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.

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.

Phenotypical variability and atypical presentations in a French cohort of Andersen-Tawil syndrome.

BACKGROUND AND PURPOSE: Andersen-Tawil syndrome (ATS) is a skeletal muscle channelopathy caused by KCNJ2 mutations, characterized by a clinical triad of periodic paralysis, cardiac arrhythmias and dysmorphism. The muscle phenotype, particularly the atypical forms with prominent permanent weakness or predominantly painful symptoms, remains incompletely characterized. METHODS: A retrospective clinical, histological, electroneuromyography (ENMG) and genetic analysis of molecularly confirmed ATS patients, diagnosed and followed up at neuromuscular reference centers in France, was conducted. RESULTS: Thirty-five patients from 27 unrelated families carrying 17 different missense KCNJ2 mutations (four novel mutations) and a heterozygous KCNJ2 duplication are reported. The typical triad was observed in 42.9% of patients. Cardiac abnormalities were observed in 65.7%: 56.5% asymptomatic and 39.1% requiring antiarrhythmic drugs. 71.4% of patients exhibited dysmorphic features. Muscle symptoms were reported in 85.7%, amongst whom 13.3% had no cardiopathy and 33.3% no dysmorphic features. Periodic paralysis was present in 80% and was significantly more frequent in men. Common triggers were exercise, immobility and carbohydrate-rich diet. Ictal serum potassium concentrations were low in 53.6%. Of the 35 patients, 45.7% had permanent weakness affecting proximal muscles, which was mild and stable or slowly progressive over several decades. Four patients presented with exercise-induced pain and myalgia attacks. Diagnostic delay was 14.4 ± 9.5 years. ENMG long-exercise test performed in 25 patients (71.4%) showed in all a decremental response up to 40%. Muscle biopsy performed in 12 patients revealed tubular aggregates in six patients (associated in two of them with vacuolar lesions), dystrophic features in one patient and non-specific myopathic features in one patient; it was normal in four patients. DISCUSSION: Recognition of atypical features (exercise-induced pain or myalgia and permanent weakness) along with any of the elements of the triad should arouse suspicion. The ENMG long-exercise test has a high diagnostic yield and should be performed. Early diagnosis is of utmost importance to improve disease prognosis.

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.

Distinctive facial features in Andersen-Tawil syndrome: A three-dimensional stereophotogrammetric analysis.

Andersen-Tawil syndrome (ATS) is a rare potassium channelopathy causing periodic paralysis, cardiac arrhythmias, and dysmorphic features. A detailed analysis of the face could facilitate diagnosis of ATS, as approximately 30% of patients do not show variants in KCNJ2 gene, and diagnosis is established by clinical findings. We aimed to characterize the face in ATS through a quantitative approach, as facial anomalies may be unnoticed on visual inspection. Facial images of 12 subjects with genetically confirmed ATS (six males, six females, age 5-67 years) were acquired through stereophotogrammetry. Using 38 soft-tissue landmarks, linear distances, angles, and ratios were calculated and expressed as z-score values, with reference to 477 healthy subjects matched for sex and age. All patients showed decreased lower facial height with shortening of philtrum (mean z-score ± SD: -1.5 ± 0.9), smaller mid and lower facial depths (-1.9 ± 0.7; -2.3 ± 0.9), short palpebral fissures (right -1.2 ± 0.4; left -1.6 ± 0.6), smaller mandibular ramus length (-2.1 ± 0.4), and increased nasal width/length ratio (1.4 ± 0.5) with smaller nostril axis length (right -1.8 ± 0.8, left -1.6 ± 0.7). Hypertelorism and low-set ears were detected in two-thirds of patients. The study quantified facial dysmorphysm in ATS, extending information about known features, and detecting unrecorded philtrum and nostril characteristics, which may be distinctive traits of the disorder.

Failure of radiofrequency catheter ablation and success of flecainide to suppress premature ventricular contractions in Andersen-Tawil syndrome: A case report.

This case report presents a 33-year-old woman with premature ventricular contractions (PVCs). Her genetic testing was positive for KCNJ2 missense mutation at chr17:68171832;NM_000891.2. This mutation was compatible with Andersen-Tawil syndrome. We made an electrophysiological study to determine origin of PVCs however at endocardial mapping there was not any focus of PVC and at epicardial mapping we ablated low voltage areas in the inferior segments of both ventricles. She was discharged with flecainide and metoprolol therapy. After 3 months, her PVC burden was significantly decreased at Holter monitoring.

Obstetric management of a patient with Andersen-Tawil syndrome: A case report.

