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PERSPECTIVES IN PHARMACOLOGY

Pharmacogenetic Considerations in Diseases of Cardiac Ion Channels

Division of Cardiology, Department of Medicine (A.A., S.M.M., G.W.A.); Graduate Program in Neuroscience (A.A.); Department of Pharmacology (G.W.A.); Weill Medical College of Cornell University, New York, New York

Received August 5, 2003; accepted September 2, 2003.

	Abstract

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
Phenotypic variation within a species arises from differences in genetic makeup between individuals. This inherent diversity empowers the species as a whole to explore and expand into new environmental niches and also to survive new stressors within an ever-changing environment. Paradoxically, one class of stressors currently challenging the human population is therapeutic drugs: medications designed to combat disease are often associated with a host of nonspecific side effects. Following earlier studies of the involvement of some cardiac ion currents in unwanted drug interactions, recent reports have identified not only the ion channel subunits involved but also a range of mutations and single nucleotide polymorphisms in ion channel genes that predispose to both drug-induced and familial cardiac arrhythmia. The tendency for individuals harboring specific, often common, gene variants to succumb to life-threatening cardiac arrhythmia, and the contribution of other factors such as drug interaction to disease etiology in these cases, are discussed here together with potential pharmacogenetic strategies for arrhythmia circumvention and therapy.

	Cardiac Ion Currents and Arrhythmia

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
The electrical activity required for cardiac contraction requires the precise, concerted action of cardiac ion channels. The depolarizing upstroke of the action potential is carried by the movement of positively charged sodium ions through voltage-gated sodium-selective channels into the cardiac myocyte cytoplasm. This depolarization initiates calcium influx and calcium release from intracellular stores, facilitating muscular contraction. Subsequently, delayed rectifier outward potassium currents govern the timing, rate, and completion of cellular repolarization (Fig. 1A). On surface electrocardiograms (ECGs) the length of the QT interval (Fig. 1B), which is measured from the onset of the QRS complex to the end of the T wave, is indicative of the time between ventricular depolarization and repolarization. The length of the interval can be influenced by several factors, including gender, age and the presence of structural heart disease. An abnormally long "corrected" QT interval (QTc, the QT interval corrected for heart rate) is indicative of a delay in ventricular repolarization. Delayed or disordered myocardial repolarization can lead to rhythm disturbances from increased automaticity or reentry circus rhythm. This electrical instability may cause ventricular tachyarrhythmias that deplete the contractile capacity, reducing cardiac output to degrees that cause syncope and sudden death (Fig. 1C). Unfortunately, there is no clear and universally accepted definition of long QT syndrome. A diagnostic schema has been proposed for clinical long QT syndrome, based on major and minor features, including QTc intervals 450 ms in men and 460 ms in women, abnormal T-wave morphology, and symptoms or family history suggesting malignant ventricular arrhythmias (Schwartz et al., 1993). This definition, however, predated genotyping for long QT syndrome and is limited by the low penetrance of QT prolongation, which has been reported in some populations of abnormal gene carriers, as well as presence of mild QT prolongation in some "normal" subjects (Priori et al., 1999).

View larger version (60K): [in this window] [in a new window]   Fig. 1. The cardiac action potential and ventricular repolarization. A, the cardiac action potential. The ion channel currents that predominate during the depolarizing and repolarizing phases of the action potential are indicated below. The rising phase (0) is mediated by influx of sodium through voltage-gated sodium channels. Outward current carried by potassium through 4-AP sensitive and Ca2+-activated potassium channels contributes to a transient repolarization of the action potential in phase 1. During the plateau (2) and repolarizing (3) phases of the action potential, calcium enters through L- and T-type calcium channels, and potassium leaves the cell through IKr and IKs potassium channels. Potassium efflux through inward rectifier potassium channels such as Kir2.1 also contributes to membrane repolarization, and the membrane potential returns to baseline in phase 4. B, a recording from lead III of a 12-lead surface ECG from a patient with normal cardiac rhythm shows a normal QT interval (solid line labeled "QT interval") of 400 ms. Labels indicate the following: P, atrial depolarization; the QRS complex, ventricular depolarization; T, ventricular repolarization. Timescale: 1 small square = 40 ms. C, a similar recording as shown in panel B, from a patient with a markedly prolonged QT interval of 700 ms; scale and labeling as shown in panel B. D, a later, continuous recording from the patient in panel C showing two premature beats (1), a pause (2), a normal beat (3), onset of torsades de pointes (4), and ventricular fibrillation (5).

