Graeme I. Bell

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.

Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus

Type 2 or non-insulin-dependent diabetes mellitus (NIDDM) is the most common form of diabetes worldwide, affecting approximately 4% of the worlds 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.

Molecular Biology of Mammalian Glucose Transporters

The oxidation of glucose represents a major source of metabolic energy for mammaliancells. However, because the plasma membrane is impermeable to polar molecules such as glucose, the cellular uptake of this important nutrient is accomplished by membrane-associated carrier proteins that bind and transfer it across the lipid bilayer. Two classes of glucose carriers have been described in mammalian cells: the Na+-glucose cotransporter and the facilitative glucose transporter. The Na+-glucose cotransporter transports glucose against its concentration gradient by coupling its uptake with the uptake of Na+ that is being transported down its concentration gradient. Facilitative glucose c rriers accelerate the transport of glucose down its concentration gradient by facilitative diffusion, a form of passive transport. cDNAs have been isolated from human tissues encoding a Na+-glucose-cotransporter protein and five functional facilitative glucosetransporter isoforms. The Na+-glucose cotransporter is expressed by absorptive epithelial cells of the small intestine and is involved in the dietary uptake of glucose. The same or a related protein may be responsible for the reabsorption of glucose by the kidney. Facilitative glucose carriers are expressed by most if not all cells. The facilitative glucose-transporter isoforms have distinct tissue distributions and biochemical properties and contribute to the precise disposal of glucose under varying physiological conditions. The GLUT1 (erythrocyte) and GLUT3 (brain) facilitative glucose-transporter isoforms may be responsible for basal or constitutive glucose uptake. The GLUT2 (liver) isoform mediates the bidirectional transport of glucose by the hepatocyte and is responsible, at least in part, for the movement of glucose out of absorptive epithelial cells into the circulation in the small intestine and kidney. This isoform may also comprise part of the glucosesensing mechanism of the insulin-producing β-cell. The subcellular localization of the GLUT4 (muscle/fat) isoform changes in response to insulin, and this isoform is responsible for most of the insulin-stimulated uptake of glucose that occurs in muscle and adipose tissue. The GLLJT5 (small intestine) facilitative glucose-transporter isoform is expressed at highest levels in the small intestine and may be involved in the transcellular transport of glucose by absorptive epithelial cells. The exon-intron organizations of the human GLUT1, GLUT2, and GLUT4 genes have been determined. In addition, the chromosomal locations of the genes encoding the Na+-dependent and facilitative glucose carriers have been determined. Restriction-fragment-length polymorphisms have also been identified at several of these loci. The isolation and characterization of cDNAs and genes for these glucose transporters will facilitate studies of their role in the pathogenesis of disorders characterized by abnormal glucose transport, including diabetes mellitus, the glucose-galactose malabsorption syndrome, and benign renal glycosuria.

A polymorphic locus near the human insulin gene is associated with insulin-dependent diabetes mellitus.

A polymorphic region flanking the human insulin gene on the short arm of chromosome 11, the insulin-gene-linked DNA polymorphism, can be described as a locus with at least three classes of alleles: a common small “class 1” allele averaging 570 base pairs, a rare intermediate “class 2” allele of about 1320 base pairs, and a large “class 3” allele averaging 2470 base pairs in size. We have determined the genotype at this locus of 393 unrelated diabetic and nondiabetic individuals. Differences were observed in the genotypie and allelic frequencies between groups of different races. Asians [17 nondiabetic, 2 with insulin-dependent diabetes mellitus (IDDM), and 8 with non-insulin-dependent diabetes mellitus (NIDDM)] exhibited the least variation in the size of this locus and 98% of the alleles in this group were class 1. A group of American blacks (32 nondiabetic, 5 with IDDM, and 40 with NIDDM) exhibited considerable variation in the size of this locus, and about 22% of the individuals examined had a genotype that included a rare class 2 allele. In neither of these two racial groups were the genotypie or allelic frequencies different between the nondiabetic and diabetic segments of these groups. However, in a group of Caucasians (83 nondiabetic, 113 with IDDM, and 76 with NIDDM), there was a significantly higher frequency of class 1 alleles and genotypes containing two class 1 alleles in the diabetic patients compared with nondiabetic controls. A strikingly higher frequency of class 1 alleles (P < 0.0001) and genotypes containing two class 1 alleles (P < 0.0001) was observed in the IDDM group compared with nondiabetic Caucasians, whereas the difference between NIDDM and controls was only near the level of statistical significance (P = 0.025). Analysis of the combined data of our Caucasian population and those reported from studies of Caucasians from Denmark and St. Louis, Missouri, continued to show an increased frequency of class 1 alleles and genotypes containing two class 1 alleles in the IDDM group (P = 0.0001) compared with the nondiabetic group; however, there were no longer differences in genotypic or allelic frequencies between NIDDM and nondiabetic groups.

Classification and nomenclature of somatostatin receptors

There is considerable controversy about the classification and nomenclature of somatostatin receptors. To date, five distinct receptor genes have been cloned and named chronologically according to their respective publication dates, but two were unfortunately given the same appellation (SSTR4). Consensually, a nomenclature for the recombinant receptors has been agreed according to IUPHAR guidelines (sst1, sst2, sst3, sst4, and sst5). However, a more informative classification is to be preferred for the future, employing all classification criteria in an integrated scheme. It is already apparent that the five recombinant receptors fall into two classes or groups, on the basis of not only structure but also pharmacological characteristics. One class (already referred to by some as SRIF1) appears to comprise sst2, sst3 and sst5 receptor subtypes. The other class (SRIF2) appears to comprise the other two recombinant receptor subtypes (sst1 and sst4). This promising approach is discussed but it is acknowledged that much more data from endogenous receptors in whole tissues are needed before further recommendations on somatostatin receptor nomenclature can be made.

