Author + information
- Received January 12, 1999
- Revision received June 24, 1999
- Accepted August 30, 1999
- Published online December 1, 1999.
- Heikki Swan, MD∗,* (, )
- Kirsi Piippo, PhD∗,
- Matti Viitasalo, MD∗,
- Päivi Heikkilä, MD†,
- Timo Paavonen, MD†,
- Katariina Kainulainen, MD∗,
- Juha Kere, MD‡,
- Pekka Keto, MD§,
- Kimmo Kontula, MD∗ and
- Lauri Toivonen, MD∗
- ↵*Reprint requests and correspondence: Dr. Heikki Swan, Helsinki University Hospital, Division of Cardiology, Department of Medicine, Haartmaninkatu 4, FIN-00290 Helsinki, Finland
The purpose of this study was to provide clinical and anatomical characteristics as well as genetic background of a malignant arrhythmogenic disorder.
An inherited autosomally dominant cardiac syndrome causing stress-induced polymorphic ventricular tachycardia and syncope in the absence of structural myocardial changes was detected in two families.
Two unrelated families with six victims of sudden death and 51 living members were evaluated. Resting and exercise electrocardiograms (ECG), echocardiography, magnetic resonance imaging (MRI), cineangiography, microscopic examination of endomyocardial biopsies and a drug testing with a class IC antiarrhythmic agent flecainide were performed. A genetic linkage analysis was carried out to map the gene locus.
Of the 24 affected individuals, 10 had succumbed with six cases of sudden death, and 14 survivors showed evidence of disease. Exercise stress test induced ventricular bigeminy or polymorphic ventricular tachycardia in affected individuals. Three children initially examined before 10 years of age developed arrhythmias during a four-year follow-up. Resting ECGs were normal in affected subjects except a slight prolongation of the QT intervals adjusted for heart rate (QTc) (430 ± 18 vs. 409 ± 19 ms, affected vs. nonaffected, p < 0.01). Administration of flecainide did not induce ECG abnormalities encountered in familial idiopathic ventricular fibrillation. Ventricular volumes, contractility and wall measurements were normal by echocardiography, right ventricular cineangiography and MRI. Histopathological examination showed no fibrosis or fatty infiltration. The cumulative cardiac mortality by the age of 30 years was 31%. The disease locus was assigned to chromosome 1q42–q43, with a maximal pairwise lod score of 4.74 in the two families combined. Only one heterozygous carrier was clinically unaffected suggesting high disease penetrance in adulthood.
A distinct cardiac disorder linked to chromosome 1q42–q43 causes exercise-induced polymorphic ventricular tachycardia in structurally normal hearts and is highly malignant. Delayed clinical manifestation necessitates repeated exercise electrocardiography to assure diagnosis in young individuals of the families.
Inherited arrhythmogenic cardiac disorders such as long QT syndrome (LQTS) (1), arrhythmogenic right ventricular dysplasia (ARVD) (2)and dilated or hypertrophic cardiomyopathies (3)are important causes of syncopal attacks and sudden death. The congenital LQTS arises from mutations of at least five different ion channel genes (4), but prolonged QT interval may also be associated with life-threatening tachyarrhythmias in cardiomyopathies and acquired heart diseases. Phenotypically and genetically heterogenic ARVD is characterized by macroscopic alterations of the right ventricle and microscopic degenerative changes of the myocardium (5–11). Hypertrophic cardiomyopathy includes genetic variants causing fatal arrhythmias with only subtle myocardial pathology (3). Thus, a multitude of diagnostic alternatives should be considered in evaluation of syncopal episodes of suspected cardiac origin.
We describe here an autosomally dominantly inherited arrhythmic syndrome, the characteristics of which do not fully match with any of the previously described disorders. The hallmarks of this syndrome include exercise-induced polymorphic ventricular tachycardia without gross or microscopic alterations of the cardiac structures, delayed clinical manifestation and a high mortality rate. We also demonstrate that the underlying gene locus is tightly linked to markers in chromosome 1q42–q43.
Two families with a total of 51 living members were evaluated (Fig. 1). Six members died suddenly at adolescence or early adulthood, and five patients were evaluated because of an exercise-related syncope. A total of 43 members of these two families underwent cardiological examination, including electrocardiographic (ECG) tests and cardiac ultrasonography. Fourteen were considered to be affected and subjected to further evaluation including cardiac catheterization and angiography, electrophysiologic testing and endomyocardial biopsy. All patients were otherwise healthy and had no medications. Informed consent was obtained from all patients and the study was carried out in accordance with the Helsinki Declaration, approved by the ethical review board.
