Cellular and molecular aspects of familial hypertrophic cardiomyopathy caused by mutations in the cardiac troponin I gene

Mutations in the cardiac troponin I (CTnI) gene occur in 5% of families with familial hypertrophic cardiomyopathy (FHC) and 20 mutations in this gene that cause FHC have now been described. The clinical manifestations of CTnI mutations that cause FHC are diverse, ranging from asymptomatic with high life expectancy to severe heart failure and sudden cardiac death. Most of these FHC mutations in CTnI result in cardiac hypertrophy unlike cardiac troponin T FHC mutations. All CTnI FHC mutations investigated in vitro affect the physiological function of CTnI, but other factors such as environmental or genetic factors (other genes that may affect the CTnI gene) are likely to be involved in influencing the severity of the phenotype produced by these mutations, since the distribution of hypertrophy among affected individuals varies within and between families. CTnI mutations mainly alter myocardial performance via changes in the Ca²⁺-sensitivity of force development and in some cases alter the muscle relaxation kinetics due to haemodynamic or physical obstructions of blood flow from the left ventricle. (Mol Cell Biochem 263: 99–114, 2004)     

Functional consequences of troponin T mutations found in hypertrophic cardiomyopathy.

Missense mutations in the cardiac thin filament protein troponin T (TnT) are a cause of familial hypertrophic cardiomyopathy (FHC). To understand how these mutations produce dysfunction, five TnTs were produced and purified containing FHC mutations found in several regions of TnT. Functional defects were diverse. Mutations F110I, E244D, and COOH-terminal truncation weakened the affinity of troponin for the thin filament. Mutation DeltaE160 resulted in thin filaments with increased calcium affinity at the regulatory site of troponin C. Mutations R92Q and F110I resulted in impaired troponin solubility, suggesting abnormal protein folding. Depending upon the mutation, the in vitro unloaded actin-myosin sliding speed showed small increases, showed small decreases, or was unchanged. COOH-terminal truncation mutation resulted in a decreased thin filament-myosin subfragment 1 MgATPase rate. The results indicate that the mutations cause diverse immediate effects, despite similarities in disease manifestations. Separable but repeatedly observed abnormalities resulting from FHC TnT mutations include increased unloaded sliding speed, increased or decreased Ca(2+) affinity, impairment of folding or sarcomeric integrity, and decreased force. Enhancement as well as impairment of contractile protein function is observed, suggesting that TnT, including the troponin tail region, modulates the regulation of cardiac contraction.     

Cardiac and skeletal muscle expression of mutant β-myosin heavy chains, degree of functional impairment and phenotypic heterogeneity in hypertrophic cardiomyopathy

Several mutations in distinct genes, all coding for sarcomeric proteins, have been reported in unrelated kindreds with familial hypertrophic cardiomyopathy (FHC). We have identified nine individuals from three families harboring two distinct mutations in one copy of the β-myosin heavy chain (β-MHC) gene. In this study, the expression of the mutant β-myosin protein isoform, isolated from slow-twitch fibers of skeletal muscle, was demonstrated by Northern and Western blot analysis; this myosin showed a decreased in vitro motility activity and produced a lower actin-activated ATPase activity. Isometric tension, measured in single slow-twitch fibers isolated from the affected individuals, also showed a significant decrease. The degree of impairment of β-myosin function, as well as the loss in isometric tension development, were strictly dependent on the amount of the isoform transcribed from the mutated allele. Interestingly, a strong correlation was also demonstrated between mutant β-myosin content and clinical features of FHC. On the other hand, we were unable to detect any correlation between mutant β-myosin expression and degree of cardiac hypertrophy, thereby strengthening the hypothesis that hypertrophy, one of the hallmarks of FHC, might not necessarily be related to the clinical evolution of this disease. These findings lend support to the notion that additional factors rather than the mutated gene may play a pathogenetic role in cardiac wall thickening, whereas the prognosis appears to be strongly related to the amount of mutant protein.     

