Notched T Waves on Holter Recordings Enhance Detection of Patients With LQT2 (HERG) Mutations
Background—The 2 genes KCNQ1 (LQT1) and HERG (LQT2), encoding cardiac potassium channels, are the most common cause of the dominant long-QT syndrome (LQTS). In addition to QT-interval prolongation, notched T waves have been proposed as a phenotypic marker of LQTS patients.
Methods and Results—The T-wave morphology of carriers of mutations in KCNQ1 (n=133) or HERG (n=57) and of 100 control subjects was analyzed from Holter ECG recordings. Averaged T-wave templates were obtained at different cycle lengths, and potential notched T waves were classified as grade 1 (G1) in case of a bulge at or below the horizontal, whatever the amplitude, and as grade 2 (G2) in case of a protuberance above the horizontal. The highest grade obtained from a template defined the notch category of the subject. T-wave morphology was normal in the majority of LQT1 and control subjects compared with LQT2 (92%, 96%, and 19%, respectively, P<0.001). G1 notches were relatively more frequent in LQT2 (18% versus 8% [LQT1] and 4% [control], P<0.01), and G2 notches were seen exclusively in LQT2 (63%). Predictors for G2 were young age, missense mutations, and core domain mutations in HERG.
Conclusions—This study provides novel evidence that Holter recording analysis is superior to the 12-lead ECG in detecting G1 and G2 T-wave notches. These repolarization abnormalities are more indicative of LQT2 versus LQT1, with G2 notches being most specific and often reflecting HERG core domain missense mutations.
Genetic studies have shown that long-QT syndrome (LQTS) is a primary electrical disease caused by mutations in specific ion channels.1 Five LQTS genes have been identified, including the potassium channel genes HERG; KCNH2 (LQT2),2 which encodes the α-subunit of the channel that underlies the rapidly activating delayed rectifier potassium current IKr; and KCNQ1 (LQT1),3 which encodes the slowly activating delayed rectifier potassium channel IKs. Most mutations have been identified in the core domain, constituted by the transmembrane domains and the pore, of KCNQ1 and HERG. Mutations in the N- and C-terminal domains, however, have also been reported.4 5 6 7 LQTS patients exhibit QT prolongation on the ECG and are at risk of arrhythmogenic syncope and sudden death. In addition to duration, T-wave morphology is often abnormal, and notched T waves have been included in diagnostic criteria.8 Furthermore, this pattern has been associated with a poor prognosis.9 Moss et al10 reported that patients with ion channel defects often display different T-wave patterns. LQT3 patients show distinctive T waves of late onset, whereas LQT1 or LQT2 patients display broad-based or low-amplitude T waves, respectively. We suggested that notched T waves on ECGs and Holter recordings in family members with a HERG A561T mutation were linked to the presence of that mutation.11 Paying attention to these ECG aspects, we reviewed all the ECG and Holter recordings of our carriers of mutations in KCNQ1 and HERG to determine the relevance of the T-wave anomalies for diagnostic and genetic testing purposes.
Clinical evaluation and blood samples were obtained after written consent in accordance with the guidelines set down by the Comité Consultatif de Protection des Personnes dans la Recherche Biomédicale de la Pitié-Salpêtrière (Paris, France). Subjects underwent detailed clinical and cardiovascular examination, including a 12-lead ECG and a 24-hour Holter recording, which were analyzed by 2 experienced observers (J.M.L., I.D.) without knowledge of the localization of the mutation. QT interval was measured on the surface ECG in lead II or V5 and corrected for heart rate (QTc) according to Bazett’s formula. All probands of the families had syncope or documented torsade de pointes and a QTc>440 ms. Diagnosis was confirmed by mutation identification. Thirty-two LQT1 and 21 LQT2 families formed the basis of this study.
