Electrophysiologic characteristics of accessory atrioventricular connections in an inherited form of Wolff-Parkinson-White syndrome

Novel PRKAG2 variant presenting as liver cirrhosis: report of a family with 2 cases and review of literature

Background

Mutations in the PRKAG2 gene encoding the 5′ Adenosine Monophosphate-Activated Protein Kinase (AMPK), specifically in its γ2 regulatory subunit, lead to Glycogen storage disease of heart, fetal congenital disorder (PRKAG2 syndrome). These mutations are rare, and their functional roles have not been fully elucidated. PRKAG2 syndrome is autosomal dominant disorder inherited with full penetrance. It is characterized by the accumulation of glycogen in the heart tissue, which is clinically manifested as hypertrophic cardiomyopathy. There is little knowledge about the characteristics of this disease. This study reports a genetic defect in an Iranian family with liver problems using targeted-gene sequencing.

Case presentation

A 4-year-old girl presented with short stature, hepatomegaly, and liver cirrhosis. As there was no specific diagnosis made based on the laboratory data and liver biopsy results, targeted-gene sequencing (TGS) was performed to detect the molecular basis of the disease. It was confirmed that this patient carried a novel heterozygous variant in the PRKAG2 gene. The echocardiography was a normal.

Conclusion

A novel heterozygous variant c.592A > T (p.Met198Leu) expands the mutational spectrum of the PRKAG2 gene in this family. Also, liver damage in patients with PRKAG2 syndrome has never been reported, which deserves further discussion.

Hypertrophic Cardiomyopathy in Children: Pathophysiology, Diagnosis, and Treatment of Non-sarcomeric Causes

Hypertrophic cardiomyopathy (HCM) is a myocardial disease characterized by left ventricular hypertrophy not solely explained by abnormal loading conditions. Despite its rare prevalence in pediatric age, HCM carries a relevant risk of mortality and morbidity in both infants and children. Pediatric HCM is a large heterogeneous group of disorders. Other than mutations in sarcomeric genes, which represent the most important cause of HCM in adults, childhood HCM includes a high prevalence of non-sarcomeric causes, including inherited errors of metabolism (i.e., glycogen storage diseases, lysosomal storage diseases, and fatty acid oxidation disorders), malformation syndromes, neuromuscular diseases, and mitochondrial disease, which globally represent up to 35% of children with HCM. The age of presentation and the underlying etiology significantly impact the prognosis of children with HCM. Moreover, in recent years, different targeted approaches for non-sarcomeric etiologies of HCM have emerged. Therefore, the etiological diagnosis is a fundamental step in designing specific management and therapy in these subjects. The present review aims to provide an overview of the non-sarcomeric causes of HCM in children, focusing on the pathophysiology, clinical features, diagnosis, and treatment of these rare disorders.

Ablation of Accessory Pathways with Challenging Anatomy

Although catheter ablation of accessory pathways is deemed highly safe and effective, peculiar location of these pathways might lead to complex and potentially hazardous procedures requiring ablation in anatomic regions such as para-Hisian area, coronary sinus, and epicardial surface. The electrophysiologist should know these possible scenarios to plan the best strategy for safe and effective ablation of these uncommon accessory pathways.

Genetic Infiltrative Cardiomyopathies

Infiltrative cardiomyopathies are characterized by abnormal accumulation or deposition of substances in cardiac tissue leading to cardiac dysfunction. These can be inherited, resulting from mutations in specific genes, which engender a diverse array of extracardiac features but overlapping cardiac phenotypes. This article provides an overview of each inherited infiltrative cardiomyopathy, describing the causative genes, the pathologic mechanisms involved, the resulting cardiac manifestations, and the therapies currently offered or being developed.