Electrocardiographic and echocardiographic examinations
Standard ECG was recorded at rest (50 mm/s, 0.1 mV/mm). QT interval was measured from lead II and adjusted for heart rate (QTc) according to the Bazett’s formula (12). QTc’s were compared to age- and gender-matched healthy controls.
Bicycle ergometer test was performed with continuous recording of leads II, aVF, V1–3,5. Maximum achieved load and heart rate at which ventricular bigeminy first appeared were recorded. QRS morphology was used to determine the origin of ventricular arrhythmias according to criteria obtained from ventricular tachycardia mapping (13).
Signal-averaged ECG was recorded (Marquette Electronics, Milwaukee, Wisconsin) using Frank XYZ leads and a band pass filtering of 40 to 250 Hz. Simpson’s criteria were applied for late potentials (14).
A 24-hour ambulatory ECG was recorded on an outpatient basis. Mean heart rate, number of ventricular premature complexes and ventricular tachycardias (≥3 consecutive ventricular complexes) were calculated.
Acute effects of intravenous flecainide (2 mg/kg), a class IC sodium channel blocking agent, were tested in four patients to unmask the potential ECG abnormalities encountered in familial idiopathic ventricular fibrillation (Brugada syndrome) (15,16). A 12-lead ECG was obtained prior to, during and 15, 30, 60 and 120 min after the administration of the flecainide.
Echocardiography with a 2.5 MHz transducer was performed using parasternal long- and short-axis and apical four-chamber views. Left ventricular dimensions and wall thickness were measured from the M-mode recordings, Doppler echocardiography was used to exclude any valvular stenosis or regurgitation.
Phenotypic classification of family members
Patients with frequent ventricular premature complexes (>10 per min) or ventricular tachycardia during exercise stress test were considered affected. Individuals <18 years of age with normal exercise stress test findings were classified as unknown. Of the deceased family members, those whose unexplainable death had occurred under the age of 30 years were also considered affected.
Electrophysiologic testing, right ventricular cineangiography and magnetic resonance imaging (MRI)
Programmed ventricular stimulation was performed from the right ventricular apex and outflow tract using two extrastimuli with and without infusion of epinephrine (50 μg/kg/min). Right ventriculography in 30° right and 60° left anterior oblique views and coronary arteriography were carried out in 11 adult affected subjects. Right ventricular wall abnormalities were assessed according to Daliento (17), and end-diastolic and end-systolic volumes were calculated according to Ferlinz (18). Cineangiograms of 10 patients with paroxysmal supraventricular tachycardia served as a reference material in a blinded analysis by two cardiologists independently. In addition, six patients underwent MRI. The MRI heart studies were performed using a 1.5 Tesla superconducting Siemens unit (Magnetom Vision, Siemens, Erlangen, Germany), a phased-array body coil and ECG triggering. T1-weighted spin echo axial and sagittal images were used to analyze the right ventricular anterior wall and a gradient-echo axial and sagittal cine sequence was used to evaluate the possible right ventricular dyskinesia. A breath-hold technique was combined to improve the image quality.
Right ventricular endomyocardial biopsy
Right ventricular endomyocardial biopsy specimens were obtained from 12 patients. Biopsies from patients less than six weeks after heart transplantation and no signs of rejection or infection were used for reference. Formalin-fixed, paraffin-embedded tissue samples were stained with hematoxylin-eosin, van Gieson and Masson’s trichrome. Extent of myocardial fibrosis (van Gieson and Masson staining), lipid degeneration, nuclear changes and interstitial inflammation were scored blindly by two pathologists. Inflammation was confirmed by immunohistochemistry using a leukocyte common antigen monoclonal antibody (DAKO, Denmark). Scores from 0 to 3, (0 indicating no and 3 advanced changes) were used.
Comparisons between groups were performed with two-tailed Student ttest for normally distributed parameters or Mann-Whitney Utest for nonnormally distributed parameters. Data are expressed as mean ± SD. A p value <0.05 was considered significant. Kaplan-Meier survival curves were constructed to assess the cardiac event-free survival and mortality.
