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

Association of ABCC8 and KCNJ11 gene variants with type 1 diabetes in south Indians

Background

Type 1 diabetes mellitus (TIDM) is a polygenic disorder with the involvement of several genetic and environmental risk factors. Mutation in genes namely ABCC8 and KCNJ11 disrupt the potentiality of KATP channel and regulates the secretion of insulin by detecting a change in the blood glucose level and consequently maintains glucose homeostasis. The present study was designed to investigate the association of ABCC8 and KCNJ11gene polymorphisms with type 1 diabetes. A case-control study was conducted enrolling 60 cases suffering from T1DM and 60 healthy controls of comparable age and sex. Gene variations were determined by PCR-RFLP and ARMS-PCR method.

Results

The ABCC8-3C > T (rs1799854) variation was found to be significantly associated with T1DM (p<0.01) and “CT” genotype was found to be predominant in T1DM with a threefold increased risk to diabetes and the association was statistically significant. However, we did not find any significant association of C>T (rs1801261) polymorphism of ABCC8 with T1DM. A significant association was observed for genetic variation at rs5219 C>T polymorphism and the frequency of TT genotype was found to be significantly higher in patients (46.7%) than in controls (21.7%), indicating the significant role of the KCNJ11 rs5219 variant in T1DM susceptibility (p<0.001), but we did not observe any significant association of G>A (rs5215) polymorphism of KCNJ11 with T1DM. In addition, haplotype analysis of the two genes revealed four haplotypes such as T-C-G-T, T-C-A-T, C-C-G-T, and T-T-G-T as risk haplotypes for type 1 diabetes (p<0.02) potentially making individual effects of these variants on the disease susceptibility, thereby indicating the synergistic role of these genes in the regulation of glucose homeostasis.

Conclusions

The present study highlights the importance of personalized medicine based on individual genetic profile.

Association between ABCC8 Ala1369Ser Polymorphism (rs757110 T/G) and Type 2 Diabetes Risk in an Iranian Population: A Case-Control Study

OBJECTIVE

Glucose metabolism increases ATP/ADP ratio within the β-cells and causes ATP-sensitive K+ (KATP) channel closure and consequently insulin secretion. The enhanced activity of the channel may be a mechanism contributing to the reduced first-phase of insulin secretion observed in T2DM. There is no study to date in the Kurdish ethnic group regarding the relationship between SNP Ala1369Ser (rs757110 T/G) of SUR1 gene and T2DM, and additionally, the results of this association in other populations are inconsistent. Therefore, our aim in this study was to explore the possible association between SNP Ala1369Ser and type 2 diabetes in an Iranian Kurdish ethnic group.

METHODS

In this study, we checked out the frequency of alleles and genotypes of SNP Ala1369Ser in T2DM individuals (207 patients; men/women: 106/101) and non-T2DM subjects (201 controls; men/women: 97/104), and their effects on anthropometric, clinical, and biochemical parameters. Genomic DNA was extracted from the leukocytes of blood specimens using a standard method. We amplified the ABCC8 rs757110 polymorphic site (T/G) using a polymerase chain reaction (PCR) method and a designed primer pair. To perform the PCR-RFLP method, the amplicons were subjected to restriction enzymes and the resulting fragments separated by gel electrophoresis.

RESULTS

The frequency of the G-allele of Ala1369Ser polymorphism was significantly (0.01) higher in the case group than the control group (19% vs. 9%, respectively). In the dominant model (TT vs. TG+GG), there was a significant relationship between this SNP and an increased risk of T2DM (P = 0.00). T2DM patients with TG+GG genotypes had significantly higher fasting plasma insulin and HOMA-IR than those who had the TT genotype (P = 0.02 and 0.01, respectively).

CONCLUSION

Our study is the first study to investigate the association between Ala1369Ser ABCC8 genetic variation and T2DM in the Kurdish population of western Iran. The obtained results clearly show that Ala1369Ser polymorphism of ABCC8 is associated with an increased risk of T2DM in this population.

A Variation in the ABCC8 Gene Is Associated with Type 2 Diabetes Mellitus and Repaglinide Efficacy in Chinese Type 2 Diabetes Mellitus Patients

Objective Previous studies have suggested that variations in the ABCC8 gene may be closely associated with T2DM susceptibility and repaglinide response. However, these results have not been entirely consistent, and there are no related studies in a Chinese population, suggesting the need for further exploration. The current study investigated the associations of the ABCC8 rs1801261 polymorphism with type 2 diabetes mellitus (T2DM) susceptibility and repaglinide therapeutic efficacy in Chinese Han T2DM patients. Methods A total of 234 T2DM patients and 105 healthy subjects were genotyped for ABCC8 rs1801261 polymorphism by a polymerase chain reaction-restriction fragment length polymorphism assay. A total of 70 patients with the same genotypes of CYP2C8*3 139Arg and OATP1B1 521TT were randomized to orally take 3 mg repaglinide per day (1 mg each time before meals) for 8 consecutive weeks. The pharmacodynamic parameters of repaglinide and biochemical indicators were then determined before and after repaglinide treatment. Results The frequency of ABCC8 rs1801261 allele was higher in T2DM patients than in the control subjects (22.6% vs.11.0%, p<0.01). After repaglinide treatment, T2DM patients carrying genotype CT showed a significantly attenuated efficacy on FPG (p<0.01) and HbA1c (p<0.01) compared with those with genotype CC. Conclusion These results suggested that the ABCC8 rs1801261 polymorphism might influence T2DM susceptibility and the therapeutic effect of repaglinide in Chinese Han T2DM patients. This study was registered in the Chinese Clinical Trial Register on May 14, 2013 (No. ChiCTR-CCC13003536).

Possible association between ABCC8 C49620T polymorphism and type 2 diabetes in a Nigerian population

BACKGROUND

The association between ABCC8 gene C49620T polymorphism and type 2 diabetes (T2D) in populations of diverse ethnic backgrounds has been reported. However, such occurrence in an African population is yet to be established. This case-control study involving 73 T2D and 75 non-diabetic (ND) patients investigated the occurrence of this polymorphism among T2D patients in Nigeria and assessed its relationship with body lipids of patients.

METHODS

Demographic and clinical characteristics of patients were collected and lipid profile indices including total cholesterol (TC), triglyceride (TG), low density lipoprotein (LDL) and high density lipoprotein (HDL) were assayed. Restriction fragment length polymorphism-PCR (RFLP-PCR) was employed to genotype the ABCC8-C49620T polymorphism using PstI restriction enzyme.

RESULTS

This study revealed significantly (p < 0.05) higher prevalence of the T allele of the ABCC8 gene in T2D patients (33.1%) compared to ND patients (28.0%). The mutant TT genotype was also higher than the CC and CT genotypes in T2D patients compared to ND patients but did not show any significant risk (p>0.05) of T2D for the unadjusted codominant, dominant and recessive models. Following age adjustment, the mutant genotypes (CT and TT) showed significant (p<0.05) risk of T2D for all the models with the recessive model presenting the greatest risk of T2D (OR: 2.39, 95% CI: 1.16-4.91, p<0.018). The TT genotype significantly (p<0.05) associated with high level of HDL and reduced levels of TC, TG and LDL in non-diabetic patients but was not associated with any of the demographic and clinical characteristics among T2D patients.

CONCLUSIONS

ABCC8 C49620T polymorphism showed possible association with T2D marked by predominance of the mutant TT genotype in T2D patients. However, the relationship between TT genotype and lipid abnormalities for possible beneficial effect on people suffering from T2D is unclear.

