Early Deviations in Coronary Flow and Myocardial Arterioles in Familial Hypertrophic Cardiomyopathy

Background: Although a majority of cases of hypertrophic cardiomyopathy (HCM) are caused by myofilament protein variants, the pathological cascade leading toward phenotypic presentation remains unclear. Relevant pathological processes shortly after the onset of myofilament dysfunction were investigated during neonatal development. It is hypothesized that altered diastolic function with impairment in vascular function and architecture precedes and promotes progressive remodeling in HCM through mechanotransduction pathways. Methods and Results: We studied disease progression during postnatal days 7-28 (P7-P28) in transgenic (TG) TG-cTnT-R92Q and non-transgenic (NTG) mice using skinned fiber mechanics, echocardiography, biochemistry, histology, and immunohistochemistry. Methods in spatial analysis and computer vision were also developed and used to investigate microvessel density and Yap signal localization. At P7, skinned myofiber bundles demonstrated increased Ca2+-sensitivity (pCa50 TG:5.97±0.04, NTG: 5.84±0.01, p=0.002) associated with cTnT-R92Q expression on the background of fetal troponin I and myosin heavy chain isoforms expression. Despite the transition to adult isoform expression between P7-P14, the increased Ca2+-sensitivity persisted through P28 in the TG mice. During this period no significant changes in cardiac gross morphology were evident, but significant diastolic dysfunction with associated coronary flow perturbation (mean diastolic velocity, TG:222.5±18.81, NTG: 338.7±28.07mm/s, p=0.001) was apparent as early as P7. It was accompanied by increased phosphorylation of phospholamban (PLN), indicative of dysregulation in Ca2+ homeostasis and localized fibrosis (percent area, TG:4.36%±0.44, NTG:2.53%±0.47, p=0.034) . By P14 there was a notable decline in small arterioles and arteriolar cross-sectional area along with an expansion of fibrosis (percent area, TG: 9.72%±0.73, NTG:2.72%±0.2, p<0.001). Crucial roles for mechanotransduction signaling in the cells of the coronary arteries was demonstrated by levels of YAP expression increasing in their endothelium with a decrease in ratio of nuclear to cytosolic YAP at P14, which reversed at P28. Despite transient increases in eNOS and NOX2 expression, no definitive role was found for oxidative stress in early HCM progression. Conclusion: Our studies reveal that early in neonatal HCM disease development the diastolic dysfunction resulting from increased myofilament Ca-sensitivity also impacts coronary flow, normal arteriogenesis and fibrotic processes which presage cardiac hypertrophy. Our data also stress the importance of the interactions between cardiac myocytes and other stromal compartments of the myocardium suggesting vasculature as a new therapeutic target for HCM.