Dilated Cardiomyopathy Mutant Tropomyosin Mice Develop Cardiac Dysfunction With Significantly Decreased Fractional Shortening and Myofilament Calcium Sensitivity

Abstract

Abstract-Mutations in striated muscle ␣-tropomyosin (␣-TM), an essential thin filament protein, cause both dilated cardiomyopathy (DCM) and familial hypertrophic cardiomyopathy. Two distinct point mutations within ␣-tropomyosin are associated with the development of DCM in humans: Glu40Lys and Glu54Lys. To investigate the functional consequences of ␣-TM mutations associated with DCM, we generated transgenic mice that express mutant ␣-TM (Glu54Lys) in the adult heart. Results showed that an increase in transgenic protein expression led to a reciprocal decrease in endogenous ␣-TM levels, with total myofilament TM protein levels remaining unaltered. Histological and morphological analyses revealed development of DCM with progression to heart failure and frequently death by 6 months. Echocardiographic analyses confirmed the dilated phenotype of the heart with a significant decrease in the left ventricular fractional shortening. Work-performing heart analyses showed significantly impaired systolic, and diastolic functions and the force measurements of cardiac myofibers revealed that the myofilaments had significantly decreased Ca 2ϩ sensitivity and tension generation. Real-time RT-PCR quantification demonstrated an increased expression of ␤-myosin heavy chain, brain natriuretic peptide, and skeletal actin and a decreased expression of the Ca 2ϩ handling proteins sarcoplasmic reticulum Ca 2ϩ -ATPase and ryanodine receptor. Furthermore, our study also indicates that the ␣-TM54 mutation decreases tropomyosin flexibility, which may influence actin binding and myofilament Ca 2ϩ sensitivity. The pathological and physiological phenotypes exhibited by these mice are consistent with those seen in human DCM and heart failure. As such, this is the first mouse model in which a mutation in a sarcomeric thin filament protein, specifically TM, leads to DCM. Key Words: mouse model Ⅲ transgenic Ⅲ myocardial contractility Ⅲ thin filament T ropomyosin (TM) is an ␣ helical coiled-coil fibrous protein that binds actin filaments providing structural stability and modulation of filament function. In striated muscle, TM along with the troponin complex regulates Ca 2ϩ -mediated actin-myosin crossbridges. Numerous mutations in many of the contractile proteins of the cardiac sarcomere have been associated with dilated and hypertrophic cardiomyopathy, where the myocardial performance is compromised. In humans, 2 dilated cardiomyopathy (DCM)-associated mutations (Glu54Lys and Glu40Lys) have been identified in ␣-tropomyosin (␣-TM) (or TPM1), 1 in contrast to the 8 distinct mutations in the same gene that are associated with familial hypertrophic cardiomyopathy (FHC). 2 The DCM mutations in ␣-TM are located in a region (amino acids 40 to 100) where half of the reported human FHC mutations occur (Glu62Gln, Ala63Val, Lys70Thr, Val95Ala); this region does not interact with troponin (Tn)T.Protein-modeling studies on the TM filaments harboring Glu54Lys and Glu40Lys substitutions show that both of them create a strong local increase in the positive cha...

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“…Within these causes of heart failure, some patterns have emerged that demonstrate potential similarities in the changes of contractile properties and disease state. For example, most DCM mutations in myofilament proteins result in decreased Ca 2+ sensitivity of force [52–56]. Additionally, perturbations in myofilament properties, including decreased Ca 2+ sensitivity, are thought to underlie (at least in part) the decreased contractility of failing myocardium post-MI [5,9–13].…”

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