Mutations in the human muscle LIM protein gene in families with hypertrophic cardiomyopathy

Muscle LIM protein interacts with cofilin 2 and regulates F-actin dynamics in cardiac and skeletal muscle

The muscle LIM protein (MLP) and cofilin 2 (CFL2) are important regulators of striated myocyte function. Mutations in the corresponding genes have been directly associated with severe human cardiac and skeletal myopathies, and aberrant expression patterns have often been observed in affected muscles. Herein, we have investigated whether MLP and CFL2 are involved in common molecular mechanisms, which would promote our understanding of disease pathogenesis. We have shown for the first time, using a range of biochemical and immunohistochemical methods, that MLP binds directly to CFL2 in human cardiac and skeletal muscles. The interaction involves the inter-LIM domain, amino acids 94 to 105, of MLP and the amino-terminal domain, amino acids 1 to 105, of CFL2, which includes part of the actin depolymerization domain. The MLP/CFL2 complex is stronger in moderately acidic (pH 6.8) environments and upon CFL2 phosphorylation, while it is independent of Ca(2+) levels. This interaction has direct implications in actin cytoskeleton dynamics in regulating CFL2-dependent F-actin depolymerization, with maximal depolymerization enhancement at an MLP/CFL2 molecular ratio of 2:1. Deregulation of this interaction by intracellular pH variations, CFL2 phosphorylation, MLP or CFL2 gene mutations, or expression changes, as observed in a range of cardiac and skeletal myopathies, could impair F-actin depolymerization, leading to sarcomere dysfunction and disease.

Diagnostic, prognostic, and therapeutic implications of genetic testing for hypertrophic cardiomyopathy

Over the last 2 decades, the pathogenic basis for the most common heritable cardiovascular disease, hypertrophic cardiomyopathy (HCM), has been investigated extensively. Affecting approximately 1 in 500 individuals, HCM is the most common cause of sudden death in young athletes. In recent years, genomic medicine has been moving from the bench to the bedside throughout all medical disciplines including cardiology. Now, genomic medicine has entered clinical practice as it pertains to the evaluation and management of patients with HCM. The continuous research and discoveries of new HCM susceptibility genes, the growing amount of data from genotype-phenotype correlation studies, and the introduction of commercially available genetic tests for HCM make it essential that the modern-day cardiologist understand the diagnostic, prognostic, and therapeutic implications of HCM genetic testing.

Expression patterns of cardiac myofilament proteins: genomic and protein analysis of surgical myectomy tissue from patients with obstructive hypertrophic cardiomyopathy

BACKGROUND

Mutations in myofilament proteins, most commonly MYBPC3-encoded myosin-binding protein C and MYH7-encoded beta-myosin heavy chain, can cause hypertrophic cardiomyopathy (HCM). Despite significant advances in structure-function relationships pertaining to the cardiac sarcomere, there is limited knowledge of how a mutation leads to clinical HCM. We, therefore, set out to study expression and localization of myofilament proteins in left ventricular tissue of patients with HCM.

METHODS AND RESULTS

Frozen surgical myectomy specimens from 47 patients with HCM were examined and genotyped for mutations involving 8 myofilament-encoding genes. Myofilament protein levels were quantified by Western blotting with localization graded from immunohistochemical staining of tissue sections. Overall, 25 of 47 (53%) patients had myofilament-HCM, including 12 with MYBPC3-HCM and 9 with MYH7-HCM. As compared with healthy heart tissue, levels of myofilament proteins were increased in patients manifesting a mutation in either gene. Patients with a frameshift mutation predicted to truncate MYBPC3 exhibited marked disturbances in protein localization as compared with missense mutations in either MYBPC3 or MYH7.

CONCLUSIONS

In this first expression study in human HCM tissue, increased myofilament protein levels in patients with either MYBPC3- or MYH7-mediated HCM suggest a poison peptide mechanism. Specifically, the mechanism of dysfunction may vary according to the genetic subgroup suggested by a distinctly abnormal distribution of myofilament proteins in patients manifesting a truncation mutation in MYBPC3.

Induction of CRP3/MLP expression during vein arterialization is dependent on stretch rather than shear stress

AIMS

Cysteine- and glycine-rich protein 3/muscle LIM-domain protein (CRP3/MLP) mediates protein-protein interaction with actin filaments in the heart and is involved in muscle differentiation and vascular remodelling. Here, we assessed the induction of CRP3/MLP expression during arterialization in human and rat veins.

METHODS AND RESULTS

Vascular CRP3/MLP expression was mainly observed in arterial samples from both human and rat. Using quantitative real time RT-PCR, we demonstrated that the CRP3/MLP expression was 10 times higher in smooth muscle cells (SMCs) from human mammary artery (h-MA) vs. saphenous vein (h-SV). In endothelial cells (ECs), CRP3/MLP was scarcely detected in either h-MA or h-SV. Using an ex vivo flow through system that mimics arterial condition, we observed induction of CRP3/MLP expression in arterialized h-SV. Interestingly, the upregulation of CRP3/MLP was primarily dependent on stretch stimulus in SMCs, rather than shear stress in ECs. Finally, using a rat vein in vivo arterialization model, early (1-14 days) CRP3/MLP immunostaining was observed predominantly in the inner layer and later (28-90 days) it appeared more scattered in the vessel layers.

CONCLUSION

Here we provide evidence that CRP3/MLP is primarily expressed in arterial SMCs and that stretch is the main stimulus for CRP3/MLP induction in veins exposed to arterial haemodynamic conditions.

