A synthetic lethal interaction between APC/C and topoisomerase poisons uncovered by proteomic screens

Mitotic Functions and Characters of KIF11 in Cancers

Mitosis mediates the accurate separation of daughter cells, and abnormalities are closely related to cancer progression. KIF11, a member of the kinesin family, plays a vital role in the formation and maintenance of the mitotic spindle. Recently, an increasing quantity of data have demonstrated the upregulated expression of KIF11 in various cancers, promoting the emergence and progression of cancers. This suggests the great potential of KIF11 as a prognostic biomarker and therapeutic target. However, the molecular mechanisms of KIF11 in cancers have not been systematically summarized. Therefore, we first discuss the functions of the protein encoded by KIF11 during mitosis and connect the abnormal expression of KIF11 with its clinical significance. Then, we elucidate the mechanism of KIF11 to promote various hallmarks of cancers. Finally, we provide an overview of KIF11 inhibitors and outline areas for future work.

Genome-Wide CRISPR Screens Reveal ZATT as a Synthetic Lethal Target of TOP2-Poison Etoposide That Can Act in a TDP2-Independent Pathway

Etoposide (ETO) is an anticancer drug that targets topoisomerase II (TOP2). It stabilizes a normally transient TOP2–DNA covalent complex (TOP2cc), thus leading to DNA double-strand breaks (DSBs). Tyrosyl-DNA phosphodiesterases two (TDP2) is directly involved in the repair of TOP2cc by removing phosphotyrosyl peptides from 5′-termini of DSBs. Recent studies suggest that additional factors are required for TOP2cc repair, which include the proteasome and the zinc finger protein associated with TDP2 and TOP2, named ZATT. ZATT may alter the conformation of TOP2cc in a way that renders the accessibility of TDP2 for TOP2cc removal. In this study, our genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) screens revealed that ZATT also has a TDP2-independent role in promoting cell survival following ETO treatment. ZATT KO cells showed relatively higher ETO sensitivity than TDP2-KO cells, and ZATT/TDP2 DKO cells displayed additive hypersensitivity to ETO treatment. The study using a series of deletion mutants of ZATT determined that the N-terminal 1–168 residues of ZATT are required for interaction with TOP2 and this interaction is critical to ETO sensitivity. Moreover, depletion of ZATT resulted in accelerated TOP2 degradation after ETO or cycloheximide (CHX) treatment, suggesting that ZATT may increase TOP2 stability and likely participate in TOP2 turnover. Taken together, this study suggests that ZATT is a critical determinant that dictates responses to ETO treatment and targeting. ZATT is a promising strategy to increase ETO efficacy for cancer therapy.

Targeting the anaphase-promoting complex/cyclosome (APC/C) enhanced antiproliferative and apoptotic response in bladder cancer

Improving the chemotherapy sensitivity of bladder cancer is a current clinical challenge. It is critical to seek out effective combination therapies that include low doses of cisplatin due to its dose-limiting toxicity. This study aims to investigate the cytotoxic effects of the combination therapy including proTAME, a small molecule inhibitor, targeting Cdc-20 and to determine the expression levels of several APC/C pathway-related genes that may play a role in the chemotherapy response of RT-4 (bladder cancer) and ARPE-19 (normal epithelial) cells. The IC20 and IC50 values were determined by MTS assay. The expression levels of apoptosis-associated (Bax and Bcl-2) and APC/C-associated (Cdc-20, Cyclin-B1, Securin, and Cdh-1) genes were assessed by qRT-PCR. Cell colonization ability and apoptosis were examined by clonogenic survival experiment and Annexin V/PI staining, respectively. Low-dose combination therapy showed a superior inhibition effect on RT-4 cells by increasing cell death and inhibiting colony formation. Triple-agent combination therapy further increased the percentage of late apoptotic and necrotic cells compared to the doublet-therapy with gemcitabine and cisplatin. ProTAME-containing combination therapies resulted in an elevation in Bax/Bcl-2 ratio in RT-4 cells, while a significant decrease was observed in proTAME-treated ARPE-19 cells. Cdc-20 expression in proTAME combined treatment groups were found to be decreased compared to their control groups. Low-dose triple-agent combination induced cytotoxicity and apoptosis in RT-4 cells effectively. It is essential to evaluate the role of APC/C pathway-associated potential biomarkers as therapeutic targets and define new combination therapy regimens to achieve improved tolerability in bladder cancer patients in the future.

Loss of RanGAP1 drives chromosome instability and rapid tumorigenesis of osteosarcoma

Chromothripsis is a catastrophic event of chromosomal instability that involves intensive fragmentation and rearrangements within localized chromosomal regions. However, its cause remains unclear. Here, we show that reduction and inactivation of Ran GTPase-activating protein 1 (RanGAP1) commonly occur in human osteosarcoma, which is associated with a high rate of chromothripsis. In rapidly expanding mouse osteoprogenitors, RanGAP1 deficiency causes chromothripsis in chr1q, instant inactivation of Rb1 and degradation of p53, consequent failure in DNA damage repair, and ultrafast osteosarcoma tumorigenesis. During mitosis, RanGAP1 anchors to the kinetochore, where it recruits PP1-γ to counteract the activity of the spindle-assembly checkpoint (SAC) and prevents TOP2A degradation, thus safeguarding chromatid decatenation. Loss of RanGAP1 causes SAC hyperactivation and chromatid decatenation failure. These findings demonstrate that RanGAP1 maintains mitotic chromosome integrity and that RanGAP1 loss drives tumorigenesis through its direct effects on SAC and decatenation and secondary effects on DNA damage surveillance.

GINS1 promotes the proliferation and migration of glioma cells through USP15-mediated deubiquitination of TOP2A

GINS1 is a GINS complex subunit that functions along with the MCM2-7 complex and Cdc45 in eukaryotic DNA replication. Despite the significance of the GINS complex in the switch between quiescence and proliferation of glioma cells inside and outside the perinecrotic niche, the biological functions and the underlying mechanism of GINS1 remain unclear. Unlike in normal cells and tissues, GINS1 expression level was significantly upregulated in glioma cells and tissues. High expression of GINS1 predicted an advanced clinical grade and a poor survival. Functional assays revealed that GINS1 aggravated glioma malignant phenotypes in vitro and in vivo. Mechanistically, this study identified that GINS1 physically interacts with TOP2A. GINS1 promotes glioma cell proliferation and migration through USP15-mediated deubiquitination of TOP2A protein. Our results delineate the clinical significance of GINS1 in glioma and the regulatory mechanisms involved in glioma cell proliferation and migration. This work provides potential therapeutic targets for glioma treatment.

