Missense Variants in Myosin Binding Protein C that Cause Hypertrophic Cardiomyopathy

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Patients with hypertrophic cardiomyopathy (HCM) experience a high symptomatic burden, heart failure and lethal arrhythmias. While HCM has been recognized as a disease of the sarcomere for >30 years, the disease mechanisms for sarcomeric gene variants are not well defined, limiting the precision and efficacy of treatment options. Heterozygous variants in the gene myosin-binding protein C (MYBPC3) cause half of all cases of familial HCM. About 15% of these are missense variants that cluster in interior protein domains C3 and C6 which have uncertain binding partners or function. Computational predictions combined with our published and preliminary experimental data support the hypothesis that missense variants in C3 and C6 domains lead to perturbation of multiple protein-protein interactions that are critical for the normal function of MyBP-C (the protein encoded by MYBPC3). In Aim 1 we will apply TurboID proximity labeling to wild-type (WT) and mutant MyBP-C. In preliminary data we have identified >200 novel and unique neighboring proteins to WT MyBP-C. Comparing C3 and C6 mutants to wild-type MyBP-C, relative abundances of sarcomeric, cytoskeletal and ribonucleoprotein complexes are reduced, while abundances of ribosomal and chaperone proteins are increased. We will explore consequences of these altered interactions by assessing changes in myosin conformation, local translation, and chaperone-mediate protein turnover. We expect to find that interactions with multiple proteins of diverse function are either strengthened or weakened by the presence of missense mutations in MyBP-C. Overcoming this perturbation in protein interactions with gene replacement by wild-type MyBP-C is the focus of Aim 2 where we will test the hypothesis that the mutant protein can be stoichiometrically replaced within the sarcomere by wild-type MyBP-C. We will transduce patient-derived inducible-pluripotent cardiomyocytes expressing C3 or C6 missense variants with adeno-associated viral vectors expressing wild-type MyBP-C or a lentiviral vector expressing a “titratable” wild-type MyBP-C-FKBP12 fusion protein that enables dose-response studies. The outcome measures will be the molar ratio of mutant to wild-type protein, and contractile and relaxation velocities. In vivo studies of gene replacement in a new Arg506Trp MYBPC3 knock-in mouse model will complement the hiPSC-CM experiments. This application explores several novel aspects of MyBP-C biology and features unique reagents and advanced proteomic techniques. Successful completion of these aims will uncover new biology in MyBP-C by defining an expanded protein neighborhood, by revealing disease mechanisms for missense MYBPC3 variants, and by testing a gene displacement strategy that leverages endogenous regulation of sarcomeric stoichiometry and could be broadly applicable to missense variants in any sarcomere gene. Our investigative team, composed of a mix of senior, highly experienced investigators and talented junior investigators with unique skill sets, is well poised to achieve these goals.