Genetic analyses have shown that long QT syndrome most often arises from malfunction of ion channel subunits. The majority of known long QT syndrome cases are precipitated by either an insufficient outflow of potassium ions through potassium channels or an excess inflow of sodium ions through sodium channels (Splawski et al., 2000). In either case, repolarization of the myocardium is delayed, which may give rise to early after depolarizations. Early after depolarizations, should they reach sufficient amplitude, are capable of triggering an action potential. If there are regional differences in refractoriness, reentry may occur producing a distinct form of polymorphic ventricular tachycardia called torsades de pointes— observed as a twisting of the QRS axis around the isoelectric line on an ECG. Occurrence of torsades de pointes predisposes to life-threatening ventricular fibrillation (Fig. 1D), syncope, and sudden cardiac arrest.

Long QT syndrome can be divided into two broad categories: inherited and acquired. Inherited long QT syndrome, classified LQT1 to 7 according to the underlying gene defect (Table 1), is most often associated with mutations in genes encoding voltage-gated ion channel subunits. Acquired long QT syndrome occurs when one or more stimuli, such as drugs that block certain cardiac ion channels, precipitate a prolonged QT interval. Recent studies show that even in so-called "acquired" long QT syndrome cases there may be an underlying genetic predisposition, discussed later in detail. Other forms of inherited arrhythmia without a prolonged QT interval can also arise from ion channel gene mutations, such as catecholaminergic ventricular tachycardia, associated with mutation of the RyR2 calcium channel gene (Priori et al., 2000). By the same token, genes other than those encoding ion channels can cause long QT syndrome— exemplified by the linkage of LQT4 to mutations in ankyrin B (Mohler et al., 2003) (Table 1).

Six ion channel genes have been associated with human long QT syndrome: the voltage-gated sodium (Na_v) channel subunit SCN5A (Wang et al., 1995), voltage-gated potassium (K_v) channel subunits KvLQT1 (KCNQ1) (Wang et al., 1996) and HERG (KCNH2) (Curran et al., 1995), and K_v channel subunits MinK (KCNE1) (Splawski et al., 1997) and MiRP1 (KCNE2) (Abbott et al., 1999). Mutations in KCNJ2, which encodes the Kir2.1 inward rectifier potassium channel, cause Andersen's Syndrome—associated with long QT syndrome in combination with periodic paralysis and dysmorphic features (Plaster et al., 2001). In familial long QT syndrome patient cohorts, mutations in K_v subunits KCNQ1 and HERG are the most common and the most often associated with symptoms such as syncope, cardiac arrest, and sudden death. In a recent compilation encompassing both previously reported and newly genotyped inherited long QT syndrome patients, 177 of 262 patients were found to harbor at least one ion channel mutation. Of the 177, 42% had KCNQ1 mutations, 45% had HERG mutations. Mutations in SCN5A (8%) were the next most prevalent, and mutations in K_v channel subunits MinK (3%) and MiRP1 (2%) were relatively uncommon (Splawski et al., 2000).

