Glucocorticoid receptor gene polymorphisms are associated with reduced first-phase glucose-stimulated insulin secretion and disposition index in women, but not in men

AIM

Glucocorticoids are efficacious anti-inflammatory agents, but, in susceptible individuals, these drugs may induce glucose intolerance and diabetes by affecting β-cell function and insulin sensitivity. We assessed whether polymorphisms in the glucocorticoid receptor gene NR3C1 associate with measures of β-cell function and insulin sensitivity derived from hyperglycaemic clamps in subjects with normal or impaired glucose tolerance.

METHODS

A cross-sectional cohort study was conducted in four academic medical centres in the Netherlands and Germany. Four hundred and forty-nine volunteers (188 men; 261 women) were recruited with normal glucose tolerance (n=261) and impaired glucose tolerance (n=188). From 2-h hyperglycaemic clamps, first- and second-phase glucose-stimulated insulin secretion, as well as insulin sensitivity index and disposition index, were calculated. All participants were genotyped for the functional NR3C1 polymorphisms N363S (rs6195), BclI (rs41423247), ER22/23EK (rs6189/6190), 9β A/G (rs6198) and ThtIIII (rs10052957). Associations between these polymorphisms and β-cell function parameters were assessed.

RESULTS

In women, but not in men, the N363S polymorphism was associated with reduced disposition index (P=1.06 10(-4) ). Also only in women, the ER22/23EK polymorphism was associated with reduced first-phase glucose-stimulated insulin secretion (P=0.011) and disposition index (P=0.003). The other single-nucleotide polymorphisms were not associated with β-cell function. Finally, none of the polymorphisms was related to insulin sensitivity.

CONCLUSION

The N363S and ER22/23EK polymorphisms of the NR3C1 gene are negatively associated with parameters of β-cell function in women, but not in men.

The heritability of beta cell function parameters in a mixed meal test design

AIMS/HYPOTHESIS

We estimated the heritability of individual differences in beta cell function after a mixed meal test designed to assess a wide range of classical and model-derived beta cell function parameters.

METHODS

A total of 183 healthy participants (77 men), recruited from the Netherlands Twin Register, took part in a 4 h protocol, which included a mixed meal test. Participants were Dutch twin pairs and their siblings, aged 20 to 49 years. All members within a family were of the same sex. Insulin sensitivity, insulinogenic index, insulin response and postprandial glycaemia were assessed, as well as model-derived parameters of beta cell function, in particular beta cell glucose sensitivity and insulin secretion rates. Genetic modelling provided the heritability of all traits. Multivariate genetic analyses were performed to test for overlap in the genetic factors influencing beta cell function, waist circumference and insulin sensitivity.

RESULTS

Significant heritabilities were found for insulinogenic index (63%), beta cell glucose sensitivity (50%), insulin secretion during the first 2 h postprandial (42-47%) and postprandial glycaemia (43-52%). Genetic factors influencing beta cell glucose sensitivity and insulin secretion during the first 30 postprandial min showed only negligible overlap with the genetic factors that influence waist circumference and insulin sensitivity.

CONCLUSIONS/INTERPRETATION

The highest heritability for postprandial beta cell function was found for the insulinogenic index, but the most specific indices of heritability of beta cell function appeared to be beta cell glucose sensitivity and the insulin secretion rate during the first 30 min after a mixed meal.

Genetic association analysis of LARS2 with type 2 diabetes

AIMS/HYPOTHESIS

LARS2 has been previously identified as a potential type 2 diabetes susceptibility gene through the low-frequency H324Q (rs71645922) variant (minor allele frequency [MAF] 3.0%). However, this association did not achieve genome-wide levels of significance. The aim of this study was to establish the true contribution of this variant and common variants in LARS2 (MAF > 5%) to type 2 diabetes risk.

METHODS

We combined genome-wide association data (n = 10,128) from the DIAGRAM consortium with independent data derived from a tagging single nucleotide polymorphism (SNP) approach in Dutch individuals (n = 999) and took forward two SNPs of interest to replication in up to 11,163 Dutch participants (rs17637703 and rs952621). In addition, because inspection of genome-wide association study data identified a cluster of low-frequency variants with evidence of type 2 diabetes association, we attempted replication of rs9825041 (a proxy for this group) and the previously identified H324Q variant in up to 35,715 participants of European descent.

RESULTS

No association between the common SNPs in LARS2 and type 2 diabetes was found. Our replication studies for the two low-frequency variants, rs9825041 and H324Q, failed to confirm an association with type 2 diabetes in Dutch, Scandinavian and UK samples (OR 1.03 [95% CI 0.95-1.12], p = 0.45, n = 31,962 and OR 0.99 [0.90-1.08], p = 0.78, n = 35,715 respectively).

CONCLUSIONS/INTERPRETATION

In this study, the largest study examining the role of sequence variants in LARS2 in type 2 diabetes susceptibility, we found no evidence to support previous data indicating a role in type 2 diabetes susceptibility.

Genetic influences on the insulin response of the beta cell to different secretagogues

AIMS/HYPOTHESIS

The aim of the present study was to estimate the heritability of the beta cell insulin response to glucose and to glucose combined with glucagon-like peptide-1 (GLP-1) or with GLP-1 plus arginine.

METHODS

This was a twin-family study that included 54 families from the Netherlands Twin Register. The participants were healthy twin pairs and their siblings of the same sex, aged 20 to 50 years. Insulin response of the beta cell was assessed by a modified hyperglycaemic clamp with additional GLP-1 and arginine. Insulin sensitivity index (ISI) was assessed by the euglycaemic-hyperinsulinaemic clamp. Multivariate structural equation modelling was used to obtain heritabilities and the genetic factors underlying individual differences in BMI, ISI and secretory responses of the beta cell.

RESULTS

The heritability of insulin levels in response to glucose was 52% and 77% for the first and second phase, respectively, 53% in response to glucose + GLP-1 and 80% in response to an additional arginine bolus. Insulin responses to the administration of glucose, glucose + GLP-1 and glucose + GLP-1 + arginine were highly correlated (0.62< r <0.79). Heritability of BMI and ISI was 74% and 60% respectively. The genetic factors that influenced BMI and ISI explained about half of the heritability of insulin levels in response to the three secretagogues. The other half was due to genetic factors specific to the beta cell.

CONCLUSIONS/INTERPRETATION

In healthy adults, genetic factors explain most of the individual differences in the secretory capacity of the beta cell. These genetic influences are partly independent from the genes that influence BMI and ISI.

Combined effects of single-nucleotide polymorphisms in GCK, GCKR, G6PC2 and MTNR1B on fasting plasma glucose and type 2 diabetes risk

AIMS/HYPOTHESIS

Variation in fasting plasma glucose (FPG) within the normal range is a known risk factor for the development of type 2 diabetes. Several reports have shown that genetic variation in the genes for glucokinase (GCK), glucokinase regulatory protein (GCKR), islet-specific glucose 6 phosphatase catalytic subunit-related protein (G6PC2) and melatonin receptor type 1B (MTNR1B) is associated with FPG. In this study we examined whether these loci also contribute to type 2 diabetes susceptibility.

METHODS

A random selection from the Dutch New Hoorn Study was used for replication of the association with FGP (2,361 non-diabetic participants). For the genetic association study we extended the study sample with 2,628 participants with type 2 diabetes. Risk allele counting was used to calculate a four-gene risk allele score for each individual.

RESULTS

Variants of the GCK, G6PC2 and MTNR1B genes but not GCKR were associated with FPG (all, p <or= 0.001; GCKR, p = 0.23). Combining these four genes in a risk allele score resulted in an increase of 0.05 mmol/l (0.04-0.07) per additional risk allele (p = 2 x 10(-13)). Furthermore, participants with less than three or more than five risk alleles showed significantly different type 2 diabetes susceptibility compared with the most common group with four risk alleles (OR 0.77 [0.65-0.93], p = 0.005 and OR 2.05 [1.50-2.80], p = 4 x 10(-6) respectively). The age at diagnosis was also significantly associated with the number of risk alleles (p = 0.009).

