A mutation in the insulin receptor gene that impairs transport of the receptor to the plasma membrane and causes insulin-resistant diabetes

Interplay of mitochondria and diabetes: Unveiling novel therapeutic strategies

The interplay between mitochondrial function and diabetes has gained significant attention due to its crucial role in the pathogenesis and progression of the disease. Mitochondria, known as the cellular powerhouses, are essential for glucose metabolism. Dysfunction of these organelles has been implicated in the development of insulin resistance and beta-cell failure, both prominent features of diabetes. This comprehensive review explores the intricate mechanisms involved, including the generation of reactive oxygen species and the impact of mitochondrial DNA (mtDNA) mutations. Moreover, the review delves into emerging therapeutic strategies that specifically target mitochondria, such as mitochondria-targeted antioxidants, agents promoting mitochondrial biogenesis, and compounds modulating mitochondrial dynamics. The potential of these novel approaches is critically evaluated, taking into account their benefits and limitations, to provide a well-rounded perspective. Ultimately, this review emphasizes the importance of advancing our understanding of mitochondrial biology to revolutionize the treatment of diabetes.

Long-range repression by ecdysone receptor on complex enhancers of the insulin receptor gene

The insulin signalling pathway is evolutionarily conserved throughout metazoans, playing key roles in development, growth, and metabolism. Misregulation of this pathway is associated with a multitude of disease states including diabetes, cancer, and neurodegeneration. The human insulin receptor gene (INSR) is widely expressed throughout development and was previously described as a 'housekeeping' gene. Yet, there is abundant evidence that this gene is expressed in a cell-type specific manner, with dynamic regulation in response to environmental signals. The Drosophila insulin-like receptor gene (InR) is homologous to the human INSR gene and was previously shown to be regulated by multiple transcriptional elements located primarily within the introns of the gene. These elements were roughly defined in ~1.5 kbp segments, but we lack an understanding of the potential detailed mechanisms of their regulation. We characterized the substructure of these cis-regulatory elements in Drosophila S2 cells, focusing on regulation through the ecdysone receptor (EcR) and the dFOXO transcription factor. By identifying specific locations of activators and repressors within 300 bp subelements, we show that some previously identified enhancers consist of relatively compact clusters of activators, while others have a distributed architecture not amenable to further reduction. In addition, these assays uncovered a long-range repressive action of unliganded EcR. The complex transcriptional circuitry likely endows InR with a highly flexible and tissue-specific response to tune insulin signalling. Further studies will provide insights to demonstrate the impact of natural variation in this gene's regulation, applicable to human genetic studies.

Long-range repression by ecdysone receptor on complex enhancers of the insulin receptor gene

The insulin signaling pathway is evolutionarily conserved throughout metazoans, playing key roles in development, growth, and metabolism. Misregulation of this pathway is associated with a multitude of disease states including diabetes, cancer, and neurodegeneration. Genome-wide association studies indicate that natural variants in putative intronic regulatory elements of the human insulin receptor gene ( INSR) are associated with metabolic conditions, however, this gene's transcriptional regulation remains incompletely studied. INSR is widely expressed throughout development and was previously described as a 'housekeeping' gene. Yet, there is abundant evidence that this gene is expressed in a cell-type specific manner, with dynamic regulation in response to environmental signals. The Drosophila insulin-like receptor gene ( InR ) is homologous to the human INSR gene and was previously shown to be regulated by multiple transcriptional elements located primarily within the introns of the gene. These elements were roughly defined in ∼1.5 kbp segments, but we lack an understanding of the potential detailed mechanisms of their regulation, as well as the integrative output of the battery of enhancers in the entire locus. Using luciferase assays, we characterized the substructure of these cis-regulatory elements in Drosophila S2 cells, focusing on regulation through the ecdysone receptor (EcR) and the dFOXO transcription factor. The direct action of EcR on Enhancer 2 reveals a bimodal form of regulation, with active repression in the absence of the ligand, and positive activation in the presence of 20E. By identifying the location of activators of this enhancer, we characterized a long-range of repression acting over at least 475 bp, similar to the action of long-range repressors found in the embryo. dFOXO and 20E have contrasting effects on some of the individual regulatory elements, and for the adjacent enhancers 2 and 3, their influence was/was not found to be additive, indicating that enhancer action on this locus can/cannot be characterized in part by additive models. Other characterized enhancers from within this locus exhibited "distributed" or "localized" modes of action, suggesting that predicting the joint functional output of multiple regulatory regions will require a deeper experimental characterization. The noncoding intronic regions of InR have demonstrated dynamic regulation of expression and cell type specificity. This complex transcriptional circuitry goes beyond the simple conception of a 'housekeeping' gene. Further studies are aimed at identifying how these elements work together in vivo to generate finely tuned expression in tissue- and temporal-specific manners, to provide a guide to understanding the impact of natural variation in this gene's regulation, applicable to human genetic studies.

Exposure to organophosphates in association with the development of insulin resistance: Evidence from in vitro, in vivo, and clinical studies

Insulin resistance is an underlying condition prior to the development of several diseases, including type 2 diabetes, cardiovascular diseases, cognitive impairment, and cerebrovascular complications. Organophosphates (OPs) are one of several factors thought to induce insulin resistance. Previous studies showed that the exposure to OPs pesticides induced insulin resistance through the impairment of hepatic glucose metabolism, pancreatic damage, and disruption of insulin signaling of both adipose tissues and skeletal muscles. Several studies reported possible mechanisms associated with OPs-induced insulin resistance in different models in in vivo studies including those in adult animals, obese animals, and offspring models, as well as in clinical studies. In addition, pharmacological interventions in OPs-induced insulin resistance have been previously investigated. This review aims to summarize and discuss all the evidence concerning OPs-induced insulin resistance in different models including in vitro, in vivo and clinical studies. The interventions of OPs-induced insulin resistance are also discussed. Any contradictory findings also considered. The information from this review will provide insight for possible therapeutic approaches to OPs-induced insulin resistance in the future.