Diagnostic yield, interpretation, and clinical utility of mutation screening of sarcomere encoding genes in Danish hypertrophic cardiomyopathy patients and relatives

The American Heart Association (AHA) recommends family screening for hypertrophic cardiomyopathy (HCM). We assessed the outcome of family screening combining clinical evaluation and screening for sarcomere gene mutations in a cohort of 90 Danish HCM patients and their close relatives, in all 451 persons. Index patients were screened for mutations in all coding regions of 10 sarcomere genes (MYH7, MYL3, MYBPC3, TNNI3, TNNT2, TPM1, ACTC, CSRP3, TCAP, and TNNC1) and five exons of TTN. Relatives were screened for presence of minor or major diagnostic criteria for HCM and tracking of DNA variants was performed. In total, 297 adult relatives (>18 years) (51.2%) fulfilled one or more criteria for HCM. A total of 38 HCM-causing mutations were detected in 32 index patients. Six patients carried two disease-associated mutations. Twenty-two mutations have only been identified in the present cohort. The genetic diagnostic yield was almost twice as high in familial HCM (53%) vs. HCM of sporadic or unclear inheritance (19%). The yield was highest in families with an additional history of HCM-related clinical events. In relatives, 29.9% of mutation carriers did not fulfil any clinical diagnostic criterion, and in 37.5% of relatives without a mutation, one or more criteria was fulfilled. A total of 60% of family members had no mutation and could be reassured and further follow-up ceased. Genetic diagnosis may be established in approximately 40% of families with the highest yield in familial HCM with clinical events. Mutation-screening was superior to clinical investigation in identification of individuals not at increased risk, where follow-up is redundant, but should be offered in all families with relatives at risk for developing HCM.

Molecular genetics of sudden cardiac death

Sudden cardiac death (SCD) is one of the most common causes of death. An important number of sudden deaths, especially in the young, are due to genetic heart disorders, both with structural and arrhythmogenic abnormalities. In recent years, significant advances have been made in understanding the genetic basis of SCD. Identification of the genetic causes of sudden death is important because close relatives are also at potential risk of having a fatal cardiac condition. A comprehensive post-mortem investigation is vital to determine the cause and manner of death and provides the opportunity to assess the potential risk to the family after appropriate genetic counselling. In this paper, we present an update of the different genetic causes of sudden death, emphasizing their importance for the forensic pathologist due to his relevant role in the diagnosis and prevention of SCD.

An FHL1-containing complex within the cardiomyocyte sarcomere mediates hypertrophic biomechanical stress responses in mice

The response of cardiomyocytes to biomechanical stress can determine the pathophysiology of hypertrophic cardiac disease, and targeting the pathways regulating these responses is a therapeutic goal. However, little is known about how biomechanical stress is sensed by the cardiomyocyte sarcomere to transduce intracellular hypertrophic signals or how the dysfunction of these pathways may lead to disease. Here, we found that four-and-a-half LIM domains 1 (FHL1) is part of a complex within the cardiomyocyte sarcomere that senses the biomechanical stress-induced responses important for cardiac hypertrophy. Mice lacking Fhl1 displayed a blunted hypertrophic response and a beneficial functional response to pressure overload induced by transverse aortic constriction. A link to the Galphaq (Gq) signaling pathway was also observed, as Fhl1 deficiency prevented the cardiomyopathy observed in Gq transgenic mice. Mechanistic studies demonstrated that FHL1 plays an important role in the mechanism of pathological hypertrophy by sensing biomechanical stress responses via the N2B stretch sensor domain of titin and initiating changes in the titin- and MAPK-mediated responses important for sarcomere extensibility and intracellular signaling. These studies shed light on the physiological regulation of the sarcomere in response to hypertrophic stress.

Molecular etiology and pathogenesis of hereditary cardiomyopathy

Cardiomyopathy is defined as a cardiac disease caused by functional abnormality of cardiac muscle, and the etiology of the functional abnormality includes both extrinsic and intrinsic factors. Cardiomyopathy caused by the intrinsic factors is defined as idiopathic or primary cardiomyopathy, and there are several clinical phenotypes, including hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). The major intrinsic factor is gene mutations, and linkage studies, as well as candidate gene approaches, have deciphered multiple disease genes for hereditary primary cardiomyopathy. Of note is that mutations in the same disease gene can be found in different clinical phenotypes of cardiomyopathy. Functional analyses of disease-related mutations have revealed that characteristic functional alterations are associated with the clinical phenotypes, such that increased and decreased Ca(2+) sensitivity because of sarcomere mutations are associated with HCM and DCM, respectively. In addition, recent data have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of the sarcomere (ie, stiff sarcomere and loose sarcomere, respectively). More recently, mutations in the components of the I region can be found in hereditary cardiomyopathy, further complicating the etiology of primary cardiomyopathy.

Targeted disruption of the mouse Csrp2 gene encoding the cysteine- and glycine-rich LIM domain protein CRP2 result in subtle alteration of cardiac ultrastructure

Background

The cysteine and glycine rich protein 2 (CRP2) encoded by the Csrp2 gene is a LIM domain protein expressed in the vascular system, particularly in smooth muscle cells. It exhibits a bimodal subcellular distribution, accumulating at actin-based filaments in the cytosol and in the nucleus. In order to analyze the function of CRP2 in vivo, we disrupted the Csrp2 gene in mice and analysed the resulting phenotype.

Results

A ~17.3 kbp fragment of the murine Csrp2 gene containing exon 3 through 6 was isolated. Using this construct we confirmed the recently determined chromosomal localization (Chromosome 10, best fit location between markers D10Mit203 proximal and D10Mit150 central). A gene disruption cassette was cloned into exon 4 and a mouse strain lacking functional Csrp2 was generated. Mice lacking CRP2 are viable and fertile and have no obvious deficits in reproduction and survival. However, detailed histological and electron microscopic studies reveal that CRP2-deficient mice have subtle alterations in their cardiac ultrastructure. In these mice, the cardiomyocytes display a slight increase in their thickness, indicating moderate hypertrophy at the cellular level. Although the expression of several intercalated disc-associated proteins such as β-catenin, N-RAP and connexin-43 were not affected in these mice, the distribution of respective proteins was changed within heart tissue.

