Context: Single-nucleotide polymorphisms (SNPs) within the G6PC2 locus are associated with fasting glucose and insulin secretion. These SNPs are not associated with type 2 diabetes risk.
Objective: Our objective was to investigate whether the impact of the SNP on variables of glucose-stimulated insulin secretion is influenced by glucose tolerance status.
Design, setting, participants, and intervention: In this cross-sectional study, we genotyped 1505 healthy Caucasian subjects [normal glucose tolerance (NGT), 1098; impaired glucose tolerance (IGT)/impaired fasting glucose (IFG), 407] for SNP rs560887 within the G6PC2 locus. A subgroup of 326 subjects underwent an iv glucose tolerance test, and 512 participants took part in a hyperinsulinemic-euglycemic clamp. For replication, SNP rs560887 was genotyped in 457 subjects (NGT, 265; IGT, 192) from four independent German and Dutch studies who underwent a hyperglycemic clamp.
Main outcome measure: Insulin secretion was evaluated.
Results: Carriers of the major G-allele exhibited increased fasting glycemia (P<0.0001). Insulin sensitivity and secretion were not associated with the SNP (P≥0.06). Glucose tolerance status and genotype interacted on insulin secretion (P=0.036), such that in NGT subjects, the minor A-allele of rs560887 was associated with decreased insulinogenic index (P=0.044), which was not the case in subjects with IFG/IGT (P=1.0). During the iv glucose tolerance test, an association of A-allele carriers with decreased first-phase insulin secretion was also observed only in NGT subjects (P=0.0053). Likewise, in the hyperglycemic clamp group, the A-allele was associated with decreased first-phase insulin secretion only in the NGT group (P=0.022) but not in the IGT group.
Conclusions: The effects of hyperglycemia on insulin secretion override the more subtle effects of genetic variation in the G6PC2 locus on insulin secretion.