A troponin T mutation that causes infantile restrictive cardiomyopathy increases Ca2+ sensitivity of force development and impairs the inhibitory properties of troponin

Impaired Relaxation in Induced Pluripotent Stem Cell-Derived Cardiomyocytes with Pathogenic TNNI3 Mutation of Pediatric Restrictive Cardiomyopathy

BACKGROUND

Restrictive cardiomyopathy (RCM) is characterized by impaired diastolic function with preserved ventricular contraction. Several pathogenic variants in sarcomere genes, including TNNI3, are reported to cause Ca2+ hypersensitivity in cardiomyocytes in overexpression models; however, the pathophysiology of induced pluripotent stem cell (iPSC)-derived cardiomyocytes specific to a patient with RCM remains unknown.

METHODS AND RESULTS

We established an iPSC line from a pediatric patient with RCM and a heterozygous TNNI3 missense variant, c.508C>T (p.Arg170Trp; R170W). We conducted genome editing via CRISPR/Cas9 technology to establish an isogenic correction line harboring wild type TNNI3 as well as a homozygous TNNI3-R170W. iPSCs were then differentiated to cardiomyocytes to compare their cellular physiological, structural, and transcriptomic features. Cardiomyocytes differentiated from heterozygous and homozygous TNNI3-R170W iPSC lines demonstrated impaired diastolic function in cell motion analyses as compared with that in cardiomyocytes derived from isogenic-corrected iPSCs and 3 independent healthy iPSC lines. The intracellular Ca2+ oscillation and immunocytochemistry of troponin I were not significantly affected in RCM-cardiomyocytes with either heterozygous or homozygous TNNI3-R170W. Electron microscopy showed that the myofibril and mitochondrial structures appeared to be unaffected. RNA sequencing revealed that pathways associated with cardiac muscle development and contraction, extracellular matrix-receptor interaction, and transforming growth factor-β were altered in RCM-iPSC-derived cardiomyocytes.

CONCLUSIONS

Patient-specific iPSC-derived cardiomyocytes could effectively represent the diastolic dysfunction of RCM. Myofibril structures including troponin I remained unaffected in the monolayer culture system, although gene expression profiles associated with cardiac muscle functions were altered.

DIA-Based Proteomic Analysis Reveals MYOZ2 as a Key Protein Affecting Muscle Growth and Development in Hybrid Sheep

Hybridization of livestock can be used to improve varieties, and different hybrid combinations produce unique breeding effects. In this study, male Southdown and Suffolk sheep were selected to hybridize with female Hu sheep to explore the effects of male parentage on muscle growth and the development of offspring. Using data-independent acquisition technology, we identified 119, 187, and 26 differentially abundant proteins (DAPs) between Hu × Hu (HH) versus Southdown × Hu (NH), HH versus Suffolk × Hu (SH), and NH versus SH crosses. Two DAPs, MYOZ2 and MYOM3, were common to the three hybrid groups and were mainly enriched in muscle growth and development-related pathways. At the myoblast proliferation stage, MYOZ2 expression decreased cell viability and inhibited proliferation. At the myoblast differentiation stage, MYOZ2 expression promoted myoblast fusion and enhanced the level of cell fusion. These findings provide new insights into the key proteins and metabolic pathways involved in the effect of male parentage on muscle growth and the development of hybrid offspring in sheep.

Focus on Paediatric Restrictive Cardiomyopathy: Frequently Asked Questions

Restrictive cardiomyopathy (RCM) is characterized by restrictive ventricular pathophysiology determined by increased myocardial stiffness. While suspicion of RCM is initially raised by clinical evaluation and supported by electrocardiographic and echocardiographic findings, invasive hemodynamic evaluation is often required for diagnosis and management of patients during follow-up. RCM is commonly associated with a poor prognosis and a high incidence of heart failure, and PH is reported in paediatric patients with RCM. Currently, only a few therapies are available for specific RCM aetiologies. Early referral to centres for advanced heart failure treatment is often necessary. The aim of this review is to address questions frequently asked when facing paediatric patients with RCM, including issues related to aetiologies, clinical presentation, diagnostic process and prognosis.

Cardiac proteomic profiling suggests that hypertrophic and dilated cardiomyopathy share a common pathogenetic pathway of the calcium signalling pathway

OBJECTIVE

Hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM) are classified as different diseases but have many similar pathogenic genes and clinical symptoms. Previous research has focused on mutated genes. This study was conducted to identify key molecular mechanisms and explore effective therapeutic targets.

METHODS

Myocardial tissue was harvested from patients with HCM (n = 3) or DCM (n = 4) during surgery. Hearts donated by healthy traffic accident victims were treated as controls (n = 4). Total proteins were extracted for liquid chromatography-tandem mass spectrometry. Differentially expressed proteins (DEPs) were annotated via GO and KEGG analyses. Selected distinguishing protein abundance was confirmed by western blotting.

RESULTS

Compared with the control group, there were 121 and 76 DEPs in the HCM and DCM groups, respectively. GO terms for these two comparisons are associated with contraction-related components and actin binding. Additionally, the most significantly upregulated and downregulated proteins were periostin and tropomyosin alpha-3 chain in both comparisons. Moreover, when comparing the HCM and DCM groups, we found 60 significant DEPs, and the GO and KEGG terms are related to the calcium signalling pathway. Expression of the calcium regulation-related protein peptidyl-prolyl cis-trans isomerase (FKBP1A) was significantly upregulated in multiple samples.

CONCLUSION

HCM and DCM have many mutual pathogenetic pathways. Calcium ion-related processes are among the most significant factors affecting disease development. For HCM and DCM, research on regulating linchpin protein expression or interfering with key calcium-related pathways may be more beneficial than genetic research.

Negative Charges Introduced Near the IT Helix of Cardiac Troponin T Stabilize the Active State of Actin Filaments

The disordered and basic C-terminal 14 residues of human troponin T (TnT) are essential for full inhibition of actomyosin ATPase activity at low Ca2+ levels and for limiting activation at saturating Ca2+. In previous studies, stepwise truncation of the C-terminal region of TnT increased activity in proportion to the number of positive charges eliminated. To define key basic residues more closely, we generated phosphomimetic-like mutants of TnT. Phosphomimetic mutants were chosen because of reports that phosphorylation of TnT, including sites within the C terminal region, depressed activity, contrary to our expectations. Four constructs were made where one or more Ser and Thr residues were replaced with Asp residues. The S275D and T277D mutants, near the IT helix and adjacent to basic residues, produced the greatest activation of ATPase rates in solution; the effects of the S275D mutant were recapitulated in muscle fiber preparations with enhanced myofilament Ca2+ sensitivity. Actin filaments containing S275D TnT were also shown to be incapable of populating the inactive state at low Ca2+ levels. Actin filaments containing both S275D/T284D were not statistically different from those containing only S275D in both solution and cardiac muscle preparation studies. Finally, actin filaments containing T284D TnT, closer to the C-terminus and not adjacent to a basic residue, had the smallest effect on activity. Thus, the effects of negative charge placement in the C-terminal region of TnT were greatest near the IT helix and adjacent to a basic residue.

