| The overarching focus of this thesis is on the molecular underpinnings of inherited cardiomyopathies. Restrictive cardiomyopathy (RCM) is a rare and malignant disease of the myocardium characterized by impaired ventricular filling and a "stiff" heart phenotype. The RCM phenotype evolves in the absence of hypertrophy and rapidly transitions to heart failure, often during childhood, with the only treatment being transplantation. By contrast, hypertrophic cardiomyopathy (HCM), with a prevalence of 1 in 500 in the general population, is characterized by gross ventricular hypertrophy, diastolic dysfunction, and premature sudden cardiac death. RCM and HCM share a common genetic locus: TNNI3, cardiac troponin I (cTnI). Cardiac TnI is a molecular switch protein essential for the Ca2+ mediated regulation of contraction. The mechanism by which distinct cardiomyopathies develop from the same TNNI3 gene remains poorly understood. Thus, the main objective of this work was to determine the primary effects of RCM and HCM mutant cTnI alleles on cardiac myocyte physiology, and to generate a complementary mouse model for assessing the RCM pathogenic process from primary functional deficit to organismal physiology. Acutely engineered adult cardiac myocytes with RCM and HCM mutant cTnIs revealed the primary and novel effects of RCM cTnI to develop a Ca 2+ independent mechanical tone and acute myocyte remodeling that coincided with the progressive incorporation of RCM cTnI into the contractile apparatus. These primary functional deficits when expressed in mouse resulted in organ level diastolic tone and diastolic dysfunction that evolved from an uncoupling between contractile function and Ca2+ signaling. RCM transgenic mouse hearts were devoid of histopathology and myocardial remodeling implying that RCM cTnI is directly influencing contractile function. This pathophysiology evolved with low levels of replacement of native cTnI with RCM mutant. Transgenic lines with moderate to high levels of replacement could not be obtained. This suggests that the mouse can only tolerate minimal amounts of RCM cTnI in vivo. The EC-uncoupling reported here points to new potential therapeutic targets in the sarcomere to redress functional deficits in the cardiomyopathic heart. |
