Coexpression of the type 2 diabetes susceptibility gene variants KCNJ11 E23K and ABCC8 S1369A alter the ATP and sulfonylurea sensitivities of the ATP-sensitive K(+) channel

The genetics of type 2 diabetes and its clinical relevance

The increasing worldwide prevalence of type 2 diabetes (T2D) motivates efforts to use genetics to define key pathways involved in disease predisposition, and thereby to improve management of the disease. Research over the past 5 years has taken the total number of genetic loci implicated in T2D susceptibility beyond 60, and the emphasis is now shifting to the translation of these genetic insights into clinical value. Clinical translation may flow from the identification of novel therapeutic targets, but opportunities also exist with respect to individual prediction, diagnostic biomarkers and therapeutic optimization. To date, the main clinical impact has been seen for relatively rare, monogenic forms of diabetes rather than common T2D. However, the advent of high throughput sequencing approaches may herald discovery of rare and low frequency variants that offer greater translational potential.

Monogenic models: what have the single gene disorders taught us?

Monogenic diabetes constitutes a heterogeneous group of single gene disorders. The molecular background and clinical picture of many of these diseases have been described. While each of these forms is much less prevalent than multifactorial type 1 and type 2 diabetes mellitus (T2DM), together they affect millions of patients worldwide. Genetic diagnosis, which has become widely available, is of great clinical importance for patients with single gene diabetes. It helps to fully understand the pathophysiology of the disease, tailor the optimal hypoglycemic treatment, and define the prognosis for the entire family. Monogenic diabetes forms can be divided into 2 large groups, resulting from impaired insulin secretion or from an abnormal response to insulin. There are several lessons we have been taught by single-gene diabetes. We learned that the gene responsible for the occurrence of diabetes can be identified if an appropriate search strategy is used. In addition, discoveries of genes responsible for monogenic disorders pointed to them as susceptibility candidates for T2DM. Moreover, establishing that some families of proteins or biological pathways, such as transcription factors or potassium channel subunits, are involved in monogenic diabetes sparked research on their involvement in multifactorial diabetes. Finally, the example of single gene diabetes, particularly HNF1A MODY and permanent neonatal diabetes associated with the KCNJ11 and ABCC8 genes, all efficiently controlled on sulfonylurea, inspires us to continue the efforts to tailor individual treatment for T2DM patients. In this review paper, we summarize the impact of single gene disease discoveries on diabetes research and clinical practice.

Interactions between genetic background, insulin resistance and β-cell function

An interaction between genes and the environment is a critical component underlying the pathogenesis of the hyperglycaemia of type 2 diabetes. The development of more sophisticated techniques for studying gene variants and for analysing genetic data has led to the discovery of some 40 genes associated with type 2 diabetes. Most of these genes are related to changes in β-cell function, with a few associated with decreased insulin sensitivity and obesity. Interestingly, using quantitative traits based on continuous measures rather than dichotomous ones, it has become evident that not all genes associated with changes in fasting or post-prandial glucose are also associated with a diagnosis of type 2 diabetes. Identification of these gene variants has provided novel insights into the physiology and pathophysiology of the β-cell, including the identification of molecules involved in β-cell function that were not previously recognized as playing a role in this critical cell.

Genetic variants in potassium channels are associated with type 2 diabetes in a Mongolian population

BACKGROUND

Recent genome-wide association studies (GWAS) have identified more than 40 common sequence variants associated with type 2 diabetes (T2D). However, the results are not always the same in populations with differing genetic backgrounds. In the present study, we evaluated a hypothesis that a North Asian population living in a geographic area with unusually harsh environmental conditions would develop unique genetic risks.

METHODS

A population-based association study was performed with 21 single-nucleotide polymorphisms (SNPs) in nine genes selected according to the results of GWAS conducted in other populations. The study participants included 393 full-heritage Mongolian individuals (177 diagnosed with T2D and 216 matched controls). Genotyping was performed by TaqMan methodology.

RESULTS

The strongest association was detected with SNPs located within the potassium channel-coding genes KCNQ1 (highest odds ratio [OR] = 1.92; P = 3.4 × 10(-5) ) and ABCC8 (OR = 1.79; P = 5 × 10(-4) ). Genetic variants identified as strongly influencing the risk of T2D in other populations (e.g. KCNJ11 or TCF7L2) did not show significant association in Mongolia.

