Department of Translational Oncology, St. Marianna University Graduate School of Medicine, 2-16-1, Sugao, Miyamae-ku, Kawasaki, 216-8511, Japan.
Department of General Surgery, The People's Hospital of Gaoming District of Foshan City, Foshan, 528500, Guangdong, China.
Sci Rep. 2021 Jan 11;11(1):360. doi: 10.1038/s41598-020-79715-y.
The nucleolus is a nuclear structure composed of ribosomal DNA (rDNA), and functions as a site for rRNA synthesis and processing. The rDNA is guanine-rich and prone to form G-quadruplex (G4), a secondary structure of DNA. We have recently found that HERC2, an HECT ubiquitin ligase, promotes BLM and WRN RecQ DNA helicases to resolve the G4 structure. Here, we report the role of HERC2 in the regulation of nucleolar localization of the helicases. Furthermore, HERC2 inactivation enhances the effects of CX-5461, an inhibitor of RNA polymerase I (Pol I)-mediated transcription of rRNA with an intrinsic G4-stabilizing activity. HERC2 depletion or homozygous deletion of the C-terminal HECT domain of HERC2 prevented the nucleolar localization of BLM and WRN, and inhibited relocalization of BLM to replication stress-induced nuclear RPA foci. HERC2 colocalized with fibrillarin and Pol I subunit RPA194, both of which are required for rRNA transcription. The HERC2 dysfunction enhanced the suppression of pre-rRNA transcription by CX-5461. These results suggest the effect of HERC2 status on the functions of BLM and WRN on rRNA transcription in the nucleolus. Since HERC2 is downregulated in numerous cancers, this effect may be clinically relevant considering the beneficial effects of CX-5461 in cancer treatments.
核仁是一种由核糖体 DNA（rDNA）组成的核结构，其功能是作为 rRNA 合成和加工的场所。rDNA 富含鸟嘌呤，容易形成 G-四链体（G4），这是 DNA 的一种二级结构。我们最近发现，一种 E3 泛素连接酶 HERC2 促进 BLM 和 WRN RecQ 解旋酶来解决 G4 结构。在这里，我们报告了 HERC2 在调节解旋酶核仁定位中的作用。此外，HERC2 失活增强了 CX-5461 的作用，CX-5461 是一种 RNA 聚合酶 I（Pol I）介导的 rRNA 转录抑制剂，具有内在的 G4 稳定活性。HERC2 缺失或 HERC2 羧基末端 HECT 结构域的纯合缺失阻止了 BLM 和 WRN 的核仁定位，并抑制了 BLM 向复制应激诱导的核 RPA 焦点的重新定位。HERC2 与核仁仁纤层蛋白和 Pol I 亚基 RPA194 共定位，这两者都是 rRNA 转录所必需的。HERC2 功能障碍增强了 CX-5461 对 pre-rRNA 转录的抑制作用。这些结果表明 HERC2 状态对 BLM 和 WRN 在核仁中 rRNA 转录功能的影响。由于 HERC2 在许多癌症中下调，考虑到 CX-5461 在癌症治疗中的有益作用，这种影响可能具有临床相关性。
