Defining a new paradigm for human arrhythmia syndromes: Phenotypic manifestations of gene mutations in ion channel- and transporter-associated proteins

Abstract

Over the past fifteen years, gene mutations in cardiac ion channels have been linked with a host of potentially fatal human arrhythmias including long QT syndrome, short QT syndrome, Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. More recently, a new paradigm for human arrhythmia has emerged based on gene mutations that affect the activity of cardiac ion channel- and transporter- associated proteins. As part of the Circulation Research thematic series on Inherited Arrhythmias, this review will focus on the emerging field of human arrhythmias due to dysfunction in cytosolic gene products (including ankyrins, yotiao, syntrophin, and caveolin-3) that regulate the activities of key membrane ion channels and transporters.

1. Introduction

Ion channels and transporters control the movement of charged ions across cell membranes. In the heart, the coordinate activities of these proteins regulate the transmembrane electrochemical gradient to control depolarization/repolarization, and thus cardiac excitability. Defects in cardiac excitability are the basis for human arrhythmia and sudden cardiac death, a leading cause of mortality in developed countries.^1–3 Normal function of ion channels and transporters requires defined biophysical properties as well as precise expression, organization, and regulation in defined membrane domains. Over the past fifteen years, geneticists, molecular cardiologists, and electrophysiologists have identified a wealth of information regarding the molecular components responsible for regulating the biophysical properties of key cardiac ion channels and transporters (Table 1).^4–6 This information has been critical for defining a number of potentially lethal heritable arrhythmia syndromes or cardiac channelopathies stemming from gain- or loss-of-function mutations in ion channels (or associated subunits) which control ventricular depolarization and repolarization. ^7–9

Table 1.

Human Long QT Syndrome-Associated Genes and Functions

LQTS Subtype	Gene	Gene Location	Protein	Protein Function	Mutant Protein Phenotype
LQT1 (RWS, JLNS)	KCNQ1	11p15.5	Kv7.1	IKs alpha subunit	Loss-of- function
LQT2 (RWS)	KCNH2	7q35-36	Kv11.1	IKr alpha subunit	Loss-of- function
LQT3 (RWS)	SCN5A	3p21	Nav1.5	INa alpha subunit	Gain-of- function
LQT4 (AnkB syndrome)	ANK2	4q25-27	Ankyrin-B	Targeting protein	Loss-of- function
LQT5 (RWS, JLNS)	KCNE1	21q22	minK	IKs beta subunit	Loss-of- function
LQT6 (RWS)	KCNE2	21q22	miRP1	IKr beta subunit	Loss-of- function
LQT7 (Anderson -Tawil syndrome, ATS1)	KCNJ2	17q23.1-24.2	Kir2.1	IK1 alpha subunit	Loss-of- function
LQT8 (Timothy syndrome, TS1)	CACNA1C	12p13.3	Cav1.2	ICa,L Alpha subunit	Gain-of- function
LQT9	CAV3	3p25	Caveolin- 3	Caveolae coat protein	Secondary gain-of- function to Nav1.5
LQT10	SCN4B	11q23	Navbeta4	Channel beta subunit	Secondary gain-of- function to Nav1.5
LQT11	AKAP9	7q21-22	Yotiao	Adaptor molecule	Loss-of- function
LQT12	SNTA1	20q11.2	Alpha1- syntrophin	Membrane scaffold	Secondary gain-of- function to Nav1.5

More recently, a new paradigm for the pathogenesis of cardiac channelopathies has emerged based not on gene defects that affect ion channel/transporter biophysical properties, but instead on mutations in genes that encode proteins affecting the expression, subcellular localization, and/or local regulation of cardiac ion channels and transporters. This review will highlight recent discoveries that underscore the critical role of ion channel accessory proteins in normal human cardiac function, as well as describe both the clinical and molecular phenotypes associated with dysfunction in ion channel interacting proteins (“ChIPs”). Notably, while ion channel β-subunits have been implicated as channelopathic substrates (Table 1) ^10–12, this class of gene products will be reviewed separately (in context with their respective sodium, calcium, and potassium channel alpha-subunits) in this Circulation Research thematic series on Inherited Arrhythmias.

