Positional cloning of a novel potassium channel gene: KVLQT1 mutations cause cardiac arrhythmias

Hearing loss secondary to novel variants of the KCNQ4 gene

PURPOSE

Heterozygous variants of the KCNQ4 gene are associated with isolated sensorineural hearing loss (DFNA2A). This study aimed to determine the frequency and clinical characteristics of pathogenic, likely pathogenic, and uncertain variants in the KCNQ4 gene among patients with sensorineural hearing loss of unknown origin in North Spain.

METHODS

We conducted a prospective observational study of patients with sensorineural hearing loss of unknown etiology at a tertiary hospital over six years. Next-generation sequencing carried out with a panel of genes was used to identify genetic variants related to both syndromic and non-syndromic hearing loss.

RESULTS

Among 370 patients, seven (1.89%) harbored pathogenic or likely pathogenic variants in the KCNQ4 gene: c.777_778delinsCC, c.626 T > G, and c.778G > C. None of these variants had been previously described. One patient also had a variant of uncertain significance (c.419 T > C). All patients exhibited progressive bilateral sensorineural hearing loss, predominantly at high frequencies, with variable onset and severity. None reported dizziness or vertigo. Five patients used hearing aids, and one received a cochlear implant with good results.

CONCLUSIONS

KCNQ4 gene variants are rare in Cantabria, present in less than 2% of patients with sensorineural hearing loss of unknown origin. Although most variants identified in our study had not been previously described, the observed phenotype aligned with the typical presentation: bilateral, progressive sensorineural hearing loss with variable onset and severity. Some patients may benefit from cochlear implants.

Generation of human induced pluripotent stem cell (hiPSC) lines from patients with extreme high and low polygenic scores for QT interval

Long QT syndrome (LQTS), an inherited cardiac arrhythmia syndrome with congenital and drug-induced presentations and known monogenic and polygenic contributions, represents a significant clinical challenge due to its complex genetic underpinning and propensity for fatal arrhythmias. In this study, we generated induced pluripotent stem cells (iPSCs) reprogrammed from peripheral blood mononuclear cells (PBMCs) of six patients with extreme polygenic scores for short and long corrected QT intervals. This patient-specific approach will enable us to better understand variable expressivity and penetrance of LQTS, using rigorously validated iPSC lines serve as a vital resource for elucidating the molecular mechanisms underlying LQTS.

Functional profiling of KCNE1 variants informs population carrier frequency of Jervell and Lange-Nielsen syndrome type 2

Congenital long-QT syndrome (LQTS) is most often associated with pathogenic variants in KCNQ1 encoding the pore-forming voltage-gated potassium channel subunit of the slow delayed rectifier current ( I Ks ). Generation of I Ks requires assembly of KCNQ1 with an auxiliary subunit encoded by KCNE1 , which is also associated with LQTS but causality of autosomal dominant disease is disputed. By contrast, KCNE1 is an accepted cause of recessive type 2 Jervell and Lange-Nielson syndrome (JLN2). The functional consequences of most KCNE1 variants have not been determined and the population prevalence of JLN2 is unknown. Methods : We determined the functional properties of 95 KCNE1 variants co-expressed with KCNQ1 in heterologous cells using high-throughput voltage-clamp recording. Experiments were conducted with each KCNE1 variant expressed in the homozygous state and then a subset was studied in the heterozygous state. The carrier frequency of JLN2 was estimated by considering the population prevalence of dysfunctional variants. Results : There is substantial overlap between disease-associated and population KCNE1 variants. When examined in the homozygous state, 68 KCNE1 variants exhibited significant differences in at least one functional property compared to WT KCNE1, whereas 27 variants did not significantly affect function. Most dysfunctional variants exhibited loss-of-function properties. We observed no evidence of dominant-negative effects. Most variants were scored as variants of uncertain significance (VUS) and inclusion of functional data resulted in revised classifications for only 14 variants. The population carrier frequency of JLN2 was calculated as 1 in 1034. Peak current density and activation voltage-dependence but no other biophysical properties were correlated with findings from a mutational scan of KCNE1. Conclusions : Among 95 disease-associated or population KCNE1 variants, many exhibit abnormal functional properties but there was no evidence of dominant-negative behaviors. Using functional data, we inferred a population carrier frequency for recessive JLN2. This work helps clarify the pathogenicity of KCNE1 variants.

Management of ultrarare inherited arrhythmia syndromes

Ultrarare inherited arrhythmia syndromes are increasingly diagnosed as a result of increased awareness as well as increased availability and reduced cost of genetic testing. Yet by definition, their rarity and heterogeneous expression make development of evidence-based management strategies more challenging, typically employing strategies garnered from similar genetic cardiac disorders. For the most part, reliance on anecdotal experiences, expert opinion, and small retrospective cohort studies is the only means to diagnose and to treat these patients. Here we review the management of specific ultrarare inherited arrhythmic syndromes together with the genetic and molecular basis, which will become increasingly important with the development of targeted therapies to correct the biologic basis of these disorders.

Targeting Kv7 Potassium Channels for Epilepsy

Voltage-gated Kv7 potassium channels, particularly Kv7.2 and Kv.7.3 channels, play a critical role in modulating susceptibility to seizures, and mutations in genes that encode these channels cause heterogeneous epilepsy phenotypes. On the basis of this evidence, activation of Kv7.2 and Kv.7.3 channels has long been considered an attractive target in the search for novel antiseizure medications. Ezogabine (retigabine), the first Kv7.2/3 activator introduced in 2011 for the treatment of focal seizures, was withdrawn from the market in 2017 due to declining use after discovery of its association with pigmentation changes in the retina, skin, and mucosae. A novel formulation of ezogabine for pediatric use (XEN496) has been recently investigated in children with KCNQ2-related developmental and epileptic encephalopathy, but the trial was terminated prematurely for reasons unrelated to safety. Among novel Kv7.2/3 openers in clinical development, azetukalner has shown dose-dependent efficacy against drug-resistant focal seizures with a good tolerability profile and no evidence of pigmentation-related adverse effects in early clinical studies, and it is now under investigation in phase III trials for the treatment of focal seizures, generalized tonic-clonic seizures, and major depressive disorder. Another Kv7.2/3 activator, BHV-7000, has completed phase I studies in healthy subjects, with excellent tolerability at plasma drug concentrations that exceed the median effective concentrations in a preclinical model of anticonvulsant activity, but no efficacy data in patients with epilepsy are available to date. Among other Kv7.2/3 activators in clinical development as potential antiseizure medications, pynegabine and CB-003 have completed phase I safety and pharmacokinetic studies, but results have not been yet reported. Overall, interest in targeting Kv7 channels for the treatment of epilepsy and for other indications remains strong. Future breakthroughs in this area could come from exploitation of mechanistic differences in the action of Kv7 activators, and from the development of molecules that combine Kv7 activation with other mechanisms of action.

A Japanese Boy with Dysmorphic Syndrome with Multiple Pituitary Hormone Deficiency and Gingival Fibromatosis Due to a Pathogenic KCNQ1 Variant

A six-year-old boy presented with short stature and gingival fibromatosis (GF). Dysmorphic features included slant optic fissures, a high-arched palate, thick earlobes, and an edematous face. Laboratory tests showed low levels of serum insulin-like growth factor-1 and serum free thyroxine but normal serum thyrotropin levels. Provocative tests suggested growth hormone deficiency, central hypocortisolemia, and hypothalamic hypothyroidism. At 12 years old, hypogonadotropic hypogonadism was observed. Next-generation sequencing revealed a heterozygous missense variant, KCNQ1 p. (P369L), in the proband and mother. The coexistence of multiple pituitary hormone deficiencies and GF helps diagnose KCNQ1-variant dysmorphic syndrome through genetic testing.

Frequency and Genotype-Dependence of intrinsic chronotropic insufficiency among patients with congenital long QT syndrome

INTRODUCTION

Long QT syndrome (LQTS) is a cardiac channelopathy characterized by QT prolongation and a potential for arrhythmic syncope, sudden cardiac arrest or deaths (SCA/SCD). It has been speculated that patients with LQTS might have a primary sinoatrial node (SAN) phenotype of chronotropic insufficiency (CI). This has not been demonstrated convincingly before because of the potentially confounding effects of beta blocker (BB) therapy. Herein, we set out to determine whether untreated patients with LQTS demonstrate intrinsic CI.

METHODS AND RESULTS

A retrospective review of all treadmill exercise stress tests (TEST) was performed on patients with one of the three most common LQTS genotypes: LQT1, LQT2, and LQT3. For each patient, the first TEST completed while off BB was analyzed. Patients with prior left cardiac sympathetic denervation (LCSD) therapy were excluded. CI was defined as having an age- and gender-predicted peak heart rate (HR) < 85% and/or a predicted HR reserve (HRR) < 80%. Overall, 463 LQTS patients (245 LQT1, 125 LQT2, and 93 LQT3) were included (267 female [58%]; mean age at time of TEST [29 ± 17 years]). Mean % predicted peak HR for all LQTS patients was 87.6% (range 42.9% - 119.1%) and mean % predicted HRR was 80% (range 19.1% - 153%). Overall, half of all LQTS patients (n = 234; 51%) displayed CI; 64% of patients with LQT1 (n = 157), 37% with LQT2 (n = 46), and 33% with LQT3 (n = 31). Patients with LQT1 were most likely to exhibit CI compared to patients with LQT2 (p < .0001) and LQT3 (p < .0001). CI was significantly more common in LQT1 compared to controls (p < .0001), while there was no difference between LQT2 (p = .5) or LQT3 and controls (p > .9). Presence of CI was not a predictor of LQTS-associated symptoms, BB side effects or likelihood of future breakthrough cardiac events (BCE).

