Alternative splicing and polyadenylation contribute to the generation of hERG1 C-terminal isoforms

KCNH2 encodes a nuclear-targeted polypeptide that mediates hERG1 channel gating and expression

KCNH2 encodes hERG1, the voltage-gated potassium channel that conducts the rapid delayed rectifier potassium current (I_Kr) in human cardiac tissue. hERG1 is one of the first channels expressed during early cardiac development, and its dysfunction is associated with intrauterine fetal death, sudden infant death syndrome, cardiac arrhythmia, and sudden cardiac death. Here, we identified a hERG1 polypeptide (hERG1_NP) that is targeted to the nuclei of immature cardiac cells, including human stem cell-derived cardiomyocytes (hiPSC-CMs) and neonatal rat cardiomyocytes. The nuclear hERG1_NP immunofluorescent signal is diminished in matured hiPSC-CMs and absent from adult rat cardiomyocytes. Antibodies targeting distinct hERG1 channel epitopes demonstrated that the hERG1_NP signal maps to the hERG1 distal C-terminal domain. KCNH2 deletion using CRISPR simultaneously abolished I_Kr and the hERG1_NP signal in hiPSC-CMs. We then identified a putative nuclear localization sequence (NLS) within the distal hERG1 C-terminus, 883-RQRKRKLSFR-892. Interestingly, the distal C-terminal domain was targeted almost exclusively to the nuclei when overexpressed HEK293 cells. Conversely, deleting the NLS from the distal peptide abolished nuclear targeting. Similarly, blocking α or β1 karyopherin activity diminished nuclear targeting. Finally, overexpressing the putative hERG1_NP peptide in the nuclei of HEK cells significantly reduced hERG1a current density, compared to cells expressing the NLS-deficient hERG1_NP or GFP. These data identify a developmentally regulated polypeptide encoded by KCNH2, hERG1_NP, whose presence in the nucleus indirectly modulates hERG1 current magnitude and kinetics.

Intronic Polyadenylation in Acquired Cancer Drug Resistance Circumvented by Utilizing CRISPR/Cas9 with Homology-Directed Repair: The Tale of Human DNA Topoisomerase IIα

Simple Summary

DNA topoisomerase IIα (170 kDa, TOP2α/170) resolves nucleic acid topological entanglements by generating transient double-strand DNA breaks. TOP2α inhibitors/poisons stabilize TOP2α-DNA covalent complexes resulting in persistent DNA damage and are frequently utilized to treat a variety of cancers. Acquired resistance to these chemotherapeutic agents is often associated with decreased TOP2α/170 expression levels. Studies have demonstrated that a reduction in TOP2α/170 results from a type of alternative polyadenylation designated intronic polyadenylation (IPA). As a consequence of IPA, variant TOP2α mRNA transcripts have been characterized that have resulted in the translation of C-terminal truncated TOP2α isoforms with altered biological activities. In this paper, an example is discussed where circumvention of acquired TOP2α-mediated drug resistance was achieved by utilizing CRISPR/Cas9 specific gene editing of an exon/intron boundary through homology directed repair (HDR) to reduce TOP2α IPA. These results illustrate the therapeutic potential of CRISPR/Cas9/HDR to impact drug resistance associated with aberrant IPA.

Abstract

Intronic polyadenylation (IPA) plays a critical role in malignant transformation, development, progression, and cancer chemoresistance by contributing to transcriptome/proteome alterations. DNA topoisomerase IIα (170 kDa, TOP2α/170) is an established clinical target for anticancer agents whose efficacy is compromised by drug resistance often associated with a reduction of nuclear TOP2α/170 levels. In leukemia cell lines with acquired resistance to TOP2α-targeted drugs and reduced TOP2α/170 expression, variant TOP2α mRNA transcripts have been reported due to IPA that resulted in the translation of C-terminal truncated isoforms with altered nuclear-cytoplasmic distribution or heterodimerization with wild-type TOP2α/170. This review provides an overview of the various mechanisms regulating pre-mRNA processing and alternative polyadenylation, as well as the utilization of CRISPR/Cas9 specific gene editing through homology directed repair (HDR) to decrease IPA when splice sites are intrinsically weak or potentially mutated. The specific case of TOP2α exon 19/intron 19 splice site editing is discussed in etoposide-resistant human leukemia K562 cells as a tractable strategy to circumvent acquired TOP2α-mediated drug resistance. This example supports the importance of aberrant IPA in acquired drug resistance to TOP2α-targeted drugs. In addition, these results demonstrate the therapeutic potential of CRISPR/Cas9/HDR to impact drug resistance associated with aberrant splicing/polyadenylation.

