The Cardiac Conduction System

Abstract

HomeCirculationVol. 123, No. 8The Cardiac Conduction System Free AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissionsDownload Articles + Supplements ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toSupplementary MaterialsFree AccessResearch ArticlePDF/EPUBThe Cardiac Conduction System David S. Park, MD, PhD and Glenn I. Fishman, MD David S. ParkDavid S. Park From the Leon H. Charney Division of Cardiology, New York University School of Medicine, New York, NY. Search for more papers by this author and Glenn I. FishmanGlenn I. Fishman From the Leon H. Charney Division of Cardiology, New York University School of Medicine, New York, NY. Search for more papers by this author Originally published1 Mar 2011https://doi.org/10.1161/CIRCULATIONAHA.110.942284Circulation. 2011;123:904–915The human heart beats 2.5 billion times during a normal lifespan, a feat accomplished by cells of the cardiac conduction system (CCS). The functional components of the CCS can be broadly divided into the impulse-generating nodes and the impulse-propagating His-Purkinje system. Human diseases of the conduction system have been identified that alter impulse generation, impulse propagation, or both. CCS dysfunction is primarily due to acquired conditions such as myocardial ischemia/infarct, age-related degeneration, procedural complications, and drug toxicity. Inherited forms of CCS disease are rare, but each new mutation provides invaluable insight into the molecular mechanisms governing CCS development and function. Applying a multidisciplinary approach, which includes human genetic screening, biophysical analysis, and transgenic mouse technology, has yielded a broad array of gene families involved in maintaining normal CCS physiology (Figure 1). In this review, we discuss gene families that have been implicated in human CCS diseases of rhythm, conduction block, accessory conduction, and development (Table). We also investigate evolving therapeutic strategies that may serve as adjuvant or replacement therapy to current implantable pacemakers.Download figureDownload PowerPointFigure 1. Cardiac conduction system cell. Genes identified in human cardiac conduction system disease are highlighted.Table. Genetic Basis of Conduction System DiseaseGene NameProteinAssociated ConditionConduction DefectMechanismReferencesIon channels SCN5ANav1.5Brugada syndrome, long QT syndrome 3Sick sinus syndrome, progressive cardiac conduction defect, atrial standstill, atrioventricular block, bundle branch blockReduced cardiac excitability and slowed conduction; loss of depolarizing current in peripheral sinoatrial node cells1–8 SCN1BScn1bBrugada syndromeProgressive cardiac conduction defect9 KCNJ2Kir2.1Andersen-Tawil syndromeAtrioventricular block, bundle branch blockProlongation of action potential duration and reduced cardiac excitability10, 11 HCN4HCN4Sinus bradycardiaReduction in pacemaker current (If)12–14Ca2+ handling proteins RYR2Ryr2CPVTSinus node dysfunction, atrioventricular block, atrial standstillAltered Ca2+ handling likely affecting the Ca2+ clock15, 16 CASQ2CalsequestrinCPVTSinus bradycardia17Gap junction proteins GJA5Cx40Atrial standstillImpaired myocyte coupling resulting in slowed conduction3Transcription factors TBX5Tbx5Holt-Oram syndromeSinus bradycardia, atrioventricular block, bundle-branch blockSpecification defect of the atrioventricular node and the ventricular cardiac conduction system18 NKX2-5Nkx2-5Atrioventricular block, bundle-branch block19Nuclear membrane proteins LMNALamin A/CEmery-Dreifuss muscular dystrophyAtrioventricular blockAltered nuclear stress mechanics and hyperactivation of MAPK signaling20Membrane adapter proteins ANKBAnkyrin-BSinus bradycardia and heart rate variabilityAltered ion channel and transporter expression and trafficking21Metabolic regulators PRKAG2G subunit of AMPKGlycogen storage diseaseWPW, atrioventricular blockAltered ventricular myocyte energetics leading to glycogen-engorged vacuoles and disruption of annulus fibrosus22, 23Transforming growth factor-β superfamily BMP2Bone morphogenetic protein 2WPWMaturation defects of annulus fibrosus24Spliceopathies DMPK 3′UTRMyotonic dystrophy type I (Steinert's disease)Atrioventricular