Familial Hypertrophic Cardiomyopathy Mutations from Different Functional Regions of Troponin T Result in Different Effects on the pH and Ca2+ Sensitivity of Cardiac Muscle Contraction

Structural dynamics of the intrinsically disordered linker region of cardiac troponin T

The cardiac troponin complex, composed of troponins I, T, and C, plays a central role in regulating the calcium-dependent interactions between myosin and the thin filament. Mutations in troponin can cause cardiomyopathies; however, it is still a major challenge to connect how changes in sequence affect troponin's function. Recent high-resolution structures of the thin filament revealed critical insights into the structure-function relationship of troponin, but there remain large, unresolved segments of troponin, including the troponin-T linker region that is a hotspot for cardiomyopathy mutations. This linker region is predicted to be intrinsically disordered, with behaviors that are not well described by traditional structural approaches; however, this proposal has not been experimentally verified. Here, we used a combination of single-molecule Förster resonance energy transfer (FRET), molecular dynamics simulations, and functional reconstitution assays to investigate the troponin-T linker region. We show that in the context of both isolated troponin and the fully regulated troponin complex, the linker behaves as a dynamic, intrinsically disordered region. This region undergoes polyampholyte expansion in the presence of high salt and distinct conformational changes during the assembly of the troponin complex. We also examine the ΔE160 hypertrophic cardiomyopathy mutation in the linker and demonstrate that it does not affect the conformational dynamics of the linker, rather it allosterically affects interactions with other troponin complex subunits, leading to increased molecular contractility. Taken together, our data clearly demonstrate the importance of disorder within the troponin-T linker and provide new insights into the molecular mechanisms driving the pathogenesis of cardiomyopathies.

Hypertrophic cardiomyopathy: an up-to-date snapshot of the clinical drug development pipeline

INTRODUCTION

Hypertrophic cardiomyopathy (HCM) is a complex cardiac disease with highly variable phenotypic expression and clinical course most often caused by sarcomeric gene mutations resulting in left ventricular hypertrophy, fibrosis, hypercontractility, and diastolic dysfunction. For almost 60 years, HCM has remained an orphan disease and still lacks a disease-specific treatment.

AREAS COVERED

This review summarizes recent preclinical and clinical trials with repurposed drugs and new emerging pharmacological and gene-based therapies for the treatment of HCM.

EXPERT OPINION

The off-label drugs routinely used alleviate symptoms but do not target the core pathophysiology of HCM or prevent or revert the phenotype. Recent advances in the genetics and pathophysiology of HCM led to the development of cardiac myosin adenosine triphosphatase inhibitors specifically directed to counteract the hypercontractility associated with HCM-causing mutations. Mavacamten, the first drug specifically developed for HCM successfully tested in a phase 3 trial, represents the major advance for the treatment of HCM. This opens new horizons for the development of novel drugs targeting HCM molecular substrates which hopefully modify the natural history of the disease. The role of current drugs in development and genetic-based approaches for the treatment of HCM are also discussed.

Allele-specific silencing by RNAi of R92Q and R173W mutations in cardiac troponin T

Autosomal dominant mutations in sarcomere proteins such as the cardiac troponin T (TNNT2) are the main genetic causes of human hypertrophic cardiomyopathy and dilated cardiomyopathy. Allele-specific silencing by RNA interference (ASP-RNAi) holds promise as a therapeutic strategy for downregulating a single mutant allele with minimal suppression of the corresponding wild-type allele. Here, we propose ASP-RNAi as a possible strategy to specifically knockdown mutant alleles coding for R92Q and R173W mutant TNNT2 proteins, identified in hypertrophic and dilated cardiomyopathy, respectively. Different siRNAs were designed and validated by luciferase reporter assay and following analysis in HEK293T cells expressing either the wild-type or mutant TNNT2 alleles. This study is the first exploration of ASP-RNAi on TNNT2-R173W and TNNT2-R92Q mutations in vitro and gives a base for further application of allele silencing as a therapeutic treatment for TNNT2-mutation-associated cardiomyopathies.

A troponin T variant linked with pediatric dilated cardiomyopathy reduces the coupling of thin filament activation to myosin and calcium binding

Dilated cardiomyopathy (DCM) is a significant cause of pediatric heart failure. Mutations in proteins that regulate cardiac muscle contraction can cause DCM; however, the mechanisms by which molecular-level mutations contribute to cellular dysfunction are not well understood. Better understanding of these mechanisms might enable the development of targeted therapeutics that benefit patient subpopulations with mutations that cause common biophysical defects. We examined the molecular- and cellular-level impacts of a troponin T variant associated with pediatric-onset DCM, R134G. The R134G variant decreased calcium sensitivity in an in vitro motility assay. Using stopped-flow and steady-state fluorescence measurements, we determined the molecular mechanism of the altered calcium sensitivity: R134G decouples calcium binding by troponin from the closed-to-open transition of the thin filament and decreases the cooperativity of myosin binding to regulated thin filaments. Consistent with the prediction that these effects would cause reduced force per sarcomere, cardiomyocytes carrying the R134G mutation are hypocontractile. They also show hallmarks of DCM that lie downstream of the initial insult, including disorganized sarcomeres and cellular hypertrophy. These results reinforce the importance of multiscale studies to fully understand mechanisms underlying human disease and highlight the value of mechanism-based precision medicine approaches for DCM.

Molecular Genetic Basis of Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a genetic disease of the myocardium characterized by a hypertrophic left ventricle with a preserved or increased ejection fraction. Cardiac hypertrophy is often asymmetrical, which is associated with left ventricular outflow tract obstruction. Myocyte hypertrophy, disarray, and myocardial fibrosis constitute the histological features of HCM. HCM is a relatively benign disease but an important cause of sudden cardiac death in the young and heart failure in the elderly. Pathogenic variants (PVs) in genes encoding protein constituents of the sarcomeres are the main causes of HCM. PVs exhibit a gradient of effect sizes, as reflected in their penetrance and variable phenotypic expression of HCM. MYH7 and MYBPC3, encoding β-myosin heavy chain and myosin binding protein C, respectively, are the two most common causal genes and responsible for ≈40% of all HCM cases but a higher percentage of HCM in large families. PVs in genes encoding protein components of the thin filaments are responsible for ≈5% of the HCM cases. Whereas pathogenicity of the genetic variants in large families has been firmly established, ascertainment causality of the PVs in small families and sporadic cases is challenging. In the latter category, PVs are best considered as probabilistic determinants of HCM. Deciphering the genetic basis of HCM has enabled routine genetic testing and has partially elucidated the underpinning mechanism of HCM as increased number of the myosin molecules that are strongly bound to actin. The discoveries have led to the development of mavacamten that targets binding of the myosin molecule to actin filaments and imparts beneficial clinical effects. In the coming years, the yield of the genetic testing is expected to be improved and the so-called missing causal gene be identified. The advances are also expected to enable development of additional specific therapies and editing of the mutations in HCM.

