- Email: [email protected]
The long QT syndromes
Progrev
Y
,,t
Pediatric Cardiology ELSEVIER
Progress
in Pediatric
Cardiology
6 (1996) 43-51
The long QT syndromes Mark W. Russell* ilepartment
of Pediatrics, Division of Pediattic Cardiology, C.S. Mott Children S Hospital, Uniuersity of Michigan Medical Center, Ann Arbor, MI, USA
Abstract The congenital long QT syndromes are a heterogeneous group of disorders that are characterized by prolongation of the QT interval on the 12-lead electrocardiogram, episodes of syncope, ventricular arrhythmias such as torsades de pointes and a high incidence of sudden death. The autosomal dominant form of congenital QT prolongation, the Romano-Ward long QT syndrome, is the most common of these disorders and has been demonstrated by linkage analysis studies to exhibit extensive genetic heterogeneity. These studies have determined that at least five different genes can cause the long QT syndrome. Positional cloning studies have led to the discovery of three genes which together appear to be responsible for nearly 90% of the autosomal dominant cases of congenital long QT syndrome. Further functional characterization of these three genes, the KVLQTl potassium channel, the HERG potassium channel and the SCNSA voltage-gated cardiac sodium channel, as well as the identification and characterization of the other long QT syndrome genes, may allow improved diagnosis and therapy for these disorders. Furthermore, the increased understanding of myocardial repolarization that is gained from characterization of these genes may lead to improved treatment for other ventricular arrhythmias, including those related to drug-induced potassium channel blockade, central nervous system insult, and, possibly, myocardial infarction.
Keywords: Long QT syndrome; death; Genetic testing
Ventricular
arrhythmias;
torsades
1. Introduction Prolongation of the QT interval, either congenital or acquired CsecondaIy to electrolyte abnormality, medication or myocardial infarction), has been associated with ventricular arrhythmias, syncope, and sudden death. Syndromes characterized by congenital QT interval prolongation include the Romano-Ward Syndrome (autosomal dominant), the Jervell-LangeNielsen Syndrome (autosomal recessive and associated with sensorineural deafness), and sporadic long QT syndrome (inheritance pattern indeterminant) [l-31. The Romano-Ward long QT syndrome comprises a group of autosomal dominant disorders that share a similar electrocardiographic and symptomatic
*Correspondance address. University of Michigan, C.S. Mott Children’s Hospital, MCHC F13101 Box 0204, 1500 E. Medical Center Drive, Ann Arbor, Michigan 48109-0204, USA. Tel: + 1 313 7645176. 3058-9813/96/$35.00 SZ 1996 Elsevier PII SlOSS-9813(96)00170-l
de pointes;
Potassium
channels;
Sodium
channels;
Sudden
phenotype (Fig. 1) including prolongation of the corrected QT interval, T wave abnormalities, ventricular arrhythmias (e.g. torsades de pointes) and a high incidence of sudden death [4]. The symptoms include episodes of syncope and sudden death, often during excitement or emotional stress. Treatment may include P-blockade, atria1 or ventricular pacing, left cervical sympathetic ganglionectomy, and, for patients with persistent symptoms, placement of implantable defibrillators [5]. However, even with appropriate therapy, there remains significant morbidity and mortality associated with the long QT syndrome. In a prospective study of 328 families, 5% of patients per year affected with the long QT syndrome had syncope and 0.9% per year suffered sudden death despite appropriate therapy [6]. The complex nature of myocardial repolarization has prevented identification of the pathophysiologic defects responsible for the prolonged QT interval in these patients using standard functional cloning
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Fig. 1. Twelve lead electrocardiograms from (A) a normal individual, (B! a patient with long QT syndrome at baseline, and (C) a different patient with long QT syndrome (ECG recorded at the onset of torsades de pointes). All recordings are from Lead 11 of a standard 12 lead electrocardiogram, recorded at a paper speed of 25 mm/set (tracings courtesy of M. Dick II).
strategies. Therefore, to identify the responsible genes and thereby increase the understanding of myocardial repolarization, positional cloning techniques were applied to the Romano-Ward syndrome (long QT syndrome). In initial linkage studies, Keating et al. [7] found that the long QT syndrome phenotype was closely linked to the Harvey ras-1 (HRASl) locus on chromosome 11~15.5 (LQTl) in several large families. However, as additional families were studied, there were several reports of long QT syndrome families in whom the syndrome was not linked to the HRASl locus [8-111. Subsequently, additional loci were identified on chromosomes 7 (LQT2), 3 (LQT3) [12] and 4 (LQT4) [13]. Furthermore, there have been families identified in whom the long QT syndrome is not linked to any of the previously-identified loci, indicating that additional genes (an LQT5 and perhaps even an LQT6) may cause the long QT syndrome [14]. Within the past year there have been rapid and remarkable advances in these positional cloning efforts, culminating in the identification of three responsible genes, the KVLQTl potassium channel (LQTl), [14] the HERG potassium channel (LQT2), [15] and the SCNSA voltage-gated sodium channel (LQT3) [16]. Together, these genes may be responsible for nearly 90% of the cases of autosomal dominant congenital long QT syndrome. These discoveries will be briefly reviewed and the implications of this research for the management of these and other patients with ventricular repolarization abnormalities will be discussed.
the long QT syndrome and are not included in linkage studies. The goal of this classification is to identify ‘pure’ populations of affected and unaffected individuals within each family. Genetic marker information in these two groups are compared to determine the chromosomal region which contains the marker information (i.e. the haplotype) that uniquely differentiates affected from unaffected individuals. The likelihood of the long QT syndrome being near, or ‘linked to,’ a particular marker in this region is calculated and expressed as the LOD (logarithm of the odds) score. Due to the genetic heterogeneity of the long QT syndrome, LOD scores have to be calculated for each family separately. A long QT syndrome family can then be categorized as linked to a particular MT gene (e.g. LQTl-linked) if the LOD scores for linkage of the syndrome to markers in that chromosomal region (e.g. chromosome 11~15.5 for the LQTl gene) are r 3. Often a long QT syndrome family is not large enough to generate a LOD score of 3 (which requires a minimum of 12 affected or unaffected offspring of the long QT syndrome patients in a particular family). In these smaller families, a LOD score between 0.7 and 3 is often ‘suggestive’ of linkage of the long QT syndrome to that marker. Linkage studies in the long QT syndrome have estimated that approximately 50% of the long QT syndrome cases are secondary to defects of the KVLQTl gene [8,11,14] while HERG gene defects
2. Linkage studies for the long QT syndrome
Table 1 Criteria for the diagnosis of the long QT syndrome
Due to the overlap in QT interval measurements between long QT syndrome patients and normal individuals, the electrocardiographic identification of each patient affected with the long QT syndrome can be difficult. Based on a study of families with LQTl-linked long QT syndrome [17], the following long QT syndrome diagnostic criteria have been adopted for linkage studies to minimize miscategorization of individuals (Table 1). These criteria divide individuals into the following categories: affected, unaffected, and indeterminant. Individuals who cannot be categorized as affected or unaffected using these criteria or whose QT interval is prolonged secondary to medication or acute illness are labeled ‘indeterminant’ for
Category
QTc measurement
Affected
QTc > 470 ms (males) QTc 2 480 ms (females) QTc 2 440 ms in patients with a strong family history and characteristic symptoms of the RWLQTS: documented torsades de pointes syncope during excitement or emotional stress onset of symptoms before age 21
Unaffected
QTc 5 410ms
Indeterminant
QTc > 410 and < 470 without symptoms QTc > 410 and < 440 regardless of symptoms or family history QTc prolongation secondary to medication or acute illness
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Table 2 Genes responsible for the long QT syndrome Percentage of LQT families
Inheritance
Chromosome location
Responsible gene
Gene function
LQTS features
LQTl
Autosomal dominant
1lp15.5
KVLQTI
Potassium channel
Sudden death w/ excitement
50
LQT2
Autosomal dominant
7q35
HERG
Potassium channel
Exacerbated by hypokalemia
30
LQn
Autosomal dominant
3~21
SCNSA
Sodium channel
Sudden death w/ sleep/bradycardia
10
LQT4
Autosomal dominant
4q25-27
?