The Role of the APC/C and Its Coactivators Cdh1 and Cdc20 in Cancer Development and Therapy

To sustain genomic stability by correct DNA replication and mitosis, cell cycle progression is tightly controlled by the cyclic activity of cyclin-dependent kinases, their binding to cyclins in the respective phase and the regulation of cyclin levels by ubiquitin-dependent proteolysis. The spindle assembly checkpoint plays an important role at the metaphase-anaphase transition to ensure a correct separation of sister chromatids before cytokinesis and to initiate mitotic exit, as an incorrect chromosome distribution may lead to genetically unstable cells and tumorigenesis. The ubiquitin ligase anaphase-promoting complex or cyclosome (APC/C) is essential for these processes by mediating the proteasomal destruction of cyclins and other important cell cycle regulators. To this end, it interacts with the two regulatory subunits Cdh1 and Cdc20. Both play a role in tumorigenesis with Cdh1 being a tumor suppressor and Cdc20 an oncogene. In this review, we summarize the current knowledge about the APC/C-regulators Cdh1 and Cdc20 in tumorigenesis and potential targeted therapeutic approaches.

NAT10 regulates mitotic cell fate by acetylating Eg5 to control bipolar spindle assembly and chromosome segregation

Cell fate of mitotic cell is controlled by spindle assembly. Deficient spindle assembly results in mitotic catastrophe leading to cell death to maintain cellular homeostasis. Therefore, inducing mitotic catastrophe provides a strategy for tumor therapy. Nucleolar acetyltransferase NAT10 has been found to regulate various cellular processes to maintain cell homeostasis. Here we report that NAT10 regulates mitotic cell fate by acetylating Eg5. NAT10 depletion results in multinuclear giant cells, which is the hallmark of mitotic catastrophe. Live-cell imaging showed that knockdown of NAT10 dramatically prolongs the mitotic time and induces defective chromosome segregation including chromosome misalignment, bridge and lagging. NAT10 binds and co-localizes with Eg5 in the centrosome during mitosis. Depletion of NAT10 reduces the centrosome loading of Eg5 and impairs the poleward movement of centrosome, leading to monopolar and asymmetrical spindle formation. Furthermore, NAT10 stabilizes Eg5 through its acetyltransferase function. NAT10 acetylates Eg5 at K771 to control Eg5 stabilization. We generated K771-Ac specific antibody and showed that Eg5 K771-Ac specifically localizes in the centrosome during mitosis. Additionally, K771 acetylation is required for the motor function of Eg5. The hyper-acetylation mimic Flag-Eg5 K771Q but not Flag-Eg5 rescued the NAT10 depletion-induced defective spindle formation and mitotic catastrophe, demonstrating that NAT10 controls mitosis through acetylating Eg5 K771. Collectively, we identify Eg5 as an important substrate of NAT10 in the control of mitosis and provide K771 as an essential acetylation site in the stabilization and motor function of Eg5. Our findings reveal that targeting the NAT10-mediated Eg5 K771 acetylation provides a potential strategy for tumor therapy.

Mechanisms to Repair Stalled Topoisomerase II-DNA Covalent Complexes

DNA topoisomerases regulate the topological state of DNA, relaxing DNA supercoils and resolving catenanes and knots that result from biologic processes, such as transcription and replication. DNA topoisomerase II (TOP2) enzymes achieve this by binding DNA and introducing an enzyme-bridged DNA double-strand break (DSB) where each protomer of the dimeric enzyme is covalently attached to the 5' end of the cleaved DNA via an active site tyrosine phosphodiester linkage. The enzyme then passes a second DNA duplex through the DNA break, before religation and release of the enzyme. However, this activity is potentially hazardous to the cell, as failure to complete religation leads to persistent TOP2 protein-DNA covalent complexes, which are cytotoxic. Indeed, this property of topoisomerase has been exploited in cancer therapy in the form of topoisomerase poisons which block the religation stage of the reaction cycle, leading to an accumulation of topoisomerase-DNA adducts. A number of parallel cellular processes have been identified that lead to removal of these covalent TOP2-DNA complexes, facilitating repair of the resulting protein-free DSB by standard DNA repair pathways. These pathways presumably arose to repair spontaneous stalled or poisoned TOP2-DNA complexes, but understanding their mechanisms also has implications for cancer therapy, particularly resistance to anti-cancer TOP2 poisons and the genotoxic side effects of these drugs. Here, we review recent progress in the understanding of the processing of TOP2 DNA covalent complexes, the basic components and mechanisms, as well as the additional layer of complexity posed by the post-translational modifications that modulate these pathways. SIGNIFICANCE STATEMENT: Multiple pathways have been reported for removal and repair of TOP2-DNA covalent complexes to ensure the timely and efficient repair of TOP2-DNA covalent adducts to protect the genome. Post-translational modifications, such as ubiquitination and SUMOylation, are involved in the regulation of TOP2-DNA complex repair. Small molecule inhibitors of these post-translational modifications may help to improve outcomes of TOP2 poison chemotherapy, for example by increasing TOP2 poison cytotoxicity and reducing genotoxicity, but this remains to be determined.

The regulation of DNA supercoiling across evolution

DNA supercoiling controls a variety of cellular processes, including transcription, recombination, chromosome replication, and segregation, across all domains of life. As a physical property, DNA supercoiling alters the double helix structure by under- or over-winding it. Intriguingly, the evolution of DNA supercoiling reveals both similarities and differences in its properties and regulation across the three domains of life. Whereas all organisms exhibit local, constrained DNA supercoiling, only bacteria and archaea exhibit unconstrained global supercoiling. DNA supercoiling emerges naturally from certain cellular processes and can also be changed by enzymes called topoisomerases. While structurally and mechanistically distinct, topoisomerases that dissipate excessive supercoils exist in all domains of life. By contrast, topoisomerases that introduce positive or negative supercoils exist only in bacteria and archaea. The abundance of topoisomerases is also transcriptionally and post-transcriptionally regulated in domain-specific ways. Nucleoid-associated proteins, metabolites, and physicochemical factors influence DNA supercoiling by acting on the DNA itself or by impacting the activity of topoisomerases. Overall, the unique strategies that organisms have evolved to regulate DNA supercoiling hold significant therapeutic potential, such as bactericidal agents that target bacteria-specific processes or anticancer drugs that hinder abnormal DNA replication by acting on eukaryotic topoisomerases specialized in this process. The investigation of DNA supercoiling therefore reveals general principles, conserved mechanisms, and kingdom-specific variations relevant to a wide range of biological questions.