Genetic linkage studies
Nine affected and 16 unaffected individuals from Family 1, and four affected and six unaffected individuals from Family 2 were included in the genetic linkage study (Fig. 1). Genomic DNA was extracted from blood samples. Highly polymorphic microsatellite markers from the Cooperative Human Linkage Center (http://www.chlc.org), Généthon (19), Genetic Location Database (20)and Whitehead Institute (http://www.genome.wi.mit.edu) databases were used in genotyping. The 31 markers used to scan chromosome 1 were approximately evenly spaced with 10 cM intervals covering the whole chromosome 1 (283 cM) and had a mean heterozygosity index of 0.77.
Linkage analyses were performed with MLINK and ILINK options of FASTLINK v. 3.0P package (21,22)and GENEHUNTER (23). The MLINK was used for calculating the two-point lod scores at different recombination fractions while the ILINK was used to define the maximal likelihood estimate of theta. For pairwise lod score calculations, equal marker allele frequencies were assumed. The multipoint scores were calculated for both parametric and nonparametric linkage analysis with GENEHUNTER using allele frequencies from the data. A penetrance of 0.9 for the disease and an affected allele frequency of 0.0002 were assumed. Multipoint lod scores were calculated in “affected only” mode in pedigree 1 while in two-point calculations the whole pedigree was included. Power calculations were performed using SIMLINK v. 4.11 (24,25).
Direct sequencing of a candidate gene TWIK-1
The gene coding for the potassium channel TWIK-1 (human two P-domain K+channel) (26)was directly sequenced by reverse transcriptase polymerase chain reaction (PCR) using lymphocyte RNA as template. In brief, RNA was isolated from fresh lymphocytes using RNAzolB (TEL-TEST, Friendswood, Texas) and was transcribed to cDNA with SuperScript™II Reverse Transcriptase (Gibco BRL, Life Technologies, Paisley, United Kingdom). The PCR was performed using cDNA as a template in standard conditions.
Characteristics of the families
In two families, there were 14 subjects in total alive who exhibited exercise-induced polymorphic ventricular arrhythmias. In addition, six family members were considered affected because of sudden death at the age of 16 to 28 years; four of them had experienced exercise- or anxiety-induced syncope prior to death. Autopsy findings were unremarkable in all six succumbed individuals. Moreover, in the generation II of Family I, there were four deceased obligate gene carriers (Fig. 1)who had died at an average age of 48 ± 9 years. Taken together, a total of 24 individuals in these two pedigrees were classified as affected (Fig. 1).
The clinical characteristics of the 14 living affected individuals are summarized in Table 1. Nine of them had experienced syncope associated with physical or emotional stress. Their age at first symptoms was 21 ± 10 years (range, 9 to 34 years). All children aged less than nine years were asymptomatic.
Resting ECG indicated sinus rhythm and normal atrioventricular conduction, QRS-complex morphology, ST segment and T-wave pattern in all patients. QTc intervals of the affected individuals (430 ± 18 ms) were slightly longer than those of controls (409 ± 19 ms, p < 0.01). Intravenous administration of flecainide did not result in development of right bundle branch block or ST segment elevation.
A total of 40 subjects were studied by exercise test. In 14 subjects, ventricular bigeminy appeared during exercise. In eight patients arrhythmia progressed to nonsustained polymorphic ventricular tachycardia (≥3 successive complexes) which in one became sustained (Fig. 2, Table 2). The average sinus rate at which the ventricular arrhythmias initially appeared was 124 ± 13 beats per min and arrhythmias disappeared rapidly after discontinuation of exercise. As judged by QRS morphology, ventricular arrhythmias originated from both left and right sides of the heart in all but one case.
The 24-h average heart rate in the ambulatory ECG registrations was 70 ± 5 beats per min. Three patients exhibited nonsustained ventricular tachycardia while the remaining had infrequent ventricular premature complexes (Table 2). Three patients had nonsustained atrial tachycardia. No disturbances of atrioventricular conduction were observed. Two affected subjects had a late potential in signal-averaged ECG. In the electrophysiologic study, ventricular tachycardia was not inducible in any patient and atrio-Hisian and His-ventricular intervals were normal (Table 2).
In echocardiography, the left ventricular end-diastolic and end-systolic indexes were normal (Table 3). Interventricular septal and posterior wall thickness was ≤11 mm in all patients. Right ventricular cineangiography revealed no structural alterations or tricuspid valve insufficiency (Table 3). One patient had a 70% stenosis of the descending left anterior coronary artery. In MRI study, the structure of the myocardium was normal in all six studied cases. No findings suggestive of right ventricular cavity or outflow tract dilation or adipose replacement of the myocardium were observed. Contraction of the right ventricle was synergistic without signs of dyskinesia.