Influence of Insulin Receptor Single Nucleotide Polymorphisms on Glycaemic Control and Formation of Anti-Insulin Antibodies in Diabetes Mellitus

Single nucleotide polymorphisms (SNPs) in insulin and insulin receptor genes may influence the interaction between the two molecules, as may anti-insulin antibodies (IAs), commonly found in patients with type 1 diabetes mellitus (T1D) or type 2 diabetes mellitus (T2D) treated with exogenous insulin. We examined the impact of two SNPs in the human insulin gene (INS), rs3842752 and rs689, and two in the insulin receptor gene (INSR) rs2245649 and rs2229429, on disease susceptibility, glycaemic control, and IAs formation in 100 T1D patients and 101 T2D patients treated with insulin. 79 individuals without diabetes were typed as healthy controls. The minor alleles of rs3842752 and rs689 in INS protected against T1D (OR: 0.50, p = 0.01 and OR: 0.44; p = 0.002, respectively). The minor alleles of both rs2245649 and rs2229429 in INSR were risk factors for poor glycaemic control (HbA1c ≥ 80 mmol/mol) in T1D (OR: 5.35, p = 0.009 and OR: 3.10, p = 0.01, respectively). Surprisingly, the minor alleles of rs2245649 and rs2229429 in INSR associated strongly with the absence of IAs in T1D (OR = 0.28, p = 0.008 and OR = 0.30, p = 0.002, respectively). In conclusion, the minor alleles of the investigated INS SNPs protect against T1D, and the minor alleles of the investigated INSR SNPs are associated with poor glycaemic control and the absence of IAs in T1D.

YIPF5 mutations cause neonatal diabetes and microcephaly: progress for precision medicine and mechanistic understanding

Identifying genes that result in monogenic diabetes can provide insights that can build a scientific foundation for precision medicine. At present, nearly 20% of neonatal diabetes cases have unknown causes. In this issue of the JCI, De Franco and Lytrivi et al. sequenced the genome of two probands with a rare neonatal diabetes subtype that also associated with microcephaly and epilepsy. The authors revealed mutations in the YIPF5 gene. YIPF5 resides in the Golgi apparatus and is thought to play a critical role in vesicular trafficking. Notably, disrupting YIPF5 in β cell-based models induced ER stress signaling and resulted in the accumulation of intracellular proinsulin. We believe that utilizing registries and biobanks to reveal other monogenic atypical forms of diabetes is an important approach to gaining insight and suggest that an insulin sensitizer may alleviate ER stress associated with YIPF5 disruption by decreasing the demand for insulin secretion.

Arg1201Gln mutation of insulin receptor impairs tyrosine kinase activity and causes insulin resistance: a case report

Type A insulin resistance syndrome (TAIRS) is a rare subtype of congenital insulin resistance (IR), which is characterized by specific clinical manifestations without clear diagnostic criteria and is easily misdiagnosed or overlooked. Herein we present a case of TAIRS with acanthosis nigricans (AN), severe IR, polycystic ovaries, hyperandrogenism and its consequence such as menstrual disturbances, acne and hirsutism. A heterozygous mutation, p.Arg1201Gln, in the insulin receptor (INSR) was detected. This mutation in the tyrosine kinase domain has been described before and shown to impair tyrosine kinase activity and is responsible for IR.

Leucine Rich Repeat Proteins: Sequences, Mutations, Structures and Diseases

Mutations in the genes encoding Leucine Rich Repeat (LRR) containing proteins are associated with over sixty human diseases; these include high myopia, mitochondrial encephalomyopathy, and Crohn's disease. These mutations occur frequently within the LRR domains and within the regions that shield the hydrophobic core of the LRR domain. The amino acid sequences of fifty-five LRR proteins have been published. They include Nod-Like Receptors (NLRs) such as NLRP1, NLRP3, NLRP14, and Nod-2, Small Leucine Rich Repeat Proteoglycans (SLRPs) such as keratocan, lumican, fibromodulin, PRELP, biglycan, and nyctalopin, and F-box/LRR-repeat proteins such as FBXL2, FBXL4, and FBXL12. For example, 363 missense mutations have been identified. Replacement of arginine, proline, or cysteine by another amino acid, or the reverse, is frequently observed. The diverse effects of the mutations are discussed based on the known structures of LRR proteins. These mutations influence protein folding, aggregation, oligomerization, stability, protein-ligand interactions, disulfide bond formation, and glycosylation. Most of the mutations cause loss of function and a few, gain of function.

Pharmacoperones as Novel Therapeutics for Diverse Protein Conformational Diseases

After synthesis, proteins are folded into their native conformations aided by molecular chaperones. Dysfunction in folding caused by genetic mutations in numerous genes causes protein conformational diseases. Membrane proteins are more prone to misfolding due to their more intricate folding than soluble proteins. Misfolded proteins are detected by the cellular quality control systems, especially in the endoplasmic reticulum, and proteins may be retained there for eventual degradation by the ubiquitin-proteasome system or through autophagy. Some misfolded proteins aggregate, leading to pathologies in numerous neurological diseases. In vitro, modulating mutant protein folding by altering molecular chaperone expression can ameliorate some misfolding. Some small molecules known as chemical chaperones also correct mutant protein misfolding in vitro and in vivo. However, due to their lack of specificity, their potential as therapeutics is limited. Another class of compounds, known as pharmacological chaperones (pharmacoperones), binds with high specificity to misfolded proteins, either as enzyme substrates or receptor ligands, leading to decreased folding energy barriers and correction of the misfolding. Because many of the misfolded proteins are misrouted but do not have defects in function per se, pharmacoperones have promising potential in advancing to the clinic as therapeutics, since correcting routing may ameliorate the underlying mechanism of disease. This review will comprehensively summarize this exciting area of research, surveying the literature from in vitro studies in cell lines to transgenic animal models and clinical trials in several protein misfolding diseases.

Imbalanced Insulin Actions in Obesity and Type 2 Diabetes: Key Mouse Models of Insulin Signaling Pathway

Since the discovery of the tyrosine kinase activity of the insulin receptor (IR), researchers have been engaged in intensive efforts to resolve physiological functions of IR and its major downstream targets, insulin receptor substrate 1 (Irs1) and Irs2. Studies conducted using systemic and tissue-specific gene-knockout mice of IR, Irs1, and Irs2 have revealed the physiological roles of these molecules in each tissue and interactions among multiple tissues. In obesity and type 2 diabetes, selective downregulation of Irs2 and its downstream actions to cause reduced insulin actions was associated with increased insulin actions through Irs1 in variety tissues. Thus, we propose the novel concept of "organ- and pathway-specific imbalanced insulin action" in obesity and type 2 diabetes, which includes and extends "selective insulin resistance." This Review focuses on recent progress in understanding insulin signaling and insulin resistance using key mouse models for elucidating pathophysiology of human obesity and type 2 diabetes.