?
Pts have bradycardia U waves
LQTS
Autosomal dominant
‘?
?
?
Jervel-LangeNielsen
Autosomal recessive
?
?
?
LQT locus
and SCNSA defects are the second and third most common causes, respectively, of the long QT syndrome [14]. A summary of the genes responsible for the long QT syndrome is presented in Table 2. To date, the long QT syndrome has been linked to chromosome 4 in only one family [13]. In this family, the LQT4 gene was localized to a greater than 10 CM interval on chromosome 4q25-27. Recently, a gene [expressed sequence tag (EST)1 demonstrating high homology to a rat 6 isoform of a calcium/calmodulin-dependent protein kinase II was mapped to this region [13]. This gene may be a good candidate for this disorder. If it is not determined to be responsible for the LQT4 form of the long QT syndrome, then positional cloning efforts will need to isolate new genes in this region to identify additional candidates. 3. LQT3 3.1. SCNSA determined to be L&T3 gene
SCNSA, the cardiac tetrodotoxin-resistant voltagegated sodium channel alpha subunit, is a large protein of 2016 amino acids and includes four homologous domains (DI-DIV), each of which contains six membrane-spanning segments (Fig. 2). It was initially cloned and characterized by Gellens et al. [18] and subsequently mapped by George et al. [19] to chromosome 3~21. Based on its chromosomal localization, its expression in cardiac myocytes, and its function, SCNSA was an excellent candidate gene for the LQT3-linked long QT syndrome. In two unrelated families, all individuals affected with the long QT syndrome inherited an identical 9 bp, in-frame deletion within the regulatory domain of the protein [16]. This mutation would be predicted to encode a functional protein lacking three amino acids, lysine-1505-
<5 10
Pts w/ sensorineural deafness
rare
proline-1506-glutamine(KPQ), in the cytoplasmic region between the homologous domains DIII and DIV (Fig. 2) [16,20]. Missense mutations in the corresponding region of the skeletal muscle voltagegated sodium channel (SCN4A) cause myotonia due to delayed channel inactivation leading to repetitive depolarizations [211. 3.2. Functional characterization of the SCNSA KPQ deletion
A recent article by Bennett et al. [22] demonstrated that the mutant SCNSA channels with the three amino acid KPQ) deletion did show a sustained inward sodium current during membrane depolarization. Single-channel recordings indicated that this mutant channel fluctuated between normal and non-inactivating gating states. This persistent inward sodium current would be predicted to prolong ventricular action potentials by prolonging the plateau phase and delaying the onset of repolarization (Fig. 3). Interestingly, long QT syndrome patients with SCNSA defects tend to have delayed onset of the T wave on the ECG, but the morphology and duration of the T wave is normal [231. This clinical observation is consistent with a prolongation of the plateau phase of the action potential with a subsequent delay in the onset of repolarization but relatively normal rate of repolarization once initiated. 3.3. Additional SCNSA mutations
Recently, additional SCNSA mutations have been identified. In one multigenerational long QT syndrome family, a highly conserved asparagine was replaced by a serine at amino acid 1325 in the linker region between the DIII-S4 and DIII-S5 regions of
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DIV
mV
time CutTents Inward INA fast 1NA slow <
,;;;
d~nl
I ca
Outward Fig. 2. Schematic representation of the predicted topology of the SCNSA sodium channel, indicating the location of the KPQ deletion associated with the long QT syndrome in four families (from Wang et al. [16] with permission).
the protein [24]. In an unrelated family, a highly conserved arginine was replaced by a histidine at amino acid 1644 in the DIV-S4 region. By comparison with known SCN4A mutations which cause paramyotonia congenita, these regions also would be predicted to be important in channel inactivation. However, further studies will be required to determine how these two new mutations cause QT prolongation. Additional LQT3-linked families are currently being evaluated to determine what other defects in this gene can cause the long QT syndrome. Characterization of these defects will aid the understanding of the function of this gene and help determine what other regions of the protein are vital to the pathogenesis of the long QT syndrome. 4. LQT2 4.1. HERG Determined to be LQT2 Gene
The HERG potassium channel (Fig. 4) is a human homologue of the Drosophila ether-a-go-go (eag) gene which encodes a CAMP- and calcium-modulated potassium channel [25]. A human member of the eag gene family, the HERG (the human ether-a-go-go related) gene was identified by Warmke and Ganetsb [26] and mapped to chromosome 7. Keating and his colleagues 1151 further localized the gene to chromosome 7q3536 and, since it mapped to the same region as the LQT2 gene, evaluated it as a candidate gene
It0 iK1
I I
I
1 I
Fig. 3. Schematic illustration of selected ion currents during an action potential in a ventricular myocyte, showing predicted alterations caused by defects in the SCNSA and HERG proteins associated with the long QT syndrome. The duration of the inward and outward currents are represented by the solid and open bars, respectively. The predicted alterations of the sodium current and the action potential by the long QT syndrome-associated SCNSA defects are represented by the solid bar enclosed by the dashed line and the dashed line, respectively. Likewise, the predicted alterations of the potassium current and the action potential caused by HERG defects are represented by the thick-outlined open bar and the thick line, respectively. These are only predicted alterations based on the observed genetic defects and the in vitro expression studies. Future studies will need to investigate how these genetic defects affect ion currents and action potentials in myocytes. I,-., calcium current; I,, - inward rectifier potassium current; I,, transient outward potassium current (from Russell and Dick I331 with permission).
for the long QT syndrome in their LQT2-linked long QT syndrome families. Using single strand conformational polymorphism (SSCP) analysis, they identified HERG gene mutations in six long QT syndrome families, [15] including three missense mutations, two frame-shift deletions, and one splice-site mutation. In at least one case, the genetic defect might not be expected to produce any protein product from the mutated gene. In these patients, only one of the two copies of the HERG gene in each myocyte would ultimately produce a functional HERG protein, resulting in roughly half the number of functional HERG potassium channels as a normal myocyte. However, the majority of mutations described would be predicted to encode for a protein product from the
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acquired long QT syndrome. Acquired prolongation of the QT interval is most commonly due to (1) hypokalemia or pharmacologic potassium channel blockade, (2) hypocalcemia, (3) CNS injury, and (4) myocardial ischemia/infarction [27]. As one of the potassium channels that is critical to myocardial repolarization, HERG may be involved in acquired QT prolongation due to hypokalemia and potassium channel blockade. 4.2. Functional characterization of the HERG channel
Fig. 4. The HERG potassium channel protein. Each potassium channel is composed of four channel proteins with the S5 and S6 transmembrane domains of each forming the pore of the channel. (From Russell and Dick; [33] with permission).