Discovery and Validation of a Compound to Target Ewing's Sarcoma

Ewing’s sarcoma, characterized by pathognomonic t (11; 22) (q24; q12) and related chromosomal ETS family translocations, is a rare aggressive cancer of bone and soft tissue. Current protocols that include cytotoxic chemotherapeutic agents effectively treat localized disease; however, these aggressive therapies may result in treatment-related morbidities including second-site cancers in survivors. Moreover, the five-year survival rate in patients with relapsed, recurrent, or metastatic disease is less than 30%, despite intensive therapy with these cytotoxic agents. By using high-throughput phenotypic screening of small molecule libraries, we identified a previously uncharacterized compound (ML111) that inhibited in vitro proliferation of six established Ewing’s sarcoma cell lines with nanomolar potency. Proteomic studies show that ML111 treatment induced prometaphase arrest followed by rapid caspase-dependent apoptotic cell death in Ewing’s sarcoma cell lines. ML111, delivered via methoxypoly(ethylene glycol)-polycaprolactone copolymer nanoparticles, induced dose-dependent inhibition of Ewing’s sarcoma tumor growth in a murine xenograft model and invoked prometaphase arrest in vivo, consistent with in vitro data. These results suggest that ML111 represents a promising new drug lead for further preclinical studies and is a potential clinical development for the treatment of Ewing’s sarcoma.

Regulation of topoisomerase II stability and activity by ubiquitination and SUMOylation: clinical implications for cancer chemotherapy

DNA topoisomerases II (TOP2) are peculiar enzymes (TOP2α and TOP2β) that modulate the conformation of DNA by momentarily breaking double-stranded DNA to allow another strand to pass through, and then rejoins the DNA phosphodiester backbone. TOP2α and TOP2β play vital roles in nearly all events involving DNA metabolism, including DNA transcription, replication, repair, and chromatin remodeling. Beyond these vital functions, TOP2 enzymes are therapeutic targets for various anticancer drugs, termed TOP2 poisons, such as teniposide, etoposide, and doxorubicin. These drugs exert their antitumor activity by inhibiting the activity of TOP2-DNA cleavage complexes (TOP2ccs) containing DNA double-strand breaks (DSBs), subsequently leading to the degradation of TOP2 by the 26S proteasome, thereby exposing the DSBs and eliciting a DNA damage response. Failure of the DSBs to be appropriately repaired leads to genomic instability. Due to this mechanism, patients treated with TOP2-based drugs have a high incidence of secondary malignancies and cardiotoxicity. While the cytotoxicity associated with TOP2 poisons appears to be TOP2α-dependent, the DNA sequence rearrangements and formation of DSBs appear to be mediated primarily through TOP2β inhibition, likely due to the differential degradation patterns of TOP2α and TOP2β. Research over the past few decades has shown that under various conditions, the ubiquitin-proteasome system (UPS) and the SUMOylation pathway are primarily responsible for regulating the stability and activity of TOP2 and are therefore critical regulators of the therapeutic effect of TOP2-targeting drugs. In this review, we summarize the current progress on the regulation of TOP2α and TOP2β by ubiquitination and SUMOylation. By fully elucidating the basic biology of these essential and complex molecular mechanisms, better strategies may be developed to improve the therapeutic efficacy of TOP2 poisons and minimize the risks of therapy-related secondary malignancy.

LxCxD motif of the APC/C coactivator subunit FZR1 is critical for interaction with the retinoblastoma protein

Retinoblastoma protein (pRB) regulates cell cycle by utilizing different regions of its pocket domain for interacting with E2F family of transcription factors and with cellular and viral proteins containing an LxCxE motif. An LxCxE-like motif, LxCxD, is present in FZR1, an adaptor protein of the multi-subunit E3 ligase complex anaphase-promoting complex/cyclosome (APC/C). The APC/C^FZR1 complex regulates the timely degradation of multiple cell cycle proteins for mitotic exit and maintains G1 state. We report that FZR1 interacts with pRB via its LxCxD motif. By using point mutations, we found that the cysteine residue in the FZR1 LxCxD motif is critical for direct interaction with pRb. The direct binding of the LxCxD motif of FZR1 to the pRB LxCxE binding pocket is confirmed by using human papillomavirus protein E7 as a competitor, both in vitro and in vivo. While mutation of the cysteine residue significantly disrupts FZR1 interaction with pRB, this motif does not affect FZR1 and core APC/C association. Expression of the FZR1 point mutant results in accumulation of S-phase kinase-associated protein 2 (SKP2) and Polo-like kinase 1 (PLK1), while p27^Kip1 and p21^Cip1 proteins are downregulated, indicating a G1 cell cycle defect. Consistently, cells containing point mutant FZR1 enter the S phase prematurely. Together our results suggest that the LxCxD motif of FZR1 is a critical determinant for the interaction between FZR1 and pRB and is important for G1 restriction.

Inter-organelle interactions between the ER and mitotic spindle facilitates Zika protease cleavage of human Kinesin-5 and results in mitotic defects

Here we identify human Kinesin-5, Kif11/HsEg5, as a cellular target of Zika protease. We show that Zika NS2B-NS3 protease targets several sites within the motor domain of HsEg5 irrespective of motor binding to microtubules. The native integral ER-membrane protease triggers mitotic spindle positioning defects and a prolonged metaphase delay in cultured cells. Our data support a model whereby loss of function of HsEg5 is mediated by Zika protease and is spatially restricted to the ER-mitotic spindle interface during mitosis. The resulting phenotype is distinct from the monopolar phenotype that typically results from uniform inhibition of HsEg5 by RNAi or drugs. In addition, our data reveal novel inter-organelle interactions between the mitotic apparatus and the surrounding reticulate ER network. Given that Kif11 is haplo-insufficient in humans, and reduced dosage results in microcephaly, we propose that Zika protease targeting of HsEg5 may be a key event in the etiology of Zika syndrome microcephaly.

