Mutations in the vasopressin V2 receptor and aquaporin-2 genes in 12 families with congenital nephrogenic diabetes insipidus

Congenital nephrogenic diabetes insipidus (CNDI) is a rare inherited disorder characterized by renal tubular insensitivity to the antidiuretic effect of arginine vasopressin (AVP). In a large majority of the cases, nephrogenic diabetes insipidus is an X-linked recessive disorder caused by mutations in the AVP V2 receptor gene (AVPR2). In the remaining cases, the disease is autosomal recessive or dominant and, for these patients, mutations in the aquaporin 2 gene (AQP2) have been reported. Fourteen probands belonging to 12 families were analyzed by single-strand conformational polymorphism and direct sequencing of the AVPR2 and AQP2 genes. Ten mutations of the AVPR2 gene (six previously reported mutations and four novel mutations: G107E, W193X, L43P, and 15delC) were identified. Three mutations of the AQP2 gene were also identified in two patients: the first patient is homozygous for the R85X mutation and the second is a compound heterozygote for V168 M and S216P mutations. Extrarenal responses to infusion of the strong V2 agonist 1-desamino-8-D-arginine vasopressin allowed AVPR2- and AQP2-associated forms of CNDI to be distinguished in three patients. This test also identified an unexpectedly high urinary osmolality (614 mosmol/kg) in a patient with a P322S mutation of AVPR2 gene and a mild form of CNDI.     

The in vivo balance between B cell clonal expansion and elimination is regulated by CD95 both on B cells and in their micro-environment

In congenital nephrogenic diabetes insipidus, the renal collecting ducts are resistant to the antidiuretic action of arginine vasopressin or to its antidiuretic analog 1-deamino[8-D-arginine] vasopressin (dDAVP). This is a rare, but now well described entity secondary to either mutations in the AVPR2 gene that codes for the vasopressin antidiuretic (V2) receptor or to mutations in the AQP2 gene that codes for the vasopressin-dependent water channel. A majority (> 90%) of congenital nephrogenic diabetes insipidus patients have AVPR2 mutations: Of 115 families with congenital nephrogenic diabetes insipidus, 105 families had AVPR2 mutations, and 10 had AQP2 mutations. When studied in vitro, most AVPR2 mutations lead to receptors that are trapped intracellularly and are unable to reach the plasma membrane. A minority of the mutant receptors reach the cell surface but are unable to bind vasopressin or to trigger an intracellular adenosine 3:5-cyclic phosphate signal properly. Most of the reported mutations are secondary to a complete loss of function of the receptor, and only a few mutations have been associated with a mild phenotype. These advances provide diagnostic tools for physicians caring for these patients because, when the disease causing mutation has been identified, carrier and perinatal testing could be done by mutation analysis.     

Observations on the California mortality studies

Since the discovery of aquaporin water channels, insight into the molecular mechanism by which rapid osmotic water occurs across cell membranes has greatly improved. Aquaporin-2 is the vasopressin-responsive water channel in the collecting duct, and vasopressin control of water permeability in the collecting duct occurs in two ways: a short-term regulation and a long-term adaptation. In congenital nephrogenic diabetes insipidus, the kidney does not respond to vasopressin. Ninety percent of these patients carry a mutation in the gene coding for the vasopressin V2 receptor located on the X chromosome. Autosomal recessive and dominant forms of nephrogenic diabetes insipidus that are caused by mutations in the aquaporin-2 gene have now been described. This review focuses on recent insight in the molecular and cellular defect in autosomal nephrogenic diabetes insipidus.     

AVPR2 variants and V2 vasopressin receptor function in nephrogenic diabetes insipidus

