Inactivation of myosin heavy chain genes in the mouse: diverse and unexpected phenotypes

Myosin heavy chain (MyHC) is a critical component of the cellular contractile apparatus. The mammalian genome contains two nonmuscle, two smooth muscle, and eight striated muscle isoforms of MyHC. Within each class of genes, there is extremely high sequence homology among different MyHC isoforms, raising the question of whether these isoforms are functionally redundant or whether they perform unique roles in cell function. Recently, strains of mice null for four different MyHC isoforms have been generated. Mice null for the nonmuscle II-B isoform experience significant prenatal lethality and surviving animals have several cardiac abnormalities [Tullio et al. (1997) Proc Natl Acad Sci USA 94:12407-12412]. Mice homozygous null for alpha cardiac MyHC are embryonic lethal, while heterozygous mice are viable but also have numerous cardiac defects [Jones et al. (1996) J Clin Invest 98:1906-1917]. Mice null for IIb or IId adult skeletal MyHC are viable but have skeletal muscle abnormalities compared to wild type mice, despite compensation of a neighboring MyHC gene [Acakpo-Satchivi et al. (1997) J Cell Biol 139:1219-1229]. Both IIb and IId null mice show significant decreases in body mass. Mean muscle mass is also significantly decreased in both null strains but the extent and the pattern of affected muscles differs between the two strains. Both strains show evidence of skeletal muscle pathology but again the pattern and extent differ between the two strains. Finally, both adult skeletal strains demonstrate distinct impairments in contractile function when compared to wild type. Together these observations support the hypothesis that the different isoforms of MyHC are functionally unique and cannot substitute for one another.