Chronotropic competence of the sinus node in congenital complete heart block

Autoimmune Calcium Channelopathies and Cardiac Electrical Abnormalities

Patients with autoimmune diseases are at increased risk for developing cardiovascular diseases, and abnormal electrocardiographic findings are common. Voltage-gated calcium channels play a major role in the cardiovascular system and regulate cardiac excitability and contractility. Particularly, by virtue of their localization and expression in the heart, calcium channels modulate pace making at the sinus node, conduction at the atrioventricular node and cardiac repolarization in the working myocardium. Consequently, emerging evidence suggests that calcium channels are targets to autoantibodies in autoimmune diseases. Autoimmune-associated cardiac calcium channelopathies have been recognized in both sinus node dysfunction atrioventricular block in patients positive for anti-Ro/La antibodies, and ventricular arrhythmias in patients with dilated cardiomyopathy. In this review, we discuss mechanisms of autoimmune-associated calcium channelopathies and their relationship with the development of cardiac electrical abnormalities.

Autoimmune channelopathies as a novel mechanism in cardiac arrhythmias

Cardiac arrhythmias confer a considerable burden of morbidity and mortality in industrialized countries. Although coronary artery disease and heart failure are the prevalent causes of cardiac arrest, in 5-15% of patients, structural abnormalities at autopsy are absent. In a proportion of these patients, mutations in genes encoding cardiac ion channels are documented (inherited channelopathies), but, to date, the molecular autopsy is negative in nearly 70% of patients. Emerging evidence indicates that autoimmunity is involved in the pathogenesis of cardiac arrhythmias. In particular, several arrhythmogenic autoantibodies targeting specific calcium, potassium, or sodium channels in the heart have been identified. Experimental and clinical studies demonstrate that these autoantibodies can promote conduction disturbances and life-threatening tachyarrhythmias by inducing substantial electrophysiological changes. In this Review, we propose the term 'autoimmune cardiac channelopathies' to define this novel pathogenic mechanism of cardiac arrhythmias, which could be more frequent and clinically relevant than previously appreciated. Indeed, pathogenic autoantibodies against ion channels are detectable not only in patients with manifest autoimmune disease, but also in apparently healthy individuals, which suggests a causal role in some cases of unexplained arrhythmias and cardiac arrest. Considering this possibility and performing specific testing in patients with 'idiopathic' rhythm disturbances could create novel treatment opportunities.

Congenital heart block maternal sera autoantibodies target an extracellular epitope on the α1G T-type calcium channel in human fetal hearts

BACKGROUND

Congenital heart block (CHB) is a transplacentally acquired autoimmune disease associated with anti-Ro/SSA and anti-La/SSB maternal autoantibodies and is characterized primarily by atrioventricular (AV) block of the fetal heart. This study aims to investigate whether the T-type calcium channel subunit α1G may be a fetal target of maternal sera autoantibodies in CHB.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate differential mRNA expression of the T-type calcium channel CACNA1G (α1G gene) in the AV junction of human fetal hearts compared to the apex (18-22.6 weeks gestation). Using human fetal hearts (20-22 wks gestation), our immunoprecipitation (IP), Western blot analysis and immunofluorescence (IF) staining results, taken together, demonstrate accessibility of the α1G epitope on the surfaces of cardiomyocytes as well as reactivity of maternal serum from CHB affected pregnancies to the α1G protein. By ELISA we demonstrated maternal sera reactivity to α1G was significantly higher in CHB maternal sera compared to controls, and reactivity was epitope mapped to a peptide designated as p305 (corresponding to aa305-319 of the extracellular loop linking transmembrane segments S5-S6 in α1G repeat I). Maternal sera from CHB affected pregnancies also reacted more weakly to the homologous region (7/15 amino acids conserved) of the α1H channel. Electrophysiology experiments with single-cell patch-clamp also demonstrated effects of CHB maternal sera on T-type current in mouse sinoatrial node (SAN) cells.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results indicate that CHB maternal sera antibodies readily target an extracellular epitope of α1G T-type calcium channels in human fetal cardiomyocytes. CHB maternal sera also show reactivity for α1H suggesting that autoantibodies can target multiple fetal targets.