CONCLUSIONS

  The strongest T2D risk-associated SNPs in Mongolians are located within two of three tested potassium channel-coding genes. Accumulated variations in these genes may be related to the exposure to harsh environmental conditions.

Pharmacogenomic analysis of ATP-sensitive potassium channels coexpressing the common type 2 diabetes risk variants E23K and S1369A

OBJECTIVES

The common ATP-sensitive potassium (KATP) channel variants E23K and S1369A, found in the KCNJ11 and ABCC8 genes, respectively, form a haplotype that is associated with an increased risk for type 2 diabetes. Our previous studies showed that KATP channel inhibition by the A-site sulfonylurea gliclazide was increased in the K23/A1369 haplotype. Therefore, we studied the pharmacogenomics of seven clinically used sulfonylureas and glinides to determine their structure-activity relationships in KATP channels containing either the E23/S1369 nonrisk or K23/A1369 risk haplotypes.

RESEARCH DESIGN AND METHODS

The patch-clamp technique was used to determine sulfonylurea and glinide inhibition of recombinant human KATP channels containing either the E23/S1369 or the K23/A1369 haplotype.

RESULTS

KATP channels containing the K23/A1369 risk haplotype were significantly less sensitive to inhibition by tolbutamide, chlorpropamide, and glimepiride (IC50 values for K23/A1369 vs. E23/S1369=1.15 vs. 0.71 μmol/l; 4.19 vs. 3.04 μmol/l; 4.38 vs. 2.41 nmol/l, respectively). In contrast, KATP channels containing the K23/A1369 haplotype were significantly more sensitive to inhibition by mitiglinide (IC50=9.73 vs. 28.19 nmol/l for K23/A1369 vs. E23/S1369) and gliclazide. Nateglinide, glipizide, and glibenclamide showed similar inhibitory profiles in KATP channels containing either haplotype.

CONCLUSION

Our results demonstrate that the ring-fused pyrrole moiety in several A-site drugs likely underlies the observed inhibitory potency of these drugs on KATP channels containing the K23/A1369 risk haplotype. It may therefore be possible to tailor existing therapy or design novel drugs that display an increased efficacy in type 2 diabetes patients homozygous for these common KATP channel haplotypes.

Reduced insulin exocytosis in human pancreatic β-cells with gene variants linked to type 2 diabetes

The majority of genetic risk variants for type 2 diabetes (T2D) affect insulin secretion, but the mechanisms through which they influence pancreatic islet function remain largely unknown. We functionally characterized human islets to determine secretory, biophysical, and ultrastructural features in relation to genetic risk profiles in diabetic and nondiabetic donors. Islets from donors with T2D exhibited impaired insulin secretion, which was more pronounced in lean than obese diabetic donors. We assessed the impact of 14 disease susceptibility variants on measures of glucose sensing, exocytosis, and structure. Variants near TCF7L2 and ADRA2A were associated with reduced glucose-induced insulin secretion, whereas susceptibility variants near ADRA2A, KCNJ11, KCNQ1, and TCF7L2 were associated with reduced depolarization-evoked insulin exocytosis. KCNQ1, ADRA2A, KCNJ11, HHEX/IDE, and SLC2A2 variants affected granule docking. We combined our results to create a novel genetic risk score for β-cell dysfunction that includes aberrant granule docking, decreased Ca(2+) sensitivity of exocytosis, and reduced insulin release. Individuals with a high risk score displayed an impaired response to intravenous glucose and deteriorating insulin secretion over time. Our results underscore the importance of defects in β-cell exocytosis in T2D and demonstrate the potential of cellular phenotypic characterization in the elucidation of complex genetic disorders.

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.

KCNJ11 gene E23K variant and therapeutic response to sulfonylureas

AIMS

Potassium inwardly rectifier 6.2 subunit (Kir6.2) of the ATP-sensitive potassium (K(ATP)) channel encoded by KCNJ11 gene is a therapeutical target for sulfonylureas. KCNJ11 E23K polymorphism was associated with type 2 diabetes in genetic association studies. The aim of the present pharmacogenetic study was to examine the effect of sulfonylurea treatment on glycemic control in relationship to KCNJ11 E23K variant.