Glucagon receptor inhibition normalizes blood glucose in severe insulin-resistant mice

Inactivating mutations in the insulin receptor results in extreme insulin resistance. The resulting hyperglycemia is very difficult to treat, and patients are at risk for early morbidity and mortality from complications of diabetes. We used the insulin receptor antagonist S961 to induce severe insulin resistance, hyperglycemia, and ketonemia in mice. Using this model, we show that glucagon receptor (GCGR) inhibition with a monoclonal antibody normalized blood glucose and β-hydroxybutyrate levels. Insulin receptor antagonism increased pancreatic β-cell mass threefold. Normalization of blood glucose levels with GCGR-blocking antibody unexpectedly doubled β-cell mass relative to that observed with S961 alone and 5.8-fold over control. GCGR antibody blockage expanded α-cell mass 5.7-fold, and S961 had no additional effects. Collectively, these data show that GCGR antibody inhibition represents a potential therapeutic option for treatment of patients with extreme insulin-resistance syndromes.

Septin 7 reduces nonmuscle myosin IIA activity in the SNAP23 complex and hinders GLUT4 storage vesicle docking and fusion

Glomerular epithelial cells, podocytes, are insulin responsive and can develop insulin resistance. Here, we demonstrate that the small GTPase septin 7 forms a complex with nonmuscle myosin heavy chain IIA (NMHC-IIA; encoded by MYH9), a component of the nonmuscle myosin IIA (NM-IIA) hexameric complex. We observed that knockdown of NMHC-IIA decreases insulin-stimulated glucose uptake into podocytes. Both septin 7 and NM-IIA associate with SNAP23, a SNARE protein involved in GLUT4 storage vesicle (GSV) docking and fusion with the plasma membrane. We observed that insulin decreases the level of septin 7 and increases the activity of NM-IIA in the SNAP23 complex, as visualized by increased phosphorylation of myosin regulatory light chain. Also knockdown of septin 7 increases the activity of NM-IIA in the complex. The activity of NM-IIA is increased in diabetic rat glomeruli and cultured human podocytes exposed to macroalbuminuric sera from patients with type 1 diabetes. Collectively, the data suggest that the activity of NM-IIA in the SNAP23 complex plays a key role in insulin-stimulated glucose uptake into podocytes. Furthermore, we observed that septin 7 reduces the activity of NM-IIA in the SNAP23 complex and thereby hinders GSV docking and fusion with the plasma membrane.

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Septin 7, nonmuscle myosin heavy chain IIA (NMHC-IIA) and SNAP23 form a complex.

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Knockdown of septin 7 increases NM-IIA activity in the SNAP23 complex.

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Insulin decreases septin 7 level and increases NM-IIA activity in the SNAP23 complex.

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Septin 7 hinders GSV docking/fusion by reducing NM-IIA activity in the SNAP23 complex.

Sequencing analysis of insulin receptor defects and detection of two novel mutations in INSR gene

Mutations in the insulin receptor gene cause the inherited insulin resistant syndromes Leprechaunism and Rabson–Mendenhall syndrome. These recessive conditions are characterized by intrauterine and post-natal growth restrictions, dysmorphic features, altered glucose homeostasis, and early demise. The insulin receptor gene (INSR) maps to the short arm of chromosome 19 and is composed of 22 exons. Here we optimize the conditions for sequencing this gene and report novel mutations in patients with severe insulin resistance.

Methods

PCR amplification of the 22 coding exons of the INSR gene was performed using M13-tailed primers. Bidirectional DNA sequencing was performed with BigDye Terminator chemistry and M13 primers and the product was analyzed on the ABI 3100 genetic analyzer. Data analysis was performed using Mutation Surveyor software comparing the sequence to a reference INSR sequence (Genbank NC_000019).

Results

We sequenced four patients with Leprechaunism or Rabson–Mendenhall syndromes as well as seven samples from normal individuals and confirmed previously identified mutations in the affected patients. Three of the four mutations identified in this group caused premature insertion of a stop codon. In addition, the INSR gene was sequenced in 14 clinical samples from patients with suspected insulin resistance and one novel mutation was found in an infant with a suspected diagnosis of Leprechaunism.

Discussion

Leprechaunism and Rabson–Mendenhall syndrome are very rare and difficult to diagnose. Diagnosis is currently based mostly on clinical criteria. Clinical availability of DNA sequencing can provide an objective way of confirming or excluding the diagnosis.

Metreleptin improves blood glucose in patients with insulin receptor mutations

CONTEXT

Rabson-Mendenhall syndrome (RMS) is caused by mutations of the insulin receptor and results in extreme insulin resistance and dysglycemia. Hyperglycemia in RMS is very difficult to treat, and patients are at risk for early morbidity and mortality from complications of diabetes.

OBJECTIVE

Our objective was to study 1-year effects of recombinant human methionyl leptin (metreleptin) in 5 patients with RMS and 10-year effects in 2 of these patients.

DESIGN AND SETTING

We conducted an open-label nonrandomized study at the National Institutes of Health.

PATIENTS

Patients were adolescents with RMS and poorly controlled diabetes.

INTERVENTION

Two patients were treated with escalating doses (0.02 up to 0.22 mg/kg/d) of metreleptin for 10 years, including 3 cycles of metreleptin withdrawal and reinitiation. In all 5 patients, 1-year effects of metreleptin (0.22 mg/kg/d) were studied.

OUTCOME MEASURES

Hemoglobin A1c (HbA1c) and body mass index (BMI) z-scores were evaluated every 6 months.

RESULTS

HbA1c decreased from 11.4% ± 1.1% at baseline to 9.3% ± 1.9% after 6 months and 9.7% ± 1.6% after 12 months of metreleptin (P = .007). In patients treated for 10 years, HbA1c declined with each cycle of metreleptin and rose with each withdrawal. BMI z-scores declined from -1.4 ± 1.8 at baseline, to -2.6 ± 1.6 after 12 months of metreleptin (P = .0006). Changes in BMI z-score correlated with changes in HbA1c (P < .0001).

