Stephan Menzel

Mutations in the hepatocyte nuclear factor-1α gene in maturity-onset diabetes of the young (MODY3)

THE disease maturity-onset diabetes of the young (MODY) is a genetically heterogeneous monogenic form of non-insulin-dependent (type 2) diabetes mellitus (NIDDM), characterized by early onset, usually before 25 years of age and often in adolescence or childhood, and by autosomal dominant inheritance1. It has been estimated that 2–5% of patients with NIDDM may have this form of diabetes mellitus2,3. Clinical studies have shown that predia-betic MODY subjects have normal insulin sensitivity but suffer from a defect in glucose-stimulated insulin secretion, suggesting that pancreatic β-cell dysfunction rather than insulin resistance is the primary defect in this disorder4,5. Linkage studies have localized the genes that are mutated in MODY on human chromosomes 20 (MODY1)6, 7 (MODY2)2 and 12 (MODY3}7, with MODY2 and MODY3 being allelic with the genes encoding glucokinase2, a key regulator of insulin secretion, and hepatocyte nuclear factor-1α (HNF-1α)8, a transcription factor involved in tissue-specific regulation of liver genes but also expressed in pancreatic islets, insulinoma cells and other tissues. Here we show that MODY1 is the gene encoding HNF-4α (gene symbol, TCP14), a member of the steroid/thyroid hormone receptor superfamily and an upstream regulator of HNF-1α expression9–11.

Altered Insulin Secretory Responses to Glucose in Diabetic and Nondiabetic Subjects With Mutations in the Diabetes Susceptibility Gene MODY3 on Chromosome 12

One form of maturity-onset diabetes of the young (MODY) results from mutations in a gene, designated MODY3, located on chromosome 12 in band q24. The present study was undertaken to define the interactions between glucose and insulin secretion rate (ISR) in subjects with mutations in MODY3. Of the 13 MODY3 subjects, six subjects with normal fasting glucose and glycosylated hemoglobin and seven overtly diabetic subjects were studied as were six nondiabetic control subjects. Each subject received graded intravenous glucose infusions on two occasions separated by a 42-h continuous intravenous glucose infusion designed to prime the β-cell to secrete more insulin in response to glucose. ISRs were derived by deconvolution of peripheral C-peptide levels. Basal glucose levels were higher and insulin levels were lower in MODY3 subjects with diabetes compared with nondiabetic subjects or with normal healthy control subjects. In response to the graded glucose infusion, ISRs were significantly lower in the diabetic subjects over a broad range of glucose concentrations. ISRs in the nondiabetic MODY3 subjects were not significantly different from those of the control subjects at plasma glucose levels <8 mmol/l. As glucose rose above this level, however, the increase in insulin secretion in these subjects was significantly reduced. Administration of glucose by intravenous infusion for 42 h resulted in a significant increase in the amount of insulin secreted over the 5–9 mmol/l glucose concentration range in the control subjects and nondiabetic MODY3 subjects (by 38 and 35%, respectively), but no significant change was observed in the diabetic MODY3 subjects. In conclusion, in nondiabetic MODY3 subjects insulin secretion demonstrates a diminished ability to respond when blood glucose exceeds 8 mmol/l. The priming effect of glucose on insulin secretion is preserved. Thus, β-cell dysfunction is present before the onset of overt hyperglycemia in this form of MODY. The defect in insulin secretion in the nondiabetic MODY3 subjects differs from that reported previously in nondiabetic MODY1 or mildly diabetic MODY2 subjects.

Mutations in the Hepatocyte Nuclear Factor-1α Gene in MODY and Early-Onset NIDDM: Evidence for a Mutational Hotspot in Exon 4

