Mutations in sarcomere protein genes can cause hypertrophic cardiomyopathy (HCM), a disorder characterized by myocyte enlargement, fibrosis, and impaired ventricular relaxation. Here, we demonstrate that sarcomere protein gene mutations activate proliferative and profibrotic signals in non-myocyte cells to produce pathologic remodeling in HCM. Gene expression analyses of non-myocyte cells isolated from HCM mouse hearts showed increased levels of RNAs encoding cell-cycle proteins, Tgf-β, periostin, and other profibrotic proteins. Markedly increased BrdU labeling, Ki67 antigen expression, and periostin immunohistochemistry in the fibrotic regions of HCM hearts confirmed the transcriptional profiling data. Genetic ablation of periostin in HCM mice reduced but did not extinguish non-myocyte proliferation and fibrosis. In contrast, administration of Tgf-β-neutralizing antibodies abrogated non-myocyte proliferation and fibrosis. Chronic administration of the angiotensin II type 1 receptor antagonist losartan to mutation-positive, hypertrophynegative (prehypertrophic) mice prevented the emergence of hypertrophy, non-myocyte proliferation, and fibrosis. Losartan treatment did not reverse pathologic remodeling of established HCM but did reduce nonmyocyte proliferation. These data define non-myocyte activation of Tgf-β signaling as a pivotal mechanism for increased fibrosis in HCM and a potentially important factor contributing to diastolic dysfunction and heart failure. Preemptive pharmacologic inhibition of Tgf-β signals warrants study in human patients with sarcomere gene mutations.

The cardiac conduction system is an anatomically discrete segment of specialized myocardium that initiates and propagates electrical impulses to coordinate myocardial contraction. To define the molecular composition of the mouse ventricular conduction system we used microdissection and transcriptional profiling by serial analysis of gene expression (SAGE). Conduction-system-specific expression for Id2, a member of the Id gene family of transcriptional repressors, was identified. Analyses of Id2-deficient mice demonstrated structural and functional conduction system abnormalities, including left bundle branch block. A 1.2 kb fragment of the Id2 promoter proved sufficient for cooperative regulation by Nkx2-5 and Tbx5 in vitro and for conduction-system-specific gene expression in vivo. Furthermore, compound haploinsufficiency of Tbx5 and Nkx2-5 or Tbx5 and Id2 prevented embryonic specification of the ventricular conduction system. We conclude that a molecular pathway including Tbx5, Nkx2-5, and Id2 coordinates specification of ventricular myocytes into the ventricular conduction system lineage.

Catecholamine-induced polymorphic ventricular tachycardia (CPVT) is a familial disorder caused by cardiac ryanodine receptor type 2 (RyR2) or calsequestrin 2 (CASQ2) gene mutations. To define how CASQ2 mutations cause CPVT, we produced and studied mice carrying a human D307H missense mutation (CASQ 307/307 ) or a CASQ2-null mutation (CASQ ΔE9/ΔE9 ). Both CASQ2 mutations caused identical consequences. Young mutant mice had structurally normal hearts but stress-induced ventricular arrhythmias; aging produced cardiac hypertrophy and reduced contractile function. Mutant myocytes had reduced CASQ2 and increased calreticulin and RyR2 (with normal phosphorylated proportions) but unchanged calstabin levels, as well as reduced total sarcoplasmic reticulum (SR) Ca 2+ , prolonged Ca 2+ release, and delayed Ca 2+ reuptake. Stress further diminished Ca 2+ transients, elevated cytosolic Ca 2+ , and triggered frequent, spontaneous SR Ca 2+ release. Treatment with Mg 2+ , a RyR2 inhibitor, normalized myocyte Ca 2+ cycling and decreased CPVT in mutant mice, indicating RyR2 dysfunction was critical to mutant CASQ2 pathophysiology. We conclude that CPVT-causing CASQ2 missense mutations function as null alleles. In the absence of CASQ2, calreticulin, a fetal Ca 2+ -binding protein normally downregulated at birth, remains a prominent SR component. Adaptive changes to CASQ2 deficiency (increased posttranscriptional expression of calreticulin and RyR2) maintained electrical-mechanical coupling, but increased RyR2 leakiness, a paradoxical response further exacerbated by stress. The central role of RyR2 dysfunction in CASQ2 deficiency unifies the pathophysiologic mechanism underlying CPVT due to RyR2 or CASQ2 mutations and suggests a therapeutic approach for these inherited cardiac arrhythmias. IntroductionCatecholamine-induced polymorphic ventricular tachycardia (CPVT) is a familial arrhythmogenic disorder characterized by adrenergic-stimulated VT that can deteriorate into ventricular fibrillation to cause sudden cardiac death (1, 2). Recurrent syncope, seizures, and cardiovascular collapse are triggered by exercise or emotional stress (2, 3). Mutations in the cardiac sarcoplasmic reticulum (SR) Ca 2+ release channel, ryanodine receptor type 2 (RyR2), or in cardiac calsequestrin 2 (CASQ2), a SR Ca 2+ -storage protein, cause CPVT (2, 4). Despite considerable knowledge about the coupling of Ca 2+ release from the SR, activation of sarcomere contraction, and Ca 2+ reuptake into the SR during relaxation (reviewed in refs. 5, 6), the mechanisms by which CASQ2 mutations alter Ca 2+ handling or cause CPVT are incompletely understood. CASQ, the most abundant Ca 2+ -buffering protein in the lumen of striated muscle SR (7,8), is encoded by the skeletal (CASQ1) and cardiac (CASQ2) calsequestrin genes (9, 10). Each molecule of CASQ2, a 415-amino acid polypeptide, binds 40-50 Ca 2+ with low affinity (K d = 1 mM), allowing cardiac SR Ca 2+ concentrations to approach 20 mM while free Ca 2+ concentrations are only 1 mM (7,(11)(12)(13). CASQ2 also part...