block, intraventricular conduction diseaseToxic mutant RNA and altered splicing factor function25CPVT indicates catecholaminergic polymorphic ventricular tachycardia; WPW, Wolff-Parkinson-White syndrome; MAPK, mitogen-activated protein kinase; AMPK, AMP-activated protein kinase; and UTR, untranslated region.Diseases of AutomaticityThe human sinoatrial node (SAN) is a crescent-shaped, intramural structure with its head located subepicardially at the junction of the right atrium and the superior vena cava and its tail extending 10 to 20 mm along the crista terminalis.26 The SAN has complex 3-dimensional tissue architecture with central and peripheral components made up of distinct ion channel and gap junction expression profiles.27 Central and peripheral cells have different action potential characteristics and conduction properties (Figure 2).27 Experimental and computational models have demonstrated that SAN heterogeneity is necessary to maintain normal automaticity and impulse conduction.28–30Download figureDownload PowerPointFigure 2. Electrophysiological heterogeneity of the sinoatrial node (SAN). The central SAN, the site of dominant pacemaking, is electronically insulated from the hyperpolarizing atrial myocardium through the differential expression of connexins and ion channels. Peripheral SAN cells are electrophysiologically intermediate between central cells and atrial cardiomyocytes. SR indicates sarcoplasmic reticulum.Pacemaker automaticity is due to spontaneous diastolic depolarization of phase 4, which depolarizes the membrane to threshold potential generating rhythmic action potentials. The current paradigm of SAN automaticity has been modeled as 2 clocks that function in concert, the “membrane voltage clock” and the “calcium clock.” The membrane voltage clock is produced by the net disequilibrium between the decay of outward potassium currents (IK) and the activation of inward currents that include, but are not limited to, background sodium-sensitive current (Ib Na), L- and T-type calcium currents (ICa,L, ICa,T), sustained inward (Ist) current, and hyperpolarization-activated current (If) (Figure 2).27,31–33The subsarcolemmal calcium clock contributes to SAN diastolic depolarization through the spontaneous, rhythmic release of Ca2+ from the sarcoplasmic reticulum (SR) via the ryanodine type 2 receptor (RYR2).34 The local intracellular calcium (Cai) elevations drive the sodium-calcium exchange current (INCX) to substitute 1 intracellular Ca2+ for 3 extracellular Na+. The net gain in positive charge results in membrane depolarization.35 The elevation of intracellular Ca2+ occurs in the latter third of diastolic depolarization and is sensitive to β-adrenergic stimulation.36 Human mutations affecting the voltage clock (SCN5A and HCN4), calcium clock (RYR2 and CASQ2), or both mechanisms (ANKB) have been identified that negatively affect sinus node function.37,38SCN5ASCN5A encodes the cardiac sodium channel (Nav1.5), which generates the fast sodium current, INa. Nav1.5 dictates the amplitude and slope of phase 0 of the cardiac action potential and therefore affects conduction velocity. More than 200 mutations have now been identified in SCN5A that cause a collection of cardiac diseases that include long QT3, Brugada syndrome, progressive cardiac conduction defect, and congenital sick sinus syndrome (SSS). To date, 14 SCN5A mutations have been linked to inherited forms of SSS.27,39Benson et al1 identified 6 SCN5A mutations in 3 kindreds with inherited SSS. The probands exhibited compound heterozygosity of the 6 SCN5A alleles (G1408R+P1298L, T220I+R1623X, delF1617+R1632H). Biophysical evaluation revealed that 2 of the 6 mutations resulted in nonfunctional channels while T220I, P1298L, and delF1617 demonstrated reduced INa current density and a hyperpolarizing shift in voltage-dependent inactivation. Smits et al40 identified a novel SCN5A mutation, E161K, in two kindreds with symptomatic sinus node dysfunction, generalized conduction disease, and Brugada syndrome. Whole-cell patch clamp revealed a 3-fold reduction in INa current density and a positive shift in voltage-dependent activation. Both a negative shift in voltage-dependent inactivation and a positive shift in voltage-dependent