Mutations in striated muscle alpha-tropomyosin (alpha-TM), an essential thin filament protein, cause both dilated cardiomyopathy (DCM) and familial hypertrophic cardiomyopathy. Two distinct point mutations within alpha-tropomyosin are associated with the development of DCM in humans: Glu40Lys and Glu54Lys. To investigate the functional consequences of alpha-TM mutations associated with DCM, we generated transgenic mice that express mutant alpha-TM (Glu54Lys) in the adult heart. Results showed that an increase in transgenic protein expression led to a reciprocal decrease in endogenous alpha-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(2+) sensitivity and tension generation. Real-time RT-PCR quantification demonstrated an increased expression of beta-myosin heavy chain, brain natriuretic peptide, and skeletal actin and a decreased expression of the Ca(2+) handling proteins sarcoplasmic reticulum Ca(2+)-ATPase and ryanodine receptor. Furthermore, our study also indicates that the alpha-TM54 mutation decreases tropomyosin flexibility, which may influence actin binding and myofilament Ca(2+) 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.