Familial bundle branch block

Targeted resequencing identifies TRPM4 as a major gene predisposing to progressive familial heart block type I

BACKGROUND

Progressive cardiac conduction disease (PCCD) is one of the most common cardiac conduction disturbances. It has been causally related to rare mutations in several genes including SCN5A, SCN1B, TRPM4, LMNA and GJA5.

METHODS AND RESULTS

In this study, by applying targeted next-generation sequencing (NGS) in 95 unrelated patients with PCCD, we have identified 13 rare variants in the TRPM4 gene, two of which are currently absent from public databases. This gene encodes a cardiac calcium-activated cationic channel which precise role and importance in cardiac conduction and disease is still debated. One novel variant, TRPM4-p.I376T, is carried by the proband of a large French 4-generation pedigree. Systematic familial screening showed that a total of 13 family members carry the mutation, including 10 out of the 11 tested affected individuals versus only 1 out of the 21 unaffected ones. Functional and biochemical analyses were performed using HEK293 cells, in whole-cell patch-clamp configuration and Western blotting. TRPM4-p.I376T results in an increased current density concomitant to an augmented TRPM4 channel expression at the cell surface.

CONCLUSIONS

This study is the first extensive NGS-based screening of TRPM4 coding variants in patients with PCCD. It reports the third largest pedigree diagnosed with isolated Progressive Familial Heart Block type I and confirms that this subtype of PCCD is caused by mutation-induced gain-of-expression and function of the TRPM4 ion channel.

Progressive Conduction Diseases

Progressive cardiac conduction disease (PCCD), a source of considerable morbidity, comprises a heterogeneous group of conditions resulting from genetic predisposition, environmental modifiers, and other physiologic determinants, including aging. The genetic factors include numerous mutations and variants within the cardiac sodium channel gene, SCN5A. The electrocardiographic phenotype has variable penetrance and is associated with appearances ranging from an isolated conduction disorder to an association with tachyarrhythmias and clinically significant cardiomyopathy. A heterozygotic Scn5a mouse model provides evidence that PCCD may lead to cardiac remodeling consistent with clinical observations in addition to slowing of intracardiac conduction. PCCD has also been associated with the altered expression of genes encoding other proteins involved in impulse propagation, including those responsible for Ca^2+- activated ion channels and cytoskeletal components, both in the presence or absence of structural abnormalities.

Genetic basis of ventricular arrhythmias

Sudden cardiac death caused by malignant ventricular arrhythmias is the most important cause of death in the industrialized world. Most of these lethal arrhythmias occur in the setting of ischemic heart disease. A significant number of sudden deaths, especially in young individuals, are caused by inherited ventricular arrhythmic disorders, however. Genetically induced ventricular arrhythmias can be divided in two subgroups: the primary electrical disorders or channelopathies, and the secondary arrhythmogenic cardiomyopathies. This article focuses on the genetic background of these electrical disorders and the current knowledge of genotype-phenotype interactions.

Haploinsufficiency in combination with aging causes SCN5A-linked hereditary Lenègre disease

OBJECTIVES

The goal of this study was to investigate the genotype-to-phenotype relationship between SCN5A gene mutation and progressive cardiac conduction defect in order to gain insights into the pathophysiologic mechanisms of the disease.

BACKGROUND

Progressive cardiac conduction defect is a frequent disease commonly attributed to degeneration and fibrosis of the His bundle and its branches. In a French family, we have identified a splicing mutation in the SCN5A gene leading to hereditary progressive cardiac conduction defect.

METHODS

We have extended the size of the pedigree and phenotyped and genotyped all family members, and also investigated in vitro the functional consequences of the mutation.

RESULTS

Among 65 potentially affected members, 25 individuals were carriers of the IVS.22+2 T-->C SCN5A mutation. In relation to aging, gene carriers exhibit various types of conduction defects. P-wave, PR, and QRS duration increased progressively with age in gene carriers and in noncarriers. Whatever the age, conduction parameters were longer in gene carriers. The widening in the QRS complex with aging was more pronounced in gene carriers older than 40 years. Functional studies show that the IVS.22+2 T-->C SCN5A mutation lead to exon 22 skipping and to a complete loss of function of the affected allele, but to a normal trafficking of the mutated gene product.

