The mixed ancestry population of South Africa has one of the highest prevalence of type 2 diabetes in South Africa and sub-Saharan Africa at large , however, genetic abnormalities that can fully account for this have not been identified. In this study, we show that PPARG Pro12 is significantly associated with insulin resistance and type 2 diabetes in this population. We observed that neither IRS1 972Arg allele nor PPARG 12Ala were associated with type 2 diabetes or insulin resistance/sensitivity, but in a model containing both the alleles and their interaction term, the presence of the PPARG Pro12 conferred a 64% risk of prevalent type 2 diabetes. Furthermore the PPARG Pro12 was associated with increased levels of 2 hour post-OGTT insulin. Overall, our findings convincingly demonstrate that PPARG Pro12Ala –IRS1 Gly972Arg interactions, PPARG Pro12 and susceptibility to environmental factors might modulate the relationship between insulin resistance and type 2 diabetes in this population.
The gene-gene interaction between IRS1 Gly972Arg and PPARG Pro12Ala is of interest because the two polymorphisms exert opposite effects on type 2 diabetes predispositions. The Gly972Arg is a functional polymorphism reported to impair insulin signaling in transfected cell lines and in human cells carrying the variant [23–25]. Although individuals carrying the Gly972Arg are reported to have a 25% increased risk for developing diabetes , genome wide association (GWAS) studies involving subjects of European descent found no association between IRS1 and type 2 diabetes [26, 27]. On the other hand, the PPARG Pro12Ala, particularly the 12Ala has been associated with a reduced risk of type 2 diabetes and insulin resistance [9, 16–20]. As such, the polymorphisms of the IRS1 and PPARG genes have been shown to interact and elevate insulin sensitivity. This was evident in a study done by Stumvoll et al.,  where the authors showed that insulin sensitivity was significantly greater in subjects with X/Ala (PPARγ2) + X/Arg (IRS1 972) than in subjects with Pro/Pro (PPARγ2) + X/Arg (IRS1) while no differences were observed in X/Ala (PPARγ2) + Gly/Gly (IRS1 972) and Pro/Pro (PPARγ2) + Gly/Gly (IRS1 972) carriers . Similarly, the interaction between the two polymorphisms has been associated with higher adiponectin levels and the greatest increase was found in subjects who were homozygous for both PPARG alanine (Ala12Ala) and IRS1 glycine (Gly972Gly) . Adiponectin is secreted by the adipose tissue and is inversely associated with obesity, insulin resistance, type 2 diabetes and cardiovascular disease [30, 31]. Taken together these reports including ours confirm the combined effect of the two SNPs on insulin resistance and type 2 diabetes.
Several epidemiological studies have demonstrated that PPARG Pro12Ala is associated with insulin sensitivity and diabetes mellitus [6–9]. In the Human Genome Epidemiology (HuGE) meta-analysis involving 32 849 type 2 diabetes cases and 47 456 controls, the Pro12Ala was associated with a 14% lower risk for developing type 2 diabetes . However, other investigators have failed to demonstrate an association between Pro12Ala and insulin sensitivity using the gold standard method for assessing insulin resistance/sensitivity, the euglycemic hyperinsulinemic clamp [32, 33]. The differences between studies have been attributed to body mass index and ethnic differences [7, 8]. The frequency of the 12Ala has been reported to be more frequent in Caucasians than in Asian populations , but conferred significantly greater protection against type 2 diabetes among Asians than Caucasians (35% vs. 15%) . However, when the authors adjusted for body mass index the differences were no longer significant . In our study, the 10.4% frequency of Pro12Ala polymorphism is comparable to that found in Caucasians and the Pro12 was strongly associated with an increased 2 hour post-OGTT insulin levels in non-diabetic subjects. Our results further add to the growing body of evidence on the association of PPARG Pro12Ala, insulin resistance and subsequent type 2 diabetes. Herein we investigated a heterogeneous population, with 32-43% Khoisan, 20 – 36% Bantu-speaking African, 21 – 28% European and 9 – 11% Asian ancestry . Our present findings require replication in a larger study involving other homogenous population before they can be considered as established in Africa.
The strengths of the present study include the use of both fasting and OGTT derived indices for assessing type 2 diabetes and insulin resistance. OGTT derived indices have been found to be of superior predictive power to simple fasting indices of IR as they take post-load glucose-insulin interaction into account . Furthermore, we made use of two independent laboratories to genotype our study population. The major limitation of our study is the statistical power of the study which was limited by the small sample size and the examination of gene-gene interaction effects reduced the sample further. In addition, we did not adjust for population stratification. Potential population stratification in unrelated sample may cause spurious positive or negative associations in population-based association studies . To minimise this type of confounding in association studies, several approaches have been suggested that utilise specific informative markers and loci to model ancestral differences between cases and controls and subsequently correct allele frequency variations at candidate loci in populations. However, markers suitable for mapping disease genes or correcting for population stratification in the mixed ancestry are not yet available. Lastly, the nature of this study is cross-sectional with high female to male participation, the latter being a common trend in South African population studies.