Mutation Load of Multiple Ion Channel Gene Mutations in Brugada Syndrome

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Brugada Syndrome: More than a Monogenic Channelopathy

Brugada syndrome (BrS) is an inherited cardiac channelopathy first diagnosed in 1992 but still considered a challenging disease in terms of diagnosis, arrhythmia risk prediction, pathophysiology and management. Despite about 20% of individuals carrying pathogenic variants in the SCN5A gene, the identification of a polygenic origin for BrS and the potential role of common genetic variants provide the basis for applying polygenic risk scores for individual risk prediction. The pathophysiological mechanisms are still unclear, and the initial thinking of this syndrome as a primary electrical disease is evolving towards a partly structural disease. This review focuses on the main scientific advancements in the identification of biomarkers for diagnosis, risk stratification, pathophysiology and therapy of BrS. A comprehensive model that integrates clinical and genetic factors, comorbidities, age and gender, and perhaps environmental influences may provide the opportunity to enhance patients' quality of life and improve the therapeutic approach.

Genetic Abnormalities of the Sinoatrial Node and Atrioventricular Conduction

The peculiar electrophysiological properties of the sinoatrial node and the cardiac conduction system are key components of the normal physiology of cardiac impulse generation and propagation. Multiple genes and transcription factors and metabolic proteins are involved in their development and regulation. In this review, we have summarized the genetic underlying causes, key clinical findings, and the latest available clinical evidence. We will discuss clinical diagnosis and management of the genetic conditions associated with conduction disorders that are more prevalent in clinical practice, for this reason, very rare genetic diseases presenting sinus node or cardiac conduction system abnormalities are not discussed.

Pharmacological Modulation and (Patho)Physiological Roles of TRPM4 Channel-Part 2: TRPM4 in Health and Disease

Transient receptor potential melastatin 4 (TRPM4) is a unique member of the TRPM protein family and, similarly to TRPM5, is Ca²⁺ sensitive and permeable for monovalent but not divalent cations. It is widely expressed in many organs and is involved in several functions; it regulates membrane potential and Ca²⁺ homeostasis in both excitable and non-excitable cells. This part of the review discusses the currently available knowledge about the physiological and pathophysiological roles of TRPM4 in various tissues. These include the physiological functions of TRPM4 in the cells of the Langerhans islets of the pancreas, in various immune functions, in the regulation of vascular tone, in respiratory and other neuronal activities, in chemosensation, and in renal and cardiac physiology. TRPM4 contributes to pathological conditions such as overactive bladder, endothelial dysfunction, various types of malignant diseases and central nervous system conditions including stroke and injuries as well as in cardiac conditions such as arrhythmias, hypertrophy, and ischemia-reperfusion injuries. TRPM4 claims more and more attention and is likely to be the topic of research in the future.

Genetic Abnormalities of the Sinoatrial Node and Atrioventricular Conduction

The peculiar electrophysiological properties of the sinoatrial node and the cardiac conduction system are key components of the normal physiology of cardiac impulse generation and propagation. Multiple genes and transcription factors and metabolic proteins are involved in their development and regulation. In this review, we have summarized the genetic underlying causes, key clinical findings, and the latest available clinical evidence. We will discuss clinical diagnosis and management of the genetic conditions associated with conduction disorders that are more prevalent in clinical practice, for this reason, very rare genetic diseases presenting sinus node or cardiac conduction system abnormalities are not discussed.

Distinct Features of Probands With Early Repolarization and Brugada Syndromes Carrying SCN5A Pathogenic Variants

BACKGROUND

Two major forms of inherited J-wave syndrome (JWS) are recognized: early repolarization syndrome (ERS) and Brugada syndrome (BrS).

OBJECTIVES

This study sought to assess the distinct features between patients with ERS and BrS carrying pathogenic variants in SCN5A.

METHODS

Clinical evaluation and next-generation sequencing were performed in 262 probands with BrS and 104 with ERS. Nav1.5 and Kv4.3 channels were studied with the use of patch-clamp techniques. A computational model was used to investigate the protein structure.

RESULTS

The SCN5A+ yield in ERS was significantly lower than in BrS (9.62% vs 22.90%; P = 0.004). Patients diagnosed with ERS displayed shorter QRS and QTc than patients with BrS. More than 2 pathogenic SCN5A variants were found in 5 probands. These patients displayed longer PR intervals and QRS duration and experienced more major arrhythmia events (MAE) compared with those carrying only a single pathogenic variant. SCN5A-L1412F, detected in a fever-induced ERS patient, led to total loss of function, destabilized the Nav1.5 structure, and showed a dominant-negative effect, which was accentuated during a febrile state. ERS-related SCN5A-G452C did not alter the inward sodium current (INa) when SCN5A was expressed alone, but when coexpressed with KCND3 it reduced peak INa by 44.52% and increased the transient outward potassium current (Ito) by 106.81%.

