Abstract P248: The DCM-Causing D230N Tropomyosin Mutation Produces an Age-Dependent Phenotype in Mice that Is Influenced by Transgene Dosage

Abstract

A recent study in two large multigenerational families demonstrated a novel mutation in tropomyosin (Tm), Asp230Asn (D230N) that caused DCM with a unique natural history resulting in a striking bimodal disease distribution. Infants and young children with the D230N Tm mutation presented with a severe, often fatal DCM while adults developed a mild to moderate clinical phenotype. Position 230 in tropomyosin is proximal to the C-terminal unwinding of tropomyosin that is essential for the Tm-Tm overlap and for the Tm-cardiac troponin T (cTnT) interaction. We hypothesized that the age-dependent remodeling is a result of temporal isoform switches in the closely linked Tm-binding partner cTnT. To directly address this hypothesis, we used the regulated in vitro motility assay to determine the biophysical effects of the D230N Tm mutation as well as the effects of cTnT isoform changes on the regulatory function of thin filaments. We have found that both wild type and D230N Tm filaments inhibit filament sliding at low calcium indicating that the D230N mutation does not completely disrupt regulation. However in the presence of high calcium the D230N Tm mutation significantly decreases the maximal velocity of filament sliding (3.790 ± 0.1333 microns/sec, n=98) as compared to the wild type filaments (4.900 ± 0.1044, microns/sec n=76). These findings support an altered Tm-cTnT interaction because at maximal calcium concentrations, the myofilament activation at the level of cardiac troponin is not properly transmitted to tropomyosin thus decreasing the activity of myosin-ATPase. In order to extend our biophysical observations and to provide translational insight, we generated a D230N Tm transgenic mouse model. In preliminary studies we demonstrate that at two months the D230N transgenic mice have evidence for early dilatation. In addition, the differences in ventricular remodeling were highly dependent on transgene dose. At six months, we observed dilatation of the ventricle and thinning of the walls compared to non-transgenic siblings. However, the differences were not as marked as those at the two-month time point. Collectively, these findings support an age-dependent DCM phenotype that is modulated by transgene dosage.