	KCNQ1 and MinK Mutations Cause Inherited Long QT Syndrome and Deafness

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
K_v channels are formed by noncovalent association of four K_v subunits, each containing six transmembrane or membrane-associated domains (S1–S6) and a membrane-embedded pore region (Fig. 2). K_v subunits also often associate with subunits that alter channel function, such as the KCNE family of single transmembrane domain subunits (Abbott et al., 1999): founder member MinK encoded by KCNE1, and the MiRPs (MinK-related peptides) encoded by KCNE2–5 (Fig. 2).

View larger version (45K): [in this window] [in a new window]   Fig. 2. Cardiac voltage-gated ion channel subunits. A, cartoon of a voltage-gated Kv channel, showing a tetramer of four subunits (blue) complexed with two MiRP subunits (red) in the plasma membrane (green). MiRP stoichiometry is based upon evidence using subunit-mixing experiments with wild-type and mutant MinK and wild-type KCNQ1 (Sesti and Goldstein, 1998); MiRP positioning in the complex is speculative. B, cross-section through the channel in panel A highlighting the ion conduction pathway and other anatomical features of the channel. C, predicted topologies of Kv and Nav channel subunits, plasma membrane in green. P, pore region. Putative transmembrane helices are numbered 1 through 6 in each Kv subunit or each of the Nav repeating units I through IV.

K_vLQT1, now more commonly referred to as KCNQ1, is a K_v channel subunit expressed in human heart and other tissues (Wang et al., 1996). Homomeric KCNQ1 channels generate a potassium-selective current with no clear native current correlates in the heart. However, currents formed by heteromeric complexes of KCNQ1 with the MinK subunit pass larger, slower activating currents that strongly resemble the cardiac I_Ks repolarization current. Thus it is thought that I_Ks is generated in vivo by MinK/KCNQ1 channel complexes (Sanguinetti et al., 1996).

Familial long QT syndrome is most often observed as one of two forms: Romano-Ward syndrome, the most common, is an autosomal dominant trait with no obvious noncardiac abnormalities that can be caused by mutations in a variety of genes (Table 1). The other, Jervell and Lange-Nielsen syndrome (JLNS), caused only by mutations in KCNQ1 or MinK, is rarer and genetically more complex. JLNS sufferers exhibit cardiac arrhythmia and profound sensorineural deafness. This is because MinK/KCNQ1 channels are also expressed in the inner ear, where they contribute to production of the potassium-rich endolymph fluid essential for maintenance of the organ of Corti in the cochlea. Although deafness in JLNS is inherited as an autosomal recessive trait (two mutant alleles are necessary for auditory dysfunction), inheritance of arrhythmia in JLNS is semidominant (one mutant allele produces a cardiac phenotype, two mutant alleles a more severe cardiac phenotype). Thus, within the same family, the same mutation in MinK or KCNQ1 can cause Romano-Ward syndrome or JLNS in different individuals depending upon how many mutant alleles are inherited (Wang et al., 1996; Neyroud et al., 1997).

Long QT syndrome-associated mutations in KCNQ1 are dispersed throughout the coding region, with particular hot spots in key functional domains such as the S4–5 linker (channel gating and interaction with MinK), the pore (the active site of ion conduction and selectivity), and the S6 helix (part of the ion conduction pathway). Mutations in subunits such as MinK can also greatly impair channel function. The most studied of these is the D76N-MinK mutation linked to both JLNS and Romano-Ward syndrome, which greatly reduces I_Ks current density in a dominant-negative fashion by a combination of reduced unitary conductance, impaired activation, and accelerated deactivation (Splawski et al., 1997; Sesti and Goldstein, 1998). Of note, the D76N variant of MinK also greatly impairs MinK/HERG currents (McDonald et al., 1997), highlighting an emerging principle that KCNE mutations may cause arrhythmia by promiscuous disruption of multiple cardiac currents (Abbott and Goldstein, 2002). Recent studies also demonstrated linkage of a rare gain-of-function KCNQ1 mutation, S140G, with familial atrial fibrillation (AF) (Chen et al., 2003) and showed that the more common SNP of MinK, 38G (as opposed to 38S), is enriched in AF patients versus controls, especially in the homozygous form (AF, 59.3%; control, 42.6%) (Lai et al., 2002).

