Fetal Cardiac Troponin Isoforms Rescue the Increased Ca2+ Sensitivity Produced by a Novel Double Deletion in Cardiac Troponin T Linked to Restrictive Cardiomyopathy

Abstract

A novel double deletion in cardiac troponin T (cTnT) of two highly conserved amino acids (Asn-100 and Glu-101) was found in a restrictive cardiomyopathic (RCM) pediatric patient. Clinical evaluation revealed the presence of left atrial enlargement and marked left ventricle diastolic dysfunction. The explanted heart examined by electron microscopy revealed myofibrillar disarray and mild fibrosis. Pedigree analysis established that this mutation arose de novo. The patient tested negative for six other sarcomeric genes. The single and double recombinant cTnT mutants were generated, and their functional consequences were analyzed in porcine skinned cardiac muscle. In the adult Tn environment (cTnT3 ؉ cardiac troponin I), the single cTnT3-⌬N100 and cTnT3-⌬E101 mutations had opposing effects on the Ca 2؉ sensitivity of force development compared with WT, whereas the double deletion cTnT3-⌬N100/⌬E101 increased the Ca 2؉ sensitivity ؉ 0.19 pCa units. In addition, cTnT3-⌬N100/⌬E101 decreased the cooperativity of force development, suggesting alterations in intrafilament proteinprotein interactions. In the fetal Tn environment, (cTnT1 ؉ slow skeletal troponin I), the single (cTnT1-⌬N110) and double (cTnT1-⌬N110/⌬E111) deletions did not change the Ca 2؉ sensitivity compared with control. To recreate the patient's heterozygous genotype, we performed a reconstituted ATPase activity assay. Thin filaments containing 50:50 cTnT3-⌬N100/ ⌬E101:cTnT3-WT also increased the myofilament Ca 2؉ sensitivity compared with WT. Co-sedimentation of thin filament proteins indicated that no significant changes occurred in the binding of Tn containing the RCM cTnT mutation to actin-Tm. This report reveals the protective role of Tn fetal isoforms as they rescue the increased Ca 2؉ sensitivity produced by a cTnT-RCM mutation and may account for the lack of lethality during gestation.

Hypertrophic Cardiomyopathy Cardiac Troponin C Mutations Differentially Affect Slow Skeletal and Cardiac Muscle Regulation

“…However, the protective role of ssTnI in cardiac muscle contraction is not a novel topic. We have previously shown that ssTnI has the ability to attenuate the functional consequences of two mutations in cTnT that are associated with restrictive cardiomyopathy (Pinto et al, 2008a , 2011b ). Others have shown that the expression of ssTnI in the murine heart has a protective effect on skinned cardiac preparations and left ventricular function during stress conditions, such as acidosis (Wolska et al, 2001 ; Urboniene et al, 2005 ).…”

The effects of slow skeletal troponin I expression in the murine myocardium are influenced by development‐related shifts in myosin heavy chain isoform

“…While ssTnI may decrease myofilament responsiveness to inotropic agents (Arteaga et al 2000), β-MHC may further alter the effect of ssTnI on submaximal contractile activation in the developing heart to prevent excessive inotropy. The MHC-TnI interaction effect may have further implications for the ssTnI impact on pH sensitivity (Westfall et al 1997) or on cardio-protective mechanisms (Arteaga et al 2005;Layland et al 2005a;Pinto et al 2011;Pound et al 2011) observed in α-MHC(ssTnI) mice.…”

Pediatric restrictive cardiomyopathy due to a heterozygous mutation of the TNNI3 gene

“…Up to now, more than 900 mutations have been confirmed to be associated with HCM. Since the first sarcomere gene mutation was identified in the TNNI3 gene in RCM in 2003 [7] , several heterozygous mutations in TNNI3 , TNNT2 , ACTC , and MYH7 have been reported to be associated with RCM [7] – [13] , indicating that RCM may also be caused by single heterozygous mutations in the genes encoding sarcomeric proteins.…”

“…Pediatric RCM is most commonly idiopathic and its molecular basis is still unclear. Recently, mutations in the sarcomeric protein genes (cardiac troponin I, TNNI3 ; cardiac troponin T, TNNT2 ; α-cardiac actin, ACTC ; β-myosin heavy chain, MYH7 ) have been identified in pediatric RCM, which suggests that sarcomeric protein mutations may be important causes of RCM [7] – [13] . Here, we performed genetic investigations of candidate genes that have been reported in RCM and identified a missense mutation in the TNNI3 gene in a 12-year-old girl with RCM.…”