Mutations in the gamma(2) subunit of AMP-activated protein kinase cause familial hypertrophic cardiomyopathy: evidence for the central role of energy compromise in disease pathogenesis

Familial hypertrophic cardiomyopathy (HCM) has been widely studied as a genetic model of cardiac hypertrophy and sudden cardiac death. HCM has been defined as a disease of the cardiac sarcomere, but mutations in the known contractile protein disease genes are not found in up to one-third of cases. Further, no consistent changes in contractile properties are shared by these mutant proteins, implying that an abnormality of force generation may not be the underlying mechanism of disease. Instead, all of the sarcomeric mutations appear to result in inefficient use of ATP, suggesting that an inability to maintain normal ATP levels may be the central abnormality. To test this hypothesis we have examined candidate genes involved in energy homeostasis in the heart. We now describe mutations in PRKAG2, encoding the gamma(2) subunit of AMP-activated protein kinase (AMPK), in two families with severe HCM and aberrant conduction from atria to ventricles in some affected individuals (pre-excitation or Wolff-Parkinson-White syndrome). The mutations, one missense and one in-frame single codon insertion, occur in highly conserved regions. Because AMPK provides a central sensing mechanism that protects cells from exhaustion of ATP supplies, we propose that these data substantiate energy compromise as a unifying pathogenic mechanism in all forms of HCM. This conclusion should radically redirect thinking about this disorder and also, by establishing energy depletion as a cause of myocardial dysfunction, should be relevant to the acquired forms of heart muscle disease that HCM models.     

Is 8860 variation a rare polymorphism or associated as a secondary effect in HCM disease?

Introduction

mtDNA defects, both deletions and point mutations, have been associated with hypertrophic cardiomyopathies. The aim of this study was to establish a spectrum for mtDNA mutations in Iranian hypertrophic cardiomyopathy (HCM) patients.

Material and methods

The control group was chosen among the special medical centre visitors who did not have hypertrophic cardiomyopathy or any related heart disease. Hypertrophic cardiomyopathy (HCM) is widely accepted as a pluricausal or multifactorial disease. Because of the linkage between energy metabolism in the mitochondria and cardiac muscle contraction, it is reasonable to assume that mitochondrial abnormalities may be responsible for some forms of HCM. Point mutations and deletions in the two hot spot regions of mtDNA were investigated by PCR and sequencing methods.

Results

Some unreported point mutations have been found in this study but no deletion was detected. Meanwhile some of these point mutations have been investigated among HCM patients for the first time.

Conclusions

A8860G transition was detected in a high proportion, raising the question whether this rare polymorphism is associated as a secondary effect in HCM disease.     

Mutation screening of the PTPN11 gene in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common inherited cardiac disease and a major cause of sudden death. It is an autosomal dominant disorder predominantly caused by mutations in genes encoding for sarcomeric proteins. Only 50-60% of HCM probands have mutations in known genes suggesting the presence of additional disease genes. Noonan and LEOPARD syndromes are characterised by multiple dysmorphia and cardiac defects with HCM present in approximately 20% of cases. Both syndromes are caused by mutations in the PTPN11 gene which codes for the protein tyrosine phosphatase SHP-2. It is suspected but unproven that the cardiac phenotype may predominate or even be present in isolation. In order to determine possible involvement of this gene in the pathogenesis of HCM, we performed mutation screening of the PTPN11 coding region in 250 selected HCM probands (200 patients without mutations in sarcomeric genes and 50 with identified mutations). No mutations in PTPN11 were identified. Our data suggests that mutations in the PTPN11 gene are not a cause of HCM in the absence of Noonan/LEOPARD syndromes.     

Danon disease: focusing on heart

Danon disease is a rare X-linked dominant lysosomal disease due to the primary deficiency of lysosome-associated membrane protein 2 (LAMP2) gene. Cardiomyopathy, skeletal myopathy and mental retardation are the typical triad of Danon disease. More than 60 LAMP2 mutations have been reported. The molecular mechanism is defects in LAMP2 protein (due to LAMP2 mutation) which causes insidious glycogen accumulation in cardiac muscle cells and resulting in cardiac hypertrophy and electrophysiological abnormalities. However, there are significant differences between the male and female Danon disease patients with regard to clinical features and cardiac manifestations. The clinical symptoms are variable, from asymptomatic to sudden cardiac death. Wolff-Parkinson-White syndrome is more common in male than female patients. Hypertrophic cardiomyopathy is predominant in male patients, whereas the similar prevalence of hypertrophic and dilated cardiomyopathy in female patients. Male patients are diagnosed usually at teenage, whereas the diagnosis and events occurred approximately 15 years later in female than male patients. Heart transplantation is the reliable treatment once the occurrence of heart failure and should be considered as early as possible due to its rapid progression.     