PATIENTS AND METHODS

One hundred and one patients with type 2 diabetes who failed to achieve HbA1c<7% on previous metformin monotherapy were included to the study. Sulfonylurea drug was given in addition to metformin. The main outcome of the study was reduction in HbA1c level (ΔHbA1c) after 6-month sulfonylurea therapy. KCNJ11 genotypes were determined by real-time PCR with melting curve analysis.

RESULTS

After 6-month treatment, KCNJ11 K-allele carriers had higher decrease in HbA1c compared with EE homozygotes in the dominant genetic model (1.04±0.10 vs. 0.79±0.12%, p=0.036). In the log-additive model, greater mean reduction in HbA1c by 0.16% (95% CI 0.01-0.32, p=0.038) per each K-allele was observed. The relationship of treatment response with KCNJ11 genotype was also significant in the biggest subgroup of patients treated with gliclazide (n=55).

CONCLUSIONS

Carriers of the KCNJ11 K-allele have better therapeutic response to gliclazide. This observation might help to identify patients who will have the highest benefit from sulfonylurea treatment.

The lessons of early-onset monogenic diabetes for the understanding of diabetes pathogenesis

Monogenic diabetes consists of different subtypes of single gene disorders comprising a large spectrum of phenotypes, namely neonatal diabetes mellitus or monogenic diabetes of infancy, dominantly inherited familial forms of early-onset diabetes (called Maturity-Onset Diabetes of the Young) and rarer diabetes-associated syndromic diseases. All these forms diagnosed at a very-young age are unrelated to auto-immunity. Their genetic dissection has revealed major genes in developmental and/or functional processes of the pancreatic β-cell physiology, and various molecular mechanisms underlying the primary pancreatic defects. Most of these discoveries have had remarkable consequences on the patients care and patient's long-term condition with outstanding examples of successful genomic medicine, which are highlighted in this chapter. Increasing evidence also shows that frequent polymorphisms in or near monogenic diabetes genes may contribute to adult polygenic type 2 diabetes. In this regard, unelucidated forms of monogenic diabetes represent invaluable models for identifying new targets of β-cell dysfunction.

The ATP-sensitive K(+) channel ABCC8 S1369A type 2 diabetes risk variant increases MgATPase activity

Pancreatic β-cell ATP-sensitive K(+) (K(ATP)) channels are composed of Kir6.2 and SUR1 subunits encoded by the KCNJ11 and ABCC8 genes, respectively. Although rare monogenic activating mutations in these genes cause overt neonatal diabetes, the common variants E23K (KCNJ11) and S1369A (ABCC8) form a tightly heritable haplotype that is associated with an increased susceptibility to type 2 diabetes (T2D) risk. However, the molecular mechanism(s) underlying this risk remain to be elucidated. A homology model of the SUR1 nucleotide-binding domains (NBDs) indicates that residue 1369 is in close proximity to the major MgATPase site. Therefore, we investigated the intrinsic MgATPase activity of K(ATP) channels containing these variants. Electrophysiological and biochemical techniques were used to study the MgATPase activity of recombinant human K(ATP) channels or glutathione S-transferase and NBD2 fusion proteins containing the E23/S1369 (nonrisk) or K23/A1369 (risk) variant haplotypes. K(ATP) channels containing the K23/A1369 haplotype displayed a significantly increased stimulation by guanosine triphosphate compared with the E23/S1369 haplotype (3.2- vs. 1.8-fold). This effect was dependent on the presence of the A1369 variant and was lost in the absence of Mg(2+) ions or in the presence of the MgATPase inhibitor beryllium fluoride. Direct biochemical assays also confirmed an increase in MgATPase activity in NBD2 fusion proteins containing the A1369 variant. Our findings demonstrate that the A1369 variant increases K(ATP) channel MgATPase activity, providing a plausible molecular mechanism by which the K23/A1369 haplotype increases susceptibility to T2D in humans homozygous for these variants.