Conclusion

We conclude that the lack of CRP2 is associated with alterations in cardiomyocyte thickness and hypertrophy.

Hypertrophic cardiomyopathy: the genetic determinants of clinical disease expression

Hypertrophic cardiomyopathy (HCM), defined clinically by the presence of unexplained left ventricular hypertrophy, is the most common inherited cardiac disorder. This condition is the major cause of sudden death in the young (<30 years of age) and in athletes. The clinical phenotype is heterogeneous, and mutations in a number of sarcomeric contractile-protein genes are responsible for causing the disease in approximately 60% of individuals with HCM. Other inherited syndromes, as well as metabolic and mitochondrial disorders, can present as clinical phenocopies and can be distinguished by their associated cardiac and noncardiac features and on the basis of their unique molecular genetics. The mode of inheritance, natural history and treatment of phenocopies can differ from those of HCM caused by mutations in sarcomere genes. Detailed clinical evaluation and mutation analysis are, therefore, important in providing an accurate diagnosis in order to enable genetic counseling, prognostic evaluation and appropriate clinical management. This Review summarizes current knowledge on the genetics, disease mechanisms, and correlations between phenotype and genotype in patients with HCM, and discusses the implications of genetic testing in routine clinical practice.

An X-linked myopathy with postural muscle atrophy and generalized hypertrophy, termed XMPMA, is caused by mutations in FHL1

We have identified a large multigenerational Austrian family displaying a novel form of X-linked recessive myopathy. Affected individuals develop an adult-onset scapulo-axio-peroneal myopathy with bent-spine syndrome characterized by specific atrophy of postural muscles along with pseudoathleticism or hypertrophy and cardiac involvement. Known X-linked myopathies were excluded by simple-tandem-repeat polymorphism (STRP) and single-nucleotide polymorphism (SNP) analysis, direct gene sequencing, and immunohistochemical analysis. STRP analysis revealed significant linkage at Xq25-q27.1. Haplotype analysis based on SNP microarray data from selected family members confirmed this linkage region on the distal arm of the X chromosome, thereby narrowing down the critical interval to 12 Mb. Sequencing of functional candidate genes led to the identification of a missense mutation within the four and a half LIM domain 1 gene (FHL1), which putatively disrupts the fourth LIM domain of the protein. Mutation screening of FHL1 in a myopathy family from the UK exhibiting an almost identical phenotype revealed a 3 bp insertion mutation within the second LIM domain. FHL1 on Xq26.3 is highly expressed in skeletal and cardiac muscles. Western-blot analysis of muscle biopsies showed a marked decrease in protein expression of FHL1 in patients, in concordance with the genetic data. In summary, we have to our knowledge characterized a new disorder, X-linked myopathy with postural muscle atrophy (XMPMA), and identified FHL1 as the causative gene. This is the first FHL protein to be identified in conjunction with a human genetic disorder and further supports the role of FHL proteins in the development and maintenance of muscle tissue. Mutation screening of FHL1 should be considered for patients with uncharacterized myopathies and cardiomyopathies.

The Drosophila muscle LIM protein, Mlp84B, is essential for cardiac function

Muscle LIM protein (MLP) is a cytoskeletal protein located at the Z-disc of sarcomeres. Mutations in the human MLP gene are associated with hypertrophic and dilated cardiomyopathy. MLP has been proposed to be a key player in the stretch-sensing response, but the molecular mechanisms underlying its function in normal and diseased cardiac muscle have not been established. A Drosophila homolog, Mlp84B, displays a similar subcellular localization at the Z-disc of sarcomeres throughout development and in the adult, suggesting Drosophila as a model to study MLP function. Here we employed genetic ablation and cardiac-specific RNA interference (RNAi) knockdown of mlp84B to investigate its role in heart function. We found that Mlp84B-deficient or heart-specific RNAi knockdown flies exhibit diastolic interval prolongation, heart rhythm abnormalities and a reduced lifespan, while showing no obvious structural phenotype. Our data demonstrate that Mlp84B is essential for normal cardiac function and establish the Drosophila model for the investigation of the mechanisms connecting defective cardiac Z-disc components to the development of cardiomyopathy.

Sarcomeric proteins and inherited cardiomyopathies

Over the last two decades, a large number of mutations have been identified in sarcomeric proteins as a cause of hypertrophic, dilated or restrictive cardiomyopathy. Functional analyses of mutant proteins in vitro have revealed several important functional changes in sarcomeric proteins that might be primarily involved in the pathogenesis of each cardiomyopathy. Creation of transgenic or knock-in animals expressing mutant proteins in their hearts confirmed that these mutations in genes for sarcomeric proteins induced distinct types of cardiomyopathies and provided useful animal models to explore the molecular pathogenic mechanisms and potential therapeutics of cardiomyopathy in vivo. In this review, I discuss the functional consequences of mutations in different sarcomeric proteins found in hypertrophic, dilated, and restrictive cardiomyopathies in conjunction with their effects on cardiac structure and function in vivo and their possible molecular and cellular mechanisms, which underlie the pathogenesis of these inherited cardiomyopathies.

Role of the sarcomeric Z-disc in the pathogenesis of cardiomyopathy

The Z-disc has traditionally been viewed as a structure required to maintain sarcomeric function and integrity. More recently, the sarcomeric Z-disc has also emerged as a nodal point in cardiomyocyte signaling and mechanotransduction. This notion is not only supported by several transgenic animal models, but also by the identification of mutations in various Z-disc proteins, resulting in dilated or hypertrophic cardiomyopathy in patients. This review will thus focus on the role of the sarcomeric Z-disc and its associated proteins in the pathogenesis of cardiomyopathy.

