KVLQT1 C-Terminal Missense Mutation Causes a Forme Fruste Long-QT Syndrome
Background KVLQT1, the gene encoding the α-subunit of a cardiac potassium channel, is the most common cause of the dominant form of long-QT syndrome (LQT1-type), the Romano-Ward syndrome (RWS). The overall phenotype of RWS is characterized by a prolonged QT interval on the ECG and cardiac ventricular arrhythmias leading to recurrent syncopes and sudden death. However, there is considerable variability in the clinical presentation, and potential severity is often difficult to evaluate. To analyze the relationship between phenotypes and underlying defects in KVLQT1, we investigated mutations in this gene in 20 RWS families originating from France.
Methods and Results By PCR-SSCP analysis, 16 missense mutations were identified in KVLQT1, 11 of them being novel. Fifteen mutations, localized in the transmembrane domains S2-S3, S4-S5, P, and S6, were associated with a high percentage of symptomatic carriers (55 of 95, or 58%) and sudden deaths (23 of 95, or 24%). In contrast, a missense mutation, Arg555Cys, identified in the C-terminal domain in 3 families, was associated with a significantly less pronounced QT prolongation (459±33 ms, n=41, versus 480±32 ms, n=70, P=.0012), and significantly lower percentages of symptomatic carriers (7 of 44, or 16%, P<.001) and sudden deaths (2 of 44, or 5%, P<.01). Most of the cardiac events occurring in these 3 families were triggered by drugs known to affect ventricular repolarization.
Conclusions Our data show a wide KVLQT1 allelic heterogeneity among 20 families in which KVLQT1 causes RWS. We describe the first missense mutation in the C-terminal domain of KVLQT1, which is clearly associated with a fruste phenotype, which could be a favoring factor of acquired LQT syndrome.
Genetic studies have shown that the autosomal dominant form of LQTS, the RWS, is a genetically heterogenous disease caused by mutations in at least four genes (LQT1 to LQT4).1 Three of them have been identified and encode ion channel subunits: KVLQT1, a potassium channel that accounts for most of the RWS cases (LQT1 type on 11p15.5),2 HERG, the potassium channel that underlies IKr (LQT2 on 7q35-36), and SCN5A, the cardiac sodium channel (LQT3 on 3p21).1 KVLQT1 also causes the recessive form of LQTS, the JLNS.3,4 RWS and JLNS have in common an abnormal repolarization visualized as a prolonged QT interval on the surface ECG, recurrent syncopes, and risk of sudden death.5 In JLNS, homozygous carriers of a KVLQT1 mutation have, in addition, a congenital deafness, whereas the heterozygous carriers are generally asymptomatic.3
To increase our understanding of the phenotype/genotype relationship of patients with RWS, we have focused our analysis on KVLQT1 mutations. So far, 15 mutations localized in the transmembrane domains have been identified in families from the United States, South Africa, and Japan.2,6-8 In this study, we identified the mutations causing RWS in 20 families originating from France, and we report the clinical data for each of them. A novel mutation in the C-terminal domain, identified in 3 families, was clearly associated with a mild clinical presentation and worsened under ventricular repolarization prolonging drugs.
Clinical evaluation and blood samples were obtained after written informed consent in accordance with the guidelines set down by the Comité Consultatif de Protection des Personnes dans la Recherche Biomédicale du Groupe Hospitalier de la Pitié-Salpêtrière (Paris). Subjects underwent detailed clinical and cardiovascular examination, including a 12-lead ECG and 24-hour Holter monitoring. The QT intervals were measured on the ECG in lead II or V5 and corrected for heart rate (QTc, in ms) by use of the Bazett formula.9 Subjects were considered to be affected when they presented with (1) syncopes or documented torsades de pointes, (2) QTc>460 ms, or (3) QTc>440 ms associated with bradycardia or abnormal T-wave pattern.10 All families originated from France, even the 2 who were living in the United States (9364) and Reunion Island (2956).
SSCP Analysis and Direct Sequencing of PCR Products
Primers designed by Wang et al2 were used to amplify the KVLQT1 region between S2 and S6 transmembrane segments from genomic DNA. The primer set (DL-DR) was previously designed to amplify part of the KVLQT1 C-terminal domain from genomic DNA.3 SSCP analysis and sequencing of the PCR products were performed as described elsewhere.3 For each abnormal SSCP pattern, the cosegregation with the disease was studied in the family. All the mutations were screened on 200 chromosomes from unrelated control subjects.
Data are given as mean±SD. Mean QTc values were compared by unpaired Student’s t test, with P<.05 considered statistically significant, and percentages by χ2 test.
By PCR-SSCP analysis, we identified 16 KVLQT1 missense mutations, which were absent in the control subjects (Table⇓). These mutations changed amino acid residues, which are highly conserved throughout evolution, from Caenorhabditis elegans to Homo sapiens.11 Eleven of them were novel, and 5 have been reported previously.2,6-8 Mutations were localized (1) in the P and S6 domains and in the S2-S3 and S4-S5 cytoplasmic loops, in agreement with the first report of Wang et al,2 and (2) in the C-terminal region where we previously identified the first JLN mutation, which was an insertion-deletion mutation leading to a premature stop codon.3
Syncopes and documented sudden deaths were taken into account to evaluate the frequency of symptoms for each mutation (Table⇑).
The highest frequency of cardiac events was detected among carriers of mutations in the transmembrane domains, especially in the S4-S5 loop (67%, 14/21), the P domain (66%, 23/35), and the S6 domain (54%, 13/24). For most of the cases, the first syncope was triggered by physical activity before age 10 years, and sudden deaths occurred in untreated patients with a history of recurrent syncopes.
