Mutations In Insulin Receptor Gene Research Articles

O-08 Mutation in insulin receptor gene with hypoglycemia symptoms

Abstract Introduction Monoallelic/heterozygous mutations in the insulin receptor INSR gene are typically associated with type A insulin resistance syndrome, but less is known about INSR-related hyperinsulinemic hypoglycaemia (HH) characterized by fasting hyperinsulinemia and postprandial hypoglycemia. Clinical Case A 38-year-old woman with a history of migraines not needing preventive medication was admitted to an endocrinology outpatient clinic because of general malaise, weakness, and reduced exercise tolerance. For several years, she has had to eat small snacks during the workday to avoid sudden hypoglycemic symptoms. As a result, she gained nearly 30 kilograms in a few years. The symptoms started after she lost 23 kg and weighed 55 kg after following a ketogenic diet in 2016. The symptoms worsened despite weight regain and dietary changes to attempt to avoid hypoglycemia. Diagnostic evaluation: Initial BMI was 31.5 kg/m2. Continuous glucose monitoring (CGM) showed low blood glucose levels during extended fasting, such as during the night or prolonged meal intervals during the workdays (Figure 1). Hypoglycemia also occurred during and after exercise. She underwent a 2-hour glucose tolerance test, a mixed-meal test and a 72-hour fasting test (Tables 1 and 2), abdominal CT, and endoscopic ultrasonography, none of which led to a specific diagnosis. The glucose profile of this patient had hypoglycemias <3.0 mmol/L only a few times, and normal findings on imaging suggest that insulinoma is unlikely. Blood samples were taken during a postprandial hypoglycemic episode at work, but the plasma glucose (3.2 mmol/L) did not meet the criteria for Whipple's triad (<2.7 mmol/L), despite pronounced symptoms that were relieved by consuming carbohydrates. Finally, gene panel for monogenic causes of hypoglycemia identified a likely pathogenic INSR mutation c.2106T>G, p. (Tyr702*), which turned out to have been found in a symptomatic third-degree relative several years ago. Isolated case reports have found a beneficial effect of metformin in decreasing hypoglycemic symptoms, although the exact mechanisms by which it may decrease symptoms are unclear. We conducted a treatment trial with increasing doses of metformin to manage the patient's symptoms and to decrease the insulin resistance that had developed as a result of the INSR mutation and pronounced weight gain. Before the treatment trial, BMI was 34.4 kg / m2. Patient responded positively to metformin and weight started to decrease. Her quality of health increased subjectively and hypoglycemic episodes decreased during work and exercise. Conclusion The differential diagnostics of hypoglycemia may be challenging. Monogenic causes for adulthood-onset hypoglycemia are very rare but should be considered in cases of prolonged hypoglycemia susceptibility impairing daily activities. Genetic testing is also warranted in cases of family history of hypoglycemia symptoms.Figure 1.Continuous glucose monitor (CGM)Patient had low blood glucose levels (2.9-3.6 mmol/L) during extended fasting, such as during the night or prolonged meal intervals during the workdays. Hypoglycemiasymptoms also occurred during and after exercise. Table 1.Laboratory Values Table 2.72-hour fasting test and 5-hour Mixed-Meal test

Diazoxide-unresponsive Hyperinsulinemic Hypoglycaemia in a Preterm Infant with Heterozygous Insulin Receptor Gene Mutation.

Homozygous or compound heterozygous mutations in insulin receptor gene (INSR) lead to marked insulin resistance and hyperglycaemia in Donohue syndrome and Rabson-Mendenhall syndrome, conditions which are associated with significant morbidity early in life. On the other hand, heterozygous INSR gene mutations result in milder phenotype known as type A insulin resistance syndrome. While presentation in adults with this condition is well reported, phenotypes in infant are less well-characterized. We herein report an infant presenting with hyperinsulinemic hypoglycaemia who did not respond to diazoxide therapy. She was subsequently found to carry heterozygous INSR gene mutation. Our patient was a female infant born at 29 weeks of gestation who developed recurrent hypoglycaemia in early infancy. Workup showed hyperinsulinism and she was started on first-line therapy with diazoxide and high-calorie feeds. However, continuous blood glucose monitoring showed post-prandial hyperglycaemia followed by rapid fall to hypogylcaemia. Whole exome sequencing was performed to investigate for diazoxide-unresponsive hyperinsulinism, which revealed a likely pathogenic mutation in the INSR gene c.1246C>T p. (R416X). This nonsense mutation was inherited from the father. With the molecular diagnosis, diazoxide was stopped and she followed a diet with low glycaemic-index food. Subsequent monitoring showed stable glucose profile. Our case highlights the importance to consider type A insulin resistance syndrome when no mutation could be identified in the ABCC8/KCNJ11 genes in diazoxide-unresponsive hyperinsulinism. With autosomal dominant inheritance, cascade screening should be performed in family members to identify those harbouring the mutation as they are at risk of early onset diabetes.