Mutations in the lamin A/C (LMNA) gene, which encodes nuclear membrane proteins, cause a variety of human conditions including dilated cardiomyopathy (DCM) with associated cardiac conduction system disease. To investigate mechanisms responsible for electrophysiologic and myocardial phenotypes caused by dominant human LMNA mutations, we performed longitudinal evaluations in heterozygous Lmna +/-mice. Despite one normal allele, Lmna +/-mice had 50% of normal cardiac lamin A/C levels and developed cardiac abnormalities. Conduction system function was normal in neonatal Lmna +/-mice but by 4 weeks of age AV nodal myocytes had abnormally shaped nuclei and active apoptosis. Telemetric and in vivo electrophysiologic studies in 10 week-old Lmna +/-mice showed atrioventricular (AV) conduction defects and both atrial and ventricular arrhythmias, analogous to those observed in humans with heterozygous LMNA mutations. Isolated myocytes from 12-month old Lmna +/-mice exhibited impaired contractility. In vivo cardiac studies of aged Lmna +/-mice revealed DCM; in some mice this occurred without Contact address: Charles I. Berul, M.D., Department of Cardiology -Children's Hospital, 300 Longwood Ave., Boston, MA 02115 USA, Phone: 617 355-6432, Fax: 617 566-5671, charles.berul@cardio.chboston.org. * These authors contributed equally to this publication. Disclosures: NonePublisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public AccessAuthor Manuscript J Mol Cell Cardiol. Author manuscript; available in PMC 2010 December 29. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript overt conduction system disease. However, neither histopathology nor serum CK levels indicated skeletal muscle pathology. These data demonstrate cardiac pathology due to heterozygous Lmna mutations reflecting a 50% reduction in lamin protein levels. Lamin haploinsufficiency caused early-onset programmed cell death of AV nodal myocytes and progressive electrophysiologic disease. While lamin haploinsufficiency was better tolerated by non-conducting myocytes, ultimately these too succumbed to diminished lamin levels leading to dilated cardiomyopathy, which presumably arose independently from conduction system disease.

Background-AMP-activated protein kinase (AMPK) regulatory ␥2 subunit (PRKAG2) mutations cause a human cardiomyopathy with cardiac hypertrophy, preexcitation, and glycogen deposition. PRKAG2 cardiomyopathy is recapitulated in transgenic mice overexpressing mutant PRKAG2 N488I in the heart (TG␥2 N488I ). AMPK is a heterotrimeric kinase consisting of 1 catalytic (␣) and 2 regulatory (␤ and ␥) subunits. Two ␣-subunit isoforms, ␣1 and ␣2, are expressed in the heart; however, the contribution of AMPK utilization of these subunits to PRKAG2 cardiomyopathy is unknown. Mice overexpressing a dominant-negative ␣2 subunit of AMPK (TG␣2 DN ) provide a tool for selectively inhibiting ␣2, but not ␣1, subunit-associated AMPK activity. Methods and Results-In compound-heterozygous TG␥2N488I /TG␣2 DN mice, AMPK activity associated with ␣2 but not ␣1 was decreased compared with TG␥2 N488I . The TG␣2 DN transgene reduced the disease phenotype of TG␥2 N488I , partially or completely normalizing the ECG, cardiac function, cardiac morphology, and exercise capacity in compoundheterozygous mice. TG␥2 N488I hearts had normal resting levels of high-energy phosphates and could improve cardiac performance during exercise. Cardiac glycogen content decreased in TG␥2 N488I mice after exercise stress, indicating availability of the stored glycogen for metabolic utilization. No differences in glycogen-metabolizing enzymes were observed. Conclusions-The PRKAG2 N488I mutation causes inappropriate AMPK activation, which leads to glycogen accumulation and conduction system disease. The accumulated glycogen can serve as an energy source, and the animals have contractile reserve during exercise. Because the dominant-negative ␣2 subunit attenuates the mutant PRKAG2 phenotype, AMPK complexes containing the ␣2 rather than the ␣1 subunit are the primary mediators of the effects of PRKAG2 mutations. (Circulation. 2005;112:3140-3148.)

