Phenotypic Variability of Andersen-Tawil Syndrome Due to Allelic Mutation c.652C>T in the KCNJ2 Gene-A New Family Case Report

Andersen-Tawil syndrome (ATS) is a multisystem channelopathy characterized by periodic paralysis, ventricular arrhythmias, prolonged QT interval, and facial dysmorphisms occurring in the first/second decade of life. High phenotypic variability and incomplete penetrance of the genes causing the disease make its diagnosis still a challenge. We describe a three-generation family with six living individuals affected by ATS. The proband is a 37-year-old woman presenting since age 16, with episodes of muscle weakness and cramps in the pre-menstrual period. The father, two brothers, one paternal uncle and one cousin also complained of cramps, muscle stiffness, and weakness. Despite normal serum potassium concentration, treatment with potassium, magnesium, and acetazolamide alleviated paralysis attacks suggesting a dyskalemic syndrome. Dysmorphic features were noted in the proband, only later. On the ECG, all but one had normal QT intervals. The affected males developed metabolic syndrome or obesity. The father had two myocardial infarctions and was implanted with an intracardiac cardioverter defibrillator (ICD). A genetic investigation by WES analysis detected the heterozygous pathogenic variant (NM_000891.2: c.652C>T, p. Arg218Trp) in the KCNJ2 gene related to ATS, confirmed by segregation studies in all affected members. Furthermore, we performed a review of cases with the same mutation in the literature, looking for similarities and divergences with our family case.

Extracellular Kir2.1C122Y Mutant Upsets Kir2.1-PIP2 Bonds and Is Arrhythmogenic in Andersen-Tawil Syndrome

BACKGROUND

Andersen-Tawil syndrome type 1 is a rare heritable disease caused by mutations in the gene coding the strong inwardly rectifying K+ channel Kir2.1. The extracellular Cys (cysteine)122-to-Cys154 disulfide bond in the channel structure is crucial for proper folding but has not been associated with correct channel function at the membrane. We evaluated whether a human mutation at the Cys122-to-Cys154 disulfide bridge leads to Kir2.1 channel dysfunction and arrhythmias by reorganizing the overall Kir2.1 channel structure and destabilizing its open state.

METHODS

We identified a Kir2.1 loss-of-function mutation (c.366 A>T; p.Cys122Tyr) in an ATS1 family. To investigate its pathophysiological implications, we generated an AAV9-mediated cardiac-specific mouse model expressing the Kir2.1C122Y variant. We employed a multidisciplinary approach, integrating patch clamping and intracardiac stimulation, molecular biology techniques, molecular dynamics, and bioluminescence resonance energy transfer experiments.

RESULTS

Kir2.1C122Y mice recapitulated the ECG features of ATS1 independently of sex, including corrected QT prolongation, conduction defects, and increased arrhythmia susceptibility. Isolated Kir2.1C122Y cardiomyocytes showed significantly reduced inwardly rectifier K+ (IK1) and inward Na+ (INa) current densities independently of normal trafficking. Molecular dynamics predicted that the C122Y mutation provoked a conformational change over the 2000-ns simulation, characterized by a greater loss of hydrogen bonds between Kir2.1 and phosphatidylinositol 4,5-bisphosphate than wild type (WT). Therefore, the phosphatidylinositol 4,5-bisphosphate-binding pocket was destabilized, resulting in a lower conductance state compared with WT. Accordingly, on inside-out patch clamping, the C122Y mutation significantly blunted Kir2.1 sensitivity to increasing phosphatidylinositol 4,5-bisphosphate concentrations. In addition, the Kir2.1C122Y mutation resulted in channelosome degradation, demonstrating temporal instability of both Kir2.1 and NaV1.5 proteins.

CONCLUSIONS

The extracellular Cys122-to-Cys154 disulfide bond in the tridimensional Kir2.1 channel structure is essential for the channel function. We demonstrate that breaking disulfide bonds in the extracellular domain disrupts phosphatidylinositol 4,5-bisphosphate-dependent regulation, leading to channel dysfunction and defects in Kir2.1 energetic stability. The mutation also alters functional expression of the NaV1.5 channel and ultimately leads to conduction disturbances and life-threatening arrhythmia characteristic of Andersen-Tawil syndrome type 1.

Transcriptome and open chromatin analysis reveals the process of myocardial cell development and key pathogenic target proteins in Long QT syndrome type 7

OBJECTIVE

Long QT syndrome type 7 (Andersen-Tawil syndrome, ATS), which is caused by KCNJ2 gene mutation, often leads to ventricular arrhythmia, periodic paralysis and skeletal malformations. The development, differentiation and electrophysiological maturation of cardiomyocytes (CMs) changes promote the pathophysiology of Long QT syndrome type 7(LQT7). We aimed to specifically reproduce the ATS disease phenotype and study the pathogenic mechanism.

METHODS AND RESULTS

We established a cardiac cell model derived from human induced pluripotent stem cells (hiPSCs) to the phenotypes and electrophysiological function, and the establishment of a human myocardial cell model that specifically reproduces the symptoms of ATS provides a reliable platform for exploring the mechanism of this disease or potential drugs. The spontaneous pulsation rate of myocardial cells in the mutation group was significantly lower than that in the repair CRISPR group, the action potential duration was prolonged, and the Kir2.1 current of the inward rectifier potassium ion channel was decreased, which is consistent with the clinical symptoms of ATS patients. Only ZNF528, a chromatin-accessible TF related to pathogenicity, was continuously regulated beginning from the cardiac mesodermal precursor cell stage (day 4), and continued to be expressed at low levels, which was identified by WGCNA method and verified with ATAC-seq data in the mutation group. Subsequently, it indicated that seven pathways were downregulated (all p < 0.05) by used single sample Gene Set Enrichment Analysis to evaluate the overall regulation of potassium-related pathways enriched in the transcriptome and proteome of late mature CMs. Among them, the three pathways (GO: 0008076, GO: 1990573 and GO: 0030007) containing the mutated gene KCNJ2 is involved that are related to the whole process by which a potassium ion enters the cell via the inward rectifier potassium channel to exert its effect were inhibited. The other four pathways are related to regulation of the potassium transmembrane pathway and sodium:potassium exchange ATPase (p < 0.05). ZNF528 small interfering (si)-RNA was applied to hiPSC-derived cardiomyocytes for CRISPR group to explore changes in potassium ion currents and growth and development related target protein levels that affect disease phenotype. Three consistently downregulated proteins (KCNJ2, CTTN and ATP1B1) associated with pathogenicity were verificated through correlation and intersection analysis.

CONCLUSION

This study uncovers TFs and target proteins related to electrophysiology and developmental pathogenicity in ATS myocardial cells, obtaining novel targets for potential therapeutic candidate development that does not rely on gene editing.