Persistent fetal sinus bradycardia associated with maternal anti-SSA/Ro and anti-SSB/La antibodies

OBJECTIVE

To study the clinical course and outcome of fetal sinus bradycardia (SB) due to maternal antibody-induced sinus node dysfunction.

METHODS

We reviewed the maternal, prenatal, and postnatal findings of fetuses with SB associated with elevated maternal anti-SSA/Ro and anti-SSB/La antibodies.

RESULTS

Of the 6 cases diagnosed prenatally, 3 had isolated SB persisting after birth and had a good prognosis. Three fetuses with SB and severe myocardial involvement (congenital complete heart block and/or endocardial fibroelastosis) succumbed in utero in spite of treatment. Postmortem histopathology in 1 fetus showed inflammatory destruction of the sinus and atrioventricular nodes. SB was detected incidentally in a 7-year-old girl. She had intermittent heart block with progressive sinus arrest requiring permanent pacemaker.

CONCLUSION

Fetal SB associated with maternal autoantibodies may persist in childhood, with a good prognosis in the absence of widespread cardiac involvement.

Rescue and worsening of congenital heart block-associated electrocardiographic abnormalities in two transgenic mice

INTRODUCTION

Congenital heart block (CHB) is a passively acquired autoimmune disease considered to be due to the transfer of maternal autoantibodies, anti-SSA/Ro -SSB/La, to the fetus resulting in atrioventricular (AV) block and sinus bradycardia. We previously established a murine model for CHB where pups born to immunized wild-type (WT) mothers exhibited electrocardiographic abnormalities similar to those seen in CHB and demonstrated inhibition of L-type Ca channels (LTCCs) by maternal antibodies. Here, we hypothesize that overexpression of LTCC should rescue, whereas knockout of LTCC should worsen the electrocardiographic abnormalities in mice.

METHODS AND RESULTS

Transgenic (TG) mice were immunized with SSA/Ro and SSB/La antigens. Pups born to immunized WT mothers had significantly greater sinus bradycardia and AV block compared to pups from nonimmunized WT. TG pups overexpressing LTCC had significantly less sinus bradycardia and AV block compared to their non-TG littermates and to pups born to immunized WT mothers. All LTCC knockout pups born to immunized mothers had sinus bradycardia, advanced degree of AV block, and decreased fetal parity. No sinus bradycardia or AV block were manifested in pups from control nonimmunized WT mothers. IgG from mothers with CHB children, but not normal IgG, completely inhibited intracellular Ca transient ([Ca](i)T) amplitude.

CONCLUSIONS

Cardiac-specific overexpression of LTCC significantly reduced the incidence of AV block and sinus bradycardia in pups exposed to anti-SSA/Ro -SSB/La autoantibodies, whereas exposure of LTCC knockout pups to these autoantibodies significantly worsened the electrocardiographic abnormalities. These findings support the hypothesis that maternal antibodies inhibit LTCC and [Ca](i)T thus contributing to the development of CHB. Altogether, the results are relevant to the development of novel therapies for CHB.

Role of calcium channels in congenital heart block

Congenital heart block (CHB) is a conduction abnormality that affects hearts of foetuses and/or newborn to mothers with autoantibodies reactive with the intracellular soluble ribonucleoproteins 48-kD La, 52-kD Ro and 60-kD Ro. CHB carries substantial mortality and morbidity, with more than 60% of affected children requiring lifelong pacemakers. Several hypotheses have been proposed to explain the pathogenesis of CHB. These can be grouped under three main hypotheses: Apoptosis, Serotoninergic and Ca channel hypothesis. Here, we discuss these hypotheses and provide recent scientific thinking that will most likely dominate the future of this field of research.

