Horst Wedekind

Molecular diagnosis in a child with sudden infant death syndrome

Although sudden infant death syndrome (SIDS) has been associated with long QT syndrome-a genetic disorder that causes arrhythmia-a causal link has not been shown. We screened genomic DNA from a child who died of SIDS and identified a de-novo mutation in KVLQT1, the gene most frequently associated with long QT syndrome. This mutation (C350T) had already been identified in an unrelated family that was affected by long QT syndrome. These results confirm the hypothesis that some deaths from SIDS are caused by long QT syndrome and support implementation of neonatal electrocardiographic screening.

Sudden infant death syndrome and long QT syndrome: an epidemiological and genetic study

Sudden infant death syndrome (SIDS) is a frequent cause of death among infants. The etiology of SIDS is unknown and several theories, including fatal ventricular arrhythmias, have been suggested. We performed an epidemiological and genetic investigation of SIDS victims to estimate the presence of inherited long QT syndrome (LQTS) as a contributor for SIDS. Forty-one consecutively collected and unrelated SIDS cases were characterized by clinical and epidemiological criteria. We performed a comprehensive gene mutation screening with single-strand conformation polymorphism analysis and sequencing techniques of the most relevant LQTS genes to assess mutation frequencies. In vitro characterization of identified mutants was subsequently performed by heterologous expression experiments in Chinese hamster ovary cells and in Xenopus laevis oocytes. A positive family history for LQTS was suspected by mild prolonged Q-T interval in family members in 2 of the 41 SIDS cases (5%). In neither case, a family history of sudden cardiac death was present nor a mutation could be identified after thorough investigation. In another SIDS case, a heterozygous missense mutation (H105L) was identified in the N-terminal region of the KCNQ1 (LQTS 1) gene. Despite absence of this mutation in the general population and a high conservational degree of the residue H105 during evolution, electrophysiological investigations failed to show a significant difference between wild-type and KCNQ1H105L/minK-mediated IKs currents. Our data suggest that a molecular diagnosis of SIDS related to LQTS genes is rare and that, even when an ion channel mutation is identified, this should be regarded with caution unless a pathophysiological relationship between SIDS and the electrophysiological characterization of the mutated ion channel has been demonstrated.

Autosomal recessive long-QT syndrome (Jervell Lange-Nielsen syndrome) is genetically heterogeneous

Jervell Lange-Nielsen syndrome (JLNS) is a recessive disorder with congenital deafness and long-QT syndrome (LQTS). Mutations in the potassium-channel gene KVLQT1 (LQTS 1) have been identified in JLNS and in autosomal-dominant LQTS as well. We performed haplotype analysis with microsatellite markers in a Lebanese family with JLNS, but failed to detect linkage at LQTS 1. Moreover, using this approach, we excluded two other ion-channel genes involved in autosomal-dominant LQTS, HERG (LQTS 2) and SCN5A (LQTS 3). Our findings indicate that JLNS is genetically heterogeneous and that, in this family, an unknown LQTS gene causes the disease.

Long QT syndrome and life threatening arrhythmia in a newborn: molecular diagnosis and treatment response

Intrauterine and neonatal manifestations of congenital long QT syndrome are associated with a high cardiac risk, particularly when atrioventricular block and excessive QT prolongation (> 600 ms1/2) are present. In a female newborn with these features, treatment with propranolol and mexiletine led to complete reduction of arrhythmia that was maintained 1.5 years later. High throughput genetic analysis found a sodium channel gene (LQT3) mutation. Disappearance of the 2:1 atrioventricular block and QTc shortening (from 740 ms1/2 to 480 ms1/2), however, was achieved when mexiletine was added to propranolol. This effect was considered to be possibly genotype related. Early onset forms of long QT syndrome may benefit from advanced genotyping.

Cardiac arrhythmias and sudden death in infancy: implication for the medicolegal investigation

Genetically transmitted diseases are an important cause of juvenile sudden cardiac death (SCD). In a considerable proportion of individuals in which a medicolegal investigation is performed, structural heart disease is absent, and the medical examiner fails to discover an adequate cause of death. In such cases, an inherited arrhythmogenic disease should be considered, which manifests with life-threatening ventricular tachycardia or SCD. Molecular diagnosis is progressively becoming an important tool for these questions. Therefore, postmortem genetic testing (“molecular autopsy”) should be considered as a part of the comprehensive medicolegal investigation in SCD cases without apparent structural heart disease. It will have implications not only for the deceased individual but also for living family members in preventing (further) cardiac events by expert counseling, appropriate lifestyle adjustment, and adequate treatment, if available.