activation result in narrowing of the INa current window.The expression of SCN5A is restricted to the peripheral SAN and absent in the central SAN, the site of dominant pacemaking (Figure 2). A proposed mechanism of how peripherally expressed SCN5A mutations affect central SAN automaticity was computer simulated by Butters et al.41SCN5A mutants (T220I, P1298L, delF1617, and E161K) were incorporated into single cell models and into a 2-D model of intact, rabbit SAN-atrium. Single cell simulations showed that SCN5A mutants reduced the pacemaking rate of peripheral cells with little impact on central nodal cells. In contrast, intact tissue simulations with the SCN5A mutants slowed the sinus rate within the central SAN. The atrium imposes a significant hyperpolarizing load on the SAN, which is counterbalanced by SCN5A expression in peripheral cells.41–43 Therefore, reduced INa in the peripheral SAN exposes the central SAN to more hyperpolarized potentials, slowing pacemaking rate. In addition, impaired action potential conduction was evident across the SAN-atrium predisposing to sinus exit block, a common feature of familial SSS.41HCN4The pacemaker, or funny current, If, is generated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel. The term funny current stems from the unique characteristics of HCN channels, which include permeability to K+ and Na+, activation at hyperpolarized membrane potentials, and modulation by cAMP. The biophysical properties of HCN channels make them ideally suited to function as modulators of the pacemaker potential. First, because they are activated at hyperpolarized membrane potentials (between −65 and −40 mV), their slow inward current contributes to diastolic depolarization; second, the main cardiac HCN channel (HCN4) is cAMP responsive, allowing If to be modulated by autonomic stimulation.44,45Mutations in HCN4, the predominant isoform in the SAN, have been identified in patients with SSS.32,45 Using candidate gene strategies, Schulze-Bahr et al12 identified a single nucleotide deletion in HCN4 (1631delC) in a patient with sinus bradycardia and chronotropic incompetence. The 1631delC mutant lacks the cyclic-nucleotide binding domain, making it unresponsive to cAMP. Additional HCN4 mutations were identified in 2 families with asymptomatic sinus bradycardia. The 2 missense mutations, S672R and G480R, resulted in channels that activate at more hyperpolarized voltages generating smaller currents during diastolic depolarization.13,14Investigations into animal models of HCN channel deficiency have shed light on their role in If current generation. HCN4 knockout mice were embryonic lethal at E10.5 to E11.5 and demonstrated a cardiomyocyte maturation defect. HCN4−/− embryonic cardiomyocytes exhibited an 85% reduction in If and were chronotropically unresponsive to cAMP.46,47 To study the loss of HCN4 expression in adult mice, tamoxifen-inducible mouse models were generated.48,49 The loss of HCN4 in SAN cells resulted in a 75% reduction in If current density. Instead of manifesting sinus bradycardia, these mice developed sinus pauses up to 300 to 500 ms. Chronotropy was also preserved. HCN4−/− SAN cells exhibited normal automaticity; however, diastolic potentials tended to drift to hyperpolarized levels at which spontaneous pacemaker activity would cease. Taken together, these results suggest that HCN4 is indispensible for SAN development and chronotropy during embryogenesis but may play only a backup role to counter membrane hyperpolarization in the adult SAN.47–49 These results are consistent with the benign bradycardic phenotype seen in the 2 families with HCN4 mutations reported by Milanesi et al13 and Nof et al.14The preservation of chronotropic competence in human and experimental models of HCN4 deficiency is suggestive of alternative mechanisms of SAN automaticity and sympathetic responsiveness. The first implication of Cai in diastolic depolarization was reported by Rubenstein and Lipsius50 when ryanodine treatment was found to slow automaticity in subsidiary pacemakers in cat right atria. A mechanistic link between Cai and SAN diastolic depolarization was subsequently reported by Huser et al,34 who proposed that subsarcolemmal Ca2+ transients released from