Identification of genetic variants in pharmacogenetic genes associated with type 2 diabetes in a Mexican-Mestizo population

Type 2 diabetes mellitus (T2DM) is one of the most prevalent chronic pathologies in the world. In developing countries, such as Mexico, its prevalence represents an important public health and research issue. Determining factors triggering T2DM are environmental and genetic. While diet, exercise and proper weight control are the first measures recommended to improve the quality of life and life expectancy of patients, pharmacological treatment is usually the next step. Within every population there are variations in interindividual drug response, which may be due to genetic background. Some of the most frequent first line T2DM treatments in developing countries are sulfonylureas (SU), whose targets are ATP-sensitive potassium channels (K_ATP). Single nucleotide polymorphisms (SNPs) of the K_ATP coding genes, potassium voltage-gated channel subfamily J member 11 (KCNJ11) and ATP binding cassette subfamily C member 8 (ABCC8) have been associated with SU response variability. To date, there is little information regarding the mechanism by which these SNPs work within Mexican populations. The present study describes the distribution of three SNPs [KCNJ11 rs5219 (E23K), ABCC8 rs757110 (S1369A) and rs1799854 (-3C/T)] among Mestizo Mexican (MM) T2DM patients, and compares it with published data on various healthy subjects and T2DM populations. Through this comparison, no difference in the KCNJ11 rs5219 and ABCC8 rs757110 allelic and genotypic frequencies in MM were observed compared with the majority of the reported populations of healthy and diabetic individuals among other ethnic groups; except for African and Colombian individuals. By contrast, ABCC8 rs1799854 genomic and allelic frequencies among MM were observed to be significantly different from those reported by the 1000 Genomes Project, and from diabetic patients within other populations reported in the literature, such as the European, Asian and Latin-American individuals [T=0.704, G=0.296; CC=0.506, CT=0.397, TT=0.097; 95% confidence interval (CI); P≤0.05]; except for South Asian and Iberian populations, which may reflect the admixture origins of the present Mexican population. This genetic similarity has not been observed in the other Latin-American groups. To the best of our knowledge, this is the first study of ABCC8 rs757110 and rs1799854 SNP frequencies in any Mexican population and, specifically with diabetic Mexicans. Knowledge of the genetic structure of different populations is key to understanding the interindividual responses to drugs, such as SU and whether genotypic differences affect clinical outcome.

Pharmacogenetic studies update in type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a silent progressive polygenic metabolic disorder resulting from ineffective insulin cascading in the body. World-wide, about 415 million people are suffering from T2DM with a projected rise to 642 million in 2040. T2DM is treated with several classes of oral antidiabetic drugs (OADs) viz. biguanides, sulfonylureas, thiazolidinediones, meglitinides, etc. Treatment strategies for T2DM are to minimize long-term micro and macro vascular complications by achieving an optimized glycemic control. Genetic variations in the human genome not only disclose the risk of T2DM development but also predict the personalized response to drug therapy. Inter-individual variability in response to OADs is due to polymorphisms in genes encoding drug receptors, transporters, and metabolizing enzymes for example, genetic variants in solute carrier transporters (SLC22A1, SLC22A2, SLC22A3, SLC47A1 and SLC47A2) are actively involved in glycemic/HbA1c management of metformin. In addition, CYP gene encoding Cytochrome P450 enzymes also play a crucial role with respect to metabolism of drugs. Pharmacogenetic studies provide insights on the relationship between individual genetic variants and variable therapeutic outcomes of various OADs. Clinical utility of pharmacogenetic study is to predict the therapeutic dose of various OADs on individual basis. Pharmacogenetics therefore, is a step towards personalized medicine which will greatly improve the efficacy of diabetes treatment.

Assignment of Functional Relevance to Genes at Type 2 Diabetes-Associated Loci Through Investigation of β-Cell Mass Deficits

Type 2 diabetes (T2D) has been associated with a large number of genomic loci, many of which encompass multiple genes without a definitive causal gene. This complexity has hindered efforts to clearly identify functional candidate genes and interpret their role in mediating susceptibility to disease. Here we examined the relevance of individual genes found at T2D-associated loci by assessing their potential contribution to a phenotype relevant to the disease state: production and maintenance of β-cell mass. Using transgenic zebrafish in which β-cell mass could be rapidly visualized in vivo, we systematically suppressed the expression of orthologs of genes found at T2D-associated genomic loci. Overall, we tested 67 orthologs, many of which had no known relevance to β-cell mass, at 62 human T2D-associated loci, including eight loci with multiple candidate genes. In total we identified 25 genes that were necessary for proper β-cell mass, providing functional evidence for their role in a physiological phenotype directly related to T2D. Of these, 16 had not previously been implicated in the regulation of β-cell mass. Strikingly, we identified single functional candidate genes at the majority of the loci for which multiple genes were analyzed. Further investigation into the contribution of the 25 genes to the adaptive capacity of β-cells suggested that the majority of genes were not required for glucose-induced expansion of β-cell mass but were significantly necessary for the regeneration of β-cells. These findings suggest that genetically programmed deficiencies in β-cell mass may be related to impaired maintenance. Finally, we investigated the relevance of our findings to human T2D onset in diabetic individuals from the Old Order Amish and found that risk alleles in β-cell mass genes were associated with significantly younger age of onset and lower body mass index. Taken together, our study offers a functional approach to assign relevance to genes at T2D-associated loci and offers experimental evidence for the defining role of β-cell mass maintenance in genetic susceptibility to T2D onset.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Evaluation of Gestational Diabetes Mellitus Risk in South Indian Women Based on MTHFR (C677T) and FVL (G1691A) Mutations

We aimed to scrutinize the extent to which single amino acid substitutions in the MTHFR and factor V Leiden (FVL) genes affect the risk of gestational diabetes mellitus (GDM) in pregnant women of South Indian descendant. This case-control study was implemented once the ethical approval has been obtained. Overall, 237 women were recruited in this study: 137 had been diagnosed with GDM and the remaining 100 women were used as normal controls or non-GDM. The diagnosis of GDM was confirmed with biochemical analysis, i.e., GCT and oral glucose tolerance tests. Five milliliters of peripheral blood was collected and used for biochemical and molecular analyses. DNA was isolated, and genotyping for MTHFR (C677T) and FVL (G1691A) mutations was performed using PCR-RFLP. FVL (G1691A) locus was not polymorphic in the investigated sample. There was no significant difference in the allele and genotype frequencies of C677T polymorphism between GDM and non-GDM women (p = 0.8892).

Disruption of ldlr causes increased LDL-c and vascular lipid accumulation in a zebrafish model of hypercholesterolemia

Hyperlipidemia and arterial cholesterol accumulation are primary causes of cardiovascular events. Monogenic forms of hyperlipidemia and recent genome-wide association studies indicate that genetics plays an important role. Zebrafish are a useful model for studying the genetic susceptibility to hyperlipidemia owing to conservation of many components of lipoprotein metabolism, including those related to LDL, ease of genetic manipulation, and in vivo observation of lipid transport and vascular calcification. We sought to develop a genetic model for lipid metabolism in zebrafish, capitalizing on one well-understood player in LDL cholesterol (LDL-c) transport, the LDL receptor (ldlr), and an established in vivo model of hypercholesterolemia. We report that morpholinos targeted against the gene encoding ldlr effectively suppressed its expression in embryos during the first 8 days of development. The ldlr morphants exhibited increased LDL-c levels that were exacerbated by feeding a high cholesterol diet. Increased LDL-c was ameliorated in morphants upon treatment with atorvastatin. Furthermore, we observed significant vascular and liver lipid accumulation, vascular leakage, and plaque oxidation in ldlr-deficient embryos. Finally, upon transcript analysis of several cholesterol-regulating genes, we observed changes similar to those seen in mammalian systems, suggesting that cholesterol regulation may be conserved in zebrafish. Taken together, these observations indicate conservation of ldlr function in zebrafish and demonstrate the utility of transient gene knockdown in embryos as a genetic model for hyperlipidemia.

