Functional analysis of the mutations in the human cardiac beta-myosin that are responsible for familial hypertrophic cardiomyopathy. Implication for the clinical outcome

MYH7 in cardiomyopathy and skeletal muscle myopathy

Myosin heavy chain gene 7 (MYH7), a sarcomeric gene encoding the myosin heavy chain (myosin-7), has attracted considerable interest as a result of its fundamental functions in cardiac and skeletal muscle contraction and numerous nucleotide variations of MYH7 are closely related to cardiomyopathy and skeletal muscle myopathy. These disorders display significantly inter- and intra-familial variability, sometimes developing complex phenotypes, including both cardiomyopathy and skeletal myopathy. Here, we review the current understanding on MYH7 with the aim to better clarify how mutations in MYH7 affect the structure and physiologic function of sarcomere, thus resulting in cardiomyopathy and skeletal muscle myopathy. Importantly, the latest advances on diagnosis, research models in vivo and in vitro and therapy for precise clinical application have made great progress and have epoch-making significance. All the great advance is discussed here.

Virus-free transfection, transient expression, and purification of human cardiac myosin in mammalian muscle cells for biochemical and biophysical assays

Myosin expression and purification is important for mechanistic insights into normal function and mutation induced changes. The latter is particularly important for striated muscle myosin II where mutations cause several debilitating diseases. However, the heavy chain of this myosin is challenging to express and the standard protocol, using C2C12 cells, relies on viral infection. This is time and work intensive and associated with infrastructural demands and biological hazards, limiting widespread use and hampering fast generation of a wide range of mutations. We here develop a virus-free method to overcome these challenges. We use this system to transfect C2C12 cells with the motor domain of the human cardiac myosin heavy chain. After optimizing cell transfection, cultivation and harvesting conditions, we functionally characterized the expressed protein, co-purified with murine essential and regulatory light chains. The gliding velocity (1.5-1.7 µm/s; 25 °C) in the in vitro motility assay as well as maximum actin activated catalytic activity (kcat; 8-9 s-1) and actin concentration for half maximal activity (KATPase; 70-80 µM) were similar to those found previously using virus based infection. The results should allow new types of studies, e.g., screening of a wide range of mutations to be selected for further characterization.

Critical Evaluation of Current Hypotheses for the Pathogenesis of Hypertrophic Cardiomyopathy

Hereditary hypertrophic cardiomyopathy (HCM), due to mutations in sarcomere proteins, occurs in more than 1/500 individuals and is the leading cause of sudden cardiac death in young people. The clinical course exhibits appreciable variability. However, typically, heart morphology and function are normal at birth, with pathological remodeling developing over years to decades, leading to a phenotype characterized by asymmetric ventricular hypertrophy, scattered fibrosis and myofibrillar/cellular disarray with ultimate mechanical heart failure and/or severe arrhythmias. The identity of the primary mutation-induced changes in sarcomere function and how they trigger debilitating remodeling are poorly understood. Support for the importance of mutation-induced hypercontractility, e.g., increased calcium sensitivity and/or increased power output, has been strengthened in recent years. However, other ideas that mutation-induced hypocontractility or non-uniformities with contractile instabilities, instead, constitute primary triggers cannot yet be discarded. Here, we review evidence for and criticism against the mentioned hypotheses. In this process, we find support for previous ideas that inefficient energy usage and a blunted Frank-Starling mechanism have central roles in pathogenesis, although presumably representing effects secondary to the primary mutation-induced changes. While first trying to reconcile apparently diverging evidence for the different hypotheses in one unified model, we also identify key remaining questions and suggest how experimental systems that are built around isolated primarily expressed proteins could be useful.

The R249Q hypertrophic cardiomyopathy myosin mutation decreases contractility in Drosophila by impeding force production

KEY POINTS

Hypertrophic cardiomyopathy (HCM) is a genetic disease that causes thickening of the heart's ventricular walls and is a leading cause of sudden cardiac death. HCM is caused by missense mutations in muscle proteins including myosin, but how these mutations alter muscle mechanical performance in largely unknown. We investigated the disease mechanism for HCM myosin mutation R249Q by expressing it in the indirect flight muscle of Drosophila melanogaster and measuring alterations to muscle and flight performance. Muscle mechanical analysis revealed R249Q decreased muscle power production due to slower muscle kinetics and decreased force production; force production was reduced because fewer mutant myosin cross-bridges were bound simultaneously to actin. This work does not support the commonly proposed hypothesis that myosin HCM mutations increase muscle contractility, or causes a gain in function; instead, it suggests that for some myosin HCM mutations, hypertrophy is a compensation for decreased contractility.

ABSTRACT

Hypertrophic cardiomyopathy (HCM) is an inherited disease that causes thickening of the heart's ventricular walls. A generally accepted hypothesis for this phenotype is that myosin heavy chain HCM mutations increase muscle contractility. To test this hypothesis, we expressed an HCM myosin mutation, R249Q, in Drosophila indirect flight muscle (IFM) and assessed myofibril structure, skinned fibre mechanical properties, and flight ability. Mechanics experiments were performed on fibres dissected from 2-h-old adult flies, prior to degradation of IFM myofilament structure, which started at 2 days old and increased with age. Homozygous and heterozygous R249Q fibres showed decreased maximum power generation by 67% and 44%, respectively. Decreases in force and work and slower overall muscle kinetics caused homozygous fibres to produce less power. While heterozygous fibres showed no overall slowing of muscle kinetics, active force and work production dropped by 68% and 47%, respectively, which hindered power production. The muscle apparent rate constant 2πb decreased 33% for homozygous but increased for heterozygous fibres. The apparent rate constant 2πc was greater for homozygous fibres. This indicates that R249Q myosin is slowing attachment while speeding up detachment from actin, resulting in less time bound. Decreased IFM power output caused 43% and 33% decreases in Drosophila flight ability and 19% and 6% drops in wing beat frequency for homozygous and heterozygous flies, respectively. Overall, our results do not support the increased contractility hypothesis. Instead, our results suggest the ventricular hypertrophy for human R249Q mutation is a compensatory response to decreases in heart muscle power output.

Hypertrophic cardiomyopathy R403Q mutation in rabbit β-myosin reduces contractile function at the molecular and myofibrillar levels

In 1990, the Seidmans showed that a single point mutation, R403Q, in the human β-myosin heavy chain (MHC) of heart muscle caused a particularly malignant form of familial hypertrophic cardiomyopathy (HCM) [Geisterfer-Lowrance AA, et al. (1990) Cell 62:999-1006.]. Since then, more than 300 mutations in the β-MHC have been reported, and yet there remains a poor understanding of how a single missense mutation in the MYH7 gene can lead to heart disease. Previous studies with a transgenic mouse model showed that the myosin phenotype depended on whether the mutation was in an α- or β-MHC backbone. This led to the generation of a transgenic rabbit model with the R403Q mutation in a β-MHC backbone. We find that the in vitro motility of heterodimeric R403Q myosin is markedly reduced, whereas the actin-activated ATPase activity of R403Q subfragment-1 is about the same as myosin from a nontransgenic littermate. Single myofibrils isolated from the ventricles of R403Q transgenic rabbits and analyzed by atomic force microscopy showed reduced rates of force development and relaxation, and achieved a significantly lower steady-state level of isometric force compared with nontransgenic myofibrils. Myofibrils isolated from the soleus gave similar results. The force-velocity relationship determined for R403Q ventricular myofibrils showed a decrease in the velocity of shortening under load, resulting in a diminished power output. We conclude that independent of whether experiments are performed with isolated molecules or with ordered molecules in the native thick filament of a myofibril, there is a loss-of-function induced by the R403Q mutation in β-cardiac myosin.