Muscle channelopathies

Muscle channelopathies encompass a wide range of mainly episodic conditions that are characterized by muscle stiffness and weakness. The myotonic conditions, characterized predominantly by stiffness, include myotonia congenita, paramyotonia congenita, and sodium channel myotonia. The periodic paralysis conditions include hypokalemic periodic paralysis, hyperkalemic periodic paralysis, and Andersen-Tawil syndrome. Clinical history is key, and diagnosis is confirmed by next-generation genetic sequencing of a panel of known genes but can also be supplemented by neurophysiology studies and MRI. As genetic testing expands, so have the spectrum of phenotypes seen including pediatric presentations and congenital myopathies. Management of these conditions requires a multidisciplinary approach with extra support needed when patients require anesthetics or when pregnant. Patients with Andersen-Tawil syndrome will also need cardiac input. Diagnosis is important as symptomatic treatment is available for all of these conditions but need to be tailored to the gene and variant of the patient.

Andersen-Tawil syndrome: deep phenotyping reveals significant cardiac and neuromuscular morbidity

Andersen-Tawil syndrome is a neurological channelopathy caused by mutations in the KCNJ2 gene that encodes the ubiquitously expressed Kir2.1 potassium channel. The syndrome is characterized by episodic weakness, cardiac arrythmias and dysmorphic features. However, the full extent of the multisystem phenotype is not well described. In-depth, multisystem phenotyping is required to inform diagnosis and guide management. We report our findings following deep multimodal phenotyping across all systems in a large case series of 69 total patients, with comprehensive data for 52. As a national referral centre, we assessed point prevalence and showed it is higher than previously reported, at 0.105 per 100 000 population in England. While the classical phenotype of episodic weakness is recognized, we found that a quarter of our cohort have fixed myopathy and 13.5% required a wheelchair or gait aid. We identified frequent fat accumulation on MRI and tubular aggregates on muscle biopsy, emphasizing the active myopathic process underpinning the potential for severe neuromuscular disability. Long exercise testing was not reliable in predicting neuromuscular symptoms. A normal long exercise test was seen in five patients, of whom four had episodic weakness. Sixty-seven per cent of patients treated with acetazolamide reported a good neuromuscular response. Thirteen per cent of the cohort required cardiac defibrillator or pacemaker insertion. An additional 23% reported syncope. Baseline electrocardiograms were not helpful in stratifying cardiac risk, but Holter monitoring was. A subset of patients had no cardiac symptoms, but had abnormal Holter monitor recordings which prompted medication treatment. We describe the utility of loop recorders to guide management in two such asymptomatic patients. Micrognathia was the most commonly reported skeletal feature; however, 8% of patients did not have dysmorphic features and one-third of patients had only mild dysmorphic features. We describe novel phenotypic features including abnormal echocardiogram in nine patients, prominent pain, fatigue and fasciculations. Five patients exhibited executive dysfunction and slowed processing which may be linked to central expression of KCNJ2. We report eight new KCNJ2 variants with in vitro functional data. Our series illustrates that Andersen-Tawil syndrome is not benign. We report marked neuromuscular morbidity and cardiac risk with multisystem involvement. Our key recommendations include proactive genetic screening of all family members of a proband. This is required, given the risk of cardiac arrhythmias among asymptomatic individuals, and a significant subset of Andersen-Tawil syndrome patients have no (or few) dysmorphic features or negative long exercise test. We discuss recommendations for increased cardiac surveillance and neuropsychometry testing.

Variable Clinical Appearance of the Kir2.1 Rare Variants in Russian Patients with Long QT Syndrome

Background: The KCNJ2 gene encodes inward rectifier Kir2.1 channels, maintaining resting potential and cell excitability. Presumably, clinical phenotypes of mutation carriers correlate with ion permeability defects. Loss-of-function mutations lead to QTc prolongation with variable dysmorphic features, whereas gain-of-function mutations cause short QT syndrome and/or atrial fibrillation. Methods: We screened 210 probands with Long QT syndrome for mutations in the KCNJ2 gene. The electrophysiological study was performed for the p.Val93Ile variant in the transfected CHO-K1 cells. Results: We found three rare genetic variants, p.Arg67Trp, p.Val93Ile, and p.R218Q, in three unrelated LQTS probands. Probands with p.Arg67Trp and p.R218Q had a phenotype typical for Andersen-Tawil (ATS), and the p.Val93Ile carrier had lone QTc prolongation. Variant p.Val93Ile was initially described as a gain-of-function pathogenic mutation causing familial atrial fibrillation. We validated electrophysiological features of this variant in CHO-K1 cells, but no family members of these patients had atrial fibrillation. Using ACMG (2015) criteria, we re-assessed this variant as a variant of unknown significance (class III). Conclusions: LQT7 is a rare form of LQTS in Russia, and accounts for 1% of the LQTS cohort. Variant p.Val93Ile leads to a gain-of-function effect in the different cell lines, but its clinical appearance is not so consistent. The clinical significance of this variant might be overestimated.

Impaired cytoplasmic domain interactions cause co-assembly defect and loss of function in the p.Glu293Lys KNCJ2 variant isolated from an Andersen-Tawil syndrome patient

AIMS

Subunit interactions at the cytoplasmic domain interface (CD-I) have recently been shown to control gating in inward rectifier potassium channels. Here we report the novel KCNJ2 variant p.Glu293Lys that has been found in a patient with Andersen-Tawil syndrome type 1 (ATS1), causing amino acid substitution at the CD-I of the inward rectifier potassium channel subunit Kir2.1. Neither has the role of Glu293 in gating control been investigated nor has a pathogenic variant been described at this position. This study aimed to assess the involvement of Glu293 in CD-I subunit interactions and to establish the pathogenic role of the p.Glu293Lys variant in ATS1.

METHODS AND RESULTS

The p.Glu293Lys variant produced no current in homomeric form and showed dominant-negative effect over wild-type (WT) subunits. Immunocytochemical labelling showed the p.Glu293Lys subunits to distribute in the subsarcolemmal space. Salt bridge prediction indicated the presence of an intersubunit salt bridge network at the CD-I of Kir2.1, with the involvement of Glu293. Subunit interactions were studied by the NanoLuc® Binary Technology (NanoBiT) split reporter assay. Reporter constructs carrying NanoBiT tags on the intracellular termini produced no bioluminescent signal above background with the p.Glu293Lys variant in homomeric configuration and significantly reduced signals in cells co-expressing WT and p.Glu293Lys subunits simultaneously. Extracellularly presented reporter tags, however, generated comparable bioluminescent signals with heteromeric WT and p.Glu293Lys subunits and with homomeric WT channels.