CONCLUSIONS

Metreleptin treatment for 12 months was associated with a 1.7% reduction in HbA1c; part of this improvement was likely mediated via decreased BMI. Metreleptin is a promising treatment option for RMS, but additional therapies are needed to achieve HbA1c targets.

BACE2 plays a role in the insulin receptor trafficking in pancreatic ß-cells

BACE1 (β-site amyloidogenic cleavage of precursor protein-cleaving enzyme 1) is a β-secretase protein that plays a central role in the production of the β-amyloid peptide in the brain and is thought to be involved in the Alzheimer's pathogenesis. In type 2 diabetes, amyloid deposition within the pancreatic islets is a pathophysiological hallmark, making crucial the study in the pancreas of BACE1 and its homologous BACE2 to understand the pathological mechanisms of this disease. The objectives of the present study were to characterize the localization of BACE proteins in human pancreas and determine their function. High levels of BACE enzymatic activity were detected in human pancreas. In normal human pancreas, BACE1 was observed in endocrine as well as in exocrine pancreas, whereas BACE2 expression was restricted to β-cells. Intracellular analysis using immunofluorescence showed colocalization of BACE1 with insulin and BACE2 with clathrin-coated vesicles of the plasma membrane in MIN6 cells. When BACE1 and -2 were pharmacologically inhibited, BACE1 localization was not altered, whereas BACE2 content in clathrin-coated vesicles was increased. Insulin internalization rate was reduced, insulin receptor β-subunit (IRβ) expression was decreased at the plasma membrane and increased in the Golgi apparatus, and a significant reduction in insulin gene expression was detected. Similar results were obtained after specific BACE2 silencing in MIN6 cells. All these data point to a role for BACE2 in the IRβ trafficking and insulin signaling. In conclusion, BACE2 is hereby presented as an important enzyme in β-cell function.

Thyroid hormone induced brown adipose tissue and amelioration of diabetes in a patient with extreme insulin resistance

CONTEXT

Brown adipose tissue (BAT) found by positron emission/computed tomography (PET-CT) using flouro-deoxyglucose (FDG) is inducible by cold exposure in men. Factors leading to increased BAT are of great interest for its potential role in the treatment of diabetes and obesity.

OBJECTIVE

We tested whether thyroid hormone (TH) levels are related to the volume and activity of BAT in a patient with a mutation in the insulin receptor gene. DESIGN/SETTING/INTERVENTION: Our work was based on the case report of a patient in an observational study at the National Institutes of Health.

PATIENT

The patient discontinued insulin and oral antidiabetics after thyroidectomy and suppressive-dose levothyroxine therapy for thyroid cancer. PET-CT uptake in BAT was confirmed by histology and molecular analysis.

OUTCOMES

PET-CT studies were performed, and we measured hemoglobin A1c and resting energy expenditure before and after levothyroxine discontinuation for thyroid cancer testing. Molecular studies of BAT and white adipose samples are presented.

RESULT

Supraclavicular and periumbilical sc adipose tissue demonstrated molecular features of BAT including uncoupling protein-1, type 2 deiodinase, and PR domain containing 16 by quantitative PCR. Activity of type 2 deiodinase activity was increased. The discontinuation of levothyroxine resulted in decreased FDG uptake and diminished volume of BAT depots accompanied by worsening of diabetic control.

CONCLUSIONS

This case demonstrates the TH effect on BAT activity and volume in this patient and an association between BAT activity and glucose levels in this patient. Because the contribution of TH on skeletal muscle energy expenditure and fuel metabolism was not assessed, an association between BAT activity and glucose homeostasis can only be suggested.

Two related cases of type A insulin resistance with compound heterozygous mutations of the insulin receptor gene

We describe two sisters with type A insulin resistance. In contrast to common situation for this genetic disorder, the sisters harbored compound heterozygous mutations in the insulin receptor gene associated with mild glucose intolerance. The cases highlight the diversity of clinical phenotypes associated with mutations of the insulin receptor gene.

Divergent pathways in COS-7 cells mediate defective internalization and intracellular routing of truncated G-CSFR forms in SCN/AML

Background

Expression of truncated G-CSFR forms in patients with SCN/AML induces hyperproliferation and prolonged cell survival. Previously, we showed that ligand internalization is delayed and degradation of truncated G-CSFR forms is defective in patients with SCN/AML.

Methodology/Principal Findings

In this study, we investigated the potential roles of dileucine and tyrosine-based motifs within the cytoplasmic domain of the G-CSFR in modulating ligand/receptor internalization. Using standard binding assays with radiolabeled ligand and COS-7 cells, substitutions in the dileucine motif or deletion of tyrosine residues in the G-CSFR did not alter internalization. Attachment of the transferrin receptor YTRF internalization motif to a truncated G-CSFR form from a patient with SCN/AML corrected defective internalization, but not receptor degradation suggesting that receptor internalization and degradation occur independently via distinct domains and/or processes.

Conclusions

Our data suggest that distinct domains within the G-CSFR mediate separate processes for receptor internalization and degradation. Our findings using standard binding assays differ from recently published data utilizing flow cytometry.

Rabson-Mendenhall syndrome: medullary sponge kidney, a new component

Rabson-Mendenhall syndrome is a rare genetic disorder characterized by severe insulin resistance, extreme hyperinsulinemia, postprandial hyperglycemia, growth retardation, and dysmorphisms. Enlargement of the kidneys and nephrocalcinosis have been described previously. We report a 10-year-old boy who presented with gross hematuria, unilateral hydronephrosis, and the initial diagnosis of bilateral extensive medullary nephrocalcinosis. Medullary sponge kidney (MSK) was included in the differential diagnosis given the ultrasound findings. Further evaluation by intravenous pyelogram confirmed the suspected bilateral MSK. Given the patient's history of hydronephrosis due to an obstructing renal stone and MSK, urine calcium excretion was assessed and found to be markedly increased at 9.5 mg/kg per day. To our knowledge, this is the first report of Rabson-Mendenhall syndrome and an association with MSK. We recommend evaluation for nephrocalcinosis, MSK, and hypercalciuria in all children diagnosed with Rabson-Mendenhall syndrome.

