Department of Radiation Oncology and Winship Cancer Institute, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Mol Cell Biol. 2011 Apr;31(8):1594-609. doi: 10.1128/MCB.00524-10. Epub 2011 Feb 14.
Many human genes exhibit evidence of initiated RNA polymerase II (Pol II) at their promoters, despite a lack of significant full-length transcript. Such genes exhibit promoter-proximal "pausing," wherein initiated Pol II accumulates just downstream of the transcription start site due to a rate-limiting step mediating the transition to elongation. The mechanisms that regulate the escape of Pol II from pausing and the relationship to chromatin structure remain incompletely understood. Recently, we showed that CpG island hypermethylation and epigenetic silencing of TMS1/ASC in human breast cancers are accompanied by a local shift from histone H4 lysine 16 acetylation (H4K16Ac) to H4 lysine 20 trimethylation (H4K20me3). Here, we show that hMOF-mediated H4K16Ac and SUV420H2-mediated H4K20me3 play opposing roles in the regulation of Pol II pausing. We found that H4K16Ac promoted the release of Pol II from pausing through the recruitment of BRD4 and pTEFb. Aberrant methylation of CpG island DNA blocked Pol II recruitment to gene promoters. Whereas the inhibition of DNA methylation allowed for the reassociation and initiation of Pol II at the TMS1 promoter, Pol II remained paused in the presence of H4K20me3. Combined inhibition of H4K20me3 and DNA methylation resulted in the rerecruitment of hMOF and subsequent H4K16Ac, release of Pol II into active elongation, and synergistic reactivation of TMS1 expression. Marking by H4K20me3 was not restricted to TMS1 but also occurred at other genes independently of DNA methylation, where it similarly imposed a block to Pol II promoter escape through a mechanism that involved the local inhibition of H4K16Ac. These data indicate that H4K20me3 invokes gene repression by antagonizing hMOF-mediated H4K16Ac and suggest that overcoming Pol II pausing might be a rate-limiting step in achieving tumor suppressor gene reactivation in cancer therapy.
许多人类基因在其启动子处表现出 RNA 聚合酶 II（Pol II）起始的证据，尽管没有显著的全长转录本。这些基因表现出启动子近端的“暂停”，其中起始的 Pol II 由于介导向延伸过渡的限速步骤，在转录起始位点的下游积累。调节 Pol II 从暂停中逃逸的机制以及与染色质结构的关系仍不完全清楚。最近，我们表明，人类乳腺癌中 TMS1/ASC 的 CpG 岛超甲基化和表观遗传沉默伴随着局部从组蛋白 H4 赖氨酸 16 乙酰化（H4K16Ac）转移到 H4 赖氨酸 20 三甲基化（H4K20me3）。在这里，我们表明 hMOF 介导的 H4K16Ac 和 SUV420H2 介导的 H4K20me3 在调节 Pol II 暂停中发挥相反的作用。我们发现 H4K16Ac 通过募集 BRD4 和 pTEFb 促进 Pol II 从暂停中释放。CpG 岛 DNA 的异常甲基化阻止了 Pol II 向基因启动子的募集。然而，DNA 甲基化的抑制允许 Pol II 在 TMS1 启动子处重新关联和起始，而 Pol II 在 H4K20me3 存在下仍然暂停。H4K20me3 和 DNA 甲基化的联合抑制导致 hMOF 的重新募集和随后的 H4K16Ac、Pol II 进入活跃延伸的释放以及 TMS1 表达的协同重新激活。H4K20me3 的标记不仅限于 TMS1，而且独立于 DNA 甲基化发生在其他基因上，在那里它通过涉及局部抑制 H4K16Ac 的机制同样对 Pol II 启动子逃逸施加阻碍。这些数据表明，H4K20me3 通过拮抗 hMOF 介导的 H4K16Ac 来引发基因抑制，并表明克服 Pol II 暂停可能是在癌症治疗中实现肿瘤抑制基因重新激活的限速步骤。