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Zika protease cleavage of Kinesin-5 impairs mitotic progression

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Inter-organelle interactions spatially control Zika proteolysis of Kinesin-5

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Native Zika protease affects mitosis differently than soluble Zika protease

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Zika protease may elicit fetal microcephaly and blindness via Kif11/Kinesin-5

Mdm2-Mediated Downmodulation of GRK2 Restricts Centrosome Separation for Proper Chromosome Congression

The timing of centrosome separation and the distance moved apart influence the formation of the bipolar spindle, affecting chromosome stability. Epidermal growth factor receptor (EGFR) signaling induces early centrosome separation through downstream G protein-coupled receptor kinase GRK2, which phosphorylates the Hippo pathway component MST2 (Mammalian STE20-like protein kinase 2), in turn allowing NIMA kinase Nek2A activation for centrosomal linker disassembly. However, the mechanisms that counterbalance centrosome disjunction and separation remain poorly understood. We unveil that timely degradation of GRK2 by the E3 ligase Mdm2 limits centrosome separation in the G2. Both knockout expression and catalytic inhibition of Mdm2 result in GRK2 accumulation and enhanced centrosome separation before mitosis onset. Phosphorylation of GRK2 on residue S670 enables a complex pattern of non-K48-linked polyubiquitin chains assembled by Mdm2, which correlate with kinase protein degradation. Remarkably, GRK2-S670A protein fails to phosphorylate MST2 despite overcoming Mdm2-dependent degradation, which results in defective centrosome separation, shorter spindles, and abnormal chromosome congression. Conversely, extra levels of wild-type kinase in the G2 cause increased inter-centrosome distances with longer spindles, also converging in congression issues. Our findings show that the signals enabling activity of the GRK2/MST2/Nek2A axis for separation also switches on Mdm2 degradation of GRK2 to ensure accurate centrosome dynamics and proper mitotic spindle functionality.

Untangling trapped topoisomerases with tyrosyl-DNA phosphodiesterases

DNA topoisomerases alleviate the torsional stress that is generated by processes that are central to genome metabolism such as transcription and DNA replication. To do so, these enzymes generate an enzyme intermediate known as the cleavage complex in which the topoisomerase is covalently linked to the termini of a DNA single- or double-strand break. Whilst cleavage complexes are normally transient they can occasionally become abortive, creating protein-linked DNA breaks that threaten genome stability and cell survival; a process promoted and exploited in the cancer clinic by the use of topoisomerase 'poisons'. Here, we review the consequences to genome stability and human health of abortive topoisomerase-induced DNA breakage and the cellular pathways that cells have adopted to mitigate them, with particular focus on an important class of enzymes known as tyrosyl-DNA phosphodiesterases.

The Insulin Receptor Adaptor IRS2 is an APC/C Substrate That Promotes Cell Cycle Protein Expression and a Robust Spindle Assembly Checkpoint

Insulin receptor substrate 2 (IRS2) is an essential adaptor that mediates signaling downstream of the insulin receptor and other receptor tyrosine kinases. Transduction through IRS2-dependent pathways is important for coordinating metabolic homeostasis, and dysregulation of IRS2 causes systemic insulin signaling defects. Despite the importance of maintaining proper IRS2 abundance, little is known about what factors mediate its protein stability. We conducted an unbiased proteomic screen to uncover novel substrates of the Anaphase Promoting Complex/Cyclosome (APC/C), a ubiquitin ligase that controls the abundance of key cell cycle regulators. We found that IRS2 levels are regulated by APC/C activity and that IRS2 is a direct APC/C target in G1 Consistent with the APC/C's role in degrading cell cycle regulators, quantitative proteomic analysis of IRS2-null cells revealed a deficiency in proteins involved in cell cycle progression. We further show that cells lacking IRS2 display a weakened spindle assembly checkpoint in cells treated with microtubule inhibitors. Together, these findings reveal a new pathway for IRS2 turnover and indicate that IRS2 is a component of the cell cycle control system in addition to acting as an essential metabolic regulator.

An interphase pool of KIF11 localizes at the basal bodies of primary cilia and a reduction in KIF11 expression alters cilia dynamics

KIF11 is a homotetrameric kinesin that peaks in protein expression during mitosis. It is a known mitotic regulator, and it is well-described that KIF11 is necessary for the formation and maintenance of the bipolar spindle. However, there has been a growing appreciation for non-mitotic roles for KIF11. KIF11 has been shown to function in such processes as axon growth and microtubule polymerization. We previously demonstrated that there is an interphase pool of KIF11 present in glioblastoma cancer stem cells that drives tumor cell invasion. Here, we identified a previously unknown association between KIF11 and primary cilia. We confirmed that KIF11 localized to the basal bodies of primary cilia in multiple cell types, including neoplastic and non-neoplastic cells. Further, we determined that KIF11 has a role in regulating cilia dynamics. Upon the reduction of KIF11 expression, the number of ciliated cells in asynchronously growing populations was significantly increased. We rescued this effect by the addition of exogenous KIF11. Lastly, we found that depleting KIF11 resulted in an increase in cilium length and an attenuation in the kinetics of cilia disassembly. These findings establish a previously unknown link between KIF11 and the dynamics of primary cilia and further support non-mitotic functions for this kinesin.

APC/C ubiquitin ligase: Functions and mechanisms in tumorigenesis

The anaphase promoting complex/ cyclosome (APC/C), is an evolutionarily conserved protein complex essential for cellular division due to its role in regulating the mitotic transition from metaphase to anaphase. In this review, we highlight recent work that has shed light on our understanding of the role of APC/C coactivators, Cdh1 and Cdc20, in cancer initiation and development. We summarize the current state of knowledge regarding APC/C structure and function, as well as the distinct ways Cdh1 and Cdc20 are dysregulated in human cancer. We also discuss APC/C inhibitors, novel approaches for targeting the APC/C as a cancer therapy, and areas for future work.

The interplay between DNA topoisomerase 2α post-translational modifications and drug resistance

The type 2 DNA topoisomerases (Top2) are conserved enzymes and biomarkers for cell proliferation. The catalytic activities of the human isoform Top2α are essential for the regulation of DNA topology during DNA replication, transcription, and chromosome segregation. Top2α is a prominent target for anti-cancer drugs and is highly regulated by post-translational modifications (PTM). Despite an increasing number of proteomic studies, the extent of PTM in cancer cells and its importance in drug response remains largely uncharacterized. In this review, we highlight the different modifications affecting the human Top2α in healthy and cancer cells, taking advantage of the structure-function information accumulated in the past decades. We also overview the regulation of Top2α by PTM, the level of PTM in cancer cells, and the resistance to therapeutic compounds targeting the Top2 enzyme. Altogether, this review underlines the importance of future studies addressing more systematically the interplay between PTM and Top2 drug resistance.

