G.P.7.10 Investigation of the patho-biology of MYH7 myopathy mutations

Abstract

The β-cardiac myosin (β-MyHC) protein is a molecular motor fundamental to both the contractile and structural properties of the muscle sarcomere. Mutations in the gene encoding β-MyHC (MYH7) cause multiple disease phenotypes: early-onset distal myopathy (MPD1), myosin storage myopathy (MSM), hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). Mutations causing HCM and DCM are spread across almost the entire gene, while those causing MPD1 and MSM are confined to exons encoding the C-terminal light-meromyosin (LMM) region of β-MyHC. How mutations located in the same region of MYH7 cause such a wide phenotypic range is as yet unknown. To investigate structural and functional effects of different β-MyHC mutants, full-length MYH7 and rod/LMM domain regions of interest were cloned for expression in mammalian cells or for recombinant protein expression. Fusion to enhanced green fluorescent protein (EGFP) allowed visualisation of the location of wildtype (WT) or mutant β-cardiac myosin in C2C12 myoblast and myotube cultures and in COS7 cells. Both WT and mutant proteins were able to arrange into ordered, striated structures in differentiated C2C12 myotubes. Analysis in COS7 cells however, suggested mutant β-MyHC showed reduced ability to form higher order structures compared to the WT protein. Secondary-structure analysis of recombinant expressed myosin tail fragments by circular dichroism (CD) revealed that both WT and mutant proteins are almost entirely α-helical. Mutant myosin tails however, tend to show a slight reduction in α-helical content. This reduction is amplified when temperature is increased to replicate physiological conditions. CD melt curve analysis indicated mutant β-MyHC proteins have decreased thermostability. Overall, the effects of the MYH7 LMM mutations on measured properties are less marked than might have been expected from missense mutations to proline or deletion or insertion of an amino-acid in a coiled coil.