CONCLUSIONS

Loss of function and dominant-negative effect confirm the causative role of p.Glu293Lys in ATS1. Co-assembly of Kir2.1 subunits is impaired in homomeric channels consisting of p.Glu293Lys subunits and is partially rescued in heteromeric complexes of WT and p.Glu293Lys Kir2.1 variants. These data point to an important role of Glu293 in mediating subunit assembly, as well as in gating of Kir2.1 channels.

Kir2.1 Interactome Mapping Uncovers PKP4 as a Modulator of the Kir2.1-Regulated Inward Rectifier Potassium Currents

Kir2.1, a strong inward rectifier potassium channel encoded by the KCNJ2 gene, is a key regulator of the resting membrane potential of the cardiomyocyte and plays an important role in controlling ventricular excitation and action potential duration in the human heart. Mutations in KCNJ2 result in inheritable cardiac diseases in humans, e.g. the type-1 Andersen-Tawil syndrome (ATS1). Understanding the molecular mechanisms that govern the regulation of inward rectifier potassium currents by Kir2.1 in both normal and disease contexts should help uncover novel targets for therapeutic intervention in ATS1 and other Kir2.1-associated channelopathies. The information available to date on protein-protein interactions involving Kir2.1 channels remains limited. Additional efforts are necessary to provide a comprehensive map of the Kir2.1 interactome. Here we describe the generation of a comprehensive map of the Kir2.1 interactome using the proximity-labeling approach BioID. Most of the 218 high-confidence Kir2.1 channel interactions we identified are novel and encompass various molecular mechanisms of Kir2.1 function, ranging from intracellular trafficking to cross-talk with the insulin-like growth factor receptor signaling pathway, as well as lysosomal degradation. Our map also explores the variations in the interactome profiles of Kir2.1WTversus Kir2.1Δ314-315, a trafficking deficient ATS1 mutant, thus uncovering molecular mechanisms whose malfunctions may underlie ATS1 disease. Finally, using patch-clamp analysis, we validate the functional relevance of PKP4, one of our top BioID interactors, to the modulation of Kir2.1-controlled inward rectifier potassium currents. Our results validate the power of our BioID approach in identifying functionally relevant Kir2.1 interactors and underline the value of our Kir2.1 interactome as a repository for numerous novel biological hypotheses on Kir2.1 and Kir2.1-associated diseases.

Skeletal Muscle Channelopathies

Skeletal muscle channelopathies are rare genetic neuromuscular conditions that include the nondystrophic myotonias and periodic paralyses. They cause disabling muscle symptoms and can limit educational potential, work opportunities, socialization, and quality of life. Effective therapy is available, making it essential to recognize and treat this group of disorders. Here, the authors highlight important aspects regarding diagnosis and management using illustrative case reports.

Bioinformatics characterisation of the (mutated) proteins related to Andersen-Tawil syndrome

In the last two decades, a group of proteins whose mutations are associated with a disease manifested by episodes of muscle weakness (periodic paralysis), changes in heart rhythm (arrhythmia), and developmental abnormalities has been under constant study. This malady is known as Andersen-Tawil syndrome, with ~60% of cases of this syndrome being caused by 16 mutations in the KCNJ2 gene [UniProt ID: P63252-01-P63252-17]. In this work, we present a computational study designed to obtain a fingerprint of Andersen-Tawil mutated proteins and differentiate them from mutated proteins associated with Brugada syndrome and from functional groups of proteins belonging to APD3, UniProt, and CPPsite databases. We show here that Andersen-Tawil mutated proteins are characterized by specific features that can be used to differentiate, with a high level of certainty (90%), proteins carrying these mutations from similar functional groups, such as mutated proteins associated with Brugada syndrome, and from different functional protein and peptide groups, such as antimicrobial peptides, Cell-Penetrating Peptides, and intrinsically disorder proteins. Therefore, our main results allow us to conjecture that it is possible to identify the group of the Andersen-Tawil mutated proteins by their "PIM profile". Furthermore, when we applied this "fingerprint PIM profile" on the UniProt database, we observed that one protein found in humans [UniProt ID: Q9NZV8], and six of all "reviewed" proteins found in living organisms, possess a very similar PIM profile as the Andersen-Tawil mutated protein group. The bioinformatics "fingerprint" of the Andersen-Tawil mutated proteins was retrieved using the in-house bioinformatics system named Polarity Index Method® and supported-at residues level- by the algorithms for the prediction of intrinsic disorder predisposition, such as PONDR® FIT, PONDR® VLXT, PONDR® VSL2, PONDR® VL3, FoldIndex, IUPred, and TopIDP.

[Analysis of clinical phenotypes and KCNJ2 gene mutations in a Chinese pedigree affected with Andersen-Tawil syndrome]

OBJECTIVE

To analyze the clinical phenotypes of a pedigree affected with periodic paralysis and explore its molecular basis.

METHODS

Clinical data and peripheral blood samples of the pedigree were collected. The proband and his father both complained of periodic paralysis and dysmorphic features. The exome of the proband was screened using Roche NimbleGen probes, and the results were confirmed by Sanger sequencing. Suspected mutations were subjected to bioinformatic and gene-disease correlation analysis.

RESULTS

A c.653G>A (p.R218Q) mutation of the KCNJ2 gene was detected in both the proband and his father. Bioinformatics analysis suggested it to be pathogenic.

CONCLUSION

The clinical manifestation of the pedigree was suggestive of Andersen-Tawil syndrome. KCNJ2 c.653G>A (p.R218Q) was the pathogenic mutation in this pedigree.

Osteogenic and Chondrogenic Master Genes Expression Is Dependent on the Kir2.1 Potassium Channel Through the Bone Morphogenetic Protein Pathway

Andersen's syndrome is a rare disorder affecting muscle, heart, and bone that is associated with mutations leading to a loss of function of the inwardly rectifying K⁺ channel Kir2.1. Although the Kir2.1 function can be anticipated in excitable cells by controlling the electrical activity, its role in non-excitable cells remains to be investigated. Using Andersen's syndrome-induced pluripotent stem cells, we investigated the cellular and molecular events during the osteoblastic and chondrogenic differentiation that are affected by the loss of the Ik1 current. We show that loss of Kir2.1 channel function impairs both osteoblastic and chondrogenic processes through the downregulation of master gene expression. This downregulation is the result of an impairment of the bone morphogenetic proteins signaling pathway through dephosphorylation of the Smad proteins. Restoring Kir2.1 channel function in Andersen's syndrome cells rescued master genes expression and restored normal osteoblast and chondrocyte behavior. Our results show that Kir2.1-mediated activity controls endochondral and intramembranous ossification signaling pathways. © 2018 American Society for Bone and Mineral Research.

