VARIABILITY OF THE PANCREATIC-ISLET BETA-CELL LIVER (GLUT-2) GLUCOSE-TRANSPORTER GENE IN NIDDM PATIENTS

The purpose of these experiments was to test the hypothesis that impaired glucose-stimulated insulin secretion in NIDDM is due to mutations in the islet beta cell/liver glucose transporter (GLUT 2) gene. Using oligonucleotide primers flanking each of the 11 exons, the structural portion of the gene was studied by PCR-SSCP analysis. DNA from African-American females (n = 48), who had gestational diabetes but developed overt NIDDM after delivery, was studied. Each SSCP variant was sequenced directly from genomic DNA. Two amino acid substitutions from the previously reported sequence were found, one in exon 3 and the other in exon 4 B. Four additional silent mutations in the coding region, and six intron mutations outside the splice junction consensus sequences, were also identified. The mutation GTC x ATC in exon 4B substituted Val197 to Ile197. This amino acid substitution was found in only one NIDDM patient in a single allele, and was not found in 52 control subjects. This residue exists in the fifth membrane spanning domain, and Val at this position is conserved in mouse and rat GLUT 2, and human GLUT 1 to GLUT 4. The other codon change in exon 3, ACT x ATT, substituted Thr110 to Ile110 in the second membrane spanning domain. To determine the frequency of this non-conservative amino acid substitution, a PCR-LCR assay was developed. This assay was simple and highly specific for detection of this single nucleotide substitution. The allelic frequency of the ATT (Ile110) in NIDDM patients (39.6 %, n = 48) and that in controls (47.1 %, n = 52) did not differ (p = 0.32, Fisher's exact test). In conclusion, we identified two variant GLUT 2 glucose transporters in a subset of NIDDM patients. The rare variant in exon 4 B may contribute to the diabetic susceptibility and awaits further investigation. However, structural abnormalities of the GLUT 2 transporter associated with NIDDM appeared to be rare and were not likely to be a major determinant of genetic susceptibility to this type of diabetes in the population studied.