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.

Cardiomyopathic troponin mutations predominantly occur at its interface with actin and tropomyosin

Reversible Ca2+ binding to troponin is the primary on-off switch of the contractile apparatus of striated muscles, including the heart. Dominant missense mutations in human cardiac troponin genes are among the causes of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy. Structural understanding of troponin action has recently advanced considerably via electron microscopy and molecular dynamics studies of the thin filament. As a result, it is now possible to examine cardiomyopathy-inducing troponin mutations in thin-filament structural context, and from that to seek new insight into pathogenesis and into the troponin regulatory mechanism. We compiled from consortium reports a representative set of troponin mutation sites whose pathogenicity was determined using standardized clinical genetics criteria. Another set of sites, apparently tolerant of amino acid substitutions, was compiled from the gnomAD v2 database. Pathogenic substitutions occurred predominantly in the areas of troponin that contact actin or tropomyosin, including, but not limited to, two regions of newly proposed structure and long-known implication in cardiomyopathy: the C-terminal third of troponin I and a part of the troponin T N terminus. The pathogenic mutations were located in troponin regions that prevent contraction under low Ca2+ concentration conditions. These regions contribute to Ca2+-regulated steric hindrance of myosin by the combined effects of troponin and tropomyosin. Loss-of-function mutations within these parts of troponin result in loss of inhibition, consistent with the hypercontractile phenotype characteristic of HCM. Notably, pathogenic mutations are absent in our dataset from the Ca2+-binding, activation-producing troponin C (TnC) N-lobe, which controls contraction by a multi-faceted mechanism. Apparently benign mutations are also diminished in the TnC N-lobe, suggesting mutations are poorly tolerated in that critical domain.

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.

TNNT2 mutations in the tropomyosin binding region of TNT1 disrupt its role in contractile inhibition and stimulate cardiac dysfunction

Muscle contraction is regulated by the movement of end-to-end-linked troponin-tropomyosin complexes over the thin filament surface, which uncovers or blocks myosin binding sites along F-actin. The N-terminal half of troponin T (TnT), TNT1, independently promotes tropomyosin-based, steric inhibition of acto-myosin associations, in vitro. Recent structural models additionally suggest TNT1 may restrain the uniform, regulatory translocation of tropomyosin. Therefore, TnT potentially contributes to striated muscle relaxation; however, the in vivo functional relevance and molecular basis of this noncanonical role remain unclear. Impaired relaxation is a hallmark of hypertrophic and restrictive cardiomyopathies (HCM and RCM). Investigating the effects of cardiomyopathy-causing mutations could help clarify TNT1's enigmatic inhibitory property. We tested the hypothesis that coupling of TNT1 with tropomyosin's end-to-end overlap region helps anchor tropomyosin to an inhibitory position on F-actin, where it deters myosin binding at rest, and that, correspondingly, cross-bridge cycling is defectively suppressed under diastolic/low Ca2+ conditions in the presence of HCM/RCM lesions. The impact of TNT1 mutations on Drosophila cardiac performance, rat myofibrillar and cardiomyocyte properties, and human TNT1's propensity to inhibit myosin-driven, F-actin-tropomyosin motility were evaluated. Our data collectively demonstrate that removing conserved, charged residues in TNT1's tropomyosin-binding domain impairs TnT's contribution to inhibitory tropomyosin positioning and relaxation. Thus, TNT1 may modulate acto-myosin activity by optimizing F-actin-tropomyosin interfacial contacts and by binding to actin, which restrict tropomyosin's movement to activating configurations. HCM/RCM mutations, therefore, highlight TNT1's essential role in contractile regulation by diminishing its tropomyosin-anchoring effects, potentially serving as the initial trigger of pathology in our animal models and humans.

Crossbridge Recruitment Capacity of Wild-Type and Hypertrophic Cardiomyopathy-Related Mutant Troponin-T Evaluated by X-ray Diffraction and Mechanical Study of Cardiac Skinned Fibers

X-ray diffraction and tension measurement experiments were conducted on rat left ventricular skinned fibers with or without “troponin-T treatment,” which exchanges the endogenous troponin T/I/C complex with exogenous troponin-T. These experiments were performed to observe the structural changes in troponin-T within a fiber elicited by contractile crossbridge formation and investigate the abnormality of hypertrophic cardiomyopathy-related troponin-T mutants. The intensity of the troponin reflection at 1/38.5 nm⁻¹ was decreased significantly by ATP addition after treatment with wild-type or mutant troponin-T, indicating that crossbridge formation affected the conformation of troponin-T. In experiments on cardiac fibers treated with the hypertrophic cardiomyopathy-related mutants E244D- and K247R-troponin-T, treatment with K247R-troponin-T did not recruit contracting actomyosin to a greater extent than wild-type-troponin-T, although a similar drop in the intensity of the troponin reflection occurred. Therefore, the conformational change in K247R-troponin-T was suggested to be unable to fully recruit actomyosin interaction, which may be the cause of cardiomyopathy.

FRET-based analysis of the cardiac troponin T linker region reveals the structural basis of the hypertrophic cardiomyopathy-causing Δ160E mutation

Mutations in the cardiac thin filament (TF) have highly variable effects on the regulatory function of the cardiac sarcomere. Understanding the molecular-level dysfunction elicited by TF mutations is crucial to elucidate cardiac disease mechanisms. The hypertrophic cardiomyopathy-causing cardiac troponin T (cTnT) mutation Δ160Glu (Δ160E) is located in a putative "hinge" adjacent to an unstructured linker connecting domains TNT1 and TNT2. Currently, no high-resolution structure exists for this region, limiting significantly our ability to understand its role in myofilament activation and the molecular mechanism of mutation-induced dysfunction. Previous regulated in vitro motility data have indicated mutation-induced impairment of weak actomyosin interactions. We hypothesized that cTnT-Δ160E repositions the flexible linker, altering weak actomyosin electrostatic binding and acting as a biophysical trigger for impaired contractility and the observed remodeling. Using time-resolved FRET and an all-atom TF model, here we first defined the WT structure of the cTnT-linker region and then identified Δ160E mutation-induced positional changes. Our results suggest that the WT linker runs alongside the C terminus of tropomyosin. The Δ160E-induced structural changes moved the linker closer to the tropomyosin C terminus, an effect that was more pronounced in the presence of myosin subfragment (S1) heads, supporting previous findings. Our in silico model fully supported this result, indicating a mutation-induced decrease in linker flexibility. Our findings provide a framework for understanding basic pathogenic mechanisms that drive severe clinical hypertrophic cardiomyopathy phenotypes and for identifying structural targets for intervention that can be tested in silico and in vitro.

