Increased Risk of Arrhythmic Events in Long-QT Syndrome With Mutations in the Pore Region of the Human Ether-a-go-go–Related Gene Potassium Channel
Background— The hereditary long-QT syndrome is characterized by prolonged ventricular repolarization and a variable clinical course with arrhythmia-related syncope and sudden death. Mutations involving the human ether-a-go-go–related gene (HERG) channel are responsible for the LQT2 form of long-QT syndrome, and in cellular expression studies these mutations are associated with reduction in the rapid component of the delayed rectifier repolarizing current (IKr). We investigated the clinical features and prognostic implications of mutations involving pore and nonpore regions of the HERG channel in the LQT2 form of this disorder.
Methods and Results— A total of 44 different HERG mutations were identified in 201 subjects, with 14 mutations located in the pore region (amino acid residues 550 through 650). Thirty-five subjects had mutations in the pore region and 166 in nonpore regions. Follow-up extended through age 40 years. Subjects with pore mutations had more severe clinical manifestations of the genetic disorder and experienced a higher frequency (74% versus 35%; P<0.001) of arrhythmia-related cardiac events occurring at earlier age than did subjects with nonpore mutations. Multivariate Cox proportional hazard regression analysis revealed that pore mutations dominated the risk, with hazard ratios in the range of 11 (P<0.0001) for QTc at 500 ms, with a 16% increase in the pore hazard ratio for each 10-ms increase in QTc.
Conclusion— Patients with mutations in the pore region of the HERG gene are at markedly increased risk for arrhythmia-related cardiac events compared with patients with nonpore mutations.
Received November 26, 2001; revision received January 4, 2002; accepted January 7, 2002.
Hereditary long-QT syndrome (LQTS) is characterized by prolonged ventricular repolarization and a variable clinical course with arrhythmia-related syncope and sudden death.1 Six ion-channel genes are known to cause this syndrome,2,3⇓ with numerous LQTS mutations identified in these genes.4 Mutations involving the human ether-a-go-go–related gene (HERG) are responsible for the LQT2 form of LQTS. In cellular expression studies, HERG mutations are associated with reduction in the rapid component of the delayed rectifier repolarizing current (IKr).5 Diminution in the repolarizing IKr current contributes to lengthening of the QT interval, the electrocardiographic phenotype in LQT2 patients.
Functional HERG channels result from the coassembly of 4 HERG subunits into a tetrameric protein that is transported to the cell membrane. Each HERG subunit contains 6 membrane-spanning domains (S1 to S6) flanked by amino (N)- and carboxyl (C)-terminus regions, with the pore region extending from S5 to S6. There are 2 proposed molecular mechanisms that may account for reduced IKr current in patients with HERG mutations6,7⇓: (1) coassembly or trafficking abnormalities, in which mutant subunits either do not coassemble with normal subunits, or if they do, are not transported to the cell membrane (in either case, the net effect can result in a 50% reduction in the number of functional channels [haplotype insufficiency]); and (2) formation of defective channels involving mutant subunits, with the altered channel protein transported to the cell membrane (the dysfunctional channel can result in >50% reduction in channel function, a so-called dominant-negative effect).
The pore region provides the potassium conductance pathway in IKr channels, and most mutations involving this region are missense mutations with dominant-negative effects on IKr.7 In contrast, most mutations in the nonpore regions of HERG are associated with coassembly or trafficking abnormalities resulting in haplotype insufficiency.7 We hypothesized that mutations involving the critical pore region would result in an increased risk of arrhythmic events when compared with mutations in other regions. To test this hypothesis, we investigated the clinical features and prognostic implications of mutations involving pore and nonpore regions of the HERG channel in the LQT2 form of LQTS.
The study population consisted of 201 subjects with genetically confirmed HERG mutations derived from 51 LQT2 families enrolled in the International Long-QT Syndrome Registry.1 The HERG mutations were identified in each subject using standard genetic tests. All subjects or their guardians provided informed consent for the genetic and clinical studies.
Each subject was categorized in terms of enrollment status, ie, either as a proband (first family member identified with LQTS) or an affected family member (individual with LQTS identified during family evaluation of the proband) because probands usually presented with symptoms and thus would be expected to have higher event rates.1,8⇓ Routine clinical and electrocardiographic parameters were acquired at the time of enrollment in the International LQTS Registry and at yearly follow-up contact. Follow-up was censored at age 41 to avoid the influence of coronary disease on cardiac events. Measured parameters on the first recorded ECG included QT, QTpeak, Tpeak to Tend, and RR interval in milliseconds,9 with QTc corrected for heart rate by Bazett (QTc=QT/√RR). Repolarization measurements were made manually in 3 leads (II, V2 and V5) with quantitative QT values reported for lead V5 because the measurements were similar in all 3 leads. The presence of notched T-wave patterns on the ECG,10 also called bifid waves or humps, was assessed only in probands. LQTS-related cardiac events were defined as syncope, aborted cardiac arrest, or unexpected sudden death without a known cause. If electrocardiographic recordings were obtained during a cardiac event, they were requested from the patient’s attending physician and made part of the Registry record when received.