	HERG and MiRP1 Mutations Associate with Inherited Long QT Syndrome

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
HERG subunits underlie the cardiac I_Kr current, crucial for ventricular repolarization in many species including man (Sanguinetti et al., 1995). Biophysical studies on HERG have revealed a peculiar attribute for a protein with classical K_v channel topology, that of inward rectification.¹ Although HERG channels open rapidly in response to membrane depolarization, they very rapidly inactivate at more positive potentials. Upon membrane repolarization, however, HERG channels rapidly recover from inactivation and pass large currents before they close (deactivate)—thus displaying an inwardly rectifying current/voltage relationship. Despite this, at physiological membrane potentials and potassium concentrations, HERG channels pass only outward current, and their peculiar properties ensure that they are poised to supply a significant repolarizing force near the end of the cardiac action potential.

A range of biophysical, pharmacological and genetic data suggests that HERG may employ KCNE subunits to generate I_Kr in some cardiac cells. HERG currents are up-regulated by coassembly with MinK in heterologous experiments, and MinK/HERG complexes are detectable in equine heart (McDonald et al., 1997; Finley et al., 2002). Furthermore, MiRP1 (KCNE2) coassembles with HERG, forming mixed complexes with reduced unitary conductance, faster deactivation and altered sensitivity to some pharmacological agents compared with homomeric HERG channels (Abbott et al., 1999). This together with increasing genetic evidence (see below) suggests that MiRP1 may form I_Kr complexes with HERG in some regions of human heart. It will be important to establish the true molecular identity of I_Kr, given that association with KCNE subunits can alter HERG pharmacology and that drug blockade of I_Kr is the primary cause of acquired arrhythmia in humans.

Mutations in both HERG and MiRP1 are associated with inherited arrhythmia in humans (Curran et al., 1995; Abbott et al., 1999; Isbrandt et al., 2002). HERG mutations are most prevalent within the intracellular domains, particularly—in contrast to KCNQ1—in the N-terminal region, although hot spots also exist in and around the pore region and in the putative cyclic nucleotide-binding domain (Splawski et al., 2000). The three reported MiRP1 mutations associated with inherited long QT syndrome—M54T, I57T, and V65M—are all within the putative transmembrane domain and are associated with various loss-of-function effects on MiRP1/HERG channel gating and conductance (Abbott et al., 1999; Isbrandt et al., 2002).

	HERG and MiRP1 Are Key Molecular Determinants in Acquired Long QT Syndrome

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
It is far more common for long QT syndrome to be acquired rather than inherited (Roden, 2001). In these instances, cardiac abnormalities are not manifested until some extrinsic factor, such as medication or metabolic abnormality, precipitates an aberrant cardiac rhythm. Acquired long QT syndrome has been an intractable clinical problem because it occurs so unpredictably; however, two lines of research are now providing hope that it does not need to remain so: identification of the I_Kr current as a key mediator of arrhythmogenic drug activity, and the growing consensus that cardiac ion channel gene variants can predispose to acquired long QT syndrome without prior clinical manifestations. Acquired long QT syndrome is thought to occur when one or more stressors reduce the "repolarization reserve", i.e., the capacity of available ion currents to repolarize the myocardium, to such an extent that timely repolarization cannot occur beat-to-beat (Roden, 2001). Probably the most common clinical presentation of this is drug-induced torsades de pointes, in which a patient who may or may not have additional risk factors develops dysrhythmia after taking a medication. Among cardiac ion currents, inhibition of I_Kr in particular is implicated in cases of acquired long QT syndrome, and there is a clear mechanistic basis for this. If one examines the list of factors that can contribute to acquired long QT syndrome, including medication, hypokalemia, female gender, slow heart rate and inherited prolongation of the QT interval, some clear links to I_Kr are evident, which are discussed below.