Andersen-Tawil syndrome (ATS) is a rare hereditary long QT syndrome type 7 caused by a missense mutation in the KCNJ2 gene. ATS is characterized by ventricular arrhythmia, periodic limb paralysis and minor external malformations. Although only three reports of pregnant women with Andersen-Tawil syndrome have been reported to date, no exacerbation of ventricular arrhythmia was observed from pre-partum to delivery in all cases compared to that before pregnancy, and it was suggested that the risk of arrhythmic events from pre-partum to delivery is not high. Unlike these previous reports, we herein present a case of Andersen-Tawil syndrome in which ventricular arrhythmias increased and sustained ventricular tachycardia was developed during labor progression. We also advise caution that pregnant patients with Andersen-Tawil syndrome may have varying times of exacerbation of the arrhythmia, and ventricular arrhythmias may be associated with painful uterine contractions.

Multivariate analysis of TU wave complex on electrocardiogram in Andersen-Tawil syndrome with KCNJ2 mutations.

BACKGROUND: The exact differences between the TU wave complex of ATS1 and that of healthy individuals remain to be investigated. We sought to characterize the TU wave complex of Andersen-Tawil syndrome type 1 (ATS1) using high frequency electrocardiogram (ECG) data. METHODS: Electrocardiograms were recorded as time series data with a 2 kHz frequency ECG amplifier in 13 patients with ATS1 (positive for KCNJ2 mutation, ATS1 group) and age-matched healthy individuals (control group). Conventional ECG parameters were measured, and principal component analysis (PCA) and independent component analysis (ICA) were applied to the TU wave complex. RESULTS: Time from T peak (Tp) to U peak (Up), time from bottom (B) to Up, and time from B to U end (BUe, U duration) (0.232 ± 0.018 vs. 0.165 ± 0.017, p < .0001), where B is the lowest point between T and U waves, were all longer in the ATS1 group than the control group. Multivariate logistic regression analysis revealed that BUe could completely differentiate the two groups. PCA ratios in the ATS1 group were significantly larger than the control group (26.5 ± 12.3 vs. 10.4 ± 6.2, p = .0005). ICA revealed 1 or 2 U-wave-specific independent components (ICs) that exclusively comprise the U wave in ATS1, whereas U waves in the control group were composed of some ICs that also comprised T waves. CONCLUSIONS: U-wave-related temporal parameters, particularly BUe, and the existence of U-wave-specific ICs, extracted in the ICA, are useful for differentiation of U waves in ATS1 from those in healthy individuals.

Clinical and neurophysiological variability in Andersen-Tawil syndrome.

INTRODUCTION: Andersen-Tawil syndrome (ATS) is characterized by a triad of periodic paralysis, ventricular arrhythmias, and dysmorphism. However, patients often lack one or more of these features. METHODS: Clinical and neurophysiological features were reviewed of five members in two families with heterozygous mutations in KCNJ2 (R218Q and R67W). RESULTS: Only one patient had all features of the triad of ATS. One patient had low-set ears, and the others had minor anomalies. Bidirectional ventricular tachycardias were seen in two patients. Two patients (R67W) never had episodes of paralysis. The long exercise test was abnormal in three patients with episodes of paralysis, but normal in two without paralytic episodes. DISCUSSION: ATS patients without skeletal muscle symptoms can have normal neurophysiological examinations. They can show variability in phenotype or the severity of arrhythmias. Such variability among patients who share the same gene mutations may result in underdiagnosis of ATS.

Short-term response to phenytoin sodium in Andersen-Tawil syndrome-1 with a cardiac-dominant phenotype.

BACKGROUND: Andersen-Tawil syndrome (ATS) is a rare familial periodic paralysis that typically also affects the heart and skeletal system. Ventricular arrhythmias (VAs) are profound and difficult to control, but minimally symptomatic. In this report, we describe an atypical phenotype of ATS in two related families. We also report our experience with phenytoin sodium for the control of resistant VAs in these patients. METHODS AND RESULTS: Between 2014 and 2018, seven siblings were diagnosed with ATS on the basis of cardiac arrhythmias and genetic evaluation. Heterozygous mutation with c.431G > C (p.G144A) in exon 2 of KCNJ2 gene was observed in all patients. Characteristic cardiac manifestations were noted in all patients but periodic paralysis or objective neurological involvement was distinctly absent. Phenytoin was considered for control of symptomatic VA in three patients. Intake of oral phenytoin (5 mg/kg/day) for 1 month completely suppressed VA (<1% in 24-h Holter monitoring) in two patients, and significantly in the third (8% per 24 h) patient. Phenytoin was well-tolerated in all three patients. CONCLUSIONS: We describe a cardiac-predominant phenotype in ATS. ATS should be suspected in patients with typical cardiac manifestations even in the absence of periodic paralysis. Our initial experience with short-term use of phenytoin for control of resistant VAs is encouraging.