Overexpression of miRNA-9 Generates Muscle Hypercontraction Through Translational Repression of Troponin-T in Drosophila melanogaster Indirect Flight Muscles

MicroRNAs (miRNAs) are small noncoding endogenous RNAs, typically 21-23 nucleotides long, that regulate gene expression, usually post-transcriptionally, by binding to the 3'-UTR of target mRNA, thus blocking translation. The expression of several miRNAs is significantly altered during cardiac hypertrophy, myocardial ischemia, fibrosis, heart failure, and other cardiac myopathies. Recent studies have implicated miRNA-9 (miR-9) in myocardial hypertrophy. However, a detailed mechanism remains obscure. In this study, we have addressed the roles of miR-9 in muscle development and function using a genetically tractable model system, the indirect flight muscles (IFMs) of Drosophila melanogaster Bioinformatics analysis identified 135 potential miR-9a targets, of which 27 genes were associated with Drosophila muscle development. Troponin-T (TnT) was identified as major structural gene target of miR-9a. We show that flies overexpressing miR-9a in the IFMs have abnormal wing position and are flightless. These flies also exhibit a loss of muscle integrity and sarcomeric organization causing an abnormal muscle condition known as "hypercontraction." Additionally, miR-9a overexpression resulted in the reduction of TnT protein levels while transcript levels were unaffected. Furthermore, muscle abnormalities associated with miR-9a overexpression were completely rescued by overexpression of TnT transgenes which lacked the miR-9a binding site. These findings indicate that miR-9a interacts with the 3'-UTR of the TnT mRNA and downregulates the TnT protein levels by translational repression. The reduction in TnT levels leads to a cooperative downregulation of other thin filament structural proteins. Our findings have implications for understanding the cellular pathophysiology of cardiomyopathies associated with miR-9 overexpression.

Molecular mechanisms and structural features of cardiomyopathy-causing troponin T mutants in the tropomyosin overlap region

Point mutations in genes encoding sarcomeric proteins are the leading cause of inherited primary cardiomyopathies. Among them are mutations in the TNNT2 gene that encodes cardiac troponin T (TnT). These mutations are clustered in the tropomyosin (Tm) binding region of TnT, TNT1 (residues 80-180). To understand the mechanistic changes caused by pathogenic mutations in the TNT1 region, six hypertrophic cardiomyopathy (HCM) and two dilated cardiomyopathy (DCM) mutants were studied by biochemical approaches. Binding assays in the absence and presence of actin revealed changes in the affinity of some, but not all, TnT mutants for Tm relative to WT TnT. HCM mutants were hypersensitive and DCM mutants were hyposensitive to Ca²⁺ in regulated actomyosin ATPase activities. To gain better insight into the disease mechanism, we modeled the structure of TNT1 and its interactions with Tm. The stability predictions made by the model correlated well with the affinity changes observed in vitro of TnT mutants for Tm. The changes in Ca²⁺ sensitivity showed a strong correlation with the changes in binding affinity. We suggest the primary reason by which these TNNT2 mutations between residues 92 and 144 cause cardiomyopathy is by changing the affinity of TnT for Tm within the TNT1 region.

Hypertrophic Cardiomyopathy: Genetics, Pathogenesis, Clinical Manifestations, Diagnosis, and Therapy

Hypertrophic cardiomyopathy (HCM) is a genetic disorder that is characterized by left ventricular hypertrophy unexplained by secondary causes and a nondilated left ventricle with preserved or increased ejection fraction. It is commonly asymmetrical with the most severe hypertrophy involving the basal interventricular septum. Left ventricular outflow tract obstruction is present at rest in about one third of the patients and can be provoked in another third. The histological features of HCM include myocyte hypertrophy and disarray, as well as interstitial fibrosis. The hypertrophy is also frequently associated with left ventricular diastolic dysfunction. In the majority of patients, HCM has a relatively benign course. However, HCM is also an important cause of sudden cardiac death, particularly in adolescents and young adults. Nonsustained ventricular tachycardia, syncope, a family history of sudden cardiac death, and severe cardiac hypertrophy are major risk factors for sudden cardiac death. This complication can usually be averted by implantation of a cardioverter-defibrillator in appropriate high-risk patients. Atrial fibrillation is also a common complication and is not well tolerated. Mutations in over a dozen genes encoding sarcomere-associated proteins cause HCM. MYH7 and MYBPC3, encoding β-myosin heavy chain and myosin-binding protein C, respectively, are the 2 most common genes involved, together accounting for ≈50% of the HCM families. In ≈40% of HCM patients, the causal genes remain to be identified. Mutations in genes responsible for storage diseases also cause a phenotype resembling HCM (genocopy or phenocopy). The routine applications of genetic testing and preclinical identification of family members represents an important advance. The genetic discoveries have enhanced understanding of the molecular pathogenesis of HCM and have stimulated efforts designed to identify new therapeutic agents.

Hypertrophic cardiomyopathy mutation in cardiac troponin T (R95H) attenuates length-dependent activation in guinea pig cardiac muscle fibers

The central region of cardiac troponin T (TnT) is important for modulating the dynamics of muscle length-mediated cross-bridge recruitment. Therefore, hypertrophic cardiomyopathy mutations in the central region may affect cross-bridge recruitment dynamics to alter myofilament Ca²⁺ sensitivity and length-dependent activation of cardiac myofilaments. Given the importance of the central region of TnT for cardiac contractile dynamics, we studied if hypertrophic cardiomyopathy-linked mutation (TnT_R94H)-induced effects on contractile function would be differently modulated by sarcomere length (SL). Recombinant wild-type TnT (TnT_WT) and the guinea pig analog of the human R94H mutation (TnT_R95H) were reconstituted into detergent-skinned cardiac muscle fibers from guinea pigs. Steady-state and dynamic contractile measurements were made at short and long SLs (1.9 and 2.3 µm, respectively). Our results demonstrated that TnT_R95H increased pCa₅₀ (-log of free Ca²⁺ concentration) to a greater extent at short SL; TnT_R95H increased pCa₅₀ by 0.11 pCa units at short SL and 0.07 pCa units at long SL. The increase in pCa₅₀ associated with an increase in SL from 1.9 to 2.3 µm (ΔpCa₅₀) was attenuated nearly twofold in TnT_R95H fibers; ΔpCa₅₀ was 0.09 pCa units for TnT_WT fibers but only 0.05 pCa units for TnT_R95H fibers. The SL dependency of rate constants of cross-bridge distortion dynamics and tension redevelopment was also blunted by TnT_R95H Collectively, our observations on the SL dependency of pCa₅₀ and rate constants of cross-bridge distortion dynamics and tension redevelopment suggest that mechanisms underlying the length-dependent activation cardiac myofilaments are attenuated by TnT_R95HNEW & NOTEWORTHY Mutant cardiac troponin T (TnT_R95H) differently affects myofilament Ca²⁺ sensitivity at short and long sarcomere length, indicating that mechanisms underlying length-dependent activation are altered by TnT_R95H TnT_R95H enhances myofilament Ca²⁺ sensitivity to a greater extent at short sarcomere length, thus attenuating the length-dependent increase in myofilament Ca²⁺ sensitivity.

