Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California 94305, USA.
Nature. 2013 May 30;497(7451):624-7. doi: 10.1038/nature12146. Epub 2013 May 22.
Recent exon-sequencing studies of human tumours have revealed that subunits of BAF (mammalian SWI/SNF) complexes are mutated in more than 20% of all human malignancies, but the mechanisms involved in tumour suppression are unclear. BAF chromatin-remodelling 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 repressive complex 2 (PRC2) across the genome. Several proteins dedicated to this multisubunit complex, including BRG1 (also known as SMARCA4) and BAF250a (also known as ARID1A), are mutated at frequencies similar to those of recognized tumour suppressors. In particular, the core ATPase BRG1 is mutated in 5-10% of childhood medulloblastomas and more than 15% of Burkitt's lymphomas. Here we show a previously unknown function of BAF complexes in decatenating newly replicated sister chromatids, a requirement for proper chromosome segregation during mitosis. We find that deletion of Brg1 in mouse cells, as well as the expression of BRG1 point mutants identified in human tumours, leads to anaphase bridge formation (in which sister chromatids are linked by catenated strands of DNA) and a G2/M-phase block characteristic of the decatenation checkpoint. Endogenous BAF complexes interact directly with endogenous topoisomerase IIα (TOP2A) through BAF250a and are required for the binding of TOP2A to approximately 12,000 sites across the genome. Our results demonstrate that TOP2A chromatin binding is dependent on the ATPase activity of BRG1, which is compromised in oncogenic BRG1 mutants. These studies indicate that the ability of TOP2A to prevent DNA entanglement at mitosis requires BAF complexes and suggest that this activity contributes to the role of BAF subunits as tumour suppressors.
最近对人类肿瘤的外显子测序研究表明，BAF（哺乳动物 SWI/SNF）复合物的亚基在超过 20%的所有人类恶性肿瘤中发生突变，但肿瘤抑制的相关机制尚不清楚。BAF 染色质重塑复合物是多态组装体，通过利用 ATP 水解提供的能量，调节转录，控制染色质结构，并在基因组上放置 Polycomb 抑制复合物 2（PRC2）。几种专门用于这种多亚基复合物的蛋白质，包括 BRG1（也称为 SMARCA4）和 BAF250a（也称为 ARID1A），其突变频率与公认的肿瘤抑制因子相似。特别是核心 ATP 酶 BRG1 在 5-10%的儿童髓母细胞瘤和超过 15%的伯基特淋巴瘤中发生突变。在这里，我们展示了 BAF 复合物在解开新复制的姐妹染色单体方面的一个先前未知的功能，这是有丝分裂过程中正确染色体分离的必需条件。我们发现，在小鼠细胞中缺失 Brg1，以及在人类肿瘤中鉴定出的 BRG1 点突变的表达，会导致后期桥的形成（其中姐妹染色单体通过 DNA 的连环链连接）和有丝分裂中期到 G2 期的阻滞，这是解连环检查点的特征。内源性 BAF 复合物通过 BAF250a 与内源性拓扑异构酶 IIα（TOP2A）直接相互作用，并且需要 BAF 复合物来结合基因组上约 12000 个位点的 TOP2A。我们的结果表明，TOP2A 染色质结合依赖于 BRG1 的 ATP 酶活性，而致癌性 BRG1 突变体则损害了这种活性。这些研究表明，TOP2A 在有丝分裂时防止 DNA 缠绕的能力需要 BAF 复合物，并表明这种活性有助于 BAF 亚基作为肿瘤抑制因子的作用。