Genetic basis of hypertrophic cardiomyopathy: from bench to the clinics

Hypertrophic cardiomyopathy (HCM) is a common inherited cardiac disorder that characterized by marked thickening of the left ventricular wall that occurs in the absence of increased external load. HCM is the most common cause of sudden cardiac death under 35 years and in addition causes heart failure. HCM is usually inherited as an autosomal dominant mutation in genes that encode protein constituents of the sarcomere. To date, more than 450 different mutations have been identified within 13 myofilament-related genes. This review focuses current research involved in the discovery of other causative genes, investigation of the mechanisms by which sarcomere genes mutations produce hypertrophy and arrhythmia, and identification of modifying factors that influence clinical expression in HCM patients. The clinical implications of molecular advances in HCM are discussed.

Genetics of hypertrophic cardiomyopathy: one, two, or more diseases?

PURPOSE OF REVIEW

Hypertrophic cardiomyopathy is the most common identifiable cause of sudden death in the young. This review details the history of hypertrophic cardiomyopathy, recent discoveries in its genetic underpinnings and important genotype-phenotype relationships described in recent studies.

RECENT FINDINGS

Since the discovery of the genetic underpinnings of hypertrophic cardiomyopathy in 1989 hundreds of mutations scattered among at least 10 sarcomeric genes confer the pathogenetic substrate for this 'disease of the sarcomere/myofilament'. More recently, the genetic spectrum of hypertrophic cardiomyopathy has expanded to encompass mutations in Z-disc-associated genes (Z-disc hypertrophic cardiomyopathy) and glycogen storage diseases mimicking hypertrophic cardiomyopathy (metabolic hypertrophic cardiomyopathy). Recent genotype-phenotype studies have discovered an important relationship between the morphology of the left ventricle, its underlying genetic substrate and the long-term outcome of this disease.

SUMMARY

Genomic medicine has entered clinical practice and the diagnostic utility of genetic testing for hypertrophic cardiomyopathy is clearly evident, but with the growing number of hypertrophic cardiomyopathy-associated genes strategic choices have to be made. With recent discoveries in genotype-phenotype relationships, especially pertaining to the echocardiographic septal shape and the underlying pathogenetic mutation, time has come to subdivide the one disease we call hypertrophic cardiomyopathy.

Dysfunction of dysferlin-deficient hearts

Mutations in the gene encoding dysferlin cause limb-girdle muscular dystrophy 2B (LGMD2B), a disorder that is believed to spare the heart. We observed dilated cardiomyopathy in two out of seven LGMD2B patients and cardiac abnormalities in three others. Cardiac biopsies showed that dysferlin was completely absent from the sarcolemma and appeared to be trapped within the cardiomyocytes. SJL/J mice (33-week-old) had diminished end-systolic pressure and reduced dP/dt; however, the hearts were histologically normal. Gene expression profiles of cardiac tissue were obtained and later confirmed by quantitative RT-PCR. Dysferlin-deficient and control mice had different gene expression patterns in terms of cardiomyocyte Z-disc and signal transduction proteins. CapZ, LIM-domain-binding protein 3 (LDB3, MLP), cypher (ZASP), desmin, and the cardiac ankyrin-repeated protein (CARP) were differentially expressed, compared to controls. Mechanical stress induced by the nonselective beta-adrenergic agonist isoproterenol (5 mg/kg body weight) given daily for 10 days resulted in reduced fractional shortening and increased cardiac fibrosis in SJL/J mice as compared to controls. Isoproterenol also caused metalloproteinase-2 upregulation in SJL/J mice. In A/J mice, the effect of isoproterenol injection was even more dramatic and lead to premature death as well as marked sarcolemmal injury as demonstrated by Evans blue dye penetration. Our data suggest that disturbances in dysferlin as well as Z-line proteins and transcription factors particularly under mechanical stress cause cardiomyopathy.

Gene expression responses over 24 h to lengthening and shortening contractions in human muscle: major changes in CSRP3, MUSTN1, SIX1, and FBXO32

Resistance training using lengthening (eccentric) contractions induces greater increases in muscle size than shortening (concentric) contractions, but the underlying molecular mechanisms are not clear. Using temporal expression profiling, we compared changes in gene expression within 24 h of an acute bout of each type of contractions conducted simultaneously in the quadriceps of different legs. Five healthy young men performed shortening contractions with one leg while the contralateral leg performed lengthening contractions. Biopsies were taken from both legs before exercise and 3, 6, and 24 h afterwards, in the fed state. Expression profiling ( n = 3) was performed using a custom-made Affymetrix MuscleChip containing probe sets of ∼3,300 known genes and expressed sequence tags expressed in skeletal muscle. We identified 51 transcripts differentially regulated between the two exercise modes. Using unsupervised hierarchical clustering, we identified four distinct clusters, three of which corresponded to unique functional categories (protein synthesis, stress response/early growth, and sarcolemmal structure). Using quantitative RT-PCR ( n = 5), we verified expression changes (lengthening/shortening) in SIX1 (3 h, −1.9-fold, P < 0.001), CSRP3 (6 h, 2.9-fold, P < 0.05), and MUSTN1 (24 h, 4.3-fold, P < 0.05). We examined whether FBXO32/atrogin-1/MAFbx, a known regulator of protein breakdown and of muscle atrophy was differentially expressed: the gene was downregulated after lengthening contractions (3 h, 2.7-fold, P < 0.05; 6 h, 3.3-fold, P < 0.05; 24 h, 2.3-fold, P < 0.05). The results suggested that lengthening and shortening contractions activated distinct molecular pathways as early as 3 h postexercise. The molecular differences might contribute to mechanisms underlying the physiological adaptations seen with training using the two modes of exercise.