mutated gene. Since functional potassium channels are homo- or heterotetramers of potassium channel proteins, this abnormal protein may complex with the normal protein to produce channels with diminished potassium channel conductance properties. This dominant negative affect of the mutant protein may further decrease the number of fully-functional HERG channels, resulting in a more severe phenotype (greater QT prolongation, increased syncope, increased episodes of torsades de pointes, and higher incidence of sudden death). This decrease in the number of fully-functional HERG channels would be expected to decrease the rate of potassium efflux from the cell and decrease the rate of repolarization (Fig. 3). In contrast to patients with SCNSA defects, patients with HERG defects tend to have T waves that are normal in onset but broad, and flattened [23]. An interesting feature of the HERG protein is that, unlike other potassium channels, it has a nucleotide binding fold which might make it responsive to intracellular levels of cyclic nucleotides such as cyclic AMP. If CAMP binding can be demonstrated to alter the potassium current through the HERG channel, then patients with a HERG gene defect may be particularly vulnerable to increases in intracellular CAMP levels. This may explain why long QT syndrome patients have increased episodes of syncope and sudden death during increased sympathetic tone which causes an increase in intracellular CAMP in ventricular myocytes. The HERG channel may also be important for
The potential role of the HERG gene in prolongation of the QT interval due to hypokalemia and due to the potassium channel blocking properties of pharmacologic agents such as quinidine and sotalol was investigated by Sanguinetti et al. [28]. These drugs have been demonstrated to inhibit the cardiac rapidly activating inward rectifier potassium current I,, [29]. In isolated myocytes, this current has been shown to have a vital role in initiating repolarization following an action potential. In their study, Sanguinetti and his colleagues injected the full-length HERG cRNA (coding RNA) sequence into Xenopus oocytes to examine the characteristics of the HERG channel (a homomultimer of four HERG proteins). In this model, the oocytes transiently express the human HERG protein at high levels such that HERG channel current is the primary determinant of changes in the membrane potential of the oocyte. The response of the channel to various ion concentrations and pharmacologic agents can be investigated in this setting without many of the confounding variables present in the intact myocardial cell (such as any regulatory pathways affecting HERG channel gating). Using this model, they determined that the HERG protein expressed alone functioned as a potassium channel. The potassium current conducted by this channel demonstrated many characteristics of the cardiac I,, current, including a ‘paradoxical’ response (see below) to extracellular potassium concentrations, and similar voltage-dependent activation and inactivation properties. However, unlike I,, HERG channel current is not blocked by methansulfonanilides. Therefore, if the HERG gene is responsible for I,, then there may be another subunit in the HERG channel complex that binds methanesulfonamides. This subunit may be another potassium channel protein which associates with the HERG protein in a heteromultimeric complex or may be an associated protein that does not form part of the pore region of the potassium channel. As determined in the Sanguinetti study, one of the unique features of the potassium current through the HERG channel is its marked, ‘paradoxical’ response
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to the extracellular potassium concentration. in contrast to other potassium channels, as extracellular potassium concentration rises, the amplitude of the potassium current, from intracellular to extracellular, increases. Therefore, since the initiation of vcntricular repolarization appears to be, at least in part, dependent upon the potassium current through the HERG channel, it is not surprising that hypokalemia causes QT prolongation and torsades de pointcs. Whereas patients with HERG gene defects would have a decreased number of HERG channels resulting in a decreased rate of potassium efflux, patients with hypokalemia might have decreased potassium efflux through each HERG channel although the total number of channels would be normal. Of note, this study determined that the HERG channel, when expressed in oocytes, is not activated by the cyclic nucleotide analogs 8-Br-CAMP or 8-BrcGMP, despite the presence of a nucleotide binding fold in the HERG protein. This suggests that, if the HERG channel responds to a cyclic nucleotide, another subunit or interacting protein may be required. Therefore, the relationship between elevated sympathetic tone and symptoms in patients with the long QT syndrome cannot yet be explained by the function of the HERG channel. However, although the Sanguinetti study [28] characterized the HERG channel activity when expressed alone, there remain many questions about how it functions in the myocardial cell. Comparisons of its function with the I,, current suggest that the HERG channel may interact with other subunits or perhaps even other. potassium channels which significantly influence the characteristics of the potassium current through the HERG channel. Some characterized potassium currents are the result of heteromultimeric assembly of different potassium channel proteins to form a potassium channel with different conductance properties than either of the potassium channels expressed alone. Therefore, there is very likely to be associated regulatory subunits of the HERG channel or heteromultimeric assembly of the HERG protein With other potassium channels which may influence the properties of the HERG channel in vivo. 5. LQTl As previously mentioned, the LQTl gene on chromosome 11~15.5 was the first of the long QT syndrome genes to be localized using linkage analysis. As additional families were studied recombination analysis was used to further restrict the region of interest for the location of the LQTl gene and exclude the known candidate genes, the KCNCl delayed rectifier potassium channel, the DRD4 dopamine receptor, tyrosine hydroxylase (TH) and HRAS [30]. Based on
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Fig. 5. Schematic illustration of the KVLQTl Potassium channel showing the location of the described mutations (from Wang et al. [14] with permission).
recombination analysis, the LQTl gene was localized to a 700000 bp region near TH [14]. New putative genes from this region were identified using exon trapping, a process in which the DNA from the region is cloned into a specialized vector and scanned for protein encoding sequences, called exons. To isolate the LQTl gene, approximately 400 exons were identified and sequenced. Of these exons, one was 53% similar to potassium channel proteins from multiple species [14]. This gene was further characterized and was determined to be highly homologous to the Drosophila Shaker potassium channel, DMSHAKEl. Mutations of this gene, named KVLQTl, were identified in 16 long QT syndrome families [14]. The mutations described included one in-frame deletion and ten different missense mutations (Fig. 5). These mutations are distributed throughout the gene with some clustering of mutations in the pore region of the protein. In each case, the genetic mutation would be predicted to encode for a protein which presumably would have diminished function compared to the normal protein. To date, no mutations have been identified that would not be predicted to produce a protein. Therefore, individuals with a genetic mutation of KVLQTl that does not produce any protein from the mutant gene are either so severely affected that they die before they can be identified as having the long QT syndrome or are so mildly affected that they cannot be detected based on the clinical criteria. If they are mildly affected they may demonstrate symptoms, such as torsades de pointes or syncope, only when the myocardial repolarization process is ‘stressed’ by another factor, such as pharmacologic agents which prolong repolarization, electrolyte abnormalities, or myocardial ischemia. As with the HERG mutations, KVLQTl mutations which produce a mutant protein might be expected to have a more severe phenotype due to the complex formation between the mutant
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and normal proteins.‘The interaction of mutant and normal proteins might be expected to markedly reduce the number of fully-functional KVLQTl channels. 6. Genotype: phenotype syndrome
interactions
in the long QT
Since the long QT syndrome can be caused by a number of different genes, the precise phenotype caused by each LQT gene is currently being evaluated. A recent study by Moss et al. 1231 delineated the ECG T wave patterns that were characteristic of the LQTl, LQT2, and LQT3 forms of the long QT syndrome. In general, patients with SCNSA defects (LQT3) had slightly slower heart rates than other family members and a T wave of normal shape that was markedly delayed in its onset. Patients with HERG defects (LQT2) had a normal T wave onset but decreased T wave amplitude and prolongation of the T wave. Defects of the KVLQTl gene produced T waves of normal amplitude but prolonged in duration and, in one family, delayed in onset (Fig. 6). However, the T wave morphologies of each group (LQTl, LQT2, or LQT3) of LQT defects overlapped significantly and would not reliably determine the gene responsible for the long QT syndrome in each family. The LQT4 form of the long QT syndrome appears to have a slightly different phenotype which may make it distinguishable from the other forms of the disorder. In the one family with LQT4-linked long QT syndrome, there is marked sinus bradycardia (resting heart rates of 40-50 beats/min) which has required pacemaker placement in nine of 21 affected patients after institution of p-blockade therapy. This family also demonstrated a high incidence of atria1 fibrillation and very prominent U waves which persisted after increasing the heart rates with atria1 pacing [13]. However, additional LQT4 families will need to be identified and characterized to determine if these characteristics are generalizable to other LQT4-linked families. Chromosome
3
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7. Implications prolongation
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for the treatment
of patients with QT
Due to the HERG gene’s important role in myocardial repolarization, steps taken to optimize potassium flux through the HERG potassium channel might help prevent ventricular arrhythmias in patients with repolarization defects either due to congenital or acquired (secondary to drugs, electrolyte disturbances, central nervous system insults, and, possibly, due to myocardial infarction) prolongation of the QT interval. Therefore, based on the study by Sanguinetti et al. [28] hyokalemia or even low normal serum potassium levels should be avoided in any patient with QT prolongation. Furthermore, clinical trials may begin to evaluate the efficacy of high potassium diets and potassium channel openers (e.g. pinacidil) in the treatment of long QT syndrome patients with HERG defects. Likewise, clinical trials are beginning to examine the efficacy of sodium channel blockade in long QT syndrome patients with SCNSA defects (Fig. 7). Treatment of a small group of long QT syndrome patients with mexilitene, a sodium channel blocker, caused normalization of the QT interval in the 7 long QT syndrome patients with SCNSA defects but no significant change in the QT intervals of patients with HERG channel defects [31]. Furthermore, patients with SCNSA defects are more likely to have syncope and sudden death during sleep and bradycardia. In the study by Schwartz et al. 1311 patients with SCNSA defects were also demonstrated to have significant shortening of their QT intervals with increased heart rate. Therefore, these patients may be best treated with sodium channel blockade using mexilitene, and if needed, atria1 pacing. Conversely, p-blockade therapy, which results in bradycardia, may actually be detrimental in long QT syndrome patients with SCNSA defects. As more is learned about the KVLQTl potassium channel function clinical trials will be developed to specifically ameliorate the pathophysiologic defects. Chromosome
7
Chromosome
11
II aVF
VS
Fig. 6. ECG recordings a HERG gene defect
from leads (chromosome
II, aVF, and 71, and one
VS in three long with a KVLQTl
QT syndrome gene defect
patients: (chromosome
one
with an SCNSA defect (chromosome 3), one I1 ). (From Moss et al. [23] with permission).