CONCLUSIONS

Patients with LQTS, particularly LQT1, demonstrate a SAN phenotype of CI. If assessing BB therapy effect by impact on peak HR, the patient's pretreatment peak HR, rather than an age- and gender-predicted maximum HR, should be used.

Advances in the design and development of chemical modulators of the voltage-gated potassium channels KV7.4 and KV7.5

INTRODUCTION

Hypertension remains a major public health concern, with significant morbidity and mortality worldwide. Despite the availability of various antihypertensive medications, blood pressure control remains suboptimal in many individuals. During the last decades, KV7.4 and KV7.5, which were already known from the view of neuronal regulation, emerged as possible important players in the regulation of vascular tone and blood pressure.

AREAS COVERED

This review covers physiological functions and current advancements in the development of KV7.4 and KV7.5 channel modulators. The authors highlight the structural elements likely to be important for the future design of KV7 subtype-selective modulators, underscoring their potential as an innovative hypertension treatment.

EXPERT OPINION

Extensive research has been focused on targeting neuronal KV7.2 and KV7.3 channels, while KV7.4 and KV7.5 attracted less attention. Many of the developed compounds represent derivatives of flupirtine or retigabine, whereby subtype channel selectivity has only been demonstrated for a handful of individual compounds. Novel substances address additional sites within the binding pocket by incorporating new functional groups. A comprehensive and systematic evaluation of a compound set with significant subtype selectivity should be performed. The discovery of new highly active, less toxic, and selective compounds, therefore, remains the goal of further research in the coming years.

Ion permeation through a narrow cavity constriction in KCNQ1 channels: Mechanism and implications for pathogenic variants

KCNQ1 potassium channels play a pivotal role in the physiology and pathophysiology of several human excitable and epithelial tissues. The latest cryo-electron microscopy (cryo-EM) structures provide unique insights into channel function and pharmacology, opening avenues for different therapeutic strategies against human diseases associated with KCNQ1 mutations. However, these structures also raise fundamental questions about the mechanisms of ion permeation. Cryo-EM structures thought to represent the open state of the channel feature a cavity region not wide enough for accommodation of hydrated K+. To understand how K+ passes through the cavity constriction, we utilized microsecond-scale molecular dynamics (MD) simulations using the KCNQ1/KCNE3 cryo-EM structure, characterized mutants at the G345 residue situated at the narrowest point of the cavity, and recorded single channels. The findings indicate that ions become partially dehydrated at the constriction, which enables permeation. MD simulations demonstrate that the constriction can impede the flow of ions through the channel's pore, a finding that is corroborated by mutational screening and single-channel recordings. Reduced channel conductance is the key mechanism underlying reported pathological KCNQ1 mutations at or near the constriction site.

Clinical Features, Long-Term Prognosis, and Clinical Management of Genotype-Negative Long QT Syndrome Patients

BACKGROUND

Approximately 15% to 20% of patients clinically diagnosed with long QT syndrome (LQTS) are genotype-negative (GEN-). Whether they have a different arrhythmic risk or should be managed differently remains unclear, often leading to incomplete treatment.

OBJECTIVES

The purpose of this study was to compare clinical aspects of GEN- and genotype-positive (GEN+) LQTS patients.

METHODS

We retrospectively evaluated 832 LQTS patients genetically screened in Japan (n = 347) and Italy (n = 485), including 698 with a disease-causing variant in the KCNQ1, KCNH2, and SCN5A genes (GEN+), and 134 without variants in these LQTS-related genes (GEN-).

RESULTS

At diagnosis, the Japanese patients were more often probands (86% vs 60%), symptomatic (39% vs 18%), and of younger age than the Italian patients; conversely, they used less β-blockers (65% vs 95%), more rarely had a family history (FH) of LQTS (42% vs 73%), and had more cardiac events during follow-up (13% vs 4%) (P < 0.001 for all comparisons). Within the Japanese cohort, the GEN- had more cardiac arrests, used less β-blockers, and had much less FH for LQTS compared their GEN+ counterpart. The Italian cohort was more homogeneous, with just more LQTS FH among the GEN+. QTc shortening (close to 30 ms in all groups) during follow-up was similar between Japanese and Italian patients, irrespective of their being GEN+ or GEN-. In both cohorts, during an average follow-up of 6 and 7 years, respectively, GEN+ and GEN- patients showed a comparable clinical outcome.

CONCLUSIONS

Arrhythmic risk is similar between GEN+ and GEN- LQTS patients; they should be managed and treated in the same way.

A Natural History Study of Timothy Syndrome

BACKGROUND

Timothy syndrome (OMIM #601005) is a rare disease caused by variants in the gene CACNA1C. Initially, Timothy syndrome was characterized by a cardiac presentation of long QT syndrome and syndactyly of the fingers and/or toes, all associated with the CACNA1C variant, Gly406Arg. However, subsequent identification of diverse variants in CACNA1C has expanded the clinical spectrum, revealing various cardiac and extra-cardiac manifestations. It remains underexplored whether individuals with the canonical Gly406Arg variants in mutually exclusive exon 8A (Timothy syndrome 1) or exon 8 (Timothy syndrome 2) exhibit overlapping symptoms. Moreover, case reports have indicated that some CACNA1C variants may produce a cardiac-selective form of Timothy syndrome often referred to as non-syndromic long QT type 8 or cardiac-only Timothy syndrome, however few reports follow up on these patients to confirm the cardiac selectivity of the phenotype over time.

METHODS

A survey was administered to the parents of patients with Timothy syndrome, querying a broad range of symptoms and clinical features. Study participants were organized into 5 separate categories based on genotype and initial diagnosis, enabling comparison between groups of patients which have been described differentially in the literature.

RESULTS

Our findings reveal that Timothy syndrome patients commonly exhibit both cardiac and extra-cardiac features, with long QT syndrome, neurodevelopmental impairments, hypoglycemia, and respiratory issues being frequently reported. Notably, the incidence of these features was similar across all patient categories, including those diagnosed with non-syndromic long QT type 8, suggesting that the 'non-syndromic' classification may be incomplete.

CONCLUSIONS

This study represents the first Natural History Study of Timothy syndrome, offering a comprehensive overview of the disease's clinical manifestations. We demonstrate that both cardiac and extra-cardiac features are prevalent across all patient groups, underscoring the syndromic nature of CACNA1C variants. While the critical role of long QT syndrome and cardiac arrhythmias in Timothy syndrome has been well recognized, our findings indicate that hypoglycemia and respiratory dysfunction also pose significant life-threatening risks, emphasizing the need for comprehensive therapeutic management of affected individuals.

The Yield of Genetic Testing and Putative Genetic Factors of Disease Heterogeneity in Long QT Syndrome Patients

Genetic overdiagnosis of long QT syndrome (LQTS) becomes a critical concern due to the high clinical significance of DNA diagnosis. Current guidelines for LQTS genetic testing recommend a limited scope and strict referral based on the Schwartz score. Nevertheless, LQTS may be underdiagnosed in patients with borderline phenotypes. We aimed to evaluate the total yield of rare variants in cardiac genes in LQTS patients. The cohort of 82 patients with LQTS referral diagnosis underwent phenotyping, Schwartz score counting, and exome sequencing. We assessed known LQTS genes for diagnostics, as per guidelines, and a broader set of genes for research. Diagnostic testing yield reached 75% in index patients; all causal variants were found in KCNQ1, KCNH2, and SCN5A genes. Research testing of 248 heart-related genes achieved a 50% yield of molecular diagnosis in patients with a low Schwartz score (<3.5). In patients with LQTS-causing variants, each additional rare variant in heart-related genes added 0.94 points to the Schwartz score (p value = 0.04), reflecting the more severe disease in such patients than in those with causal variants but without additional findings. We conclude that the current LQTS genetic diagnosis framework is highly specific but may lack sensitivity for patients with a Schwartz score <3.5. Improving referral criteria for these patients could enhance DNA diagnosis. Also, our results suggest that additional variants in cardiac genes may affect the severity of the disease in the carriers of LQTS-causing variants, which may aid in identifying new modifier genes.

Discovery of a potent, Kv7.3-selective potassium channel opener from a Polynesian traditional botanical anticonvulsant

Plants remain an important source of biologically active small molecules with high therapeutic potential. The voltage-gated potassium (Kv) channel formed by Kv7.2/3 (KCNQ2/3) heteromers is a major target for anticonvulsant drug development. Here, we screened 1444 extracts primarily from plants collected in California and the US Virgin Islands, for their ability to activate Kv7.2/3 but not inhibit Kv1.3, to select against tannic acid being the active component. We validated the 7 strongest hits, identified Thespesia populnea (miro, milo, portia tree) as the most promising, then discovered its primary active metabolite to be gentisic acid (GA). GA highly potently activated Kv7.2/3 (EC50, 2.8 nM). GA is, uniquely to our knowledge, 100% selective for Kv7.3 versus other Kv7 homomers; it requires S5 residue Kv7.3-W265 for Kv7.2/3 activation, and it ameliorates pentylenetetrazole-induced seizures in mice. Structure-activity studies revealed that the FDA-approved vasoprotective drug calcium dobesilate, a GA analog, is a previously unrecognized Kv7.2/3 channel opener. Also an active aspirin metabolite, GA provides a molecular rationale for the use of T. populnea as an anticonvulsant in Polynesian indigenous medicine and presents novel pharmacological prospects for potent, isoform-selective, therapeutic Kv7 channel activation.

