Cardiac conduction diseases: Understanding the molecular mechanisms to uncover targets for future treatments

Abstract

Introduction:

The cardiac conduction system (CCS) is crucial for maintaining adequate cardiac frequency at rest and modulation during exercise. Furthermore, the atrioventricular node and His-Purkinje system are essential for maintaining atrioventricular and interventricular synchrony and consequently maintaining an adequate cardiac output.

Areas covered:

In this review article, we examine the anatomy, physiology, and pathophysiology of the CCS. We then discuss in detail the most common genetic mutations and the molecular mechanisms of cardiac conduction disease (CCD) and provide our perspectives on future research and therapeutic opportunities in this field.

Expert opinion:

Significant advancement has been made in understanding the molecular mechanisms of CCD, including the recognition of the heterogeneous signaling at the subcellular levels of sinoatrial node, the involvement of inflammatory and autoimmune mechanisms, and the potential impact of epigenetic regulations on CCD. However, the current treatment of CCD manifested as bradycardia still relies primarily on cardiovascular implantable electronic devices (CIEDs). On the other hand, an I_f specific inhibitor was developed to treat inappropriate sinus tachycardia and sinus tachycardia in heart failure patients with reduced ejection fraction. More work is needed to translate current knowledge into pharmacologic or genetic interventions for the management of CCDs.

1. Introduction

The cardiac conduction system (CCS) comprises specialized cardiomyocytes responsible for propagating electrical impulses throughout the heart, orchestrating its contractions, and facilitating the circulation of blood throughout the body. Cardiac conduction disease (CCD) is common and depending on severity and the level at which it occurs can lead to irregular heart rhythms, potentially resulting in syncope, heart failure (HF) [1], and even sudden cardiac death [2,3]. Pacemaker implantation is required for severe symptomatic CCD. Considering the critical role of the CCS in maintaining normal cardiac activity and the potentially severe consequences of CCD, a thorough comprehension of the molecular mechanisms underlying the CCS and pathogenesis of CCD is fundamental for advancing therapeutic strategies. Over the past decades, emerging evidence from genetic factors have helped uncover the molecular basis for CCD. Therefore, this review will discuss the current knowledge of the molecular mechanisms underlying CCD, therapies employed to treat CCD, and discuss potential therapeutic targets for addressing CCD.

1.1. Anatomy of conduction system and its applications

The initial discovery of the CCS dates back to the 19^th century [4,5]. The three main components of the CCS are the sinoatrial node (SAN), atrioventricular node (AVN), and the His-Purkinje System (HPS) (Figure 1). Anatomically, the SAN is located at the junction of the superior vena cava and the right atrium with multiple directional extensions. The head and proximal body of the SAN are typically subepicardial, whereas the distal body and tail extend downward and obliquely into crista terminalis and toward the eustachian ridge [6]. The AVN is localized at the apex of the Koch’s triangle, which is located at the base of the right atrium bounded by coronary sinus (CS) ostium, tendon of Todaro, and the septal leaflet of the tricuspid valve [7]. The human AVN consists of the compact node (CN) and lower nodal bundle (LNB). The rightward nodal extension extends from the LNB toward the CS and functions as the ‘slow’ pathway (SP). The leftward nodal extension extends from the CN toward the CS and acts as the ‘fast’ pathway (FP) [7]. The HPS constitutes the furthest segment of the CCS. The His bundle originates from the AVN, extends to the interventricular septum, and then divides into the left and right branches, reaching the myocardium of the ventricles via the Purkinje fibers, which consist of parallel fibers and form a complex and dense network [8]. The anatomic integrity of the CCS is necessary for its synchronized electrical activity.

Figure 1.

Anatomy of the conduction system. (A) Micro-computed tomography (CT) images of the conduction tissue from a human heart. (B) Conduction system images overlaid on the human heart. (C) and (D) present the same model as viewed from the left. Abbreviations: Ao: aorta; AVN: AV node; HB: His bundle; LA: left atrium; LBB: left bundle branch; LPN: left Purkinje network; LV: left ventricle; MA: mitral annulus; RA: right atrium; RBB: right bundle branch; RPN: right Purkinje network; RV: right ventricle; SAN: sinoatrial node. (Adapted with permission from Stephenson et al. [134]).

Recent technological advancements, including contrast-enhanced micro-computed tomography (micro-CT) imaging, have expanded our understanding of the CCS anatomy. This progress has shifted our perspective from 2D to a more comprehensive 3D view at a cellular resolution. Micro-CT imaging allows for the development of mathematical models for cardiac electrical activation by defining cardiac microstructure and cardiomyocyte orientation [9]. It also establishes the links between CCS morphological remodeling and conduction disorders, providing insight into cardiac surgery and ablation therapies. Furthermore, intravenously-injected engineered CCS-targeted antibody (Cntn2)-fluorescent dye conjugates have been introduced to provide improved in vivo delineation of CCS structures [10]. Conjugation of antibodies specific to CCS pacemaker cell markers with diverse cargo molecules has been proposed to facilitate the targeted transportation of therapeutic agents to distinct CCS subcomponents, which serve to advance precision molecular targeting therapy for a spectrum of CCDs and other arrhythmic conditions [10].

1.2. CCS physiology

The CCS consists of specialized myocytes with pacemaker activity, which are responsible for the spontaneous generation of electric signals and propagation throughout the heart, leading to rhythmic and synchronized myocardial contractions. Each component is characterized by unique molecular markers (Figure 2). The SAN initiates electrical impulse formation, serving as the heart’s natural pacemaker, while the AVN slows down propagation, and the HPS rapidly transmits signals through the ventricles. The automaticity of CCS pacemaker cells relies on the dynamic coupling between the surface ‘membrane clock’ (M clock) and intracellular ‘Ca²⁺ clock’ [11]. The ‘membrane clock’ is mediated by a number of voltage-sensitive membrane currents, including L-type and T-type Ca²⁺ current (I_CaL, I_CaT), delayed rectifier K⁺ current (I_K), funny current (I_f) and Na⁺/Ca²⁺ exchanger (NCX, I_NCX). The ‘Ca²⁺ clock’ refers to the Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) via ryanodine receptor 2 (RyR2) during late diastolic depolarization [12]. With improved high-resolution imaging techniques, emerging evidence has unveiled more complex and heterogeneous signaling, particularly in subcellular Ca²⁺ signals, across different cell clusters within the SAN [13,14]. While some areas within the SAN exhibit pacemaker activity, other areas may exhibit subthreshold Ca²⁺ release, thus displaying no conduction [15]. The interplay between the firing and non-firing cells within the SAN, modulated by the autonomic nervous system — referred to as ‘tonic entrainment’ — controls the overall function of the SAN [14,15]. These studies set the stage for developing new paradigms in SAN physiology.