with
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9. Future research p
(k6)
T
,
0 Control m Mexiletine
NS
(n=7)
Fig. 7. Bar graph showing QTc values in control conditions and acute oral drug testing with mexilitene in patients with SCNSA defects (LQT3, n = 6) and in patients with HERG defects (LQT2, n = 7) (from Schwartz et al. [31] with permission).
Prior studies of the effectiveness of the p-blockade in long QT syndrome patients [4] may have been composed of a relatively large number of patients of KVLQTl defects, since mutations of this gene appear to account for 50% of the long QT syndrome cases. Therefore, P-blockade may be a relatively effective treatment in these patients. Whether or not potassium channel openers or other more specific therapy will be more effective remains to be determined. 8. Genetic testing for the long QT syndrome
These clinical trials indicate that appropriate therapy for a particular patient may strongly depend on which LQT gene is responsible for the syndrome in their family. This will depend on genetic testing, either by linkage analysis in large families or mutation detection in small families and sporadic cases. Once a family’s particular genetic defect has been identified, it can easily be traced through the family to determine who is affected and therefore who is at risk for sudden death. Due to the marked overlap in the corrected QT intervals between normal individuals and patients affected with the long QT syndrome, not all individuals at risk for sudden death can be identified by electrocardiographic testing, even when done during exercise. Furthermore, these apparently normal individuals can pass the disorder onto their offspring who may be more severely affected but may not be screened with an ECG if their parents were not identified as having a genetic mutation. In our studies we have confirmed the findings of Vincent et al. [17] that there are clinically normal individuals (QTc intervals of 0.41-0.42 s) with genetic mutations who passed the genetic defect onto their offspring (unpublished observations). In these families, the genetic defect was not detected until one of the children had syncope or a cardiac arrest.
Long QT syndrome patients with the exact same genetic defect can have very different QT interval measurements and symptomatology. This observation suggests that other genes may strongly interact with the LOT genes, influencing their clinical expression. These genes may be extremely important for (1) understanding the spectrum of symptoms in long QT syndrome patients, (2) identifying other genes that may be vital to myocardial repolarization, and (3) suggesting new avenues for therapy, not only in patients with the long QT syndrome but with other ventricular arrhythmias as well. For patients with HERG and KVLQTl potassium channel defects there is a 2:l ratio of females to males diagnosed with clinical long QT syndrome. For both of these genes, mutant genes should be inherited by an equal number of males and females. Therefore, there must be a sex-specific influence on the expression of these two genes. Determining why females are more likely to diagnosed with the long QT syndrome and are more likely to be symptomatic even at comparable QTc measurements to males [32] may be central to understanding what factors modify expression of the LQT genes. Now that the genetic heterogeneity of this syndrome is recognized, it is critical that the long QT syndrome be viewed as five (or more> distinct genetic disorders that share a similar electrocardiographic feature, namely, QT interval prolongation [33]. These patients may require very different therapy depending on which gene is responsible for their QT prolongation. Based on preliminary studies, patients with sodium channel (SCNSA) appear to have QT interval shortening when treated with mexilitene and when increasing their heart rate. ,&blockade therapy in these patients may be detrimental since they tend to have increased episodes of sudden death during sleep and bradycardia. Patients with HERG defects should avoid hypokalemia and may be helped by high potassium diets and, potentially, potassium channel openers. Based on these preliminary clinical trials, genetic testing may be a critical step in determining the appropriate course of therapy for patients with long QT syndrome. Continued characterization of the genes responsible for this disorder may improve the diagnosis and treatment not only of patients with the long QT syndrome but also of patients with other ventricular arrhythmias, including those occurring secondary to myocardial ischemia/infarction. Acknowledgements
I would like to thank Dr. MacDonald Dick II for thoughtful review of this manuscript. This work has
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been supported by a grant (MO1 RR000421 from the General Clinical Research Center at the University of Michigan. References
[I81 Gellens
al. Primary structure and cardiac tetrodotoxin-inchannel. Proc Natl Acad
[191
HA et al. Assignment of voltage-gated Na+ to band 3~21. Cytogenet
111 Roman0
C, Gemme G, Pongiglione R. Artimie cardiache rare dell’eta pediatrica. Clin Pediatr 1963;45:658-683. Dl Ward OC. New familial cardiac syndrome in children. J Irish Med Assoc 1964;54:103-106. F. Congenital deaf-mutism, functio[31 Jervell A, Lange-Nielsen nal heart diseases with prolongation of the QT interval and sudden death. Am Heart J 1957;54:59-68. C, Sala S. 141 Schwartz PJ, Bonazzi 0, Locati E, Napolitano Pathogenesis and therapy of the idiopathic long QT syndrome. Ann NY Acad Sci 1992;644:112-141. A et al. The long QT 151 Garson A Jr, Dick II M, Fournier syndrome in children. An international study of 287 patients. Circulation 1993;87:1866-1872. 161 Moss AJ, Schwartz PJ, Crampton RS et al. The long QT syndrome. Prospective longitudinal study of 328 families. Circulation 1991$4:1136-l 144. D, Dunn C, Timothy K, Vincent GM, [71 Keating M, Atkinson Leppert M. Linkage of a cardiac arrhythmia, the long QT and the Harvey ras-1 gene. Science syndrome, 1991;252:704-706. long QT [81 Towbin JA, Pagotto L, Siu B et al. Romano-Ward syndrome (RWLQTS): Evidence of genetic heterogeneity. Pediatr Res 1992;31:23. J, Kalman YM, Medina A et al. Evidence of [91 Benhorin genetic heterogeneity in the long QT syndrome. Science 1993;260:1960-1961. [lOI Curran M, Atkinson D, Timothy K et al. Locus heterogeneity of autosomal dominant long QT syndrome. J Clin Invest 1993;92:799-803. [Ill Towbin JA, Moss AJ, Robinson J et al. Evidence of genetic heterogeneity in Romano-Ward long QT syndrome: Analysis of 23 families. Circulation 1994;90:2635-2644. [121 Jiang C, Atkinson D, Towbin JA et al. Two long QT syndrome loci map to chromosomes 3 and 7 with evidence for further heterogeneity. Nature Genet 1994;8:141-147. [131 Schott J-J, Charpentier F, Peltiers S et al. Mapping of a gene for long QT syndrome to chromosome 4q25-27. Am J Hum Genet 1995;57:1114-1122. [141 Wang Q, Curran ME, Splawski I et al. Positional cloning of a novel potassium channel gene: KVLQTl mutations cause cardiac arrhythmias. Nature Genet 1996;12:17-23. [151 Curran ME, Splawski I, Timothy K, Vincent GM, Green ED, Keating MT. A molecular basis for cardiac arrhythmia: HERG mutations cause long QT syndrome. Cell 1995;80:795-803. iI61 Wang Q, Shen J, Splawski I et al. SCNSA mutations associated with an inherited cardiac arrhythmia, Long QT syndrome. Cell 1995;80:805-811. [I71 Vincent GM, Timothy K, Leppert M, Keating M. The spectrum of symptoms and QT intervals in carriers of the gene for the long-QT syndrome. N Engl J Med 1992;327:846-852.