The LQT syndromes--current status of molecular mechanisms.

Molekulargenetische Erkenntnisse bei angeborenen Herzrhythmusstörungen wurden, im Vergleich beispielsweise zu den familiären Kardiomyopathien, relativ spät bekannt, was zu einem Teil auf eine hohe Sterblichkeit und einen frühzeitigen Krankheitsbeginn dieser Erkrankungen zurückzuführen ist. Die Durchführung von genetischen Kopplungsanalysen, für die große Familien mit vielen Merkmalsträgern notwendig sind, war zunächst erschwert. 1991 wurde erstmals ein Genort auf dem Chromosom 11p15.5 beschrieben, der mit angeborenem QT-Syndrom assoziiert ist. Das ursächliche Gen KCNQ1, das eine Untereinheit eines Kaliumkanals kodiert, wurde erst 1996 identifiziert. Ferner wurden vier weitere Genloci für QT-Syndrom beschrieben und an drei von vier Mutationen in Genen, die Ionenkanaluntereinheiten kodieren, identifiziert. Zusammenfassend ist das QT-Syndrom eine genetisch heterogene Krankheit kardialer Ionenkanäle. Die Kenntnis der verursachenden Gene für QT-Syndrom hat außerdem zur Identifizierung wichtiger Komponenten geführt, die Ionenströme regulieren und an der Repolarisation beteiligt sind. Mittels In-vitro-Expression wurde gezeigt, daß die strukturell veränderten Ionenkanäle entweder eine Reduktion des Kaliumstromes I (K) während der Phase III des Aktionspotentials oder einen veränderten Natriumstrom während der Phase 0 bedingen. Mittlerweile ist die genomische Struktur der vier LQT-Gene fast vollständig bekannt; Mutationsdetektion mittels SSCP-Analysen und anschließender Sequenzierung hat das diagnostische Spektrum für diese Erkrankung erweitert und ist besonders bei der Diagnosestellung von Patienten, die keine eindeutigen EKG-Veränderungen haben, hilfreich. Die Kenntnis der krankheitsursächlichen Mutation ermöglicht eine präsymptomatische Diagnostik innerhalb einer betroffenen Familie. Derzeit werden Genotyp-Phänotyp-Untersuchungen durchgeführt, um die Penetranz und klinische Expressivität der Erkrankung zu bestimmen und weiteren Aufschlußüber inter- und intrafamiliäre Unterschiede in der Krankheitsmanifestation zu bekommen. In diesem Kapitel wird der jetzige Stand der Molekulargenetik, Elektrophysiologie und klinischen, genotypbezogenen Untersuchungen dargestellt. Our knowledge on the molecular genetics of inherited cardiac arrhythmias is very recent in comparison to the advances of genetics achieved in other inherited cardiac disorders. This is related to the high mortality and early disease onset of these arrhythmias resulting in mostly small nucleus families. Thus, traditional genetic linkage studies that are based on the genetic information obtained from large multi-generation families were made difficult. In 1991, the first chromosomal locus for congenital long-QT (LQT) syndrome was identified on chromosome 11p15.5 (LQT1 locus) by linkage analysis. Meanwhile, the disease-causing gene at the LQT1 locus (KCNQ1), a gene encoding a K+ channel subunit of the IKs channel, and three other, major genes, all encoding cardiac ion channel components, have been identified. Taken together, LQT syndrome turned out to be a heterogeneous channelopathy. Moreover, the power of linkage studies to reveal the genetic causes of the LQT syndrome was also important to identify unknown but fundamental channel components that contribute to the ion currents tuning ventricular repolarization. In-vitro-expression of the altered ion channel genes demonstrated in each case that the altered ion channel function produces prolongation of the action potential and thus the increasing propensity to ventricular tachyarrhythmias. Since these ion channels are pharmacological targets of many antiarrhythmic (and other) drugs, individual and potentially deleterious drug responses may be related to genetic variation in ion channel genes. Very recently, also in acquired LQT syndrome, which is a frequent clinical disorder in cardiology a genetic basis has been proposed in part since mutations in LQT genes have been specifically found. The discovery of ion channel defects in LQT syndrome represents the major achievement in our understanding and implies potential therpeutic options. The knowledge of the genomic structure of the LQT genes now offers the possibility to detect the underlying genetic defect in 80–90% of all patients. With this specific information, containing the type of ion channel (Na+ versus K+ channel) and electrophysiological alteratio n by the mutation (loss-of-function versus change-of-function mutation), gene-directed, elective drug therapies have been initiated in genotyped LQT patients. Based on preliminary data, that were supported by in vitro studies, this approach may be useful in recompensating the characteristic phenotypes in some LQT patients. Mutation detection is a new diagnostic tool which may become of more increasing importance in patients with a normal QTc or just a borderline prolongation of the QTc interval at presentation. These patients represent approximately 40% of all familial cases. Moreover, LQT3 syndrome and idiopathic ventricular fibrillation are allelic disorders and genetically overlap. In both mutations in the LQT3 gene SCN5A encoding the Na+ channel alpha-subunit for INa have been reported. Thus, the clinical nosology of inherited arrhythmias may be reconsidered after elucidation of the underlying molecular bases. Meanwhile, genotype-phenotype correlation in large families are on the way to evaluate intergene, interfamilial, and intrafamilial differences in the clinical phenotype reflecting gene specific, gene-site specific, and individual consequences of a given mutation. LQT syndrome is phenotypically heterogeneous due to the reduced penetrance and variable expressivity associated with the mutations. This paper discusses the current data on molecular genetics and genotype-phenotype correlations and the implications for diagnosis and treatment.