Atomic force microscopy identifies the alteration of rheological properties of the cardiac fibroblasts in idiopathic restrictive cardiomyopathy

Restrictive cardiomyopathy (RCM) is a rare disease characterized by increased ventricular stiffness and preserved ventricular contraction. Various sarcomere gene variants are known to cause RCM; however, more than a half of patients do not harbor such pathogenic variants. We recently demonstrated that cardiac fibroblasts (CFs) play important roles in inhibiting the diastolic function of cardiomyocytes via humoral factors and direct cell–cell contact regardless of sarcomere gene mutations. However, the mechanical properties of CFs that are crucial for intercellular communication and the cardiomyocyte microenvironment remain less understood. In this study, we evaluated the rheological properties of CFs derived from pediatric patients with RCM and healthy control CFs via atomic force microscopy. Then, we estimated the cellular modulus scale factor related to the cell stiffness, fluidity, and Newtonian viscosity of single cells based on the single power-law rheology model and analyzed the comprehensive gene expression profiles via RNA-sequencing. RCM-derived CFs showed significantly higher stiffness and viscosity and lower fluidity compared to healthy control CFs. Furthermore, RNA-sequencing revealed that the signaling pathways associated with cytoskeleton elements were affected in RCM CFs; specifically, cytoskeletal actin-associated genes (ACTN1, ACTA2, and PALLD) were highly expressed in RCM CFs, whereas several tubulin genes (TUBB3, TUBB, TUBA1C, and TUBA1B) were down-regulated. These results implies that the signaling pathways associated with cytoskeletal elements alter the rheological properties of RCM CFs, particularly those related to CF–cardiomyocyte interactions, thereby leading to diastolic cardiac dysfunction in RCM.

Cardiomyocyte Dysfunction in Inherited Cardiomyopathies

Inherited cardiomyopathies form a heterogenous group of disorders that affect the structure and function of the heart. Defects in the genes encoding sarcomeric proteins are associated with various perturbations that induce contractile dysfunction and promote disease development. In this review we aimed to outline the functional consequences of the major inherited cardiomyopathies in terms of myocardial contraction and kinetics, and to highlight the structural and functional alterations in some sarcomeric variants that have been demonstrated to be involved in the pathogenesis of the inherited cardiomyopathies. A particular focus was made on mutation-induced alterations in cardiomyocyte mechanics. Since no disease-specific treatments for familial cardiomyopathies exist, several novel agents have been developed to modulate sarcomere contractility. Understanding the molecular basis of the disease opens new avenues for the development of new therapies. Furthermore, the earlier the awareness of the genetic defect, the better the clinical prognostication would be for patients and the better the prevention of development of the disease.

A Heterozygous Mutation in Cardiac Troponin T Promotes Ca2+ Dysregulation and Adult Cardiomyopathy in Zebrafish

Cardiomyopathies are a group of heterogeneous diseases that affect the muscles of the heart, leading to early morbidity and mortality in young and adults. Genetic forms of cardiomyopathy are caused predominantly by mutations in structural components of the cardiomyocyte sarcomeres, the contractile units of the heart, which includes cardiac Troponin T (TnT). Here, we generated mutations with CRISPR/Cas9 technology in the zebrafish tnnt2a gene, encoding cardiac TnT, at a mutational “hotspot” site to establish a zebrafish model for genetic cardiomyopathies. We found that a heterozygous tnnt2a mutation deleting Arginine at position 94 and Lysine at position 95 of TnT causes progressive cardiac structural changes resulting in heart failure. The cardiac remodeling is presented by an enlarged atrium, decreased ventricle size, increased myocardial stress as well as increased fibrosis. As early as five days post fertilization, larvae carrying the TnT RK94del mutation display diastolic dysfunction and impaired calcium dynamics related to increased Ca²⁺ sensitivity. In conclusion, adult zebrafish with a heterozygous TnT-RK94del mutation develop cardiomyopathy as seen in patients with TnT mutations and therefore represent a promising model to study disease mechanisms and to screen for putative therapeutic compounds.

Cardiac Fibroblasts Play Pathogenic Roles in Idiopathic Restrictive Cardiomyopathy

BACKGROUND

Restrictive cardiomyopathy (RCM) is characterized by impaired ventricular relaxation. Although several mutations were reported in some patients, no mutations were identified in cardiomyocyte expressing genes of other patients, indicating that pathological mechanisms underlying RCM could not be determined by cardiomyocytes only. Cardiac fibroblasts (CFs) are a major cell population in the heart; however, the pathological roles of CFs in cardiomyopathy are not fully understood.Methods and Results:This study established 4 primary culture lines of CFs from RCM patients and analyzed their cellular physiology, the effects on the contraction and relaxation ability of healthy cardiomyocytes under co-culture with CFs, and RNA sequencing. Three of four patients hadTNNI3mutations. There were no significant alterations in cell proliferation, apoptosis, migration, activation, and attachment. However, when CFs from RCM patients were co-cultured with healthy cardiomyocytes, the relaxation velocity of cardiomyocytes was significantly impaired both under direct and indirect co-culture conditions. RNA sequencing revealed that gene expression profiles of CFs in RCM were clearly distinct from healthy CFs. The differential expression gene analysis identified that several extracellular matrix components and cytokine expressions were dysregulated in CFs from RCM patients.

CONCLUSIONS

The comprehensive gene expression patterns were altered in RCM-derived CFs, which deteriorated the relaxation ability of cardiomyocytes. The specific changes in extracellular matrix composition and cytokine secretion from CFs might affect pathological behavior of cardiomyocytes in RCM.

Genetic Restrictive Cardiomyopathy: Causes and Consequences-An Integrative Approach

The sarcomere as the smallest contractile unit is prone to alterations in its functional, structural and associated proteins. Sarcomeric dysfunction leads to heart failure or cardiomyopathies like hypertrophic (HCM) or restrictive cardiomyopathy (RCM) etc. Genetic based RCM, a very rare but severe disease with a high mortality rate, might be induced by mutations in genes of non-sarcomeric, sarcomeric and sarcomere associated proteins. In this review, we discuss the functional effects in correlation to the phenotype and present an integrated model for the development of genetic RCM.

Mutation location of HCM-causing troponin T mutations defines the degree of myofilament dysfunction in human cardiomyocytes

BACKGROUND

The clinical outcome of hypertrophic cardiomyopathy patients is not only determined by the disease-causing mutation but influenced by a variety of disease modifiers. Here, we defined the role of the mutation location and the mutant protein dose of the troponin T mutations I79N, R94C and R278C.

METHODS AND RESULTS

We determined myofilament function after troponin exchange in permeabilized single human cardiomyocytes as well as in cardiac patient samples harboring the R278C mutation. Notably, we found that a small dose of mutant protein is sufficient for the maximal effect on myofilament Ca2+-sensitivity for the I79N and R94C mutation while the mutation location determines the magnitude of this effect. While incorporation of I79N and R94C increased myofilament Ca2+-sensitivity, incorporation of R278C increased Ca2+-sensitivity at low and intermediate dose, while it decreased Ca2+-sensitivity at high dose. All three cTnT mutants showed reduced thin filament binding affinity, which coincided with a relatively low maximal exchange (50.5 ± 5.2%) of mutant troponin complex in cardiomyocytes. In accordance, 32.2 ± 4.0% mutant R278C was found in two patient samples which showed 50.0 ± 3.7% mutant mRNA. In accordance with studies that showed clinical variability in patients with the exact same mutation, we observed variability on the functional single cell level in patients with the R278C mutation. These differences in myofilament properties could not be explained by differences in the amount of mutant protein.