Human γ2-AMPK Mutations

In humans, dominant mutations in the gene encoding the regulatory γ2-subunit of AMP-activated protein kinase (PRKAG2) result in a highly penetrant phenotype dominated by cardiac features: left ventricular hypertrophy, ventricular pre-excitation, atrial tachyarrhythmia, cardiac conduction disease, and myocardial glycogen storage. The discovery of a link between the cell's fundamental energy sensor, AMPK, and inherited cardiac disease catalyzed intense interest into the biological role of AMPK in the heart. In this chapter, we provide an introduction to the spectrum of human disease resulting from pathogenic variants in PRKAG2, outlining its discovery, clinical genetics, and current perspectives on its pathogenesis and highlighting mechanistic insights derived through the evaluation of disease models. We also present a clinical perspective on the major components of the cardiomyopathy associated with mutations in PRKAG2, together with less commonly described extracardiac features, its prognosis, and principles of management.

Multiple accessory pathways in the young: the impact of structural heart disease

BACKGROUND

The presence of multiple accessory pathways (MultAP) is described in structural heart disease (SHD) such as Ebstein's anomaly and cardiomyopathies. Structural defects can impact the tolerability of tachyarrhythmia and can complicate both medical management and ablation. In a large cohort of pediatric patients with and without SHD undergoing invasive electrophysiology study, we examined the prevalence of MultAP and the effect of both MultAP and SHD on ablation outcomes.

METHODS

Accessory pathway number and location, presence of SHD, ablation success, and recurrence were analyzed in consecutive patients from our center over a 16-year period.

RESULTS

In 1088 patients, 1228 pathways (36% retrograde only) were mapped to the right side (TV) in 18%, septum (S) in 39%, and left side (MV) in 43%. MultAP were present in 111 pts (10%), involving 250 distinct pathways. SHD tripled the risk of MultAP (26% SHD vs 8% no SHD, P < .001). Multivariable adjusted risk factors for MultAP included Ebstein's (OR 8.7[4.4-17.5], P < .001) and cardiomyopathy (OR 13.3[5.1-34.5], P < .001). Of 1306 ablation attempts, 94% were acutely successful with an 8% recurrence rate. Ablation success was affected by SHD (85% vs 95% for no SHD, P < .01) but not by MultAP (91% vs 94% for single, P = .24). Recurrence rate was higher for SHD (17% SHD vs 8% no SHD, P < .05) and MultAP (19% MultAP vs 8% single, P < .001).

CONCLUSIONS

MultAP are found in 10% of pediatric patients, and are more common in SHD compared to those with normal hearts. Both the presence of MultAP and SHD negatively influence ablation outcomes.

The history of the wolff-Parkinson-white syndrome

While Drs Wolff, Parkinson, and White fully described the syndrome in 1930, prior case reports had described the essentials. Over the ensuing century this syndrome has captivated the interest of anatomists, clinical cardiologists, and cardiac surgeons. Stanley Kent described lateral muscular connections over the atrioventricular (AV) groove which he felt were the normal AV connections. The normal AV connections were, however, clearly described by His and Tawara. True right-sided AV connections were initially described by Wood et al., while Öhnell first described left free wall pathways. David Scherf is thought to be the first to describe our current understanding of the pathogenesis of the WPW syndrome in terms of a re-entrant circuit involving both the AV node-His axis as well as the accessory pathway. This hypothesis was not universally accepted, and many theories were applied to explain the clinical findings. The basics of our understanding were established by the brilliant work of Pick, Langendorf, and Katz who by using careful deductive analysis of ECGs were able to define the basic pathophysiological processes. Subsequently, Wellens and Durrer applied invasive electrical stimulation to the heart in order to confirm the pathophysiological processes. Sealy and his colleagues at Duke University Medical Center were the first to successfully surgically divide an accessory pathway and ushered in the modern era of therapy for these patients. Morady and Scheinman were the first to successfully ablate an accessory pathway (posteroseptal) using high-energy direct-current shocks. Subsequently Jackman, Kuck, Morady, and a number of groups proved the remarkable safety and efficiency of catheter ablation for pathways in all locations using radiofrequency energy. More recently, Gollob et al. first described the gene responsible for a familial form of WPW. The current ability to cure patients with WPW is due to the splendid contributions of individuals from diverse disciplines throughout the world.