Why Is HERG So Often Implicated in Drug-Induced Torsades de Pointes? HERG and I_Kr are highly sensitive to block by a broad spectrum of drugs: antidepressants (amitriptyline, imipramine), antipsychotics (chlorpromazine, haloperidol), antihistamines (terenadine, astemizole), anti-anginal agents (bepridil), and antibiotics (sulfamethoxazole, clarithromycin, erythromycin) (Abbott et al., 1999; Kass and Cabo, 2000; Mitcheson et al., 2000; Sesti et al., 2000; Roden, 2001). Class Ia and class III antiarrhythmic drugs may themselves cause long QT syndrome; a drug concentration that slightly prolongs the action potential plateau and is antiarrhythmic for some patients may be sufficient to induce LQT and act as a proarrhythmic in others (Kass and Cabo, 2000). A mechanism has been put forward for the molecular basis of the unusual avidity of HERG for a wide range of pharmacological agents (Mitcheson et al., 2000). First, unlike most other K_v channels, HERG lacks two specific proline residues that normally produce a kink that limits the volume of the inner cavity of the channel. Therefore, the larger HERG cavity volume may allow relatively large drugs to enter and prevent the channel from conducting potassium. Second, HERG has two aromatic residues that face the inner cavity of the channel; results of alanine-scanning mutagenesis have led to the conclusion that these residues facilitate the interaction of HERG with aromatic groups on drugs in the inner cavity by a -stacking interaction, again favoring drug binding and channel blockade. Third, the unusual gating of HERG channels may also increase the chances of drugs being trapped within the inner cavity. Thus, unwanted clinical side effects from a wide spectrum of medications are mediated through blockade of I_Kr largely because of the unusual properties of the HERG pore-forming subunit.

Ion Channel Gene Variants That Predispose to I_Kr-Associated Drug-Induced Torsades de Pointes. The peculiar structural and functional attributes of HERG most likely underlie its drug susceptibility, but this does not explain why some individuals are affected and not others. Recent studies have identified a genetic basis for variable tolerance to channel-blocking drugs: sequence variants in ion channel genes. Genetic analyses of cohorts of patients exhibiting drug-induced arrhythmia show that 10 to 15% of them harbor ion channel gene mutations or SNPs, a significant enrichment compared with the control population (Yang et al., 2002). In contrast to inherited long QT syndrome, the most frequent mutations found so far in acquired torsades de pointes populations are within KCNE genes and mostly in MiRP1 (KCNE2) (Abbott et al., 1999; Sesti et al., 2000; Yang et al., 2002). Gene variants associated with acquired arrhythmia can be considered as one of three categories, which we describe here as indirect, direct, and compound. Indirect mutations are those that impair I_Kr at baseline, do not affect drug sensitivity, but are associated with arrhythmia after drug administration because of superimposition of inherited impairment and coincidental drug blockade of I_Kr, e.g., A116V-MiRP1 with quinidine (Sesti et al., 2000). Direct mutations are perhaps the most insidious variants—those that do not affect I_Kr predrug but increase sensitivity to drug blockade (Priori et al., 1999). These cause no detectable phenotype before drug administration, either as QT prolongation on the patient's ECG or by in vitro analysis in absence of drug. An example is the T8A-MiRP1 SNP, present in 1.6% of the U.S. population, which was identified in a patient who developed prolonged QTc after taking the commonly prescribed antibiotic combination of sulfamethoxazole and trimethoprim. T8A-MiRP1 increases 4-fold the sensitivity of MiRP1/HERG channels to blockade by sulfamethoxazole, without altering affinity for trimethoprim (Sesti et al., 2000). The existence of "silent" drug-susceptible SNPs such as this argues for preprescription genotyping when feasible, although other factors such as differences in drug metabolism and adrenergic stimulation, or SNPs in other subunits, probably play a role in the disease etiology of T8A-associated long QT syndrome. Otherwise, many more individuals would have succumbed to T8A-related arrhythmias, given the common occurrence of this variant and the widespread use of sulfamethoxazole. Alternatively, it may be necessary for an individual to be T8A/T8A homozygous before significant drug sensitivity is observed, a genotype predicted to occur in only 0.026% of the U.S. population. Compound mutations are those that both impair channel function at baseline, and also increase sensitivity to I_Kr blockade—an example is Q9E-MiRP1, which was discovered in a female after clarithromycin treatment precipitated torsades de pointes. Q9E impairs channel gating and also increases sensitivity to clarithromycin blockade (Abbott et al., 1999). In a recent study, two subunit mutations, one in HERG and one in KCNQ1, were found in acquired arrhythmia patients but not in controls; both mutations reduced current density at baseline but differences in drug sensitivity were not discussed. Furthermore in a preliminary study, a MinK SNP (D85N) that alters I_Ks kinetics was found to be enriched in acquired arrhythmia patients (7%) versus controls (2–4%) (Yang et al., 2002).

