Replication bypass of the acrolein-mediated deoxyguanine DNA-peptide cross-links by DNA polymerases of the DinB family

Endogenous Cellular Metabolite Methylglyoxal Induces DNA-Protein Cross-Links in Living Cells

Methylglyoxal (MGO) is an electrophilic α-oxoaldehyde generated endogenously through metabolism of carbohydrates and exogenously due to autoxidation of sugars, degradation of lipids, and fermentation during food and drink processing. MGO can react with nucleophilic sites within proteins and DNA to form covalent adducts. MGO-induced advanced glycation end-products such as protein and DNA adducts are thought to be involved in oxidative stress, inflammation, diabetes, cancer, renal failure, and neurodegenerative diseases. Additionally, MGO has been hypothesized to form toxic DNA-protein cross-links (DPC), but the identities of proteins participating in such cross-linking in cells have not been determined. In the present work, we quantified DPC formation in human cells exposed to MGO and identified proteins trapped on DNA upon MGO exposure using mass spectrometry-based proteomics. A total of 265 proteins were found to participate in MGO-derived DPC formation including gene products engaged in telomere organization, nucleosome assembly, and gene expression. In vitro experiments confirmed DPC formation between DNA and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), as well as histone proteins H3.1 and H4. Collectively, our study provides the first evidence for MGO-mediated DNA-protein cross-linking in living cells, prompting future studies regarding the relevance of these toxic lesions in cancer, diabetes, and other diseases linked to elevated MGO levels.

Abasic site-peptide cross-links are blocking lesions repaired by AP endonucleases

Apurinic/apyrimidinic (AP) sites are abundant DNA lesions arising from spontaneous hydrolysis of the N-glycosidic bond and as base excision repair (BER) intermediates. AP sites and their derivatives readily trap DNA-bound proteins, resulting in DNA-protein cross-links. Those are subject to proteolysis but the fate of the resulting AP-peptide cross-links (APPXLs) is unclear. Here, we report two in vitro models of APPXLs synthesized by cross-linking of DNA glycosylases Fpg and OGG1 to DNA followed by trypsinolysis. The reaction with Fpg produces a 10-mer peptide cross-linked through its N-terminus, while OGG1 yields a 23-mer peptide attached through an internal lysine. Both adducts strongly blocked Klenow fragment, phage RB69 polymerase, Saccharolobus solfataricus Dpo4, and African swine fever virus PolX. In the residual lesion bypass, mostly dAMP and dGMP were incorporated by Klenow and RB69 polymerases, while Dpo4 and PolX used primer/template misalignment. Of AP endonucleases involved in BER, Escherichia coli endonuclease IV and its yeast homolog Apn1p efficiently hydrolyzed both adducts. In contrast, E. coli exonuclease III and human APE1 showed little activity on APPXL substrates. Our data suggest that APPXLs produced by proteolysis of AP site-trapped proteins may be removed by the BER pathway, at least in bacterial and yeast cells.

Bypass of Abasic Site-Peptide Cross-Links by Human Repair and Translesion DNA Polymerases

DNA-protein cross-links remain the least-studied type of DNA damage. Recently, their repair was shown to involve proteolysis; however, the fate of the peptide remnant attached to DNA is unclear. Particularly, peptide cross-links could interfere with DNA polymerases. Apurinuic/apyrimidinic (AP) sites, abundant and spontaneously arising DNA lesions, readily form cross-links with proteins. Their degradation products (AP site-peptide cross-links, APPXLs) are non-instructive and should be even more problematic for polymerases. Here, we address the ability of human DNA polymerases involved in DNA repair and translesion synthesis (POLβ, POLλ, POLη, POLκ and PrimPOL) to carry out synthesis on templates containing AP sites cross-linked to the N-terminus of a 10-mer peptide (APPXL-I) or to an internal lysine of a 23-mer peptide (APPXL-Y). Generally, APPXLs strongly blocked processive DNA synthesis. The blocking properties of APPXL-I were comparable with those of an AP site, while APPXL-Y constituted a much stronger obstruction. POLη and POLκ demonstrated the highest bypass ability. DNA polymerases mostly used dNTP-stabilized template misalignment to incorporate nucleotides when encountering an APPXL. We conclude that APPXLs are likely highly cytotoxic and mutagenic intermediates of AP site-protein cross-link repair and must be quickly eliminated before replication.

Synthesis and polymerase bypass studies of DNA-peptide and DNA-protein conjugates

DNA-peptide (DpCs) and DNA-protein cross-links (DPCs) are DNA lesions formed when polypeptides and nuclear proteins become covalently trapped on DNA strands. DNA-protein cross-links are of enormous size and hence pose challenges to cell survival by blocking DNA replication, transcription, and repair. However, DPCs can undergo proteolytic degradation via various pathways to give shorter polypeptide chains (DpCs). The resulting DpC lesions are efficiently bypassed by translesion synthesis (TLS) DNA polymerases like κ, η, δ, etc., although polymerase bypass efficiency as well as correct base insertion depends heavily on size, sequence context, and position of peptides in DpCs. This chapter explores various synthetic methods to generate these lesions including detailed experimental procedures for the construction of DpCs and DPCs via reductive amination and oxime ligation. Further we describe biochemical experiments to investigate the effects of these lesions on DNA polymerase activity and fidelity.

DNA polymerases η and κ bypass N2-guanine-O6-alkylguanine DNA alkyltransferase cross-linked DNA-peptides

DNA-protein cross-links are formed when proteins become covalently trapped with DNA in the presence of exogenous or endogenous alkylating agents. If left unrepaired, they inhibit transcription as well as DNA unwinding during replication and may result in genome instability or even cell death. The DNA repair protein O6-alkylguanine DNA-alkyltransferase (AGT) is known to form DNA cross-links in the presence of the carcinogen 1,2-dibromoethane, resulting in G:C to T:A transversions and other mutations in both bacterial and mammalian cells. We hypothesized that AGT-DNA cross-links would be processed by nuclear proteases to yield peptides small enough to be bypassed by translesion (TLS) polymerases. Here, a 15-mer and a 36-mer peptide from the active site of AGT were cross-linked to the N2 position of guanine via conjugate addition of a thiol containing a peptide dehydroalanine moiety. Bypass studies with DNA polymerases (pols) η and κ indicated that both can accurately bypass the cross-linked DNA peptides. The specificity constant (kcat/Km) for steady-state incorporation of the correct nucleotide dCTP increased by 6-fold with human (h) pol κ and 3-fold with hpol η, with hpol η preferentially inserting nucleotides in the order dC > dG > dA > dT. LC-MS/MS analysis of the extension product also revealed error-free bypass of the cross-linked 15-mer peptide by hpol η. We conclude that a bulky 15-mer AGT peptide cross-linked to the N2 position of guanine can retard polymerization, but that overall fidelity is not compromised because only correct bases are inserted and extended.

