Analysis of the gene sequences of the insulin receptor and the insulin-sensitive glucose transporter (GLUT-4) in patients with common-type non-insulin-dependent diabetes mellitus

Mutation in mitochondrial tRNA(Leu)(UUR) gene in a large pedigree with maternally transmitted type II diabetes mellitus and deafness

Non-insulin-dependent (type II) diabetes mellitus (NIDDM) is characterized by hyperglycaemia and insulin resistance, and affects nearly 5% of the general population. Inherited factors are important for its development, but the genes involved are unknown. We have identified a large pedigree in which NIDDM, in combination with a sensorineural hearing loss, is maternally inherited. The maternal inheritance and the observed decrease in mitochondrial enzyme activities of the respiratory chain indicate a genetic defect in the mitochondrial DNA. An A to G transition was identified at nucleotide 3,243, a conserved position in the mitochondrial gene for tRNA(Leu)(UUR). This mutation cosegregates with the disease in this family and is absent in controls, and indicates that a point mutation in mitochondrial DNA is a pathogenetic factor for NIDDM.

Alternatively spliced variants of the insulin receptor protein. Expression in normal and diabetic human tissues

Two insulin receptor mRNA transcripts resulting from alternative splicing of exon 11 in the receptor gene are expressed in a highly regulated tissue-specific fashion. To date, there is no information about the relative abundance of the protein isoforms encoded by these mRNAs in tissues of normal or diabetic subjects. We employed an antibody raised against the peptide sequence encoded by exon 11 to develop a specific immunoprecipitation assay that is capable of determining the fraction of receptors that include this amino acid sequence. The assay is based on the relative ability of the exon 11 specific monoclonal antibody (alpha IR alpha) compared to a nonspecific anti-receptor antiserum (B-2) to immunoprecipitate solubilized receptors that are first labeled with 125I-insulin. The assay was validated using standard curves generated with samples composed of known ratios of the two receptor isoforms. Our results in general confirm observations regarding the relative abundance of the two mRNA species in human tissues, with marked predominance of the exon 11+ isoform in liver, and the exon 11- isoform in leukocytes. Similar amounts of both variants are present in placenta, skeletal muscle, and adipose tissue. In studies with this assay using skeletal muscle extracts from control and noninsulin-dependent diabetes mellitus (NIDDM) subjects, as well as in studies of the two mRNAs in control versus NIDDM muscle using a quantitative polymerase chain reaction assay, we could find no significant difference between control and diabetic subjects. This data contradicts a recent report claiming that normal individuals have only the exon 11- mRNA transcript in their skeletal muscle, whereas NIDDM subjects have similar expression of both mRNAs. Given the emerging evidence that functional differences exist between the two receptor isoforms, these studies are relevant to our understanding of insulin receptor function in health and disease.

Facilitative glucose transporters: regulatory mechanisms and dysregulation in diabetes

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A variant insulin promoter in non-insulin-dependent diabetes mellitus

To test the hypothesis that alterations in regulatory regions of the insulin gene occur in a subset of patients with non-insulin-dependent diabetes mellitus (NIDDM), the promoter region was studied by polymerase chain reaction (PCR) amplification directly from genomic DNA, followed by high-resolution polyacrylamide gel electrophoresis under nondenaturing conditions. By using this method a previously identified HincII polymorphism (GTTGAC to GTTGAG at position-56) in American Blacks was readily detected, indicating that single base changes could be observed. In the course of screening the insulin promoter from 40 American Black subjects with NIDDM, an apparent larger allele was found in two individuals. Both patients were shown to have in addition to a normal allele, a larger allele containing an 8-bp repeat, TGGTCTAA from positions -322 to -315 of the insulin promoter. To facilitate rapid screening for the 8-bp repeat, a high-resolution agarose gel electrophoretic analysis was adopted. DNA from American Black NIDDM subjects (n = 100) and nondiabetic subjects (n = 100) was PCR amplified and analyzed. The 8-bp repeat was present in five NIDDM subjects, and one nondiabetic subject. DNA from Mauritius Creoles, also of African ancestry, was analyzed, and the 8-bp repeat was present in 3 of 41 NIDDM subjects, and 0 of 41 nondiabetic subjects. Analysis of glucose metabolism in three presumed normal sibs of an NIDDM patient with an 8-bp repeat revealed that one sib had overt diabetes, and two sibs were glucose intolerant, but there was no consistent segregation of the insulin promoter variant with the diabetes phenotype. The variant promoter was not present in 35 Caucasian NIDDM patients or in 40 Pima Indians. To test the biological consequences of the 8-bp repeat sequence in the insulin promoter, a normal and variant promoter were subcloned into a luciferase plasmid, and reporter gene activity assessed by transient transfection into mouse insulinoma (beta TC1) and hamster insulinoma (HIT) cells. The promoter activity of the variant allele was found to be reduced to 37.9 +/- 10.3% of the activity of the normal promoter in HIT cells (P less than 0.01, n = 4), and 49.1 +/- 6.4% in beta TC1 cells (P less than 0.01, n = 6). These data thus suggest that a naturally occurring variant of the insulin promoter may contribute to the diabetes phenotype in 5-6% of Black NIDDM patients.