DNA–protein crosslink repair

The interplay of homology-directed repair pathways in the repair of zebularine-induced DNA-protein crosslinks in Arabidopsis

DNA-protein crosslinks (DPCs) are highly toxic DNA lesions represented by proteins covalently bound to the DNA. Persisting DPCs interfere with fundamental genetic processes such as DNA replication and transcription. Cytidine analog zebularine (ZEB) has been shown to crosslink DNA METHYLTRANSFERASE1 (MET1). Recently, we uncovered a critical role of the SMC5/6-mediated SUMOylation in the repair of DPCs. In an ongoing genetic screen, we identified two additional candidates, HYPERSENSITIVE TO ZEBULARINE 2 and 3, that were mapped to REGULATOR OF TELOMERE ELONGATION 1 (RTEL1) and polymerase TEBICHI (TEB), respectively. By monitoring the growth of hze2 and hze3 plants in response to zebularine, we show the importance of homologous recombination (HR) factor RTEL1 and microhomology-mediated end-joining (MMEJ) polymerase TEB in the repair of MET1-DPCs. Moreover, genetic interaction and sensitivity assays showed the interdependency of SMC5/6 complex, HR, and MMEJ in the homology-directed repair of MET1-DPCs in Arabidopsis. Altogether, we provide evidence that MET1-DPC repair in plants is more complex than originally expected.

A synthetic methylotrophic Escherichia coli as a chassis for bioproduction from methanol

Methanol synthesized from captured greenhouse gases is an emerging renewable feedstock with great potential for bioproduction. Recent research has raised the prospect of methanol bioconversion to value-added products using synthetic methylotrophic Escherichia coli, as its metabolism can be rewired to enable growth solely on the reduced one-carbon compound. Here we describe the generation of an E. coli strain that grows on methanol at a doubling time of 4.3 h-comparable to many natural methylotrophs. To establish bioproduction from methanol using this synthetic chassis, we demonstrate biosynthesis from four metabolic nodes from which numerous bioproducts can be derived: lactic acid from pyruvate, polyhydroxybutyrate from acetyl coenzyme A, itaconic acid from the tricarboxylic acid cycle and p-aminobenzoic acid from the chorismate pathway. In a step towards carbon-negative chemicals and valorizing greenhouse gases, our work brings synthetic methylotrophy in E. coli within reach of industrial applications.

Dimerization-dependent serine protease activity of FAM111A prevents replication fork stalling at topoisomerase 1 cleavage complexes

FAM111A, a serine protease, plays roles in DNA replication and antiviral defense. Missense mutations in the catalytic domain cause hyper-autocleavage and are associated with genetic disorders with developmental defects. Despite the enzyme's biological significance, the molecular architecture of the FAM111A serine protease domain (SPD) is unknown. Here, we show that FAM111A is a dimerization-dependent protease containing a narrow, recessed active site that cleaves substrates with a chymotrypsin-like specificity. X-ray crystal structures and mutagenesis studies reveal that FAM111A dimerizes via the N-terminal helix within the SPD. This dimerization induces an activation cascade from the dimerization sensor loop to the oxyanion hole through disorder-to-order transitions. Dimerization is essential for proteolytic activity in vitro and for facilitating DNA replication at DNA-protein crosslink obstacles in cells, while it is dispensable for autocleavage. These findings underscore the role of dimerization in FAM111A's function and highlight the distinction in its dimerization dependency between substrate cleavage and autocleavage.

Isolation and detection of DNA-protein crosslinks in mammalian cells

DNA-protein crosslinks (DPCs) are toxic DNA lesions wherein a protein is covalently attached to DNA. If not rapidly repaired, DPCs create obstacles that disturb DNA replication, transcription and DNA damage repair, ultimately leading to genome instability. The persistence of DPCs is associated with premature ageing, cancer and neurodegeneration. In mammalian cells, the repair of DPCs mainly relies on the proteolytic activities of SPRTN and the 26S proteasome, complemented by other enzymes including TDP1/2 and the MRN complex, and many of the activities involved are essential, restricting genetic approaches. For many years, the study of DPC repair in mammalian cells was hindered by the lack of standardised assays, most notably assays that reliably quantified the proteins or proteolytic fragments covalently bound to DNA. Recent interest in the field has spurred the development of several biochemical methods for DPC analysis. Here, we critically analyse the latest techniques for DPC isolation and the benefits and drawbacks of each. We aim to assist researchers in selecting the most suitable isolation method for their experimental requirements and questions, and to facilitate the comparison of results across different laboratories using different approaches.

From cultivation to cancer: formation of N-nitrosamines and other carcinogens in smokeless tobacco and their mutagenic implications

Tobacco use is a major cause of preventable morbidity and mortality globally. Tobacco products, including smokeless tobacco (ST), generally contain tobacco-specific N-nitrosamines (TSNAs), such as N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-butanone (NNK), which are potent carcinogens that cause mutations in critical genes in human DNA. This review covers the series of biochemical and chemical transformations, related to TSNAs, leading from tobacco cultivation to cancer initiation. A key aim of this review is to provide a greater understanding of TSNAs: their precursors, the microbial and chemical mechanisms that contribute to their formation in ST, their mutagenicity leading to cancer due to ST use, and potential means of lowering TSNA levels in tobacco products. TSNAs are not present in harvested tobacco but can form due to nitrosating agents reacting with tobacco alkaloids present in tobacco during certain types of curing. TSNAs can also form during or following ST production when certain microorganisms perform nitrate metabolism, with dissimilatory nitrate reductases converting nitrate to nitrite that is then released into tobacco and reacts chemically with tobacco alkaloids. When ST usage occurs, TSNAs are absorbed and metabolized to reactive compounds that form DNA adducts leading to mutations in critical target genes, including the RAS oncogenes and the p53 tumor suppressor gene. DNA repair mechanisms remove most adducts induced by carcinogens, thus preventing many but not all mutations. Lastly, because TSNAs and other agents cause cancer, previously documented strategies for lowering their levels in ST products are discussed, including using tobacco with lower nornicotine levels, pasteurization and other means of eliminating microorganisms, omitting fermentation and fire-curing, refrigerating ST products, and including nitrite scavenging chemicals as ST ingredients.

Aldehyde-Associated Mutagenesis─Current State of Knowledge

Aldehydes are widespread in the environment, with multiple sources such as food and beverages, industrial effluents, cigarette smoke, and additives. The toxic effects of exposure to several aldehydes have been observed in numerous studies. At the molecular level, aldehydes damage DNA, cross-link DNA and proteins, lead to lipid peroxidation, and are associated with increased disease risk including cancer. People genetically predisposed to aldehyde sensitivity exhibit severe health outcomes. In various diseases such as Fanconi's anemia and Cockayne syndrome, loss of aldehyde-metabolizing pathways in conjunction with defects in DNA repair leads to widespread DNA damage. Importantly, aldehyde-associated mutagenicity is being explored in a growing number of studies, which could offer key insights into how they potentially contribute to tumorigenesis. Here, we review the genotoxic effects of various aldehydes, focusing particularly on the DNA adducts underlying the mutagenicity of environmentally derived aldehydes. We summarize the chemical structures of the aldehydes and their predominant DNA adducts, discuss various methodologies, in vitro and in vivo, commonly used in measuring aldehyde-associated mutagenesis, and highlight some recent studies looking at aldehyde-associated mutation signatures and spectra. We conclude the Review with a discussion on the challenges and future perspectives of investigating aldehyde-associated mutagenesis.