BACKGROUND: The AVPR2 gene encodes the type 2 vasopressin receptor, a member of the vasopressin/oxytocin receptor subfamily of G protein-coupled receptors. Disruption of AVPR2 causes X-linked congenital nephrogenic diabetes insipidus (NDI), yet the functional significance of most gene sequence variations found in association with NDI has not been proven. The large number of naturally occurring AVPR2 mutations constitutes a model system for studying the structure-function relationship of G protein-coupled receptors. This analysis can be aided by examining amino acid sequence variation and conservation among evolutionarily disparate members of the subfamily. METHODS: Twenty-five new NDI patients were evaluated by DNA sequencing for mutations in AVPR2. Receptors encoded by eighteen NDI alleles were tested for physiologic signaling activity in response to varying concentrations of arginine vasopressin (AVP) in a sensitive cell culture assay. Seventeen amino acid sequences from the vasopressin/oxytocin receptor subfamily were aligned and conserved residues were identified and correlated with the locations of NDI associated variations. RESULTS: Twenty-four variant alleles were found among the 25 new patients. Thirteen had no prior family history of expressed NDI. All 18 of the NDI-associated AVPR2 alleles tested for function demonstrated diminished response to stimulation with AVP. Twelve failed to respond at all, whereas six signaled only at high AVP concentrations. Evolutionarily conserved residues clustered in the transmembrane domains and in the first and second extracellular loops, and NDI-associated missense mutations appeared mostly in the conserved domains. CONCLUSIONS: Sporadic cases are frequent and they usually represent the X-linked rather than the autosomal form of NDI. Genetic and functional testing can confirm this in individual cases. Mutations in this study affecting ligand binding domains tend to retain partial signaling in vitro, whereas those that introduce a charged residue in a transmembrane domain are inactive. The minimal partial signaling observed in cultured cells is unlikely to correlate with clinically significant urine concentrating ability. Other AVPR2 mutations with milder effects on receptor function probably exist, but may not be expressed clinically as typical NDI.     

Application of mRNA differential display to investigate gene expression in thermotolerant cells of Saccharomyces cerevisiae

In nephrogenic diabetes insipidus, the kidney is unable to concentrate urine despite normal or elevated concentrations of the antidiuretic hormone arginine vasopressin (AVP). In congenital nephrogenic diabetes insipidus (NDI), the obvious clinical manifestations of the disease, that is polyuria and polydipsia, are present at birth and need to be immediately recognized to avoid severe episodes of dehydration. Most (>90%) congenital NDI patients have mutations in the AVPR2 gene, the Xq28 gene coding for the vasopressin V2 (antidiuretic) receptor. In <10% of the families studied, congenital NDI has an autosomal recessive inheritance and mutations of the aquaporin-2 gene (AQP2), ie, the vasopressin-sensitive water channel, have been identified. When studied in vitro, most AVPR2 mutations lead to receptors that are trapped intracellularly and are unable to reach the plasma membrane. A minority of the mutant receptors reach the cell surface but are unable to bind AVP or to trigger an intracellular cyclic adenosine-monophosphate (cAMP) signal. Similarly AQP2 mutant proteins are trapped intracellularly and cannot be expressed at the luminal membrane. The acquired form of NDI is much more common than the congenital form, is almost always less severe, and is associated with downregulation of AQP2. The advances described here are examples of "bedside physiology" and provide diagnostic tools for physicians caring for these patients.     

The abnormal quinine drinking aversion in the Brattleboro rat with diabetes insipidus is reversed by the vasopressin agonist DDAVP: a possible role for vasopressin in the motivation to drink

The Brattleboro rat with hypothalamic diabetes insipidus (BDI) has an abnormal aversion to drinking quinine-adulterated water compared with normal rats of the parent Long Evans (LE) strain. This BDI animal tolerates marked hypovolemia and decreased body weight in preference to drinking the quinine-adulterated fluid, indicative of a reduced motivation to drink. Acute or chronic treatment of BDI rats with desamino-8D arginine vasopressin (DDAVP) restored to normal their drinking response to quinine solution. Partial restoration of fluid turnover in BDI rats with hydrochlorothiazide, which has an antidiuretic effect in diabetes insipidus (when vasopressin is absent), failed to abolish the abnormal drinking response to quinine-adulterated solution in 8 out of 12 animals. In contrast, induction of diabetes mellitus in LE rats, which resulted in a marked polydipsia and polyuria even though vasopressin was still present, did not impair the drinking response to quinine solutions. These results suggest that the abnormal drinking response to quinine-adulterated fluid in BDI rats is reversed by treatment with the vasopressin V2-receptor agonist DDAVP but is unlikely to be a consequence of the restoration of fluid turnover to normal levels by a renal action. A possible central action involving vasopressin and the motivation to drink is discussed.     

