Transcription-Replication Conflicts as a Source of Genome Instability

Transcription and replication both require large macromolecular complexes to act on a DNA template, yet these machineries cannot simultaneously act on the same DNA sequence. Conflicts between the replication and transcription machineries (transcription-replication conflicts, or TRCs) are widespread in both prokaryotes and eukaryotes and have the capacity to both cause DNA damage and compromise complete, faithful replication of the genome. This review will highlight recent studies investigating the genomic locations of TRCs and the mechanisms by which they may be prevented, mitigated, or resolved. We address work from both model organisms and mammalian systems but predominantly focus on multicellular eukaryotes owing to the additional complexities inherent in the coordination of replication and transcription in the context of cell type-specific gene expression and higher-order chromatin organization.

Confronting the loss of trophic support

Classic experiments with peripheral sympathetic neurons established an absolute dependence upon NGF for survival. A forgotten problem is how these neurons become resistant to deprivation of trophic factors. The question is whether and how neurons can survive in the absence of trophic support. However, the mechanism is not understood how neurons switch their phenotype to lose their dependence on trophic factors, such as NGF and BDNF. Here, we approach the problem by considering the requirements for trophic support of peripheral sympathetic neurons and hippocampal neurons from the central nervous system. We developed cellular assays to assess trophic factor dependency for sympathetic and hippocampal neurons and identified factors that rescue neurons in the absence of trophic support. They include enhanced expression of a subunit of the NGF receptor (Neurotrophin Receptor Homolog, NRH) in sympathetic neurons and an increase of the expression of the glucocorticoid receptor in hippocampal neurons. The results are significant since levels and activity of trophic factors are responsible for many neuropsychiatric conditions. Resistance of neurons to trophic factor deprivation may be relevant to the underlying basis of longevity, as well as an important element in preventing neurodegeneration.

Rupture of a non sinus of Valsalva aneurysm during pregnancy: Case report and review of literatures

Ruptured Sinus of Valsalva aneurysms during pregnancy is rare and presents a threat both to the mother and the fetus. We report a case of ruptured nonsinus of Valsalva aneurysms in a 26-year-old woman diagnosed at 32⁺⁴ weeks of gestation. A successful elective lower-segment cesarean section was conducted under general anesthesia. A successful surgical correction of the ruptured aneurysm under cardiopulmonary bypass (CPB) was performed with patch repair after 13 days. A multidisciplinary approach with respect to the pregnant patient's diagnosis, indications, and timing of surgery is necessary in ensuring the best possible outcomes for both the mother and the child.

Targeting SARS-CoV-2 Proteases for COVID-19 Antiviral Development

The emergence of severe acute respiratory syndrome (SARS-CoV-2) in 2019 marked the third occurrence of a highly pathogenic coronavirus in the human population since 2003. As the death toll surpasses 5 million globally and economic losses continue, designing drugs that could curtail infection and disease progression is critical. In the US, three highly effective Food and Drug Administration (FDA)-authorized vaccines are currently available, and Remdesivir is approved for the treatment of hospitalized patients. However, moderate vaccination rates and the sustained evolution of new viral variants necessitate the ongoing search for new antivirals. Several viral proteins have been prioritized as SARS-CoV-2 antiviral drug targets, among them the papain-like protease (PLpro) and the main protease (Mpro). Inhibition of these proteases would target viral replication, viral maturation, and suppression of host innate immune responses. Knowledge of inhibitors and assays for viruses were quickly adopted for SARS-CoV-2 protease research. Potential candidates have been identified to show inhibitory effects against PLpro and Mpro, both in biochemical assays and viral replication in cells. These results encourage further optimizations to improve prophylactic and therapeutic efficacy. In this review, we examine the latest developments of potential small-molecule inhibitors and peptide inhibitors for PLpro and Mpro, and how structural biology greatly facilitates this process.

Single-cell transcriptomics identifies Gadd45b as a regulator of herpesvirus-reactivating neurons

Single-cell RNA sequencing (scRNA-seq) is a powerful technique for dissecting the complexity of normal and diseased tissues, enabling characterization of cell diversity and heterogeneous phenotypic states in unprecedented detail. However, this technology has been underutilized for exploring the interactions between the host cell and viral pathogens in latently infected cells. Herein, we use scRNA-seq and single-molecule sensitivity fluorescent in situ hybridization (smFISH) technologies to investigate host single-cell transcriptome changes upon the reactivation of a human neurotropic virus, herpes simplex virus-1 (HSV-1). We identify the stress sensor growth arrest and DNA damage-inducible 45 beta (Gadd45b) as a critical antiviral host factor that regulates HSV-1 reactivation events in a subpopulation of latently infected primary neurons. We show that distinct subcellular localization of Gadd45b correlates with the viral late gene expression program, as well as the expression of the viral transcription factor, ICP4. We propose that a hallmark of a "successful" or "aborted" HSV-1 reactivation state in primary neurons is determined by a unique subcellular localization signature of the stress sensor Gadd45b.

[Role of the ES-62 protein derived from Acanthocheilonema viteae in regulation of immune dysregulation diseases: a review]

ES-62 is a phosphorylcholine-containing, 62 kDa glycoprotein derived from the excretory-secretory product of Acanthocheilonema viteae, which is effective for the prevention and treatment of immune dysregulation diseases through triggering activation of immune cells, such as dendritic cells, mononuclear macrophages and regulatory B cells and mediating immune responses. Recently, the role of the ES-62 protein in the management of allergic, autoimmune and metabolic diseases has been paid much attention. This review summarizes the regulatory role of the ES-62 protein in immune dysregulation diseases and the underlying mechanisms, so as to provide insights into future experimental studies.

[Analysis of components of proteins from Echinococcus granulosus cyst fluid]

OBJECTIVE

To analyze the components of proteins from Echinococcus granulosus cyst fluid using the shotgun method, and to identify the active components with potential regulatory effects for immune dysregulation diseases.

METHODS

The E. granulosus cyst fluid was collected aseptically from the hepatic cysts of patients with cystic echinococcosis, and characterized by liquid chromatography (LC) tandem mass spectrometry (MS/MS) following digestion with trypsin. The protein data were searched using the software MaxQuant version 1.6.1.0 and the cellular components, molecular functions, and biological processes of the identified proteins were analyzed using the Gene Ontology (GO) method.

