (RsaI, ins56, Glu188Gly and C8246T) were associated with obesity. Lean carriers of the C8246T CC-genotype had higher serum leptin levels compared to CT or TT carriers (9.7+/-6.6 vs 6.7+/-4.4 microg/l, P=0.003 for leptin levels adjusted for age, sex and BMI in regression analysis), especially lean females (P=0.004) and lean female carriers with the C8246T(CC)/RsaI(--or +-) genotype combinations (P<0.0005). Neither the C8246T CC-genotype nor the C8246T(CC)/RsaI(--or +-) were associated with serum leptin levels in obese subjects. CONCLUSIONS: Monogenic forms of obesity due to mutations in POMC and MC4R are rare in Swedish obese patients. Polymorphisms in POMC are associated with variation in serum leptin levels within the normal range in healthy lean but not in obese individuals.