Sharlene M. Day

Dystrophic heart failure blocked by membrane sealant poloxamer

Dystrophin deficiency causes Duchenne muscular dystrophy (DMD) in humans, an inherited and progressive disease of striated muscle deterioration that frequently involves pronounced cardiomyopathy. Heart failure is the second leading cause of fatalities in DMD. Progress towards defining the molecular basis of disease in DMD has mostly come from studies on skeletal muscle, with comparatively little attention directed to cardiac muscle. The pathophysiological mechanisms involved in cardiac myocytes may differ significantly from skeletal myofibres; this is underscored by the presence of significant cardiac disease in patients with truncated or reduced levels of dystrophin but without skeletal muscle disease. Here we show that intact, isolated dystrophin-deficient cardiac myocytes have reduced compliance and increased susceptibility to stretch-mediated calcium overload, leading to cell contracture and death, and that application of the membrane sealant poloxamer 188 corrects these defects in vitro. In vivo administration of poloxamer 188 to dystrophic mice instantly improved ventricular geometry and blocked the development of acute cardiac failure during a dobutamine-mediated stress protocol. Once issues relating to optimal dosing and long-term effects of poloxamer 188 in humans have been resolved, chemical-based membrane sealants could represent a new therapeutic approach for preventing or reversing the progression of cardiomyopathy and heart failure in muscular dystrophy.

Chronic Iron Administration Increases Vascular Oxidative Stress and Accelerates Arterial Thrombosis

Background—Iron overload has been implicated in the pathogenesis of ischemic cardiovascular events. However, the effects of iron excess on vascular function and the thrombotic response to vascular injury are not well understood. Methods and Results—We examined the effects of chronic iron dextran administration (15 mg over 6 weeks) on thrombosis, systemic and vascular oxidative stress, and endothelium-dependent vascular reactivity in mice. Thrombus generation after photochemical carotid artery injury was accelerated in iron-loaded mice (mean time to occlusive thrombosis, 20.4±8.5 minutes; n=10) compared with control mice (54.5±35.5 minutes, n=10, P =0.009). Iron loading had no effect on plasma clotting, vessel wall tissue factor activity, or ADP-induced platelet aggregation. Acute administration of dl-cysteine, a reactive oxygen species scavenger, completely abrogated the effects of iron loading on thrombus formation, suggesting that iron accelerated thrombosis through a pro-oxidant mechanism. Iron loading enhanced both systemic and vascular reactive oxygen species production. Endothelium-dependent vasorelaxation was impaired in iron-loaded mice, indicating reduced NO bioavailability. Conclusions—Moderate iron loading markedly accelerates thrombus formation after arterial injury, increases vascular oxidative stress, and impairs vasoreactivity. Iron-induced vascular dysfunction may contribute to the increased incidence of ischemic cardiovascular events that have been associated with chronic iron overload.

Prevention of Sudden Cardiac Death With Implantable Cardioverter-Defibrillators in Children and Adolescents With Hypertrophic Cardiomyopathy