Overexpression and purification of human myosins from transiently and stably transfected suspension adapted HEK293SF-3F6 cells

The myosin family of motor proteins is an attractive target of therapeutic small-molecule protein inhibitors and modulators. Milligrams of protein quantities are required to conduct proper biophysical and biochemical studies to understand myosin functions. Myosin protein expression and purification represent a critical starting point towards this goal. Established utilization of Dictyostelium discoideum, Drosophila melanogaster, insect and mouse cells for myosin expression and purification is limited, cost, labor and time inefficient particularly for (full-length) human myosins. Here we are presenting detailed protocols for production of several difficult-to-purify recombinant human myosins in efficient quantities up to 1 mg of protein per liter of cell culture. This is the first time that myosins have been purified in large scales from suspension adapted transiently and stably expressing human cells. The method is also useful for expressing other human proteins in quantities sufficient to perform extensive biochemical and biophysical characterization.

The Molecular Mechanisms of Mutations in Actin and Myosin that Cause Inherited Myopathy

The discovery that mutations in myosin and actin genes, together with mutations in the other components of the muscle sarcomere, are responsible for a range of inherited muscle diseases (myopathies) has revolutionized the study of muscle, converting it from a subject of basic science to a relevant subject for clinical study and has been responsible for a great increase of interest in muscle studies. Myopathies are linked to mutations in five of the myosin heavy chain genes, three of the myosin light chain genes, and three of the actin genes. This review aims to determine to what extent we can explain disease phenotype from the mutant genotype. To optimise our chances of finding the right mechanism we must study a myopathy where there are a large number of different mutations that cause a common phenotype and so are likely to have a common mechanism: a corollary to this criterion is that if any mutation causes the disease phenotype but does not correspond to the proposed mechanism, then the whole mechanism is suspect. Using these criteria, we consider two cases where plausible genotype-phenotype mechanisms have been proposed: the actin “A-triad” and the myosin “mesa/IHD” models.

Ensemble force changes that result from human cardiac myosin mutations and a small-molecule effector

Cardiomyopathies due to mutations in human β-cardiac myosin are a significant cause of heart failure, sudden death, and arrhythmia. To understand the underlying molecular basis of changes in the contractile system's force production due to such mutations and search for potential drugs that restore force generation, an in vitro assay is necessary to evaluate cardiac myosin's ensemble force using purified proteins. Here, we characterize the ensemble force of human α- and β-cardiac myosin isoforms and those of β-cardiac myosins carrying left ventricular non-compaction (M531R) and dilated cardiomyopathy (S532P) mutations using a utrophin-based loaded in vitro motility assay and new filament-tracking software. Our results show that human α- and β-cardiac myosin, as well as the mutants, show opposite mechanical and enzymatic phenotypes with respect to each other. We also show that omecamtiv mecarbil, a previously discovered cardiac-specific myosin activator, increases β-cardiac myosin force generation.

Chaperone-enhanced purification of unconventional myosin 15, a molecular motor specialized for stereocilia protein trafficking

Unconventional myosin 15 is a molecular motor expressed in inner ear hair cells that transports protein cargos within developing mechanosensory stereocilia. Mutations of myosin 15 cause profound hearing loss in humans and mice; however, the properties of this motor and its regulation within the stereocilia organelle are unknown. To address these questions, we expressed a subfragment 1-like (S1) truncation of mouse myosin 15, comprising the predicted motor domain plus three light-chain binding sites. Following unsuccessful attempts to express functional myosin 15-S1 using the Spodoptera frugiperda (Sf9)-baculovirus system, we discovered that coexpression of the muscle-myosin-specific chaperone UNC45B, in addition to the chaperone heat-shock protein 90 (HSP90) significantly increased the yield of functional protein. Surprisingly, myosin 15-S1 did not bind calmodulin with high affinity. Instead, the IQ domains bound essential and regulatory light chains that are normally associated with class II myosins. We show that myosin 15-S1 is a barbed-end-directed motor that moves actin filaments in a gliding assay (∼ 430 nm · s(-1) at 30 °C), using a power stroke of 7.9 nm. The maximum ATPase rate (k(cat) ∼ 6 s(-1)) was similar to the actin-detachment rate (k(det) = 6.2 s(-1)) determined in single molecule optical trapping experiments, indicating that myosin 15-S1 was rate limited by transit through strongly actin-bound states, similar to other processive myosin motors. Our data further indicate that in addition to folding muscle myosin, UNC45B facilitates maturation of an unconventional myosin. We speculate that chaperone coexpression may be a simple method to optimize the purification of other myosin motors from Sf9 insect cells.

Faster cross-bridge detachment and increased tension cost in human hypertrophic cardiomyopathy with the R403Q MYH7 mutation

The first mutation associated with hypertrophic cardiomyopathy (HCM) is the R403Q mutation in the gene encoding β-myosin heavy chain (β-MyHC). R403Q locates in the globular head of myosin (S1), responsible for interaction with actin, and thus motor function of myosin. Increased cross-bridge relaxation kinetics caused by the R403Q mutation might underlie increased energetic cost of tension generation; however, direct evidence is absent. Here we studied to what extent cross-bridge kinetics and energetics are related in single cardiac myofibrils and multicellular cardiac muscle strips of three HCM patients with the R403Q mutation and nine sarcomere mutation-negative HCM patients (HCMsmn). Expression of R403Q was on average 41 ± 4% of total MYH7 mRNA. Cross-bridge slow relaxation kinetics in single R403Q myofibrils was significantly higher (P < 0.0001) than in HCMsmn myofibrils (0.47 ± 0.02 and 0.30 ± 0.02 s(-1), respectively). Moreover, compared to HCMsmn, tension cost was significantly higher in the muscle strips of the three R403Q patients (2.93 ± 0.25 and 1.78 ± 0.10 μmol l(-1) s(-1) kN(-1) m(-2), respectively) which showed a positive linear correlation with relaxation kinetics in the corresponding myofibril preparations. This correlation suggests that faster cross-bridge relaxation kinetics results in an increase in energetic cost of tension generation in human HCM with the R403Q mutation compared to HCMsmn. Therefore, increased tension cost might contribute to HCM disease in patients carrying the R403Q mutation.

To understand muscle you must take it apart

Striated muscle is an elegant system for study at many levels. Much has been learned about the mechanism of contraction from studying the mechanical properties of intact and permeabilized (or skinned) muscle fibers. Structural studies using electron microscopy, X-ray diffraction or spectroscopic probes attached to various contractile proteins were possible because of the highly ordered sarcomeric arrangement of actin and myosin. However, to understand the mechanism of force generation at a molecular level, it is necessary to take the system apart and study the interaction of myosin with actin using in vitro assays. This reductionist approach has lead to many fundamental insights into how myosin powers muscle contraction. In addition, nature has provided scientists with an array of muscles with different mechanical properties and with a superfamily of myosin molecules. Taking advantage of this diversity in myosin structure and function has lead to additional insights into common properties of force generation. This review will highlight the development of the major assays and methods that have allowed this combined reductionist and comparative approach to be so fruitful. This review highlights the history of biochemical and biophysical studies of myosin and demonstrates how a broad comparative approach combined with reductionist studies have led to a detailed understanding of how myosin interacts with actin and uses chemical energy to generate force and movement in muscle contraction and motility in general.

Muscle dysfunction in hypertrophic cardiomyopathy: what is needed to move to translation?

