A Requirement for the Lysine Methyl Transferase SMYD1 in Myoblast Differentiation

The SMYD (SET and MYND domain) family of lysine methyltransferases harbors a unique structure in which the methyltransferase (SET) domain is intervened by a zinc finger protein-protein interaction (MYND) domain. SMYD family proteins methylate both histone and non-histone substrates and participate in diverse biological processes including transcriptional regulation, DNA repair, proliferation, and apoptosis. Smyd1 is unique among the five family members in that it is specifically expressed in striated muscles. Complete deletion of Smyd1 in mice caused embryonic lethality at E10.5 due to defects in heart development prior to proper onset of skeletal myogenesis program. Smyd1 is expressed in the skeletal muscle lineage throughout myogenesis and in mature myofibers, shuttling from nucleus to cytosol during myoblast differentiation. Because of this expression pattern, we hypothesized that Smyd1 plays multiple roles at different stages of myogenesis. The goal of our study was to decipher Smyd1’s role in mammalian skeletal muscle development. This was achieved by using Myf5-cre to knock Smyd1 out of earliest skeletal muscle precursor cells, thereby providing clues to its nuclear function. We found that Smyd1 is dispensable for initiation of skeletal myogenesis and Smyd1 conditional knock-out (CKO) embryos appear unaffected during the primary myogenic wave. However, by the second myogenic wave, both primary and secondary muscle fibers in Smyd1 CKO embryos had declined in number with no change in myoblast proliferation or apoptosis as compared to control embryos. A number of skeletal muscle-specific genes were found to be down-regulated at the mRNA level in Smyd1 CKOs by the secondary wave of myogenesis. Also, Smyd1 CKO embryos exhibited significantly higher percentage of Myog+ cells in the EDL during the second wave of myogenesis, indicating a block in downstream differentiation. Mutant embryos showed perinatal lethality and subcutaneous edema. Down-regulation of Smyd1 in C2C12 skeletal myoblast cell-line led to poorly differentiated, thinner myotubes, thereby recapitulating our observations in-vivo. Smyd1 appears important for proper down-stream differentiation of skeletal myoblasts.