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Arrhythmogenic KCNE Gene Variants: Current Knowledge and Future Challenges

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Arrhythmogenic KCNE Gene Variants: Current Knowledge and Future Challenges UC Irvine UC Irvine Previously Published Works Title Arrhythmogenic KCNE gene variants: current knowledge and future challenges. Permalink https://escholarship.org/uc/item/7f24t3m5 Journal Frontiers in genetics, 5(JAN) ISSN 1664-8021 Authors Crump, Shawn M Abbott, Geoffrey W Publication Date 2014-01-24 DOI 10.3389/fgene.2014.00003 License https://creativecommons.org/licenses/by/4.0/ 4.0 Peer reviewed eScholarship.org Powered by the California Digital Library University of California REVIEW ARTICLE published: 24 January 2014 doi: 10.3389/fgene.2014.00003 Arrhythmogenic KCNE gene variants: current knowledge and future challenges Shawn M. Crump and Geoffrey W. Abbott* Bioelectricity Laboratory, Department of Pharmacology, Department of Physiology and Biophysics, School of Medicine, University of California, Irvine, CA, USA Edited by: There are twenty-five known inherited cardiac arrhythmia susceptibility genes, all of Junjie Xiao, Shanghai University, which encode either ion channel pore-forming subunits or proteins that regulate aspects China of ion channel biology such as function, trafficking, and localization. The human KCNE Reviewed by: gene family comprises five potassium channel regulatory subunits, sequence variants Junjie Xiao, Shanghai University, China in each of which are associated with cardiac arrhythmias. KCNE gene products exhibit Lei Zhang, Massachusetts General promiscuous partnering and in some cases ubiquitous expression, hampering efforts Hospital, USA to unequivocally correlate each gene to specific native potassium currents. Likewise, *Correspondence: deducing the molecular etiology of cardiac arrhythmias in individuals harboring rare KCNE Geoffrey W. Abbott, Bioelectricity gene variants, or more common KCNE polymorphisms, can be challenging. In this Laboratory, Department of Pharmacology, Department of review we provide an update on putative arrhythmia-causing KCNE gene variants, and Physiology and Biophysics, School discuss current thinking and future challenges in the study of molecular mechanisms of of Medicine, University of California, KCNE-associated cardiac rhythm disturbances. 360 Medical Surge II, Irvine, CA 92697, USA Keywords: MinK-related peptide, MiRP, Long QT Syndrome, atrial fibrillation, Brugada Syndrome e-mail: [email protected] INTRODUCTION to tissues outside the heart, including polarized epithelia. In con- A quarter of a century ago, Takumi and colleagues discovered a trast, more than sixty KCNE gene variants have been suggested to fraction of rat kidney mRNA that generated an unusual, slow- associate with human cardiac arrhythmias. In this mini-review we activating K+-selective current when injected into Xenopus laevis summarize current knowledge on arrhythmia-associated KCNE oocytes (Takumi et al., 1988). The protein product required for gene variants and discuss the difficulties in establishing causal- this slow current has been variously termed “minimal potas- ity and molecular etiology when dealing with rare diseases and sium channel” (MinK), “IsK” (for slow potassium current), and promiscuous regulatory proteins. more recently KCNE1—the gene name KCNE1 now being most Kv channels play a central role in active repolarization of commonly also used when referring to the protein product, for all excitable cells, including cardiac myocytes. In human ventri- simplicity. We now know that KCNE1 is the founding member of cles, three types of Kv channel in particular are important for a five-strong family of single transmembrane domain potassium timely myocyte repolarization, and also for the action potential channel ancillary (β) subunits (Figures 1, 2)thatdonotform morphology optimal for rhythmic contractions, incorporating a currents alone but are essential for generation of some native K+ plateau phase followed by relatively steep phase 3 repolarization currents by virtue of formation of heteromeric ion channel com- (Figure 1B). During an action potential, membrane depolar- + + plexes with voltage-gated potassium (Kv) channel pore-forming α ization primarily from Na influx through voltage-gated Na + subunits (Abbott and Goldstein, 1998). Because KCNE1 was rel- channels is counteracted by a transient outward K (Ito) current, atively quickly found to be a molecular correlate of the slowly producing the initial repolarization “notch.” Subsequently, slower + + activating ventricular myocyte K current, IKs (Freeman and delayed rectifier-generated outward K currents (IKr and IKs) + + Kass, 1993), study of the KCNE family as a whole has historically counteract inward Ca2 flux through voltage-gated Ca2 chan- been focused primarily on the heart. This is especially true for the nels, modulating