Hypertrophic cardiomyopathy (HCM) is caused by mutations in sarcomere genes. As such, HCM provides remarkable opportunities to study how changes to the heart's molecular motor apparatus may influence cardiac structure and function. Although the genetic basis of HCM is well-described, there is much more limited understanding of the precise consequences of sarcomere mutations--how they remodel the heart, and how these changes lead to the dramatic clinical consequences associated with HCM. More precise characterization of the mechanisms leading from sarcomere mutation to altered cardiac muscle function is critical to gain insight into fundamental disease biology and phenotypic evolution. Such knowledge will help foster development of novel treatment strategies aimed at correcting and preventing disease development in HCM.

Molecular consequences of the R453C hypertrophic cardiomyopathy mutation on human β-cardiac myosin motor function

Cardiovascular disorders are the leading cause of morbidity and mortality in the developed world, and hypertrophic cardiomyopathy (HCM) is among the most frequently occurring inherited cardiac disorders. HCM is caused by mutations in the genes encoding the fundamental force-generating machinery of the cardiac muscle, including β-cardiac myosin. Here, we present a biomechanical analysis of the HCM-causing mutation, R453C, in the context of human β-cardiac myosin. We found that this mutation causes a ∼30% decrease in the maximum ATPase of the human β-cardiac subfragment 1, the motor domain of myosin, and a similar percent decrease in the in vitro velocity. The major change in the R453C human β-cardiac subfragment 1 is a 50% increase in the intrinsic force of the motor compared with wild type, with no appreciable change in the stroke size, as observed with a dual-beam optical trap. These results predict that the overall force of the ensemble of myosin molecules in the muscle should be higher in the R453C mutant compared with wild type. Loaded in vitro motility assay confirms that the net force in the ensemble is indeed increased. Overall, this study suggests that the R453C mutation should result in a hypercontractile state in the heart muscle.

Cell-intrinsic functional effects of the α-cardiac myosin Arg-403-Gln mutation in familial hypertrophic cardiomyopathy

Human familial hypertrophic cardiomyopathy is the most common Mendelian cardiovascular disease worldwide. Among the most severe presentations of the disease are those in families heterozygous for the mutation R403Q in β-cardiac myosin. Mice heterozygous for this mutation in the α-cardiac myosin isoform display typical familial hypertrophic cardiomyopathy pathology. Here, we study cardiomyocytes from heterozygous 403/+ mice. The effects of the R403Q mutation on force-generating capabilities and dynamics of cardiomyocytes were investigated using a dual carbon nanofiber technique to measure single-cell parameters. We demonstrate the Frank-Starling effect at the single cardiomyocyte level by showing that cell stretch causes an increase in amplitude of contraction. Mutant 403/+ cardiomyocytes exhibit higher end-diastolic and end-systolic stiffness than +/+ cardiomyocytes, whereas active force generation capabilities remain unchanged. Additionally, 403/+ cardiomyocytes show slowed relaxation dynamics. These phenotypes are consistent with increased end-diastolic and end-systolic chamber elastance, as well as diastolic dysfunction seen at the level of the whole heart. Our results show that these functional effects of the R403Q mutation are cell-intrinsic, a property that may be a general phenomenon in familial hypertrophic cardiomyopathy.

Rare, nonsynonymous variant in the smooth muscle-specific isoform of myosin heavy chain, MYH11, R247C, alters force generation in the aorta and phenotype of smooth muscle cells

RATIONALE

Mutations in myosin heavy chain (MYH11) cause autosomal dominant inheritance of thoracic aortic aneurysms and dissections. At the same time, rare, nonsynonymous variants in MYH11 that are predicted to disrupt protein function but do not cause inherited aortic disease are common in the general population and the vascular disease risk associated with these variants is unknown.

OBJECTIVE

To determine the consequences of the recurrent MYH11 rare variant, R247C, through functional studies in vitro and analysis of a knock-in mouse model with this specific variant, including assessment of aortic contraction, response to vascular injury, and phenotype of primary aortic smooth muscle cells (SMCs).

METHODS AND RESULTS

The steady state ATPase activity (actin-activated) and the rates of phosphate and ADP release were lower for the R247C mutant myosin than for the wild-type, as was the rate of actin filament sliding in an in vitro motility assay. Myh11(R247C/R247C) mice exhibited normal growth, reproduction, and aortic histology but decreased aortic contraction. In response to vascular injury, Myh11(R247C/R247C) mice showed significantly increased neointimal formation due to increased SMC proliferation when compared with the wild-type mice. Primary aortic SMCs explanted from the Myh11(R247C/R247C) mice were dedifferentiated compared with wild-type SMCs based on increased proliferation and reduced expression of SMC contractile proteins. The mutant SMCs also displayed altered focal adhesions and decreased Rho activation, associated with decreased nuclear localization of myocardin-related transcription factor-A. Exposure of the Myh11(R247C/R247C) SMCs to a Rho activator rescued the dedifferentiated phenotype of the SMCs.

CONCLUSIONS

These results indicate that a rare variant in MYH11, R247C, alters myosin contractile function and SMC phenotype, leading to increased proliferation in vitro and in response to vascular injury.

Mechanisms of disease: hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is the most-common monogenically inherited form of heart disease, characterized by thickening of the left ventricular wall, contractile dysfunction, and potentially fatal arrhythmias. HCM is also the most-common cause of sudden cardiac death in individuals younger than 35 years of age. Much progress has been made in the elucidation of the genetic basis of HCM, resulting in the identification of more than 900 individual mutations in over 20 genes. Interestingly, most of these genes encode sarcomeric proteins, such as myosin-7 (also known as cardiac muscle β-myosin heavy chain; MYH7), cardiac myosin-binding protein C (MYBPC3), and cardiac muscle troponin T (TNNT2). However, the molecular events that ultimately lead to the clinical phenotype of HCM are still unclear. We discuss several potential pathways, which include altered calcium cycling and sarcomeric calcium sensitivity, increased fibrosis, disturbed biomechanical stress sensing, and impaired cardiac energy homeostasis. An improved understanding of the pathological mechanisms involved will result in greater specificity and success of therapies for patients with HCM.

How do mutations in contractile proteins cause the primary familial cardiomyopathies?

In this article, the available evidence about the functional effects of the contractile protein mutations that cause hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) is assessed. The molecular mechanism of the contractile apparatus of cardiac muscle and its regulation by Ca(2+) and PKA phosphorylation have been extensively studied. Therefore, when a number of point mutations in the contractile protein genes were found to cause the well-defined phenotypes of HCM and DCM, it was expected that the diseases could be explained at the molecular level. However, the search for a distinctive molecular phenotype did not yield rapid results. Now that a substantial number of mutations that cause HCM or DCM have been investigated in physiologically relevant systems and with a range of experimental techniques, a pattern is emerging. In the case of HCM, the hypothesis that the major effect of mutations is to increase myofibrillar Ca(2+)-sensitivity seems to be well established, but the mechanisms by which an increase in myofibrillar Ca(2+)-sensitivity induces hypertrophy remain obscure. In contrast, DCM mutations are not correlated with a specific effect on Ca(2+)-sensitivity. It has recently been proposed that DCM mutations uncouple troponin I phosphorylation from Ca(2+)-sensitivity changes, albeit based on only a few mutations so far. A plausible link between uncoupling and DCM has been proposed via blunting of the response to α-adrenergic stimulation.

Hypertrophic cardiomyopathy: from genetics to treatment

BACKGROUND

Hypertrophic cardiomyopathy (HCM) is the prototypic form of pathological cardiac hypertrophy. HCM is an important cause of sudden cardiac death in the young and a major cause of morbidity in the elderly.

DESIGN

We discuss the clinical implications of recent advances in the molecular genetics of HCM.