The fully activated open state of KCNQ1 controls the cardiac "fight-or-flight" response

The cardiac KCNQ1 + KCNE1 (IKs) channel regulates heart rhythm under both normal and stress conditions. Under stress, the β-adrenergic stimulation elevates the intracellular cyclic adenosine monophosphate (cAMP) level, leading to KCNQ1 phosphorylation by protein kinase A and increased IKs, which shortens action potentials to adapt to accelerated heart rate. An impaired response to the β-adrenergic stimulation due to KCNQ1 mutations is associated with the occurrence of a lethal congenital long QT syndrome (type 1, also known as LQT1). However, the underlying mechanism of β-adrenergic stimulation of IKs remains unclear, impeding the development of new therapeutics. Here, we find that the unique properties of KCNQ1 channel gating with two distinct open states are key to this mechanism. KCNQ1's fully activated open (AO) state is more sensitive to cAMP than its intermediate open state. By enhancing the AO state occupancy, the small molecules ML277 and C28 are found to effectively enhance the cAMP sensitivity of the KCNQ1 channel, independent of KCNE1 association. This finding of enhancing AO state occupancy leads to a potential novel strategy to rescue the response of IKs to β-adrenergic stimulation in LQT1 mutants. The success of this approach is demonstrated in cardiac myocytes and also in a high-risk LQT1 mutation. In conclusion, the present study not only uncovers the key role of the AO state in IKs channel phosphorylation, but also provides a target for antiarrhythmic strategy.

Sudden death in epilepsy: the overlap between cardiac and neurological factors

People with epilepsy are at risk of premature death, of which sudden unexpected death in epilepsy (SUDEP), sudden cardiac death (SCD) and sudden arrhythmic death syndrome (SADS) are the primary, partly overlapping, clinical scenarios. We discuss the epidemiologies, risk factors and pathophysiological mechanisms for these sudden death events. We reviewed the existing evidence on sudden death in epilepsy. Classification of sudden death depends on the presence of autopsy and expertise of the clinician determining aetiology. The definitions of SUDEP, SCD and SADS lead to substantial openings for overlap. Seizure-induced arrhythmias constitute a minority of SUDEP cases. Comorbid cardiovascular conditions are the primary determinants of increased SCD risk in chronic epilepsy. Genetic mutations overlap between the states, yet whether these are causative, associated or incidentally present is often unclear. Risk stratification for sudden death in people with epilepsy requires a multidisciplinary approach, including a review of clinical history, toxicological analysis and complete autopsy with histologic and, preferably, genetic examination. We recommend pursuing genetic testing of relatives of people with epilepsy who died suddenly, mainly if a post-mortem genetic test contained a Class IV/V (pathogenic/likely pathogenic) gene variant. Further research may allow more precise differentiation of SUDEP, SCD and SADS and the development of algorithms for risk stratification and preventative strategies.

KCNQ1 suppression-replacement gene therapy in transgenic rabbits with type 1 long QT syndrome

BACKGROUND AND AIMS

Type 1 long QT syndrome (LQT1) is caused by pathogenic variants in the KCNQ1-encoded Kv7.1 potassium channels, which pathologically prolong ventricular action potential duration (APD). Herein, the pathologic phenotype in transgenic LQT1 rabbits is rescued using a novel KCNQ1 suppression-replacement (SupRep) gene therapy.

METHODS

KCNQ1-SupRep gene therapy was developed by combining into a single construct a KCNQ1 shRNA (suppression) and an shRNA-immune KCNQ1 cDNA (replacement), packaged into adeno-associated virus serotype 9, and delivered in vivo via an intra-aortic root injection (1E10 vg/kg). To ascertain the efficacy of SupRep, 12-lead electrocardiograms were assessed in adult LQT1 and wild-type (WT) rabbits and patch-clamp experiments were performed on isolated ventricular cardiomyocytes.

RESULTS

KCNQ1-SupRep treatment of LQT1 rabbits resulted in significant shortening of the pathologically prolonged QT index (QTi) towards WT levels. Ventricular cardiomyocytes isolated from treated LQT1 rabbits demonstrated pronounced shortening of APD compared to LQT1 controls, leading to levels similar to WT (LQT1-UT vs. LQT1-SupRep, P < .0001, LQT1-SupRep vs. WT, P = ns). Under β-adrenergic stimulation with isoproterenol, SupRep-treated rabbits demonstrated a WT-like physiological QTi and APD90 behaviour.

CONCLUSIONS

This study provides the first animal-model, proof-of-concept gene therapy for correction of LQT1. In LQT1 rabbits, treatment with KCNQ1-SupRep gene therapy normalized the clinical QTi and cellular APD90 to near WT levels both at baseline and after isoproterenol. If similar QT/APD correction can be achieved with intravenous administration of KCNQ1-SupRep gene therapy in LQT1 rabbits, these encouraging data should compel continued development of this gene therapy for patients with LQT1.

Decoding polyubiquitin regulation of KV7. 1 functional expression with engineered linkage-selective deubiquitinases

Protein posttranslational modification with distinct polyubiquitin linkage chains is a critical component of the 'ubiquitin code' that universally regulates protein expression and function to control biology. Functional consequences of diverse polyubiquitin linkages on proteins are mostly unknown, with progress hindered by a lack of methods to specifically tune polyubiquitin linkages on individual proteins in live cells. Here, we bridge this gap by exploiting deubiquitinases (DUBs) with preferences for hydrolyzing different polyubiquitin linkages: OTUD1 - K63; OTUD4 - K48; Cezanne - K11; TRABID - K29/K33; and USP21 - non-specific. We developed a suite of engineered deubiquitinases (enDUBs) comprised of DUB catalytic domains fused to a GFP-targeted nanobody and used them to investigate polyubiquitin linkage regulation of an ion channel, YFP-KCNQ1. Mass spectrometry of YFP-KCNQ1 expressed in HEK293 cells indicated channel polyubiquitination with K48 (72%) and K63 (24%) linkages being dominant. NEDD4-2 and ITCH both decreased KCNQ1 functional expression but with distinctive polyubiquitination signatures. All enDUBs reduced KCNQ1 ubiquitination but yielded unique effects on channel expression, surface density, ionic currents, and subcellular localization. The pattern of outcomes indicates K11, K29/K33, and K63 chains mediate net KCNQ1-YFP intracellular retention, but achieved in different ways: K11 promotes ER retention/degradation, enhances endocytosis, and reduces recycling; K29/K33 promotes ER retention/degradation; K63 enhances endocytosis and reduces recycling. The pattern of enDUB effects on KCNQ1-YFP differed in cardiomyocytes, emphasizing ubiquitin code mutability. Surprisingly, enDUB-O4 decreased KCNQ1-YFP surface density suggesting a role for K48 in forward trafficking. Lastly, linkage-selective enDUBs displayed varying capabilities to rescue distinct trafficking-deficient long QT syndrome type 1 mutations. The results reveal distinct polyubiquitin chains control different aspects of KCNQ1 functional expression, demonstrate ubiquitin code plasticity, and introduce linkage-selective enDUBs as a potent tool to help demystify the polyubiquitin code.

Targeting the IKs Channel PKA Phosphorylation Axis to Restore Its Function in High-Risk LQT1 Variants

BACKGROUND

The KCNQ1+KCNE1 (IKs) potassium channel plays a crucial role in cardiac adaptation to stress, in which β-adrenergic stimulation phosphorylates the IKs channel through the cyclic adenosine monophosphate (cAMP)/PKA (protein kinase A) pathway. Phosphorylation increases the channel current and accelerates repolarization to adapt to an increased heart rate. Variants in KCNQ1 can cause long-QT syndrome type 1 (LQT1), and those with defective cAMP effects predispose patients to the highest risk of cardiac arrest and sudden death. However, the molecular connection between IKs channel phosphorylation and channel function, as well as why high-risk LQT1 mutations lose cAMP sensitivity, remain unclear.

METHODS

Regular patch clamp and voltage clamp fluorometry techniques were utilized to record pore opening and voltage sensor movement of wild-type and mutant KCNQ1/IKs channels. The clinical phenotypic penetrance of each LQT1 mutation was analyzed as a metric for assessing their clinical risk. The patient-specific-induced pluripotent stem-cell model was used to test mechanistic findings in physiological conditions.

RESULTS

By systematically elucidating mechanisms of a series of LQT1 variants that lack cAMP sensitivity, we identified molecular determinants of IKs channel regulation by phosphorylation. These key residues are distributed across the N-terminus of KCNQ1 extending to the central pore region of IKs. We refer to this pattern as the IKs channel PKA phosphorylation axis. Next, by examining LQT1 variants from clinical databases containing 10 579 LQT1 carriers, we found that the distribution of the most high-penetrance LQT1 variants extends across the IKs channel PKA phosphorylation axis, demonstrating its clinical relevance. Furthermore, we found that a small molecule, ML277, which binds at the center of the phosphorylation axis, rescues the defective cAMP effects of multiple high-risk LQT1 variants. This finding was then tested in high-risk patient-specific induced pluripotent stem cell-derived cardiomyocytes, where ML277 remarkably alleviates the beating abnormalities.

