The Ubiquitin Ligase TRAIP: Double-Edged Sword at the Replisome

Cdc48/p97 segregase: Spotlight on DNA-protein crosslinks

The ATP-dependent molecular chaperone Cdc48 (in yeast) and its human counterpart p97 (also known as VCP), are essential for a variety of cellular processes, including the removal of DNA-protein crosslinks (DPCs) from the DNA. Growing evidence demonstrates in the last years that Cdc48/p97 is pivotal in targeting ubiquitinated and SUMOylated substrates on chromatin, thereby supporting the DNA damage response. Along with its cofactors, notably Ufd1-Npl4, Cdc48/p97 has emerged as a central player in the unfolding and processing of DPCs. This review introduces the detailed structure, mechanism and cellular functions of Cdc48/p97 with an emphasis on the current knowledge of DNA-protein crosslink repair pathways across several organisms. The review concludes by discussing the potential therapeutic relevance of targeting p97 in DPC repair.

NEIL3: A unique DNA glycosylase involved in interstrand DNA crosslink repair

Endonuclease VIII-like 3 (NEIL3) is a versatile DNA glycosylase that repairs a diverse array of chemical modifications to DNA. Unlike other glycosylases, NEIL3 has a preference for lesions within single-strand DNA and at single/double-strand DNA junctions. Beyond its canonical role in base excision repair of oxidized DNA, NEIL3 initiates replication-dependent interstrand DNA crosslink repair as an alternative to the Fanconi Anemia pathway. This review outlines our current understanding of NEIL3's biological functions, role in disease, and three-dimensional structure as it pertains to substrate specificity and catalytic mechanism.

Novel Processes Associated with the Repair of Interstrand Cross-Links Derived from Abasic Sites in Duplex DNA: Roles for the Base Excision Repair Glycosylase NEIL3 and the SRAP Protein HMCES

Recent studies have defined a novel pathway for the repair of interstrand cross-links derived from the reaction of an adenine residue with an apurinic/apyrimidinic (AP) site on the opposing strand of DNA (dA-AP ICL). Stalling of a replication fork at the dA-AP ICL triggers TRAIP-dependent ubiquitylation of the CMG helicase that recruits the base excision repair glycosylase NEIL3 to the lesion. NEIL3 unhooks the dA-AP ICL to regenerate the native adenine residue on one strand and an AP site on the other strand. Covalent capture of the abasic site by the SRAP protein HMCES protects against genomic instability that would result from cleavage of the abasic site in the context of single-stranded DNA at the replication fork. After repair synthesis moves the HMCES-AP adduct into the context of double-stranded DNA, the DNA-protein cross-link is resolved by a nonproteolytic mechanism involving dissociation of thiazolidine attachment. The AP site in duplex DNA is then repaired by the base excision repair pathway.

TRAIP suppresses bladder cancer progression by catalyzing K48-linked polyubiquitination of MYC

TRAF-interacting protein (TRAIP), an E3 ligase containing a RING domain, has emerged as a significant contributor to maintaining genome integrity and is closely associated with cancer. Our study reveals that TRAIP shows reduced expression in bladder cancer (BLCA), which correlates with an unfavorable prognosis. In vitro and in vivo, TRAIP inhibits proliferation and migration of BLCA cells. MYC has been identified as a novel target for TRAIP, wherein direct interaction promotes K48-linked polyubiquitination at neighboring K428 and K430 residues, ultimately resulting in proteasome-dependent degradation and downregulation of MYC transcriptional activity. This mechanism effectively impedes the progression of BLCA. Restoring MYC expression reverses suppressed proliferation and migration of BLCA cells induced by TRAIP. Moreover, our results suggest that MYC may bind to the transcriptional start region of TRAIP, thereby exerting regulatory control over TRAIP transcription. Consequently, this interaction establishes a negative feedback loop that regulates MYC expression, preventing excessive levels. Taken together, this study reveals a mechanism that TRAIP inhibits proliferation and migration of BLCA by promoting ubiquitin-mediated degradation of MYC.