Pathogenesis of Hypertrophic Cardiomyopathy is Mutation Rather Than Disease Specific: A Comparison of the Cardiac Troponin T E163R and R92Q Mouse Models

Background

In cardiomyocytes from patients with hypertrophic cardiomyopathy, mechanical dysfunction and arrhythmogenicity are caused by mutation‐driven changes in myofilament function combined with excitation‐contraction (E‐C) coupling abnormalities related to adverse remodeling. Whether myofilament or E‐C coupling alterations are more relevant in disease development is unknown. Here, we aim to investigate whether the relative roles of myofilament dysfunction and E‐C coupling remodeling in determining the hypertrophic cardiomyopathy phenotype are mutation specific.

Methods and Results

Two hypertrophic cardiomyopathy mouse models carrying the R92Q and the E163R TNNT2 mutations were investigated. Echocardiography showed left ventricular hypertrophy, enhanced contractility, and diastolic dysfunction in both models; however, these phenotypes were more pronounced in the R92Q mice. Both E163R and R92Q trabeculae showed prolonged twitch relaxation and increased occurrence of premature beats. In E163R ventricular myofibrils or skinned trabeculae, relaxation following Ca²⁺ removal was prolonged; resting tension and resting ATPase were higher; and isometric ATPase at maximal Ca²⁺ activation, the energy cost of tension generation, and myofilament Ca²⁺ sensitivity were increased compared with that in wild‐type mice. No sarcomeric changes were observed in R92Q versus wild‐type mice, except for a large increase in myofilament Ca²⁺ sensitivity. In R92Q myocardium, we found a blunted response to inotropic interventions, slower decay of Ca²⁺ transients, reduced SERCA function, and increased Ca²⁺/calmodulin kinase II activity. Contrarily, secondary alterations of E‐C coupling and signaling were minimal in E163R myocardium.

Conclusions

In E163R models, mutation‐driven myofilament abnormalities directly cause myocardial dysfunction. In R92Q, diastolic dysfunction and arrhythmogenicity are mediated by profound cardiomyocyte signaling and E‐C coupling changes. Similar hypertrophic cardiomyopathy phenotypes can be generated through different pathways, implying different strategies for a precision medicine approach to treatment.

Clinically Divergent Mutation Effects on the Structure and Function of the Human Cardiac Tropomyosin Overlap

The progression of genetically inherited cardiomyopathies from an altered protein structure to clinical presentation of disease is not well understood. One of the main roadblocks to mechanistic insight remains a lack of high-resolution structural information about multiprotein complexes within the cardiac sarcomere. One example is the tropomyosin (Tm) overlap region of the thin filament that is crucial for the function of the cardiac sarcomere. To address this central question, we devised coupled experimental and computational modalities to characterize the baseline function and structure of the Tm overlap, as well as the effects of mutations causing divergent patterns of ventricular remodeling on both structure and function. Because the Tm overlap contributes to the cooperativity of myofilament activation, we hypothesized that mutations that enhance the interactions between overlap proteins result in more cooperativity, and conversely, those that weaken interaction between these elements lower cooperativity. Our results suggest that the Tm overlap region is affected differentially by dilated cardiomyopathy-associated Tm D230N and hypertrophic cardiomyopathy-associated human cardiac troponin T (cTnT) R92L. The Tm D230N mutation compacts the Tm overlap region, increasing the cooperativity of the Tm filament, contributing to a dilated cardiomyopathy phenotype. The cTnT R92L mutation causes weakened interactions closer to the N-terminal end of the overlap, resulting in decreased cooperativity. These studies demonstrate that mutations with differential phenotypes exert opposite effects on the Tm-Tn overlap, and that these effects can be directly correlated to a molecular level understanding of the structure and dynamics of the component proteins.

Cardiac Troponin and Tropomyosin: Structural and Cellular Perspectives to Unveil the Hypertrophic Cardiomyopathy Phenotype

Inherited myopathies affect both skeletal and cardiac muscle and are commonly associated with genetic dysfunctions, leading to the production of anomalous proteins. In cardiomyopathies, mutations frequently occur in sarcomeric genes, but the cause-effect scenario between genetic alterations and pathological processes remains elusive. Hypertrophic cardiomyopathy (HCM) was the first cardiac disease associated with a genetic background. Since the discovery of the first mutation in the β-myosin heavy chain, more than 1400 new mutations in 11 sarcomeric genes have been reported, awarding HCM the title of the “disease of the sarcomere.” The most common macroscopic phenotypes are left ventricle and interventricular septal thickening, but because the clinical profile of this disease is quite heterogeneous, these phenotypes are not suitable for an accurate diagnosis. The development of genomic approaches for clinical investigation allows for diagnostic progress and understanding at the molecular level. Meanwhile, the lack of accurate in vivo models to better comprehend the cellular events triggered by this pathology has become a challenge. Notwithstanding, the imbalance of Ca²⁺ concentrations, altered signaling pathways, induction of apoptotic factors, and heart remodeling leading to abnormal anatomy have already been reported. Of note, a misbalance of signaling biomolecules, such as kinases and tumor suppressors (e.g., Akt and p53), seems to participate in apoptotic and fibrotic events. In HCM, structural and cellular information about defective sarcomeric proteins and their altered interactome is emerging but still represents a bottleneck for developing new concepts in basic research and for future therapeutic interventions. This review focuses on the structural and cellular alterations triggered by HCM-causing mutations in troponin and tropomyosin proteins and how structural biology can aid in the discovery of new platforms for therapeutics. We highlight the importance of a better understanding of allosteric communications within these thin-filament proteins to decipher the HCM pathological state.

Cardiac troponin structure-function and the influence of hypertrophic cardiomyopathy associated mutations on modulation of contractility

Cardiac troponin (cTn) acts as a pivotal regulator of muscle contraction and relaxation and is composed of three distinct subunits (cTnC: a highly conserved Ca(2+) binding subunit, cTnI: an actomyosin ATPase inhibitory subunit, and cTnT: a tropomyosin binding subunit). In this mini-review, we briefly summarize the structure-function relationship of cTn and its subunits, its modulation by PKA-mediated phosphorylation of cTnI, and what is known about how these properties are altered by hypertrophic cardiomyopathy (HCM) associated mutations of cTnI. This includes recent work using computational modeling approaches to understand the atomic-based structural level basis of disease-associated mutations. We propose a viewpoint that it is alteration of cTnC-cTnI interaction (rather than the Ca(2+) binding properties of cTn) per se that disrupt the ability of PKA-mediated phosphorylation at cTnI Ser-23/24 to alter contraction and relaxation in at least some HCM-associated mutations. The combination of state of the art biophysical approaches can provide new insight on the structure-function mechanisms of contractile dysfunction resulting cTnI mutations and exciting new avenues for the diagnosis, prevention, and even treatment of heart diseases.