Role of troponin T in disease

Several striated muscle myopathies have been directly linked to mutations in contractile and associated proteins. Troponin T (TnT) is one of the three subunits that form troponin (Tn) which together with tropomyosin is responsible for the regulation of striated muscle contraction. All three subunits of cardiac Tn as well as tropomyosin have been associated with hypertrophic cardiomyopathy (HCM). However, TnT accounts for most of the mutations that cause HCM in these regulatory proteins. To date 30 mutations have been identified in the cardiac TnT (CTnT) gene that results in familial HCM (FHC). The CTnT gene has also been associated with familial dilated cardiomyopathy (DCM). CTnT deficiency is lethal due to impaired cardiac development. A recessive nonsense mutation in the gene encoding slow skeletal TnT has been associated with an unusual, severe form of nemaline myopathy among the Old Order Amish. How each mutation leads to the diverse clinical symptoms associated with FHC, DCM or nemaline myopathy is unclear. However, the use of animal model systems, in particular transgenic mice, has significantly increased our knowledge of normal and myopathic muscle physiology. In this review, we focus on the role of TnT in muscle physiology and disease. (Mol Cell Biochem 263: 115–129, 2004)     

Computational simulation of hypertrophic cardiomyopathy mutations in troponin I: influence of increased myofilament calcium sensitivity on isometric force, ATPase and [Ca2+]i

Familial hypertrophic cardiomyopathy (FHC) is an inherited disease that is characterized by ventricular hypertrophy, cardiac arrhythmias and increased risk of premature sudden death. FHC is caused by autosomal-dominant mutations in genes for a number of sarcomeric proteins; many mutations in Ca(2+)-regulatory proteins of the cardiac thin filament are associated with increased Ca(2+) sensitivity of myofilament function. Computational simulations were used to investigate the possibility that these mutations could affect the Ca(2+) transient and mechanical response of a myocyte during a single cardiac cycle. We used existing experimental data for specific mutations of cardiac troponin I that exhibit increased Ca(2+) sensitivity in physiological and biophysical assays. The simulated Ca(2+) transients were used as input for a three-dimensional half-sarcomere biomechanical model with filament compliance to predict the resulting force. Mutations with the highest Ca(2+) affinity (lowest K(m)) values, exhibit the largest decrease in peak Ca(2+) assuming a constant influx of Ca(2+) into the cytoplasm; they also prolong Ca(2+) removal but have little effect on diastolic Ca(2+). Biomechanical model results suggest that these cTnI mutants would increase peak force despite the decrease in peak [Ca(2+)](i). There is a corresponding increase in net ATP hydrolysis, with no change in tension cost (ATP hydrolyzed per unit of time-integrated tension). These simulations suggest that myofilament-initiated hypertrophic signaling could be associated with decreased [Ca(2+)](i), increased stress/strain, and/or increased ATP flux.     

Independent origin of identical beta cardiac myosin heavy-chain mutations in hypertrophic cardiomyopathy.

The origins of the beta cardiac myosin heavy-chain (MHC) gene missense mutations that cause familial hypertrophic cardiomyopathy (FHC) in 14 families have been evaluated. Of eight different mutations, four were present in single families, while four occurred in two or more families. To investigate the origins of the four shared mutations, we defined the beta cardiac MHC haplotypes of each of the mutation-bearing chromosomes by determining the alleles present at three intragenic polymorphic loci. Two of the mutations (Arg453Cys and Val606Met) have arisen independently in each of three families, being found on different chromosomal backgrounds. A third mutation (Gly584Arg) is associated with identical haplotypes in two families with Portuguese ancestors, suggesting a founder effect. Haplotype analysis was uninformative for the fourth mutation (Arg403Gln). Thus, FHC-causing mutations have arisen independently in at least 12 of the 14 families studied, suggesting that the majority have arisen relatively recently as new mutations. This finding predicts the prevalence of disease-causing beta cardiac MHC mutations to be comparable in all population groups.     