Completing genome replication outside of S phase

Mitotic DNA synthesis (MiDAS) is an unusual form of DNA replication that occurs during mitosis. Initially, MiDAS was characterized as a process associated with intrinsically unstable loci known as common fragile sites that occurs after cells experience DNA replication stress (RS). However, it is now believed to be a more widespread "salvage" mechanism that is called upon to complete the duplication of any under-replicated genomic region. Emerging data suggest that MiDAS is a DNA repair process potentially involving two or more pathways working in parallel or sequentially. In this review, we introduce the causes of RS, regions of the human genome known to be especially vulnerable to RS, and the strategies used to complete DNA replication outside of S phase. Additionally, because MiDAS is a prominent feature of aneuploid cancer cells, we will discuss how targeting MiDAS might potentially lead to improvements in cancer therapy.

TRAIP serves as a potential prognostic biomarker and correlates with immune infiltrates in lung adenocarcinoma

BACKGROUND

Lung adenocarcinoma (LUAD) is one of the major types of lung cancer with high morbidity and mortality. The TRAF-interacting protein (TRAIP) is a ring-type E3 ubiquitin ligase which has been recently identified to play pivotal roles in various cancers. However, the expression and function of TRAIP in LUAD remain elusive.

METHODS

In this study, we used bioinformatic tools as well as molecular experiments to explore the exact role of TRAIP and the underlying mechanism.

RESULTS

Data mining across the UALCAN, GEPIA and GTEx, GEO and HPA databases revealed that TRAIP was significantly overexpressed in LUAD tissues than that in adjacent normal tissues. Kaplan-Meier curve showed that high TRAIP expression was associated with poor overall survival (OS) and relapse-free survival (RFS). Univariate and multivariate cox regression analysis revealed that TRAIP was an independent risk factor in LUAD. And the TRAIP-based nomogram further supported the prognostic role of TRAIP in LUAD. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis demonstrated that TRAIP-associated genes were mainly involved in DNA replication, cell cycle and other processes. The immune infiltration analysis indicated that TRAIP expression was tightly correlated with the infiltration of diverse immune cell types, including B cell, CD8 + T cell, neutrophil and dendritic cell. Moreover, TRAIP expression was observed to be significantly associated with tumor infiltrating lymphocytes (TILs) and immune checkpoint molecules. In vitro experiments further confirmed knockdown of TRAIP inhibited cell migration and invasion, as well as decreasing chemokine production and inhibiting M2-like macrophage recruitment. Lastly, CMap analysis identified 10 small molecule compounds that may target TRAIP, providing potential therapies for LUAD.

CONCLUSIONS

Collectively, our study found that TRAIP is an oncogenic gene in LUAD, which may be a potential prognostic biomarker and promising therapeutic target for LUAD.

DTX3L induced NLRP3 ubiquitination inhibit R28 cell pyroptosis in OGD/R injury

Ischemia/reperfusion (I/R) injury is one of the most common etiologies in many diseases. Retinal I/R leads to cytokine storm, resulting in tissue damage and cell death. Pyroptosis, a novel type of regulated cell death, occurs after cellular I/R injury. In this study, we established an oxygen glucose deprivation (OGD/R) cellular model (R28) to simulate retinal I/R injury. We conducted an LDH assay, and EthD-III and PI staining procedures to confirm pyroptosis. Mass spectrometry and bioinformatics analysis were used to identify the possible proteins interacting with NLRP3. Co-IP and various molecular biology techniques were used to investigate the possible modes regulating NLRP3 by DTX3L. EthD-III, PI staining and LDH assays demonstrated pyroptosis induced by OGD/R injury, mediated via NLRP3 pathway. Mass spectrometry and bioinformatics analysis screened out three candidate proteins interacting with NLRP3, and further Co-IP experiment indicated that DTX-3L may interact with NLRP3 to regulate its protein levels after injury. Co-IP experiments and various molecular biology methods demonstrated that DTX3L ubiquitinates NLRP3 resulting in pyroptosis after R28 OGD/R injury. Further, NLRP3 LRR and DTX3L RING domains interact with each other. Our study demonstrated that DTX3L may ubiquitinate NLRP3 to regulate OGD/R-induced pyroptosis globally in R28 cells.