Phase transitions in the multi-cellular regulatory behavior of pancreatic islet excitability

The pancreatic islets of Langerhans are multicellular micro-organs integral to maintaining glucose homeostasis through secretion of the hormone insulin. β-cells within the islet exist as a highly coupled electrical network which coordinates electrical activity and insulin release at high glucose, but leads to global suppression at basal glucose. Despite its importance, how network dynamics generate this emergent binary on/off behavior remains to be elucidated. Previous work has suggested that a small threshold of quiescent cells is able to suppress the entire network. By modeling the islet as a Boolean network, we predicted a phase-transition between globally active and inactive states would emerge near this threshold number of cells, indicative of critical behavior. This was tested using islets with an inducible-expression mutation which renders defined numbers of cells electrically inactive, together with pharmacological modulation of electrical activity. This was combined with real-time imaging of intracellular free-calcium activity [Ca²⁺]_i and measurement of physiological parameters in mice. As the number of inexcitable cells was increased beyond ∼15%, a phase-transition in islet activity occurred, switching from globally active wild-type behavior to global quiescence. This phase-transition was also seen in insulin secretion and blood glucose, indicating physiological impact. This behavior was reproduced in a multicellular dynamical model suggesting critical behavior in the islet may obey general properties of coupled heterogeneous networks. This study represents the first detailed explanation for how the islet facilitates inhibitory activity in spite of a heterogeneous cell population, as well as the role this plays in diabetes and its reversal. We further explain how islets utilize this critical behavior to leverage cellular heterogeneity and coordinate a robust insulin response with high dynamic range. These findings also give new insight into emergent multicellular dynamics in general which are applicable to many coupled physiological systems, specifically where inhibitory dynamics result from coupled networks.

As science has successfully broken down the elements of many biological systems, the network dynamics of large-scale cellular interactions has emerged as a new frontier. One way to understand how dynamical elements within large networks behave collectively is via mathematical modeling. Diabetes, which is of increasing international concern, is commonly caused by a deterioration of these complex dynamics in a highly coupled micro-organ called the islet of Langerhans. Therefore, if we are to understand diabetes and how to treat it, we must understand how coupling affects ensemble dynamics. While the role of network connectivity in islet excitation under stimulatory conditions has been well studied, how connectivity also suppresses activity under fasting conditions remains to be elucidated. Here we use two network models of islet connectivity to investigate this process. Using genetically altered islets and pharmacological treatments, we show how suppression of islet activity is solely dependent on a threshold number of inactive cells. We found that the islet exhibits critical behavior in the threshold region, rapidly transitioning from global activity to inactivity. We therefore propose how the islet and multicellular systems in general can generate a robust stimulated response from a heterogeneous cell population.

Association study of the ABCC8 gene variants with type 2 diabetes in south Indians

BACKGROUND:

The ABCC8 gene which encodes the sulfonylurea receptor plays a major role in insulin secretion and is a potential candidate for type 2 diabetes. The -3c → t (rs1799854) and Thr759Thr (C → T, rs1801261) single nucleotide polymorphisms (SNPs) of the ABCC8 gene have been associated with type 2 diabetes in many populations. The present study was designed to investigate the association of these two SNPs in an Asian Indian population from south India.

MATERIALS AND METHODS:

A total of 1,300 subjects, 663 normal glucose tolerant (NGT) and 637 type 2 diabetic subjects were randomly selected from the Chennai Urban Rural Epidemiology Study (CURES). The -3c → t and Thr759Thr were genotyped in these subjects using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and a few variants were confirmed by direct sequencing.

RESULTS:

The frequency of the ‘t’ allele of the -3c → t SNP was found to be 0.27 in NGT and 0.29 in type 2 diabetic subjects (P = 0.44). There was no significant difference in the genotypic frequency between the NGT and type 2 diabetic group (P = 0.18). Neither the genotypic frequency nor the allele frequency of the Thr759Thr polymorphism was found to differ significantly between the NGT and type 2 diabetic groups.

CONCLUSION:

The -3c → t and the Thr759Thr polymorphisms of the ABCC8 gene were not associated with type 2 diabetes in this study. However, an effect of these genetic variants on specific unidentified sub groups of type 2 diabetes cannot be excluded.

Association of adiponectin (AdipoQ) and sulphonylurea receptor (ABCC8) gene polymorphisms with Type 2 Diabetes in North Indian population of Punjab

In Type 2 Diabetes (T2D), adiponectin (AdipoQ) and sulphonylurea receptor genes (ABCC8) are important targets for candidate gene association studies. The single nucleotide polymorphisms (SNPs) in these genes have been associated with features of the metabolic syndrome across various populations. The present case-control study undertaken in the population of Punjab, evaluates the association of +45T>G polymorphism in AdipoQ gene; and Exon16-3C>T as well as Exon18C>T polymorphisms in ABCC8 gene with T2D. These SNPs were genotyped in 200 T2D cases and 200 non-diabetic healthy controls using the PCR-RFLP method. The frequency of the minor G-allele for AdipoQ+45(T>G) polymorphism was significantly higher in T2D cases (29.0%) than in controls (21.5%) [P=0.02, OR=1.49 (1.07-2.04)]. The genetic model analysis revealed that the G-allele cumulatively provides nearly 1.59-1.78 fold increased risk to T2D under the additive (P=0.009; OR=1.59, 1.12-2.25 at 95% CI), dominant (TG/GG vs. TT) (P=0.034, OR=1.64, 1.04-2.56 at 95% CI) and codominant model (TG vs. TT/GG) (P=0.014; OR=1.78, 1.12-2.82 at 95% CI) after adjusting for confounding factors. However, no difference in the distribution of genotype and allele frequencies was observed for both the ABCC8 polymorphisms. The distribution of obesity profiles (BMI, WC and WHR) was also significantly different between cases and controls (P<0.05). Higher BMI and central obesity were observed to increase the risk of T2D. G-allele of +45(T>G) polymorphism in the adiponectin gene appears to be associated with increased risk of T2D, but the polymorphisms in sulphonylurea receptor gene do not seem to be associated with T2D in the population of Punjab.

Phytochemicals targeting genes relevant for type 2 diabetes

Nutrigenomic approaches based on ethnopharmacology and phytotherapy concepts have revealed that type 2 diabetes mellitus (T2DM) may be susceptible to dietary intervention. Interaction between bioactive food components and the genome may influence cell processes and modulate the onset and progression of the disease. T2DM, characterized by insulin resistance and beta cell dysfunction, is one of the leading causes of death and disability. Despite the great advances that have been made in the understanding and management of this complex, multifactorial disease, T2DM has become a worldwide epidemic in the 21st century. Population and family studies have revealed a strong genetic component of T2DM, and a number of candidate genes have been identified in humans. Variations in the gene sequences such as single nucleotide polymorphisms, explain the individual differences in traits like disease susceptibility and response to treatment. A clear understanding of how nutrients affect the expression of genes should facilitate the development of individualized intervention and, eventually, treatment strategies for T2DM. Review of the literature identified many phytochemicals/extracts from traditional medicinal plants that can target diabetogenic genes. This review focuses on the genetic aspects of T2DM, nutrient modification of genes relevant for diabetes, and future prospects of nutritional therapy of T2DM.

Individualized therapy for type 2 diabetes: clinical implications of pharmacogenetic data

Type 2 diabetes mellitus (T2DM) is characterized by insulin resistance, abnormally elevated hepatic glucose production, and reduced glucose-stimulated insulin secretion. Treatment with antihyperglycemic agents is initially successful in type 2 diabetes, but it is often associated with a high secondary failure rate, and the addition of insulin is eventually necessary for many patients, in order to restore acceptable glycemic control and to reduce the risk of development and progression of disease complications. Notably, even patients who appear to have similar requirements of antidiabetic regimens show great variability in drug disposition, glycemic response, tolerability, and incidence of adverse effects during treatment. Pharmacogenomics is a promising area of investigation and involves the search for genetic polymorphisms that may explain the interindividual variability in antidiabetic therapy response. The initial positive results portend that genomic efforts will be able to shed important light on variability in pharmacologic traits. In this review, we summarize the current understanding of genetic polymorphisms that may affect the responses of subjects with T2DM to antidiabetic treatment. These genes belong to three major classes: genes involved in drug metabolism and transporters that influence pharmacokinetics (including the cytochrome P450 [CYP] superfamily, the organic anion transporting polypeptide [OATP] family, and the polyspecific organic cation transporter [OCT] family); genes encoding drug targets and receptors (including peroxisome proliferator-activated receptor gamma [PPARG], the adenosine triphosphate [ATP]-sensitive potassium channel [K(ATP)], and incretin receptors); and genes involved in the causal pathway of T2DM that are able to modify the effects of drugs (including adipokines, transcription factor 7-like 2 (T cell specific, HMG-box) [TCF7L2], insulin receptor substrate 1 [IRS1], nitric oxide synthase 1 (neuronal) adaptor protein [NOS1AP], and solute carrier family 30 (zinc transporter), member 8 [SLC30A8]). In addition to these three major classes, we also review the available evidence on novel genes (CDK5 regulatory subunit associated protein 1-like 1 [CDKAL1], insulin-like growth factor 2 mRNA binding protein 2 [IGF2BP2], potassium voltage-gated channel, KQT-like subfamily, member 1 [KCNQ1], paired box 4 [PAX4] and neuronal differentiation 1 [NEUROD1] transcription factors, ataxia telangiectasia mutated [ATM], and serine racemase [SRR]) that have recently been proposed as possible modulators of therapeutic response in subjects with T2DM.