M, George A, Chen L et functional expression of the human sensitive voltage dependent sodium Sci USA 1992;89:554-558. George AL, Varkony TA, Drabkin the human heart tetrodotoxin-resistant channel a-subunit gene (SCNSAI Cell Genet 1995;68:67-70.
DO1
Towbin JA. New revelations about the long-QT syndrome. N Engl J Med 1995;333:384-385. WI Fontaine B, Khurana TS, Hoffman EP et al. Hyperkalemic periodic paralysis and the skeletal muscle sodium channel gene. Science 1990;250:1000-1002. [221 Bennett PB, Yazawa K, Makita N, George AL. Molecular mechanism for an inherited cardiac arrhythmia. Nature 1995;376:683-685. [231 Moss AJ, Zareba W, Benhorin J et al. ECG T-wave patterns in genetically distinct forms of the hereditary long QT syndrome. Circulation 1995;92:2929-2934. 1241Wang Q, Shen J, Li Z et al. Cardiac sodium channel mutations in patients with long QT syndrome, an inherited cardiac arrhythmia. Hum Mol Genet 1995;4:1603-1607.
[251 Bruggeman
A, Pardo LA, Struhmer W, Pongs 0. Ether-a-gogo encodes a voltage-gated channel permeable to k+ and Ca2+ and modulated by CAMP. Nature 1993;365:445-448.
Ml
Warmke JE, Ganetsky B. A family of potassium channel genes related to eag in Drosophlla and mammals. Proc Natl Acad Sci USA 1994;91:3438-3442.
[271 Jackman
WM. Friday KJ Anderson JL, Aliot EM, Clark M, Lazzara R. The long QT syndromes: A critical review, new clinical observations and a unifying hypothesis. Prog Cardiovast Dis 1988;XXXI(2):115-172.
[281 Sanguinetti
WI [301
[311
[=I
MC, Jiang C, Curran ME, Keating MT. A mechanistic link between an inherited and acquired cardiac arrhythmia: HERG encodes the I,, potassium channel. Cell 1995;81:299-307. Shibasaki T. Conductance and kinetics of delayed rectifier potassium channels in nodal cells of the rabbit heart. J Physiol 1987;387:227-250. Russell MW, Dick II M, Campbell RM et al. Localization of the Romano-Ward long QT syndrome gene, LQTl, to the interval between tyrosine hydroxylase (TH) and DllS1349. Am J Hum Genet 1995;57:503-507. Schwartz P, Priori SG, Locati EH et al. Long QT syndrome patients with mutations of the SCNSA and HERG genes have differential responses to Na+ channel blockade and to increases in heart rate. Implications for gene-specific therapy. Circulation 1995;92:3381-3386. Zareba W, Moss AJ, le Cessie S et al. Risk of cardiac in family members of patients with long QT syndrome. Coil Cardiol 1995;26:1685-1689.
[331 Russell congenital press).
events J Am
MW, Dick II M. The molecular genetics of the long QT syndromes. Curr Opin Cardiol 1996;ll (in
Y
,,t
Pediatric Cardiology ELSEVIER
Progress
in Pediatric
Cardiology
6 (1996) 43-51
The long QT syndromes Mark W. Russell* ilepartment
of Pediatrics, Division of Pediattic Cardiology, C.S. Mott Children S Hospital, Uniuersity of Michigan Medical Center, Ann Arbor, MI, USA
Abstract The congenital long QT syndromes are a heterogeneous group of disorders that are characterized by prolongation of the QT interval on the 12-lead electrocardiogram, episodes of syncope, ventricular arrhythmias such as torsades de pointes and a high incidence of sudden death. The autosomal dominant form of congenital QT prolongation, the Romano-Ward long QT syndrome, is the most common of these disorders and has been demonstrated by linkage analysis studies to exhibit extensive genetic heterogeneity. These studies have determined that at least five different genes can cause the long QT syndrome. Positional cloning studies have led to the discovery of three genes which together appear to be responsible for nearly 90% of the autosomal dominant cases of congenital long QT syndrome. Further functional characterization of these three genes, the KVLQTl potassium channel, the HERG potassium channel and the SCNSA voltage-gated cardiac sodium channel, as well as the identification and characterization of the other long QT syndrome genes, may allow improved diagnosis and therapy for these disorders. Furthermore, the increased understanding of myocardial repolarization that is gained from characterization of these genes may lead to improved treatment for other ventricular arrhythmias, including those related to drug-induced potassium channel blockade, central nervous system insult, and, possibly, myocardial infarction.
Keywords: Long QT syndrome; death; Genetic testing
Ventricular
arrhythmias;
torsades
1. Introduction Prolongation of the QT interval, either congenital or acquired CsecondaIy to electrolyte abnormality, medication or myocardial infarction), has been associated with ventricular arrhythmias, syncope, and sudden death. Syndromes characterized by congenital QT interval prolongation include the Romano-Ward Syndrome (autosomal dominant), the Jervell-LangeNielsen Syndrome (autosomal recessive and associated with sensorineural deafness), and sporadic long QT syndrome (inheritance pattern indeterminant) [l-31. The Romano-Ward long QT syndrome comprises a group of autosomal dominant disorders that share a similar electrocardiographic and symptomatic
*Correspondance address. University of Michigan, C.S. Mott Children’s Hospital, MCHC F13101 Box 0204, 1500 E. Medical Center Drive, Ann Arbor, Michigan 48109-0204, USA. Tel: + 1 313 7645176. 3058-9813/96/$35.00 SZ 1996 Elsevier PII SlOSS-9813(96)00170-l
de pointes;
Potassium
channels;
Sodium
channels;
Sudden
phenotype (Fig. 1) including prolongation of the corrected QT interval, T wave abnormalities, ventricular arrhythmias (e.g. torsades de pointes) and a high incidence of sudden death [4]. The symptoms include episodes of syncope and sudden death, often during excitement or emotional stress. Treatment may include P-blockade, atria1 or ventricular pacing, left cervical sympathetic ganglionectomy, and, for patients with persistent symptoms, placement of implantable defibrillators [5]. However, even with appropriate therapy, there remains significant morbidity and mortality associated with the long QT syndrome. In a prospective study of 328 families, 5% of patients per year affected with the long QT syndrome had syncope and 0.9% per year suffered sudden death despite appropriate therapy [6]. The complex nature of myocardial repolarization has prevented identification of the pathophysiologic defects responsible for the prolonged QT interval in these patients using standard functional cloning
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Fig. 1. Twelve lead electrocardiograms from (A) a normal individual, (B! a patient with long QT syndrome at baseline, and (C) a different patient with long QT syndrome (ECG recorded at the onset of torsades de pointes). All recordings are from Lead 11 of a standard 12 lead electrocardiogram, recorded at a paper speed of 25 mm/set (tracings courtesy of M. Dick II).
strategies. Therefore, to identify the responsible genes and thereby increase the understanding of myocardial repolarization, positional cloning techniques were applied to the Romano-Ward syndrome (long QT syndrome). In initial linkage studies, Keating et al. [7] found that the long QT syndrome phenotype was closely linked to the Harvey ras-1 (HRASl) locus on chromosome 11~15.5 (LQTl) in several large families. However, as additional families were studied, there were several reports of long QT syndrome families in whom the syndrome was not linked to the HRASl locus [8-111. Subsequently, additional loci were identified on chromosomes 7 (LQT2), 3 (LQT3) [12] and 4 (LQT4) [13]. Furthermore, there have been families identified in whom the long QT syndrome is not linked to any of the previously-identified loci, indicating that additional genes (an LQT5 and perhaps even an LQT6) may cause the long QT syndrome [14]. Within the past year there have been rapid and remarkable advances in these positional cloning efforts, culminating in the identification of three responsible genes, the KVLQTl potassium channel (LQTl), [14] the HERG potassium channel (LQT2), [15] and the SCNSA voltage-gated sodium channel (LQT3) [16]. Together, these genes may be responsible for nearly 90% of the cases of autosomal dominant congenital long QT syndrome. These discoveries will be briefly reviewed and the implications of this research for the management of these and other patients with ventricular repolarization abnormalities will be discussed.