The molecular genetics of sulfonylurea receptors in the pathogenesis and treatment of insulin secretory disorders and type 2 diabetes

Sulfonylurea receptors (SURs) form an integral part of the ATP-sensitive potassium (K(ATP)) channel complex that is present in most excitable cell types. K(ATP) channels couple cellular metabolism to electrical activity and provide a wide range of cellular functions including stimulus secretion coupling in pancreatic β cells. K(ATP) channels are composed of SURs and inward rectifier potassium channel (Kir6.x) subunits encoded by the ABCC8/9 and KCNJ8/11 genes, respectively. Recent advances in the genetics, molecular biology, and pharmacology of SURs have led to an increased understanding of these channels in the etiology and treatment of rare genetic insulin secretory disorders. Furthermore, common genetic variants in these genes are associated with an increased risk for type 2 diabetes. In this review we summarize the molecular biology, pharmacology, and physiology of SURs and K(ATP) channels, highlighting recent advances in their genetics and understanding of rare insulin secretory disorders and susceptibility to type 2 diabetes.

Pharmacogenetics in type 2 diabetes: potential implications for clinical practice

Pharmacogenetic research aims to study how genetic variation may influence drug efficacy and/or toxicity; pharmacogenomics expands this quest to the entire genome. Pharmacogenetic findings may help to uncover new drug targets, illuminate pathophysiology, clarify disease heterogeneity, aid in the fine-mapping of genetic associations, and contribute to personalized treatment. In diabetes, there is precedent for the successful application of pharmacogenetic concepts to monogenic forms of the disease, such as maturity onset diabetes of the young or neonatal diabetes. Whether similar insights will be produced for the common form of type 2 diabetes remains to be seen. With recent advances in genetic approaches, the successive application of candidate gene studies, large-scale genotyping studies and genome-wide association studies has begun to generate suggestive results that may lead to changes in clinical practice. However, many potential barriers to the translation of pharmacogenetic discoveries to the clinical management of diabetes still remain. Here, we offer a contemporary overview of the field in its current state, identify potential obstacles, and highlight future directions.

Pharmacogenomics of oral antidiabetic medications: current data and pharmacoepigenomic perspective

Type 2 diabetes mellitus (T2DM) is an increasingly prevalent disease. Several classes of drugs are currently available to treat T2DM patients; however, clinical response to these drugs often exhibits significant variation among individuals. For the oral antidiabetic drug classes of sulfonylureas, nonsulfonylurea insulin secretagogs, biguanides and thiazolidinediones, pharmacogenomic evidence has accumulated demonstrating an association between specific gene polymorphisms and interindividual variability in their therapeutic and adverse reaction effects. These polymorphisms are in genes of molecules involved in metabolism, transport and therapeutic mechanisms of the aforementioned drugs. Overall, it appears that pharmacogenomics has the potential to improve the management of T2DM and help clinicians in the effective prescribing of oral antidiabetic medications. Although pharmacogenomics can explain some of the heterogeneity in dose requirements, response and incidence of adverse effects of drugs between individuals, it is now clearly understood that much of the diversity in drug effects cannot be solely explained by studying the genomic diversity. Epigenomics, the field that focuses on nongenomic modifications that influence gene expression, may expand the scope of pharmacogenomics towards optimization of drug therapy. Therefore, pharmacoepigenomics, the combined analysis of genetic variations and epigenetic modifications, holds promise for the realization of personalized medicine. Although pharmacoepigenomics has so far been evaluated mainly in cancer pharmacotherapy, studies on epigenomic modifications during T2DM development provide useful data on the potential of pharmacoepigenomics to elucidate the mechanisms underlying interindividual response to oral antidiabetic treatment. In summary, the present article focuses on available data from pharmacogenomic studies of oral antidiabetic drugs and also provides an overview of T2DM epigenomic research, which has the potential to boost the development of pharmacoepigenomics in antidiabetic treatment.

Effect of genetic polymorphisms on the pharmacokinetics and efficacy of glimepiride in a Korean population

BACKGROUNDS

Glimepiride is a commonly used sulfonylurea hypoglycemic agent. There is considerable interindividual variation in the response to sulfonylurea for patients with type 2 diabetes. The purpose of this study was to investigate whether genetic variations influence the efficacy of glimepiride in healthy Korean subjects.

METHODS

A single 2-mg oral dose of glimepiride was administered to 46 healthy volunteers. Serial blood sampling for 12h after oral dosing was performed for determination of plasma glimepiride, glucose and insulin levels. We tested the association of seven single nucleotide polymorphisms (SNPs) in four candidate genes with the efficacy of glimepiride.

