An intrinsic requirement for Smyd1 in mouse cardiac and skeletal muscle
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Smyd1 is the founder of a gene family whose members contain split SET and MYND Domains. Smyd1 has several SET dependent lysine methyl-transferase substrates, including multiple histone lysines and at least one non-histone protein, skNAC. The MYND domain of Smyd1 is required for protein interactions, such as that with skNAC. Conventional Smyd1 knockouts die at E10 due to cardiac defects, including an enrichment of cardiac jelly, a decrease in trabeculation, and the loss of ventricular septation. dHand, a transcription factor specific for right ventricular development, and Irx4, a ventricle specific gene, are down regulated. I have shown that an approximately one kb stretch of DNA sequence upstream of the muscle specific first exon of Smyd1 is sufficient to drive expression of a reporter in transgenic mice. Cardiac specific expression is mediated by a proximal Mef2 binding site whereas skeletal muscle expression is dependent on E-boxes. I have fully analyzed this stretch of sequence via computational methods and made predictions on other potential regulatory factors. Through the use of Cre mediated conditional knockouts, I have shown that the phenotype of the conventional knockout was not due to the introduction of the Neomycin cassette at the gene locus or due to cell non-autonomous effects on the heart. Smyd1 is not only essential for cardiac septation, but throughout embryonic cardiac development, during embryonic skeletal muscle development, and in adult cardiac tissue. Conditionally deficient Smyd1 embryonic hearts are less affected than conventional Smyd1 knockouts, but are embryonically lethal and show poor trabeculation, cardiac hemorrhaging, and a pericardial edema. I detail that the Nkx2.5-Cre mediated Smyd1 deletion phenocopies the skNAC conventional knockout and that both knockouts have similar changes in the expression levels of several genes. Furthermore, when Smyd1 is conditionally removed from adult cardiac tissue, survival rates are diminished. Surprisingly a skeletal muscle specific CKO of Smyd1 mediated by Myogenin-Cre has resulted in perinatal lethality, with a visible phenotype as early as E15. Evident in the phenotype is a large edema between the epithelium and skeletal muscle, fewer myoblasts, decreased muscle mass, increased degenerating cells, and a potentially defective differentiation process.