Figure 2.

Diagram of the cardiac conduction system with unique markers. Gene makers of different CCS components from scRNA-seq data. SAN: sinoatrial node; AVN/His: atrioventricular node/His bundle; PF: Purkinje fiber. (Adapted with permission from Goodyer et al. [10,135]).

1.3. Conduction system modulation by the autonomic nervous system

The heart is innervated by the cardiac autonomic nervous system (ANS), which can be divided into three levels: intracardiac neurons, intrathoracic extracardiac neurons and the central nervous system (CNS) [16]. The extracardiac ANS comprises parasympathetic and sympathetic components, which extend into the pericardial space and form synapses with cardiac ganglia. Most of these ganglia interconnect to form ganglionated plexi (GP) on the surface of atria and ventricles. Among these, the right atrial GP regulates the SAN, while the inferior vena cava-inferior atrial GP controls the AVN [17].

The ANS plays a crucial role in modulating cardiac electrophysiology and has a chronotropic effect. The sympathetic neurons release norepinephrine (NE), which binds to α- and β-adrenergic receptors on cardiomyocytes, shortening the action potential (AP) and reducing transmural dispersion of repolarization, thus acting as a proarrhythmic factor for both atrial and ventricular myocytes [18]. Additionally, NE also mediates a positive chronotropic effect by activating Gs protein-coupled receptors on SAN pacemaker cells, elevating second messenger cAMP levels in SAN pacemaker cells, resulting in an augmented slope of the sinus diastolic depolarization, firing frequency, and heart rate (HR) [19]. The parasympathetic neurons release acetylcholine (Ach), which activates cholinergic receptors on cardiomyocytes, leading to prolonged AP in ventricle myocytes, reduced effective refractory (ERP) period, increased electrophysiological heterogeneity, and promotes early afterdepolarization in atrial myocytes. The distinct effects of parasympathetic neurons on the duration of AP and ERP determine their antiarrhythmic impact on ventricles and proarrhythmic influence on atria [18]. Furthermore, Ach regulates a negative chronotropic effect by reducing intracellular cAMP levels, and activating potassium channels, thereby reducing HR [19].

2. Sinus node disease

Abnormalities in SAN impulse formation and/or propagation can lead to sinus node dysfunction (SND), which encompasses several related conditions including sinus bradycardia, sinus pauses, SAN exit block, and tachycardia-bradycardia syndrome [6]. Degenerative fibrosis and age-related remodeling are thought to play a primary role in the development of SND, which has the greatest incidence in patients between 70 and 80 years of age [20]. Conversely, increased automaticity of the SAN can lead to inappropriate sinus tachycardia (IST), which is defined as an average sinus HR exceeding 90 bpm over 24 hours or a HR exceeding 100 bpm at rest [21]. In contrast to SND, IST usually affects younger females [21].

2.1. Molecular mechanisms of SND

Cases of familial SND has been linked to genetic mutations affecting ion channels or their associated proteins involved in the ‘M clock’, such as HCN4, SCN5A, KCNJ5, and GNB2, or the ‘Ca²⁺ clock’, such as CACNA1D, RYR2, or CASQ2 (Table 1, Figure 3) [6]. Other novel mutations affecting ankyrin-B (ANK2), connexin 45 (GJC1), and myosin heavy chain 6 (MYH6) protein have also been implicated with SND [6]. SND is also commonly seen with other inherited arrhythmia disorders, such as long QT syndrome, Brugada syndrome, or catecholaminergic polymorphic ventricular tachycardia, the latter of which frequently demonstrates concomitant sinus bradycardia as the only abnormality seen on resting ECG [6].

Table 1.

List of proteins involved in cardiac conduction disorders.