Mechanisms of DNA-protein cross-link formation and repair

Covalent binding of DNA to proteins produces DNA-protein cross-links (DPCs). DPCs are formed as intermediates of enzymatic processes, generated from the reactions of protein nucleophiles with DNA electrophiles, and produced by endogenous and exogenous cross-linking agents. DPCs are heterogeneous due to the variations of DNA conjugation sites, flanking DNA structures, protein sizes, and cross-link bonds. Unrepaired DPCs are toxic because their bulky sizes physically block DNA replication and transcription, resulting in impaired genomic integrity. Compared to other types of DNA lesions, DPC repair is less understood. Emerging evidence suggests a general repair model that DPCs are proteolyzed by the proteasome and/or DPC proteases, followed by the peptide removal through canonical repair pathways. Herein, we first describe the recently discovered DPCs. We then review the mechanisms of DPC proteolysis with the focus on recently identified DPC proteases. Finally, distinct pathways that bypass or remove the cross-linked peptides following proteolysis are discussed.

Enzymatic bypass of an N6-deoxyadenosine DNA-ethylene dibromide-peptide cross-link by translesion DNA polymerases

Unrepaired DNA-protein cross-links, due to their bulky nature, can stall replication forks and result in genome instability. Large DNA-protein cross-links can be cleaved into DNA-peptide cross-links, but the extent to which these smaller fragments disrupt normal replication is not clear. Ethylene dibromide (1,2-dibromoethane) is a known carcinogen that can cross-link the repair protein O6-alkylguanine-DNA alkyltransferase (AGT) to the N6 position of deoxyadenosine (dA) in DNA, as well as four other positions in DNA. We investigated the effect of a 15-mer peptide from the active site of AGT, cross-linked to the N6 position of dA, on DNA replication by human translesion synthesis DNA polymerases (Pols) η, ⍳, and κ. The peptide-DNA cross-link was bypassed by the three polymerases at different rates. In steady-state kinetics, the specificity constant (kcat/Km) for incorporation of the correct nucleotide opposite to the adduct decreased by 220-fold with Pol κ, tenfold with pol η, and not at all with Pol ⍳. Pol η incorporated all four nucleotides across from the lesion, with the preference dT > dC > dA > dG, while Pol ⍳ and κ only incorporated the correct nucleotide. However, LC-MS/MS analysis of the primer-template extension product revealed error-free bypass of the cross-linked 15-mer peptide by Pol η. We conclude that a bulky 15-mer peptide cross-linked to the N6 position of dA can retard polymerization and cause miscoding but that overall fidelity is not compromised because only correct pairs are extended.

A hand-off of DNA between archaeal polymerases allows high-fidelity replication to resume at a discrete intermediate three bases past 8-oxoguanine

During DNA replication, the presence of 8-oxoguanine (8-oxoG) lesions in the template strand cause the high-fidelity (HiFi) DNA polymerase (Pol) to stall. An early response to 8-oxoG lesions involves ‘on-the-fly’ translesion synthesis (TLS), in which a specialized TLS Pol is recruited and replaces the stalled HiFi Pol for lesion bypass. The length of TLS must be long enough for effective bypass, but it must also be regulated to minimize replication errors by the TLS Pol. The exact position where the TLS Pol ends and the HiFi Pol resumes (i.e. the length of the TLS patch) has not been described. We use steady-state and pre-steady-state kinetic assays to characterize lesion bypass intermediates formed by different archaeal polymerase holoenzyme complexes that include PCNA123 and RFC. After bypass of 8-oxoG by TLS PolY, products accumulate at the template position three base pairs beyond the lesion. PolY is catalytically poor for subsequent extension from this +3 position beyond 8-oxoG, but this inefficiency is overcome by rapid extension of HiFi PolB1. The reciprocation of Pol activities at this intermediate indicates a defined position where TLS Pol extension is limited and where the DNA substrate is handed back to the HiFi Pol after bypass of 8-oxoG.

How to fix DNA-protein crosslinks

Proteins that act on DNA, or are in close proximity to it, can become inadvertently crosslinked to DNA and form highly toxic lesions, known as DNA-protein crosslinks (DPCs). DPCs are generated by different chemotherapeutics, environmental or endogenous sources of crosslinking agents, or by lesions on DNA that stall the catalytic cycle of certain DNA processing enzymes. These bulky adducts impair processes on DNA such as DNA replication or transcription, and therefore pose a serious threat to genome integrity. The large diversity of DPCs suggests that there is more than one canonical mechanism to repair them. Indeed, many different enzymes have been shown to act on DPCs by either processing the protein, the DNA or the crosslink itself. In addition, the cell cycle stage or cell type are likely to dictate pathway choice. In recent years, a detailed understanding of DPC repair during S phase has started to emerge. Here, we review the current knowledge on the mechanisms of replication-coupled DPC repair, and describe and also speculate on possible pathways that remove DPCs outside of S phase. Moreover, we highlight a recent paradigm shifting finding that indicates that DPCs are not always detrimental, but can also play a protective role, preserving the genome from more deleterious forms of DNA damage.

