A missense mutation in a ubiquitously expressed protein, vinculin, confers susceptibility to hypertrophic cardiomyopathy

The R975W mutation, in the alternatively spliced exon 19 of vinculin (VCL) which yields the isoform metavinculin, was associated previously with hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM), and shown to alter in vivo organization of intercalated discs. We tested the hypothesis that alterations in the ubiquitously expressed, VCL-encoded protein, vinculin, may provide a pathogenic substrate for HCM. Comprehensive mutational analysis of VCL's 22 translated exons was performed in a cohort of 228 unrelated patients with genotype negative HCM, having no identifiable mutations in 12 HCM-associated myofilament/Z-disc-encoding genes. A novel missense mutation, L277M-VCL, involving a conserved residue was identified in a patient with severely obstructive, mid-ventricular hypertrophy. This mutation was not detected in 400 reference alleles. Immunohistochemical analysis of the proband's myectomy specimen demonstrated markedly reduced vinculin levels in the intercalated discs. We provide the first report of a cardiomyopathy associated mutation in vinculin. Despite its ubiquitous expression, the HCM-associated VCL mutation clinically yielded a cardiac-specific phenotype.     

Upregulation of redox-regulating protein,thioredoxin, in endomyocardial biopsy samples of patients with myocarditis and cardiomyopathies

An important role of redox regulation in myocardial diseases and heart failure has been postulated. Thioredoxin (TRX) is a redox-regulating protein. Recent studies indicated a possible association between plasma TRX concentrations and the severity of heart failure. Accordingly, we investigated the myocardial expression of TRX in patients with myocarditis and cardiomyopathies. Four cases of hypertrophic cardiomyopathy (HCM), 10 of dilated cardiomyopathy (DCM), 6 of myocarditis, and 5 of controls were studied. Right and left ventricular endomyocardial biopsy samples were obtained at the diagnostic cardiac catheterization. The samples were processed for immunohistological staining for TRX, which was done by the indirect immunoperoxidase technique. 8-hydoxy-2-deoxyguanosine (8-OHdG), one of the major DNA base-modified products, was also detected for an established marker for oxidative stress. TRX immunoreactivity was none or trivial in control specimens. Positive TRX staining was found in 6 cases; 3 in active myocarditis and 3 in DCM. The positive staining was found in infiltrating cells and damaged myocytes in the perinecrotic lesions. Damaged myocytes were also positive for 8-OHdG. All the 3 cases of DCM positive for TRX stain showed severe left ventricular hypertrophy on electrocardiogram and highly elevated left ventricular end-diastolic pressure (> 24 mmHg), suggesting the overload of oxidative stress by hemodynamic impairment. Myocardial TRX was upregulated in myocarditis and cardiomyopathies with active necrotic stage associated with DNA damage, which may reflect the oxidative stress overload in hemodynamically uncontrolled status.     

Cardiac expression of urocortin (Ucn) in diseased heart; preliminary results on possible involvement of Ucn in pathophysiology of cardiac diseases

Recently, several studies reported that urocortin (Ucn) had beneficial effects on cardiovascular system and was expressed both in the normal heart and in the heart of dilated cardiomyopathy (DCM), yet the relationship between high expression of Ucn and pathophysiology of Ucn in diseased heart has been discussed. Thus, the present study was designed to elucidate the expression of Ucn in the diseased heart by immunohistochemical approach using endomyocardial biopsy specimens. The involvement of immunoreactive Ucn in pathophysiology of cardiac disease was evaluated using endomyocardial biopsy specimens obtained from the patients with some heart diseases, including DCM and hypertrophic cardiomyopathy (HCM). Ucn was detected in all endomyocardial biopsy specimens of ventricular tissue obtained from the patients with such cardiac diseases, a specimens of atrial tissue, and normal heart specimens obtained from autopsy cases. In DCM patients, left ventricular end-diastolic pressure significantly elevated in severely stained group. On the contrary, in HCM patients, left ventricular ejection fraction was higher in the severely stained group. Ucn was expressed more abundantly in the diseased heart, especially in HCM and DCM, than in the normal heart. In conclusion, such close relationship between Ucn expression in the heart and cardiac function indicated that clinical features of Ucn resembled those of norepinephrine and Ucn could play a certain pathophysiological roles in the cardiac diseases.     