Ubx5-Cdc48 assists the protease Wss1 at DNA-protein crosslink sites in yeast

DNA-protein crosslinks (DPCs) pose a serious threat to genome stability. The yeast proteases Wss1, 26S proteasome, and Ddi1 are safeguards of genome integrity by acting on a plethora of DNA-bound proteins in different cellular contexts. The AAA ATPase Cdc48/p97 is known to assist Wss1/SPRTN in clearing DNA-bound complexes; however, its contribution to DPC proteolysis remains unclear. Here, we show that the Cdc48 adaptor Ubx5 is detrimental in yeast mutants defective in DPC processing. Using an inducible site-specific crosslink, we show that Ubx5 accumulates at persistent DPC lesions in the absence of Wss1, which prevents their efficient removal from the DNA. Abolishing Cdc48 binding or complete loss of Ubx5 suppresses sensitivity of wss1∆ cells to DPC-inducing agents by favoring alternate repair pathways. We provide evidence for cooperation of Ubx5-Cdc48 and Wss1 in the genotoxin-induced degradation of RNA polymerase II (RNAPII), a described candidate substrate of Wss1. We propose that Ubx5-Cdc48 assists Wss1 for proteolysis of a subset of DNA-bound proteins. Together, our findings reveal a central role for Ubx5 in DPC clearance and repair.

SMC5/6 complex-mediated SUMOylation stimulates DNA-protein cross-link repair in Arabidopsis

DNA-protein cross-links (DPCs) are highly toxic DNA lesions consisting of proteins covalently attached to chromosomal DNA. Unrepaired DPCs physically block DNA replication and transcription. Three DPC repair pathways have been identified in Arabidopsis (Arabidopsis thaliana) to date: the endonucleolytic cleavage of DNA by the structure-specific endonuclease MUS81; proteolytic degradation of the crosslinked protein by the metalloprotease WSS1A; and cleavage of the cross-link phosphodiester bonds by the tyrosyl phosphodiesterases TDP1 and TDP2. Here we describe the evolutionary conserved STRUCTURAL MAINTENANCE OF CHROMOSOMEs SMC5/6 complex as a crucial component involved in DPC repair. We identified multiple alleles of the SMC5/6 complex core subunit gene SMC6B via a forward-directed genetic screen designed to identify the factors involved in the repair of DPCs induced by the cytidine analog zebularine. We monitored plant growth and cell death in response to DPC-inducing chemicals, which revealed that the SMC5/6 complex is essential for the repair of several types of DPCs. Genetic interaction and sensitivity assays showed that the SMC5/6 complex works in parallel to the endonucleolytic and proteolytic pathways. The repair of zebularine-induced DPCs was associated with SMC5/6-dependent SUMOylation of the damage sites. Thus, we present the SMC5/6 complex as an important factor in plant DPC repair.

Functions and evolution of FAM111 serine proteases

Proteolysis plays fundamental and regulatory roles in diverse cellular processes. The serine protease FAM111A (FAM111 trypsin-like peptidase A) emerged recently as a protease involved in two seemingly distinct processes: DNA replication and antiviral defense. FAM111A localizes to nascent DNA and plays a role at the DNA replication fork. At the fork, FAM111A is hypothesized to promote DNA replication at DNA-protein crosslinks (DPCs) and protein obstacles. On the other hand, FAM111A has also been identified as a host restriction factor for mutants of SV40 and orthopoxviruses. FAM111A also has a paralog, FAM111B, a serine protease with unknown cellular functions. Furthermore, heterozygous missense mutations in FAM111A and FAM111B cause distinct genetic disorders. In this review, we discuss possible models that could explain how FAM111A can function as a protease in both DNA replication and antiviral defense. We also review the consequences of FAM111A and FAM111B mutations and explore possible mechanisms underlying the diseases. Additionally, we propose a possible explanation for what drove the evolution of FAM111 proteins and discuss why some species have two FAM111 proteases. Altogether, studies of FAM111 proteases in DNA repair, antiviral defense, and genetic diseases will help us elucidate their functions and the regulatory mechanisms.

Targeting DNA-Protein Crosslinks via Post-Translational Modifications

Covalent binding of proteins to DNA forms DNA-protein crosslinks (DPCs), which represent cytotoxic DNA lesions that interfere with essential processes such as DNA replication and transcription. Cells possess different enzymatic activities to counteract DPCs. These include enzymes that degrade the adducted proteins, resolve the crosslinks, or incise the DNA to remove the crosslinked proteins. An important question is how DPCs are sensed and targeted for removal via the most suited pathway. Recent advances have shown the inherent role of DNA replication in triggering DPC removal by proteolysis. However, DPCs are also efficiently sensed and removed in the absence of DNA replication. In either scenario, post-translational modifications (PTMs) on DPCs play essential and versatile roles in orchestrating the repair routes. In this review, we summarize the current knowledge of the mechanisms that trigger DPC removal via PTMs, focusing on ubiquitylation, small ubiquitin-related modifier (SUMO) conjugation (SUMOylation), and poly (ADP-ribosyl)ation (PARylation). We also briefly discuss the current knowledge gaps and emerging hypotheses in the field.

Mechanisms and Regulation of DNA-Protein Crosslink Repair During DNA Replication by SPRTN Protease

DNA-protein crosslinks (DPCs) are deleterious DNA lesions that occur when proteins are covalently crosslinked to the DNA by the action of variety of agents like reactive oxygen species, aldehydes and metabolites, radiation, and chemotherapeutic drugs. Unrepaired DPCs are blockades to all DNA metabolic processes. Specifically, during DNA replication, replication forks stall at DPCs and are vulnerable to fork collapse, causing DNA breakage leading to genome instability and cancer. Replication-coupled DPC repair involves DPC degradation by proteases such as SPRTN or the proteasome and the subsequent removal of DNA-peptide adducts by nucleases and canonical DNA repair pathways. SPRTN is a DNA-dependent metalloprotease that cleaves DPC substrates in a sequence-independent manner and is also required for translesion DNA synthesis following DPC degradation. Biallelic mutations in SPRTN cause Ruijs-Aalfs (RJALS) syndrome, characterized by hepatocellular carcinoma and segmental progeria, indicating the critical role for SPRTN and DPC repair pathway in genome maintenance. In this review, we will discuss the mechanism of replication-coupled DPC repair, regulation of SPRTN function and its implications in human disease and cancer.

Comparative transcriptome and metabolome analyses reveal the methanol dissimilation pathway of Pichia pastoris

BACKGROUND

Pichia pastoris (Komagataella phaffii) is a model organism widely used for the recombinant expression of eukaryotic proteins, and it can metabolize methanol as its sole carbon and energy source. Methanol is oxidized to formaldehyde by alcohol oxidase (AOX). In the dissimilation pathway, formaldehyde is oxidized to CO₂ by formaldehyde dehydrogenase (FLD), S-hydroxymethyl glutathione hydrolase (FGH) and formate dehydrogenase (FDH).