the SR (ie, Ca2+ sparks) lead to activation of the Na+-Ca2+ exchanger (NCX), resulting in membrane depolarization. Bogdanov et al35 confirmed this mechanism in isolated rabbit SAN cells and directly linked Ca2+ sparks to late diastolic depolarization via the NCX. Vinogradova et al36,51 then showed that β-adrenergic augmentation of rabbit SAN chronotropy is dependent on SR Ca2+ release as treatment with ryanodine blunted the pacemaker rate response to isoproterenol. Recent work by Joung et al38 examined the effect of adrenergic stimulation on intact canine SAN preparations using dual optical mapping of transmembrane potential (Vm) and Cai. Adrenergic stimulation resulted in a robust increase and superior shift in late diastolic Cai elevations that colocalized with the primary pacemaking site.RYR2 and CASQ2It is therefore interesting that mutations in SR calcium handling proteins are associated with sinus node dysfunction. Sinus bradycardia is a common feature of catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited arrhythmia syndrome characterized by bidirectional or polymorphic ventricular tachycardia induced by adrenergic stress in the absence of structural heart disease. Two CPVT-linked genes have been identified in humans, the cardiac ryanodine receptor, RYR2, and cardiac calsequestrin, CASQ2. The cardiac ryanodine receptor is the main calcium release channel on the SR in cardiomyocytes. Calsequestrin is the major calcium storage protein in the SR and complexes with RYR2. Mutations in these proteins result in altered calcium release characteristics from the SR leading to catecholamine-induced spontaneous SR Ca2+ release, resulting in delayed afterdepolarizations and triggered activity.52,53Postma et al16 identified 13 RYR2 missense mutations in 12 families with CPVT. A total of 54 family members were found to be carriers, and all had resting sinus bradycardia off medications. A variant of CPVT with extended features of dilated cardiomyopathy, sinus node dysfunction, progressive atrioventricular block, atrial fibrillation, and atrial standstill was reported recently. Linkage analysis and long-range polymerase chain reaction revealed an in-frame, genomic deletion involving RYR2 exon3.15 Sinus bradycardia is also a characteristic feature of CPVT due to CASQ2 mutations. Postma et al identified 3 nonsense mutations in CASQ2 (a nonsense R33X, a splicing 532+1 G>A, and a 1-bp deletion, 62delA) in 3 kindreds with CPVT; all manifested sinus bradycardia on baseline ECG.17 The association of sinus bradycardia with mutations in SR calcium handling proteins lends further support for the calcium clock hypothesis in sinoatrial pacemaking; however, the mechanism of sinus node dysfunction in CPVT will need further study.ANKBMutations in ankyrin-B (ANKB) are associated with long QT type 454 and familial sinus node dysfunction.21 ANKB is required for the proper targeting of ion channels and transporters to specific membrane domains. Mouse models haploinsufficient for ANKB develop profound sinus bradycardia and resting heart rate variability. Biochemical analysis revealed reduced expression and improper targeting of the NCX (NCX1), Na/K-ATPase (NKA), IP3 receptor (IP3R), and Cav1.3. ICa,L and INCX were significantly reduced in AnkB+/− SAN cells.21 Therefore, ANKB mutations likely cause sinus node dysfunction by reducing ionic currents involved in both clock mechanisms.Diseases of Conduction BlockConduction block can occur at any level of the CCS and can manifest as sinoatrial exit block, atrioventricular block, infra-Hisian block, or bundle branch block. Impaired conduction can be caused by ion channel defects that alter action potential shape or by defective coupling between cardiomyocytes. Inherited defects in cardiac conduction have been linked to mutations in SCN5A and SCN1B (both affect phase 0) and KCNJ2 (affects phase 3 and 4).The cardiac sodium channel consists of the pore-forming α-subunit (encoded by SCN5A) and a modulatory β-subunit (encoded by SCN1B). The α-subunit contains a voltage sensor that allows for rapid activation in response to membrane depolarization. After depolarization, the sodium channel undergoes a period of inactivation, in which it is refractory to further impulses. SCN5A