Syndrome of Congenital Insulin Resistance Caused by a Novel INSR Gene Mutation

Mutations in the INSR gene result in rare inherited syndromes causing insulin resistance, such as leprechaunism (Donohue syndrome), Rabson-Mendenhall syndrome and insulin resistance type A. Leprechaunism is an autosomal recessive disorder associated with extreme insulin resistance that leads to hyperinsulinemia, impaired glucose homeostasis, fasting hypoglycemia and postprandial hyperglycemia. Impaired insulin action causes prenatal and postnatal growth retardation. Lipoatrophy, dysmorphic facies, hypertrichosis, macrogenitosomia and hypertrophy of internal organs are also present. A male infant with congenital insulin resistance was born at term after a normal pregnancy with a weight of 1905 g (<3 c), a length of 48 cm (<3 c), and an Apgar score of 10. Intrauterine growth retardation, transient hypoglycemia, pneumonia, urinary tract infection and heart defects [patent foramen ovale (PFO); patent ductus arteriosus (PDA)] were diagnosed after birth. At 5 weeks of age, he was admitted to the regional hospital with severe fever, diarrhea and dehydration. Hyperglycemia was observed (672 mg/dL), and insulin was administered. He was referred to a hospital at 7 weeks of age for suspected neonatal diabetes and hypertrophic cardiomyopathy. The physical examination revealed a loud systolic heart murmur, tachycardia, tachypnea, dysmorphic facies, hypertrichosis, acanthosis nigricans, hypotonia, swollen nipples and enlarged testicles. Glycemic fluctuations (50-250 mg/dL) were observed. The serum insulin concentration was high (maximum 1200 IU/mL) at normoglycemia. Ultrasound of the heart confirmed progressive hypertrophic cardiomyopathy. Leprechaunism was confirmed by genetic analysis of INSR, in which a novel c.320C>G; p. Thr107Arg homozygous missense mutation was found in exon 2.

Type A Insulin Resistance Syndrome- Novel insulin receptor gene mutation and familiar phenotypic variability

Type A Insulin Resistance Syndrome is due to heterozygous mutations in the insulin receptor (INSR) gene or its signaling pathway. We present a premenarcheal 14 year-old girl with normal BMI, severe hirsutism, acanthosis nigricans, clitoral hypertrophy, deep voice, enlarged polycystic ovaries, severe hyperinsulinemia and biochemical hyperandrogenism. We identified a novel heterozygous missense variant in the tyrosine kinase domain of INSR (p.Leu1150Pro) and an heterozygous missense variant in SH2B adapter protein 1 involved in the insulin pathway (p.Ala663Val). Interestingly, the patients’ mother and brother had the same INSR mutation although of a milder phenotype, reason why their IR went undiagnosed.

Hyperinsulinemia and Insulin Receptor Gene Mutation in Nonobese Healthy Subjects in Japan.

Context:Hyperinsulinemia is often observed in obese people, owing to their insulin resistance accompanied by visceral fat accumulation, but the frequency of hyperinsulinemia in nonobese people is not well known. Mutations in the insulin receptor gene are known to cause insulin resistance and hyperinsulinemia in type A insulin resistance syndrome, Rabson-Mendenhall syndrome, and Donohue syndrome. However, insulin receptor gene abnormalities have not been investigated in asymptomatic hyperinsulinemic subjects.Purpose:The aim of the current study was to investigate the prevalence of hyperinsulinemia in nonobese Japanese subjects and to examine the involvement of insulin receptor gene mutations.Methods:We enrolled 11,046 subjects who received health checkups. From these, we extracted nonobese subjects (body mass index <25 kg/m2) who exhibited hyperinsulinemia (serum fasting immunoreactive insulin ≥15 µU/mL). Genetic analysis was performed for the insulin receptor gene in 11 nonobese subjects with hyperinsulinemia.Results:The prevalence of hyperinsulinemia without apparent diabetes in nonobese subjects was 0.4% (33/8630). In the 11 analyzed subjects, two novel heterozygous nonsense mutations were detected [c.2106 T>G (p.Y702X) and c.2779-2780 GC>A]. The prevalence of insulin receptor gene mutations was 18.2% (2/11).Conclusions:To our knowledge, this is the first report of the prevalence of hyperinsulinemia in nonobese healthy subjects. We identified two novel mutations in the insulin receptor gene. These findings indicate that mutations in the insulin receptor gene may be related to fasting hyperinsulinemia, and insulin receptor gene screening may be useful for determining the cause of unexplained hyperinsulinemia.