RESULTS

The transcriptome and metabolome of P. pastoris were determined under methanol cultivation when its dissimilation pathway cut off. Firstly, Δfld and Δfgh were significantly different compared to the wild type (GS115), with a 60.98% and 23.66% reduction in biomass, respectively. The differential metabolites between GS115 and Δfld were mainly enriched in ABC transporters, amino acid biosynthesis, and protein digestion and absorption. Secondly, comparative transcriptome between knockout and wild type strains showed that oxidative phosphorylation, glycolysis and the TCA cycle were downregulated, while alcohol metabolism, proteasomes, autophagy and peroxisomes were upregulated. Interestingly, the down-regulation of the oxidative phosphorylation pathway was positively correlated with the gene order of dissimilation pathway knockdown. In addition, there were significant differences in amino acid metabolism and glutathione redox cycling that raised our concerns about formaldehyde sorption in cells.

CONCLUSIONS

This is the first time that integrity of dissimilation pathway analysis based on transcriptomics and metabolomics was carried out in Pichia pastoris. The blockage of dissimilation pathway significantly down-regulates the level of oxidative phosphorylation and weakens the methanol assimilation pathway to the point where deficiencies in energy supply and carbon fixation result in inefficient biomass accumulation and genetic replication. In addition, transcriptional upregulation of the proteasome and autophagy may be a stress response to resolve formaldehyde-induced DNA-protein crosslinking.

Contribution of proteases to the hallmarks of aging and to age-related neurodegeneration

Protein quality control ensures the degradation of damaged and misfolded proteins. Derangement of proteostasis is a primary cause of aging and age-associated diseases. The ubiquitin-proteasome and autophagy-lysosome play key roles in proteostasis but, in addition to these systems, the human genome encodes for ~600 proteases, also known as peptidases. Here, we examine the role of proteases in aging and age-related neurodegeneration. Proteases are present across cell compartments, including the extracellular space, and their substrates encompass cellular constituents, proteins with signaling functions, and misfolded proteins. Proteolytic processing by proteases can lead to changes in the activity and localization of substrates or to their degradation. Proteases cooperate with the autophagy-lysosome and ubiquitin-proteasome systems but also have independent proteolytic roles that impact all hallmarks of cellular aging. Specifically, proteases regulate mitochondrial function, DNA damage repair, cellular senescence, nutrient sensing, stem cell properties and regeneration, protein quality control and stress responses, and intercellular signaling. The capacity of proteases to regulate cellular functions translates into important roles in preserving tissue homeostasis during aging. Consequently, proteases influence the onset and progression of age-related pathologies and are important determinants of health span. Specifically, we examine how certain proteases promote the progression of Alzheimer's, Huntington's, and/or Parkinson's disease whereas other proteases protect from neurodegeneration. Mechanistically, cleavage by proteases can lead to the degradation of a pathogenic protein and hence impede disease pathogenesis. Alternatively, proteases can generate substrate byproducts with increased toxicity, which promote disease progression. Altogether, these studies indicate the importance of proteases in aging and age-related neurodegeneration.

The protease SPRTN and SUMOylation coordinate DNA-protein crosslink repair to prevent genome instability

DNA-protein crosslinks (DPCs) are a specific type of DNA lesion in which proteins are covalently attached to DNA. Unrepaired DPCs lead to genomic instability, cancer, neurodegeneration, and accelerated aging. DPC proteolysis was recently identified as a specialized pathway for DPC repair. The DNA-dependent protease SPRTN and the 26S proteasome emerged as two independent proteolytic systems. DPCs are also repaired by homologous recombination (HR), a canonical DNA repair pathway. While studying the cellular response to DPC formation, we identify ubiquitylation and SUMOylation as two major signaling events in DNA replication-coupled DPC repair. DPC ubiquitylation recruits SPRTN to repair sites, promoting DPC removal. DPC SUMOylation prevents DNA double-strand break formation, HR activation, and potentially deleterious genomic rearrangements. In this way, SUMOylation channels DPC repair toward SPRTN proteolysis, which is a safer pathway choice for DPC repair and prevention of genomic instability.

Transcription-coupled DNA double-strand break repair

The genomic DNA is constantly under attack by cellular and/or environmental factors. Fortunately, the cell is armed to safeguard its genome by various mechanisms such as nucleotide excision, base excision, mismatch and DNA double-strand break repairs. While these processes maintain the integrity of the genome throughout, DNA repair occurs preferentially faster at the transcriptionally active genes. Such transcription-coupled repair phenomenon plays important roles to maintain active genome integrity, failure of which would interfere with transcription, leading to an altered gene expression (and hence cellular pathologies/diseases). Among the various DNA damages, DNA double-strand breaks are quite toxic to the cells. If DNA double-strand break occurs at the active gene, it would interfere with transcription/gene expression, thus threatening cellular viability. Such DNA double-strand breaks are found to be repaired faster at the active gene in comparison to its inactive state or the inactive gene, thus supporting the existence of a new phenomenon of transcription-coupled DNA double-strand break repair. Here, we describe the advances of this repair process.

An updated mode of action and human relevance framework evaluation for Formaldehyde-Related nasal tumors

Formaldehyde is a reactive aldehyde naturally present in all plant and animal tissues and a critical component of the one-carbon metabolism pathway. It is also a high production volume chemical used in the manufacture of numerous products. Formaldehyde is also one of the most well-studied chemicals with respect to environmental fate, biology, and toxicology-including carcinogenic potential, and mode of action (MOA). In 2006, a published MOA for formaldehyde-induced nasal tumors in rats concluded that nasal tumors were most likely driven by cytotoxicity and regenerative cell proliferation, with possible contributions from direct genotoxicity. In the past 15 years, new research has better informed the MOA with the publication of in vivo genotoxicity assays, toxicogenomic analyses, and development of ultra-sensitive methods to measure endogenous and exogenous formaldehyde-induced DNA adducts. Herein, we review and update the MOA for nasal tumors, with particular emphasis on the numerous studies published since 2006. These new studies further underscore the involvement of cytotoxicity and regenerative cell proliferation, and further inform the genotoxic potential of inhaled formaldehyde. The data lend additional support for the use of mechanistic data for the derivation of toxicity criteria and/or scientifically supported approaches for low-dose extrapolation for the risk assessment of formaldehyde.

DNA-protein crosslink proteases in genome stability

Proteins covalently attached to DNA, also known as DNA-protein crosslinks (DPCs), are common and bulky DNA lesions that interfere with DNA replication, repair, transcription and recombination. Research in the past several years indicates that cells possess dedicated enzymes, known as DPC proteases, which digest the protein component of a DPC. Interestingly, DPC proteases also play a role in proteolysis beside DPC repair, such as in degrading excess histones during DNA replication or controlling DNA replication checkpoints. Here, we discuss the importance of DPC proteases in DNA replication, genome stability and their direct link to human diseases and cancer therapy.