CONCLUSIONS

These findings point to SCN5A as a major susceptibility gene in ERS as much as it is in BrS, whereas the lower SCN5A+ ratio in ERS indicates the difference in underlying electrophysiology. These findings also identify the first case of fever-induced ERS and demonstrate a critical role of Ito in JWS and a higher risk for MAE in JWS probands carrying multiple pathogenic variants in SCN5A.

Mutations in MYBPC3 and MYH7 in Association with Brugada Type 1 ECG Pattern: Overlap between Brugada Syndrome and Hypertrophic Cardiomyopathy?

Brugada syndrome (BrS) is an inherited disorder with high allelic and genetic heterogeneity clinically characterized by typical coved-type ST segment elevation at the electrocardiogram (ECG), which may occur either spontaneously or after provocative drug testing. BrS is classically described as an arrhythmic condition occurring in a structurally normal heart and is associated with the risk of ventricular fibrillation and sudden cardiac death (SCD). We studied five patients with spontaneous or drug-induced type 1 ECG pattern, variably associated with symptoms and a positive family history through a Next Generation Sequencing panels approach, which includes genes of both channelopathies and cardiomyopathies. We identified variants in MYBPC3 and in MYH7, hypertrophic cardiomyopathy (HCM) genes (MYBPC3: p.Lys1065Glnfs*12 and c.1458-1G > A, MYH7: p.Arg783His, p.Val1213Met, p.Lys744Thr). Our data propose that Brugada type 1 ECG may be an early electrocardiographic marker of a concealed structural heart disease, possibly enlarging the genotypic overlap between Brugada syndrome and cardiomyopathies.

Functional Characterization of Two Novel Mutations in SCN5A Associated with Brugada Syndrome Identified in Italian Patients

Background. Brugada syndrome (BrS) is an autosomal dominantly inherited cardiac disease characterized by “coved type” ST-segment elevation in the right precordial leads, high susceptibility to ventricular arrhythmia and a family history of sudden cardiac death. The SCN5A gene, encoding for the cardiac voltage-gated sodium channel Nav1.5, accounts for ~20–30% of BrS cases and is considered clinically relevant. Methods. Here, we describe the clinical findings of two Italian families affected by BrS and provide the functional characterization of two novel SCN5A mutations, the missense variant Pro1310Leu and the in-frame insertion Gly1687_Ile1688insGlyArg. Results. Despite being clinically different, both patients have a family history of sudden cardiac death and had history of arrhythmic events. The Pro1310Leu mutation significantly reduced peak sodium current density without affecting channel membrane localization. Changes in the gating properties of expressed Pro1310Leu channel likely account for the loss-of-function phenotype. On the other hand, Gly1687_Ile1688insGlyArg channel, identified in a female patient, yielded a nearly undetectable sodium current. Following mexiletine incubation, the Gly1687_Ile1688insGlyArg channel showed detectable, albeit very small, currents and biophysical properties similar to those of the Nav1.5 wild-type channel. Conclusions. Overall, our results suggest that the degree of loss-of-function shown by the two Nav1.5 mutant channels correlates with the aggressive clinical phenotype of the two probands. This genotype-phenotype correlation is fundamental to set out appropriate therapeutical intervention.

Standard ECG in Brugada Syndrome as a Marker of Prognosis: From Risk Stratification to Pathophysiological Insights

Background

The 12‐lead ECG plays a key role in the diagnosis of Brugada syndrome (BrS). Since the spontaneous type 1 ECG pattern was first described, several other ECG signs have been linked to arrhythmic risk, but results are conflicting.

Methods and Results

We performed a systematic review to clarify the associations of these specific ECG signs with the risk of syncope, sudden death, or equivalents in patients with BrS. The literature search identified 29 eligible articles comprising overall 5731 patients. The ECG findings associated with an incremental risk of syncope, sudden death, or equivalents (hazard ratio ranging from 1.1–39) were the following: localization of type 1 Brugada pattern (in V2 and peripheral leads), first‐degree atrioventricular block, atrial fibrillation, fragmented QRS, QRS duration >120 ms, R wave in lead aVR, S wave in L1 (≥40 ms, amplitude ≥0.1 mV, area ≥1 mm²), early repolarization pattern in inferolateral leads, ST‐segment depression, T‐wave alternans, dispersion of repolarization, and Tzou criteria.