CONCLUSIONS

Our findings not only elucidate the molecular mechanism of PKA-dependent IKs channel phosphorylation but also provide an effective antiarrhythmic strategy for patients with high-risk LQT1 variants.

A broad survey of choanoflagellates revises the evolutionary history of the Shaker family of voltage-gated K+ channels in animals

The Shaker family of voltage-gated K+ channels has been thought of as an animal-specific ion channel family that diversified in concert with nervous systems. It comprises four functionally independent gene subfamilies (Kv1-4) that encode diverse neuronal K+ currents. Comparison of animal genomes predicts that only the Kv1 subfamily was present in the animal common ancestor. Here, we show that some choanoflagellates, the closest protozoan sister lineage to animals, also have Shaker family K+ channels. Choanoflagellate Shaker family channels are surprisingly most closely related to the animal Kv2-4 subfamilies which were believed to have evolved only after the divergence of ctenophores and sponges from cnidarians and bilaterians. Structural modeling predicts that the choanoflagellate channels share a T1 Zn2+ binding site with Kv2-4 channels that is absent in Kv1 channels. We functionally expressed three Shakers from Salpingoeca helianthica (SheliKvT1.1-3) in Xenopus oocytes. SheliKvT1.1-3 function only in two heteromultimeric combinations (SheliKvT1.1/1.2 and SheliKvT1.1/1.3) and encode fast N-type inactivating K+ channels with distinct voltage dependence that are most similar to the widespread animal Kv1-encoded A-type Shakers. Structural modeling of the T1 assembly domain supports a preference for heteromeric assembly in a 2:2 stoichiometry. These results push the origin of the Shaker family back into a common ancestor of metazoans and choanoflagellates. They also suggest that the animal common ancestor had at least two distinct molecular lineages of Shaker channels, a Kv1 subfamily lineage predicted from comparison of animal genomes and a Kv2-4 lineage predicted from comparison of animals and choanoflagellates.

The fully activated open state of KCNQ1 controls the cardiac "fight-or-flight" response

The cardiac KCNQ1+KCNE1 (I Ks ) channel regulates heart rhythm in both normal and stress conditions. Under stress, the β-adrenergic stimulation elevates the intracellular cAMP level, leading to KCNQ1 phosphorylation by protein kinase A and increased I Ks , which shortens action potentials to adapt to accelerated heart rate. An impaired response to the β-adrenergic stimulation due to KCNQ1 mutations is associated with the occurrence of a lethal congenital long QT syndrome (type 1, also known as LQT1). However, the underlying mechanism of β-adrenergic stimulation of I Ks remains unclear, impeding the development of new therapeutics. Here we find that the unique properties of KCNQ1 channel gating with two distinct open states are key to this mechanism. KCNQ1's fully activated open (AO) state is more sensitive to cAMP than its' intermediate open (IO) state. By enhancing the AO state occupancy, the small molecules ML277 and C28 are found to effectively enhance the cAMP sensitivity of the KCNQ1 channel, independent of KCNE1 association. This finding of enhancing AO state occupancy leads to a potential novel strategy to rescue the response of I Ks to β-adrenergic stimulation in LQT1 mutants. The success of this approach is demonstrated in cardiac myocytes and also in a high-risk LQT1 mutation. In conclusion the present study not only uncovers the key role of the AO state in I Ks channel phosphorylation, but also provides a new target for anti-arrhythmic strategy.

Significance statement

The increase of I Ks potassium currents with adrenalin stimulation is important for "fight-or-flight" responses. Mutations of the IKs channel reducing adrenalin responses are associated with more lethal form of the type-1 long-QT syndrome (LQT). The alpha subunit of the IKs channel, KCNQ1 opens in two distinct open states, the intermediate-open (IO) and activated-open (AO) states, following a two-step voltage sensing domain (VSD) activation process. We found that the AO state, but not the IO state, is responsible for the adrenalin response. Modulators that specifically enhance the AO state occupancy can enhance adrenalin responses of the WT and LQT-associated mutant channels. These results reveal a mechanism of state dependent modulation of ion channels and provide an anti-arrhythmic strategy.

Clinical characterization of type 1 long QT syndrome caused by C-terminus Kv7.1 variants

BACKGROUND

Variants in the KCNQ1 gene, encoding the α-subunit of the slow component of delayed rectifier K+ channel Kv7.1, cause long QT syndrome (LQTS) type 1. The location of variants may be one of the factors in determining prognosis. However, detailed genotype-phenotype relationships associated with C-terminus variants remain unelucidated.

OBJECTIVE

We investigated the clinical characteristics and variant-specific arrhythmic risks in patients with LQTS carrying Kv7.1 C-terminus variants.

METHODS

The study comprises 202 consecutive patients with LQTS (98 probands and 104 family members) who carry a rare heterozygous variant in the Kv7.1 C-terminus. Their clinical characteristics and arrhythmic events were investigated.

RESULTS

We identified 36 unique C-terminus variants (25 missense and 11 non-missense). The p.R366W variant was identified in 8 families, and p.T587M was identified in 21 families in large numbers from northwestern Japan. As for the location of the variant, we found that the variants in highly conserved regions and nonhelical domains were associated with longer QTc intervals compared with the variants in other regions. Both p.R366W and p.T587M variants are located in the highly conserved and functionally pivotal regions close to helices A and D, which are associated with calmodulin binding and channel assembly (tetramerization), respectively. The probands carrying p.T587M and p.R366W variants had worse arrhythmia outcomes compared with those with other C-terminus variants. The haplotype analysis of p.T587M families was suggestive of a founder effect.

CONCLUSION

The arrhythmic risk of C-terminus variants in Kv7.1 in LQTS is not homogeneous, and locations of variants can be a determining factor for prognosis.

A method of identifying the high-risk mutations of sudden cardiac death at KCNQ1 and KCNH2 genes

Sudden Cardiac Death (SCD) often shows negative anatomy results after a systemic autopsy and the gene mutations of potassium channel play a key role in the etiology of SCD. We established a feasible system to detect SCD-related mutations and investigated the mutations at KCNQ1 and KCNH2 genes in the Chinese population. We established a mutation detection system combined with multiplex PCR, SNaPshot technique, and capillary electrophoresis. We genotyped 101 putative mutations at KCNQ1 and KCNH2 genes in 60 SCD of negative anatomy and 50 controls using the established assay and compared Odd Ratio (OR). Four coding variants were identified in the KCNQ1 gene: S546S, I145I, P448R, and G643S. The mutations of I145I and S546S did not differ significantly in the SCD compared with controls. 21 SCD individuals (35 %) and 1 control individual (2 %) showed a genotype of C/G at P448R (OR = 17.5, 95 % CI [2.40-127.82]). 24 SCD individuals (40 %) and 1 control individual (2 %) showed a genotype of C/G at G643S (OR = 20.0, 95 % CI [2.75-145.25]). We established a robust assay for rapid screening the putative SCD-related mutations in KCNQ1 and KCNH2 genes. The new assay in our study is easily amenable to the majority of laboratories without the need for new specialized equipment. Our method will meet the increasing requirement of mutation screening for SCD in regular DNA laboratories and will help screen mutations in those dead of SCD and their relatives.

KCNQ1 is an essential mediator of the sex-dependent perception of moderate cold temperatures

Low temperatures and cooling agents like menthol induce cold sensation by activating the peripheral cold receptors TRPM8 and TRPA1, cation channels belonging to the TRP channel family, while the reduction of potassium currents provides an additional and/or synergistic mechanism of cold sensation. Despite extensive studies over the past decades to identify the molecular receptors that mediate thermosensation, cold sensation is still not fully understood and many cold-sensitive peripheral neurons do not express the well-established cold sensor TRPM8. We found that the voltage-gated potassium channel KCNQ1 (Kv7.1), which is defective in cardiac LQT1 syndrome, is, in addition to its known function in the heart, a highly relevant and sex-specific sensor of moderately cold temperatures. We found that KCNQ1 is expressed in skin and dorsal root ganglion neurons, is sensitive to menthol and cooling agents, and is highly sensitive to moderately cold temperatures, in a temperature range at which TRPM8 is not thermosensitive. C-fiber recordings from KCNQ1-/- mice displayed altered action potential firing properties. Strikingly, only male KCNQ1-/- mice showed substantial deficits in cold avoidance at moderately cold temperatures, with a strength of the phenotype similar to that observed in TRPM8-/- animals. While sex-dependent differences in thermal sensitivity have been well documented in humans and mice, KCNQ1 is the first gene reported to play a role in sex-specific temperature sensation. Moreover, we propose that KCNQ1, together with TRPM8, is a key instrumentalist that orchestrates the range and intensity of cold sensation.

Glial KCNQ K+ channels control neuronal output by regulating GABA release from glia in C. elegans

KCNQs are voltage-gated K+ channels that control neuronal excitability and are mutated in epilepsy and autism spectrum disorder (ASD). KCNQs have been extensively studied in neurons, but their function in glia is unknown. Using voltage, calcium, and GABA imaging, optogenetics, and behavioral assays, we show here for the first time in Caenorhabditis elegans (C. elegans) that glial KCNQ channels control neuronal excitability by mediating GABA release from glia via regulation of the function of L-type voltage-gated Ca2+ channels. Further, we show that human KCNQ channels have the same role when expressed in nematode glia, underscoring conservation of function across species. Finally, we show that pathogenic loss-of-function and gain-of-function human KCNQ2 mutations alter glia-to-neuron GABA signaling in distinct ways and that the KCNQ channel opener retigabine exerts rescuing effects. This work identifies glial KCNQ channels as key regulators of neuronal excitability via control of GABA release from glia.