Identical anatomical location of accessory pathway in a family with Wolff-Parkinson-White syndrome

Identical location of accessory pathways in family members with Wolff-Parkinson-White syndrome, although theoretically possible, has never been described. A 37-year-old woman and her 18-year-old son were referred for electrophysiological study due to fast rate palpitations and pre-excitation on baseline electrocardiogram. After mapping during pre-excitation, successful radiofrequency application was located at the right free wall in both patients, in an identical anatomical position, on the infero-lateral aspect of the tricuspid ring.

Nodoventricular Accessory Pathways in PRKAG2 -Dependent Familial Preexcitation Syndrome Reveal a Disorder in Cardiac Development

Background— Familial preexcitation syndrome is linked to mutations in PRKAG2 . Previous studies on the R302Q mutation have provided evidence for a remarkably high proportion of otherwise rare accessory pathways with atrioventricular (AV) node-like conduction properties (Mahaim fibers). Yet, histopathologic proof is still lacking. We aimed to provide such proof.

Methods and Results— We retrospectively studied the medical records of 17 members of a 5-generation family. Five subjects died prematurely. The R302Q mutation was found in 8 living subjects and 2 deceased subjects (obligate carriers). Cardiac hypertrophy was found in 7 mutation carriers. ECGs compatible with preexcitation were found in 13 subjects and AV block at varying degrees in 5 subjects. All mutation carriers had electrocardiographic evidence of preexcitation, AV block, or both. Three individuals had high-grade AV block with preexcited conducted beats. Electrophysiological studies in 3 individuals revealed bypasses with AV node–like properties. Histopathologic studies of 1 suddenly deceased mutation carrier revealed concentric hypertrophy of the left ventricle with extensive myocardial disarray associated with slight interstitial fibrosis but no lysosomal-bound glycogen. Moreover, there were 3 small nodoventricular tracts (Mahaim fibers) passing through the central fibrous body and connecting the AV node with the working myocardium of the interventricular septum.

Conclusions— Preexcitation associated with the R302Q mutation in PRKAG2 is associated with Mahaim fibers. These findings support the novel insight that PRKAG2 may be involved in the development of the cardiac conduction system.

AMP-Activated Protein Kinase in the Heart

AMP-activated protein kinase (AMPK) is a heterotrimeric enzyme that is expressed in most mammalian tissues including cardiac muscle. Among the multiple biological processes influenced by AMPK, regulation of fuel supply and energy-generating pathways in response to the metabolic needs of the organism is fundamental and likely accounts for the remarkable evolutionary conservation of this enzyme complex. By regulating the activity of acetyl-coenzyme A carboxylase, AMPK affects levels of malonyl-coenzyme A, a key energy regulator in the cell. AMPK is generally quiescent under normal conditions but is activated in response to hormonal signals and stresses sufficient to produce an increase in AMP/ATP ratio, such as hypoglycemia, strenuous exercise, anoxia, and ischemia. Once active, muscle AMPK enhances uptake and oxidative metabolism of fatty acids as well as increases glucose transport and glycolysis. Data from AMPK deficiency models suggest that AMPK activity might influence the pathophysiology and therapy of diabetes and increase heart tolerance to ischemia. Effects that are not as well understood include AMPK regulation of transcription. Different AMPK isoforms are found in distinct locations within the cell and have distinct functions in different tissues. A principal mode of AMPK activation is phosphorylation by upstream kinases (eg, LKB1). These kinases have a fundamental role in cell-cycle regulation and protein synthesis, suggesting involvement in a number of human disorders including cardiac hypertrophy, apoptosis, cancer, and atherosclerosis. The physiological role played by AMPK during health and disease is far from being clearly defined. Naturally occurring mutations affecting the nucleotide-sensing modules in the regulatory gamma subunit of AMPK lead to enzyme dysregulation and inappropriate activation under resting conditions. Glycogen accumulation ensues, leading to human disease manifesting as cardiac hypertrophy, accessory atrioventricular connections, and degeneration of the physiological conduction system. Whether AMPK is a key participant or bystander in other disease states and whether its selective manipulation may significantly benefit these conditions remain important questions.