CONCLUSIONS

A combined risk allele score for single-nucleotide polymorphisms in four known FPG loci is significantly associated with FPG and HbA(1c) in a Dutch population-based sample of non-diabetic participants. Carriers of low or high numbers of risk alleles show significantly different risks for type 2 diabetes compared with the reference group.

The molecular basis and clinical characteristics of Maternally Inherited Diabetes and Deafness (MIDD), a recently recognized diabetic subtype

Diabetes mellitus comprises a number of diseases with hyperglycemia as hallmark. Currently, multiple genetic factors are being recognized which contribute to the development of diabetes or which may modulate its clinical expression. This review presents an overview of our current knowledge on a diabetic subtype which associates with a single mutation in mitochondrial DNA. Based on the triad of Maternal Inheritance, Diabetes and Deafness we propose the name Maternally Inherited Diabetes and Deafness (MIDD) for this syndrome. In Northwestern Europe MIDD affects approximately 1.3% of all diabetic individuals.

Variants of CDKAL1 and IGF2BP2 affect first-phase insulin secretion during hyperglycaemic clamps

AIMS/HYPOTHESIS

Genome-wide association studies have recently identified novel type 2 diabetes susceptibility gene regions. We assessed the effects of six of these regions on insulin secretion as determined by a hyperglycaemic clamp.

METHODS

Variants of the HHEX/IDE, CDKAL1, SLC30A8, IGF2BP2 and CDKN2A/CDKN2B genes were genotyped in a cohort of 146 participants with NGT and 126 with IGT from the Netherlands and Germany, who all underwent a hyperglycaemic clamp at 10 mmol/l glucose.

RESULTS

Variants of CDKAL1 and IGF2BP2 were associated with reductions in first-phase insulin secretion (34% and 28%, respectively). The disposition index was also significantly reduced. For gene regions near HHEX/IDE, SLC30A8 and CDKN2A/CDKN2B we did not find significant associations with first-phase insulin secretion (7-18% difference between genotypes; all p > 0.3). None of the variants showed a significant effect on second-phase insulin secretion in our cohorts (2-8% difference between genotypes, all p > 0.3). Furthermore, the gene variants were not associated with the insulin sensitivity index.

CONCLUSIONS

Variants of CDKAL1 and IGF2BP2 attenuate the first phase of glucose-stimulated insulin secretion but show no effect on the second phase of insulin secretion. Our results, based on hyperglycaemic clamps, provide further insight into the pathogenic mechanism behind the association of these gene variants with type 2 diabetes.

Loss of 50% of excess weight using a very low energy diet improves insulin-stimulated glucose disposal and skeletal muscle insulin signalling in obese insulin-treated type 2 diabetic patients

AIMS/HYPOTHESIS

Both energy restriction (ER) per se and weight loss improve glucose metabolism in obese insulin-treated type 2 diabetic patients. Short-term ER decreases basal endogenous glucose production (EGP) but not glucose disposal. In contrast the blood glucose-lowering mechanism of long-term ER with substantial weight loss has not been fully elucidated. The aim of this study was to investigate the effect of loss of 50% of excess weight [50% excess weight reduction (EWR)] on EGP, whole-body insulin sensitivity and the disturbed myocellular insulin-signalling pathway in ten obese insulin-treated type 2 diabetic patients.

METHODS

A euglycaemic-hyperinsulinaemic clamp with stable isotopes ([6,6-(2)H2]glucose and [2H5]glycerol) combined with skeletal muscle biopsies was performed during a very low energy diet (VLED; 1,883 kJ/day) on day 2 and again after 50% EWR. Oral blood glucose-lowering agents and insulin were discontinued 3 weeks prior to the VLED and at the start of the VLED, respectively.

RESULTS

Loss of 50% EWR (20.3+/-2.2 kg from day 2 to day of 50% EWR) normalised basal EGP and improved insulin sensitivity, especially insulin-stimulated glucose disposal (18.8+/-2.0 to 39.1+/-2.8 micromol kg fat-free mass(-1) min(-1), p=0.001). The latter was accompanied by improved insulin signalling at the level of the recently discovered protein kinase B/Akt substrates AS160 and PRAS40 along with a decrease in intramyocellular lipid (IMCL) content.

CONCLUSIONS/INTERPRETATION

Considerable weight loss in obese, insulin-treated type 2 diabetic patients normalises basal EGP and improves insulin sensitivity resulting from an improvement in insulin signal transduction in skeletal muscle. The decrease in IMCL might contribute to this effect.

Fatty acid-induced mitochondrial uncoupling in adipocytes as a key protective factor against insulin resistance and beta cell dysfunction: a new concept in the pathogenesis of obesity-associated type 2 diabetes mellitus

Type 2 diabetes is associated with excessive food intake and a sedentary lifestyle. Local inflammation of white adipose tissue induces cytokine-mediated insulin resistance of adipocytes. This results in enhanced lipolysis within these cells. The fatty acids that are released into the cytosol can be removed by mitochondrial beta-oxidation. The flux through this pathway is normally limited by the rate of ADP supply, which in turn is determined by the metabolic activity of the adipocyte. It is expected that the latter does not adapt to an increased rate of lipolysis. We propose that elevated fatty acid concentrations in the cytosol of adipocytes induce mitochondrial uncoupling and thereby allow mitochondria to remove much larger amounts of fatty acids. By this, release of fatty acids out of adipocytes into the circulation is prevented. When the rate of fatty acid release into the cytosol exceeds the beta-oxidation capacity, cytosolic fatty acid concentrations increase and induce mitochondrial toxicity. This results in a decrease in beta-oxidation capacity and the entry of fatty acids into the circulation. Unless these released fatty acids are removed by mitochondrial oxidation in active muscles, these fatty acids result in ectopic triacylglycerol deposits, induction of insulin resistance, beta cell damage and diabetes. Thiazolidinediones improve mitochondrial function within adipocytes and may in this way alleviate the burden imposed by the excessive fat accumulation associated with the metabolic syndrome. Thus, the number and activity of mitochondria within adipocytes contribute to the threshold at which fatty acids are released into the circulation, leading to insulin resistance and type 2 diabetes.

Dynamics of insulin signalling in liver during hyperinsulinemic euglycaemic clamp conditions in vivo and the effects of high-fat feeding in male mice

Insulin is an important regulator of hepatic carbohydrate, lipid, and protein metabolism, and the regulation of these processes by insulin is disturbed under conditions of insulin resistance and type 2 diabetes. Despite these alterations, the impact of insulin resistance on insulin signalling in the liver is not well defined. Variations in time and dose of insulin stimulation as well as plasma glucose levels may underlie this. The present study aimed at determining the dynamics of activation of hepatic insulin signalling in vivo at insulin concentrations resembling those achieved after a meal, and addressing the effects of high-fat feeding. An unexpected finding of this study was the biphasic activation pattern of the IRS-PI3K-PKB/Akt pathway. Our findings indicate that the first burst of activation contributes to regulation of glucose metabolism. The physiological function of the second peak is still unknown, but may involve regulation of protein synthesis. Finally, high-fat feeding caused hepatic insulin resistance, as illustrated by a reduced suppression of hepatic glucose production. A sustained increased phosphorylation of the serine/threonine kinases p70S6kinase and Jun N-terminal kinase in the absence of insulin may underlie the abrogated phosphorylation of the IRS proteins and their downstream targets.