Genetic screening of calcium regulation genes in familial hypertrophic cardiomyopathy

Genes encoding Ca(2+) regulatory proteins responsible for Ca(2+) homeostasis have been suggested as possible candidates for FHC. Mutations in sarcomere genes account for approximately 50% of all FHC cases indicating other genes, including those involved in Ca(2+) handling, may account for the remainder. The aim of this study was to identify causative mutations in genes involved in Ca(2+) regulation in patients with familial hypertrophic cardiomyopathy (FHC). An Australian cohort of 252 unrelated familial hypertrophic cardiomyopathy patients were screened for mutations in the Ca(2+) regulatory genes, sorcin (SRI), calstabin (FKBP1B), calsequestrin (CASQ2), phospholamban (PLN), sarcolipin (SLN), calreticulin (CALR3) and calmodulin (CALM). A total of 17 exonic DNA variants were identified in the 7 Ca(2+) regulatory genes studied, of which 4 were considered of pathogenic significance. Two novel mutations in the CALR3 gene were identified (Lys82Arg, Arg73Gln) and one truncation mutation in the PLN gene (Leu39Ter). A variant was also identified in the CASQ2 gene (Asp63Glu). These four variants were all novel, resulted in changes in conserved amino acids and were not identified in a normal population. In conclusion, mutations in Ca(2+) handling genes are an infrequent but important cause of FHC. DNA variants in Ca(2+) genes may also be involved as modifying factors in phenotype development. Further evaluation of the role of defects in Ca(2+) regulation will shed light on the molecular pathogenesis of FHC.

[Pathophysiology of heart failure]

Chronic heart failure is a clinical syndrome and the final common pathway of different cardiac diseases. Heart failure is accompanied by activation of the renin-angiotensin-aldosterone-system and the adrenergic nervous system. In addition, recent data emphasize important roles of maladaptive intracellular signaling pathways, decreased capillary density, altered calcium handling, metabolic changes, genetic polymorphisms, and programmed cell death in the failing heart. In this context, traditional pathophysiological concepts, e. g. concerning the role of cardiac hypertrophy, had to be given up. Thus, an increasingly complex scenario emerges with interdependent changes on the biochemical, molecular, metabolic, and cellular level. Novel therapeutic strategies may soon be based on these new pathophysiological concepts.

The Drosophila muscle LIM protein, Mlp84B, cooperates with D-titin to maintain muscle structural integrity

Muscle LIM protein (MLP) is a cytoskeletal LIM-only protein expressed in striated muscle. Mutations in human MLP are associated with cardiomyopathy; however, the molecular mechanism by which MLP functions is not established. A Drosophila MLP homolog, mlp84B, displays many of the same features as the vertebrate protein, illustrating the utility of the fly for the study of MLP function. Animals lacking Mlp84B develop into larvae with a morphologically intact musculature, but the mutants arrest during pupation with impaired muscle function. Mlp84B displays muscle-specific expression and is a component of the Z-disc and nucleus. Preventing nuclear retention of Mlp84B does not affect its function, indicating that Mlp84B site of action is likely to be at the Z-disc. Within the Z-disc, Mlp84B is colocalized with the N-terminus of D-titin, a protein crucial for sarcomere organization and stretch mechanics. The mlp84B mutants phenotypically resemble weak D-titin mutants. Furthermore, reducing D-titin activity in the mlp84B background leads to pronounced enhancement of the mlp84B muscle defects and loss of muscle structural integrity. The genetic interactions between mlp84B and D-titin reveal a role for Mlp84B in maintaining muscle structural integrity that was not obvious from analysis of the mlp84B mutants themselves, and suggest Mlp84B and D-titin cooperate to stabilize muscle sarcomeres.

Reversal of calcium cycling defects in advanced heart failure toward molecular therapy

Heart failure is a growing major cause of human morbidity and mortality worldwide. A wave of new insights from diverse laboratories has begun to uncover new therapeutic strategies that affect the molecular pathways within cardiomyocytes that drive heart failure progression. Using an integrative approach that employs insights from genetic-based studies in mouse and humans and in vivo somatic gene transfer studies, we have uncovered a new link between stress signals mediated by mechanical stretch and defects in sarcoplasmic reticulum (SR) calcium cycling. An intrinsic mechanical stress sensing system is embedded in the Z disc of cardiomyocytes, and defects in stretch responses can lead to heart failure progression and associated increases in wall stress. Reversal of the chronic increases in wall stress by promoting SR calcium cycling can prevent and partially reverse heart failure progression in multiple genetic and acquired model systems of heart failure in both small and large animals. We propose that reversal of advanced heart failure is possible by targeting the defects in SR calcium cycling, which may be a final common pathway for the progression of many forms of heart failure.

Mutations in JPH2-encoded junctophilin-2 associated with hypertrophic cardiomyopathy in humans

Junctophilin-2 (JPH2) is a cardiac specific member of the junctophilins, a newly characterized family of junctional membrane complex proteins important in physically approximating the plasmalemmal L-type calcium channel and the sarcoplasmic reticulum ryanodine receptor for calcium-induced calcium release. JPH2 knockout mice showed disrupted calcium transients, altered junctional membrane complex formation, cardiomyopathy, and embryonic lethality. Furthermore, JPH2 gene expression is down-regulated in murine cardiomyopathy models. To this end, we explored JPH2 as a novel candidate gene for the pathogenesis of hypertrophic cardiomyopathy (HCM) in humans. Using polymerase chain reaction, denaturing high performance liquid chromatography, and direct DNA sequencing, comprehensive open reading frame/splice site mutational analysis of JPH2 was performed on DNA obtained from 388 unrelated patients with HCM. HCM-associated JPH2 mutations were engineered and functionally characterized using immunocytochemistry, cell morphometry measurements, and live cell confocal calcium imaging. Three novel HCM-susceptibility mutations: S101R, Y141H and S165F, which localize to key functional domains, were discovered in 3/388 unrelated patients with HCM and were absent in 1000 ethnic-matched reference alleles. Functionally, each human mutation caused (i) protein reorganization of junctophilin-2, (ii) perturbations in intracellular calcium signaling, and (iii) marked cardiomyocyte hyperplasia. The molecular and functional evidence implicates defective junctophilin-2 and disrupted calcium signaling as a novel pathogenic mechanism for HCM and establishes HCM as the first human disease associated with genetic defects in JPH2. Whether susceptibility for other cardiomyopathies, such as dilated cardiomyopathy, can be conferred by mutations in JPH2 warrants investigation.

