mSWI/SNF (BAF) Complexes Facilitate Decatentation of DNA by Topoisomerase IIa
Recent exon sequencing studies of human tumors have revealed that subunits of mSWI/SNF or BAF complexes are mutated in more than 20% of human malignancies, yet the mechanisms involved in tumor suppression is unclear. BAF chromatin remodeling complexes are polymorphic assemblies that use energy provided by ATP hydrolysis to regulate transcription through the control of chromatin structure and the placement of Polycomb (PcG) across the genome. Several proteins dedicated to this large multi-subunit complex, including SMARCA4 (BRG1) and BAF250A (ARID1A), are mutated at frequencies similar to that of many recognized tumor suppressors. In particular, the core ATPase BRG1 is mutated in 5-10% of childhood medulloblastoma (MB) and greater than 15% of Burkitt's Lymphoma (BL). Here we find a novel function of BAF complexes in decatenating newly replicated sister chromatids, which is necessary for proper chromosome segregation during mitosis. We find that deletion of Brg1, as well as the expression of Brg1 point mutants identified in human tumors leads to anaphase bridge formation (sister chromatids linked by catenated strands of DNA), and a G2/M phase block characteristic of the decatenation checkpoint. Endogenous BAF complexes directly interact with endogenous TopoIIa through BAF250a and are required for TopoIIa binding to about 12,000 sites over the genome. Our results indicate that TopoIIa's chromatin binding is dependent on the ATPase activity of Brg1, which is compromised in oncogenic Brg1 mutants. These studies indicate that the ability of TopoIIa to prevent DNA entanglement at mitosis requires BAF complexes and suggest that this activity contributes to the role of BAF subunits as tumor suppressors. Overall design: Examination of sites of TopoIIa activity in WT and Brg1-/- ES cells as defined by TopoIIa-DNA covalent adducts formed in the presence of etoposide