2. Ankyrin-based ion channel targeting pathways and human arrhythmia

2.1 Ankyrin-B (ANK2) and type 4 long QT syndrome (“ankyrin-B syndrome”)

Ankyrins are adapter proteins with key roles in membrane protein targeting in erythrocytes, epithelial cells, neurons, and cardiac myocytes.⁴ Vertebrate heart contains three ankyrin gene products termed ankyrin-R (ANK1, 8p11.1), ankyrin-B (ANK2, 4q25-27), and ankyrin-G (ANK3, 10q21). Canonical cardiac ankyrins harbor a large N-terminal domain comprised of thirty-three amino acid ANK repeats (ANK repeats found in >125 different human polypeptides), a large central domain, a death domain, and a C-terminal domain.⁴ The large N-terminal domain (termed the “membrane-binding domain”) associates with high affinity (K_D ~low nM) with multiple cardiac ion channels and transporters including voltage-gated sodium channels (Na_v1.5)⁵, the Na/Ca exchanger^{6, 7}, the Na/K ATPase ⁸, Kir6.2^{9, 10}, and the inositol 1,4,5 trisphosphate (IP3) receptor (Figure 1).^{11, 12} The ankyrin membrane-binding domain also facilitates interaction of ankyrin with cell adhesion molecules including neurofascin. ⁴ The central domain of ankyrin associates with members of the beta-spectrin family of actin-associated molecules as well as signaling proteins including the regulatory subunit of protein phosphatase 2A (Figure 1). ^{13, 14} Finally, the ankyrin death domain and C-terminal domain display intermolecular interactions with chaperone (Hdj1) and cytoskeletal proteins (obscurin), as well as regulate intramolecular association with the ankyrin N-terminus (Figure 1). ^15–17

Figure 1. Ankyrin-B organizes membrane ion channels and transporters in ventricular cardiomyocytes.

Model depicts multifunctional role for ankyrin-B in targeting and clustering Na/Ca exchanger, Na/K ATPase, and IP3 receptor at transverse-tubule/sarcoplasmic reticulum domains in ventricular cardiomyocytes. Ankyrin-B also organizes structural (obscurin, betaII spectrin) and signaling (protein phosphatase 2A) in myocytes.

The role of ankyrins for normal cardiac function is well illustrated by animal and cell models lacking ankyrin gene products. Ankyrin polypeptides have unique cellular binding partners and non-redundant functions in myocyte biology. In myocardium, ANK2 –encoded ankyrin-B directly associates with three functionally-related ion channels and transporters including the Na/Ca exchanger and Na/K ATPase at the plasma membrane, as well as the IP3 receptor calcium-release channel found at on the sarcoplasmic reticulum (SR).⁸ While mice homozygous for an ankyrin-B null mutation are neonatal lethal¹⁸, myocytes derived from postnatal day 1 mice display striking defects in the cellular expression and membrane localization of Na/Ca exchanger, Na/K ATPase, and IP3 receptor.^{19, 20} Mice heterozygous for an ankyrin-B null mutation (ankyrin-B^+/−) are viable and show significant defects in ion channel and transporter membrane targeting. However, these targeting defects are selective to specific cellular membranes. For example, Na/Ca exchanger and Na/K ATPase isoforms are found at both the peripheral sarcolemma as well as specialized transverse-tubule (T-tubule) membrane invaginations.⁸ Loss of ankyrin-B (localized to T-tubules, but not on peripheral sarcolemma) affects only the membrane localization of T-tubule Na/Ca exchanger and Na/K ATPase. ⁸

Functionally, loss of ankyrin-B-associated Na/Ca exchanger, Na/K ATPase, and IP3 receptor results in defects in intracellular Na⁺ and Ca²⁺ regulation and cellular afterdepolarizations in response to catecholamines.²⁰ In fact, similar to the actions of cardiac glycosides, loss of ankyrin-B affects the membrane activity of the Na/K ATPase, resulting in intracellular Na⁺ accumulation. This increase in cytosolic [Na⁺] directly inhibits forward-mode activity of the Na/Ca exchanger (already affected itself by ankyrin-B loss) reflected by increased cytosolic [Ca²⁺] accumulation. This Ca²⁺ is rapidly sequestered into the internal sarcoplasmic reticulum (SR) resulting in increased SR Ca²⁺ stores and increased cellular contractility at baseline. ²¹ However, in response to catecholaminergic stimulation (i.e. isoproterenol), ankyrin-B^+/− SR exhibits spontaneous calcium release, triggering cellular afterdepolarizations (likely via residual membrane Na/Ca exchanger). Consistent with these findings, ankyrin-B^+/− mice display minor ECG phenotypes at rest (mild QT prolongation and sinus bradycardia), but show sustained polymorphic ventricular arrhythmia following catecholaminergic stimulation.^{8, 20, 22, 23}

