The Bromodomains of the mammalian SWI/SNF (mSWI/SNF) ATPases Brahma (BRM) and Brahma Related Gene 1 (BRG1) promote chromatin interaction and are critical for skeletal muscle differentiation

Skeletal muscle regeneration is mediated by myoblasts that undergo epigenomic changes to determine the gene expression program of differentiated myofibers. mSWI/SNF chromatin remodeling enzymes coordinate with lineage-figuring out transcription factors to determine the epigenome of differentiated myofibers. Bromodomains bind to acetylated lysines on histone N-terminal tails along with other proteins. The mutually exclusive ATPases of mSWI/SNF complexes, BRG1 and BRM, contain bromodomains with undefined functional importance in skeletal muscle differentiation. Medicinal inhibition of mSWI/SNF bromodomain function while using small molecule PFI-3 reduced differentiation in cell culture as well as in vivo through decreased myogenic gene expression, while growing cell cycle-related gene expression and the amount of cells residing in the cell cycle. Comparative gene expression analysis with data from myoblasts depleted of BRG1 or BRM demonstrated that bromodomain function was needed for any subset of BRG1- and BRM-dependent gene expression. Reduced binding of BRG1 and BRM after PFI-3 treatment demonstrated the bromodomain is needed for stable chromatin binding at target gene promoters to change gene expression. Our findings show mSWI/SNF ATPase bromodomains permit stable binding from the mSWI/SNF ATPases to promoters needed for cell cycle exit and establishment of muscle-specific gene PFI-3 expression.