RESULTS

The E. granulosus cyst fluid separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) had a relative molecular mass of 25 to 70 kDa. LS-MS/MS analysis identified 37 proteins, including 32 known proteins and 5 unknown proteins. At least 4 proteins were preliminarily found to exhibit potential regulatory effects for immune dysregulation diseases, including antigen B, glutathione-S-transferase (GST), thioredoxin peroxidase (TPX) and malate dehydrogenase (MDH). GO enrichment analysis showed that the identified proteins had 149 molecular functions and were involved in 341 biological processes.

CONCLUSIONS

E. granulosus cyst fluid has a variety of protein components, and four known proteins are preliminarily identified to be associated with immune dysregulation diseases.

A basal-level activity of ATR links replication fork surveillance and stress response

Mammalian cells use diverse pathways to prevent deleterious consequences during DNA replication, yet the mechanism by which cells survey individual replisomes to detect spontaneous replication impediments at the basal level, and their accumulation during replication stress, remain undefined. Here, we used single-molecule localization microscopy coupled with high-order-correlation image-mining algorithms to quantify the composition of individual replisomes in single cells during unperturbed replication and under replicative stress. We identified a basal-level activity of ATR that monitors and regulates the amounts of RPA at forks during normal replication. Replication-stress amplifies the basal activity through the increased volume of ATR-RPA interaction and diffusion-driven enrichment of ATR at forks. This localized crowding of ATR enhances its collision probability, stimulating the activation of its replication-stress response. Finally, we provide a computational model describing how the basal activity of ATR is amplified to produce its canonical replication stress response.

A molecular sensor determines the ubiquitin substrate specificity of SARS-CoV-2 papain-like protease

The SARS-CoV-2 papain-like protease (PLpro) is a target for antiviral drug development. It is essential for processing viral polyproteins for replication and functions in host immune evasion by cleaving ubiquitin (Ub) and ubiquitin-like protein (Ubl) conjugates. While highly conserved, SARS-CoV-2 and SARS-CoV PLpro have contrasting Ub/Ubl substrate preferences. Using a combination of structural analyses and functional assays, we identify a molecular sensor within the S1 Ub-binding site of PLpro that serves as a key determinant of substrate specificity. Variations within the S1 sensor specifically alter cleavage of Ub substrates but not of the Ubl interferon-stimulated gene 15 protein (ISG15). Significantly, a variant of concern associated with immune evasion carries a mutation in the S1 sensor that enhances PLpro activity on Ub substrates. Collectively, our data identify the S1 sensor region as a potential hotspot of variability that could alter host antiviral immune responses to newly emerging SARS-CoV-2 lineages.

WEE1i-ATRi combination therapy: a promising low-dose treatment for CCNE1-amplified gynecologic cancers

CCNE1 amplification is an oncogenic driver for many gynecologic cancers and is associated with poor patient outcomes. In this issue, Xu et al.1 identify a combination therapy that is responsive to high CCNE1-copy number ovarian and endometrial cancers using PDX models.

Fused in sarcoma regulates DNA replication timing and kinetics

Fused in sarcoma (FUS) encodes an RNA-binding protein with diverse roles in transcriptional activation and RNA splicing. While oncogenic fusions of FUS and transcription factor DNA-binding domains are associated with soft tissue sarcomas, dominant mutations in FUS can cause amyotrophic lateral sclerosis. FUS has also been implicated in genome maintenance. However, the underlying mechanisms of its actions in genome stability are unknown. Here, we applied gene editing, functional reconstitution, and integrated proteomics and transcriptomics to illuminate roles for FUS in DNA replication and repair. Consistent with a supportive role in DNA double-strand break repair, FUS-deficient cells exhibited subtle alterations in the recruitment and retention of double-strand break-associated factors, including 53BP1 and BRCA1. FUS-/- cells also exhibited reduced proliferative potential that correlated with reduced speed of replication fork progression, diminished loading of prereplication complexes, enhanced micronucleus formation, and attenuated expression and splicing of S-phase-associated genes. Finally, FUS-deficient cells exhibited genome-wide alterations in DNA replication timing that were reversed upon re-expression of FUS complementary DNA. We also showed that FUS-dependent replication domains were enriched in transcriptionally active chromatin and that FUS was required for the timely replication of transcriptionally active DNA. These findings suggest that alterations in DNA replication kinetics and programming contribute to genome instability and functional defects in FUS-deficient cells.

Risk Factors for Maternal and Perinatal Complications during Pregnancy among Women with Tetralogy of Fallot

Background

Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart disease during pregnancy. Studies of risk factors are of great significance to maternal and fetal outcomes in patients with TOF.

Aims

To identify predictive risk factors for maternal and perinatal complications in women with TOF.

Subjects and Methods

78 patients with TOF who delivered at Shanghai Obstetrical Cardiology Intensive Care Center between January 1993 and December 2017 were retrospective reviewed. A logistic regression model was used to identify risk factors for maternal and perinatal complications.

Results

There was no maternal death, five patients developed cardiac failure, sustained arrhythmias requiring treatments were recorded among 2 patients. Factors identified for maternal complications included previous cardiac events and oxygen saturation <90%. Oxygen saturation <90% was found to be independent predictor of maternal cardiac complications (RR = 21.455, 95%CI 2.186-210.572, P = 0.009). The perinatal survival rate was 87.18%, there were 52 term deliveries (66.67%). Perinatal poor outcomes include 9 therapeutic abortions (11.54%), 1 neonatal death (1.28%), 16 premature births (20.51%), 18 small for gestational age children (23.08%), 3 neonatal asphyxia (3.85%), and 3 neonatal cardiac malformations (3.85%). Factors identified for perinatal complications included without cardiac surgery, higher hemoglobin values, higher hematocrit values, oxygen saturation <90%, right ventricular hypertrophy, pulmonary stenosis, ventricular septal defect, and pulmonary hypertension. Oxygen saturation <90% was found to be independent predictor of perinatal complications (RR = 8.270, 95%CI 1.374-49.790, P = 0.021).

Conclusions

Oxygen saturation <90% is associated with maternal and perinatal risks. Women with TOF whose oxygen saturation <90% are not recommended for pregnancy because of high maternal and perinatal complications.