The ARK Assay Is a Sensitive and Versatile Method for the Global Detection of DNA-Protein Crosslinks

DNA-protein crosslinks (DPCs) are a frequent form of DNA lesion and are strongly inhibitive in diverse DNA transactions. Despite recent developments, the biochemical detection of DPCs remains a limiting factor for the in-depth mechanistic understanding of DPC repair. Here, we develop a sensitive and versatile assay, designated ARK, for the quantitative analysis of DPCs in cells. ARK uses sequential chaotropic and detergent-based isolation of DPCs and substantially enhances sample purity, resulting in a 5-fold increase in detection sensitivity and a 10-fold reduction in background reading. We validate the ARK assay with genetic mutants with established deficiencies in DPC repair and demonstrate its robustness by using common DPC-inducing reagents, including formaldehyde, camptothecin, and etoposide. In addition, we show that the Fanconi anemia pathway contributes to the repair of DPCs. Thus, ARK is expected to facilitate various studies aimed at understanding both fundamental biology and translational applications of DNA-protein crosslink repair.

Hu et al. develop a protocol to analyze DNA-protein crosslinking (DPC) damage. Designated the ARK assay, this method outperforms widely used assays by allowing the detection of global DPCs with improved sensitivity and expanded readout. Defective DPC repair is detected in Fanconi anemia mutant cells by this protocol.

Converting Escherichia coli to a Synthetic Methylotroph Growing Solely on Methanol

Methanol, being electron rich and derivable from methane or CO₂, is a potentially renewable one-carbon (C1) feedstock for microorganisms. Although the ribulose monophosphate (RuMP) cycle used by methylotrophs to assimilate methanol differs from the typical sugar metabolism by only three enzymes, turning a non-methylotrophic organism to a synthetic methylotroph that grows to a high cell density has been challenging. Here we reprogrammed E. coli using metabolic robustness criteria followed by laboratory evolution to establish a strain that can efficiently utilize methanol as the sole carbon source. This synthetic methylotroph alleviated a so far uncharacterized hurdle, DNA-protein crosslinking (DPC), by insertion sequence (IS)-mediated copy number variations (CNVs) and balanced the metabolic flux by mutations. Being capable of growing at a rate comparable with natural methylotrophs in a wide range of methanol concentrations, this synthetic methylotrophic strain illustrates genome editing and evolution for microbial tropism changes and expands the scope of biological C1 conversion.

The Chromatin Response to Double-Strand DNA Breaks and Their Repair

Cellular DNA is constantly being damaged by numerous internal and external mutagenic factors. Probably the most severe type of insults DNA could suffer are the double-strand DNA breaks (DSBs). They sever both DNA strands and compromise genomic stability, causing deleterious chromosomal aberrations that are implicated in numerous maladies, including cancer. Not surprisingly, cells have evolved several DSB repair pathways encompassing hundreds of different DNA repair proteins to cope with this challenge. In eukaryotic cells, DSB repair is fulfilled in the immensely complex environment of the chromatin. The chromatin is not just a passive background that accommodates the multitude of DNA repair proteins, but it is a highly dynamic and active participant in the repair process. Chromatin alterations, such as changing patterns of histone modifications shaped by numerous histone-modifying enzymes and chromatin remodeling, are pivotal for proficient DSB repair. Dynamic chromatin changes ensure accessibility to the damaged region, recruit DNA repair proteins, and regulate their association and activity, contributing to DSB repair pathway choice and coordination. Given the paramount importance of DSB repair in tumorigenesis and cancer progression, DSB repair has turned into an attractive target for the development of novel anticancer therapies, some of which have already entered the clinic.

Fanconi anemia and the underlying causes of genomic instability

Fanconi anemia (FA) is a rare genetic disorder, characterized by birth defects, progressive bone marrow failure, and a predisposition to cancer. This devastating disease is caused by germline mutations in any one of the 22 known FA genes, where the gene products are primarily responsible for the resolution of DNA interstrand cross-links (ICLs), a type of DNA damage generally formed by cytotoxic chemotherapeutic agents. However, the identity of endogenous mutagens that generate DNA ICLs remains largely elusive. In addition, whether DNA ICLs are indeed the primary cause behind FA phenotypes is still a matter of debate. Recent genetic studies suggest that naturally occurring reactive aldehydes are a primary source of DNA damage in hematopoietic stem cells, implicating that they could play a role in genome instability and FA. Emerging lines of evidence indicate that the FA pathway constitutes a general surveillance mechanism for the genome by protecting against a variety of DNA replication stresses. Therefore, understanding the DNA repair signaling that is regulated by the FA pathway, and the types of DNA lesions underlying the FA pathophysiology is crucial for the treatment of FA and FA-associated cancers. Here, we review recent advances in our understanding of the relationship between reactive aldehydes, bone marrow dysfunction, and FA biology in the context of signaling pathways triggered during FA-mediated DNA repair and maintenance of the genomic integrity. Environ. Mol. Mutagen. 2020. © 2020 Wiley Periodicals, Inc.

Cisplatin reacts with histone H1 and the adduct forms a ternary complex with DNA

Cisplatin is an anticancer drug widely used in clinics; it induces the apoptosis of cancer cells by targeting DNA. However, its interaction with proteins has been found to be crucial in modulating the pre and post-target activity. Nuclear DNA is tightly assembled with histone proteins to form nucleosomes in chromatin; this can impede the drug to access DNA. On the other hand, the linker histone H1 is considered 'the gate to nucleosomal DNA' due to its exposed location and dynamic conformation; therefore, this protein can influence the platination of DNA. In this study, we performed a reaction of cisplatin with histone H1 and investigated the interaction of the H1/cisplatin adduct with DNA. The reactions were conducted on the N-terminal domains of H1.4 (sequence 1-90, H1N90) and H1.0 (sequence 1-7, H1N7). The results show that H1 readily reacts with cisplatin and generates bidentate and tridentate adducts, with methionine and glutamate residues as the preferential binding sites. Chromatographic and NMR analyses show that the platination rate of H1 is slightly higher than that of DNA and the platinated H1 can form H1-cisplatin-DNA ternary complexes. Interestingly, cisplatin is more prone to form H1-Pt-DNA ternary complexes than trans-oriented platinum agents. The formation of H1-cisplatin-DNA ternary complexes and their preference for cis- over trans-oriented platinum agents suggest an important role of histone H1 in the mechanism of action of cisplatin.

Wss1 homolog from Candida albicans and its role in DNA-protein crosslink tolerance

Candida albicans is an opportunistic yeast that can cause life-threatening systemic infection in immunocompromised individuals. During infections, C. albicans has to cope with genotoxic stresses generated by the host immune system. DNA-protein crosslink (DPC), the covalent linkage of proteins with DNA, is one type of DNA damages that can be caused by the host immune response. DPCs are bulky lesions that interfere with the progression of replication and transcription machineries, and hence threaten genomic integrity. Accordingly, either a DPC tolerance mechanism or a DPC repair pathway is essential for C. albicans to maintain genomic stability and survive in the host. Here, we identified Wss1 (weak suppressor of Smt3) in C. albicans (CaWss1) using bioinformatics, genetic complementation, and biochemical studies. We showed that CaWss1 promotes cell survival under genotoxic stress conditions that generate DPCs and that the catalytic metalloprotease domain of CaWss1 is essential for its cellular function. Interactions of CaWss1 with Cdc48 and small ubiquitin-like modifier, although not strictly required, contribute to the function of CaWss1 in the suppression of the growth defects under DPC-inducing conditions. This report is the first investigation of the role of CaWss1 in DPC tolerance in C. albicans.