The roles of polymerases ν and θ in replicative bypass of O6- and N2-alkyl-2'-deoxyguanosine lesions in human cells

Exogenous and endogenous chemicals can react with DNA to produce DNA lesions that may block DNA replication. Not much is known about the roles of polymerase (Pol) ν and Pol θ in translesion synthesis (TLS) in cells. Here we examined the functions of these two polymerases in bypassing major-groove O6-alkyl-2'-deoxyguanosine (O6-alkyl-dG) and minor-groove N2-alkyl-dG lesions in human cells, where the alkyl groups are ethyl, n-butyl (nBu), and, for O6-alkyl-dG, pyridyloxobutyl. We found that Pol ν and Pol θ promote TLS across major-groove O6-alkyl-dG lesions. O6-alkyl-dG lesions mainly induced G→A mutations that were modulated by the two TLS polymerases and the structures of the alkyl groups. Simultaneous ablation of Pol ν and Pol θ resulted in diminished mutation frequencies for all three O6-alkyl-dG lesions. Depletion of Pol ν alone reduced mutations only for O6-nBu-dG, and sole loss of Pol θ attenuated the mutation rates for O6-nBu-dG and O6-pyridyloxobutyl-dG. Replication across the two N2-alkyl-dG lesions was error-free, and Pol ν and Pol θ were dispensable for their replicative bypass. Together, our results provide critical knowledge about the involvement of Pol ν and Pol θ in bypassing alkylated guanine lesions in human cells.

The Impact of Minor-Groove N2-Alkyl-2'-deoxyguanosine Lesions on DNA Replication in Human Cells

Endogenous metabolites and exogenous chemicals can induce covalent modifications on DNA, producing DNA lesions. The N² of guanine was shown to be a common alkylation site in DNA; however, not much is known about the influence of the size of the alkyl group in N²-alkyldG lesions on cellular DNA replication or how translesion synthesis (TLS) polymerases modulate DNA replication past these lesions in human cells. To answer these questions, we employ a robust shuttle vector method to investigate the impact of four N²-alkyldG lesions (i.e., with the alkyl group being a methyl, ethyl, n-propyl, or n-butyl group) on DNA replication in human cells. We find that replication through the N²-alkyldG lesions was highly efficient and accurate in HEK293T cells or isogenic CRISPR-engineered cells with deficiency in polymerase (Pol) ζ or Pol η. Genetic ablation of Pol ι, Pol κ, or Rev1, however, results in decreased bypass efficiencies and elicits substantial frequencies of G → A transition and G → T transversion mutations for these lesions. Moreover, further depletion of Pol ζ in Pol κ- or Pol ι-deficient cells gives rise to elevated rates of G → A and G → T mutations and substantially decreased bypass efficiencies. Cumulatively, we demonstrate that the error-free replication past the N²-alkyldG lesions is facilitated by a specific subset of TLS polymerases, and we find that longer alkyl chains in these lesions induce diminished bypass efficiency and fidelity in DNA replication.

5-Formylcytosine mediated DNA-protein cross-links block DNA replication and induce mutations in human cells

5-Formylcytosine (5fC) is an epigenetic DNA modification introduced via TET protein-mediated oxidation of 5-methyl-dC. We recently reported that 5fC form reversible DNA–protein conjugates (DPCs) with histone proteins in living cells (Ji et al. (2017) Angew. Chem. Int. Ed., 56:14130–14134). We now examined the effects of 5fC mediated DPCs on DNA replication. Synthetic DNA duplexes containing site-specific DPCs between 5fC and lysine-containing proteins and peptides were subjected to primer extension experiments in the presence of human translesion synthesis DNA polymerases η and κ. We found that DPCs containing histones H2A or H4 completely inhibited DNA replication, but the replication block was removed when the proteins were subjected to proteolytic digestion. Cross-links to 11-mer or 31-mer peptides were bypassed by both polymerases in an error-prone manner, inducing targeted C→T transitions and –1 deletions. Similar types of mutations were observed when plasmids containing 5fC-peptide cross-links were replicated in human embryonic kidney (HEK) 293T cells. Molecular simulations of the 11-mer peptide-dC cross-links bound to human polymerases η and κ revealed that the peptide fits well on the DNA major groove side, and the modified dC forms a stable mismatch with incoming dATP via wobble base pairing in the polymerase active site.

Mammalian DNA Polymerase Kappa Activity and Specificity

DNA polymerase (pol) kappa is a Y-family translesion DNA polymerase conserved throughout all domains of life. Pol kappa is special6 ized for the ability to copy DNA containing minor groove DNA adducts, especially N²-dG adducts, as well as to extend primer termini containing DNA damage or mismatched base pairs. Pol kappa generally cannot copy DNA containing major groove modifications or UV-induced photoproducts. Pol kappa can also copy structured or non-B-form DNA, such as microsatellite DNA, common fragile sites, and DNA containing G quadruplexes. Thus, pol kappa has roles both in maintaining and compromising genomic integrity. The expression of pol kappa is altered in several different cancer types, which can lead to genome instability. In addition, many cancer-associated single-nucleotide polymorphisms have been reported in the POLK gene, some of which are associated with poor survival and altered chemotherapy response. Because of this, identifying inhibitors of pol kappa is an active area of research. This review will address these activities of pol kappa, with a focus on lesion bypass and cellular mutagenesis.

Error-prone replication of a 5-formylcytosine-mediated DNA-peptide cross-link in human cells

DNA-protein cross-links can interfere with chromatin architecture, block DNA replication and transcription, and interfere with DNA repair. Here we synthesized a DNA 23-mer containing a site-specific DNA-peptide cross-link (DpC) by cross-linking an 11-mer peptide to the DNA epigenetic mark 5-formylcytosine in synthetic DNA and used it to generate a DpC-containing plasmid construct. Upon replication of the DpC-containing plasmid in HEK 293T cells, approximately 9% of progeny plasmids contained targeted mutations and 5% semitargeted mutations. Targeted mutations included C→T transitions and C deletions, whereas semitargeted mutations included several base substitutions and deletions near the DpC lesion. To identify DNA polymerases involved in DpC bypass, we comparatively studied translesion synthesis (TLS) efficiency and mutagenesis of the DpC in a series of cell lines with TLS polymerase knockouts or knockdowns. Knockdown of either hPol ι or hPol ζ reduced the mutation frequency by nearly 50%. However, the most significant reduction in mutation frequency (50%-70%) was observed upon simultaneous knockout of hPol η and hPol κ with knockdown of hPol ζ, suggesting that these TLS polymerases play a critical role in error-prone DpC bypass. Because TLS efficiency of the DpC construct was not significantly affected in TLS polymerase-deficient cells, we examined a possible role of replicative DNA polymerases in their bypass and determined that hPol δ and hPol ϵ can accurately bypass the DpC. We conclude that both replicative and TLS polymerases can bypass this DpC lesion in human cells but that mutations are induced mainly by TLS polymerases.