Sustained activation of the mammalian target of rapamycin nutrient sensing pathway is associated with hepatic insulin resistance, but not with steatosis, in mice

AIMS/HYPOTHESIS

Activation of nutrient sensing through mammalian target of rapamycin (mTOR) has been linked to the pathogenesis of insulin resistance. We examined activation of mTOR-signalling in relation to insulin resistance and hepatic steatosis in mice.

MATERIALS AND METHODS

Chronic hepatic steatosis and hepatic insulin resistance were induced by high-fat feeding of male C57BL/6Jico mice for 6 weeks. In addition, acute hepatic steatosis in the absence of insulin resistance was induced by pharmacological blockade of beta-oxidation using tetradecylglycidic acid (TDGA). mTOR signalling was examined in liver homogenates.

RESULTS

High-fat feeding caused obesity (p<0.001), hepatic steatosis (p<0.05) and hepatic insulin resistance (p<0.05). The phosphorylation of mTOR and its downstream targets p70S6 kinase and S6 ribosomal protein was two-fold higher in mice on a high-fat diet than in mice fed standard chow (all p<0.05) and associated with enhanced rates of protein synthesis. Acute induction of hepatic steatosis with TDGA had no effect on mTOR activity. The increased activity of the mTOR pathway in livers from mice on a high-fat diet could not be ascribed to diet-induced alterations in known modulators of mTOR activity such as circulating plasma leucine levels, phosphorylation of protein kinase B and AMP-activated protein kinase, and changes in mitochondrial function.

CONCLUSIONS/INTERPRETATION

High-fat diet induces increase of the mTOR nutrient sensing pathway in association with hepatic insulin resistance, but not with hepatic lipid accumulation as such.

Mitochondrial diabetes and its lessons for common Type 2 diabetes

Multiple pathogenic pathways are able to deregulate glucose homoeostasis leading to diabetes. The 3243A>G mutation in the mtDNA (mitochondrial DNA)-encoded tRNALeu,UUR gene was found by us to be associated with a particular diabetic subtype, designated MIDD (maternally inherited diabetes and deafness). This mutation causes an imbalance in the mitochondrion between proteins encoded by the nuclear and mitochondrial genomes, resulting in a gradual deterioration of glucose homoeostasis during life. Remarkably, carriers of the 3243A>G mutation are generally not obese. The mutation also results in enhanced radical production by mitochondria. We propose that this mutation leads to the development of diabetes due to an inappropriate storage of triacylglycerols within adipocytes. The result is a fatty acid-induced deterioration of pancreatic β-cell function. In combination with an enhanced radical production in the β-cell due to the mutation, this leads to an age-dependent, accelerated decline in insulin production. In common Type 2 (non-insulin-dependent) diabetes, which is generally associated with obesity, a decline in mitochondrial function in adipose cells seems to result in an inappropriate scavenging of fatty acids by β-oxidation. As a consequence, a systemic overload with fatty acids occurs, leading to an enhanced decline in β-cell function due to lipotoxicity.

Diabetes-associated mitochondrial DNA mutation A3243G impairs cellular metabolic pathways necessary for beta cell function

AIMS/HYPOTHESIS

Mitochondrial DNA (mtDNA) mutations cause several diseases, including mitochondrial inherited diabetes and deafness (MIDD), typically associated with the mtDNA A3243G point mutation on tRNALeu gene. The common hypothesis to explain the link between the genotype and the phenotype is that the mutation might impair mitochondrial metabolism expressly required for beta cell functions. However, this assumption has not yet been tested.

METHODS

We used clonal osteosarcoma cytosolic hybrid cells (namely cybrids) harbouring mitochondria derived from MIDD patients and containing either exclusively wild-type or mutated (A3243G) mtDNA. According to the importance of mitochondrial metabolism in beta cells, we studied the impact of the mutation on key parameters by comparing stimulation of these cybrids by the main insulin secretagogue glucose and the mitochondrial substrate pyruvate.

RESULTS

Compared with control mtDNA from the same patient, the A3243G mutation markedly modified metabolic pathways leading to a high glycolytic rate (2.8-fold increase), increased lactate production (2.5-fold), and reduced glucose oxidation (-83%). We also observed impaired NADH responses (-56%), negligible mitochondrial membrane potential, and reduced, only transient ATP generation. Moreover, cybrid cells carrying patient-derived mutant mtDNA exhibited deranged cell calcium handling with increased cytosolic loads (1.4-fold higher), and elevated reactive oxygen species (2.6-fold increase) under glucose deprivation.

CONCLUSIONS/INTERPRETATION

The present study demonstrates that the mtDNA A3243G mutation impairs crucial metabolic events required for proper cell functions, such as coupling of glucose recognition to insulin secretion.

Cardiac dysfunction induced by high-fat diet is associated with altered myocardial insulin signalling in rats

AIMS/HYPOTHESIS

Diabetic cardiomyopathy (DCM) is common in type 2 diabetes. In DCM, insulin resistance may alter cardiac substrate supply and utilisation leading to changes in myocardial metabolism and cardiac function. In rats, exposure to excessive alimentary fat, inducing a type 2 diabetic phenotype, may result in myocardial insulin resistance and cardiac functional changes resembling DCM.

MATERIALS AND METHODS

Rats received high-fat (HFD) or low-fat (LFD) diets for 7 weeks. Prior to killing, insulin or saline was injected i.p. Contractile function and insulin signalling were assessed in papillary muscles and ventricular lysates, respectively.

RESULTS

Fasting and post-load blood glucose levels were increased in HFD- vs LFD-rats (all p < 0.02). Mean heart weight, but not body weight, was increased in HFD-rats (p < 0.01). HFD-hearts showed structural changes and triglyceride accumulation. HFD-muscles developed higher baseline and maximum forces, but showed impaired recovery from higher workloads. Insulin-associated modulation of Ca2+-induced force augmentation was abolished in HFD-muscles. HFD reduced insulin-stimulated IRS1-associated phosphatidylinositol 3'-kinase activity and phosphorylation of protein kinase B, glycogen synthase kinase-3beta, endothelial nitric oxide synthase, and forkhead transcription factors by 40-60% (all p < 0.05). Insulin-mediated phosphorylation of phospholamban, a critical regulator of myocardial contractility, was decreased in HFD-hearts (p < 0.05).

CONCLUSIONS/INTERPRETATION

HFD induced a hypertrophy-like cardiac phenotype, characterised by a higher basal contractile force, an impaired recovery from increased workloads and decreased insulin-mediated protection against Ca2+ overload. Cardiac dysfunction was associated with myocardial insulin resistance and phospholamban hypophosphorylation. Our data suggest that myocardial insulin resistance, resulting from exposure to excessive alimentary fat, may contribute to the pathogenesis of diabetes-related heart disease.

[Molecular diagnosis on indication of maturity onset diabetes of the young; results from 184 patients]

OBJECTIVE

To describe the results of mutation analysis of the genes involved in maturity onset diabetes of the young (MODY) types 1-3.

DESIGN

Descriptive.

METHOD

In the period July 2000-October 2003 the DNA from 184 possible MODY patients was analysed for the presence of mutations of the genes involved in MODY types 1, 2 and 3. The patients fulfilled at least one of the following criteria: diabetes mellitus had been diagnosed before the age of 25, or at least before the age of 40, there was a family history of diabetes mellitus at an early age, there were no characteristics to indicate diabetes mellitus type 1 or 2.

RESULTS

In the blood of 65 patients (35%) a pathogenic gene mutation was found. A total of 45 patients had a mutation in the HNF-1alpha-gene (which is linked to MODY3), 11 in the glucokinase gene (MODY2) and 9 in the HNF-4alpha-gene (MODY1). Of all the HNF-1alpha-gene mutations, the insertion of a C in codon 291 was the most frequently seen (in 11 families). A mutation in exon 9 of the HNF-1alpha-gene was also shown in 9 apparently non-related families, which probably was a founder mutation.

