title: The role of cardiac troponin T quantity and function in cardiac development and dilated cardiomyopathy.
abstract: 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.
  
  
  RESULTS
  We ablated Tnnt2 to produce heterozygous Tnnt2(+/-) mice, and crossbreeding produced homozygous null Tnnt2(-/-) embryos. We also generated transgenic mice overexpressing wildtype (TG(WT)) or DCM mutant (TG(K210Delta)) Tnnt2. Crossbreeding produced mice lacking one allele of Tnnt2, but carrying wildtype (Tnnt2(+/-)/TG(WT)) or mutant (Tnnt2(+/-)/TG(K210Delta)) 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(+/-)/TG(K210Delta) mice had severe DCM, TG(K210Delta) 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(+/-)/TG(K210Delta) relative to TG(K210Delta) mice (2.42+/-0.08, p = 0.03). Tnnt2(+/-)/TG(K210Delta) muscle showed Ca(2+) desensitization (pCa(50) = 5.34+/-0.08 vs. 5.58+/-0.03 at sarcomere length 1.9 microm, 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 Ca(2+) 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.
authors.affiliation: Cardiovascular Institute, Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania, United States of America. ahmadf@upmc.edu
authors.forename: Ferhaan
authors.initials: F
authors.lastname: Ahmad
authors.forename: Sanjay K
authors.initials: SK
authors.lastname: Banerjee
authors.forename: Michele L
authors.initials: ML
authors.lastname: Lage
authors.forename: Xueyin N
authors.initials: XN
authors.lastname: Huang
authors.forename: Stephen H
authors.initials: SH
authors.lastname: Smith
authors.forename: Samir
authors.initials: S
authors.lastname: Saba
authors.forename: Jennifer
authors.initials: J
authors.lastname: Rager
authors.forename: David A
authors.initials: DA
authors.lastname: Conner
authors.forename: Andrzej M
authors.initials: AM
authors.lastname: Janczewski
authors.forename: Kimimasa
authors.initials: K
authors.lastname: Tobita
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Tinney
authors.forename: Ivan P
authors.initials: IP
authors.lastname: Moskowitz
authors.forename: Antonio R
authors.initials: AR
authors.lastname: Perez-Atayde
authors.forename: Bradley B
authors.initials: BB
authors.lastname: Keller
authors.forename: Michael A
authors.initials: MA
authors.lastname: Mathier
authors.forename: Sanjeev G
authors.initials: SG
authors.lastname: Shroff
authors.forename: Christine E
authors.initials: CE
authors.lastname: Seidman
authors.forename: J G
authors.initials: JG
authors.lastname: Seidman
mesh_terms: D000818:Animals; D002311:Cardiomyopathy, Dilated / Q000235:genetics* / Q000378:metabolism; D002312:Cardiomyopathy, Hypertrophic / Q000378:metabolism; D004452:Echocardiography; D004622:Embryo, Mammalian / Q000378:metabolism; D006321:Heart / Q000196:embryology*; D051379:Mice; D018345:Mice, Knockout; D008822:Mice, Transgenic; D009206:Myocardium / Q000378:metabolism; D010641:Phenotype; D020107:Troponin T / Q000235:genetics* / Q000378:metabolism / Q000502:physiology*
chemical_list: D020107:Troponin T