the strength of contraction and duration of the study of the role of KCNE gene variants in human disease. action potential plateau (Sanguinetti and Jurkiewicz, 1990; Niwa Although in the new millennium the role of various KCNE and Nerbonne, 2010). subunits in epithelia has been extensively explored, this work has been largely conducted using mouse models (Arrighi et al., KCNE REGULATION OF hERG: THE α SUBUNIT UNDERLYING 2001; Dedek and Waldegger, 2001; Barriere et al., 2003; Rivas and VENTRICULAR IKr + Francis, 2005; Roepke et al., 2006, 2009, 2011a,b; Preston et al., The “rapidly activating” K current (IKr)isthepredom- 2010). The existing evidence from human genetics of the neces- inant human ventricular repolarization current under nor- sity for KCNE proteins in extracardiac tissue (Schulze-Bahr et al., mal circumstances. IKr is generated by channels comprising a 1997; Tyson et al., 1997) probably represents the tip of the ice- tetramer of hERG α subunits, encoded by the KCNH2 gene. berg in terms of the actual importance of human KCNE proteins KCNH2 gene mutations are (together with KCNQ1)oneof the top two identified inherited causes of the cardiac arrhyth- Abbreviations: AF, atrial fibrillation; BrS, Brugada syndrome; CHO, Chinese hamster ovary; diLQTS, drug-induced Long QT Syndrome; IKs, cardiac-delayed mia Long QT Syndrome (LQTS), which results from delayed rectifier-like K+ current; JLNS, Jervell and Lange-Nielsen syndrome. ventricular myocyte repolarization, manifests as a prolonged www.frontiersin.org January 2014 | Volume 5 | Article 3 | 1 Crump and Abbott Arrhythmogenic KCNE gene variants FIGURE 1 | KCNE subunits and the ventricular myocyte action Extracellular side is uppermost in each case. (B) Upper, a ventricular potential. (A) Upper, transmembrane topology of Kv α and KCNE action potential waveform indicating the major ionic currents that subunits with transmembrane segments numbered; lower, one contribute to its morphology and duration; lower, a human surface ECG suggested stoichiometry of a KCNE-containing Kv channel complex. waveform showing the QT interval. FIGURE 2 | Human KCNE1-KCNE5 protein sequence alignments and changes (changes involving >1 amino acid are omitted). In cases where an gene variants. Image of aligned sequences generated using amino acid substitution is associated with LQTS in addition to another http://www.uniprot.org/align. Colors highlight inherited or sporadic arrhythmia, only the latter is color-coded (see Table S1 for full information). non-synonymous mutations or polymorphisms resulting in single amino acid The predicted transmembrane domain for each subunit is outlined in red. electrocardiogram QT interval (Figure 1B), and can cause lethal function via protein maturation/trafficking defects rather than ventricular fibrillation (Curran et al., 1995; Sanguinetti et al., channel conduction or gating defects (Anderson et al., 2006). 1995). hERG channels exhibit unusual properties that influence Second, upon membrane depolarization, hERG channels open both cardiac electrical function and arrhythmogenesis. First, the and then rapidly inactivate. As the membrane begins to repo- majority of pathologic KCNH2 gene mutations cause loss of larize, hERG recovers rapidly from inactivation but deactivates Frontiers in Genetics | Epigenomics and Epigenetics January 2014 | Volume 5 | Article 3 | 2 Crump and Abbott Arrhythmogenic KCNE gene variants slowly. This creates an atypical mode of inward rectification disease associations are relatively rare and the subunits involved (classic inward rectifier K+ channels being generated instead by exhibit promiscuous partnering, even human genetics does not tetramers of two-transmembrane domain α subunits) (Smith automatically uncover the precise functional role of a regulatory et al., 1996). It ensures that hERG channels pass robust currents subunit. Mouse models have been useful in discovering physio- relatively late in the ventricular action potential, to speed phase logical and arrhythmogenic roles for KCNE subunits and their 3 repolarization, without curtailing the preceding plateau phase. disruption (Temple et al., 2005; Roepke et al., 2008; Hu et al., Third, hERG is highly susceptible to block by drugs from a wide 2013) but come with the caveat, especially for the heart, that there range of chemical structures, making it the bane of pharmaceuti- is a big divide between mouse and human heart in terms of phys- cal companies attempting to bring to market otherwise efficacious iology, the primary repolarizing currents, and their molecular drugs that fail safety standards because they inhibit hERG and underpinnings (Nerbonne et al., 2001). therefore are predicted (or demonstrated) to cause drug-induced LQTS (diLQTS) (Sanguinetti et al., 1995; Chen et al., 2002). KCNE MODULATION OF KCNQ1: THE α SUBUNIT hERG channels are modulated by both KCNE1 and KCNE2
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