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

A novel mutation in the HCN4 gene causes symptomatic sinus bradycardia in Moroccan Jews

OBJECTIVES

to conduct a clinical, genetic, and functional analysis of 3 unrelated families with familial sinus bradycardia (FSB).

BACKGROUND

mutations in the hyperpolarization-activated nucleotide-gated channel (HCN4) are known to be associated with FSB.

METHODS AND RESULTS

three males of Moroccan Jewish descent were hospitalized: 1 survived an out-of-hospital cardiac arrest and 2 presented with weakness and presyncopal events. All 3 had significant sinus bradycardia, also found in other first-degree relatives, with a segregation suggesting autosomal-dominant inheritance. All had normal response to exercise and normal heart structure. Sequencing of the HCN4 gene in all patients revealed a C to T transition at nucleotide position 1,454, which resulted in an alanine to valine change (A485V) in the ion channel pore found in most of their bradycardiac relatives, but not in 150 controls. Functional expression of the mutated ion channel in Xenopus oocytes and in human embryonic kidney 293 cells revealed profoundly reduced function and synthesis of the mutant channel compared to wild-type.

CONCLUSIONS

we describe a new mutation in the HCN4 gene causing symptomatic FSB in 3 unrelated individuals of similar ethnic background that may indicate unexplained FSB in this ethnic group. This profound functional defect is consistent with the symptomatic phenotype.

The Contribution of HCN4 to normal sinus node function in humans and animal models

Although sinus node bradycardia is a very common clinical condition, the cellular mechanisms contributing to abnormal sinus node function are not clearly delineated. In recent publications, mutations in the hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channels have been associated with sinus bradycardia. These channels are thought to be crucial in generating the spontaneous sinus node action potential, in accelerating the heart rate during sympathetic drive, and decelerating heart rate during vagal stimulation. Humans carrying HCN4 mutations indeed display significant bradycardia. Recent studies generating HCN4 knock out mice suggested that although HCN4 is crucial in early development, other mechanisms may also play a role in the accelerated heat rate achieved during sympathetic drive. In this review, we focus on genotype-phenotype correlation of these mutations and discuss the relative contribution of various ion channels to sinus node function. We also discuss the importance of HCN in treating clinical conditions such as brady- and tachycardia.

Point mutation in the HCN4 cardiac ion channel pore affecting synthesis, trafficking, and functional expression is associated with familial asymptomatic sinus bradycardia

BACKGROUND

The hyperpolarization-activated nucleotide-gated channel--HCN4 plays a major role in the diastolic depolarization of sinus atrial node cells. Mutant HCN4 channels have been found to be associated with inherited sinus bradycardia.

METHODS AND RESULTS

Sixteen members of a family with sinus bradycardia were evaluated. Evaluation included a clinical questionnaire, 12-lead ECGs, Holter monitoring, echocardiography, and treadmill exercise testing. Eight family members (5 males) were classified as affected. All affected family members were asymptomatic with normal exercise capacity during long-term follow-up. Electrophysiological testing performed on 2 affected family members confirmed significant isolated sinus node dysfunction. Segregation analysis suggested autosomal-dominant inheritance. Direct sequencing of the exons encoding HCN4 revealed a missense mutation, G480R, in the ion channel pore domain in all affected family members. Function analysis, including expression of HCN4 wild-type and G480R in Xenopus oocytes and human embryonic kidney 293 cells, revealed that mutant channels were activated at more negative voltages compared with wild-type channels. Synthesis and expression of the wild-type and mutant HCN4 channel on the plasma membrane tested in human embryonic kidney 293 cells using biotinylation and Western blot analysis demonstrated a reduction in synthesis and a trafficking defect in mutant compared with wild-type channels.

CONCLUSIONS

We describe an inherited, autosomal-dominant form of sinus node dysfunction caused by a missense mutation in the HCN4 ion channel pore. Despite its critical location, this mutation carries a favorable prognosis without the need for pacemaker implantation during long-term follow-up.

Familial Pseudo-Wolff-Parkinson-White Syndrome

INTRODUCTION

PRKAG2 plays a role in regulating metabolic pathways, and mutations in this gene are associated with familial ventricular preexcitation, hypertrophic cardiomyopathy, and atrioventricular conduction disturbances. Clinico-pathologic and experimental data suggest the hypothesis of a glycogen storage disease.

OBJECTIVE

To report a unique pattern of clinical features observed in individuals with a mutant PRKAG2 from two unrelated families.

METHODS AND RESULTS

We studied two large families and found a total of 20 affected individuals showing a combination of sinus bradycardia, short PR interval, RBBB, intra and infrahisian conduction disturbances often requiring a pacemaker, and atrial tachyarrhythmias. Three individuals died suddenly at a young age. No patient had the Wolff-Parkinson-White (WPW) syndrome, and only two patients (10%) had myocardial hypertrophy. We performed screening of the exons and exon-intron boundaries of PRKAG2. Genetic analysis revealed a missense mutation (Arg302Gln) in the affected individuals from both families. This mutation had been described before and has been associated with the familial form of the WPW syndrome and with a high prevalence of left ventricular hypertrophy.