Diabetes mellitus and genetically programmed defects in beta-cell function.

The pathways that control insulin secretion and regulate pancreatic β-cell mass are crucial in the development of diabetes mellitus. Maturity-onset diabetes of the young comprises a number of single-gene disorders affecting pancreatic β-cell function, and the consequences of mutations in these genes are so serious that diabetes develops in childhood or adolescence. A genetic basis for the more common form of type 2 diabetes, which affects 10–20% of adults in many developed countries, is less clear cut. It is also characterized by abnormal β-cell function, but other tissues are involved as well. However, in both forms identification of causative and susceptibility genes are providing new insight into the control of insulin action and secretion, as well as suggesting new treatments for diabetes.

Isolation and nucleotide sequence of a cDNA encoding the precursor of mouse nerve growth factor.

Nerve growth factor (NGF) is a polypeptide that enhances survival, nerve fibre outgrowth and neurotransmitter biosynthesis in sympathetic and sensory neurones1–3. Administration of antibodies against NGF to developing animals leads to atrophy of the sympathetic system4. NGF is not normally detectable in innervated tissues but ablation of the innervating neurones leads to the production of measurable NGF in the target tissue5. After transplantation of the denervated tissue, reinnervation occurs, then NGF decreases to undetectable levels. Thus NGF seems to act as a neurotrophic messenger and its level is regulated by innervating neurones. Because of the minute levels present it is very difficult to study NGF biosynthesis in innervated tissue. However, NGF can be isolated from male mouse submaxillary glands, where it exists in inexplicably high levels6. Its amino acid sequence has been determined7, and the synthesis of NGF and its larger precursors has been demonstrated in cultured submaxillary glands8. We report here the nucleotide sequence of a submaxillary cDNA encoding the mouse NGF precursor (preproNGF). In contrast to previous suppositions8 the NGF moiety is situated near the carboxy-terminus of the polyprotein precursor. It is flanked at the amino-terminus by 187 amino acids which may be cleaved at dibasic residues to generate three peptides; there are only two additional amino acids at the carboxy-terminus.

Loci on chromosomes 2 (NIDDM1) and 15 interact to increase susceptibility to diabetes in Mexican Americans

Complex disorders such as diabetes, cardiovascular disease, asthma, hypertension and psychiatric illnesses account for a large and disproportionate share of health care costs, but remain poorly characterized with respect to aetiology. The transmission of such disorders is complex, reflecting the actions and interactions of multiple genetic and environmental factors. Genetic analyses that allow for the simultaneous consideration of susceptibility from multiple regions may improve the ability to map genes for complex disorders, but such analyses are currently computationally intensive and narrowly focused. We describe here an approach to assessing the evidence for statistical interactions between unlinked regions that allows multipoint allele–sharing analysis to take the evidence for linkage at one region into account in assessing the evidence for linkage over the rest of the genome. Using this method, we show that the interaction of genes on chromosomes 2 (NIDDM1) and 15 (near CYP19) makes a contribution to susceptibility to type 2 diabetes in Mexican Americans from Starr County, Texas.

Insulin gene mutations as a cause of permanent neonatal diabetes

We report 10 heterozygous mutations in the human insulin gene in 16 probands with neonatal diabetes. A combination of linkage and a candidate gene approach in a family with four diabetic members led to the identification of the initial INS gene mutation. The mutations are inherited in an autosomal dominant manner in this and two other small families whereas the mutations in the other 13 patients are de novo. Diabetes presented in probands at a median age of 9 weeks, usually with diabetic ketoacidosis or marked hyperglycemia, was not associated with β cell autoantibodies, and was treated from diagnosis with insulin. The mutations are in critical regions of the preproinsulin molecule, and we predict that they prevent normal folding and progression of proinsulin in the insulin secretory pathway. The abnormally folded proinsulin molecule may induce the unfolded protein response and undergo degradation in the endoplasmic reticulum, leading to severe endoplasmic reticulum stress and potentially β cell death by apoptosis. This process has been described in both the Akita and Munich mouse models that have dominant-acting missense mutations in the Ins2 gene, leading to loss of β cell function and mass. One of the human mutations we report here is identical to that in the Akita mouse. The identification of insulin mutations as a cause of neonatal diabetes will facilitate the diagnosis and possibly, in time, treatment of this disorder.

Molecular biology of opioid receptors

Opium and its derivatives are potent analgesics that also have many other pharmacological effects in the nervous system. These agents and the endogenous opioid peptides exert their effects by interacting with high-affinity receptors. Complementary DNAs encoding the delta, kappa and mu opioid receptors have been isolated and characterized. These receptors, which are members of the superfamily of seven-transmembrane spanning receptors, share a high degree of amino acid sequence similarity with approximately 50% of the residues being identical. The cloned opioid receptors mediate agonist inhibition of cyclic AMP formation and have pharmacological properties similar to the endogenous proteins. The cloning of these receptors will facilitate the development of new clinically useful compounds as well as studies of the molecular basis of tolerance and drug addiction.