RESULTS

Pharmacodynamic profiles for plasma glucose and insulin showed no statistically significant differences among genotype groups, and parameters were not different from one another. There were no association of the KCNJ11, NOS1AP, TCF7L2 and ABCC8 gene polymorphisms and the efficacy of glimepiride.

CONCLUSIONS

Knowledge of these polymorphisms provides no clinical useful information for the pharmacogenetic therapeutic approach for Korean patients with type 2 diabetes.

Type 2 diabetes and obesity: genomics and the clinic

Type 2 diabetes (T2D) and obesity represent major challenges for global public health. They are at the forefront of international efforts to identify the genetic variation contributing to complex disease susceptibility, and recent years have seen considerable success in identifying common risk-variants. Given the clinical impact of molecular diagnostics in rarer monogenic forms of these diseases, expectations have been high that genetic discoveries will transform the prospects for risk stratification, development of novel therapeutics and personalised medicine. However, so far, clinical translation has been limited. Difficulties in defining the alleles and transcripts mediating association effects have frustrated efforts to gain early biological insights, whilst the fact that variants identified account for only a modest proportion of observed familiarity has limited their value in guiding treatment of individual patients. Ongoing efforts to track causal variants through fine-mapping and to illuminate the biological mechanisms through which they act, as well as sequence-based discovery of lower-frequency alleles (of potentially larger effect), should provide welcome acceleration in the capacity for clinical translation. This review will summarise recent advances in identifying risk alleles for T2D and obesity, and existing contributions to understanding disease pathology. It will consider the progress made in translating genetic knowledge into clinical utility, the challenges remaining, and the realistic potential for further progress.

Current understanding of K ATP channels in neonatal diseases: focus on insulin secretion disorders

ATP-sensitive potassium (K(ATP)) channels are cell metabolic sensors that couple cell metabolic status to electric activity, thus regulating many cellular functions. In pancreatic beta cells, K(ATP) channels modulate insulin secretion in response to fluctuations in plasma glucose level, and play an important role in glucose homeostasis. Recent studies show that gain-of-function and loss-of-function mutations in K(ATP) channel subunits cause neonatal diabetes mellitus and congenital hyperinsulinism respectively. These findings lead to significant changes in the diagnosis and treatment for neonatal insulin secretion disorders. This review describes the physiological and pathophysiological functions of K(ATP) channels in glucose homeostasis, their specific roles in neonatal diabetes mellitus and congenital hyperinsulinism, as well as future perspectives of K(ATP) channels in neonatal diseases.

ABCC8 polymorphism (Ser1369Ala): influence on severe hypoglycemia due to sulfonylureas

AIMS

Sulfonylureas are categorized according to their binding sites of the ATP-sensitive K+ channel (K(ATP) channel) complex in pancreatic β-cells. The binding sites are classified as A, B and A + B site (both A and B sites), respectively. The Ser1369Ala variant in the sulfonylurea receptor gene ABCC8 which encodes a subunit of the K(ATP) channel complex has been demonstrated to be associated with the hypoglycemic effect of gliclazide, which binds to the A site. However, the hypoglycemic effect of the Ser1369Ala variant on treatment with A + B binding site sulfonylureas, such as glimepiride or glibenclamide, is still uncertain.

MATERIALS & METHODS

In a case-control study, 32 patients with Type 2 diabetes admitted to hospital with severe hypoglycemia and 125 consecutive Type 2 diabetic outpatients without severe hypoglycemia were enrolled. We determined the genotypes of the ABCC8 polymorphism (Ser1369Ala) in the patients with or without severe hypoglycemia. All of the patients were taking glimepiride or glibenclamide.

RESULTS

In the patients treated with glimepiride or glibenclamide, we found no significant differences in the distribution of the Ser1369Ala genotype between patients with or without severe hypoglycemia (p = 0.26). Moreover, the Ala1369 minor allele tended to be less frequent in the hypoglycemic group (31 vs 43%; OR: 1.65; 95% CI: 0.92-2.96; p = 0.09).