FAM111A protects replication forks from protein obstacles via its trypsin-like domain

Persistent protein obstacles on genomic DNA, such as DNA-protein crosslinks (DPCs) and tight nucleoprotein complexes, can block replication forks. DPCs can be removed by the proteolytic activities of the metalloprotease SPRTN or the proteasome in a replication-coupled manner; however, additional proteolytic mechanisms may exist to cope with the diversity of protein obstacles. Here, we show that FAM111A, a PCNA-interacting protein, plays an important role in mitigating the effect of protein obstacles on replication forks. This function of FAM111A requires an intact trypsin-like protease domain, the PCNA interaction, and the DNA-binding domain that is necessary for protease activity in vivo. FAM111A, but not SPRTN, protects replication forks from stalling at poly(ADP-ribose) polymerase 1 (PARP1)-DNA complexes trapped by PARP inhibitors, thereby promoting cell survival after drug treatment. Altogether, our findings reveal a role of FAM111A in overcoming protein obstacles to replication forks, shedding light on cellular responses to anti-cancer therapies.

Function and evolution of the DNA-protein crosslink proteases Wss1 and SPRTN

Covalent DNA-protein crosslinks (DPCs) are highly toxic DNA adducts, which interfere with faithful DNA replication. The proteases Wss1 and SPRTN degrade DPCs and have emerged as crucially important DNA repair enzymes. Their protective role has been described in various model systems ranging from yeasts, plants, worms and flies to mice and humans. Loss of DPC proteases results in genome instability, cellular arrest, premature ageing and cancer predisposition. Here we discuss recent insights into the function and molecular mechanism of these enzymes. Furthermore, we present an in-depth phylogenetic analysis of the Wss1/SPRTN protease continuum. Remarkably flexible domain architectures and constantly changing protein-protein interaction motifs indicate ongoing evolutionary dynamics. Finally, we discuss recent data, which suggest that further partially-overlapping proteolytic systems targeting DPCs exist in eukaryotes. These new developments raise interesting questions regarding the division of labour between different DPC proteases and the mechanisms and principles of repair pathway choice.

Molecular mechanisms of topoisomerase 2 DNA-protein crosslink resolution

The compaction of DNA and the continuous action of DNA transactions, including transcription and DNA replication, create complex DNA topologies that require Type IIA Topoisomerases, which resolve DNA topological strain and control genome dynamics. The human TOP2 enzymes catalyze their reactions via formation of a reversible covalent enzyme DNA-protein crosslink, the TOP2 cleavage complex (TOP2cc). Spurious interactions of TOP2 with DNA damage, environmental toxicants and chemotherapeutic "poisons" perturbs the TOP2 reaction cycle, leading to an accumulation of DNA-protein crosslinks, and ultimately, genomic instability and cell death. Emerging evidence shows that TOP2-DNA protein crosslink (DPC) repair entails multiple strand break repair activities, such as removal of the poisoned TOP2 protein and rejoining of the DNA ends through homologous recombination (HR) or non-homologous end joining (NHEJ). Herein, we discuss the molecular mechanisms of TOP2-DPC resolution, with specific emphasis on the recently uncovered ZATTZnf451-licensed TDP2-catalyzed TOP2-DPC reversal mechanism.

Molecular basis of abasic site sensing in single-stranded DNA by the SRAP domain of E. coli yedK

HMCES and yedK were recently identified as sensors of abasic sites in ssDNA. In this study, we present multiple crystal structures captured in the apo-, nonspecific-substrate-binding, specific-substrate-binding, and product-binding states of yedK. In combination with biochemical data, we unveil the molecular basis of AP site sensing in ssDNA by yedK. Our results indicate that yedK has a strong preference for AP site-containing ssDNA over native ssDNA and that the conserved Glu105 residue is important for identifying AP sites in ssDNA. Moreover, our results reveal that a thiazolidine linkage is formed between yedK and AP sites in ssDNA, with the residues that stabilize the thiazolidine linkage important for the formation of DNA-protein crosslinks between yedK and the AP sites. We propose that our findings offer a unique platform to develop yedK and other SRAP domain-containing proteins as tools for detecting abasic sites in vitro and in vivo.

DNA- and DNA-Protein-Crosslink Repair in Plants

DNA-crosslinks are one of the most severe types of DNA lesions. Crosslinks (CLs) can be subdivided into DNA-intrastrand CLs, DNA-interstrand CLs (ICLs) and DNA-protein crosslinks (DPCs), and arise by various exogenous and endogenous sources. If left unrepaired before the cell enters S-phase, ICLs and DPCs pose a major threat to genomic integrity by blocking replication. In order to prevent the collapse of replication forks and impairment of cell division, complex repair pathways have emerged. In mammals, ICLs are repaired by the so-called Fanconi anemia (FA) pathway, which includes 22 different FANC genes, while in plants only a few of these genes are conserved. In this context, two pathways of ICL repair have been defined, each requiring the interaction of a helicase (FANCJB/RTEL1) and a nuclease (FAN1/MUS81). Moreover, homologous recombination (HR) as well as postreplicative repair factors are also involved. Although DPCs possess a comparable toxic potential to cells, it has only recently been shown that at least three parallel pathways for DPC repair exist in plants, defined by the protease WSS1A, the endonuclease MUS81 and tyrosyl-DNA phosphodiesterase 1 (TDP1). The importance of crosslink repair processes are highlighted by the fact that deficiencies in the respective pathways are associated with diverse hereditary disorders.

SPRTN protease and checkpoint kinase 1 cross-activation loop safeguards DNA replication

The SPRTN metalloprotease is essential for DNA-protein crosslink (DPC) repair and DNA replication in vertebrate cells. Cells deficient in SPRTN protease exhibit DPC-induced replication stress and genome instability, manifesting as premature ageing and liver cancer. Here, we provide a body of evidence suggesting that SPRTN activates the ATR-CHK1 phosphorylation signalling cascade during physiological DNA replication by proteolysis-dependent eviction of CHK1 from replicative chromatin. During this process, SPRTN proteolyses the C-terminal/inhibitory part of CHK1, liberating N-terminal CHK1 kinase active fragments. Simultaneously, CHK1 full length and its N-terminal fragments phosphorylate SPRTN at the C-terminal regulatory domain, which stimulates SPRTN recruitment to chromatin to promote unperturbed DNA replication fork progression and DPC repair. Our data suggest that a SPRTN-CHK1 cross-activation loop plays a part in DNA replication and protection from DNA replication stress. Finally, our results with purified components of this pathway further support the proposed model of a SPRTN-CHK1 cross-activation loop.

Cells deficient in SPRTN protease activity exhibit severe DNA-protein crosslink induced replication stress and genome instability. Here the author reveal a functional link between the SPRTN protease and the CHK1 kinase during physiological DNA replication.