Category	Gene	Protein	Function	Animal models	Human disease
Ion channels	SCN5A	Sodium voltage-gated channel alpha subunit 5 (Nav1.5)	Fast depolarization and impulse conduction maintenance	Mouse: LQT3, AVB, SND	BrS, LQT3, AVB, SND
	SCN1B	Sodium voltage-gated channel beta subunit 1	Sodium current	Mouse: LQTS	AVB, AF, BrS
	TRPM4	Transient receptor potential cation channel subfamily M member 4	Cell membrane depolarization and repolarization	Mouse: AVB, BBB	Complete heart block, PFHBI, IVF, RBBB, BrS, LQTS, AVB
	HCN4	Hyperpolarization activated cyclic nucleotide gated potassium channel 4	Responsible for hyperpolarization activated funny current (If), pacemaker cell automaticity	Mouse: SND	SND, BrS, AVB, IST, early onset AF
	KCNJ5	G protein-activated inward rectifier potassium channel 4 (GIRK-4)	Modulation of HR in response to vagal stimulation	Mouse, Xenopus oocytes, Chinese hamster ovary (CHO) cells: SND	SND, LQTS, Andersen-Tawil syndrome
	KCNE1	Potassium voltage-gated channel subfamily E regulatory subunit 1	Slowly activated component of the delayed rectifying K+ current	Mouse: LQTS, AF	LQTS
	KCNH2	Potassium voltage-gated channel subfamily H member 2	Conduct the delayed-rectifier K+ currents	Mouse: LQTS	LQTS
	KCNQ1	Potassium voltage-gated channel subfamily Q member 1	Conduct the delayed-rectifier K+ currents	Mouse: LQTS	LQTS, AVB
	KCNJ2	Potassium inwardly rectifying channel subfamily J member 2	Stabilize the rest membrane potential	N.A.	Andersen-Tawil Syndrome, LQTS, AVB
	CACNA1C	L-type calcium channel (Cav1.2)	Inward depolarizing current	N.A.	AVB, BrS, LQTS
	CACNA1D	L-type calcium channel subunit (Cav1.3)	Involved in automaticity of SAN through ICa,L	Mouse: SND	SND
	CACNA1G	Calcium voltage-gated channel subunit alpha1 G (Cav3.1)	T-type Ca2+ channel current	Mouse: Bradycardia, AVB	N.A.
	CACNA2D2	Calcium channel, voltage-dependent, alpha 2/delta subunit 2	Voltage-gated L-type Ca2+ channel current	Mouse: bradycardia	N.A.
Ca2+ handling proteins	RYR2	Ryanodine receptor 2	Ca2+ release of from SR	Mouse: CPVT, SND	Complete AVB, SND, CPVT, early onset AF, ARVC
	CALM2	Calmodulin 2	Ca2+ sensing and modulation	Mouse: LQTS	LQTS, AVB
	ANK2	Ankyrin 2	Adaptor protein, Ca2+ handling protein localization and organization	Mouse: AVB, SND	LQT4, SND, CPVT, AF, arrhythmogenic cardiomyopathy
	CASQ2	Calsequestrin 2	Ca2+ binding protein; plays a role in calcium homeostasis within the SR	Mouse: SND	CPVT, SND, early onset AF
Gap junctions	GJA5	Gap junction protein alpha 5, Connexin 40	Cell-to-cell electrical conduction	Mouse: AVB	PFHBI
	GJC1	Gap junction protein gamma 1, Connexin 45		Mouse: AVB, SND	Progressive AVB, SND, atrial standstill, early onset AF
	GJA1	Gap junction protein alpha 1, Connexin 43		N.A.	N.A.
	Gjd3/GJD3	Gap junction protein delta, Connexin Cx30.2/31.9		Mouse: AVB	N.A.
Transcription factors	NKX2.5	NK2 homeobox 5	CCS development	Mouse: AVB	AVB
	TBX2/3/5/20	T-box 2/3/5/20		Mouse: CCS malformation	AVB, AF
	GATA4/6	GATA binding protein 4/6		Mouse: CCS malformation	AF
	BMP2	Bone morphogenetic protein 2		Mouse: CCS malformation	AF, WPWS, ventricular arrhythmias
	SHOX2	Short stature homeobox 2 protein	Homeodomain transcriptional factor; essential for SAN development	Mouse: SND	Early onset AF and SND
	ETV1	ETS variant transcription factor 1	Regulator of fast conduction physiology for pectinated atrial myocardium and ventricular conduction system cardiomyocytes	Mouse: atrial and ventricular conduction defects, and bundle branch block	Bundle branch block, heart block
Nuclear envelope protein	LMNA	Lamin A/C	Nuclear maintenance, chromatin distribution and gene regulation	Mouse: AVB, SND, ventricular arrhythmias	AVB, SND, AF, ventricle arrhythmias, DCM
	EMD	Emerin		Mouse: AVB	AVB
Miscellaneous	GNB2	B2 subunit of the heterotrimeric G-protein complex	Activates GIRK channels	Xenopus oocytes: SND	SND, AVB
	MYH6	Myosin heavy chain 6	Sarcomeric protein involved in myocardial contraction	Mouse: SND	HCM, DCM, SND, AF
	RGS6	Regulator of G protein signaling 6	Modulator of parasympathetic innervation in the heart	Mouse: exaggerated bradycardia	N.A.
	SMOC2	SPARC related modular calcium binding 2	Endothelial cell proliferation and migration and angiogenesis in non-cardiac tissues	Mouse: No cardiac electrophysiology change [133] Rat: heart failure	N.A.
	PCDH17	Protocadherin 17	Aggravate myocardial infarction and cardiac hypertrophy	Mouse: aggravate cardiac hypertrophy	N.A.
	SLITRK5	SLIT and NTRK like family member 5	Enriched in AVN	N.A.	N.A.
	SLIT2	Slit guidance ligand 2	Promotes cardiac fibrosis	Mouse: low penetrant bicuspid aortic valves	Valvular heart disease complicated by AF
	IGFBP5	Insulin like growth factor binding protein 5	Cardiac fibrosis activation	N.A.	SND, AF
	CPNE5		Membrane trafficking	Mouse: SND	HR variability
	NTM	Neurotrimin	IgLON immunoglobulin domain-containing cell adhesion molecules	N.A.	N.A.

AF: atrial fibrillation; ARVC: arrhythmogenic right ventricular cardiomyopathy; AVB: atrioventricular block; BBB: bundle branch block; BrS: Brugada syndrome; CCS: cardiac conduction system; CPVT: catecholaminergic polymorphic ventricular tachycardia; DCM: dilated cardiomyopathy; HCM: hypertrophic cardiomyopathy; HR: heart rate; IST: inappropriate sinus tachycardia; IVF: idiopathic ventricular fibrillation; LQTS: Long QT syndrome; PFHBI: progressive familial heart block type I; RBBB: right bundle branch block; SAN: sinoatrial node; SANDD: sinoatrial node dysfunction and deafness; SND: sinus node dysfunction; WPWS: Wolff-Parkinson-White syndrome.

Figure 3: SAN cell diagram with key proteins involved in normal SAN function.

Mutant proteins include ion channels (SCN5a, CACNAD1, KCNJ5, HCN4), calcium-handling proteins (RYR-2, CASQ2) , transcription factors (SHOX2); nuclear envelope proteins (Lamin A/C) and other protein (GNB2, MYH6) have the potential to cause sinus node pathology. These mutant proteins are labeled in red text. Abbreviations: SR: sarcoplasmic reticulum; NCX: Na⁺/Ca²⁺ exchanger; CaM: Calmodulin; GPCR: G protein-coupled receptors; I_CaL: L-type calcium current; I_CaT: T-type calcium current; I_Na: sodium current; I_K: potassium current; I_f: funny current; SAN: sinoatrial node. (Figure created in Biorender.com)

Other novel channels recently implicated in animal models of SND include small conductance K⁺ (SK) channels and large conductance Ca²⁺ activated K⁺ (BK) channels, which play an important role in SAN repolarization in response to intracellular Ca²⁺ [22,23]. Ca²⁺ homeostasis has been shown to be altered with aging and in other pathological cardiac conditions such as HF and atrial fibrillation (AF), offering another potential mechanism for the development of SND [6]. While BK channel mutations have been previously identified in patients with neurological disorders, it was only recently that a novel mutation in the KCNMA1 gene, encoding the alpha subunit of the channel, was identified in a family affected by SND and AF [24]. SK channels represent a potential SAN selective target for the treatment of SND as these channels are highly expressed in atrial tissue but not in ventricular myocytes [22]. Administration of apamin, a SK blocker, in NCX knockout mice with abnormal cellular Ca²⁺ cycling was shown to reduce SK channel-mediated pauses, showing potential feasibility for the treatment of SND [22].

2.2. Epigenetic factors

Non-coding microRNAs (miRNAs) are small single stranded RNAs that inhibit gene expression at the post-transcriptional level and have been shown to be important in regulating normal SAN function [25]. Petkova et al. demonstrated that the SAN exhibits a unique miRNA expression profile that is significantly different from that of the adjacent atrial muscle [25]. Of particular interest is the identification of miR-486–3p, which is expressed at a significantly lower level in the SAN and is shown to control the expression of HCN4. Transfected ex vivo murine SAN tissue with miR-486–3p showed significantly reduced HCN4 mRNA expression and a reduction in HR by 35% [25]. Increased SAN expression of miR-486–3p has also been demonstrated in human HF hearts, providing a potential mechanism for the development of SND in patient with HF [26]. miR-423–5p is another miRNA that has been shown to be upregulated in rodent models in response to physical training and is thought to be a potential mechanism for the development of SND in veteran athletes [27]. Furthermore, D’Souza et al. demonstrated that administration of anti-miR-423–5p reversed training-induced bradycardia in swim-trained rats [27]. miR-1976, which targets the Cav1.2 and Ca_v1.3 subunits of L-type voltage-gated Ca²⁺ channels, has been shown to be upregulated in patients with age-related SND as compared to healthy controls [28]. Transgenic miR-1976 mice were seen to develop SND and have decreased Cav1.2 and Ca_v1.3 protein expression, suggesting the potential role of miR-1976 as a therapeutic target for SND [28].