CONCLUSION

The MODY subtype was found in one third of the selected patients. This diagnosis may have implications in the clinical management of the patient.

The Gly482Ser variant in the peroxisome proliferator-activated receptor gamma coactivator-1 is not associated with diabetes-related traits in non-diabetic German and Dutch populations

The peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) is involved in regulation of fatty acid oxidation, skeletal muscle fiber type specificity, and gluconeogenesis. The prevalent Gly482Ser variant in PGC-1 was shown to be associated with type 2 diabetes in some but not all studies. Moreover, it is unclear whether it influences prediabetic subphenotypes in non-diabetic populations. We studied the association of this variant with glucose tolerance (oral glucose tolerance test), insulin sensitivity (euglycemic hyperinsulinemic clamp) of glucose disposal and antilipolysis, insulin secretion (hyperglycemic clamp, 10 mM), maximal oxygen consumption (VO(2)max, bicycle ergometry), and intramyocellular lipids (magnetic resonance spectroscopy, tibialis and soleus muscle) in a normal glucose tolerant German cohort (n = 423) and a normal (n = 65) and impaired glucose tolerant (n = 94) cohort from the Netherlands. No statistically significant association with an examined phenotype was detected in any of the study cohorts. Specifically, VO(2)max and the soleus-to-tibialis ratio of intramyocellular lipid contents as a surrogate parameter of fiber type specificity was not different between the genotype groups. We conclude, that the Gly482Ser variant in PGC-1 is not associated with diabetes-related traits or skeletal muscle fiber type composition in a non-diabetic German and Dutch population.

Mutations in the NSD1 gene in patients with Sotos syndrome associate with endocrine and paracrine alterations in the IGF system

OBJECTIVE

To investigate the effect of nuclear receptor Su-var, 3-9, enhancer of zeste, trithorax (SET) domain-containing protein 1 (NSD1) gene alteration in patients with Sotos syndrome on plasma IGFs and IGF-binding proteins (IGFBPs), as well as on the IGF/IGFBP system activity at the tissue level.

DESIGN

Twenty-nine patients suspected of Sotos syndrome were divided into two groups: patients with heterozygous deletions or mutations in the NSD1 gene (NSD1(+/-)) (n=11) and subjects without (NSD1(+/+)) (n=18). Plasma samples (n=29) and skin fibroblasts (n=23) were obtained. The results of both groups were compared and related to reference values.

METHODS

IGF-I, IGF-II, IGFBP-2, IGFBP-3, IGFBP-4 and IGFBP-6 levels were determined by RIAs. The mitogenic response of fibroblasts to IGFs was investigated by [methyl-(3)H]thymidine incorporation. IGFBP-3 levels in the culture media were measured by RIA. IGFBP-3 mRNA expression was determined by real time RT-PCR.

RESULTS

NSD1(+/-) patients showed significantly altered levels of IGF-I (mean-1.2 SDS), IGF-II (-1.2), IGFBP-3 (-1.7), IGFBP-4 (-0.4), IGFBP-2 (+0.8) and IGFBP-6 (+1.5). The NSD1(+/+) patients did not differ from the reference, with the exception of the mean IGFBP-3 level (-1.3). Basal proliferation and mitogenic response to IGFs was diminished in NSD1(+/-) fibroblasts compared with NSD1(+/+) (basal, P=0.02; IGF-I, P<0.001; IGF-II, P=0.02). Compared with control fibroblasts, only the mitogenic response was diminished (basal, P=0.07; IGF-I, P=0.04; IGF-II, P=0.04). A trend of higher IGFBP-3 secretion after IGF-I stimulation (P=0.09) and 3.5-5 times higher mRNA expression of IGFBP-3 in basal conditions was found in NSD1(+/-) fibroblasts in comparison to controls.

CONCLUSIONS

NSD1(+/-) patients show endocrine and paracrine changes in the IGF system. These changes may contribute to the abnormal growth pattern.

A Novel 7301-bp Deletion in Mitochondrial DNA in a Patient with Kearns-Sayre Syndrome, Diabetes Mellitus, and Primary Amenorrhoea

We report a 27-year-old woman with a form of mitochondrial myopathy including chronic progressive external opthalmoplegia, retinal pigmentary dystrophy, cerebellar ataxia, and cardiac conduction block (Kearns-Sayre syndrome). At age 13 years a cardiac pacemaker was implanted. She also had sensineural hearing loss, delayed puberty, and primary amenorrhoea. She was weelchair-bound since the age of 20 years. At age 27, insulin-dependent diabetes mellitus and osteoporosis were diagnosed. Insulin treatment was started and associated endocrinopathies were investigated. DNA analysis identified a novel 7301-bp deletion in mitochondrial DNA, ranging from position 6530 to 13 831 corroborating the diagnosis of Kearns-Sayre syndrome.

Mitochondrial diabetes, diabetes and the thiamine-responsive megaloblastic anaemia syndrome and MODY-2. Diseases with common pathophysiology?

Diabetes represents a conglomerate of diseases with chronic hyperglycaemia as hallmark. The present review discusses those diabetic cases that associate with variants in genes that affect the magnitude of the glycolytic flux and oxidative disposal of glucose by mitochondria in pancreatic beta-cells. These genetic variants result in an attenuated secretion of insulin in response to glucose stimulation. The diabetic states that associate with these genetic variants are MODY 2, thiamine responsive anaemia syndrome (TRAS) and mitochondrial diabetes. These disease states highlight the critical contribution of the carbohydrate flux through glycolysis and mitochondria and its coupling to ATP production in determining insulin secretion.

A case of a de novo A3243G mutation in mitochondrial DNA in a patient with diabetes and deafness

A female individual with symptoms of the Maternally Inherited Diabetes and Deafness syndrome (MIDD) was diagnosed positive for the A3243G mutation in her mitochondrial DNA. Heteroplasmy levels were 18% in DNA from leucocytes and 55% in oral mucosa DNA. This finding corroborates the diagnosis of MIDD. Normally, this mutation is present in all the individuals within the maternal lineage of the pedigree. In this particular pedigree the mutation was undetectable in the mother of the proband and her three brothers. Paternity testing using polymorphic chromosomal DNA markers supported the assumed family relationship. We conclude that we are dealing in this proband with the de novo appearance of the A3243G mutation that has reached high heteroplasmy values in at least two tissues within one generation. This observation supports the hypothesis that during embryogenesis mitochondrial DNA goes through a genetic bottleneck with a limited number of segregating units.

Mitochondrial diabetes mellitus

This review discusses the current insight by which mutations in mitochondrial DNA (mtDNA) contribute to the development of particular disease states with emphasis on diabetes mellitus. Mitochondria are the power factories of the cells and produce ATP by oxidizing reducing equivalents via the respiratory chain. These reducing equivalents originate mainly from the citric acid cycle that also occurs within the mitochondria. Human mitochondria contain their own genetic material in the form of circular DNA that encodes for only a fraction of the mitochondrial components. The other mitochondrial components are nuclear encoded. Pathogenic mutations in mtDNA can affect the activity of the respiratory chain, thereby leading to the reduced generation of ATP. However, mitochondria not only produce ATP but they also regulate cytosolic concentrations of signaling molecules such as calcium and iron ions. The metabolic processes within mitochondria such as the citric acid cycle determine the concentration of metabolites that can also act as signalling molecules. Furthermore, the respiratory chain and mitochondrion-associated monoamine oxidase are major producers of reactive oxygen radicals. As a result, mutations in mtDNA can deregulate multiple processes within cells and the balance of this deregulation may contribute to the clinical phenotype.

