An ATP-binding mutation (G334D) in KCNJ11 is associated with a sulfonylurea-insensitive form of developmental delay, epilepsy, and neonatal diabetes

Interaction between mutations in the slide helix of Kir6.2 associated with neonatal diabetes and neurological symptoms

ATP-sensitive potassium (K(ATP)) channels regulate insulin secretion from pancreatic beta-cells. Gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause neonatal diabetes. We report two novel mutations on the same haplotype (cis), F60Y and V64L, in the slide helix of Kir6.2 in a patient with neonatal diabetes, developmental delay and epilepsy. Functional analysis revealed the F60Y mutation increases the intrinsic channel open probability (Po(0)), thereby indirectly producing a marked decrease in channel inhibition by ATP and an increase in whole-cell K(ATP) currents. When expressed alone, the V64L mutation caused a small reduction in apparent ATP inhibition, by enhancing the ability of MgATP to stimulate channel activity. The V64L mutation also ameliorated the deleterious effects on the F60Y mutation when it was expressed on the same (but not a different) subunit. These data indicate that F60Y is the pathogenic mutation and reveal that interactions between slide helix residues can influence K(ATP) channel gating.

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.

Glibenclamide unresponsiveness in a Brazilian child with permanent neonatal diabetes mellitus and DEND syndrome due to a C166Y mutation in KCNJ11 (Kir6.2) gene

Heterozygous activating mutations of KCNJ11 (Kir6.2) are the most common cause of permanent neonatal diabetes mellitus (PNDM) and several cases have been successfully treated with oral sulfonylureas. We report on the attempted transfer of insulin therapy to glibenclamide in a 4-year old child with PNDM and DEND syndrome, bearing a C166Y mutation in KCNJ11. An inpatient transition from subcutaneous NPH insulin (0.2 units/kg/d) to oral glibenclamide (1 mg/kg/d and 1.5 mg/kg/d) was performed. Glucose and C-peptide responses stimulated by oral glucose tolerance test (OGTT), hemoglobin A1c levels, the 8-point self-measured blood glucose (SMBG) profile and the frequency of hypoglycemia episodes were analyzed, before and during treatment with glibenclamide. Neither diabetes control nor neurological improvements were observed. We concluded that C166Y mutation was associated with a form of PNDM insensitive to glibenclamide.

Improved diabetic control during oral sulfonylurea treatment in two children with permanent neonatal diabetes mellitus

Permanent neonatal diabetes mellitus (PND), defined as diabetes diagnosed in the first 6 months of age and requiring life-long insulin therapy, is a rare disorder of unknown etiology. Activating mutations of the KCNJ11 gene, which encodes the Kir6.2 subunit of the ATP-dependent potassium channel in beta-cells, have been found to cause 30-58% of cases of PND. Sulfonylurea treatment in theses patients reduces or eliminates the need for exogenous insulin. We report two Taiwanese boys who were diagnosed with PND at 1 and 4.5 months of age. They had been treated with exogenous insulin for 6 and 15 years, respectively. In September 2006, they were both found to have a KCNJ11 mutation (valine-to-methionine at codon 59; V59M). Glibenclamide successfully increased the basal C-peptide level, lowered HbA(1c), and reduced blood sugar excursions. In one patient, the insulin dose was reduced to 0.2 U/kg/day, and the other was able to discontinue insulin altogether. These two cases from Taiwan add to the experience with similar mutations reported in Caucasians.

Adjacent mutations in the gating loop of Kir6.2 produce neonatal diabetes and hyperinsulinism

K(ATP) channels regulate insulin secretion from pancreatic beta-cells. Loss- and gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause hyperinsulinism of infancy and neonatal diabetes, respectively. We report two novel mutations in the gating loop of Kir6.2 which cause neonatal diabetes with developmental delay (T293N) and hyperinsulinism (T294M). These mutations increase (T293N) or decrease (T294M) whole-cell K(ATP) currents, accounting for the different clinical phenotypes. The T293N mutation increases the intrinsic channel open probability (Po((0))), thereby indirectly decreasing channel inhibition by ATP and increasing whole-cell currents. T294M channels exhibit a dramatically reduced Po((0)) in the homozygous but not in the pseudo-heterozygous state. Unlike wild-type channels, hetT294M channels were activated by MgADP in the absence but not in the presence of MgATP; however, they are activated by MgGDP in both the absence and presence of MgGTP. These mutations demonstrate the importance of the gating loop of Kir channels in regulating Po((0)) and further suggest that Mg-nucleotide interaction with SUR1 may reduce ATP inhibition at Kir6.2.

