Individuals with impaired glucose tolerance have increased proinsulin levels, despite normal glucose or C-peptide levels. In the Diabetes Prevention Program (DPP), increased proinsulin levels predicted type 2 diabetes and proinsulin levels were significantly reduced following treatment with metformin, lifestyle modification or troglitazone compared with placebo. Genetic and physiological studies suggest a role for the zinc transporter gene SLC30A8 in diabetes risk, possibly through effects on insulin-processing in beta cells. We hypothesised that the risk allele at the type 2 diabetes-associated missense polymorphism rs13266634 (R325W) in SLC30A8 would predict proinsulin levels in individuals at risk of type 2 diabetes and may modulate response to preventive interventions.
We genotyped rs13266634 in 3,007 DPP participants and examined its association with fasting proinsulin and fasting insulin at baseline and at 1 year post-intervention.
We found that increasing dosage of the C risk allele at SLC30A8 rs13266634 was significantly associated with higher proinsulin levels at baseline (p=0.002) after adjustment for baseline insulin. This supports the hypothesis that risk alleles at SLC30A8 mark individuals with insulin-processing defects. At the 1 year analysis, proinsulin levels decreased significantly in all groups receiving active intervention and were no longer associated with SLC30A8 genotype (p=0.86) after adjustment for insulin at baseline and 1 year. We found no genotype × treatment interactions at 1 year.
In prediabetic individuals, genotype at SLC30A8 predicts baseline proinsulin levels independently of insulin levels, but does not predict proinsulin levels after amelioration of insulin sensitivity at 1 year.
Keywords: Diabetes Prevention Program, Genetic association, Proinsulin, Single nucleotide polymorphisms, SLC30A8, Zinc transporter