Progressive Familial Heart Block Type Research Articles

Sudden cardiac death in young athletes: exome sequencing results

BACKGROUND: Ranking risks and prevention of sudden cardiac death in professional young athletes is a crucial unresolved problem in sports medicine. Intense physical exertion can lead to adaptive changes in the cardiovascular system, which can mask inherited diseases with predominant myocardial involvement, such as hypertrophic cardiomyopathy. Undiagnosed cases of such diseases can be a risk factor for fatal outcomes associated with training and competitive activities, making this problem extremely relevant.
 AIM: To conduct a diagnostic search for possible molecular causes of cardiovascular diseases associated with a high risk of sudden death in young athletes.
 MATERIALS AND METHODS: Whole-exome sequencing of DNA extracted from two groups of biomaterials was performed: nine autopsy samples of heart tissues from young athletes who died during intense physical exertion and three venous blood samples from active athletes of the Russian national team. Obtained data were subjected to bioinformatic analysis and clinical interpretation.
 RESULTS: In total, 12 DNA samples were analyzed using a panel of 277 genes associated with the development of cardiovascular diseases with a high risk of sudden death. In 4 of 12 samples (33.3%), variants possibly related to the phenotype were found upon clinical interpretation of the sequencing results. A pathogenic variant was found in the MYBPC3 gene, leading to the development of hypertrophic cardiomyopathy (OMIM: 115197), and a potential pathogenic variant was found in the TRPM4 gene, associated with progressive familial heart block type IB (OMIM: 618531). Moreover, two variants with uncertain clinical significance were found in CAV3 and SCN1B.
 CONCLUSION: Currently, data from high-throughput sequencing are collected to identify the molecular basis of cardiovascular diseases with a high risk of sudden cardiac death. Identifying young athletes who are carriers of pathogenic and possibly pathogenic gene variants is beneficial for preventing fatal outcomes and personalizing medical support for professional athletes.

Familial Complete Atrioventricular Block (FCAVB) DDDR Pacemaker Implant with Synchromax

We present the case of a family with complete atrioventricular block involving several generations, with autosomal dominant Mendelian inheritance, where one of those long-term affected presents heart failure. This entity corresponds to progressive familial heart block type I, mainly described in South Africa. Permanent Pacemaker implant using Synchromax was successful.

Abstract 506: Trpm4 Contributes to Ca 2+ -dependent Triggered Arrhythmias in Pathological Conditions

Rationale: TRPM4 is a Ca 2+ -activated non-selective cation channel that is abundantly expressed in the heart. Mutations in the Trpm4 gene are associated with human cardiac conduction disorders, including Progressive Familial Heart Block type I (PFHBI) and Brugada Syndrome. However, the mechanistic role and in vivo significance of TRPM4 in the triggering of cardiac arrhythmias is still completely unclear. Objective: To investigate the role of TRPM4 during pathological Ca 2+ handling in the heart. Methods and results: Using three in vivo pro-arrhythmic assays, we found that Trpm4 -/- mice show a reduced arrhythmic burden compared to control mice. First, aconitine intoxication resulted in severe cardiac arrhythmias, both in WT and Trpm4 -/- animals, but Trpm4 -/- mice developed significantly less ventricular ectopic beats (VEBs) and showed a lower arrhythmic score. Second, during ischemia-reperfusion, induced by 30 min of LAD ligation, significantly more WT animals developed arrhythmias compared to Trpm4 -/- mice. Third, catecholaminergic polymorphic ventricular tachycardia (CPVT) mice, carrying mutations in RyR2, were subjected to a stress test. Significantly more RyR +/R2474S -Trpm4 +/+ animals developed arrhythmias compared to RyR +/R2474S -Trpm4 -/- mice. Conclusion: Our data establish that TRPM4 represents a novel target in the prevention and treatment of cardiac arrhythmias.

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

BackgroundProgressive 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 resultsIn 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. ConclusionsThis 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.