Decreased HCN4 expression has also been partially attributed to the reactive oxygen species (ROS)-mediated activation of class II histone deacetylase 4 (HDAC4) in the context of the ischemia-induced SND [29,30]. Similarly, p21-activated kinase 1 (Pak1) can also regulate HCN expression in a SAN mouse model. Pereira et al. have shown that Pak1 deficient mice have an attenuated HR due to reduced HCN4 expression in the right atria [30]. SND in these Pak1 deficient mice could be rescued by class II HDAC inhibition, suggesting Pak1 as a potential therapeutic target for SND [30]. Other epigenetic mechanisms such as altered DNA methylation and chromatin remodeling have been associated with AF progression [31]; given the similar pathophysiology underlying AF and SND, it is possible that the same mechanisms could play a role in SND development, however, further work is still needed to elucidate these epigenetic mechanisms in SND.

2.3. Fibrosis

Fibrosis plays an integral role in the pathogenesis of SND through the promotion of a source- sink mismatch between the SAN pacemaker cells and the surrounding atrial myocardium [6]. Natriuretic peptides (NPs) are seen to play a central role in the fibrosis pathway and are normally upregulated in HF as a compensatory mechanism to promote diuresis and vasodilation. NPs have also been shown to play a direct role in SAN automaticity by potently reducing HR through decreased spontaneous firing in the SAN and slowed conduction in the SAN and atrial myocardium by activating NP receptor-C (NPR-C) [32]. Studies of NPR-C knockout mice have demonstrated a prolonged SAN recovery time in knockout mice, indicative of SND, as well as markedly increased susceptibility to AF [32]. While further studies are still needed to elucidate the complete mechanisms underlying fibrosis related SND, therapeutic targeting of the fibrosis pathway may have potential implications in attenuating the development of SND and atrial arrhythmias.

2.4. Autoimmunity

There is an increasing recognition of the role of autoimmunity in the pathogenesis of SND. Anti-Ro/SSA autoantibodies, which demonstrate cross-reactivity with L-type and T-type Ca²⁺ channels, have long been implicated in the development of congenital heart block [33]. However, the development of sinus bradycardia due to anti-Ro/SSA antibodies has also been observed even before the appearance of overt AVB in human newborns and experimental animal models, suggesting a continuum of conduction system disorders due to anti-Ro/SSA antibodies [33,34]. In particular, the direct binding of anti-Ro/SSA antibodies to the Cav1.3 subunit of the L-type Ca²⁺ channel is thought to underlie the development of autoimmune-associated sinus bradycardia in fetal sinus nodal cells [35]. Bay K8644, a Ca²⁺ channel activator, has been shown to subsequently rescue inhibition of I_Ca,L [35]. Chronic exposure to anti-Ro/SSA antibodies leads to Ca²⁺ channel downregulation and fibrosis of the fetal conduction system, leading to irreversible damage [33]. Although the adult conduction system is rarely a target for anti-Ro/SSA antibodies due to electrophysiological differences compared to the fetal conduction system [33], up to 50% of cases of isolated third-degree AVB of unknown origin in adults have been associated with anti-RO/SSA antibodies [36]. This raises the possibility of similar findings with isolated SND in adults,. Anti-muscarinic M2 receptor antibodies have also been associated with SND in adults, with one small study reporting anti-M2 receptor antibodies in 75% of patients with primary SND [37]. Anti-M2 receptor antibodies have been shown to exhibit an agonist activity on the M2 receptor, decreasing cyclic adenosine monophosphate (cAMP) and I_Ca,L, leading to a decreased HR, which can be rescued by atropine [38]. However, the mere presence of autoantibodies does not seem to be sufficient for the development of SND, as these autoantibodies can also be observed in healthy individuals [33]. The precise mechanisms for the development of autoimmunity related SND remain to be elucidated but may represent potential therapeutic targets for the treatment of SND.

2.5. Molecular mechanisms of inappropriate sinus tachycardia

The pathophysiology of IST is not well established but is likely to be multifactorial. In addition to intrinsic SAN abnormalities, increased sympathetic activity, depressed parasympathetic activity, impaired baroreflex control, inflammation, abnormal neurohormonal modulation, and anti-β-adrenergic receptor antibodies have all been hypothesized to contribute to the development of IST [21]. However, the relative contributions of each of these factors in the development of IST remains debated.

To date, the only mutation reported to be associated with familial IST is a gain-of-function mutation in HCN4 (R524Q) [26]. The mutation, which is located in the C-linker region, renders HCN4 channels more sensitive to cAMP activation and thus increase SAN automaticity [26]. Petkova et al. demonstrated the feasibility of reducing HCN4 expression and lowering the intrinsic HR of ex-vivo rat SAN tissue through transfection with miR-486–3p [25]. While no studies have evaluated the expression profile of miRNA in IST patients, it is conceivable that IST patients may express a pathologic miRNA profile which may promote the development of IST through altering the expression of genes responsible for normal SAN pacemaker activity.

In addition to enhanced intrinsic SAN automaticity, SAN re-entry is hypothesized to be another mechanism for IST [27]. Glukhov et al. demonstrated, in a canine model, that extensive SAN fibrosis facilitates intranodal unidirectional block upon autonomic stimulation, which can then initiate SAN re-entry [28]. Interestingly SAN micro-reentry was capable of producing both tachycardia and paradoxical bradycardia due to exit block [39].

IST is a common observation in patients with post-COVID-19 syndrome, affecting approximately 7–20% of patients [40]. The degree of COVID-19 severity is not predictive of the development of IST as the vast majority of patients were only affected by mild COVID-19 [41]. Other pathogens such as influenza virus and Epstein-Barr virus have also been associated as a precipitating event for IST in some patients [42]. The precise mechanisms for post-infectious IST remain poorly understood, but may be related to autonomic imbalance, endothelial dysfunction, and possibly neuroendocrine modulation [21,40].