FANCI functions as a repair/apoptosis switch in response to DNA crosslinks

Cells counter DNA damage through repair or apoptosis, yet a direct mechanism for this choice has remained elusive. When facing interstrand crosslinks (ICLs), the ICL-repair protein FANCI heterodimerizes with FANCD2 to initiate ICL excision. We found that FANCI alternatively interacts with a pro-apoptotic factor, PIDD1, to enable PIDDosome (PIDD1-RAIDD-caspase-2) formation and apoptotic death. FANCI switches from FANCD2/repair to PIDD1/apoptosis signaling in the event of ICL-repair failure. Specifically, removing key endonucleases downstream of FANCI/FANCD2, increasing ICL levels, or allowing damaged cells into mitosis (when repair is suppressed) all suffice for switching. Reciprocally, apoptosis-committed FANCI reverts from PIDD1 to FANCD2 after a failed attempt to assemble the PIDDosome. Monoubiquitination and deubiquitination at FANCI K523 impact interactor selection. These data unveil a repair-or-apoptosis switch in eukaryotes. Beyond ensuring the removal of unrepaired genomes, the switch's bidirectionality reveals that damaged cells can offset apoptotic defects via de novo attempts at lesion repair.

Hyperactive CDK2 Activity in Basal-like Breast Cancer Imposes a Genome Integrity Liability that Can Be Exploited by Targeting DNA Polymerase ε

Knowledge of fundamental differences between breast cancer subtypes has driven therapeutic advances; however, basal-like breast cancer (BLBC) remains clinically intractable. Because BLBC exhibits alterations in DNA repair enzymes and cell-cycle checkpoints, elucidation of factors enabling the genomic instability present in this subtype has the potential to reveal novel anti-cancer strategies. Here, we demonstrate that BLBC is especially sensitive to suppression of iron-sulfur cluster (ISC) biosynthesis and identify DNA polymerase epsilon (POLE) as an ISC-containing protein that underlies this phenotype. In BLBC cells, POLE suppression leads to replication fork stalling, DNA damage, and a senescence-like state or cell death. In contrast, luminal breast cancer and non-transformed mammary cells maintain viability upon POLE suppression but become dependent upon an ATR/CHK1/CDC25A/CDK2 DNA damage response axis. We find that CDK1/2 targets exhibit hyperphosphorylation selectively in BLBC tumors, indicating that CDK2 hyperactivity is a genome integrity vulnerability exploitable by targeting POLE.

Activity profiling and crystal structures of inhibitor-bound SARS-CoV-2 papain-like protease: A framework for anti-COVID-19 drug design

Viral papain-like cysteine protease (PLpro, NSP3) is essential for SARS-CoV-2 replication and represents a promising target for the development of antiviral drugs. Here, we used a combinatorial substrate library and performed comprehensive activity profiling of SARS-CoV-2 PLpro. On the scaffold of the best hits from positional scanning, we designed optimal fluorogenic substrates and irreversible inhibitors with a high degree of selectivity for SARS PLpro. We determined crystal structures of two of these inhibitors in complex with SARS-CoV-2 PLpro that reveals their inhibitory mechanisms and provides a molecular basis for the observed substrate specificity profiles. Last, we demonstrate that SARS-CoV-2 PLpro harbors deISGylating activity similar to SARSCoV-1 PLpro but its ability to hydrolyze K48-linked Ub chains is diminished, which our sequence and structure analysis provides a basis for. Together, this work has revealed the molecular rules governing PLpro substrate specificity and provides a framework for development of inhibitors with potential therapeutic value or drug repurposing.

Roles for MDC1 in cancer development and treatment

The DNA damage response (DDR) is necessary to maintain genome integrity and prevent the accumulation of oncogenic mutations. Consequently, proteins involved in the DDR often serve as tumor suppressors, carrying out the crucial task of keeping DNA fidelity intact. Mediator of DNA damage checkpoint 1 (MDC1) is a scaffold protein involved in the early steps of the DDR. MDC1 interacts directly with γ-H2AX, the phosphorylated form of H2AX, a commonly used marker for DNA damage. It then propagates the phosphorylation of H2AX by recruiting ATM kinase. While the function of MDC1 in the DDR has been reviewed previously, its role in cancer has not been reviewed, and numerous studies have recently identified a link between MDC1 and carcinogenesis. This includes MDC1 functioning as a tumor suppressor, with its loss serving as a biomarker for cancer and contributor to drug sensitivity. Studies also indicate that MDC1 operates outside of its traditional role in DDR, and functions as a co-regulator of nuclear receptor transcriptional activity, and that mutations in MDC1 are present in tumors and can also cause germline predisposition to cancer. This review will discuss reports that link MDC1 to cancer and identify MDC1 as an important player in tumor formation, progression, and treatment. We also discuss mechanisms by which MDC1 levels are regulated and how this contributes to tumor formation.

Engineered unnatural ubiquitin for optimal detection of deubiquitinating enzymes

Herein we present a workflow for design and synthesis of novel selective Ub-based tools for DUBs. Selectivity is achieved by incorporation of unnatural amino acids into the Ub C-terminal epitope.

Deubiquitinating enzymes (DUBs) are responsible for removing ubiquitin (Ub) from its protein conjugates. DUBs have been implicated as attractive therapeutic targets in the treatment of viral diseases, neurodegenerative disorders and cancer. The lack of selective chemical tools for the exploration of these enzymes significantly impairs the determination of their roles in both normal and pathological states. Commercially available fluorogenic substrates are based on the C-terminal Ub motif or contain Ub coupled to a fluorophore (Z-LRGG-AMC, Ub-AMC); therefore, these substrates suffer from lack of selectivity. By using a hybrid combinatorial substrate library (HyCoSuL) and a defined P2 library containing a wide variety of nonproteinogenic amino acids, we established a full substrate specificity profile for two DUBs—MERS PLpro and human UCH-L3. Based on these results, we designed and synthesized Ub-based substrates and activity-based probes (ABPs) containing selected unnatural amino acids located in the C-terminal Ub motif. Biochemical analysis and cell lysate experiments confirmed the activity and selectivity of engineered Ub-based substrates and probes. Using this approach, we propose that for any protease that recognizes Ub and Ub-like substrates, a highly active and selective unnatural substrate or probe can be engineered.

