Functional assessment of compound mutations in the KCNQ1 and KCNH2 genes associated with long QT syndrome

doi:10.1016/j.hrthm.2005.07.025

Heart Rhythm

Volume 2, Issue 11, November 2005, Pages 1238-1249

https://doi.org/10.1016/j.hrthm.2005.07.025 Get rights and content

Background

Long QT syndrome (LQTS) is a cardiovascular disorder characterized by prolonged QTc time, syncope, or sudden death caused by torsades de pointes and ventricular fibrillation. We investigated the clinical and electrophysiologic phenotype of individual mutations and the compound mutations in a family in which different genotypes could be found.

Objectives

The purpose of this study was to determine the impact of genotype-based diagnostic assessment in LQTS.

Methods

We used cascade screening and functional analyses to investigate the phenotype in a family with LQTS. The contributions of the compound mutations in the KCNQ1 and KCNH2 genes (KCNQ1 R591H, KCNH2 R328C) were analyzed by heterologous expression in Xenopus laevis oocytes using two-electrode voltage clamp and by confocal imaging.

Results

KCNH2 R328C did not show any functional phenotype whereas KCNQ1 R591H resulted in severe reduction of current. Neither wild-type nor mutant channels affected each other functionally in coexpression experiments. Therefore, a direct interaction between KCNQ1 and KCNH2 was ruled out under these conditions.

Conclusion

Assessment of novel mutational findings in LQTS should include accurate genetic and functional analysis. Notably, appropriate studies are needed if two or more mutations in different genes are present in one proband. Our findings prompt reconsideration of the impact of compound mutations in LQTS families and reinforce the need for thorough functional evaluation of novel ion channel mutations before assignment of pathogenic status.

Introduction

Long QT syndrome (LQTS) is a cardiac disease characterized by prolongation of the QT interval on ECG. Patients suffer from syncopal episodes due to ventricular arrhythmias such as torsades de pointes and are at risk for sudden death.¹ LQTS is genetically heterogeneous. It is caused by mutations in genes coding for cardiac ion channel subunits involved in the cardiac action potential, that is, KCNQ1 (LQT1), KCNH2 (LQT2), KCNE1 (LQT5), KCNE2 (LQT6), and SCN5A (LQT3).² KCNQ1 and KCNH2 code for α-subunits of voltage-gated potassium channels (KCNQ1 [Kv7.1] and HERG1 [Kv11.1], respectively), and SCN5A encodes a sodium channel α-subunit. KCNE1 and KCNE2 code for β-subunits also termed minK and MiRP1. KCNE1 and KCNE2 combine with KCNQ1 and HERG1 to constitute the repolarizing potassium currents in the cardiac action potential, I_Ks and I_Kr.3, 4 Mutations in ANK2 (LQT4), coding for the membrane receptor ankyrin B, and mutations in the genes KCNJ2 and CACNA1C, coding for the ion channel α-subunits involved in I_Kl and I_CaL, can cause LQTS.5, 6, 7 Although LQTS is known to cause severe cardiac arrhythmias, not all mutation carriers have symptoms, and only a few die of cardiac events.⁸ It has been postulated that a large proportion of LQTS carriers appear unaffected due to low penetrance.⁹ Further diversity appears to arise from an increasing number of reports of compound heterozygosity within one or even two different LQTS genes.10, 11, 12, 13 Schwartz et al¹⁴ suggested that compound mutations increase the risk of arrhythmia. Westenskow et al¹² demonstrated that LQTS-associated compound mutations may cause a more severe phenotype and are common. Only one study to date has reported mutations in both KCNQ1 and KCNH2, including functional analyses.¹³

In this report, we used cascade screening and functional studies to evaluate the contributions from the concomitant KCNQ1 (R591H) and HERG1 (R328C) mutations in a family with LQTS. Whereas the mutation in KCNQ1 led to a dramatic reduction in current and was sufficient to explain the phenotype, the HERG1 mutation appeared to have no impact. Mimicking the compound state in the patient by coexpression experiments demonstrated that the reduction in the current density was equivalent to the effect of the KCNQ1 mutation alone.

Section snippets

Patients

Among patients referred to Statens Serum Institut from St. George’s Hospital, London, United Kingdom, for molecular/genetic workup for LQTS, a British family with mutations in both KCNH2 and KCNQ1 was selected for careful clinical and electrophysiologic assessment. Subjects gave informed consent. The clinical evaluation included history, family history, examination, 12-lead ECG, and exercise and ambulatory ECG monitoring whenever possible. The QT interval was corrected using Bazett’s formula,

Phenotypic characterization

The pedigree of the family is shown in Figure 1. The proband (18.1) presented at age 58 years with multiple syncopal episodes and a single episode of aborted cardiac arrest due to ventricular tachycardia. She was diagnosed with LQTS and treated with β-adrenergic blockers and an implantable cardioverter-defibrillator (ICD). The proband had four children. The youngest child was a boy with a diagnosis of status epilepticus who died suddenly at age 12 years. DNA material from the dead boy was

Discussion

In this study, the sequences of all exons of the KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 genes in a British family with LQTS were analyzed. The proband was heterozygous for missense mutations in two individual genes, namely, R591H in KCNQ1 and R328C in KCNH2. The KCNQ1 mutation has been described.¹⁷ The mutation in KCNH2 was considered novel but now has been described as LQTS associated.¹⁸ Neither had been functionally characterized. The KCNH2 mutation had been identified in only two unrelated

Conclusion

Molecular screening of all family members of genotyped patients to minimize the risk of sudden death in mutation carriers is appropriate. Nevertheless, analysis of the functional effects of any mutant in order to fully understand the pathogenetic impact and severity of the disease is crucial. Only then can appropriate clinical management proceed.

Acknowledgments

We thank Birthe Lynderup, Jette Rasmussen, and Mette Hougaard for excellent technical assistance.

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Citation Excerpt :
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: This work was supported by The John and Birthe Meyer Foundation, The Velux Foundation, The NovoNordisk Foundation, The Danish Heart Foundation, and The Beckett Foundation.

^⁎: The first two authors share first authorship.

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Original-experimental geneticFunctional assessment of compound mutations in the KCNQ1 and KCNH2 genes associated with long QT syndrome

Background

Objectives

Methods

Results

Conclusion

Introduction

Section snippets

Patients

Phenotypic characterization

Discussion

Conclusion

Acknowledgments

Cell

Cell

Clin Chim Acta

Heart Rhythm

J Biol Chem

Cell

J Biol Chem

J Biol Chem

J Biol Chem

J Mol Cell Cardiol

Genetics, molecular mechanisms and management of long QT syndrome

Ann Med

Spectrum of mutations in long-QT syndrome genes. KVLQT1, HERG, SCN5A, KCNE1, and KCNE2

Circulation

Coassembly of K(V)LQT1 and minK (IsK) proteins to form cardiac I(Ks) potassium channel

Nature

A minK-HERG1 complex regulates the cardiac potassium current I(Kr)

Nature

Original-experimental genetic
Functional assessment of compound mutations in the KCNQ1 and KCNH2 genes associated with long QT syndrome