FUNCTIONAL-ANALYSIS OF MYOSIN MISSENSE MUTATIONS IN FAMILIAL HYPERTROPHIC CARDIOMYOPATHY

To analyze potential functional consequences of myosin heavy chain (MH C) mutations identified in patients with familial hypertrophic cardiom yopathy (FHC), we have assessed the stability of the mutant MHCs and t heir ability to form thick filaments. Constructs encoding wild-type ra t alpha MHC and seven corresponding FHC missense mutants were transfec ted into COS cells. Immunoblot analysis suggested that FHC mutations d o not grossly alter protein stability. Wild-type alpha MHC transfected into COS cells forms structures previously shown to be arrays of thic k filaments, which also resemble myosin structures observed early in d ifferentiation of muscle cells. Surprisingly, up to 29% of COS cells t ransfected with the FHC mutants failed to form filamentous structures. To assess whether this phenotype was specific for the FHC mutants and not generalizable to any myosin mutation, COS cells were transfected with a construct encoding an MHC with a 168-amino acid deletion of the hinge/rod region. This deletion construct formed filamentous structur es with the same frequency as wild-type MHC. Biochemical analysis of o ne FHC mutant (Arg-249 --> Gin) demonstrates that the structures forme d by the mutant are solubilized at a lower ionic strength than those f ormed by wild-type MHC. We conclude that although the FHC mutant MHC i s not labile, its assembly properties may be impaired.