A molecular basis for familial hypertrophic cardiomyopathy: a beta cardiac myosin heavy chain gene missense mutation

A point mutation in exon 13 of the beta cardiac myosin heavy chain (MHC) gene is present in all individuals affected with familial hypertrophic cardiomyopathy (FHC) from a large kindred. This missense mutation converts a highly conserved arginine residue (Arg-403) to a glutamine. Affected individuals from an unrelated family lack this missense mutation, but instead have an alpha/beta cardiac MHC hybrid gene. Identification of two unique mutations within cardiac MHC genes in all individuals with FHC from two unrelated families demonstrates that defects in the cardiac MHC genes can cause this disease. The pathology resulting from a missense mutation at residue 403 further suggests that a critical function of myosin is disrupted by this mutation.     

Prolonged Ca2+ and force transients in myosin RLC transgenic mouse fibers expressing malignant and benign FHC mutations

Clinical studies have revealed that mutations in the ventricular myosin regulatory light chain (RLC) lead to the development of familial hypertrophic cardiomyopathy (FHC), an autosomal dominant disease characterized by left ventricular hypertrophy, myofibrillar disarray and sudden cardiac death. While mutations in other contractile proteins have been studied widely by others, there is no report elucidating the mechanism(s) associated with FHC-linked RLC mutations. In this study, we have assessed the functional consequences of two RLC mutations, R58Q and N47K, in transgenic mice. Clinical phenotypes associated with these mutations included inter-ventricular hypertrophy, abnormal ECG findings and the R58Q mutation caused multiple cases of premature sudden cardiac death. Simultaneous measurements of the ATPase and force in transgenic skinned papillary muscle fibers from mutated versus control mice showed an increase in the Ca(2+) sensitivity of ATPase and steady-state force only in R58Q fibers. The calculated energy cost or rate of dissociation of force generating myosin cross-bridges (ATPase/force ratio) plotted as a function of activation state was the same in all groups of fibers. Both mutations caused prolonged [Ca(2+)] transients in electrically stimulated intact papillary muscles; however, the R58Q mutation also resulted in a significantly prolonged force transient. Our results suggest that the phenotypes of FHC observed in patients harboring these RLC mutations correlate with the extent of physiological changes monitored in transgenic fibers. Cardiac hypertrophy observed in patients is most likely caused by the activation of compensatory mechanisms ensuing from higher workloads due to incomplete relaxation as evidenced by prolonged [Ca(2+)] transients for both N47K and R58Q fibers. Furthermore, the poor prognosis of the R58Q patients may be associated with more severe diastolic dysfunction due to the slower off-rate of Ca(2+) from troponin C leading to longer force and [Ca(2+)] transients and increased Ca(2+) sensitivity of ATPase and force.     

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

Restrictive cardiomyopathy (RCM) is a rare disorder characterized by impaired ventricular filling with decreased diastolic volume. We are reporting the functional effects of the first cardiac troponin T (CTnT) mutation linked to infantile RCM resulting from a de novo deletion mutation of glutamic acid 96. The mutation was introduced into adult and fetal isoforms of human cardiac TnT (HCTnT3-DeltaE96 and HCTnT1-DeltaE106, respectively) and studied with either cardiac troponin I (CTnI) or slow skeletal troponin I (SSTnI). Skinned cardiac fiber measurements showed a large leftward shift in the Ca(2+) sensitivity of force development with no differences in the maximal force. HCTnT1-DeltaE106 showed a significant increase in the activation of actomyosin ATPase with either CTnI or SSTnI, whereas HCTnT3-DeltaE96 was only able to increase the ATPase activity with CTnI. Both mutants showed an impaired ability to inhibit the ATPase activity. The capacity of the CTnI.CTnC and SSTnI.CTnC complexes to fully relax the fibers after TnT displacement was also compromised. Experiments performed using fetal troponin isoforms showed a less severe impact compared with the adult isoforms, which is consistent with the cardioprotective role of SSTnI and the rapid onset of RCM after birth following the isoform switch. These data indicate that troponin mutations related to RCM may have specific functional phenotypes, including large leftward shifts in the Ca(2+) sensitivity and impaired abilities to inhibit ATPase and to relax skinned fibers. All of this would account for and contribute to the severe diastolic dysfunction seen in RCM.     