Conclusions

At least 12 features of standard ECG are associated with a higher risk of sudden death in BrS. A multiparametric risk assessment approach based on ECG parameters associated with clinical and genetic findings could help improve current risk stratification scores of patients with BrS and warrants further investigation.

Registration

URL: https://www.crd.york.ac.uk/prospero/. Unique identifier: CRD42019123794.

Evaluating the Use of Genetics in Brugada Syndrome Risk Stratification

The evolution of the current dogma surrounding Brugada syndrome (BrS) has led to a significant debate about the real usefulness of genetic testing in this syndrome. Since BrS is defined by a particular electrocardiogram (ECG) pattern, after ruling out certain possible causes, this disease has come to be defined more for what it is not than for what it is. Extensive research is required to understand the effects of specific individual variants, including modifiers, rather than necessarily grouping together, for example, “all SCN5A variants” when trying to determine genotype-phenotype relationships, because not all variants within a particular gene act similarly. Genetic testing, including whole exome or whole genome testing, and family segregation analysis should always be performed when possible, as this is necessary to advance our understanding of the genetics of this condition. All considered, BrS should no longer be considered a pure autosomal dominant disorder, but an oligogenic condition. Less common patterns of inheritance, such as recessive, X–linked, or mitochondrial may exist. Genetic testing, in our opinion, should not be used for diagnostic purposes. However, variants in SCN5A can have a prognostic value. Patients should be diagnosed and treated per the current guidelines, after an arrhythmologic examination, based on the presence of the specific BrS ECG pattern. The genotype characterization should come in a second stage, particularly in order to guide the familial diagnostic work-up. In families in which an SCN5A pathogenic variant is found, genetic testing could possibly contribute to the prognostic risk stratification.

Single-cell transcriptomics trajectory and molecular convergence of clinically relevant mutations in Brugada syndrome

Brugada syndrome (BrS) is a rare, inherited arrhythmia with high risk of sudden cardiac death. To evaluate the molecular convergence of clinically relevant mutations and to identify developmental cardiac cell types that are associated with BrS etiology, we collected 733 mutations represented by 16 sodium, calcium, potassium channels, and regulatory and structural genes related to BrS. Among the clinically relevant mutations, 266 are unique singletons and 88 mutations are recurrent. We observed an over-representation of clinically relevant mutations (∼80%) in SCN5A gene and also identified several candidate genes, including GPD1L, TRPM4, and SCN10A. Furthermore, protein domain enrichment analysis revealed that a large proportion of the mutations impacted ion transport domains in multiple genes, including SCN5A, TRPM4, and SCN10A. A comparative protein domain analysis of SCN5A further established a significant (P = 0.04) enrichment of clinically relevant mutations within ion transport domain, including a significant (P = 0.02) mutation hotspot within 1321-1380 residue. The enrichment of clinically relevant mutations within SCN5A ion transport domain is stronger (P = 0.00003) among early onset of BrS. Our spatiotemporal cellular heart developmental (prenatal to adult) trajectory analysis applying single-cell transcriptome identified the most frequently BrS-mutated genes (SCN5A and GPD1L) are significantly upregulated in the prenatal cardiomyocytes. A more restrictive cellular expression trajectory is prominent in the adult heart ventricular cardiomyocytes compared to prenatal. Our study suggests that genomic and proteomic hotspots in BrS converge into ion transport pathway and cardiomyocyte as a major BrS-associated cell type that provides insight into the complex genetic etiology of BrS.NEW & NOTEWORTHY Brugada syndrome is a rare inherited arrhythmia with high risk of sudden cardiac death. We present the findings for a molecular convergence of clinically relevant mutations and identification of a single-cell transcriptome-derived cardiac cell types that are associated with the etiology of BrS. Our study suggests that genomic and proteomic hotspots in BrS converge into ion transport pathway and cardiomyocyte as a major BrS-associated cell type that provides insight into the complex genetic etiology of BrS.