Role of myotonic dystrophy protein kinase (DMPK) in glucose homeostasis and muscle insulin action

Myotonic dystrophy 1 (DM1) is caused by a CTG expansion in the 3′-unstranslated region of the DMPK gene, which encodes a serine/threonine protein kinase. One of the common clinical features of DM1 patients is insulin resistance, which has been associated with a pathogenic effect of the repeat expansions. Here we show that DMPK itself is a positive modulator of insulin action. DMPK-deficient (dmpk^−/−) mice exhibit impaired insulin signaling in muscle tissues but not in adipocytes and liver, tissues in which DMPK is not expressed. Dmpk^−/− mice display metabolic derangements such as abnormal glucose tolerance, reduced glucose uptake and impaired insulin-dependent GLUT4 trafficking in muscle. Using DMPK mutants, we show that DMPK is required for a correct intracellular trafficking of insulin and IGF-1 receptors, providing a mechanism to explain the molecular and metabolic phenotype of dmpk^−/− mice. Taken together, these findings indicate that reduced DMPK expression may directly influence the onset of insulin-resistance in DM1 patients and point to dmpk as a new candidate gene for susceptibility to type 2-diabetes.

[Molecular diagnosis of diabetes mellitus]

Diabetes mellitus comprises a heterogeneous group of disorders characterized by chronic hyperglycemia. Type 1 and type 2 diabetes result from alterations of various genes, each having partial and additive effects. Thus, the inheritance pattern is rather complex, and environmental factors play an important role in the manifestation and clinical course of the disease. There is no genetic test to diagnose diabetes mellitus type 1 or type 2. However, certain susceptibility genes and genetic variations can be examined for specific scientific questions. Furthermore, defined genetic defects exist of pancreatic beta-cell function (maturity-onset diabetes of the young, mitochondrial diabetes) and insulin action (e.g. insulin resistance syndromes and lipodystrophy syndromes) resembling monogenic disorders. In these cases, genetic tests are crucial for the correct classification of the type of diabetes, genetic counseling, and initiation of the appropriate therapy regimen.

Genetics of Type 2 diabetes

Type 2 diabetes (T2D) has become a health-care problem worldwide, with the rise in disease prevalence being all the more worrying as it not only affects the developed world but also developing nations with fewer resources to cope with yet another major disease burden. Furthermore, the problem is no longer restricted to the ageing population, as young adults and children are also being diagnosed with T2D. In recent years, there has been a surge in the number of genetic studies of T2D in attempts to identify some of the underlying risk factors. In this review, I highlight the main genes known to cause uncommon monogenic forms of diabetes (e.g. maturity-onset diabetes of the young--MODY--and insulin resistance syndromes), as well as describe some of the main approaches used to identify genes involved in the more common forms of T2D that result from the interaction between environmental risk factors and predisposing genotypes. Linkage and candidate gene studies have been highly successful in the identification of genes that cause the monogenic variants of diabetes and, although progress in the more common forms of T2D has been slow, a number of genes have now been reproducibly associated with T2D risk in multiple studies. These are discussed, as well as the main implications that the diabetes gene discoveries will have in diabetes treatment and prevention.

Clinical course of genetic diseases of the insulin receptor (type A and Rabson-Mendenhall syndromes): a 30-year prospective

The interaction of insulin with its cell surface receptor is the first step in insulin action and the first identified target of insulin resistance. The insulin resistance in several syndromic forms of extreme insulin resistance has been shown to be caused by mutations in the receptor gene. We studied 8 female patients with the type A form of extreme insulin resistance and 3 patients (2 male and 1 female) with the Rabson-Mendenhall syndrome and followed the natural history of these patients for up to 30 years. The 11 patients ranged in age from 7 to 32 years at presentation. All 11 patients had extreme insulin resistance, acanthosis nigricans, and hyperandrogenism in the female patients, and all but 1 were of normal body weight. This phenotype strongly predicts mutations in the insulin receptor: of the 8 patients studied, 7 were found to have mutations. Similar results from the literature are found in other patients with type A and Rabson-Mendenhall syndromes and leprechaunism. The hyperandrogenic state resulting from hyperinsulinemia and insulin resistance in these patients was extreme: 6 of 8 patients had ovarian surgery to correct the polycystic ovarian syndrome and elevation of serum testosterone. By contrast, a larger group of insulin-resistant patients who were obese with hyperandrogenism, insulin resistance, and acanthosis nigricans (HAIR-AN syndrome) did not have a high probability of mutations in the insulin receptor. The morbidity and mortality of these patients were high: 3 of 11 died, 9 of 11 were diabetic and 1 had impaired glucose tolerance, and 7 of 9 patients had 1 or more severe complication of diabetes. Our literature review revealed that the mortality of leprechaunism is so high that the term leprechaunism should be restricted to infants or young children under 2 years of age. Analogous to patients with the common forms of type 2 diabetes, these patients had a heterogeneous course. In 2 patients who were able to maintain extremely high endogenous insulin production, the fasting blood glucose remained normal even though post-glucose-challenge levels were elevated. Most patients, however, required large doses of exogenous insulin to ameliorate the severe hyperglycemia. Preliminary results of a recent study suggest that recombinant leptin administration may benefit these patients with severe insulin resistance.

Mosaic analysis of insulin receptor function

Insulin promotes both metabolism and growth. However, it is unclear whether insulin-dependent growth is merely a result of its metabolic actions. Targeted ablation of insulin receptor (Insr) has not clarified this issue, because of early postnatal lethality. To examine this question, we generated mice with variable cellular mosaicism for null Insr alleles. Insr ablation in approximately 80% of cells caused extreme growth retardation, lipoatrophy, and hypoglycemia, a clinical constellation that resembles the human syndrome of leprechaunism. Insr ablation in 98% of cells, while resulting in similar growth retardation and lipoatrophy, caused diabetes without beta-cell hyperplasia. The growth retardation was associated with a greater than 60-fold increase in the expression of hepatic insulin-like growth factor binding protein-1. These findings indicate that insulin regulates growth independently of metabolism and that the number of insulin receptors is an important determinant of the specificity of insulin action.