Use of CRISPR/Cas9 with homology-directed repair to silence the human topoisomerase IIα intron-19 5’ splice site: Generation of etoposide resistance in human leukemia K562 cells

DNA Topoisomerase IIα (TOP2α/170) is an enzyme essential for proliferating cells. For rapidly multiplying malignancies, this has made TOP2α/170 an important target for etoposide and other clinically active anticancer drugs. Efficacy of these agents is often limited by chemoresistance related to alterations in TOP2α/170 expression levels. Our laboratory recently demonstrated reduced levels of TOP2α/170 and overexpression of a C-terminal truncated 90-kDa isoform, TOP2α/90, due to intronic polyadenylation (IPA; within intron 19) in an acquired etoposide-resistant K562 clonal cell line, K/VP.5. We previously reported that this isoform heterodimerized with TOP2α/170 and was a determinant of acquired resistance to etoposide. Optimization of the weak TOP2α exon 19/intron 19 5′ splice site in drug-resistant K/VP.5 cells by gene-editing restored TOP2α/170 levels, diminished TOP2α/90 expression, and circumvented drug resistance. Conversely, in the present study, silencing of the exon 19/intron 19 5′ splice site in parental K562 cells by CRISPR/Cas9 with homology-directed repair (HDR), and thereby forcing intron 19 retention, was used to induce resistance by disrupting normal RNA processing (i.e., gene knockout), and to further evaluate the role of TOP2α/170 and TOP2α/90 isoforms as resistance determinants. Gene-edited clones were identified by quantitative polymerase chain reaction (qPCR) and verified by Sanger sequencing. TOP2α/170 mRNA/protein expression levels were attenuated in the TOP2α gene-edited clones which resulted in resistance to etoposide as assessed by reduced etoposide-induced DNA damage (γH2AX, Comet assays) and growth inhibition. RNA-seq and qPCR studies suggested that intron 19 retention leads to decreased TOP2α/170 expression by degradation of the TOP2α edited mRNA transcripts. Forced expression of TOP2α/90 in the gene-edited K562 cells further decreased etoposide-induced DNA damage in support of a dominant negative role for this truncated isoform. Together results support the important role of both TOP2α/170 and TOP2α/90 as determinants of sensitivity/resistance to TOP2α-targeting agents.

The ERG1 K+ Channel and Its Role in Neuronal Health and Disease

The ERG1 potassium channel, encoded by KCNH2, has long been associated with cardiac electrical excitability. Yet, a growing body of work suggests that ERG1 mediates physiology throughout the human body, including the brain. ERG1 is a regulator of neuronal excitability, ERG1 variants are associated with neuronal diseases (e.g., epilepsy and schizophrenia), and ERG1 serves as a potential therapeutic target for neuronal pathophysiology. This review summarizes the current state-of-the-field regarding the ERG1 channel structure and function, ERG1’s relationship to the mammalian brain and highlights key questions that have yet to be answered.

Regulation of Kv11.1 Isoform Expression by Polyadenylate Binding Protein Nuclear 1

The Kv11.1 voltage-gated potassium channel, encoded by the KCNH2 gene, conducts the rapidly activating delayed rectifier current in the heart. KCNH2 pre-mRNA undergoes alternative polyadenylation to generate two C-terminal Kv11.1 isoforms in the heart. Utilization of a poly(A) signal in exon 15 produces the full-length, functional Kv11.1a isoform, while intron 9 polyadenylation generates the C-terminally truncated, nonfunctional Kv11.1a-USO isoform. The relative expression of Kv11.1a and Kv11.1a-USO isoforms plays an important role in the regulation of Kv11.1 channel function. In this study, we tested the hypothesis that the RNA polyadenylate binding protein nuclear 1 (PABPN1) interacts with a unique 22 nt adenosine stretch adjacent to the intron 9 poly(A) signal and regulates KCNH2 pre-mRNA alternative polyadenylation and the relative expression of Kv11.1a C-terminal isoforms. We showed that PABPN1 inhibited intron 9 poly(A) activity using luciferase reporter assays, tandem poly(A) reporter assays, and RNA pulldown assays. We also showed that PABPN1 increased the relative expression level of the functional Kv11.1a isoform using RNase protection assays, immunoblot analyses, and patch clamp recordings. Our present findings suggest a novel role for the RNA-binding protein PABPN1 in the regulation of functional and nonfunctional Kv11.1 isoform expression.