Spectrum of Nondystrophic Skeletal Muscle Channelopathies in Children

BACKGROUND

The nondystrophic skeletal muscle channelopathies are a group of disorders caused by mutations of various voltage-gated ion channel genes, including nondystrophic myotonia and periodic paralysis.

METHODS

We identified patients with a diagnosis of muscle channelopathy from our neuromuscular database in a tertiary care pediatric center from 2005 to 2015. We then performed a retrospective review of their medical records for demographic characteristics, clinical features, investigations, treatment, and follow-up.

RESULTS

Thirty-three patients were identified. Seventeen had nondystrophic myotonia. Seven of them had chloride channelopathy (four Becker disease and three Thomsen disease). Warm-up phenomenon and muscle hypertrophy were common clinical manifestations in this subgroup. Ten patients had sodium channelopathy (four paramyotonia congenita and six other sodium channel myotonia). Stiffness of the facial muscles was an important presenting symptom, and eyelid myotonia was a common clinical finding in this subgroup. The majority of these patients had electrical myotonia. Mexiletine was effective in controlling the symptoms in patients who had received treatment. Sixteen children had periodic paralysis (four hyperkalemic periodic paralysis, eight hypokalemic periodic paralysis, and four Andersen-Tawil syndrome). Acetazolamide was commonly used to prevent paralytic attacks and was found to be effective.

CONCLUSIONS

Nondystrophic muscle channelopathies present with diverse clinical manifestations (myotonia, muscle hypertrophy, proximal weakness, swallowing difficulties, and periodic paralysis). Cardiac arrhythmias are potentially life threatening in Andersen-Tawil syndrome. Timely identification of these disorders is helpful for effective symptomatic management and genetic counseling.

Bioelectric signalling via potassium channels: a mechanism for craniofacial dysmorphogenesis in KCNJ2-associated Andersen-Tawil Syndrome

KEY POINTS

Xenopus laevis craniofacial development is a good system for the study of Andersen-Tawil Syndrome (ATS)-associated craniofacial anomalies (CFAs) because (1) Kcnj2 is expressed in the nascent face; (2) molecular-genetic and biophysical techniques are available for the study of ion-dependent signalling during craniofacial morphogenesis; (3) as in humans, expression of variant Kcnj2 forms in embryos causes a muscle phenotype; and (4) variant forms of Kcnj2 found in human patients, when injected into frog embryos, cause CFAs in the same cell lineages. Forced expression of WT or variant Kcnj2 changes the normal pattern of Vmem (resting potential) regionalization found in the ectoderm of neurulating embryos, and changes the normal pattern of expression of ten different genetic regulators of craniofacial development, including markers of cranial neural crest and of placodes. Expression of other potassium channels and two different light-activated channels, all of which have an effect on Vmem , causes CFAs like those induced by injection of Kcnj2 variants. In contrast, expression of Slc9A (NHE3), an electroneutral ion channel, and of GlyR, an inactive Cl(-) channel, do not cause CFAs, demonstrating that correct craniofacial development depends on a pattern of bioelectric states, not on ion- or channel-specific signalling. Using optogenetics to control both the location and the timing of ion flux in developing embryos, we show that affecting Vmem of the ectoderm and no other cell layers is sufficient to cause CFAs, but only during early neurula stages. Changes in Vmem induced late in neurulation do not affect craniofacial development. We interpret these data as strong evidence, consistent with our hypothesis, that ATS-associated CFAs are caused by the effect of variant Kcnj2 on the Vmem of ectodermal cells of the developing face. We predict that the critical time is early during neurulation, and the critical cells are the ectodermal cranial neural crest and placode lineages. This points to the potential utility of extant, ion flux-modifying drugs as treatments to prevent CFAs associated with channelopathies such as ATS.

ABSTRACT

Variants in potassium channel KCNJ2 cause Andersen-Tawil Syndrome (ATS); the induced craniofacial anomalies (CFAs) are entirely unexplained. We show that KCNJ2 is expressed in Xenopus and mouse during the earliest stages of craniofacial development. Misexpression in Xenopus of KCNJ2 carrying ATS-associated mutations causes CFAs in the same structures affected in humans, changes the normal pattern of membrane voltage potential regionalization in the developing face and disrupts expression of important craniofacial patterning genes, revealing the endogenous control of craniofacial patterning by bioelectric cell states. By altering cells' resting potentials using other ion translocators, we show that a change in ectodermal voltage, not tied to a specific protein or ion, is sufficient to cause CFAs. By adapting optogenetics for use in non-neural cells in embryos, we show that developmentally patterned K(+) flux is required for correct regionalization of the resting potentials and for establishment of endogenous early gene expression domains in the anterior ectoderm, and that variants in KCNJ2 disrupt this regionalization, leading to the CFAs seen in ATS patients.

Identification of the KCNJ2 Mutation in a Korean Family with Andersen-Tawil Syndrome and Developmental Delay

Andersen-Tawil syndrome is a rare autosomal dominant disease characterized by the clinical triad of periodic paralysis, long QT with ventricular arrhythmias, and dysmorphic facial or skeletal features. However, the phenotypic heterogeneity and poor disease awareness of this syndrome can hinder an accurate and timely diagnosis. In this study, we describe a Korean family with Andersen-Tawil syndrome with a G215D mutation of the KCNJ2 gene revealed by diagnostic exome sequencing. Two sisters had severe growth restriction, characteristic facial anomalies, and developmental delay. The father carried the same mutation with similar characteristic facial features and short stature. This family lacked periodic paralysis. This report highlights the importance of an exome study for unusual clinical manifestations, such as preand postnatal growth restriction, developmental delay, and the lack of a critical diagnostic clue, such as periodic paralysis.

[Andersen-Tawil syndrome. Efficacy of class IC drugs]

A case of a rare disease - Andersen-Tawil the syndrome (ATS) is presented. Diagnosis of ATS, 7-th molecular-genetic variant of long QT syndrome was made basing on the characteristic clinical picture (periodic stress induced syncopal states), data of ECG and its Holter monitoring (pronounced QT prolongation, bouts of polymorphic bidirectional ventricular tachycardia), typical dysmorphic features (low-set ears, small mandible, brachydactyly, fifth-digit clinodactyly). However mutation of the KCNJ2 gene typical for this variant was not detected. Problems of pathogenesis, diagnostics, and treatment of the disease are discussed with special stress on class IC antiarrhythmic drugs.