Expression of proto-oncogene c-kit receptor in rats (Rattus norvegicus) and identification of a mutant mRNA transcript implicated in spermatogenic failure

The pleiotropic proto-oncogene c-kit receptor, implicated in hematopoiesis and melanogenesis, is also known to play an important role in germ cell proliferation and differentiation although the mechanisms for the latter remain unknown. We studied c-kit expression by RT-PCR in various tissues of both fertile and infertile Brown Norway rats. Using different sets of primers, several regions from within the extracellular domain were amplified, cloned, and sequenced. One set of primers, in addition to the expected 352-bp amplicon, revealed a 276-bp transcript, although its biological functions remain unknown. These two transcripts showed varying levels of expression in different tissues of infertile rats against nearly uniform expression in the fertile animals. Significantly, the 352 bp testis transcript showed mutational hotspots from nucleotide 84-266 in the infertile rats. Analysis of testis and brain genomic DNA from these infertile rats showed mutations only in the testis suggesting this to be a postzygotic event. In contrast, no mutation was detected in the genomic DNA of testis and brain of the fertile rats. Protein expression studies showed complete absence of the cytoplasmic kinase domain and soluble c-kit protein in one of the infertile rats. Histological examination of testis of these infertile animals showed stem cell depletion resulting in fewer germ cells. Based on these results, we infer that 352-bp mutant mRNA transcript is implicated in the spermatogenic failure.

Deletion of V335 from the L2 domain of the insulin receptor results in a conformationally abnormal receptor that is unable to bind insulin and causes Donohue's syndrome in a human subject

An infant with Donohue's syndrome (leprechaunism) was found to be homozygous for an in-frame trinucleotide deletion within the insulin receptor gene resulting in the deletion of valine 335. When transiently transfected into Chinese hamster ovary cells, mutant receptor was produced in a mature form, but at significantly lower levels compared with wild-type receptor. Cell surface biotinylation experiments revealed that significant amounts of the DeltaV335 receptor were expressed on the cell surface. Despite this, cells expressing this receptor showed no significant insulin binding or ligand-induced receptor autophosphorylation. Although the DeltaV335 receptor was capable of being immunoprecipitated with antibodies directed against the beta-subunit of the receptor, the mutant receptor could not be recognized by a panel of antibodies directed against different epitopes of the alpha-subunit, suggesting that the loss of V335 results in a major conformational alteration in the receptor alpha-subunit. This would be predicted by the positioning of V335 at a critical location within a strand that provides the main rigid scaffold for the two beta-sheet faces of the L2 domain of the receptor. The severe biochemical and clinical consequences of this novel mutation, which occur despite substantial expression on the cell surface, emphasize the crucial role of the L2 domain in ligand binding by the insulin receptor.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

Preserved pancreatic beta-cell development and function in mice lacking the insulin receptor-related receptor

Receptors of the insulin/insulinlike growth factor (IGF) family have been implicated in the regulation of pancreatic beta-cell growth and insulin secretion. The insulin receptor-related receptor (IRR) is an orphan receptor of the insulin receptor gene (Ir) subfamily. It is expressed at considerably higher levels in beta cells than either insulin or IGF-1 receptors, and it has been shown to engage in heterodimer formation with insulin or IGF-1 receptors. To address whether IRR plays a physiologic role in beta-cell development and regulation of insulin secretion, we have characterized mice lacking IRR and generated a combined knockout of Ir and Irr. We report that islet morphology, beta-cell mass, and secretory function are not affected in IRR-deficient mice. Moreover, lack of IRR does not impair compensatory beta-cell hyperplasia in insulin-resistant Ir(+/-) mice, nor does it affect beta-cell development and function in Ir(-/-) mice. We conclude that glucose-stimulated insulin secretion and embryonic beta-cell development occur normally in mice lacking Irr.

Alternative glycosylation of the insulin receptor prevents oligomerization and acquisition of insulin-dependent tyrosine kinase activity

Glucose deprivation leads to the synthesis of an aberrantly glycosylated ('alternative') and inefficiently processed form of the insulin proreceptor in 3T3-L1 adipocytes. To further explore the effect of aberrant (rather than absent) N-linked glycosylation of the insulin receptor, we examined the relationship of processing to function. Our studies show that the alternative form of the proreceptor does not oligomerize nor does it acquire the ability to undergo insulin-sensitive autophosphorylation. This along with an interaction with the glucose-regulated stress protein GRP78/BiP implies inappropriate folding/dimerization and retention in the ER. Glucose refeeding causes the post-translational modification of the alternative form of the proreceptor to a novel 'intermediate' form which is independent of new protein synthesis. As little as 100 microM glucose (or mannose) can induce this modification. In vitro digestion of the alternative and intermediate proreceptors with SPC1/furin shows that both the alpha- and beta-subunit domains are glycosylated, albeit aberrantly. This implies that the aberrantly glycosylated proreceptor could serve as a substrate for SPC1 in a physiological setting if the receptor was able to interact with the enzyme in the appropriate compartment (i.e., the trans-Golgi network). Based on inhibitor studies, however, both the alternative and intermediate forms of the proreceptor appear to be primarily targeted to the proteasome for degradation.

Effect of alternative glycosylation on insulin receptor processing

The mature insulin receptor is a cell surface heterotetrameric glycoprotein composed of two alpha- and two beta-subunits. In 3T3-L1 adipocytes as in other cell types, the receptor is synthesized as a single polypeptide consisting of uncleaved alpha- and beta-subunits, migrating as a 190-kDa glycoprotein. To examine the importance of N-linked glycosylation on insulin receptor processing, we have used glucose deprivation as a tool to alter protein glycosylation. Western blot analysis shows that glucose deprivation led to a time-dependent accumulation of an alternative proreceptor of 170 kDa in a subcellular fraction consistent with endoplasmic reticulum localization. Co-precipitation assays provide evidence that the alternative proreceptor bound GRP78, an endoplasmic reticulum molecular chaperone. N-Glycosidase F treatment shows that the alternative proreceptor contained N-linked oligosaccharides. Yet, endoglycosidase H insensitivity indicates an aberrant oligosaccharide structure. Using pulse-chase methodology, we show that the synthetic rate was similar between the normal and alternative proreceptor. However, the normal proreceptor was processed into alpha- and beta-subunits (t((1)/(2)) = 1.3 +/- 0.6 h), while the alternative proreceptor was degraded (t((1)/(2)) = 5.1 +/- 0.6 h). Upon refeeding cells that were initially deprived of glucose, the alternative proreceptor was processed to a higher molecular weight form and gained sensitivity to endoglycosidase H. This "intermediate" form of the proreceptor was also degraded, although a small fraction escaped degradation, resulting in cleavage to the alpha- and beta-subunits. These data provide evidence for the first time that glucose deprivation leads to the accumulation of an alternative proreceptor, which can be post-translationally glycosylated with the readdition of glucose inducing both accelerated degradation and maturation.