EVALUATION OF EXON 17 OF INSULIN RECEPTOR (INSR) GENE AND ITS RELATIONSHIP WITH DIABETES TYPE 2 IN AN IRANIAN POPULATION

Mutations in insulin receptor gene cause the inherited insulin resistant syndrome, especially diabetic diseases. Here were optimized the conditions for sequencing of a partial fragment of INSR gene, exon 17. We sequenced fourteen fragments from a diabetic sample and a control group chosen from an Iranian population. In this study we analyzed eleven sequences of diabetic’s patients by DNAMAN program (DEMO 8.0), deposited in Genbank with accession numbers LC055416, LC055417, LC055419, LC055421, LC055423, LC055424, LC055425, LC055426, LC055496, LC055497, and LC055498. From our control group, three sequences were deposited in Genbank with accession numbers of LC055418, LC055420, and LC055422. Results showed that there were variation between the sequences of exon 17 of INSR gene in diabetics and control group. Variations were observed in fragments at the beginning and the end of exon 17 among diabetics population. Moreover we found subjected SNPs between diabetic’s patients that haven’t been reported by other researchers. In this study we concluded that mutations in exon 17 of INSR gene contributed in diabetic diseases. Keyword: Insulin receptor gene, Exon 17, Diabetes, Sequencing, Control group

A Syndrome of Insulin Resistance Resembling Donohue Syndrome with Patent Ductus Arteriosus

Donohue syndrome, a rare autosomal recessive disorder, is associated with the mutation of the insulin receptor gene in the short arm of the 19th chromosome. It is very rare that a syndrome of insulin resistance resembles Donohue syndrome with patent ductus arteriosus. A 14-year-old girl, whose parents were consanguineous, was often admitted for abdominal pain over the past 8 years. She presented not only polydipsia, polyphagia, and weight loss but also small elfin face, distended abdomen, enlarged clitoris, hypertrichosis, acanthosis nigricans of the neck, decreased subcutaneous fat and 3/6 continuous murmur radiating to the right shoulder with thrill on 2nd to 3rd left sternal border. In addition, she had hyperglycemia, hyperinsulinism, and patent ductus arteriosus by laboratory examination and echocardiography, respectively. This child shows insulin resistance resembling Donohue syndrome with patent ductus arteriosus. Most of patients with Donohue syndrome die prematurely. The child surviving for a long time may have a milder form of Donohue syndrome and may be due to a less severe form of the defective gene. Chromosomal abnormalities may be also associated with this disease.

Six cases with severe insulin resistance (SIR) associated with mutations of insulin receptor: Is a Bartter-like syndrome a feature of congenital SIR?

Biallelic insulin receptor (INSR) gene mutations cause congenital syndromes of severe insulin resistance (SIR) known as Donohue syndrome (DS) and Rabson-Mendenhall syndrome (RMS). At presentation, DS and RMS are difficult to differentiate since they share many clinical features; however, while patients with DS usually die within 1 year of birth, individuals classified as RMS can reach adult age. INSR mutations can be also found in pubertal females with hyperinsulinism, hyperandrogenism, and acanthosis nigricans (type A SIR). We studied the INSR gene in five subjects with congenital SIR and in a patient with type A SIR. Nine biallelic INSR gene mutations (eight novels, including an in-frame deletion of INSR signal peptide) were identified in patients with congenital SIR; a heterozygous, spontaneous INSR mutation was detected in the patient with type A SIR. Two probands, presenting severe hirsutism at birth, died at the age of 3 months and were classified as DS, while other 2, currently 2 and 3 years old, were diagnosed with RMS (patients 3 and 4). The fifth patient with congenital SIR died when 14 months old. Nephrocalcinosis, hyperaldosteronism, hyperreninemia, and hypokalemia, in the absence of hypertension, were discovered in patients 3 and 5 when 24 and 4 months old, respectively. Patient 3, now 3 years/3 months old, still shows hyperreninemic hyperaldosteronism requiring potassium supplementation. We conclude that renal abnormalities resembling antenatal Bartter's syndrome type II, recently reported also by others, is a common observation in patients with congenital SIR.