2.6. Pharmacologic treatment

While isolated SND and IST are generally benign conditions, affected patients can often have debilitating symptoms that adversely affect their quality of life. Pharmacologic treatment for SND is generally reserved for the acute treatment of bradycardia leading to hemodynamic instability. In this situation, current ACC/AHA/HRS guidelines recommend atropine, a muscarinic acetylcholine receptor blocker, to increase the sinus rate [43]. If ineffective, isoproterenol, dopamine, dobutamine, or epinephrine can be considered for acute management. However, since SND is generally a chronic, indolent condition rarely acutely life threatening, acute pharmacologic management for SND is rarely needed. There is little role for pharmacologic treatment in the chronic management of SND, for which permanent pacemaker implantation is the only established long term therapy. However, in patients with SND who decline or are not candidates for pacemaker implantation, theophylline can be effective at increasing the HR and reducing symptoms [43]. Theophylline exerts a positive chronotropic effect primarily through the blockade of the A1 adenosine receptor, which decreases I_K via inhibiting G protein-activated inward rectifier K⁺ channel (GIRK) channels [44,45]. Cilostazol, a selective phosphodiesterase-III inhibitor that increase cAMP and HCN channels activation and thus intracellular Ca²⁺ cycling, has also been shown in small studies to be effective in improving HR and symptoms [30]. [46,47].

Pharmacologic treatment is used as first-line management of IST. The most commonly utilized medications for IST are beta-blockers and calcium channel blockers. Nevertheless, these medications are generally ineffective and can be poorly tolerated by patients [21]. Ivabradine, which blocks HCN channels and inhibits I_f, has recently emerged as an effective treatment for IST [48]. Ivabradine is generally well tolerated with minimal effects on blood pressure and is more effective at symptom relief as compared to beta-blockers [21,49]. However, the use of ivabradine is limited due to its high cost and contraindication in pregnant patients [21]. In cases of medically refractory IST, SAN modification targeting the superior aspect of the crista terminalis through either an endocardial or epicardial approach can be attempted. However arrhythmia recurrence is common and there is a risk of inadvertent SAN and phrenic nerve injury with more extensive ablation [21]. Hybrid SAN sparing ablation techniques which result in regional sympathetic denervation have recently been described to be an effective alternative to SAN modification, with less risk of SAN injury [21]. Novel technologies such as pulsed field ablation, may also facilitate safer SAN modification by providing more cardio-selective ablation. Nevertheless, management of refractory IST can be challenging and further studies to delineate the optimal invasive approach are needed.

2.7. Pacemakers

Permanent pacemaker implantation is the definitive treatment for symptomatic SND and is highly effective. Single chamber atrial pacemakers or dual chamber pacemakers can be utilized, depending on the presence of concurrent atrioventricular block (AVB). A single chamber ventricular pacemakers is also an option, but this method of pacing is not preferred due to lack of AV synchrony, increased risk of AF development, and the possible deleterious effects of chronic right ventricular pacing [43]. In SND patients with intact AV conduction, there is a future risk of developing AVB given the similar underlying degenerative changes underlying both conditions. Estimates of the risk of developing AVB after pacemaker implantation ranges from 3% to 35% at 5 years of follow up [43]. Subsequently, dual chamber pacemakers are by far the most utilized device for SND in the US, accounting for more than 80% of device implants, to avoid the need for a future device upgrade in case of AVB development [6].

Although effective, drawbacks with conventional electronic pacemakers include the risk of mechanical complications, infections, and the need for generator exchanges due to their finite battery life. There has been a growing interest in developing biological pacemakers which would overcome many of these limitations. Such biological pacemakers would also be capable of providing autonomic responsiveness to deliver more natural pacing [50,51]. The earliest method of producing a biological pacemaker was a gene-based approach where adenoviral vectors are used to overexpress genes encoding ion channels involved in pacemaking, such as HCN2 [50]. While animal models have demonstrated the feasibility of this approach to produce ectopic pacemaker activity by functional re-engineering of normal cardiomyocytes, the potential pro-arrhythmic effects of overexpressed ion channels, transient nature of adenoviral transgene expression, and long-term safety of gene editing remain obstacles [51]. Cell-based approaches have also been described, where a cluster of pacemaker cells derived from human embryonic stem cells or induced pluripotent stem cells (iPSCs) are implanted into the heart to elicit ectopic pacemaker activity [51]. While promising, methods to faithfully differentiate iPSCs into pacemaker cells with high efficiency are still needed for this approach. There are also safety concerns regarding the potential for immature cells to migrate or differentiate into other unintended cell types [51]. Somatic reprogramming is another novel approach to producing a biological pacemaker, where overexpression of transcription factors, such as Tbx18 or Shox2, via adenoviral vectors is utilized to reprogram adult cardiomyocytes into induced SAN cells [51]. These induced SAN cells by somatic reprogramming demonstrate phenotypic and functional characteristics of native SAN cells. While promising, the development of biological pacemakers is still in its infancy, with much work needed before translation of preclinical studies to human clinical trials.

3. Atrioventricular node disease

3.1. Molecular mechanism of AV node disease

Disruptions within the AVN can lead to the occurrence of AVN disease, including AVB [52] and atrioventricular nodal reentrant tachycardia (AVNRT) [53]. AVB is classified into 3 main degrees based on the level of AVB: first-degree, second-degree (Mobitz type 1, Mobitz type 2, 2:1 AVB, and high-grade AVB), and third-degree (complete) AVB [43] . AVB is predominantly attributed to acquired conditions, including aging, post-surgery complication, exposure to toxins, and pathological diseases. Inherited AVB is closely associated with genetic variations. However, in approximately 50% of cases, the cause of severe AVB requiring pacemaker implantation in individuals under the age of 50 remains unknown [54]. Here we list several molecular mechanisms that contribute to AVB (Table 1, Figure 4).

Figure 4: AVN cell diagram with key proteins involved in normal AVN function.

Mutations in some of the proteins are found to contribute to the development of AVB in patients or animal models. The mutant proteins include ion channels (SCN5A, TRPM4, HCN4), calcium-handling proteins (RYR2, CaM, ANK2), gap junctions (GJA5, GJC1), transcription factors (NKX2.5, TBX2/3/5/20, GATA4/6, BMP2) and nuclear envelope proteins (Lamin A/C, EMD). These mutant proteins are labeled in red text. Abbreviations: SR: sarcoplasmic reticulum; NCX: Na⁺/Ca²⁺ exchanger; CaM: Calmodulin; I_CaL: L-type calcium current; I_CaT: T-type calcium current; I_Na: sodium current; I_K: potassium current; I_f: funny current; AVN: atrioventricular node. (Figure created in Biorender.com).