CONCLUSIONS

Our findings demonstrate that hereditary Lenègre disease is caused by a haploinsufficiency mechanism, which in combination with aging leads to progressive alteration in conduction velocity.

Progressive cardiac conduction defect and cardiomyopathy in siblings with syncope

Three siblings evaluated for syncope were found to have left bundle-branch block. Progression to complete heart block occurred in all. Pacemaker implantation eliminated syncopal epidoses in each case. Echocardiographic manifestations of cardiomyopathy were present in each child despite normal roentgenographic heart size. Careful family study identified no other affected members with conduction defects. These children area believed to represent a form of familial cardiomyopathy in which clinical manifestations of cardiac conduction system disease predominate.

Genetic aspects of arrhythmias

Advances in the treatment and prevention of heart disease have led to consistently declining morbidity and mortality rates over the past 30 years. Despite these advances, therapy remains largely palliative. The development of curative therapies is limited by our lack of knowledge of the basic mechanisms of disease. In the next decade, we will probably change many of these current approaches from treating the crisis to preventing the disease. Molecular biology and genetics have elucidated several basic pathways. It is hoped that targeted therapies will prevent or arrest many of these cardiac diseases, in particular, arrhythmias and sudden death. With the discovery of the genes causing familial diseases like long QT, hypertrophic cardiomyopathy, and Brugada syndrome, we have identified several substrates responsible for triggering malignant arrhythmias.

Hereditary bundle branch defect: right bundle branch blocks of different causes have different morphologic characteristics

Hereditary bundle branch defect is an autosomal dominant genetic disease that, in a large Lebanese family, was mapped to the long arm of chromosome 19. Affected individuals have various combinations of conduction defects such as right bundle branch block, left or right QRS frontal-axis deviation, or atrioventricular blocks. We now further characterize this disease with the presentation of a two-decade follow-up and analysis of electrocardiographic features and mutation-carrier status. The conduction block may be overt in the first year of life, and among affected individuals, there is a worsening of the conduction block in 5% to 15% of cases, leading to complete atrioventricular block and possibly to sudden death. A group of individuals had QRS anomalies in right precordial leads such as rsr's', rss', or rSr', which may account for partial right bundle branch blocks. In this group, which we referred to as having an "r' pattern," 53% were actually mutation carriers, and 19% evolved toward a complete fascicular block. By contrast, mutation carriers with a normal electrocardiogram remained normal. The QRS morphologic appearance in the right precordial leads of affected individuals and r' pattern mutation carriers is notable for the absence or weakness of negative forces resulting in a rsR' or rR' morphology. In addition, an r' pattern is highly suggestive of a mutation carrier status in the presence of a broad r wave in aVR and s in V6 or a frontal-axis deviation. Finally, mutation carriers demonstrate a conduction block significantly more often in males than females (75% and 50%, respectively). This incomplete penetrance and slow evolution suggest that the actual prevalence of hereditary bundle branch defect is very much underestimated.

Familial cardiomyopathy underlies syndrome of right bundle branch block, ST segment elevation and sudden death

OBJECTIVES

We sought to assess whether structural heart disease underlies the syndrome of right bundle branch block, persistent ST segment elevation and sudden death.

BACKGROUND

Ventricular fibrillation and sudden death may occur in patients with a distinctive electrocardiographic (ECG) pattern of right bundle branch block and persistent ST segment elevation in the right precordial leads.

METHODS

Sixteen members of a family affected by this syndrome underwent noninvasive cardiac evaluation, including electrocardiography, Holter ambulatory ECG monitoring, stress testing, echocardiography and signal-averaged electrocardiography; two patients had electrophysiologic and angiographic study. Endomyocardial biopsy was performed in one living patient, and postmortem examination, including study of the specialized conduction system, was performed in one victim of sudden death.