Effects of a cardiomyopathy-causing troponin t mutation on thin filament function and structure

Familial hypertrophic cardiomyopathy (FHC) is caused by missense or premature truncation mutations in proteins of the cardiac contractile apparatus. Mutant proteins are incorporated into the thin filament or thick filament and eventually produce cardiomyopathy. However, it has been unclear how the several, genetically identified defects in protein structure translate into impaired protein and muscle function. We have studied the basis of FHC caused by premature truncation of the most frequently implicated thin filament target, troponin T. Electron microscope observations showed that the thin filament undergoes normal structural changes in response to Ca(2+) binding. On the other hand, solution studies showed that the mutation alters and destabilizes troponin binding to the thin filament to different extents in different regulatory states, thereby affecting the transitions among states that regulate myosin binding and muscle contraction. Development of hypertrophic cardiomyopathy can thus be traced to a defect in the primary mechanism controlling cardiac contraction, switching between different conformations of the thin filament.     

A molecular basis for familial hypertrophic cardiomyopathy: an alpha/beta cardiac myosin heavy chain hybrid gene

An alpha/beta cardiac myosin heavy chain (MHC) hybrid gene is coinherited with familial hypertrophic cardiomyopathy (FHC) in one kindred. FHC is a disease of the heart muscle characterized by a thickening of the left ventricular wall with myocyte and myofibrillar disarray that is inherited as an autosomal dominant trait. We demonstrate here and in the accompanying article that the cardiac MHC genes, which encode integral myofibrillar components, are mutated in all affected individuals from two unrelated families with FHC. In one kindred, an unequal crossover event during meiosis may have produced the alpha/beta cardiac MHC hybrid gene that is present in affected individuals. We conclude that mutations in the cardiac MHC genes can cause FHC.     

Familial hypertrophic cardiomyopathy-linked alterations in Ca2+ binding of human cardiac myosin regulatory light chain affect cardiac muscle contraction

The ventricular isoform of human cardiac regulatory light chain (HCRLC) has been shown to be one of the sarcomeric proteins associated with familial hypertrophic cardiomyopathy (FHC), an autosomal dominant disease characterized by left ventricular and/or septal hypertrophy, myofibrillar disarray, and sudden cardiac death. Our recent studies have demonstrated that the properties of isolated HCRLC could be significantly altered by the FHC mutations and that their detrimental effects depend upon the specific position of the missense mutation. This report reveals that the Ca(2+) sensitivity of myofibrillar ATPase activity and steady-state force development are also likely to change with the location of the specific FHC HCRLC mutation. The largest effect was seen for the two FHC mutations, N47K and R58Q, located directly in or near the single Ca(2+)-Mg(2+) binding site of HCRLC, which demonstrated no Ca(2+) binding compared with wild-type and other FHC mutants (A13T, F18L, E22K, P95A). These two mutants when reconstituted in porcine cardiac muscle preparations increased Ca(2+) sensitivity of myofibrillar ATPase activity and force development. These results suggest the importance of the intact Ca(2+) binding site of HCRLC in the regulation of cardiac muscle contraction and imply its possible role in the regulatory light chain-linked pathogenesis of FHC.     

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

To understand the molecular function of troponin T (TnT) in the Ca(2+) regulation of muscle contraction as well as the molecular pathogenesis of familial hypertrophic cardiomyopathy (FHC), eight FHC-linked TnT mutations, which are located in different functional regions of human cardiac TnT (HCTnT), were produced, and their structural and functional properties were examined. Circular dichroism spectroscopy demonstrated different secondary structures of these TnT mutants. Each of the recombinant HCTnTs was incorporated into porcine skinned fibers along with human cardiac troponin I (HCTnI) and troponin C (HCTnC), and the Ca(2+) dependent isometric force development of these troponin-replaced fibers was determined at pH 7.0 and 6.5. All eight mutants altered the contractile properties of skinned cardiac fibers. E244D potentiated the maximum force development without changing Ca(2+) sensitivity. In contrast, the other seven mutants increased the Ca(2+) sensitivity of force development but not the maximal force. R92L, R92W, and R94L also decreased the change in Ca(2+) sensitivity of force development observed on lowering the pH from 7 to 6.5, when compared with wild type TnT. The examination of additional mutants, H91Q and a double mutant H91Q/R92W, suggests that mutations in a region including residues 91-94 in HCTnT can perturb the proper response of cardiac contraction to changes in pH. These results suggest that different regions of TnT may contribute to the pathogenesis of TnT-linked FHC through different mechanisms.     