Long QT Syndrome Type 2: Emerging Strategies for Correcting Class 2 KCNH2 (hERG) Mutations and Identifying New Patients

Significant advances in our understanding of the molecular mechanisms that cause congenital long QT syndrome (LQTS) have been made. A wide variety of experimental approaches, including heterologous expression of mutant ion channel proteins and the use of inducible pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from LQTS patients offer insights into etiology and new therapeutic strategies. This review briefly discusses the major molecular mechanisms underlying LQTS type 2 (LQT2), which is caused by loss-of-function (LOF) mutations in the KCNH2 gene (also known as the human ether-à-go-go-related gene or hERG). Almost half of suspected LQT2-causing mutations are missense mutations, and functional studies suggest that about 90% of these mutations disrupt the intracellular transport, or trafficking, of the KCNH2-encoded Kv11.1 channel protein to the cell surface membrane. In this review, we discuss emerging strategies that improve the trafficking and functional expression of trafficking-deficient LQT2 Kv11.1 channel proteins to the cell surface membrane and how new insights into the structure of the Kv11.1 channel protein will lead to computational approaches that identify which KCNH2 missense variants confer a high-risk for LQT2.

Long QT Syndrome Type 2: Emerging Strategies for Correcting Class 2 KCNH2 (hERG) Mutations and Identifying New Patients

Significant advances in our understanding of the molecular mechanisms that cause congenital long QT syndrome (LQTS) have been made. A wide variety of experimental approaches, including heterologous expression of mutant ion channel proteins and the use of inducible pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from LQTS patients offer insights into etiology and new therapeutic strategies. This review briefly discusses the major molecular mechanisms underlying LQTS type 2 (LQT2), which is caused by loss-of-function (LOF) mutations in the KCNH2 gene (also known as the human ether-à-go-go-related gene or hERG). Almost half of suspected LQT2-causing mutations are missense mutations, and functional studies suggest that about 90% of these mutations disrupt the intracellular transport, or trafficking, of the KCNH2-encoded Kv11.1 channel protein to the cell surface membrane. In this review, we discuss emerging strategies that improve the trafficking and functional expression of trafficking-deficient LQT2 Kv11.1 channel proteins to the cell surface membrane and how new insights into the structure of the Kv11.1 channel protein will lead to computational approaches that identify which KCNH2 missense variants confer a high-risk for LQT2.

Regulation of Kv11.1 potassium channel C-terminal isoform expression by the RNA-binding proteins HuR and HuD

The potassium voltage-gated channel subfamily H member 2 (KCNH2) gene encodes the Kv11.1 potassium channel, which conducts the rapidly activating delayed rectifier current in the heart. KCNH2 pre-mRNA undergoes alternative polyadenylation and forms a functional, full-length Kv11.1a isoform if exon 15 is polyadenylated or a nonfunctional, C-terminally truncated Kv11.1a-USO isoform if intron 9 is polyadenylated. The molecular mechanisms that regulate Kv11.1 isoform expression are poorly understood. In this study, using HEK293 cells and reporter gene expression, pulldown assays, and RNase protection assays, we identified the RNA-binding proteins Hu antigen R (HuR) and Hu antigen D (HuD) as regulators of Kv11.1 isoform expression. We show that HuR and HuD inhibit activity at the intron 9 polyadenylation site. When co-expressed with the KCNH2 gene, HuR and HuD increased levels of the Kv11.1a isoform and decreased the Kv11.1a-USO isoform in the RNase protection assays and immunoblot analyses. In patch clamp experiments, HuR and HuD significantly increased the Kv11.1 current. siRNA-mediated knockdown of HuR protein decreased levels of the Kv11.1a isoform and increased those of the Kv11.1a-USO isoform. Our findings suggest that the relative expression levels of Kv11.1 C-terminal isoforms are regulated by the RNA-binding HuR and HuD proteins.

Regulation of Isoform Expression by Blocking Polyadenylation Signal Sequences with Morpholinos

Alternative polyadenylation is increasingly being recognized as an important layer of gene regulation. Antisense-mediated modulation of alternative polyadenylation represents an attractive strategy for the regulation of gene expression as well as potential therapeutic applications. In this chapter, we describe methods to upregulate the functional Kv11.1 isoform expression by blocking intronic polyadenylation signal sequences with antisense morpholinos.