Genetic mutations of the HERG amino acid sequence were characterized by specific location and coding effect (missense, nonsense, splice site, or frameshift). The pore region of the HERG channel was defined as the area extending from S5 to the mid-portion of S6 involving amino acid residues 550 through 650.4 A schematic representation of the location of the mutations on the HERG potassium channel-α subunit is presented in Figure 1.
Differences in the characteristics between subjects with and without pore mutations were evaluated by standard statistical methods. The cumulative probability of a first cardiac event was assessed by the Kaplan-Meier method and the log-rank statistic. The Cox proportional-hazards survivorship model with frailty terms11 adjusting for family membership and for mutation locations across pore and nonpore regions was used to evaluate the independent contribution of clinical and genetic (pore versus nonpore mutations) factors to cardiac events from birth through age 40 years.
A total of 44 different HERG mutations were identified in 201 subjects, with 14 mutations located in the pore region, 14 in the N-terminus region, 4 in nonpore membrane-spanning segments, and 12 in the C-terminus region (Figure 1). The numbers of subjects by mutation and location are presented in Table 1.12–18⇓⇓⇓⇓⇓⇓ Missense mutations accounted for 73% of the 44 mutations. Cellular electrophysiological effects have been reported in 15 of the 44 different HERG mutations, and the functional effects of these 15 mutations are summarized and referenced in Table 1. The 15 mutations for which functional expression studies have been reported were all associated with reduced or absent IKr current: 3 pore mutations had a dominant-negative effect on IKr current, 8 N-terminus mutations had accelerated deactivation, and 4 C-terminus mutations had trafficking defects.
Of the 201 study subjects, 35 had mutations residing in the pore region, and 166 had mutations in nonpore regions. The clinical characteristics of the study population are presented in Table 2. Subjects with pore mutations were more likely to be probands and had more severe clinical manifestations of LQTS (more therapy initiated for LQTS, a younger age at the first cardiac event, more frequent malignant arrhythmias, and an overall higher frequency of LQTS-related cardiac events) than did subjects with nonpore mutations.
The QTc intervals were longer with pore than with nonpore mutations. Other quantitative repolarization characteristics, including T-wave amplitude and T-wave duration, were similar in those with pore and nonpore mutations (data not shown). The frequencies of torsade de pointes and notched T-wave patterns were twice as high in those with pore than in those with nonpore mutations (Table 2).
Clinical Course by Mutation Location
Kaplan-Meier cumulative cardiac event curves from birth through age 40 years for those with mutations involving the pore, N-terminus, and C-terminus regions are presented in Figure 2. The difference in outcome by mutation location is significant (P<0.0001), with the risk for first cardiac events dominated by subjects with pore mutations.
Because the clinical course of subjects with C- and N-terminus mutations was similar (Figure 2), subjects with these 2 mutations plus the 4 subjects who had nonpore membrane-spanning mutations were combined into a nonpore group and were compared with the pore group in the multivariate Cox survivorship analyses. Significant interactions (P<0.025) exist between mutation location (pore versus nonpore) and both enrollment status (proband or affected family member) and QTc duration. These interactions are presented in Table 3, with QTc centered about 500 ms for ease of interpretation. Among subjects with nonpore mutations, probands had a 4.7-fold greater risk for first cardiac events than affected family members. Subjects with pore mutations had a considerably higher risk, with similar high risk among affected family members and probands (hazard ratios of 10.7 and 11.7, respectively, compared with family members with nonpore mutations). QTc had no effect on risk among subjects with nonpore mutations but did modulate the risk among subjects with pore mutations, with a 16% increase in risk per 10-ms increase in QTc. The cumulative probability of a first cardiac event in each of the 4 risk groups identified in the Cox model was estimated; by age 40 years, 30% of the lowest-risk group (family members with nonpore mutations) would have a cardiac event, whereas >90% of those with pore mutations would experience a cardiac event.
The major finding from this study is that LQT2 patients with mutations in the pore region of the HERG gene are at considerably greater risk for arrhythmia-related cardiac events than are patients with nonpore mutations. This increased risk with pore mutations persists throughout the first 40 years of life, with the risk modulated by QTc duration.