the long QT syndrome and are not included in linkage studies. The goal of this classification is to identify ‘pure’ populations of affected and unaffected individuals within each family. Genetic marker information in these two groups are compared to determine the chromosomal region which contains the marker information (i.e. the haplotype) that uniquely differentiates affected from unaffected individuals. The likelihood of the long QT syndrome being near, or ‘linked to,’ a particular marker in this region is calculated and expressed as the LOD (logarithm of the odds) score. Due to the genetic heterogeneity of the long QT syndrome, LOD scores have to be calculated for each family separately. A long QT syndrome family can then be categorized as linked to a particular MT gene (e.g. LQTl-linked) if the LOD scores for linkage of the syndrome to markers in that chromosomal region (e.g. chromosome 11~15.5 for the LQTl gene) are r 3. Often a long QT syndrome family is not large enough to generate a LOD score of 3 (which requires a minimum of 12 affected or unaffected offspring of the long QT syndrome patients in a particular family). In these smaller families, a LOD score between 0.7 and 3 is often ‘suggestive’ of linkage of the long QT syndrome to that marker. Linkage studies in the long QT syndrome have estimated that approximately 50% of the long QT syndrome cases are secondary to defects of the KVLQTl gene [8,11,14] while HERG gene defects
2. Linkage studies for the long QT syndrome
Table 1 Criteria for the diagnosis of the long QT syndrome
Due to the overlap in QT interval measurements between long QT syndrome patients and normal individuals, the electrocardiographic identification of each patient affected with the long QT syndrome can be difficult. Based on a study of families with LQTl-linked long QT syndrome [17], the following long QT syndrome diagnostic criteria have been adopted for linkage studies to minimize miscategorization of individuals (Table 1). These criteria divide individuals into the following categories: affected, unaffected, and indeterminant. Individuals who cannot be categorized as affected or unaffected using these criteria or whose QT interval is prolonged secondary to medication or acute illness are labeled ‘indeterminant’ for
Category
QTc measurement
Affected
QTc > 470 ms (males) QTc 2 480 ms (females) QTc 2 440 ms in patients with a strong family history and characteristic symptoms of the RWLQTS: documented torsades de pointes syncope during excitement or emotional stress onset of symptoms before age 21
Unaffected
QTc 5 410ms
Indeterminant
QTc > 410 and < 470 without symptoms QTc > 410 and < 440 regardless of symptoms or family history QTc prolongation secondary to medication or acute illness
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Table 2 Genes responsible for the long QT syndrome Percentage of LQT families
Inheritance
Chromosome location
Responsible gene
Gene function
LQTS features
LQTl
Autosomal dominant
1lp15.5
KVLQTI
Potassium channel
Sudden death w/ excitement
50
LQT2
Autosomal dominant
7q35
HERG
Potassium channel
Exacerbated by hypokalemia
30
LQn
Autosomal dominant
3~21
SCNSA
Sodium channel
Sudden death w/ sleep/bradycardia
10
LQT4
Autosomal dominant
4q25-27
?
?
Pts have bradycardia U waves
LQTS
Autosomal dominant
‘?
?
?
Jervel-LangeNielsen
Autosomal recessive
?
?
?
LQT locus
and SCNSA defects are the second and third most common causes, respectively, of the long QT syndrome [14]. A summary of the genes responsible for the long QT syndrome is presented in Table 2. To date, the long QT syndrome has been linked to chromosome 4 in only one family [13]. In this family, the LQT4 gene was localized to a greater than 10 CM interval on chromosome 4q25-27. Recently, a gene [expressed sequence tag (EST)1 demonstrating high homology to a rat 6 isoform of a calcium/calmodulin-dependent protein kinase II was mapped to this region [13]. This gene may be a good candidate for this disorder. If it is not determined to be responsible for the LQT4 form of the long QT syndrome, then positional cloning efforts will need to isolate new genes in this region to identify additional candidates. 3. LQT3 3.1. SCNSA determined to be L&T3 gene
SCNSA, the cardiac tetrodotoxin-resistant voltagegated sodium channel alpha subunit, is a large protein of 2016 amino acids and includes four homologous domains (DI-DIV), each of which contains six membrane-spanning segments (Fig. 2). It was initially cloned and characterized by Gellens et al. [18] and subsequently mapped by George et al. [19] to chromosome 3~21. Based on its chromosomal localization, its expression in cardiac myocytes, and its function, SCNSA was an excellent candidate gene for the LQT3-linked long QT syndrome. In two unrelated families, all individuals affected with the long QT syndrome inherited an identical 9 bp, in-frame deletion within the regulatory domain of the protein [16]. This mutation would be predicted to encode a functional protein lacking three amino acids, lysine-1505-
<5 10
Pts w/ sensorineural deafness
rare
proline-1506-glutamine(KPQ), in the cytoplasmic region between the homologous domains DIII and DIV (Fig. 2) [16,20]. Missense mutations in the corresponding region of the skeletal muscle voltagegated sodium channel (SCN4A) cause myotonia due to delayed channel inactivation leading to repetitive depolarizations [211. 3.2. Functional characterization of the SCNSA KPQ deletion
A recent article by Bennett et al. [22] demonstrated that the mutant SCNSA channels with the three amino acid KPQ) deletion did show a sustained inward sodium current during membrane depolarization. Single-channel recordings indicated that this mutant channel fluctuated between normal and non-inactivating gating states. This persistent inward sodium current would be predicted to prolong ventricular action potentials by prolonging the plateau phase and delaying the onset of repolarization (Fig. 3). Interestingly, long QT syndrome patients with SCNSA defects tend to have delayed onset of the T wave on the ECG, but the morphology and duration of the T wave is normal [231. This clinical observation is consistent with a prolongation of the plateau phase of the action potential with a subsequent delay in the onset of repolarization but relatively normal rate of repolarization once initiated. 3.3. Additional SCNSA mutations
Recently, additional SCNSA mutations have been identified. In one multigenerational long QT syndrome family, a highly conserved asparagine was replaced by a serine at amino acid 1325 in the linker region between the DIII-S4 and DIII-S5 regions of
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DIV
mV
time CutTents Inward INA fast 1NA slow <
,;;;
d~nl
I ca
Outward Fig. 2. Schematic representation of the predicted topology of the SCNSA sodium channel, indicating the location of the KPQ deletion associated with the long QT syndrome in four families (from Wang et al. [16] with permission).
the protein [24]. In an unrelated family, a highly conserved arginine was replaced by a histidine at amino acid 1644 in the DIV-S4 region. By comparison with known SCN4A mutations which cause paramyotonia congenita, these regions also would be predicted to be important in channel inactivation. However, further studies will be required to determine how these two new mutations cause QT prolongation. Additional LQT3-linked families are currently being evaluated to determine what other defects in this gene can cause the long QT syndrome. Characterization of these defects will aid the understanding of the function of this gene and help determine what other regions of the protein are vital to the pathogenesis of the long QT syndrome. 4. LQT2 4.1. HERG Determined to be LQT2 Gene
The HERG potassium channel (Fig. 4) is a human homologue of the Drosophila ether-a-go-go (eag) gene which encodes a CAMP- and calcium-modulated potassium channel [25]. A human member of the eag gene family, the HERG (the human ether-a-go-go related) gene was identified by Warmke and Ganetsb [26] and mapped to chromosome 7. Keating and his colleagues 1151 further localized the gene to chromosome 7q3536 and, since it mapped to the same region as the LQT2 gene, evaluated it as a candidate gene
It0 iK1
I I
I
1 I
Fig. 3. Schematic illustration of selected ion currents during an action potential in a ventricular myocyte, showing predicted alterations caused by defects in the SCNSA and HERG proteins associated with the long QT syndrome. The duration of the inward and outward currents are represented by the solid and open bars, respectively. The predicted alterations of the sodium current and the action potential by the long QT syndrome-associated SCNSA defects are represented by the solid bar enclosed by the dashed line and the dashed line, respectively. Likewise, the predicted alterations of the potassium current and the action potential caused by HERG defects are represented by the thick-outlined open bar and the thick line, respectively. These are only predicted alterations based on the observed genetic defects and the in vitro expression studies. Future studies will need to investigate how these genetic defects affect ion currents and action potentials in myocytes. I,-., calcium current; I,, - inward rectifier potassium current; I,, transient outward potassium current (from Russell and Dick I331 with permission).