Reviews of Translational Medicine and Genomics in Cardiovascular Disease: New Disease Taxonomy and Therapeutic Implications

The enduring subdivision of cardiomyopathies into hypertrophic (HCM), dilated (DCM), and restrictive (RCM) categories reflects the emphasis of traditional classifications on morphology. Rapid advances in the genetic interrogation of these disorders have redefined their taxonomy and revealed potential conflicts between the old and new classifications. Hypertrophic cardiomyopathy has been redefined as a disease of perturbed sarcomere function. Dilated cardiomyopathy is a disease that results from more varied perturbations, including, but not limited to, defects of the cytoskeleton. Positional cloning and candidate gene approaches have been successful in identifying >40 disease loci, many of which have led to disease genes in HCM, DCM, RCM, and arrhythmogenic right ventricular cardiomyopathy. These findings provide mechanistic insights, permit genetic screening, and to a limited extent, facilitate prognostication. Although single gene analyses rapidly focus down to the underlying mechanistic pathways, they do not take account of all relevant variation in the human genome. Correspondingly, advances in genomics, through microarrays, have facilitated characterization of these broader downstream elements. As well as refining the taxonomic reclassification of cardiomyopathies, these genomic approaches, coupled with functional studies, have identified novel potential therapeutic targets, such as cardiac energetics, calcium handling, and apoptosis. We review the successes and pitfalls of genetic and genomic approaches to cardiomyopathy and their impact on current and future clinical care.

Novel structures for alpha-actinin:F-actin interactions and their implications for actin-membrane attachment and tension sensing in the cytoskeleton

We have applied correspondence analysis to electron micrographs of 2-D rafts of F-actin cross-linked with alpha-actinin on a lipid monolayer to investigate alpha-actinin:F-actin binding and cross-linking. More than 8000 actin crossover repeats, each with one to five alpha-actinin molecules bound, were selected, aligned, and grouped to produce class averages of alpha-actinin cross-links with approximately 9-fold improvement in the stochastic signal-to-noise ratio. Measurements and comparative molecular models show variation in the distance separating actin-binding domains and the angle of the alpha-actinin cross-links. Rafts of F-actin and alpha-actinin formed predominantly polar 2-D arrays of actin filaments, with occasional insertion of filaments of opposite polarity. Unique to this study are the numbers of alpha-actinin molecules bound to successive crossovers on the same actin filament. These "monofilament"-bound alpha-actinin molecules may reflect a new mode of interaction for alpha-actinin, particularly in protein-dense actin-membrane attachments in focal adhesions. These results suggest that alpha-actinin is not simply a rigid spacer between actin filaments, but rather a flexible cross-linking, scaffolding, and anchoring protein. We suggest these properties of alpha-actinin may contribute to tension sensing in actin bundles.

Genetic basis of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) was the first cardiovascular disorder in which a genetic basis was identified. The disease is characterized by a marked thickening of the left ventricle and is the most common structural cause of sudden cardiac death in those aged under 35 years. HCM is primarily a disease of the sarcomere with over 250 mutations identified currently within 13 sarcomere-related genes. At present, genetic screening is available for the genes shown to cause HCM most frequently, with a mutation pick-up rate of up to 60%. Current research is focused on the identification of additional causative genes and elucidation into signaling mechanisms involved in HCM pathogenesis, as well as investigation of modifying factors that can alter the clinical phenotype in HCM. The unifying goal of these studies is to improve our understanding of disease pathogenesis in HCM, thereby facilitating the process of new diagnostic and therapeutic approaches in patients, ultimately leading to disease prevention and possible curative treatment.

Cardiac dysfunction and heart failure are associated with abnormalities in the subcellular distribution and amounts of oligomeric muscle LIM protein

Prolonged hemodynamic overload results in cardiac hypertrophy and failure with detrimental changes in myocardial gene expression and morphology. Cysteine-rich protein 3 or muscle LIM protein (MLP) is thought to be a mechanosensor in cardiac myocytes. Therefore, the subcellular location of MLP may have functional implications in health and disease. Our hypothesis is that MLP becomes mislocalized after prolonged overload, resulting in impaired mechanosensing in cardiac myocytes. Using the techniques of biochemical subcellular fractionation and immunocytochemistry, we found MLP exhibits oligomerization in the membrane and cytoskeleton of cultured cardiac rat neonatal myocytes. Nuclear MLP was always monomeric. MLP translocated to the nucleolus in response to 10% cyclic stretch at 1 Hz for 48 h. This was associated with a threefold increase in S6 ribosomal protein ( P < 0.01; n = 3 cultures). Adenoviral overexpression of MLP also resulted in a twofold increase in S6 protein, suggesting that MLP can activate ribosomal protein synthesis in the nucleolus. In ventricles from aortic-banded and myocardially infarcted rat hearts, nuclear MLP increased by twofold ( P < 0.01; n = 7) along with a significant decrease in the nonnuclear oligomeric fraction. The ratio of nuclear to nonnuclear MLP increased threefold in both groups ( P < 0.01; n = 7). In failing human hearts, there was almost a complete loss of oligomeric MLP. Using a flag-tagged adenoviral MLP, we demonstrate that the COOH terminus is required for oligomerization and that this is a precursor to stretch sensing and subsequent nuclear translocation. Therefore, reduced oligomeric MLP in the costamere and cytoskeleton may contribute to impaired mechanosensing in heart failure.