Skin fibroblasts of children with idiopathic short stature show an increased mitogenic response to IGF-I and secrete more IGFBP-3

OBJECTIVE AND PATIENTS

To study differences in cellular parameters of GH and IGF-I responsiveness in skin fibroblasts of 14 children with idiopathic short stature (ISS) treated with recombinant human GH and 13 children with normal height. Secondly, to investigate whether these cellular parameters can predict the growth response to GH treatment in children with ISS.

DESIGN AND MEASUREMENTS

The mitogenic responsiveness to GH and IGF-I was investigated by 3H-Thymidine incorporation. Insulin-like growth factor binding protein-3 (IGFBP-3) levels in the media were measured by radioimmunoassay (RIA).

RESULTS

No significant mitogenic responses were observed to various doses of GH (1000, 5000 or 50.000 ng/ml) in children with ISS or controls. ISS fibroblasts showed an increased mitogenic response to IGF-I (10 ng/ml) compared to controls (mean +/- SD 5.9 +/- 2.4- vs. 4.2 +/- 1.5-fold stimulation, P < 0.05), and GH enhanced this effect in both groups. IGFBP-3 secretion was increased in ISS fibroblasts when compared to controls under all conditions examined (basal, 200 and 5000 ng/ml GH, 10 ng/ml IGF-I for 24 and 48 h). High IGFBP-3 levels were related to low mitogenic responses to IGF-I or to GH + IGF-I in children with ISS (r = -0.7, P < 0.05), but not in controls. Within the ISS group, an enhanced mitogenic response to IGF-I in vitro was related to more extreme short stature before GH treatment (r = -0.70, P < 0.05) and to a relatively impaired response to high dose GH treatment in vivo (r = -0.52, P < 0.05).

CONCLUSION

The demonstration of high IGFBP-3 levels and enhanced mitogenic response to IGF-I shows that ISS fibroblasts have different cellular characteristics compared to controls of normal height. It is hypothesized that in ISS an alteration of the signal transduction pathway between the GH receptor and IGFBP-3 synthesis results in a local imbalance with high IGFBP-3 levels and lower IGF-I availability for the IGF-I receptor. This may be reflected by an increased IGF-I responsiveness in vitro which is associated with an impaired capacity to grow in vivo.

Molecular and clinical aspects of mitochondrial diabetes mellitus

This review provides a compact overview on the contribution of mutations in mtDNA to the pathogenesis of diabetes mellitus, with emphasis on the A3243G mutation in the tRNA(Leu, UUR) gene. This mutation associates in most individuals with maternally inherited diabetes and deafness (MIDD) whereas in some other carriers the MELAS syndrome or a progressive kidney failure is seen. Possible pathogenic mechanisms are discussed especially the question why particular mutations in mtDNA associate with distinct clinical entities. Mutations in mtDNA can affect the ATP production, thereby leading to particular clinical phenotypes such as muscle weakness. On the other hand mtDNA mutations may also alter the intracellular concentration of mitochondrial metabolites which can act as signalling molecules, such as Ca or glutamate. This situation may contribute to the development of particular phenotypes that are associated with distinct mtDNA mutations.

Hyperosmotic stress activates the insulin receptor in CHO cells

Stress factors, such as osmotic stress and genotoxic agents, activate stress kinases, whereas growth factors preferentially stimulate the structurally homologous mitogen-activated protein kinases, ERK1/2. Hyperosmolarity also has insulin-mimicking action as reflected by ERK1/2 activation and by the stimulation of glucose uptake in adipocytes. We examined to what extent hyperosmolarity activates components of the insulin receptor (IR) signalling pathway. CHO cells expressing the human IR were treated with 500 mM NaCl or 700 mM sorbitol and the activation of insulin signalling intermediates was studied. Hyperosmolarity induced tyrosine phosphorylation of the IR beta-subunit, and the adaptor proteins p52-Shc, p66-Shc, and IRS1. Furthermore, the stress kinases JNK and p38 were activated. When CHO cells were transfected with a kinase-dead IR (K1030R) mutant, hyperosmolarity did not induce tyrosine phosphorylation of the IR, indicating that hyperosmolarity induced IR autophosphorylation directly, rather than inducing phosphorylation by an exogenous tyrosine kinase. A partially purified and detergent-solubilized IR was not phosphorylated in response to hyperosmolarity, suggesting that hyperosmolarity activates the receptor only when present in the plasma membrane. In cells stably expressing the kinase-dead IR, IRS1 and Shc Tyr phosphorylation was abrogated, indicating that the hyperosmolarity signalling was dependent on an active IR tyrosine kinase. In contrast, the stress kinases p38 and JNK were normally activated by hyperosmolarity in the IR-K1030R mutant. We conclude that, at least in CHO cells, hyperosmolarity signals partially through IR autophosphorylation and subsequent activation of the IR downstream targets. This may be responsible for some of the insulin-mimicking effects of hyperosmolarity. The activation of stress kinases by hyperosmolarity occurs independent of the IR.

Identification of the gene altered in Berardinelli-Seip congenital lipodystrophy on chromosome 11q13

Congenital generalized lipodystrophy, or Berardinelli-Seip syndrome (BSCL), is a rare autosomal recessive disease characterized by a near-absence of adipose tissue from birth or early infancy and severe insulin resistance. Other clinical and biological features include acanthosis nigricans, hyperandrogenism, muscular hypertrophy, hepatomegaly, altered glucose tolerance or diabetes mellitus, and hypertriglyceridemia. A locus (BSCL1) has been mapped to 9q34 with evidence of heterogeneity. Here, we report a genome screen of nine BSCL families from two geographical clusters (in Lebanon and Norway). We identified a new disease locus, designated BSCL2, within the 2.5-Mb interval flanked by markers D11S4076 and D11S480 on chromosome 11q13. Analysis of 20 additional families of various ethnic origins led to the identification of 11 families in which the disease cosegregates with the 11q13 locus; the remaining families provide confirmation of linkage to 9q34. Sequence analysis of genes located in the 11q13 interval disclosed mutations in a gene homologous to the murine guanine nucleotide-binding protein (G protein), gamma3-linked gene (Gng3lg) in all BSCL2-linked families. BSCL2 is most highly expressed in brain and testis and encodes a protein (which we have called seipin) of unknown function. Most of the variants are null mutations and probably result in a severe disruption of the protein. These findings are of general importance for understanding the molecular mechanisms underlying regulation of body fat distribution and insulin resistance.

[From gene to disease; 'maturity-onset diabetes of the young' (MODY), monogenetic inheritable forms of diabetes mellitus]

Maturity-onset diabetes of the young (MODY) exhibits an autosomal dominant pattern of inheritance and can be divided in at least five subtypes (MODY 1 to 5), each subtype being caused by mutations in a specific gene. The unambiguous molecular diagnosis of the specific MODY subtype facilitates an early diagnosis of diabetes and can help to reduce the development of diabetic complications. Furthermore, MODY2 patients generally have a milder clinical course and fewer complications than MODY3 patients, who consequently require a more aggressive therapeutic approach.

[From gene to disease; mutation in mitochondrial DNA and maternally inherited diabetes mellitus with deafness (MIDD)]

MIDD is a maternally inherited disorder with diabetes and impaired hearing due to a reduced perception of high tones. The disorder is caused by an A to G mutation at position 3243 in mitochondrial DNA. Approximately 1.3% of insulin-dependent diabetic patients in the Netherlands has this mutation. The main defect in these patients seems to be a reduced secretion of insulin by the pancreas in response to glucose stimulation.