CONCLUSIONS

Using troponin exchange in single human cardiomyocytes, we show that TNNT2 mutation-induced changes in myofilament Ca2+-sensitivity depend on mutation location, while all mutants show reduced thin filament binding affinity. The specific mutation-effect observed for R278C could not be translated to myofilament function of cardiomyocytes from patients, and is most likely explained by other (post)-translational troponin modifications. Overall, our studies illustrate that mutation location underlies variability in myofilament Ca2+-sensitivity, while only the R278C mutation shows a highly dose-dependent effect on myofilament function.

Pediatric Restrictive Cardiomyopathies

Restrictive cardiomyopathy (RCM) is the least frequent phenotype among pediatric heart muscle diseases, representing only 2.5-3% of all cardiomyopathies diagnosed during childhood. Pediatric RCM has a poor prognosis, high incidence of pulmonary hypertension (PH), thromboembolic events, and sudden death, is less amenable to medical or surgical treatment with high mortality rates. In this scenario, heart transplantation remains the only successful therapeutic option. Despite a shared hemodynamic profile, characterized by severe diastolic dysfunction and restrictive ventricular filling, with normal ventricle ejection fraction and wall thickness, RCM recognizes a broad etiological spectrum, consisting of genetic/familial and acquired causes, each of which has a distinct pathophysiology and natural course. Hence, the aim of this review is to cover the causes, clinical presentation, diagnostic evaluation, treatment, and prognosis of pediatric RCM.

Eliminating the First Inactive State and Stabilizing the Active State of the Cardiac Regulatory System Alters Behavior in Solution and in Ordered Systems

Calcium binding to troponin C (TnC) is insufficient for full activation of myosin ATPase activity by actin-tropomyosin-troponin. Previous attempts to investigate full activation utilized ATP-free myosin or chemically modified myosin to stabilize the active state of regulated actin. We utilized the Δ14-TnT and the A8V-TnC mutants to stabilize the activated state at saturating Ca²⁺ and to eliminate one of the inactive states at low Ca²⁺. The observed effects differed in solution studies and in the more ordered in vitro motility assay and in skinned cardiac muscle preparations. At saturating Ca²⁺, full activation with Δ14-TnT·A8V-TnC decreased the apparent K_M for actin-activated ATPase activity compared to bare actin filaments. Rates of in vitro motility increased at both high and low Ca²⁺ with Δ14-TnT; the maximum shortening speed at high Ca²⁺ increased 1.8-fold. Cardiac muscle preparations exhibited increased Ca²⁺ sensitivity and large increases in resting force with either Δ14-TnT or Δ14-TnT·A8V-TnC. We also observed a significant increase in the maximal rate of tension redevelopment. The results of full activation with Ca²⁺ and Δ14-TnT·A8V-TnC confirmed and extended several earlier observations using other means of reaching full activation. Furthermore, at low Ca²⁺, elimination of the first inactive state led to partial activation. This work also confirms, in three distinct experimental systems, that troponin is able to stabilize the active state of actin-tropomyosin-troponin without the need for high-affinity myosin binding. The results are relevant to the reason for two inactive states and for the role of force producing myosin in regulation.

The intrinsically disordered C terminus of troponin T binds to troponin C to modulate myocardial force generation

Aberrant regulation of myocardial force production represents an early biomechanical defect associated with sarcomeric cardiomyopathies, but the molecular mechanisms remain poorly defined. Here, we evaluated the pathogenicity of a previously unreported sarcomeric gene variant identified in a pediatric patient with sporadic dilated cardiomyopathy, and we determined a molecular mechanism. Trio whole-exome sequencing revealed a de novo missense variant in TNNC1 that encodes a p.I4M substitution in the N-terminal helix of cardiac troponin C (cTnC). Reconstitution of this human cTnC variant into permeabilized porcine cardiac muscle preparations significantly decreases the magnitude and rate of isometric force generation at physiological Ca2+-activation levels. Computational modeling suggests that this inhibitory effect can be explained by a decrease in the rates of cross-bridge attachment and detachment. For the first time, we show that cardiac troponin T (cTnT), in part through its intrinsically disordered C terminus, directly binds to WT cTnC, and we find that this cardiomyopathic variant displays tighter binding to cTnT. Steady-state fluorescence and NMR spectroscopy studies suggest that this variant propagates perturbations in cTnC structural dynamics to distal regions of the molecule. We propose that the intrinsically disordered C terminus of cTnT directly interacts with the regulatory N-domain of cTnC to allosterically modulate Ca2+ activation of force, perhaps by controlling the troponin I switching mechanism of striated muscle contraction. Alterations in cTnC-cTnT binding may compromise contractile performance and trigger pathological remodeling of the myocardium.

Basic residues within the cardiac troponin T C terminus are required for full inhibition of muscle contraction and limit activation by calcium

Striated muscle is activated by myosin- and actin-linked processes, with the latter being regulated through changes in the position of tropomyosin relative to the actin surface. The C-terminal region of cardiac troponin T (TnT), a tropomyosin-associated protein, is required for full TnT inactivation at low Ca²⁺ and for limiting its activation at saturating Ca²⁺ Here, we investigated whether basic residues in this TnT region are involved in these activities, whether the TnT C terminus undergoes Ca²⁺-dependent conformational changes, and whether these residues affect cardiac muscle contraction. We generated a human cardiac TnT variant in which we replaced seven C-terminal Lys and Arg residues with Ala and added a Cys residue at either position 289 or 275 to affix a fluorescent probe. At pCa 3.7, actin filaments containing high-alanine TnT had an elevated ATPase rate like that obtained when the last TnT 14 residues were deleted. Acrylodan-tropomyosin fluorescence changes and S1-actin binding kinetics revealed that at pCa 8, the high-alanine TnT-containing filaments did not enter the first inactive state. FRET analyses indicated that the C-terminal TnT region approached Cys-190 of tropomyosin as actin filaments transitioned to the inactive B state; that transition was abolished with high-alanine TnT. High-alanine TnT-containing cardiac muscle preparations had increased Ca²⁺ sensitivity of both steady-state isometric force and sinusoidal stiffness as well as increased maximum steady-state isometric force and sinusoidal stiffness. We conclude that C-terminal basic residues in cardiac TnT are critical for the regulation of cardiac muscle contraction.

