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 Ca 2+ desensitization occurs in DCM; and whether absence of cTnT impairs early embryonic cardiogenesis. Methods and Findings: We ablated Tnnt2 to produce heterozygousTnnt2 +/2 mice, and crossbreeding produced homozygous null Tnnt2 2/2 embryos. We also generated transgenic mice overexpressing wildtype (TG WT ) or DCM mutant (TG K210D ) Tnnt2. Crossbreeding produced mice lacking one allele of Tnnt2, but carrying wildtype (Tnnt2 +/2 /TG WT ) or mutant (Tnnt2 +/2 /TG K210D ) transgenes.Tnnt2 +/2 mice relative towildtype hadsignificantly reducedtranscript(0.8260.06[SD] vs. 1.0060.12 arbitraryunits; p=0.025),butnot protein (1.0160.20 vs. 1.0060.13 arbitrary units;p=0.44). Tnnt2 +/2 mice had normal hearts(histology, mass, left ventricular end diastolic diameter [LVEDD], fractional shortening [FS]). Moreover, whereas Tnnt2 +/2 /TG K210D mice had severe DCM, TG K210D mice had only mild DCM (FS 1864 vs. 2967%; p,0.01). The difference in severity of DCM may be attributable to a greater ratio of mutant to wildtype Tnnt2 transcript in Tnnt2 +/2 /TG K210D relative to TG K210D mice (2.4260.08, p=0.03). Tnnt2 +/2 /TG K210D muscle showed Ca 2+ 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 Tnnt2 2/2 embryos had normally looped hearts, but thin