Update of mutations in the genes encoding the pancreatic beta-cell K(ATP) channel subunits Kir6.2 (KCNJ11) and sulfonylurea receptor 1 (ABCC8) in diabetes mellitus and hyperinsulinism

The beta-cell ATP-sensitive potassium (K(ATP)) channel is a key component of stimulus-secretion coupling in the pancreatic beta-cell. The channel couples metabolism to membrane electrical events bringing about insulin secretion. Given the critical role of this channel in glucose homeostasis it is therefore not surprising that mutations in the genes encoding for the two essential subunits of the channel can result in both hypo- and hyperglycemia. The channel consists of four subunits of the inwardly rectifying potassium channel Kir6.2 and four subunits of the sulfonylurea receptor 1 (SUR1). It has been known for some time that loss of function mutations in KCNJ11, which encodes for Kir6.2, and ABCC8, which encodes for SUR1, can cause oversecretion of insulin and result in hyperinsulinism of infancy, while activating mutations in KCNJ11 and ABCC8 have recently been described that result in the opposite phenotype of diabetes. This review focuses on reported mutations in both genes, the spectrum of phenotypes, and the implications for treatment on diagnosing patients with mutations in these genes.

Secondary consequences of beta cell inexcitability: identification and prevention in a murine model of K(ATP)-induced neonatal diabetes mellitus

ATP-insensitive K(ATP) channel mutations cause neonatal diabetes mellitus (NDM). To explore the mechanistic etiology, we generated transgenic mice carrying an ATP-insensitive mutant K(ATP) channel subunit. Constitutive expression in pancreatic beta cells caused neonatal hyperglycemia and progression to severe diabetes and growth retardation, with loss of islet insulin content and beta cell architecture. Tamoxifen-induced expression in adult beta cells led to diabetes within 2 weeks, with similar secondary consequences. Diabetes was prevented by transplantation of normal islets under the kidney capsule. Moreover, the endogenous islets maintained normal insulin content and secretion in response to sulfonylureas, but not glucose, consistent with reduced ATP sensitivity of beta cell K(ATP) channels. In NDM, transfer to sulfonylurea therapy is less effective in older patients. This may stem from poor glycemic control or lack of insulin because glibenclamide treatment prior to tamoxifen induction prevented diabetes and secondary complications in mice but failed to halt disease progression after diabetes had developed.

The K(ATP) channel and neonatal diabetes

ATP-sensitive potassium (K(ATP)) channels play a key role in glucose-dependent insulin secretion in pancreatic beta-cells. Recently, activating mutations in beta-cell K(ATP) channels were found to be an important cause of neonatal diabetes. In some patients, these mutations may also affect K(ATP) channel function in muscles, nerves and brain which can result in a severe disease termed DEND syndrome (Developmental delay, Epilepsy and Neonatal Diabetes). This review focuses on mutations in the pore-forming K(ATP) channel subunit (Kir6.2) that cause neonatal diabetes and discusses recent advances in our understanding of clinical features of neonatal diabetes, its underlying molecular mechanisms and their impact on treatment.

Modeling K(ATP) channel gating and its regulation

ATP-sensitive potassium (K(ATP)) channels couple cell metabolism to plasmalemmal potassium fluxes in a variety of cell types. The activity of these channels is primarily determined by intracellular adenosine nucleotides, which have both inhibitory and stimulatory effects. The role of K(ATP) channels has been studied most extensively in pancreatic beta-cells, where they link glucose metabolism to insulin secretion. Many mutations in K(ATP) channel subunits (Kir6.2, SUR1) have been identified that cause either neonatal diabetes or congenital hyperinsulinism. Thus, a mechanistic understanding of K(ATP) channel behavior is necessary for modeling beta-cell electrical activity and insulin release in both health and disease. Here, we review recent advances in the K(ATP) channel structure and function. We focus on the molecular mechanisms of K(ATP) channel gating by adenosine nucleotides, phospholipids and sulphonylureas and consider the advantages and limitations of various mathematical models of macroscopic and single-channel K(ATP) currents. Finally, we outline future directions for the development of more realistic models of K(ATP) channel gating.