RESULTS

The current diagnosis of HCM is neither adequately sensitive nor specific. Partial elucidation of the molecular genetic basis of HCM has raised interest in genetic-based diagnosis and management. Over a dozen causal genes have been identified. MYH7 and MYBPC3 mutations account for about 50% of cases. The remaining known causal genes are uncommon and some are rare. Advances in DNA sequencing techniques have made genetic screening practical. The difficulty, particularly in the sporadic cases and in small families, is to discern the causal from the non-causal variants. Overall, the causal mutations alone have limited implications in risk stratification and prognostication, as the clinical phenotype arises from complex and often non-linear interactions between various determinants.

CONCLUSIONS

The clinical phenotype of 'HCM' results from mutations in sarcomeric proteins and subsequent activation of multiple cellular constituents including signal transducers. We advocate that HCM, despite its current recognition and management as a single disease entity, involves multiple partially independent mechanisms, despite similarity in the ensuing phenotype. To treat HCM effectively, it is necessary to delineate the underlying fundamental mechanisms that govern the pathogenesis of the phenotype and apply these principles to the treatment of each subset of clinically recognized HCM.

Myosin individualized: single nucleotide polymorphisms in energy transduction

BACKGROUND

Myosin performs ATP free energy transduction into mechanical work in the motor domain of the myosin heavy chain (MHC). Energy transduction is the definitive systemic feature of the myosin motor performed by coordinating in a time ordered sequence: ATP hydrolysis at the active site, actin affinity modulation at the actin binding site, and the lever-arm rotation of the power stroke. These functions are carried out by several conserved sub-domains within the motor domain. Single nucleotide polymorphisms (SNPs) affect the MHC sequence of many isoforms expressed in striated muscle, smooth muscle, and non-muscle tissue. The purpose of this work is to provide a rationale for using SNPs as a functional genomics tool to investigate structurefunction relationships in myosin. In particular, to discover SNP distribution over the conserved sub-domains and surmise what it implies about sub-domain stability and criticality in the energy transduction mechanism.

RESULTS

An automated routine identifying human nonsynonymous SNP amino acid missense substitutions for any MHC gene mined the NCBI SNP data base. The routine tested 22 MHC genes coding muscle and non-muscle isoforms and identified 89 missense mutation positions in the motor domain with 10 already implicated in heart disease and another 8 lacking sequence homology with a skeletal MHC isoform for which a crystallographic model is available. The remaining 71 SNP substitutions were found to be distributed over MHC with 22 falling outside identified functional sub-domains and 49 in or very near to myosin sub-domains assigned specific crucial functions in energy transduction. The latter includes the active site, the actin binding site, the rigid lever-arm, and regions facilitating their communication. Most MHC isoforms contained SNPs somewhere in the motor domain.

CONCLUSIONS

Several functional-crucial sub-domains are infiltrated by a large number of SNP substitution sites suggesting these domains are engineered by evolution to be too-robust to be disturbed by otherwise intrusive sequence changes. Two functional sub-domains are SNP-free or relatively SNP-deficient but contain many disease implicated mutants. These sub-domains are apparently highly sensitive to any missense substitution suggesting they have failed to evolve a robust sequence paradigm for performing their function.

Rescue of familial cardiomyopathies by modifications at the level of sarcomere and Ca2+ fluxes

Cardiomyopathies are a heterogeneous group of diseases of the myocardium associated with mechanical and/or electrical dysfunction that frequently show inappropriate ventricular hypertrophy or dilation. Current data suggest that numerous mutations in several genes can cause cardiomyopathies, and the severity of their phenotypes is also influenced by modifier genes. Two major types of inherited cardiomyopathies include familial hypertrophic cardiomyopathy (FHC) and dilated cardiomyopathy (DCM). FHC typically involves increased myofilament Ca(2+) sensitivity associated with diastolic dysfunction, whereas DCM often results in decreased myofilament Ca(2+) sensitivity and systolic dysfunction. Besides alterations in myofilament Ca(2+) sensitivity, alterations in the levels of Ca(2+)-handling proteins have also been described in both diseases. Recent work in animal models has attempted to rescue FHC and DCM via modifications at the myofilament level, altering Ca(2+) homeostasis by targeting Ca(2+)-handling proteins, such as the sarcoplasmic reticulum ATPase and phospholamban, or by interfering with the products of different modifiers genes. Although attempts to rescue cardiomyopathies in animal models have shown great promise, further studies are needed to validate these strategies in order to provide more effective and specific treatments.

Experimental therapies in hypertrophic cardiomyopathy

The quintessential clinical diagnostic phenotype of human hypertrophic cardiomyopathy (HCM) is primary cardiac hypertrophy. Cardiac hypertrophy is also a major determinant of mortality and morbidity including the risk of sudden cardiac death (SCD) in patients with HCM. Reversal and attenuation of cardiac hypertrophy and its accompanying fibrosis is expected to improve morbidity as well as decrease the risk of SCD in patients with HCM.The conventionally used pharmacological agents in treatment of patients with HCM have not been shown to reverse or attenuate established cardiac hypertrophy and fibrosis. An effective treatment of HCM has to target the molecular mechanisms that are involved in the pathogenesis of the phenotype. Mechanistic studies suggest that cardiac hypertrophy in HCM is secondary to activation of various hypertrophic signaling molecules and, hence, is potentially reversible. The hypothesis is supported by the results of genetic and pharmacological interventions in animal models. The results have shown potential beneficial effects of angiotensin II receptor blocker losartan, mineralocorticoid receptor blocker spironolactone, 3-hydroxy-3-methyglutaryl-coenzyme A reductase inhibitors simvastatin and atorvastatin, and most recently, N-acetylcysteine (NAC) on reversal or prevention of hypertrophy and fibrosis in HCM. The most promising results have been obtained with NAC, which through multiple thiol-responsive mechanisms completely reversed established cardiac hypertrophy and fibrosis in three independent studies. Pilot studies with losartan and statins in humans have established the feasibility of such studies. The results in animal models have firmly established the reversibility of established cardiac hypertrophy and fibrosis in HCM and have set the stage for advancing the findings in the animal models to human patients with HCM through conducting large-scale efficacy studies.

Decreased contractility due to energy deprivation in a transgenic rat model of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is associated with cardiac hypertrophy, diastolic dysfunction, and sudden death. Recently, it has been suggested that inefficient energy utilization could be a common molecular pathway of HCM-related mutations. We have previously generated transgenic Sprague-Dawley rats overexpressing a truncated cardiac troponin T (DEL-TNT) molecule, displaying typical features of HCM such as diastolic dysfunction and an increased susceptibility to ventricular arrhythmias. We now studied these rats using 31P magnetic resonance spectroscopy (MRS). MRS demonstrated that cardiac energy metabolism was markedly impaired, as indicated by a decreased phosphocreatine to ATP ratio (-31%, p < 0.05). In addition, we assessed contractility of isolated cardiomyocytes. While DEL-TNT and control cardiomyocytes showed no difference under baseline conditions, DEL-TNT cardiomyocytes selectively exhibited a decrease in fractional shortening by 28% after 1 h in glucose-deprived medium (p < 0.05). Moreover, significant decreases in contraction velocity and relaxation velocity were observed. To identify the underlying molecular pathways, we performed transcriptional profiling using real-time PCR. DEL-TNT hearts exhibited induction of several genes critical for cardiac energy supply, including CD36, CPT-1/-2, and PGC-1alpha. Finally, DEL-TNT rats and controls were studied by radiotelemetry after being stressed by isoproterenol, revealing a significantly increased frequency of arrhythmias in transgenic animals. In summary, we demonstrate profound energetic alterations in DEL-TNT hearts, supporting the notion that inefficient cellular ATP utilization contributes to the pathogenesis of HCM.

