Type 2 or non-insulin-dependent diabetes mellitus (NIDDM) is the most common form of diabetes worldwide, affecting approximately 4% of the world's adult population. It is multifactorial in origin with both genetic and environmental factors contributing to its development. A genome-wide screen for type 2 diabetes genes carried out in Mexican Americans localized a susceptibility gene, designated NIDDM1, to chromosome 2. Here we describe the positional cloning of a gene located in the NIDDM1 region that shows association with type 2 diabetes in Mexican Americans and a Northern European population from the Botnia region of Finland. This putative diabetes-susceptibility gene encodes a ubiquitously expressed member of the calpain-like cysteine protease family, calpain-10 (CAPN10). This finding suggests a novel pathway that may contribute to the development of type 2 diabetes.

Insulin resistance is a key component in the pathogenesis of polycystic ovary syndrome (PCOS) and type 2 diabetes. Polymorphisms in the genes encoding the insulin receptor substrate (IRS) proteins, IRS-1 (Gly(972)Arg) and IRS-2 (Gly(1057)Asp), influence susceptibility to type 2 diabetes. This study was undertaken to assess the influence of these polymorphisms on insulin resistance, glucose tolerance, and androgen levels in nondiabetic PCOS women. We studied 227 PCOS subjects including 126 and 48 nondiabetic white and African-American subjects, respectively. The IRS-1 Gly(972)Arg allele frequencies were identical in whites and African-Americans [0.95 (Gly) and 0.05 (Arg)]. The IRS-2 Gly(1057)Asp allele frequencies were 0.85 (Gly) and 0.15 (Asp) in African-Americans and 0.59 (Gly) and 0.41 (Asp) in whites. There was no association of IRS-1 genotype with any clinical or hormonal measure in nondiabetic white or African-American PCOS subjects. However, nondiabetic subjects with the IRS-2 Gly/Gly genotype had significantly higher 2-h oral glucose tolerance test glucose levels compared with those with Gly/Asp and Asp/Asp genotypes in whites or Gly/Asp genotype in African-Americans (there were no Asp/Asp subjects in our modest size African-American sample). These results suggest that the IRS-2 Gly(1057)Asp polymorphism influences blood glucose levels in nondiabetic white and African-American women with PCOS. Thus, individuals with the common IRS-2 Gly/Gly genotype may be at increased risk of developing type 2 diabetes.

The type 3 form of maturity-onset diabetes of the young (MODY3) results from mutations in the gene encoding the transcription factor, hepatocyte nuclear factor-1alpha (HNF-1alpha). The mechanism by which mutations in only one allele of the HNF-1alpha gene impair pancreatic beta-cell function is unclear. The functional form of HNF-1alpha is a dimer--either a homodimer or a heterodimer with the structurally related protein HNF-1beta--that binds to and activates transcription of the genes whose expression it regulates. HNF-1alpha is composed of three functional domains: an amino-terminal dimerization domain (amino acids 1-32), a DNA-binding domain with POU-like and homeodomain-like motifs (amino acids 150-280), and a COOH-terminal transactivation domain (amino acids 281-631). Because the dimerization domain is intact in many of the mutant forms of HNF-1alpha found in MODY subjects, these mutant proteins may impair pancreatic beta-cell function by forming nonproductive dimers with wild-type protein, thereby inhibiting its activity; that is, they are dominant-negative mutations. This hypothesis was tested by comparing the functional properties of the frameshift mutation P291fsinsC, the most common mutation identified to date in MODY3 patients, and wild-type HNF-1alpha. P291fsinsC-HNF-1alpha showed no transcriptional transactivation activity in HeLa cells, which lack endogenous HNF-1alpha. Overexpression of P291fsinsC-HNF-1alpha in MIN6 cells, a mouse beta-cell line, resulted in an approximately 40% inhibition of the endogenous HNF-1alpha activity in a dosage-dependent manner. Furthermore, heterodimer formation between wild-type and P291fsinsC mutant proteins were observed by electrophoretic mobility shift assay. These data suggest that the P291fsinsC mutation in HNF-1alpha functions as a dominant-negative mutation. However, other mutations, such as those in the promoter region and dimerization domain, may represent loss of function mutations. Thus mutations in the HNF-1alpha gene may lead to beta-cell dysfunction by two different mechanisms.

Mutations in the HNF-1alpha gene may affect the normal islet function by different molecular mechanisms.

To clarify the regeneration process of pancreatic beta-cells, we established a new mouse model of diabetes induced by selective perfusion of alloxan after clamping the superior mesenteric artery. In this model, diabetes could be induced by the destruction of beta-cells in alloxan-perfused segments, while beta-cells in nonperfused segments were spared. Intraperitoneal glucose tolerance tests showed glucose intolerance, which gradually ameliorated and was completely normalized in 1 year with a concomitant increase of insulin content in the pancreas. Histological examination showed neo-islet formation in the alloxan-perfused segment and the proliferation of spared beta-cells in the nonperfused segment. In the alloxan-perfused segment, despite a marked reduction of islets in size and number at an early stage, both the number of islets, including islet-like cell clusters (ICCs), and the relative islet area significantly increased at a later stage. Increased single beta-cells and ICCs were located in close contact with duct cell lining, suggesting that they differentiated from duct cells and that such extra-islet precursor cells may be important for beta-cell regeneration in beta-cell-depleted segment. In addition to beta-cells, some nonhormone cells in ICCs were positive for nuclear insulin promoter factor 1, which indicated that most, if not all, nonhormone cells positive for this factor were beta-cell precursors. In the nonperfused segment, the islet area increased significantly, and the highest 5-bromo-2-deoxyuridine-labeling index in beta-cells was observed at day 5, while the number of islets did not increase significantly. This indicated that the regeneration of islet endocrine cells occurs mostly through the proliferation of preexisting intra-islet beta-cells in the nonperfused segment. In conclusion, the regeneration process of beta-cells varied by circumstance. Our mouse model is useful for studying the mechanism of regeneration, since differentiation and proliferation could be analyzed separately in one pancreas.