A molecular screening strategy based on beta-myosin heavy chain, cardiac myosin binding protein C and troponin T genes in Italian patients with hypertrophic cardiomyopathy

BACKGROUND

Mutations causing hypertrophic cardiomyopathy (HCM) have been described in nine different genes of the sarcomere. Three genes account for most known mutations: beta-myosin heavy chain (MYH7), cardiac myosin binding protein C (MYBPC3) and cardiac troponin T (TNNT2). Their prevalence in Italian HCM patients is unknown. Thus, we prospectively assessed a molecular screening strategy of these three genes in a consecutive population with HCM from two Italian centres.

METHODS

Comprehensive screening of MYBPC3, MYH7 and TNNT2 was performed in 88 unrelated HCM patients by denaturing high-performance liquid chromatography and automatic sequencing.

RESULTS

We identified 32 mutations in 50 patients (57%); 16 were novel. The prevalence rates for MYBPC3, MYH7 and TNNT2 were 32%, 17% and 2%, respectively. MYBPC3 mutations were 18, including two frameshift, five splice-site and two nonsense. All were 'private' except insC1065 and R502Q, present in three and two patients, respectively. Moreover, E258K was found in 14% of patients, suggesting a founder effect. MYH7 mutations were 12, all missense; seven were novel. In TNNT2, only two mutations were found. In addition, five patients had a complex genotype [i.e. carried a double MYBPC3 mutation (n = 2), or were double heterozygous for mutations in MYBPC3 and MYH7 (n = 3)].

CONCLUSIONS

The first comprehensive evaluation of MYBPC3, MYH7 and TNNT2 in an Italian HCM population allowed a genetic diagnosis in 57% of the patients. These data support a combined analysis of the three major sarcomeric genes as a rational and cost-effective initial approach to the molecular screening of HCM.

Echocardiographic-determined septal morphology in Z-disc hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) can be classified into at least four major anatomic subsets based upon the septal contour, and the location and extent of hypertrophy: reverse curvature-, sigmoidal-, apical-, and neutral contour-HCM. Here, we sought to identify genetic determinants for sigmoidal-HCM and hypothesized that Z-disc-HCM may be associated preferentially with a sigmoidal phenotype. Utilizing PCR, DHPLC, and direct DNA sequencing, we performed mutational analysis of five genes encoding cardiomyopathy-associated Z-disc proteins. The study cohort consisted of 239 unrelated patients with HCM previously determined to be negative for mutations in the eight genes associated with myofilament-HCM. Blinded to the Z-disc genotype status, the septal contour was graded qualitatively using standard transthoracic echocardiography. Thirteen of the 239 patients (5.4%) had one of 13 distinct HCM-associated Z-disc mutations involving residues highly conserved across species and absent in 600 reference alleles: LDB3 (6), ACTN2 (3), TCAP (1), CSRP3 (1), and VCL (2). For this subset with Z-disc-associated HCM, the septal contour was sigmoidal in 11 (85%) and apical in 2 (15%). While Z-disc-HCM is uncommon, it is equal in prevalence to thin filament-HCM. In contrast to myofilament-HCM, Z-disc-HCM is associated preferentially with sigmoidal morphology.

Mutations in the EGF-CFC gene cryptic are an infrequent cause of congenital heart disease

Cryptic (CFC1), a member of the epidermal growth factor-Cripto/FRL-1/Cryptic (EGF-CFC) gene family, is involved in the evolutionarily conserved establishment of left-right lateral asymmetry. Inactivation of Cfc1 in mice results in laterality defects and complex cardiac malformations. Similarly, mutations in the human CFC1 gene have been identified in patients with heterotaxy syndrome. The cardiac defects in humans resemble those in mice lacking Cfc1. We postulated that some patients with isolated cardiac malformations could also have mutations in the CFC1 gene. Our analysis of the CFC1 gene in 167 patients with congenital heart disease revealed a novel A145T missense variant in 3 patients with type II atrial septal defect. Furthermore, we found the previously characterized R78W polymorphism in another patient with type II atrial septal defect. However, the A145T sequence alteration was also identified in 3 controls, suggesting that this variant is a polymorphism. We conclude that CFC1 variants could be a rare cause of congenital heart disease in patients without laterality defects.

PICOT inhibits cardiac hypertrophy and enhances ventricular function and cardiomyocyte contractility

Multiple signaling pathways involving protein kinase C (PKC) have been implicated in the development of cardiac hypertrophy. We observed that a putative PKC inhibitor, PICOT (PKC-Interacting Cousin Of Thioredoxin) was upregulated in response to hypertrophic stimuli both in vitro and in vivo. This suggested that PICOT may act as an endogenous negative feedback regulator of cardiac hypertrophy through its ability to inhibit PKC activity, which is elevated during cardiac hypertrophy. Adenovirus-mediated gene transfer of PICOT completely blocked the hypertrophic response of neonatal rat cardiomyocytes to enthothelin-1 and phenylephrine, as demonstrated by cell size, sarcomere rearrangement, atrial natriuretic factor expression, and rates of protein synthesis. Transgenic mice with cardiac-specific overexpression of PICOT showed that PICOT is a potent inhibitor of cardiac hypertrophy induced by pressure overload. In addition, PICOT overexpression dramatically increased the ventricular function and cardiomyocyte contractility as measured by ejection fraction and end-systolic pressure of transgenic hearts and peak shortening of isolated cardiomyocytes, respectively. Intracellular Ca(2+) handing analysis revealed that increases in myofilament Ca(2+) responsiveness, together with increased rate of sarcoplasmic reticulum Ca(2+) reuptake, are associated with the enhanced contractility in PICOT-overexpressing cardiomyocytes. The inhibition of cardiac remodeling by of PICOT with a concomitant increase in ventricular function and cardiomyocyte contractility suggests that PICOT may provide an efficient modality for treatment of cardiac hypertrophy and heart failure.