The common C49620T polymorphism in the sulfonylurea receptor gene SUR1 (ABCC8) in patients with gestational diabetes and subsequent glucose metabolism abnormalities

AIM

The aim of this study is to investigate the relationship between the common C49620T polymorphism in the sulfonylurea receptor (SUR1) gene and glucose metabolism, β-cell secretory function and insulin resistance in women with a history of gestational diabetes (GDM).

MATERIAL AND METHODS

Study group included 199 women, diagnosed GDM within the last 5-12 years and control group of comparable 50 women in whom GDM was excluded during pregnancy. Blood glucose and insulin levels were measured during oral glucose tolerance test. Indices of insulin resistance (HOMA-IR) and β-cell function (HOMA %B) were calculated. In all patients, the C49620T polymorphism in intron 15 of the SUR1 gene was determined.

RESULTS

The distribution of the studied polymorphism in the two groups did not differ from each other (χ(2) = 0.34, P = 0.8425). No association between the distribution of polymorphisms and coexisting glucose metabolism disorders (χ(2) = 7,13, P = 0, 3043) was found. No association was also observed between the polymorphism and HOMA %B or HOMA-IR.

CONCLUSIONS

The polymorphism C49620T in the SUR1 gene is not associated with insulin resistance and/or insulin secretion in women with a history of GDM and does not affect the development of GDM, or the development of glucose intolerance in the studied population.

ABCC8 polymorphisms are associated with triglyceride concentration in type 2 diabetics on sulfonylurea therapy

BACKGROUND

The failure of therapy with oral hypoglycemic drugs leads to not only poorly regulated glycemic status, but also dyslipidemia and increased body weight and body mass index (BMI). Sulfonylureas act as insulin secretagogues by binding to the sulfonylurea receptor (SUR-1) encoded by the gene ABCC8. The aim of this study was to explore whether there is an association of ABCC8 polymorphisms SUR1 exon 16 (-3C/T), SUR-1 exon 31 (Arg1273Arg), and SUR-1 exon 33 (S1369A) with lipid concentration and BMI in type 2 diabetics on sulfonylurea therapy.

MATERIALS AND METHODS

This study included 251 unrelated type 2 diabetics on sulfonylurea therapy. Height and weight were measured for BMI calculation. All polymorphisms were detected by polymerase chain reaction-restriction fragment length polymorphism methods. Lipid concentrations and BMI were measured at inclusion into the study and after 6 months of follow-up.

RESULTS

Wild-type allele carriers for the SUR-1 exon 31 polymorphism (Arg1273Arg) had a significantly higher triglyceride (TG) concentration when compared with the carriers of two variant alleles (p=0.023). Polymorphic allele carriers of the SUR-1 exon 16 (-3C/T) polymorphism were more frequent in the subgroup of patients with the TG concentration increase after 6 months (p for genotype and allelic differences: 0.024 and 0.015, respectively).

CONCLUSION

ABCC8 polymorphisms in exon 16 and 31 are associated with the TG concentration in type 2 diabetics on sulfonylurea therapy.

Metabolic control in type 2 diabetes is associated with sulfonylurea receptor-1 (SUR-1) but not with KCNJ11 polymorphisms

BACKGROUND AND AIMS

Sulfonylureas are hypoglycemic agents used for promotion of insulin secretion in type 2 diabetics (T2D). They bind to sulfonylurea receptor-1 (SUR-1), which is a functional subunit of the ATP-sensitive potassium channel (K(ATP)). The other component of the potassium channel is Kir6.2, encoded by gene KCNJ11. Polymorphisms in these genes may lead to modulated response to sulfonylurea therapy. The aim of this study was to determine a relationship between SUR-1 [exon 16 (-3C/T), exon 31 (Arg1273Arg; AGG-->AGA) and exon 33 (S1369A)] and KCNJ11 (E23K) polymorphisms and the following parameters of metabolic control in T2D: fasting plasma glucose (FPG), postprandial glucose (PPG) and HbA1c in Caucasian T2D of European origin.

METHODS

A total of 228 unrelated patients with T2D on sulfonylurea therapy were included in the study. Genotyping of all polymorphisms was performed by PCR-RFLP method. Biochemical parameters were determined using standard laboratory methods.

RESULTS

There was no difference in FPG and PPG concentration in any of the genotype subgroups. However, diabetics with wild-type C/C genotype of the SUR-1 exon 16 polymorphism had significantly lower HbA1c concentration compared to the patients with variant T/T genotype [6.9 (6.2-7.7) mmol/L vs. 8.1 (6.7-8.8) mmol/L; p=0.009]. Also, patients with wild-type G/G genotype of the SUR-1 exon 31 polymorphism had significantly higher HbA1c concentration compared to the patients with variant A/A genotype [7.8 (6.9-8.8) mmol/L vs. 6.3 (5.7-6.8) mmol/L; p<0.001].

CONCLUSIONS

SUR-1 exon 16 and exon 31 polymorphisms are significantly associated with HbA1c concentration.

Pharmacogenetics of glucose-lowering drug treatment: a systematic review

Intensive blood glucose lowering can significantly reduce the risk of micro- and macrovascular complications in patients with diabetes mellitus. However, 30% of all treated patients do not achieve optimal blood glucose levels. Genetic factors may influence the response to glucose-lowering medication. A search of MEDLINE-indexed literature published between January 1966 and July 2007 revealed 37 studies reporting data on genetic polymorphisms and response to glucose-lowering drugs. Most studies involving cytochrome P450 (CYP) genes had small sample sizes (21 studies <50 subjects) and were among healthy volunteers. Multiple studies indicated that the CYP2C9 *3 allele (Ile359Leu polymorphism) was associated with decreased clearance of sulfonylurea drugs. Supporting this, one study reported an increased insulin secretion in CYP2C9*3 allele carriers when using the sulfonylurea agent glyburide. The CYP2C9*3 allele was also associated with a decreased clearance of meglitinides, whereas the CYP2C8*3 (Arg139Lys; Lys399Arg) variant increased the clearance of meglitinides. Polymorphisms in genes encoding the inwardly rectifying potassium channel Kir6.2 (KCNJ11) and the insulin receptor substrate-1 (IRS1) were reported to be associated with an increased risk of (secondary) failure to respond to sulfonylurea therapy. A significant decrease in fasting plasma glucose and hemoglobin A(1c) (HbA(1c)) in response to rosiglitazone was seen in subjects carrying the Pro12Ala polymorphism of the peroxisome proliferator-activated receptor-gamma (PPARG) gene. Conversely, carriers of this polymorphism also had a higher conversion to diabetes mellitus when treated with acarbose; this effect was also seen in adiponectin (ADIPOQ) gene polymorphism carriers. Future studies with adequate sample sizes in which several SNPs in multiple candidate genes are genotyped in patients with diabetes should provide reliable information on genetic variants and response to glucose-lowering drugs.