Coincidence of Andersen-Tawil syndrome and Marfan syndrome: A case report.

We report on a 44-year-old woman with coincidence of two genetic disorders: Andersen-Tawil syndrome and Marfan syndrome. In both, life-threatening arrhythmias could occur. A 44-year-old woman presented acute ascending aortic dissection with aortic arch involvement and chronic thoracic descending and abdominal aortic dissection. Clinical and genetic examination confirmed Marfan syndrome (MFS) diagnosis. Due to repolarization disorder in ECG and premature ventricular contractions in Holter ECG, the sequencing data were analyzed again and mutation in KCNJ2 gene was identified. The case showed that coincidence of Andersen-Tawil syndrome (ATS) and MFS did not provoke life-threatening arrhythmias. Complication was rather caused by expression of FBN1 mutation.

Andersen's syndrome mutants produce a knockdown of inwardly rectifying K+ channel in mouse skeletal muscle in vivo.

Andersen's syndrome (AS) is a rare autosomal disorder that has been defined by the triad of periodic paralysis, cardiac arrhythmia, and developmental anomalies. AS has been directly linked to over 40 different autosomal dominant negative loss-of-function mutations in the KCNJ2 gene, encoding for the tetrameric strong inward rectifying K+ channel KIR2.1. While KIR2.1 channels have been suggested to contribute to setting the resting membrane potential (RMP) and to control the duration of the action potential (AP) in skeletal and cardiac muscle, the mechanism by which AS mutations produce such complex pathophysiological symptoms is poorly understood. Thus, we use an adenoviral transduction strategy to study in vivo subcellular distribution of wild-type (WT) and AS-associated mutant KIR2.1 channels in mouse skeletal muscle. We determined that WT and D71V AS mutant KIR2.1 channels are localized to the sarcolemma and the transverse tubules (T-tubules) of skeletal muscle fibers, while the ∆314-315 AS KIR2.1 mutation prevents proper trafficking of the homo- or hetero-meric channel complexes. Whole-cell voltage-clamp recordings in individual skeletal muscle fibers confirmed the reduction of inwardly rectifying K+ current (IK1) after transduction with ∆314-315 KIR2.1 as compared to WT channels. Analysis of skeletal muscle function revealed reduced force generation during isometric contraction as well as reduced resistance to muscle fatigue in extensor digitorum longus muscles transduced with AS mutant KIR2.1. Together, these results suggest that KIR2.1 channels may be involved in the excitation-contraction coupling process required for proper skeletal muscle function. Our findings provide clues to mechanisms associated with periodic paralysis in AS.

Different responses to exercise between Andersen-Tawil syndrome and catecholaminergic polymorphic ventricular tachycardia.

Aims: Andersen-Tawil Syndrome (ATS) and catecholaminergic polymorphic ventricular tachycardia (CPVT) are both inherited arrhythmic disorders characterized by bidirectional ventricular tachycardia (VT). The aim of this study was to evaluate the diagnostic value of exercise stress tests for differentiating between ATS and CPVT. Methods and results: We included 26 ATS patients with KCNJ2 mutations from 22 families and 25 CPVT patients with RyR2 mutations from 22 families. We compared the clinical and electrocardiographic (ECG) characteristics, responses of ventricular arrhythmias (VAs) to exercise testing, and the morphology of VAs between ATS and CPVT patients. Ventricular arrhythmias were more frequently observed at baseline in ATS patients compared with CPVT patients [the ratio of ventricular premature beats (VPBs)/sinus: 0.83 ± 1.87 vs. 0.06 ± 0.30, P = 0.01]. At peak exercise, VAs were suppressed in ATS patients, whereas they were increased in CPVT patients (0.14 ± 0.40 vs. 1.94 ± 2.71, P < 0.001). Twelve-lead ECG showed that all 25 VPBs and 15 (94%) of 16 bidirectional VTs were right bundle branch block (RBBB) morphology in ATS patients, whereas 19 (86%) of 22 VPBs had left bundle branch block (LBBB), and 12 (71%) of 17 bidirectional VT had LBBB and RBBB morphologies in CPVT patients. Conclusion: In patients with ATS, VAs with RBBB morphology were frequently observed at baseline and suppressed at peak exercise. In contrast, exercise provoked VAs with mainly LBBB morphology in patients with CPVT. In adjunct to clinical and baseline ECG assessments, exercise testing might be useful for making the diagnosis of ATS vs. CPVT, both characterized by bidirectional VT.