中国人群肥厚型心肌病MYH7基因突变的研究与展望

肥厚型心肌病( hypertrophic cardiomyopathy, HCM)是一种以左心室或室间隔不对称性肥厚为基本特征的最常见遗传性心脏病,该病是青少年及年轻运动员猝死的最主要原因。先天基因缺陷是HCM主要的分子遗传学基础,其遗传方式遵循常染色体显性遗传模式。与HCM发病相关的基因较多,其中编码心脏β-肌球蛋白重链(cardiac beta-myosin heavy chain, MYH7)基因为HCM最主要的致病基因。通过查阅文献,检索到目前已报道的587例中国HCM患者被进行MYH7基因突变筛查,统计结果表明：共有150例中国HCM患者由MYH7基因外显子上的66个致病性突变所致。对已报道的中国HCM人群MYH7基因的突变特点进行总结和分析,这将为中国HCM人群的诊断、预防和治疗提供参考依据。     

Molecular mechanisms of inherited cardiomyopathies

Cardiomyopathies are diseases of heart muscle that may result from a diverse array of conditions that damage the heart and other organs and impair myocardial function, including infection, ischemia, and toxins. However, they may also occur as primary diseases restricted to striated muscle. Over the past decade, the importance of inherited gene defects in the pathogenesis of primary cardiomyopathies has been recognized, with mutations in some 18 genes having been identified as causing hypertrophic cardiomyopathy (HCM) and/or dilated cardiomyopathy (DCM). Defining the role of these genes in cardiac function and the mechanisms by which mutations in these genes lead to hypertrophy, dilation, and contractile failure are major goals of ongoing research. Pathophysiological mechanisms that have been implicated in HCM and DCM include the following: defective force generation, due to mutations in sarcomeric protein genes; defective force transmission, due to mutations in cytoskeletal protein genes; myocardial energy deficits, due to mutations in ATP regulatory protein genes; and abnormal Ca2+ homeostasis, due to altered availability of Ca2+ and altered myofibrillar Ca2+ sensitivity. Improved understanding that will result from these studies should ultimately lead to new approaches for the diagnosis, prognostic stratification, and treatment of patients with heart failure.     

Review: Metabolic cardiomyopathy and conduction system defects in children

Metabolic cardiomyopathies include amino acid, lipid and mitochondrial disorders, as well as storage diseases. A number of metabolic disorders are associated with both myopathy and cardiomyopathy. These include the glycogen storage diseases, ie, acid maltase deficiency (infantile, childhood, and adult onset), McArdle disease, and debrancher and brancher deficiencies. Disorders of lipid metabolism include systemic carnitine deficiency and abnormalities of carnitine palmitoyltransferase (CPT), long-chain acyl-CoA dehydrogenase, and multiple acyl-CoA dehydrogenase. Disorders of mitochondrial metabolism affect complex I, II, III, IV and V, in addition to multiple respiratory chain defects. These may cause either hypertrophic or dilated cardiomyopathy. In addition, cardiomyopathy is frequently a component part of the storage disorders, including mucopolysaccharidosis, mucolipidosis, Fabry disease, gangliosidosis, and neuronal ceroid lipofuscinosis. Primary hypertrophic cardiomyopathy is caused by mutations in one of the genes that encode proteins of the cardiac sarcomere. Mutations in different genes are attended by different prognoses and different risks of sudden death. Mutations of the genes for myosin binding protein C (MBPC) and tropomyosin have low penetrance and cause mild forms of primary hypertrophic cardiomyopathy, while mutations of the troponin T and B-myosin genes carry a worse prognosis. Conduction disorders result in cardiac arrhythmias that may be fatal. Histiocytoid cardiomyopathy is usually an autosomal recessive disorder that results in the presence of abnormal Purkinje cells that interfere with normal cardiac conduction. Other conduction defects include arrhythmogenic right ventricular dysplasia (ARVD), congenital heart block, noncompaction of the left ventricle, and long Q-T syndrome (LQTS). The genetic loci for LQTS reside usually in the potassium channel, and, less frequently, in the sodium channel (channelopathies). Although the histological appearance of some of these disorders may be diagnostic, molecular analysis is necessary to define clearly the particular type of cardiomyopathy.     