DNA Break Repair in Plants and Its Application for Genome Engineering

Genome engineering is a biotechnological approach to precisely modify the genetic code of a given organism in order to change the context of an existing sequence or to create new genetic resources, e.g., for obtaining improved traits or performance. Efficient targeted genome alterations are mainly based on the induction of DNA double-strand breaks (DSBs) or adjacent single-strand breaks (SSBs). Naturally, all organisms continuously have to deal with DNA-damaging factors challenging the genetic integrity, and therefore a wide range of DNA repair mechanisms have evolved. A profound understanding of the different repair pathways is a prerequisite to control and enhance targeted gene modifications. DSB repair can take place by nonhomologous end joining (NHEJ) or homology-dependent repair (HDR). As the main outcome of NHEJ-mediated repair is accompanied by small insertions and deletions, it is applicable to specifically knock out genes or to rearrange linkage groups or whole chromosomes. The basic requirement for HDR is the presence of a homologous template; thus this process can be exploited for targeted integration of ectopic sequences into the plant genome. The development of different types of artificial site-specific nucleases allows for targeted DSB induction in the plant genome. Such synthetic nucleases have been used for both qualitatively studying DSB repair in vivo with respect to mechanistic differences and quantitatively in order to determine the role of key factors for NHEJ and HR, respectively. The conclusions drawn from these studies allow for a better understanding of genome evolution and help identifying synergistic or antagonistic genetic interactions while supporting biotechnological applications for transiently modifying the plant DNA repair machinery in favor of targeted genome engineering.

Oxidation of 8-Oxo-7,8-dihydro-2'-deoxyguanosine Leads to Substantial DNA-Histone Cross-Links within Nucleosome Core Particles

8-Oxo-7,8-dihydro-2'-deoxyguanosine(8-oxodGuo) is a common primary product of cellular oxidative DNA damage. 8-OxodGuo is more readily oxidized than 2'-deoxyguanosine (dG); a two-electron oxidation generates a highly reactive intermediate (OG^ox), which forms covalent adducts with nucleophiles, including OH^-, free amines, and the side chains of amino acids such as lysine. We determined here that K₃Fe(CN)₆ oxidation of 8-oxodGuo in nucleosome core particles (NCPs) produces high yields, quantitative (i.e., 100%) in some cases, of DNA-protein cross-links (DPCs). The efficiency of DPC formation was closely related to 8-oxodGuo base pairing and location within the NCP and was only slightly decreased by adding the DNA-protective polyamine spermine to the system. Using NCPs that contained histone mutants, we determined that DPCs result predominantly from OG^ox trapping by the N-terminal histone amine. The DPCs were stable under physiological conditions and therefore could have important biological consequences. For instance, the essentially quantitative yield of DPCs at some positions within NCPs would reduce the yield of the mutagenic DNA lesions spiroiminodihydantoin and guanidinohydantoin produced from the common intermediate OG^ox, which in turn would affect mutation signatures of oxidative stress in a position-dependent manner. In summary, our findings indicate that site-specific incorporation of 8-oxodGuo into NCPs, followed by its oxidation, leads to DPCs with an efficiency depending on 8-oxodGuo location and orientation. Given that 8-oxodGuo formation is widespread in genomic DNA and that DPC formation is highly efficient, DPCs may occur in eukaryotic cells and may affect several important biological processes.

The Cockayne syndrome protein B is involved in the repair of 5-AZA-2'-deoxycytidine-induced DNA lesions

The Cockayne Syndrome Protein B (CSB) plays an essential role in Transcription-Coupled Nucleotide Excision Repair (TC-NER) by recruiting repair proteins once transcription is blocked with a DNA lesion. In fact, CSB-deficient cells are unable to recover from transcription-blocking DNA lesions. 5-Aza-2′-deoxycytidine (5-azadC) is a nucleoside analogue that covalently traps DNA methyltransferases (DNMTs) onto DNA. This anticancer drug has a double mechanism of action: it reverts aberrant hypermethylation in tumour-suppressor genes, and it induces DNA damage. We have recently reported that Homologous Recombination and XRCC1/PARP play an important role in the repair of 5-azadC-induced DNA damage. However, the mechanisms involved in the repair of the DNMT adducts induced by azadC remain poorly understood. In this paper, we show for the first time the importance of CSB in the repair of azadC-induced DNA lesions. We propose a model in which CSB initiates a signalling pathway to repair transcription blocks induced by incorporated 5-azadC. Indeed, CSB-deficient cells treated with 5-azadC show a delay in the repair of trapped DNMT1, increased levels of DNA damage and reduced survival.

Loss of Cohesin Subunit Rec8 Switches Rad51 Mediator Dependence in Resistance to Formaldehyde Toxicity in Ustilago maydis

DNA-protein cross-links (DPCs) are frequently occurring lesions that provoke continual threats to the integrity of the genome by interference with replication and transcription. Reactive aldehydes generated from endogenous metabolic processes or produced in the environment are sources that trigger cross-linking of DNA with associated proteins. DNA repair pathways in place for removing DPCs, or for bypassing them to enable completion of replication, include homologous recombination (HR) and replication fork remodeling (FR) systems. Here, we surveyed a set of mutants defective in known HR and FR components to determine their contribution toward maintaining resistance to chronic formaldehyde (FA) exposure in Ustilago maydis, a fungus that relies on the BRCA2-family member Brh2 as the principal Rad51 mediator in repair of DNA strand breaks. We found that, in addition to Brh2, Rad52 was also vital for resistance to FA. Deleting the gene for Rec8, a kleisin subunit of cohesin, eliminated the requirement for Brh2, but not Rad52, in FA resistance. The Rad51K133R mutant variant that is able to bind DNA but unable to dissociate from it was able to support resistance to FA. These findings suggest a model for DPC repair and tolerance that features a specialized role for Rad52, enabling Rad51 to access DNA in its noncanonical capacity of replication fork protection rather than DNA strand transfer.

DNA protein crosslink proteolysis repair: From yeast to premature ageing and cancer in humans

DNA-protein crosslinks (DPCs) are a specific type of DNA lesion consisting of a protein covalently and irreversibly bound to DNA, which arise after exposure to physical and chemical crosslinking agents. DPCs can be bulky and thereby pose a barrier to DNA replication and transcription. The persistence of DPCs during S phase causes DNA replication stress and genome instability. The toxicity of DPCs is exploited in cancer therapy: many common chemotherapeutics kill cancer cells by inducing DPC formation.

Recent work from several laboratories discovered a specialized repair pathway for DPCs, namely DPC proteolysis (DPCP) repair. DPCP repair is carried out by replication-coupled DNA-dependent metalloproteases: Wss1 in yeast and SPRTN in metazoans. Mutations in SPRTN cause premature ageing and liver cancer in humans and mice; thus, defective DPC repair has great clinical ramifications. In the present review, we will revise the current knowledge on the mechanisms of DPCP repair and on the regulation of DPC protease activity, while highlighting the most significant unresolved questions in the field. Finally, we will discuss the impact of faulty DPC repair on disease and cancer therapy.