Mitotic DNA synthesis in response to replication stress requires the sequential action of DNA polymerases zeta and delta in human cells

Oncogene activation creates DNA replication stress (RS) in cancer cells, which can generate under-replicated DNA regions (UDRs) that persist until cells enter mitosis. UDRs also have the potential to generate DNA bridges in anaphase cells or micronuclei in the daughter cells, which could promote genomic instability. To suppress such damaging changes to the genome, human cells have developed a strategy to conduct 'unscheduled' DNA synthesis in mitosis (termed MiDAS) that serves to rescue under-replicated loci. Previous studies have shown that MiDAS proceeds via a POLD3-dependent pathway that shows some features of break-induced replication. Here, we define how human cells utilize both DNA gap filling (REV1 and Pol ζ) and replicative (Pol δ) DNA polymerases to complete genome duplication following a perturbed S-phase. We present evidence for the existence of a polymerase-switch during MiDAS that is required for new DNA synthesis at UDRs. Moreover, we reveal that, upon oncogene activation, cancer cell survival is significantly compromised when REV1 is depleted, suggesting that REV1 inhibition might be a feasible approach for the treatment of some human cancers.

DNA replication: Mechanisms and therapeutic interventions for diseases

Accurate and integral cellular DNA replication is modulated by multiple replication-associated proteins, which is fundamental to preserve genome stability. Furthermore, replication proteins cooperate with multiple DNA damage factors to deal with replication stress through mechanisms beyond their role in replication. Cancer cells with chronic replication stress exhibit aberrant DNA replication and DNA damage response, providing an exploitable therapeutic target in tumors. Numerous evidence has indicated that posttranslational modifications (PTMs) of replication proteins present distinct functions in DNA replication and respond to replication stress. In addition, abundant replication proteins are involved in tumorigenesis and development, which act as diagnostic and prognostic biomarkers in some tumors, implying these proteins act as therapeutic targets in clinical. Replication-target cancer therapy emerges as the times require. In this context, we outline the current investigation of the DNA replication mechanism, and simultaneously enumerate the aberrant expression of replication proteins as hallmark for various diseases, revealing their therapeutic potential for target therapy. Meanwhile, we also discuss current observations that the novel PTM of replication proteins in response to replication stress, which seems to be a promising strategy to eliminate diseases.

ZNF212 promotes genomic integrity through direct interaction with TRAIP

TRAIP is a key factor involved in the DNA damage response (DDR), homologous recombination (HR) and DNA interstrand crosslink (ICL) repair. However, the exact functions of TRAIP in these processes in mammalian cells are not fully understood. Here we identify the zinc finger protein 212, ZNF212, as a novel binding partner for TRAIP and find that ZNF212 colocalizes with sites of DNA damage. The recruitment of TRAIP or ZNF212 to sites of DNA damage is mutually interdependent. We show that depletion of ZNF212 causes defects in the DDR and HR-mediated repair in a manner epistatic to TRAIP. In addition, an epistatic analysis of Zfp212, the mouse homolog of human ZNF212, in mouse embryonic stem cells (mESCs), shows that it appears to act upstream of both the Neil3 and Fanconi anemia (FA) pathways of ICLs repair. We find that human ZNF212 interacted directly with NEIL3 and promotes its recruitment to ICL lesions. Collectively, our findings identify ZNF212 as a new factor involved in the DDR, HR-mediated repair and ICL repair though direct interaction with TRAIP.