requires membrane repolarization to relieve the inactivated state. The inward rectifier potassium channel, Kir2.1, encoded by KCNJ2, maintains the resting membrane potential. Therefore, proper functioning of Nav1.5 and Kir2.1 is necessary for normal cardiac excitability.SCN5AProgressive cardiac conduction defect, or Lev-Lenègre disease, is characterized by age-related, fibrosclerotic degeneration of the His-Purkinje system.6 Impulse propagation through the proximal ventricular conduction system progressively declines, resulting in bundle branch blocks and eventually complete atrioventricular block. An inherited form of Lev-Lenègre disease is associated with loss of function mutations in SCN5A and can exist alone or as overlap syndromes with Brugada or long QT syndrome 3.6 Inherited progressive cardiac conduction defect is associated with a high risk of complete atrioventricular block and Stoke-Adams syncope without ventricular dysrhythmia.7 Schott et al8 identified a mutation in SCN5A that cosegregates with Lenègre disease in a large French family. Affected individuals had variable degrees of conduction block requiring pacemaker implantation in 4 family members because of syncope or complete heart block. Linkage analysis and candidate gene sequencing identified a T>C substitution at position +2 of the donor splice site of intron 22 (IVS22+2 T>C), which results in a mutant lacking the voltage-sensitive segment.8 Functional analysis demonstrated no transient inward sodium current in response to depolarization, consistent with a loss-of-function mutation.6SCN1BThe majority of patients with Brugada and conduction disease do not have SCN5A mutations. Therefore, modifiers of Nav1.5 expression or function have become the target of candidate gene sequencing approaches. Watanabe et al9 identified SCN1B mutations in 3 families with conduction disease with or without Brugada syndrome. Coexpression of mutant β-subunits with Nav1.5 resulted in diminished sodium current.KCNJ2Mutations in KCNJ2 have been found in a rare autosomal dominant condition called Andersen-Tawil syndrome, characterized by periodic paralysis, dysmorphic features, polymorphic ventricular tachycardia, and cardiac conduction disease.10,11 ECG evaluation of 96 patients with Andersen-Tawil syndrome from 33 unrelated kindreds revealed conduction defects at multiple levels from the atrioventricular node to the distal conduction system.55 Cardiomyocytes expressing a dominant-negative subunit of Kir2.1 exhibited a 95% reduction in IK1, resulting in significant action potential prolongation. Mouse models of Andersen-Tawil syndrome exhibited a slower heart rate and significant slowing of conduction.56,57Diseases of Accessory ConductionWolff-Parkinson-White (WPW) syndrome is characterized by preexcitation of ventricular myocardium via an accessory pathway (bundle of Kent) that bypasses the normal slow conduction through the atrioventricular node. Ventricular preexcitation is common, with a disease prevalence of 1.5 to 3 per 1000 people.22,58 Histological evaluation of Kent bundles resected from human subjects displayed features of typical ventricular myocytes with expression of connexin 43 (Cx43).59 The expression of high-conductance gap junctions in bypass tracts enables them to preexcite ventricular myocardium, manifesting as a short PR and a slurred QRS complex, or “delta wave,” on the ECG. The vast majority of WPW cases are sporadic, and the underlying mechanism remains unknown; however, rare inherited forms have been reported. Vidaillet et al60 determined that 3.4% of probands with WPW had 1 or more first-degree relatives with accessory conduction.PRKAG2A familial form of WPW with an autosomal dominant mode of transmission was identified in 2 families. Thirty-one affected individuals had evidence of preexcitation and cardiac hypertrophy. A missense mutation in PRKAG2 was identified that results in a constitutively active form of the γ2 regulatory subunit of AMP-activated protein kinase.22,23 Under normal conditions, AMP-activated protein kinase responds to energy-depleted states by increasing glucose uptake and promoting glycolysis. Transgenic mice expressing a heart-restricted, constitutively active mutant, PRKAG2N488I, recapitulated