We have recently shown that mutations in the gene encoding the transcription factor hepatocyte nuclear factor (HNF)-1α are the cause of one form of maturity-onset diabetes of the young (MODY3). Here, we report the exon-intron organization and partial sequence of the human HNF-1α gene. In addition, we have screened the ten exons and flanking introns of this gene for mutations in a group of 25 unrelated white subjects from Germany who presented with NIDDM before 35 years of age and had a first-degree relative with NIDDM. Mutations were identified in nine of these individuals, suggesting that mutations in the HNF-1α gene are a common cause of diabetes in German subjects with early-onset NIDDM and a family history of diabetes. Thus, screening for mutations in this gene may be indicated in subjects with early-onset NIDDM. Interestingly, three of the nine mutations occurred at the same site in exon 4 with insertion of a C in a polyCtract, centered around codon 290 (designated Pro291fsinsC), thereby resulting in a frameshift during translation and premature termination. Analyses of linked DNA polymorphisms in the HNF-1α gene indicated that the Pro291fsinsC mutation was present on a different haplotype in each subject, implying that the polyC tract represents a mutational hot spot. We have also identified the mutation in the HNF-1α gene in the Jutland pedigree, one of the original MODY pedigrees reported in the literature, as being a T→G substitution in codon 241, resulting in the replacement of a conserved Cys by Gly (C241G). The information on the sequence of the HNF-1α gene and its promoter region will facilitate the search for mutations in other subjects and studies of the role of the gene in determining normal β-cell function.

Isolation of a cDNA Clone Encoding a KATP Channel–Like Protein Expressed in Insulin-Secreting Cells, Localization of the Human Gene to Chromosome Band 21q22.1, and Linkage Studies With NIDDM

The metabolism of glucose in insulin-secreting cells leads to closure of ATP-sensitive K+ channels (KATP), an event that initiates the insulin secretory process. Defects in insulin secretion are a common feature of non-insulin-dependent diabetes mellitus (NIDDM), and the β-cell KATP that couples metabolism and membrane potential is a candidate for contributing to the development of this clinically and genetically heterogeneous disorder. We screened a hamster insulinoma cDNA library by low-stringency hybridization with a probe coding for the G-protein-coupled inwardly rectifying K+ channel GIRK1/KGA and isolated clones encoding a protein, KATP-2, whose sequence is 90% similar to that of the recently described KATP-1, an ATP-sensitive K+ channel expressed in heart and other tissues. RNA blotting showed that KATP mRNA was present in insulin-secreting cells and brain but not in heart. To assess the contribution of KATP-2 to the development of NIDDM, the human KATP-2 gene (symbol KCNJ7) was isolated and mapped to chromosome band 21q22.1 by fluorescence in situ hybridization. A simple tandem repeat DNA polymorphism, D21S1255, was identified in the region of the KATP-2 gene, and linkage studies between this marker and NIDDM were carried out in a group of Mexican-American sib pairs with NIDDM. There was no evidence for linkage between D21S1255 and NIDDM, indicating that KATP-2 is not a major susceptibility gene in this population.

Localization of MODY3 to a 5-cM Region of Human Chromosome 12

Maturity-onset diabetes of the young (MODY) is a heterogeneous disorder that appears to be characterized by a primary defect in insulin secretion. Mutations in an unknown locus (MODY1) on chromosome 20 and the glucokinase gene (MODY2) on chromosome 7 can cause this form of non-insulin-dependent diabetes. Recent genetic studies have identified a third locus on chromosome 12 (MODY3) that is linked to MODY in a group of French families. We have identified three families from Denmark, Germany, and the U.S. (Michigan) showing evidence of linkage with MODY3 and a family from Japan showing suggestive evidence. Analysis of key recombinants in these families localized MODY3 to a 5-cM interval between the markers D12S86 and D12S807/D12S820.

Searching for NIDDM susceptibility genes: studies of genes with triplet repeats expressed in skeletal muscle

SummaryThe expansion of trinucleotide repeats has been associated with late-onset neurodegenerative disorders. Although the genes harbouring the triplet expansions may be widely expressed, the pathological expression of these diseases is restricted to specific tissues. Non-insulin-dependent diabetes mellitus (NIDDM) shares several features with diseases resulting from such dynamic mutations including late-onset and specific but limited sites of tissue pathology — muscle, fat, liver and insulin-secreting pancreatic beta cells. In order to examine the contribution of genes containing polymorphic CAG/CTG repeats to the development of NIDDM, we screened an adult human skeletal muscle cDNA library for expressed sequences containing tandem repeats of CAG and/or CTG. Ten different loci with polymorphic CAG/CTG repeats were identified, of which seven had a heterozygosity greater than 0.20. There was no evidence for linkage between these seven loci and NIDDM in a group of affected Mexican-American sib pairs. Nor was there a significant difference in the distribution of alleles between Caucasian patients with NIDDM and normal healthy control subjects or evidence for repeat expansion in diabetic subjects. Thus, muscle genes with polymorphic CAG/CTG repeats do not appear to play a significant role in the development of NIDDM.