An historical approach to the diagnostic biomarkers of acute coronary syndrome

Suspected acute myocardial infarction (AMI) is one of the leading causes of admission to the emergency departments in Western countries but also an increasing cause in many other nations. The diagnosis of AMI involves the evaluation of clinical signs and symptoms, electrocardiographic assessment, and measurement of cardiac circulating biomarkers. In the last sixty years, the use of laboratory markers has changed considerably. Early biomarkers assessment has entailed testing for total enzyme activity of aspartate aminotransferase (AST), lactate dehydrogenase (LDH) and creatine kinase (CK). Advances in electrophoresis allowed the identification of more cardio-specific isoenzymes of both CK and LDH, thus leading to the introduction of the CK-MB and LDH-1 activity assays. Soon thereafter, the development of immunoassays, as well as technical advances in automation, allowed the measurements of the CK-MB in mass rather than in activity and myoglobin. Currently, cardiac troponins have the highest sensitivity and specificity for myocardial necrosis and represent the biochemical gold standard for diagnosing AMI. This review provides a chronology of the major events which marked the evolution of cardiac biomarkers testing and the development of the relative assays from the first introduction of AST in the 1950s to the last high sensitivity troponin immunoassays in the 2010s.

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.

The effect of cardiomyopathy mutation (R97L) in mouse cardiac troponin T on the muscle length-mediated recruitment of crossbridges is modified divergently by α- and β-myosin heavy chain

Hypertrophic cardiomyopathy mutations in cardiac troponin T (TnT) lead to sudden cardiac death. Augmented myofilament Ca(2+) sensitivity is a common feature in TnT mutants, but such observations fail to provide a rational explanation for severe cardiac phenotypes. To better understand the mutation-induced effect on the cardiac phenotype, it is imperative to determine the effects on dynamic contractile features such as the muscle length (ML)-mediated activation against α- and β-myosin heavy chain (MHC) isoforms. α- and β-MHC are not only differentially expressed in rodent and human hearts, but they also modify ML-mediated activation differently. Mouse analog of human TnTR94L (TnTR97L) or wild-type TnT was reconstituted into de-membranated muscle fibers from normal (α-MHC) and transgenic (β-MHC) mouse hearts. TnTR97L augmented myofilament Ca(2+) sensitivity by a similar amount in α- and β-MHC fibers. However, TnTR97L augmented the negative impact of strained crossbridges on other crossbridges (γ) by 22% in α-MHC fibers, but attenuated γ by 21% in β-MHC fibers. TnTR97L decreased the magnitude of ML-mediated recruitment of crossbridges (ER) by 37% in α-MHC fibers, but increased ER by 35% in β-MHC fibers. We provide a mechanistic basis for the TnTR97L-induced effects in α- and β-MHC fibers and discuss the relevance to human hearts.

Gene regulation, alternative splicing, and posttranslational modification of troponin subunits in cardiac development and adaptation: a focused review

Troponin plays a central role in regulating the contraction and relaxation of vertebrate striated muscles. This review focuses on the isoform gene regulation, alternative RNA splicing, and posttranslational modifications of troponin subunits in cardiac development and adaptation. Transcriptional and posttranscriptional regulations such as phosphorylation and proteolysis modifications, and structure-function relationships of troponin subunit proteins are summarized. The physiological and pathophysiological significances are discussed for impacts on cardiac muscle contractility, heart function, and adaptations in health and diseases.

Allosteric effects of cardiac troponin TNT1 mutations on actomyosin binding: a novel pathogenic mechanism for hypertrophic cardiomyopathy

The majority of hypertrophic cardiomyopathy mutations in (cTnT) occur within the alpha-helical tropomyosin binding TNT1 domain. A highly charged region at the C-terminal end of TNT1 unwinds to create a flexible "hinge". While this region has not been structurally resolved, it likely acts as an extended linker between the two cTnT functional domains. Mutations in this region cause phenotypically diverse and often severe forms of HCM. Mechanistic insight, however, has been limited by the lack of structural information. To overcome this limitation, we evaluated the effects of cTnT 160-163 mutations using regulated in vitro motility (R-IVM) assays and transgenic mouse models. R-IVM revealed that cTnT mutations Δ160E, E163R and E163K disrupted weak electrostatic actomyosin binding. Reducing the ionic strength or decreasing Brownian motion rescued function. This is the first observation of HCM-linked mutations in cTnT disrupting weak interactions between the thin filament and myosin. To evaluate the in vivo effects of altering weak actomyosin binding we generated transgenic mice expressing Δ160E and E163R mutant cTnT and observed severe cardiac remodeling and profound myofilament disarray. The functional changes observed in vitro may contribute to the structural impairment seen in vivo by destabilizing myofilament structure and acting as a constant pathophysiologic stress.

A Drosophila melanogaster model of diastolic dysfunction and cardiomyopathy based on impaired troponin-T function

RATIONALE

Regulation of striated muscle contraction is achieved by Ca2+ -dependent steric modulation of myosin cross-bridge cycling on actin by the thin filament troponin-tropomyosin complex. Alterations in the complex can induce contractile dysregulation and disease. For example, mutations between or near residues 112 to 136 of cardiac troponin-T, the crucial TnT1 (N-terminal domain of troponin-T)-tropomyosin-binding region, cause cardiomyopathy. The Drosophila upheld(101) Glu/Lys amino acid substitution lies C-terminally adjacent to this phylogenetically conserved sequence.

OBJECTIVE

Using a highly integrative approach, we sought to determine the molecular trigger of upheld(101) myofibrillar degeneration, to evaluate contractile performance in the mutant cardiomyocytes, and to examine the effects of the mutation on the entire Drosophila heart to elucidate regulatory roles for conserved TnT1 regions and provide possible mechanistic insight into cardiac dysfunction.

METHODS AND RESULTS

Live video imaging of Drosophila cardiac tubes revealed that the troponin-T mutation prolongs systole and restricts diastolic dimensions of the heart, because of increased numbers of actively cycling myosin cross-bridges. Elevated resting myocardial stiffness, consistent with upheld(101) diastolic dysfunction, was confirmed by an atomic force microscopy-based nanoindentation approach. Direct visualization of mutant thin filaments via electron microscopy and 3-dimensional reconstruction resolved destabilized tropomyosin positioning and aberrantly exposed myosin-binding sites under low Ca2+ conditions.

CONCLUSIONS

As a result of troponin-tropomyosin dysinhibition, upheld(101) hearts exhibited cardiac dysfunction and remodeling comparable to that observed during human restrictive cardiomyopathy. Thus, reversal of charged residues about the conserved tropomyosin-binding region of TnT1 may perturb critical intermolecular associations required for proper steric regulation, which likely elicits myopathy in our Drosophila model.