3.1.1. Ion channels

Multiple mutations affecting ion channels contribute to the development of AVB, encompassing sodium channels (e.g. SCN5A, SCN1B), calcium channels (e.g. CACNA1C, CACNA1G), potassium channels (e.g. KCNH2, KCNJ2, KCNQ1), funny channels (e.g. HCN4), and other channels (e.g. TRPM4). SCN5A encodes voltage-gated sodium channel Type 5 (Nav1.5) and plays a critical role in fast depolarization and impulse conduction maintenance (Figure 4). Loss-of-function SCN5A mutations contribute to progressive CCDs by reducing atrioventricular and intraventricular depolarization reserve [55], while gain-of-function SCN5A mutations can lead to enhanced late sodium current and intracellular Ca²⁺ levels, resulting in AVB [56]. CACNA1C encodes the L- type calcium channel (Cav1.2), responsible for an inward depolarizing current. Mutations in CACNA1C have been identified in patients with AVB and long QT syndrome [57,58]. CACNA1G encodes the T-type calcium channel (Cav3.1). SAN and AVN specific Cav3.1 knockout mice display intrinsic bradycardia and prolonged AV conduction [59]. KCNQ1 and KCNH2 encode the voltage-gated potassium channels (Kv7.1 and Kv11.1), which conduct the delayed-rectifier potassium currents. KCNJ2 encodes the inwardly rectifying potassium channel (KIR2.1), critical for stabilizing the resting membrane potential. Mutations in KCNJ2 are associated with advanced AVB in Andersen-Tawil syndrome [60]. HCN4 gene encodes the α-subunits of pacemaker I_f channels, and studies have shown that HCN4 mutations cause a loss-of-function on I_f channels, leading to AVB [61]. TRPM4 encodes a nonselective Ca²⁺- and voltage-activated channel, which is Ca²⁺ impermeable but allows for the influx of Na⁺ and efflux of K⁺, leading to cell membrane depolarization and repolarization [62]. TRPM4 has been reported to be associated with complete heart block [63] and progressive familial heart block type I (PFHBI) [64].

3.1.2. Ca²⁺ handling proteins

The normal functioning of Ca²⁺ handling proteins serve as the foundation for regulation of the Ca²⁺ clock, which couples with ion channels to sustain the automaticity of AVN cells [65,66]. RyR2 is located at the SR and is activated when there is a rapid increase in intercellular Ca²⁺ levels, leading to a substantial release of Ca²⁺ from the SR in the form of a Ca²⁺ spark (Figure 4). A RYR2 mutation has been identified in patients with complete AVB [67]. CALM2 encodes the Ca²⁺ sensing protein calmodulin. CALM2 mutations have been found to cause long QT syndrome and 2:1 AVB due to impaired Ca²⁺ affinity and Ca²⁺-dependent inactivation of L-type voltage-gated Ca²⁺ channel [68]. ANK2 is an adaptor protein, allowing the proper localization and organization of calcium handling proteins (Figure 4). At present, although there are no reports on mutations of ANK2 associated with human AVB, mice with Ank2 loss-of-function mutation displayed complete AVB, SND and ventricular arrhythmias, which may be due to reduced binding affinity between ANK2 and NCX1 and prolonged AP duration [69].

3.1.3. Gap junctions

The gap junction proteins connexin 40 (Cx40, encoded by GJA5), connexin 45 (Cx45, encoded by GJC1), connexin 30.2/31.9 (Cx30.2/Cx31.9, encoded by Gjd3/GJD3), and connexin 43 (Cx43, encoded by GJA1) are responsible for intercellular electric coupling in the AVN (Figure 5). Cx40 has been reported as a genetic cause of PFHBI [70], and Cx45 mutation can lead to progressive AVB in patients [71]. ZO-1 (Zonula occludens-1) is a plasma membrane-associated scaffolding protein, stabilizing gap junctions Cx45 and Cx40, and ion channel Nav1.5 at the cytoplasmic surface. Cardiac specific deficiency of ZO-1 in mice results in varied degrees of AVB due to reduced Cx45 expression and dislocation of Nav1.5 [72,73]. Cx30.2/Cx31.9 is responsible for slowing down electric impulse transmission through the AVN, and Cx30.2-deficient mice exhibit 2^nd/3^rd degree AVB [74]. Cx43 is primarily distributed in atrial and ventricular working cardiomyocytes, and it is also expressed heterogeneously throughout the CCS [75]. Cx43 may play a role in facilitating the rapid propagation (fast pathway) of the AVN [76]. Notably, Cx43 has been identified to be expressed in AVN macrophages, establishing connections between macrophages and AVN pacemakers, thereby facilitating cardiac conduction. Macrophage specific Cx3 knockout mice exhibit delayed AV conduction [77].

Figure 5. Molecular and anatomic illustration of AVN and bundle branches.

A. Summary of connexin expression at the atrioventricular (AV) junction. Traffic light colors summarize the level of expression of higher conductance connexin isoforms: connexin 40 (Cx40) and connexin 43 (Cx43). CN: compact node; CS: coronary sinus; FO: fossa ovalis; INE: inferior nodal extension; IVC: inferior vena cava; LBBB: left bundle branch; PB: penetrating bundle; RBBB: right bundle branch. (Adapted from Temple et al. [136]) B. Longitudinal dissociation of conduction fibers within the His bundle. Proximally, conduction fibers are predestined to form left (green and magenta) and right (blue) bundle branches. Lesions within the His bundle can, therefore, lead to left bundle branch block (left). Pacing distal to the lesion allows for restoration of normal conduction (right). AVN indicates atrioventricular node (Adapted with permission from Tan et al. [116]).

3.1.4. Transcription factors

The AVN originates from the atrioventricular canal (AVC), which forms in the early stages of heart development. Transcription factors, such as NKX2.5, TBX2/3/5/20, GATA4/6, and BMP2 play a significant role in AVN formation and function (Figure 4) [78]. The key cardiac transcription factors NKX2.5 [79] and TBX5 [80] mutations have been investigated as the genetic cause of progressive AVB in patients.

3.1.5. Nuclear envelope proteins

LaminA/C [81] and emerin [82] are nuclear membrane proteins, which are very important in nuclear maintenance, chromatin distribution and gene regulation (Figure 4). Patients who carry LMNA mutations present with varied degree of AVB [83,84]. A clear understanding of the mechanistic link between these proteins and AVB remains elusive.

