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—Mutant contractile protein genes that cause familial hypertrophic cardiomyopathy (FHC) are presumed to encode mutant proteins that interfere with contractile function. However, it has generally not been possible to show mutant protein expression and incorporation into the sarcomere in vivo. This study aimed to assess whether a mutant α-fast tropomyosin (TM) responsible for FHC is actually expressed and determines abnormal contractile function. Since α-fast TM is expressed in heart and skeletal muscle, samples from vastus lateralis muscles were studied from two FHC patients carrying an Asp¹⁷⁵Asn α-fast TM mutation and two healthy control subjects. TM isoforms from whole biopsy samples and single fibers were identified by gel electrophoresis and Western blot analysis. An additional faster-migrating TM band was observed in both FHC patients. The aberrant TM was identified as the Asp¹⁷⁵Asn α-fast TM …