基于靶基因文库的高通量测序平台建立肥厚型心肌病患者致病基因突变检测方法

目的 基于靶基因文库,应用下一代半导体高通量测序平台,建立快速、 准确的肥厚型心肌病(hypertrophic cardiomyopathy,HCM)常见致病基因突变检测方法 ,有利于HCM患者的早期预防及临床分子诊断.方法 选择国内外公认的与HCM致病相关的常见基因(MYH7、MYBPC3、TNNT2、TNNI3、A...     

Inherited cardiomyopathies mimicking arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) represents an inherited cardiomyopathy that manifests clinically with malignant ventricular arrhythmias, sudden cardiac death, and less commonly heart failure. The condition is characterized by replacement of the myocardium, primarily of the right ventricle, with fibrofatty tissue. Extensive fibrofatty replacement of the myocardium has been previously thought to be pathognomonic of ARVC; however, this report details two other forms of inherited cardiomyopathy, namely hypertrophic cardiomyopathy (HCM) and the PRKAG2 cardiac syndrome, that were found to have significant fibrofatty myocardial replacement at pathologic examination. This report represents the first documentation of inherited cardiomyopathies mimicking ARVC and highlights the concept that other cardiac conditions can be associated with fibrofatty replacement of the myocardium.     

A DNA resequencing array for pathogenic mutation detection in hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a heterogeneous autosomal dominant cardiac disorder with a prevalence of 1 in 500. Over 450 different pathogenic mutations in at least 16 genes have been identified so far. The large allelic and genetic heterogeneity of HCM requires high-throughput, rapid, and affordable mutation detection technologies to efficiently integrate molecular screening into clinical practice. We developed a custom DNA resequencing array that contains both strands of all coding exons (160), splice-site junctions, and 5'UTR regions of 12 genes that have been clearly implicated in HCM (MYH7, MYBPC3, TNNT2, TPM1, TNNI3, MYL3, MYL2, CSRP3, PLN, ACTC, TNNC1, and PRKAG2). We analyzed a first series of 38 unrelated patients with HCM (17 familial, 21 sporadic). A total of 953,306 bp across the 38 patients were sequenced with a mean nucleotide call rate of 96.92% (range: 93-99.9%). Pathogenic mutations (single nucleotide substitutions) in MYH7, MYBPC3, TNNI3, and MYL3 (six known and six novel) were identified in 60% (10/17) of familial HCM and 10% of sporadic cases (2/21). The high-throughput HCM resequencing array is the most rapid and cost-effective tool for molecular testing of HCM to date; it thus has considerable potential in diagnostic and predictive testing, and prognostic stratification.     

Compound heterozygous or homozygous truncating MYBPC3 mutations cause lethal cardiomyopathy with features of noncompaction and septal defects

Familial hypertrophic cardiomyopathy (HCM) is usually caused by autosomal dominant pathogenic mutations in genes encoding sarcomeric or sarcomere-associated cardiac muscle proteins. The disease mainly affects adults, although young children with severe HCM have also been reported. We describe four unrelated neonates with lethal cardiomyopathy, and performed molecular studies to identify the genetic defect. We also present a literature overview of reported patients with compound heterozygous or homozygous pathogenic MYBPC3 mutations and describe their clinical characteristics. All four children presented with feeding difficulties, failure to thrive, and dyspnea. They died from cardiac failure before age 13 weeks. Features of left ventricular noncompaction were diagnosed in three patients. In the fourth, hypertrabeculation was not a clear feature, but could not be excluded. All of them had septal defects. Two patients were compound heterozygotes for the pathogenic c.2373dup p.(Trp792fs) and c.2827C>T p.(Arg943*) mutations, and two were homozygous for the c.2373dup and c.2827C>T mutations. All patients with biallelic truncating pathogenic mutations in MYBPC3 reported so far (n=21) were diagnosed with severe cardiomyopathy and/or died within the first few months of life. In 62% (13/21), septal defects or a patent ductus arteriosus accompanied cardiomyopathy. In contrast to heterozygous pathogenic mutations, homozygous or compound heterozygous truncating pathogenic MYBPC3 mutations cause severe neonatal cardiomyopathy with features of left ventricular noncompaction and septal defects in approximately 60% of patients.     