Is Wolff-Parkinson-White syndrome a genetic disease?

Family studies, and more recent molecular genetic investigations, indicate that the Wolff-Parkinson-White (WPW) syndrome and associated preexcitation disorders can have a substantial genetic component. Because preexcitation disorders are sometimes inherited as single gene disorders, key mechanistic insights can be gained that are expected to be relevant also to the more common multifactorial forms of these traits. Potentially, such insights will inform the future management of these conditions. Where WPW is inherited as a familial trait, with or without associated cardiac defects or a systemic syndrome, there are clinical genetic ramifications that are already of practical importance.

History of Wolff-Parkinson-White syndrome

While Drs. Wolff, Parkinson, and White fully described the syndrome that bears their names in 1930, prior case reports had already described the essentials. Over the ensuing century this syndrome has captivated the interest of anatomists, clinical cardiologists, and cardiac surgeons. Stanley Kent described lateral muscular connections over the atrioventricular (AV) groove, which he felt were the normal AV connections. The normal AV connections were, however, clearly described by His and Tawara. True right-sided AV connections were initially described by Wood et al., while Ohnell first described left free wall pathways. David Scherf is thought to be the first to describe our current understanding of the pathogenesis of the Wolff-Parkinson-White (WPW) syndrome in terms of a reentrant circuit involving both the AV node--His axis as well as the accessory pathway. This hypothesis was not universally accepted and many theories were applied to explain the clinical findings. The basics of our understandings were established by the brilliant work of Pick, Langendorf, and Katz who by using careful deductive analysis of ECGs were able to define the basic pathophysiological processes. Subsequently, Wellens and Durrer applied invasive electrical stimulation to the heart in order to confirm the pathophysiological processes. Sealy and his colleagues at Duke University Medical Center were the first to successfully surgically divide an accessory pathway and ushered in the modern area for curative therapy for these patients. Morady and Scheinman were the first to successfully ablate an accessory pathway (posteroseptal) using high-energy direct-current shocks. Subsequently, Jackman, Kuck, Morady, and a number of groups proved the remarkable safety and efficiency of catheter ablation for pathways in all locations using radiofrequency energy. More recently, Gallob et al. first described the gene responsible for a familial form of WPW. The current ability to cure patients with WPW is due to the splendid contributions of individuals from diverse disciplines from throughout the world.

Familial Atrioventricular Nodal Reentry Tachycardia

Dual atrioventricular nodal pathways, the substrate responsible for atrioventricular node reentry tachycardia (AVNRT), are thought to be randomly occurring congenital anomalies. This article describes 14 patients in six families, each with two or three first-degree relatives with paroxysmal supraventricular tachycardia. Electrophysiological evidence of dual atrioventricular nodal pathways was established in all 13 patients studied, AVNRT was induced in 12 (92%), and radiofrequency ablation of the slow pathway was curative in all cases. The data suggest a hereditary contribution to the development of atrioventricular nodal pathways and AVNRT. The pattern of inheritance appears to be autosomal dominant.

Electrophysiologic characterization and postnatal development of ventricular pre-excitation in a mouse model of cardiac hypertrophy and Wolff-Parkinson-White syndrome

OBJECTIVES

We sought to characterize an animal model of the Wolff-Parkinson-White (WPW) syndrome to help elucidate the mechanisms of accessory pathway formation.

BACKGROUND

Patients with mutations in PRKAG2 manifest cardiac hypertrophy and ventricular pre-excitation; however, the mechanisms underlying the development and conduction of accessory pathways remain unknown.

METHODS

We created transgenic mice overexpressing either the Asn488Ile mutant (TG(N488I)) or wild-type (TG(WT)) human PRKAG2 complementary deoxyribonucleic acid under a cardiac-specific promoter. Both groups of transgenic mice underwent intracardiac electrophysiologic, electrocardiographic (ECG), and histologic analyses.