Activity profiling and structures of inhibitor-bound SARS-CoV-2-PLpro protease provides a framework for anti-COVID-19 drug design

In December 2019, the first cases of a novel coronavirus infection causing COVID-19 were diagnosed in Wuhan, China. Viral Papain-Like cysteine protease (PLpro, NSP3) is essential for SARS-CoV-2 replication and represents a promising target for the development of antiviral drugs. Here, we used a combinatorial substrate library containing natural and a wide variety of nonproteinogenic amino acids and performed comprehensive activity profiling of SARS-CoV-2-PLpro. On the scaffold of best hits from positional scanning we designed optimal fluorogenic substrates and irreversible inhibitors with a high degree of selectivity for SARS PLpro variants versus other proteases. We determined crystal structures of two of these inhibitors (VIR250 and VIR251) in complex with SARS-CoV-2-PLpro which reveals their inhibitory mechanisms and provides a structural basis for the observed substrate specificity profiles. Lastly, we demonstrate that SARS-CoV-2-PLpro harbors deISGylating activities similar to SARS-CoV-1-PLpro but its ability to hydrolyze K48-linked Ub chains is diminished, which our sequence and structure analysis provides a basis for. Altogether this work has revealed the molecular rules governing PLpro substrate specificity and provides a framework for development of inhibitors with potential therapeutic value or drug repositioning.

Transcription-replication conflicts as a source of common fragile site instability caused by BMI1-RNF2 deficiency

Common fragile sites (CFSs) are breakage-prone genomic loci, and are considered to be hotspots for genomic rearrangements frequently observed in cancers. Understanding the underlying mechanisms for CFS instability will lead to better insight on cancer etiology. Here we show that Polycomb group proteins BMI1 and RNF2 are suppressors of transcription-replication conflicts (TRCs) and CFS instability. Cells depleted of BMI1 or RNF2 showed slower replication forks and elevated fork stalling. These phenotypes are associated with increase occupancy of RNA Pol II (RNAPII) at CFSs, suggesting that the BMI1-RNF2 complex regulate RNAPII elongation at these fragile regions. Using proximity ligase assays, we showed that depleting BMI1 or RNF2 causes increased associations between RNAPII with EdU-labeled nascent forks and replisomes, suggesting increased TRC incidences. Increased occupancy of a fork protective factor FANCD2 and R-loop resolvase RNH1 at CFSs are observed in RNF2 CRISPR-KO cells, which are consistent with increased transcription-associated replication stress in RNF2-deficient cells. Depleting FANCD2 or FANCI proteins further increased genomic instability and cell death of the RNF2-deficient cells, suggesting that in the absence of RNF2, cells depend on these fork-protective factors for survival. These data suggest that the Polycomb proteins have non-canonical roles in suppressing TRC and preserving genomic integrity.

Using Primary SCG Neuron Cultures to Study Molecular Determinants of HSV-1 Latency and Reactivation

We describe a primary neuronal culture system suitable for molecular characterization of herpes simplex virus type 1 (HSV-1) infection, latency, and reactivation. While several alternative models are available, including infections of live animal or explanted ganglia, these are complicated by the presence of multiple cell types, including immune cells, and difficulties in manipulating the neuronal environment. The highly pure neuron culture system described here can be readily manipulated and is ideal for molecular studies that focus exclusively on the relationship between the virus and host neuron, the fundamental unit of latency. As such this model allows for detailed investigations of both viral and neuronal factors involved in the establishment and maintenance of HSV-1 latency and in viral reactivation induced by defined stimuli.

Role of Y-family translesion DNA polymerases in replication stress: Implications for new cancer therapeutic targets

DNA replication stress, defined as the slowing or stalling of replication forks, is considered an emerging hallmark of cancer and a major contributor to genomic instability associated with tumorigenesis (Macheret and Halazonetis, 2015). Recent advances have been made in attempting to target DNA repair factors involved in alleviating replication stress to potentiate genotoxic treatments. Various inhibitors of ATR and Chk1, the two major kinases involved in the intra-S-phase checkpoint, are currently in Phase I and II clinical trials [2]. In addition, currently approved inhibitors of Poly-ADP Ribose Polymerase (PARP) show synthetic lethality in cells that lack double-strand break repair such as in BRCA1/2 deficient tumors [3]. These drugs have also been shown to exacerbate replication stress by creating a DNA-protein crosslink, termed PARP 'trapping', and this is now thought to contribute to the therapeutic efficacy. Translesion synthesis (TLS) is a mechanism whereby special repair DNA polymerases accommodate and tolerate various DNA lesions to allow for damage bypass and continuation of DNA replication (Yang and Gao, 2018). This class of proteins is best characterized by the Y-family, encompassing DNA polymerases (Pols) Kappa, Eta, Iota, and Rev1. While best studied for their ability to bypass physical lesions on the DNA, there is accumulating evidence for these proteins in coping with various natural replication fork barriers and alleviating replication stress. In this mini-review, we will highlight some of these recent advances, and discuss why targeting the TLS pathway may be a mechanism of enhancing cancer-associated replication stress. Exacerbation of replication stress can lead to increased genome instability, which can be toxic to cancer cells and represent a therapeutic vulnerability.

TOP2β-Dependent Nuclear DNA Damage Shapes Extracellular Growth Factor Responses via Dynamic AKT Phosphorylation to Control Virus Latency

The mTOR pathway integrates both extracellular and intracellular signals and serves as a central regulator of cell metabolism, growth, survival, and stress responses. Neurotropic viruses, such as herpes simplex virus-1 (HSV-1), also rely on cellular AKT-mTORC1 signaling to achieve viral latency. Here, we define a novel genotoxic response whereby spatially separated signals initiated by extracellular neurotrophic factors and nuclear DNA damage are integrated by the AKT-mTORC1 pathway. We demonstrate that endogenous DNA double-strand breaks (DSBs) mediated by Topoisomerase 2β-DNA cleavage complex (TOP2βcc) intermediates are required to achieve AKT-mTORC1 signaling and maintain HSV-1 latency in neurons. Suppression of host DNA-repair pathways that remove TOP2βcc trigger HSV-1 reactivation. Moreover, perturbation of AKT phosphorylation dynamics by downregulating the PHLPP1 phosphatase led to AKT mis-localization and disruption of DSB-induced HSV-1 reactivation. Thus, the cellular genome integrity and environmental inputs are consolidated and co-opted by a latent virus to balance lifelong infection with transmission.