The molecular basis of nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus (NDI) is characterized by resistance of the kidney to the action of arginine-vasopressin (AVP); it may be due to genetic or acquired causes. Recent advances in molecular genetics have allowed the identification of the genes involved in congenital NDI. While inactivating mutations of the vasopressin V2 receptor are responsible for X-linked NDI, autosomal recessive NDI is caused by inactivating mutations of the vasopressin-regulated water channel aquaporin-2 (AQP-2). About 70 different mutations of the V2 receptor have been reported, most of them missense mutations. The functionally characterized mutants show a loss of function due to defects in their synthesis, processing, intracellular transport, AVP binding, or interaction with the G protein/adenylyl cyclase system. Thirteen different mutations of the AQP-2 gene have been reported. Functional studies of three AQP-2 mutations reveal impaired cellular routing as the main defect. The great number of different mutations with various functional defects hinders the development of a specific therapy. Gene therapy may, however, eventually become applicable to the congenital forms of NDI. At present all gene-therapeutic approaches lack safety and efficiency, which is of particular relevance in a disease that is treatable by an adequate water intake. The progress with regard to the molecular basis of antidiuresis contributes to the understanding of acquired forms of NDI on a molecular level. Recent data show that lithium dramatically reduces the expression of AQP-2. Likewise, hypokalemia reduces the expression of this water channel. The exact mechanisms leading to this reduced expression of AQP-2 remain to be determined.     

Comprehensive evaluation of long-term trends in occupational exposure: Part 1. Description of the database

The regulation of water excretion by the kidney is one of the few physiologic processes that are prominent in everyday life. This process predominantly occurs in renal collecting duct cells, where transcellular water reabsorption is induced after binding of the pituitary hormone arginine-vasopressin to its vasopressin type-2 receptor and the subsequent insertion of aquaporin-2 (AQP2) water channels in the apical membrane of these cells. Removal of the hormone triggers endocytosis of AQP2 and restores the water-impermeable state of the collecting duct cells. Nephrogenic diabetes insipidus is characterized by the inability of the kidney to concentrate urine in response to vasopressin; the vasopressin type-2 receptor and the AQP2 water channel have both been shown to be involved in this disease. This article focuses on mutations in the vasopressin V2 receptor and aquaporin-2 water channel identified in nephrogenic diabetes insipidus patients, and on the effects of these mutations on the transport and function of these proteins upon expression in cell systems.     

Functional studies of twelve mutant V2 vasopressin receptors related to nephrogenic diabetes insipidus: molecular basis of a mild clinical phenotype

X-linked nephrogenic diabetes insipidus (NDI) is a rare disease with defective renal and extrarenal arginine vasopressin V2 receptor responses due to mutations in the AVPR2 gene in Xq28. To study the cause of loss of function of mutant V2 receptors, we expressed 12 mutations (N55H, L59P, L83Q, V88M, 497CC-->GG, deltaR202, I209F, 700delC, 908insT, A294P, P322H, P322S) in COS-7 cells. Eleven of these, including P322H, were characterized by a complete loss of function, but the mutation P322S demonstrated a mild clinical and in vitro phenotype. This was characterized by a late diagnosis without any growth or developmental delay and a significant increase in urine osmolality after intravenous 1-deamino[D-Arg8]AVP administration. In vitro, the P322S mutant was able to partially activate the Gs/adenylyl cyclase system in contrast to the other V2R mutants including P322H, which were completely inactive in this regard. This showed not only that Pro 322 is important for proper V2R coupling, but also that the degree of impairment is strongly dependent on the identity of the substituting amino acid. Three-dimensional modeling of the P322H and P322S mutant receptors suggested that the complete loss of function of the P322H receptor could be due, in part, to hydrogen bond formation between the His 322 side chain and the carboxyl group of Asp 85, which does not occur in the P322S receptor.     

Diabetes insipidus

Diabetes insipidus, characterized by the excretion of copious volumes of unconcentrated urine, results from a deficiency in the action of the antidiuretic hormone arginine vasopressin and can be caused by any of four fundamentally different defects, including impaired secretion (neurohypophyseal diabetes insipidus), impaired renal response (nephrogenic diabetes insipidus), excessive fluid intake (primary polydipsia), or increased metabolism of the hormone (gestational diabetes insipidus). Differentiation between their causes, pathophysiology, and treatment methods is essential for effective management and is best achieved by a combination of hormonal, clinical, and neuroradiologic observations. Understanding of the genetic forms has advanced greatly and may soon lead to improved methods of prevention, diagnosis, and treatment.     

