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Familial hypertrophic cardiomyopathy (FHC) is associated with mutations in 11 genes encoding sarcomeric proteins. Most families present mutations in MYBPC3 and MYH7 encoding cardiac myosin-binding protein C and beta-myosin heavy chain. The consequences of MYH7 mutations have been extensively studied at the molecular level, but controversial results have been obtained with either reduced or augmented myosin motor function depending on the type or homogeneity of myosin studied. In the present study, we took advantage of the accessibility to an explanted heart to analyze for the first time the properties of human homozygous mutant myosin. The patient exhibited eccentric hypertrophy with severely impaired ejection fraction leading to heart transplantation, and carries a homozygous mutation in MYH7 (R403W) and a heterozygous variant in MYBPC3 (V896M). In situ analysis of the left ventricular tissue showed myocyte disarray and hypertrophy plus interstitial fibrosis. In vitro motility assays showed a small, but significant increase in sliding velocity of fluorescent-labeled actin filaments over human mutant cardiac myosin-coated surface compared to control (+18%; P0.001). Mutant myosin exhibited a large increase in maximal actin-activated ATPase activity (+114%; P0.05) and Km for actin (+87%; P0.05) when compared to control. These data show disproportionate enhancement of mechanical and enzymatic properties of human mutant myosin. This suggests inefficient ATP utilization and reduced mechanical efficiency in the myocardial tissue of the patient, which could play an important role in the development of FHC phenotype.

Background—Hypertrophic cardiomyopathy is an autosomal-dominant disorder in which 10 genes and numerous mutations have been reported. The aim of the present study was to perform a systematic screening of these genes in a large population, to evaluate the distribution of the disease genes, and to determine the best molecular strategy in clinical practice.Methods and Results—The entire coding sequences of 9 genes (MYH7,MYBPC3,TNNI3,TNNT2,MYL2,MYL3,TPM1,ACTC, andTNNC1) were analyzed in 197 unrelated index cases with familial or sporadic hypertrophic cardiomyopathy. Disease-causing mutations were identified in 124 index patients (≈63%), and 97 different mutations, including 60 novel ones, were identified. The cardiac myosin-binding protein C (MYBPC3) and β-myosin heavy chain (MYH7) genes accounted for 82% of families with identified mutations (42% and 40%, respectively). Distribution of the genes varied according to the prognosis (P=0.036). Moreover, a mutation was found in 15 of 25 index cases with “sporadic” hypertrophic cardiomyopathy (60%). Finally, 6 families had patients with more than one mutation, and phenotype analyses suggested a gene dose effect in these compound-heterozygous, double-heterozygous, or homozygous patients.Conclusion—These results might have implications for genetic diagnosis strategy and, subsequently, for genetic counseling. First, on the basis of this experience, the screening of already known mutations is not helpful. The analysis should start by testingMYBPC3andMYH7and then focus onTNNI3,TNNT2, andMYL2. Second, in particularly severe phenotypes, several mutations should be searched. Finally, sporadic cases can be successfully screened.