Congenital heart block: identification of autoantibody binding site on the extracellular loop (domain I, S5-S6) of alpha(1D) L-type Ca channel

Congenital heart block (CHB) is an autoimmune disease associated with autoantibodies against intracellular ribonucleoproteins SSB/La and SSA/Ro. The hallmark of CHB is complete atrioventricular block. We have recently established that anti-SSA/Ro -SSB/La autoantibodies inhibit alpha(1D) L-type Ca current, I(Ca-L), and cross-react with the alpha(1D) Ca channel protein. This study aims at identifying the possible binding sites on alpha(1D) protein for autoantibodies from sera of mothers with CHB children. GST fusion proteins of the extracellular regions between the transmembrane segments (S5-S6) of each of the four alpha(1D) Ca channel protein domains I-IV were prepared and tested for reactivity with sera from mothers with CHB children and controls using ELISA. Sera containing anti-Ro/La autoantibodies from 118 mothers with CHB children and from 15 mothers with anti-Ro/La autoantibodies but have healthy children, and from 28 healthy mothers without anti-Ro/La autoantibodies and healthy children were evaluated. Seventeen of 118 (14.4%) sera from mothers with CHB children reacted with the extracellular loop of domain I S5-S6 region (E1). In contrast, only 2 of 28 (7%) of sera from healthy mothers (-anti-Ro/La) and healthy children reacted with E1 loop and none (0 of 15) of sera from healthy mothers (+anti-Ro/La) and healthy children reacted with the E1 loop. Preincubation of E1 loop with the positive sera decreased the O.D reading establishing the specificity of the response. Electrophysiological characterization of the ELISA positive sera and purified IgG showed inhibition (44.1% and 49.8%, respectively) of the alpha(1D) I(Ca-L) expressed in tsA201 cells. The inhibition was abolished when the sera were pre-incubated with E1 fusion protein. The results identified the extracellular loop of domain I S5-S6 of L-type Ca channel alpha(1D) subunit as a target for autoantibodies from a subset of mothers with CHB children. This novel finding provides insights into the potential development of therapeutic peptides that could bind to the pathogenic antibodies and prevent CHB.

Exercise testing in the presence of complete heart block

A 28-yr-old female presented for preoperative evaluation. The resting ECG revealed sinus arrhythmia with complete heart block with a junctional escape rhythm and a ventricular rate of 43 bpm. The patient was in no apparent distress, and resting blood pressure was 80/50 mm Hg. Physical examination was largely unremarkable, with the exception of a soft, nontender, movable abdominal mass. The patient complained of mild dyspnea with stair climbing, but she was otherwise asymptomatic. Although high-grade atrioventricular (AV) block is considered a relative contraindication for exercise testing, this patient was referred for an exercise ECG stress test to assist in determining whether a electronic pacemaker or other therapy was necessary. A symptom-limited maximal ECG treadmill test was performed using the Bruce protocol. The resting HR before the test was 47 bpm, and resting blood pressure was 70/50 mm Hg. Beginning with the first stage of the test, sinus rhythm with a first-degree AV block was observed. The patient achieved 10.2 METs, with an HRpeak of 122 bpm and a peak blood pressure of 122/70 mm Hg. No ectopy was noted during exercise, no ST segment changes occurred during exercise or recovery, and the patient remained asymptomatic. The test was terminated because of the patient's inability to keep pace with the treadmill. On the basis of these results, no medical therapy or electronic pacemaker was recommended at this time, and the patient was permitted to undergo surgery for uterine fibroid tumors. This case illustrates that although complete heart block is a relative contraindication to exercise testing in some patients, the benefits of testing outweigh the risks.

Novel molecular mechanism involving alpha1D (Cav1.3) L-type calcium channel in autoimmune-associated sinus bradycardia

BACKGROUND

Congenital heart block (CHB) is an autoimmune disease that affects fetuses/infants born to mothers with anti-Ro/La antibodies (positive IgG). Although the hallmark of CHB is complete atrioventricular block, sinus bradycardia has been reported recently in animal models of CHB. Interestingly, knockout of the neuroendocrine alpha1D Ca channel in mice results in significant sinus bradycardia and atrioventricular block, a phenotype reminiscent to that seen in CHB. Here, we tested the hypothesis that the alpha1D Ca channel is a novel target for positive IgG.

