Targeted disruption of skeletal myosin heavy chain genes in the mouse
Acakpo-Satchivi, Leslie Jean-Regis
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Skeletal myosin heavy-chain (MyHC) genes are part of a family of which there are seven known isoforms expressed in mammalian striated muscle. These include genes expressed in developing muscle (MyHC-emb, MyHC-pn), and those expressed in adult muscle (slow skeletal MyHC-beta, fast skeletal MyHC-IIa, IIb and MyHC IId, and extraocular muscle MyHCeo). The expression of these genes is regulated in both a temporal and tissue specific fashion and the expression of a distinct isoform in a particular class of muscle is not permanent. Innervation patterns and altered physiologic stimuli such as dis-use, and mechanical overload are known to cause class switches.;The genes for all the skeletal myosin heavy-chain genes are mapped to human chromosome 17 and mouse chromosome 11 and individual members share very high levels of homology at the DNA and protein level. The importance of the different MyHC isoforms and their role in normal development and muscle function is not known.;To assess the individual roles of these isoforms, we have targeted two of the adult fast skeletal myosin heavy chain genes (MyHC-IIb, and IId/x) in embryonic stem cells. We have generated chimeric mice for both, and now have mice of both lines heterozygous and homozygous for the mutations.;In the MyHC-IIb knockout line, heterozygous mice are phenotypically indistinguishable from wild type. Homozygotes display a very subtle muscle phenotype detectable at the histologic and physiologic levels. These mice thrive and do not suffer from any gross abnormalities in muscle function. We conclude from these studies that the MyHC-IIb isoform is not essential to the normal growth and development of the animal.;Homozygous MyHC-IId/x mice, however, were found to weigh significantly less than their wild type littermates, and display varying degrees of muscle weakness. Analysis of these mice revealed signs of histologic and physiologic abnormalities. In addition, a small percentage of female homozygous MyHC-IId/x knockout mice develop severe kyphosis after 4-6 months of age. We conclude from these studies that the IId/x gene is essential to the normal growth and development of the animal.;We have also constructed a YAC contig spanning the skeletal MyHC gene cluster on mouse chromosome 11. This map allowed the deduction of the organization of the six genes present in the cluster, and provides some information pertaining to their relative transcriptional orientations.