Abstract 182: Characterization of Electrophysiological and Conduction Parameters of the Heart in Trpm4-deficient Mice

TRPM4 is a Ca2+-activated non-selective cation channel that belongs to the family of the Transient Receptor Potential (TRP) ion channels. Importantly, TRPM4 is impermeable for Ca2+ and is involved in different Ca2+-dependent cell functions, such as exocytosis, contraction and cell death. Trpm4 is known to be expressed in atrial and ventricular cardiomyocytes. The interest in the functional role of TRPM4 in the heart has risen further by the discovery of Trpm4 mutations that are linked to cardiac conduction disorders, including Progressive Familial Heart Block type I (PFHBI) and Brugada Syndrome. Both gain-of-function and loss-of-function mutation were described in patients with cardiac conduction diseases. Recently, our group showed that TRPM4 plays a role during the late repolarization phase of the action potential in murine ventricular cardiomyocytes and that deletion of the Trpm4 gene leads to shorter ventricular action potentials. To characterize if deletion of Trpm4 has an effect on the conduction properties of the heart, an in depth electrophysiological study was performed in living mice. An octapolar catheter was inserted into the right atrium and ventricle of the heart to measure intracardial electrograms. The atrial-His (AH) and His-ventricular (HV) intervals were calculated and no differences were found between WT and Trpm4-deficient mice. Additionally, more detailed conduction parameters of the heart were determined by use of programmed electrical stimulation (PES) protocols. Sinus node recovery time (SNRT) was not different between WT and Trpm4-deficient mice. Effective refractory period of atrium (AERP), AV node (AVNERP) and ventricle (VERP) were in the same range in WT and Trpm4-deficient mice. Wenckebach periodicity, the parameter for AV nodal conduction, was also not different between WT and Trpm4-deficient mice. These results suggest that deletion of Trpm4 has no effects on the conduction properties of the murine heart.

P134Increased beta-adrenergic inotropy in ventricular myocardium from Trpm4 knockout mice

TRPM4 have an extensive role in regulating intracellular calcium dependent cell functions in many cells including mast cells, chromaffin cells and recently in neurons. TRPM4 proteins constitute calcium activated, but calcium impermeable, non-selective cation channels and are expressed both in atrial and ventricular cardiomyocytes. The Trpm4 gene has recently been associated with several disorders, including cardiac conduction diseases such as Progressive Familial Heart Block Type 1, Right Bundle Branch Block and Brugada Syndrome. The physiological function of TRPM4 in the heart remains however incompletely understood. The objective of this study was to establish the role of TRPM4 in left ventricular muscle function. We used TRPM4 knockout mice and performed patch-clamp experiments, membrane potential measurements, microfluorometry, contractility measurements and in vivo pressure-volume loop analysis. We demonstrate that TRPM4 proteins are functionally present in mouse ventricular myocytes and are activated upon calcium induced calcium release. Measurements of action potential duration show that TRPM4 is active during the repolarization of the ventricular action potential. We show a significantly decreased time for 50 % and 90 % repolarization in Trpm4 knockout ventricular myocytes. We also provide evidence that this change in action potential shape leads to an increased driving force for the L-type calcium current during the action potential. Moreover, in vivo pressure-volume analysis and in vitro contraction strength measurement on isolated papillary muscles displays an increased β-adrenergic inotropic response in TRPM4 knockout mice. Taking together these data, our results show that functional TRPM4 proteins are novel determinants of the inotropic effect of β-adrenergic stimulation on the ventricular heart muscle.

