Objective: Reduced circulating adiponectin levels contribute to the aetiology of insulin resistance. Adiponectin circulates in three different isoforms: high molecular weight (HMW), medium molecular weight (MMW) and low molecular weight (LMW) isoforms. The genetics of adiponectin isoforms is mostly unknown. Our aim was to investigate whether and to which extent circulating adiponectin isoforms are heritable and whether they share common genetic backgrounds with insulin resistance-related traits.
Methods: In a family-based sample of 640 nondiabetic White Caucasians from Italy, serum adiponectin isoforms concentrations were measured by ELISA. Three single nucleotide polymorphisms (SNPs) in the ADIPOQ gene previously reported to affect total adiponectin levels (rs17300539, rs1501299 and rs677395) were genotyped. The heritability of adiponectin isoform levels was assessed by variance component analysis. A linear mixed effects model was used to test the association between SNPs and adiponectin isoforms. Bivariate analyses were conducted to study genetic correlations between adiponectin isoforms levels and other insulin resistance-related traits.
Results: All isoforms were highly heritable (h(2) = 0.60-0.80, P = 1.0 x 10(-13)-1.0 x 10(-23)). SNPs rs17300539, rs1501299 and rs6773957 explained a significant proportion of HMW variance (2-9%, P = 1.0 x 10(-3)-1.0 x 10(-5)). In a multiple-SNP model, only rs17300539 and rs1501299 remained associated with HMW adiponectin (P = 3.0 x 10(-4) and 2.0 x 10(-2)). Significant genetic correlations (P = 1.0 x 10(-2)-1.0 x 10(-5)) were observed between HMW adiponectin and fasting insulin, homeostasis model assessment of insulin resistance, HDL cholesterol and the metabolic syndrome score. Only rs1501299 partly accounted for these genetic correlations.
Conclusion: Circulating levels of adiponectin isoforms are highly heritable. The genetic control of HMW adiponectin is shared in part with insulin resistance-related traits and involves, but is not limited to, the ADIPOQ locus.