In 1995, type 4 long QT syndrome (LQT4, MIM#600919), was linked to human chromosome 4q25-27 by Schott and colleagues using a large four generation French kindred.²⁴ Affected patients displayed prolonged QTc, catecholaminergic polymorphic ventricular arrhythmia, and sudden death. Eight years later, an ANK2 mis-sense mutation was identified as the causal mutation in the original LQT4 kindred. ^{20, 25} This ANK2-E1425G missense mutation localized to the C-terminal region of ankyrin-B, was present in affected individuals, was absent in unaffected family members, or in subsequent studies in the general population and results in loss-of-function when characterized in primary cardiomyocytes.^{29, 31, 32} Further experiments have characterized this mutation (ankyrin-B E1425G) as a loss-of-function mutation in primary cardiomyocytes. ²⁰ Interestingly, affected patients also displayed non-ventricular phenotypes including sinus bradycardia, atrial fibrillation, and conduction defects.^{29, 31, 32, 35}

Following the initial characterization of the original LQT4 proband, additional large kindreds with ANK2 loss-of-function variants have been identified. To date, nine loss-of-function variants have been identified throughout ANK2.²⁵ Phenotypes associated with ANK2 loss of function mutations range from severe (sinus node dysfunction, atrial fibrillation, polymorphic ventricular arrhythmia, sudden death) to mild (sinus bradycardia).²³ A clear unifying set of clinical phenotypes of ANK2 mutation carriers are sinus node dysfunction and atrial fibrillation. In fact, in the large kindreds identified, the penetrance of sinus node disease in individuals harboring an ANK2 loss of function variant is over 75%.^{35, 36} QT prolongation is primarily found only in individuals with the most severe ANK2 mutations (found in initial French kindred). ²⁰ Based on the relative absence of abnormal repolarization/QT prolongation among ANK2 mutation positive subjects, type 4 long QT syndrome is now more appropriately termed “ankyrin-B syndrome”. From the standpoint of a clinician or geneticist, it is noteworthy that the ANK2 gene is large, complex (>750 kb) ²⁶ and riddled with benign polymorphisms, both common and rare. ²⁷ Thus, in the absence of functional data, caution must be used to discriminate simple polymorphisms from bona fide disease-causing mutations. Finally beyond monogenic disease, ankyrin variability has been linked with cardiovascular disease in large animal models of heart disease, as well as with arrhythmia susceptibility in large human populations. Large animal models of coronary artery occlusion display striking loss of ankyrin-B in the infarct border zone.²⁸ Moreover, the ANK2 locus has been identified as a key locus for the regulation of repolarization in large human populations.²⁹ Clearly, future experiments will be critical to define additional roles for ankyrin in the regulation of cardiac membrane biology and in human cardiovascular disease. However, collectively these findings with ankyrin-B define a new paradigm for arrhythmogenesis stemming from defects in non-ion channels and transporters. In fact, these initial findings in 2003, have paved the way for a wealth of new gene defects in atypical arrhythmia genes. Beyond arrhythmias, cardiac ankyrin-B data suggest that ankyrins may play an active role in regulating membrane protein trafficking when compared with classic roles of ankyrin in the erythrocyte. In fact, recent findings link ankyrin-B in heart with a class of endosome-associated proteins, the EH domain proteins, that play key roles in membrane protein trafficking, endocytosis, and recycling in other cell systems. ³⁰