The familial hypertrophic cardiomyopathy-associated myosin mutation R403Q accelerates tension generation and relaxation of human cardiac myofibrils

The R403Q mutation in beta-myosin heavy chain was the first mutation to be identified as responsible for familial hypertrophic cardiomyopathy (FHC). In spite of extensive work on the functional sequelae of this mutation, the mechanism by which the mutant protein causes the disease has not been definitely identified. Here we directly compare contraction and relaxation mechanics of single myofibrils from left ventricular samples of one patient carrying the R403Q mutation to those from a healthy control heart. Tension generation and relaxation following sudden increase and decrease in [Ca(2+)] were much faster in the R403Q myofibrils with relaxation rates being the most affected parameters. The results show that the R403Q mutation leads to an apparent gain of protein function but a greater energetic cost of tension generation. Increased energy cost of tension generation may be central to the FHC disease process, help explain some unresolved clinical observations, and carry significant therapeutic implications.

Functional effects of the hypertrophic cardiomyopathy R403Q mutation are different in an alpha- or beta-myosin heavy chain backbone

The R403Q mutation in the beta-myosin heavy chain (MHC) was the first mutation to be linked to familial hypertrophic cardiomyopathy (FHC), a primary disease of heart muscle. The initial studies with R403Q myosin, isolated from biopsies of patients, showed a large decrease in myosin motor function, leading to the hypothesis that hypertrophy was a compensatory response. The introduction of the mouse model for FHC (the mouse expresses predominantly alpha-MHC as opposed to the beta-isoform in larger mammals) created a new paradigm for FHC based on finding enhanced motor function for R403Q alpha-MHC. To help resolve these conflicting mechanisms, we used a transgenic mouse model in which the endogenous alpha-MHC was largely replaced with transgenically encoded beta-MHC. A His(6) tag was cloned at the N terminus of the alpha-and beta-MHC to facilitate protein isolation by Ni(2+)-chelating chromatography. Characterization of the R403Q alpha-MHC by the in vitro motility assay showed a 30-40% increase in actin filament velocity compared with wild type, consistent with published studies. In contrast, the R403Q mutation in a beta-MHC backbone showed no enhancement in velocity. Cleavage of the His-tagged myosin by chymotrypsin made it possible to isolate homogeneous myosin subfragment 1 (S1), uncontaminated by endogenous myosin. We find that the actin-activated MgATPase activity for R403Q alpha-S1 is approximately 30% higher than for wild type, whereas the enzymatic activity for R403Q beta-S1 is reduced by approximately 10%. Thus, the functional consequences of the mutation are fundamentally changed depending upon the context of the cardiac MHC isoform.

Structural effects of the slow/b-cardiac myosin heavy chain R453C mutation in cardiac and skeletal muscle

OBJECTIVES

Hypertrophic cardiomyopathy (HCM) represents an important cause of sudden cardiac death particularly in otherwise healthy young individuals. In some families, HCM is caused by distinct mutations of the cardiac beta myosin heavy chain gene (MYH7).

DESIGN

We have analyzed the expression of the malignant MYH7Arg453Cys mutation, in cardiac and skeletal muscle, and related it to morphological alterations.

RESULTS

Morphological investigation revealed hypertrophic cardiomyocytes but regularly arranged myofibrils. Skeletal muscle showed no sign of structural alterations.

CONCLUSIONS

Our results indicate that cardiomyocyte hypertrophy is secondary, due to impaired function, and that the mutation causes no structural alteration in myofibrillar structure in cardiac or skeletal muscle.

Myosin dynamics on the millisecond time scale

Myosin is a motor protein associating with actin and ATP. It translates along actin filaments against a force by transduction of free energy liberated with ATP hydrolysis. Various myosin crystal structures define time points during ATPase showing the protein undergoes large conformation change during transduction over a cycle with approximately 10 ms periodicity. The protein conformation trajectory between two intermediates in the cycle is surmised by non-equilibrium Monte Carlo simulation utilizing free-energy minimization. The trajectory shows myosin transduction of free energy to mechanical work giving evidence for: (i) a causal relationship between product release and work production in the native isoform that is correctly disrupted in a chemically modified protein, (ii) the molecular basis of ATP-sensitive tryptophan fluorescence enhancement and acrylamide quenching, (iii) an actin-binding site peptide containing the free-energy barrier to ATPase product release defining the rate limiting step and, (iv) a scenario for actin-activation of myosin ATPase.

The R403Q myosin mutation implicated in familial hypertrophic cardiomyopathy causes disorder at the actomyosin interface

Background

Mutations in virtually all of the proteins comprising the cardiac muscle sarcomere have been implicated in causing Familial Hypertrophic Cardiomyopathy (FHC). Mutations in the β-myosin heavy chain (MHC) remain among the most common causes of FHC, with the widely studied R403Q mutation resulting in an especially severe clinical prognosis. In vitro functional studies of cardiac myosin containing the R403Q mutation have revealed significant changes in enzymatic and mechanical properties compared to wild-type myosin. It has been proposed that these molecular changes must trigger events that ultimately lead to the clinical phenotype.

Principal Findings

Here we examine the structural consequences of the R403Q mutation in a recombinant smooth muscle myosin subfragment (S1), whose kinetic features have much in common with slow β-MHC. We obtained three-dimensional reconstructions of wild-type and R403Q smooth muscle S1 bound to actin filaments in the presence (ADP) and absence (apo) of nucleotide by electron cryomicroscopy and image analysis. We observed that the mutant S1 was attached to actin at highly variable angles compared to wild-type reconstructions, suggesting a severe disruption of the actin-myosin interaction at the interface.

Significance

These results provide structural evidence that disarray at the molecular level may be linked to the histopathological myocyte disarray characteristic of the diseased state.

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.

Hypertrophic and dilated cardiomyopathy mutations differentially affect the molecular force generation of mouse α-cardiac myosin in the laser trap assay

Point mutations in cardiac myosin, the heart's molecular motor, produce distinct clinical phenotypes: hypertrophic (HCM) and dilated (DCM) cardiomyopathy. Do mutations alter myosin's molecular mechanics in a manner that is predictive of the clinical outcome? We have directly characterized the maximal force-generating capacity (Fmax) of two HCM (R403Q, R453C) and two DCM (S532P, F764L) mutant myosins isolated from homozygous mouse models using a novel load-clamped laser trap assay. Fmaxwas 50% (R403Q) and 80% (R453C) greater for the HCM mutants compared with the wild type, whereas Fmaxwas severely depressed for one of the DCM mutants (65% S532P). Although Fmaxwas normal for the F764L DCM mutant, its actin-activated ATPase activity and actin filament velocity ( Vactin) in a motility assay were significantly reduced (Schmitt JP, Debold EP, Ahmad F, Armstrong A, Frederico A, Conner DA, Mende U, Lohse MJ, Warshaw D, Seidman CE, Seidman JG. Proc Natl Acad Sci USA 103: 14525–14530, 2006.). These Fmaxdata combined with previous Vactinmeasurements suggest that HCM and DCM result from alterations to one or more of myosin's fundamental mechanical properties, with HCM-causing mutations leading to enhanced but DCM-causing mutations leading to depressed function. These mutation-specific changes in mechanical properties must initiate distinct signaling cascades that ultimately lead to the disparate phenotypic responses observed in HCM and DCM.