CONCLUSION

Our findings suggest that the Ser1369Ala variant is not a major predictive factor of severe hypoglycemia due to glimepiride or glibenclamide, both of which bind to the A + B site. It is likely that severe hypoglycemia due to A + B binding site sulfonylureas will be mediated by other factors, and not the Ala1369 minor allele.

Milk signalling in the pathogenesis of type 2 diabetes

The presented hypothesis identifies milk consumption as an environmental risk factor of Western diet promoting type 2 diabetes (T2D). Milk, commonly regarded as a valuable nutrient, exerts important endocrine functions as an insulinotropic, anabolic and mitogenic signalling system supporting neonatal growth and development. The presented hypothesis substantiates milk's physiological role as a signalling system for pancreatic β-cell proliferation by milk's ability to increase prolactin-, growth hormone and incretin-signalling. The proposed mechanism of milk-induced postnatal β-cell mass expansion mimics the adaptive prolactin-dependent proliferative changes observed in pregnancy. Milk signalling down-regulates the key transcription factor FoxO1 leading to up-regulation of insulin promoter factor-1 which stimulates β-cell proliferation, insulin secretion as well as coexpression of islet amyloid polypeptide (IAPP). The recent finding that adult rodent β-cells only proliferate by self-duplication is of crucial importance, because permanent milk consumption beyond the weaning period may continuously over-stimulate β-cell replication thereby accelerating the onset of replicative β-cell senescence. The long-term use of milk may thus increase endoplasmic reticulum (ER) stress and toxic IAPP oligomer formation by overloading the ER with cytotoxic IAPPs thereby promoting β-cell apoptosis. Both increased β-cell proliferation and β-cell apoptosis are hallmarks of T2D. This hypothesis gets support from clinical states of hyperprolactinaemia and progeria syndromes with early onset of cell senescence which are both associated with an increased incidence of T2D and share common features of milk signalling. Furthermore, the presented milk hypothesis of T2D is compatible with the concept of high ER stress in T2D and the toxic oligomer hypothesis of T2D and may explain the high association of T2D and Alzheimer disease.

Physiologic characterization of type 2 diabetes-related loci

For the past two decades, genetics has been widely explored as a tool for unraveling the pathogenesis of diabetes. Many risk alleles for type 2 diabetes and hyperglycemia have been detected in recent years through massive genome-wide association studies and evidence exists that most of these variants influence pancreatic β-cell function. However, risk alleles in five loci seem to have a primary impact on insulin sensitivity. Investigations of more detailed physiologic phenotypes, such as the insulin response to intravenous glucose or the incretion hormones, are now emerging and give indications of more specific pathologic mechanisms for diabetes-related risk variants. Such studies have shed light on the function of some loci but also underlined the complex nature of disease mechanism. In the future, sequencing-based discovery of low-frequency variants with higher impact on intermediate diabetes-related traits is a likely scenario and identification of new pathways involved in type 2 diabetes predisposition will offer opportunities for the development of novel therapeutic and preventative approaches.

From genetic association to molecular mechanism

Over the past 3 years, there has been a dramatic increase in the number of confirmed type 2 diabetes (T2D) susceptibility loci, most arising through the implementation of genome-wide association studies (GWAS). However, progress toward the understanding of disease mechanisms has been slowed by modest effect sizes and the fact that most GWAS signals map away from coding sequence: the presumption is that their effects are mediated through regulation of nearby transcripts, but the identities of the genes concerned are often far from clear. In this review we describe the progress that has been made to date in translating association signals into molecular mechanisms with a focus on the most tractable signals (eg, KCNJ11/ABCC8, SLC30A8, GCKR) and those in which human, animal, and cellular models (FTO, TCF7L2, G6PC2) have provided insights into the role in T2D pathogenesis. Finally, the challenges for the field with the advent of genome-scale next-generation resequencing efforts are discussed.