Megalobrama amblycephala cardiac troponin T variants: molecular cloning, expression and response to nitrite

Cardiac troponin T (TNNT2), as a member of troponin superfamily, plays important roles during early cardiogenesis, and contraction and relaxation of myocardial cells. In this study, two alternatively spliced variants of Megalobrama amblycephala TNNT2 were identified showing a difference of 19 amino acids in the N-terminal hypervariable region. The longer cDNA (TNNT2-1) was 1,118 bp, encoding 284 amino acid residues, contained conserved central tropomyosin-binding region, cardiac specific signal and C-terminal segments except the N-terminal hypervariable region. The TNNT2 transcripts first appeared at 16 hours post-fertilization (hpf) peaking at 28 hpf (onset of heartbeat). In addition, strong expression of TNNT2 was found in the cardiac muscle. After nitrite exposure, the increased TNNT2 expression levels in the heart indicated that nitrite might induce cardiac injury. Results of semi-quantitative RT-PCR indicated that the two alternatively spliced variants existed in early development stages since their first appearance at 16 hpf and heart, spleen, headkiney of M. amblycephala. The shorter transcript (TNNT2-2) was proved to be dominant in the embryos and heart of M. amblycephala, furthermore, the increase of TNNT2 expression level in the heart after nitrite exposure was mainly caused by TNNT2-2.

HCM-linked ∆160E cardiac troponin T mutation causes unique progressive structural and molecular ventricular remodeling in transgenic mice

Hypertrophic cardiomyopathy (HCM) is a primary disease of the cardiac muscle, and one of the most common causes of sudden cardiac death (SCD) in young people. Many mutations in cardiac troponin T (cTnT) lead to a complex form of HCM with varying degrees of ventricular hypertrophy and ~65% of all cTnT mutations occur within or flanking the elongated N-terminal TNT1 domain. Biophysical studies have predicted that distal TNT1 mutations, including Δ160E, cause disease by a novel, yet unknown mechanism as compared to N-terminal mutations. To begin to address the specific effects of this commonly observed cTnT mutation we generated two independent transgenic mouse lines carrying variant doses of the mutant transgene. Hearts from the 30% and 70% cTnT Δ160E lines demonstrated a highly unique, dose-dependent disruption in cellular and sarcomeric architecture and a highly progressive pattern of ventricular remodeling. While adult ventricular myocytes isolated from Δ160E transgenic mice exhibited dosage-independent mechanical impairments, decreased sarcoplasmic reticulum calcium load and SERCA2a calcium uptake activity, the observed decreases in calcium transients were dosage-dependent. The latter findings were concordant with measures of calcium regulatory protein abundance and phosphorylation state. Finally, studies of whole heart physiology in the isovolumic mode demonstrated dose-dependent differences in the degree of cardiac dysfunction. We conclude that the observed clinical severity of the cTnT Δ160E mutation is caused by a combination of direct sarcomeric disruption coupled to a profound dysregulation of Ca(2+) homeostasis at the cellular level that results in a unique and highly progressive pattern of ventricular remodeling.

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.

Mutations in the cardiac troponin T gene show various prognoses in Japanese patients with hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant disorder resulting from mutations in genes for at least 15 various sarcomere-related proteins including cardiac β-myosin heavy chain, cardiac myosin-binding protein C, and cardiac troponin T. The troponin T gene (TNNT2) mutation has the third incidence of familial HCM, and the genotype–phenotype correlation of this gene still remains insufficient in Japanese familial HCM. Therefore, in the present study, we focused on screening the TNNT2 mutation in 173 unrelated Japanese patients with familial HCM, and found three reported mutations and a new mutation of TNNT2 in 11 individuals from four families. In these families, two individuals from one family had double mutations, Arg130Cys and Phe110Ile, six individuals from two other families had an Arg92Trp mutation, and one individual of another family had a new mutation, Ile79Thr, of TNNT2. The phenotype of each family was often different from reported cases, even if they had the same genetic mutation. In addition, families with the same genetic mutation showed a similar trend in the phenotype, but it was not exactly the same. However, sudden death in youth was observed in all of these families. Although the type of genetic mutation is not useful for predicting prognosis in HCM, the possibility of sudden cardiac death remains. Therefore, the prognosis of individuals bearing the TNNT2 mutation with familial HCM should be more carefully observed from birth.

Molecular effects of familial hypertrophic cardiomyopathy-related mutations in the TNT1 domain of cTnT

Familial hypertrophic cardiomyopathy (FHC) is one of the most common genetic causes of heart disease. Approximately 15% of FHC-related mutations are found in cTnT [cardiac troponin (cTn) T]. Most of the cTnT FHC-related mutations are in or flanking the N-tail TNT1 domain that directly interacts with overlapping tropomyosin (Tm). We investigate two sets of cTnT mutations at opposite ends of TNT1, mutations in residue 92 in the Tm-Tm overlap region of TNT1 and mutations in residues 160 and 163 in the C-terminal portion of TNT1 adjacent to the cTnT H1-H2 linker. Though all the mutations are located within TNT1, they have widely different phenotypes clinically and biophysically. Using a complete atomistic model of the cTn-Tm complex, we identify mechanisms by which the effects of TNT1 mutations propagate to the cTn core and site II of cTnC, where calcium binding and dissociation occurs. We find that mutations in TNT1 alter the flexibility of TNT1, which is inversely proportional to the cooperativity of calcium activation of the thin filament. Further, we identify a pathway of propagation of structural and dynamic changes from TNT1 to site II of cTnC, including TNT1, cTnT linker, I-T arm, regulatory domain of cTnI, the D-E linker of cTnC, and site II cTnC. Mutationally induced changes at site II of cTnC alter calcium coordination that corresponds to biophysical measurements of calcium sensitivity. Finally, we compare this pathway of mutational propagation with that of the calcium activation of the thin filament and find that they are identical but opposite in direction.

Evaluating pathogenicity of rare variants from dilated cardiomyopathy in the exome era

BACKGROUND

Human exome sequencing is a recently developed tool to aid in the discovery of novel coding variants. Now broadly applied, exome sequencing data sets provide a novel opportunity to evaluate the allele frequencies of previously published pathogenic rare variants.

METHODS AND RESULTS

We examined the exome data set from the National Heart, Lung and Blood Institute Exome Sequencing Project and compared this data set with a catalog of 197 previously published rare variants reported as causative of dilated cardiomyopathy (DCM) from familial and sporadic cases. Of these 197, 33 (16.8%) were also present in the Exome Sequencing Project database, raising the question of whether they were uncommon polymorphisms. Supporting functional data has been published for 14 of the 33 (42%), suggesting they are unlikely to be false-positives. The frequencies of these functional variants in the Exome Sequencing Project data set ranged from 0.02 to 1.33% (median 0.04%), which when applied as a cutoff to filter variants in a DCM pedigree identified an additional DCM candidate gene. A greater proportion of sporadic DCM cases had variants that were present in the Exome Sequencing Project data set versus novel variants (ie, not in the Exome Sequencing Project; 44% versus 21%; P=0.002), suggesting some of the variants identified as disease causing in sporadic DCM are either false-positives or low penetrance alleles in human populations.