Monogenic β-cell dysfunction in children: clinical phenotypes, genetic etiology and mutational pathways

Monogenic diabetes accounts for 1–2% of all cases of diabetes mellitus and presentation is often in childhood. Recognizing the clinical features of monogenic β-cell dysfunction prevents misdiagnosis and allows for more effective management and genetic counseling. Monogenic β-cell dysfunction is a diverse collection of clinical phenotypes underpinned by common mutational pathways. Mutations affecting the glycolytic glucokinase enzyme, the mitochondria, the KATP channels and transcription factors have been known for some time. Until recently, the role of endoplasmic reticulum stress was underestimated in the pathogenesis of diabetes. It is becoming increasingly clear that endoplasmic reticulum stress is an important etiological factor in the development of monogenic and polygenic diabetes. In this article, we aim to define the etiology of pediatric monogenic β-cell dysfunction and provide guidance on the investigation and management of children presenting with monogenic β-cell dysfunction.

Seven mutations in the human insulin gene linked to permanent neonatal/infancy-onset diabetes mellitus

Permanent neonatal diabetes mellitus (PNDM) is a rare disorder usually presenting within 6 months of birth. Although several genes have been linked to this disorder, in almost half the cases documented in Italy, the genetic cause remains unknown. Because the Akita mouse bearing a mutation in the Ins2 gene exhibits PNDM associated with pancreatic beta cell apoptosis, we sequenced the human insulin gene in PNDM subjects with unidentified mutations. We discovered 7 heterozygous mutations in 10 unrelated probands. In 8 of these patients, insulin secretion was detectable at diabetes onset, but rapidly declined over time. When these mutant proinsulins were expressed in HEK293 cells, we observed defects in insulin protein folding and secretion. In these experiments, expression of the mutant proinsulins was also associated with increased Grp78 protein expression and XBP1 mRNA splicing, 2 markers of endoplasmic reticulum stress, and with increased apoptosis. Similarly transfected INS-1E insulinoma cells had diminished viability compared with those expressing WT proinsulin. In conclusion, we find that mutations in the insulin gene that promote proinsulin misfolding may cause PNDM.

A Kir6.2 mutation causing severe functional effects in vitro produces neonatal diabetes without the expected neurological complications

AIMS/HYPOTHESIS

Heterozygous activating mutations in the pancreatic ATP-sensitive K+ channel cause permanent neonatal diabetes mellitus (PNDM). This results from a decrease in the ability of ATP to close the channel, which thereby suppresses insulin secretion. PNDM mutations that cause a severe reduction in ATP inhibition may produce additional symptoms such as developmental delay and epilepsy. We identified a heterozygous mutation (L164P) in the pore-forming (Kir6.2) subunit of the channel in three unrelated patients and examined its functional effects.

METHODS

The patients (currently aged 2, 8 and 20 years) developed diabetes shortly after birth. The two younger patients attempted transfer to sulfonylurea therapy but were unsuccessful (up to 1.1 mg kg(-1) day(-1)). They remain insulin dependent. None of the patients displayed neurological symptoms. Functional properties of wild-type and mutant channels were examined by electrophysiology in Xenopus oocytes.

RESULTS

Heterozygous (het) and homozygous L164P K(ATP) channels showed a marked reduction in channel inhibition by ATP. Consistent with its predicted location within the pore, L164P enhanced the channel open state, which explains the reduction in ATP sensitivity. HetL164P currents exhibited greatly increased whole-cell currents that were unaffected by sulfonylureas. This explains the inability of sulfonylureas to ameliorate the diabetes of affected patients.

CONCLUSIONS/INTERPRETATION

Our results provide the first demonstration that mutations such as L164P, which produce a severe reduction in ATP sensitivity, do not inevitably cause developmental delay or neurological problems. However, the neonatal diabetes of these patients is unresponsive to sulfonylurea therapy. Functional analysis of PNDM mutations can predict the sulfonylurea response.

Neonatal diabetes mellitus

An explosion of work over the last decade has produced insight into the multiple hereditary causes of a nonimmunological form of diabetes diagnosed most frequently within the first 6 months of life. These studies are providing increased understanding of genes involved in the entire chain of steps that control glucose homeostasis. Neonatal diabetes is now understood to arise from mutations in genes that play critical roles in the development of the pancreas, of beta-cell apoptosis and insulin processing, as well as the regulation of insulin release. For the basic researcher, this work is providing novel tools to explore fundamental molecular and cellular processes. For the clinician, these studies underscore the need to identify the genetic cause underlying each case. It is increasingly clear that the prognosis, therapeutic approach, and genetic counseling a physician provides must be tailored to a specific gene in order to provide the best medical care.