Aberrant Activation of β-Catenin Signaling Drives Glioma Tumorigenesis via USP1-Mediated Stabilization of EZH2

Aberrant activation of β-catenin signaling is a critical driver for tumorigenesis, but the mechanism underlying this activation is not completely understood. In this study, we demonstrate a critical role of β-catenin signaling in stabilization of enhancer of zeste homolog 2 (EZH2) and control of EZH2-mediated gene repression in oncogenesis. β-Catenin/TCF4 activated the transcription of the deubiquitinase USP1, which then interacted with and deubiquitinated EZH2 directly. USP1-mediated stabilization of EZH2 promoted its recruitment to the promoters of CDKN1B, RUNX3, and HOXA5, resulting in enhanced enrichment of histone H3K27me3 and repression of target gene expression. In human glioma specimens, expression levels of nuclear β-catenin, USP1, and EZH2 correlated with one another. Depletion of β-catenin/USP1/EZH2 repressed glioma cell proliferation in vitro and tumor formation in vivo. Our findings indicate that a β-catenin-USP1-EZH2 axis orchestrates the interplay between dysregulated β-catenin signaling and EZH2-mediated gene epigenetic silencing during glioma tumorigenesis. SIGNIFICANCE: These findings identify the β-catenin-USP1-EZH2 signaling axis as a critical mechanism for glioma tumorigenesis that may serve as a new therapeutic target in glioblastoma.

Translesion polymerase kappa-dependent DNA synthesis underlies replication fork recovery

DNA replication stress is often defined by the slowing or stalling of replication fork progression leading to local or global DNA synthesis inhibition. Failure to resolve replication stress in a timely manner contribute toward cell cycle defects, genome instability and human disease; however, the mechanism for fork recovery remains poorly defined. Here, we show that the translesion DNA polymerase (Pol) kappa, a DinB orthologue, has a unique role in both protecting and restarting stalled replication forks under conditions of nucleotide deprivation. Importantly, Pol kappa-mediated DNA synthesis during hydroxyurea (HU)-dependent fork restart is regulated by both the Fanconi Anemia (FA) pathway and PCNA polyubiquitination. Loss of Pol kappa prevents timely rescue of stalled replication forks, leading to replication-associated genomic instability, and a p53-dependent cell cycle defect. Taken together, our results identify a previously unanticipated role for Pol kappa in promoting DNA synthesis and replication stress recovery at sites of stalled forks.

Solid pseudopapillary neoplasm of pancreas in pregnancy treated with tumor enucleation: Case report and review of the literature

Solid pseudopapillary neoplasm of pancreas (SPN) during pregnancy is rare and presents a threat both to the mother and the fetus. We report a case of SPN in a 26-year-old woman diagnosed at 21 weeks of gestation. Tumor enucleation was successfully performed by a general surgeon. A healthy female infant was delivered at 39 weeks and 5 days of gestation vaginally without complications. Our report provides an example that tumor enucleation of SPN during the second trimester could be successfully performed during pregnancy. A multidisciplinary approach with respect to the pregnant patient's diagnosis, indications, and timing of surgery is necessary in ensuring the best possible outcomes for both the mother and the child.

Transoral vertical ramus osteotomy fixed with Kirschner pins

Transoral vertical ramus osteotomy (VRO) has been condemned because the condyle has the potential to sag, and because it needs lengthy maxillomandibular fixation. We have therefore introduced a simple method of fixation, and examined its effectiveness and complications. After the osteotomy, the proximal and distal segments are trimmed to adapt to each other. Four Kirschner (K) pins 0.9mm in diameter are inserted percutaneously from the proximal to the distal segment while the condyle is positioned in the glenoid fossa. This is followed by a brief period of maxillomandibular fixation. We have reviewed the records of 95 patients who had unilateral or bilateral vertical ramus osteotomy fixed with K pins, after which the mean (SD) period of fixation was 19 (11) days. Fixation failed in two patients because excursion of the jaw was either too heavy or too early. The fixations were redone. All other fixations remained stable, including the 20 dual-jaw procedures in which VRO preceded maxillary osteotomy. The mean (SD) maximal mouth opening at final follow-up was 44 (7) mm, and in only one patient was it less than 30mm. Numbness of the lip or chin developed in seven patients, five of whom had other anterior mandibular procedures. Four patients had discomfort on palpation of the site of the pins, and one required removal. The new method was effective, and resulted in few complications within its limitations.

Applications of systems modelling in obesity research

Obesity is a complex system problem involving a broad spectrum of policy, social, economic, cultural, environmental, behavioural, and biological factors and the complex interrelated, cross-sector, non-linear, dynamic relationships among them. Systems modelling is an innovative approach with the potential for advancing obesity research. This study examined the applications of systems modelling in obesity research published between 2000 and 2017, examined how the systems models were developed and used in obesity studies and discussed related gaps in current research. We focused on the applications of two main systems modelling approaches: system dynamics modelling and agent-based modelling. The past two decades have seen a growing body of systems modelling in obesity research. The research topics ranged from micro-level to macro-level energy-balance-related behaviours and policies (19 studies), population dynamics (five studies), policy effect simulations (eight studies), environmental (10 studies) and social influences (15 studies) and their effects on obesity rates. Overall, systems analysis in public health research is still in its early stages, with limitations linked to model validity, mixed findings and its actual use in guiding interventions. Challenges in theory and modelling practices need to be addressed to realize the full potential of systems modelling in future obesity research and interventions.

The COP9 signalosome inhibits Cullin-RING E3 ubiquitin ligases independently of its deneddylase activity

The COP9 signalosome inhibits the activity of Cullin-RING E3 ubiquitin ligases by removing Nedd8 modifications from their Cullin subunits. Neddylation renders these complexes catalytically active, but deneddylation is also necessary for them to exchange adaptor subunits and avoid auto-ubiquitination. Although deneddylation is thought to be the primary function of the COP9 signalosome, additional activities have been ascribed to some of its subunits. We recently showed that COP9 subunits protect the transcriptional repressor and tumor suppressor Capicua from two distinct modes of degradation. Deneddylation by the COP9 signalosome inactivates a Cullin 1 complex that ubiquitinates Capicua following its phosphorylation by MAP kinase in response to Epidermal Growth Factor Receptor signaling. The CSN1b subunit also stabilizes unphosphorylated Capicua to control its basal level, independently of the deneddylase function of the complex. Here we further examine the importance of deneddylation for COP9 functions in vivo. We use an uncleavable form of Nedd8 to show that preventing deneddylation does not reproduce the effects of loss of COP9. In contrast, in the presence of COP9, conjugation to uncleavable Nedd8 renders Cullins unable to promote the degradation of their substrates. Our results suggest that irreversible neddylation prolongs COP9 binding to and inhibition of Cullin-based ubiquitin ligases.