3.1.6. Inflammation and autoimmune factors

Accumulating studies demonstrate that inflammation factors are crucial players in the development of CCD. The inflammatory cytokines, such as TNF-α, IL-1, IL-6 and IL-17, exert their effects either by directly influencing cardiomyocytes or indirectly through systemic pathways [85]. Overexpression of TNF-α in mice leads to increased collagen deposition, resulting in impaired AVN conduction, manifesting as prolonged PR interval, AVN Wenckebach, and extended effective and functional refractory periods [87]. Injection of IL-6 in guinea pigs can induce bradycardia and slow AV conduction. When combined with azithromycin and hydroxychloroquine, it can exacerbate these effects to complete AVB [86]. Inflammation can affect AV conduction via IL-6-mediated suppression of Cx43 expression in AVN myocytes and macrophages [87]. A population-based genome-wide association study of 16,468 individuals revealed that single-nucleotide polymorphisms near IL17D are associated with PR segment duration, this association is strengthened by the higher expression level of IL17D in the AVN compared to the left ventricle [88]. AVB is common in patients with COVID-19 infection, and elevated levels of IL-6 and IL-10 can predict the risk of arrhythmias in these patients [89,90]. Emerging evidence has shown that autoimmune factors play a role in AVB, with some autoantibodies seen to target specific ion channels, blocking the related currents. Lazzerini et al. proposed the term “autoimmune cardiac channelopathies” to describe the pathogenic mechanism underlying cardiac arrhythmias, observed both in patients with autoimmune diseases and in otherwise healthy individuals experiencing idiopathic rhythm disturbances [91]. Congenital AVB (CAVB), diagnosed in utero or during the neonate period, is mediated by anti-Ro/SSA and/or anti-La/SSB autoantibodies. Transplacental passage of these autoantibodies lead to complete AVB via inflammation, fibrosis, and calcification within the AVN [92]. Moreover, a population-based cross-sectional study demonstrated that the anti-Ro/SSA antibodies are also associated with AVB in adults [93]. The mechanism lies in the inhibition of the L-type calcium channel (Cav1.2) by anti-Ro/SSA antibodies [36]. Additionally, Korkmaz et al. discovered that inducing an autoimmune response against the voltage-gated sodium channel Nav1.5 provokes complete AVB and SND, likely due to reduced expression levels and inhibition of Nav1.5 by autoantibodies, leading to decreased Na⁺ current [94].

3.1.7. Epigenetic factors

In recent years, epigenetic factors have emerged as critical players in AVB. Mesirca et al. discovered that in animal models of training-induced AVB, the expression of Cav1.2 and HCN4 channels is reduced, a process directly regulated by miR-211–5p and miR-432 [95].

3.2. Pharmacologic Treatment

To date, there are limited pharmacologic therapies available for the effective treatment of AVB.

Hydroxychloroquine (HCQ) is a well-established medication used to treat malaria and autoimmune disease, such as systemic lupus erythematosus (SLE). Its role in the treatment of SLE involves the inhibition of Toll-like receptors (TLR) activation by suppressing endosomal acidification [96]. Macrophage TLR activation plays a crucial role in maternal anti-SSA/Ro- mediated CAVB. HCQ has been proved to be effective in reducing the recurrence of CAVB in fetuses of mothers and adults who are exposed to Anti-SSA/Ro [97,98].

Glucocorticoids are powerful anti-inflammatory and immunomodulatory agents. Emerging evidence has shown the effectiveness of glucocorticoid therapy for anti-Ro/SSA associated AVB [99]. Santos-Pardo et al reported that methylprednisolone can reverse complete AVB in an adult patient with positive anti-Ro/SSA antibodies [100]. Moreover, Lazzerini et al also demonstrated the efficacy of steroid therapy in achieving rapid treatment outcomes in AVB patients with circulating anti-Ro/SSA antibodies based on a cohort of 13 AVN patients [36].

SPM-354, a potent bitopic antagonist targeting the sphingosine-1-phosphate (S1P) receptor subtype S1P3, competes with both the natural ligand and the selective S1P3 agonist for binding. Fingolimod (FTY720) is an analogue of S1P that primarily targets S1P1 receptors. FTY720 has been employed in the treatment of relapsing multiple sclerosis. However, its use may lead to atrioventricular conduction disease for the first dose, including AVB [101]. The induction of AVB by FTY720 requires the presence of S1P3. Notably, SPM-354 has demonstrated the potential to rescue S1P induced complete heart block and bradycardia, as well as fully reverse FTY720- induced bradycardia. But it partially restores the prolonged PR interval in animal models [102]. Further clinical trials are needed to confirm these findings in humans.

3.3. Pacemakers and denervation

According to the 2018 ACC/AHA/HRS guideline on bradycardia and conduction delay, the indication for pacing involves acquired second-degree Mobitz type II AVB, high-grade AVB and complete AVB not caused by reversible or physiologic reasons, as well as other types of AVB with correlated symptoms [43]. Because of the proven effectiveness, versatile application, well-established procedure, and long-term reliability, transvenous pacemakers are the first line treatment for patients with CCD, particularly in cases of high-degree AVB. In recent years, leadless pacemakers have emerged as options for certain patients. Among them, dual-chamber leadless pacemaker stands out as the most promising option for patients with AVB. The dual-chamber leadless pacemaker provides atrial pacing and consistent AV synchrony. An international, multicenter, single-group clinical trial has demonstrated that the dual-chamber leadless pacemaker system successfully met the primary safety and performance endpoints at 3 months with a similar incidence of acute complications compared to transvenous dual-chamber pacemakers [103].

It has been demonstrated that vagal hyperactivity may be the primary contributor to AVB [104]. Therefore, cardiac denervation-ganglionated plexus ablation has emerged as a promising therapeutic approach in patients with functional AVB. Recent studies have shown that anatomically guided GP targeting is effective in reducing AVB occurrence, preventing AVB related syncope, and inducing enduring autonomic modifications [104,105].

4. His-Purkinje System disease

Purkinje cells (PCs) have been implicated in a variety of cardiac diseases including cardiac arrhythmias and HF syndromes. However, its exact role in cardiac arrhythmogenesis is still not fully explored [106].

4.1. Molecular mechanism of HPS disease

Logantha et al have shown that PCs and repolarization abnormalities contribute to the significant QRS and QT prolongation in rabbits with surgically induced HF [107]. Imaging studies via micro-CT demonstrated increase in volume and length of Purkinje network in rabbits with HF [107]. In addition, compared to ventricular myocytes, PCs had significant dysregulation of mRNA levels of ion channels, connexins, pro-inflammatory markers as well as markers of fibrosis [107].

Ion channel mutations have been described in heritable His-Purkinje conduction slowing and/or block. Loss-of-function mutations of the alpha subunit of the sodium channel Nav1.5 (SCN5A) have been described to be associated with His-Purkinje conduction disease as well as hyper excitability [108]. In addition, mutations of the beta-1 modifier subunit of the cardiac sodium channel have been reported to cause conduction delay and bundle branch block even in the absence of SCN5A mutations [109]. Studies have demonstrated that loss-of-function of SCN5A in PCs leads to reduction of cardiac Na⁺ current via altered Na⁺ channel inactivation and that decrease in its expression leads to slow conduction and prolongation of AP duration [108,110].