The G53D mutation in Kir6.2 (KCNJ11) is associated with neonatal diabetes and motor dysfunction in adulthood that is improved with sulfonylurea therapy

CONTEXT

Mutations in the Kir6.2 subunit (KCNJ11) of the ATP-sensitive potassium channel (KATP) underlie neonatal diabetes mellitus. In severe cases, Kir6.2 mutations underlie developmental delay, epilepsy, and neonatal diabetes (DEND). All Kir6.2 mutations examined decrease the ATP inhibition of KATP, which is predicted to suppress electrical activity in neurons (peripheral and central), muscle, and pancreas. Inhibitory sulfonylureas (SUs) have been used successfully to treat diabetes in patients with activating Kir6.2 mutations. There are two reports of improved neurological features in SU-treated DEND patients but no report of such improvement in adulthood.

OBJECTIVE

The objective of the study was to determine the molecular basis of intermediate DEND in a 27-yr-old patient with a KCNJ11 mutation (G53D) and the patient's response to SU therapy.

DESIGN

The G53D patient was transferred from insulin to gliclazide and then to glibenclamide over a 160-d period. Motor function was assessed throughout. Electrophysiology assessed the effect of the G53D mutation on KATP activity.

RESULTS

The G53D patient demonstrated improved glycemic control and motor coordination with SU treatment, although glibenclamide was more effective than gliclazide. Reconstituted G53D channels exhibit reduced ATP sensitivity, which is predicted to suppress electrical activity in vivo. G53D channels coexpressed with SUR1 (the pancreatic and neuronal isoform) exhibit high-affinity block by gliclazide but are insensitive to block when coexpressed with SUR2A (the skeletal muscle isoform). High-affinity block by glibenclamide is present in G53D channels coexpressed with either SUR1 or SUR2A.

CONCLUSION

The results demonstrate that SUs can resolve motor dysfunction in an adult with intermediate DEND and that this improvement is due to inhibition of the neuronal but not skeletal muscle KATP.

Clinical implications of a molecular genetic classification of monogenic beta-cell diabetes

Monogenic diabetes resulting from mutations that primarily reduce beta-cell function accounts for 1-2% of diabetes cases, although it is often misdiagnosed as either type 1 or type 2 diabetes. Knowledge of the genetic etiology of diabetes enables more-appropriate treatment, better prediction of disease progression, screening of family members and genetic counseling. We propose that the old clinical classifications of maturity-onset diabetes of the young and neonatal diabetes are obsolete and that specific genetic etiologies should be sought in four broad clinical situations because of their specific treatment implications. Firstly, diabetes diagnosed before 6 months of age frequently results from mutation of genes that encode Kir6.2 (ATP-sensitive inward rectifier potassium channel) or sulfonylurea receptor 1 subunits of an ATP-sensitive potassium channel, and improved glycemic control can be achieved by treatment with high-dose sulfonylureas rather than insulin. Secondly, patients with stable, mild fasting hyperglycemia detected particularly when they are young could have a glucokinase mutation and might not require specific treatment. Thirdly, individuals with familial, young-onset diabetes that does not fit with either type 1 or type 2 diabetes might have mutations in the transcription factors HNF-1alpha (hepatocyte nuclear factor 1-alpha) or HNF-4alpha, and can be treated with low-dose sulfonylureas. Finally, extrapancreatic features, such as renal disease (caused by mutations in HNF-1beta) or deafness (caused by a mitochondrial m.3243A>G mutation), usually require early treatment with insulin.

Functional analysis of two Kir6.2 (KCNJ11) mutations, K170T and E322K, causing neonatal diabetes

Heterozygous activating mutations in Kir6.2 (KCNJ11), the pore-forming subunit of the adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channel, are a common cause of neonatal diabetes (ND). We assessed the functional effects of two Kir6.2 mutations associated with ND: K170T and E322K. K(ATP) channels were expressed in Xenopus oocytes, and the heterozygous state was simulated by coexpression of wild-type and mutant Kir6.2 with SUR1 (the beta cell type of sulphonylurea receptor (SUR)). Both mutations reduced the sensitivity of the K(ATP) channel to inhibition by MgATP and enhanced whole-cell K(ATP) currents. In pancreatic beta cells, such an increase in the K(ATP) current is expected to reduce insulin secretion and thereby cause diabetes. The E322K mutation was without effect when Kir6.2 was expressed in the absence of SUR1, suggesting that this residue impairs coupling to SUR1. This is consistent with its predicted location on the outer surface of the tetrameric Kir6.2 pore. The kinetics of K170T channel opening and closing were altered by the mutation, which may contribute to the lower ATP sensitivity. Neither mutation affected the sensitivity of the channel to inhibition by the sulphonylurea tolbutamide, suggesting that patients carrying these mutations may respond to these drugs.