Modulation of insulin-stimulated glycogen synthesis by Src Homology Phosphatase 2

We have examined the requirement of the protein tyrosine phosphatase Src Homology Phosphatase 2 (SHP2) for insulin-stimulated glycogen synthesis. To this end, 3T3L1 fibroblasts were stably transfected with either wild type or a catalytically inactive C463A-mutant of SHP2, and analysed for insulin-induced glycogen synthesis, tyrosine phosphorylation of the insulin receptor and IRS-1, and activation of phosphatidylinositol 3'-kinase (PI 3'-kinase). Glycogen synthesis was stimulated 9.1+/-0.9-fold by insulin in untransfected cells. In cells expressing the dominant-negative C463A-SHP2 mutant, the stimulation of glycogen synthesis by insulin was strongly enhanced (18.7+/-2.7-fold stimulation), while this response was impaired in cells overexpressing wild-type SHP2 (6.6+/-1.1-fold stimulation). When exploring the early post-receptor signalling pathways that contribute to glycogen synthesis, we found that insulin stimulated the tyrosine phosphorylation of IRS-1, and the activation of IRS-1-associated PI 3'-kinase more strongly in C463A-SHP2 expressing 3T3L1-cells (18.1+/-4.7-fold) than in parental 3T3L1 cells (6.8+/-0.5-fold). In 3T3L1 cells overexpressing wild-type SHP2, the insulin stimulation of IRS-1 tyrosine phosphorylation and the activation of PI 3'-kinase (4.5+/-1.0-fold) were impaired. An enhanced activity of SHP2 leads to negative modulation of insulin signalling by reducing the tyrosine phosphorylation of IRS-1 and the concomitant activation of PI 3'-kinase. This results in an impaired ability of insulin to stimulate glycogen synthesis.

Identification of Ser(1275) and Ser(1309) as autophosphorylation sites of the human insulin receptor in intact cells

In a previous report we described Ser(1275) and Ser(1309) as autophosphorylation sites of the human insulin receptor (IR) tyrosine kinase (TK) in vitro. The question remained whether the observed phosphorylation was exclusive for the in vitro activated receptor or a more general, mechanism of the activated receptor in situ. In this study, we determined the intrinsic activity of the IR to phosphorylate both serine residues in intact cells. For this purpose CHO-09 and NIH-3T3 derived cell-lines expressing the human IR were metabolically labelled with [(32)P]orthophosphate, followed by hormone stimulation of the receptor. The IR was isolated by immunoprecipitation and SDS-PAGE and subsequently analysed for serine phosphorylation by phosphopeptide mapping of HPLC-purified tryptic phosphopeptides. Activation of the IR in the intact cell appeared to result in phosphate incorporation into Ser(1275) and Ser(1309), providing strong evidence that both serine residues are phosphorylation sites of the activated receptor in intact cells.

IRS-1 tyrosine phosphorylation reflects insulin-induced metabolic and mitogenic responses in 3T3-L1 pre-adipocytes

We determined the involvement of Tyr-1158 within the regulatory loop of the insulin receptor (IR) in the generation of insulin-specific responses in situ. For this purpose chimeric receptors with an epidermal growth factor (EGF) receptor extracellular domain and an IR cytoplasmic domain (EIR) were constructed, which allow activation of the cytoplasmic IR domain without activation of endogenous wt-IRs. Tyr-1158 of the chimera EIR was exchanged for Phe, creating a mutant chimeric receptor (EIR-Y1158F). Chimeric receptors were expressed in 3T3-L1 pre-adipocytes, which do not show insulin-specific responses upon EGF stimulation. We found that pre-adipocytes expressing EIR-Y1158F were impaired in their ability to stimulate glycogen synthesis and DNA synthesis upon maximal stimulation with EGF. EIR-Y1158F was impaired in its ability to phosphorylate insulin receptor substrate (IRS)-1 and induce downstream signals of IRS-1 phosphorylation, such as the association of IRS-1 with phosphatidyl-inositol-3'-kinase and the activation of protein kinase B (Akt). In contrast with the phosphorylation of IRS-1, the phosphorylation of IRS-2 and extracellular regulated protein kinase-1/-2 was normal in EIR-Y1158F expressing cells. These observations suggest that the level of IRS-1 phosphorylation rather than the level of IRS-2 phosphorylation mediates insulin-induced glycogen synthesis and DNA synthesis in 3T3-L1 pre-adipocytes.

HLA-DQ polymorphism and degree of heteroplasmy of the A3243G mitochondrial DNA mutation in maternally inherited diabetes and deafness

AIM

Maternally inherited diabetes and deafness (MIDD) associates with a mutation at position 3243 in mitochondrial DNA. Phenotypic expression of MIDD includes Type 1-like and Type 2-like diabetes. This study examined whether HLA-DQ phenotype and the degree of heteroplasmy in leucocyte and oral mucosa DNA influence clinical expression of the 3242 mutation.

METHODS

In a group of 20 unrelated probands with MIDD, eight with Type 1- like diabetes, 12 with Type 2-like diabetes, HLA-DQ type and degree of heteroplasmy for the 3243 mutation were determined. HLA-DQA1/DQB1 phenotypes were categorized as predisposing, neutral or protective for autoimmune-mediated Type 1 diabetes.

RESULTS

No differences were observed between Type 1 and Type 2-like MIDD groups with respect to the cumulative frequency of protective and predisposing HLA-DQ types. Predisposing HLA-DQ types are more prevalent in MIDD patients than in the control population (P < 0.05). Degrees of heteroplasmy for the 3243 mutation showed large variations in patients, ranging from 1 to 52% in leucocyte DNA. A strong correlation was seen between heteroplasmy in leucocyte DNA and DNA from oral mucosa cells (r = 0.89, P < 0.001). No correlation was observed between the degree of heteroplasmy and diabetic phenotype, even when group size was extended with diabetic relatives of patients with MIDD. The age of diagnosis of diabetes was not correlated with heteroplasmy, but the degree of heteroplasmy tended to decrease with age.

CONCLUSIONS

The phenotype of diabetes in MIDD appears to be independent of HLA-DQ phenotype and degree of heteroplasmy in leucocyte and oral mucosa DNA indicating that other, as yet unknown, factors modulate clinical expression of the 3243 mutation.

Reduced second phase insulin secretion in carriers of a sulphonylurea receptor gene variant associating with Type II diabetes mellitus

AIMS/HYPOTHESIS

The sulphonylurea receptor is a subunit of the ATP-sensitive potassium channel in the pancreatic beta cell. Mutations at nt -3 of the splice acceptor site of exon 16 and a silent mutation in exon 18 of the gene for the sulphonylurea receptor (SUR1) associate with Type II (non-insulin-dependent) diabetes mellitus in several independent populations. We investigated whether these gene variants associate with changes in the pattern of glucose-stimulated insulin secretion.

METHODS

Subjects who had normal glucose tolerance (n = 67) and subjects with an impaired glucose tolerance (n = 94), originating from two independent studies, were included in the study. Beta-cell function and insulin sensitivity were assessed by the hyperglycaemic clamp.

RESULTS

Frequencies of the exon 16 -3t allele in the normal and impaired glucose tolerant groups were 46% and 44% respectively (p = NS). The more rare exon 18 T allele showed frequencies of 5 and 7% respectively (p = NS). We observed an approximately 25% reduced second-phase insulin secretion in carriers of the exon 16 -3t allele in both groups (p < 0.05). Estimates of insulin sensitivity did not show differences between carriers and non-carriers. The variant in exon 18 and the combined presence of variants in exon 16 and exon 18 were not associated with differences in insulin secretion or insulin sensitivity in our study groups.

CONCLUSION/INTERPRETATION

The diabetes associated exon 16 -3t variant of the SUR1 gene associates with a functional change of the beta cell as reflected by reduced second-phase insulin secretion in response to a standardized hyperglycaemia in normal and impaired glucose tolerant subjects.