OBJECTIVES The aim of this study was to determine the efficacy of implantable cardioverter-defibrillators (ICDs) in children and adolescents with hypertrophic cardiomyopathy (HCM). BACKGROUND HCM is the most common cause of sudden death in the young. The availability of ICDs over the past decade for HCM has demonstrated the potential for sudden death prevention, predominantly in adult patients. METHODS A multicenter international registry of ICDs implanted (1987 to 2011) in 224 unrelated children and adolescents with HCM judged at high risk for sudden death was assembled. Patients received ICDs for primary (n = 188) or secondary (n = 36) prevention after undergoing evaluation at 22 referral and nonreferral institutions in the United States, Canada, Europe, and Australia. RESULTS Defibrillators were activated appropriately to terminate ventricular tachycardia or ventricular fibrillation in 43 of 224 patients (19%) over a mean of 4.3 ± 3.3 years. ICD intervention rates were 4.5% per year overall, 14.0% per year for secondary prevention after cardiac arrest, and 3.1% per year for primary prevention on the basis of risk factors (5-year cumulative probability 17%). The mean time from implantation to first appropriate discharge was 2.9 ± 2.7 years (range to 8.6 years). The primary prevention discharge rate terminating ventricular tachycardia or ventricular fibrillation was the same in patients who underwent implantation for 1, 2, or ≥3 risk factors (12 of 88 [14%], 10 of 71 [14%], and 4 of 29 [14%], respectively, p = 1.00). Extreme left ventricular hypertrophy was the most common risk factor present (alone or in combination with other markers) in patients experiencing primary prevention interventions (17 of 26 [65%]). ICD-related complications, particularly inappropriate shocks and lead malfunction, occurred in 91 patients (41%) at 17 ± 5 years of age. CONCLUSIONS In a high-risk pediatric HCM cohort, ICD interventions terminating life-threatening ventricular tachyarrhythmias were frequent. Extreme left ventricular hypertrophy was most frequently associated with appropriate interventions. The rate of device complications adds a measure of complexity to ICD decisions in this age group.

Loss of H3K4 methylation destabilizes gene expression patterns and physiological functions in adult murine cardiomyocytes

Histone H3 lysine 4 (H3K4me) methyltransferases and their cofactors are essential for embryonic development and the establishment of gene expression patterns in a cell-specific and heritable manner. However, the importance of such epigenetic marks in maintaining gene expression in adults and in initiating human disease is unclear. Here, we addressed this question using a mouse model in which we could inducibly ablate PAX interacting (with transcription-activation domain) protein 1 (PTIP), a key component of the H3K4me complex, in cardiac cells. Reducing H3K4me3 marks in differentiated cardiomyocytes was sufficient to alter gene expression profiles. One gene regulated by H3K4me3 was Kv channel-interacting protein 2 (Kcnip2), which regulates a cardiac repolarization current that is downregulated in heart failure and functions in arrhythmogenesis. This regulation led to a decreased sodium current and action potential upstroke velocity and significantly prolonged action potential duration (APD). The prolonged APD augmented intracellular calcium and in vivo systolic heart function. Treatment with isoproterenol and caffeine in this mouse model resulted in the generation of premature ventricular beats, a harbinger of lethal ventricular arrhythmias. These results suggest that the maintenance of H3K4me3 marks is necessary for the stability of a transcriptional program in differentiated cells and point to an essential function for H3K4me3 epigenetic marks in cellular homeostasis.

Rad GTPase Deficiency Leads to Cardiac Hypertrophy

Background— Rad (Ras associated with diabetes) GTPase is the prototypic member of a subfamily of Ras-related small G proteins. The aim of the present study was to define whether Rad plays an important role in mediating cardiac hypertrophy. Methods and Results— We document for the first time that levels of Rad mRNA and protein were decreased significantly in human failing hearts (n=10) compared with normal hearts (n=3; P<0.01). Similarly, Rad expression was decreased significantly in cardiac hypertrophy induced by pressure overload and in cultured cardiomyocytes with hypertrophy induced by 10 &mgr;mol/L phenylephrine. Gain and loss of Rad function in cardiomyocytes significantly inhibited and increased phenylephrine-induced hypertrophy, respectively. In addition, activation of calcium-calmodulin–dependent kinase II (CaMKII), a strong inducer of cardiac hypertrophy, was significantly inhibited by Rad overexpression. Conversely, downregulation of CaMKII&dgr; by RNA interference technology attenuated the phenylephrine-induced hypertrophic response in cardiomyocytes in which Rad was also knocked down. To further elucidate the potential role of Rad in vivo, we generated Rad-deficient mice and demonstrated that they were more susceptible to cardiac hypertrophy associated with increased CaMKII phosphorylation than wild-type littermate controls. Conclusions— The present data document for the first time that Rad is a novel mediator that inhibits cardiac hypertrophy through the CaMKII pathway. The present study will have significant implications for understanding the mechanisms of cardiac hypertrophy and setting the basis for the development of new strategies for treatment of cardiac hypertrophy.