Sister chromatid exchanges induced by perturbed replication can form independently of BRCA1, BRCA2 and RAD51

Sister chromatid exchanges (SCEs) are products of joint DNA molecule resolution, and are considered to form through homologous recombination (HR). Indeed, SCE induction upon irradiation requires the canonical HR factors BRCA1, BRCA2 and RAD51. In contrast, replication-blocking agents, including PARP inhibitors, induce SCEs independently of BRCA1, BRCA2 and RAD51. PARP inhibitor-induced SCEs are enriched at difficult-to-replicate genomic regions, including common fragile sites (CFSs). PARP inhibitor-induced replication lesions are transmitted into mitosis, suggesting that SCEs can originate from mitotic processing of under-replicated DNA. Proteomics analysis reveals mitotic recruitment of DNA polymerase theta (POLQ) to synthetic DNA ends. POLQ inactivation results in reduced SCE numbers and severe chromosome fragmentation upon PARP inhibition in HR-deficient cells. Accordingly, analysis of CFSs in cancer genomes reveals frequent allelic deletions, flanked by signatures of POLQ-mediated repair. Combined, we show PARP inhibition generates under-replicated DNA, which is processed into SCEs during mitosis, independently of canonical HR factors.

Traip controls mushroom body size by suppressing mitotic defects

Microcephaly is a failure to develop proper brain size and neuron number. Mutations in diverse genes are linked to microcephaly, including several with DNA damage repair (DDR) functions; however, it is not well understood how these DDR gene mutations limit brain size. One such gene is TRAIP, which has multiple functions in DDR. We characterized the Drosophila TRAIP homolog nopo, hereafter traip, and found that traip mutants (traip-) have a brain-specific defect in the mushroom body (MB). traip- MBs were smaller and contained fewer neurons, but no neurodegeneration, consistent with human primary microcephaly. Reduced neuron numbers in traip- were explained by premature loss of MB neuroblasts (MB-NBs), in part via caspase-dependent cell death. Many traip- MB-NBs had prominent chromosome bridges in anaphase, along with polyploidy, aneuploidy or micronuclei. Traip localization during mitosis is sufficient for MB development, suggesting that Traip can repair chromosome bridges during mitosis if necessary. Our results suggest that proper brain size is ensured by the recently described role for TRAIP in unloading stalled replication forks in mitosis, which suppresses DNA bridges and premature neural stem cell loss to promote proper neuron number.

Mre11 exonuclease activity promotes irreversible mitotic progression under replication stress

Mre11 is a versatile exo-/endonuclease involved in multiple aspects of DNA replication and repair, such as DSB end processing and checkpoint activation. We previously demonstrated that forced mitotic entry drives replisome disassembly at stalled replication forks in Xenopus egg extracts. Here, we examined the effects of various chemical inhibitors using this system and discovered a novel role of Mre11 exonuclease activity in promoting mitotic entry under replication stress. Mre11 activity was necessary for the initial progression of mitotic entry in the presence of stalled forks but unnecessary in the absence of stalled forks or after mitotic entry. In the absence of Mre11 activity, mitotic CDK was inactivated by Wee1/Myt1-dependent phosphorylation, causing mitotic exit. An inhibitor of Wee1/Myt1 or a nonphosphorylatable CDK1 mutant was able to partially bypass the requirement of Mre11 for mitotic entry. These results suggest that Mre11 exonuclease activity facilitates the processing of stalled replication forks upon mitotic entry, which attenuates the inhibitory pathways of mitotic CDK activation, leading to irreversible mitotic progression and replisome disassembly.

Tools for Decoding Ubiquitin Signaling in DNA Repair

The maintenance of genome stability requires dedicated DNA repair processes and pathways that are essential for the faithful duplication and propagation of chromosomes. These DNA repair mechanisms counteract the potentially deleterious impact of the frequent genotoxic challenges faced by cells from both exogenous and endogenous agents. Intrinsic to these mechanisms, cells have an arsenal of protein factors that can be utilised to promote repair processes in response to DNA lesions. Orchestration of the protein factors within the various cellular DNA repair pathways is performed, in part, by post-translational modifications, such as phosphorylation, ubiquitin, SUMO and other ubiquitin-like modifiers (UBLs). In this review, we firstly explore recent advances in the tools for identifying factors involved in both DNA repair and ubiquitin signaling pathways. We then expand on this by evaluating the growing repertoire of proteomic, biochemical and structural techniques available to further understand the mechanistic basis by which these complex modifications regulate DNA repair. Together, we provide a snapshot of the range of methods now available to investigate and decode how ubiquitin signaling can promote DNA repair and maintain genome stability in mammalian cells.