Andersen cardiodysrhythmic periodic paralysis with KCNJ2 mutations: a novel mutation in the pore selectivity filter residue

Andersen cardiodysrhythmic periodic paralysis or Andersen-Tawil syndrome includes the distinct clinical features of periodic paralysis, cardiac arrhythmia, and facial and skeletal dysmorphisms and exhibits autosomal dominant inheritance. Mutations in the KCNJ2 gene, which encodes the human inward rectifier potassium channel Kir2.1, have been identified in the majority of cases. Despite well-established clinical and molecular characteristics, treatment is still case oriented, and timely diagnosis could be delayed because of the low incidence and phenotypic heterogeneity of this disease. This article describes the clinical and molecular features of 3 cases of Andersen-Tawil syndrome in 2 families. One of the mutations (G144D) was located in the pore selectivity filter residue (which is mutated recurrently) and was considered novel. Intermittent muscle weakness in childhood warrants careful evaluation of cardiac dysrhythmia and skeletal anomalies.

[QTU pattern in a patient with the Anderson-Tawil syndrome]

The Andersen-Tawil syndrome (ATS) is an autosomal dominant or sporadic disorder characterised by periodic paralysis, cardiac arrhythmias and dysmorphic features. Mutations in KCNJ2, which encodes the inward rectifier K+ channel Kir2.1 can be found in approximately 70% of ATS cases. The genetic mechanism in the remaining 30% of patients is still unknown. We present ECG of a 19-year-old man with the Andersen-Tawil syndrome. The standard ECG revealed prominent U wave with the ATS TU pattern.

T75M-KCNJ2 mutation causing Andersen–Tawil syndrome enhances inward rectification by changing Mg2+ sensitivity

Andersen-Tawil syndrome (ATS) is a multisystem inherited disease exhibiting periodic paralysis, cardiac arrhythmias, and dysmorphic features. In this study, we characterized the KCNJ2 channels with an ATS mutation (T75M) which is associated with cardiac phenotypes of bi-directional ventricular tachycardia, syncope, and QT(c) prolongation. Confocal imaging of GFP-KCNJ2 fusion proteins showed that the T75M mutation impaired membrane localization of the channel protein, which was restored by co-expression of WT channels with T75M channels. Whole-cell patch-clamp experiments in CHO-K1 cells showed that the T75M mutation produced a loss-of-function of the channel. When both WT and the T75M were co-expressed, the T75M mutation showed dominant-negative effects on inward rectifier K+ current densities, with prominent suppression of outward currents at potentials between 0 mV and +80 mV over the E(K). Inside-out patch experiments in HEK293T cells revealed that co-expression of WT and the T75M channels enhanced voltage-dependent block of the channels by internal Mg2+, resulting in enhanced inward rectification at potentials 50 mV more positive than the E(K). We suggest that the T75M mutation causes dominant-negative suppression of the co-expressed WT KCNJ2 channels. In addition, the T75M mutation caused alteration of gating kinetics of the mutated KCNJ2 channels, i.e., increased sensitivity to intracellular Mg2+ and resultant enhancement of inward rectification. The data presented suggest that the mutation may influence clinical features, but it does not directly show this.

[Potassium channels, genetic and acquired diseases]

INTRODUCTION

K(+) channels allow the passive and selective transport of K(+) ions through the membranes. They control K(+) homeostasis, neuronal and muscular excitabilities, and neurotransmitter and hormone release.

EXEGESIS

K(+) channels are composed of pore-forming subunits associated with regulatory subunits. Many different K(+) channels have been identified.

CONCLUSION

This diversity is stressed by the growing number of genetic and acquired diseases associated with these channels.

Impaired interaction between the slide helix and the C-terminus of Kir2.1: a novel mechanism of Andersen syndrome

OBJECTIVE

Andersen syndrome (AS) is a rare genetic disease caused by mutations of the potassium channel Kir2.1 (KCNJ2). We identified two unrelated patients with mutations in the slide helix of Kir2.1 leading to AS. The functional consequences of these two mutations, Y68D and D78Y, were studied and compared with previously reported slide helix mutations.

METHODS

Channel function and surface expression were studied by voltage clamp recordings and a chemiluminescence assay in Xenopus laevis oocytes and by patch clamp recordings and fluorescence microscopy in HEK293 cells. In addition, a phosphatidylinositol bisphosphate (PIP(2)) binding assay and a yeast-two-hybrid assay were used to characterize the molecular mechanisms by which slide helix mutations cause AS.

RESULTS

Neither mutant channel produced any current, but both had dominant negative effects on Kir2.2, Kir2.3, and Kir2.4 channels. We show that Y68D, D78Y, and previously reported AS mutations are clustered on the hydrophilic, cytosolic side of the slide helix and traffic normally to the plasma membrane. The in vitro lipid binding assay indicated that Y68D or D78Y N-terminal peptides bind PIP(2) similar to wild-type peptides. Yeast-two-hybrid assays showed that AS-associated mutations disturb the interaction between the slide helix and the C-terminal domain of the channel protein.

CONCLUSION

Our experiments indicate a new disease-causing mechanism independent of trafficking and PIP(2) binding defects. Our findings suggest that the hydrophilic side of the slide helix interacts with a specific domain of the C-terminus facing the membrane. This interaction, which may be required for normal gating both in homomeric and heteromeric Kir2 channels, is disturbed by several mutations causing AS.

Management and treatment of Andersen-Tawil syndrome (ATS)

Andersen-Tawil syndrome (ATS) is characterized by periodic paralysis, cardiac arrhythmias, and distinct facial and skeletal features. The majority of patients with ATS (ATS1) have point mutations in the KCNJ2 gene, which encodes the inward-rectifying potassium channel known as Kir2.1. The skeletal muscle and cardiac symptoms are accounted for, in most cases, by a dominant negative effect of the mutations on potassium channel current, resulting in prolonged depolarization of the action potential. Mechanisms of disruption of channel function include abnormal trafficking and assembly of second messengers such as phosphatidylinositol 4,5-bisphosphate, abnormal gating of the channel, and incorrect folding of the Kir2.1 protein. Less apparent is the mechanism by which these mutations account for the typical facial and skeletal abnormalities. The concomitant involvement of cardiac and skeletal muscle in ATS poses unique treatment and management challenges. Because of differences in cardiac and skeletal muscle physiology, drugs that may have a beneficial effect on cardiac function may have a detrimental effect on skeletal muscle and vice versa. We review the clinical, laboratory, and genetic features of this disorder with particular emphasis on treatment and management.

