Genetic Variants in Promoters and Coding Regions of the Muscle Glycogen Synthase and the Insulin-Responsive GLUT4 Genes in NIDDM

To examine the hypothesis that variants in the regulatory or coding regions of the glycogen synthase (GS) and insulin-responsive glucose transporter (GLUT4) genes contribute to insulin-resistant glucose processing of muscle from non-insulin-dependent diabetes mellitus (NIDDM) patients, promoter regions and regions of importance for translation, as well as coding sequences of the two genes, were studied using single-strand conformation polymorphism (SSCP) analysis and DNA sequencing. The genetic analyses were performed in subgroups of 52 Caucasian NIDDM patients and 25 age-matched healthy volunteers. By applying inverse polymerase chain reaction and direct DNA sequencing, 532 base pairs (bp) of the GS promoter were identified and the transcriptional start site determined by primer extension. SSCP scanning of the promoter region detected five single nucleotide substitutions, positioned at 42, –16, –43, –143, and –250. The three most common variants could be excluded for having major impact on allele-specific GS mRNA expression in muscle. Scanning of GS cDNA revealed one frequent silent polymorphism at codon 342. Moreover, SSCP analysis of ∼900 bp of the promoter, the 5′-untranslated region, and the coding region of the GLUT4 gene showed four polymorphisms, all single nucleotide substitutions, positioned at –581, 1, 30, and 582. None of the three changes in the regulatory region of the gene had any major influence on expression of the GLUT4 gene in muscle. The variant at 582 in the GLUT4 cDNA was a silent polymorphism at codon 130. Southern blotting of both gene loci did not detect any major abnormalities. These findings, altogether, suggest that genetic abnormalities in the GS and GLUT4 genes are unlikely to be major contributors to the insulin-resistant glucose utilization in muscle among Caucasian NIDDM patients.