Cellular electrophysiological effects of only a small percentage of known HERG mutations have been reported in the literature. The in vivo clinical findings of increased cardiac events in patients with pore mutations are consistent with the known in vitro electrophysiological effects of the reported HERG mutations, with pore mutations having a greater negative effect on IKr current than nonpore mutations.6,7⇓ The interaction involving pore mutations and QTc indicates that the clinical risk associated with pore mutations is higher with longer QTc intervals, a finding suggesting that the effects of pore mutations on IKr, QTc, and risk are more complicated than originally thought. Of interest, Mitcheson et al19 recently identified the inner cavity of the HERG pore as the vulnerable structural site for drug-induced LQTS.
In the present study, patients with pore mutations received more LQTS-related therapies than did patients with nonpore mutations—almost certainly because they were experiencing more clinical events. High-risk individuals with HERG pore mutations and prolonged QTc intervals may benefit from an implanted cardioverter-defibrillator, but we are reluctant to make a definitive recommendation until the findings are corroborated by other investigators.
The present study involved a limited number of different HERG mutations and only a small number of subjects with each specific mutation. Thus, we had limited power to show any risk heterogeneity within designated pore and nonpore channel regions. Missense mutations made up 94% of the pore mutations, thus limiting our ability to evaluate risk by mutation type within the pore region. More patients per mutation and a greater spectrum of HERG mutations are needed to examine the clinical risks associated with mutations involving the selectivity filter, voltage-sensing regions, and regulators of protein conformation.
Among subjects with nonpore mutations, probands were at greater risk than affected family members (Table 3), probably because of enrollment selection bias, as previously demonstrated.1,8⇓ Probands usually were brought to medical attention because of a cardiac event, whereas affected family members were identified during family screening procedures and were more likely to be asymptomatic at the time of enrollment in the Registry.
The present findings of risk heterogeneity between pore and nonpore mutations add to the expanding phenotype-genotype associations being reported in LQTS for mutations at different locations within specific cardiac ion-channel genes. For example, mutations in different locations of the SCN5A sodium-channel gene have been associated with 3 different arrhythmogenic cardiac disorders: the LQT3 form of LQTS,20 Brugada syndrome,21 and conduction defects.22 In this study, we have shown that mutations in different locations of the HERG potassium-channel gene are associated with different levels of risk for arrhythmic cardiac events in LQT2, with the greatest risk related to mutations in the critical pore region of the channel.
This study was supported in part by research grants HL-33843 and HL-51618 from the National Institutes of Health, Bethesda, Md.
This article originally appeared Online on January 28, 2002 (Circulation. 2002;105:r7–r12).
- ↵Moss AJ, Schwartz PJ, Crampton RS, et al. The long-QT syndrome: prospective longitudinal study of 328 families. Circulation. 1991; 84: 1136–1144.
- ↵Splawski I, Shen J, Timothy KW, et al. Spectrum of mutations in long-QT syndrome genes: KVLQT1, HERG, SCN5A, KCNE1, and KCNE2. Circulation. 2000; 102: 1178–1185.
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- ↵Moss AJ, Zareba W, Hall WJ, et al. Effectiveness and limitations of beta-blocker therapy in congenital long-QT syndrome. Circulation. 2000; 101: 616–623.
- ↵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.
- ↵Therneau TM, Grambsch PM. Modeling Survival Data: Extending the Cox Model. New York, NY: Springer; 2000.
- ↵Kagan A, Yu Z, Fishman GI, et al. The dominant negative LQT2 mutation A561V reduces wild-type HERG expression. J Biol Chem. 2000; 275: 11241–11248.
- ↵Huang FD, Chen J, Lin M, et al. Long-QT syndrome–associated missense mutations in the pore helix of the HERG potassium channel. Circulation. 2001; 104: 1071–1075.
- ↵Nakajima T, Furukawa T, Tanaka T, et al. Novel mechanism of HERG current suppression in LQT2: shift in voltage dependence of HERG inactivation. Circ Res. 1998; 83: 415–422.
- ↵Zhou Z, Gong Q, Epstein ML, et al. HERG channel dysfunction in human long-QT syndrome: intracellular transport and functional defects. J Biol Chem. 1998; 273: 21061–21066.
- ↵Chen J, Zou A, Splawski I, et al. Long-QT syndrome-associated mutations in the Per-Arnt-Sim (PAS) domain of HERG potassium channels accelerate channel deactivation. J Biol Chem. 1999; 274: 10113–10118.
- ↵Ficker E, Thomas D, Viswanathan PC, et al. Novel characteristics of a misprocessed mutant HERG channel linked to hereditary long-QT syndrome. Am J Physiol Heart Circ Physiol. 2000; 279: H1748–H1756.
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