Cardiomyopathy in Children: Classification and Diagnosis: A Scientific Statement From the American Heart Association

In this scientific statement from the American Heart Association, experts in the field of cardiomyopathy (heart muscle disease) in children address 2 issues: the most current understanding of the causes of cardiomyopathy in children and the optimal approaches to diagnosis cardiomyopathy in children. Cardiomyopathies result in some of the worst pediatric cardiology outcomes; nearly 40% of children who present with symptomatic cardiomyopathy undergo a heart transplantation or die within the first 2 years after diagnosis. The percentage of children with cardiomyopathy who underwent a heart transplantation has not declined over the past 10 years, and cardiomyopathy remains the leading cause of transplantation for children >1 year of age. Studies from the National Heart, Lung, and Blood Institute-funded Pediatric Cardiomyopathy Registry have shown that causes are established in very few children with cardiomyopathy, yet genetic causes are likely to be present in most. The incidence of pediatric cardiomyopathy is ≈1 per 100 000 children. This is comparable to the incidence of such childhood cancers as lymphoma, Wilms tumor, and neuroblastoma. However, the published research and scientific conferences focused on pediatric cardiomyopathy are sparcer than for those cancers. The aim of the statement is to focus on the diagnosis and classification of cardiomyopathy. We anticipate that this report will help shape the future research priorities in this set of diseases to achieve earlier diagnosis, improved clinical outcomes, and better quality of life for these children and their families.

Pediatric Cardiomyopathies

Pediatric cardiomyopathies are rare diseases with an annual incidence of 1.1 to 1.5 per 100 000. Dilated and hypertrophic cardiomyopathies are the most common; restrictive, noncompaction, and mixed cardiomyopathies occur infrequently; and arrhythmogenic right ventricular cardiomyopathy is rare. Pediatric cardiomyopathies can result from coronary artery abnormalities, tachyarrhythmias, exposure to infection or toxins, or secondary to other underlying disorders. Increasingly, the importance of genetic mutations in the pathogenesis of isolated or syndromic pediatric cardiomyopathies is becoming apparent. Pediatric cardiomyopathies often occur in the absence of comorbidities, such as atherosclerosis, hypertension, renal dysfunction, and diabetes mellitus; as a result, they offer insights into the primary pathogenesis of myocardial dysfunction. Large international registries have characterized the epidemiology, cause, and outcomes of pediatric cardiomyopathies. Although adult and pediatric cardiomyopathies have similar morphological and clinical manifestations, their outcomes differ significantly. Within 2 years of presentation, normalization of function occurs in 20% of children with dilated cardiomyopathy, and 40% die or undergo transplantation. Infants with hypertrophic cardiomyopathy have a 2-year mortality of 30%, whereas death is rare in older children. Sudden death is rare. Molecular evidence indicates that gene expression differs between adult and pediatric cardiomyopathies, suggesting that treatment response may differ as well. Clinical trials to support evidence-based treatments and the development of disease-specific therapies for pediatric cardiomyopathies are in their infancy. This compendium summarizes current knowledge of the genetic and molecular origins, clinical course, and outcomes of the most common phenotypic presentations of pediatric cardiomyopathies and highlights key areas where additional research is required.

CLINICAL TRIAL REGISTRATION

URL: http://www.clinicaltrials.gov. Unique identifiers: NCT02549664 and NCT01912534.

Hypertrophic Cardiomyopathy Cardiac Troponin C Mutations Differentially Affect Slow Skeletal and Cardiac Muscle Regulation

Mutations in TNNC1—the gene encoding cardiac troponin C (cTnC)—that have been associated with hypertrophic cardiomyopathy (HCM) and cardiac dysfunction may also affect Ca²⁺-regulation and function of slow skeletal muscle since the same gene is expressed in both cardiac and slow skeletal muscle. Therefore, we reconstituted rabbit soleus fibers and bovine masseter myofibrils with mutant cTnCs (A8V, C84Y, E134D, and D145E) associated with HCM to investigate their effects on contractile force and ATPase rates, respectively. Previously, we showed that these HCM cTnC mutants, except for E134D, increased the Ca²⁺ sensitivity of force development in cardiac preparations. In the current study, an increase in Ca²⁺ sensitivity of isometric force was only observed for the C84Y mutant when reconstituted in soleus fibers. Incorporation of cTnC C84Y in bovine masseter myofibrils reduced the ATPase activity at saturating [Ca²⁺], whereas, incorporation of cTnC D145E increased the ATPase activity at inhibiting and saturating [Ca²⁺]. We also tested whether reconstitution of cardiac fibers with troponin complexes containing the cTnC mutants and slow skeletal troponin I (ssTnI) could emulate the slow skeletal functional phenotype. Reconstitution of cardiac fibers with troponin complexes containing ssTnI attenuated the Ca²⁺ sensitization of isometric force when cTnC A8V and D145E were present; however, it was enhanced for C84Y. In summary, although the A8V and D145E mutants are present in both muscle types, their functional phenotype is more prominent in cardiac muscle than in slow skeletal muscle, which has implications for the protein-protein interactions within the troponin complex. The C84Y mutant warrants further investigation since it drastically alters the properties of both muscle types and may account for the earlier clinical onset in the proband.

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Molecular genetics and pathogenesis of cardiomyopathy

Cardiomyopathy is defined as a disease of functional impairment in the cardiac muscle and its etiology includes both extrinsic and intrinsic factors. Cardiomyopathy caused by the intrinsic factors is called as primary cardiomyopathy of which two major clinical phenotypes are hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Genetic approaches have revealed the disease genes for hereditary primary cardiomyopathy and functional studies have demonstrated that characteristic functional alterations induced by the disease-associated mutations are closely related to the clinical types, such that increased and decreased Ca(2+) sensitivities of muscle contraction are associated with HCM and DCM, respectively. In addition, recent studies have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of sarcomere, respectively. Moreover, functional analysis of mutations in the other components of cardiac muscle have suggested that the altered response to metabolic stresses is associated with cardiomyopathy, further indicating the heterogeneity in the etiology and pathogenesis of cardiomyopathy.

Experimental models of inherited cardiomyopathy and its therapeutics

Cardiomyopathy is a disease of myocardium categorized into three major forms, hypertrophic (HCM), dilated (DCM) and restrictive cardiomyopathy (RCM), which has recently been demonstrated to be a monogenic disease due to mutations in various proteins expressed in cardiomyocytes. Mutations in HCM and RCM typically increase the myofilament sensitivity to cytoplasmic Ca²⁺, leading to systolic hyperfunction and diastolic dysfunction. In contrast, mutations in DCM typically decrease the myofilament sensitivity to cytoplasmic Ca²⁺ and/or force generation/transmission, leading to systolic dysfunction. Creation of genetically-manipulated transgenic and knock-in animals expressing mutant proteins exogenously and endogenously, respectively, in their hearts provides valuable animal models to discover the molecular and cellular mechanisms for pathogenesis and promising therapeutic strategy in vivo. Recently, cardiomyocytes have been differentiated from patient’s induced pluripotent stem cells as a model of inherited cardiomyopathies in vitro. In this review, we provide overview of experimental models of cardiomyopathies with a focus on revealed molecular and cellular pathogenic mechanisms and potential therapeutics.

Pathogenesis associated with a restrictive cardiomyopathy mutant in cardiac troponin T is due to reduced protein stability and greatly increased myofilament Ca2+ sensitivity

BACKGROUND

Dilated and hypertrophic cardiomyopathy mutations in troponin can blunt effects of protein kinase A (PKA) phosphorylation of cardiac troponin I (cTnI), decreasing myofilament Ca2+-sensitivity; however this effect has never been tested for restrictive cardiomyopathy (RCM) mutants. This study explores whether an RCM cardiac troponin T mutant (cTnT-ΔE96) interferes with convergent PKA regulation and if TnT instability contributes to greatly enhanced Ca2+-sensitivity in skinned fibers.