DNA-protein cross-links: Formidable challenges to maintaining genome integrity

DNA is associated with proteins that are involved in its folding and transaction processes. When cells are exposed to chemical cross-linking agents or free radical-generating ionizing radiation, DNA-associated proteins are covalently trapped within the DNA to produce DNA-protein cross-links (DPCs). DPCs produced by these agents contain cross-linked proteins in an undisrupted DNA strand. Some DNA-metabolizing enzymes that form covalent reaction intermediates can also be irreversibly trapped in the presence of inhibitors or DNA damage to give rise to abortive DPCs. The abortive DPCs often contain cross-linked proteins attached to the 5' or 3' end of a DNA strand break. In vitro studies show that steric hindrance caused by cross-linked proteins impedes the progression of DNA helicases and polymerases and of RNA polymerases. The modes and consequences by which DPCs impede replication and transcription processes are considerably different from those with conventional DNA lesions. Thus, DPCs are formidable challenges to maintaining genome integrity and faithful gene expression. Current models of DPC repair involve direct and indirect removal of DPCs. The direct mechanism works for DPCs that contain topoisomerase 2 attached to the 5' end of DNA. The Mre11-Rad50-Nbs1 complex cleaves the site internal to the DPC and directly releases a DPC-containing oligonucleotide. The indirect mechanism involves degradation of cross-linked proteins by proteasomes or the recently identified DPC proteases Wss1 and Sprtn to relieve steric hindrance of DPCs. The resulting peptide-cross-links might be processed by translesion synthesis or other canonical repair mechanisms: however, the exact mechanism remains to be elucidated.

Variable termination sites of DNA polymerases encountering a DNA-protein cross-link

DNA-protein cross-links (DPCs) are important DNA lesions induced by endogenous crosslinking agents such as formaldehyde or acetaldehyde, as well as ionizing radiation, cancer chemotherapeutic drugs, and abortive action of some enzymes. Due to their very bulky nature, they are expected to interfere with DNA and RNA synthesis and DNA repair. DPCs are highly genotoxic and the ability of cells to deal with them is relevant for many chemotherapeutic interventions. However, interactions of DNA polymerases with DPCs have been poorly studied due to the lack of a convenient experimental model. We have used NaBH4-induced trapping of E. coli formamidopyrimidine-DNA glycosylase with DNA to construct model DNA polymerase substrates containing a DPC in single-stranded template, or in the template strand of double-stranded DNA, or in the non-template (displaced) strand of double-stranded DNA. Nine DNA polymerases belonging to families A, B, X, and Y were studied with respect to their behavior upon encountering a DPC: Klenow fragment of E. coli DNA polymerase I, Thermus aquaticus DNA polymerase I, Pyrococcus furiosus DNA polymerase, Sulfolobus solfataricus DNA polymerase IV, human DNA polymerases β, κ and λ, and DNA polymerases from bacteriophages T4 and RB69. Although none were able to fully bypass DPCs in any context, Family B DNA polymerases (T4, RB69) and Family Y DNA polymerase IV were able to elongate the primer up to the site of the cross-link if a DPC was located in single-stranded template or in the displaced strand. In other cases, DNA synthesis stopped 4-5 nucleotides before the site of the cross-link in single-stranded template or in double-stranded DNA if the polymerases could displace the downstream strand. We suggest that termination of DNA polymerases on a DPC is mostly due to the unrelieved conformational strain experienced by the enzyme when pressing against the cross-linked protein molecule.

Cold atmospheric-pressure plasma induces DNA-protein crosslinks through protein oxidation

Reactive oxygen and nitrogen species (ROS and RNS) generated by cold atmospheric-pressure plasma could damage genomic DNA, although the precise types of these DNA damage induced by plasma are poorly characterized. Understanding plasma-induced DNA damage will help to elucidate the biological effect of plasma and guide the application of plasma in ROS-based therapy. In this study, it was shown that ROS and RNS generated by physical plasma could efficiently induce DNA-protein crosslinks (DPCs) in bacteria, yeast, and human cells. An in vitro assay showed that plasma treatment resulted in the formation of covalent DPCs by activating proteins to crosslink with DNA. Mass spectrometry and hydroperoxide analysis detected oxidation products induced by plasma. DPC formation were alleviated by singlet oxygen scavenger, demonstrating the importance of singlet oxygen in this process. These results suggested the roles of DPC formation in DNA damage induced by plasma, which could improve the understanding of the biological effect of plasma and help to develop a new strategy in plasma-based therapy including infection and cancer therapy.

Site-specific covalent capture of human O6-alkylguanine-DNA-alkyltransferase using single-stranded intrastrand cross-linked DNA

A methodology is reported to conjugate human O⁶-alkylguanine-DNA-alkyltransferase (hAGT) to the 3'-end of DNA in excellent yields with short reaction times by using intrastrand cross-linked (IaCL) DNA probes. This strategy exploited the substrate specificity of hAGT to generate the desired DNA-protein covalent complex. IaCL DNA linking two thymidine residues, or linking a thymidine residue to a 2'-deoxyguanosine residue (either in a 5'→3' or 3'→5' fashion), lacking a phosphodiester linkage at the cross-linked site, were prepared using a phosphoramidite strategy followed by solid-phase synthesis. All duplexes containing the model IaCL displayed a reduction in thermal stability relative to unmodified control duplexes. The O⁴-thymidine-alkylene-O⁴-thymidine and the (5'→3') O⁶-2'-deoxyguanosine-alkylene-O⁴-thymidine IaCL DNA adducts were not repaired by any of the AGTs evaluated (human AGT and Escherichia coli homologues, OGT and Ada-C). The (5'→3') O⁴-thymidine-alkylene-O⁶-2'-deoxyguanosine IaCL DNA containing a butylene or heptylene tethers were efficiently repaired by the human variant, whereas Ada-C was capable of modestly repairing the heptylene IaCL adduct. The IaCL strategy has expanded the toolbox for hAGT conjugation to DNA strands, without requiring the presence of a complementary DNA sequence. Finally, hAGT was functionalized with a fluorescently-labelled DNA sequence to demonstrate the applicability of this conjugation method.

AGT Activity Towards Intrastrand Crosslinked DNA is Modulated by the Alkylene Linker

DNA oligomers containing dimethylene and trimethylene intrastrand crosslinks (IaCLs) between the O4 and O6 atoms of neighboring thymidine (T) and 2'-deoxyguanosine (dG) residues were prepared by solid-phase synthesis. UV thermal denaturation (T_m ) experiments revealed that these IaCLs had a destabilizing effect on the DNA duplex relative to the control. Circular dichroism spectroscopy suggested these IaCLs induced minimal structural distortions. Susceptibility to dealkylation by reaction with various O⁶ -alkylguanine DNA alkyltransferases (AGTs) from human and Escherichia coli was evaluated. It was revealed that only human AGT displayed activity towards the IaCL DNA, with reduced efficiency as the IaCL shortened (from four to two methylene linkages). Changing the site of attachment of the ethylene linkage at the 5'-end of the IaCL to the N3 atom of T had minimal influence on duplex stability and structure, and was refractory to AGT activity.

Formation and repair of DNA-protein crosslink damage

DNA is constantly exposed to a wide array of genotoxic agents, generating a variety of forms of DNA damage. DNA-protein crosslinks (DPCs)-the covalent linkage of proteins with a DNA strand-are one of the most deleterious and understudied forms of DNA damage, posing as steric blockades to transcription and replication. If not properly repaired, these lesions can lead to mutations, genomic instability, and cell death. DPCs can be induced endogenously or through environmental carcinogens and chemotherapeutic agents. Endogenously, DPCs are commonly derived through reactions with aldehydes, as well as through trapping of various enzymatic intermediates onto the DNA. Proteolytic cleavage of the protein moiety of a DPC is a general strategy for removing the lesion. This can be accomplished through a DPC-specific protease and and/or proteasome-mediated degradation. Nucleotide excision repair and homologous recombination are each involved in repairing DPCs, with their respective roles likely dependent on the nature and size of the adduct. The Fanconi anemia pathway may also have a role in processing DPC repair intermediates. In this review, we discuss how these lesions are formed, strategies and mechanisms for their removal, and diseases associated with defective DPC repair.