Identifying susceptibility variants for type 2 diabetes

The etiology of type 2 diabetes (T2D) is complex and remains poorly understood. Differences in individual susceptibility to this condition reflect the action of multiple variants, each of which confers a modest effect, and their interactions with a variety of environmental exposures. Several complementary approaches to the identification of the etiological variants have been adopted, though, for all, association analyses provide the final common pathway. The genes and/or chromosomal regions studied have been selected on the basis of their presumed biological relevance to diabetes, known involvement in monogenic forms, or animal models of the condition and/or signals arising from whole-genome linkage scans. These association studies have featured a wide variety of designs and analytical approaches, but reliable biological insights have been few, largely because of difficulties in obtaining reproducible findings. However, in recent years, several examples of robustly replicated associations have emerged, largely as a result of an emphasis on the need for improved power and more appropriate analysis and interpretation. New strategies for the large-scale identification of T2D susceptibility variants are now becoming possible, including the prospect of genuine genome-wide association scans, but caution in their design, analysis, and interpretation remains essential.

Association studies of variants in the genes involved in pancreatic beta-cell function in type 2 diabetes in Japanese subjects

Because impaired insulin secretion is characteristic of type 2 diabetes in Asians, including Japanese, the genes involved in pancreatic beta-cell function are candidate susceptibility genes for type 2 diabetes. We examined the association of variants in genes encoding several transcription factors (TCF1, TCF2, HNF4A, ISL1, IPF1, NEUROG3, PAX6, NKX2-2, NKX6-1, and NEUROD1) and genes encoding the ATP-sensitive K(+) channel subunits Kir6.2 (KCNJ11) and SUR1 (ABCC8) with type 2 diabetes in a Japanese cohort of 2,834 subjects. The exon 16 -3c/t variant rs1799854 in ABCC8 showed a significant association (P = 0.0073), and variants in several genes showed nominally significant associations (P < 0.05) with type 2 diabetes. Although the E23K variant rs5219 in KCNJ11 showed no association with diabetes in Japanese (for the K allele, odds ratio [OR] 1.08 [95% CI 0.97-1.21], P = 0.15), 95% CI around the OR overlaps in meta-analysis of European populations, suggesting that our results are not inconsistent with the previous studies. This is the largest association study so far conducted on these genes in Japanese and provides valuable information for comparison with other ethnic groups.

The Genetic Basis of Type 2 Diabetes

Type 2 Diabetes results from a complex physiologic process that includes the pancreatic beta cells, peripheral glucose uptake in muscle, the secretion of multiple cytokines and hormone-like molecules from adipocytes, hepatic glucose production, and likely the central nervous system. Consistent with the complex web of physiologic defects, the emerging picture of the genetics will involve a large number of risk susceptibility genes, each individually with relatively small effect (odds ratios below 1.2 in most cases). The challenge for the future will include cataloging and confirming the genetic risk factors, and understanding how these risk factors interact with each other and with the known environmental and lifestyle risk factors that increase the propensity to type 2 diabetes.

Analysis of separate and combined effects of common variation in KCNJ11 and PPARG on risk of type 2 diabetes

The separate and combined effects of the PPARG Pro(12)Ala polymorphism and the KCNJ11 Glu(23)Lys polymorphisms on risk of type 2 diabetes were investigated in relatively large-scale, case-control studies. Separate effects of the variants were examined among 1187/1461 type 2 diabetic patients and 4791/4986 middle-aged, glucose-tolerant subjects. The combined analysis involved 1164 type 2 diabetic patients and 4733 middle-aged, glucose-tolerant subjects. In the separate analyses, the K allele of the KCNJ11 Glu(23)Lys associated with type 2 diabetes (odds ratio, 1.19; P = 0.0002), whereas the PPARG Pro(12)Ala showed no significant association with type 2 diabetes. The combined analysis indicated that the two polymorphisms acted in an additive manner to increase the risk of type 2 diabetes, and we found no evidence for a synergistic interaction between them. Analysis of a model with equal additive effects of the two variants showed that the odds ratio for type 2 diabetes increased with 1.14/risk allele (P = 0.003). Together, the two polymorphisms conferred a population-attributable risk for type 2 diabetes of 28%. In conclusion, our results showed no evidence of a synergistic interaction between the KCNJ11 Glu(23)Lys and PPARG Pro(12)Ala polymorphisms, but indicated that they may act in an additive manner to increase the risk of type 2 diabetes.

From congenital hyperinsulinism to diabetes mellitus: the role of pancreatic beta-cell KATP channels

Pancreatic beta-cell adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels play a pivotal role in linking glucose metabolism to regulated insulin secretion. K(ATP) channels are hetero-octameric complexes comprising two subunits Kir6.2 and sulfonylurea receptor 1 (SUR1). Changes in the intracellular concentration of nucleotides (ATP) cause alterations in the resting and opening state of the K(ATP) channels. Loss-of-function mutations in the genes encoding the two subunits of K(ATP) channels lead to the most common form of congenital hyperinsulinism (CHI). This causes persistent and severe hypoglycemia in the neonatal and infancy period. CHI can cause mental retardation and epilepsy if not treated properly. On the other hand, now there is evidence of an association between polymorphisms in the Kir6.2 gene and type 2 diabetes mellitus, mutations in the Kir6.2 gene and neonatal diabetes mellitus, and mutations in the SUR1 gene and diabetes mellitus. Interestingly, for reasons that are unclear at present, mice knockout models of K(ATP) channels are different from the human phenotype of CHI. This article is a review focusing on how abnormalities in the pancreatic beta-cell K(ATP) channels can lead to severe hypoglycemia on the one hand and diabetes mellitus on the other.

Genetics of Type 2 diabetes

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

Common variants in the ATP-sensitive K+ channel genes KCNJ11 (Kir6.2) and ABCC8 (SUR1) in relation to glucose intolerance: population-based studies and meta-analyses

AIMS

To evaluate the relation between common variants in the ATP-sensitive K+ channel genes and glucose intolerance.

METHODS

We conducted a meta-analysis of reported association studies in Caucasian populations for common variants in the ABCC8 (exons 16 and 18) and the KCNJ11 (E23K) gene and examined sources of heterogeneity in the results. The meta-analysis was based on 7768-10216 subjects (depending on the gene variant), and included two new population-based studies in the Netherlands with 725 cases and 742 controls.

RESULTS

For the KCNJ11 variant, the summary odds ratio (OR) for glucose intolerance was 1.12 (1.01-1.23, P=0.03) for the EK genotype and 1.44 (1.17-1.78, P=0.0007) for the KK genotype, as compared with the EE genotype. For the ABCC8 exon 16 variant, the OR was 1.06 (0.94-1.19, P=0.34) for ct and 0.93 (0.71-1.20, P=0.56) for tt, as compared with the cc genotype. For ABCC8 exon 18, the OR was 1.20 (0.97-1.49, P=0.10) for CT/TT, as compared with the CC genotype. Studies of the ABCC8 variants that were published first or had smaller sample sizes (for the exon 18 variant) showed stronger associations, which may indicate publication bias. For the ABCC8 exon 18 and the KCNJ11 variant, associations were stronger for studies of clinical diabetes than newly detected glucose intolerance. The population attributable risk for clinical Type 2 diabetes was 6.2% for the KCNJ11 KK genotype and 10.1% for the KCNJ11 EK and KK genotype combined.

CONCLUSIONS

The common KCNJ11 E23K gene variant, but not the ABCC8 exon 16 or exon 18 variant, was consistently associated with Type 2 diabetes.

Genetics of type 2 diabetes mellitus: status and perspectives

Throughout the last decade, molecular genetic studies of non-autoimmune diabetes mellitus have contributed significantly to our present understanding of this disease's complex aetiopathogenesis. Monogenic forms of diabetes (maturity-onset diabetes of the young, MODY) have been identified and classified into MODY1-6 according to the mutated genes that by being expressed in the pancreatic beta-cells confirm at the molecular level the clinical presentation of MODY as a predominantly insulin secretory deficient form of diabetes mellitus. Genomewide linkage studies of presumed polygenic type 2 diabetic populations indicate that loci on chromosomes 1q, 5q, 8p, 10q, 12q and 20q contain susceptibility genes. Yet, so far, the only susceptibility gene, calpain-10 (CAPN10), which has been identified using genomewide linkage studies, is located on chromosome 2q37. Mutation analyses of selected 'candidate' susceptibility genes in various populations have also identified the widespread Pro12Ala variant of the peroxisome proliferator-activated receptor-gamma and the common Glu23Lys variant of the ATP-sensitive potassium channel, Kir6.2 (KCNJ11). These variants may contribute significantly to the risk type 2 diabetes conferring insulin resistance of liver, muscle and fat (Pro12Ala) and a relative insulin secretory deficiency (Glu23Lys). It is likely that, in the near future, the recent more detailed knowledge of the human genome and insights into its haploblocks together with the developments of high-throughput and cheap genotyping will facilitate the discovery of many more type 2 diabetes gene variants in study materials, which are statistically powered and phenotypically well characterized. The results of these efforts are likely to be the platform for major progress in the development of personalized antidiabetic drugs with higher efficacy and few side effects.