Hypertrophic cardiomyopathy: two homozygous cases with "typical" hypertrophic cardiomyopathy and three new mutations in cases with progression to dilated cardiomyopathy

About 10% of cases of hypertrophic cardiomyopathy (HCM) evolve into dilated cardiomyopathy (DCM) with unknown causes. We studied 11 unrelated patients (pts) with HCM who progressed to DCM (group A) and 11 who showed "typical" HCM (group B). Mutational analysis of the beta-myosin heavy chain (MYH7), myosin-binding protein C (MYBPC3), and cardiac troponin T (TNNT2) genes demonstrated eight mutations affecting MYH7 or MYBPC3 gene, five of which were new mutations. In group A-pts, the first new mutation occurred in the myosin head-rod junction and the second occurred in the light chain-binding site. The third new mutation leads to a MYBPC3 lacking titin and myosin binding sites. In group B, two pts with severe HCM carried two homozygous MYBPC3 mutations and one with moderate hypertrophy was a compound heterozygous for MYBPC3 gene. We identified five unreported mutations, potentially "malignant" defects as for the associated phenotypes, but no specific mutations of HCM/DCM.     

ndufa7 plays a critical role in cardiac hypertrophy

Cardiac hypertrophy is a common pathological change in patients with progressive cardiac function failure, which can be caused by hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM) or arterial hypertension. Despite years of study, there is still limited knowledge about the underlying molecular mechanisms for cardiac hypertrophy. NDUFA7, a subunit of NADH:ubiquinone oxidoreductase (complex I), has been reported to be a novel HCM associated gene. However, the biological role of NDUFA7 in heart remains unknown. In this study, we found that NDUFA7 exhibited high expression in the heart, and its level was significantly decreased in mice model of cardiac hypertrophy. Moreover, we demonstrated that ndufa7 knockdown in developing zebrafish embryos resulted in cardiac development and functional defects, associated with increased expression of pathological hypertrophy biomarkers nppa (ANP) and nppb (BNP). Mechanistic study demonstrated that ndufa7 depletion promoted ROS production and calcineurin signalling activation. Moreover, NDUFA7 depletion contributed to cardiac cell hypertrophy. Together, these results report for the first time that ndufa7 is implicated in pathological cardiac hypertrophy.     

Effect of rs4646994 polymorphism of angiotensin-converting enzyme on the risk of nonischemic cardiomyopathy

Background: Angiotensin-converting enzyme (ACE) gene polymorphisms have recently been shown to be associated with risk of developing left ventricular hypertrophy (LVH). However, the results were controversial. We aimed to conduct this meta-analysis to further confirm the association between ACE rs4646994 polymorphism and hypertrophic cardiomyopathy (HCM)/dilated cardiomyopathy (DCM).Methods: PubMed, Embase, the Chinese National Knowledge Information, and Wanfang databases were searched for eligible studies. The Newcastle–Ottawa Scale (NOS) was used to evaluate the quality of included studies. Then we evaluated the association between ACE gene mutation and HCM/DCM by calculating odds ratios (ORs) and 95% confidence intervals (95% CIs). Subgroup analysis was further performed to explore situations in specialized subjects. Sensitivity analysis and publication bias was assessed to confirm the study reliability.Results: There were 13 studies on DCM (2004 cases and 1376 controls) and 16 studies on HCM (2161 controls and 1192 patients). ACE rs4646994 polymorphism was significantly associated with DCM in all genetic models. However, in HCM, four genetic models (allele model, homozygous model, heterozygous model, and dominant model) showed significant association between ACE rs4646994 polymorphism and DCM. In subgroup analysis, we found that ACE rs4646994 polymorphism was significantly associated with DCM/HCM in Asian population. Finally, we also conducted a cumulative meta-analysis, which indicates that the results of our meta-analysis are highly reliable.Conclusion: ACE rs4646994 polymorphism increases the risk of DCM/HCM in Asians, but not in Caucasians. More case–control studies are needed to strengthen our conclusions and to assess the gene–gene and gene–environment interactions between ACE rs4646994 polymorphism and DCM/HCM.     