RESULTS

On the ECG, approximately 50% of TG(N488I) mice displayed sinus bradycardia and features suggestive of pre-excitation, not seen in TG(WT) mice. The electrophysiologic studies revealed a distinct atrioventricular (AV) connection apart from the AV node, using programmed stimulation. In TG(N488I) mice with pre-excitation, procainamide blocked bypass tract conduction, whereas adenosine infusion caused AV block in TG(WT) mice but not TG(N488I) mice with pre-excitation. Serial ECGs in 16 mice pups revealed no differences at birth. After one week, two of eight TG(N488I) pups had ECG features of pre-excitation, increasing to seven of eight pups by week 4. By nine weeks, one TG(N488I) mouse with WPW syndrome lost this phenotype, whereas TG(WT) pups never developed pre-excitation. Histologic investigation revealed postnatal development of myocardial connections through the annulus fibrosum of the AV valves in young TG(N488I) but not TG(WT) mice.

CONCLUSIONS

Transgenic mice overexpressing the Asn488Ile PRKAG2 mutation recapitulate an electrophysiologic phenotype similar to humans with this mutation. This includes procainamide-sensitive, adenosine-resistant accessory pathways induced in postnatal life that may rarely disappear later in life.

Proposal for a new definition of congenital complete atrioventricular block

The classic old definition of congenital heart block by Yater (1929) is still generally accepted: 'Heart block established in a young patient. There must be some evidence of the existence of the slow pulse at a fairly early age and absence of a history of any infection which might cause the condition after birth: notably diphtheria, rheumatic fever, chorea and congenital syphilis'. However, other definitions are used. We systematically reviewed 1825 cases from 38 separate studies. We conclude that complete AV blocks detected in utero in the absence of structural abnormalities differ from blocks detected later in life with respect to pathogenesis (they are generally associated with maternal anti-Ro/SSA antibodies), poorer childhood prognosis, increased risk of developing late-onset dilated cardiomyopathy, different maternal clinical features and increased risk of recurrence in future pregnancies. For these reasons we propose a new modern definition of congenital complete AV block which might be acceptable to cardiologists, rheumatologists, pediatricians and obstetricians: 'an AV block is defined as congenital if it is diagnosed in utero, at birth or within the neonatal period (0-27 days after birth)'.

Transgenic mice overexpressing mutant PRKAG2 define the cause of Wolff-Parkinson-White syndrome in glycogen storage cardiomyopathy

BACKGROUND

Mutations in the gamma2 subunit (PRKAG2) of AMP-activated protein kinase produce an unusual human cardiomyopathy characterized by ventricular hypertrophy and electrophysiological abnormalities: Wolff-Parkinson-White syndrome (WPW) and progressive degenerative conduction system disease. Pathological examinations of affected human hearts reveal vacuoles containing amylopectin, a glycogen-related substance.

METHODS AND RESULTS

To elucidate the mechanism by which PRKAG2 mutations produce hypertrophy with electrophysiological abnormalities, we constructed transgenic mice overexpressing the PRKAG2 cDNA with or without a missense N488I human mutation. Transgenic mutant mice showed elevated AMP-activated protein kinase activity, accumulated large amounts of cardiac glycogen (30-fold above normal), developed dramatic left ventricular hypertrophy, and exhibited ventricular preexcitation and sinus node dysfunction. Electrophysiological testing demonstrated alternative atrioventricular conduction pathways consistent with WPW. Cardiac histopathology revealed that the annulus fibrosis, which normally insulates the ventricles from inappropriate excitation by the atria, was disrupted by glycogen-filled myocytes. These anomalous microscopic atrioventricular connections, rather than morphologically distinct bypass tracts, appeared to provide the anatomic substrate for ventricular preexcitation.

CONCLUSIONS

Our data establish PRKAG2 mutations as a glycogen storage cardiomyopathy, provide an anatomic explanation for electrophysiological findings, and implicate disruption of the annulus fibrosis by glycogen-engorged myocytes as the cause of preexcitation in Pompe, Danon, and other glycogen storage diseases.