METHODS

Force of contraction in skinned cardiac porcine fiber and spectroscopic studies were performed.

RESULTS

A decrease of -0.26 and -0.25 pCa units in Ca2+-sensitivity of contraction after PKA incubation was observed for skinned fibers incorporated with WT or cTnT-ΔE96, respectively. To further assess whether cTnT-ΔE96 interferes solely with transmission of cTnI phosphorylation effects, skinned fibers were reconstituted with PKA pseudo-phosphorylated cTnI (cTnI-SS/DD.cTnC). Fibers displaced with cTnT-WT, reconstituted with cTnI-SS/DD.cTnC decreased Ca2+-sensitivity of force (pCa50=5.61) compared to control cTnI-WT.cTnC (pCa50=5.75), similarly affecting cTnT-ΔE96 (pCa50=6.03) compared to control \cTnI-WT.cTnC (pCa50=6.14). Fluorescence studies measuring cTnC(IAANS) Ca2+-affinity changes due to cTnT-ΔE96 indicated that higher complexity (thin filament) better recapitulates skinned fiber Ca2+ sensitive changes. Circular dichroism revealed reduced α-helicity and earlier thermal unfolding for cTnT-ΔE96 compared to WT.

CONCLUSIONS

Although ineffective in decreasing myofilament Ca2+-sensitivity to normal levels, cTnT-ΔE96 does not interfere with PKA cTnI phosphorylation mediated effects; 2) cTnT-ΔE96 requires actin to increase cTnC Ca2+-affinity; and 3) deletion of E96 reduces cTnT stability, likely disrupting crucial thin filament interactions.

GENERAL SIGNIFICANCE

The pathological effect of cTnT-ΔE96 is largely manifested by dramatic myofilament Ca2+-sensitization which still persists even after PKA phosphorylation mediated Ca2+-desensitization.

A novel arginine to tryptophan (R144W) mutation in troponin T (cTnT) gene in an indian multigenerational family with dilated cardiomyopathy (FDCM)

Cardiomyopathy is a major cause of heart failure and sudden cardiac death; several mutations in sarcomeric protein genes have been associated with this disease. Our aim in the present study is to investigate the genetic variations in Troponin T (cTnT) gene and its association with dilated cardiomyopathy (DCM) in south-Indian patients. Analyses of all the exons and exon-intron boundaries of cTnT in 147 DCM and in 207 healthy controls had revealed a total of 15 SNPs and a 5 bp INDEL; of which, polymorphic SNPs were compared with the HapMap population data. Interestingly, a novel R144W mutation, that substitutes polar-neutral tryptophan for a highly conserved basic arginine in cTnT, altering the charge drastically, was identified in a DCM, with a family history of sudden-cardiac death (SCD). This mutation was found within the tropomyosin (TPM1) binding domain, and was evolutionarily conserved across species, therefore it is expected to have a significant impact on the structure and function of the protein. Family studies had revealed that the R144W is co-segregating with disease in the family as an autosomal dominant trait, but it was completely absent in 207 healthy controls and in 162 previously studied HCM patients. Further screening of the proband and three of his family members (positive for R144W mutant) with eight other genes β-MYH7, MYBPC3, TPM1, TNNI3, TTN, ACTC, MYL2 and MYL3, did not reveal any disease causing mutation, proposing the absence of compound heterozygosity. Therefore, we strongly suggest that the novel R144W unique/private mutant identified in this study is associated with FDCM. This is furthermore signifying the unique genetic architecture of Indian population.

Validation of biomarkers for loin meat quality (M. longissimus) of pigs

The aim of this study was to validate previously reported associations between microarray gene expression levels and pork quality traits using real-time PCR. Meat samples and meat quality data from 100 pigs were collected from a different pig breed to the one tested by microarray (Large White versus Pietrain) and a different country of origin (Denmark versus Germany). Ten genes (CARP, MB, CSRP3, TNNC1, VAPB, TNNI1, HSPB1, TNNT1, TIMP-1, RAD-like) were chosen from the original microarray study on the basis of the association between gene expression levels and the meat quality traits meat %, back fat, pH24, drip loss %, colour a*, colour b*, colour L*, WB-SF, SFA, MUFA, PUFA. Real-time PCR detection methods were developed for validation of all ten genes, confirming association with drip loss (two of two genes), ultimate pH (three of four genes), a* (redness) (two of six genes) and L*(lightness) (two of four genes). Furthermore, several new correlations for MUFA and PUFA were established due to additional meat quality trait information on fatty acid composition not available for the microarray study. Regression studies showed that the maximum explanation of the phenotypic variance of the meat quality traits was 50% for the ultimate pH trait using these ten genes only. Additional studies showed that the gene expression of several of the genes was correlated with each other. We conclude that the genes initially selected from the microarray study were robust, explaining variances of the genes for the meat quality traits.

Analysis of the molecular pathogenesis of cardiomyopathy-causing cTnT mutants I79N, ΔE96, and ΔK210

Three troponin T (TnT) mutants that cause hypertrophic, restrictive, and dilated cardiomyopathy (I79N, ΔE96, and ΔK210, respectively), were examined using the thin-filament extraction/reconstitution technique. Effects of Ca(2+), ATP, phosphate, and ADP concentrations on force and its transients were studied at 25°C. Maximal Ca(2+) tension (THC) and Ca(2+)-activatable tension (Tact), respectively, were similar among I79N, ΔE96, and WT, whereas ΔK210 led to a significantly lower THC (∼20% less) and Tact (∼25% less) than did WT. In pCa solution containing 8 mM Pi and ionic strength adjusted to 200 mM, the Ca(2+) sensitivity (pCa50) of I79N (5.63 ± 0.02) and ΔE96 (5.60 ± 0.03) was significantly greater than that of WT (5.45 ± 0.04), but the pCa50 of ΔK210 (5.54 ± 0.04) remained similar to that of WT. Five equilibrium constants were deduced using sinusoidal analysis. All three mutants showed significantly lower K0 (ADP association constant) and larger K4 (equilibrium constant of force generation step) relative to the corresponding values for WT. I79N and ΔK210 were associated with a K2 (equilibrium constant of cross-bridge detachment step) significantly lower than that of ΔE96 and WT. These results demonstrated that at pCa 4.66, the force/cross-bridge is ∼18% less in I79N and ∼41% less in ΔK210 than that in WT. These results indicate that the molecular pathogenesis of the cardiac TnT mutation-related cardiomyopathies is different for each mutation.

Inherited cardiomyopathies caused by troponin mutations

Genetic investigations of cardiomyopathy in the recent two decades have revealed a large number of mutations in the genes encoding sarcomeric proteins as a cause of inherited hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), or restrictive cardiomyopathy (RCM). Most functional analyses of the effects of mutations on cardiac muscle contraction have revealed significant changes in the Ca(2+)-regulatory mechanism, in which cardiac troponin (cTn) plays important structural and functional roles as a key regulatory protein. Over a hundred mutations have been identified in all three subunits of cTn, i.e., cardiac troponins T, I, and C. Recent studies on cTn mutations have provided plenty of evidence that HCM- and RCM-linked mutations increase cardiac myofilament Ca(2+) sensitivity, while DCM-linked mutations decrease it. This review focuses on the functional consequences of mutations found in cTn in terms of cardiac myofilament Ca(2+) sensitivity, ATPase activity, force generation, and cardiac troponin I phosphorylation, to understand potential molecular and cellular pathogenic mechanisms of the three types of inherited cardiomyopathy.