[A preliminary study on the clinical characteristics of Parkinson's disease patients with restless legs syndrome]

Objective: To study the clinical characteristics of Parkinson's disease (PD) patients with restless legs syndrome (RLS). Methods: Ninety-nine PD patients and eighty-nine control group were included into this study and assessed for RLS by the question 6 of the non-motor symptoms questionnaire (NMSquest). The PD patients were divided into the RLS group and non RLS group, according to their answers to the NMSquest.The severity of motor symptoms, anxiety, depression and quality of life were evaluated according Unified Parkinson Disease Rating Scale (UPDRS), Beck Anxiety Inventory (BAI), Beck Depression Inventory (BDI), Parkinson' s Disease Quality Of Life Questionnaire(PDQ-39), respectively. Results: (1)18.2%(18/81)of the PD patients experienced RLS, the incidence was significantly higher than the general population (P<0.001). (2) The RLS patients were more often women (66.7% vs 33.3%, P=0.033 ). (3) The UPDRS-Ⅱ score(20±7, P=0.008), UPDRS-Ⅲ score(43±14, P=0.015), BDI score(23±13, P=0.002), BAI score(18±8, P=0.012), PDQ-39 score(75±26, P=0.000)in RLS group were significantly higher compared with non RLS group. (4) RLS showed no association with the course of the disease, the age, the PD onset age and levodopa dose equivalents. Conclusions: RLS more likely appears in PD patients, especially in women. The PD patients with RLS commonly suffer from serious motor symptom, low quality of life, anxiety and depression.

SENP8 limits aberrant neddylation of NEDD8 pathway components to promote cullin-RING ubiquitin ligase function

NEDD8 is a ubiquitin-like modifier most well-studied for its role in activating the largest family of ubiquitin E3 ligases, the cullin-RING ligases (CRLs). While many non-cullin neddylation substrates have been proposed over the years, validation of true NEDD8 targets has been challenging, as overexpression of exogenous NEDD8 can trigger NEDD8 conjugation through the ubiquitylation machinery. Here, we developed a deconjugation-resistant form of NEDD8 to stabilize the neddylated form of cullins and other non-cullin substrates. Using this strategy, we identified Ubc12, a NEDD8-specific E2 conjugating enzyme, as a substrate for auto-neddylation. Furthermore, we characterized SENP8/DEN1 as the protease that counteracts Ubc12 auto-neddylation, and observed aberrant neddylation of Ubc12 and other NEDD8 conjugation pathway components in SENP8-deficient cells. Importantly, loss of SENP8 function contributes to accumulation of CRL substrates and defective cell cycle progression. Thus, our study highlights the importance of SENP8 in maintaining proper neddylation levels for CRL-dependent proteostasis.

DOI:http://dx.doi.org/10.7554/eLife.24325.001

How SUMOylation Fine-Tunes the Fanconi Anemia DNA Repair Pathway

Fanconi anemia (FA) is a rare human genetic disorder characterized by developmental defects, bone marrow failure and cancer predisposition, primarily due to a deficiency in the repair of DNA interstrand crosslinks (ICLs). ICL repair through the FA DNA repair pathway is a complicated multi-step process, involving at least 19 FANC proteins and coordination of multiple DNA repair activities, including homologous recombination, nucleotide excision repair and translesion synthesis (TLS). SUMOylation is a critical regulator of several DNA repair pathways, however, the role of this modification in controlling the FA pathway is poorly understood. Here, we summarize recent advances in the fine-tuning of the FA pathway by small ubiquitin-like modifier (SUMO)-targeted ubiquitin ligases (STUbLs) and other SUMO-related interactions, and discuss the implications of these findings in the design of novel therapeutics for alleviating FA-associated condition, including cancer.

ATR-mediated phosphorylation of FANCI regulates dormant origin firing in response to replication stress

Excess dormant origins bound by the minichromosome maintenance (MCM) replicative helicase complex play a critical role in preventing replication stress, chromosome instability, and tumorigenesis. In response to DNA damage, replicating cells must coordinate DNA repair and dormant origin firing to ensure complete and timely replication of the genome; how cells regulate this process remains elusive. Herein, we identify a member of the Fanconi anemia (FA) DNA repair pathway, FANCI, as a key effector of dormant origin firing in response to replication stress. Cells lacking FANCI have reduced number of origins, increased inter-origin distances, and slowed proliferation rates. Intriguingly, ATR-mediated FANCI phosphorylation inhibits dormant origin firing while promoting replication fork restart/DNA repair. Using super-resolution microscopy, we show that FANCI co-localizes with MCM-bound chromatin in response to replication stress. These data reveal a unique role for FANCI as a modulator of dormant origin firing and link timely genome replication to DNA repair.

Co-opting the Fanconi anemia genomic stability pathway enables herpesvirus DNA synthesis and productive growth

DNA damage associated with viral DNA synthesis can result in double-strand breaks that threaten genome integrity and must be repaired. Here, we establish that the cellular Fanconi anemia (FA) genomic stability pathway is exploited by herpes simplex virus 1 (HSV-1) to promote viral DNA synthesis and enable its productive growth. Potent FA pathway activation in HSV-1-infected cells resulted in monoubiquitination of FA effector proteins FANCI and FANCD2 (FANCI-D2) and required the viral DNA polymerase. FANCD2 relocalized to viral replication compartments, and FANCI-D2 interacted with a multisubunit complex containing the virus-encoded single-stranded DNA-binding protein ICP8. Significantly, whereas HSV-1 productive growth was impaired in monoubiquitination-defective FA cells, this restriction was partially surmounted by antagonizing the DNA-dependent protein kinase (DNA-PK), a critical enzyme required for nonhomologous end-joining (NHEJ). This identifies the FA-pathway as a cellular factor required for herpesvirus productive growth and suggests that FA-mediated suppression of NHEJ is a fundamental step in the viral life cycle.