Other Factors That Contribute to I_Kr-Associated Acquired Arrhythmia. There are at least two other risk factors for acquired arrhythmia that can be mechanistically linked to I_Kr: gender and hypokalemia (Roden, 2001). Women are more vulnerable to torsades de pointes caused by I_Kr-blocking drugs than men (Makkar et al., 1993), although two recent, apparently contradictory, studies are illustrative of the further complexity of this gender-based susceptibility. A HERG SNP, K897T, is relatively common (16–25%, depending on ethnicity and/or study group size). In a study of middle-aged Finns, K897T was associated with significantly increased QTc in women but not men (Pietila et al., 2002). In contrast, in a study of German Caucasians, K897T was associated with shortened QTc, and more significantly in women than men (Bezzina et al., 2003). The reports suggest that other genetic and/or environmental factors contribute to I_Kr phenotype, highlighting the potential complexity of pharmacogenetic prediction of factors predisposing to cardiac arrhythmia.

Another predisposing factor for acquired arrhythmia is hypokalemia. In several studies of patients with torsades de pointes, hypokalemia was considered a contributory factor in the context of missense mutations in either HERG (S818L and V822M) (Berthet et al., 1999) or MiRP1 (Q9E) (Abbott et al., 1999). This has been attributed to several properties of HERG (Roden, 2001); the most well characterized of these being that contrary to what one would expect, outward HERG current is decreased by reduction of extracellular potassium near physiological levels despite an increase in electrochemical gradient (Sanguinetti et al., 1995). Therefore hypokalemic patients receiving I_Kr blockers such as quinidine and dofetilide are at increased risk for acquired long QT syndrome (Berthet et al., 1999) because of an already reduced repolarization reserve (Roden, 2001). Increasing serum potassium in patients suffering from hypokalemia-induced torsades de pointes shortens the QT interval and attenuates abnormalities in myocardial repolarization.

	Lethal Mutations and Subtle SNPs in the SCN5A Gene

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
Voltage-gated sodium channel genes encode large plasma membrane proteins organized into four covalently linked, similar repeating units each resembling superficially K_v subunits (Fig. 2C). The SCN5A cardiac Na⁺ channel is primarily responsible for the depolarizing Phase 0 of the cardiac action potential (Fig. 1A). Arrhythmia-associated SCN5A mutations are most often gain-of-function mutations, causing increased net inward current flux of Na⁺ ions through SCN5A channels and preventing timely repolarization. The most common mechanism for sodium channel gain-of-function is a destabilization of the inactivated state, causing either delayed inactivation or an increase in plateau current. Aside from long QT syndrome, other familial cardiac arrhythmias are known although the underlying gene defect is not always understood. One well understood example is the Brugada syndrome, a form of idiopathic ventricular fibrillation again caused by SCN5A mutations (Chen et al., 1998). This disorder manifests as J-point and ST segment elevation in the right precordial leads of the ECG, without significant prolongation of the QT interval. Patients with the Brugada syndrome may be susceptible to ventricular arrhythmias after treatment with drugs that block I_Na, specifically class I antiarrhythmic agents such as flecainide, which is used to accentuate the ECG abnormalities of suspected Brugada patients to aid diagnosis (Gasparini et al., 2003). SCN5A mutations are capable of producing a wide range of clinical symptoms; whereas LQT and Brugada syndrome sufferers experience life-threatening arrhythmias, other SCN5A mutations can cause cardiac-conduction disease, light-headedness, or syncope.