Corticosteroid-exacerbated symptoms in an Andersen's syndrome kindred

Periodic paralysis, cardiac arrhythmia and bone features are the hallmark of Andersen's syndrome (AS), a rare disorder caused by mutations in the KCNJ2 gene that encodes for the inward rectifier K(+)-channel Kir2.1. Rest following strenuous physical activity, carbohydrate ingestion, emotional stress and exposure to cold are the precipitating triggers. Most of the mutations act in a dominant-negative fashion, either through a trafficking dysfunction or through Kir2.1-phosphatidyl inositol bisphosphate binding defect. We have identified two families that were diagnosed with periodic paralysis and cardiac abnormalities, but only discrete development features. The proband in one of the two families reported having his symptoms occurring twice within the day following corticosteroids ingestion, and alleviated after stopping the corticosteroid treatment. Electromyographic evaluations pointed out to a typical hypokalemic periodic paralysis pattern. Molecular screening of the KCNJ2 gene identified two mutations leading to C54F and T305P substitutions in the Kir2.1 protein. Functional expression in mammalian cells revealed a loss-of-function of the mutated channels and a dominant-negative effect when both mutants and wild-type channels are present in the same cell. However, channel trafficking and assembly are not affected. Substitutions at these residues may interfere with phosphatidyl inositol bisphosphate binding to Kir2.1 channels. Sensitivity of our patients to multiple corticosteroid administrations shows that care must be taken in the use of such treatments in AS patients. Taken together, our data suggest the inclusion of the KCNJ2 gene in the molecular screening of patients with periodic paralysis, even when the classical AS dysmorphic features are not present.

Sudden cardiac death in Andersen-Tawil syndrome

Andersen-Tawil syndrome (ATS) is an autosomal dominant or sporadic disorder characterized by periodic paralysis, dysmorphic features, and ventricular arrhythmias. Although ventricular tachycardia burden is quite high sudden cardiac death in ATS is rare. We describe a case with sudden cardiac death due to electrical storm a few days after ICD implantation in KCNJ2 mutation-negative ATS.

Mutations of KCNJ2 gene associated with Andersen-Tawil syndrome in Korean families

Mutations of the KCNJ2 gene are a major underlying cause of Andersen-Tawil syndrome (ATS), a rare autosomal dominant inherited disorder that is characterized by periodic paralysis, cardiac arrhythmias, and developmental dysmorphic features. The KCNJ2 gene encodes an inward rectifying K(+) channel protein, Kir2.1, which plays an important role in maintaining the homeostasis of channel current in various cell types. We have identified two missense mutations of KCNJ2 (R218Q and M307I) in two Korean families diagnosed with ATS. The M307I mutation is a novel mutation, located at the intracellular C-terminal domain, which is known to be concerned with putative phosphatidylinositol 4,5-bisphosphate (PIP(2)) binding and channel trafficking.

Genotype-phenotype correlations ofKCNJ2 mutations in Japanese patients with Andersen-Tawil syndrome

Andersen-Tawil syndrome (ATS) is a rare inherited disorder characterized by periodic paralysis, mild dysmorphic features, and QT or QU prolongation with ventricular arrhythmias in electrocardiograms (ECGs). Mutations of KCNJ2, encoding the human inward rectifying potassium channel Kir 2.1, have been identified in patients with ATS. We aimed to clarify the genotype-phenotype correlations in ATS patients. We screened 23 clinically diagnosed ATS patients from 13 unrelated Japanese families. Ten different forms of KCNJ2 mutations were identified in the 23 ATS patients included in this study. Their ECGs showed normal QTc intervals and abnormal U waves with QUc prolongation and a variety of ventricular arrhythmias. Especially, bidirectional ventricular tachycardia (VT) was observed in 13 of 23 patients (57%). Periodic paralysis was seen in 13 of 23 carriers (57%), dysmorphic features in 17 (74%), and seizures during infancy in 4 (17%). Functional assays for the two novel KCNJ2 mutations (c. 200G>A (p. R67Q) and c. 436G>A (p. G146S)) displayed no functional inward rectifying currents in a heterologous expression system and showed strong dominant negative effects when co-expressed with wild-type KCNJ2 channels (91% and 84% reduction at -50 mV respectively compared to wild-type alone). Immunocytochemistry and confocal imaging revealed normal trafficking for mutant channels. In our study, all of the clinically diagnosed ATS patients had KCNJ2 mutations and showed a high penetrance with regard to the typical cardiac phenotypes: predominant U wave and ventricular arrhythmias, typically bidirectional VT.

An andersen-Tawil syndrome mutation in Kir2.1 (V302M) alters the G-loop cytoplasmic K+ conduction pathway

Loss-of-function mutations in the inward rectifier potassium channel, Kir2.1, cause Andersen-Tawil syndrome (ATS-1), an inherited disorder of periodic paralysis and ventricular arrhythmias. Here, we explore the mechanism by which a specific ATS-1 mutation (V302M) alters channel function. Val-302 is located in the G-loop, a structure that is believed to form a flexible barrier for potassium permeation at the apex of the cytoplasmic pore. Consistent with a role in stabilizing the G-loop in an open conformation, we found the V302M mutation specifically renders the channel unable to conduct potassium without altering subunit assembly or attenuating cell surface expression. As predicted by the position of the Val-302 side chain in the crystal structure, amino acid substitution analysis revealed that channel activity and phosphatidylinositol 4,5-bisphosphate (PIP2) sensitivity are profoundly sensitive to alterations in the size, shape, and hydrophobicity of side chains at the Val-302 position. The observations establish that the Val-302 side chain is a critical determinant of potassium conduction through the G-loop. Based on our functional studies and the cytoplasmic domain crystal structure, we suggest that Val-302 may influence PIP2 gating indirectly by translating PIP2 binding to conformational changes in the G-loop pore.

Andersen syndrome: an association of periodic paralysis, cardiac arrhythmia and dysmorphic abnormalities

Andersen syndrome (AS) is a rare disease characterized by the presence of periodic paralysis (PP), cardiac arrhythmia and dysmorphic abnormalities. We report herein the first Brazilian patient presenting AS who also had obesity, obstructive sleep apnea (OSA) and daytime sleepiness. Clinical and genetic evaluation of six family members demonstrated that four had dysmorphic abnormalities but none had PP or cardiac arrhythmia. Sequencing of KCNJ2 revealed the R218W mutation in the index patient and her 6-year-old daughter, who presented dysmorphic abnormalities (micrognathia, clinodactyly of fourth and fifth fingers, short stature) and OSA. Three relatives had clinodactyly as the only manifestation but the R218W mutation was absent, suggesting that this characteristic may be influenced by another gene. OSA accompanied by dysmorphic features may be related to AS.