Characterization of a novel mitochondrial DNA deletion in a patient with a variant of the Pearson marrow-pancreas syndrome

We have recently diagnosed a patient with anaemia, severe tubulopathy, and diabetes mellitus. As the clinical characteristics resembled Pearson marrow-pancreas syndrome, despite the absence of malfunctioning of the exocrine pancreas in this patient, we have performed DNA analysis to seek for deletions in mtDNA. DNA analysis showed a novel heteroplasmic deletion in mtDNA of 8034bp in length, with high proportions of deleted mtDNA in leukocytes, liver, kidney, and muscle. No deletion could be detected in mtDNA of leukocytes from her mother and young brother, indicating the sporadic occurrence of this deletion. During culture, skin fibroblasts exhibited a rapid decrease of heteroplasmy indicating a selection against the deletion in proliferating cells. We estimate that per cell division heteroplasmy levels decrease by 0.8%. By techniques of fluorescent in situ hybridisation (FISH) and mitochondria-mediated transformation of rho(o) cells we could show inter- as well as intracellular variation in the distribution of deleted mtDNA in a cell population of cultured skin fibroblasts. Furthermore, we studied the mitochondrial translation capacity in cybrid cells containing various proportions of deleted mtDNA. This result revealed a sharp threshold, around 80%, in the proportion of deleted mtDNA, above which there was strong depression of overall mitochondrial translation, and below which there was complementation of the deleted mtDNA by the wild-type DNA. Moreover, catastrophic loss of mtDNA occurred in cybrid cells containing 80% deleted mtDNA.

The diabetes-associated 3243 mutation in the mitochondrial tRNA(Leu(UUR)) gene causes severe mitochondrial dysfunction without a strong decrease in protein synthesis rate

Cells harboring patient-derived mitochondria with an A-to-G transition at nucleotide position 3243 of their mitochondrial DNA display severe loss of respiration when compared with cells containing the wild-type adenine but otherwise identical mitochondrial DNA sequence. The amount and degree of leucylation of tRNA(Leu(UUR)) were both found to be highly reduced in mutant cells. Despite the low level of leucyl-tRNA(Leu(UUR)), the rate of mitochondrial translation was not seriously affected by this mutation. Therefore, decrease of mitochondrial protein synthesis as such does not appear to be a necessary prerequisite for loss of respiration. Rather, the mitochondrially encoded proteins seem subject to elevated degradation, leading to a severe reduction in their steady state levels. Our results favor a scheme in which the 3243 mutation causes loss of respiration through accelerated protein degradation, leading to a disequilibrium between the levels of mitochondrial and nuclear encoded respiratory chain subunits and thereby a reduction of functional respiratory chain complexes. The possible mechanisms underlying the pathogenesis of mitochondrial diabetes is discussed.

Hypoglycaemia associated with the production of insulin-like growth factor II and insulin-like growth factor binding protein 6 by a haemangiopericytoma

Non-islet-cell tumour-induced hypoglycaemia (NICTH) is, in most cases, attributable to tumour production of insulin-like growth factor II (IGF-II). Tumour-derived IGF-II has a higher than normal molecular weight (big 'IGF-II') and an impaired ability to form the normal ternary 150 kD complex with IGF binding protein-3 (IGFBP-3) and the acid-labile subunit (ALS). Consequently, tumoral IGF-II circulates mainly in smaller binary complexes which have a higher bioavailability than the ternary complex. We had the opportunity to analyze IGFs and IGF-related factors in both pre- and post-operative blood, tumour tissue and tumour cyst fluid from a patient with a disseminated haemangiopericytoma and severe hypoglycaemia. In addition, the effect of serum and tumour cyst fluid on autophosphorylation of the insulin receptor was examined. Patient serum contained low levels of IGF-I, IGFBP-3 and ALS, while the concentrations of IGFBP-2 and IGFBP-6 were markedly elevated. The total level of circulating IGF-II was within the normal range, but Biogel P-60 gel filtration of patient serum revealed that 77% of the IGF-II was present in high molecular weight forms (normal: 10-15%), which decreased to 53% after partial removal of the tumour. Most of the IGF-II immunoreactivity in pre- and post-operative patient serum was associated with 50-60 kD complexes with only a minimal contribution (<10%) from the 150 kD complex. Tumour cyst fluid contained excessive amounts of both big IGF-II and IGFBP-6. Northern blot analysis of total mRNA isolated from the tumour demonstrated high expression of the IGF-II gene and abundant 1.1 kb IGFBP-6 transcript, while the genes encoding IGFBP-3, -4 and -5 were only weakly expressed and mRNA of IGFBP-1, -2 and IGF-I could not be detected. mRNAs for the IGF type II receptor could be easily demonstrated, whereas those for the insulin- and IGF type I receptor were hardly detectable. In contrast to patient serum tumour cyst fluid strongly stimulated the insulin receptor in vitro. The present study suggests an important role of the simultaneous production of IGF-II and IGFBP-6 in the pathophysiology of tumour-induced hypoglycaemia.

The insulin receptor tyrosine kinase domain in a chimaeric epidermal growth factor-insulin receptor generates Ca2+ signals through the PLC-gamma1 pathway

The receptors for insulin (IR) and epidermal growth factor (EGFR) are members of the tyrosine kinase receptor (TKR) family. Despite homology of their cytosolic TK domains, both receptors induce different cellular responses. Tyrosine phosphorylation of insulin receptor substrate (IRS) molecules is a specific IR post-receptor response. The EGFR specifically activates phospholipase C-gamma1 (PLC-gamma1). Recruitment of substrate molecules with Src homology 2 (SH2) domains or phosphotyrosine binding (PTB) domains to phosphotyrosines in the receptor is one of the factors creating substrate specificity. In addition, it has been shown that the TK domains of the IR and EGFR show preferences to phosphorylate distinct peptides in vitro, suggesting additional mechanisms of substrate recognition. We have examined to what extent the substrate preference of the TK domain contributes to the specificity of the receptor in vivo. For this purpose we determined whether the IR TK domain, in situ, is able to tyrosine-phosphorylate substrates normally used by the EGFR. A chimaeric receptor, consisting of an EGFR in which the juxtamembrane and tyrosine kinase domains were exchanged by their IR counterparts, was expressed in CHO-09 cells lacking endogenous EGFR. This receptor was found to activate PLC-gamma1, indicating that the IR TK domain, in situ, is able to tyrosine phosphorylate substrates normally used by the EGFR. These findings suggest that the IR TK domain, in situ, has a low specificity for selection and phosphorylation of non-cognate substrates.

Variants in the sulphonylurea receptor gene: association of the exon 16-3t variant with Type II diabetes mellitus in Dutch Caucasians

AIMS/HYPOTHESIS

We have analysed to what extent two previously reported single nucleotide polymorphisms in the sulphonylurea receptor gene (SUR1) are associated with Type II (non-insulin-dependent) diabetes mellitus in The Netherlands. Furthermore, we estimated haplotype frequencies in control and diabetic populations, including data extracted from three other studies.

METHODS

Subjects with Type II diabetes (n = 388) and normoglycaemic subjects (n = 336) were randomly selected from two population-based studies, the Hoorn and Rotterdam studies. DNA was typed for variants in exon 16 (-3c-->t variant in the splice acceptor site) and exon 18 (Thr759Thr, ACC-->ACT).

RESULTS

The genotype frequencies in both populations were similar. We observed an association of the exon 16-3t variant with Type II diabetes (allele frequencies 0.41 % vs 0.48 % in NGT and Type II diabetes, respectively, p = 0.01). There was no association between Type II diabetes and the variant in exon 18 or the combination of both variants (p > 0.5). A strong linkage disequilibrium between the exon 16 and exon 18 variants was observed in the diabetic groups but not, or less pronounced, in the control groups from the different studies. Haplotype estimation shows that several different risk haplotypes exist in different Caucasian populations.