After identification of the mutation by polymerase chain reaction–single-strand conformation polymorphism analysis and direct sequencing of polymerase chain reaction products,5 7 all the family members were screened and separated into carriers and noncarriers of the mutation. The mutated nucleotides for the 9 new HERG missense mutations were G131T, G172A, C340T, C1001T, G1704T, A1724G, G1825A, G1862A, and G2879A for C44F, E58K, P114S, P334L, W568C, E575G, D609N, S621N, and S960N, respectively.
Twenty-Four-Hour Holter Recordings
All subjects underwent a 24-hour Holter ECG recording (Delmar Avionics) with an XYZ configuration. Analog data from leads X (ECG lead I equivalent) and Y (ECG lead V2 equivalent) were digitized at 200 Hz and analyzed by the Elatec Holter System (Ela Medical). QRS-T templates were obtained as previously described.12 Briefly, QRS-T complexes were classified according to the value of the preceding RR interval every 25 ms and then averaged. To provide a wide range of RR intervals for analysis and to take into account a possible role of heart rate or the autonomic nervous system in the occurrence of notched T waves, 2 different periods of analysis were defined according to the mean hourly heart rate and the patient’s diary. The first period consisted of the 8 consecutive diurnal hours with fastest heart rate and the second of the 4 consecutive sleeping hours with lowest heart rate. The totality of these templates was considered for morphological analysis. Notched T waves, also called bifid waves or humps, were defined as a bulge or protuberance just beyond the apex or on the descending limb of an upright T wave.9 13 Classification of T-wave morphology was adapted from Lehmann et al,13 with T waves assigned to 1 of 3 categories: grade 0, no deflection (G0); grade 1, a perceptible bulge at or below the apex, whatever the amplitude (G1); and grade 2, a distinct protuberance above the apex (G2)9 13 (Figure 1a⇓). The highest grade observed over the range of templates for an individual defined the notch category for that individual. QRS-T–wave templates were considered to be inverted if the T wave was negative and to be broad-based when the base of the T wave was >50% of the QT interval duration (working definition).
Univariate analyses were carried out with the t test for quantitative data. To take into account familial correlations within the same family, multivariate analyses were carried out by the hierarchical mixed models (SAS Proc Mixed) for quantitative data. A randomized familial effect was introduced at the intercept level. Multivariate analyses were performed by generalized estimating equations (SAS Proc Genmod) for qualitative data with the logit function. Results from quantitative data analyses are expressed as mean±SD and from qualitative data as odds ratios (OR) and their 95% confidence interval (95% CI). A significant level of P<0.05 was required for an explanatory variable to remain in the models.
A total of 190 mutation carriers in KCNQ1 (n=133) and HERG (n=57) and 100 noncarrier control subjects from the 53 families with LQTS were studied. Holter recordings were obtained in every subject, and ECG recordings were present in all but 7 cases (LQT1, n=127; LQT2, n=56; controls, n=100). LQT patients were similar with regard to female sex, prior cardiac events, and β-blocking therapy. LQT2 patients were significantly younger than LQT1 patients, with 42% <15 years old versus 28%, respectively. QTc intervals were longer in LQT2 patients despite a similar proportion of core domain mutations (47% versus 62%, respectively, P<0.06). The control population was similar in terms of age (30% of subjects <15 years) and sex (Table 1⇓).
Twenty-three KCNQ1 missense mutations were identified in 133 subjects. Most of them have been reported previously by us or other groups.4 These missense mutations changed conserved amino acid residues. Nineteen were found in the core domain (25 families, n=83) and 2 in the C-terminal domain (4 families, n=43). In addition, 2 frameshift mutations were evidenced in 3 families (n=7).