In 2002, Splawski and colleagues reported a S1102Y SNP in the SCN5A gene that is associated with increased risk of cardiac arrhythmia. At the cellular level, the S1102Y SNP increases the rate of activation of the sodium channel, as well as allowing greater peak amplitude and a larger sustained current than the wild-type protein (Splawski et al., 2002). The proband was a 36-year-old African-American woman with idiopathic dilated cardiomyopathy and hypokalemia who developed long QT syndrome while on the antiarrhythmic drug amiodarone. When the frequency of Y1102 was analyzed in the general U.S. population using control samples, it was found to be substantially more prevalent in West African, Caribbean, and African-American populations. Though most carriers will not develop arrhythmia, the Y1102 SNP appears to increase the likelihood of arrhythmia when superimposed upon other risk factors. Thus, as with MiRP1, SCN5A sequence variability represents a tangible molecular basis for predisposition to acquired arrhythmia (see Table 1).

	Gene-Guided Therapies

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 
Several proposed experimental strategies for therapy are emerging based on current knowledge of the molecular basis of cardiac ion channel disorders. A subset of long QT syndrome-associated mutations, most commonly in HERG, reduces I_Kr by defective HERG trafficking. HERG blockers such as E-4031 can actually increase I_Kr current density in these cases by acting as "pharmacological chaperones" that stabilize HERG, reducing misfolding and retention, and more recent studies have identified agents such as fexofenadine that rescue HERG mutants without channel block (Rajamani et al., 2002). Fexofenadine (terfenadine carboxylate), a metabolite of terfenadine, is a H1 receptor antagonist currently used to treat allergies, but it also binds to HERG channels with only weak channel block, unlike terfenadine, which was withdrawn from sales because of high-affinity HERG block. A pharmacological approach has also been used to rescue long QT syndrome-associated mutant I_Ks channels, using stilbenes and fenamates that bind to I_Ks channels and restore the allosteric interplay between MinK and KCNQ1 necessary for correct channel function (Abitbol et al., 1999). Application of these therapies to patients will require genotyping to identify mutants that would benefit from specific rescue strategies.

Another gene-guided approach is gene therapy; although this has been hampered by practical difficulties in safe and specific delivery, experimental reports describe the possible future applicability of this to human ion channel disorders. Ectopic expression of MiRP2, which forms a constitutively open potassium channel with KCNQ1, enhances repolarization in guinea pigs and thereby reduces the QT interval (Mazhari et al., 2002). The use of Q9E-MiRP1 has also been explored as a potential therapy for reentrant atrial cardiac arrhythmias (Burton et al., 2003). Blockade of Q9E-MiRP1/HERG channels with clarithromycin gives repolarization-delaying effects similar to those of class III antiarrhythmics (Abbott et al., 1999). Thus it is proposed that ectopic expression of Q9E-MiRP1 in affected atrial tissue combined with application of low doses of clarithromycin could delay atrial repolarization enough to block reentrant atrial arrhythmias without affecting less-sensitive wild-type I_Kr channels in the ventricle, therefore avoiding the unwanted side effect of ventricular long QT syndrome.