DNA replication studies of N-nitroso compound-induced O 6-alkyl-2'-deoxyguanosine lesions in Escherichia coli

N-Nitroso compounds (NOCs) are common DNA-alkylating agents, are abundantly present in food and tobacco, and can also be generated endogenously. Metabolic activation of some NOCs can give rise to carboxymethylation and pyridyloxobutylation/pyridylhydroxybutylation of DNA, which are known to be carcinogenic and can lead to gastrointestinal and lung cancer, respectively. Herein, using the competitive replication and adduct bypass (CRAB) assay, along with MS- and NMR-based approaches, we assessed the cytotoxic and mutagenic properties of three O ⁶-alkyl-2'-deoxyguanosine (O ⁶-alkyl-dG) adducts, i.e. O ⁶-pyridyloxobutyl-dG (O ⁶-POB-dG) and O ⁶-pyridylhydroxybutyl-dG (O ⁶-PHB-dG), derived from tobacco-specific nitrosamines, and O ⁶-carboxymethyl-dG (O ⁶-CM-dG), induced by endogenous N-nitroso compounds. We also investigated two neutral analogs of O ⁶-CM-dG, i.e. O ⁶-aminocarbonylmethyl-dG (O ⁶-ACM-dG) and O ⁶-hydroxyethyl-dG (O ⁶-HOEt-dG). We found that, in Escherichia coli cells, these lesions mildly (O ⁶-POB-dG), moderately (O ⁶-PHB-dG), or strongly (O ⁶-CM-dG, O ⁶-ACM-dG, and O ⁶-HOEt-dG) impede DNA replication. The strong blockage effects of the last three lesions were attributable to the presence of hydrogen-bonding donor(s) located on the alkyl functionality of these lesions. Except for O ⁶-POB-dG, which also induced a low frequency of G → T transversions, all other lesions exclusively stimulated G → A transitions. SOS-induced DNA polymerases played redundant roles in bypassing all the O ⁶-alkyl-dG lesions investigated. DNA polymerase IV (Pol IV) and Pol V, however, were uniquely required for inducing the G → A transition for O ⁶-CM-dG exposure. Together, our study expands our knowledge about the recognition of important NOC-derived O ⁶-alkyl-dG lesions by the E. coli DNA replication machinery.

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.

Replication and repair of a reduced 2΄-deoxyguanosine-abasic site interstrand cross-link in human cells

Apurinic/apyrimidinic (AP) sites, or abasic sites, which are a common type of endogenous DNA damage, can forge interstrand DNA–DNA cross-links via reaction with the exocyclic amino group on a nearby 2΄-deoxyguanosine or 2΄-deoxyadenosine in the opposite strand. Here, we utilized a shuttle vector method to examine the efficiency and fidelity with which a reduced dG–AP cross-link-containing plasmid was replicated in cultured human cells. Our results showed that the cross-link constituted strong impediments to DNA replication in HEK293T cells, with the bypass efficiencies for the dG- and AP-containing strands being 40% and 20%, respectively. While depletion of polymerase (Pol) η did not perturb the bypass efficiency of the lesion, the bypass efficiency was markedly reduced (to 1–10%) in the isogenic cells deficient in Pol κ, Pol ι or Pol ζ, suggesting the mutual involvement of multiple translesion synthesis polymerases in bypassing the lesion. Additionally, replication of the cross-linked AP residue in HEK293T cells was moderately error-prone, inducing a total of ∼26% single-nucleobase substitutions at the lesion site, whereas replication past the cross-linked dG component occurred at a mutation frequency of ∼8%. Together, our results provided important insights into the effects of an AP-derived interstrand cross-link on the efficiency and accuracy of DNA replication in human cells.

Human Y-Family DNA Polymerase κ Is More Tolerant to Changes in Its Active Site Loop than Its Ortholog Escherichia coli DinB

DNA damage is a constant threat and can be bypassed in a process called translesion synthesis, which is typically carried out by Y-family DNA polymerases. Y-family DNA polymerases are conserved in all domains of life and tend to have specificity for certain types of DNA damage. Escherichia coli DinB and its human ortholog pol κ can bypass specific minor groove deoxyguanine adducts efficiently and are inhibited by major groove adducts, as Y-family DNA polymerases make contacts with the minor groove side of the DNA substrate and lack contacts with the major groove at the nascent base pair. DinB is inhibited by major groove adducts more than pol κ, and they each have active site loops of different lengths, with four additional amino acids in the DinB loop. We previously showed that the R35A active site loop mutation in DinB allows for bypass of the major groove adduct N⁶-furfuryl-dA. These observations led us to investigate the different active site loops by creating loop swap chimeras of DinB with a pol κ loop and vice versa by changing the loop residues in a stepwise fashion. We then determined their activity with undamaged DNA or DNA containing N²-furfuryl-dG or N⁶-furfuryl-dA. The DinB proteins with the pol kappa loop have low activity on all templates but have decreased misincorporation compared to either wild-type protein. The kappa proteins with the DinB loop retain activity on all templates and have decreased misincorporation compared to either wild-type protein. We assessed the thermal stability of the proteins and observed an increase in stability in the presence of all DNA templates and additional increases generally only in the presence of the undamaged and N²-furfuryl-dG adduct and dCTP, which correlates with activity. Overall we find that pol κ is more tolerant to changes in the active site loop than DinB.

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.