Eighteen HERG mutations were identified in 57 subjects: 15 missense mutations and 3 mutations leading to a premature truncation of the protein. Among the missense mutations, 8 were in the core domain (11 families, n=27) and 7 in the C- and N- terminal domains (7 families, n=23) (Figure 2⇓). Six have been described previously, and 9 were novel: C44F, E58K, P114S, P334L, W568C, E575G, D609N, S621N, and S960N. All these mutations were absent in 150 unrelated control subjects. In addition, a novel 1-bp insertion (1009insT, n=2) in the N-terminal domain (codon 336) induced a premature stop codon at position 355 before the S1 transmembrane domain. A splicing mutation (2592+1G→A, n=3) and a 1-bp insertion (3108insG, n=2) induced a putative premature truncation of the C-terminal end of the channel.7
Holter Recordings in Control Subjects
The majority of control subjects had normal T-wave morphology (96%). G1 notched T waves were found in only 4% of the patients, all older than 15 years. No G2 notches were detected.
Holter and ECG Recordings in LQT1 Patients
Figure 1b⇑ displays the typical T-wave morphology obtained in leads X and Y at all cycle lengths in LQT1 patients. In the majority of cases, T-wave morphology was normal (122 of 133, 92%). G1 notches were found in only 11 of 133 (8%), with only 5 of 11 patients <15 years old (Table 1⇑). No correlation was found between notch grade and age, sex, QTc, type or localization of the mutation, treatment, and prior cardiac event (Figure 3a⇓). No G2 was evidenced in any heart rate or period analyzed. The T wave was broad-based in all cases but 1 (132 of 133) and was always positive in both leads X and Y.
Holter Recordings in LQT2 Patients
A notched T-wave pattern (G1 or G2) was found in lead X in a high proportion of LQT2 patients (46 of 57, 81%) and was associated with inverted T waves in lead Y in 83% of them (38 of 46) (Figure 1c⇑). G1 notches were present in 10 of 57 (18%), with only 2 patients <15 years old. G1 notches were relatively more frequent in LQT2 (18% versus 8% [LQT1] and 4% [control], P<0.01). G2 notches were found in 36 of 57 (63%) (Table 1⇑). Furthermore, taking into account the type and localization of the mutation, these abnormalities were significantly more frequent in case of core missense mutations in the core domain, with a G2 pattern evidenced in 25 of 27 (92%) of the carriers (Table 2⇓, Figures 1c⇑ and 4⇓). A consistent G2 pattern at all cycle lengths of the Holter recording was found in the majority of patients carrying core mutations (20 of 27, 74%).
In contrast, in LQT2 patients with missense mutations localized in the N- or C-terminal domain or frameshift mutations, T-wave anomalies were significantly less frequent (Table 2⇑, Figure 5⇓). In addition, among the 11 patients with G2, only 5 had a consistent G2 pattern at all cycle lengths.
Grade classification was independent of QTc duration (Figure 3b⇑). From univariate analysis, younger age, missense mutation, and core domain mutation were significant predictors of the presence of G2, at variance from prior cardiac events, β-blocking therapy, and female sex (Table 3⇓). At multivariate analysis, only a core missense domain mutation (OR 30.1; 95% CI 3.8 to 238.8; P<0.002) and young age (OR 8.8; 95% CI 1.7 to 46.1; P<0.02) remained significantly and positively associated with the presence of G2 on the Holter recordings in lead X.
Taking into account the control group and the LQT1 and LQT2 populations, the specificity of G2 was very high for LQT2 (100%), with a sensitivity of 63% and a positive predictive value of 100%. In contrast, G1 and G2 T waves gave a lower specificity (94%) and positive predictive value (75%) but a higher sensitivity (80%) for LQT2.
Comparison Between ECG Findings and Holter Recordings in LQT2 Patients
On the surface ECG, the T-wave morphology was normal (G0) in 43% of the LQT2 patients (n=24 of 56). G1 (n=6, 11%) and G2 (n=26, 46%) T waves were found mainly in the 3 left precordial leads. A subset of the 24 patients with G0 at baseline ECG had higher grade classification at Holter recording: G1 (n=7) or G2 (n=11). In addition, 4 of 6 patients with G1 at ECG had G2 morphology at Holter. Altogether, G2 notches were evidenced at Holter recording in 15 of 30 patients who did not have G2 at surface ECG, thus demonstrating the superiority of Holter recording over ECG for detection of T-wave notches.