Psychological Issues in Genetic Testing for Inherited Cardiovascular Diseases

Clinical genetic testing is now available for many cardiovascular diseases, including hypertrophic cardiomyopathy (HCM), familial dilated cardiomyopathy, and arrhythmogenic right ventricular cardiomyopathy, channelopathies such as long-QT syndrome (LQTS), aortic diseases such as Marfan syndrome, and disorders of cholesterol metabolism such as familial hypercholesterolemia (FH). Utility and interpretation of these tests can be complicated by incomplete penetrance (not everyone with a gene mutation will have the disease), variable expressivity (varying symptoms and presentation in those who have the mutation), reduced detection rates (gene mutations cannot be identified in all individuals with the condition), extensive allelic and locus heterogeneity (multiple genes with multiple mutations), and limited genotype-phenotype correlations. These factors can introduce confusion and uncertainty when test results are conveyed to the individual being tested. With the rapid expansion in knowledge of the genetic basis for inherited cardiovascular diseases, and the evolution and increasing availability of clinical genetic testing, there is a compelling need to understand and address the psychological implications of genetic testing in cardiovascular diseases. This article has been organized to reflect the issues of clinically affected individuals and unaffected at-risk adults and children. Most of the relevant literature on this topic is in HCM and LQTS, so discussion of these diseases will be disproportionate to others. However, these genetic prototypes have many similarities with other inherited cardiovascular diseases, and concepts discussed are likely to be generally applicable. When relevant, unique issues for an individual disease will be highlighted. Genetic counseling is the process of integrating family and medical histories, providing education, and promoting informed choices and adaptation to having, or being at risk of developing, a genetic condition.1 Counselors also play a fundamental role in interpretation of the results and conveying the implications of positive, negative, and uncertain molecular test results to the index patient and his/her family …

Redox-Sensitive Sulfenic Acid Modification Regulates Surface Expression of the Cardiovascular Voltage-Gated Potassium Channel Kv1.5

Rationale: Kv1.5 (KCNA5) is expressed in the heart, where it underlies the IKur current that controls atrial repolarization, and in the pulmonary vasculature, where it regulates vessel contractility in response to changes in oxygen tension. Atrial fibrillation and hypoxic pulmonary hypertension are characterized by downregulation of Kv1.5 protein expression, as well as with oxidative stress. Formation of sulfenic acid on cysteine residues of proteins is an important, dynamic mechanism for protein regulation under oxidative stress. Kv1.5 is widely reported to be redox-sensitive, and the channel possesses 6 potentially redox-sensitive intracellular cysteines. We therefore hypothesized that sulfenic acid modification of the channel itself may regulate Kv1.5 in response to oxidative stress. Objective: To investigate how oxidative stress, via redox-sensitive modification of the channel with sulfenic acid, regulates trafficking and expression of Kv1.5. Methods and Results: Labeling studies with the sulfenic acid–specific probe DAz and horseradish peroxidase–streptavidin Western blotting demonstrated a global increase in sulfenic acid–modified proteins in human patients with atrial fibrillation, as well as sulfenic acid modification to Kv1.5 in the heart. Further studies showed that Kv1.5 is modified with sulfenic acid on a single COOH-terminal cysteine (C581), and the level of sulfenic acid increases in response to oxidant exposure. Using live-cell immunofluorescence and whole-cell voltage-clamping, we found that modification of this cysteine is necessary and sufficient to reduce channel surface expression, promote its internalization, and block channel recycling back to the cell surface. Moreover, Western blotting demonstrated that sulfenic acid modification is a trigger for channel degradation under prolonged oxidative stress. Conclusions: Sulfenic acid modification to proteins, which is elevated in diseased human heart, regulates Kv1.5 channel surface expression and stability under oxidative stress and diverts channel from a recycling pathway to degradation. This provides a molecular mechanism linking oxidative stress and downregulation of channel expression observed in cardiovascular diseases.