Nonsense-Mediated mRNA Decay of hERG Mutations in Long QT Syndrome

Long QT syndrome type 2 (LQT2) is caused by mutations in the human ether-à-go-go related gene (hERG), which encodes the Kv11.1 potassium channel in the heart. Over 30% of identified LQT2 mutations are nonsense or frameshift mutations that introduce premature termination codons (PTCs). Contrary to intuition, the predominant consequence of LQT2 nonsense and frameshift mutations is not the production of truncated proteins, but rather the degradation of mutant mRNA by nonsense-mediated mRNA decay (NMD), an RNA surveillance mechanism that selectively eliminates the mRNA transcripts that contain PTCs. In this chapter, we describe methods to study NMD of hERG nonsense and frameshift mutations in long QT syndrome.

Upregulation of functional Kv11.1a isoform expression by modified U1 small nuclear RNA

The KCNH2 or human ether-a go-go-related gene (hERG) encodes the Kv11.1 potassium channel that conducts the rapidly activating delayed rectifier potassium current in the heart. The expression of Kv11.1 C-terminal isoforms is directed by the alternative splicing and polyadenylation of intron 9. Splicing of intron 9 leads to the formation of a functional, full-length Kv11.1a isoform and polyadenylation of intron 9 results in the production of a non-functional, C-terminally truncated Kv11.1a-USO isoform. The relative expression of Kv11.1a and Kv11.1a-USO plays an important role in regulating Kv11.1 channel function. In the heart, only one-third of KCNH2 pre-mRNA is processed to Kv11.1a due to the weak 5' splice site of intron 9. We previously showed that the weak 5' splice site is caused by sequence deviation from the consensus, and that mutations toward the consensus sequence increased the efficiency of intron 9 splicing. It is well established that 5' splice sites are recognized by complementary base-paring with U1 small nuclear RNA (U1 snRNA). In this study, we modified the sequence of U1 snRNA to increase its complementarity to the 5' splice site of KCNH2 intron 9 and observed a significant increase in the efficiency of intron 9 splicing. RNase protection assay and western blot analysis showed that modified U1 snRNA increased the expression of the functional Kv11.1a isoform and concomitantly decreased the expression of the non-functional Kv11.1a-USO isoform. In patch-clamp experiments, modified U1 snRNA significantly increased Kv11.1 current. Our findings suggest that relative expression of Kv11.1 C-terminal isoforms can be regulated by modified U1 snRNA.

Molecular pathogenesis of long QT syndrome type 2

The molecular mechanisms underlying congenital long QT syndrome (LQTS) are now beginning to be understood. New insights into the etiology and therapeutic strategies are emerging from heterologous expression studies of LQTS-linked mutant proteins, as well as inducible pluripotent stem cell derived cardiomyocytes (iPSC-CMs) from LQTS patients. This review focuses on the major molecular mechanism that underlies LQTS type 2 (LQT2). LQT2 is caused by loss of function (LOF) mutations in KCNH2 (also known as the human Ether-à-go-go-Related Gene or hERG). Most LQT2-linked mutations are missense mutations and functional studies suggest that ~90% of them disrupt the intracellular transport (trafficking) of KCNH2-encoded Kv11.1 proteins to the cell membrane. Trafficking deficient LQT2 mutations disrupt Kv11.1 protein folding and misfolded Kv11.1 proteins are retained in the endoplasmic reticulum (ER) until they are degraded in the ER associated degradation pathway (ERAD). This review focuses on the quality control mechanisms in the ER that contribute to the folding and ERAD of Kv11.1 proteins; the mechanism for ER export of Kv11.1 proteins in the secretory pathway; different subclasses of trafficking deficient LQT2 mutations; and strategies being developed to mitigate or correct trafficking deficient LQT2-related phenotypes.

Upregulation of functional Kv11.1 isoform expression by inhibition of intronic polyadenylation with antisense morpholino oligonucleotides

The KCNH2 gene encodes the Kv11.1 potassium channel that conducts the rapidly activating delayed rectifier current in the heart. KCNH2 pre-mRNA undergoes alternative processing; intron 9 splicing leads to the formation of a functional, full-length Kv11.1a isoform, while polyadenylation within intron 9 generates a non-functional, C-terminally truncated Kv11.1a-USO isoform. The relative expression of Kv11.1 isoforms plays an important role in the regulation of Kv11.1 channel function and the pathogenesis of long QT syndrome. In this study, we identified cis-acting elements that are required for KCNH2 intron 9 poly(A) signal activity. Mutation of these elements decreased Kv11.1a-USO expression and increased the expression of Kv11.1a mRNA, protein and channel current. More importantly, blocking these elements by antisense morpholino oligonucleotides shifted the alternative processing of KCNH2 intron 9 from the polyadenylation to the splicing pathway, leading to the predominant production of Kv11.1a and a significant increase in Kv11.1 current. Our findings indicate that the expression of the Kv11.1a isoform can be upregulated by an antisense approach. Antisense inhibition of KCNH2 intronic polyadenylation represents a novel approach to increase Kv11.1 channel function.