The electrophysiologic effects of KCNQ1 extend beyond expression of IKs: evidence from genetic and pharmacologic block

AIMS

While variants in KCNQ1 are the commonest cause of the congenital long QT syndrome, we and others find only a small IKs in cardiomyocytes from human-induced pluripotent stem cells (iPSC-CMs) or human ventricular myocytes.

METHODS AND RESULTS

We studied population control iPSC-CMs and iPSC-CMs from a patient with Jervell and Lange-Nielsen (JLN) syndrome due to compound heterozygous loss-of-function (LOF) KCNQ1 variants. We compared the effects of pharmacologic IKs block to those of genetic KCNQ1 ablation, using JLN cells, cells homozygous for the KCNQ1 LOF allele G643S, or siRNAs reducing KCNQ1 expression. We also studied the effects of two blockers of IKr, the other major cardiac repolarizing current, in the setting of pharmacologic or genetic ablation of KCNQ1: moxifloxacin, associated with a very low risk of drug-induced long QT, and dofetilide, a high-risk drug. In control cells, a small IKs was readily recorded but the pharmacologic IKs block produced no change in action potential duration at 90% repolarization (APD90). In contrast, in cells with genetic ablation of KCNQ1 (JLN), baseline APD90 was markedly prolonged compared with control cells (469 ± 20 vs. 310 ± 16 ms). JLN cells displayed increased sensitivity to acute IKr block: the concentration (μM) of moxifloxacin required to prolong APD90 100 msec was 237.4 [median, interquartile range (IQR) 100.6-391.6, n = 7] in population cells vs. 23.7 (17.3-28.7, n = 11) in JLN cells. In control cells, chronic moxifloxacin exposure (300 μM) mildly prolonged APD90 (10%) and increased IKs, while chronic exposure to dofetilide (5 nM) produced greater prolongation (67%) and no increase in IKs. However, in the siRNA-treated cells, moxifloxacin did not increase IKs and markedly prolonged APD90.

CONCLUSION

Our data strongly suggest that KCNQ1 expression modulates baseline cardiac repolarization, and the response to IKr block, through mechanisms beyond simply generating IKs.

A Natural History Study of Timothy Syndrome

Timothy syndrome (OMIM #601005) is a rare disease caused by variants in the gene CACNA1C . Timothy syndrome patients were first identified as having a cardiac presentation of Long QT and syndactyly of the fingers and/or toes, and an identical variant in CACNA1C , Gly406Arg. However, since this original identification, more individuals harboring diverse variants in CACNA1C have been identified and have presented with various cardiac and extra-cardiac symptoms. Furthermore, it has remained underexplored whether individuals harboring canonical Gly406Arg variants in mutually exclusive exon 8A (Timothy syndrome 1) or exon 8 (Timothy syndrome 2) have additional symptoms. Here, we describe the first Natural History Study for Timothy syndrome, providing a thorough resource describing the current understanding of disease manifestation in Timothy syndrome patients. Parents of Timothy syndrome children were queried regarding a wide-ranging set of symptoms and features via a survey. Importantly, we find that in addition to cardiac concerns, Timothy syndrome patients commonly share extra-cardiac features including neurodevelopmental impairments, hypoglycemia, and respiratory problems. Our work expands the current understanding of the disorder to better inform the care of Timothy syndrome patients.

Epigenetic Regulation of Cardiomyocyte Maturation by Arginine Methyltransferase CARM1

BACKGROUND

During the neonatal stage, the cardiomyocyte undergoes a constellation of molecular, cytoarchitectural, and functional changes known collectively as cardiomyocyte maturation to increase myocardial contractility and cardiac output. Despite the importance of cardiomyocyte maturation, the molecular mechanisms governing this critical process remain largely unexplored.

METHODS

We leveraged an in vivo mosaic knockout system to characterize the role of Carm1, the founding member of protein arginine methyltransferase, in cardiomyocyte maturation. Using a battery of assays, including immunohistochemistry, immuno-electron microscopy imaging, and action potential recording, we assessed the effect of loss of Carm1 function on cardiomyocyte cell growth, myofibril expansion, T-tubule formation, and electrophysiological maturation. Genome-wide transcriptome profiling, H3R17me2a chromatin immunoprecipitation followed by sequencing, and assay for transposase-accessible chromatin with high-throughput sequencing were used to investigate the mechanisms by which CARM1 (coactivator-associated arginine methyltransferase 1) regulates cardiomyocyte maturation. Finally, we interrogated the human syntenic region to the H3R17me2a chromatin immunoprecipitation followed by sequencing peaks for single-nucleotide polymorphisms associated with human heart diseases.

RESULTS

We report that mosaic ablation of Carm1 disrupts multiple aspects of cardiomyocyte maturation cell autonomously, leading to reduced cardiomyocyte size and sarcomere thickness, severe loss and disorganization of T tubules, and compromised electrophysiological maturation. Genomics study demonstrates that CARM1 directly activates genes that underlie cardiomyocyte cytoarchitectural and electrophysiological maturation. Moreover, our study reveals significant enrichment of human heart disease-associated single-nucleotide polymorphisms in the human genomic region syntenic to the H3R17me2a chromatin immunoprecipitation followed by sequencing peaks.

CONCLUSIONS

This study establishes a critical and multifaceted role for CARM1 in regulating cardiomyocyte maturation and demonstrates that deregulation of CARM1-dependent cardiomyocyte maturation gene expression may contribute to human heart diseases.

Catheter Ablation for Channelopathies: When Is Less More?

Ventricular fibrillation (VF) is a common cause of sudden cardiac death in patients with channelopathies, particularly in the young population. Although pharmacological treatment, cardiac sympathectomy, and implantable cardioverter defibrillators (ICD) have been the mainstay in the management of VF in patients with channelopathies, they are associated with significant adverse effects and complications, leading to poor quality of life. Given these drawbacks, catheter ablation has been proposed as a therapeutic option for patients with channelopathies. Advances in imaging techniques and modern mapping technologies have enabled increased precision in identifying arrhythmia triggers and substrate modification. This has aided our understanding of the underlying pathophysiology of ventricular arrhythmias in channelopathies, highlighting the roles of the Purkinje network and the epicardial right ventricular outflow tract in arrhythmogenesis. This review explores the role of catheter ablation in managing the most common channelopathies (Brugada syndrome, congenital long QT syndrome, short QT syndrome, and catecholaminergic polymorphic ventricular tachycardia). While the initial results for ablation in Brugada syndrome are promising, the long-term efficacy and durability of ablation in different channelopathies require further investigation. Given the genetic and phenotypic heterogeneity of channelopathies, future studies are needed to show whether catheter ablation in patients with channelopathies is associated with a reduction in VF, and psychological distress stemming from recurrent ICD shocks, particularly relative to other available therapeutic options (e.g., quinidine in high-risk Brugada patients).

Clinical Genetic Testing for Atrial Fibrillation: Are We There Yet?

Important progress has been made toward unravelling the complex genetics underlying atrial fibrillation (AF). Initial studies were aimed to identify monogenic causes; however, it has become increasingly clear that the most common predisposing genetic substrate for AF is polygenic. Despite intensive investigations, there is robust evidence for rare variants for only a limited number of genes and cases. Although the current yield for genetic testing in early onset AF might be modest, there is an increasing appreciation that genetic culprits for potentially life-threatening ventricular cardiomyopathies and channelopathies might initially present with AF. The potential clinical significance of this recognition is highlighted by evidence that suggests that identification of a pathogenic or likely pathogenic rare variant in a patient with early onset AF is associated with an increased risk of death. These findings suggest that it might be warranted to screen patients with early onset AF for these potentially more sinister cardiac conditions. Beyond facilitating the early identification of genetic culprits associated with potentially malignant phenotypes, insight into underlying AF genetic substrates might improve the selection of patients for existing therapies and guide the development of novel ones. Herein, we review the evidence that links genetic factors to AF, then discuss an approach to using genetic testing for early onset AF patients in the present context, and finally consider the potential value of genetic testing in the foreseeable future. Although further work might be necessary before recommending uniform integration of genetic testing in cases of early onset AF, ongoing research increasingly highlights its potential contributions to clinical care.

KCNQ5 Controls Perivascular Adipose Tissue-Mediated Vasodilation

BACKGROUND

Small arteries exhibit resting tone, a partially contracted state that maintains arterial blood pressure. In arterial smooth muscle cells, potassium channels control contraction and relaxation. Perivascular adipose tissue (PVAT) has been shown to exert anticontractile effects on the blood vessels. However, the mechanisms by which PVAT signals small arteries, and their relevance remain largely unknown. We aimed to uncover key molecular components in adipose-vascular coupling.

METHODS

A wide spectrum of genetic mouse models targeting Kcnq3, Kcnq4, and Kcnq5 genes (Kcnq3-/-, Kcnq4-/-, Kcnq5-/-, Kcnq5dn/dn, Kcnq4-/-/Kcnq5dn/dn, and Kcnq4-/-/Kcnq5-/-), telemetry blood pressure measurements, targeted lipidomics, RNA-Seq profiling, wire-myography, patch-clamp, and sharp-electrode membrane potential measurements was used.