Clinical aspects of ventricular arrhythmias associated with QT prolongation

QT interval prolongation is a risk factor in a number of cardiovascular as well as non-cardiovascular diseases. Apart from this, abnormal, i.e. excessive, QT prolongation is typical for patients with acquired as well as congenital long QT syndrome. In these syndromes, prolongation of repolarization is often associated with severe, potentially life-threatening, ventricular tachyarrhythmias of the type torsade de pointes (TdP). While the congenital long QT syndrome has recently been identified as an ion channelopathy, the mechanisms underlying acquired long QT syndrome, which is most often induced by drugs prolonging myocardial repolarization, are far from understood. Recent studies have yielded only a small number of individual cases in whom the clinical setting has suggested an acquired form of the syndrome and genetic analysis revealed a familial form. In order to prevent an unwanted exposure to risk, physicians prescribing agents that may prolong repolarization need to be aware of their potential to cause excessive QT prolongation and TdP. A clearer delineation of the factors predisposing to abnormal prolongation of repolarization and TdP, and a more precise quantification of the torsadogenic potency of individual drugs appear mandatory in order to prevent, or at least minimize, the incidence of this potentially fatal adverse effect of certain drugs.

Molecular genetics of arrhythmias – a new paradigm

The molecular genetic background of inherited cardiac arrhythmias has only recently been uncovered. This late development in comparison to other inherited cardiac disorders has partly been due to the high mortality and early disease onset of these arrhythmias resulting in mostly small nucleus families. Thus, traditional genetic linkage studies, which are based on the genetic information obtained from large multi-generation families, were made difficult. Inherited arrhythmogenic disorders can be divided into ‘primary electrical disorders’ (e.g., long-QT [LQT] syndrome) in which a detectable, organic heart disease is not evident, and into inherited diseases of the myocardial structure (e.g., hypertrophic cardiomyopathies) in which the arrhythmias occur combined with the structural alterations. To date, all inherited arrhythmogenic disorders in which the causative genes have been identified turned out to be channelopathies, since the genes encode channel subunits that regulate important ion currents that tune the cardiac action potential. The discovery of the genetic bases of the LQT syndrome became a now methodologic paradigm; because with the use of ‘classical’ genetic linkage strategies (named [positional] candidate strategies) not only the causative genes have been found, but moreover, functional components with a previously unknown but fundamental role for a normal repolarization process were discovered. Disease mutations turned out to be not only a family-specific event with a distinct phenotype and the potential of an additional diagnostic tool, but also, when expressed in heterologous expression systems, characterize the defective ion channel in a topological way and lead to a more specific understanding of ion channel function. Most, if not all, primary electrical cardiac disorders show a high genetic diversity. For the LQT syndromes, sixth disease loci and the responsible gene have been recently discovered (socalled locus or genetic heterogeneity). Within all disease genes, the mutations are spread over the entire gene (allelic heterogeneity); in addition, more than one disease mutation may be present. This complexity requires, at least, complete mutation analysis of all LQT genes before medical advice should be given. Meanwhile, genotype-phenotype correlations in large families are being used to evaluate intergene, interfamilial and intrafamilial differences in the clinical phenotype, reflecting