Cell Cycle-Dependent Control and Roles of DNA Topoisomerase II

Type II topoisomerases are ubiquitous enzymes in all branches of life that can alter DNA superhelicity and unlink double-stranded DNA segments during processes such as replication and transcription. In cells, type II topoisomerases are particularly useful for their ability to disentangle newly-replicated sister chromosomes. Growing lines of evidence indicate that eukaryotic topoisomerase II (topo II) activity is monitored and regulated throughout the cell cycle. Here, we discuss the various roles of topo II throughout the cell cycle, as well as mechanisms that have been found to govern and/or respond to topo II function and dysfunction. Knowledge of how topo II activity is controlled during cell cycle progression is important for understanding how its misregulation can contribute to genetic instability and how modulatory pathways may be exploited to advance chemotherapeutic development.

The anaphase-promoting complex/cyclosome: a new promising target in diffuse large B-cell lymphoma and mantle cell lymphoma

Background

The aggressive B-cell non-Hodgkin lymphomas diffuse large B-cell lymphoma (DLBCL) and mantle cell lymphoma (MCL) are characterised by a high proliferation rate. The anaphase-promoting complex/cyclosome (APC/C) and its co-activators Cdc20 and Cdh1 represent an important checkpoint in mitosis. Here, the role of the APC/C and its co-activators is examined in DLBCL and MCL.

Methods

The expression and prognostic value of Cdc20 and Cdh1 was investigated using GEP data and immunohistochemistry. Moreover, the therapeutic potential of APC/C targeting was evaluated using the small-molecule inhibitor proTAME and the underlying mechanisms of action were investigated by western blot.

Results

We demonstrated that Cdc20 is highly expressed in DLBCL and aggressive MCL, correlating with a poor prognosis in DLBCL. ProTAME induced a prolonged metaphase, resulting in accumulation of the APC/C-Cdc20 substrate cyclin B1, inactivation/degradation of Bcl-2 and Bcl-xL and caspase-dependent apoptosis. In addition, proTAME strongly enhanced the anti-lymphoma effect of the clinically relevant agents doxorubicin and venetoclax.

Conclusion

We identified for the first time APC/C as a new, promising target in DLBCL and MCL. Moreover, we provide evidence that Cdc20 might be a novel, independent prognostic factor in DLBCL and MCL.

Hyperphosphorylation of CDH1 in Glioblastoma Cancer Stem Cells Attenuates APC/CCDH1 Activity and Pharmacologic Inhibition of APC/CCDH1/CDC20 Compromises Viability

Glioblastoma (GBM) is the most common and lethal primary brain tumor and remains incurable. This is in part due to the cellular heterogeneity within these tumors, which includes a subpopulation of treatment-resistant cells called cancer stem-like cells (CSC). We previously identified that the anaphase-promoting complex/cylosome (APC/C), a key cell-cycle regulator and tumor suppressor, had attenuated ligase activity in CSCs. Here, we assessed the mechanism of reduced activity, as well as the efficacy of pharmacologically targeting the APC/C in CSCs. We identified hyperphosphorylation of CDH1, but not pseudosubstrate inhibition by early mitotic inhibitor 1 (EMI1), as a major mechanism driving attenuated APC/C^CDH1 activity in the G₁-phase of the cell cycle in CSCs. Small-molecule inhibition of the APC/C reduced viability of both CSCs and nonstem tumor cells (NSTCs), with the combination of proTAME and apcin having the biggest impact. Combinatorial drug treatment also led to the greatest mitotic arrest and chromosomal abnormalities. IMPLICATIONS: Our findings demonstrate how the activity of the APC/C^CDH1 tumor suppressor is reduced in CSCs and also validates small-molecule inhibition of the APC/C as a promising therapeutic target for the treatment of GBM.

Targeting mitosis exit: A brake for cancer cell proliferation

The transition from mitosis to interphase, referred to as mitotic exit, is a critical mitotic process which involves activation and inactivation of multiple mitotic kinases and counteracting protein phosphatases. Loss of mitotic exit checkpoints is a common feature of cancer cells, leading to mitotic dysregulation and confers cancer cells with oncogenic characteristics, such as aberrant proliferation and microtubule-targeting agent (MTA) resistance. Since MTA resistance results from cancer cells prematurely exiting mitosis (mitotic slippage), blocking mitotic exit is believed to be a promising anticancer strategy. Moreover, based on this theory, simultaneous inhibition of mitotic exit and additional cell cycle phases would likely achieve synergistic antitumor effects. In this review, we divide the molecular regulators of mitotic exit into four categories based on their different regulatory functions: 1) the anaphase-promoting complex/cyclosome (APC/C, a ubiquitin ligase), 2) cyclin B, 3) mitotic kinases and phosphatases, 4) kinesins and microtubule-binding proteins. We also review the regulators of mitotic exit and propose prospective anticancer strategies targeting mitotic exit, including their strengths and possible challenges to their use.

PRL-3 Promotes Ubiquitination and Degradation of AURKA and Colorectal Cancer Progression via Dephosphorylation of FZR1

The oncogenic phosphatase PRL-3 is highly expressed in metastatic colorectal cancer but not in nonmetastatic colorectal cancer or noncolorectal cancer metastatic cancers. Although the proinvasive capacity of PRL-3 has been validated in multiple types of cancer, its impact on colorectal cancer progression and the underlying mechanisms remain poorly understood. Here, we report that overexpressed PRL-3 stimulates G₂-M arrest, chromosomal instability (CIN), self-renewal, and growth of colorectal cancer cells in xenograft models, while colorectal cancer cell proliferation is decreased. PRL-3-induced G₂-M arrest was associated with decreased expression of Aurora kinase A (AURKA). PRL-3-promoted slow proliferation, CIN, self-renewal, and growth in xenografts were counteracted by ectopic expression of AURKA. Conversely, knockdown of PRL-3 resulted in low proliferation, S-phase arrest, impaired self-renewal, increased apoptosis, and diminished xenograft growth independently of AURKA. Analysis of colorectal cancer specimens showed that expression of PRL-3 was associated with high status of CIN and poor prognosis, which were antagonized by expression of AURKA. PRL-3 enhanced AURKA ubiquitination and degradation in a phosphatase-dependent fashion. PRL-3 interacted with AURKA and FZR1, a regulatory component of the APC/C^FZR1 complex. Destabilization of AURKA by PRL-3 required PRL-3-mediated dephosphorylation of FZR1 and assembly of the APC/C^FZR1 complex. Our study suggests that PRL-3-regulated colorectal cancer progression is collectively determined by distinct malignant phenotypes and further reveals PRL-3 as an essential regulator of APC/C^FZR1 in controlling the stability of AURKA. SIGNIFICANCE: Dephosphorylation of FZR1 by PRL-3 facilitates the activity of APC/C^FZR1 by destabilizing AURKA, thus influencing aggressive characteristics and overall progression of colorectal cancer.