Mutagenicity of a Model DNA-Peptide Cross-Link in Human Cells: Roles of Translesion Synthesis DNA Polymerases

DNA-protein cross-links are formed upon exposure of cellular DNA to various agents, including antitumor drugs, UV light, transition metals, and reactive oxygen species. They are thought to contribute to cancer, aging, and neurodegenerative diseases. It has been proposed that DNA-protein cross-links formed in cells are subject to proteolytic degradation to the corresponding DNA-peptide cross-links (DpCs). To investigate the effects of DpCs on DNA replication, we have constructed plasmid DNA containing a 10-mer Myc peptide covalently linked to C7 of 7-deaza-dG, a hydrolytically stable mimic of N7-dG lesions. Following transfection in human embryonic kidney cells (HEK 293T), progeny plasmids were recovered and sequenced. Translesion synthesis (TLS) past DpC was 76% compared to that of the unmodified control. The DpC induced 20% targeted G → A and G → T plus 15% semitargeted mutations, notably at a guanine (G₅) five bases 3' to the lesion site. Proteolytic digestion of the DpC reduced the mutation frequency considerably, indicating that the covalently attached 10-mer peptide was responsible for the observed mutations. TLS efficiency and targeted mutations were reduced upon siRNA knockdown of pol η, pol κ, or pol ζ, indicating that they participate in error-prone bypass of the DpC lesion. However, the semitargeted mutation at G₅ was only reduced upon knockdown of pol ζ, suggesting its critical role in this type of mutations. Our results indicate that DpCs formed at the N7 position of guanine can induce both targeted and semitargeted mutations in human cells and that the TLS polymerases play a critical role in their error-prone bypass.

Bypass of DNA-Protein Cross-links Conjugated to the 7-Deazaguanine Position of DNA by Translesion Synthesis Polymerases

DNA-protein cross-links (DPCs) are bulky DNA lesions that form both endogenously and following exposure to bis-electrophiles such as common antitumor agents. The structural and biological consequences of DPCs have not been fully elucidated due to the complexity of these adducts. The most common site of DPC formation in DNA following treatment with bis-electrophiles such as nitrogen mustards and cisplatin is the N7 position of guanine, but the resulting conjugates are hydrolytically labile and thus are not suitable for structural and biological studies. In this report, hydrolytically stable structural mimics of N7-guanine-conjugated DPCs were generated by reductive amination reactions between the Lys and Arg side chains of proteins/peptides and aldehyde groups linked to 7-deazaguanine residues in DNA. These model DPCs were subjected to in vitro replication in the presence of human translesion synthesis DNA polymerases. DPCs containing full-length proteins (11-28 kDa) or a 23-mer peptide blocked human polymerases η and κ. DPC conjugates to a 10-mer peptide were bypassed with nucleotide insertion efficiency 50-100-fold lower than for native G. Both human polymerase (hPol) κ and hPol η inserted the correct base (C) opposite the 10-mer peptide cross-link, although small amounts of T were added by hPol η. Molecular dynamics simulation of an hPol κ ternary complex containing a template-primer DNA with dCTP opposite the 10-mer peptide DPC revealed that this bulky lesion can be accommodated in the polymerase active site by aligning with the major groove of the adducted DNA within the ternary complex of polymerase and dCTP.

Involvement of translesion synthesis DNA polymerases in DNA interstrand crosslink repair

DNA interstrand crosslinks (ICLs) covalently join the two strands of a DNA duplex and block essential processes such as DNA replication and transcription. Several important anti-tumor drugs such as cisplatin and nitrogen mustards exert their cytotoxicity by forming ICLs. However, multiple complex pathways repair ICLs and these are thought to contribute to the development of resistance towards ICL-inducing agents. While the understanding of many aspects of ICL repair is still rudimentary, studies in recent years have provided significant insights into the pathways of ICL repair. In this perspective we review the recent advances made in elucidating the mechanisms of ICL repair with a focus on the role of TLS polymerases. We describe the emerging models for how these enzymes contribute to and are regulated in ICL repair, discuss the key open questions and examine the implications for this pathway in anti-cancer therapy.

Mechanisms of mutagenesis: DNA replication in the presence of DNA damage

Environmental mutagens cause DNA damage that disturbs replication and produces mutations, leading to cancer and other diseases. We discuss mechanisms of mutagenesis resulting from DNA damage, from the level of DNA replication by a single polymerase to the complex DNA replisome of some typical model organisms (including bacteriophage T7, T4, Sulfolobus solfataricus, Escherichia coli, yeast and human). For a single DNA polymerase, DNA damage can affect replication in three major ways: reducing replication fidelity, causing frameshift mutations, and blocking replication. For the DNA replisome, protein interactions and the functions of accessory proteins can yield rather different results even with a single DNA polymerase. The mechanism of mutation during replication performed by the DNA replisome is a long-standing question. Using new methods and techniques, the replisomes of certain organisms and human cell extracts can now be investigated with regard to the bypass of DNA damage. In this review, we consider the molecular mechanism of mutagenesis resulting from DNA damage in replication at the levels of single DNA polymerases and complex DNA replisomes, including translesion DNA synthesis.

The contribution of translesion synthesis polymerases on geminiviral replication

Geminiviruses multiply primarily in the plant phloem, but never in meristems. Their Rep protein can activate DNA synthesis in differentiated cells. However, when their single-stranded DNA is injected into the phloem by insects, no Rep is present for inducing initial complementary strand replication. Considering a contribution of translesion synthesis (TLS) polymerases in plants, four of them (Polη, Polζ, Polκ, Rev1) are highly and constitutively expressed in differentiated tissues like the phloem. Two geminiviruses (Euphorbia yellow mosaic virus, Cleome leaf crumple virus), inoculated either biolistically or by whiteflies, replicated in Arabidopsis thaliana mutant lines of these genes to the same extent as in wild type plants. Comparative deep sequencing of geminiviral DNAs, however, showed a high exchange rate (10(-4)-10(-3)) similar to the phylogenetic variation described before and a significant difference in nucleotide substation rates if Polη and Polζ were absent, with a differential response to the viral DNA components.