RESULTS

Five years before a fatal cardiac arrest, the proband had been resuscitated from sudden cardiac arrest due to recorded ventricular fibrillation. Serial ECGs showed a prolonged PR interval, right bundle branch block, left-axis deviation and persistent ST segment elevation in the right precordial leads, in the absence of clinical heart disease. Postmortem investigation disclosed right ventricular dilation and myocardial atrophy with adipose replacement of the right ventricular free wall as well as sclerotic interruption of the right bundle branch. A variable degree of right bundle branch block and upsloping right precordial ST segment was observed in seven family members; four of the seven had structural right ventricular abnormalities on echocardiography and late potentials on signal-averaged electrocardiography. A sib of the proband also had a prolonged HV interval, inducible ventricular tachycardia and fibrofatty replacement on endomyocardial biopsy.

CONCLUSIONS

An autosomal dominant familial cardiomyopathy, mainly involving the right ventricle and the conduction system, accounted for the ECG changes and the electrical instability of the syndrome.

An isolated cardiac conduction disease maps to chromosome 19q

Isolated cardiac conduction disease is an autosomal dominant defect that includes various combinations of bundle branch or fascicular blocks. These defects can cause sudden death due to a complete heart block. We used a genome-wide screening approach with polymorphic (CA)n repeat markers to determine the chromosomal position of the gene defect implicated in this disorder. The analyses were carried out on a large Lebanese kindred, which included individuals with either a complete or incomplete right bundle branch block (RBBB) with a vertical-axis deviation (< or = -30 or > or = +100). Linkage to the disease locus was detected with the polymorphic marker D19S604 on the q arm of chromosome 19 (19q13.3) with a multipoint lod score of 7.18. Additionally, we were able to exclude the flanking loci D19S606 and D19S571, which are 13 cM apart because of recombination events in three affected individuals. The histidine-rich calcium-binding protein gene is found in this region and is an attractive candidate gene on the basis of its physiological properties and a tight linkage. There is no expansion in two exon 1 regions known for a variable number of triplet repeats.

Hereditary bundle branch system defect. A new genetic entity?

A familial survey demonstrated mendelian inheritance in three large kindreds with conduction abnormalities and heart block. The trait was autosomal dominant, with varying expressivity and penetrance, apparent male preponderance, and congenital onset. Manifestations included right bundle branch block, left axis deviation, and right bundle branch block associated with left axis deviation. Complete heart block proved almost always to be a late event, and developed in all documented cases from bilateral bundle branch block. An r' pattern, most likely representing a right ventricular conduction delay, is discussed.

Familial occurrence of sinus bradycardia, short PR interval, intraventricular conduction defects, recurrent supraventricular tachycardia, and cardiomegaly

Four members of a family presenting with sinus bradycardia, a short P-R interval, intraventricular conduction defects, recurrent supraventricular tachycardia (SVT), syncope, and cardiomegaly had His bundle studies and were found to have markedly shortened A-H intervals (30 to 55 msec.) with normal H-V times (35 to 50 msec.). Right atrial pacing at rates as high as 170 to 215 per minute failed to increase the A-H or H-V intervals significantly. The data are compatible with the presence of an A-V nodal bypass tract (James bundle) or even complete absence of an A-V node. Ventricular pacing and spontaneous ventricular premature beats resulted in a short ventriculoatrial conduction time (110 msec.) suggesting that if A-V nodal bypass tracts exist, they are utilized in an antegrade and retrograde fashion. None of the features of WPW syndrome was present. The mechanism of syncope in the mother and daughter was intermittent third-degree heart block. Both went on to develop permanent complete heart block despite electrophysiologic studies demonstrating 1:1 A-V conduction at extremely rapid atrial pacing rates and both required implantation of permanent pacemakers. The mechanism of syncope in the two brothers was possibly marked sinus bradycardia, but transient complete heart block has not been ruled out. Permanent pacemaker therapy was recommended for both. The nature of the cardiomegaly, which was mild in three patients, is not known. Although not well documented, several maternal relatives have had enlarged hearts, SVT, complete heart block, and syncope.

Hereditary progressive atrioventricular conduction defect

New data on genetics, including an extensive pedigree and certain aspects of natural history, have been compiled on Family S, which is characterized by a hereditary progressive atrioventricular (A-V) conduction defect. Concordance analysis of heart block in affected parents and their offspring suggests as a working hypothesis transmission of diathesis for the defect by means of a Mendelian autosomal dominant factor. Regression of the defect in several relatives, including reversion from first degree heart block to normal A-V conduction, defies explanation at this time. However, rapport established with the unusually large (1,067 members) and cooperative kindred will permit longitudinal evaluation of these findings.