Andersen's syndrome mutation effects on the structure and assembly of the cytoplasmic domains of Kir2.1

Kir2.1 channels play a key role in maintaining the correct resting potential in eukaryotic cells. Recently, specific amino acid mutations in the Kir2.1 inwardly rectifying potassium channel have been found to cause Andersen's Syndrome in humans. Here, we have characterized individual Andersen's Syndrome mutants R218Q, G300V, E303K, and delta314-315 and have found multiple effects on the ability of the cytoplasmic domains in Kir2.1 channels to form proper tetrameric assemblies. For the R218Q mutation, we identified a second site mutation (T309K) that restored tetrameric assembly but not function. We successfully crystallized and solved the structure (at 2.0 A) of the N- and C-terminal cytoplasmic domains of Kir2.1-R218Q/T309K(S). This new structure revealed multiple conformations of the G-loop and CD loop, providing an explanation for channels that assemble but do not conduct ions. Interestingly, Glu303 forms both intra- and intersubunit salt bridges, depending on the conformation of the G-loop, suggesting that the E303K mutant stabilizes both closed and open G-loop conformations. In the Kir2.1-R218Q/T309K(S) structure, we discovered that the DE loop forms a hydrophobic pocket that binds 2-methyl-2,4-pentanediol, which is located near the putative G(betagamma)-activation site of Kir3 channels. Finally, we observed a potassium ion bound to the cytoplasmic domain for this class of K+ channels.

Electrophysiological mechanisms of ventricular arrhythmias in relation to Andersen-Tawil syndrome under conditions of reduced IK1: a simulation study

Patients with Andersen-Tawil syndrome (ATS) mostly have mutations on the KCNJ2 gene, producing loss of function or dominant-negative suppression of the inward rectifier K(+) channel Kir2.1. However, clinical manifestations of ATS including dysmorphic features, periodic paralysis (hypo-, hyper-, or normokalemic), long QT, and ventricular arrhythmias (VAs) are considerably variable. Using a modified dynamic Luo-Rudy simulation model of cardiac ventricular myocytes, we attempted to elucidate mechanisms of VA in ATS by analyzing effects of the inward rectifier K(+) channel current (I(K1)) on the action potential (AP). During pacing at 1.0 Hz with extracellular K(+) concentration ([K(+)](o)) at 4.5 mM, a stepwise 10% reduction of Kir2.1 channel conductance progressively prolonged the terminal repolarization phase of the AP along with gradual depolarization of the resting membrane potential (RMP). At 90% reduction, early afterdepolarizations (EADs) became inducible and RMP was depolarized to -52.0 mV (control: -89.8 mV), followed by emergence of spontaneous APs. Both EADs and spontaneous APs were facilitated by a decrease in [K(+)](o) and suppressed by an increase in [K(+)](o). Simulated beta-adrenergic stimulation enhanced delayed afterdepolarizations (DADs) and could also facilitate EADs as well as spontaneous APs in the setting of low [K(+)](o) and reduced Kir2.1 channel conductance. In conclusion, the spectrum of VAs in ATS may include 1) triggered activity mediated by EADs and/or DADs and 2) abnormal automaticity manifested as spontaneous APs. These VAs can be aggravated by a decrease in [K(+)](o) and beta-adrenergic stimulation and may potentially induce torsade de pointes and cause sudden death. In patients with ATS, the hypokalemic form of periodic paralysis should have the highest propensity to VAs, especially during physical activity.

Andersen-Tawil syndrome: definition of a neurocognitive phenotype

BACKGROUND

The Andersen-Tawil syndrome (ATS) is a potassium ion channelopathy caused by mutations in the KCNJ2 gene. It is characterized by periodic paralysis, cardiac arrhythmias, and distinctive features; the effect of KCNJ2 mutations on the CNS has never been studied.

OBJECTIVE

To define a potential CNS phenotype in ATS using standardized methods.

METHODS

Ten subjects with KCNJ2 mutations and their unaffected siblings were evaluated at the University of California San Francisco General Clinical Research Center. A comprehensive battery of neurocognitive tests was administered to ATS subjects and their unaffected siblings, followed by pairwise analysis of the resultant differences in scores. An EEG was obtained for all ATS subjects.

RESULTS

There was no EEG evidence of subclinical seizure activity in any subject. ATS subjects universally had more school difficulties than their siblings, despite similar IQ between the two groups. On formal neurocognitive testing, there was no difference between ATS subjects and their siblings on tests of verbal and visual memory. Assessment of executive functioning revealed ATS subjects scored 1.93 points lower than their siblings on tests of Design Fluency (95% CI -3.46, 0.01; p = 0.052) and made 1.9 more errors (95% CI 0.46, 2.54; p = 0.005). Subjects with ATS scored an average of 5 points lower than their siblings on tests of matrix reasoning (95% CI -8.67, -1.33; p = 0.008). On tests of general ability, ATS subjects achieved much lower scores than their siblings, with an average difference of 9.13 points for reading (95% CI -12.46, 3.21; p = 0.056) and 23.4 points for mathematics (95% CI -42.53, -4.22; p = 0.017).

CONCLUSION

Mutations in KCNJ2 are associated with a distinct neurocognitive phenotype, characterized by deficits in executive function and abstract reasoning.

Functional and clinical characterization of a mutation in KCNJ2 associated with Andersen-Tawil syndrome

BACKGROUND

Andersen-Tawil syndrome (ATS) is a rare inherited disorder, characterised by periodic paralysis, cardiac dysarrhythmias, and dysmorphic features, and is caused by mutations in the gene KCNJ2, which encodes the inward rectifier potassium channel, Kir2.1. This study sought to analyse KCNJ2 in patients with familial ATS and to determine the functional characteristics of the mutated gene.

METHODS AND RESULTS

We screened a family with inherited ATS for the mutation in KCNJ2, using direct DNA sequencing. A missense mutation (T75R) of Kir2.1, located in the highly conserved cytoplasmic N-terminal domain, was identified in three affected members of this family. Using the Xenopus oocyte expression system and whole cell voltage clamp analyses, we found that the T75R mutant was non-functional and possessed a strong dominant negative effect when co-expressed with the same amount of wild type Kir2.1. Transgenic (Tg) mice expressing the mutated form of Kir2.1 in the heart had prolonged QTc intervals compared with mice expressing the wild type protein. Ventricular tachyarrhythmias were observed in 5 of 14 T75R-Tg mice compared with 1 of 7 Wt-Tg and none of 6 non-transgenic littermates. In three of five T75R-Tg mice with ventricular tachycardia, their ECG disclosed bidirectional tachycardia as in our proband.

CONCLUSIONS

The in vitro studies revealed that the T75R mutant of Kir2.1 had a strong dominant negative effect in the Xenopus oocyte expression system. It still preserved the ability to co-assemble and traffic to the cell membrane in mammalian cells. For in vivo studies, the T75R-Tg mice had bidirectional ventricular tachycardia after induction and longer QT intervals.

Cellular basis for electrocardiographic and arrhythmic manifestations of Andersen-Tawil syndrome (LQT7)

BACKGROUND

Andersen-Tawil syndrome, a skeletal muscle syndrome associated with periodic paralysis and long QT intervals on the ECG, has been linked to defects in KCNJ2, the gene encoding for the inward rectifier potassium channel (I(K1).)