the human WPW phenotype of cardiac hypertrophy, preexcitation, and conduction defects. The predominant histological finding was ventricular myocyte engorgement with glycogen-laden vacuoles. The disruption of the annulus fibrosus by vacuolated ventricular myocytes resulted in the preexcitation phenotype.61 Using a mouse model of reversible glycogen-storage defect, Wolf et al62 demonstrated that the cardiomyopathy and CCS degeneration seen in PRKAG2N488I mice were reversible processes.BMP2Lalani et al24 reported a novel WPW syndrome associated with microdeletion of the bone morphogenetic protein-2 (Bmp2) region within 20p12.3 that is characterized by variable cognitive deficits and dysmorphic features. The BMPs are members of the transforming growth factor-β superfamily and play a critical role in cardiac development. Mice with cardiac deletion of BMP receptor type Ia (Bmpr1a) were embryonic lethal before E18.5 because of abnormal development of trabecular and compact myocardium, interventricular septum, and endocardial cushion.63 More restricted deletion of Bmpr1a in the atrioventricular canal resulted in defective atrioventricular valve formation and maturation defects in the annulus fibrosus, resulting in preexcitation.64,65Diseases of CCS DevelopmentCongenital heart disease is the most common form of birth defect, affecting 1% to 2% of live births.66 Arrhythmias may result from defective CCS specification/patterning, malformation or displacement of the conduction system, altered hemodynamics, prolonged hypoxic states, or postoperative sequelae.67,68 Developmentally, the conduction system derives from myocardial precursor cells within the fetal heart.69–71 The process by which conduction cells are specified or recruited into a “conduction” versus “working myocyte” lineage is determined by the regional expression of transcription factors.69–74 The main transcription factors identified in human CCS development are the T-box and homeobox factors.TBX5Holt-Oram syndrome is an autosomal dominant condition characterized by preaxial radial ray limb deformities (defects of the radius, carpal bones, and/or thumbs) and cardiac septation defects. The septal defects are typically ostium secundum atrial septal defects, muscular ventricular septal defects, and atrioventricular canal defects. Patients with Holt-Oram syndrome manifest variable degrees of CCS dysfunction, such as sinus bradycardia and atrioventricular block, even in the absence of overt structural heart disease. In 1997, Basson et al18 screened 2 families with Holt-Oram syndrome and identified mutations in the T-box transcription factor, TBX5. The T-box transcription factors can function as transcriptional activators or repressors and are known to be critical regulators of cardiac specification and differentiation. Seven TBX family members are expressed in the developing heart, 3 of which (TBX1, TBX5, TBX20) have been linked to human congenital heart disease.75Mice deficient in Tbx5 were embryonic lethal at E10.5 because of arrested development of the atria and left ventricle. Tbx5+/− mice recapitulated the upper limb and cardiac manifestations of human Holt-Oram syndrome, including the conduction abnormalities.72,76 Significant maturation defects in the atrioventricular canal and ventricular conduction system were present.76 Moskowitz et al76 demonstrated that Tbx5+/− mice have maturation failure of the atrioventricular canal manifesting as persistent atrioventricular rings around the tricuspid and mitral valves. Patterning defects were noted in the His bundle and bundle branches, including complete absence of right bundle branch formation in some cases. Expression of CCS-enriched markers, such as atrial natriuretic factor and Cx40, were found to be significantly downregulated, implicating TBX5 as a transcriptional activator of these genes. TBX5 and the homeobox transcription factor NKX2-5 were found to act synergistically in upregulating atrial natriuretic factor and Cx40 expression.76NKX2-5NKX2-5, a member of the homeodomain family, plays a central role in CCS induction and maintenance. Members of the homeodomain family all share a conserved 60–amino acid DNA binding motif known as the homeobox. These transcription factors are essential