RESULTS

We show that PVAT causes smooth muscle cell KV7.5 family of voltage-gated potassium (K+) channels to hyperpolarize the membrane potential. This effect relaxes small arteries and regulates blood pressure. Oxygenation of polyunsaturated fats generates oxylipins, a superclass of lipid mediators. We identified numerous oxylipins released by PVAT, which potentiate vasodilatory action in small arteries by opening smooth muscle cell KV7.5 family of voltage-gated potassium (K+) channels.

CONCLUSIONS

Our results reveal a key molecular function of the KV7.5 family of voltage-gated potassium (K+) channels in the adipose-vascular coupling, translating PVAT signals, particularly oxylipins, to the central physiological function of vasoregulation. This novel pathway opens new therapeutic perspectives.

Modulation of potassium channels by transmembrane auxiliary subunits via voltage-sensing domains

Voltage-gated K+ (KV ) and Ca2+ -activated K+ (KCa ) channels are essential proteins for membrane repolarization in excitable cells. They also play important physiological roles in non-excitable cells. Their diverse physiological functions are in part the result of their auxiliary subunits. Auxiliary subunits can alter the expression level, voltage dependence, activation/deactivation kinetics, and inactivation properties of the bound channel. KV and KCa channels are activated by membrane depolarization through the voltage-sensing domain (VSD), so modulation of KV and KCa channels through the VSD is reasonable. Recent cryo-EM structures of the KV or KCa channel complex with auxiliary subunits are shedding light on how these subunits bind to and modulate the VSD. In this review, we will discuss four examples of auxiliary subunits that bind directly to the VSD of KV or KCa channels: KCNQ1-KCNE3, Kv4-DPP6, Slo1-β4, and Slo1-γ1. Interestingly, their binding sites are all different. We also present some examples of how functionally critical binding sites can be determined by introducing mutations. These structure-guided approaches would be effective in understanding how VSD-bound auxiliary subunits modulate ion channels.

A conditional mutation in a wheat (Triticum aestivum L.) gene regulating root morphology

KEY MESSAGE

Characterisation and genetic mapping of a key gene defining root morphology in bread wheat. Root morphology is central to plants for the efficient uptake up of soil water and mineral nutrients. Here we describe a conditional mutant of hexaploid wheat (Triticum aestivum L.) that when grown in soil with high Ca2+ develops a larger rhizosheath accompanied with shorter roots than the wild type. In wheat, rhizosheath size is a reliable surrogate for root hair length and this was verified in the mutant which possessed longer root hairs than the wild type when grown in high Ca2+ soil. We named the mutant Stumpy and showed it to be due to a single semi-dominant mutation. The short root phenotype at high Ca2+ was due to reduced cellular elongation which might also explain the long root hair phenotype. Analysis of root cell walls showed that the polysaccharide composition of Stumpy roots is remodelled when grown at non-permissive (high) Ca2+ concentrations. The mutation mapped to chromosome 7B and sequencing of the 7B chromosomes in both wild type and Stumpy identified a candidate gene underlying the Stumpy mutation. As part of the process to determine whether the candidate gene was causative, we identified wheat lines in a Cadenza TILLING population with large rhizosheaths but accompanied with normal root length. This finding illustrates the potential of manipulating the gene to disconnect root length from root hair length as a means of developing wheat lines with improved efficiency of nutrient and water uptake. The Stumpy mutant will be valuable for understanding the mechanisms that regulate root morphology in wheat.

Voltage-gated potassium channels KCNQs: Structures, mechanisms, and modulations

KCNQ (Kv7) channels are voltage-gated, phosphatidylinositol 4,5-bisphosphate- (PIP2-) modulated potassium channels that play essential roles in regulating the activity of neurons and cardiac myocytes. Hundreds of mutations in KCNQ channels are closely related to various cardiac and neurological disorders, such as long QT syndrome, epilepsy, and deafness, which makes KCNQ channels important drug targets. During the past several years, the application of single-particle cryo-electron microscopy (cryo-EM) technique in the structure determination of KCNQ channels has greatly advanced our understanding of their molecular mechanisms. In this review, we summarize the currently available structures of KCNQ channels, analyze their special voltage gating mechanism, and discuss their activation mechanisms by both the endogenous membrane lipid and the exogenous synthetic ligands. These structural studies of KCNQ channels will guide the development of drugs targeting KCNQ channels.

Activation of Kv7 channels normalizes hyperactivity of the VTA-NAcLat circuit and attenuates methamphetamine-induced conditioned place preference and sensitization in mice

The brain circuit projecting from the ventral tegmental area (VTA) to the nucleus accumbens lateral shell (NAcLat) has a key role in methamphetamine (MA) addiction. As different dopamine (DA) neuron subpopulations in the VTA participate in different neuronal circuits, it is a challenge to isolate these DA neuron subtypes. Using retrograde tracing and Patch-seq, we isolated DA neurons in the VTA-NAcLat circuit in MA-treated mice and performed gene expression profiling. Among the differentially expressed genes, KCNQ genes were dramatically downregulated. KCNQ genes encode Kv7 channel proteins, which modulate neuronal excitability. Injection of both the Kv7.2/3 agonist ICA069673 and the Kv7.4 agonist fasudil into the VTA attenuated MA-induced conditioned place preference and locomotor sensitization and decreased neuronal excitability. Increasing Kv7.2/3 activity decreased neural oscillations, synaptic plasticity and DA release in the VTA-NacLat circuit in MA-treated mice. Furthermore, overexpression of only Kv7.3 channels in the VTA-NacLat circuit was sufficient to attenuate MA-induced reward behavior and decrease VTA neuron excitability. Activation of Kv7 channels in the VTA may become a novel treatment strategy for MA abuse.

Development of automated patch clamp assays to overcome the burden of variants of uncertain significance in inheritable arrhythmia syndromes

Advances in next-generation sequencing have been exceptionally valuable for identifying variants in medically actionable genes. However, for most missense variants there is insufficient evidence to permit definitive classification of variants as benign or pathogenic. To overcome the deluge of Variants of Uncertain Significance, there is an urgent need for high throughput functional assays to assist with the classification of variants. Advances in parallel planar patch clamp technologies has enabled the development of automated high throughput platforms capable of increasing throughput 10- to 100-fold compared to manual patch clamp methods. Automated patch clamp electrophysiology is poised to revolutionize the field of functional genomics for inheritable cardiac ion channelopathies. In this review, we outline i) the evolution of patch clamping, ii) the development of high-throughput automated patch clamp assays to assess cardiac ion channel variants, iii) clinical application of these assays and iv) where the field is heading.

Arrhythmia-associated calmodulin variants interact with KCNQ1 to confer aberrant membrane trafficking and function

Missense variants in calmodulin (CaM) predispose patients to arrhythmias associated with high mortality rates ("calmodulinopathy"). As CaM regulates many key cardiac ion channels, an understanding of disease mechanism associated with CaM variant arrhythmias requires elucidating individual CaM variant effects on distinct channels. One key CaM regulatory target is the KCNQ1 (KV7.1) voltage-gated potassium channel that carries the IKs current. Yet, relatively little is known as to how CaM variants interact with KCNQ1 or affect its function. Here, we take a multipronged approach employing a live-cell fluorescence resonance energy transfer binding assay, fluorescence trafficking assay, and functional electrophysiology to characterize >10 arrhythmia-associated CaM variants for effect on KCNQ1 CaM binding, membrane trafficking, and channel function. We identify one variant (G114W) that exhibits severely weakened binding to KCNQ1 but find that most other CaM variants interact with similar binding affinity to KCNQ1 when compared with CaM wild-type over physiological Ca2+ ranges. We further identify several CaM variants that affect KCNQ1 and IKs membrane trafficking and/or baseline current activation kinetics, thereby delineating KCNQ1 dysfunction in calmodulinopathy. Lastly, we identify CaM variants with no effect on KCNQ1 function. This study provides extensive functional data that reveal how CaM variants contribute to creating a proarrhythmic substrate by causing abnormal KCNQ1 membrane trafficking and current conduction. We find that CaM variant regulation of KCNQ1 is not uniform with effects varying from benign to significant loss of function, suggesting how CaM variants predispose patients to arrhythmia via the dysregulation of multiple cardiac ion channels. Classification: Biological, Health, and Medical Sciences, Physiology.

The Physiological Function of nNOS-Associated CAPON Proteins and the Roles of CAPON in Diseases

In this review, the structure, isoform, and physiological role of the carboxy-terminal PDZ ligand of neuronal nitric oxide synthase (CAPON) are summarized. There are three isoforms of CAPON in humans, including long CAPON protein (CAPON-L), short CAPON protein (CAPON-S), and CAPON-S' protein. CAPON-L includes three functional regions: a C-terminal PDZ-binding motif, carboxypeptidase (CPE)-binding region, and N-terminal phosphotyrosine (PTB) structural domain. Both CAPON-S and CAPON-S' only contain the C-terminal PDZ-binding motif. The C-terminal PDZ-binding motif of CAPON can bind with neuronal nitric oxide synthase (nNOS) and participates in regulating NO production and neuronal development. An overview is given on the relationship between CAPON and heart diseases, diabetes, psychiatric disorders, and tumors. This review will clarify future research directions on the signal pathways related to CAPON, which will be helpful for studying the regulatory mechanism of CAPON. CAPON may be used as a drug target, which will provide new ideas and solutions for treating human diseases.