CONCLUSIONS

Rare nonsynonymous variants identified in DCM subjects also present at very low frequencies in public databases are likely relevant for DCM. Allele frequencies >0.04% are of less certain pathogenicity, especially if identified in sporadic cases, although this cutoff should be viewed as preliminary.

Correlation of molecular and functional effects of mutations in cardiac troponin T linked to familial hypertrophic cardiomyopathy: an integrative in silico/in vitro approach

Nearly 70% of all of the known cTnT mutations that cause familial hypertrophic cardiomyopathy fall within the TNT1 region that is critical to cTn-Tm binding. The high resolution structure of this domain has not been determined, and this lack of information has hindered structure-function analysis. In the current study, a coupled computational experimental approach was employed to correlate changes in cTnT dynamics to basic function using the regulated in vitro motility assay (R-IVM). An in silico approach to calculate forces in terms of a bending coordinate was used to precisely identify decreases in bending forces at residues 105 and 106 within the proposed cTnT "hinge" region. Significant functional changes were observed in multiple functional properties, including a decrease in the cooperativity of calcium activation, the calcium sensitivity of sliding speed, and maximum sliding speed. Correlation of the computational and experimental findings revealed an association between TNT1 flexibility and the cooperativity of thin filament calcium activation where an increase in flexibility led to a decrease in cooperativity. Further analysis of the primary sequence of the TNT1 region revealed a unique pattern of conserved charged TNT1 residues altered by the R92W and R92L mutations and may represent the underlying "structure" modulating this central functional domain. These data provide a framework for further integrated in silico/in vitro approaches that may be extended into a high-throughput predictive screen to overcome the current structural limitations in linking molecular phenotype to genotype in thin filament cardiomyopathies.

Computational Characterization of Mutations in Cardiac Troponin T Known to Cause Familial Hypertrophic Cardiomyopathy

Cardiac Troponin T (cTnT) is a central modulator of thin filament regulation of myofilament activation. The lack of structural data for the TNT1 tail domain, a proposed α-helical region, makes the functional implications of the FHC mutations difficult to determine. Studies have suggested that flexibility of TNT1 is important in normal protein-protein interactions within the thin filament. Our groups have previously shown through Molecular Dynamics (MD) simulations that some FHC mutations, Arg92Leu(R92L) and Arg92Trp(R92W), result in increased flexibility at a critical hinge region 12 residues distant from the mutation. To explain this distant effect and its implications for FHC mutations, we characterized the dynamics of wild type and mutational segments of cTnT using MD. Our data shows an opening of the helix between residues 105-110 in mutants. Consequently, the dihedral angles of these residues correspond to non-α-helical regions on Ramachandran plots. We hypothesize the removal of a charged residue decreases electrostatic repulsion between the point mutation and surrounding residues resulting in local helical compaction. Constrained ends of the helix and localized compaction results in expansion within the nearest non-polar helical turn from the mutation site, residues 105-109.

Genetics of inherited cardiomyopathy

During the past two decades, numerous disease-causing genes for different cardiomyopathies have been identified. These discoveries have led to better understanding of disease pathogenesis and initial steps in the application of mutation analysis in the evaluation of affected individuals and their family members. As knowledge of the genetic abnormalities, and insight into cellular and organ biology has grown, so has appreciation of the level of complexity of interaction between genotype and phenotype across disease states. What were initially thought to be one-to-one gene-disease correlates have turned out to display important relational plasticity dependent in large part on the genetic and environmental backgrounds into which the genes of interest express. The current state of knowledge with regard to genetics of cardiomyopathy represents a starting point to address the biology of disease, but is not yet developed sufficiently to supplant clinically based classification systems or, in most cases, to guide therapy to any significant extent. Future work will of necessity be directed towards elucidation of the biological mechanisms of both rare and common gene variants and environmental determinants of plasticity in the genotype-phenotype relationship with the ultimate goal of furthering our ability to identify, diagnose, risk stratify, and treat this group of disorders which cause heart failure and sudden death in the young.

Thin filament mutations: developing an integrative approach to a complex disorder

Sixteen years ago, mutations in cardiac troponin (Tn)T and α-tropomyosin were linked to familial hypertrophic cardiomyopathy, thus transforming the disorder from a disease of the β-myosin heavy chain to a disease of the cardiac sarcomere. From the outset, studies suggested that mutations in the regulatory thin filament caused a complex, heterogeneous pattern of ventricular remodeling with wide variations in clinical expression. To date, the clinical heterogeneity is well matched by an extensive array of nearly 100 independent mutations in all components of the cardiac thin filament. Significant advances in our understanding of the biophysics of myofilament activation, coupled to the emerging evidence that thin filament linked cardiomyopathies are progressive, suggests that a renewed focus on the most proximal events in both the molecular and clinical pathogenesis of the disease will be necessary to achieve the central goal of using genotype information to manage affected patients. In this review, we examine the existing biophysical and clinical evidence in support of a more proximal definition of thin filament cardiomyopathies. In addition, new high-resolution, integrated approaches are presented to help define the way forward as the field works toward developing a more robust link between genotype and phenotype in this complex disorder.

Late onset sporadic dilated cardiomyopathy caused by a cardiac troponin T mutation

Mutations in TNNT2, encoding cardiac troponin T, commonly shows early onset, aggressive dilated cardiomyopathy (DCM). This observation may influence the decision of whether to undertake clinical genetic testing for TNNT2 in later onset DCM. Further, the trigger for late onset DCM remains enigmatic. A 70-year-old woman, previously healthy with a left ventricular ejection fraction of 50%-55% at age 69, presented with DCM of unknown cause and a 4-month history progressive heart failure requiring cardiac transplantation. Clinical genetic testing revealed a novel TNNT2 R139H mutation but no relevant variants in 18 other DCM genes. Her explanted heart showed partial fatty replacement in the right ventricle. Sequencing for five arrhythmogenic right ventricular dysplasia genes was negative. Functional studies in porcine cardiac skinned fibers reconstituted with the mutant R139H troponin T protein showed decreased Ca(2+) sensitivity at pH 7, characteristic of DCM. Because fatty infiltration may acidify the myocellular environment, maximal force development examined at pH 6.5 was diminished, suggesting a possible environmental trigger. We conclude that the TNNT2 R139H mutation was likely to be disease causing. Further, later age of onset may not be relevant to exclude genetic testing for TNNT2 mutations.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Hypertrophic cardiomyopathy: from genetics to treatment

BACKGROUND

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

DESIGN

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

RESULTS

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

CONCLUSIONS

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

Experimental therapies in hypertrophic cardiomyopathy

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

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.

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.