Molecular genetics of sudden cardiac death in small animals - a review

Sudden cardiac death in small animals is uncommon but often occurs due to cardiac conduction defects or myocardial diseases. Primary cardiac conduction defects are mainly caused by mutations in genes involved in impulse conduction processes (e.g., gap-junction genes and transcription factors) or repolarisation processes (e.g., ion-channel genes), whereas primary cardiomyopathies are mainly caused by defective force generation or force transmission due to gene mutations in either sarcomeric or cytoskeleton proteins. Although over 50 genes have been identified in humans directly or indirectly related to sudden cardiac death, no genetic aetiologies have been identified in small animals. Sudden cardiac deaths have been also reported in German Shepherds and Boxers. A better understanding of molecular genetic aetiologies for sudden cardiac death will be required for future study toward unveiling aetiology in sudden cardiac death in small animals.

Mechanical stress-strain sensors embedded in cardiac cytoskeleton: Z disk, titin, and associated structures

Cardiac muscle is equipped with intricate intrinsic mechanisms to regulate adaptive remodeling. Recent and extensive experimental findings powered by novel strategies for screening protein-protein interactions, improved imaging technologies, and versatile transgenic mouse methodologies reveal that Z disks and titin filaments possess unexpectedly complicated sensory and modulatory mechanisms for signal reception and transduction. These mechanisms employ molecules such as muscle-enriched LIM domain proteins, PDZ-LIM domain proteins, myozenin gene family members, titin-associated ankyrin repeat family proteins, and muscle-specific ring finger proteins, which have been identified as potential molecular sensor components. Moreover, classic transmembrane signaling processes, including mitogen-activated kinase, protein kinase C, and calcium signaling, also involve novel interactions with the Z disk/titin network. This compartmentalization of signaling complexes permits alteration of receptor-dependent transcriptional regulation by direct sensing of intrinsic stress. Newly identified mechanical stress sensors are not limited to Z-disk region and to I-band and M-band regions of titin but are also embedded in muscle-specific membrane systems such as the costamere, intercalated disks, and caveolae-like microdomains. This review summarizes current knowledge of this rapidly developing area with focus on how the heart adjusts physiological remodeling process to meet with mechanical demands and how this process fails in cardiac pathologies.

Genotype-phenotype relationships involving hypertrophic cardiomyopathy-associated mutations in titin, muscle LIM protein, and telethonin

BACKGROUND

TTN-encoded titin, CSRP3-encoded muscle LIM protein, and TCAP-encoded telethonin are Z-disc proteins essential for the structural organization of the cardiac sarcomere and the cardiomyocyte's stretch sensor. All three genes have been established as cardiomyopathy-associated genes for both dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM). Here, we sought to characterize the frequency, spectrum, and phenotype associated with HCM-associated mutations in these three genes in a large cohort of unrelated patients evaluated at a single tertiary outpatient center.

METHODS

DNA was obtained from 389 patients with HCM (215 male, left ventricular wall thickness of 21.6+/-6 mm) and analyzed for mutations involving all translated exons of CSRP3 and TCAP and targeted HCM-associated exons (2, 3, 4, and 14) of TTN using polymerase chain reaction (PCR), denaturing high performance liquid chromatography (DHPLC), and direct DNA sequencing. Clinical data were extracted from patient records and maintained independent of the genotype.

RESULTS

Overall, 16 patients (4.1%) harbored a Z-disc mutation: 12 had a MLP mutation and 4 patients a TCAP mutation. No TTN mutations were detected. Seven patients were also found to have a concomitant myofilament mutation. Seven patients with a MLP-mutation were found to harbor the DCM-associated, functionally characterized W4R mutation. W4R-MLP was also noted in a single white control subject. Patients with MLP/TCAP-associated HCM clinically mimicked myofilament-HCM.

CONCLUSIONS

Approximately 4.1% of unrelated patients had HCM-associated MLP or TCAP mutations. MLP/TCAP-HCM phenotypically mirrors myofilament-HCM and is more severe than the subset of patients who still remain without a disease-causing mutation. The precise role of W4R-MLP in the pathogenesis of either DCM or HCM warrants further investigation.

The sarcomeric Z-disc: a nodal point in signalling and disease

The perception of the Z-disc in striated muscle has undergone significant changes in the past decade. Traditionally, the Z-disc has been viewed as a passive constituent of the sarcomere, which is important only for the cross-linking of thin filaments and transmission of force generated by the myofilaments. The recent discovery of multiple novel molecular components, however, has shed light on an emerging role for the Z-disc in signal transduction in both cardiac and skeletal muscles. Strikingly, mutations in several Z-disc proteins have been shown to cause cardiomyopathies and/or muscular dystrophies. In addition, the elusive cardiac stretch receptor appears to localize to the Z-disc. Various signalling molecules have been shown to interact with Z-disc proteins, several of which shuttle between the Z-disc and other cellular compartments such as the nucleus, underlining the dynamic nature of Z-disc-dependent signalling. In this review, we provide a systematic view on the currently known Z-disc components and the functional significance of the Z-disc as an interface between biomechanical sensing and signalling in cardiac and skeletal muscle functions and diseases.