The mean QTc value was determined for carriers of mutations located in the S2-S3 loop (476±25 ms, n=12), the S4-S5 loop (490±46 ms, n=11), the P domain (483±32 ms, n=27), and the S6 domain (472±27 ms, n=20). No significant difference was found between these subgroups. The mean QTc value for all the carriers of a mutation in the transmembrane domains was 480±32 ms (n=70).
In contrast to the mutations in the transmembrane domains, the C-terminal mutation, Arg555Cys, which was identified in 3 families, clearly caused a milder phenotype (Figure, Table⇑). Indeed, among the 44 carriers of the Arg555Cys mutation, only 5 living subjects experienced syncope, and 2 died suddenly (Figure). One of the sudden deaths occurred in a 38-year-old woman treated with terfenadine (F8006, II5). The other sudden death occurred in a 35-year-old woman with a history of palpitation and dizziness. She also experienced two syncopes at the age of 18 years at arousal while she was being treated for tonsillitis (F1387, III11). Among the 5 other syncopal events in these families, 3 occurred under disopyramide (F1387, III13), meflaquine (F8006, II2), and diuretics (F1822, III2), which are drugs known to modify ventricular repolarization. It is noteworthy that no syncopal episode occurred before the age of 10 in these families, in contrast to what was observed for mutations in the transmembrane domains (Table⇑). The QTc intervals of the Arg555Cys mutation carriers were often borderline or even normal (<440 ms) (Figure) but were variable at follow-up. The QTc values were significantly lower than in the case of mutations in the transmembrane domains (459±33 ms, n=41 versus 480±32 ms, n=70; P=.0012), or in the S4-S5 loop (490±46 ms, n=11, P=.014), or in the P domain (483±32 ms, n=27, P=.0035).
This is the first report of a missense mutation located in the C-terminal region of KVLQT1 in RWS families. Numerous missense mutations in voltage-gated ionic channels have previously been reported in human diseases in addition to LQTS. Hyperkalemic periodic paralysis, paramyotonia congenita, and sodium channel myotonia are skeletal muscle sodium channel disorders (SCN4A), hypokalemic paralysis is caused by mutations in the voltage-gated calcium channel (CACNL1A3), and episodic ataxia is caused by mutations in a neuronal voltage-gated potassium channel (KCNA1).12-14 It is noteworthy that no missense mutations in the C-terminal domains of these various channels have been described so far. An important feature is that the mutation Arg555Cys is associated with a less severe phenotype than the mutations previously or currently reported occurring in the transmembrane regions of the α-subunit. Indeed, the percentage of syncopes and sudden deaths among the carriers of the C-terminal mutation is significantly lower compared with the carriers of the other mutations (16% versus 58%, P<.001). In addition, the occurrence of sudden deaths is significantly lower (5% versus 24%, P<.01). The most severe phenotypes were associated with transmembrane domain mutations located in the P domain and the S4-S5 loop. Nevertheless, the phenotype of the S2-S3 loop mutations seems less severe and could represent intermediate forms between typical and fruste forms of LQTS. These observations need to be verified in larger series.
The phenotypic differences between the Arg555Cys mutation and the other mutations are in agreement with our recent in vitro data,11 in which mutated proteins have been produced and expressed in COS cells, in the presence of IsK as described in References 15 and 1615 16 . The mutated Arg555Cys subunit formed a functional channel, although it exhibited abnormal gating properties: the voltage dependence of the activation was strongly shifted to more positive values, and deactivation kinetics were accelerated. In contrast, all other mutated subunits formed nonfunctional channels in the homozygous state.
Carriers of the Arg555Cys mutation have a minor or no QTc prolongation and are generally devoid of emotion or exercise-induced syncopes, in particular in the first decade of life, but may experience drug-induced major QTc prolongation and arrhythmias. The current standard LQT diagnostic criteria17 did not allow diagnosis of such patients. We thus used a lower cutoff value of 440 ms, but associated with two other important ECG parameters, bradycardia or abnormal T-wave pattern.10 Detection of gene carriers is clinically relevant, because severe symptoms and sudden death can be triggered in these patients by drugs known to induce ventricular repolarization prolongation.18-20 In conclusion, a follow-up of clearly identified RW mutation carriers and a better knowledge of the phenotype associated with each mutation are essential for proper patient management and counseling.
Selected Abbreviations and Acronyms
|JLNS||=||Jervell and Lange-Nielsen syndrome|
|PCR||=||polymerase chain reaction|
|SSCP||=||single-strand conformational polymorphism|
This work was supported by the Institut National de la Santé et de la Recherche Médicale (Clinical Research Network No. 494012), the Association Française contre les Myopathies, the Direction de la Recherche Clinique des Hôpitaux de Paris (PHRC P-920308), and the European Community (BMH4-CT96-0028). We are indebted to the family members, without whose participation this work could not have been done. We thank Dr L. Bailly, Dr M. Cavailles, Dr P. Chavernac, Dr P. Chevalier, Dr P. Cosnay, Dr J.C. Daubert, Dr J.M. Davy, Dr J.P. Favier, Dr F. Heitz, Dr M. Komajda, Dr D. Lamaison, Dr A. Leenhardt, Dr V. Lucet, Dr J. Poinsot, and Dr J. Victor for their participation in clinical examinations and Dr J. Barhanin for helpful discussion.
- Received June 26, 1997.
- Revision received August 8, 1997.
- Accepted August 12, 1997.
- Copyright © 1997 by American Heart Association
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