Congenital nephrogenic diabetes insipidus: a difficult diagnosis?

In five patients (a boy aged 10 years, a boy aged 3 months, his brother aged 1 week, the brother of the mother of the last-mentioned two boys who had died at the age of one, and a girl of kindergarten age) congenital nephrogenic diabetes insipidus was diagnosed. This rare syndrome (prevalence 1:500,000) is caused by renal insensitivity to the antidiuretic hormone arginine vasopressin. In infancy the symptoms of this disorder are aspecific, and the main symptoms of the disease, polyuria and polydipsia, often remain unnoticed at this young age. A simple anamnesis and a few laboratory tests should suggest the diagnosis. Early diagnosis and genetic counselling are possible as the molecular effects involved have been elucidated.     

The vasopressin precursor is not processed in the hypothalamus of Wolfram syndrome patients with diabetes insipidus: evidence for the involvement of PC2 and 7B2

Wolfram syndrome (WS) is characterized by optic atrophy, insulin-dependent diabetes mellitus, vasopressin (VP)-sensitive diabetes insipidus, and neurosensory hearing loss. Here we report a disturbance in VP precursor processing in the supraoptic and paraventricular nuclei of WS patients. In these patients with diabetes insipidus we could hardly detect any cellular immunoreactivity for processed VP in the supraoptic and paraventricular nuclei. On the other hand, in the paraventricular nucleus a considerable number of cells immunoreactive for the VP precursor were present. In addition, the proprotein convertase PC2 and the molecular chaperone 7B2 were absent. As expression of PC2 and 7B2 was detected in the nearby nucleus basalis of Meynert of one WS patient and in the anterior lobe of the other WS patient, the absence of the two proteins in the paraventricular nucleus was not due to mutations in their genes. These results indicate that in WS patients with diabetes insipidus, not only does VP neuron loss occur in the supraoptic nucleus, but there is also a defect in VP precursor processing.     

Correlation between the age-dependent changes in caloric response and in pure tone hearing

Function and biochemical properties of the V2 vasopressin receptor (V2R) mutant R337ter, identified in patients suffering from X-linked recessive nephrogenic diabetes insipidus, were investigated by expression in COS.M6 or HEK293 cells. Binding assays and measurements of adenylyl cyclase activity failed to detect function for the truncated receptor, although metabolic labeling demonstrated normal levels of protein synthesis. ELISA assays performed on cells expressing the receptors tagged at the amino terminus with the HA epitope failed to detect V2R R337ter on the plasma membrane. Treatment with endoglycosidase H revealed that the receptor was present only as a precursor form because the mature R337ter V2R, resistant to endoglycosidase H treatment, was not detected. The precursor of V2R-R337ter had a longer half-life than that of the wild type V2R, suggesting that arrested maturation may slow the degradation of the precursor. Unrelated experiments had demonstrated that V2R-G345ter, containing eight additional amino acids, was expressed on the plasma membrane and functioned normally. Receptor truncations longer than 337ter revealed that four of the eight amino acids identified initially provided the minimum length required for the protein to acquire cell surface expression. This was shown by the production of mature receptor (V2R-341ter) detectable in SDS-PAGE, which mediated arginine vasopressin stimulation of adenylyl cyclase activity and bound ligand. In addition, the identity of amino acid 340 was found to play a role in this phenomenon. In conclusion, these data demonstrate that the V2R R337ter is nonfunctional because it does not reach the plasma membrane and that the minimal protein length required for translocation of the V2R to the cell surface is sufficient to confer function to the receptor protein. They also suggest the existence of a protein quality control in the endoplasmic reticulum independent of glycosylation.     

Vasopressin administration in the first month of life: effects of growth and water metabolism in hypothalamic diabetes insipidus rats

Rats homozygous for the mutant gene for diabetes insipidus (Brattleboro strain) are stunted in growth compared to rats heterozygous for the mutant gene and normal rats without the mutant gene. The hypothesis was tested that normal growth depends upon the presence of vasopressin. It was expected that replacement therapy of vasopressin rats homozygous for diabetes insipidus would make possible a normal growth rate similar to that of rats heterozygous for diabetes insipidus. Rats heterozygous and homozygous for diabetes insipidus were treated with 0.25 U (Days 0-9) and 0.5 U (Days 10-29) of vasopressin during the first month of life. During the treatment period, vasopressin significantly increased the urine osmolatities of the homozygous rats demonstrating the renal effectiveness of the vasopressin. The results showed that remedial vasopressin administration could not produce normal growth rates in homozygous rats and may be detrimental. Six weeks following vasopressin treatment, homozygous, diabetes insipidus rats which had received vasopressin had increased 24 hr water intakes and decreased urine osmolalities compared to control, homozygous rats, Heterozygous rats also had decreased urine osmolalities resulting from vasopressin six weeks after the cessation of vasopressin treatment.     