The functional properties of human slow skeletal troponin T isoforms in cardiac muscle regulation

Human slow skeletal troponin T (HSSTnT) shares a high degree of homology with cardiac TnT (CTnT). Although the presence of HSSTnT has not been confirmed in the heart at the protein level, detectable levels of HSSTnT mRNA have been found. Whether HSSTnT isoforms are expressed transiently remains unknown. Because transient re-expression of HSSTnT may be a potential mechanism of regulating function, we explored the effect of HSSTnT on the regulation of cardiac muscle. At least three HSSTnT isoforms have been found to exist in slow skeletal muscle: HSSTnT1 (+exons 5 and 12), HSSTnT2 (+exon 5, -exon 12), and HSSTnT3 (-exons 5 and 12). Another isoform, HSSTnT hypothetical (Hyp) (-exon 5, +exon 12), has only been found at the mRNA level. Compared with HCTnT3 (adult isoform), Tn complexes containing HSSTnT1, -2, and -3 did not alter the actomyosin ATPase activation and inhibition in the presence and absence of Ca(2+), respectively. HSSTnTHyp was not evaluated as it did not form a Tn complex under a variety of conditions. Porcine papillary skinned fibers displaced with HSSTnT1, -2, or -3 and reconstituted with human cardiac troponin I and troponin C (HCTnI·TnC) complex showed a decrease in the Ca(2+) sensitivity of force development and an increase in maximal recovered force (HSSTnT1 and -3) compared with HCTnT3. In contrast, HSSTnTHyp showed an increase in the Ca(2+) sensitivity of force development. This suggests that re- or overexpression of specific SSTnT isoforms might have therapeutic potential in the failing heart because they increase the maximal force of contraction. In addition, circular dichroism and proteolytic digestion experiments revealed structural differences between HSSTnT isoforms and HCTnT3 and that HSSTnT1 is more susceptible to calpain and trypsin proteolysis than the other HSSTnTs. Overall, HSSTnT isoforms despite being homologues of CTnT may display distinct functional properties in muscle regulation.

Insights into restrictive cardiomyopathy from clinical and animal studies

Cardiomyopathies are diseases that primarily affect the myocardium, leading to serious cardiac dysfunction and heart failure. Out of the three major categories of cardiomyopathies (hypertrophic, dilated and restrictive), restrictive cardiomyopathy (RCM) is less common and also the least studied. However, the prognosis for RCM is poor as some patients dying in their childhood. The molecular mechanisms behind the disease development and progression are not very clear and the treatment of RCM is very difficult and often ineffective. In this article, we reviewed the recent progress in RCM research from the clinical studies and the translational studies done on diseased transgenic animal models. This will help for a better understanding of the mechanisms underlying the etiology and development of RCM and for the design of better treatments for the disease.

A mutation in TNNC1-encoded cardiac troponin C, TNNC1-A31S, predisposes to hypertrophic cardiomyopathy and ventricular fibrillation

Defined as clinically unexplained hypertrophy of the left ventricle, hypertrophic cardiomyopathy (HCM) is traditionally understood as a disease of the cardiac sarcomere. Mutations in TNNC1-encoded cardiac troponin C (cTnC) are a relatively rare cause of HCM. Here, we report clinical and functional characterization of a novel TNNC1 mutation, A31S, identified in a pediatric HCM proband with multiple episodes of ventricular fibrillation and aborted sudden cardiac death. Diagnosed at age 5, the proband is family history-negative for HCM or sudden cardiac death, suggesting a de novo mutation. TnC-extracted cardiac skinned fibers were reconstituted with the cTnC-A31S mutant, which increased Ca(2+) sensitivity with no effect on the maximal contractile force generation. Reconstituted actomyosin ATPase assays with 50% cTnC-A31S:50% cTnC-WT demonstrated Ca(2+) sensitivity that was intermediate between 100% cTnC-A31S and 100% cTnC-WT, whereas the mutant increased the activation of the actomyosin ATPase without affecting the inhibitory qualities of the ATPase. The secondary structure of the cTnC mutant was evaluated by circular dichroism, which did not indicate global changes in structure. Fluorescence studies demonstrated increased Ca(2+) affinity in isolated cTnC, the troponin complex, thin filament, and to a lesser degree, thin filament with myosin subfragment 1. These results suggest that this mutation has a direct effect on the Ca(2+) sensitivity of the myofilament, which may alter Ca(2+) handling and contribute to the arrhythmogenesis observed in the proband. In summary, we report a novel mutation in the TNNC1 gene that is associated with HCM pathogenesis and may predispose to the pathogenesis of a fatal arrhythmogenic subtype of HCM.

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.

Fetal cardiac troponin isoforms rescue the increased Ca2+ sensitivity produced by a novel double deletion in cardiac troponin T linked to restrictive cardiomyopathy: a clinical, genetic, and functional approach

A novel double deletion in cardiac troponin T (cTnT) of two highly conserved amino acids (Asn-100 and Glu-101) was found in a restrictive cardiomyopathic (RCM) pediatric patient. Clinical evaluation revealed the presence of left atrial enlargement and marked left ventricle diastolic dysfunction. The explanted heart examined by electron microscopy revealed myofibrillar disarray and mild fibrosis. Pedigree analysis established that this mutation arose de novo. The patient tested negative for six other sarcomeric genes. The single and double recombinant cTnT mutants were generated, and their functional consequences were analyzed in porcine skinned cardiac muscle. In the adult Tn environment (cTnT3 + cardiac troponin I), the single cTnT3-ΔN100 and cTnT3-ΔE101 mutations had opposing effects on the Ca(2+) sensitivity of force development compared with WT, whereas the double deletion cTnT3-ΔN100/ΔE101 increased the Ca(2+) sensitivity + 0.19 pCa units. In addition, cTnT3-ΔN100/ΔE101 decreased the cooperativity of force development, suggesting alterations in intrafilament protein-protein interactions. In the fetal Tn environment, (cTnT1 + slow skeletal troponin I), the single (cTnT1-ΔN110) and double (cTnT1-ΔN110/ΔE111) deletions did not change the Ca(2+) sensitivity compared with control. To recreate the patient's heterozygous genotype, we performed a reconstituted ATPase activity assay. Thin filaments containing 50:50 cTnT3-ΔN100/ΔE101:cTnT3-WT also increased the myofilament Ca(2+) sensitivity compared with WT. Co-sedimentation of thin filament proteins indicated that no significant changes occurred in the binding of Tn containing the RCM cTnT mutation to actin-Tm. This report reveals the protective role of Tn fetal isoforms as they rescue the increased Ca(2+) sensitivity produced by a cTnT-RCM mutation and may account for the lack of lethality during gestation.