PRKAG2 cardiac syndrome: familial ventricular preexcitation, conduction system disease, and cardiac hypertrophy

Genetic studies of families with inherited cardiac rhythm disturbances have established a molecular basis for ventricular arrhythmogenic disorders. Genes responsible for the long QT syndrome, Brugada syndrome, and polymorphic ventricular tachycardia have been identified. The elucidation of genetic defects responsible for more commonly occurring supraventricular rhythm disturbances have not been as forthcoming, with the exception of SCN5A mutations known to cause conduction system disease. Recently, we identified the genetic cause of a familial arrhythmogenic syndrome characterized by ventricular preexcitation and tachyarrhythmias (Wolff-Parkinson-White syndrome), progressive conduction system disease, and cardiac hypertrophy. The causative gene was shown to be the gamma-2 regulatory subunit (PRKAG2) of AMP-activated protein kinase. The role of AMP-activated protein kinase in the regulation of the glucose metabolic pathway in muscle suggests that genetic defects in PRKAG2 may induce a previously undescribed cardiac glycogenosis syndrome.

Constitutively active AMP kinase mutations cause glycogen storage disease mimicking hypertrophic cardiomyopathy

Mutations in PRKAG2, the gene for the gamma 2 regulatory subunit of AMP-activated protein kinase, cause cardiac hypertrophy and electrophysiologic abnormalities, particularly preexcitation (Wolff-Parkinson-White syndrome) and atrioventricular conduction block. To understand the mechanisms by which PRKAG2 defects cause disease, we defined novel mutations, characterized the associated cardiac histopathology, and studied the consequences of introducing these mutations into the yeast homologue of PRKAG2, Snf4. Although the cardiac pathology caused by PRKAG2 mutations Arg302Gln, Thr400Asn, and Asn488Ile include myocyte enlargement and minimal interstitial fibrosis, these mutations were not associated with myocyte and myofibrillar disarray, the pathognomonic features of hypertrophic cardiomyopathy caused by sarcomere protein mutations. Instead PRKAG2 mutations caused pronounced vacuole formation within myocytes. Several lines of evidence indicated these vacuoles were filled with glycogen-associated granules. Analyses of the effects of human PRKAG2 mutations on Snf1/Snf4 kinase function demonstrated constitutive activity, which could foster glycogen accumulation. Taken together, our data indicate that PRKAG2 mutations do not cause hypertrophic cardiomyopathy but rather lead to a novel myocardial metabolic storage disease, in which hypertrophy, ventricular pre-excitation and conduction system defects coexist.

Evaluation and treatment of other arrhythmic causes of syncope in children and adolescents with an apparently normal heart: Wolff-Parkinson-White syndrome and right ventricular cardiomyopathy

Syncope could be a symptom of tachyarrhythmias related to the Wolff-Parkinson-White syndrome, or the consequence of the ventricular tachycardias seen in patients with Arrhythmogenic Right Ventricular Cardiomyopathy. Syncope should be considered the consequence of atrial fibrillation or flutter, with rapid conduction over the accessory atrioventricular connection in Wolff-Parkinson-White syndrome, and these patients are at risk of presenting with ventricular fibrillation and sudden death. Radiofrequency ablation of the anomalous, accessory connection, which can be performed with high success and low complication rates, should be the first line of treatment for symptomatic children and adolescents with Wolff-Parkinson-White. Arrhythmogenic Right Ventricular Cardiomyopathy is a rare disorder of the cardiac muscle affecting predominantly, although not exclusively, the right ventricle. Clinical presentation varies from asymptomatic cases to patients with severe symptoms related to life-threatening arrhythmias, right ventricular failure, or congestive heart failure with involvement of both ventricles. The clinical diagnosis is difficult. A set of major and minor criteria has been proposed to help to identify patients with this disease. Without an identified cause, the treatment of patients with Arrhythmogenic Right Ventricular Cardiomyopathy is symptomatic. Medical management of the associated congestive heart failure, pharmacologic treatment of the arrhythmias, radiofrequency ablation and implantable cardioverter-defibrillator therapy should all be considered.