CONCLUSION/INTERPRETATION

The exon 16-3t allele of the SUR1 gene is associated with Type II diabetes in the Netherlands. Based on estimated haplotype frequencies in different Caucasian populations we conclude that multiple haplotypes on the SUR1 gene seem to confer a risk for developing Type II diabetes in Caucasians.

Altered beta-cell characteristics in impaired glucose tolerant carriers of a GAA trinucleotide repeat polymorphism in the frataxin gene

Friedreich's ataxia is associated with GAA trinucleotide repeat expansions in the frataxin gene. In the general population, these trinucleotide expansions are variable in length, and three types of expansions are seen: short, intermediate, and long repeats. Friedreich's ataxia patients are generally homozygous for the long repeats and exhibit diabetes as pronounced comorbidity. Ristow et al. recently reported an association between the intermediate-length normal allele in the frataxin gene and type 2 diabetes. We have investigated in 94 subjects with impaired glucose tolerance (IGT) as to whether the length of the GAA trinucleotide repeat polymorphism in the frataxin gene associates with parameters reflecting beta-cell function. A hyperglycemic clamp at 10 mmol/l glucose for 3 h was used to quantitate beta-cell characteristics. Carriers of one or two intermediate repeat alleles (n = 32) had a 50% higher median first- phase insulin response to glucose than the noncarriers. Furthermore, they needed less time to reach peak insulin. An analysis of the distribution of the various repeat lengths in elderly type 2 diabetic (n = 179) and control subjects (n = 183), with the same age and ethnic background, did not provide evidence for an association of the intermediate-length repeat allele with type 2 diabetes in Dutch Caucasians.

Functional and morphological abnormalities of mitochondria harbouring the tRNA(Leu)(UUR) mutation in mitochondrial DNA derived from patients with maternally inherited diabetes and deafness (MIDD) and progressive kidney disease

AIMS/HYPOTHESIS

An A to G transition at nucleotide position 3243 in the mitochondrial tRNA Leu(UUR) gene has been identified in patients with maternally inherited diabetes and deafness, as well as in patients with mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes, chronic progressive external ophpthalmoplegia, cardiomyopathy and progressive kidney disease. Variations in the mitochondrial DNA haplotype as well as differences in the degree and distribution of heteroplasmy in a certain tissue are factors that may contribute to the variety in phenotypical expression of the 3243 tRNA(Leu)(UUR) mutation. We have done morphological and functional experiments on mitochondria carrying the 3243 mutation derived from patients with either maternally inherited diabetes and deafness or progressive kidney disease to prove the pathogenicity of the 3243 mutation and to examine whether the mtDNA haplotype modulates the pathobiochemistry of this mutation.

METHODS

We constructed clonal cell lines that contain predominantly mutated or exclusively wild-type mtDNA with a distinct mtDNA haplotype by the methodology of mitochondria-mediated transformation. Cells lacking mitochondrial DNA (rho(o)) were used as recipients and donor mitochondria were derived from fibroblasts of a patient with either maternally inherited diabetes and deafness or progressive kidney disease. The fibroblasts from these clinically distinct patients carry different mitochondrial DNA haplotypes with the 3243 mutation in heteroplasmic form.

RESULTS

Heteroplasmy in the clonal cybrid cells ranged from 0 to 100%, reflecting the heterogeneity of the mitochondrial donor cell. Cybrid cells containing predominantly mutant mitochondrial DNA showed lactic acidosis, poor respiration and marked defects in mitochondrial morphology and respiratory chain complex I and IV activities. No differences were observed in the extent of the mitochondrial dysfunction between the mutant cells derived from the two donors.

CONCLUSION/INTERPRETATION

These results provide evidence for a pathogenic effect of the tRNA(Leu)(UUR) mutation in maternally inherited diabetes and deafness and progressive kidney disease, and show no evidence of a contribution of the mitochondrial DNA haplotype as a modulating the biochemical expression of the mutation.

The DD genotype of the ACE gene polymorphism is associated with progression of diabetic nephropathy to end stage renal failure in IDDM

BACKGROUND

The insertion-deletion (I/D) polymorphism of the angiotensin converting enzyme gene is a diallelic polymorphism that constitutes a genetic influence on the progression of renal diseases such as IgA nephropathy. Patients with the DD genotype have an accelerated progression towards end stage renal failure in these diseases. The role of the I/D polymorphism in the pathogenesis of diabetic nephropathy in IDDM is unresolved.

PATIENTS AND METHODS

We therefore set out to study the contribution of the I/D polymorphism in 79 patients (age 39.5 +/- 7.6 years (mean +/- SD) with end stage renal failure due to diabetic nephropathy, who were recipients of a combined kidney-pancreas transplantation (n = 60), or who were on the waiting list for such a procedure (n = 19). The control series consisted of 82 patients (age 39.5 +/- 9.6 years) without microalbuminuria after fifteen years of IDDM.

RESULTS

The ACE genotype distribution in patients was not in accordance with the Hardy-Weinberg equilibrium due to a significant overrepresentation of the DD genotype (X2 = 8.9, p = 0.01). This resulted in a significant increase of the D-allele frequency in the cases compared to controls (X2 = 4.9, p = 0.03). The presence of one D-allele did not increase the risk of end stage renal failure (odds ratio ID/II = 1.0, 95% CI 0.4-2.2). The presence of the DD genotype increased the risk of end stage renal failure twofold compared to the other genotypes (odds ratio 2.1, 95% CI 1.1-4.0). The risk estimate seemed slightly higher in patients with good metabolic control (odds ratio 2.6, 95% CI 1.0-7.1), than in patients with poor control (odds ratio 1.6, 95% CI 0.59-4.3).

CONCLUSION

It is concluded that the risk of end-stage renal failure in patients with IDDM is twofold increased in patients with the DD genotype as compared to patients with other genotypes.

Prevalence of variants in candidate genes for type 2 diabetes mellitus in The Netherlands: the Rotterdam study and the Hoorn study

We have analyzed the association of variants in the genes for amylin, insulin receptor, insulin receptor substrate-1 (IRS-1), and coagulation factor V with type 2 diabetes mellitus. Random samples of subjects with type 2 diabetes and controls were taken from two population-based studies, the Hoorn and Rotterdam studies, to reduce the risk of artifactual associations. No association was found for variants in the genes for amylin, IRS-1, and coagulation factor V, nor was there any evidence for epistatic interactions between these gene variants. A significant difference in the frequency of the Arg972 allele of the IRS-1 gene was observed between control subjects from Hoorn and Rotterdam (9.4% vs. 18.6%; P < 0.05). The insulin receptor Met985 variant was found at frequencies of 4.4% and 1.8%, respectively, in type 2 diabetic (n = 433) and normoglycemic patients (n = 799; P < 0.02). Inclusion of data from two other studies yielded a summarized odds ratio of 1.87 (95% confidence interval, 1.06-3.29; P = 0.03). We conclude that the association between the Met985 variant in the insulin receptor gene and type 2 diabetes, which we previously reported in the Rotterdam study, is supported by thejoint analysis with a second population-based study and other studies. The large regional differences in allele frequency of the Arg972 allele of IRS-1 gene makes genetic association studies of this gene less reliable.

Search for mitochondrial DNA mutations in migraine subgroups

It has been suggested that mitochondrial mutations cause migraine(-like) symptoms. The presence of mtDNA mutations (3243, 3271, 11084, and deletions) was investigated in three migraine subgroups (maternally transmitted migraine with and without aura, migrainous infarction, and nonfamilial hemiplegic migraine). No mutations were found. These mutations and deletions probably are not involved in the migraine subgroups studied, although an investigation of other material (e.g., muscle tissue) would have shown this with more certainty.