Progression from hypertrophic to dilated cardiomyopathy in mice that express a mutant myosin transgene

A mouse model of hypertrophic cardiomyopathy (HCM) was created by expression of a cardiac alpha-myosin transgene including the R(403)Q mutation and a deletion of a segment of the actin-binding domain. HCM mice show early histopathology and hypertrophy, with progressive hypertrophy in females and ventricular dilation in older males. To test the hypothesis that dilated cardiomyopathy (DCM) is part of the pathological spectrum of HCM, we studied chamber morphology, exercise tolerance, hemodynamics, isolated heart function, adrenergic sensitivity, and embryonic gene expression in 8- to 11-mo-old male transgenic animals. Significantly impaired exercise tolerance and both systolic and diastolic dysfunction were seen in vivo. Contraction and relaxation parameters of isolated hearts were also decreased, and lusitropic responsiveness to the beta-adrenergic agonist isoproterenol was modestly reduced. Myocardial levels of the G protein-coupled beta-adrenergic receptor kinase 1 (beta-ARK1) were increased by more than twofold over controls, and total beta-ARK1 activity was also significantly elevated. Induction of fetal gene expression was also observed in transgenic hearts. We conclude that transgenic male animals have undergone cardiac decompensation resulting in a DCM phenotype. This supports the idea that HCM and DCM may be part of a pathological continuum rather than independent diseases.     

Evaluation of ultrasound lung comets by hand-held echocardiography

Background

Most patients with hypertrophic cardiomyopathy (HCM) have asymmetric septal hypertrophy and among them, 25% present dynamic subaortic obstruction. Apical HCM is unusual and mid-ventricular HCM is the most infrequent presentation, but both variants may be associated to an apical aneurysm. An even more rare presentation is the coexistece mid-ventricular and apical HCM. This case is a combination of obstructive HCM with mid-ventricular HCM and an apical aneurysm, which to date, has not been reported in the literature.

Case presentation

The patient is a 49 year-old lady who presents a combination of septal asymmetric hypertrophic cardiomyopathy (HCM) and midventricular HCM, a subaortic gradient of 65 mm Hg and a midventricular gradient of 20 mm Hg, plus an apical aneurysm. Her clinical presentation was an acute myocardial infarction in June 2005. One month after hospital discharge, the electrocardiogram (ECG) showed a right bundle branch block (RBBB) with no Q waves or ST segment elevation. Coronary angiography revealed normal coronary arteries, left ventricular hypertrophy and an apical aneurysm.

Conclusion

This case is a rare example of an asymptomatic patient with subaortic and mid-ventricular hypertrophic cardiomyopathy, who presents a myocardial infarction and normal coronary arteries, and during the course of her disease develops an apical aneurysm.     

Comparative biochemical analysis suggests that vinculin and metavinculin cooperate in muscular adhesion sites

Metavinculin, the muscle-specific splice variant of the cell adhesion protein vinculin, is characterized by a 68-amino acid insert within the C-terminal tail domain. The findings that mutations within this region correlate with hereditary idiopathic dilated cardiomyopathy in man suggest a specific contribution of metavinculin to the molecular architecture of muscular actin-membrane attachment sites, the nature of which, however, is still unknown. In mice, metavinculin is expressed in smooth and skeletal muscle, where it co-localizes with vinculin in dense plaques and costameres, respectively, but is of conspicuously low abundance in the heart. Immunoprecipitates suggest that both isoforms are present in the same complex. On the molecular level, both vinculin isoforms are regulated via an intramolecular head-tail interaction, with the metavinculin tail domain having a lower affinity for the head as compared with the vinculin tail. In addition, metavinculin displays impaired binding to acidic phospholipids and reduced homodimerization. Only in the presence of phospholipid-activated vinculin tail, the metavinculin tail domain is readily incorporated into heterodimers. Mutational analysis revealed that the metavinculin insert significantly alters binding of the C-terminal hairpin loop to acidic phospholipids. In summary, our data lead to a model in which unfurling of the metavinculin tail domain is impaired by the negative charges of the 68-amino acid insert, thus requiring vinculin to fully activate the metavinculin molecule. As a consequence, microfilament anchorage may be modulated at muscular adhesion sites through heterodimer formation.     

Genetic variation in exon 5 of troponin - I gene in hypertrophic cardiomyopathy cases

BACKGROUND:

Cardiomyopathies are a heterogeneous group of heart muscle disorders and are classified as 1) Hypertrophic Cardiomyopathy (HCM) 2) Dilated cardiomyopathy (DCM) 3) Restrictive cardiomyopathy (RCM) and 4) Arrhythmogenic right ventricular dysplasia (ARVD) as per WHO classification, of which HCM and DCM are common. HCM is a complex but relatively common form of inherited heart muscle disease with prevalence of 1 in 500 individuals and is commonly associated with sarcomeric gene mutations. Cardiac muscle troponin I (TNNI-3) is one such sarcomeric protein and is a subunit of the thin filament-associated troponin-tropomyosin complex involved in calcium regulation of skeletal and cardiac muscle contraction. Mutations in this gene were found to be associated with a history of sudden cardiac death in HCM patients.