The molecular mechanisms and pharmacotherapy of ATP-sensitive potassium channel gene mutations underlying neonatal diabetes

Neonatal diabetes mellitus (NDM) is a monogenic disorder caused by mutations in genes involved in regulation of insulin secretion from pancreatic β-cells. Mutations in the KCNJ11 and ABCC8 genes, encoding the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel Kir6.2 and SUR1 subunits, respectively, are found in ∼50% of NDM patients. In the pancreatic β-cell, K(ATP) channel activity couples glucose metabolism to insulin secretion via cellular excitability and mutations in either KCNJ11 or ABCC8 genes alter K(ATP) channel activity, leading to faulty insulin secretion. Inactivation mutations decrease K(ATP) channel activity and stimulate excessive insulin secretion, leading to hyperinsulinism of infancy. In direct contrast, activation mutations increase K(ATP) channel activity, resulting in impaired insulin secretion, NDM, and in severe cases, developmental delay and epilepsy. Many NDM patients with KCNJ11 and ABCC8 mutations can be successfully treated with sulfonylureas (SUs) that inhibit the K(ATP) channel, thus replacing the need for daily insulin injections. There is also strong evidence indicating that SU therapy ameliorates some of the neurological defects observed in patients with more severe forms of NDM. This review focuses on the molecular and cellular mechanisms of mutations in the K(ATP) channel that underlie NDM. SU pharmacogenomics is also discussed with respect to evaluating whether patients with certain K(ATP) channel activation mutations can be successfully switched to SU therapy.

Pharmacogenetics of Anti-Diabetes Drugs

A variety of treatment modalities exist for individuals with type 2 diabetes mellitus (T2D). In addition to dietary and physical activity interventions, T2D is also treated pharmacologically with nine major classes of approved drugs. These medications include insulin and its analogues, sulfonylureas, biguanides, thiazolidinediones (TZDs), meglitinides, α-glucosidase inhibitors, amylin analogues, incretin hormone mimetics, and dipeptidyl peptidase 4 (DPP4) inhibitors. Pharmacological treatment strategies for T2D are typically based on efficacy, yet favorable responses to such therapeutics are oftentimes variable and difficult to predict. Characterization of drug response is expected to substantially enhance our ability to provide patients with the most effective treatment strategy given their individual backgrounds, yet pharmacogenetic study of diabetes medications is still in its infancy. To date, major pharmacogenetic studies have focused on response to sulfonylureas, biguanides, and TZDs. Here, we provide a comprehensive review of pharmacogenetics investigations of these specific anti-diabetes medications. We focus not only on the results of these studies, but also on how experimental design, study sample issues, and definition of 'response' can significantly impact our interpretation of findings. Understanding the pharmacogenetics of anti-diabetes medications will provide critical baseline information for the development and implementation of genetic screening into therapeutic decision making, and lay the foundation for "individualized medicine" for patients with T2D.

Sulfonylurea pharmacogenomics in Type 2 diabetes: the influence of drug target and diabetes risk polymorphisms

The sulfonylureas stimulate insulin release from pancreatic beta cells, and have been a cornerstone of Type 2 diabetes pharmacotherapy for over 50 years. Although sulfonylureas are effective antihyperglycemic agents, interindividual variability exists in drug response (i.e., pharmacodynamics), disposition (i.e., pharmacokinetics) and adverse effects. The field of pharmacogenomics has been applied to sulfonylurea clinical studies in order to elucidate the genetic underpinnings of this response variability. Historically, most studies have sought to determine the influence of polymorphisms in drug-metabolizing enzyme genes on sulfonylurea pharmacokinetics in humans. More recently, polymorphisms in sulfonylurea drug target genes and diabetes risk genes have been implicated as important determinants of sulfonylurea pharmacodynamics in patients with Type 2 diabetes. As such, the purpose of this review is to discuss sulfonylurea pharmacogenomics in the setting of Type 2 diabetes, specifically focusing on polymorphisms in drug target and diabetes risk genes, and their relationship with interindividual variability in sulfonylurea response and adverse effects.

The role of the KATP channel in glucose homeostasis in health and disease: more than meets the islet

ATP-sensitive potassium (K(ATP)) channels are critical for the maintenance of glucose homeostasis. They are essential for glucose-stimulated insulin secretion from pancreatic beta-cells, contribute to the mechanisms by which hypoglycaemia stimulates glucagon release from pancreatic alpha-cells, and are involved in glucose uptake into skeletal muscle, glucose production and release from the liver, and feeding behaviour. Not surprisingly, loss- or gain-of-function mutations in K(ATP) channel genes have profound effects, giving rise to congenital hyperinsulinaemia and neonatal diabetes respectively. This symposium review focuses on our current understanding of the role of the K(ATP) channel in glucose homeostasis in health and disease.