in organogenesis, dictating tissue specification and differentiation. Loss of the Nkx2-5 homolog, tinman, in the fruit fly results in failure of cardiogenesis.77 The role of NKX2-5 in human CCS development was established when mutations were identified in 4 families with nonsyndromic congenital heart disease and atrioventricular conduction block.19 Pedigree analysis was consistent with an autosomal dominant mode of transmission, and the most common structural abnormality was secundum atrial septal defects. Other associated structural heart defects included ventricular septal defects, tetralogy of Fallot, pulmonary atresia, redundant mitral valve, left ventricular hypertrophy, and subvalvular aortic stenosis.19 ECG analysis revealed atrioventricular conduction defects that were not strictly dependent on underlying structural heart disease. Fourteen individuals required pacemaker implantation. Genome-wide linkage analysis and candidate gene sequencing identified 3 mutations in NKX2-5.19 Currently, >60 Nkx2-5 single-nucleotide substitutions have been identified in patients with congenital heart disease, emphasizing its critical role in cardiogenesis and conduction system specification and maintenance.Mice deficient in Nkx2-5 die in utero at E9-10 because of arrested cardiac development in the linear heart tube stage. The hearts of these mice undergo partial looping morphogenesis, lack endocardial cushions and trabeculae, and have underdeveloped atrioventricular canals.78 Jay et al79 reported complete absence of atrioventricular node primordial cells in another model of Nkx2-5 deficiency (Nkx2-5neo/neo) in a background of minK-LacZ, a reporter mouse that labels the SAN, atrioventricular node, and proximal ventricular conduction system. Heterozygous Nkx2-5 mice exhibit an overall reduction in the size of the CCS from the atrioventricular node to the distal Purkinje network. Histological evaluation of the Nkx2-5+/neo atrioventricular node revealed that compact nodal cells (N region) that are Cx40−/Cx45+ are distinctly absent, whereas the nodo-His region that is Cx40+/Cx45+ remains intact.79 Immunostaining for Cx40 revealed that the density of Purkinje fibers was reduced.79Surface electrograms of Nkx2-5+/neo mice showed PR and QRS prolongation. Intracardiac electrograms were notable for a diminutive His depolarization amplitude, consistent with reduced size of the His bundle, and for prolonged 1:1 and 2:1 atrioventricular cycle lengths and atrioventricular node effective refractory periods suggestive of atrioventricular node dysfunction. His-ventricular intervals were unchanged likely because of sufficient expression of Cx40. Similarities in atrioventricular node dysfunction between humans and mice with Nkx2-5 haploinsufficiency point to the conserved role of NKX2-5 in the induction and maintenance of the mammalian CCS.79–81NKX2-5, TBX5, and the inhibitor of differentiation 2 (Id2) are now known to function together as a conduction system transcriptome specifying the murine ventricular CCS.74 The coexpression of NKX2-5 and TBX5 in the developing ventricular conduction system results in regionally restricted expression of Id2, ANF, and Cx40. Id2 is believed to function as an inhibitor of muscle differentiation allowing specification toward a conduction lineage.74 Combined haploinsufficiency of Nkx2-5 and Tbx5 (Nkx2-5+/−/Tbx5+/−) results in developmental failure of the ventricular conduction system with complete absence of the His bundle and bundle branches. Loss of CCS markers, like Cx40, in mutant hearts results in marked widening of the QRS complex on surface ECG.74 A human correlate of compound heterozygosity of Nkx2-5 and Tbx5 has not yet been identified.Conduction Disease Associated With Neuromuscular DisordersNeuromuscular disorders represent a diverse collection of diseases that commonly present with cardiac involvement. Mutations have been identified in genes involved in the cytoskeleton, nuclear envelope, and mitochondrial function. Cardiac involvement typically manifests as dilated or hypertrophic cardiomyopathy, atrioventricular conduction defects, and atrial and ventricular dysrhythmias.82EMD and LMNAMutations affecting the nuclear envelope have been associated with significant CCS