Upregulation of transient receptor potential melastatin 4 (TRPM4) in ventricular fibroblasts from heart failure patients

The transient receptor potential melastatin 4 (TRPM4) is a Ca2+-activated nonselective monovalent cation channel belonging to the TRP channel superfamily. TRPM4 is widely expressed in various tissues and most abundantly expressed in the heart. TRPM4 plays a critical role in cardiac conduction. Patients carrying a gain-of-function or loss-of-function mutation of TRPM4 display impaired cardiac conduction. Knockout or over-expression of TRPM4 in mice recapitulates conduction defects in patients. Moreover, recent studies have indicated that TRPM4 plays a role in hypertrophy and heart failure. Whereas the role of TRPM4 mediated by cardiac myocytes has been well investigated, little is known about TRPM4 and its role in cardiac fibroblasts. Here we show that in human left ventricular fibroblasts, TRPM4 exhibits typical Ca2+-activation characteristics, linear current-voltage (I-V) relation, and monovalent permeability. TRPM4 currents recorded in fibroblasts from heart failure patients (HF) are more than 2-fold bigger than those from control individuals (CTL). The enhanced functional TRPM4 in HF is not resulted from changed channel properties, as TRPM4 currents from both HF and CTL fibroblasts demonstrate similar sensitivity to intracellular calcium activation and extracellular 9-phenanthrol (9-phen) blockade. Consistent with enhanced TRPM4 activity, the protein level of TRPM4 is about 2-fold higher in HF than that of CTL hearts. Moreover, TRPM4 current in CTL fibroblasts is increased after 24 hours of TGFβ1 treatment, implying that TRPM4 in vivo may be upregulated by fibrogenesis promotor TGFβ1. The upregulated TRPM4 in HF fibroblasts suggests that TRPM4 may play a role in cardiac fibrogenesis under various pathological conditions.

Inherited Cardiac Arrhythmia Syndromes: Focus on Molecular Mechanisms Underlying TRPM4 Channelopathies

The Transient Receptor Potential Melastatin 4 (TRPM4) is a transmembrane N-glycosylated ion channel that belongs to the large family of TRP proteins. It has an equal permeability to Na⁺ and K⁺ and is activated via an increase of the intracellular calcium concentration and membrane depolarization. Due to its wide distribution, TRPM4 dysfunction has been linked with several pathophysiological processes, including inherited cardiac arrhythmias. Many pathogenic variants of the TRPM4 gene have been identified in patients with different forms of cardiac disorders such as conduction defects, Brugada syndrome, and congenital long QT syndrome. At the cellular level, these variants induce either gain- or loss-of-function of TRPM4 channels for similar clinical phenotypes. However, the molecular mechanisms associating these functional alterations to the clinical phenotypes remain poorly understood. The main objective of this article is to review the major cardiac TRPM4 channelopathies and recent advances regarding their genetic background and the underlying molecular mechanisms.

TRP Channels: Current Perspectives in the Adverse Cardiac Remodeling

Calcium is an important second messenger required not only for the excitation-contraction coupling of the heart but also critical for the activation of cell signaling pathways involved in the adverse cardiac remodeling and consequently for the heart failure. Sustained neurohumoral activation, pressure-overload, or myocardial injury can cause pathologic hypertrophic growth of the heart followed by interstitial fibrosis. The consequent heart’s structural and molecular adaptation might elevate the risk of developing heart failure and malignant arrhythmia. Compelling evidences have demonstrated that Ca²⁺ entry through TRP channels might play pivotal roles in cardiac function and pathology. TRP proteins are classified into six subfamilies: TRPC (canonical), TRPV (vanilloid), TRPM (melastatin), TRPA (ankyrin), TRPML (mucolipin), and TRPP (polycystin), which are activated by numerous physical and/or chemical stimuli. TRP channels participate to the handling of the intracellular Ca²⁺ concentration in cardiac myocytes and are mediators of different cardiovascular alterations. This review provides an overview of the current knowledge of TRP proteins implication in the pathologic process of some frequent cardiac diseases associated with the adverse cardiac remodeling such as cardiac hypertrophy, fibrosis, and conduction alteration.

Functional characterization of TRPM4 variants identified in sudden unexpected natural death

BACKGROUND

The TRPM4 gene encodes the subunit of the Ca²⁺-activated nonselective cation channel, which is enriched in the specialized cardiac conduction system and Purkinje fibers. To date, several putative disease-causing variants in TRPM4 have been reported to be associated with cardiac arrhythmia and progressive conduction disease. Here, we report the functional effects of previously uncharacterized variants of uncertain significance (VUS) that we have found while performing a "genetic autopsy" in individuals who have suffered sudden unexpected death (SUD) in the New York City area.

