Elsevier

Heart Rhythm

Volume 5, Issue 4, April 2008, Pages 553-561
Heart Rhythm

Original-clinical genetic
Recurrent intrauterine fetal loss due to near absence of HERG: Clinical and functional characterization of a homozygous nonsense HERG Q1070X mutation

https://doi.org/10.1016/j.hrthm.2008.01.020Get rights and content

Background

Inherited arrhythmias may underlie intrauterine and neonatal arrhythmias. Resolving the molecular genetic nature of these rare cases provides significant insight into the role of the affected proteins in arrhythmogenesis and (extra-) cardiac development.

Objective

The purpose of this study was to perform clinical, molecular, and functional studies of a consanguineous Arabian family with repeated early miscarriages and two intrauterine fetal losses in the early part of the third trimester of pregnancy due to persistent arrhythmias.

Methods

In-depth clinical investigation was performed in two siblings, both of whom developed severe arrhythmia during the second trimester of pregnancy. Homozygosity mapping with microsatellite repeat polymorphic markers encompassing various cardiac ion channel genes linked to electrical instability of the heart was performed. Screening of the candidate gene in the homozygous locus was performed. Biochemical and electrophysiologic analysis was performed to elucidate the function of the mutated gene.

Results

Screening of the HERG gene in the homozygous locus detected a homozygous nonsense mutation Q1070X in the HERG C-terminus in affected children. Biochemical and functional analysis of the Q1070X mutant showed that although the mutant HERG had the ability to traffic to the plasma membrane and to form functional channels, it was destroyed by the nonsense-mediated decay (NMD) pathway before its translation. NMD leads to near absence of HERG in homozygous Q1070X mutation carriers, causing debilitating arrhythmias (prior to birth) in homozygous carriers but no apparent phenotype in heterozygous carriers.

Conclusion

Homozygous HERG Q1070X is equivalent to near functional knockout of HERG. Clinical consequences appear early, originating during the early stages of embryonic life. The NMD pathway renders HERG Q1070X functionless before it can form a functional ion channel.

Introduction

Congenital long QT syndrome is an inherited disorder defined by prolongation of the QT interval. Patients with long QT syndrome are predisposed to the ventricular tachyarrhythmia torsade de pointes, which leads to recurrent syncope or sudden cardiac death. Long QT syndrome affects an estimated one in 2000 people worldwide.1 The molecular basis of long QT syndrome is heterogeneous. It is caused by mutations in one of several genes, including KCNQ1, HERG, KCNJ2, KCNE1, and KCNE2 encoding potassium channel subunits, the cardiac sodium channel gene SCN5A, L-type calcium channel gene CACNA1C, and ANK2, a membrane-anchoring gene.2 Mutations involving the gene HERG are responsible for long QT syndrome type 2 (LQT2).2, 3 In cellular expression studies, HERG mutations are associated with reduction in the rapid component of the delayed rectifier repolarizing current IKr.4 Diminution in the repolarizing IKr current contributes to lengthening of the QT interval, the ECG hallmark of LQT2.

Cardiac IKr channels are composed of HERGa and HERGb subunits,5 which differ in their amino terminal sequence. Functional HERG channels result from coassembly of four HERG subunits (HERGa and HERGb) into a tetrameric protein. Proposed molecular mechanisms that may account for reduced IKr current in patients with HERG mutations are disruption of protein synthesis, protein trafficking, gating, or permeation, and degradation of mutant mRNA by the nonsense-mediated mRNA decay (NMD) pathway.6, 7, 8 Homozygous missense mutations in HERG are associated with a severe form of long QT syndrome,9, 10 with symptoms appearing in the early years of life. Natural human knockout of HERGa, the larger isoform, has more severe effects, with intrauterine fetal death and severe fetal arrhythmias in newborns diagnosed in utero with tachyarrhythmias.11 Biallelic nonsense mutations affecting both HERG isoforms (HERGa and HERGb) have not been described.

We studied a family with recurrent stillbirths and neonatal/intrauterine long QT syndrome in whom the disease locus was mapped to HERG and a HERG mutation segregating with disease was identified, but cell surface expression and electrophysiology were normal. Because the mutation truncated the distal C-terminus of the protein, we examined whether loss of interaction with the HERG C-terminus interacting protein 14-3-3ϵ contributed to the phenotype. We also investigated the hypothesis that Q1070X results in accelerated mRNA degradation and thus failure of protein translation, the phenomenon of “nonsense-mediated decay” recently implicated as a mechanism for loss of function in ion channel and other diseases.

Section snippets

Clinical data

The study was performed according to a protocol approved by the local ethics committee. The patients studied were the offspring of asymptomatic, consanguineous, Arabian parents (see Figure 1A). The mother had two miscarriages at 8 (II:1) weeks and 10 (II:2) weeks of pregnancy, followed by delivery of a stillborn (II:3) hydropic baby with reported irregular heartbeat at week 29 of pregnancy. Autopsy, complete workup of the stillbirth, and chromosome culture were performed. The fourth pregnancy

II:5

Fetal bradycardia with a heart rate of approximately 90 bpm without fetal hydrops was noted at 22 weeks, and the patient (I:2) was referred to a tertiary fetal medicine unit. Fetal echocardiography performed at 23+2 weeks of gestation showed persistent ventricular tachycardia and hydrops with mild ascites, pericardial and pleural effusions, and skin edema. Ventricular rates were 210–220 bpm, and atrial rates were 120–130 bpm. The fetal heart was structurally normal; however, important

Discussion

TheHERG gene encodes IKr, the rapid component of the delayed rectifier current. Heterozygous missense mutations and mutations giving rise to premature truncations are causally involved in LQT2. The HERG nonsense mutation Q1070X described in this study is expected to lead to a premature termination codon at the C-terminal region of HERG channels. The parents in this study are consanguineous, and both are heterozygous for the same mutation. The heterozygous carriers display virtually no clinical

Conclusion

This study, which was based on homozygosity for the nonsense Q1070X mutant, demonstrates that degradation of the mutant HERG mRNA by NMD causes almost complete loss of HERG, leading to a severe exclusively cardiac phenotype that presumably includes miscarriages and stillbirth. Because our data show that the Q1070X mutant can form HERG channels with near-normal function, interventions to prevent degradation of mutant mRNA transcripts might be a therapeutic approach for homozygous HERG Q1070X

Acknowledgments

We thank Dr. Abdulraouf Al Saeedi and Dr. Saad Alyousef, Prince Sultan Cardiac Center, Riyadh, Saudi Arabia, for expert consultations and F. Hossenia-Salehi for laboratory assistance.

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  • Cited by (0)

    Dr. Wilde was supported by the Netherlands Heart Foundation (NHS 2003T302). Dr. Zhou is supported by National Institutes of Health Grant HL68854.

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