The role of cardiac troponin T quantity and function in cardiac development and dilated cardiomyopathy
F Ahmad, SK Banerjee, ML Lage, XN Huang… - PloS one, 2008 - journals.plos.org
F Ahmad, SK Banerjee, ML Lage, XN Huang, SH Smith, S Saba, J Rager, DA Conner…
PloS one, 2008•journals.plos.orgBackground 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. Methods and Findings We ablated Tnnt2 to produce heterozygous Tnnt2+ …
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. Methods and Findings We ablated Tnnt2 to produce heterozygous Tnnt2+ …
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.
Methods and Findings
We ablated Tnnt2 to produce heterozygous Tnnt2+/− mice, and crossbreeding produced homozygous null Tnnt2−/− embryos. We also generated transgenic mice overexpressing wildtype (TGWT) or DCM mutant (TGK210Δ) Tnnt2. Crossbreeding produced mice lacking one allele of Tnnt2, but carrying wildtype (Tnnt2+/−/TGWT) or mutant (Tnnt2+/−/TGK210Δ) transgenes. Tnnt2+/− mice relative to wildtype had significantly reduced transcript (0.82±0.06[SD] vs. 1.00±0.12 arbitrary units; p = 0.025), but not protein (1.01±0.20 vs. 1.00±0.13 arbitrary units; p = 0.44). Tnnt2+/− mice had normal hearts (histology, mass, left ventricular end diastolic diameter [LVEDD], fractional shortening [FS]). Moreover, whereas Tnnt2+/−/TGK210Δ mice had severe DCM, TGK210Δ mice had only mild DCM (FS 18±4 vs. 29±7%; p<0.01). The difference in severity of DCM may be attributable to a greater ratio of mutant to wildtype Tnnt2 transcript in Tnnt2+/−/TGK210Δ relative to TGK210Δ mice (2.42±0.08, p = 0.03). Tnnt2+/−/TGK210Δ muscle showed Ca2+ desensitization (pCa50 = 5.34±0.08 vs. 5.58±0.03 at sarcomere length 1.9 µm, p<0.01), but no difference in maximum force generation. Day 9.5 Tnnt2−/− embryos had normally looped hearts, but thin ventricular walls, large pericardial effusions, noncontractile hearts, and severely disorganized sarcomeres.
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+ 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.
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