BAF complexes facilitate decatenation of DNA by topoisomerase IIα

Area of Science:

• Cell Biology
• Genetics
• Cancer Research

Background:

• BAF (BRG1-associated factor) complexes, also known as mammalian SWI/SNF, are chromatin remodelers involved in transcription regulation.
• Mutations in BAF subunits, including BRG1 (SMARCA4) and BAF250a (ARID1A), occur in over 20% of human malignancies, but their tumor suppressor mechanisms are not fully understood.
• BRG1, a core ATPase subunit, is frequently mutated in childhood medulloblastomas and Burkitt's lymphomas.

Purpose of the Study:

• To investigate the previously unknown function of BAF complexes in mitosis.
• To elucidate the role of BAF complexes in chromosome segregation and their connection to tumor suppression.
• To determine the mechanism by which BAF complexes interact with topoisomerase IIα (TOP2A).

Main Methods:

• Deletion of Brg1 in mouse cells and expression of human tumor-derived BRG1 mutants.
• Analysis of cell cycle progression, focusing on the G2/M phase and anaphase bridge formation.
• Chromatin immunoprecipitation assays to assess TOP2A binding to genomic sites.
• Co-immunoprecipitation to study the interaction between BAF complexes and TOP2A.

Main Results:

• Loss or mutation of Brg1 leads to anaphase bridge formation and a G2/M-phase block, indicating impaired decatenation.
• BAF complexes, via BAF250a, directly interact with TOP2A.
• BAF complexes are essential for TOP2A binding to approximately 12,000 genomic sites.
• TOP2A chromatin binding is dependent on BRG1's ATPase activity, which is compromised in oncogenic BRG1 mutants.

Conclusions:

• BAF complexes play a critical role in decatenating newly replicated sister chromatids, a process vital for proper chromosome segregation.
• The ATPase activity of BRG1 is required for BAF-mediated TOP2A chromatin binding and function.
• Impairment of this BAF-TOP2A interaction due to BRG1 mutations contributes to the tumor suppressor function of BAF subunits.