2.2 Ankyrin-G (ANK3) and Brugada syndrome

While not as well characterized as ankyrin-B, a second ankyrin gene product, ankyrin-G encoded by ANK3 (10q21) has critical roles in ion channel trafficking and regulation in heart and human arrhythmia. Specifically, ankyrin-G directly associates with, and targets Na_v1.5 to the intercalated disc of ventricular cardiomyocytes (Figure 2).^{5, 31} Primary cardiomyocytes deficient in ankyrin-G expression display i) decreased Na_v1.5 expression, ii) decreased Na_v1.5 membrane targeting, and iii) reduced cardiomyocyte I_Na.³¹ This pathway is specific to ankyrin-G/Na_v1.5 as myocytes lacking the similar ankyrin-B display normal Na_v1.5 targeting, and ankyrin-G-deficient cardiomyocytes display normal targeting and function of Ca_v1.2.³¹ Relevant to humans with heritable arrhythmia syndromes, ankyrin-G directly associates with Na_v1.5 via a nine residue motif on the Na_v1.5 DII-DIII cytoplasmic loop (Figure 2). ⁵ Notably, individuals with mutations localizing to this nine amino acid motif in the SCN5A-encoded Na_v1.5 display Brugada syndrome (BrS) due to lack of ankyrin-G binding and defective Na_v1.5 membrane trafficking. ⁵ Together, these findings support a central role of ankyrin-based ion channel targeting pathways for normal cardiac physiology, as well as demonstrate the link between ankyrin dysfunction and human arrhythmia. Future central unanswered questions related to ankyrin-G/Na_v1.5 function include identification of the mechanism for ankyrin-G targeting in myocytes, as well as defining the cellular pathways that discriminate ankyrin-G- and ankyrin-B-specific protein trafficking pathways.

Figure 2. Cardiac voltage-gated sodium channel macromolecular complex.

Model illustrates cardiac voltage-gated sodium channel alpha (Nav1.5, black) and beta (white) subunits, as well as associated complex members including ankyrin-G, alpha1 syntrophin, nNOS, and PMCA4b. Nav1.5 is also associated with caveolin-3 although the mechanism for the interaction is still unclear.

3. α1 syntrophin, cardiac voltage-gated Na⁺ channel regulation and LQT12

Voltage-gated Na⁺ channels are critical for the rapid upstroke of the cardiac action potential, and thus cardiomyocyte excitability. ³² The role of the primary cardiac voltage-gated Na_v channel (Na_v1.5) for human cardiovascular function is clearly demonstrated by with the cardiac channelopathies that are secondary to Na_v1.5 gain (LQT3)- or loss-of-function (BrS).^33–37 Since the discovery of Na_v1.5 in the heart, decades of research have focused on solving the mechanisms underlying basic structure/function relationships for cardiac I_Na. However, the mechanisms underlying local regulation of Na_v1.5 have largely remained unanswered, primarily due to the lack of physiologically-relevant model systems to study in vivo I_Na regulation. As noted above, Na_v1.5 is targeted to the intercalated disc by ankyrin-G-based cellular pathways and Na_v1.5 mutations that affect this interaction are associated with BrS. ⁵ However, more recently human mutations have been discovered in additional ChIPs that define novel membrane regulatory pathways for this critical cardiac ion channel.

Syntrophins are a family of cytoplasmic adapter proteins (~500 residues) that link the extracellular matrix to the intracellular actin-based cytoskeleton via the dystrophin-associated protein complex. Over a decade ago, syntrophins (via an internal PDZ domain) were demonstrated to interact with the C-terminus of voltage-gated Na_v channels including Na_v1.4 and Na_v1.5 (Figure 2).^{38, 39} In fact, mdx mice lacking dystrophin display cardiac defects and loss of cardiomyocyte Na_v1.5 expression, most likely due to aberrant regulation of syntrophin.³⁹ Subsequently, syntrophins have been linked with a host of membrane proteins and signaling molecules including the plasma membrane calcium ATPase (PMCA) and neuronal nitric oxide synthase (nNOS) ^40–42 (Figure 2). Functionally, this connection is relevant for cardiac arrhythmias as NO directly affects persistent (or late) I_Na where pathogenic mutations in SCN5A that accentuate late I_Na gives rise to LQT3. Moreover, variants in the nNOS regulatory protein, NOS1AP (CAPON), have been linked with QT interval regulation in large genome wide association studies.^{43, 44} Based on these collective data, Ackerman and Makielski performed direct sequencing on the primary cardiac isoform of syntrophin (α1 syntrophin, SNTA1; 20q11.2) in a large cohort of “genotype negative” long QT patients. This screen resulted in the identification of a novel missense mutation (A390V) in α1 syntrophin in a patient presenting with syncope and QTc prolongation (now termed type 12 long QT syndrome; MIM#601017).⁴⁵ While this mutation in a highly conserved region of α1 syntrophin did not directly affect Na_v1.5 association, it decreased α1 syntrophin binding activity for PMCAb and nNOS. ⁴⁵ Furthermore, SNTA1-A390V increased Na_v1.5 nitrosylation and persistent I_Na in cardiomyocytes⁵⁶, thus serving as a logical trigger for human arrhythmia (akin to LQT3-causative mutations in SCN5A⁴⁶). It is possible that α1 syntrophin dysfunction may regulate additional cellular parameters beyond Na_v1.5, including PMCA function, or even cardiomyocyte structural elements associated with the syntrophin/dystrophin macromolecular complex.