Autosomal recessive nephrogenic diabetes insipidus caused by an aquaporin-2 mutation

Vasopressin V2 receptors, expressed from an x-chromosomal gene, are involved in antidiuresis, but also in release of coagulation factor VIII and von Willebrand factor (vWF). The present study describes autosomal recessive nephrogenic diabetes insipidus (NDI) in a large cluster of patients in Israel's Lower-Galilee. Evidence for an intact V2 receptor was concluded by their normal increase in factor VIII and vWF after desmopressin infusion. Thus, in these patients a defect in the pathway beyond the V2 receptor was suspected. The recent cloning of the human Aquaporin-2 gene enabled us to test this gene as a candidate for such a postreceptor defect. Direct sequencing of the Aquaporin-2 gene revealed a G298T substitution causing a Gly100Stop nonsense mutation in the third transmembrane region. Because this putative disease-causing mutation was identified in index patients of different families, we suggest that all patients are descendants of a common ancestor. Thus, this new entity is characterized by an autosomal recessive NDI. The differential response of clotting factors and urine osmolality to desmopressin may provide a simple tool for clinical diagnosis of a V2-postreceptor defect. The early stop-codon of Aquaporin-2 results in complete resistance to vasopressin antidiuretic effect.     

Novel mutations in aquaporin-2 gene in female siblings with nephrogenic diabetes insipidus: evidence of disrupted water channel function

Novel mutations of the aquaporin-2 (AQP2) gene have been detected in Japanese female siblings with autosomal-recessive nephrogenic diabetes insipidus. The patients were compound heterozygote for point mutations at nucleotide position 374 (C374T) and at position 523 (G523A) in exon 2 of the AQP2 gene, resulting in substitution of methionine for threonine at codon 125 (T125M) and arginine for glycine at codon 175 (G175R). The water permeability (Pf) of oocytes injected with wild-type complementary RNA increased 9.0-fold compared with the Pf of water-injected oocytes, whereas the increases in the Pf of oocytes injected with T125M and G175R complementary RNA were only 1.7-fold and 1.5-fold, respectively. Immunoblot and immunocytochemistry indicated that the plasma membrane expressions of T125M and G175R AQP2 proteins were comparable to that of the wild-type, suggesting that although neither the T125M nor G175R mutation had a significant effect on plasma membrane expression, they both distorted the structure and function of the aqueous pore of AQP2. These results provide evidence that the nephrogenic diabetes insipidus in patients with T125M and G175R mutations is attributable not to the misrouting of AQP2, but to the disrupted water channel function.     

Opioid dependence in 15 patients after a work injury

A 50-year-old Japanese man had been suffering from polydipsia and polyuria for 2 months without any other specific symptoms. His daily urinary output reached 5 liters. On admission, no abnormalities of the kidneys, heart, thyroid, adrenals, pituitary or hypothalamus were detected by laboratory tests and MRI of the head. Pure psychogenic polydipsia was ruled out because his urine volume did not decrease sufficiently with 18 h of water deprivation and the subsequent injection of aqueous vasopressin. Plasma arginine vasopressin (AVP) levels against plasma osmolality remained within the normal range during the test. These results indicated that diabetes insipidus in this case was caused by renal insensitivity to AVP. The symptoms disappeared spontaneously, and marked improvement was observed in a second water deprivation test 1 month later, although the maximum urine concentration was still subnormal. The combination of both latent insufficiency of AVP secretion and impairment of the renal countercurrent system induced by psychogenic polydipsia was speculated as a possible mechanism for the transient nephrogenic diabetes insipidus in this case.     