[Molecular mechanisms of genetic damages of the myocardium in cardiomyopathy]

The review highlighted problems of reorganization of myocardical contractile and cytoskeletal proteins in cardiomyopathy (CM). The role of the genetic factors coding contractile proteins, proteins of thin and thick filaments, and also extracellular matrix proteins in processes of formation and development of hypertrophic (HCM) and dilated (DCM) cardiomyopathy are analyzed. The mechanisms responsible for the changes in cardiac proteins on regulation involved into force generation, its transfer, recycling ATP, impairments in transmembranal signals, that finally lead to cardiac cell dysfunction determining various manifestations of CM are considered.

Predicting cardiomyopathic phenotypes by altering Ca2+ affinity of cardiac troponin C

Cardiac diseases associated with mutations in troponin subunits include hypertrophic cardiomyopathy (HCM), dilated cardiomyopathy (DCM), and restrictive cardiomyopathy (RCM). Altered calcium handling in these diseases is evidenced by changes in the Ca(2+) sensitivity of contraction. Mutations in the Ca(2+) sensor, troponin C (TnC), were generated to increase/decrease the Ca(2+) sensitivity of cardiac skinned fibers to create the characteristic effects of DCM, HCM, and RCM. We also used a reconstituted assay to determine the mutation effects on ATPase activation and inhibition. One mutant (A23Q) was found with HCM-like properties (increased Ca(2+) sensitivity of force and normal levels of ATPase inhibition). Three mutants (S37G, V44Q, and L48Q) were identified with RCM-like properties (a large increase in Ca(2+) sensitivity, partial loss of ATPase inhibition, and increased basal force). Two mutations were identified (E40A and I61Q) with DCM properties (decreased Ca(2+) sensitivity, maximal force recovery, and activation of the ATPase at high [Ca(2+)]). Steady-state fluorescence was utilized to assess Ca(2+) affinity in isolated cardiac (c)TnCs containing F27W and did not necessarily mirror the fiber Ca(2+) sensitivity. Circular dichroism of mutant cTnCs revealed a trend where increased alpha-helical content correlated with increased Ca(2+) sensitivity in skinned fibers and vice versa. The main findings from this study were as follows: 1) cTnC mutants demonstrated distinct functional phenotypes reminiscent of bona fide HCM, RCM, and DCM mutations; 2) a region in cTnC associated with increased Ca(2+) sensitivity in skinned fibers was identified; and 3) the F27W reporter mutation affected Ca(2+) sensitivity, maximal force, and ATPase activation of some mutants.

Cardiac troponin mutations and restrictive cardiomyopathy

Mutations in sarcomeric proteins have recently been established as heritable causes of Restrictive Cardiomyopathy (RCM). RCM is clinically characterized as a defect in cardiac diastolic function, such as, impaired ventricular relaxation, reduced diastolic volume and increased end-diastolic pressure. To date, mutations have been identified in the cardiac genes for desmin, α-actin, troponin I and troponin T. Functional studies in skinned muscle fibers reconstituted with troponin mutants have established phenotypes consistent with the clinical findings which include an increase in myofilament Ca²⁺ sensitivity and basal force. Moreover, when RCM mutants are incorporated into reconstituted myofilaments, the ability to inhibit the ATPase activity is reduced. A majority of the mutations cluster in specific regions of cardiac troponin and appear to be mutational “hot spots”. This paper highlights the functional and clinical characteristics of RCM linked mutations within the troponin complex.

Correcting diastolic dysfunction by Ca2+ desensitizing troponin in a transgenic mouse model of restrictive cardiomyopathy

Several cardiac troponin I (cTnI) mutations are associated with restrictive cardiomyopathy (RCM) in humans. We have created transgenic mice (cTnI(193His) mice) that express the corresponding human RCM R192H mutation. Phenotype of this RCM animal model includes restrictive ventricles, biatrial enlargement and sudden cardiac death, which are similar to those observed in RCM patients carrying the same cTnI mutation. In the present study, we modified the overall cTnI in cardiac muscle by crossing cTnI(193His) mice with transgenic mice expressing an N-terminal truncated cTnI (cTnI-ND) that enhances relaxation. Protein analyses determined that wild type cTnI was replaced by cTnI-ND in the heart of double transgenic mice (Double TG), which express only cTnI-ND and cTnI R193H in cardiac myocytes. The presence of cTnI-ND effectively rescued the lethal phenotype of RCM mice by reducing the mortality rate. Cardiac function was significantly improved in Double TG mice when measured by echocardiography. The hypersensitivity to Ca(2+) and the prolonged relaxation of RCM cTnI(193His) cardiac myocytes were completely reversed by the presence of cTnI-ND in RCM hearts. The results demonstrate that myofibril hypersensitivity to Ca(2+) is a key mechanism that causes impaired relaxation in RCM cTnI mutant hearts and Ca(2+) desensitization by cTnI-ND can correct diastolic dysfunction and rescue the RCM phenotypes, suggesting that Ca(2+) desensitization in myofibrils is a therapeutic option for treatment of diastolic dysfunction without interventions directed at the systemic beta-adrenergic-PKA pathways.

Evolving molecular diagnostics for familial cardiomyopathies: at the heart of it all

Cardiomyopathies are an important and heterogeneous group of common cardiac diseases. An increasing number of cardiomyopathies are now recognized to have familial forms, which result from single-gene mutations that render a Mendelian inheritance pattern, including hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy, arrhythmogenic right ventricular cardiomyopathy and left ventricular noncompaction cardiomyopathy. Recently, clinical genetic tests for familial cardiomyopathies have become available for clinicians evaluating and treating patients with these diseases, making it necessary to understand the current progress and challenges in cardiomyopathy genetics and diagnostics. In this review, we summarize the genetic basis of selected cardiomyopathies, describe the clinical utility of genetic testing for cardiomyopathies and outline the current challenges and emerging developments.

Molecular basis of hereditary cardiomyopathy: abnormalities in calcium sensitivity, stretch response, stress response and beyond

Cardiomyopathy is caused by functional abnormality of cardiac muscle. The functional abnormality involved in its etiology includes both extrinsic and intrinsic factors, and cardiomyopathy caused by the intrinsic factors is called as idiopathic or primary cardiomyopathy. There are several clinical types of primary cardiomyopathy including hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Linkage studies and candidate gene approaches have explored the disease genes for hereditary primary cardiomyopathy. The most notable finding was that mutations in the same disease gene can be found in different clinical types of cardiomyopathy. Functional analyses of disease-related mutations have revealed that characteristic functional alterations are associated with the clinical types, such that increased and decreased Ca(2+) sensitivity due to sarcomere mutations are associated with HCM and DCM, respectively. In addition, our recent studies have suggested that mutations in the Z-disc components found in HCM and DCM may result in increased and decreased stiffness of sarcomere; that is, stiff sarcomere and loose sarcomere, respectively, and hence altered stretch response. More recently, mutations in the components of I region were found in hereditary cardiomyopathy and the functional analyses of the mutations suggested that the altered stress response was associated with cardiomyopathy, further complicating the etiology and pathogenesis. However, elucidation of genetic etiology and functional alterations caused by the mutations shed lights on the new therapeutic approaches to hereditary cardiomyopathy, such that treatment of DCM with a Ca(2+) sensitizer prevented the disease in a mouse model.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

A functional and structural study of troponin C mutations related to hypertrophic cardiomyopathy