Identification of Kv11.1 isoform switch as a novel pathogenic mechanism of long-QT syndrome

BACKGROUND

The KCNH2 gene encodes the Kv11.1 potassium channel that conducts the rapidly activating delayed rectifier current in the heart. The relative expression of the full-length Kv11.1a isoform and the C-terminally truncated Kv11.1a-USO isoform plays an important role in regulation of channel function. The formation of C-terminal isoforms is determined by competition between the splicing and alternative polyadenylation of KCNH2 intron 9. It is not known whether changes in the relative expression of Kv11.1a and Kv11.1a-USO can cause long-QT syndrome.

METHODS AND RESULTS

We identified a novel KCNH2 splice site mutation in a large family. The mutation, IVS9-2delA, is a deletion of the A in the AG dinucleotide of the 3' acceptor site of intron 9. We designed an intron-containing full-length KCNH2 gene construct to study the effects of the mutation on the relative expression of Kv11.1a and Kv11.1a-USO at the mRNA, protein, and functional levels. We found that this mutation disrupted normal splicing and resulted in exclusive polyadenylation of intron 9, leading to a switch from the functional Kv11.1a to the nonfunctional Kv11.1a-USO isoform in HEK293 cells and HL-1 cardiomyocytes. We also showed that IVS9-2delA caused isoform switch in the mutant allele of mRNA isolated from patient lymphocytes.

CONCLUSIONS

Our findings indicate that the IVS9-2delA mutation causes a switch in the expression of the functional Kv11.1a isoform to the nonfunctional Kv11.1a-USO isoform. Kv11.1 isoform switch represents a novel mechanism in the pathogenesis of long-QT syndrome.

Position of premature termination codons determines susceptibility of hERG mutations to nonsense-mediated mRNA decay in long QT syndrome

The degradation of human ether-a-go-go-related gene (hERG, KCNH2) transcripts containing premature termination codon (PTC) mutations by nonsense-mediated mRNA decay (NMD) is an important mechanism of long QT syndrome type 2 (LQT2). The mechanisms governing the recognition of PTC-containing hERG transcripts as NMD substrates have not been established. We used a minigene system to study two frameshift mutations, R1032Gfs 25 and D1037Rfs 82. R1032Gfs 25 introduces a PTC in exon 14, whereas D1037Rfs 82 causes a PTC in the last exon (exon 15). We showed that R1032Gfs 25, but not D1037Rfs 82, reduced the level of mutant mRNA compared to the wild-type minigene in an NMD-dependent manner. The deletion of intron 14 prevented degradation of R1032Gfs 25 mRNA indicating that a downstream intron is required for NMD. The recognition and elimination of PTC-containing transcripts by NMD required that the mutation be positioned >54-60 nt upstream of the 3'-most exon-exon junction. Finally, we used a full-length hERG splicing-competent construct to show that inhibition of downstream intron splicing by antisense morpholino oligonucleotides inhibited NMD and rescued the functional expression of a third LQT2 mutation, Y1078. The present study defines the positional requirements for the susceptibility of LQT2 mutations to NMD and posits that the majority of reported LQT2 nonsense and frameshift mutations are potential targets of NMD.

Isoform-specific dominant-negative effects associated with hERG1 G628S mutation in long QT syndrome

Background

Mutations in the human ether-a-go-go-related gene 1 (hERG1) cause type 2 long QT syndrome (LQT2). The hERG1 gene encodes a K⁺ channel with properties similar to the rapidly activating delayed rectifying K⁺ current in the heart. Several hERG1 isoforms with unique structural and functional properties have been identified. To date, the pathogenic mechanisms of LQT2 mutations have been predominantly described in the context of the hERG1a isoform. In the present study, we investigated the functional consequences of the LQT2 mutation G628S in the hERG1b and hERG1a_USO isoforms.