Hyperinsulinaemic Hypoglycaemia: Genetic Mechanisms, Diagnosis and Management

Hyperinsulinaemic hypoglycaemia (HH) is characterized by unregulated insulin secretion from pancreatic β-cells. Untreated hypoglycaemia in infants can lead to seizures, developmental delay, and subsequent permanent brain injury. Early identification and meticulous managementof these patients is vital to prevent neurological insult. Mutations in eight different genes (ABCC8, KCNJ11, GLUD1, CGK, HADH, SLC16A1, HNF4A and UCP2) have been identified to date in patients with congenital forms of hyperinsulinism (CHI). The most severe forms of CHI are due to mutations in ABCC8 and KCJN11, which encode the two components of pancreatic β-cell ATP-sensitive potassium channel. Recent advancement in understanding the genetic aetiology, histological characterisation into focal and diffuse variety combined with improved imaging (such as fluorine 18 L-3, 4-dihydroxyphenylalanine positron emission tomography 18F-DOPA-PET scanning) and laparoscopic surgical techniques have greatly improved management. In adults, HH can be due to an insulinoma, pancreatogenous hypoglycaemic syndrome, post gastric-bypass surgery for morbid obesity as well as to mutations in insulin receptor gene. This review provides an overview of the molecular basis of CHI and outlines the clinical presentation, diagnostic criteria, and management of these patients.Conflict of interest:None declared.

Hyperinsulinaemic hypoglycaemia: genetic mechanisms, diagnosis and management

Hyperinsulinaemic hypoglycaemia (HH) is due to the unregulated secretion of insulin from pancreatic β-cells. A rapid diagnosis and appropriate management of these patients is essential to prevent the potentially associated complications like epilepsy, cerebral palsy and neurological impairment. The molecular basis of HH involves defects in key genes (ABCC8, KCNJ11, GLUD1, GCK, HADH, SLC16A1, HNF4A and UCP2) which regulate insulin secretion. The most severe forms of HH are due to loss of function mutations in ABCC8/KCNJ11 which encode the SUR1 and KIR6.2 components respectively of the pancreatic β-cell K(ATP) channel. At a histological level there are two major forms (diffuse and focal) each with a different genetic aetiology. The diffuse form is inherited in an autosomal recessive (or dominant) manner whereas the focal form is sporadic in inheritance and is localised to a small region of the pancreas. The focal form can now be accurately localised pre-operatively using a specialised positron emission tomography scan with the isotope Fluroine-18L-3, 4-dihydroxyphenyalanine (18F-DOPA-PET). Focal lesionectomy can provide cure from the hypoglycaemia. However the diffuse form is managed medically or by near total pancreatectomy (with high risk of diabetes mellitus). Recent advances in molecular genetics, imaging with 18F-DOPA-PET/CT and novel surgical techniques have changed the clinical approach to patients with HH.

Two Novel Insulin Receptor Gene Mutations in a Patient with Rabson-Mendenhall Syndrome: The First Korean Case Confirmed by Biochemical, and Molecular Evidence

Rabson-Mendenhall syndrome (RMS) is a rare syndrome manifested by extreme insulin resistance with hyperinsulinemia, acanthosis nigricans, tooth dysplasia and growth retardation. Our patient was first noted at the age of 8 months due to pigmentations on skin-folded areas. Initial laboratory tests showed normal fasting glucose (69 mg/dL). Fasting insulin level was severely elevated, up to 554.6 µIU/mL, and c-peptide level was increased, up to 13.81 ng/mL. However, hemoglobin A1c was within normal range (4.8%). He is now 11 yr old. His growth development followed the 5-10th percentile and oral hypoglycemic agents are being administered. The last laboratory results showed insulin 364.1 µIU/mL, C-peptide 4.30 ng/mL, and hemoglobin A1c 7.6%. The boy was a compound heterozygote for the c.90C > A and c.712G > A mutations of the insulin receptor gene, INSR, which are nonsense and missense mutations. In summary, we report the first Korean case of RMS, which was confirmed by two novel mutations of the INSR.

