Ubiquitin-specific protease 15 (USP15) is a widely expressed deubiquitylase that has been implicated in diverse cellular processes in cancer. Here we identify topoisomerase II (TOP2A) as a novel protein that is regulated by USP15. TOP2A accumulates during G2 and functions to decatenate intertwined sister chromatids at prophase, ensuring the replicated genome can be accurately divided into daughter cells at anaphase. We show that USP15 is required for TOP2A accumulation, and that USP15 depletion leads to the formation of anaphase chromosome bridges. These bridges fail to decatenate, and at mitotic exit form micronuclei that are indicative of genome instability. We also describe the cell cycle-dependent behaviour for two major isoforms of USP15, which differ by a short serine-rich insertion that is retained in isoform-1 but not in isoform-2. Although USP15 is predominantly cytoplasmic in interphase, we show that both isoforms move into the nucleus at prophase, but that isoform-1 is phosphorylated on its unique S229 residue at mitotic entry. The micronuclei phenotype we observe on USP15 depletion can be rescued by either USP15 isoform and requires USP15 catalytic activity. Importantly, however, an S229D phospho-mimetic mutant of USP15 isoform-1 cannot rescue either the micronuclei phenotype, or accumulation of TOP2A. Thus, S229 phosphorylation selectively abrogates this role of USP15 in maintaining genome integrity in an isoform-specific manner. Finally, we show that USP15 isoform-1 is preferentially upregulated in a panel of non-small cell lung cancer cell lines, and propose that isoform imbalance may contribute to genome instability in cancer. Our data provide the first example of isoform-specific deubiquitylase phospho-regulation and reveal a novel role for USP15 in guarding genome integrity.

中文翻译：

---

去泛素化酶USP15调节拓扑异构酶IIα以维持基因组完整性。

泛素特异性蛋白酶15（USP15）是一种广泛表达的去泛素化酶，已与癌症的多种细胞过程有关。在这里，我们确定拓扑异构酶II（TOP2A）是一种受USP15调控的新型蛋白质。TOP2A在G2期间积聚并起着在前期将交织的姐妹染色单体分离的作用，从而确保复制的基因组可以在后期精确地分为子细胞。我们表明，USP15是TOP2A积累所必需的，并且USP15的耗竭导致后期染色体桥的形成。这些桥无法分界，并在有丝分裂出口形成微核，表明基因组不稳定。我们还描述了USP15的两个主要同工型的细胞周期依赖性行为，这与保留在同工型1中而不是同工型2中的富含丝氨酸的短插入不同。尽管USP15在相间主要是胞质，但我们显示两种同工型都在前期移入细胞核，但是同工型1在有丝分裂进入时在其独特的S229残基上被磷酸化。我们观察到的USP15耗竭的微核表型可以通过任一USP15异构体来挽救，并需要USP15催化活性。但是重要的是，USP15亚型-1的S229D磷酸化模拟突变体不能挽救微核表型或TOP2A的积累。因此，S229磷酸化选择性地消除了USP15以同工型特异性方式维持基因组完整性的作用。最后，我们表明，USP15亚型-1在非小细胞肺癌细胞系中优先上调，并提出亚型不平衡可能导致癌症的基因组不稳定。