Abstract 257: Determining the Potential Role of cTnT Isoform Switching in the Development of Early Childhood Tropomyosin-linked DCM

Abstract

An oft noted component of sarcomeric DCM is the observation that patients within families carrying the same primary mutation exhibit significant phenotypic variability. This lack of a distinct link between genotype and phenotype has complicated clinical management. Recently two unrelated multigenerational families were identified with the tropomyosin (Tm) mutation Asp230Asn (D230N), exhibiting a striking “bimodal” distribution of severity. In these families, many children (<1 year) with D230N Tm presented with a severe form of DCM that led to sudden, often fatal CHF, while adults developed a mild to moderate DCM in mid-life. Of note, children who survived the initial presentation often recovered significant systolic function into young adulthood. To explain this improvement, despite the persistence of D230N Tm, we hypothesized that the phenotype is modified by other thin filament (TF) isoforms. Thus we propose the age-dependent remodeling seen in children with D230N Tm is a result of temporal isoform switches involving a closely linked Tm binding partner cardiac Troponin T (cTnT). We have shown that D230N Tm leads to a more stable (rigid) filament primarily at the C-terminus of Tm near the Tm overlap, a crucial region for TF regulatory function that cTnT modulates. Myofilaments from D230N Tm mice exhibited a small decrease in calcium sensitivity of force development that was significantly reduced in the presence of cTnT1, supportive of a modulatory role. We assessed cardiac performance in our novel D230N Tm x cTnT1 double transgenic (DTg) mice, % FS was similarly reduced for D230N Tm and DTg mice at 2 months likely due to persistent endogenous expression of cTnT1. Divergent cardiac remodeling occurred at 4 months at which point DTg mice had significantly reduced % FS compared to D230N, indicating that additive exposure to cTnT1 is detrimental to the function of D230N hearts. Additionally, In vitro studies on non-failing and failing human heart tissue found that RNA levels of cTnT1 are significantly higher in failing hearts. Thus modulation by cTnT1 could be a more general mechanism for the progressive remodeling seen in heart failure.