The Role of Cardiac Troponin T Quantity and Function in Cardiac Development and Dilated Cardiomyopathy

Background: Hypertrophic (HCM) and dilated (DCM) cardiomyopathies result from sarcomeric protein mutations, including cardiac troponin T (cTnT, TNNT2). We determined whether TNNT2 mutations cause cardiomyopathies by altering cTnT function or quantity; whether the severity of DCM is related to the ratio of mutant to wildtype cTnT; whether Ca2+ desensitization occurs in DCM; and whether absence of cTnT impairs early embryonic cardiogenesis.\ud Methods and Findings: We ablated Tnnt2 to produce heterozygous Tnnt2+/2 mice, and crossbreeding produced homozygous null Tnnt22/2 embryos. We also generated transgenic mice overexpressing wildtype (TGWT) or DCM mutant (TGK210D) Tnnt2. Crossbreeding produced mice lacking one allele of Tnnt2, but carrying wildtype (Tnnt2+/2/TGWT) or mutant (Tnnt2+/2/TGK210D) transgenes. Tnnt2+/2 mice relative to wildtype had significantly reduced transcript (0.8260.06[SD] vs. 1.0060.12 arbitrary units; p = 0.025), but not protein (1.0160.20 vs. 1.0060.13 arbitrary units; p = 0.44). Tnnt2+/2 mice had normal hearts (histology, mass,\ud left ventricular end diastolic diameter [LVEDD], fractional shortening [FS]). Moreover, whereas Tnnt2+/2/TGK210D mice had severe DCM, TGK210D mice had only mild DCM (FS 1864 vs. 2967%; p,0.01). The difference in severity of DCM may be\ud attributable to a greater ratio of mutant to wildtype Tnnt2 transcript in Tnnt2+/2/TGK210D relative to TGK210D mice (2.4260.08, p = 0.03). Tnnt2+/2/TGK210D muscle showed Ca2+ desensitization (pCa50 = 5.3460.08 vs. 5.5860.03 at sarcomere length 1.9 mm, p,0.01), but no difference in maximum force generation. Day 9.5 Tnnt22/2 embryos had normally looped hearts, but thin ventricular walls, large pericardial effusions, noncontractile hearts, and severely disorganized sarcomeres.\ud Conclusions: Absence of one Tnnt2 allele leads to a mild deficit in transcript but not protein, leading to a normal cardiac phenotype. DCM results from abnormal function of a mutant protein, which is associated with myocyte Ca2+\ud desensitization. The severity of DCM depends on the ratio of mutant to wildtype Tnnt2 transcript. cTnT is essential for sarcomere formation, but normal embryonic heart looping occurs without contractile activity