Microscopic examination of the endomyocardial biopsies showed no significant differences in the extent of fibrosis, adiposity, nuclear changes and inflammation between patients and controls (Table 4). Iron, Kongo and periodic acid-Schiff staining were negative in all samples excluding relevant storage diseases.
Localization of the arrhythmogenic disorder to chromosome 1q42–q43
Linkage of the arrhythmic syndrome to gene loci underlying the LQTS (KVLQT1, HERG and SCN5A) as well as to the putative fourth gene in chromosome 4 (27–30)was excluded. Sequencing of the whole coding sequence of the minK gene (31,32)revealed no mutations (data not shown).
Highly polymorphic microsatellite markers, located approximately every 10 cM, were used in genotyping 24 members of Family 1, starting with chromosome 1. A promising positive signal was obtained with the marker D1S179. To assign the positive region more precisely, a denser map including highly polymorphic markers both centromeric (D1S2800) and telomeric (D1S235, ACTN2, D1S2680, D1S2670, AFM214Xe11, D1S184) to D1S179 was used to type Families 1 and 2. When the results from both pedigrees were combined, a maximal pairwise lod score of 4.74 at theta = 0 was revealed with the marker D1S2670 (Table 5), and a maximal multipoint lod score of 4.62, NPL score of 17.10 and p value of 0.0005 were achieved with the marker D1S2680 (Fig. 3). A lod score exceeding the value 3.0 was achieved in pedigree 1 alone (Zmax= 4.13) but not in the small pedigree 2 (Zmax= 1.64) (Table 5). The expected maximal (mean) lod scores based on simulation calculations were 5.63 (2.48) for pedigree 1 and 2.07 (0.73) for pedigree 2.
Recombinations between the disease and marker D1S2800 were observed in pedigree 1 and in markers D1S184 and AFM214Xe11 in both pedigrees, limiting the disease locus telomeric of D1S2800 but centromeric of AFM214xe11 (Table 6). The haplotype shared by all affected subjects in pedigree 1 was 10-1-1-1-4 (D1S179-D1S235-ACTN2-D1S2680-D1S2670) while the corresponding haplotype in pedigree 2 was 6-1-1-5-3. One unaffected individual (age 30 years) in pedigree 1 had inherited the affected haplotype from his affected mother. We hypothesized that this subject is a carrier of the mutant gene and lack of symptoms is due to incomplete penetrance of the gene. In conclusion, our data assigned the disease locus between markers D1S2800 and AFM214xe11 in a region of approximately 9 cM in size, close to the telomere of chromosome 1 (1q42–q43).
Exclusion of TWIK-1 as a candidate gene by direct sequencing
The chromosomal area 1q42–q43 contains a putative candidate gene TWIK-1 (33)which is supposed to be responsible for background potassium current in several tissues and for phase IV in cardiac action potential. We sequenced the entire coding region of the TWIK-1 gene; however, no mutations or polymorphisms were identified (data not shown).
Treatment, follow-up and mortality
Since the recognition of the inherited and malignant nature of the syndrome, all affected patients with ventricular arrhythmias have been on beta-adrenergic blocking therapy. During the average treatment period of 8 ± 6 years, one patient experienced an episode of syncope and one cardiac arrest, later treated with an implantable cardiac defibrillator. None of the children aged less than 10 years showed any arrhythmias in their ambulatory ECG recordings or exercise tests. However, when reexamined up to four years after the first survey, three children at the age of 11 to 13 years exhibited ventricular arrhythmias on exercise test.
Kaplan-Meier curves for cumulative symptom free survival and mortality of all heterozygous carriers are indicated in Figure 4. Considering the two families together, the cumulative mortality by the age of 30 years was 31%.
We have characterized an arrhythmogenic cardiac disorder which is inherited in an autosomal dominant fashion and shows linkage to chromosome 1q42–q43 markers. The patients suffer exercise-related syncope and sudden death. The pathognomonic feature appeared to be a polymorphic, exercise-induced ventricular tachycardia without evidence of structural cardiac alterations. The QT interval may be slightly prolonged. The penetrance of the putative mutant gene is high, and the syndrome shows significant mortality.