Translesion DNA Synthesis

All living organisms are continually exposed to agents that damage their DNA, which threatens the integrity of their genome. As a consequence, cells are equipped with a plethora of DNA repair enzymes to remove the damaged DNA. Unfortunately, situations nevertheless arise where lesions persist, and these lesions block the progression of the cell's replicase. In these situations, cells are forced to choose between recombination-mediated "damage avoidance" pathways or a specialized DNA polymerase (pol) to traverse the blocking lesion. The latter process is referred to as Translesion DNA Synthesis (TLS). As inferred by its name, TLS not only results in bases being (mis)incorporated opposite DNA lesions but also bases being (mis)incorporated downstream of the replicase-blocking lesion, so as to ensure continued genome duplication and cell survival. Escherichia coli and Salmonella typhimurium possess five DNA polymerases, and while all have been shown to facilitate TLS under certain experimental conditions, it is clear that the LexA-regulated and damage-inducible pols II, IV, and V perform the vast majority of TLS under physiological conditions. Pol V can traverse a wide range of DNA lesions and performs the bulk of mutagenic TLS, whereas pol II and pol IV appear to be more specialized TLS polymerases.

DNA-Protein Cross-Links: Formation, Structural Identities, and Biological Outcomes

Noncovalent DNA-protein interactions are at the heart of normal cell function. In eukaryotic cells, genomic DNA is wrapped around histone octamers to allow for chromosomal packaging in the nucleus. Binding of regulatory protein factors to DNA directs replication, controls transcription, and mediates cellular responses to DNA damage. Because of their fundamental significance in all cellular processes involving DNA, dynamic DNA-protein interactions are required for cell survival, and their disruption is likely to have serious biological consequences. DNA-protein cross-links (DPCs) form when cellular proteins become covalently trapped on DNA strands upon exposure to various endogenous, environmental and chemotherapeutic agents. DPCs progressively accumulate in the brain and heart tissues as a result of endogenous exposure to reactive oxygen species and lipid peroxidation products, as well as normal cellular metabolism. A range of structurally diverse DPCs are found following treatment with chemotherapeutic drugs, transition metal ions, and metabolically activated carcinogens. Because of their considerable size and their helix-distorting nature, DPCs interfere with the progression of replication and transcription machineries and hence hamper the faithful expression of genetic information, potentially contributing to mutagenesis and carcinogenesis. Mass spectrometry-based studies have identified hundreds of proteins that can become cross-linked to nuclear DNA in the presence of reactive oxygen species, carcinogen metabolites, and antitumor drugs. While many of these proteins including histones, transcription factors, and repair proteins are known DNA binding partners, other gene products with no documented affinity for DNA also participate in DPC formation. Furthermore, multiple sites within DNA can be targeted for cross-linking including the N7 of guanine, the C-5 methyl group of thymine, and the exocyclic amino groups of guanine, cytosine, and adenine. This structural complexity complicates structural and biological studies of DPC lesions. Two general strategies have been developed for creating DNA strands containing structurally defined, site-specific DPCs. Enzymatic methodologies that trap DNA modifying proteins on their DNA substrate are site specific and efficient, but do not allow for systematic studies of DPC lesion structure on their biological outcomes. Synthetic methodologies for DPC formation are based on solid phase synthesis of oligonucleotide strands containing protein-reactive unnatural DNA bases. The latter approach allows for a wider range of protein substrates to be conjugated to DNA and affords a greater flexibility for the attachment sites within DNA. In this Account, we outline the chemistry of DPC formation in cells, describe our recent efforts to identify the cross-linked proteins by mass spectrometry, and discuss various methodologies for preparing DNA strands containing structurally defined, site specific DPC lesions. Polymerase bypass experiments conducted with model DPCs indicate that the biological outcomes of these bulky lesions are strongly dependent on the peptide/protein size and the exact cross-linking site within DNA. Future studies are needed to elucidate the mechanisms of DPC repair and their biological outcomes in living cells.

Quantification of pyrophosphate as a universal approach to determine polymerase activity and assay polymerase inhibitors

The importance of DNA polymerases in biology and biotechnology, and their recognition as potential therapeutic targets, drives development of methods for deriving kinetic characteristics of polymerases and their propensity to perform polynucleotide synthesis over modified DNA templates. Among various polymerases, translesion synthesis (TLS) polymerases enable cells to avoid the cytotoxic stalling of replicative DNA polymerases at chemotherapy-induced DNA lesions, thereby leading to drug resistance. Identification of TLS inhibitors to overcome drug-resistance necessitates the development of appropriate high-throughput assays. Since polymerase-mediated DNA synthesis involves the release of inorganic pyrophosphate (PPi), we established a universal and fast method for monitoring the progress of DNA polymerases based on the quantification of PPi with a fluorescence-based assay that we coupled to in vitro primer extension reactions. The established assay has a nanomolar detection limit in PPi and enables the evaluation of single nucleotide incorporation and DNA synthesis progression kinetics. The results demonstrated that the developed assay is a reliable method for monitoring TLS and identifying nucleoside and nucleotide-based TLS inhibitors.

Error-prone translesion synthesis past DNA-peptide cross-links conjugated to the major groove of DNA via C5 of thymidine

DNA-protein cross-links (DPCs) are exceptionally bulky, structurally diverse DNA adducts formed in cells upon exposure to endogenous and exogenous bis-electrophiles, reactive oxygen species, and ionizing radiation. If not repaired, DPCs can induce toxicity and mutations. It has been proposed that the protein component of a DPC is proteolytically degraded, giving rise to smaller DNA-peptide conjugates, which can be subject to nucleotide excision repair and replication bypass. In this study, polymerase bypass of model DNA-peptide conjugates structurally analogous to the lesions induced by reactive oxygen species and DNA methyltransferase inhibitors was examined. DNA oligomers containing site-specific DNA-peptide conjugates were generated by copper-catalyzed [3 + 2] Huisgen cyclo-addition between an alkyne-functionalized C5-thymidine in DNA and an azide-containing 10-mer peptide. The resulting DNA-peptide conjugates were subjected to steady-state kinetic experiments in the presence of recombinant human lesion bypass polymerases κ and η, followed by PAGE-based assays to determine the catalytic efficiency and the misinsertion frequency opposite the lesion. We found that human polymerase κ and η can incorporate A, G, C, or T opposite the C5-dT-conjugated DNA-peptide conjugates, whereas human polymerase η preferentially inserts G opposite the lesion. Furthermore, HPLC-ESI(-)-MS/MS sequencing of the extension products has revealed that post-lesion synthesis was highly error-prone, resulting in mutations opposite the adducted site or at the +1 position from the adduct and multiple deletions. Collectively, our results indicate that replication bypass of peptides conjugated to the C5 position of thymine by human translesion synthesis polymerases leads to large numbers of base substitution and frameshift mutations.