for the long QT syndrome in their LQT2-linked long QT syndrome families. Using single strand conformational polymorphism (SSCP) analysis, they identified HERG gene mutations in six long QT syndrome families, [15] including three missense mutations, two frame-shift deletions, and one splice-site mutation. In at least one case, the genetic defect might not be expected to produce any protein product from the mutated gene. In these patients, only one of the two copies of the HERG gene in each myocyte would ultimately produce a functional HERG protein, resulting in roughly half the number of functional HERG potassium channels as a normal myocyte. However, the majority of mutations described would be predicted to encode for a protein product from the
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HERG potassium Extracellular
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47
acquired long QT syndrome. Acquired prolongation of the QT interval is most commonly due to (1) hypokalemia or pharmacologic potassium channel blockade, (2) hypocalcemia, (3) CNS injury, and (4) myocardial ischemia/infarction [27]. As one of the potassium channels that is critical to myocardial repolarization, HERG may be involved in acquired QT prolongation due to hypokalemia and potassium channel blockade. 4.2. Functional characterization of the HERG channel
Fig. 4. The HERG potassium channel protein. Each potassium channel is composed of four channel proteins with the S5 and S6 transmembrane domains of each forming the pore of the channel. (From Russell and Dick; [33] with permission).
mutated gene. Since functional potassium channels are homo- or heterotetramers of potassium channel proteins, this abnormal protein may complex with the normal protein to produce channels with diminished potassium channel conductance properties. This dominant negative affect of the mutant protein may further decrease the number of fully-functional HERG channels, resulting in a more severe phenotype (greater QT prolongation, increased syncope, increased episodes of torsades de pointes, and higher incidence of sudden death). This decrease in the number of fully-functional HERG channels would be expected to decrease the rate of potassium efflux from the cell and decrease the rate of repolarization (Fig. 3). In contrast to patients with SCNSA defects, patients with HERG defects tend to have T waves that are normal in onset but broad, and flattened [23]. An interesting feature of the HERG protein is that, unlike other potassium channels, it has a nucleotide binding fold which might make it responsive to intracellular levels of cyclic nucleotides such as cyclic AMP. If CAMP binding can be demonstrated to alter the potassium current through the HERG channel, then patients with a HERG gene defect may be particularly vulnerable to increases in intracellular CAMP levels. This may explain why long QT syndrome patients have increased episodes of syncope and sudden death during increased sympathetic tone which causes an increase in intracellular CAMP in ventricular myocytes. The HERG channel may also be important for
The potential role of the HERG gene in prolongation of the QT interval due to hypokalemia and due to the potassium channel blocking properties of pharmacologic agents such as quinidine and sotalol was investigated by Sanguinetti et al. [28]. These drugs have been demonstrated to inhibit the cardiac rapidly activating inward rectifier potassium current I,, [29]. In isolated myocytes, this current has been shown to have a vital role in initiating repolarization following an action potential. In their study, Sanguinetti and his colleagues injected the full-length HERG cRNA (coding RNA) sequence into Xenopus oocytes to examine the characteristics of the HERG channel (a homomultimer of four HERG proteins). In this model, the oocytes transiently express the human HERG protein at high levels such that HERG channel current is the primary determinant of changes in the membrane potential of the oocyte. The response of the channel to various ion concentrations and pharmacologic agents can be investigated in this setting without many of the confounding variables present in the intact myocardial cell (such as any regulatory pathways affecting HERG channel gating). Using this model, they determined that the HERG protein expressed alone functioned as a potassium channel. The potassium current conducted by this channel demonstrated many characteristics of the cardiac I,, current, including a ‘paradoxical’ response (see below) to extracellular potassium concentrations, and similar voltage-dependent activation and inactivation properties. However, unlike I,, HERG channel current is not blocked by methansulfonanilides. Therefore, if the HERG gene is responsible for I,, then there may be another subunit in the HERG channel complex that binds methanesulfonamides. This subunit may be another potassium channel protein which associates with the HERG protein in a heteromultimeric complex or may be an associated protein that does not form part of the pore region of the potassium channel. As determined in the Sanguinetti study, one of the unique features of the potassium current through the HERG channel is its marked, ‘paradoxical’ response
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to the extracellular potassium concentration. in contrast to other potassium channels, as extracellular potassium concentration rises, the amplitude of the potassium current, from intracellular to extracellular, increases. Therefore, since the initiation of vcntricular repolarization appears to be, at least in part, dependent upon the potassium current through the HERG channel, it is not surprising that hypokalemia causes QT prolongation and torsades de pointcs. Whereas patients with HERG gene defects would have a decreased number of HERG channels resulting in a decreased rate of potassium efflux, patients with hypokalemia might have decreased potassium efflux through each HERG channel although the total number of channels would be normal. Of note, this study determined that the HERG channel, when expressed in oocytes, is not activated by the cyclic nucleotide analogs 8-Br-CAMP or 8-BrcGMP, despite the presence of a nucleotide binding fold in the HERG protein. This suggests that, if the HERG channel responds to a cyclic nucleotide, another subunit or interacting protein may be required. Therefore, the relationship between elevated sympathetic tone and symptoms in patients with the long QT syndrome cannot yet be explained by the function of the HERG channel. However, although the Sanguinetti study [28] characterized the HERG channel activity when expressed alone, there remain many questions about how it functions in the myocardial cell. Comparisons of its function with the I,, current suggest that the HERG channel may interact with other subunits or perhaps even other. potassium channels which significantly influence the characteristics of the potassium current through the HERG channel. Some characterized potassium currents are the result of heteromultimeric assembly of different potassium channel proteins to form a potassium channel with different conductance properties than either of the potassium channels expressed alone. Therefore, there is very likely to be associated regulatory subunits of the HERG channel or heteromultimeric assembly of the HERG protein With other potassium channels which may influence the properties of the HERG channel in vivo. 5. LQTl As previously mentioned, the LQTl gene on chromosome 11~15.5 was the first of the long QT syndrome genes to be localized using linkage analysis. As additional families were studied recombination analysis was used to further restrict the region of interest for the location of the LQTl gene and exclude the known candidate genes, the KCNCl delayed rectifier potassium channel, the DRD4 dopamine receptor, tyrosine hydroxylase (TH) and HRAS [30]. Based on
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Fig. 5. Schematic illustration of the KVLQTl Potassium channel showing the location of the described mutations (from Wang et al. [14] with permission).