Structure of a human replisome shows the organisation and interactions of a DNA replication machine

The human replisome is an elaborate arrangement of molecular machines responsible for accurate chromosome replication. At its heart is the CDC45-MCM-GINS (CMG) helicase, which, in addition to unwinding the parental DNA duplex, arranges many proteins including the leading-strand polymerase Pol ε, together with TIMELESS-TIPIN, CLASPIN and AND-1 that have key and varied roles in maintaining smooth replisome progression. How these proteins are coordinated in the human replisome is poorly understood. We have determined a 3.2 Å cryo-EM structure of a human replisome comprising CMG, Pol ε, TIMELESS-TIPIN, CLASPIN and AND-1 bound to replication fork DNA. The structure permits a detailed understanding of how AND-1, TIMELESS-TIPIN and Pol ε engage CMG, reveals how CLASPIN binds to multiple replisome components and identifies the position of the Pol ε catalytic domain. Furthermore, the intricate network of contacts contributed by MCM subunits and TIMELESS-TIPIN with replication fork DNA suggests a mechanism for strand separation.

DNA damage responses that enhance resilience to replication stress

During duplication of the genome, eukaryotic cells may experience various exogenous and endogenous replication stresses that impede progression of DNA replication along chromosomes. Chemical alterations in template DNA, imbalances of deoxynucleotide pools, repetitive sequences, tight DNA-protein complexes, and conflict with transcription can negatively affect the replication machineries. If not properly resolved, stalled replication forks can cause chromosome breaks leading to genomic instability and tumor development. Replication stress is enhanced in cancer cells due, for example, to the loss of DNA repair genes or replication-transcription conflict caused by activation of oncogenic pathways. To prevent these serious consequences, cells are equipped with diverse mechanisms that enhance the resilience of replication machineries to replication stresses. This review describes DNA damage responses activated at stressed replication forks and summarizes current knowledge on the pathways that promote faithful chromosome replication and protect chromosome integrity, including ATR-dependent replication checkpoint signaling, DNA cross-link repair, and SLX4-mediated responses to tight DNA-protein complexes that act as barriers. This review also focuses on the relevance of replication stress responses to selective cancer chemotherapies.

Mechanisms for Maintaining Eukaryotic Replisome Progression in the Presence of DNA Damage

The eukaryotic replisome coordinates template unwinding and nascent-strand synthesis to drive DNA replication fork progression and complete efficient genome duplication. During its advancement along the parental template, each replisome may encounter an array of obstacles including damaged and structured DNA that impede its progression and threaten genome stability. A number of mechanisms exist to permit replisomes to overcome such obstacles, maintain their progression, and prevent fork collapse. A combination of recent advances in structural, biochemical, and single-molecule approaches have illuminated the architecture of the replisome during unperturbed replication, rationalised the impact of impediments to fork progression, and enhanced our understanding of DNA damage tolerance mechanisms and their regulation. This review focusses on these studies to provide an updated overview of the mechanisms that support replisomes to maintain their progression on an imperfect template.

DNA2 in Chromosome Stability and Cell Survival-Is It All about Replication Forks?

The conserved nuclease-helicase DNA2 has been linked to mitochondrial myopathy, Seckel syndrome, and cancer. Across species, the protein is indispensable for cell proliferation. On the molecular level, DNA2 has been implicated in DNA double-strand break (DSB) repair, checkpoint activation, Okazaki fragment processing (OFP), and telomere homeostasis. More recently, a critical contribution of DNA2 to the replication stress response and recovery of stalled DNA replication forks (RFs) has emerged. Here, we review the available functional and phenotypic data and propose that the major cellular defects associated with DNA2 dysfunction, and the links that exist with human disease, can be rationalized through the fundamental importance of DNA2-dependent RF recovery to genome duplication. Being a crucial player at stalled RFs, DNA2 is a promising target for anti-cancer therapy aimed at eliminating cancer cells by replication-stress overload.