Temporal and mutation-specific alterations in Ca2+ homeostasis differentially determine the progression of cTnT-related cardiomyopathies in murine models

Naturally occurring mutations in cardiac troponin T (cTnT) result in a clinical subset of familial hypertrophic cardiomyopathy. To determine the mechanistic links between thin-filament mutations and cardiovascular phenotypes, we have generated and characterized several transgenic mouse models carrying cTnT mutations. We address two central questions regarding the previously observed changes in myocellular mechanics and Ca(2+) homeostasis: 1) are they characteristic of all severe cTnT mutations, and 2) are they primary (early) or secondary (late) components of the myocellular response? Adult left ventricular myocytes were isolated from 2- and 6-mo-old transgenic mice carrying missense mutations at residue 92, flanking the TNT1 NH(2)-terminal tail domain. Results from R92L and R92W myocytes showed mutation-specific alterations in contraction and relaxation indexes at 2 mo with improvements by 6 mo. Alterations in Ca(2+) kinetics remained consistent with mechanical data in which R92L and R92W exhibited severe diastolic impairments at the early time point that improved with increasing age. A normal regulation of Ca(2+) kinetics in the context of an altered baseline cTnI phosphorylation suggested a pathogenic mechanism at the myofilament level taking precedence for R92L. The quantitation of Ca(2+)-handling proteins in R92W mice revealed a synergistic compensatory mechanism involving an increased Ser16 and Thr17 phosphorylation of phospholamban, contributing to the temporal onset of improved cellular mechanics and Ca(2+) homeostasis. Therefore, independent cTnT mutations in the TNT1 domain result in primary mutation-specific effects and a differential temporal onset of altered myocellular mechanics, Ca(2+) kinetics, and Ca(2+) homeostasis, complex mechanisms which may contribute to the clinical variability in cTnT-related familial hypertrophic cardiomyopathy mutations.

Functional aberration of myofibrils by cardiomyopathy-causing mutations in the coiled-coil region of the troponin-core domain

Two cardiomyopathy-causing mutations, E244D and K247R, in human cardiac troponin T (TnT) are located in the coiled-coil region of the Tn-core domain. To elucidate effects of mutations in this region on the regulatory function of Tn, we measured Ca(2+)-dependent ATPase activity of myofibrils containing various mutants of TnT at these residues. The results confirmed that the mutant E244D increases the maximum ATPase activity without changing the Ca(2+)-sensitivity. The mutant K247R was shown for the first time to have the effect similar to the mutant E244D. Furthermore, various TnT mutants (E244D, E244M, E244A, E244K, K247R, K247E, and K247A) showed various effects on the maximum ATPase activity while the Ca(2+)-sensitivity was unchanged. Molecular dynamics simulations of the Tn-core containing these TnT mutants suggested that the hydrogen-bond network formed by the side chains of neighboring residues around residues 244 and 247 is important for Tn to function properly.

Cardiac troponin T mutations promote life-threatening arrhythmias

Mutations in contractile proteins in heart muscle can cause anatomical changes that result in cardiac arrhythmias and sudden cardiac death. However, a conundrum has existed because mutations in one such contractile protein, a so-called Ca2+ sensor troponin T (TnT), can promote ventricular rhythm disturbances even in the absence of hypertrophy or fibrosis. Thus, these mutations must enhance abnormal electrophysiological events via alternative means. In this issue of the JCI, Baudenbacher et al. report a novel mechanism to explain this puzzle (see the related article beginning on page 3893). They show that a selected TnT mutation in the adult mouse heart can markedly increase the sensitivity of cardiac muscle myofilaments to Ca2+ and enhance the susceptibility to arrhythmia, even in the absence of anatomical deformities. As these same mutations can cause some forms of arrhythmias in humans, these findings are of both basic and translational significance.

Cardiac troponin T mutation in familial cardiomyopathy with variable remodeling and restrictive physiology

We identified a unique family with autosomal dominant heart disease variably expressed as restrictive cardiomyopathy (RCM), hypertrophic cardiomyopathy (HCM), and dilated cardiomyopathy (DCM), and sought to identify the molecular defect that triggered divergent remodeling pathways. Polymorphic DNA markers for nine sarcomeric genes for DCM and/or HCM were tested for segregation with disease. Linkage to eight genes was excluded, but a cardiac troponin T (TNNT2) marker cosegregated with the disease phenotype. Sequencing of TNNT2 identified a heterozygous missense mutation resulting in an I79N substitution, inherited by all nine affected family members but by none of the six unaffected relatives. Mutation carriers were diagnosed with RCM (n = 2), non-obstructive HCM (n = 3), DCM (n = 2), mixed cardiomyopathy (n = 1), and mild concentric left ventricular hypertrophy (n = 1). Endomyocardial biopsy in the proband revealed non-specific fibrosis, myocyte hypertrophy, and no myofibrillar disarray. Restrictive Doppler filling patterns, atrial enlargement, and pulmonary hypertension were observed among family members regardless of cardiomyopathy subtype. Mutation of a sarcomeric protein gene can cause RCM, HCM, and DCM within the same family, underscoring the necessity of comprehensive morphological and physiological cardiac assessment in familial cardiomyopathy screening.

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.

Functional consequences of the human cardiac troponin I hypertrophic cardiomyopathy mutation R145G in transgenic mice

In this study, we addressed the functional consequences of the human cardiac troponin I (hcTnI) hypertrophic cardiomyopathy R145G mutation in transgenic mice. Simultaneous measurements of ATPase activity and force in skinned papillary fibers from hcTnI R145G transgenic mice (Tg-R145G) versus hcTnI wild type transgenic mice (Tg-WT) showed a significant decrease in the maximal Ca(2+)-activated force without changes in the maximal ATPase activity and an increase in the Ca(2+) sensitivity of both ATPase and force development. No difference in the cross-bridge turnover rate was observed at the same level of cross-bridge attachment (activation state), showing that changes in Ca(2+) sensitivity were not due to changes in cross-bridge kinetics. Energy cost calculations demonstrated higher energy consumption in Tg-R145G fibers compared with Tg-WT fibers. The addition of 3 mm 2,3-butanedione monoxime at pCa 9.0 showed that there was approximately 2-4% of force generating cross-bridges attached in Tg-R145G fibers compared with less than 1.0% in Tg-WT fibers, suggesting that the mutation impairs the ability of the cardiac troponin complex to fully inhibit cross-bridge attachment under relaxing conditions. Prolonged force and intracellular [Ca(2+)] transients in electrically stimulated intact papillary muscles were observed in Tg-R145G compared with Tg-WT. These results suggest that the phenotype of hypertrophic cardiomyopathy is most likely caused by the compensatory mechanisms in the cardiovascular system that are activated by 1) higher energy cost in the heart resulting from a significant decrease in average force per cross-bridge, 2) slowed relaxation (diastolic dysfunction) caused by prolonged [Ca(2+)] and force transients, and 3) an inability of the cardiac TnI to completely inhibit activation in the absence of Ca(2+) in Tg-R145G mice.