[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.

Characterization of a novel KCNJ2 sequence variant detected in Andersen-Tawil syndrome patients.

BACKGROUND: Mutations in the KCNJ2 gene encoding the ion channel Kir2.1 have been linked to the Andersen-Tawil syndrome (ATS). Molecular genetic screening performed in a family exhibiting clinical ATS phenotypes unmasked a novel sequence variant (c.434A > G, p.Y145C) in this gene. The aim of this study was to investigate the effect of this variant on Kir2.1 ion channel functionality. METHODS: Mutant as well as wild type GFP tagged Kir2.1 channels were expressed in HEK293 cells. In order to examine the effect of the new variant, electrophysiological measurements were performed using patch clamp technique. Cellular localization of the mutant in comparison to the wild type ion channel was analyzed by confocal laser scanning microscopy. RESULTS: The currents of cells expressing only mutant channels or a mixture of wild type and mutant were significantly reduced compared to those expressing wild type (WT) channels (p < 0.01). Whereas WT expressing cells exhibited at -120 mV an averaged current of -4.5 ± 1.9 nA, the mutant generates only a current of -0.17 ± 0.07 nA. A co-expression of mutant and WT channel generates only a partial rescue of the WT current. Confocal laser scanning microscopy indicated that the novel variant is not interfering with synthesis and/or protein trafficking. CONCLUSIONS: The detected sequence variant causes loss-of-function of the Kir2.1 channel and explains the clinical phenotypes observed in Andersen-Tawil syndrome patients.

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.

Clinical features and long exercise test in Chinese patients with Andersen-Tawil syndrome.

INTRODUCTION: Andersen-Tawil syndrome (ATS) is a rare multisystem channelopathy characterized by periodic paralysis, ventricular arrhythmias, and developmental dysmorphology. There are few reports concerning ATS in the Chinese population. We analyzed clinical features and evaluated the long exercise test as a tool for diagnosis of periodic paralysis in ATS. METHODS: Direct sequencing of KCNJ2 was performed in 12 subjects from mainland China with suspected ATS. Clinical features, therapeutic responses, and long exercise tests (LET) were retrospectively analyzed. RESULTS: Twelve patients were genetically confirmed to have ATS. A small mandible and clinodactyly were demonstrated in all patients. Premature ventricular contractions were the most prevalent form of cardiac arrhythmia. The LET revealed an early amplitude decrement. CONCLUSIONS: Chinese ATS patients shared some common clinical features with reported subjects in other countries. An early amplitude decrement in LET may be useful for diagnosis of ATS. Muscle Nerve 54: 1059-1063, 2016.

Coexistence of Andersen-Tawil Syndrome with Polymorphisms in hERG1 Gene (K897T) and SCN5A Gene (H558R) in One Family.

BACKGROUND: Andersen-Tawil Syndrome (ATS) is a channelopathy caused by mutations in KCNJ2 gene. It is characterized by symptoms of ventricular arrhythmias, periodic paralysis or muscle weakness, and dysmorphic features. ATS can present with the triad of symptoms, any combination or none of them. Risk factors for dangerous arrhythmias are unknown. The study assessed the impact of K897T polymorphism in hERG1 gene and H558R polymorphism in SCN5A gene coexisting with R218Q mutation in KCNJ2 in one family on clinical manifestation. METHODS: Family members underwent clinical assessment, ECG and genotyping. Holter monitoring was performed in mutation carriers and additionally in one family member with no mutation, but with K897T polymorphism. RESULTS: Proband with ATS mutation, K897T and H558R polymorphisms and proband's sister with ATS mutation and K897T polymorphism presented following symptoms: loss of consciousness, bidirectional and polymorphic ventricular tachycardia and about 5000 ventricular extrasystoles. Symptoms presented by the member with only the ATS mutation and by member with ATS mutation and H558R polymorphism were not as severe. U wave appeared in all examined family members regardless of the mutation presence. Studied individuals with ATS mutation had the T-peak-U-peak interval longer than 200 ms. In all ATS mutation carriers it was longer than in family members with no mutation. T-peak-T-end interval was the longest (>120 ms) in members with coexisting mutation and K897T polymorphism. CONCLUSION: ATS severity possibly depends on other genes' polymorphisms. In the presented family, it could depend on the presence of K897T polymorphism in hERG1.