Methods

A double-stable, mammalian expression system was developed to characterize isoform-specific dominant-negative effects of G628S-containing channels when co-expressed at equivalent levels with wild-type hERG1a. Western blot and co-immunoprecipitation studies were performed to study the trafficking and co-assembly of wild-type and mutant hERG1 isoforms. Patch-clamp electrophysiology was performed to characterize hERG1 channel function and the isoform-specific dominant-negative effects associated with the G628S mutation.

Conclusions

The non-functional hERG1a-G628S and hERG1b-G628S channels co-assembled with wild-type hERG1a and dominantly suppressed hERG1 current. In contrast, G628S-induced dominant-negative effects were absent in the context of the hERG1a_USO isoform. hERG1a_USO-G628S channels did not appreciably associate with hERG1a and did not significantly suppress hERG1 current when co-expressed at equivalent ratios or at ratios that approximate those found in cardiac tissue. These results suggest that the dominant-negative effects of LQT2 mutations may primarily occur in the context of the hERG1a and hERG1b isoforms.

Deciphering hERG channels: molecular basis of the rapid component of the delayed rectifier potassium current

The rapid component of the delayed rectifier potassium current (I(Kr)), encoded by the ether-a-go-go-related gene (ERG1, officially denominated as KCNH2), is a major contributor to repolarization in the mammalian heart. Acute (e.g. drug-induced) and chronic (e.g. inherited genetic disorder) disruptions of this current can lead to prolongation of the action potential and potentiate occurrence of lethal arrhythmias. Many cardiac and non-cardiac drugs show high affinity for the I(Kr) channel and it is therefore extensively studied during safety pharmacology. The unique biophysical and pharmacological properties of the I(Kr) channel are largely recapitulated by expressing the human variant (hERG1a) in overexpressing systems. hERG1a channels are tetramers consisting of four 1159 amino acid long proteins and have electrophysiological properties similar, but not identical, to native I(Kr). In the search for an explanation to the discrepancies between I(Kr) and hERG1a channels, two alternative hERG1 proteins have been found. Alternative transcription of hERG1 leads to a protein with a 56 amino acid shorter N-terminus, known as hERG1b. hERG1b can form channels alone or coassemble with hERG1a. Alternative splicing leads to an alternate C-terminus and a protein known as hERGuso. hERGuso and hERG1b regulate hERG1a channel trafficking, functional expression and channel kinetics. Expression of hERGuso leads to a reduced number of channels at the plasma membrane and thereby reduces current density. On the contrary, co-assembly with hERG1b alters channel kinetics resulting in more available channels and a larger current. These findings have implication for understanding mechanisms of disease, acute and chronic drug effects, and potential gender differences.

Nonsense-mediated mRNA decay caused by a frameshift mutation in a large kindred of type 2 long QT syndrome

BACKGROUND

Nonsense and frameshift mutations are common in congenital long QT syndrome type 2 (LQT2). We previously demonstrated that hERG nonsense mutations cause degradation of mutant mRNA by nonsense-mediated mRNA decay (NMD) and are associated with mild clinical phenotypes. The impact of NMD on the expression of hERG frameshift mutations and their phenotypic severity is not clear.

OBJECTIVE

The purpose of this study was to examine the role of NMD in the pathogenesis of a hERG frameshift mutation, P926AfsX14, identified in a large LQT2 kindred and characterize genotype-phenotype correlations.

METHODS

Genetic screening was performed among family members. Phenotyping was performed by assessment of ECGs and LQTS-related cardiac events. The functional effect of P926AfsX14 was studied using hERG cDNA and minigene constructs expressed in HEK293 cells.

RESULTS

Significant cardiac events occurred in carriers of the P926AfsX14 mutation. When expressed from cDNA, the P926AfsX14 mutant channel was only mildly defective. However, when expressed from a minigene, the P926AfsX14 mutation caused a significant reduction in mutant mRNA, protein, and hERG current. Inhibition of NMD by RNA interference knockdown of up-frameshift protein 1 partially restored expression of mutant mRNA and protein and led to a significant increase in hERG current in the mutant cells. These results suggest that NMD is involved in the pathogenic mechanism of the P926AfsX14 mutation.

CONCLUSION

Our findings suggest that the hERG frameshift mutation P926AfsX14 primarily results in degradation of mutant mRNA by the NMD pathway rather than production of truncated proteins. When combined with environmental triggers and genetic modifiers, LQT2 frameshift mutations associated with NMD can manifest with a severe clinical phenotype.