LAUSR

We investigate the molecular mechanisms that induce hypertrophic cardiomyopathy (HCM) in the 10-20% of patients who carry missense mutations in MYBPC3, the gene coding for cardiac myosin-binding protein C (cMyBP-C). Our starting point is the striking observation that single-amino-acid changes in this 1274-residue-long structural protein cause HCM in a similar manner to truncations, in which a significant fraction of the protein is missing. We have curated a database of naturally occurring missense variants in MYBPC3 according to their clinical presentation and studied several potential drivers of the disease. Using bioinformatics tools, we found that most pathogenic variants are not predicted to induce gross changes in thermodynamical stability or RNA splicing, two typical properties affected in other monogenic diseases. cMyBP-C slows down muscle contraction by establishing mechanical tethers between myofilaments. Hence, we hypothesized that pathogenic mutations can alter the mechanical properties of cMyBP-C leading to reduced braking ability and hypercontractility, a hallmark of HCM. To test this hypothesis, we have produced several variants of the C3 domain of cMyBP-C, a central domain of the protein without known protein interactors. As expected, most mutants retain close-to-wild-type structure and thermodynamic stability. However, pathogenic mutants show diminished mechanical stability, as determined by single-molecule atomic force microscopy experiments in force-clamp mode. Application of Bell's model to our experimental data show that pathogenic mutants unfold up to 2 times faster than wild-type protein at physiological forces. This mechanical alteration would lead to softer cMyBP-C tethers. Hence, our results support the new idea that nanomechanical phenotypes in cMyBP-C can trigger the development of HCM. This mechanical perspective can explain why missense mutations and truncations, which induce full loss of the cMyBP-C tether, converge in similar clinical outcomes.