Recently four new hypertrophic cardiomyopathy mutations in cardiac troponin C (cTnC) (A8V, C84Y, E134D, and D145E) were reported, and their effects on the Ca(2+) sensitivity of force development were evaluated (Landstrom, A. P., Parvatiyar, M. S., Pinto, J. R., Marquardt, M. L., Bos, J. M., Tester, D. J., Ommen, S. R., Potter, J. D., and Ackerman, M. J. (2008) J. Mol. Cell. Cardiol. 45, 281-288). We performed actomyosin ATPase and spectroscopic solution studies to investigate the molecular properties of these mutations. Actomyosin ATPase activity was measured as a function of [Ca(2+)] utilizing reconstituted thin filaments (TFs) with 50% mutant and 50% wild type (WT) and 100% mutant cardiac troponin (cTn) complexes: A8V, C84Y, and D145E increased the Ca(2+) sensitivity with only A8V demonstrating lowered Ca(2+) sensitization at the 50% ratio when compared with 100%; E134D was the same as WT at both ratios. Of these four mutants, only D145E showed increased ATPase activation in the presence of Ca(2+). None of the mutants affected ATPase inhibition or the binding of cTn to the TF measured by co-sedimentation. Only D145E increased the Ca(2+) affinity of site II measured by 2-(4'-(2''-iodoacetamido)phenyl)aminonaphthalene-6-sulfonic acid fluorescence in isolated cTnC or the cTn complex. In the presence of the TF, only A8V was further sensitized to Ca(2+). Circular dichroism measurements in different metal-bound states of the isolated cTnCs showed changes in the secondary structure of A8V, C84Y, and D145E, whereas E134D was the same as WT. PyMol modeling of each cTnC mutant within the cTn complex revealed potential for local changes in the tertiary structure of A8V, C84Y, and D145E. Our results indicate that 1) three of the hypertrophic cardiomyopathy cTnC mutants increased the Ca(2+) sensitivity of the myofilament; 2) the effects of the mutations on the Ca(2+) affinity of isolated cTnC, cTn, and TF are not sufficient to explain the large Ca(2+) sensitivity changes seen in reconstituted and fiber assays; and 3) changes in the secondary structure of the cTnC mutants may contribute to modified protein-protein interactions along the sarcomere lattice disrupting the coupling between the cross-bridge and Ca(2+) binding to cTnC.

Clinical and functional characterization of TNNT2 mutations identified in patients with dilated cardiomyopathy

BACKGROUND

A key issue for cardiovascular genetic medicine is ascertaining if a putative mutation indeed causes dilated cardiomyopathy (DCM). This is critically important as genetic DCM, usually presenting with advanced, life-threatening disease, may be preventable with early intervention in relatives known to carry the mutation.

METHODS AND RESULTS

We recently undertook bidirectional resequencing of TNNT2, the cardiac troponin T gene, in 313 probands with DCM. We identified 6 TNNT2 protein-altering variants in 9 probands, all who had early onset, aggressive disease. Additional family members of mutation carriers were then studied when available. Four of the 9 probands had DCM without a family history, and 5 probands had familial DCM. Only 1 mutation (Lys210del) could be attributed as definitively causative from previous reports. Four of the 5 missense mutations were novel (Arg134Gly, Arg151Cys, Arg159Gln, and Arg205Trp), and one was previously reported with hypertrophic cardiomyopathy (Glu244Asp). Based on the clinical, pedigree, and molecular genetic data, these 5 mutations were considered possibly or likely disease causing. To further clarify their potential pathophysiologic impact, we undertook functional studies of these mutations in cardiac myocytes reconstituted with mutant troponin T proteins. We observed decreased Ca(2+) sensitivity of force development, a hallmark of DCM, in support of the conclusion that these mutations are disease causing.

CONCLUSIONS

We conclude that the combination of clinical, pedigree, molecular genetic, and functional data strengthen the interpretation of TNNT2 mutations in DCM.

Challenging current paradigms related to cardiomyopathies. Are changes in the Ca2+ sensitivity of myofilaments containing cardiac troponin C mutations (G159D and L29Q) good predictors of the phenotypic outcomes?

Two novel mutations (G159D and L29Q) in cardiac troponin C (CTnC) associate their phenotypic outcomes with dilated (DCM) and hypertrophic cardiomyopathy (HCM), respectively. Current paradigms propose that sarcomeric mutations associated with DCM decrease the myofilament Ca2+ sensitivity, whereas those associated with HCM increase it. Therefore, we incorporated the mutant CTnCs into skinned cardiac muscle in order to determine if their effects on the Ca2+ sensitivities of tension and ATPase activity coincide with the current paradigms and phenotypic outcomes. The G159D-CTnC decreases the Ca2+ sensitivity of tension and ATPase activation and reduces the maximal ATPase activity when incorporated into regulated actomyosin filaments. Under the same conditions, the L29Q-CTnC has no effect. Surprisingly, changes in the apparent G159D-CTnC Ca2+ affinity measured by tension in fibers do not occur in the isolated CTnC, and large changes measured in the isolated L29Q-CTnC do not manifest in the fiber. These counterintuitive findings are justified through a transition in Ca2+ affinity occurring at the level of cardiac troponin and higher, implying that the true effects of these mutations become apparent as the hierarchical level of the myofilament increases. Therefore, the contractile apparatus, representing a large cooperative machine, can provide the potential for a change (G159D) or no change (L29Q) in the Ca2+ regulation of contraction. In accordance with the clinical outcomes and current paradigms, the desensitization of myofilaments from G159D-CTnC is expected to weaken the contractile force of the myocardium, whereas the lack of myofilament changes from L29Q-CTnC may preserve diastolic and systolic function.

Molecular etiology and pathogenesis of hereditary cardiomyopathy

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

Impaired relaxation is the main manifestation in transgenic mice expressing a restrictive cardiomyopathy mutation, R193H, in cardiac TnI

Transgenic mice were generated to express a restrictive cardiomyopathy (RCM) human cardiac troponin I (cTnI) R192H mutation in the heart (cTnI(193His) mice). The objective of this study was to assess cardiac function during the development of diastolic dysfunction and to gain insight into the pathophysiological impact of the RCM cTnI mutation. Cardiac function and pathophysiological changes were monitored in cTnI193His mice and wild-type littermates for a period of 12 mo. It progressed gradually from abnormal relaxation to diastolic dysfunction characterized with high-resolution echocardiography by a reversed E-to-A ratio, increased deceleration time, and prolonged isovolumetric relaxation time. At the age of 12 mo, cardiac output in cTnI(193His) mice was significantly declined, and some transgenic mice showed congestive heart failure. The negative impact of cTnI193His on ventricular contraction and relaxation was further demonstrated in isolated mouse working heart preparations. The main morphological change in cTnI193His myocytes was shortened cell length. Dobutamine stimulation increased heart rate in cTnI193His mice but did not improve CO. The cTnI193His mice had a phenotype similar to that in human RCM patients carrying the cTnI mutation characterized morphologically by enlarged atria and restricted ventricles and functionally by diastolic dysfunction and diastolic heart failure. The results demonstrate a critical role of the COOH-terminal domain of cTnI in the diastolic function of cardiac muscle.