METHODS AND RESULTS

Reverse transcription-polymerase chain reaction, confocal indirect immunostaining, and Western blot data established the expression of the alpha1D Ca channel in the human fetal heart. The effect of positive IgG on alpha1D Ca current (I(Ca-L)) was characterized in heterologous expression systems (tsA201 cells and Xenopus oocytes) because of the unavailability of alpha1D-specific modulators. alpha1D I(Ca-L) activated at negative potentials (between -60 and -50 mV). Positive IgG inhibited alpha1D I(Ca-L) in both expression systems. This inhibition was rescued by a Ca channel activator, Bay K8644. No effect on alpha1D I(Ca-L) was observed with negative IgG and denatured positive IgG. Western blot data showed that positive IgG binds directly to alpha1D Ca channel protein.

CONCLUSIONS

The data are the first to demonstrate (1) expression of the alpha1D Ca channel in human fetal heart, (2) inhibition of alpha1D I(Ca-L) by positive IgG, and (3) direct cross-reactivity of positive IgG with the alpha1D Ca channel protein. Given that alpha1D I(Ca-L) activates at voltages within the pacemaker's diastolic depolarization, inhibition of alpha1D I(Ca-L) in part may account for autoimmune-associated sinus bradycardia. In addition, Bay K8644 rescue of alpha1D I(Ca-L) inhibition opens new directions in the development of pharmacotherapeutic approaches in the management of CHB.

Effect of maternal autoantibodies on fetal cardiac conduction: an experimental murine model

The pathogenesis of congenital heart block (CHB) remains unclear. The occurrence rate of neonatal CHB is low, even in murine models of lupus erythematosus. The assessment of heart block in murine maternal lupus models by measuring atrioventricular conduction in neonatal offspring is potentially confounded by fetal wastage. We therefore sought to develop a murine CHB model with a superior immune response and to use embryonic Doppler echocardiography to observe conduction system damage in the fetus. Mature 8-wk-old female C3H/HeJ mice (n=43) were immunized with 60 kD Ro, 48 kD La, or recombinant calreticulin autoantigens. ELISA confirmed that significant serum autoantibodies developed in all three immunized groups when compared with controls. Starting at 13 d of gestation, a significantly lower fetal heart rate (HR) and a higher percentage of fetal bradycardia/atrioventricular block (AVB, nonadvanced second degree) were observed in all immunized groups, compared with controls. There was 9-18% nonadvanced second-degree AVB in immunized groups and 0% in controls at <18 d of gestation. Neonatal electrocardiograms demonstrated only 1 degrees AVB in immunized groups. Maternal immunization with 60 kD Ro, 48 kD La, or recombinant calreticulin autoantigens resulted in AVB in a significant percentage of fetuses, however, lesser degrees of AVB were seen at birth. Significant fetal bradycardia and AVB may be missed by assessment only at birth in murine models of CHB due to fetal wastage.

Anti-SSA/Ro antibodies and the heart: more than complete congenital heart block? A review of electrocardiographic and myocardial abnormalities and of treatment options

Apart from complete and incomplete congenital heart block (CHB), new cardiac manifestations related to anti-SSA/Ro antibodies have been reported in children born to mothers bearing these antibodies. These manifestations include transient fetal first-degree heart block, prolongation of corrected QT (QTc) interval, sinus bradycardia, late-onset cardiomyopathy, endocardial fibroelastosis and cardiac malformations. Anti-SSA/Ro antibodies are not considered pathogenic to the adult heart, but a prolongation of the QTc interval has recently been reported in adult patients and is still a matter of debate. Treatment of CHB is not well established and needs to be assessed carefully. The risks and benefits of prenatal fluorinated steroids are discussed.

Outcome of pregnancies in patients with anti-SSA/Ro antibodies: A study of 165 pregnancies, with special focus on electrocardiographic variations in the children and comparison with a control group

OBJECTIVE

Aside from congenital heart block (CHB), sinus bradycardia and prolongation of the corrected QT (QTc) interval have been reported in infants born to mothers with anti-SSA antibodies. To assess the pathologic nature of these manifestations, this study focused on electrocardiographic (EKG) variations in these children, comparing them with findings in a control group.

METHODS

We studied 165 consecutive pregnancies in 106 anti-SSA-positive women with connective tissue diseases (CTDs). EKGs obtained on 58 children of this group were compared with those obtained on 85 infants born to mothers with CTD who were negative for both anti-SSA and anti-SSB.