A large number of inherited cardiac disorders cause arrhythmic sudden death. Many of these syndromes have been assigned to specific chromosomal loci and, in some cases, even the underlying genes have been cloned. These malignant disorders include LQTS (4), ARVD (7–11), Brugada syndrome (15), hypertrophic cardiomyopathy (34)and dilated cardiomyopathies (35–43). The clinical, electrophysiological and absent histopathologic characteristics make the chromosome 1q42–q43 linked disorder different from all the currently known syndromes.
The observed normality of cardiac structures suggests pathogenetic mechanisms dissimilar to those in dilated or hypertrophic cardiomyopathy. The normal average QTc interval, the nature and age-dependent manifestation and the chromosomal locus assigned to the syndrome are likely to differentiate the arrhythmic disorder from the LQTS. Finally, the absence of right bundle branch block and ST segment elevation, even after provocative testing with sodium channel blocking agent flecainide, makes it distinct from the Brugada syndrome which results from mutations in cardiac sodium channel gene SCN5A (44).
Arrhythmias are seldom induced during exercise tests in the LQTS but are frequently encountered in ARVD. Indeed, exercise-induced ventricular tachycardia originating from the right side of the heart is among the diagnostic criteria of the disease (45). In striking contrast to ARVD, our patients did not show T-wave abnormalities (6)or right ventricular wall abnormalities as judged by cineangiography and MRI, and the polymorphic appearance of the ventricular tachycardia was clearly different from monomorphic tachycardia with left bundle branch block pattern encountered in arrythmogenic right ventricular dysplasia (6). In addition, right ventricular endomyocardial biopsies did not show inflammatory (46)or degenerative changes (5,6,47)typical of ARVD. Rare occurrence of late potentials (48)and response to beta-adrenergic antagonists further emphasise the difference to ARVD.
A variant form of ARVD and later designated as ARVD2 characterized by occurrence of normal resting ECGs, effort-induced polymorphic tachycardias and normal-sized right ventricle was described by Nava and coworkers in a single Italian family (49). As histological studies showed large areas of fatty-fibrous replacement in the subepicardial layer of the right ventricle, muscle-specific alpha-actinin genes were considered likely candidates, and indeed, linkage of the alpha-actinin ACTN2 gene to chromosome 1q42–q43 was demonstrated in the family (7). At present, it is not possible to exclude the possibility that the syndromes described by Rampazzo et al. (7)and by us represent allelic although different forms of the same underlying molecular pathology.
Another syndrome causing stress or emotion induced syncope is the catecholaminergic polymorphic ventricular tachycardia, first described in 1975 (50)and recently reviewed by Leenhardt et al. (51). In this syndrome any form of increased adrenergic stimulation can induce polymorphic ventricular premature complexes followed by bidirectional and polymorphic tachycardia (51). A familial history of syncope or sudden death is reported in 30% of cases. In distinction to the disorder characterized by us, first symptoms occur before the age of 10 years in the majority of the patients (51).
The consistent arousal of arrhythmias in the absence of structural alterations in heterozygous carriers suggests a cardiac electric disorder as an etiological mechanism. Therefore, of the potential candidate genes for the inherited polymorphic ventricular tachycardia, we sequenced the protein coding region of the TWIK-1 gene but failed to detect any mutations or polymorphism. Another ion channel gene of potential interest in chromosome 1q42–q43 is that encoding the human cardiac muscle ryanodine receptor-calcium release channel (Ryr-2) (52). This channel is a very large protein which is located in the sarcoplasmic reticulum and mediates the Ca2+efflux during the excitation-contraction coupling in cardiac muscle.
Conclusions and clinical implications
Our data suggest that a specific inherited form of human polymorphic ventricular tachycardia mapped to chromosome 1q42–q43 is distinct in its characteristics from other previously identified hereditary arrhythmias. The possibility of this malignant syndrome should not be ignored in cases with a positive family history of syncope or sudden death but with no structural cardiac abnormalities. In suspected cases, an exercise stress test should be performed and even repeated, if required, in adolescent cases. Since many inherited arrhythmic disorders may present with only subtle abnormalities and evade recognition, molecular genetic confirmation of diagnosis will be of utmost importance.
☆ This study was supported by a grant from the Finnish Foundation for Cardiovascular Research, the Medical Council of the Finnish Academy, the Instrumentarium Science Foundation and the Sigrid Juselius Foundation.
- arrhythmogenic right ventricular dysplasia
- electrocardiogram, electrocardiographic
- long QT syndrome
- magnetic resonance imaging
- polymerase chain reaction
- Received January 12, 1999.
- Revision received June 24, 1999.
- Accepted August 30, 1999.
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