Impact of nine common type 2 diabetes risk polymorphisms in Asian Indian Sikhs: PPARG2 (Pro12Ala), IGF2BP2, TCF7L2 and FTO variants confer a significant risk.

BACKGROUND: Recent genome-wide association (GWA) studies have identified several unsuspected genes associated with type 2 diabetes (T2D) with previously unknown functions. In this investigation, we have examined the role of 9 most significant SNPs reported in GWA studies: [peroxisome proliferator-activated receptor gamma 2 (PPARG2; rs 1801282); insulin-like growth factor two binding protein 2 (IGF2BP2; rs 4402960); cyclin-dependent kinase 5, a regulatory subunit-associated protein1-like 1 (CDK5; rs7754840); a zinc transporter and member of solute carrier family 30 (SLC30A8; rs13266634); a variant found near cyclin-dependent kinase inhibitor 2A (CDKN2A; rs10811661); hematopoietically expressed homeobox (HHEX; rs 1111875); transcription factor-7-like 2 (TCF7L2; rs 10885409); potassium inwardly rectifying channel subfamily J member 11(KCNJ11; rs 5219); and fat mass obesity-associated gene (FTO; rs 9939609)]. METHODS: We genotyped these SNPs in a case-control sample of 918 individuals consisting of 532 T2D cases and 386 normal glucose tolerant (NGT) subjects of an Asian Sikh community from North India. We tested the association between T2D and each SNP using unconditional logistic regression before and after adjusting for age, gender, and other covariates. We also examined the impact of these variants on body mass index (BMI), waist to hip ratio (WHR), fasting insulin, and glucose and lipid levels using multiple linear regression analysis. RESULTS: Four of the nine SNPs revealed a significant association with T2D; PPARG2 (Pro12Ala) [odds ratio (OR) 0.12; 95% confidence interval (CI) (0.03-0.52); p = 0.005], IGF2BP2 [OR 1.37; 95% CI (1.04-1.82); p = 0.027], TCF7L2 [OR 1.64; 95% CI (1.20-2.24); p = 0.001] and FTO [OR 1.46; 95% CI (1.11-1.93); p = 0.007] after adjusting for age, sex and BMI. Multiple linear regression analysis revealed significant association of two of nine investigated loci with diabetes-related quantitative traits. The 'C' (risk) allele of CDK5 (rs 7754840) was significantly associated with decreased HDL-cholesterol levels in both NGT (p = 0.005) and combined (NGT and T2D) (0.005) groups. The less common 'C' (risk) allele of TCF7L2 (rs 10885409) was associated with increased LDL-cholesterol (p = 0.010) in NGT and total and LDL-cholesterol levels (p = 0.008; p = 0.003, respectively) in combined cohort. CONCLUSION: To our knowledge, this is first study reporting the role of some recently emerged loci with T2D in a high risk population of Asian Indian origin. Further investigations are warranted to understand the pathway-based functional implications of these important loci in T2D pathophysiology in different ethnicities.

Replication of association between a common variant near melanocortin-4 receptor gene and obesity-related traits in Asian Sikhs.

Recent genome-wide association studies (GWAS) in Asian Indians reported strong associations of variants near melanocortin-4 receptor (MC4R) and MLX interacting protein-like (MLXIPL) genes with insulin resistance and several obesity-related quantitative traits (QTs). Here, we evaluated the association of two variants (rs12970134 and rs4450508) near MC4R and a nonsynonymous (Gln241His) variant (rs3812316) in MLXIPL gene with type 2 diabetes (T2D) and obesity-related QTs in our case-control cohort (n = 1,528; 745 T2D cases and 783 controls) from a Sikh population from North India. We have successfully replicated the association of MC4R (rs12970134) with BMI (P = 0.0005), total weight (WT) (P = 0.001), and waist circumference (WC) (P = 0.001). These associations remained significant after controlling for multiple testing by applying Bonferroni's correction. However, our data did not confirm the association of rs3812316 in the MLXIPL gene with triglyceride (TG) levels. These observations demonstrate that the genetic variation in MC4R locus can have a moderate contribution in the regional fat deposition and development of central obesity in Asian Indians.

The Khatri Sikh Diabetes Study (SDS): study design, methodology, sample collection, and initial results.

Non-insulin-dependent diabetes mellitus, or type 2 diabetes (T2DM), has become a major public health problem in India. The high prevalence, relatively young age of onset, and strong familial aggregation of T2DM in some Indian communities remains to be explained. Many of the traditional risk factors established for European populations do not appear to be present in Asian Indians. Phase I of the Sikh Diabetes Study (SDS) was launched to build the population resources required to initiate a large-scale genetic epidemiological study of diabetes in an Asian Indian population. The SDS is focused on the Khatri Sikh population of North India. In all, 1,892 subjects were enrolled to participate in the family-based study; 1,623 of these subjects belong to 324 families, each of which has at least 2 siblings affected with T2DM. The sample included 1,288 individuals affected with T2DM (siblings, parents, or relatives) and 335 unaffected siblings, parents, or relatives. The remaining 269 subjects were unrelated nondiabetic control subjects, including unaffected spouses of probands or siblings. This primarily nonvegetarian, nonsmoking endogamous caste group has presented an unusual clinical picture of uneven distribution of adiposity and a high rate of T2DM with secondary complications. Such well-characterized population isolates may offer unique advantages in mapping genes for common complex diseases.