DUB-resistant ubiquitin to survey ubiquitination switches in mammalian cells

The ubiquitin-modification status of proteins in cells is highly dynamic and maintained by specific ligation machineries (E3 ligases) that tag proteins with ubiquitin or by deubiquitinating enzymes (DUBs) that remove the ubiquitin tag. The development of tools that offset this balance is critical in characterizing signaling pathways that utilize such ubiquitination switches. Herein, we generated a DUB-resistant ubiquitin mutant that is recalcitrant to cleavage by various families of DUBs both in vitro and in mammalian cells. As a proof-of-principle experiment, ectopic expression of the uncleavable ubiquitin stabilized monoubiquitinated PCNA in the absence of DNA damage and also revealed a defect in the clearance of the DNA damage response at unprotected telomeres. Importantly, a proteomic survey using the uncleavable ubiquitin identified ubiquitinated substrates, validating the DUB-resistant ubiquitin expression system as a valuable tool for interrogating cell signaling pathways.

The auto-generated fragment of the Usp1 deubiquitylase is a physiological substrate of the N-end rule pathway

Deamidation of N-terminal Gln by the Ntaq1 Nt(Q)-amidase is a part of the Arg/N-end rule pathway, a ubiquitin-dependent proteolytic system. Here we identify Gln-Usp1(Ct), the C-terminal fragment of the autocleaved Usp1 deubiquitylase, as the first physiological Arg/N-end rule substrate that is targeted for degradation through deamidation of N-terminal Gln. Usp1 regulates genomic stability, in part through the deubiquitylation of monoubiquitylated PCNA, a DNA polymerase processivity factor. The autocleaved Usp1 remains a deubiquitylase because its fragments remain associated with Uaf1, an enhancer of Usp1 activity, until the Gln-Usp1(Ct) fragment is selectively destroyed by the Arg/N-end rule pathway. We also show that metabolic stabilization of Gln-Usp1(Ct) results in a decreased monoubiquitylation of PCNA and in a hypersensitivity of cells to ultraviolet irradiation. Thus, in addition to its other functions in DNA repair and chromosome segregation, the Arg/N-end rule pathway regulates genomic stability through the degradation-mediated control of the autocleaved Usp1 deubiquitylase.

Insights into phosphorylation-dependent mechanisms regulating USP1 protein stability during the cell cycle

Tight regulation of the cell cycle and DNA repair machinery is essential for maintaining genome stability. The APC/CCdh1 ubiquitin ligase complex is a key regulator of protein stability during the G 1 phase of the cell cycle. APC/CCdh1 regulates and promotes the degradation of proteins involved in both cell cycle regulation and DNA repair. In a recent study, we identified a novel APC/CCdh1 substrate, the ubiquitin protease USP1. USP1 is a critical regulator of both the Fanconi anemia (FA) and translesion synthesis (TLS) DNA repair pathways. Here, we provide additional mechanistic insights into the regulation of USP1 during the cell cycle. Specifically, we demonstrate that USP1 is phosphorylated in mitosis by cyclin-dependent kinases (Cdks), and that this phosphorylation event may prevent premature degradation of USP1 during normal cell cycle progression. Finally, we provide a unifying hypothesis integrating the role of G 1-specific proteolysis of USP1 with the regulation of the transcriptional repressors, Inhibitor of DNA-binding (ID) proteins.

Expression of anti-neuroexcitation peptide III of scorpion Buthus martensii Karsch BmK ANEP III in plants

Anti-neuroexcitation peptide III of Buthus martensii Karsch (BmK ANEP III) has better anti-epileptic and anticonvulsive effects in the test animal models. The present study is aimed at developing transgenic tomato and tobacco lines overproducing the ANEP III protein. Using the molecular cloning technique, the plant expression vector pBI-ANEP III was constructed successfully. The ANEP III expression cassette included a double CaMV 35S promoter with omega enhancers, the ANEP III gene with the Kozak sequence, the ER retention signal and the NOS terminator. Recombinant plasmids were transferred into Agrobacterium tumefaciens EHA105 by freeze-thaw transformation methods. By the Agrobacterium-mediated leaf disc transformation method, tobacco (Nicotiana tabacum) and tomato (Lycopersicum esculentum) lines were transformed. Transformants were screened and confirmed by PCR, RT-PCR and western blotting analysis. It was demonstrated that the ANEP III gene was successfully expressed in the genomic DNA of transgenic plants. The ANEP III protein was detected by immunofluorescence analysis, and the results confirmed the high amount of ANEP III protein, being 0.81 and 1.08% of total soluble proteins in transgenic tobacco and tomato. The study of plants with high expression levels of ANEP III has an important theoretical and practical significance and provides valuable information for establishing a new, economical and effective system for industrial protein production.

APC/CCdh1-dependent proteolysis of USP1 regulates the response to UV-mediated DNA damage

APC/C^Cdh1-dependent degradation of USP1 allows for PCNA monoubiquitination and subsequent recruitment of trans-lesion synthesis polymerase to UV repair sites.

Targeted protein destruction of critical cellular regulators during the G1 phase of the cell cycle is achieved by anaphase-promoting complex/cyclosome^Cdh1 (APC/C^Cdh1), a multisubunit E3 ubiquitin ligase. Cells lacking Cdh1 have been shown to accumulate deoxyribonucleic acid (DNA) damage, suggesting that it may play a previously unrecognized role in maintaining genomic stability. The ubiquitin-specific protease 1 (USP1) is a known critical regulator of DNA repair and genomic stability. In this paper, we report that USP1 was degraded in G1 via APC/C^Cdh1. USP1 levels were kept low in G1 to provide a permissive condition for inducing proliferating cell nuclear antigen (PCNA) monoubiquitination in response to ultraviolet (UV) damage before DNA replication. Importantly, expression of a USP1 mutant that cannot be degraded via APC/C^Cdh1 inhibited PCNA monoubiquitination during G1, likely compromising the recruitment of trans-lesion synthesis polymerase to UV repair sites. Thus, we propose a role for APC/C^Cdh1 in modulating the status of PCNA monoubiquitination and UV DNA repair before S phase entry.

Nuclear export of the NF-κB inhibitor IκBα is required for proper B cell and secondary lymphoid tissue formation

The N-terminal nuclear export sequence (NES) of inhibitor of nuclear factor kappa B (NF-κB) alpha (IκBα) promotes NF-κB export from the cell nucleus to the cytoplasm, but the physiological role of this export regulation remains unknown. Here we report the derivation and analysis of genetically targeted mice harboring a germline mutation in IκBα NES. Mature B cells in the mutant mice displayed nuclear accumulation of inactive IκBα complexes containing a NF-κB family member, cRel, causing their spatial separation from the cytoplasmic IκB kinase. This resulted in severe reductions in constitutive and canonical NF-κB activities, synthesis of p100 and RelB NF-κB members, noncanonical NF-κB activity, NF-κB target gene induction, and proliferation and survival responses in B cells. Consequently, mice displayed defective B cell maturation, antibody production, and formation of secondary lymphoid organs and tissues. Thus, IκBα nuclear export is essential to maintain constitutive, canonical, and noncanonical NF-κB activation potentials in mature B cells in vivo.