Ligand activation mechanisms of human KCNQ2 channel

The human voltage-gated potassium channel KCNQ2/KCNQ3 carries the neuronal M-current, which helps to stabilize the membrane potential. KCNQ2 can be activated by analgesics and antiepileptic drugs but their activation mechanisms remain unclear. Here we report cryo-electron microscopy (cryo-EM) structures of human KCNQ2-CaM in complex with three activators, namely the antiepileptic drug cannabidiol (CBD), the lipid phosphatidylinositol 4,5-bisphosphate (PIP2), and HN37 (pynegabine), an antiepileptic drug in the clinical trial, in an either closed or open conformation. The activator-bound structures, along with electrophysiology analyses, reveal the binding modes of two CBD, one PIP2, and two HN37 molecules in each KCNQ2 subunit, and elucidate their activation mechanisms on the KCNQ2 channel. These structures may guide the development of antiepileptic drugs and analgesics that target KCNQ2.

Estimating the probability of early afterdepolarizations and predicting arrhythmic risk associated with long QT syndrome type 1 mutations

Early afterdepolarizations (EADs) are action potential (AP) repolarization abnormalities that can trigger lethal arrhythmias. Simulations using biophysically detailed cardiac myocyte models can reveal how model parameters influence the probability of these cellular arrhythmias; however, such analyses can pose a huge computational burden. We have previously developed a highly simplified approach in which logistic regression models (LRMs) map parameters of complex cell models to the probability of ectopic beats. Here, we extend this approach to predict the probability of EADs (P(EAD)) as a mechanistic metric of arrhythmic risk. We use the LRM to investigate how changes in parameters of the slow-activating delayed rectifier current (IKs) affect P(EAD) for 17 different long QT syndrome type 1 (LQTS1) mutations. In this LQTS1 clinical arrhythmic risk prediction task, we compared P(EAD) for these 17 mutations with two other recently published model-based arrhythmia risk metrics (AP morphology metric across populations of myocyte models and transmural repolarization prolongation based on a one-dimensional [1D] tissue-level model). These model-based risk metrics yield similar prediction performance; however, each fails to stratify clinical risk for a significant number of the 17 studied LQTS1 mutations. Nevertheless, an interpretable ensemble model using multivariate linear regression built by combining all of these model-based risk metrics successfully predicts the clinical risk of 17 mutations. These results illustrate the potential of computational approaches in arrhythmia risk prediction.

A generic binding pocket for small molecule IKs activators at the extracellular inter-subunit interface of KCNQ1 and KCNE1 channel complexes

The cardiac IKs ion channel comprises KCNQ1, calmodulin, and KCNE1 in a dodecameric complex which provides a repolarizing current reserve at higher heart rates and protects from arrhythmia syndromes that cause fainting and sudden death. Pharmacological activators of IKs are therefore of interest both scientifically and therapeutically for treatment of IKs loss-of-function disorders. One group of chemical activators are only active in the presence of the accessory KCNE1 subunit and here we investigate this phenomenon using molecular modeling techniques and mutagenesis scanning in mammalian cells. A generalized activator binding pocket is formed extracellularly by KCNE1, the domain-swapped S1 helices of one KCNQ1 subunit and the pore/turret region made up of two other KCNQ1 subunits. A few residues, including K41, A44 and Y46 in KCNE1, W323 in the KCNQ1 pore, and Y148 in the KCNQ1 S1 domain, appear critical for the binding of structurally diverse molecules, but in addition, molecular modeling studies suggest that induced fit by structurally different molecules underlies the generalized nature of the binding pocket. Activation of IKs is enhanced by stabilization of the KCNQ1-S1/KCNE1/pore complex, which ultimately slows deactivation of the current, and promotes outward current summation at higher pulse rates. Our results provide a mechanistic explanation of enhanced IKs currents by these activator compounds and provide a map for future design of more potent therapeutically useful molecules.

Lem2 is essential for cardiac development by maintaining nuclear integrity

AIMS

Nuclear envelope integrity is essential for the compartmentalization of the nucleus and cytoplasm. Importantly, mutations in genes encoding nuclear envelope (NE) and associated proteins are the second highest cause of familial dilated cardiomyopathy. One such NE protein that causes cardiomyopathy in humans and affects mouse heart development is Lem2. However, its role in the heart remains poorly understood.

METHODS AND RESULTS

We generated mice in which Lem2 was specifically ablated either in embryonic cardiomyocytes (Lem2 cKO) or in adult cardiomyocytes (Lem2 iCKO) and carried out detailed physiological, tissue, and cellular analyses. High-resolution episcopic microscopy was used for three-dimensional reconstructions and detailed morphological analyses. RNA-sequencing and immunofluorescence identified altered pathways and cellular phenotypes, and cardiomyocytes were isolated to interrogate nuclear integrity in more detail. In addition, echocardiography provided a physiological assessment of Lem2 iCKO adult mice. We found that Lem2 was essential for cardiac development, and hearts from Lem2 cKO mice were morphologically and transcriptionally underdeveloped. Lem2 cKO hearts displayed high levels of DNA damage, nuclear rupture, and apoptosis. Crucially, we found that these defects were driven by muscle contraction as they were ameliorated by inhibiting myosin contraction and L-type calcium channels. Conversely, reducing Lem2 levels to ∼45% in adult cardiomyocytes did not lead to overt cardiac dysfunction up to 18 months of age.

CONCLUSIONS

Our data suggest that Lem2 is critical for integrity at the nascent NE in foetal hearts, and protects the nucleus from the mechanical forces of muscle contraction. In contrast, the adult heart is not detectably affected by partial Lem2 depletion, perhaps owing to a more established NE and increased adaptation to mechanical stress. Taken together, these data provide insights into mechanisms underlying cardiomyopathy in patients with mutations in Lem2 and cardio-laminopathies in general.

Modulation of Kv7 Channel Currents by Echinocystic Acid

Modulation of KCNQ-encoded voltage-gated potassium Kv7/M channel function represents an attractive strategy to treat neuronal excitability disorders such as epilepsy, pain, and depression. The Kv7 channel group includes five subfamily members (Kv7.1-Kv7.5). Pentacyclic triterpenes display extensive pharmacological activities including antitumor, anti-inflammatory, and antidepression effects. In this study, we investigated the effects of pentacyclic triterpenes on Kv7 channels. Our results show that echinocystic acid, ursonic acid, oleanonic acid, demethylzeylasteral, corosolic acid, betulinaldehyde, acetylursolic acid, and α-boswellic acid gradually exert decreasing degrees of Kv7.2/Kv7.3 channel current inhibition. Echinocystic acid was the most potent inhibitor, with a half-maximal inhibitory concentration (IC50) of 2.5 µM. It significantly shifted the voltage-dependent activation curve in a positive direction and slowed the time constant of activation for Kv7.2/Kv7.3 channel currents. Furthermore, echinocystic acid nonselectively inhibited Kv7.1-Kv7.5 channels. Taken together, our findings indicate that echinocystic acid is a novel and potent inhibitor that could be used as a tool to further understand the pharmacological functions of neuronal Kv7 channels. SIGNIFICANCE STATEMENT: Pentacyclic triterpenes reportedly have multiple potential therapeutic uses such as anticancer, anti-inflammatory, antioxidant, and antidepression effects. In the present study, we show that echinocystic acid, ursonic acid, oleanonic acid, and demethylzeylasteral inhibit Kv7.2/Kv7.3 channels to varying degrees. Of these, echinocystic acid was the most potent Kv7.2/Kv7.3 current inhibitor and inhibited Kv7.1-Kv7.5 currents in a nonselective manner.

KCNH2 variants in a family with epilepsy and long QT syndrome: A case report and literature review

OBJECTIVE

Genes associated with Long QT syndromes (LQTS), such as KCNQ1, KCNH2, and SCN5A, are common causes of epilepsy. The Arg 744* variant of KCNH2 has been previously reported in people with epilepsy or LQTS, but none of these patients were reported to simultaneously suffer from epilepsy and LQTS. Herein, we report the case of a family with epilepsy and cardiac disorders.

METHOD

The proband, a 25-year-old woman, with a family history of epilepsy and LQTS was followed at West China Hospital. The proband experienced her first seizure at the age of seven. Video electroencephalograms (vEEGs) showed epileptic discharges. Her 24-h dynamic electrocardiograms 2 (ECGs) showed QTc prolongation. The proband's mother, who is 50 years old, had her first generalized tonic-clonic seizure (GTCS) at the age of 18 years old. After she gave birth at the age of 25, the frequency of seizures increased, so antiepileptic therapy was initiated. When she was 28 years old, she complained of palpitations and syncope for the first time, and QTc prolongation was detected on her 24-h dynamic ECGs. The proband's grandmother also had complaints of palpitations and syncope at the age of 73. Her 24-h dynamic ECGs indicated supraventricular arrhythmia, with the lowest heart rate being 41 bpm, so she agreed to a pacemaker. Considering the young patient's family history, blood samples of the patient and her parents were collected for genetic analysis.

RESULTS

A heterozygous variant of KCNH2 [c.2230 (exon9) C>T, p. Arg744Ter, 416, NM_000238, rs189014161] was found in the proband and her mother. According to the guidelines of the American College of Medical Genetics and Genomics and the Association for Molecular Pathology, we classified the KCNH2 variant as pathogenic.