Use and interpretation of genetic tests in cardiovascular genetics

Our understanding of the genetic basis of many Mendelian forms of cardiovascular disease has advanced significantly in the last 5–10 years. There are now many professional society guidelines that recommend genetic testing for a variety of hereditary cardiovascular diseases including long QT syndrome, hypertrophic cardiomyopathy, and arrhythmogenic right ventricular cardiomyopathy (ARVC).1–3 The number of genes associated with cardiac conditions continues to increase, and the number of clinically available genetic tests for cardiac conditions has expanded rapidly in recent years (table 1). View this table: Table 1 Genetic tests for hereditary cardiac conditions. Genetic tests for hereditary cardiac conditions typically involve sequencing some or all of the various genes associated with a given condition. The number of genes included and the sequencing methodology used may vary by laboratory. Some laboratories also offer analyses to look for duplications or deletions in the associated genes Clinical genetic testing can be highly valuable in the management of families with hereditary disease. Determining which family members inherited the genetic predisposition to cardiac disease allows us to separate those in need of lifelong clinical evaluations from those who need no further evaluations beyond those recommended for the general population. This strategy is particularly valuable in inherited cardiovascular diseases where definitive clinical diagnosis of at-risk relatives is limited by incomplete penetrance, variable age of onset and, in some cases, insensitivity of clinical testing.4–7 Recent guidelines and expert opinions have gone beyond simply recommending genetic testing; they emphasise important points for the judicious use of genetic testing such as performing genetic testing on the most clearly affected person in the family, careful genetic counselling regarding the implications of positive, negative or uncertain results, and consideration of referral to a specialised centre due to the complexity of such genetic evaluations.1 8 9 To further elucidate principles and approaches critical to the …

The ubiquitin proteasome system in human cardiomyopathies and heart failure

Maintenance of protein quality control is a critical function of the ubiquitin proteasome system (UPS). Evidence is rapidly mounting to link proteasome dysfunction with a multitude of cardiac diseases, including ischemia, reperfusion, atherosclerosis, hypertrophy, heart failure, and cardiomyopathies. Recent studies have demonstrated a remarkable level of complexity in the regulation of the UPS in the heart and suggest that our understanding of how UPS dysfunction might contribute to the pathophysiology of such a wide range of cardiac afflictions is still very limited. Whereas experimental systems, including animal models, are invaluable for exploring mechanisms and establishing pathogenicity of UPS dysfunction in cardiac disease, studies using human heart tissue provide a vital adjunct for establishing clinical relevance of experimental findings and promoting new hypotheses. Accordingly, this review will focus on UPS dysfunction in human dilated and hypertrophic cardiomyopathies and highlight areas rich for further study in this expanding field.

Tuning cardiac performance in ischemic heart disease and failure by modulating myofilament function.

The cardiac myofilaments are composed of highly ordered arrays of proteins that coordinate cardiac contraction and relaxation in response to the rhythmic waves of [Ca2+] during the cardiac cycle. Several cardiac disease states are associated with altered myofilament protein interactions that contribute to cardiac dysfunction. During acute myocardial ischemia, the sensitivity of the myofilaments to activating Ca2+ is drastically reduced, largely due to the effects of intracellular acidosis on the contractile machinery. Myofilament Ca2+ sensitivity remains compromised in post-ischemic or “stunned” myocardium even after complete restoration of blood flow and intracellular pH, likely because of covalent modifications of or proteolytic injury to contractile proteins. In contrast, myofilament Ca2+ sensitivity can be increased in chronic heart failure, owing in part to decreased phosphorylation of troponin I, the inhibitory subunit of the troponin regulatory complex. We highlight, in this paper, the central role of the myofilaments in the pathophysiology of each of these distinct disease entities, with a particular focus on the molecular switch protein troponin I. We also discuss the beneficial effects of a genetically engineered cardiac troponin I, with a histidine button substitution at C-terminal residue 164, for a variety of pathophysiologic conditions, including hypoxia, ischemia, ischemia–reperfusion and chronic heart failure.