Functional characterization of insulin receptor gene mutations contributing to Rabson-Mendenhall syndrome - phenotypic heterogeneity of insulin receptor gene mutations

Rabson-Mendenhall syndrome (RMS) is a rare disorder that presents as severe insulin resistance as a result of mutations present in the insulin receptor (INSR). A Chinese girl with RMS presented with profound diabetes, hyperinsulinemia, acanthosis nigricans, hirsutism, and abnormalities of teeth and nails. Direct sequencing of the patient's INSR detected heterozygote mutations at Arg83Gln (R83Q) and Ala1028Val (A1028V), with the former representing a novel mutation. Functional studies of Chinese hamster ovary (CHO) cells transfected with wild-type (WT) and mutant forms of INSR were performed to evaluate the effects of these mutations on receptor expression and activation. Receptor expression, insulin binding activity, and phosphorylation of the R83Q variant were comparable to WT. In contrast, expression of the A1028V receptor was much lower than that of WT INSR, and impairment of insulin binding and autophosphorylation were nearly commensurate with the decrease in expression detected. Reductions in the phosphorylation of IRS-1, Akt, and Erk1/2 (60%, 40%, and 50% of WT, respectively) indicate that the A1028V receptor contributes to impaired signal transduction. In conclusion, INSR mutations associated with RMS were identified. Moreover, the A1028V mutation associated with a decrease in expression of INSR potentially accounts for loss of function of the INSR.

Clinical and genetic analysis of the insulin receptor gene in a Chinese patient with extreme insulin resistance

Several types of mutations in insulin receptor gene have been identified in patients with type A insulin resistance. A 21-year old girl was diagnosed with diabetic retinopathy and cataract after 6 years of uncontrolled diabetes. Three nucleotide substitution mutations were detected, which may be associated with the patient's extreme insulin resistance.

Insulin Receptor and its Relationship with Different Forms of Insulin Resistance

SummaryInsulin plays an important role in maintaining the whole organism’s homeostasis. The presence of insulin receptors in all vertebrates and invertebrates cells reflects the diversity of regulatory processes in which this hormone is involved. Furthermore, many different factors may influence the level of insulin receptor expression. These factors include e.g. the sole insulin or stage of development. Mutations in the receptor may lead to the development of insulin resistance. These mutations differ in the level of severity and are frequently associated with diabetes mellitus, hypertension, cardiovascular disorders, heart failure, metabolic syndrome and infertility in women. More than 50 mutations in insulin receptor gene have already been characterized. These mutations are associated with rare forms of insulin resistance like leprechaunism, insulin resistance type A or Rabson-Mendenhall syndrome. Molecular analysis of insulin receptor gene may lead to a better understanding of molecular mechanisms underlying various types of insulin resistance and help to develop more efficient treatment.

Phenotypical variety of insulin resistance in a family with a novel mutation of the insulin receptor gene

A novel mutation of insulin receptor gene (INSR gene) was identified in a three generation family with phenotypical variety. Proband was a 12-year-old Japanese girl with type A insulin resistance. She showed diabetes mellitus with severe acanthosis nigricans and hyperinsulinemia without obesity. Using direct sequencing, a heterozygous nonsense mutation causing premature termination at amino acid 331 in the alpha subunit of INSR gene (R331X) was identified. Her father, 40 years old, was not obese but showed impaired glucose tolerance. Her paternal grandmother, 66 years old, has been suffered from diabetes mellitus for 15 years. Interestingly, they had the same mutation. One case of leprechaunism bearing homozygous mutation at codon 331 was identified. These findings led to the hypothesis that R331X may contribute to the variation of DM in the general population in Japan. An extensive search was done in 272 participants in a group medical examination that included 92 healthy cases of normoglycemia and 180 cases already diagnosed type 2 DM or detected hyperglycemia. The search, however, failed to detect any R331X mutation in this local population. In addition, the proband showed low level C-peptide/insulin molar ratio, indicating that this ratio is considered to be a useful index for identifying patients with genetic insulin resistance. In conclusion, a nonsense mutation causing premature termination after amino acid 331 in the alpha subunit of the insulin receptor was identified in Japanese diabetes patients. Further investigations are called for to address the molecular mechanism.

Molecular biology of insulin resistance.