Deletion of a Critical Internalization Domain in the G-CSFR in Acute Myelogenous Leukemia Preceded by Severe Congenital Neutropenia

Acquired mutations in the granulocyte colony-stimulating factor receptor (G-CSFR) occur in a subset of patients with severe congenital neutropenia (SCN) who develop acute myelogenous leukemia (AML). These mutations affect one allele and result in hyperproliferative responses to G-CSF, presumably through a dominant-negative mechanism. Here we show that a critical domain in the G-CSFR that mediates ligand internalization is deleted in mutant G-CSFR forms from patients with SCN/AML. Deletion of this domain results in impaired ligand internalization, defective receptor downmodulation, and enhanced growth signaling. These results explain the molecular basis for G-CSFR mutations in the pathogenesis of the dominant-negative phenotype and hypersensitivity to G-CSF in SCN/AML.

Deletion of a Critical Internalization Domain in the G-CSFR in Acute Myelogenous Leukemia Preceded by Severe Congenital Neutropenia

Acquired mutations in the granulocyte colony-stimulating factor receptor (G-CSFR) occur in a subset of patients with severe congenital neutropenia (SCN) who develop acute myelogenous leukemia (AML). These mutations affect one allele and result in hyperproliferative responses to G-CSF, presumably through a dominant-negative mechanism. Here we show that a critical domain in the G-CSFR that mediates ligand internalization is deleted in mutant G-CSFR forms from patients with SCN/AML. Deletion of this domain results in impaired ligand internalization, defective receptor downmodulation, and enhanced growth signaling. These results explain the molecular basis for G-CSFR mutations in the pathogenesis of the dominant-negative phenotype and hypersensitivity to G-CSF in SCN/AML.

Alanine-scanning mutagenesis of a C-terminal ligand binding domain of the insulin receptor alpha subunit

A recent affinity labeling study has suggested that amino acids 704-717 of the C terminus of the insulin receptor represent a contact site for insulin. To determine whether these amino acids are part of a ligand binding site, we have performed alanine-scanning mutagenesis of this region. Mutant cDNAs encoding recombinant secreted receptors were transiently expressed in 293 EBNA cells, and their insulin binding properties were evaluated. Of the 14 residues in this region only 4 amino acids, Asp-707, Val-712, Pro-716, and Arg-717, could be mutated to alanine without compromising insulin binding. The reduction in affinity resulting from the individual mutation of the remaining amino acids varied from an increase in Kd to 3.69 x 10(-9) M (Asn-711) to greater than 10(-6) M (Thr-704, Phe-705, Glu-706, and His-710); the Kd of native secreted recombinant receptor is 0.56 x 10(-9) M.

Mutant insulin receptors in syndromes of insulin resistance

To date, mutations of the insulin receptor remain the only well-established causes of severe insulin resistance. There is a broad correlation between the extent of impairment of signal transduction seen when the mutant receptors are expressed in vitro with the severity of the clinical phenotype. Thus leprechaunism, Rabson-Mendenhall syndrome and Type A insulin resistance appear to represent points on a continuum of severity of receptor dysfunction, rather than completely distinct syndromes. In other syndromes of insulin resistance, insulin receptor abnormalities remain the exception. However, functional studies of expressed naturally occurring insulin receptor mutations have acted as experiments of nature and greatly aided attempts to dissect the structure-function relationships of the receptor. The next few years will no doubt begin to reveal the contributions made by defects in the post-receptor signalling cascade to the syndromes of insulin resistance in man.

Intermolecular phosphorylation between insulin holoreceptors does not stimulate substrate kinase activity

We photocoupled benzoylphenylalanineB25, B29 epsilon-biotin insulin (BBpa-insulin) to native insulin receptors to obtain a uniform receptor population with covalently bound, non-dissociable ligand. We employed BBpa-insulin-bound and autophosphorylated (activated) receptor to phosphorylate substrate insulin receptor under conditions where the substrate receptor never interacts with insulin. The substrate receptor becomes phosphorylated in this inter-receptor fashion and reaches a phosphorylation state 50% of the maximal obtainable by autophosphorylation. However, this phosphorylation does not activate the substrate receptor to any measurable degree. We conclude that intermolecular phosphorylation of the insulin holoreceptors is unlikely to be of physiological significance.

Replacements of leucine 87 in human insulin receptor alter affinity for insulin

In a previous analysis, we identified a point mutation that substituted Pro (CCG) for Leu (CTG) at amino acid 87 in the alpha-subunit of the insulin receptor (IR) in a Japanese patient with leprechaunism. In the present study, we transfected either the wild type (Leu-87) or the mutant (Pro-87) IR cDNA into NIH3T3 cells. Pulse-chase in nonreducing conditions revealed that the dimerization of Pro-87 IR was slightly impaired. However, cell surface biotinylation showed that Pro-87 IR was transported to the cell surface. The Pro-87 IR reduced the insulin binding affinity to about 15% of Leu-87 IR, and the dissociation of insulin in Pro-87 IR was more rapid than in Leu-87 IR. The autophosphorylation of Pro-87 IR was less sensitive to insulin than that of Leu-87 IR, suggesting the reduced insulin binding affinity. Site-directed mutagenesis at amino acid 87 was performed to substitute Ile or Ala for Leu. Both mutant IRs were transported to the cell surface and labeled by cell surface biotinylation. The Ile-87 IR enhanced the insulin binding affinity about 4-fold. The insulin binding affinity of Ala-87 IR was reduced by 85% relative to that of Leu-87 IR. In addition, the dissociation of insulin in Ile-87 IR was slower than in Leu-87 IR, but in Ala-87 IR it was more rapid. These results provide the first direct evidence for a critical role of Leu-87 in binding insulin.

Human diabetes and obesity: tracking down the genes

Diabetes mellitus and obesity are heterogeneous disorders that affect up to 5-8% of the population of Western Europe. The importance of genetic factors in these disorders is well established; however, the genes involved and the defects leading to the manifestation of disease are largely unknown. Family studies are a powerful tool with which to localize chromosomal regions linked to a genetic disorder. A genome-wide search for the genes associated with susceptibility to diabetes mellitus and obesity, combined with a more-specific, candidate-gene approach, should enable the identification of the loci involved in these diseases. Once regions linked to disease are identified, positional-cloning techniques can be used to track down the gene(s) responsible.