gene specific, gene-site specific and individual consequences of a given mutation. A widespread phenotypic heterogenity even within mutation carriers in the same family raises the importance of modifying factors and genes that are mostly unknown to date. The reduced penetrance and variable expressivity associated with the LQT mutations remain still to be explained. First insights into the complex actions of mutations are being extracted, from expression data; these preliminary results may lead to potential implications for a specific (gene-site directed) therapy. This paper discusses the current data on molecular genetics and genotype-phenotype correlations in LQT syndrome and related disorders and the potential implications for diagnosis and treatment.

Clinical Value of Electrocardiographic Parameters in Genotyped Individuals with Familial Long QT Syndrome

MOENNIG, G., et al.: Clinical Value of Electrocardiographic Parameters in Genotyped Individuals with Familial Long QT Syndrome. Rate corrected QT interval (QTc) and QT dispersion (QTd) have been suggested as markers of an increased propensity to arrhythmic events and efficacy of therapy in patients with long QT syndrome (LQTS). To evaluate whether QTc and QTd correlate to genetic status and clinical symptoms in LQTS patients and their relatives, ECGs of 116 genotyped individuals were analyzed. JTc and QTc were longest in symptomatic patients (n = 28). Both QTd and JTd were significantly higher in symptomatic patients than in asymptomatic (n = 29) or unaffected family members (n = 59). The product of QTd/JTd and QTc/JTc was significantly different among all three groups. Both dispersion and product put additional and independent power on identification of mutation carriers when adjusted for sex and age in a logistic regression analysis. Thus, symptomatic patients with LQTS show marked inhomogenity of repolarization in the surface ECG. QT dispersion and QT product might be helpful in finding LQTS mutation carriers and might serve as additional ECG tools to identify asymptomatic LQTS patients.

Effective long-term control of cardiac events with β-blockers in a family with a common LQT1 mutation

The congenital long QT syndrome (LQTS) is characterized by a prolonged QT interval on the surface electrocardiogram and an increased risk of recurrent syncope and sudden cardiac death. Mutations in seven genes have been identified as the molecular basis of LQTS. β‐blockers are the treatment of choice to reduce cardiac symptoms. However, long‐term follow‐up of genotyped families with LQTS has been rarely reported. We have clinically followed a four‐generation family with LQTS being treated with β‐blocker therapy over a period of 23 years. Seven family members were carriers of two amino acid alterations in cis (V254M‐V417M) in the cardiac potassium channel gene KCNQ1. Voltage‐clamp recordings of mutant KCNQ1 protein in Xenopus oocytes showed that only the V254M mutation reduced the IKs current and that the effect of the V417M variant was negligible. The family exhibited the complete clinical spectrum of the disease, from asymptomatic patients to victims of sudden death before β‐blocker therapy. There was no significant reduction in QTc (556 ± 40 ms½ before therapy, 494 ± 20 ms½ during 17 years of treatment; n = 5 individuals). Of nine family members, one female died suddenly before treatment, three females of the second generation were asymptomatic, and four individuals of the third and fourth generation were symptomatic. All mutation carriers were treated with β‐blockers and remained asymptomatic for a follow‐up up to 23 years. Long‐term follow‐up of a LQT1 family with a common mutation (V254M) being on β‐blocker therapy was effective and safe. This study underscores the importance of long‐term follow‐up in families with specific LQT mutations to provide valuable information for clinicians for an appropriate antiarrhythmic treatment.