Mechanism of repair of acrolein- and malondialdehyde-derived exocyclic guanine adducts by the α-ketoglutarate/Fe(II) dioxygenase AlkB

The structurally related exocyclic guanine adducts α-hydroxypropano-dG (α-OH-PdG), γ-hydroxypropano-dG (γ-OH-PdG), and M₁dG are formed when DNA is exposed to the reactive aldehydes acrolein and malondialdehyde (MDA). These lesions are believed to form the basis for the observed cytotoxicity and mutagenicity of acrolein and MDA. In an effort to understand the enzymatic pathways and chemical mechanisms that are involved in the repair of acrolein- and MDA-induced DNA damage, we investigated the ability of the DNA repair enzyme AlkB, an α-ketoglutarate/Fe(II) dependent dioxygenase, to process α-OH-PdG, γ-OH-PdG, and M₁dG in both single- and double-stranded DNA contexts. By monitoring the repair reactions using quadrupole time-of-flight (Q-TOF) mass spectrometry, it was established that AlkB can oxidatively dealkylate γ-OH-PdG most efficiently, followed by M₁dG and α-OH-PdG. The AlkB repair mechanism involved multiple intermediates and complex, overlapping repair pathways. For example, the three exocyclic guanine adducts were shown to be in equilibrium with open-ring aldehydic forms, which were trapped using (pentafluorobenzyl)hydroxylamine (PFBHA) or NaBH₄. AlkB repaired the trapped open-ring form of γ-OH-PdG but not the trapped open-ring of α-OH-PdG. Taken together, this study provides a detailed mechanism by which three-carbon bridge exocyclic guanine adducts can be processed by AlkB and suggests an important role for the AlkB family of dioxygenases in protecting against the deleterious biological consequences of acrolein and MDA.

Synthesis of site-specific DNA-protein conjugates and their effects on DNA replication

DNA–protein cross-links (DPCs) are bulky, helix-distorting DNA lesions that form in the genome upon exposure to common antitumor drugs, environmental/occupational toxins, ionizing radiation, and endogenous free-radical-generating systems. As a result of their considerable size and their pronounced effects on DNA–protein interactions, DPCs can interfere with DNA replication, transcription, and repair, potentially leading to mutagenesis, genotoxicity, and cytotoxicity. However, the biological consequences of these ubiquitous lesions are not fully understood due to the difficulty of generating DNA substrates containing structurally defined, site-specific DPCs. In the present study, site-specific cross-links between the two biomolecules were generated by copper-catalyzed [3 + 2] Huisgen cycloaddition (click reaction) between an alkyne group from 5-(octa-1,7-diynyl)-uracil in DNA and an azide group within engineered proteins/polypeptides. The resulting DPC substrates were subjected to in vitro primer extension in the presence of human lesion bypass DNA polymerases η, κ, ν, and ι. We found that DPC lesions to the green fluorescent protein and a 23-mer peptide completely blocked DNA replication, while the cross-link to a 10-mer peptide was bypassed. These results indicate that the polymerases cannot read through the larger DPC lesions and further suggest that proteolytic degradation may be required to remove the replication block imposed by bulky DPC adducts.

DNA polymerase IV mediates efficient and quick recovery of replication forks stalled at N2-dG adducts

Escherichia coli DNA polymerase IV (Pol IV, also known as DinB) is a Y-family DNA polymerase capable of catalyzing translesion DNA synthesis (TLS) on certain DNA lesions, and accumulating data suggest that Pol IV may play an important role in copying various kinds of spontaneous DNA damage including N²-dG adducts and alkylated bases. Pol IV has a unique ability to coexist with Pol III on the same β clamp and to positively dissociate Pol III from β clamp in a concentration-dependent manner. Reconstituting the entire process of TLS in vitro using E. coli replication machinery and Pol IV, we observed that a replication fork stalled at (−)-trans-anti-benzo[a]pyrene-N²-dG lesion on the leading strand was efficiently and quickly recovered via two sequential switches from Pol III to Pol IV and back to Pol III. Our results suggest that TLS by Pol IV smoothes the way for the replication fork with minimal interruption.

The DinB•RecA complex of Escherichia coli mediates an efficient and high-fidelity response to ubiquitous alkylation lesions

Alkylation DNA lesions are ubiquitous, and result from normal cellular metabolism as well as from treatment with methylating agents and chemotherapeutics. DNA damage tolerance by translesion synthesis DNA polymerases has an important role in cellular resistance to alkylating agents. However, it is not yet known whether Escherichia coli (E. coli) DNA Pol IV (DinB) alkylation lesion bypass efficiency and fidelity in vitro are similar to those inferred by genetic analyses. We hypothesized that DinB-mediated bypass of 3-deaza-3-methyladenine, a stable analog of 3-methyladenine, the primary replication fork-stalling alkylation lesion, would be of high fidelity. We performed here the first kinetic analyses of E. coli DinB•RecA binary complexes. Whether alone or in a binary complex, DinB inserted the correct deoxyribonucleoside triphosphate (dNTP) opposite either lesion-containing or undamaged template; the incorporation of other dNTPs was largely inefficient. DinB prefers undamaged DNA, but the DinB•RecA binary complex increases its catalytic efficiency on lesion-containing template, perhaps as part of a regulatory mechanism to better respond to alkylation damage. Notably, we find that a DinB derivative with enhanced affinity for RecA, either alone or in a binary complex, is less efficient and has a lower fidelity than DinB or DinB•RecA. This finding contrasts our previous genetic analyses. Therefore, mutagenesis resulting from alkylation lesions is likely limited in cells by the activity of DinB•RecA. These two highly conserved proteins play an important role in maintaining genomic stability when cells are faced with ubiquitous DNA damage. Kinetic analyses are important to gain insights into the mechanism(s) regulating TLS DNA polymerases.