Familial hypertrophic cardiomyopathy: genes, mutations and animal models. A review

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease, which may afflict as many as 1 in 500 subjects (0.2%), being probably the most common hereditary cardiovascular disease and the most common cause of sudden cardiac death (SCD). Hypertrophic cardiomyopathy is characterized by the presence of unexplained left ventricular hypertrophy (in absence of hypertension, valvular disease, etc), which is usually asymmetric and involves the ventricular septum. Molecular genetic studies have identified eleven genes that code proteins of the sarcomere that are associated with the HCM; the beta-myosin heavy chain gene (MYH7), alpha-myosin heavy chain (MYH6), cardiac troponin T (TNNT2); cardiac troponin C (TNNC1), alpha-tropomyosin (TPM1), myosin binding protein-C (MYBPC3), cardiac troponin (TNNI3), essential and regulatory light chain genes (MYL3 and MYL2, respectively), cardiac alpha-actin gene (ACTC) and titin (TTN). The objective of this paper is the revision of the current state of the knowledge on (1) the organization and mutations of the HCM causing genes and their proteins and (2) the animal models developed for the study of the genes, mutations and proteins in the hypertrophic cardiomyopathy.     

Beyond the sarcomere: CSRP3 mutations cause hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a frequent genetic cardiac disease and the most common cause of sudden cardiac death in young individuals. Most of the currently known HCM disease genes encode sarcomeric proteins. Previous studies have shown an association between CSRP3 missense mutations and either dilated cardiomyopathy (DCM) or HCM, but all these studies were unable to provide comprehensive genetic evidence for a causative role of CSRP3 mutations. We used linkage analysis and identified a CSRP3 missense mutation in a large German family affected by HCM. We confirmed CSRP3 as an HCM disease gene. Furthermore, CSRP3 missense mutations segregating with HCM were identified in four other families. We used a newly designed monoclonal antibody to show that muscle LIM protein (MLP), the protein encoded by CSRP3, is mainly a cytosolic component of cardiomyocytes and not tightly anchored to sarcomeric structures. Our functional data from both in vitro and in vivo analyses suggest that at least one of MLP's mutated forms seems to be destabilized in the heart of HCM patients harbouring a CSRP3 missense mutation. We also present evidence for mild skeletal muscle disease in affected persons. Our results support the view that HCM is not exclusively a sarcomeric disease and also suggest that impaired mechano-sensory stress signalling might be involved in the pathogenesis of HCM.     

A new phenotype of severe dilated cardiomyopathy associated with a mutation in the LAMP2 gene previously known to cause hypertrophic cardiomyopathy in the context of Danon disease

Danon disease is a rare X-linked cardiac and skeletal muscle disorder with multisystem clinical manifestations. Genetic defects at the lysosome-associated membrane 2 protein (LAMP2) are the cause of the disorder. Due to the rarity of the disease, there is limited progress in understanding the correlation between genotype and phenotype, and explaining the large variability of the clinical features of the disease. In this study, we report two patients, twin sisters, referred to our hospital for end stage heart failure due to dilated cardiomyopathy, requiring heart transplant evaluation. Genetic analysis, using targeted next generation sequencing, showed that the proband carried a LAMP2 missense variant, c.928G?>?A. The mutation was also detected in her twin sister by sanger sequencing. This variant has already been reported by other investigators and was correlated with the clinical triad of Danon disease i.e. hypertrophic cardiomyopathy, mental retardation and peripheral myopathy. The new phenotype of dilated cardiomyopathy associated with this mutation, confirms the phenotypic heterogeneity of the particular mutation, as well as of Danon disease.     

Axial distribution of myosin binding protein-C is unaffected by mutations in human cardiac and skeletal muscle

Myosin binding protein-C (MyBP-C), a major thick filament associated sarcomeric protein, plays an important functional and structural role in regulating sarcomere assembly and crossbridge formation. Missing or aberrant MyBP-C proteins (both cardiac and skeletal) have been shown to cause both cardiac and skeletal myopathies, thereby emphasising its importance for the normal functioning of the sarcomere. Mutations in cardiac MyBP-C are a major cause of hypertrophic cardiomyopathy (HCM), while mutations in skeletal MyBP-C have been implicated in a disease of skeletal muscle-distal arthrogryposis type 1 (DA-1). Here we report the first detailed electron microscopy studies on human cardiac and skeletal tissues carrying MyBP-C gene mutations, using samples obtained from HCM and DA-1 patients. We have used established image averaging methods to identify and study the axial distribution of MyBP-C on the thick filament by averaging profile plots of the A-band of the sarcomere from electron micrographs of human cardiac and skeletal myopathy specimens. Due to the difficulty of obtaining normal human tissue, we compared the distribution to the A-band structure in normal frog skeletal, rat cardiac muscle and in cardiac muscle of MyBP-C-deficient mice. Very similar overall profile averages were obtained from the C-zones in cardiac HCM samples and skeletal DA-1 samples with MyBP-C gene mutations, suggesting that mutations in MyBP-C do not alter its mean axial distribution along the thick filament.     