SIGNIFICANCE

This study expands the clinical phenotype of the Arg 744* KCNH2 pathogenic variant. In the context of channelopathies, because of the genetic susceptibility of the brain and the heart, the risk of comorbidity should be considered. This also indicates the importance of precise antiepileptic drug (AED) management and regular ECG monitoring for patients with channelopathies.

Bioinformatic Identification of Potential RNA Alterations on the Atrial Fibrillation Remodeling from Human Pulmonary Veins

Atrial fibrillation (AF) is the most frequent persistent arrhythmia. Many genes have been reported as a genetic background for AF. However, most transcriptome analyses of AF are limited to the atrial samples and have not been evaluated by multiple cardiac regions. In this study, we analyzed the expression levels of protein-coding and long noncoding RNAs (lncRNAs) in six cardiac regions by RNA-seq. Samples were donated from six subjects with or without persistent AF for left atria, left atrial appendages, right atria, sinoatrial nodes, left ventricles, right ventricles, and pulmonary veins (PVs), and additional four right atrial appendages samples were collected from patients undergoing mitral valve replacement. In total, 23 AF samples were compared to 23 non-AF samples. Surprisingly, the most influenced heart region in gene expression by AF was the PV, not the atria. The ion channel-related gene set was significantly enriched upon analysis of these significant genes. In addition, some significant genes are cancer-related lncRNAs in PV in AF. A co-expression network analysis could detect the functional gene clusters. In particular, the cancer-related lncRNA, such as SAMMSON and FOXCUT, belong to the gene network with the cancer-related transcription factor FOXC1. Thus, they may also play an aggravating role in the pathogenesis of AF, similar to carcinogenesis. In the least, this study suggests that (1) RNA alteration is most intense in PVs and (2) post-transcriptional gene regulation by lncRNA may contribute to the progression of AF. Through the screening analysis across the six cardiac regions, the possibility that the PV region can play a role other than paroxysmal triggering in the pathogenesis of AF was demonstrated for the first time. Future research with an increase in the number of PV samples will lead to a novel understanding of the pathophysiology of AF.

Mechanism of external K+ sensitivity of KCNQ1 channels

KCNQ1 voltage-gated K+ channels are involved in a wide variety of fundamental physiological processes and exhibit the unique feature of being markedly inhibited by external K+. Despite the potential role of this regulatory mechanism in distinct physiological and pathological processes, its exact underpinnings are not well understood. In this study, using extensive mutagenesis, molecular dynamics simulations, and single-channel recordings, we delineate the molecular mechanism of KCNQ1 modulation by external K+. First, we demonstrate the involvement of the selectivity filter in the external K+ sensitivity of the channel. Then, we show that external K+ binds to the vacant outermost ion coordination site of the selectivity filter inducing a diminution in the unitary conductance of the channel. The larger reduction in the unitary conductance compared to whole-cell currents suggests an additional modulatory effect of external K+ on the channel. Further, we show that the external K+ sensitivity of the heteromeric KCNQ1/KCNE complexes depends on the type of associated KCNE subunits.

Progress in genetics of type 2 diabetes and diabetic complications

Type 2 diabetes results from a complex interaction between genetic and environmental factors. Precision medicine for type 2 diabetes using genetic data is expected to predict the risk of developing diabetes and complications and to predict the effects of medications and life-style intervention more accurately for individuals. Genome-wide association studies (GWAS) have been conducted in European and Asian populations and new genetic loci have been identified that modulate the risk of developing type 2 diabetes. Novel loci were discovered by GWAS in diabetic complications with increasing sample sizes. Large-scale genome-wide association analysis and polygenic risk scores using biobank information is making it possible to predict the development of type 2 diabetes. In the ADVANCE clinical trial of type 2 diabetes, a multi-polygenic risk score was useful to predict diabetic complications and their response to treatment. Proteomics and metabolomics studies have been conducted and have revealed the associations between type 2 diabetes and inflammatory signals and amino acid synthesis. Using multi-omics analysis, comprehensive molecular mechanisms have been elucidated to guide the development of targeted therapy for type 2 diabetes and diabetic complications.

Initiation of ventricular arrhythmia in the acquired long QT syndrome

AIMS

Long QT syndrome (LQTS) carries a risk of life-threatening polymorphic ventricular tachycardia (Torsades de Pointes, TdP) and is a major cause of premature sudden cardiac death. TdP is induced by R-on-T premature ventricular complexes (PVCs), thought to be generated by cellular early-afterdepolarisations (EADs). However, EADs in tissue require cellular synchronisation, and their role in TdP induction remains unclear. We aimed to determine the mechanism of TdP induction in rabbit hearts with acquired LQTS (aLQTS).

METHODS AND RESULTS

Optical mapping of action potentials (APs) and intracellular Ca2+ was performed in Langendorff-perfused rabbit hearts (n = 17). TdP induced by R-on-T PVCs was observed during aLQTS (50% K+/Mg++ & E4031) conditions in all hearts (P < 0.0001 vs. control). Islands of AP prolongation bounded by steep voltage gradients (VGs) were consistently observed before arrhythmia and peak VGs were more closely related to the PVC upstroke than EADs, both temporally (7 ± 5 ms vs. 44 ± 27 ms, P < 0.0001) and spatially (1.0 ± 0.7 vs. 3.6 ± 0.9 mm, P < 0.0001). PVCs were initiated at estimated voltages of ∼ -40 mV and had upstroke dF/dtmax and Vm-Ca2+ dynamics compatible with ICaL activation. Computational simulations demonstrated that PVCs could arise directly from VGs, through electrotonic triggering of ICaL. In experiments and the model, sub-maximal L-type Ca2+ channel (LTCC) block (200 nM nifedipine and 90% gCaL, respectively) abolished both PVCs and TdP in the continued presence of aLQTS.

CONCLUSION

These data demonstrate that ICaL activation at sites displaying steep VGs generates the PVCs which induce TdP, providing a mechanism and rationale for LTCC blockers as a novel therapeutic approach in LQTS.

COVID-19 mRNA Vaccines: The Molecular Basis of Some Adverse Events

Each injection of any known vaccine results in a strong expression of pro-inflammatory cytokines. This is the result of the innate immune system activation, without which no adaptive response to the injection of vaccines is possible. Unfortunately, the degree of inflammation produced by COVID-19 mRNA vaccines is variable, probably depending on genetic background and previous immune experiences, which through epigenetic modifications could have made the innate immune system of each individual tolerant or reactive to subsequent immune stimulations.We hypothesize that we can move from a limited pro-inflammatory condition to conditions of increasing expression of pro-inflammatory cytokines that can culminate in multisystem hyperinflammatory syndromes following COVID-19 mRNA vaccines (MIS-V). We have graphically represented this idea in a hypothetical inflammatory pyramid (IP) and we have correlated the time factor to the degree of inflammation produced after the injection of vaccines. Furthermore, we have placed the clinical manifestations within this hypothetical IP, correlating them to the degree of inflammation produced. Surprisingly, excluding the possible presence of an early MIS-V, the time factor and the complexity of clinical manifestations are correlated to the increasing degree of inflammation: symptoms, heart disease and syndromes (MIS-V).

Gene mutations in comorbidity of epilepsy and arrhythmia

Epilepsy is one of the most common neurological disorders, and sudden unexpected death in epilepsy (SUDEP) is the most severe outcome of refractory epilepsy. Arrhythmia is one of the heterogeneous factors in the pathophysiological mechanism of SUDEP with a high incidence in patients with refractory epilepsy, increasing the risk of premature death. The gene co-expressed in the brain and heart is supposed to be the genetic basis between epilepsy and arrhythmia, among which the gene encoding ion channel contributes to the prevalence of "cardiocerebral channelopathy" theory. Nevertheless, this theory could only explain the molecular mechanism of comorbid arrhythmia in part of patients with epilepsy (PWE). Therefore, we summarized the mutant genes that can induce comorbidity of epilepsy and arrhythmia and the possible corresponding treatments. These variants involved the genes encoding sodium, potassium, calcium and HCN channels, as well as some non-ion channel coding genes such as CHD4, PKP2, FHF1, GNB5, and mitochondrial genes. The relationship between genotype and clinical phenotype was not simple linear. Indeed, genes co-expressed in the brain and heart could independently induce epilepsy and/or arrhythmia. Mutant genes in brain could affect cardiac rhythm through central or peripheral regulation, while in the heart it could also affect cerebral electrical activity by changing the hemodynamics or internal environment. Analysis of mutations in comorbidity of epilepsy and arrhythmia could refine and expand the theory of "cardiocerebral channelopathy" and provide new insights for risk stratification of premature death and corresponding precision therapy in PWE.

The Advantages, Challenges, and Future of Human-Induced Pluripotent Stem Cell Lines in Type 2 Long QT Syndrome

Type 2 long QT syndrome (LQT2) is the second most common subtype of long QT syndrome and is caused by mutations in KCHN2 encoding the rapidly activating delayed rectifier potassium channel vital for ventricular repolarization. Sudden cardiac death is a sentinel event of LQT2. Preclinical diagnosis by genetic testing is potentially life-saving.Traditional LQT2 models cannot wholly recapitulate genetic and phenotypic features; therefore, there is a demand for a reliable experimental model. Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) meet this challenge. This review introduces the advantages of the hiPSC-CM model over the traditional model and discusses how hiPSC-CM and gene editing are used to decipher mechanisms of LQT2, screen for cardiotoxicity, and identify therapeutic strategies, thus promoting the realization of precision medicine for LQT2 patients.