TRPM4 channels in the cardiovascular system

The non-selective Transient Receptor Potential Melastatin 4 (TRPM4) cation channel is abundantly expressed in cardiac cells, being involved in several aspects of cardiac rhythmicity, including cardiac conduction, pace making and action-potential repolarization. Dominantly inherited mutations in the TRPM4 gene are associated with the cardiac bundle-branch disorder progressive familial heart block type I (PFHBI) and isolated cardiac conduction disease (ICCD) giving rise to atrio-ventricular conduction block (AVB), right bundle branch block, bradycardia, and the Brugada syndrome. The mutant phenotypes closely resemble those associated with mutations in the SCN5A gene, encoding the voltage-gated Na(+) channel NaV1.5. These observations and the unexpected partnership with sulfonylurea-receptors (SURs) makes the TRPM4 channel a promising novel target for treatment of cardiac disorders.

A Connexin40 Mutation Associated With a Malignant Variant of Progressive Familial Heart Block Type I

Progressive familial heart block type I (PFHBI) is a hereditary arrhythmia characterized by progressive conduction disturbances in the His-Purkinje system. PFHBI has been linked to genes such as SCN5A that influence cardiac excitability but not to genes that influence cell-to-cell communication. Our goal was to explore whether nucleotide substitutions in genes coding for connexin proteins would associate with clinical cases of PFHBI and if so, to establish a genotype-cell phenotype correlation for that mutation. We screened 156 probands with PFHBI. In addition to 12 sodium channel mutations, we found a germ line GJA5 (connexin40 [Cx40]) mutation (Q58L) in 1 family. Heterologous expression of Cx40-Q58L in connexin-deficient neuroblastoma cells resulted in marked reduction of junctional conductance (Cx40-wild type [WT], 22.2±1.7 nS, n=14; Cx40-Q58L, 0.56±0.34 nS, n=14; P<0.001) and diffuse localization of immunoreactive proteins in the vicinity of the plasma membrane without formation of gap junctions. Heteromeric cotransfection of Cx40-WT and Cx40-Q58L resulted in homogenous distribution of proteins in the plasma membrane rather than in membrane plaques in ≈50% of cells; well-defined gap junctions were observed in other cells. Junctional conductance values correlated with the distribution of gap junction plaques. Mutation Cx40-Q58L impairs gap junction formation at cell-cell interfaces. This is the first demonstration of a germ line mutation in a connexin gene that associates with inherited ventricular arrhythmias and emphasizes the importance of Cx40 in normal propagation in the specialized conduction system.

Enhanced impact of SCN5A mutation associated with long QT syndrome in fetal splice isoform

Congenital long QT syndrome (LQTS) is an inherited syndrome characterized by prolongation of the QT interval on the electrocardiogram and an increased susceptibility to life-threatening ventricular arrhythmias. Mutations in the SCN5A gene, which encodes the α-subunit of the cardiac Na+ channel, represent the third most common cause of LQTS, after mutations in potassium channel genes KCNQ1 and KCNH2. Moreover, mutations in SCN5A have been linked to other types of inherited channelopathies, including the Brugada syndrome, progressive familial heart block type 1, sick sinus syndrome type 1, idiopathic ventricular fibrillation, familiar atrial standstill, dilated cardiomyopathy type 1E, and sudden infant death syndrome (SIDS).

Impaired endocytosis of the ion channel TRPM4 is associated with human progressive familial heart block type I

Progressive familial heart block type I (PFHBI) is a progressive cardiac bundle branch disease in the His-Purkinje system that exhibits autosomal-dominant inheritance. In 3 branches of a large South African Afrikaner pedigree with an autosomal-dominant form of PFHBI, we identified the mutation c.19G-->A in the transient receptor potential cation channel, subfamily M, member 4 gene (TRPM4) at chromosomal locus 19q13.3. This mutation predicted the amino acid substitution p.E7K in the TRPM4 amino terminus. TRPM4 encodes a Ca2+-activated nonselective cation (CAN) channel that belongs to the transient receptor potential melastatin ion channel family. Quantitative analysis of TRPM4 mRNA content in human cardiac tissue showed the highest expression level in Purkinje fibers. Cellular expression studies showed that the c.19G-->A missense mutation attenuated deSUMOylation of the TRPM4 channel. The resulting constitutive SUMOylation of the mutant TRPM4 channel impaired endocytosis and led to elevated TRPM4 channel density at the cell surface. Our data therefore revealed a gain-of-function mechanism underlying this type of familial heart block.

