The Swi2/Snf2 bromodomain is required for the displacement of SAGA and the octamer transfer of SAGA-acetylated nucleosomes

The SWI/SNF and SAGA chromatin-modifying complexes contain bromodomains that help anchor these complexes to acetylated promoter nucleosomes. To study the importance of bromodomains in these complexes, we have compared the chromatin-remodeling and octamer-transfer activity of the SWI/SNF complex to a mutant complex that lacks the Swi2/Snf2 bromodomain. Here we show that the SWI/SNF complex can remodel or transfer SAGA-acetylated nucleosomes more efficiently than the Swi2/Snf2 bromodomain-deleted complex. These results demonstrate that the Swi2/Snf2 bromodomain is important for the remodeling as well as for the octamer-transfer activity of the complex on H3-acetylated nucleosomes. Moreover, we show that, although the wild-type SWI/SNF complex displaces SAGA that is bound to acetylated nucleosomes, the bromodomain mutant SWI/SNF complex is less efficient in SAGA displacement. Thus, the Swi2/Snf2 bromodomain is required for the full functional activity of SWI/SNF on acetylated nucleosomes and is important for the displacement of SAGA from acetylated promoter nucleosomes.     

Epigenetic and pharmacological control of pigmentation via Bromodomain Protein 9 (BRD9)

Lineage-specific differentiation programs are activated by epigenetic changes in chromatin structure. Melanin-producing melanocytes maintain a gene expression program ensuring appropriate enzymatic conversion of metabolites into the pigment, melanin, and transfer to surrounding cells. During neuroectodermal development, SMARCA4 (BRG1), the catalytic subunit of SWItch/Sucrose Non-Fermentable (SWI/SNF) chromatin remodeling complexes, is essential for lineage specification. SMARCA4 is also required for development of multipotent neural crest precursors into melanoblasts, which differentiate into pigment-producing melanocytes. In addition to the catalytic domain, SMARCA4 and several SWI/SNF subunits contain bromodomains which are amenable to pharmacological inhibition. We investigated the effects of pharmacological inhibitors of SWI/SNF bromodomains on melanocyte differentiation. Strikingly, treatment of murine melanoblasts and human neonatal epidermal melanocytes with selected bromodomain inhibitors abrogated melanin synthesis and visible pigmentation. Using functional genomics, iBRD9, a small molecule selective for the bromodomain of BRD9 was found to repress pigmentation-specific gene expression. Depletion of BRD9 confirmed a requirement for expression of pigmentation genes in the differentiation program from melanoblasts into pigmented melanocytes and in melanoma cells. Chromatin immunoprecipitation assays showed that iBRD9 disrupts the occupancy of BRD9 and the catalytic subunit SMARCA4 at melanocyte-specific loci. These data indicate that BRD9 promotes melanocyte pigmentation whereas pharmacological inhibition of BRD9 is repressive.     

A cooperative activation loop among SWI/SNF, γ‐H2AX and H3 acetylation for DNA double‐strand break repair

Although recent studies highlight the importance of histone modifications and ATP‐dependent chromatin remodelling in DNA double‐strand break (DSB) repair, how these mechanisms cooperate has remained largely unexplored. Here, we show that the SWI/SNF chromatin remodelling complex, earlier known to facilitate the phosphorylation of histone H2AX at Ser‐139 (S139ph) after DNA damage, binds to γ‐H2AX (the phosphorylated form of H2AX)‐containing nucleosomes in S139ph‐dependent manner. Unexpectedly, BRG1, the catalytic subunit of SWI/SNF, binds to γ‐H2AX nucleosomes by interacting with acetylated H3, not with S139ph itself, through its bromodomain. Blocking the BRG1 interaction with γ‐H2AX nucleosomes either by deletion or overexpression of the BRG1 bromodomain leads to defect of S139ph and DSB repair. H3 acetylation is required for the binding of BRG1 to γ‐H2AX nucleosomes. S139ph stimulates the H3 acetylation on γ‐H2AX nucleosomes, and the histone acetyltransferase Gcn5 is responsible for this novel crosstalk. The H3 acetylation on γ‐H2AX nucleosomes is induced by DNA damage. These results collectively suggest that SWI/SNF, γ‐H2AX and H3 acetylation cooperatively act in a feedback activation loop to facilitate DSB repair.