This retrospective study provides evidence that Holter recording, by exploring a wide range of cycle lengths, is superior to the 12-lead ECG in detecting G1 and G2 T-wave notches. These repolarization abnormalities are more indicative of LQT2 versus LQT1, with G2 notches being most specific and often reflecting HERG core domain missense mutations. Furthermore, G1 notches have potential diagnostic value as a clue for LQT2 mutations and are specially helpful in those individuals with N- or C-terminal missense mutations or frameshift mutations, where the prevalence of G2 was lower.
In LQTS, in addition to QT-interval prolongation, a notched T-wave constitutes one of the most striking ECG abnormalities outlined since the initial reports9 13 14 15 and was a diagnostic criterion.8 We previously reported consistent notched T waves on ECG and Holter recordings in all affected members of a family with a HERG mutation in the core domain, associated with deeply inverted T waves in left precordial leads.11 After identification of 17 other LQT2 mutations and numerous LQT1 mutations, we reanalyzed these various ECG aspects.
In the 133 LQT1 genotyped patients of our population, broad-based T waves were found repeatedly on both ECG and Holter analysis, in agreement with the first report on ECG findings by Moss et al.10 These patients had, for the most part, a normal T-wave morphology. We did not find any patient with G2 even in case of prolonged QTc interval and mutations in the core domain of KCNQ1. G1 notches were rarely evidenced unassociated with an inverted T wave in older patients and could merely reflect very subtle cardiac anomalies related to age.
For LQT2 patients, we confirmed our initial findings for an important number of familial cases, and we further refined the LQT2 phenotype through Holter recordings. Indeed, G2 T waves on Holter recordings were frequently found in LQT2 patients carrying a missense mutation. Furthermore, G2 notches were detected at all cycle lengths in most of the cases of missense mutations in the core domain of HERG. Compared with surface ECG, Holter recording provides a larger number of QRS-T complexes, which are obtained at a wide range of cycle lengths, and the use of averaging allows us to reduce the signal-to-noise ratio and to improve the quality of the ECG tracing. In our study, G2 at all cycle lengths was a strong predictor of missense mutations in the core domain of HERG. We studied only a limited number of mutations, however, and although our data are highly suggestive of G2 notches being more likely to manifest with core missense mutations and G1 notches with other types and localizations of mutations, further studies with a larger number of subjects are needed to confirm our preliminary results. In addition, the absence of Holter data from LQT3 subjects and rare variants of LQTS is a limitation to our findings.
It is noteworthy that G2 notches were less frequent in patients with frameshift mutations or missense mutations in the N- and C-terminal domains and could be detected on Holter recordings only at certain heart rates. For these particular cases, Holter recording, which explores a wide range of heart rates, improved detection of notched T-waves compared with ECG. To better characterize such patterns obtained on Holter recording, however, quantitative and more refined methods, such as wavelet analysis, may be more informative.
The strong association of this phenotypic trait with a genetic defect was foreseen by Lehmann et al,13 who, in a study of nongenotyped LQTS families, identified notched T waves in a significant proportion of blood relatives with prolonged or even borderline (0.42 to 0.46 second) QTc intervals. We found it interesting to report that the notched T-wave pattern in lead X is associated with inverted T waves in lead Y in a large proportion of the LQT2 patients. This combination was never found in our LQT1 patients or in control subjects and could contribute to identification of true G1 notches in LQT2 versus false-positive G1 notches in LQT1 and control subjects.
From a practical point of view, consistent G2 T waves at all heart rates were found in 74% of LQT2 patients with core missense mutations and in 17% in the other group and can be detected on all types of Holter analyzer, provided that signal averaging according to heart rate is available. Holter recordings of related family members, especially children, can often be helpful to improve G2 T-wave detection.