Familial, isolated, complete right bundle-branch block

An electrocardiographic study of the families of two children with isolated complete right bundle-branch block is presented. Several cases of classical complete right bundle-branch block were disclosed (17/69 and 3/22 respectively) in three generations of the two families. Personal interviews and parish registers showed that the two families were related, being descendants 8 generations back from a glass-blower who has emigrated to Sweden in the eighteenth century. It is concluded that complete right bundle-branch block can be an autosomal, genetic anomaly which is inherited dominantly but with reduced penetrance and/or expressivity.

Familial atrioventricular heart block; an autosomal dominant trait

A family of 28 individuals spanning four generations was investigated because of a finding of complete heart block in five members and the existence of a low degree of atrioventricular (A-V) heart block in a sixth member. The disorder was characterized by 1) adult onset in all, 2) complete A-V heart block in five and first degree A-V heart block in one, 3) sinus bradycardia in three, 4) atrial fibrillation in five, 5) abnormal QRS complex in five, 6) ventricular tachycardia in three, 7) left ventricular enlargement in all, and 8) mitral insufficiency in five. Proximal location of the A-V heart block was suggested by the fact that atropine caused acceleration of the ventricular rate and by the presence of a His bundle potential preceding the QRS complexes. Involvement of the distal conducting system was indicated by the widened QRS complex and a prolonged H-V interval. Pathologic examination in one case showed extensive sinus node fibrosis and interruption of the A-V node-His bundle connection. This disorder is probably due to an autosomal dominant trait.

Familial atrial dysrhythmia with A-V block. Intracellular microelectrode, clinical electrophysiologic, and morphologic observations

This study is of a family with a syndrome of atrial fibrillation or flutter with advanced or complete atrioventricular (A-V) block, involving four members in two generations. Less serious arrhythmias were documented in another 15 members (three generations). Inheritance was by autosomal dominance with varying degrees of expression. An atrial biopsy was obtained in one of the members, a 40-year-old female who had atrial flutter with advanced A-V block proximal to the His bundle (H-V interval of 39 msec). Intracellular action potential (IAP) recordings from this tissue revealed: 1) partially depolarized cells with depressed IAP amplitude, 2) phase IV diastolic depolarization with spontaneous firing, 3) diminished excitability and responsiveness, and 4) decremental conduction with local block and re-entry. Histological findings revealed vacuolar degeneration, hypertrophy, and early necrosis of atrial cells.

In conclusion, the multiple IAP and pathologic abnormalities provide a basis for atrial dysrhythmia in this family. The etiology of the disease is unknown.

The electrocardiogram in Duchenne progressive muscular dystrophy

Interpretation of the electrocardiogram in the Duchenne form of progressive muscular dystrophy (PMD) is still debated in the literature. The group studied consisted of 106 patients with the Duchenne type of PMD, aged 3 to 29 years. The duration of the disease varied from a few months to 15 years. One hundred one patients had abnormal electrocardiograms. These were of four types: type a (64 cases) showed tall R waves in V 1 and deep Q waves in V 4-6 ; type b (24 cases), RSr' and deep Q waves in V 5-6 ; and type c (13 cases), polyphasic R waves in V 1 .

The view that the electrocardiographic abnormalities in Duchenne dystrophy are a manifestation of the dystrophic process in the myocardium is discussed. The authors' material, however, failed to support this view but did support the view that the electrocardiographic abnormalities have an heredofamilial character.

Congenital Familial Cardiac Conduction Defects

A family in which several members of three separate generations have evidence of congenital familial conduction disturbance has been described. The occurrence of Adams-Stokes seizures with congenital AV block was also noted. From the available information it is clear that the conduction disturbance was congenital in two generations, and the family history suggests that three or four generations were probably involved. The data suggest that in this family the congenital cardiac conduction defects were transmitted as an autosomal dominant trait with incomplete penetrance.