Insulin-like growth factors: clinical experience in ovarian function

Insulin, insulin-like growth factor, and insulin-like growth factor binding proteins have been shown to play major roles in the modulation of both normal and disturbed ovarian physiology. Identification of many of the actions of these peptides was initially characterized using animal models. However, an increasing body of evidence has emerged to clarify their contributions in human reproductive function. It is clear that at various stages of folliculogenesis, local steroid production acts in concert with intraovarian peptides to promote dominant follicle development. This review will discuss the physiologic role(s) of the insulin-insulin-like growth factor-insulin-like growth factor binding protein family in reproductive function and disorders of androgen excess.

Homozygosity for a null allele of the insulin receptor gene in a patient with leprechaunism

Mutations in the insulin receptor gene can cause genetic syndromes associated with extreme insulin resistance. We have investigated a patient with leprechaunism (leprechaun/Qatar-1) born of a consanguineous marriage. Postnatally, the proband had episodes of severe hypoglycemia and hyperinsulinernia, with blood glucose levels ranging from 0.9 to 9.9 mmol/L. The C peptide concentration with 1880 nmol/L, and the total insulin concentration was 1409 mU/L. The patient died outside the hospital at the age of four months. All 22 exons of the patient's insulin receptor gene were screened for mutations using denaturing gradient gel electrophoresis. Thereafter, the nucleotide sequences of selected exons were determined directly. The patient was homozygous for a mutation in exon 13; thirteen base pairs were deleted and replaced by a 5 b.p. sequence. This mutation shifts the reading frame and introduces a premature chain termination codon downstream in exon 13. Thus, the mutant allele is predicted to be a null allele that encodes a truncated receptor lacking both transmembrane and tyrosine kinase domains.

Absence of insulin receptor gene mutations in three insulin-resistant women with the polycystic ovary syndrome

Women with polycystic ovary syndrome (PCOS) are markedly insulin-resistant, but the molecular mechanisms of these changes and their relationship to the hyperandrogenic state remain to be clarified. Mutations have recently been identified in the insulin receptor gene of patients with extreme forms of insulin resistance associated with hyperandrogenism (eg, type A insulin resistance), and these mutations account for the insulin resistance in such patients. We performed this study to determine whether mutations in the coding portion of the insulin receptor gene were responsible for insulin resistance in PCOS. Insulin binding studies using cultured skin fibroblasts of three obese (body mass index > 27 kg/m2) women with PCOS (ie, mild hyperandrogenemia and chronic anovulation of unknown etiology) and documented insulin resistance showed no apparent abnormalities in either the number or affinity of insulin binding sites. Direct sequencing of all 22 exons of the insulin receptor gene from two of the women with PCOS did not reveal any mutations. Furthermore, both alleles of the gene were expressed at equal levels. In a third insulin-resistant PCOS woman, there was no evidence for a mutation in the coding portion of the insulin receptor gene as determined by denaturing gradient gel electrophoresis (DGGE). We conclude that the insulin resistance in these PCOS women was caused by a defect extrinsic to the insulin receptor.

Decreased expression of insulin-sensitive glucose transporter mRNA (GLUT-4) in adipose tissue of non-insulin-dependent diabetic and obese patients: evaluation by a simplified quantitative PCR assay

Impaired cellular uptake and utilization of glucose is the hallmark of non-insulin-dependent-diabetes (NIDDM). We have developed a quantitative assay to probe the expression of glucose-transporter genes in tissues derived from patients with NIDDM. Using the polymerase chain reaction (PCR), we assessed levels of expression of the insulin responsive glucose transporter GLUT-4 in adipose tissue of patients with NIDDM and in obese patients. We report that expression of GLUT-4 is reduced in NIDDM and in obesity associated with hyperinsulinemia and insulin resistance. These results suggest that reduction of GLUT-4 levels in the adipose cell plays an important role in the pathogenesis of insulin resistance, an early feature of NIDDM.

Functional properties of a heterozygous mutation (Arg1174-->Gln) in the tyrosine kinase domain of the insulin receptor from a type A insulin resistant patient

We analysed the biochemical properties of insulin receptors of a Type A insulin resistant patient with a single heterozygous point mutation substituting Gln for Arg1174. Insulin binding capacity and affinity to Epstein-Barr virus transformed lymphocytes was normal. Quantitative analysis of autophosphorylation and substrate phosphorylation of soluble insulin receptors isolated from patient cells revealed no differences in the basal state whereas in the presence of insulin autophosphorylation activity was only 30% of control receptors. The stimulation of substrate phosphorylation and down-regulation of receptors on patient cells after chronic exposure to insulin was diminished when compared to controls. We conclude that the heterozygous Arg1174 mutation does not perturb basal kinase activity but specifically interferes with the kinase activation by insulin and that the mutation has a dominant negative effect on the wild type kinase.

Homozygosity for a new mutation (Ile119-->Met) in the insulin receptor gene in five sibs with familial insulin resistance

Mutations in the insulin receptor gene can cause genetic syndromes such as leprechaunism that are associated with extreme insulin resistance. We have investigated a patient with leprechaunism born of a consanguineous marriage. All 22 exons of the insulin receptor gene were screened for mutations using denaturing gradient gel electrophoresis. Thereafter, the nucleotide sequences of selected exons were determined directly. The patient was homozygous for a point mutation in exon 2 of the insulin receptor gene which results in the substitution of methionine for isoleucine at codon 119. Thus, the mutant allele encodes a receptor that has a mutation in the putative insulin binding domain. Accordingly, the mutant receptor would be predicted not to transduce the insulin signal effectively. In spite of a homozygous abnormality of the insulin receptor gene and many of the clinical features of severe insulin resistance, the proband's clinical syndrome was noticeably different from previously described patients with leprechaunism who usually die within the first six months of life. There are a total of nine children in the family, five of whom are homozygous for the Ile119-->Met mutation in the insulin receptor gene, and are clinically affected with varying degrees of severity. Four unaffected sibs are clinically normal; two are heterozygous carriers of the mutant allele, one is homozygous for the normal allele, and one unaffected sib was not available for molecular studies.