recombination analysis, the LQTl gene was localized to a 700000 bp region near TH [14]. New putative genes from this region were identified using exon trapping, a process in which the DNA from the region is cloned into a specialized vector and scanned for protein encoding sequences, called exons. To isolate the LQTl gene, approximately 400 exons were identified and sequenced. Of these exons, one was 53% similar to potassium channel proteins from multiple species [14]. This gene was further characterized and was determined to be highly homologous to the Drosophila Shaker potassium channel, DMSHAKEl. Mutations of this gene, named KVLQTl, were identified in 16 long QT syndrome families [14]. The mutations described included one in-frame deletion and ten different missense mutations (Fig. 5). These mutations are distributed throughout the gene with some clustering of mutations in the pore region of the protein. In each case, the genetic mutation would be predicted to encode for a protein which presumably would have diminished function compared to the normal protein. To date, no mutations have been identified that would not be predicted to produce a protein. Therefore, individuals with a genetic mutation of KVLQTl that does not produce any protein from the mutant gene are either so severely affected that they die before they can be identified as having the long QT syndrome or are so mildly affected that they cannot be detected based on the clinical criteria. If they are mildly affected they may demonstrate symptoms, such as torsades de pointes or syncope, only when the myocardial repolarization process is ‘stressed’ by another factor, such as pharmacologic agents which prolong repolarization, electrolyte abnormalities, or myocardial ischemia. As with the HERG mutations, KVLQTl mutations which produce a mutant protein might be expected to have a more severe phenotype due to the complex formation between the mutant
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and normal proteins.‘The interaction of mutant and normal proteins might be expected to markedly reduce the number of fully-functional KVLQTl channels. 6. Genotype: phenotype syndrome
interactions
in the long QT
Since the long QT syndrome can be caused by a number of different genes, the precise phenotype caused by each LQT gene is currently being evaluated. A recent study by Moss et al. 1231 delineated the ECG T wave patterns that were characteristic of the LQTl, LQT2, and LQT3 forms of the long QT syndrome. In general, patients with SCNSA defects (LQT3) had slightly slower heart rates than other family members and a T wave of normal shape that was markedly delayed in its onset. Patients with HERG defects (LQT2) had a normal T wave onset but decreased T wave amplitude and prolongation of the T wave. Defects of the KVLQTl gene produced T waves of normal amplitude but prolonged in duration and, in one family, delayed in onset (Fig. 6). However, the T wave morphologies of each group (LQTl, LQT2, or LQT3) of LQT defects overlapped significantly and would not reliably determine the gene responsible for the long QT syndrome in each family. The LQT4 form of the long QT syndrome appears to have a slightly different phenotype which may make it distinguishable from the other forms of the disorder. In the one family with LQT4-linked long QT syndrome, there is marked sinus bradycardia (resting heart rates of 40-50 beats/min) which has required pacemaker placement in nine of 21 affected patients after institution of p-blockade therapy. This family also demonstrated a high incidence of atria1 fibrillation and very prominent U waves which persisted after increasing the heart rates with atria1 pacing [13]. However, additional LQT4 families will need to be identified and characterized to determine if these characteristics are generalizable to other LQT4-linked families. Chromosome
3
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Due to the HERG gene’s important role in myocardial repolarization, steps taken to optimize potassium flux through the HERG potassium channel might help prevent ventricular arrhythmias in patients with repolarization defects either due to congenital or acquired (secondary to drugs, electrolyte disturbances, central nervous system insults, and, possibly, due to myocardial infarction) prolongation of the QT interval. Therefore, based on the study by Sanguinetti et al. [28] hyokalemia or even low normal serum potassium levels should be avoided in any patient with QT prolongation. Furthermore, clinical trials may begin to evaluate the efficacy of high potassium diets and potassium channel openers (e.g. pinacidil) in the treatment of long QT syndrome patients with HERG defects. Likewise, clinical trials are beginning to examine the efficacy of sodium channel blockade in long QT syndrome patients with SCNSA defects (Fig. 7). Treatment of a small group of long QT syndrome patients with mexilitene, a sodium channel blocker, caused normalization of the QT interval in the 7 long QT syndrome patients with SCNSA defects but no significant change in the QT intervals of patients with HERG channel defects [31]. Furthermore, patients with SCNSA defects are more likely to have syncope and sudden death during sleep and bradycardia. In the study by Schwartz et al. 1311 patients with SCNSA defects were also demonstrated to have significant shortening of their QT intervals with increased heart rate. Therefore, these patients may be best treated with sodium channel blockade using mexilitene, and if needed, atria1 pacing. Conversely, p-blockade therapy, which results in bradycardia, may actually be detrimental in long QT syndrome patients with SCNSA defects. As more is learned about the KVLQTl potassium channel function clinical trials will be developed to specifically ameliorate the pathophysiologic defects. Chromosome
7
Chromosome
11
II aVF
VS
Fig. 6. ECG recordings a HERG gene defect
from leads (chromosome
II, aVF, and 71, and one
VS in three long with a KVLQTl
QT syndrome gene defect
patients: (chromosome
one
with an SCNSA defect (chromosome 3), one I1 ). (From Moss et al. [23] with permission).
with
50
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9. Future research p
(k6)
T
,
0 Control m Mexiletine
NS
(n=7)
Fig. 7. Bar graph showing QTc values in control conditions and acute oral drug testing with mexilitene in patients with SCNSA defects (LQT3, n = 6) and in patients with HERG defects (LQT2, n = 7) (from Schwartz et al. [31] with permission).
Prior studies of the effectiveness of the p-blockade in long QT syndrome patients [4] may have been composed of a relatively large number of patients of KVLQTl defects, since mutations of this gene appear to account for 50% of the long QT syndrome cases. Therefore, P-blockade may be a relatively effective treatment in these patients. Whether or not potassium channel openers or other more specific therapy will be more effective remains to be determined. 8. Genetic testing for the long QT syndrome
These clinical trials indicate that appropriate therapy for a particular patient may strongly depend on which LQT gene is responsible for the syndrome in their family. This will depend on genetic testing, either by linkage analysis in large families or mutation detection in small families and sporadic cases. Once a family’s particular genetic defect has been identified, it can easily be traced through the family to determine who is affected and therefore who is at risk for sudden death. Due to the marked overlap in the corrected QT intervals between normal individuals and patients affected with the long QT syndrome, not all individuals at risk for sudden death can be identified by electrocardiographic testing, even when done during exercise. Furthermore, these apparently normal individuals can pass the disorder onto their offspring who may be more severely affected but may not be screened with an ECG if their parents were not identified as having a genetic mutation. In our studies we have confirmed the findings of Vincent et al. [17] that there are clinically normal individuals (QTc intervals of 0.41-0.42 s) with genetic mutations who passed the genetic defect onto their offspring (unpublished observations). In these families, the genetic defect was not detected until one of the children had syncope or a cardiac arrest.
Long QT syndrome patients with the exact same genetic defect can have very different QT interval measurements and symptomatology. This observation suggests that other genes may strongly interact with the LOT genes, influencing their clinical expression. These genes may be extremely important for (1) understanding the spectrum of symptoms in long QT syndrome patients, (2) identifying other genes that may be vital to myocardial repolarization, and (3) suggesting new avenues for therapy, not only in patients with the long QT syndrome but with other ventricular arrhythmias as well. For patients with HERG and KVLQTl potassium channel defects there is a 2:l ratio of females to males diagnosed with clinical long QT syndrome. For both of these genes, mutant genes should be inherited by an equal number of males and females. Therefore, there must be a sex-specific influence on the expression of these two genes. Determining why females are more likely to diagnosed with the long QT syndrome and are more likely to be symptomatic even at comparable QTc measurements to males [32] may be central to understanding what factors modify expression of the LQT genes. Now that the genetic heterogeneity of this syndrome is recognized, it is critical that the long QT syndrome be viewed as five (or more> distinct genetic disorders that share a similar electrocardiographic feature, namely, QT interval prolongation [33]. These patients may require very different therapy depending on which gene is responsible for their QT prolongation. Based on preliminary studies, patients with sodium channel (SCNSA) appear to have QT interval shortening when treated with mexilitene and when increasing their heart rate. ,&blockade therapy in these patients may be detrimental since they tend to have increased episodes of sudden death during sleep and bradycardia. Patients with HERG defects should avoid hypokalemia and may be helped by high potassium diets and, potentially, potassium channel openers. Based on these preliminary clinical trials, genetic testing may be a critical step in determining the appropriate course of therapy for patients with long QT syndrome. Continued characterization of the genes responsible for this disorder may improve the diagnosis and treatment not only of patients with the long QT syndrome but also of patients with other ventricular arrhythmias, including those occurring secondary to myocardial ischemia/infarction. Acknowledgements
I would like to thank Dr. MacDonald Dick II for thoughtful review of this manuscript. This work has
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been supported by a grant (MO1 RR000421 from the General Clinical Research Center at the University of Michigan. References
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