The Missense E258K-MyBP-C Mutation Increases the Energy Cost of Tension Generation in Both Ventricular and Atrial Tissue from HCM Patients

Mutations in MYBPC3, the gene coding for cardiac myosin-binding protein-C (cMyBP-C) are the most common cause of Hypertrophic CardioMyopathy (HCM). The E258K-MyBP-C is a highly penetrant missense mutation with poorly understood molecular mechanisms. Mechanics and kinetics of contraction as well the energetic cost of tension generation were investigated using left ventricular (LV) and atrial tissue from three E258K HCM patients and compared to those from controls (donor hearts, aortic stenosis patients, and HCM patients negative for sarcomeric protein mutations). Kinetics of tension generation and relaxation were measured in single LV and atrial myofibrils mounted in a force recording apparatus (15 °C), maximally Ca2+ -activated (pCa 4.5) and fully relaxed (pCa 9.0) by rapid solution switching (<10 ms). Maximal ATPase and isometric active tension were simultaneously measured in Triton-permeabilized LV and atrial strips. In E258K, maximal tension of both ventricular and atrial myofibrils was reduced compared to controls. The rate of tension generation following maximal Ca2+ activation (kACT) was faster in both ventricular and atrial E258K myofibrils compared to controls. The rate of isometric relaxation (slow kREL) was also faster in E258K myofibrils, suggesting faster cross-bridge detachment and increased energy cost of tension generation. Direct measurements in ventricular and atrial skinned strips confirmed that tension cost was 2-3 fold higher in E258K preparations compared to controls. We conclude that the E258K mutation primarily alters apparent cross-bridge kinetics and impairs sarcomere energetics. In vitro, the mutation induces similar kinetic and energetic effects in both atrial and LV sarcomeres. The smaller impact of the mutation on atrial muscle function compared to LV muscle in vivo is likely due to the different loading conditions of the two chambers.