Spartan deficiency causes accumulation of Topoisomerase 1 cleavage complexes and tumorigenesis

Germline mutations in SPRTN cause Ruijs–Aalfs syndrome (RJALS), a disorder characterized by genome instability, progeria and early onset hepatocellular carcinoma. Spartan, the protein encoded by SPRTN, is a nuclear metalloprotease that is involved in the repair of DNA–protein crosslinks (DPCs). Although Sprtn hypomorphic mice recapitulate key progeroid phenotypes of RJALS, whether this model expressing low amounts of Spartan is prone to DPC repair defects and spontaneous tumors is unknown. Here, we showed that the livers of Sprtn hypomorphic mice accumulate DPCs containing Topoisomerase 1 covalently linked to DNA. Furthermore, these mice exhibited DNA damage, aneuploidy and spontaneous tumorigenesis in the liver. Collectively, these findings provide evidence that partial loss of Spartan impairs DPC repair and tumor suppression.

Mechanisms of DNA-protein crosslink repair

Covalent DNA-protein crosslinks (DPCs, also known as protein adducts) of topoisomerases and other proteins with DNA are highly toxic DNA lesions. Of note, chemical agents that induce DPCs include widely used classes of chemotherapeutics. Their bulkiness blocks virtually every chromatin-based process and makes them intractable for repair by canonical repair pathways. Distinct DPC repair pathways employ unique points of attack and are crucial for the maintenance of genome stability. Tyrosyl-DNA phosphodiesterases (TDPs) directly hydrolyse the covalent linkage between protein and DNA. The MRE11-RAD50-NBS1 (MRN) nuclease complex targets the DNA component of DPCs, excising the fragment affected by the lesion, whereas proteases of the spartan (SPRTN)/weak suppressor of SMT3 protein 1 (Wss1) family target the protein component. Loss of these pathways renders cells sensitive to DPC-inducing chemotherapeutics, and DPC repair pathways are thus attractive targets for combination cancer therapy.

DNA-dependent protease activity of human Spartan facilitates replication of DNA-protein crosslink-containing DNA

Mutations in SPARTAN are associated with early onset hepatocellular carcinoma and progeroid features. A regulatory function of Spartan has been implicated in DNA damage tolerance pathways such as translesion synthesis, but the exact function of the protein remained unclear. Here, we reveal the role of human Spartan in facilitating replication of DNA–protein crosslink-containing DNA. We found that purified Spartan has a DNA-dependent protease activity degrading certain proteins bound to DNA. In concert, Spartan is required for direct DPC removal in vivo; we also show that the protease Spartan facilitates repair of formaldehyde-induced DNA–protein crosslinks in later phases of replication using the bromodeoxyuridin (BrdU) comet assay. Moreover, DNA fibre assay indicates that formaldehyde-induced replication stress dramatically decreases the speed of replication fork movement in Spartan-deficient cells, which accumulate in the G2/M cell cycle phase. Finally, epistasis analysis mapped these Spartan functions to the RAD6-RAD18 DNA damage tolerance pathway. Our results reveal that Spartan facilitates replication of DNA–protein crosslink-containing DNA enzymatically, as a protease, which may explain its role in preventing carcinogenesis and aging.

Structural analysis of Wss1 protein from saccharomyces cerevisiae

Wss1 is a DNA-protein crosslinks (DPCs) repair protein, which is responsible for degradation of the protein components in DPCs. In this investigation, crystal structure of the protease domain from saccharomyces cerevisiae Wss1 (ScWss1) was solved and was compared with the known crystal structure of Schizosaccharomyces prombe Wss1 (SpWss1). It is found that the cleft near zinc ion to be the most conserved core region of Wss1 and that the electronic surface distributions vary greatly between the two homologs. Solution architecture of the full-length ScWss1 was further investigated by small-angle X-ray scattering (SAXS), which indicated the protein contains a flexible region inside. Finally, based on the structural information, a mechanism was proposed about how the enzyme is activated by DNA substrates.

Radiation-induced DNA-protein cross-links: Mechanisms and biological significance

Ionizing radiation produces various DNA lesions such as base damage, DNA single-strand breaks (SSBs), DNA double-strand breaks (DSBs), and DNA-protein cross-links (DPCs). Of these, the biological significance of DPCs remains elusive. In this article, we focus on radiation-induced DPCs and review the current understanding of their induction, properties, repair, and biological consequences. When cells are irradiated, the formation of base damage, SSBs, and DSBs are promoted in the presence of oxygen. Conversely, that of DPCs is promoted in the absence of oxygen, suggesting their importance in hypoxic cells, such as those present in tumors. DNA and protein radicals generated by hydroxyl radicals (i.e., indirect effect) are responsible for DPC formation. In addition, DPCs can also be formed from guanine radical cations generated by the direct effect. Actin, histones, and other proteins have been identified as cross-linked proteins. Also, covalent linkages between DNA and protein constituents such as thymine-lysine and guanine-lysine have been identified and their structures are proposed. In irradiated cells and tissues, DPCs are repaired in a biphasic manner, consisting of fast and slow components. The half-time for the fast component is 20min-2h and that for the slow component is 2-70h. Notably, radiation-induced DPCs are repaired more slowly than DSBs. Homologous recombination plays a pivotal role in the repair of radiation-induced DPCs as well as DSBs. Recently, a novel mechanism of DPC repair mediated by a DPC protease was reported, wherein the resulting DNA-peptide cross-links were bypassed by translesion synthesis. The replication and transcription of DPC-bearing reporter plasmids are inhibited in cells, suggesting that DPCs are potentially lethal lesions. However, whether DPCs are mutagenic and induce gross chromosomal alterations remains to be determined.

Recognition but no repair of abasic site in single-stranded DNA by human ribosomal uS3 protein residing within intact 40S subunit

Isolated human ribosomal protein uS3 has extra-ribosomal functions including those related to base excision DNA repair, e.g. AP lyase activity that nicks double-stranded (ds) DNA 3΄ to the abasic (AP) site. However, the ability of uS3 residing within ribosome to recognize and cleave damaged DNA has never been addressed. Here, we compare interactions of single-stranded (ss) DNA and dsDNA bearing AP site with human ribosome-bound uS3 and with the isolated protein, whose interactions with ssDNA were not yet studied. The AP lyase activity of free uS3 was much higher with ssDNA than with dsDNA, whereas ribosome-bound uS3 was completely deprived of this activity. Nevertheless, an exposed peptide of ribosome-bound uS3 located far away from the putative catalytic center previously suggested for isolated uS3 cross-linked to full-length uncleaved ssDNA, but not to dsDNA. In contrast, free uS3 cross-linked mainly to the 5΄-part of the damaged DNA strand after its cleavage at the AP site. ChIP-seq analysis showed preferential uS3 binding to nucleolus-associated chromatin domains. We conclude that free and ribosome-bound uS3 proteins interact with AP sites differently, exhibiting their non-translational functions in DNA repair in and around the nucleolus and in regulation of DNA damage response in looped DNA structures, respectively.