Altered regulation of cardiac muscle contraction by troponin T mutations that cause familial hypertrophic cardiomyopathy

To study the effect of troponin (Tn) T mutations that cause familial hypertrophic cardiomyopathy (FHC) on cardiac muscle contraction, wild-type, and the following recombinant human cardiac TnT mutants were cloned and expressed: I79N, R92Q, F110I, E163K, R278C, and intron 16(G(1) --> A) (In16). These TnT FHC mutants were reconstituted into skinned cardiac muscle preparations and characterized for their effect on maximal steady state force activation, inhibition, and the Ca(2+) sensitivity of force development. Troponin complexes containing these mutants were tested for their ability to regulate actin-tropomyosin(Tm)-activated myosin-ATPase activity. TnT(R278C) and TnT(F110I) reconstituted preparations demonstrated dramatically increased Ca(2+) sensitivity of force development, while those with TnT(R92Q) and TnT(I79N) showed a moderate increase. The deletion mutant, TnT(In16), significantly decreased both the activation and the inhibition of force, and substantially decreased the activation and the inhibition of actin-Tm-activated myosin-ATPase activity. ATPase activation was also impaired by TnT(F110I), while its inhibition was reduced by TnT(R278C). The TnT(E163K) mutation had the smallest effect on the Ca(2+) sensitivity of force; however, it produced an elevated activation of the ATPase activity in reconstituted thin filaments. These observed changes in the Ca(2+) regulation of force development caused by these mutations would likely cause altered contractility and contribute to the development of FHC.     

Altered myocardial gene expression reveals possible maladaptive processes in heterozygous and homozygous cardiac myosin-binding protein C knockout mice

Familial hypertrophic cardiomyopathy (FHC) is an autosomal dominant disease characterized by left ventricular hypertrophy (LVH) predominantly affecting the interventricular septum. Cardiac myosin-binding protein C (cMyBP-C) mutations are common causes of FHC. Gene expression profiling was performed in left ventricles of 9-week-old wild-type mice, heterozygous cMyBP-C KO mice displaying asymmetric septal hypertrophy, and homozygous mice developing eccentric LVH. Knocking out one or two cMyBP-C genes leads primarily to gene expression changes indicating an increased energy demand, activation of the JNK and p38 parts of the MAPK pathway and deactivation of the ERK part, and induction of apoptosis. Altered gene expression for processes related to cardiac structure, contractile proteins, and protein turnover was also identified. Many of the changes were more pronounced in the homozygous KO mice. These alterations point to physiological and pathological adaptations in the prehypertrophic heterozygous KO mice and the hypertrophic homozygous mice.     

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

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

Abnormal heart rate regulation in murine hearts with familial hypertrophic cardiomyopathy-related cardiac troponin T mutations

Mutations in cardiac troponin T (cTnT), Δ160E and R92Q, have been linked to familial hypertrophic cardiomyopathy (FHC), and some studies have indicated that these mutations can lead to a high incidence of sudden cardiac death in the relative absence of significant ventricular hypertrophy. Alterations in autonomic function have been documented in patients with hypertrophic cardiomyopathy. We hypothesize that alterations in autonomic function may contribute to mutation-specific clinical phenotypes in cTnT-related FHC. Heart rate (HR) variability (HRV) has been used to assess autonomic function from an electrocardiograph. Nontransgenic, Δ160E, or R92Q mice were implanted with radiofrequency transmitters to obtain continuous electrocardiograph recordings during 24-h baseline and 30-min recordings after β-adrenergic receptor drug injections. Although Δ160E mice did not differ from nontransgenic mice for any 24-h HRV measurements, R92Q mice had impaired HR regulation, as measured by a decrease in the SD of the R-R interval, a decrease in the low frequency-to-high frequency ratio, a decrease in normalized low frequency, and an increase in normalized high frequency. β-Adrenergic receptor density measurements and HRV analysis after drug injections did not reveal any significant differences for Δ160E or R92Q mice versus nontransgenic mice. Arrhythmia analysis revealed both an increased incidence of heart block in R92Q mice at baseline and frequency of premature ventricular contractions after isoproterenol injections in Δ160E and R92Q mice. In addition, Δ160E and R92Q mice exhibited a prolonged P duration after drug injections. Therefore, between two independent and clinically severe cTnT mutations within the same functional domain, only R92Q mice exhibited altered autonomic function, whereas both mutations demonstrated abnormalities in conduction and ventricular ectopy.     