OBJECTIVES

The purpose of this study was to examine the cellular mechanisms underlying the ECG and arrhythmic manifestations of Andersen-Tawil syndrome.

METHODS

To investigate the effects of KCNJ2 loss-of-function mutations responsible for Andersen-Tawil syndrome, we used barium chloride (BaCl(2)) to inhibit I(K1) in arterially perfused wedge preparation. Transmembrane action potentials (APs) were simultaneously recorded from endocardial, midmyocardial, and epicardial cells, together with a transmural ECG.

RESULTS

BaCl(2) (1 to 30 microM) produced a concentration-dependent prolongation of the QT interval, secondary to a homogeneous prolongation of AP duration of the three cell types. QT interval was prolonged without an increase in transmural dispersion of repolarization (TDR). Low extracellular potassium (2.0 mM), isoproterenol (20-50 nM), and an abrupt increase in temperature (36 degrees C-39 degrees C) in the presence of 10 microM BaCl(2) did not significantly increase TDR but increased ectopic extrasystolic activity. Early afterdepolarizations were not observed under any condition. Spontaneous torsades de pointes arrhythmias were never observed, nor could they be induced with programmed electrical stimulation under any of the conditions studied.

CONCLUSION

Our results provide an understanding of why QT prolongation associated with Andersen-Tawil syndrome is relatively benign in the clinic and provide further support for the hypothesis that the increase in TDR, rather than QT interval, is responsible for development of torsades de pointes.

Andersen-Tawil syndrome: Prospective cohort analysis and expansion of the phenotype

Andersen-Tawil syndrome (ATS) is an autosomal dominant multisystem disorder characterized by developmental, cardiac, and neuromuscular abnormalities. Approximately 70% of patients have mutations in KCNJ2, resulting in dysfunction of the inward-rectifying potassium channel Kir2.1. Variable expression complicates the diagnosis of ATS, which in many cases, is not made until years after the first recognized symptom. To better define the distinctive clinical features of ATS and facilitate earlier diagnosis, we conducted a prospective, standardized evaluation of 10 subjects with confirmed KCNJ2 mutations. Detailed anthropometric, neurological, and cardiac evaluations were performed. Using this approach, we identified novel skeletal and dental findings and proposed additional diagnostic criteria for ATS dysmorphology.

Trafficking-competent and trafficking-defectiveKCNJ2 mutations in Andersen syndrome

Mutations in KCNJ2, the gene encoding the human inward rectifier potassium channel Kir2.1, have been identified in Andersen syndrome (or Andersen-Tawil syndrome), an inherited disorder characterized by periodic paralysis, cardiac arrhythmias, and dysmorphic features. We identified and characterized two novel KCNJ2 mutations (c.220A>G/p.T74A and c.443G>C/p.G144A) associated with Andersen syndrome. Heterologous expression of a recombinant wild type human KCNJ2 cDNA (WT-KCNJ2) in HEK-293 cells results in robust inward rectifying currents, but we did not observe measurable currents from cells expressing either mutant. Cells co-transfected with WT-KCNJ2 and either mutant exhibited substantially lower whole-cell current amplitude consistent with a dominant-negative suppression of WT-KCNJ2 by the mutant channels. Both p.T74A and p.G144A exhibit robust plasma membrane expression, but a third previously reported allele (p.C101R) exhibited impaired trafficking. Our results demonstrate functional consequences of two novel trafficking-competent KCNJ2 mutations associated with Andersen syndrome and expand our knowledge of allelic diversity in this disease.

Andersen-Tawil syndrome: New potassium channel mutations and possible phenotypic variation

OBJECTIVE

To evaluate clinical, genetic, and electrophysiologic features of patients with Andersen-Tawil syndrome (ATS) in the United Kingdom.

METHODS

Clinical and neurophysiologic evaluation was conducted of 11 families suspected to have ATS. Molecular genetic analysis of each proband was performed by direct DNA sequencing of the entire coding region of KCNJ2. Control samples were screened by direct DNA sequencing. The electrophysiologic consequences of several new mutations were studied in an oocyte expression system.

RESULTS

All 11 ATS families harbored pathogenic mutations in KCNJ2 with six mutations not previously reported. Some unusual clinical features including renal tubular defect, CNS involvement, and dental and phonation abnormalities were observed. Five mutations (T75M, D78G, R82Q, L217P, and G300D) were expressed, all of which resulted in nonfunctional channels when expressed alone, and co-expression with wild-type (WT) KCNJ2 demonstrated a dominant negative effect.

CONCLUSION

Six new disease-causing mutations in KCNJ2 were identified, one of which was in a PIP2 binding site. Molecular expression studies indicated that five of the mutations exerted a dominant negative effect on the wild-type allele. KCNJ2 mutations are an important cause of ATS in the UK.

Electrocardiographic features in Andersen-Tawil syndrome patients with KCNJ2 mutations: characteristic T-U-wave patterns predict the KCNJ2 genotype

BACKGROUND

The ECG features of Andersen-Tawil syndrome (ATS) patients with KCNJ2 mutations (ATS1) have not been systematically assessed. This study aimed to define ECG features of KCNJ2 mutation carriers, to determine whether characteristic T-U-wave patterns exist, and to establish whether T-U patterns predict the ATS1 genotype.

METHODS AND RESULTS

In phase I, evaluation of T-U morphology in ECGs of 39 KCNJ2 mutation carriers identified characteristic T-U patterns: prolonged terminal T downslope, wide T-U junction, and biphasic and enlarged U waves. In phase II, ATS1 genotype prediction by T-U pattern was evaluated in the next 147 ECGs (57 other KCNJ2 mutation carriers, 61 unaffected family members, and 29 ATS patients without KCNJ2 mutations), with a sensitivity of 84% and specificity of 97%. Characteristic T-U patterns were present in 91% (87/96), in whom an enlarged U wave was predominant (73%). In phase III, QTc, QUc, and T- and U-wave duration/amplitude were compared in the 96 ATS1, 29 non-KCNJ2 ATS, and 75 normal subjects. In ATS1 patients, QUc, U-wave duration and amplitude, and QTc were all increased (P<0.001), but median QTc and interquartile range (IQR) were just 440 ms (IQR, 28 ms) compared with 420 ms (IQR, 20 ms) in normal subjects and 425 ms (IQR, 48 ms) in ATS non-KCNJ2 patients.

CONCLUSIONS

In ATS1 patients, gene-specific T-U-wave patterns resulting from decreased IK1 owing to KCNJ2 mutations can aid diagnosis and direct genotyping. The normal QTc, distinct ECG, and other clinical features distinguish ATS1 from long-QT syndrome, and it is best designated as ATS1 rather than LQT7.