5. Caveolae, Caveolin-3, and LQT9

Caveolae are 50–100 nm flask-shaped invaginations of the plasma membrane implicated in membrane endocytosis, cholesterol homeostasis, tumorigenesis and cellular signaling. ⁴⁷ Three genes (CAV1, CAV2, CAV3) encode three distinct caveolin polypeptides termed caveolins 1–3 that constitute the primary coat proteins of caveolae. Caveolin polypeptides share similar domain organization consisting of an amino terminal domain, a scaffolding domain, a hydrophobic domain, and finally a C-terminal domain. In striated muscle, including heart and skeletal muscle, caveolin 3 (~200 amino acids, 3p25) is the primary gene product, and beyond heart has been linked with human disease including muscular dystrophy, rippling muscle disease, and idiopathic hyperCKemia.^48–51 In biochemical assays, caveolin 3 associates with cardiac membrane proteins including Na_v1.5 (Figure 2), K_v1.5, HCN4, Na/Ca exchanger, IP3 receptor, TRP channels, Ca_v1.2, and PMCA, although many questions remain regarding the mechanism for caveolin/membrane protein association. ^52–60 Additionally, caveolin 3 interacts with signaling molecules including most prominently small G-protein subunits, beta adrenergic receptors, protein phosphatase 2A, protein kinase A, eNOS, protein kinase C, and adenylyl cyclase. ^61–67 Together, caveolae and constituents including ion channels, signaling molecules, and regulatory caveolae-coat proteins are hypothesized to represent local, and highly tunable, and signaling modules. Notably, caveolin 3-deficient mice display abnormal cardiac transverse-tubule organization, mild to moderate cardiomyopathy, and skeletal muscle myopathy. ^68–71 Based upon the potential role for caveolae and caveolin 3 in organizing ion channel organization and regulation in cardiomyocytes⁶⁰, Vatta and colleagues screened “genotype negative” LQT probands for mutations in the two CAV3 exons encoding caveolin 3. Four CAV3 mutations (now termed type 9 long QT syndrome; MIM#611818) were identified in this screen, and located across the polypeptide. ⁷² Based on prior experiments linking caveolin with cardiac voltage-gated Na⁺ channels⁶⁰, the potential link between caveolin 3 gene mutations and Na_v1.5 were further investigated. Relevant for an in vivo functional association, caveolin 3 and Na_v1.5 are co-expressed in heart, co-localize to cardiomyocyte membrane domains, and associate in cardiac lysates. ^{60, 72} Moreover, caveolin 3’s N-terminal domain is required for the interaction (direct or indirect) with Na_v1.5 (Figure 2). ⁷² Functionally, overexpression of Na_v1.5 with human caveolin 3 mutations resulted in a nearly three-fold increase in persistent (late) I_Na, consistent with the phenotype of Na_v1.5-associated LQT3. ⁷² However, as caveolin 3 mutations did not directly block the association of Na_v1.5 with caveolin 3, the precise molecular mechanism(s) responsible for caveolin 3-mediated LQT9 remain unsolved. It is possible that these mutations may interfere with the recruitment of Na_v1.5 channels to the lipid-rich caveolae membrane invaginations. Alternatively, these mutations may interfere with signaling between caveolae-enriched ion channels and a network of integrated signaling molecules. Nonetheless, these findings, supported by more recent data linking caveolin 3 mutations to sudden infant death syndrome⁷³ strongly support the role of local ion channel signaling and targeting for normal cardiac excitability.

