Abnormal Contractile Function in Transgenic Mice Expressing a Familial Hypertrophic Cardiomyopathy-linked Troponin T (I79N) Mutation

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The cardiac troponin T (TnT) I79N mutation has been linked to familial hypertrophic cardiomyopathy and a high incidence of sudden death, despite causing little or no cardiac hypertrophy. In skinned fibers, I79N increased myofilamental calcium sensitivity (Miller, T., Szczesna, D., Housmans, P. R., Zhao, J., deFreitas, F., Gomes, A. V., Culbreath, L., McCue concentration of the perfusate; systolic function was significantly increased in Tg-I79N hearts at 0.5 and 1 mmol/liter. At higher Ca 2؉ concentrations, systolic function was not different, but diastolic dysfunction became manifest as increased end-diastolic pressure and time to 90% relaxation. In vivo measurements by echocardiography and Doppler confirmed that base-line systolic function was significantly higher in Tg-I79N mice without evidence for diastolic dysfunction. Inotropic stimulation with isoproterenol resulted only in a modest contractile response but caused significant mortality in Tg-I79N mice. Doppler studies ruled out aortic outflow obstruction and were consistent with increased chamber stiffness. We conclude that in vivo, the increased myofilament Ca 2؉ sensitivity due to the I79N mutation enhances base-line contractility but leads to cardiac dysfunction during inotropic stimulation. Mutations in cardiac troponin T (TnT)1 have been implicated in familial hypertrophic cardiomyopathy (FHC) (1-5). Individuals with cardiac TnT mutations appear to have a high incidence of sudden cardiac death at a young age, although heterozygote individuals have either little or no cardiac hypertrophy (1, 3, 4). At present, there is no clear understanding as to why TnT mutations in particular pose a high risk for sudden death, as opposed to, for example, mutations in the myosin heavy chain, which usually cause a much greater degree of cardiac hypertrophy. Sudden cardiac death of FHC patients is often caused by ventricular tachyarrhythmias (6), but its cause remains unknown for patients with TnT mutations. In fact, the clinical features of hypertrophic cardiomyopathy have been established mostly without knowledge of the genotype and may not apply to patients carrying specific TnT mutations. Given the paucity of clinical information, a transgenic mouse model provides the opportunity to study the functional consequences of a TnT mutation in an in vivo system.To investigate the mechanisms of how a TnT mutation alters cardiac function and lead to sudden cardiac death, we have generated transgenic mice expressing the human cardiac TnT-I79N mutant (Tg-I79N). Similar to humans carrying this mutation, Tg-I79N mice show no cardiac hypertrophy (7). We found a large increase in Ca 2ϩ sensitivity of the skinned cardiac fibers from Tg-I79N mice compared with fibers from transgenic mice expressing human wild-type TnT (Tg-WT), but maximal developed force was significantly lower in cardiac fibers from Tg-I79N mice (7).In this study, we examined the effect of the I79N mutation on cardiac performance and electrophysiological properties of the whole heart, in vitro and in vivo. We fou...

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confidence: 83%

Section: Methodsmentioning

confidence: 99%

Inotropic Stimulation Induces Cardiac Dysfunction in Transgenic Mice Expressing a Troponin T (I79N) Mutation Linked to Familial Hypertrophic Cardiomyopathy

Knollmann¹,

Blatt²,

Horton

et al. 2001

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“…In a few cases, some FHC mutants show no change in Ca 2ϩ sensitivity. Fiber studies in transgenic mice harboring the TnT-FHC mutations (I79N and R92Q) also show increased Ca 2ϩ sensitivity, which possibly leads to altered cardiac energetics through increased cost of force production in vivo (29,30). In addition to changes in Ca 2ϩ sensitivity, FHC TnT mutations lead to alterations in ATPase activity, troponin solubility and folding, thin filament sliding speed, and the Ca 2ϩ affinity of troponin C (27,28,31).…”

Section: Discussionmentioning

confidence: 99%

Different Functional Properties of Troponin T Mutants That Cause Dilated Cardiomyopathy

