Abstract EC105: A Mouse Model Carrying The R502W Mutation In Cardiac Myosin Binding Protein C (cMyBP-C) Leads To Hypertrophic Myocardial Remodeling In The Absence Of <cMyBP-C> Haploinsufficiency

Background: Familial cardiomyopathies are caused by inherited genetic mutations resulting in anatomical and/or mechanical alterations that compromise cardiac function. The missense variant R502W in cardiac myosin-binding protein C (cMyBP-C) is the most frequent mutation leading to hypertrophy cardiomyopathy (HCM) in humans. Protein and mRNA levels have been proposed not to be altered by this mutation, which points towards pathomechanisms not involving cMyBP-C haploinsufficiency. To test the hypothesis that R502W variant induces HCM remodeling despite preserved cMyBP-C levels, CRISPR/Cas9-based genetic engineering was used to generate a knock-in (KI) mouse model harboring the R502W mutation in murine cMyBP-C. Methods: Histology, echocardiography and magnetic resonance imaging were employed to characterize the cardiac phenotype of KI mice. Protein and cMyBP-C mRNA levels were quantified by Western Blot and qPCR. Immunofuorescence in neonatal cardiomyocytes was used to analyze protein localization. Alteration of myosin conformations was assessed in permeabilized myofibrils using Mant-ATP pulse-chase experiments. Transcriptomic analysis was performed to confirm upregulation of hypertrophic cardiac remodeling programs. Results: We observe higher heart weight to tibia length ratio starting at 18 weeks old homozygous KI mice, which is accompanied by thicker trabeculae, altered ventricle geometry, reduced left ventricular systolic and diastolic function. Fibrosis and hypertrophy at the cardiomyocyte level were detected at 18 weeks of age. Biochemical and immunofluorescence analyses demonstrate that cMyBP-C mRNA, protein levels and localization are not altered by the mutation. Myosin conformations appear largely unaffected in homozygous KI mice. Conclusion: We propose that pathogenicity of the R502W mutation stems from mechanisms that do not involve cMyBP-C haploinsufficiency. We expect that further investigation of R502W mice will shed light on the molecular triggers of HCM caused by cMyBP-C point mutations not triggering protein haploinsufficiency.