Insulin resistance is an essential feature of a great variety of clinical disorders, like diabetes mellitus, obesity, essential hypertension, and is primarily due to a defect in hormone action at the cellular level. In the past decade application of novel research techniques including recombinant DNA technology have paved the way to understand the mechanisms of insulin action and its alterations at the molecular level. The first step in insulin action is the activation of the insulin receptor. The insulin receptor is a tetrameric protein consisting of two extracellular alpha- and two transmembrane beta-subunits. Binding of insulin to the alpha-subunit causes autophosphorylation of the intracellular beta-subunit region on tyrosine residues thereby activating the receptor. How the hormonal signal is subsequently transduced within the cell is still quiet unclear. The activated insulin receptor appears to couple to cytosolic receptor substrates which can affect different signaling cascades eliciting the pleiotropic hormone response on cell metabolism and growth. Most proteins involved in the signal transduction pathway of insulin are not known yet, but each of them might play a role in the various forms of insulin resistance. Taking the insulin receptor as an exemplary protein involved in insulin action we review molecular mechanisms regulating insulin receptor activity, gene expression, and the role of natural occurring insulin receptor gene mutations in patients with insulin resistant diabetes mellitus. It is outlined how the combination of both clinical medicine and molecular biology not only helps to understand insulin action and the pathogenesis of insulin resistance, but also leads to new avenues in the differential diagnosis, therapy, and possibly prevention of this heterogenous but most frequent metabolic and endocrine disorder.

Protein Tyrosine Phosphatase Regulation in Fibroblasts from Patients with an Insulin Receptor Gene Mutation

Tyrosine phosphorylation of the insulin receptor is the initial event following receptor binding to insulin, and it induces further tyrosine phosphorylation of various intracellular molecules. This signaling is countered by protein tyrosine phosphatases (PTPases), which reportedly are associated with insulin resistance that can be reduced by regulation of PTPases. Protein tyrosine phosphatase 1B (PTP1B) and leukocyte antigen-related PTPase (LAR) are the PTPases implicated most frequently in insulin resistance and diabetes mellitus. Here, we show that PTP1B and LAR are expressed in human fibroblasts, and we examine the regulation of PTPase activity in fibroblasts from patients with an insulin receptor gene mutation as an in vitro model of insulin resistance. Total PTPase activity was significantly lower in the cytosolic and membrane fractions of fibroblasts with mutations compared with controls (p<0.05). Insulin stimulation of fibroblasts with mutations resulted in a significantly smaller increase in PTP1B activity compared with stimulation of wild-type fibroblasts (p<0.05). This indicates that insulin receptor gene mutations blunt increases in PTPase activity in response to insulin, possibly via a negative feedback mechanism. Our data suggest that the PTPase activity in patients with insulin receptor gene mutation and severe insulin resistance may differ from that in ordinary type 2 diabetes.

Type A Insulin Resistance Syndrome Revealing a Novel Lamin A Mutation

Particular forms of polycystic ovary syndrome with severe hyperandrogenism, acanthosis nigricans, and marked insulin resistance, defining the type A insulin resistance syndrome, are due to insulin receptor gene mutations. However, the majority of affected individuals do not have such mutation, arguing for the genetic heterogeneity of this syndrome. The familial partial lipodystrophy of the Dunnigan type, one of the diseases due to mutations in the lamin A/C (LMNA) gene, is characterized by a lipodystrophic phenotype and shares some clinical and metabolic features with the type A syndrome. We describe here the case of a nonobese 24-year-old woman affected with type A syndrome without clinical lipodystrophy. We linked this phenotype to a novel heterozygous missense mutation in the LMNA, predicting a G602S amino acid substitution in lamin A. This mutation cosegregated with impaired glucose tolerance, insulin resistance, and acanthosis nigricans in the absence of clinical lipodystrophy in the family. The skin fibroblasts from the proband exhibited nuclear alterations similar to those described in other laminopathies, and showed several defects in the insulin transduction pathway. This study further extends the vast range of diseases linked to LMNA mutations and identifies another genetic cause for the type A insulin resistance syndrome.

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.

Genetic modifiers of the insulin resistance phenotype in mice.

Insulin resistance can result from genetic interactions among susceptibility alleles. To identify genetic loci predisposing to insulin resistance, we used crosses between different strains of mice with a targeted null allele of the insulin receptor gene. On the genetic background of B6 mice, the insulin receptor gene mutation causes mild hyperinsulinemia. In contrast, on the genetic background of 129/Sv mice, the same mutation causes severe hyperinsulinemia, suggesting that the 129/Sv strain harbors alleles that interact with the insulin receptor mutation and predispose to insulin resistance. As a first step to identify these alleles, we generated an F2 intercross between insulin receptor heterozygous mutant mice on B6 and 129/Sv backgrounds (B6IR x 129IR) and performed a genome-wide scan with polymorphic markers at a 20-cM resolution. We report the identification of loci on chromosomes 2 (logarithm of odds [LOD] 5.58) and 10 (LOD 5.58) that show significant evidence for linkage to plasma insulin levels as a quantitative trait. These findings indicate that targeted mutations in knockout mice can be used to unravel the complex genetic interactions underlying insulin resistance.