Gene-Targeted Mice with the Human Troponin T R141W Mutation Develop Dilated Cardiomyopathy with Calcium Desensitization

Most studies of the mechanisms leading to hereditary dilated cardiomyopathy (DCM) have been performed in reconstituted in vitro systems. Genetically engineered murine models offer the opportunity to dissect these mechanisms in vivo. We generated a gene-targeted knock-in murine model of the autosomal dominant Arg141Trp (R141W) mutation in Tnnt2, which was first described in a human family with DCM. Mice heterozygous for the mutation (Tnnt2(R141W/+)) recapitulated the human phenotype, developing left ventricular dilation and reduced contractility. There was a gene dosage effect, so that the phenotype in Tnnt2(R141W/+)mice was attenuated by transgenic overexpression of wildtype Tnnt2 mRNA transcript. Male mice exhibited poorer survival than females. Biomechanical studies on skinned fibers from Tnnt2(R141W/+) hearts showed a significant decrease in pCa(50) (-log[Ca(2+)] required for generation of 50% of maximal force) relative to wildtype hearts, indicating Ca(2+) desensitization. Optical mapping studies of Langendorff-perfused Tnnt2(R141W/+) hearts showed marked increases in diastolic and peak systolic intracellular Ca(2+) ([Ca(2+)](i)), and prolonged systolic rise and diastolic fall of [Ca(2+)](i). Perfused Tnnt2(R141W/+) hearts had slower intrinsic rates in sinus rhythm and reduced peak heart rates in response to isoproterenol. Tnnt2(R141W/+) hearts exhibited a reduction in phosphorylated phospholamban relative to wildtype mice. However, crossing Tnnt2(R141W/+) mice with phospholamban knockout (Pln(-/-)) mice, which exhibit increased Ca(2+) transients and contractility, had no effect on the DCM phenotype. We conclude that the Tnnt2 R141W mutation causes a Ca(2+) desensitization and mice adapt by increasing Ca(2+)-transient amplitudes, which impairs Ca(2+) handling dynamics, metabolism and responses to β-adrenergic activation.