A UAF1-containing multisubunit protein complex regulates the Fanconi anemia pathway

The deubiquitinating enzyme USP1 controls the cellular levels of the DNA damage response protein Ub-FANCD2, a key protein of the Fanconi anemia DNA repair pathway. Here we report the purification of a USP1 multisubunit protein complex from HeLa cells containing stoichiometric amounts of a WD40 repeat-containing protein, USP1 associated factor 1 (UAF1). In vitro reconstitution of USP1 deubiquitinating enzyme activity, using either ubiquitin-7-amido-4-methylcoumarin (Ub-AMC) or purified monoubiquitinated FANCD2 protein as substrates, demonstrates that UAF1 functions as an activator of USP1. UAF1 binding increases the catalytic turnover (kcat) but does not increase the affinity of the USP1 enzyme for the substrate (KM). Moreover, we show that DNA damage results in an immediate shutoff of transcription of the USP1 gene, leading to a rapid decline in the USP1/UAF1 protein complex. Taken together, our results describe a mechanism of regulation of the deubiquitinating enzyme, USP1, and of DNA repair.

FANCI is a second monoubiquitinated member of the Fanconi anemia pathway

Activation of the Fanconi anemia (FA) DNA damage-response pathway results in the monoubiquitination of FANCD2, which is regulated by the nuclear FA core ubiquitin ligase complex. A FANCD2 protein sequence-based homology search facilitated the discovery of FANCI, a second monoubiquitinated component of the FA pathway. Biallelic mutations in the gene coding for this protein were found in cells from four FA patients, including an FA-I reference cell line.

Tunable S-band erbium-doped triple-ring laser with single-longitudinal-mode operation

We propose and demonstrate a tunable and stable single-longitudinal-mode (SLM) erbium fiber laser with a passive triple-ring cavity structure in S-band operation. The proposed laser is fundamentally structured by using three different lengths of ring cavities, which serve as the mode filters. When a mode-restricting intracavity fiber Fabry-Perot tunable filter (FFP-TF) is combined, the proposed resonator can guarantee a tunable and stable SLM laser oscillation. Moreover, the performances of the output power, wavelength stability, tuning range, and side-mode suppression ratio (SMSR) are studied.

Calcium-dependent regulation of NEMO nuclear export in response to genotoxic stimuli

The mechanisms involved in activation of the transcription factor NF-kappaB by genotoxic agents are not well understood. Previously, we provided evidence that a regulatory subunit of the IkappaB kinase (IKK) complex, NF-kappaB essential modulator (NEMO)/IKKgamma, is a component of a nuclear signal that is generated after DNA damage to mediate NF-kappaB activation. Here, we found that etoposide (VP16) and camptothecin induced increases in intracellular free calcium levels at 60 min after stimulation of CEM T leukemic cells. Inhibition of calcium increases by calcium chelators, BAPTA-AM and EGTA-AM, abrogated NF-kappaB activation by these agents in several cell types examined. Conversely, thapsigargin and ionomycin attenuated the BAPTA-AM effects and promoted NF-kappaB activation by the genotoxic stimuli. Analyses of nuclear NEMO levels in VP16-treated cells suggested that calcium was required for nuclear export of NEMO. Inhibition of the nuclear exporter CRM1 by leptomycin B did not interfere with NEMO nuclear export. Similarly, deficiency of a plausible calcium-dependent nuclear export receptor, calreticulin, failed to prevent NF-kappaB activation by VP16. However, temperature inactivation of the Ran guanine nucleotide exchange factor RCC1 in the tsBN2 cell line harboring a temperature-sensitive mutant of RCC1 blocked NF-kappaB activation induced by genotoxic stimuli. Overexpression of Ran in this cell model showed that DNA damage stimuli induced formation of a complex between Ran and NEMO, suggesting that RCC1 regulated NF-kappaB activation through the modulation of RanGTP. Indeed, evidence for VP16-inducible interaction between Ran-GTP and NEMO could be obtained by means of glutathione S-transferase (GST) pull-down assays using GST fused to the Ran binding domain of RanBP2, which specifically interacts with the GTP-bound form of Ran. BAPTA-AM did not alter these interactions, suggesting that calcium is a necessary step beyond the formation of a Ran-GTP-NEMO complex in the nucleus. These results suggest that calcium has a unique role in genotoxic stress-induced NF-kappaB signaling by regulating nuclear export of NEMO subsequent to the formation of a nuclear export complex composed of Ran-GTP, NEMO, and presumably, an undefined nuclear export receptor.

Regulation of DNA repair by ubiquitylation

The process of ubiquitylation is best known for its role in targeting proteins for degradation by the proteasome. However, recent studies of DNA-repair and DNA-damage-response pathways have significantly broadened the scope of the role of ubiquitylation to include non-proteolytic functions of ubiquitin. These pathways involve the monoubiquitylation of key DNA-repair proteins that have regulatory functions in homologous recombination and translesion DNA synthesis, and involve the polyubiquitylation of nucleotide-excision-repair proteins.

Regulation of monoubiquitinated PCNA by DUB autocleavage

Monoubiquitination is a reversible post-translational protein modification that has an important regulatory function in many biological processes, including DNA repair. Deubiquitinating enzymes (DUBs) are proteases that are negative regulators of monoubiquitination, but little is known about their regulation and contribution to the control of conjugated-substrate levels. Here, we show that the DUB ubiquitin specific protease 1 (USP1) deubiquitinates the DNA replication processivity factor, PCNA, as a safeguard against error-prone translesion synthesis (TLS) of DNA. Ultraviolet (UV) irradiation inactivates USP1 through an autocleavage event, thus enabling monoubiquitinated PCNA to accumulate and to activate TLS. Significantly, the site of USP1 cleavage is immediately after a conserved internal ubiquitin-like diglycine (Gly-Gly) motif. This mechanism is reminiscent of the processing of precursors of ubiquitin and ubiquitin-like modifiers by DUBs. Our results define a regulatory mechanism for protein ubiquitination that involves the signal-induced degradation of an inhibitory DUB.