CONCLUSION

PRKAG2 mutations are responsible for a diverse phenotype and not only the familial form of the WPW syndrome. Familial occurrence of right bundle branch block, sinus bradycardia, and short PR interval should raise suspicion of a mutant PRKAG2 gene.

Radiofrequency catheter ablation in familial paroxysmal supraventricular tachycardia due to accessory atrioventricular pathways

Radiofrequency catheter ablation (RF-CA) has been widely used to cure paroxysmal supraventricular tachycardia (PSVT). However, its use has never been reported in familial PSVT caused by an accessory atrioventricular pathway (AP), which is known as one of the typical familial cardiovascular diseases. Two cases of using RF-CA for familial PSVT due to APs are presented, in a brother and sister, supporting a potential genetic role in the developmental failure to lose the atrioventricular connection during fetal life. The sister, a 24-year-old woman, had intermittent episodes of palpitation accompanied by chest pain for 2 years. An electrophysiologic study (EPS) confirmed her clinical tachycardia was atrioventricular reentrant tachycardia (AVRT) due to a left lateral concealed AP, which was subsequently successfully ablated with RF-CA. The brother, a 22-year-old man, had a 5-year history of paroxysmal palpitation. A resting electrocardiogram showed a right bundle branch block and left axis deviation with a delta wave. During his EPS, AVRT was reproducibly induced and a manifest AP was localized and then ablated at the left posteroseptal site, resulting in disappearance of the delta wave. PSVT, however, recurred 1 month later and during a repeat EPS the tachycardia was proved to be AVRT due to a right anterior concealed AP. The right anterior AP was successfully ablated with RF-CA. Both patients remained asymptomatic for more than 3 years following the successful ablation procedures.

Familial atrioventricular heart block of adult onset: electrocardiogram and HLA typing analysis

A family was investigated because of heart block and sinus arrhythmia. The electrical characteristics were: (1) adult onset in all members; (2) complete heart block with atrial fibrillation in 2, and first- or second-degree heart block in 7 members; and (3) sinus arrhythmia in 3 members. Human leukocyte antigen (HLA) typing was performed. HLA A2, B39, Cw7, and DR12 were positive in 4 of 5 members in the heart block group. In the sinus arrhythmia group, HLA DR12 was positive in all members. In the normal group, none of these HLA typings was positive except one. These findings indicate a tighter relationship between heart block and the HLA locus than previously thought.

Prevalence of intraventricular block in the general population: the Framingham Study

QRS intervals in the ECGs of members of the Framingham Heart Study cohort and offspring were measured to provide an estimate of the prevalence of intraventricular block in the general population. Intervals of greater than or equal to 0.09 second appear in men twice as commonly as in women, are rare before age 50 to 60, and shift from a predominance of right bundle branch block in the young to an indeterminate pattern in the elderly. Complete intraventricular block (QRS interval greater than or equal to 0.12 second) is seen in 11% of elderly men and 5% of elderly women. Aside from age and sex, logistic regression indicates strong associations with concurrent manifestations of coronary heart disease, congestive heart failure, and atrioventricular block, as well as hypertension, left ventricular hypertrophy, and ventricular extrasystoles. Among those subjects free of clinical coronary disease and congestive heart failure, associations between QRS interval and age, sex, atrioventricular block, and ECG left ventricular hypertrophy remain significant by multivariate analysis. Whether people with prolonged QRS intervals need special monitoring or attention cannot be told from these data.

Familial combined sinus node and atrioventricular conduction dysfunctions

Eighteen members of a family were investigated for cardiac conduction abnormalities following the discovery of a second-degree atrioventricular block in a fetus at 35 weeks of gestation. The conduction disturbance was diagnosed by ultrasonography. Seven of the family members were diagnosed as suffering from sinus node dysfunction and/or various degrees of atrioventricular block. Three of them were children aged 9 months to 6 years and all were asymptomatic. The symptomatic family members were two adults. One of them had a pacemaker inserted for a complete atrioventricular block and Adam-Stokes attacks while the other had had several fainting attacks. The clinical, electrocardiographic and ultrasonographic findings of the family members are presented. Previous reports in literature have documented the dominant transmission of familial sinus node dysfunctions or of familial cardiac conduction disturbances. The coexistence of both dysfunctions in the same family has not been emphasized in previous reports. A review of these previously reported families, as well as our patients, suggests the need of investigation and follow-up of the families in which one member of any age is diagnosed as suffering from an "idiopathic" disturbance of the cardiac conduction system.

Sinuatrial disease in young people

A survey at three cardiac centres disclosed nine patients under the age of 25 years with sinus node dysfunction in the absence of other forms of heart disease. All were male and seven were above the 90th centile for height. Ambulatory monitoring was performed on all the first-degree relatives of six of the patients and three families showed conducting system disturbances in the form of sinuatrial disorders or delayed atrioventricular conduction. A genetic factor may be involved in the aetiology of sinuatrial disease in young people.