AIM:

Therefore the present study aims to identify for mutations associated with troponin I gene in a set of HCM patients from Indian population.

MATERIALS AND METHODS:

Mutational analyses of 92 HCM cases were carried out following PCR based SSCP analysis.

RESULTS:

The study revealed band pattern variation in 3 cases from a group of 92 HCM patients. This band pattern variation, on sequencing revealed base changes, one at nt 2560 with G>T transversion in exon-5 region with a wobble and others at nt 2479 and nt 2478 with G>C and C>G transversions in the intronic region upstream of the exon 5 on sequencing. Further analysis showed that one of the probands showed apical form of hypertrophy, two others showing asymmetric septal hypertrophy. Two of these probands showed family history of the condition.

CONCLUSIONS:

Hence, the study supports earlier reports of involvement of TNNI-3 in the causation of apical and asymmetrical forms of hypertrophy.     

Metavinculin: New insights into functional properties of a muscle adhesion protein

Metavinculin is a muscle-specific splice variant of the ubiquitously expressed cytoskeletal adaptor protein vinculin. Both proteins are thought to be co-expressed in all muscle types where they co-localize to microfilament-associated adhesion sites. It has been shown that a metavinculin-specific insertion of 68 amino acids alters the biochemical properties of the five-helix bundle in the tail domain. Here, we demonstrate that the metavinculin-specific helix H1′ plays an important role for protein stability of the tail domain, since a point mutation in this helix, R975W, which is associated with the occurrence of dilated cardiomyopathy in man, further decreases thermal stability of the metavinculin tail domain. In striated muscle progenitor cells (myoblasts), both, metavinculin and the R975W mutant show significantly reduced, albeit distinctive residency and exchange rates in adhesion sites as compared to vinculin. In contrast to previous studies, we show that metavinculin is localized in a muscle fiber type-dependent fashion to the costameres of striated muscle, reflecting the individual metabolic and physiological status of a given muscle fiber. Metavinculin expression is highest in fast, glycolytic muscle fibers and virtually absent in M. diaphragmaticus, a skeletal muscle entirely lacking fast, glycolytic fibers. In summary, our data suggest that metavinculin enrichment in attachment sites of muscle cells leads to higher mechanical stability of adhesion complexes allowing for greater shear force resistance.     

The enhancement of Raf-1 kinase activity by knockdown of Spry2 is associated with high sensitivity to paclitaxel in v-Ha-ras-transformed NIH 3T3 fibroblasts

The aim of the current study was to determine the frequency of mutations in the beta-myosin heavy chain gene (MYH7) in a cohort of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) and their families, and to investigate correlations between genotype and phenotype. About 130 consecutive patients diagnosed with HCM or DCM (69 with HCM and 61 with DCM) attending the cardiology clinic of Post Graduate Institute of Medical Education and Research were screened for mutations in the MYH7 gene. The control group for genetic studies consisted of 100 healthy subjects. We report 14 mutations in 6 probands (5 probands in HCM and 1 proband in DCM) and their family members. Out of these 6 mutations, 3 are new and are being reported for the first time. One known mutation (p.Gly716Arg) was found to be "de novo" which resulted in severe asymmetric septal hypertrophy (31 mm) and resulted in the sudden cardiac death (SCD) of the proband at the age of 21 years. Further, a DCM causing novel mutation p.Gly377Ser was identified which resulted in the milder phenotype. The present study shows that there is genetic and phenotypic heterogeneity of cardiomyopathies in Indian population. Further, the location and type of mutation in a given sarcomeric gene determines the severity and phenotypic plasticity in cardiomyopathies.     

Identification and functional analysis of a caveolin-3 mutation associated with familial hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are caused by mutations in 14 and 15 different disease genes, respectively, in a part of the patients and the disease genes for cardiomyopathy overlap in part with that for limb-girdle muscular dystrophy (LGMD). In this study, we examined an LGMD gene encoding caveolin-3 (CAV3) for mutation in the patients with HCM or DCM. A Thr63Ser mutation was identified in a sibling case of HCM. Because the mutation was found at the residue that is involved in the LGMD-causing mutations, we investigate the functional change due to the Thr63Ser mutation as compared with the LGMD mutations by examining the distribution of GFP-tagged CAV3 proteins. It was observed that the Thr63Ser mutation reduced the cell surface expression of caveolin-3, albeit the change was mild as compared with the LGMD mutations. These observations suggest that HCM is a clinical spectrum of CAV3 mutations.     