Notched T Waves and HERG Mutations
Mutations in HERG lead to a decreased contribution of IKr to cardiac repolarization by multiple mechanisms.16 It is conceivable that missense mutations in functional domains of HERG, like the core domain, induce a major decrease in IKr due to dominant-negative effects and/or altered gating properties and produce a more severe ECG phenotype with a greater prolongation of QTc interval and T-wave morphology alteration, such as G2 notches. In contrast, frameshift mutations or missense mutations in the N- or C-terminal domains, which may result in protein processing defects leading to failure of the channel protein to undergo normal transport to the cell surface, induce a milder phenotype with less consistent morphological alterations of the ECG and milder QTc prolongation. In our patients, G2 notched T waves were found in only 11 of 30 such cases, and QTc prolongation was milder (471±31 ms).
Notched T Waves in HERG and Electrophysiological Mechanism
Because the presence of notched T waves appears to be an indicator of genetic defects in HERG, one can speculate that this pattern is related to IKr decrease. In an arterially perfused wedge of canine ventricle allowing the simultaneous recording of transmural ECG and transmembrane action potentials, d-sotalol and chromanol 293B were used to mimic LQT2 and LQT1 defects, respectively. In the LQT2 model, the IKr blocker d-sotalol caused a preferential prolongation of the M-cell action potential duration and resulted in the interruption of the descending limb of the T wave,17 consistent with the high prevalence of notched T waves in our patients who were carriers of genetic defects in HERG. Such notched T waves have also been observed in vivo in humans after IKr blockers18 and in normal dogs under d-sotalol.19 In the presence of IKr block, the slope of phase 3 of the action potential is reduced, leading to much smaller transmural voltage gradients. These changes have 2 effects: a smaller T-wave amplitude and a transient crossing over of the opposing voltage gradient, thus generating the notched T wave.17 In contrast, chromanol 293B, a specific IKs blocker used to mimic defects in KCNQ1, prolonged the action potential duration homogeneously, resulting in a prolonged QT interval with a broad-based T wave characteristic of our LQT1 population and others.20
Prognosis and Notched T Waves
A higher prevalence of notched T waves on 12-lead ECGs in symptomatic than in asymptomatic nongenotyped LQTS patients has been reported.9 In that study, notched T waves were found in 62% of LQTS patients and could possibly reflect a high proportion of LQT2 patients.9 Interestingly, in our study, G2 notches were found only in the LQT2 group and were associated with a missense mutation in the core domain of HERG and a Holter recording at a young age. These conditions could represent potentially severe forms of LQT2.21 22 We could not observe any relation between prior cardiac events and the presence of G2, however, which in turn signifies no additional risk of events. This apparent discrepancy could be explained by the fact that we analyzed data from all family members (including numerous asymptomatic carriers) in the setting of an identified HERG mutation, at variance from other studies in which patients were included and characterized according to their clinical status.9 13 Thus, detection of G2 or G1 T-wave notches by Holter recordings in known or suspected LQTS patients, especially in case of a borderline QTc, suggests the presence of a HERG mutation causing LQTS. In addition, we did not evidence G2 in our LQT1 population, even in patients with a history of cardiac events.
In conclusion, this study provides novel evidence that Holter recording analysis is superior to the 12-lead ECG in detecting G1 and G2 T-wave notches. These repolarization abnormalities are more indicative of LQT2 versus LQT1, with G2 notches being most specific and often reflecting HERG core domain missense mutations. This confirms pathophysiological approaches.17 19 From a practical point of view, detection of notched T waves can be used as a diagnostic tool as well as to direct genetic testing in favor of HERG.
This work was supported by the Institut National de la Santé et de la Recherche Médicale (Progres No. 4P009D), the Association Française contre les Myopathies (AFM, France), and the Fondation Leducq. We are indebted to the family members, without whose participation this work could not have been done. We thank Drs J.M. Davy, V. Lucet, and I. Durand for their participation in clinical examinations and Drs K. Schwartz and J. Weissenburger for helpful discussions.
- Received August 14, 2000.
- Revision received October 13, 2000.
- Accepted October 16, 2000.
- Copyright © 2001 by American Heart Association
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