Aneuploid acute myeloid leukemia exhibits a signature of genomic alterations in the cell cycle and protein degradation machinery

Background

Aneuploidy occurs in more than 20% of acute myeloid leukemia (AML) cases and correlates with an adverse prognosis.

Methods

To understand the molecular bases of aneuploid acute myeloid leukemia (A‐AML), this study examined the genomic profile in 42 A‐AML cases and 35 euploid acute myeloid leukemia (E‐AML) cases.

Results

A‐AML was characterized by increased genomic complexity based on exonic variants (an average of 26 somatic mutations per sample vs 15 for E‐AML). The integration of exome, copy number, and gene expression data revealed alterations in genes involved in DNA repair (eg, SLX4IP, RINT1, HINT1, and ATR) and the cell cycle (eg, MCM2, MCM4, MCM5, MCM7, MCM8, MCM10, UBE2C, USP37, CK2, CK3, CK4, BUB1B, NUSAP1, and E2F) in A‐AML, which was associated with a 3‐gene signature defined by PLK1 and CDC20 upregulation and RAD50 downregulation and with structural or functional silencing of the p53 transcriptional program. Moreover, A‐AML was enriched for alterations in the protein ubiquitination and degradation pathway (eg, increased levels of UHRF1 and UBE2C and decreased UBA3 expression), response to reactive oxygen species, energy metabolism, and biosynthetic processes, which may help in facing the unbalanced protein load. E‐AML was associated with BCOR/BCORL1 mutations and HOX gene overexpression.

Conclusions

These findings indicate that aneuploidy‐related and leukemia‐specific alterations cooperate to tolerate an abnormal chromosome number in AML, and they point to the mitotic and protein degradation machineries as potential therapeutic targets.

Aneuploid acute myeloid leukemia (A‐AML) is associated with genomic and transcriptional alterations in the cell cycle and protein degradation pathways. The upregulation of PLK1 and CDC20 and the downregulation of RAD50 and of a p53‐related signature are hallmarks of A‐AML.

An E2-ubiquitin thioester-driven approach to identify substrates modified with ubiquitin and ubiquitin-like molecules

Covalent modifications of proteins with ubiquitin and ubiquitin-like molecules are instrumental to many biological processes. However, identifying the E3 ligase responsible for these modifications remains a major bottleneck in ubiquitin research. Here, we present an E2-thioester-driven identification (E2~dID) method for the targeted identification of substrates of specific E2 and E3 enzyme pairs. E2~dID exploits the central position of E2-conjugating enzymes in the ubiquitination cascade and provides in vitro generated biotinylated E2~ubiquitin thioester conjugates as the sole source for ubiquitination in extracts. This enables purification and mass spectrometry-based identification of modified proteins under stringent conditions independently of the biological source of the extract. We demonstrate the sensitivity and specificity of E2-dID by identifying and validating substrates of APC/C in human cells. Finally, we perform E2~dID with SUMO in S. cerevisiae, showing that this approach can be easily adapted to other ubiquitin-like modifiers and experimental models.

Mitosis inhibitors in anticancer therapy: When blocking the exit becomes a solution

Current microtubule-targeting agents (MTAs) remain amongst the most important antimitotic drugs used against a broad range of malignancies. By perturbing spindle assembly, MTAs activate the spindle assembly checkpoint (SAC), which induces mitotic arrest and subsequent apoptosis. However, besides toxic side effects and resistance, mitotic slippage and failure in triggering apoptosis in various cancer cells are limiting factors of MTAs efficacy. Alternative strategies to target mitosis without affecting microtubules have, thus, led to the identification of small molecules, such as those that target spindle Kinesins, Aurora and Polo-like kinases. Unfortunately, these so-called second-generation of antimitotics, encompassing mitotic blockers and mitotic drivers, have failed in clinical trials. Our recent understanding regarding the mechanisms of cell death during a mitotic arrest pointed out apoptosis as the main variable, providing an opportunity to control the cell fates and influence the effectiveness of antimitotics. Here, we provide an overview on the second-generation of antimitotics, and discuss possible strategies that exploit SAC activity, mitotic slippage/exit and apoptosis induction, in order to improve the efficacy of anticancer strategies that target mitosis.

Synthetic Lethality and Cancer - Penetrance as the Major Barrier

Synthetic lethality has long been proposed as an approach for targeting genetic defects in tumours. Despite a decade of screening efforts, relatively few robust synthetic lethal targets have been identified. Improved genetic perturbation techniques, including CRISPR/Cas9 gene editing, have resulted in renewed enthusiasm for searching for synthetic lethal effects in cancer. An implicit assumption behind this enthusiasm is that the lack of reproducibly identified targets can be attributed to limitations of RNAi technologies. We argue here that a bigger hurdle is that most synthetic lethal interactions (SLIs) are not highly penetrant, in other words they are not robust to the extensive molecular heterogeneity seen in tumours. We outline strategies for identifying and prioritising SLIs that are most likely to be highly penetrant.

Identification of the APC/C co-factor FZR1 as a novel therapeutic target for multiple myeloma

Multiple Myeloma (MM) is a haematological neoplasm characterised by the clonal proliferation of malignant plasma cells in the bone marrow. The success of proteasome inhibitors in the treatment of MM has highlighted the importance of the ubiquitin proteasome system (UPS) in the pathogenesis of this disease. In this study, we analysed gene expression of UPS components to identify novel therapeutic targets within this pathway in MM. Here we demonstrate how this approach identified previously validated and novel therapeutic targets. In addition we show that FZR1 (Fzr), a cofactor of the multi-subunit E3 ligase complex anaphase-promoting complex/cyclosome (APC/C), represents a novel therapeutic target in myeloma. The APC/C associates independently with two cofactors, Fzr and Cdc20, to control cell cycle progression. We found high levels of FZR1 in MM primary cells and cell lines and demonstrate that expression is further increased on adhesion to bone marrow stromal cells (BMSCs). Specific knockdown of either FZR1 or CDC20 reduced viability and induced growth arrest of MM cell lines, and resulted in accumulation of APC/C^Fzr substrate Topoisomerase IIα (TOPIIα) or APC/C^Cdc20 substrate Cyclin B. Similar effects were observed following treatment with proTAME, an inhibitor of both APC/C^Fzr and APC/C^Cdc20. Combinations of proTAME with topoisomerase inhibitors, etoposide and doxorubicin, significantly increased cell death in MM cell lines and primary cells, particularly if TOPIIα levels were first increased through pre-treatment with proTAME. Similarly, combinations of proTAME with the microtubule inhibitor vincristine resulted in enhanced cell death. This study demonstrates the potential of targeting the APC/C and its cofactors as a therapeutic approach in MM.