Clinical, biochemical and molecular findings in a patient with X-linked liver glycogenosis followed for 40 years

Phosphorylase kinase (PHK) is a regulatory enzyme in glycogen metabolism. Mutations in the gene encoding the alpha subunit of PHK (PHKA2) have been shown to be responsible for X-linked liver glycogenosis (XLG). XLG, a frequent type of glycogen storage disease, is characterised by hepatomegaly and growth retardation. Two subtypes of XLG have been described: XLG type I patients have a clear-cut PHK deficiency in liver and blood cells, whereas XLG type II patients have a normal or residual activity. Here, we present clinical, biochemical and molecular findings on a liver glycogenosis patient in whom the diagnosis XLG II only became clear after enzyme assays in the liver and identification of the disease-causing mutation. A missense mutation replacing arginine at amino acid position 186 by histidine (R186H) was identified in the PHKA2 gene. Mutations of the same arginine residue have been previously found in at least four other unrelated XLG II patients. CONCLUSION: Arginine at position 186 of the alpha subunit seems to play an important role in the structure or the regulation of PHK. In patients with XLG having normal or residual PHK activity where XLG II is suspected, the identification of mutations in PHKA2 leads to the final classification.     

Clinical characteristics of familial amyotrophic lateral sclerosis with Cu/Zn superoxide dismutase gene mutations

We report clinical characteristics of familial amyotrophic lateral sclerosis (FALS) with 4 different missense point mutations in exons 2, 4, and 5 of the Cu/Zn superoxide dismutase (SOD) gene, that result in amino acid substitutions of histidine46 by arginine (H46R), leucine84 by valine (L84V), isoleucine104 by phenylalanine (I104F), and valine148 by isoleucine (V148I), in 5 Japanese families. Although features of progressive neurogenic muscular atrophy were common in patients of these families, patients of each family showed characteristic clinical features. FALS patients with the H46R mutation showed a benign clinical course and stereotype progression of muscular weakness and atrophy beginning from the legs. In FALS with the L84V mutation, while the clinical course of the disease was similar, the age at onset was younger in men than women. The patients with I104F showed wide ranges of age at onset and duration with ophthalmoparesis and sensory involvement in one patient. Those with the V148I mutation showed younger age at onset and variable first symptoms within the family. Although lower motor sign was evident in all cases, hyperreflexia varied from 0 to 100% among patients with the different mutations, and the Babinski sign was not observed in any case. Bulbar palsy was frequent with I104F, but not with H46R. SOD activity of the red blood cells was severely reduced with I104F and V148I, but was slightly reduced with H46R. These results suggest that familial ALS with different mutations of the Cu/Zn SOD gene each showed clinical characteristics, and that genetic mutations and clinical features are well correlated in familial ALS.     

Swiss journey through the clinical and genetic characteristics of diabetes in young patients

The aim of this study is to understand better the genetic causes of type II diabetes and the phenotypic consequences of the genetic changes. We first investigated the relative prevalence of the different forms of diabetes in young adults and their clinical features. 51 non-obese patients were identified in whom diabetes had been diagnosed before age 40; cases of typical insulin-dependent type I diabetes were excluded. A search for mutations of the glucokinase and HNF-1 alpha genes and for mitochondrial DNA was made, anti-islet and anti-GAD antibodies were determined and HLA class II genotyping was performed. Patients were subdivided on clinical grounds into a MODY (maturity onset diabetes of the young) group (n = 19) and a non-MODY group (n = 32). MODY is a form of diabetes which has an autosomal dominant inheritance for which 3 genes have already been implicated (MODY1, HNF-4 gene; MODY2, glucokinase gene, and MODY3, HNF-1 alpha gene). In the MODY group we identified 3 patients with MODY2, 1 with MODY3, 1 with the 3243 mitochondrial mutation and a further patient with autoimmune diabetes. In the non-MODY group we found 5 patients with autoimmune diabetes and 1 with MODY2. No clinical parameter was helpful in classifying patients in one of these subclasses of diabetes; however, glucagon stimulated C-peptide was useful in discriminating between MODY2 patients and the others. Young and lean non-insulin-dependent diabetic patients thus constitute a very heterogeneous group, though presenting similar clinical features. In the second study we analyzed hepatic glucose metabolism in patients with a mutation of the glucokinase gene expressed in both liver and islet beta cells. We found that endogenous glucose production is inadequately inhibited by hyperglycemia, a fact which contributes to the pathogenesis of hyperglycemia in these patients.