Interferon restores replication fork stability and cell viability in BRCA-defective cells via ISG15

DNA replication and repair defects or genotoxic treatments trigger interferon (IFN)-mediated inflammatory responses. However, whether and how IFN signaling in turn impacts the DNA replication process has remained elusive. Here we show that basal levels of the IFN-stimulated gene 15, ISG15, and its conjugation (ISGylation) are essential to protect nascent DNA from degradation. Moreover, IFN treatment restores replication fork stability in BRCA1/2-deficient cells, which strictly depends on topoisomerase-1, and rescues lethality of BRCA2-deficient mouse embryonic stem cells. Although IFN activates hundreds of genes, these effects are specifically mediated by ISG15 and ISGylation, as their inactivation suppresses the impact of IFN on DNA replication. ISG15 depletion significantly reduces cell proliferation rates in human BRCA1-mutated triple-negative, whereas its upregulation results in increased resistance to the chemotherapeutic drug cisplatin in mouse BRCA2-deficient breast cancer cells, respectively. Accordingly, cells carrying BRCA1/2 defects consistently show increased ISG15 levels, which we propose as a novel, in-built mechanism of drug resistance linked to BRCAness.

DNA复制与修复缺陷或遗传毒性处理可触发干扰素（IFN）介导的炎症应答。然而，干扰素信号通路是否以及如何反向调控DNA复制过程，迄今仍未明晰。本研究证实，干扰素刺激基因15（IFN-stimulated gene 15, ISG15）的基础表达水平及其ISG化修饰（ISGylation）对保护新生DNA免受降解不可或缺。此外，干扰素处理可恢复BRCA1/2缺陷细胞的复制叉稳定性，该过程严格依赖拓扑异构酶1（topoisomerase-1），同时可挽救BRCA2缺陷小鼠胚胎干细胞的致死表型。尽管干扰素可激活数百个基因，但上述效应仅由ISG15及其ISG化修饰所介导：抑制二者的表达可阻断干扰素对DNA复制的调控作用。具体而言，ISG15缺失可显著降低BRCA1突变型人类三阴性乳腺癌细胞的增殖速率，而上调ISG15则可增强小鼠BRCA2缺陷型乳腺癌细胞对化疗药物顺铂（cisplatin）的耐药性。相应地，携带BRCA1/2缺陷的细胞始终呈现ISG15水平升高的特征，我们据此提出，ISG15水平升高是一种与BRCAness表型相关的新型内源性耐药机制。