Current status of the E23K Kir6.2 polymorphism: implications for type-2 diabetes

The ATP-sensitive potassium (KATP) channel couples membrane excitability to cellular metabolism and is a critical mediator in the process of glucose-stimulated insulin secretion. Increasing numbers of KATP channel polymorphisms are being described and linked to altered insulin secretion indicating that genes encoding this ion channel could be susceptibility markers for type-2 diabetes. Genetic variation of KATP channels may result in altered beta-cell electrical activity, glucose homeostasis, and increased susceptibility to type-2 diabetes. Of particular interest is the Kir6.2 E23K polymorphism, which is linked to increased susceptibility to type-2 diabetes in Caucasian populations and may also be associated with weight gain and obesity, both of which are major diabetes risk factors. This association highlights the potential contribution of both genetic and environmental factors to the development and progression of type-2 diabetes. In addition, the common occurrence of the E23K polymorphism in Caucasian populations may have conferred an evolutionary advantage to our ancestors. This review will summarize the current status of the association of KATP channel polymorphisms with type-2 diabetes, focusing on the possible mechanisms by which these polymorphisms alter glucose homeostasis and offering insights into possible evolutionary pressures that may have contributed to the high prevalence of KATP channel polymorphisms in the Caucasian population.

Haplotype structure and genotype-phenotype correlations of the sulfonylurea receptor and the islet ATP-sensitive potassium channel gene region

The genes for the sulfonylurea receptor (SUR1; encoded by ABCC8) and its associated islet ATP-sensitive potassium channel (Kir6.2; encoded by KCNJ11) are adjacent to one another on human chromosome 11. Multiple studies have reported association of the E23K variant of Kir6.2 with risk of type 2 diabetes. Whether and how E23K itself-or other variant(s) in either of these two closely linked genes-influences type 2 diabetes remains to be fully determined. To better understand genotype-phenotype correlation at this important candidate gene locus, we 1) characterized haplotype structures across the gene region by typing 77 working, high-frequency markers spanning 207 kb and both genes; 2) performed association studies of E23K and nearby markers in >3,400 patients (type 2 diabetes and control) not previously reported in the literature; and 3) analyzed the resulting data for measures of insulin secretion. These data independently replicate the association of E23K with type 2 diabetes with an odds ratio (OR) in the new data of 1.17 (P = 0.003) as compared with an OR of 1.14 provided by meta-analysis of previously published, nonoverlapping data (P = 0.0002). We find that the E23K variant in Kir6.2 demonstrates very strong allelic association with a coding variant (A1369S) in the neighboring SUR1 gene (r(2) > 0.9) across a range of population samples, making it difficult to distinguish which gene and polymorphism in this region are most likely responsible for the reported association. We show that E23K is also associated with decreased insulin secretion in glucose-tolerant control subjects, supporting a mechanism whereby beta-cell dysfunction contributes to the common form of type 2 diabetes. Like peroxisome proliferator-activated receptor gamma, the SUR1/Kir6.2 gene region both contributes to the inherited risk of type 2 diabetes and encodes proteins that are targets for hypoglycemic medications, providing an intriguing link between the underlying mechanism of disease and validated targets for pharmacological treatment.

The inherited basis of diabetes mellitus: implications for the genetic analysis of complex traits

Diabetes encompasses a heterogeneous group of diseases, each with a substantial genetic component. We review the division of diabetes into different subtypes based on clinical phenotype, the fruitful pursuit of genes underlying monogenic forms of the disease, the successes and drawbacks of whole-genome linkage scans in type 1 and type 2 diabetes, and the recent identification of several diabetes genes by large association studies. We use the lessons learned from this extensive body of evidence to illustrate general implications for the genetic analysis of complex traits.

Candidate gene association study in type 2 diabetes indicates a role for genes involved in beta-cell function as well as insulin action

Type 2 diabetes is an increasingly common, serious metabolic disorder with a substantial inherited component. It is characterised by defects in both insulin secretion and action. Progress in identification of specific genetic variants predisposing to the disease has been limited. To complement ongoing positional cloning efforts, we have undertaken a large-scale candidate gene association study. We examined 152 SNPs in 71 candidate genes for association with diabetes status and related phenotypes in 2,134 Caucasians in a case-control study and an independent quantitative trait (QT) cohort in the United Kingdom. Polymorphisms in five of 15 genes (33%) encoding molecules known to primarily influence pancreatic beta-cell function-ABCC8 (sulphonylurea receptor), KCNJ11 (KIR6.2), SLC2A2 (GLUT2), HNF4A (HNF4alpha), and INS (insulin)-significantly altered disease risk, and in three genes, the risk allele, haplotype, or both had a biologically consistent effect on a relevant physiological trait in the QT study. We examined 35 genes predicted to have their major influence on insulin action, and three (9%)-INSR, PIK3R1, and SOS1-showed significant associations with diabetes. These results confirm the genetic complexity of Type 2 diabetes and provide evidence that common variants in genes influencing pancreatic beta-cell function may make a significant contribution to the inherited component of this disease. This study additionally demonstrates that the systematic examination of panels of biological candidate genes in large, well-characterised populations can be an effective complement to positional cloning approaches. The absence of large single-gene effects and the detection of multiple small effects accentuate the need for the study of larger populations in order to reliably identify the size of effect we now expect for complex diseases.

Large-scale association studies of variants in genes encoding the pancreatic beta-cell KATP channel subunits Kir6.2 (KCNJ11) and SUR1 (ABCC8) confirm that the KCNJ11 E23K variant is associated with type 2 diabetes

The genes ABCC8 and KCNJ11, which encode the subunits sulfonylurea receptor 1 (SUR1) and inwardly rectifying potassium channel (Kir6.2) of the beta-cell ATP-sensitive potassium (K(ATP)) channel, control insulin secretion. Common polymorphisms in these genes (ABCC8 exon 16-3t/c, exon 18 T/C, KCNJ11 E23K) have been variably associated with type 2 diabetes, but no large ( approximately 2,000 subjects) case-control studies have been performed. We evaluated the role of these three variants by studying 2,486 U.K. subjects: 854 with type 2 diabetes, 1,182 population control subjects, and 150 parent-offspring type 2 diabetic trios. The E23K allele was associated with diabetes in the case-control study (odds ratio [OR] 1.18 [95% CI 1.04-1.34], P = 0.01) but did not show familial association with diabetes. Neither the exon 16 nor the exon 18 ABCC8 variants were associated with diabetes (1.04 [0.91-1.18], P = 0.57; 0.93 [0.71-1.23], P = 0.63, respectively). Meta-analysis of all case-control data showed that the E23K allele was associated with type 2 diabetes (K allele OR 1.23 [1.12-1.36], P = 0.000015; KK genotype 1.65 [1.34-2.02], P = 0.000002); but the ABCC8 variants were not associated. Our results confirm that E23K increases risk of type 2 diabetes and show that large-scale association studies are important for the identification of diabetes susceptibility alleles.

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.

A new subtype of autosomal dominant diabetes attributable to a mutation in the gene for sulfonylurea receptor 1

BACKGROUND

ATP-sensitive potassium (KATP) channels are major regulators of glucose-induced insulin secretion in pancreatic beta cells. We have described a dominant heterozygous mutation--E1506K--in the sulfonylurea receptor 1 (SUR1) gene (ABCC8) in a Finnish family, which leads to congenital hyperinsulinaemia due to reduction of K(ATP)-channel activity. We aimed to characterise glucose metabolism in adults heterozygous for the E1506K mutation.

