Systematic search for single nucleotide polymorphisms in the insulin gene: evidence for a high frequency of -23T-->A in Japanese subjects

It has recently been shown that the A/A genotype at g.-23 of the insulin gene correlates with impaired insulin secretion in response to body weight gain in subjects of European descent. To examine whether there are single nucleotide polymorphisms (SNPs) in the insulin gene associated with type 2 diabetes, all exons with their flanking sequences for 113 Japanese type 2 diabetic patients and 99 nondiabetic control subjects were analyzed using PCR direct sequencing. We have only found g.-23T --> A, 806G --> C, 1128T --> C, and 1141A --> C, which have previously been reported in alpha (A-C-C-C) and beta (T-G-T-A) alleles. The allele frequency of -23T --> A in control Japanese subjects was 97.4%, whereas that in Europeans is about 30%. The A/A genotype was found in 94 of 99 Japanese subjects (94.9%) and the allele frequencies of 806G --> C, 1128T --> C, and 1141A --> C were all 96.5%. The estimated haplotype frequencies were (A-C-C-C) (96.0%), (T-G-T-A) (2.0%), (A-G-T-A) (1.5%), and (T-C-C-C) (0.5%). No association of these SNPs or haplotypes with type 2 diabetes was evident. Thus, the A/A genotype at the g.-23 of insulin gene was generally high in Japanese subjects, which could account for the fact that they typically secrete lower levels of insulin.