Uncovering the roles of rare variants in common disease through whole-genome sequencing

Although genome-wide association (GWA) studies for common variants have thus far succeeded in explaining only a modest fraction of the genetic components of human common diseases, recent advances in next-generation sequencing technologies could rapidly facilitate substantial progress. This outcome is expected if much of the missing genetic control is due to gene variants that are too rare to be picked up by GWA studies and have relatively large effects on risk. Here, we evaluate the evidence for an important role of rare gene variants of major effect in common diseases and outline discovery strategies for their identification.

Replication of association between polymorphisms of the pancreatic ATP-sensitive potassium channel and susceptibility to type 2 diabetes in two Russian urban populations

The KCNJ11 and ABCC8 genes encode components of the pancreatic ATP-sensitive potassium (KATP) channel. Previously, we reported association of the KCNJ11 E23K and ABCC8 R1273R G/A variants with type 2 diabetes (T2D) in a small Russian population sample (n=244). Here we replicated association between these genetic variants and T2D in a larger cohort (588 diabetic and 597 non-diabetic subjects). Using the ANCOVA analysis, Odds Ratios (ORs) and relationships between the carriage of a genotype and biochemical parameters of the patients were assessed and then adjusted for confounders (age, gender, HbA1c, hypertension, and obesity). The KCNJ11 K23 variant and the ABCC8 R1273R allele A showed association with higher risk of T2D (adjusted OR of 1.41 and 2.03, P<0.0001, respectively). Diabetic patients homozygous for K/K had lower 2h insulin (Padjusted=0.044). The ABCC8 A/A variant was associated with increased 2h serum insulin in diabetic and non-diabetic subjects (Padjusted=0.027 and 0.033, respectively). The carriage of the risk variant K/K of KCNJ11 E23K or A/A of ABCC8 G/A R1273R was associated with reduced response to nonsulfonylurea and sulfonylurea blockers of the pancreatic KATP channel. Adjusted attributable population risk was 3.0% (KCNJ11 E23K) and 4.8% (ABCC8 G/A) suggesting for the modest effects of these genetic variants on diabetes susceptibility.

K(ATP) channelopathies in the pancreas

Adenosine-triphosphate-sensitive potassium channels (KATP) are regulated by adenosine nucleotides, and, thereby, couple cellular metabolism with electrical activity in multiple tissues including the pancreatic beta-cell. The critical involvement of KATP in insulin secretion is confirmed by the demonstration that inactivating and activating mutations in KATP underlie persistent hyperinsulinemia and neonatal diabetes mellitus, respectively, in both animal models and humans. In addition, a common variant in KATP represents a risk factor in the etiology of type 2 diabetes. This review focuses on the mechanistic basis by which KATP mutations underlie insulin secretory disorders and the implications of these findings for successful clinical intervention.

RD Lawrence Lecture 2009. Old genes, new tricks: learning about blood glucose regulation from naturally occurring genetic variation in humans

The study of rare monogenic forms of diabetes and pancreatic B-cell dysfunction provides an unrivalled opportunity to link a specific change in gene function with precise cellular consequences and clinical phenotype in humans. Over the past 20 years there has been considerable success in determining the genetic aetiology of a number of rare monogenic forms of diabetes, which has had a significant impact on both our understanding of normal physiology and on translational medicine. The impact of these discoveries has been substantial, with insights into both developmental biology and normal physiology. There are clear examples where determining the genetic aetiology for individuals with rare monogenic subtypes of diabetes has led to improved treatment. Although formerly in the shadow of the monogenic diabetes field, over the past 3 years there has been staggering progress in our understanding of the genetic basis of Type 2 diabetes. This has been largely as a result of genome-wide association studies and has seen the list of 'diabetes susceptibility genes' increase from three to close to 20. There is now encouraging evidence to support a potential role for genetics in determining the response of individuals with Type 2 diabetes to different therapeutic options. One of the challenges that lies ahead is determining how the non-coding genetic variants exert their pathogenicity. It is possible that parallels can be drawn from functional work on rare regulatory mutations causing monogenic forms of diabetes. However, it is more likely that comprehensive approaches will be necessary.