Elsevier

Heart Rhythm

Volume 16, Issue 11, November 2019, Pages 1698-1706
Heart Rhythm

Experimental
A de novo gain-of-function KCND3 mutation in early repolarization syndrome

https://doi.org/10.1016/j.hrthm.2019.05.033Get rights and content

Background

Early repolarization syndrome (ERS) is characterized by J-point elevation on electrocardiograms and ventricular fibrillation (VF). Early repolarization arises from augmentation of the transmural electrical gradient in the cardiac action potential; therefore, the transient outward potassium current (Ito) has been regarded as a key candidate current for elucidating the mechanism of ERS. KCND3 encoding Kv4.3, an α-subunit of the Ito channel, is considered as one of target genes.

Objective

The purpose of this study was to search for novel KCND3 mutations associated with ERS and to clarify the pathogenesis.

Methods

We performed genetic screening for 11 unrelated probands with ERS and analyzed the electrophysiological properties of detected mutations by patch-clamp methods.

Results

A novel de novo KCND3 heterozygous mutation, Gly306Ala (c.917g>c), was found in 1 proband. The proband was a 12-year-old boy, who suffered VF storm and showed significant J-point elevation in multiple leads. Intravenous isoproterenol and subsequent administration of quinidine were effective in preventing VF recurrence and restored the J-point elevation. In electrophysiological analysis, cultured cells expressing mutant Kv4.3 showed significantly increased current densities, slow inactivation, and slow recovery from inactivation compared to wild type. Extracellular application of quinidine significantly restored the inactivation time course in mutant Kv4.3. A simulation study confirmed the relationship between the novel KCND3 mutation and early repolarization on electrocardiograms.

Conclusion

A novel KCND3 heterozygous mutation was found to be associated with ERS. The pathogenesis can be explained by the increased Ito. Genetic screening for KCND3 could be useful for understanding the pathogenesis and selecting effective treatment.

Introduction

Early repolarization (ER) seen on electrocardiograms (ECGs) had been considered nonpathologic for ventricular arrhythmias. However, Haissaguerre et al demonstrated that patients with ER experienced a higher recurrence of idiopathic ventricular fibrillation (VF) than those without ER.1 After this pioneering report, several population studies confirmed the clinical relationship between ER and the occurrence of fatal arrhythmias.2, 3 Therefore, early repolarization syndrome (ERS) is characterized by ER and fatal arrhythmias, including idiopathic VF.4

In humans, the transient outward potassium current (Ito) is larger in the subepicardial layer than in the subendocardial one, creating a transmural electrical gradient in the early phase of the action potential (AP).5 It has been hypothesized that ER is caused by augmentation of this transmural electrical gradient. In an experiment using canine ventricular wedge preparations, an Ito agonist, NS5806, amplified the electrical gradient of the AP, resulting in J-point elevation on pseudo-ECGs and fatal arrhythmias; conversely, an Ito inhibitor, 4-aminopyrodine, suppressed J-point elevation and arrhythmogenicity arising from NS5806.6 Therefore, Ito has been regarded as a key candidate current for elucidating the mechanism underlying ERS. The Ito channel comprises α- and β-subunits, and the α-subunit is Kv4.3 encoded by KCND3.7, 8 Hence, KCND3 has been considered as one of the target genes for ERS. However, apart from 1 recent case report,9 there has been no electrophysiological analysis of KCND3 mutations in ERS patients. We aimed to search for novel KCND3 mutations associated with ERS in a cohort of patients and to clarify the pathogenesis of ERS by electrophysiological analysis.

Section snippets

Patients

The study population comprised 11 consecutive, unrelated probands who had suffered unexplained cardiac arrest with ERS. The ERS was diagnosed by the presence of J-point elevation ≥0.1 mV in >2 inferolateral leads on 12-lead ECG.4 The patients were referred to our laboratories from all over Japan for genetic evaluation. This study was conducted in accordance with the principles outlined in the Declaration of Helsinki and approved by our institutional ethics review board (reference number

Genetic analysis and phenotypic characterization

In 1 of the 11 probands (9%), we identified a heterozygous missense mutation in KCND3, Gly306Ala (c.917 g>c) (Figure 1A). Codon 306 is located in segment 4 of Kv4.3 (Figure 1B), and the amino acid glycine is highly conserved among species (Figure 1C). This mutation has not previously been reported to be associated with ERS or other inherited arrhythmias, and it was not reported in the genomic variation databases. PolyPhen-2, SIFT, and CADD all predicted that the mutant protein would be severely

Discussion

We identified a novel and de novo KCND3 heterozygous mutation, Gly306Ala (c.917g>c), in a sporadic case of ERS. The proband showed obvious J-point elevation during bradycardia and suffered VF storm. Quinidine administration was dramatically effective for preventing recurrence of VF. We demonstrated that the mutation induced gain-of-function changes in Kv4.3, an α-subunit of the Ito channel. The significant characteristics of the mutant Kv4.3 were represented by the markedly slow inactivation

Conclusion

A novel heterozygous Gly306Ala mutation in KCND3 was found to be associated with ERS by electrophysiological analysis and simulation study. The pathogenesis of ERS can be explained by the increased Ito. Although KCND3 mutation carriers seem to be rare in ERS, genetic screening for KCND3 could be useful in understanding the pathogenesis of the disease and selecting the effective treatment.

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    This work was supported by research grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grant Numbers 15H04818 to Dr Horie and 15K09689 to Dr Ohno); Health Science Research Grants from the Ministry of Health, Labour and Welfare of Japan for Clinical Research on Measures for Intractable Diseases (Grant Number H27-032 to Dr Horie); and Japan Agency for Medical Research and Development (Grant Number JP17ek0109202 to Dr Ohno).

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