Long-Range Effects of Familial Hypertrophic Cardiomyopathy Mutations E180G and D175N on the Properties of Tropomyosin

Cardiac α-tropomyosin (Tm) single-site mutations D175N and E180G cause familial hypertrophic cardiomyopathy (FHC). Previous studies have shown that these mutations increase both Ca(2+) sensitivity and residual contractile activity at low Ca(2+) concentrations, which causes incomplete relaxation during diastole resulting in hypertrophy and sarcomeric disarray. However, the molecular basis for the cause and the difference in the severity of the manifested phenotypes of disease are not known. In this work we have (1) used ATPase studies using reconstituted thin filaments in solution to show that these FHC mutants result in an increase in Ca(2+) sensitivity and an increased residual level of ATPase, (2) shown that both FHC mutants increase the rate of cleavage at R133, ～45 residues N-terminal to the mutations, when free and bound to actin, (3) shown that for Tm-E180G, the increase in the rate of cleavage is greater than that for D175N, and (4) shown that for E180G, cleavage also occurs at a new site 53 residues C-terminal to E180G, in parallel with cleavage at R133. The long-range decreases in dynamic stability due to these two single-site mutations suggest increases in flexibility that may weaken the ability of Tm to inhibit activity at low Ca(2+) concentrations for D175N and to a greater degree for E180G, which may contribute to differences in the severity of FHC.     

Invited Review: pathophysiology of cardiac muscle contraction and relaxation as a result of alterations in thin filament regulation.

Cardiac muscle contraction depends on the tightly regulated interactions of thin and thick filament proteins of the contractile apparatus. Mutations of thin filament proteins (actin, tropomyosin, and troponin), causing familial hypertrophic cardiomyopathy (FHC), occur predominantly in evolutionarily conserved regions and induce various functional defects that impair the normal contractile mechanism. Dysfunctional properties observed with the FHC mutants include altered Ca(2+) sensitivity, changes in ATPase activity, changes in the force and velocity of contraction, and destabilization of the contractile complex. One apparent tendency observed in these thin filament mutations is an increase in the Ca(2+) sensitivity of force development. This trend in Ca(2+) sensitivity is probably induced by altering the cross-bridge kinetics and the Ca(2+) affinity of troponin C. These in vitro defects lead to a wide variety of in vivo cardiac abnormalities and phenotypes, some more severe than others and some resulting in sudden cardiac death.     

Troponin and cardiomyopathy

The troponin complex was discovered over thirty years ago and since then much insight has been gained into how this complex senses fluctuating levels of Ca²⁺ and transmits this signal to the myofilament. Advances in genetics methods have allowed identification of mutations that lead to the phenotypically distinct cardiomyopathies: hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM) and dilated cardiomyopathy (DCM). This review serves to highlight key in vivo studies of mutation effects that have followed many years of functional studies and discusses how these mutations alter energetics and promote the characteristic remodeling associated with cardiomyopathic diseases. Studies have been performed that examine alterations in signaling and genomic methods have been employed to isolate upregulated proteins, however these processes are complex as there are multiple roads to hypertrophy or dilation associated with genetic cardiomyopathies. This review suggests future directions to explore in the troponin field that would heighten our understanding of the complex regulation of cardiac muscle contraction.