RESULTS

No statistically significant difference was seen between the 2 study groups with regard to gestational age, prematurity, birth weight, age of the children at the time of EKG, age of the mothers, or treatments received by the mothers during their pregnancies. Seven of 137 children developed cutaneous neonatal lupus syndrome; 1 child developed CHB (CHB risk of 1 in 99 [1%] if only the first prospectively observed pregnancy in women without a history of CHB is included in the analysis). For EKGs recorded during the first 2 months of life, the mean +/- SD PR interval was 96 +/- 16 msec in the anti-SSA-positive group and 96 +/- 13 msec in the anti-SSA-negative group (P = 0.84), with mean QTc values of 397 +/- 27 and 395 +/- 25 msec (P = 0.57) and mean heart rates of 141 +/- 23 and 137 +/- 21 beats per minute (P = 0.20), respectively. No difference in the PR interval, QTc interval, or heart rate was observed for EKGs obtained between 2 and 4 months of life. When EKGs obtained at 0-2 months were compared with those obtained at 2-4 months, a physiologic prolongation of the QTc interval was observed in both study groups. No sudden infant death or symptomatic arrhythmia occurred during the first year of life.

CONCLUSION

The EKG findings in children of anti-SSA-positive and anti-SSA-negative mothers were not significantly different. Our results suggest that the prolongation of the QTc interval and sinus bradycardia that have recently been reported in children of mothers with anti-SSA antibodies occur independently of the anti-SSA antibodies. The pathologic nature of these EKG variations was not confirmed by our controlled study.

Incidence and spectrum of neonatal lupus erythematosus: a prospective study of infants born to mothers with anti-Ro autoantibodies

OBJECTIVE

Neonatal lupus erythematosus (NLE) is characterized by complete congenital heart block (CCHB), cutaneous rash, and laboratory abnormalities in infants born to mothers with autoantibodies directed against SSA/Ro, SSB/La, or both. We carried out a prospective study to determine the incidence of individual NLE features.

STUDY DESIGN

The study was performed in two centers: Toronto, Canada, and Milano, Italy. Mothers had been referred for the presence of anti-SSA/Ro autoantibodies, regardless of their diagnosis. All the children were seen at least once within the first 6 months of life for clinical evaluation and laboratory testing. The study group consisted of 128 infants born from 124 pregnancies in 112 women with anti-Ro antibodies with or without anti-La antibodies.

RESULTS

There were two cases of CCHB for an overall percentage of 1.6%. Twenty-one children (16%) developed cutaneous NLE. Laboratory testing showed hematologic abnormalities in 27% of the babies and elevation of liver enzymes in 26%.

CONCLUSIONS

Mothers with autoimmune diseases and anti-Ro antibodies are at risk of delivering a child with NLE but at a low risk of delivering a child with CCHB. Infants born to mothers with anti-Ro or anti-La antibodies should be monitored for other features of NLE in addition to CCHB.

Optical mapping of activation patterns in an animal model of congenital heart block

Congenital heart block (CHB) is associated with high mortality and affects children of mothers with autoantibodies (IgG) to ribonucleoproteins SSB/La and SSA/Ro. IgG from mothers of children with CHB (positive IgG) was used to assess activation patterns in both the right atrium (RA) and right ventricle (RV) of Langendorff-perfused young rabbit hearts. Optical action potentials (AP) were obtained by using a 124-site photodiode array with 4-[-[2-(di-n-butylamino)-6-naphthyl]vinyl]pyridinium. Optical APs were recorded to simultaneously image activation patterns from the RA and RV. Perfusion of positive IgG (800--1,200 micro resulted in sinus bradycardia and varying degrees of heart block. Activation maps revealed marked conduction delay at the sinoatrial junction but only minor changes in overall atrial and ventricular activation patterns. No conduction disturbances were seen in the presence of IgG from mothers with healthy children. In conclusion, besides atrioventricular (AV) block, positive IgG induces sinus bradycardia. These results establish that the sequelae of CHB are associated with impaired intrasinus and/or sinoatrial conduction. The findings raise the possibility that sinus bradycardia in the developing heart may indicate the potential for AV conduction disturbances.