The Walter B. Cannon Physiology in Perspective Lecture, 2007. ATP-sensitive K+ channels and disease: from molecule to malady

This essay is based on a lecture given to the American Physiological Society in honor of Walter B. Cannon, an advocate of homeostasis. It focuses on the role of the ATP-sensitive potassium K(+) (K(ATP)) channel in glucose homeostasis and, in particular, on its role in insulin secretion from pancreatic beta-cells. The beta-cell K(ATP) channel comprises pore-forming Kir6.2 and regulatory SUR1 subunits, and mutations in either type of subunit can result in too little or too much insulin release. Here, I review the latest information on the relationship between K(ATP) channel structure and function, and consider how mutations in the K(ATP) channel genes lead to neonatal diabetes or congenital hyperinsulinism.

A mutation in the ATP-binding site of the Kir6.2 subunit of the KATP channel alters coupling with the SUR2A subunit

Mutations in the pore-forming subunit of the ATP-sensitive K(+) (K(ATP)) channel Kir6.2 cause neonatal diabetes. Understanding the molecular mechanism of action of these mutations has provided valuable insight into the relationship between the structure and function of the K(ATP) channel. When Kir6.2 containing a mutation (F333I) in the putative ATP-binding site is coexpressed with the cardiac type of regulatory K(ATP) channel subunit, SUR2A, the channel sensitivity to ATP inhibition is reduced and the intrinsic open probability (P(o)) is increased. However, the extent of macroscopic current activation by MgADP was unaffected. Here we examine rundown and MgADP activation of wild-type and Kir6.2-F333I/SUR2A channels using single-channel recording, noise analysis and spectral analysis. We also compare the effect of mutating the adjacent residue, G334, on rundown and MgADP activation. All three approaches indicated that rundown of Kir6.2-F333I/SUR2A channels is due to a reduction in the number of active channels in the patch and that MgADP reactivation involves recruitment of inactive channels. In contrast, rundown and MgADP reactivation of wild-type and Kir6.2-G334D/SUR2A channels, and of Kir6.2-F333I/SUR1 channels, involve a gradual change in P(o). Our results suggest that F333 in Kir6.2 interacts functionally with SUR2A to modulate channel rundown and MgADP activation. This interaction is fairly specific as it is not disturbed when the adjacent residue (G334) is mutated. It is also not a consequence of the enhanced P(o) of Kir6.2-F333I/SUR2A channels, as it is not found for other mutant channels with high P(o) (Kir6.2-I296L/SUR2A).

A mutation in the TMD0-L0 region of sulfonylurea receptor-1 (L225P) causes permanent neonatal diabetes mellitus (PNDM)

OBJECTIVE

We sought to examine the molecular mechanisms underlying permanenent neonatal diabetes mellitus (PNDM) in a patient with a heterozygous de novo L225P mutation in the L0 region of the sulfonylurea receptor (SUR)1, the regulatory subunit of the pancreatic ATP-sensitive K(+) channel (K(ATP) channel).

RESEARCH DESIGN AND METHODS

The effects of L225P on the properties of recombinant K(ATP) channels in transfected COS cells were assessed by patch-clamp experiments on excised membrane patches and by macroscopic Rb-flux experiments in intact cells.

RESULTS

L225P-containing K(ATP) channels were significantly more active in the intact cell than in wild-type channels. In excised membrane patches, L225P increased channel sensitivity to stimulatory Mg nucleotides without altering intrinsic gating or channel inhibition by ATP in the absence of Mg(2+). The effects of L225P were abolished by SUR1 mutations that prevent nucleotide hydrolysis at the nucleotide binding folds. L225P did not alter channel inhibition by sulfonylurea drugs, and, consistent with this, the patient responded to treatment with oral sulfonylureas.

CONCLUSIONS

L225P underlies K(ATP) channel overactivity and PNDM by specifically increasing Mg-nucleotide stimulation of the channel, consistent with recent reports of mechanistically similar PNDM-causing mutations in SUR1. The mutation does not affect sulfonylurea sensitivity, and the patient is successfully treated with sulfonylureas.