Ubiquitin, the centrosome, and chromosome segregation

For over a century, the abnormal movement or number of centrosomes has been linked with errors of chromosomes distribution in mitosis. While not essential for the formation of the mitotic spindle, the presence and location of centrosomes has a major influence on the manner in which microtubules interact with the kinetochores of replicated sister chromatids and the accuracy with which they migrate to resulting daughter cells. A complex network has evolved to ensure that cells contain the proper number of centrosomes and that their location is optimal for effective attachment of emanating spindle fibers with the kinetochores. The components of this network are regulated through a series of post-translational modifications, including ubiquitin and ubiquitin-like modifiers, which coordinate the timing and strength of signaling events key to the centrosome cycle. In this review, we examine the role of the ubiquitin system in the events relating to centriole duplication and centrosome separation, and discuss how the disruption of these functions impacts chromosome segregation.

Emerging power of proteomics for delineation of intrinsic tumor subtypes and resistance mechanisms to anti-cancer therapies

INTRODUCTION

Despite extreme genetic heterogeneity, tumors often show similar alterations in the expression, stability, and activation of proteins important in oncogenic signaling pathways. Thus, classifying tumor samples according to shared proteomic features may help facilitate the identification of cancer subtypes predictive of therapeutic responses and prognostic for patient outcomes. Meanwhile, understanding mechanisms of intrinsic and acquired resistance to anti-cancer therapies at the protein level may prove crucial to devising reversal strategies. Areas covered: Herein, we review recent advances in quantitative proteomic technology and their applications in studies to identify intrinsic tumor subtypes of various tumors, to illuminate mechanistic aspects of pharmacological and oncogenic adaptations, and to highlight interaction targets for anti-cancer compounds and cancer-addicted proteins. Expert commentary: Quantitative proteomic technologies are being successfully employed to classify tumor samples into distinct intrinsic subtypes, to improve existing DNA/RNA based classification methods, and to evaluate the activation status of key signaling pathways.

RNF168 and USP10 regulate topoisomerase IIα function via opposing effects on its ubiquitylation

Topoisomerase IIα (TOP2α) is essential for chromosomal condensation and segregation, as well as genomic integrity. Here we report that RNF168, an E3 ligase mutated in the human RIDDLE syndrome, interacts with TOP2α and mediates its ubiquitylation. RNF168 deficiency impairs decatenation activity of TOP2α and promotes mitotic abnormalities and defective chromosomal segregation. Our data also indicate that RNF168 deficiency, including in human breast cancer cell lines, confers resistance to the anti-cancer drug and TOP2 inhibitor etoposide. We also identify USP10 as a deubiquitylase that negatively regulates TOP2α ubiquitylation and restrains its chromatin association. These findings provide a mechanistic link between the RNF168/USP10 axis and TOP2α ubiquitylation and function, and suggest a role for RNF168 in the response to anti-cancer chemotherapeutics that target TOP2.

Ubiquitin ligase RNF20/40 facilitates spindle assembly and promotes breast carcinogenesis through stabilizing motor protein Eg5

Whether transcriptional regulators are functionally involved in mitosis is a fundamental question in cell biology. Here we report that the RNF20/40 complex, a major ubiquitin ligase catalysing histone H2B monoubiquitination, interacts with the motor protein Eg5 during mitosis and participates in spindle assembly. We show that the RNF20/40 complex monoubiquitinates and stabilizes Eg5. Loss of RNF20/40 results in spindle assembly defects, cell cycle arrest and apoptosis. Consistently, depletion of either RNF20/40 or Eg5 suppresses breast cancer in vivo. Significantly, RNF20/40 and Eg5 are concurrently upregulated in human breast carcinomas and high Eg5 expression is associated with poorer overall survival of patients with luminal A, or B, breast cancer. Our study uncovers an important spindle assembly role of the RNF20/40 complex, and implicates the RNF20/40-Eg5 axis in breast carcinogenesis, supporting the pursuit of these proteins as potential targets for breast cancer therapeutic interventions.

Eg5 has a role in spindle assembly and has been associated with tumorigenesis but it is not clear how its activity is regulated. Here, the authors show that the E3 ligase RNF20/40 regulates mitotic spindle assembly by regulating the stability of Eg5 through mono-ubiquitination of K745.

Insights into APC/C: from cellular function to diseases and therapeutics

Anaphase-promoting complex/cyclosome (APC/C) is a multifunctional ubiquitin-protein ligase that targets different substrates for ubiquitylation and therefore regulates a variety of cellular processes such as cell division, differentiation, genome stability, energy metabolism, cell death, autophagy as well as carcinogenesis. Activity of APC/C is principally governed by two WD-40 domain proteins, Cdc20 and Cdh1, in and beyond cell cycle. In the past decade, the results based on numerous biochemical, 3D structural, mouse genetic and small molecule inhibitor studies have largely attracted our attention into the emerging role of APC/C and its regulation in biological function, human diseases and potential therapeutics. This review will aim to summarize some recently reported insights into APC/C in regulating cellular function, connection of its dysfunction with human diseases and its implication of therapeutics.

The mitotic kinesin KIF11 is a driver of invasion, proliferation, and self-renewal in glioblastoma

The proliferative and invasive nature of malignant cancers drives lethality. In glioblastoma, these two processes are presumed mutually exclusive and hence termed "go or grow." We identified a molecular target that shuttles between these disparate cellular processes-the molecular motor KIF11. Inhibition of KIF11 with a highly specific small-molecule inhibitor stopped the growth of the more treatment-resistant glioblastoma tumor-initiating cells (TICs, or cancer stem cells) as well as non-TICs and impeded tumor initiation and self-renewal of the TIC population. Targeting KIF11 also hit the other arm of the "go or grow" cell fate decision by reducing glioma cell invasion. Administration of a KIF11 inhibitor to mice bearing orthotopic glioblastoma prolonged their survival. In its role as a shared molecular regulator of cell growth and motility across intratumoral heterogeneity, KIF11 is a compelling therapeutic target for glioblastoma.