Adaptations of cytoarchitecture in human dilated cardiomyopathy

Hypertrophic cardiomyopathy is characterised by a histological phenotype of myocyte disarray, but heart tissue samples from patients with dilated cardiomyopathy (DCM) often look comparatively similar to those from healthy individuals apart from conspicuous regions of fibrosis and necrosis. We have previously investigated subcellular alterations in the cytoarchitecture of mouse models of dilated cardiomyopathy and found that both the organisation and composition of the intercalated disc, i.e. the specialised type of cell–cell contact in the heart, is altered. There is also is a change in the composition of the M-band of the sarcomere due to an expression shift towards the more extensible embryonic heart (EH)-myomesin isoform. Analysis of human samples from the Sydney Human Heart Tissue Bank have revealed similar structural findings and also provided evidence for a dramatic change in overall cardiomyocyte size control, which has also been seen in the mouse. Together these changes in cytoarchitecture probably contribute to the decreased functional output that is seen in DCM.     

Myocyte cytoskeletal disorganization and right heart failure in hypoxia-induced neonatal pulmonary hypertension

Previous studies have demonstrated that environmentally or genetically induced changes in the intracellular proteins that compose the cytoskeleton can contribute to heart failure. Because neonatal right ventricular myocytes are immature and are in the process of significant cytoskeletal change, we hypothesized that they may be particularly susceptible to pressure stress. Newborn calves exposed to hypobaric hypoxia (barometric pressure = 430 mmHg) for 14 days developed severe pulmonary hypertension (pulmonary arterial pressure = 101 +/- 6 vs. 27 +/- 1 mmHg) and right heart failure compared with age-matched controls. Light microscopy showed partial loss of myocardial striations in the failing neonatal right but not left ventricles and in neither ventricle of adolescent cattle dying of altitude-induced right heart failure. In neonatal calves, immunohistochemical analysis of the cytoskeletal proteins (vinculin, metavinculin, desmin, vimentin, and cadherin) showed selectively, within the failing right ventricles, patchy areas characterized by loss and disorganization of costameres and intercalated discs. Within myocytes from the failing ventricles, vinculin and desmin were observed to redistribute diffusely within the cytosol, metavinculin appeared in disorganized clumps, and vimentin immunoreactivity was markedly decreased. Western blot analysis of the failing right ventricular myocardium showed, compared with control, vinculin and desmin to be little changed in total content but redistributed from insoluble (structural) to soluble (cytosolic) fractions; metavinculin total content was markedly decreased, tubulin content increased, particularly in the structural fraction, and cadherin total content and distribution were unchanged. We conclude that hypoxic pulmonary hypertensive-induced neonatal right ventricular failure is associated with disorganization of the cytoskeletal architecture.     

Biventricular / left ventricular pacing in hypertrophic obstructive cardiomyopathy: an overview

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant inherited genetic disease characterized by compensatory pathological left ventricle (LV) hypertrophy due to sarcomere dysfunction. In an important proportion of patients with HCM, the site and extent of cardiac hypertrophy results in severe obstruction to LV outflow tract (LVOT), contributing to disabling symptoms and increasing the risk of sudden cardiac death (SCD). In patients with progressive and/or refractory symptoms despite optimal pharmacological treatment, invasive therapies that diminish or abolish LVOT obstruction relieve heart failure-related symptoms, improve quality of life and could be associated with long-term survival similar to that observed in the general population. The gold standard in this respect is surgical septal myectomy, which might be supplementary associated with a reduction in SCD. Percutaneous techniques, particularly alcohol septal ablation (ASA) and more recently radiofrequency (RF) septal ablation, can achieve LVOT gradient reduction and symptomatic benefit in a large proportion of HOCM patients at the cost of a supposedly limited septal myocardial necrosis and a 10-20% risk of chronic atrioventricular block. After an initial period of enthusiasm, standard DDD pacing failed to show in randomized trials significant LVOT gradient reductions and objective improvement in exercise capacity. However, case reports and recent small pilot studies suggested that atrial synchronous LV or biventricular (biV) pacing significantly reduce LVOT obstruction and improve symptoms (acutely as well as long-term) in a large proportion of severely symptomatic HOCM patients not suitable to other gradient reduction therapies. Moreover, biV/LV pacing in HOCM seems to be associated with significant LV reverse remodelling.     