METHODS AND RESULTS

We have identified thirteen uncommon missense VUS in TRPM4 by testing 95 targeted genes implicated in channelopathy and cardiomyopathy in 330 cases of SUD. In several cases there were co-existing VUS in one or more other genes that were tested. We selected four TRPM4 VUS (C20S, A380V, L595V and I1082S) for functional characterization, since these cases lacked detectable variants in other genes of our testing panel. Two of the cases were infants, one was a child and one an adult. RNA-seq data analysis showed that the longer TRPM4b splice variant is predominantly expressed in adult and fetal human heart. We therefore used site-directed mutagenesis to introduce these variants in a TRPM4b cDNA. HEK293 cells were transfected with the cDNAs and patch clamping was performed to assess the functional consequences of the TRPM4 mutants. The TRPM4 current was recorded in excised patches and was significantly reduced by each of the mutants. The total protein level of TRPM4-C20S was markedly decreased, whereas the A380V and L595V mutants exhibited decreased surface expression. The TRPM4-A380V current rapidly desensitized following patch excision.

CONCLUSIONS

Each of the VUS tested caused a defect in TRPM4 channel function via distinctly different mechanisms, hence, it lays the foundation for further co-segregation family studies and animal studies of the TRPM4 variants.

TRPM4 mutations to cause autosomal recessive and not autosomal dominant Brugada type 1 syndrome

Cardiac channelopathies, mainly Long QT and Brugada syndromes, are genetic disorders for which genotype/phenotypes relationships remains to be improved. To provide new insights into the Brugada syndrome pathophysiology, a mutational study was performed on a 64-year-old man presented with isolated exertional dyspnea (NYHA class: II-III), hypertension, chronic kidney disease, coronary disease, an electrocardiogram suggesting a Brugada type 1-like pattern with ST-segment elevation in leads V1-V2. Molecular diagnosis study was performed using molecular strategy based on the sequencing of a panel of 19 Brugada-associated genes. The proband was carrier of 2 TRPM4 null alleles [IVS9+1G > A and p. Trp525X] resulting in the absence of functional hTRPM4 proteins. Due to this unexpected genotype, meta-analysis of previously reported TRPM4 variations associated with cardiac pathologies was performed using ACMG guidelines. All were detected in a heterozygous status. This additional meta-analysis indicated that most of them could not be considered definitely as pathogen. In conclusion, our study reports, for the first time, identification of compound heterozygous TRPM4 null mutations in a proband with, at an arrhythmogenic level, only a Brugada type 1-like electrocardiogram. By combining the genotype/phenotype relationship of this case and analysis of previously reported TRPM4 variations, we suggest that loss-of-function TRPM4 variations, in a heterozygous status, could not be considered as pathogenic or likely pathogenic mutations in cardiac channelopathies such as Long QT syndrome or Brugada syndrome.

Four TRPM4 Cation Channel Mutations Found in Cardiac Conduction Diseases Lead to Altered Protein Stability

Transient receptor potential melastatin member 4 (TRPM4), a non-selective cation channel, mediates cell membrane depolarization in immune response, insulin secretion, neurological disorders, and cancer. Pathological variants in TRPM4 gene have been linked to several cardiac phenotypes such as complete heart block (CHB), ventricular tachycardia, and Brugada syndrome (BrS). Despite recent findings regarding the functional implications of TRPM4 in cardiac diseases, the molecular and cellular mechanisms leading to altered conduction are poorly understood. In the present study, we identify and characterize four novel TRPM4 variants found in patients with CHB or ventricular fibrillation. Three of them, p.A101T, p.S1044C and a double variant p.A101T/P1204L, led to a decreased expression and function of the channel. On the contrary, the variant p.Q854R showed an increase in TRPM4 current. Recent evidence indicates that altered degradation rate of mutant proteins represents a pathogenic mechanism underlying genetic diseases. In consequence, protein turnover of WT-TRPM4 and TRPM4 variants overexpressed in HEK293 cells was analyzed using cycloheximide, an inhibitor of protein biosynthesis. Upon addition of cycloheximide, WT-TRPM4 decayed with a half-life of ~20 h, while loss-of-expression variants showed a ~30% increase in degradation rate, with a half-life close to 12 h. Together, the gain-of-expression variant showed a higher stability and a doubled half-life compared to WT-TRPM4. In conclusion, decreased or increased protein expression of several TRPM4 variants linked to cardiac conduction disorders or ventricular arrhythmias were found to be caused by altered TRPM4 half-life compared to the WT form.