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.

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.

Andersen-Tawil Syndrome With Novel Mutation in KCNJ2: Case Report

Andersen-Tawil syndrome (ATS) is a rare autosomal dominant disorder characterized by a classic symptom triad: periodic paralysis, ventricular arrhythmias associated with prolonged QT interval, and dysmorphic skeletal and facial features. Pathogenic variants of the inwardly rectifying potassium channel subfamily J member 2 (KCNJ2) gene have been linked to the ATS. Herein, we report a novel KCNJ2 causative variant in a proband and her father showing different ATS-associated symptoms. A 15-year-old girl was referred because of episodic weakness and periodic paralysis in both legs for 2-3 months. The symptoms occurred either when she was tired or after strenuous exercise. These attacks made walking or climbing stairs difficult and lasted from one to several days. She had a short stature (142 cm, <3rd percentile) and weighed 40 kg. The proband also showed orbital hypertelorism, dental crowding, mandibular hypoplasia, fifth-digit clinodactyly, and small hands. Scoliosis in the thoracolumbar region was detected by chest X-ray. Since she was 7 years old, she had been treated for arrhythmia-associated long QT interval and underwent periodic echocardiography. Brain MRI revealed cerebrovascular abnormalities indicating absence or hypoplasia of bilateral internal carotid arteries, and compensation of other collateral vessels was observed. There were no specific findings related to intellectual development. The proband's father also had a history of periodic paralysis similar to the proband. He did not show any cardiac symptoms. Interestingly, he was diagnosed with hyperthyroidism during an evaluation for paralytic symptoms. Clinical exome sequencing revealed a novel heterozygous missense variant: Chr17(GRCh37):g.68171593A>T, NM_000891.2:c.413A>T, p.(Glu138Val) in KCNJ2 in the proband and the proband's father.

Andersen-Tawil Syndrome Is Associated With Impaired PIP2 Regulation of the Potassium Channel Kir2.1

Andersen-Tawil syndrome (ATS) type-1 is associated with loss-of-function mutations in KCNJ2 gene. KCNJ2 encodes the tetrameric inward-rectifier potassium channel Kir2.1, important to the resting phase of the cardiac action potential. Kir-channels' activity requires interaction with the agonist phosphatidylinositol-4,5-bisphosphate (PIP₂). Two mutations were identified in ATS patients, V77E in the cytosolic N-terminal "slide helix" and M307V in the C-terminal cytoplasmic gate structure "G-loop." Current recordings in Kir2.1-expressing HEK cells showed that each of the two mutations caused Kir2.1 loss-of-function. Biotinylation and immunostaining showed that protein expression and trafficking of Kir2.1 to the plasma membrane were not affected by the mutations. To test the functional effect of the mutants in a heterozygote set, Kir2.1 dimers were prepared. Each dimer was composed of two Kir2.1 subunits joined with a flexible linker (i.e. WT-WT, WT dimer; WT-V77E and WT-M307V, mutant dimer). A tetrameric assembly of Kir2.1 is expected to include two dimers. The protein expression and the current density of WT dimer were equally reduced to ~25% of the WT monomer. Measurements from HEK cells and Xenopus oocytes showed that the expression of either WT-V77E or WT-M307V yielded currents of only about 20% compared to the WT dimer, supporting a dominant-negative effect of the mutants. Kir2.1 sensitivity to PIP₂ was examined by activating the PIP₂ specific voltage-sensitive phosphatase (VSP) that induced PIP₂ depletion during current recordings, in HEK cells and Xenopus oocytes. PIP₂ depletion induced a stronger and faster decay in Kir2.1 mutant dimers current compared to the WT dimer. BGP-15, a drug that has been demonstrated to have an anti-arrhythmic effect in mice, stabilized the Kir2.1 current amplitude following VSP-induced PIP₂ depletion in cells expressing WT or mutant dimers. This study underlines the implication of mutations in cytoplasmic regions of Kir2.1. A newly developed calibrated VSP activation protocol enabled a quantitative assessment of changes in PIP₂ regulation caused by the mutations. The results suggest an impaired function and a dominant-negative effect of the Kir2.1 variants that involve an impaired regulation by PIP₂. This study also demonstrates that BGP-15 may be beneficial in restoring impaired Kir2.1 function and possibly in treating ATS symptoms.