METHODS

Glucose tolerance was assessed by an oral glucose tolerance test, insulin secretion by the intravenous glucose tolerance test and hyperglycaemic clamp, and insulin sensitivity by hyperinsulinaemic euglycaemic clamp in 11 people heterozygous for the E1506K mutation and 19 controls.

FINDINGS

Four people who were heterozygous for the SUR1 E1506K mutation had diabetes, five had impaired glucose tolerance, one had impaired fasting glucose, and one had normal glucose tolerance. Although glucose-induced, first-phase insulin secretion was normal in children younger than 10 years of age who were heterozygous for the SUR1 E1506K mutation (n=2; 66 and 334 pmol/L), it fell rapidly after puberty (n=3; 12-32 pmol/L), and was almost completely lost in adulthood (n=11; 12-32 pmol/L). Furthermore, these heterozygous people had a substantial reduction in maximum glucose-stimulated insulin secretion during hyperglycemic clamp (carriers without diabetes 422 pmol/L; carriers with diabetes 97 pmol/L). By contrast, insulin sensitivity (M/I value) was normal in carriers of the E1506K mutation who did not have diabetes and was reduced by 15% in those who were heterozygous with diabetes (0.07 in those without diabetes and 0.05 in those with the disorder; not significantly different from controls).

INTERPRETATION

Heterozygous E1506K substitution in the SUR1 gene causes congenital hyperinsulinism in infancy, loss of insulin secretory capacity in early adulthood, and diabetes in middle-age. This variant represents a new subtype of autosomal dominant diabetes.

Variations in insulin secretion in carriers of the E23K variant in the KIR6.2 subunit of the ATP-sensitive K(+) channel in the beta-cell

An association between type 2 diabetes and genetic variation in the KIR6.2 gene has been reported in several populations. Based on in vitro studies with cell lines expressing the Glu(23)Lys (E23K) mutation, it was recently suggested that this mutation might result in altered insulin secretion. We have examined glucose-stimulated insulin secretion in relation to this KIR6.2 gene variant in two independent Dutch cohorts. Subjects with normal (n = 65) or impaired (n = 94) glucose tolerance underwent 3-h hyperglycemic clamps at 10 mmol/l glucose. We did not observe significant differences in first- or second-phase insulin secretion between carriers and noncarriers of the gene variant in either of the study populations (all P > 0.45). Furthermore, we found no evidence for a significant interaction with disease-associated gene variants in the sulfonylurea receptor (SUR1) gene. We conclude that the E23K mutation in the KIR6.2 gene is not associated with detectable alterations in glucose-stimulated insulin secretion in two independent populations from the Netherlands.

Diabetes and insulin secretion: whither KATP?

The critical involvement of ATP-sensitive potassium (KATP) channels in insulin secretion is confirmed both by the demonstration that mutations that reduce KATP channel activity underlie many if not most cases of persistent hyperinsulinemia, and by the ability of sulfonylureas, which inhibit KATP channels, to enhance insulin secretion in type II diabetics. By extrapolation, we contend that mutations that increase beta-cell KATP channel activity should inhibit glucose-dependent insulin secretion and underlie, or at least predispose to, a diabetic phenotype. In transgenic animal models, this prediction seems to be borne out. Although earlier genetic studies failed to demonstrate a linkage between KATP mutations and diabetes in humans, recent studies indicate significant association of KATP channel gene mutations or polymorphisms and type II diabetes. We suggest that further efforts to understand the involvement of KATP channels in diabetes are warranted.

Genetics of type 2 diabetes and insulin resistance: knowledge from human studies

Both type 2 diabetes mellitus (T2DM) and insulin resistance are complex traits in which multiple gene effects and metabolic and environmental factors combine to contribute to the overall pathogenesis of these conditions. This complexity has complicated the search for susceptibility genes and has led to different but complementary approaches being used for the detection of gene effects. These include the study of monogenic cases of insulin resistance and T2DM, association studies of candidate genes and genome-wide scans. The peroxisome proliferator-activated receptor gamma (PPARgamma) and calpain-10 (CAPN10) genes have recently been identified as T2DM susceptibility genes, and the lessons learnt from these studies are helping to shape future strategies to search for additional susceptibility genes in T2DM and insulin resistance.

Genetically defined pancreatic beta cell failure

The major subtypes of diabetes mellitus (DM) are complex diseases in which the interactions between genetic susceptibility and metabolic, immunologic and environmental factors ultimately lead to the clinical phenotype. The unraveling of these factors and the study of their contribution to the pathogenesis of DM is a process of unimaginable complexity. As a result, despite the individual differences in DM phenotype, most patients with DM receive treatment according to generalized treatment protocols, only discriminating between Type 1 and Type 2 DM and not related to the individual genetic background of the patients. Nevertheless, in recent years, much progress has been made in the search for genetic factors in DM. As an example, the recognition of monogenetic defects in beta cell function has lead to the definition of novel DM subtypes, which have important implications for the individual therapeutic approach for these patients and the understanding of the etiology of DM.

Bases Genéticas do Diabetes Mellitus Tipo 2

A patogênese do diabetes mellitus tipo 2 (DM2) é complexa, associando fatores genéticos e fatores ambientais. A hiperglicemia é secundária à combinação de defeitos tanto na sensibilidade à insulina quanto na disfunção das células beta-pancreáticas. Vários estudos estabeleceram claramente a importância dos fatores genéticos na predisposição ao DM2. No momento, conhecemos alguns genes implicados em formas monogênicas de diabetes (MODY, diabetes mitocondrial). No entanto, nas formas mais comuns da doença de caráter poligênico, conhecemos apenas poucos genes que são associados à doença de uma forma reprodutível nos diferentes grupos populacionais estudados. Cada um destes poligenes apresenta um papel isolado muito pequeno, atuando na modulação de fenótipos associados ao diabetes. Nestas formas tardias poligênicas de DM2 é evidente a importância dos fatores ambientais que modulam a expressão clínica da doença. Nesta revisão abordamos os avanços mais relevantes das bases genéticas do DM2.

Perspective: the search for genes for type 2 diabetes in the post-genome era

Twin and family studies have demonstrated a strong genetic component to type 2 diabetes mellitus (T2DM), but mapping the susceptibility genes that account for this risk has proved difficult. At least seven single gene defects are known to cause T2DM, often with early onset and insulin deficiency, but these causes account for 5% or less of all T2DM. A large number of candidate genes have been evaluated for typical T2DM, but few have been confirmed in multiple studies, and among these, the effect on individual risk is modest. A large number of genome-wide scans have been published in the last few years, and at least four regions show evidence in multiple studies. However, only NIDDM1 has been mapped to a single gene, and that gene (calpain 10) appears to have a major role only in selected populations. Work is ongoing in many laboratories and multiple populations to map additional regions, but T2DM and other complex diseases have proved recalcitrant to current methodology. In addition to the ongoing progress in completing the genome sequence and in developing a comprehensive map of single nucleotide polymorphisms, new statistical models will be needed to incorporate the multiple loci with modest effect and the known environmental interactions that characterize the susceptibility to T2DM.

The genetics of type 2 diabetes

Type 2 diabetes mellitus is not a single disease but a genetically heterogeneous group of metabolic disorders sharing glucose intolerance. The precise underlying biochemical defects are unknown and almost certainly include impairments of both insulin secretion and action. The rapidly increasing prevalence of T2D world wide makes it a major cause of morbidity and mortality. Understanding the genetic aetiology of T2D will facilitate its diagnosis, treatment and prevention. The results of linkage and association studies to date demonstrate that, as with other common diseases, multiple genes are involved in the susceptibility to T2D, each making a modest contribution to the overall risk. The completion of the draft human genome sequence and a brace of novel tools for genomic analysis promise to accelerate progress towards a more complete molecular description of T2D.