Actin mutations in hypertrophic and dilated cardiomyopathy cause inefficient protein folding and perturbed filament formation

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are the most common hereditary cardiac conditions. Both are frequent causes of sudden death and are often associated with an adverse disease course. Alpha-cardiac actin is one of the disease genes where different missense mutations have been found to cause either HCM or DCM. We have tested the hypothesis that the protein-folding pathway plays a role in disease development for two actin variants associated with DCM and six associated with HCM. Based on a cell-free coupled translation assay the actin variants could be graded by their tendency to associate with the chaperonin TCP-1 ring complex/chaperonin containing TCP-1 (TRiC/CCT) as well as their propensity to acquire their native conformation. Some variant proteins are completely stalled in a complex with TRiC and fail to fold into mature globular actin and some appear to fold as efficiently as the wild-type protein. A fraction of the translated polypeptide became ubiquitinated and detergent insoluble. Variant actin proteins overexpressed in mammalian cell lines fail to incorporate into actin filaments in a manner correlating with the degree of misfolding observed in the cell-free assay; ranging from incorporation comparable to wild-type actin to little or no incorporation. We propose that effects of mutations on folding and fiber assembly may play a role in the molecular disease mechanism.     

Mammoth interatrial septal aneurysm in the ICE age

Background

Subaortic and midventricular hypertrophic cardiomyopathy in a patient with extreme segmental hypertrophy exceeding the usual maximum wall thickness reported in the literature is a rare phenomenon.

Case Presentation

A 19-year-old man with recently diagnosed hypertrophic cardiomyopathy (HCM) was referred for sudden death risk assessment. The patient had mild exertional dyspnea (New York Heart Association functional class II), but without syncope or chest pain. There was no family history of HCM or sudden death. A two dimensional echocardiogram revealed an asymmetric type of LV hypertrophy; anterior ventricular septum = 49 mm; posterior ventricular septum = 20 mm; anterolateral free wall = 12 mm; and posterior free wall = 6 mm. The patient had 2 types of obstruction; a LV outflow obstruction due to systolic anterior motion of both mitral leaflets (Doppler-estimated 38 mm Hg gradient at rest); and a midventricular obstruction (Doppler-estimated 43 mm Hg gradient), but without apical aneurysm or dyskinesia. The patient had a normal blood pressure response on exercise test and no episodes of non-sustained ventricular tachycardia in 24-h ECG recording. Cardiac MRI showed a gross late enhancement at the hypertrophied septum. Based on the extreme degree of LV hypertrophy and the myocardial hyperenhancement, an implantation of a cardioverter-defibrillator was recommended prophylactically for primary prevention of sudden death.

Conclusion

Midventricular HCM is an infrequent phenotype, but may be associated with an apical aneurysm and progression to systolic dysfunction (end-stage HCM).     

Cardiomyopathy Mutations in Metavinculin Disrupt Regulation of Vinculin-Induced F-Actin Assemblies

Debilitating heart conditions, notably dilated and hypertrophic cardiomyopathies (CMs), are associated with point mutations in metavinculin, a larger isoform of the essential cytoskeletal protein vinculin. Metavinculin is co-expressed with vinculin at sub-stoichiometric ratios in cardiac tissues. CM mutations in the metavinculin tail domain (MVt) occur within the extra 68-residue insert that differentiates it from the vinculin tail domain (Vt). Vt binds actin filaments (F-actin) and promotes vinculin dimerization to bundle F-actin into thick fibers. While MVt binds to F-actin in a similar manner to Vt, MVt is incapable of F-actin bundling and inhibits Vt-mediated F-actin bundling. We performed F-actin co-sedimentation and negative-stain EM experiments to dissect the coordinated roles of metavinculin and vinculin in actin fiber assembly and the effects of three known metavinculin CM mutations. These CM mutants were found to weakly induce the formation of disordered F-actin assemblies. Notably, they fail to inhibit Vt-mediated F-actin bundling and instead promote formation of large assemblies embedded with linear bundles. Computational models of MVt bound to F-actin suggest that MVt undergoes a conformational change licensing the formation of a protruding sub-domain incorporating the insert, which sterically prevents dimerization and bundling of F-actin by Vt. Sub-domain formation is destabilized by CM mutations, disrupting this inhibitory mechanism. These findings provide new mechanistic insights into the ability of metavinculin to tune actin organization by vinculin and suggest that dysregulation of this process by CM mutants could underlie their malfunction in disease.