Functional studies of individual myosin molecules

The "conventional" isoform of myosin that polymerizes into filaments (myosin II) is the molecular motor powering contraction in all three types of muscle. Considerable attention has been paid to the developmental progression, isoform distribution, and mutations that affect myocardial development, function, and adaptation. Optical trap (laser tweezer) experiments and various types of high-resolution fluorescence microscopy, capable of interrogating individual protein motors, are revealing novel and detailed information about their functionally relevant nanometer motions and pico-Newton forces. Single-molecule laser tweezer studies of cardiac myosin isoforms and their mutants have helped to elucidate the pathogenesis of familial hypertrophic cardiomyopathies. Surprisingly, some disease mutations seem to enhance myosin function. More broadly, the myosin superfamily includes more than 20 nonfilamentous members with myriad cellular functions, including targeted organelle transport, endocytosis, chemotaxis, cytokinesis, modulation of sensory systems, and signal transduction. Widely varying genetic, developmental and functional disorders of the nervous, pigmentation, and immune systems have been described in accordance with these many roles. Compared to the collective nature of myosin II, some myosin family members operate with only a few partners or even alone. Individual myosin V and VI molecules can carry cellular vesicular cargoes much farther distances than their own size. Laser tweezer mechanics, single-molecule fluorescence polarization, and imaging with nanometer precision have elucidated the very different mechano-chemical properties of these isoforms. Critical contributions of nonsarcomeric myosins to myocardial development and adaptation are likely to be discovered in future studies, so these techniques and concepts may become important in cardiovascular research.

Sarcomeric proteins and familial hypertrophic cardiomyopathy: linking mutations in structural proteins to complex cardiovascular phenotypes

Hypertrophic Cardiomyopathy (HCM) is a relatively common primary cardiac disorder defined as the presence of a hypertrophied left ventricle in the absence of any other diagnosed etiology. HCM is the most common cause of sudden cardiac death in young people which often occurs without precedent symptoms. The overall clinical phenotype of patients with HCM is broad, ranging from a complete lack of cardiovascular symptoms to exertional dyspnea, chest pain, and sudden death, often due to arrhythmias. To date, 270 independent mutations in nine sarcomeric protein genes have been linked to Familial Hypertrophic Cardiomyopathy (FHC), thus the clinical variability is matched by significant genetic heterogeneity. While the final clinical phenotype in patients with FHC is a result of multiple factors including modifier genes, environmental influences and genotype, initial screening studies had suggested that individual gene mutations could be linked to specific prognoses. Given that the sarcomeric genes linked to FHC encode proteins with known functions, a vast array of biochemical, biophysical and physiologic experimental approaches have been applied to elucidate the molecular mechanisms that underlie the pathogenesis of this complex cardiovascular disorder. In this review, to illustrate the basic relationship between protein dysfunction and disease pathogenesis we focus on representative gene mutations from each of the major structural components of the cardiac sarcomere: the thick filament (beta MyHC), the thin filament (cTnT and Tm) and associated proteins (MyBP-C). The results of these studies will lead to a better understanding of FHC and eventually identify targets for therapeutic intervention.

Biological, biochemical, and kinetic effects of mutations of the cardiomyopathy loop of Dictyostelium myosin II: importance of ALA400

The cardiomyopathy (CM)-loop of the heavy chain of class-II myosins begins with a highly conserved Arg residue (whose mutation in human beta-cardiac myosin II results in familial hypertrophic cardiomyopathy). The CM-loop of Dictyostelium myosin II (Arg397-Gln407) is essential for its biological functions and biochemical activities. We found that the CM-loop of smooth muscle myosin II substituted partially, and the CM-loop of beta-cardiac myosin II less well, for growth, capping of surface receptors and development, and the actin-activated MgATPase and in vitro motility activities of purified myosins. There was little correlation between the biochemical and biological activities of the two chimeras and 19 point mutants, but only the five mutants with k cat/K actin values equivalent to wild-type myosin supported essentially full biological function. The three point mutations of Arg397 equivalent to those that result in hypertrophic cardiomyopathy in humans had minimal biological effects and different biochemical effects. The A400V mutation rendered full-length wild-type myosin almost completely inactive, both in vitro and in vivo, and the reverse V400A mutation in the cardiac CM-loop chimera restored almost full activity, even though the sequence still differed from wild-type in 7 of 11 positions. Transient kinetic studies of acto-subfragment-1 (S1) showed that the chimeras and the Ala/Val, Val/Ala mutations do not affect the equilibrium or the association and dissociation rate constants for either ATP or ADP binding to acto-S1 or the rate of ATP-induced dissociation of acto-S1. We conclude that the Ala/Val, Val/Ala mutations affect the release of Pi from acto-S1.ADP.Pi. In addition, Val at position 400 substantially reduces the affinity of actin for S1 in the absence of nucleotide.

Molecular and phenotypic effects of heterozygous, homozygous, and compound heterozygote myosin heavy-chain mutations

Autosomal dominant familial hypertrophic cardiomyopathy (FHC) has variable penetrance and phenotype. Heterozygous mutations in MYH7 encoding beta-myosin heavy chain are the most common causes of FHC, and we proposed that "enhanced" mutant actin-myosin function is the causative molecular abnormality. We have studied individuals from families in which members have two, one, or no mutant MYH7 alleles to examine for dose effects. In one family, a member homozygous for Lys207Gln had cardiomyopathy complicated by left ventricular dilatation, systolic impairment, atrial fibrillation, and defibrillator interventions. Only one of five heterozygous relatives had FHC. Leu908Val and Asp906Gly mutations were detected in a second family in which penetrance for Leu908Val heterozygotes was 46% (21/46) and 25% (3/12) for Asp906Gly. Despite the low penetrance, hypertrophy was severe in several heterozygotes. Two individuals with both mutations developed severe FHC. The velocities of actin translocation (V(actin)) by mutant and wild-type (WT) myosins were compared in the in vitro motility assay. Compared with WT/WT, V(actin) was 34% faster for WT/D906G and 21% for WT/L908V. Surprisingly V(actin) for Leu908Val/Asp906Gly and Lys207Gln/Lys207Gln mutants were similar to WT. The apparent enhancement of mechanical performance with mutant/WT myosin was not observed for mutant/mutant myosin. This suggests that V(actin) may be a poor predictor of disease penetrance or severity and that power production may be more appropriate, or that the limited availability of double mutant patients prohibits any definitive conclusions. Finally, severe FHC in heterozygous individuals can occur despite very low penetrance, suggesting these mutations alone are insufficient to cause FHC and that uncharacterized modifying mechanisms exert powerful influences.

Differential cross-bridge kinetics of FHC myosin mutations R403Q and R453C in heterozygous mouse myocardium

The kinetic effects of the cardiac myosin point mutations R403Q and R453C, which underlie lethal forms of familial hypertrophic cardiomyopathy (FHC), were assessed using isolated myosin and skinned strips taken from heterozygous (R403Q/+ and R453C/+) male mouse hearts. Compared with wild-type (WT) mice, actin-activated ATPase was increased by 38% in R403Q/+ and reduced by 45% in R453C/+, maximal velocity of regulated thin filament ( VRTF) in the in vitro motility assay was increased by 8% in R403Q/+ and was not different in R453C/+, myosin concentration at half-maximal VRTF was reduced by 30% in R403Q/+ and not different in R453C/+, and the characteristic frequency for oscillatory work production ( b frequency), determined by sinusoidal analysis in the skinned strip at maximal calcium activation, was 27% lower in R403Q/+ and 18% higher in R453C/+. The calcium sensitivity for isometric tension in the skinned strip was not different in R403Q/+ (pCa50 5.64 ± 0.02) and significantly enhanced in R453C/+ (5.82 ± 0.03) compared with WT (5.58 ± 0.02). We conclude that isolated myosin and skinned strips of R403Q/+ and R453C/+ myocardium show marked differences in cross-bridge kinetic parameters and in calcium sensitivity of force production that indicate different functional roles associated with the location of each point mutation at the molecular level.