Andersen-Tawil Syndrome and Hypothyroidism: A Case Report with an Unusual Association

Andersen-Tawil syndrome (ATS) is an autosomal dominant disorder, characterized by the triad of muscular paralysis, skeletal, and craniofacial anomalies and prolonged QT interval on echocardiogram with a tendency toward malignant ventricular arrhythmia. Although the patient may express one or two of the three components of triad, hypothyroidism is an endocrine disorder resulting in the delayed eruption of teeth, defective mineralization of bone and teeth, and speech and hearing deformity. Here, we report a case of ATS with hypothyroidism. To the best of authors' knowledge, no such association has been reported in the literature.

Asymptomatic ventricular tachycardia: diagnostic pitfalls of Andersen-Tawil syndrome-a case report

Background

Andersen–Tawil syndrome (ATS) is a rare arrhythmia disorder caused by a mutation in the KCNJ2 gene. Typical presentation includes a triad of cardiac arrhythmia, dysmorphia, and periodic paralysis. However, KCNJ2 mutations can mimic other disorders such as catecholaminergic polymorphic ventricular tachycardia (CPVT) making treatment challenging.

Case summary

A 9-year-old asymptomatic female patient presented with an irregular heart rate noted at a well-child visit. Physical examination revealed short stature and facial dysmorphism. An initial rhythm strip showed intermittent runs of non-sustained bidirectional ventricular tachycardia with a prolonged QT interval of 485 ms at rest. Exercise testing showed no significant increase in ectopy from baseline at higher heart rates. Cardiac imaging was normal, and the burden of ventricular ectopy was significantly reduced on a beta-blocker and Class IC antiarrhythmic combination. Genetic testing marked a D71N mutation in the KCNJ2 gene.

Discussion

Clinical distinction between ATS and CPVT is a challenge. Genetic testing in the above patient attributed a likely pathogenic variant for both ATS and CPVT to a single D71N mutation in the KCNJ2 gene. Further evaluation revealed no clinical CPVT, emphasizing the need for cautious interpretation of genetic results in inherited arrhythmia disorders.

Periodic paralysis

The periodic paralyses are a group of skeletal muscle channelopathies characterizeed by intermittent attacks of muscle weakness often associated with altered serum potassium levels. The underlying genetic defects include mutations in genes encoding the skeletal muscle calcium channel Ca_v1.1, sodium channel Na_v1.4, and potassium channels K_ir2.1, K_ir3.4, and possibly K_ir2.6. Our increasing knowledge of how mutant channels affect muscle excitability has resulted in better understanding of many clinical phenomena which have been known for decades and sheds light on some of the factors that trigger attacks. Insights into the pathophysiology are also leading to new therapeutic approaches.

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.

Ion channelopathies in human induced pluripotent stem cell derived cardiomyocytes: a dynamic clamp study with virtual IK1

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) are widely used in studying basic mechanisms of cardiac arrhythmias that are caused by ion channelopathies. Unfortunately, the action potential profile of hiPSC-CMs-and consequently the profile of individual membrane currents active during that action potential-differs substantially from that of native human cardiomyocytes, largely due to almost negligible expression of the inward rectifier potassium current (IK1). In the present study, we attempted to "normalize" the action potential profile of our hiPSC-CMs by inserting a voltage dependent in silico IK1 into our hiPSC-CMs, using the dynamic clamp configuration of the patch clamp technique. Recordings were made from single hiPSC-CMs, using the perforated patch clamp technique at physiological temperature. We assessed three different models of IK1, with different degrees of inward rectification, and systematically varied the magnitude of the inserted IK1. Also, we modified the inserted IK1 in order to assess the effects of loss- and gain-of-function mutations in the KCNJ2 gene, which encodes the Kir2.1 protein that is primarily responsible for the IK1 channel in human ventricle. For our experiments, we selected spontaneously beating hiPSC-CMs, with negligible IK1 as demonstrated in separate voltage clamp experiments, which were paced at 1 Hz. Upon addition of in silico IK1 with a peak outward density of 4-6 pA/pF, these hiPSC-CMs showed a ventricular-like action potential morphology with a stable resting membrane potential near -80 mV and a maximum upstroke velocity >150 V/s (n = 9). Proarrhythmic action potential changes were observed upon injection of both loss-of-function and gain-of-function IK1, as associated with Andersen-Tawil syndrome type 1 and short QT syndrome type 3, respectively (n = 6). We conclude that injection of in silico IK1 makes the hiPSC-CM a more reliable model for investigating mechanisms underlying cardiac arrhythmias.