The negative elongation factor (NELF) is a four-subunit protein complex (NELF-E, NELF-A, NELF-B and NELF-C/D) that negatively regulates transcription elongation of RNA polymerase II (Pol II). Interestingly, upregulation of NELF-E subunit promotes hepatocellular carcinoma (HCC) and pancreatic cancer. In addition, we have previously shown that NELF complex fosters double-strand break (DSB)-induced transcription silencing and promotes homology-directed repair (HDR). However, the mechanisms underlying NELF-E regulation of HDR of DSBs remain unknown. Here, we show that NELF-E interacts with BRCA1 and promotes its recruitment to laser-microirradiated sites and facilitates ionizing radiation-induced foci (IRIF) of BRCA1 in HCC cells (Hep3B). The reduction in BRCA1 IRIF is accompanied by decreased RAD51 IRIF. A corollary to this, NELF-E-deficient Hep3B cells exhibit defective HDR of chromosomal DSBs induced by CRISPR-Cas9 system. Consequently, the disruption of NELF complex integrity, by NELF-E downregulation, sensitizes Hep3B cells to PARP inhibition. Altogether, our results suggest that NELF promotes HDR by facilitating BRCA1 and RAD51 IRIF formation and identify NELF complex as a novel synthetic lethal partner of PARP1.

负延伸因子 (NELF) 是一种四亚基蛋白复合物 (NELF-E、NELF-A、NELF-B 和 NELF-C/D)，可负向调节 RNA 聚合酶 II (Pol II) 的转录延伸。有趣的是，NELF-E 亚基的上调促进了肝细胞癌 (HCC) 和胰腺癌。此外，我们之前已经表明 NELF 复合物促进双链断裂 (DSB) 诱导的转录沉默并促进同源定向修复 (HDR)。然而，DSB 的 HDR 的 NELF-E 调节机制仍然未知。在这里，我们展示了 NELF-E 与 BRCA1 相互作用并促进其向激光微辐照部位的募集，并促进 HCC 细胞 (Hep3B) 中 BRCA1 的电离辐射诱导病灶 (IRIF)。BRCA1 IRIF 的减少伴随着 RAD51 IRIF 的减少。一个推论，NELF-E 缺陷的 Hep3B 细胞表现出由 CRISPR-Cas9 系统诱导的染色体 DSB 的 HDR 缺陷。因此，NELF-E 下调对 NELF 复合物完整性的破坏使 Hep3B 细胞对 PARP 抑制敏感。总之，我们的结果表明 NELF 通过促进 BRCA1 和 RAD51 IRIF 的形成来促进 HDR，并将 NELF 复合物鉴定为 PARP1 的新型合成致死伙伴。