Connexins, the proteins that form gap junctions, are highly expressed in the conduction system and exhibit specific spatio-temporal isoform expression within the conduction system (Figure 5A). In mice knock-out models, the lack of Cx40 has been shown to be associated with delayed conduction at the HPS and bundle branch block [111]. Variations in Cx40 and Cx43 have been shown to cause bundle branch block. Polymorphism in Cx43 is associated with left bundle branch block (LBBB) and its presence is associated with a genetical susceptibility for LBBB development [112]. mRNA expression of subunits involved in AP propagation (Nav1.5, Cx40, and Cx43) and repolarization in PCs is significantly downregulated in cases of HF, which leads to slowing of conduction velocity and impulse propagation and potentially electrical dyssynchrony [113].

4.2. Pharmacologic Treatment

In cases of HF, optimizing guideline directed medical therapy to improve hemodynamic profile has been reported to reverse structural as well as electrical pathologic cardiac remodeling [107,114,115]. However, molecular mechanisms are still uncertain. Future clinical studies are needed to examine the effect of guideline directed medical therapy on electrical reverse remodeling.

4.3. Cardiac resynchronization therapy (biventricular pacing and conduction system pacing)

LBBB has been shown to result in electrical and mechanical dyssynchrony [116]. LBB committed fibers appear to be longitudinally dissociated within the His bundle (Figure 5B). As with LBBB, chronic right ventricular pacing leads to dyssynchronous mechanical contraction (ventricular dyssynchrony), which over time can lead to adverse remodeling and dyssynchrony-induced cardiomyopathy in a subset of patients [117]. Cardiac resynchronization therapy (CRT) via biventricular pacing (BVP) and conduction system pacing (CSP) via His-bundle pacing (HBP) or left bundle branch area pacing (LBBP) are forms of cardiac physiologic pacing intended to improve ventricular contraction synchrony [117]. BVP aims to shorten the ventricular activation time in patients with LBBB and dyssynchronous ventricular activation [118,119]. BVP has been shown to result in improved mortality, reduced hospitalization, improved quality of life, improved exercise tolerance, and improved NYHA functional status [117,120–127]. CSP via HBP has been shown to improve cardiovascular outcomes [128]. When compared to BVP, HBP produced significantly shorter left ventricular (LV) activation time and QRS duration as well as improved acute hemodynamic response [129,130]. LBBP (Figure 5B) has been shown to be more technically feasible compared to HBP. LBBP has been shown to have very promising procedural success rates as well as improved cardiovascular outcomes and LV synchrony [131]. Several retrospective observational studies have shown improved all-cause mortality and HF hospitalization with CSP compared to BVP [132]. Ongoing large randomized clinical trials will further investigate the effects of CSP compared BVP.

5. Expert Opinion

Significant advancements have led to increased understanding of the ion channels required for maintaining the rhythmic contractions of the heart and the molecular pathways governing the function of the conduction system cells. There is a growing recognition of intricate subcellular signaling and intercellular communications within the SAN and AVN, adding complexity to the physiological controls of CCS. These emerging concepts presents both challenges and opportunities for developing therapeutic targets. Furthermore, our understanding of the genetic factors contributing to CCS disease is expanding. Factors such as inflammatory signaling, autoimmune mechanisms, and structural changes in the CCS, including fibrosis or altered number and size of conduction cells, also play a significant role in the development of CCS disease. Understanding these changes is crucial for developing targeted treatments.

A significant challenge in this field was the relatively small number of cells and with some degree the ability to access them. However, with the advent of advanced imaging techniques, computational modeling, and molecular biology tools, there are substantial opportunities to deepen our understanding of CCS diseases and develop more effective treatments. The rapid advancements in single-cell and spatial transcriptomics now enable us to gain insights into the intricate and complex information related to the cellular composition, signal transduction, and intercellular communications within the CCS, under both physiological and pathophysiological conditions.

Despite these advancements, there has been no significant translation of this knowledge into clinical practice. There are no pharmacological or genetic therapies for CCS disease manifested as bradycardia. The current approach is using CIED across the board to supplant the CCS disease without addressing the underlying etiology. The development of an I_f-specific inhibitor marks a significant step in pharmacological treatment, particularly for conditions like IST and sinus tachycardia in heart failure patients with reduced ejection fraction. This suggests a potential path forward in developing drug therapies that target specific molecular pathways implicated in CCS disorders.

Although efforts have been made to develop biological pacemakers, with the hope of treating SND, progress in this area has slowed in recent years. This slowdown is partially due to concerns regarding the safety of cell therapy, cost efficiency, and potential arrhythmogenic effects. With the advancement of synthesized RNA technology and the expanding applicability of modified RNA in vivo, the strategy for somatic reprogramming could be further optimized and tested. This may facilitate the development of functional SAN and AVN cells in preclinical models. Moreover, advancements in regenerative medicine might offer new avenues for treating CCS diseases, particularly in cases where structural remodeling of the heart is a significant factor. Cell therapy strategies do show promise. However, the main limitations are due to ensuring homogeneity, maturity, consistent phenotype and minimizing immune responses by using autologous cell types. Tissue engineering represents another avenue for developing biological pacemakers that is supported by developments in 3-dimensional printing technologies and hydrogels to ensure proper compartmentalization. Ultimately, the ideal solution would be either confined reprograming of tissues to establish stable, mature, reliable pacemaker cells or engineered organoid to supplant diseased conduction tissue.

In conclusion, while there have been significant advancements in understanding CCS physiology and pathophysiology, more work needs to be done to translate current knowledge into pharmacological or genetic interventions for the management of CCS diseases. Continued research and collaboration across disciplines will be key to unlocking new therapeutic strategies for these diseases.

Article highlights.

• The microstructure of the conduction system, including the sinoatrial node, atrioventricular node, and the His-Purkinje system, has been characterized using high resolution imaging.

• Sinus node dysfunction may be due to genetic (e.g. mutations affecting ion channels or their associated proteins involved in the ‘M clock’ or the ‘Ca²⁺ clock’) and epigenetic (e.g. non-coding microRNAs) factors, autoimmunity (e.g. Anti-Ro/SSA autoantibodies) or fibrosis.

• Ivabradine, a HCN channel inhibitor, has emerged as an effective treatment for inappropriate sinus tachycardia.

• Atrioventricular block may be due to genetic and epigenetic factors, and autoimmunity.

• Inflammatory cytokines (e.g. TNF-α, IL-1, IL-6 and IL-17) may play a role in certain types of AVN disease.

• Sodium channels and connexins have been linked to His-Purkinje disease.

• Pacemakers remain the mainstay for the treatment of symptomatic bradycardias albeit biological pacemakers show promise. Biventricular pacing and conduction system pacing are effective treatments in patients with left ventricular dyssynchrony due to intra- and infrahisian conduction system disease.