Rationale High-myofilament Ca2+-sensitivity has been proposed as trigger of disease pathogenesis in familial hypertrophic cardiomyopathy (HCM) based on in vitro and transgenic mice studies. However, myofilament Ca2+-sensitivity depends on protein phosphorylation and muscle length, and at present, data in human are scarce. Objective To investigate whether high-myofilament Ca2+-sensitivity and perturbed length-dependent activation are characteristics for human HCM with mutations in thick- and thin-filament proteins. Methods and Results Cardiac samples from patients with HCM harboring mutations in genes encoding thick (MYH7, MYBPC3) and thin (TNNT2, TNNI3, TPM1) filament proteins were compared with sarcomere mutation-negative HCM and nonfailing donors. Cardiomyocyte force measurements showed higher myofilament Ca2+-sensitivity in all HCM samples and low phosphorylation of protein kinase A (PKA)-targets compared with donors. After exogenous PKA treatment, myofilament Ca2+-sensitivity was either similar (MYBPC3mut, TPM1mut, sarcomere mutation-negative HCM), higher (MYH7mut, TNNT2mut), or even significantly lower (TNNI3mut) compared with donors. Length-dependent activation was significantly smaller in all HCM than in donor samples. PKA treatment increased phosphorylation of PKA-targets in HCM myocardium and normalized length-dependent activation to donor values in sarcomere mutation-negative HCM and HCM with truncating MYBPC3 mutations, but not in HCM with missense mutations. Replacement of mutant by wild-type troponin in TNNT2mut and TNNI3mut corrected length-dependent activation to donor values. Conclusions High-myofilament Ca2+-sensitivity is a common characteristic of human HCM and partly reflects hypophosphorylation of PKA-targets compared with donors. Length-dependent sarcomere activation is perturbed by missense mutations, possibly via post-translational modifications other than PKA-hypophosphorylation or altered protein–protein interactions, and represents a common pathomechanism in HCM.

Background-Hypertrophic cardiomyopathy (HCM), typically characterized by asymmetrical left ventricular hypertrophy, frequently is caused by mutations in sarcomeric proteins. We studied if changes in sarcomeric properties in HCM depend on the underlying protein mutation. Methods and Results-Comparisons were made between cardiac samples from patients carrying a MYBPC3 mutation (MYBPC3 mut ; nϭ17), mutation negative HCM patients without an identified sarcomere mutation (HCM mn ; nϭ11), and nonfailing donors (nϭ12). All patients had normal systolic function, but impaired diastolic function. Protein expression of myosin binding protein C (cMyBP-C) was significantly lower in MYBPC3 mut by 33Ϯ5%, and similar in HCM mn compared with donor. cMyBP-C phosphorylation in MYBPC3 mut was similar to donor, whereas it was significantly lower in HCM mn . Troponin I phosphorylation was lower in both patient groups compared with donor. Force measurements in single permeabilized cardiomyocytes demonstrated comparable sarcomeric dysfunction in both patient groups characterized by lower maximal force generating capacity in MYBPC3 mut and HCM mn, compared with donor (26.4Ϯ2.9, 28.0Ϯ3.7, and 37.2Ϯ2.3 kN/m 2 , respectively), and higher myofilament Ca 2ϩ -sensitivity (EC 50 ϭ2.5Ϯ0.2, 2.4Ϯ0.2, and 3.0Ϯ0.2 mol/L, respectively). The sarcomere length-dependent increase in Ca Key Words: cardiomyopathy Ⅲ myofilament proteins Ⅲ mutation Ⅲ myocardial contraction H ypertrophic cardiomyopathy (HCM), most often caused by mutations in genes encoding sarcomeric proteins, is a major cause of morbidity and mortality affecting Ϸ1:500 people worldwide at a relatively young age. 1,2 It often is characterized by asymmetrical left ventricular (LV) hypertrophy, predominantly involving the interventricular septum, occurring in the absence of other cardiac or systemic disease (such as hypertension or aortic stenosis). Clinical presentation is very heterogeneous in HCM as some patients reach old age with virtually no complaints, while others progress to end-stage heart failure or die at a young age from sudden cardiac arrest. To develop a targeted treatment to prevent or delay HCM, it is highly relevant to understand the pathophysiology of this disease. Clinical Perspective on p 46During the last 2 decades, many disease causing mutations have been identified, mainly in genes encoding sarcomeric proteins. 3,4 Despite improved genetic testing the causal gene mutation remains unidentified in over 40% of HCM patients. 5 Furthermore, the pathophysiological mechanism leading from a Recently we have provided evidence for sarcomeric dysfunction in manifest HCM patients with truncating MYBPC3 founder mutations (c.2373dupG and c.2864_2865delCT). 12 The sarcomeric dysfunction included a reduction in maximal force generating capacity and a higher myofilament Ca 2ϩ -sensitivity compared with nonfailing human myocardium, which may be the result of altered sarcomeric protein composition as we observed haploinsufficiency (ie, reduced cardiac myosin binding protein C [cMyBP...

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Background To maintain the balance between the demand of the body and supply (cardiac output), cardiac performance is tightly regulated via the parasympathetic and sympathetic nervous systems. In heart failure, cardiac output (supply) is decreased due to pathologic remodelling of the heart. To meet the demands of the body, the sympathetic system is activated and catecholamines stimulate b-adrenergic receptors (b-ARs) to increase contractile performance and cardiac output. Although this is beneficial in the acute phase, chronic b-ARs stimulation initiates a cascade of alterations at the cellular level, resulting in a diminished contractile performance of the heart.