Sarcomere protein gene mutations cause hypertrophic cardiomyopathy (HCM), a disease with distinctive histopathology and increased susceptibility to cardiac arrhythmias and risk for sudden death. Myocyte disarray (disorganized cell-cell contact) and cardiac fibrosis, the prototypic but protean features of HCM histopathology, are presumed triggers for ventricular arrhythmias that precipitate sudden death events. To assess relationships between arrhythmias and HCM pathology without confounding human variables, such as genetic heterogeneity of disease-causing mutations, background genotypes, and lifestyles, we studied cardiac electrophysiology, hypertrophy, and histopathology in mice engineered to carry an HCM mutation. Both genetically outbred and inbred HCM mice had variable susceptibility to arrhythmias, differences in ventricular hypertrophy, and variable amounts and distribution of histopathology. Among inbred HCM mice, neither the extent nor location of myocyte disarray or cardiac fibrosis correlated with ex vivo signal conduction properties or in vivo electrophysiologically stimulated arrhythmias. In contrast, the amount of ventricular hypertrophy was significantly associated with increased arrhythmia susceptibility. These data demonstrate that distinct somatic events contribute to variable HCM pathology and that cardiac hypertrophy, more than fibrosis or disarray, correlates with arrhythmic risk. We suggest that a shared pathway triggered by sarcomere gene mutations links cardiac hypertrophy and arrhythmias in HCM.fibrosis ͉ somatic modifiers ͉ disarray ͉ electrophysiology ͉ left ventrical wall thickness

The cardiac conduction system can be anatomically, developmentally, and molecularly distinguished from the working myocardium. Abnormalities in cardiac conduction can occur due to a variety of factors, including developmental and congenital defects, acquired injury or ischemia of portions of the conduction system, or less commonly due to inherited diseases that alter cardiac conduction system function. So called "idiopathic" conduction system degeneration may have familial clustering, and therefore is consistent with a hereditary basis. This "Molecular Perspectives" will highlight several diverse mechanisms of isolated conduction system disease as well as conduction system degeneration associated with other cardiac and non-cardiac disorders. The first part of this review focuses on channelopathies associated with conduction system disease. Human genetic studies have identified mutations in the sodium channel SCN5A gene causing tachyarrhythmia disorders, as well as progressive cardiac conduction system diseases, or overlapping syndromes. Next, the importance of embryonic developmental genes such as homeobox and T-box transcription factors are highlighted in conduction system development and function. Conduction system diseases associated with multisystem disorders, such as muscular and myotonic dystrophies, will be described. Last, a new glycogen storage cardiomyopathy associated with ventricular preexcitation and progressive conduction system degeneration will be reviewed. There are a myriad of mutations identified in genes encoding cardiac transcription factors, ion channels, gap junctions, energy metabolism regulators, lamins and other structural proteins. Understanding of the molecular and ionic mechanisms underlying cardiac conduction is essential for the appreciation of the pathogenesis of conduction abnormalities in structurally normal and altered hearts.

Hypertrophic cardiomyopathy (HCM) is the most common inherited heart disease and defined by unexplained isolated progressive myocardial hypertrophy, systolic and diastolic ventricular dysfunction, arrhythmias, sudden cardiac death and histopathologic changes, such as myocyte disarray and myocardial fibrosis. Mutations in genes encoding for proteins of the contractile apparatus of the cardiomyocyte, such as β-myosin heavy chain and myosin binding protein C, have been identified as cause of the disease.Disease is caused by altered biophysical properties of the cardiomyocyte, disturbed calcium handling, and abnormal cellular metabolism. Mutations in sarcomere genes can also activate other signaling pathways via transcriptional activation and can influence non-cardiac cells, such as fibroblasts. Additional environmental, genetic and epigenetic factors result in heterogeneous disease expression. The clinical course of the disease varies greatly with some patients presenting during childhood while others remain asymptomatic until late in life. Patients can present with either heart failure symptoms or the first symptom can be sudden death due to malignant ventricular arrhythmias. The morphological and pathological heterogeneity results in prognosis uncertainty and makes patient management challenging. Current standard therapeutic measures include the prevention of sudden death by prohibition of competitive sport participation and the implantation of cardioverter-defibrillators if indicated, as well as symptomatic heart failure therapies or cardiac transplantation. There exists no causal therapy for this monogenic autosomal-dominant inherited disorder, so that the focus of current management is on early identification of asymptomatic patients at risk through molecular diagnostic and clinical cascade screening of family members, optimal sudden death risk stratification, and timely initiation of preventative therapies to avoid disease progression to the irreversible adverse myocardial remodeling stage. Genetic diagnosis allowing identification of asymptomatic affected patients prior to clinical disease onset, new imaging technologies, and the establishment of international guidelines have optimized treatment and sudden death risk stratification lowering mortality dramatically within the last decade. However, a thorough understanding of underlying disease pathogenesis, regular clinical follow-up, family counseling, and preventative treatment is required to minimize morbidity and mortality of affected patients. This review summarizes current knowledge about molecular genetics and pathogenesis of HCM secondary to mutations in the sarcomere and provides an overview about current evidence and guidelines in clinical patient management. The overview will focus on clinical staging based on disease mechanism allowing timely initiation of preventative measures. An outlook about so far experimental treatments and potential for future therapies will be provided.