Venkatraman

Harada

Gomes

et al. 2003

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The effects of Troponin T (TnT) mutants R141W and ⌬K210, the only two currently known mutations in TnT that cause dilated cardiomyopathy(DCM) independent of familial hypertrophic cardiomyopathy (FHC), and TnT-K273E, a mutation that leads to a progression from FHC to DCM, were investigated. Studies on the Ca 2؉ sensitivity of force development in porcine cardiac fibers demonstrated that TnT-⌬K210 caused a significant decrease in Ca 2؉ sensitivity, whereas the TnT-R141W did not result in any change in Ca 2؉ sensitivity when compared with human cardiac wild-type TnT (HCWTnT). TnT-⌬K210 also caused a decrease in maximal force when compared with HCWTnT and TnT-R141W. In addition, the TnT-⌬K210 mutant decreased maximal ATPase activity in the presence of Ca 2؉ . However, the TnT-K273E mutation caused a significant increase in Ca 2؉ sensitivity but behaved similarly to HCWTnT in actomyosin activation assays. Inhibition of ATPase activity in reconstituted actin-activated myosin ATPase assays was similar for all three TnT mutants and HCWTnT. Additionally, circular dichroism studies suggest that the secondary structure of all three TnT mutants was similar to that of the HCWTnT. These results suggest that a rightward shift in Ca 2؉ sensitivity is not the only determinant for the phenotype of DCM.

“…Extensive details of this quantitative computer model can be found in the methods and appendix sections of Miller et al (35) along with the additional features mentioned above. The terms used in this computer model are briefly described below.…”

Section: Displacement Of the Endogenous Tn Complex In Porcine Skinnedmentioning

confidence: 99%

Functional Analysis of a Troponin I (R145G) Mutation Associated with Familial Hypertrophic Cardiomyopathy

Lang¹,

Gomes²,

Zhao³

et al. 2002

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Familial hypertrophic cardiomyopathy has been associated with several mutations in the gene encoding human cardiac troponin I (HCTnI). A missense mutation in the inhibitory region of TnI replaces an arginine residue at position 145 with a glycine and cosegregates with the disease. Results from several assays indicate that the inhibitory function of HCTnI R145G is significantly reduced. When HCTnI R145G was incorporated into whole troponin, Tn R145G (HCTnT⅐HCTnI R145G ⅐HCTnC), only partial inhibition of the actin-tropomyosin-myosin ATPase activity was observed in the absence of Ca 2؉ compared with wild type Tn (HCTnT⅐HCTnI⅐HCTnC). Maximal activation of actin-tropomyosin-myosin ATPase in the presence of Ca 2؉ was also decreased in Tn R145G when compared with Tn. Using skinned cardiac muscle fibers, we determined that in comparison with the wild type complex 1) the complex containing HCTnI R145G only inhibited 84% of Ca 2؉ -unregulated force, 2) the recovery of Ca 2؉ -activated force was decreased, and 3) there was a significant increase in the Ca 2؉ sensitivity of force development. Computer modeling of troponin C and I variables predicts that the primary defect in TnI caused by these mutations would lead to diastolic dysfunction. These results suggest that severe diastolic dysfunction and somewhat decreased contractility would be prominent clinical features and that hypertrophy could arise as a compensatory mechanism. Familial hypertrophic cardiomyopathy (FHC)1 has been linked to mutations in genes of nine different sarcomeric proteins. These mutations have been found in the genes for ␣-myosin heavy chain (1), cardiac myosin essential light chain and cardiac myosin regulatory light chain (2), ␣-tropomyosin, cardiac troponin T (TnT) (3), cardiac myosin-binding protein C (4, 5), troponin I (TnI) (6), ␣-actin (7), as well as titin (8), and possibly troponin C (TnC) (9). This disease has recently gained significant attention due to several highly publicized reports of sudden death and fainting spells in young athletes who were asymptomatic and otherwise healthy individuals. In general, patients with FHC demonstrate an increase in heart muscle mass and sometimes an irregular echocardiogram (10). Kimura et al. (6) reported five missense mutations in TnI, R145G, R145Q, R162W, G203S, and K206Q, that cosegregate with FHC (Fig. 1). Three other TnI FHC mutants (S199N, Lys-183 deletion, and an exon 8 deletion mutant encompassing the stop codon of the cardiac TnI gene) have recently been discovered (11, 12). Functionally TnI is the inhibitory subunit of the troponin (Tn) complex that controls the interaction between actin and myosin in a Ca 2ϩ -dependent manner (13-15). Studies using proteolytic fragments of fast skeletal TnI identified the central TnI sequence (residues 96 -116) as being responsible for its inhibitory activity. Residues 104 -115 of fast skeletal TnI (comparable to residues 136 -147 in cardiac TnI) formed the minimum sequence necessary for inhibition of muscle contraction (16 -19). Two of these mutations occ...