Cardiac and skeletal myopathies: can genotype explain phenotype?

The inherited muscle diseases, skeletal muscle nemaline myopathy and cardiac muscle hypertrophic myopathy (HCM) have been recognised for decades. Recently it has become apparent that mutations in almost any protein component of the sarcomere could cause myopathy. Thus changes in many sarcomeric protein genes can produce a common phenotype. Several recent publications indicate the opposite property: mutations in one sarcomeric protein can produce different muscle disease phenotypes. The most dramatic example of this property is actin, mutations in which are associated with hypertrophic cardiomyopathy, dilated cardiomyopathy, nemaline myopathy and actin myopathy.     

Myocardial contractile and metabolic properties of familial hypertrophic cardiomyopathy caused by cardiac troponin I gene mutations: a simulation study

Familial hypertrophic cardiomyopathy (FHC) is an inherited disease that is caused by sarcomeric protein gene mutations. The mechanism by which these mutant proteins cause disease is uncertain. Experimentally, cardiac troponin I (CTnI) gene mutations mainly alter myocardial performance via increases in the Ca(2+) sensitivity of cardiac contractility. In this study, we used an integrated simulation that links electrophysiology, contractile activity and energy metabolism of the myocardium to investigate alterations in myocardial contractile function and energy metabolism regulation as a result of increased Ca(2+) sensitivity in CTnI mutations. Simulation results reproduced the following typical features of FHC: (1) slower relaxation (diastolic dysfunction) caused by prolonged [Ca(2+)](i) and force transients; (2) higher energy consumption with the increase in Ca(2+) sensitivity; and (3) reduced fatty acid oxidation and enhanced glucose utilization in hypertrophied heart metabolism. Furthermore, the simulation indicated that in conditions of high energy consumption (that is, more than an 18.3% increase in total energy consumption), the myocardial energetic metabolic network switched from a net consumer to a net producer of lactate, resulting in a low coupling of glucose oxidation to glycolysis, which is a common feature of hypertrophied hearts. This study provides a novel systematic myocardial contractile and metabolic analysis to help elucidate the pathogenesis of FHC and suggests that the alterations in resting heart energy supply and demand could contribute to disease progression.     

Friedreich's ataxia reveals a mechanism for coordinate regulation of oxidative metabolism via feedback inhibition of the SIRT3 deacetylase

Friedreich's ataxia (FRDA) is the most common inherited human ataxia and is caused by a deficiency in the mitochondrial protein frataxin. Clinically, patients suffer from progressive spinocerebellar degeneration, diabetes and a fatal cardiomyopathy, associated with mitochondrial respiratory chain defects. Recent findings have shown that lysine acetylation regulates mitochondrial function and intermediary metabolism. However, little is known about lysine acetylation in the setting of pathologic energy stress and mitochondrial dysfunction. We tested the hypothesis that the respiratory chain defects in frataxin deficiency alter mitochondrial protein acetylation. Using two conditional mouse models of FRDA, we demonstrate marked hyperacetylation of numerous cardiac mitochondrial proteins. Importantly, this biochemical phenotype develops concurrently with cardiac hypertrophy and is caused by inhibition of the NAD(+)-dependent SIRT3 deacetylase. This inhibition is caused by an 85-fold decrease in mitochondrial NAD(+)/NADH and direct carbonyl group modification of SIRT3, and is reversed with excess SIRT3 and NAD(+) in vitro. We further demonstrate that protein hyperacetylation may be a common feature of mitochondrial disorders caused by respiratory chain defects, notably, cytochrome oxidase I (COI) deficiency. These findings suggest that SIRT3 inhibition and consequent protein hyperacetylation represents a negative feedback mechanism limiting mitochondrial oxidative pathways when respiratory metabolism is compromised, and thus, may contribute to the lethal cardiomyopathy in FRDA.