Fbxo30 Regulates Mammopoiesis by Targeting the Bipolar Mitotic Kinesin Eg5

Fbxo30 is an orphan member of the F-box protein family with no known substrate or function. Here we report that, while Fbxo30(-/-) mice exhibit normal development, growth, lifespan, and fertility, the females fail to nurture their offspring as a result of defective mammopoiesis. Mass spectrometry analysis of Fbxo30-associated proteins revealed that Fbxo30 specifically interacts with the bipolar spindle kinesin EG5 (encoded by Kif11). As a result, Fbxo30 targets Eg5 for ubiquitinylation and controls its oscillation during the cell cycle. Correlated with EG5 dysregulation, Fbxo30(-/-) mammary epithelial cells exhibit multiple defects in centrosome homeostasis, mitotic spindle formation, and proliferation. Effects on proliferation, centrosome homeostasis, and mammopoiesis in the Fbxo30(-/-) mice were rescued through normalization of Eg5 activity using shRNA and/or an EG5 inhibitor. Our data reveal the Fbxo30-Eg5 interaction as a critical checkpoint in mammopoiesis and a critical role for ubiquitinylation-regulated Eg5 oscillation in the cell cycle.

The role of APC/C(Cdh1) in replication stress and origin of genomic instability

It has been proposed that the APC/C(Cdh1) functions as a tumor suppressor by maintaining genomic stability. However, the exact nature of genomic instability following loss of Cdh1 is unclear. Using biochemistry and live cell imaging of single cells we found that Cdh1 knockdown (kd) leads to strong nuclear stabilization of the substrates cyclin A and B and deregulated kinetics of DNA replication. Restoration of the Cdh1-dependent G2 DNA damage checkpoint did not result in G2 arrest but blocked cells in prometaphase, suggesting that these cells enter mitosis despite incomplete replication. This results in DNA double-strand breaks, anaphase bridges, cytokinesis defects and tetraploidization. Tetraploid cells are the source of supernumerary centrosomes following Cdh1-kd, leading to multipolar mitosis or centrosome clustering, in turn resulting in merotelic attachment and lagging chromosomes. Whereas some of these events cause apoptosis during mitosis, surviving cells may accumulate chromosomal aberrations.

Increased Rrm2 gene dosage reduces fragile site breakage and prolongs survival of ATR mutant mice

In S. cerevisiae, deletion of the checkpoint kinase Mec1 (ATR) is viable upon mutations that increase the activity of the ribonucleotide reductase (RNR) complex. Lopez-Contreras et al. show that cells from mice carrying extra alleles of the RNR regulatory subunit RRM2 present supraphysiological RNR activity and reduced chromosomal breakage at fragile sites. Increased Rrm2 gene dosage also extends the life span of ATR mutant mice.

In Saccharomyces cerevisiae, absence of the checkpoint kinase Mec1 (ATR) is viable upon mutations that increase the activity of the ribonucleotide reductase (RNR) complex. Whether this pathway is conserved in mammals remains unknown. Here we show that cells from mice carrying extra alleles of the RNR regulatory subunit RRM2 (Rrm2^TG) present supraphysiological RNR activity and reduced chromosomal breakage at fragile sites. Moreover, increased Rrm2 gene dosage significantly extends the life span of ATR mutant mice. Our study reveals the first genetic condition in mammals that reduces fragile site expression and alleviates the severity of a progeroid disease by increasing RNR activity.

The ubiquitin ligase APC/C(Cdh1) puts the brakes on DNA-end resection

DNA double-strand breaks (DSBs) are highly deleterious lesions and their misrepair can promote genomic instability, a hallmark of cancer. DNA-end resection is a cell cycle-regulated mechanism that is required for the faithful repair of DSBs. We recently discovered that the anaphase-promoting complex/cyclosome-Cdh1 (APC/C^Cdh1) ubiquitin ligase is responsible for the timely degradation of CtBP-interacting protein (CtIP), a key DNA-end resection factor, providing a new layer of regulation of DSB repair in human cells.

APC/C(Cdh1) controls CtIP stability during the cell cycle and in response to DNA damage

Human cells have evolved elaborate mechanisms for responding to DNA damage to maintain genome stability and prevent carcinogenesis. For instance, the cell cycle can be arrested at different stages to allow time for DNA repair. The APC/C(C) (dh1) ubiquitin ligase mainly regulates mitotic exit but is also implicated in the DNA damage-induced G2 arrest. However, it is currently unknown whether APC/C(C) (dh1) also contributes to DNA repair. Here, we show that Cdh1 depletion causes increased levels of genomic instability and enhanced sensitivity to DNA-damaging agents. Using an integrated proteomics and bioinformatics approach, we identify CtIP, a DNA-end resection factor, as a novel APC/C(C) (dh1) target. CtIP interacts with Cdh1 through a conserved KEN box, mutation of which impedes ubiquitylation and downregulation of CtIP both during G1 and after DNA damage in G2. Finally, we find that abrogating the CtIP-Cdh1 interaction results in delayed CtIP clearance from DNA damage foci, increased DNA-end resection, and reduced homologous recombination efficiency. Combined, our results highlight the impact of APC/C(C) (dh1) on the maintenance of genome integrity and show that this is, at least partially, achieved by controlling CtIP stability in a cell cycle- and DNA damage-dependent manner.

TopoIIα prevents telomere fragility and formation of ultra thin DNA bridges during mitosis through TRF1-dependent binding to telomeres

Telomeres are repetitive nucleoprotein structures at the ends of chromosomes. Like most genomic regions consisting of repetitive DNA, telomeres are fragile sites prone to replication fork stalling and generation of chromosomal instability. In particular, abrogation of the TRF1 telomere binding protein leads to stalled replication forks and aberrant telomere structures known as “multitelomeric signals”. Here, we report that TRF1 deficiency also leads to the formation of “ultra-fine bridges” (UFB) during mitosis, and to an increased time to complete mitosis mediated by the spindle assembly checkpoint proteins (SAC). We find that topoisomerase IIα (TopoIIα), an enzyme essential for resolution of DNA replication intermediates, binds telomeres in a TRF1-mediated manner. Indeed, similar to TRF1 abrogation, TopoIIα downregulation leads to telomere fragility and UFB, suggesting that these phenotypes are due to decreased TopoIIα at telomeres. We find that SAC proteins bind telomeres in vivo, and that this is disrupted upon TRF1 deletion. These findings suggest that TRF1 links TopoIIα and SAC proteins in a pathway that ensures correct telomere replication and mitotic segregation, unveiling how TRF1 protects from telomere fragility and mitotic defects.