Cardiomyopathy mutations in the tail of beta cardiac myosin modify the coiled-coil structure and affect integration into thick filaments in muscle sarcomeres in adult cardiomyocytes.

It is unclear why mutations in the filament forming tail of myosin heavy chain (MHC) cause hypertrophic (HCM) or dilated (DCM) cardiomyopathy as these mutations should not directly affect contraction. To investigate this, we first investigated the impact of five HCM-causing (N1327K, E1356K, R1382W, E1555K and R1768K) and one DCM-causing (R1500W) tail mutations on their ability to incorporate into muscle sarcomeres in vivo. We used adenoviral delivery to express full-length wild type or mutant eGFP-MHC in isolated adult cardiomyocytes. Three mutations (N1327K, E1356K and E1555K) reduced eGFP-MHC incorporation into muscle sarcomeres while the remainder had no effect. No mutations significantly affected contraction. Fluorescence recovery after photobleaching (FRAP) showed that fluorescence recovery for the mutation that incorporated least well (N1327K), was significantly faster than that of WT with half times of 25.1 ± 1.8 and 32.2 ± 2.5 minutes (mean ± S.E.M.) respectively. Next, we determined the effects of each mutation on the helical properties of wild type and 7 mutant peptides (7, 11 or 15 heptads long) from the myosin tail by circular dichroism. R1382W and E1768K slightly increased the α-helical nature of peptides. The remaining mutations reduced α-helical content, with N1327K showing the greatest reduction. Only peptides containing residues 1301-1329 were highly α-helical suggesting that this region helps in initiation of coiled coil. These results suggest that small effects of mutations on helicity translate into a reduced ability to incorporate into sarcomeres, which may elicit compensatory hypertrophy.