The myosin cardiac loop participates functionally in the actomyosin interaction

The motor protein myosin in association with actin transduces chemical free energy in ATP into work in the form of actin translation against an opposing force. Mediating the actomyosin interaction in myosin is an actin binding site distributed among several peptides on the myosin surface including surface loops contributing to affinity and actin regulation of myosin ATPase. A structured surface loop on beta-cardiac myosin, the cardiac or C-loop, was recently demonstrated to affect myosin ATPase and was indirectly implicated in the actomyosin interaction. The C-loop is a conserved feature of all myosin isoforms with crystal structures, suggesting that it is an essential part of the core energy transduction machinery. It is shown here that proteolytic digestion of the C-loop in beta-cardiac myosin eliminates actin-activated myosin ATPase and reduces actomyosin affinity in rigor more than 100-fold. Studies of C-loop function in smooth muscle myosin were also undertaken using site-directed mutagenesis. Mutagenesis of a single charged residue in the C-loop of smooth muscle myosin alters actomyosin affinity and doubles myosin in vitro motility and actin-activated ATPase velocities, thereby involving a charged region of the loop in the actomyosin interaction. It appears likely that the C-loop is an essential electrostatic binding site for actin involved in modulation of actomyosin affinity and regulation of actomyosin ATPase velocity.

F-actin and myosin II binding domains in supervillin

Detergent-resistant membranes contain signaling and integral membrane proteins that organize cholesterol-rich domains called lipid rafts. A subset of these detergent-resistant membranes (DRM-H) exhibits a higher buoyant density ( approximately 1.16 g/ml) because of association with membrane skeleton proteins, including actin, myosin II, myosin 1G, fodrin, and an actin- and membrane-binding protein called supervillin (Nebl, T., Pestonjamasp, K. N., Leszyk, J. D., Crowley, J. L., Oh, S. W., and Luna, E. J. (2002) J. Biol. Chem. 277, 43399-43409). To characterize interactions among DRM-H cytoskeletal proteins, we investigated the binding partners of the novel supervillin N terminus, specifically amino acids 1-830. We find that the supervillin N terminus binds directly to myosin II, as well as to F-actin. Three F-actin-binding sites were mapped to sequences within amino acids approximately 280-342, approximately 344-422, and approximately 700-830. Sequences with combinations of these sites promote F-actin cross-linking and/or bundling. Supervillin amino acids 1-174 specifically interact with the S2 domain in chicken gizzard myosin and nonmuscle myosin IIA (MYH-9) but exhibit little binding to skeletal muscle myosin II. Direct or indirect binding to filamin also was observed. Overexpression of supervillin amino acids 1-174 in COS7 cells disrupted the localization of myosin IIB without obviously affecting actin filaments. Taken together, these results suggest that supervillin may mediate actin and myosin II filament organization at cholesterol-rich membrane domains.

Hypertrophic cardiomyopathy:a paradigm for myocardial energy depletion

Genetic analysis of hypertrophic cardiomyopathy (HCM), a mendelian form of cardiac hypertrophy, indicates that the primary defect is in sarcomeric function. However, the initial proposal that depressed myocardial contraction leads to a 'compensatory' hypertrophy has proven inconsistent with laboratory and clinical evidence. Drawing on observations of mutant contractile protein function, together with mouse models and clinical studies, we propose that sarcomeric HCM mutations lead to inefficient ATP utilization. The suggestion that energy depletion underlies HCM is supported by the HCM-like phenotype found with mutations in a variety of metabolic genes. A central role for compromised energetics would also help explain the unresolved clinical observations of delayed onset and asymmetrical hypertrophy in HCM, and would have implications for therapy in HCM and, potentially, in more-common forms of cardiac hypertrophy and failure.

Heterologous expression of wild-type and mutant beta-cardiac myosin changes the contractile kinetics of cultured mouse myotubes

The properties of myosin expressed in muscle are a major determinant of muscle performance. In this study we used a novel approach to examine the functional impact of changes in myosin heavy chain (MHC) isoform expression, as well as the consequences of expressing the mutant MHC implicated in familial hypertrophic cardiomyopathy (FHC). Cultured mouse myoblasts that normally express fast embryonic myosin were untransfected, or stably transfected with a plasmid expressing either wild-type (cWT) or mutant (D778G or G741R) beta-cardiac myosin. After differentiation for 5-7 days, cWT or mutant beta-cardiac myosin was expressed at 25 % of total myosin in the myotube. We measured time-to-peak shortening (ttp), time for half-relaxation (t0.5), the maximum velocity of shortening (Vmax) at 1 Hz stimulation, and the tetanic fusion frequency. Expression of cWT beta-cardiac myosin significantly increased ttp and t0.5 and decreased the fusion frequency compared with untransfected myotubes. However, when we compared myotubes expressing mutant beta-cardiac myosin with those expressing cWT beta-cardiac myosin, we found that ttp and t0.5 were significantly decreased, and Vmax was increased for the D778G mutant, whereas ttp, t0.5 and Vmax were unchanged for the G741R mutant. The fusion frequency was increased for both mutant myosins. Our data support the conclusion that the impact of the slower myosin isoform dominates when both slow and fast isoforms are present. This work suggests that FHC associated with either D778G or G741R mutation in MHC is an 'energy cost' disease, but that the phenotype of D778G is more severe than that of G741R.

Hypertrophic cardiomyopathy: from gene defect to clinical disease

Major advances have been made over the last decade in our understanding of the molecular basis of several cardiac conditions. Hypertrophic cardiomyopathy (HCM) was the first cardiac disorder in which a genetic basis was identified and as such, has acted as a paradigm for the study of an inherited cardiac disorder. HCM can result in clinical symptoms ranging from no symptoms to severe heart failure and premature sudden death. HCM is the commonest cause of sudden death in those aged less than 35 years, including competitive athletes. At least ten genes have now been identified, defects in which cause HCM. All of these genes encode proteins which comprise the basic contractile unit of the heart, i.e. the sarcomere. While much is now known about which genes cause disease and the various clinical presentations, very little is known about how these gene defects cause disease, and what factors modify the expression of the mutant genes. Studies in both cell culture and animal models of HCM are now beginning to shed light on the signalling pathways involved in HCM, and the role of both environmental and genetic modifying factors. Understanding these mechanisms will ultimately improve our knowledge of the basic biology of heart muscle function, and will therefore provide new avenues for treating cardiovascular disease in man.

[Molecular genetics of cardiomyopathies]

In the last decade our understanding of cardiac pathophysiology has experienced significant advances linked to major advances in molecular genetics. Although many genes are associated today with cardiac diseases, the genetics of both hypertrophic cardiomyopathy and dilated cardiomyopathy have generated great interest. The familial nature of the disease in some patients has been very useful in this regard. In addition, there are also excellent experimental models to study the implications of the genetic abnormalities. Altogether the study of the molecular genetics of the cardiomyopathies should provide not only prognostic information but also new therapeutic alternatives.

Cardiomiopatia hipertrófica, atividade física e morte súbita

A morte súbita em atletas é um evento dramático e tem sido demonstrado que, na maioria das vezes, se deve a doenças cardiovasculares congênitas ou adquiridas. O objetivo deste artigo é abordar alguns aspectos da cardiomiopatia hipertrófica, que é a principal causa de morte súbita em atletas jovens, e sugerir medidas para prevenir essa condição.

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.