Impact of sulfonylurea receptor 1 genetic variability on non-insulin-dependent diabetes mellitus prevalence and treatment: a population study

The high affinity sulfonylurea receptor 1 (SUR1) is involved in the metabolism of glucose in pancreatic beta-cells. We investigated the impact of the SUR1 intron 16-3t-->c polymorphism on non-insulin-dependent diabetes mellitus (NIDDM) prevalence in a large representative sample of French men and women, 35-64 years old, and explored potential relationships between the SUR1 intron 16 -t-->c polymorphism and sulfonylurea therapy efficiency. This study took place in Lille (northern), Strasbourg (eastern), and Toulouse (southern France). One hundred and twenty-two subjects with NIDDM were registered. We stratified NIDDM subjects according to their medical treatment: sulfonylureas (n = 70) versus other treatments (n = 50). From the three populations, a control group was selected (n = 1,250). Subjects carrying the cc intron 16 genotype had an increased risk of NIDDM [odds ratio (OR) = 1.76, 95% confidence interval (CI) 1.10-2.80; P = 0.017]. Subjects bearing at least one -3c allele and treated with sulfonylurea agents had fasting plasma triglyceride concentrations 35% lower than subjects that were tt homozygous (P = 0.026), whereas no difference could be detected between genotypes in NIDDM subjects treated with other treatments. The SUR1 intron 16 -3t-->c polymorphism was associated with an increased susceptibility to NIDDM in this population study, and seems to modulate the sulfonylurea therapy efficiency on hypertriglyceridemia reduction. This observation may help to better target the various therapies available for treatment of NIDDM.

Decreased fasting and oral glucose stimulated C-peptide in nondiabetic subjects with sequence variants in the sulfonylurea receptor 1 gene

The high-affinity sulfonylurea receptor 1 (SUR1) plays an important role in regulating insulin secretion. In the Québec Family Study, we genotyped 731 individuals (685 nondiabetic [ND] subjects) for the SUR1 gene IVS15-3c-->t and exon 18 Thr759(ACC-->ACT) polymorphisms using polymerase chain reaction-restriction fragment-length polymorphism analysis. Phenotypes measured were fasting plasma glucose (GLU), fasting plasma insulin (INS), and fasting C-peptide (CPEP), as well as oral glucose tolerance test (OGTT) responses; they were adjusted for age, sex, waist circumference, and the sum of six skinfold thicknesses. In ND subjects, exon 18 Thr759(ACC-->ACT) T allele carriers (T+) had lower CPEP (P = 0.022, -12.8%) and acute C-peptide responses (area above basal in first 30 min [CP30]) (P = 0.051, -12.4%) than noncarriers (T-). Also, in those with the cT/tC haplotype (from both IVS15-3c-->t and exon 18 Thr759[ACC-->ACT] polymorphisms), CPEP (P = 0.005, -21.2%), CP30 (P = 0.034, -19.2%), and total C-peptide responses (P = 0.016, -20.2%) were lower than that in cT- subjects. In overweight individuals (BMI >25 kg/m2), differences between carriers and noncarriers of the T or cT alleles were greater for GLU (P = 0.023-0.034), CPEP (P = 0.021-0.015), acute OGTT insulin response (P = 0.014-0.019), and CP30 (P = 0.034-0.019). These results suggest that the T and cT allele variants are associated with lower insulin secretion parameters, particularly in female and overweight subjects, adding evidence to the role of SUR1 sequence variants in decreased insulin secretion.

Role of common sequence variants in insulin secretion in familial type 2 diabetic kindreds: the sulfonylurea receptor, glucokinase, and hepatocyte nuclear factor 1alpha genes

OBJECTIVE

We have demonstrated high heritability of insulin secretion measured as acute insulin response to glucose times insulin sensitivity (disposition index). Furthermore, we showed that obese normoglycemic family members of a type 2 diabetic proband failed to compensate for the insulin resistance of obesity by increasing insulin secretion. In this study, we tested the primary hypotheses that previously described variants in the pancreatic sulfonylurea receptor gene (SUR1 or ABCC8), glucokinase (GCK) gene, or hepatocyte nuclear factor 1alpha (TCF1 or HNF1alpha) gene contribute to the inherited deficiencies of insulin secretion and beta-cell compensation to insulin resistance, as well as the secondary hypotheses that these variants altered insulin sensitivity.

RESEARCH DESIGN AND METHODS

We typed 124 nondiabetic members of 26 familial type 2 diabetic kindreds who had undergone tolbutamide-modified intravenous glucose tolerance tests for two variants of the ABCC8 (sulfonylurea) gene, two variants of the GCK gene, and one common amino acid variant in the TCF1 (HNF1alpha) gene. All family members were classified as normal or having impaired glucose tolerance based on oral glucose tolerance testing. We used minimal model analysis to calculate the insulin sensitivity index (S1) and glucose effectiveness (SG), and acute insulin response to glucose was calculated as the mean insulin excursion above baseline during the first 10 min after the glucose bolus. Disposition index (DI), a measure of beta-cell compensation for insulin sensitivity, was calculated as insulin sensitivity times acute insulin response. Effects of polymorphisms were determined using mixed effects models that incorporated family membership and by a likelihood analysis that accounted for family structure through polygenic inheritance.

RESULTS

An intronic variant of the ABCC8 gene just upstream of exon 16 was a significant determinant of both DI and an analogous index based on acute insulin response to tolbutamide. Surprisingly, heterozygous individuals showed the lowest indexes, whereas the DI in the two homozygous states did not differ significantly. Neither the exon 18 variant nor the variants in the GCK and TCF1 genes were significant in this model. However, combined genotypes of ABCC8 exon 16 and 18 variants again significantly predicted both indexes of glucose and tolbutamide-stimulated insulin secretion. Unexpectedly, a variant in the 3' untranslated region of the GCK gene interacted significantly with BMI to predict insulin sensitivity.

CONCLUSIONS

The exon 16 variant of the ABCC8 gene reduced beta-cell compensation to the decreased insulin sensitivity in the heterozygous state. This may explain the observation from several groups of an association of the ABCC8 variants in diabetes and is consistent with other studies showing a role of ABCC8 variants in pancreatic beta-cell function. However, our study focused on individuals from relatively few families. Ascertainment bias, family structure, and other interacting genes might have influenced our unexpected result. Additional studies are needed to replicate our observed deficit in beta-cell compensation in individuals heterozygous for ABCC8 variants. Likewise, the role of the GCK 3' variant in the reduced insulin sensitivity of obesity will require further study.

Association studies of variants in promoter and coding regions of beta-cell ATP-sensitive K-channel genes SUR1 and Kir6.2 with Type 2 diabetes mellitus (UKPDS 53)

AIMS

The beta-cell ATP-sensitive potassium channel consists of two subunits, SUR1 and Kir6.2. Population association studies have shown that three variants in SUR1 and one in Kir6.2 are associated with Type 2 diabetes. These polymorphisms do not result in a functional change or affect splicing, suggesting that they could be in linkage disequilibrium with a pathogenic mutation. The present study aimed firstly to screen the promoter regions of SUR1 and Kir6.2 to determine whether mutations in linkage disequilibrium with the silent variants lie in regulatory regions, which might lead to changes in gene expression. Secondly, novel and previously described variants associated with Type 2 diabetes (SUR1 exon 16-3t, exon 18 T, and Kir6.2 E23K) were investigated in the UKPDS cohort.

METHODS

The promoter sequences of both genes were screened by single-stranded conformational polymorphism analysis for variants associated with Type 2 diabetes. The previously reported variants were evaluated in 364 Type 2 diabetic and 328 normoglycaemic control subjects.

RESULTS

Two variants were detected in the SUR1 promoter, a three base insertion (caa) at -522 bp and a single base substitution at - 679 bp (c-->g). Neither of the variants were associated with diabetes, nor were they in a sequence consensus region for transcription factors. No association with diabetes was observed for either SUR1 variant. However, in contrast, analysis of the Kir6.2 E23K variant showed that the KK homozygosity was more frequent in Type 2 diabetic than control subjects. Variants were not associated with clinical characteristics nor did they affect response to sulphonylurea therapy

CONCLUSION

There is no support at present for mutations in either Kir6.2 or SUR1 promoter sequences contributing to Type 2 diabetes. However, the minimal promoter region of SUR1 has yet to be investigated. The E23K variant of Kir6.2 is associated with Type 2 diabetes mellitus in the UKPDS cohort.