A third strategy, and one that is already being tested in cases of inherited arrhythmia, is the use of the genetic lesion to guide the choice of antiarrhythmic drug. An example is the indication of flecainide treatment for patients carrying the SCN5A-KPQ mutation that causes abnormal, repetitive reopenings of the cardiac sodium channel and thus a delay in repolarization. Flecainide, a potent open sodium channel blocker, reduced the average QTc interval in five male KPQ patients by almost 100 ms, consistent with blockade of the abnormal reopenings that occur during the plateau phase in SCN5A-KPQ sodium channels (Windle et al., 2001).

A further potential strategy that exploits knowledge of the molecular etiology of drug-channel interactions in arrhythmia, and one that may prove the most beneficial to public health overall, is that of avoidance of specific drug-gene variant interactions by tailoring drugs and drug prescriptions based on gene mutations or common SNPs harbored by individuals requiring arrhythmia therapy. This approach relies upon patient genotyping and the existence of a database of known adverse drug-gene interactions. At present, widespread genotyping is hampered by practical considerations; a nonbiased approach to genetic profiling requires the sequencing of tens of thousands of allelic variants per individual, too costly and time-consuming with present technologies. However, a candidate-based approach to genotyping may in some cases be both indicated and not so impracticable. In previous studies of acquired long QT syndrome, 10% of cases were associated with variants in either MinK or MiRP1 (Yang et al., 2002), which can be rapidly sequenced because of their small size (coding region of 400 bases), and thus, certain therapies avoided relatively easily in individuals harboring potentially malevolent SNPs in these genes.

Since the discovery in the 1990s of relatively rare ion channel gene mutations that cause inherited arrhythmia, more recent studies have described a genetic disease etiology that represents a far wider public health concern: common SNPs that increase the risk of cardiac arrhythmia. The ability to detect and act upon SNPs, however, represents an opportunity to respond to a bona fide risk indicator before disease onset. Recently, an SNP Consortium was launched to produce a public resource of SNPs in the human genome (http://snp.cshl.org). Originally, the goal was to discover 300,000 SNPs in 2 years; by late 2001, 1.4 million SNPs were released into the public domain, and this number continues to grow. Ready availability of genetic information is already beginning to aid diagnoses and guide treatment options for patients with cardiac arrhythmia and may lead to bespoke drugs designed by pharmaceutical companies to best suit patients with more common SNPs. Thus, whereas therapeutic drugs expose the inherent vulnerability of many individuals to emergent stressors, research into the mechanisms behind adverse drug-polymorphism interactions will ultimately lead to a gene-guided design of compounds that are not only safer but also more effective because of our increased understanding of drug-protein interactions.

	Footnotes

 
G.W.A. is supported financially by an American Heart Association National Scientist Development Grant and a Greenberg Atrial Fibrillation Grant.

DOI: 10.1124/jpet.103.054569.

ABBREVIATIONS: ECG, electrocardiogram; MiRP, MinK-related peptide; JLNS, Jervell and Lange-Nielsen syndrome; AF, atrial fibrillation; K_v channel, voltage-gated potassium channel; Na_v channel, voltage-gated sodium channel; SNP, single nucleotide polymorphism.

¹ Most inwardly rectifying K⁺ channels are formed by coassembly of four principal subunits each with only two transmembrane domains and a membrane-embedded pore region; they possess no intrinsic voltage sensor and are inwardly rectifying because of blockade at depolarized voltages by intracellular moieties such as Mg²⁺ ions or polyamines.

Address correspondence to: Dr. Geoffrey W. Abbott, Division of Cardiology, Dept. of Medicine, Weill Medical College of Cornell University, Starr 463, 520 East 70th Street, New York, NY 10021. E-mail: gwa2001{at}med.cornell.edu

	References

Top Abstract Cardiac Ion Currents and... KCNQ1 and MinK Mutations... HERG and MiRP1 Mutations... HERG and MiRP1 Are... Lethal Mutations and Subtle... Gene-Guided Therapies References

 

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