Molecular genetics and pathogenesis of hypertrophic cardiomyopathy

Advances in molecular genetics of hypertrophic cardiomyopathy (HCM) have led to identification of mutations in 11 genes coding for sarcomeric proteins. In addition, mutations in gene coding for the gamma subunit of AMP-activated protein kinase and triplet-repeat syndromes, as well as in mitochondrial DNA have been identified in patients with HCM. Mutations in genes coding for the beta-myosin heavy chain, myosin binding protein-C, and cardiac troponin T account for approximately 2/3 of all HCM cases. Accordingly, HCM is considered a disease of contractile sarcomeric proteins. Genotype-phenotype correlation studies show mutations and the genetic background affect the phenotypic expression of HCM. The final phenotype is the result of interactions between the causal genes, genetic background (modifier genes), and probably the environmental factors. The molecular pathogenesis of HCM is not completely understood. The initial defects caused by the mutant proteins are diverse. However, despite their diversity, they converge into common final pathway of impaired cardiac myocyte function. The latter leads to an increased myocyte stress and subsequent activation of stress-responsive signaling kinases and trophic factors, which activate the transcriptional machinery inducing cardiac hypertrophy, interstitial fibrosis and myocyte disarray, the pathological characteristics of HCM. Studies in transgenic animal models show that cardiac hypertrophy, interstitial fibrosis, and myocyte disarray are potentially reversible. These findings raise the possibility of reversal of evolving phenotype or prevention of phenotypes in human patients with HCM. Elucidation of the molecular genetic basis and the pathogenesis of HCM could provide the opportunity for genetic based diagnosis, risk stratification, and implementation of preventive and therapeutic measures in those who have inherited the causal mutations for HCM.

R403Q and L908V mutant beta-cardiac myosin from patients with familial hypertrophic cardiomyopathy exhibit enhanced mechanical performance at the single molecule level

Familial hypertrophic cardiomyopathy (FHC) is a disease of the sarcomere. In the beta-myosin heavy chain gene, which codes for the mechanical enzyme myosin, greater than 40 point mutations have been found that are causal for this disease. We have studied the effect of two mutations, the R403Q and L908V, on myosin molecular mechanics. In the in vitro motility assay, the mutant myosins produced a 30% greater velocity of actin filament movement (v(actin)). At the single molecule level, v(actin) approximately d/t(on), where d is the myosin unitary step displacement and t(on) is the step duration. Laser trap studies were performed at 10 microM MgATP to estimate d and t(on) for the normal and mutant myosin molecules. The increase in v(actin) can be explained by a significant decrease in the average t(on)'s in both the R403Q and L908V mutants (approximately 30 ms) compared to controls (approximately 40 ms), while d was not different for all myosins tested (approximately 7 nm). Thus the mutations affect the kinetics of the cross-bridge cycle without any effect on myosin's inherent motion and force generating capacity. Based on these studies, the primary signal for the hypertrophic response appears to be an apparent gain in function of the individual mutant myosin molecules.

The molecular genetic basis for hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM), a relatively common disease, is diagnosed clinically by unexplained cardiac hypertrophy and pathologically by myocyte hypertrophy, disarray, and interstitial fibrosis. HCM is the most common cause of sudden cardiac death (SCD) in the young and a major cause of morbidity and mortality in elderly. Hypertrophy and fibrosis are the major determinants of morbidity and SCD. More than 100 mutations in nine genes, all encoding sarcomeric proteins have been identified in patients with HCM, which had led to the notion that HCM is a disease of contractile sarcomeric proteins. The beta -myosin heavy chain (MyHC), cardiac troponin T (cTnT) and myosin binding protein-C (MyBP-C) are the most common genes accounting for approximately 2/3 of all HCM cases. Genotype-phenotype correlation studies suggest that mutations in the beta -MyHC gene are associated with more extensive hypertrophy and a higher risk of SCD as compared to mutations in genes coding for other sarcomeric proteins, such as MyBP-C and cTnT. The prognostic significance of mutations is related to their hypertrophic expressivity and penetrance, with the exception of those in the cTnT, which are associated with mild hypertrophic response and a high incidence of SCD. However, there is a significant variability and factors, such as modifier genes and probably the environmental factors affect the phenotypic expression of HCM. The molecular pathogenesis of HCM is not completely understood. In vitro and in vivo studies suggest that mutations impart a diverse array of functional defects including reduced ATPase activity of myosin, acto-myosin interaction, cross-bridging kinetics, myocyte contractility, and altered Ca2+ sensitivity. Hypertrophy and other clinical and pathological phenotypes are considered compensatory phenotypes secondary to functional defects. In summary, the molecular genetic basis of HCM has been identified, which affords the opportunity to delineate its pathogenesis. Understanding the pathogenesis of HCM could provide for genetic based diagnosis, risk stratification, treatment and prevention of cardiac phenotypes.

A novel missense mutation, Leu390Val, in the cardiac beta-myosin heavy chain associated with pronounced septal hypertrophy in two families with hypertrophic cardiomyopathy

OBJECTIVE

An examination of the genetic background and phenotypic presentation of familial hypertrophic cardiomyopathy (FHC) with respect to specific mutations in the MYH7-gene encoding the cardiac beta-myosin heavy chain.

SETTINGS

Two families (n = 22) from a cohort of 67 families with FHC were studied at the National University Hospital, Rigshospitalet, Copenhagen.

METHODS

Clinical, non-invasive examinations of all included family members followed by molecular genetic analysis including PCR-single strand conformation polymorphism/heteroduplex (SSCP/HD) analysis and sequencing of exon 3-23 of the MYH7-gene.

RESULTS

We found FHC associated with a missense mutation in two families, i.e. a C > G transversion at position g10124 and a G > T transversion at position g10126 causing the change of a leucine residue at codon 390 to a valine residue. The mutation is located in the actin-binding region of the beta-myosin heavy chain. The leucine residue is evolutionarily conserved in vertebrate myosins. In the two families, the phenotypic presentations in the clinically affected were characterized by asymmetric septal hypertrophy (septum diameter 18.8 (5.0) mm (mean (SD)) with only minor involvement of the left ventricular free wall (posterior wall diameter 11.0 (2.2) mm). Furthermore, the left ventricular systolic and diastolic functions were well preserved, even at a high age. The symptomatic status of the clinically affected patients depended on the presence or absence of a concomitant left ventricular outflow tract gradient.

CONCLUSIONS

We report a novel missense mutation associated with FHC caused by a double nucleotide transversion. The penetrance of the mutation was not complete, but in clinically affected patients the mutation gives rise to an echocardiographic phenotype, predominantly characterized by pronounced septal hypertrophy.

Inactivation of myosin heavy chain genes in the mouse: diverse and unexpected phenotypes

Myosin heavy chain (MyHC) is a critical component of the cellular contractile apparatus. The mammalian genome contains two nonmuscle, two smooth muscle, and eight striated muscle isoforms of MyHC. Within each class of genes, there is extremely high sequence homology among different MyHC isoforms, raising the question of whether these isoforms are functionally redundant or whether they perform unique roles in cell function. Recently, strains of mice null for four different MyHC isoforms have been generated. Mice null for the nonmuscle II-B isoform experience significant prenatal lethality and surviving animals have several cardiac abnormalities [Tullio et al. (1997) Proc Natl Acad Sci USA 94:12407-12412]. Mice homozygous null for alpha cardiac MyHC are embryonic lethal, while heterozygous mice are viable but also have numerous cardiac defects [Jones et al. (1996) J Clin Invest 98:1906-1917]. Mice null for IIb or IId adult skeletal MyHC are viable but have skeletal muscle abnormalities compared to wild type mice, despite compensation of a neighboring MyHC gene [Acakpo-Satchivi et al. (1997) J Cell Biol 139:1219-1229]. Both IIb and IId null mice show significant decreases in body mass. Mean muscle mass is also significantly decreased in both null strains but the extent and the pattern of affected muscles differs between the two strains. Both strains show evidence of skeletal muscle pathology but again the pattern and extent differ between the two strains. Finally, both adult skeletal strains demonstrate distinct impairments in contractile function when compared to wild type. Together these observations support the hypothesis that the different isoforms of MyHC are functionally unique and cannot substitute for one another.