4. Yotiao, IKs, and the cardiac channelopathies LQT1, LQT5, and LQT11

Originally identified in brain, yotiao is a member of a large family of protein kinase A (PKA)- anchoring proteins (AKAPs) with critical roles in excitable cell signaling. ⁷⁴ Moreover, the ~1600 residue protein encoded by AKAP9 (7q21-22) also associates with protein phosphatase 1 (PP1), adenylyl cyclase (AC) ⁷⁵, and phosphodiesterase 4D3 (PDE4D3).⁷⁶ In neurons, yotiao-dependent targeting of PP1 and PKA to the post-synaptic density is critical for N-methyl-D-aspartate (NMDA) receptor activity ⁷⁷. In heart, AKAP-associated PKA activity has been linked with the activities of at least three different ion channel complexes including I_KS ⁷⁸, the cardiac ryanodine receptor (RyR2) ⁷⁹, and the L-type calcium channel.⁸⁰ Cardiac I_KS is encoded by KCNQ1 (I_KS Kv7.1 alpha subunit; 11p15.5) and KCNE1 (I_KS beta subunit also called mink, 21q22.12) and human mutations in either subunit may cause LQT1 (MIM#192500) or LQT5 (Figure 3; MIM#176261). Notably, I_KS activity is tightly regulated by PKA-dependent phosphorylation, and this phosphorylation is tuned by both local PKA and PP1 signaling cascades.⁷⁸ In heart, yotiao directly associates with Kv7.1 to recruit both PKA and PP1 to regulate I_KS phosphorylation and gating (Figure 3). ⁷⁸ Moreover, the LQT1-causative mutation, G589D abolishes Kv7.1’s association with yotiao thereby disrupting the channel’s sensitivity to beta-adrenergic regulation.⁸¹

Figure 3. Cardiac IKs and IKr complexes in heart.

In addition to interactions of channel alpha and beta subunits for both complexes, the Kv7.1 alpha subunit encoded by KCNQ1 also associates with yotiao (AKAP9) in heart which recruits signaling molecules protein phosphatase 1 (PP1), protein kinase A (PKA), and phosphodiesterase 4D3 (PDE4D3) to the IKs complex. In brain, yotiao also associates with specific isoforms of adenylyl cyclase (AC).

Additional proof for the critical role of yotiao in normal cardiac excitability comes from recent studies that identify mutations in AKAP9-encoded yotiao in patients with previously genetically elusive LQTS. Specifically, recent findings from Kass and Ackerman have identified AKAP9-S1570L which markedly diminishes the interaction of yotiao with KCNQ1, resulting in reduction in PKA-dependent phosphorylation of Kv7.1, and thus striking inhibition of I_KS regulation by cAMP.⁸² Similar to models of LQT1 -associated mutations in KCNQ1 that block yotiao binding and I_KS phospho-regulation, yotiao mutations are also predicted to prolong the action potential and increase arrhythmia susceptibility (now termed type 11 long QT syndrome). ⁸² In support of this notion, the S1570L proband displayed a clinical LQTS phenotypes (now referred to as type 11 long QT syndrome; MIM#611820) similar to LQT1 mutations that block KCNQ1 association with yotiao (female, QT_c 485 ms presenting with syncope and family history of LQTS).⁸² Together, these two related studies have revealed the critical importance of local regulation of ion channels by accessory ChIPs, as well as demonstrate the power of combining clinical genetics and molecular/cellular cardiology to drive disease discovery.

Summary and future directions

As illustrated above, our insights into the molecular mechanisms underlying human arrhythmia have grown exponentially over the past decade. In addition to classical ion channels and transporters, mutations in associated proteins such as ankyrins, syntrophin, and yotiao (and channel beta subunits) are now fully implicated in the spectrum of the “cardiac channelopathies”. Furthermore, these findings have set the stage for the identification of additional “atypical” or “non-conventional” arrhythmia and cardiovascular disease gene products including cardiac enzymes (GPD1L for example)⁸³, and proteins of the nuclear lamina^{84, 85}, dyadic cleft⁸⁶, and caveolae⁷². While it is true that these gene mutations represent insight into rare human arrhythmias arrhythmia syndromes, affecting only a small proportion of the population, these new findings have provided incredible insight into the key molecules underlying normal myocyte cell biology and physiology, as well as defined exciting new therapeutic targets and pathways to target in more general cardiovascular disease. We predict that the next decade will witness the emergence of novel pathogenic substrates emanating from all components of the cardiac channelsome including not only additional ChIPs but other signaling molecules, transcription factors, and even microRNAs.

Non-standard abbreviations

AKAP

A Kinase Anchoring Protein

LQTS

Long QT Syndrome

CPVT

Catecholaminergic Polymorphic Ventricular Tachycardia

CaMKII

Calcium-calmodulin dependent protein kinase II

PKA

Protein Kinase A

CHiPs

channel interacting proteins

EH

epsin homology

SR

sarcoplasmic reticulum

PMCA

plasma membrane calcium ATPase