Endocytic control of ion channel density as a target for cardiovascular disease

Ion channels encoded by the human ether-a-go-go-related gene (HERG) give rise to the rapidly activating delayed rectifier K+ current (IKr), the perturbation of which causes ventricular arrhythmias associated with inherited and acquired long QT syndrome. Electrolyte imbalances, such as reduced serum K+ levels (hypokalemia), also trigger these potentially fatal arrhythmias. In this issue of the JCI, Guo et al. report that physiological levels of serum K+ are required to maintain normal HERG surface density in HEK 293 cells and IKr in rabbit cardiomyocytes. They found that hypokalemia evoked HERG channel ubiquitination, enhanced internalization via endocytosis, and ultimately degradation at the lysosome, thus identifying unbridled turnover as a mechanism of hypokalemia-induced arrhythmia. But too little channel turnover can also cause disease, as suggested by Kruse et al. in a study also in this issue. The authors identified mutations in TRPM4--a nonselective cation channel--in a large family with progressive familial heart block type I and showed that these mutations prevented channel internalization (see the related articles beginning on pages 2745 and 2737, respectively).

A gene locus for progressive familial heart block type II (PFHBII) maps to chromosome 1q32.2-q32.3

Cardiac conduction defects that are associated with dilated cardiomyopathy (DCM) are generally considered to be sporadic clinical entities, although familial forms of disorders with these clinical features have been identified in a number of families in different countries. An autosomal dominant cardiac disorder characterised by conduction abnormalities and DCM, termed progressive familial heart block type II (PFHBII) (OMIM 140400), has been described in a South African Caucasian family of Northern European descent. Known candidate loci for isolated conduction disorders, isolated DCM and conduction disorders complicated by DCM were excluded from disease causation in this family by linkage analysis, with the exception of the DCM-associated (CMD1D) locus on chromosome 1q32, where a maximum multipoint lod score of 3.7 in the interval between D1S3753 and D1S414, was generated. This region encompassed the troponin T gene (TNNT2), however, genetic fine mapping and haplotype analysis excluded TNNT2 as cause of PFHBII and placed the disease-causative gene within a 3.9 cM (2.85 Mb) interval, flanked by D1S70 and D1S505. Analysis of KCNH1, KIAA0205, LAMB3 and PPP2R5A, which map within the critical interval, indicated that the PFHBII-causative mutation does not lie within the coding regions or splice junctions of these plausible candidate genes. The data indicate the existence of a novel locus involved in the pathogenesis of cardiac conduction abnormalities and DCM.

Calumenin, a Ca2+-binding protein retained in the endoplasmic reticulum with a novel carboxyl-terminal sequence, HDEF.

We have identified and characterized a cDNA encoding a novel Ca2+-binding protein named calumenin from mouse heart by the signal sequence trap method. The deduced amino acid sequence (315 residues) of calumenin contains an amino-terminal signal sequence and six Ca2+-binding (EF-hand) motifs and shows homology with reticulocalbin, Erc-55, and Cab45. These proteins seem to form a new subset of the EF-hand protein family expressed in the lumen of the endoplasmic reticulum (ER) and Golgi apparatus. Purified calumenin had Ca2+-binding ability. The carboxyl-terminal tetrapeptide His-Asp-Glu-Phe was shown to be responsible for retention of calumenin in ER by the retention assay, immunostaining with a confocal laser microscope, and the deglycosylation assay. This is the first report indicating that the Phe residue is included in the ER retention signal. Calumenin is expressed most strongly in heart of adult and 18.5-day embryos. The calumenin gene (Calu) was mapped at the proximal portion of mouse chromosome 7.