Advances in understanding the complex mechanisms of DNA interstrand cross-link repair

DNA interstrand cross-links (ICLs) are lesions caused by a variety of endogenous metabolites, environmental exposures, and cancer chemotherapeutic agents that have two reactive groups. The common feature of these diverse lesions is that two nucleotides on opposite strands are covalently joined. ICLs prevent the separation of two DNA strands and therefore essential cellular processes including DNA replication and transcription. ICLs are mainly detected in S phase when a replication fork stalls at an ICL. Damage signaling and repair of ICLs are promoted by the Fanconi anemia pathway and numerous posttranslational modifications of DNA repair and chromatin structural proteins. ICLs are also detected and repaired in nonreplicating cells, although the mechanism is less clear. A unique feature of ICL repair is that both strands of DNA must be incised to completely remove the lesion. This is accomplished in sequential steps to prevent creating multiple double-strand breaks. Unhooking of an ICL from one strand is followed by translesion synthesis to fill the gap and create an intact duplex DNA, harboring a remnant of the ICL. Removal of the lesion from the second strand is likely accomplished by nucleotide excision repair. Inadequate repair of ICLs is particularly detrimental to rapidly dividing cells, explaining the bone marrow failure characteristic of Fanconi anemia and why cross-linking agents are efficacious in cancer therapy. Herein, recent advances in our understanding of ICLs and the biological responses they trigger are discussed.

Ring-opening of the γ-OH-PdG adduct promotes error-free bypass by the Sulfolobus solfataricus DNA polymerase Dpo4

Acrolein, a mutagenic aldehyde, reacts with deoxyguanosine (dG) to form 3-(2'-deoxy-β-d-erythro-pentofuranosyl)-5,6,7,8-tetrahydro-8-hydroxypyrimido[1,2-a] purin-10(3H)-one (γ-OH-PdG). When placed opposite deoxycytosine (dC) in DNA, γ-OH-PdG undergoes ring-opening to the N(2)-(3-oxopropyl)-dG. Ring-opening of the adduct has been hypothesized to facilitate nonmutagenic bypass, particularly by DNA polymerases of the Y family. This study examined the bypass of γ-OH-PdG by Sulfolobus solfataricus Dpo4, the prototypic Y-family DNA polymerase, using templates that contained the adduct in either the 5'-CXG-3' or the 5'-TXG-3' sequence context. Although γ-OH-PdG partially blocked Dpo4-catalyzed DNA synthesis, full primer extension was observed, and the majority of bypass products were error-free. Conversion of the adduct into an irreversibly ring-opened derivative prior to reaction facilitated bypass and further improved the fidelity. Structures of ternary Dpo4·DNA·dNTP complexes were determined with primers that either were positioned immediately upstream of the lesion (preinsertion complexes) or had a 3'-terminal dC opposite the lesion (postinsertion complexes); the incoming nucleotides, either dGTP or dATP, were complementary to the template 5'-neighbor nucleotide. In both postinsertion complexes, the adduct existed as ring-opened species, and the resulting base-pair featured Watson-Crick hydrogen bonding. The incoming nucleotide paired with the 5'-neighbor template, while the primer 3'-hydroxyl was positioned to facilitate extension. In contrast, γ-OH-PdG was in the ring-closed form in both preinsertion complexes, and the overall structure did not favor catalysis. These data provide insights into γ-OH-PdG chemistry during replication bypass by the Dpo4 DNA polymerase and may explain why γ-OH-PdG-induced mutations due to primer-template misalignment are uncommon.

Discrimination against major groove adducts by Y-family polymerases of the DinB subfamily

Y-family DNA polymerases bypass DNA adducts in a process known as translesion synthesis (TLS). Y-family polymerases make contacts with the minor groove side of the DNA substrate at the nascent base pair. The Y-family polymerases also contact the DNA major groove via the unique little finger domain, but they generally lack contacts with the major groove at the nascent base pair. Escherichia coli DinB efficiently and accurately copies certain minor groove guanosine adducts. In contrast, we previously showed that the presence in the DNA template of the major groove-modified base 1,3-diaza-2-oxophenothiazine (tC) inhibits the activity of E. coli DinB. Even when the DNA primer is extended up to three nucleotides beyond the site of the tC analog, DinB activity is strongly inhibited. These findings prompted us to investigate discrimination against other major groove modifications by DinB and its orthologs. We chose a set of pyrimidines and purines with modifications in the major groove and determined the activity of DinB and several orthologs with these substrates. DinB, human pol kappa, and Sulfolobus solfataricus Dpo4 show differing specificities for the major groove adducts pyrrolo-dC, dP, N(6)-furfuryl-dA, and etheno-dA. In general, DinB was least efficient for bypass of all of these major groove adducts, whereas Dpo4 was most efficient. DinB activity was essentially completely inhibited by the presence of etheno-dA, while pol kappa activity was strongly inhibited. All three of these DNA polymerases were able to bypass N(6)-furfuryl-dA with modest efficiency, with DinB being the least efficient. We also determined that the R35A variant of DinB enhances bypass of N(6)-furfuryl-dA but not etheno-dA. In sum, we find that whereas DinB is specific for bypass of minor groove adducts, it is specifically inhibited by major groove DNA modifications.

Human polymerase kappa uses a template-slippage deletion mechanism, but can realign the slipped strands to favour base substitution mutations over deletions

Polymerases belonging to the DinB class of the Y-family translesion synthesis DNA polymerases have a preference for accurately and efficiently bypassing damaged guanosines. These DinB polymerases also generate single-base (-1) deletions at high frequencies with most occurring on repetitive 'deletion hotspot' sequences. Human DNA polymerase kappa (hPolκ), the eukaryotic DinB homologue, displays an unusual efficiency for to extend from mispaired primer termini, either by extending directly from the mispair or by primer-template misalignment. This latter property explains how hPolκ creates single-base deletions in non-repetitive sequences, but does not address how deletions occur in repetitive deletion hotspots. Here, we show that hPolκ uses a classical Streisinger template-slippage mechanism to generate -1 deletions in repetitive sequences, as do the bacterial and archaeal homologues. After the first nucleotide is added by template slippage, however, hPolκ can efficiently realign the primer-template duplex before continuing DNA synthesis. Strand realignment results in a base-substitution mutation, minimizing generation of more deleterious frameshift mutations. On non-repetitive sequences, we find that nucleotide misincorporation is slower if the incoming nucleotide can correctly basepair with the nucleotide immediately 5' to the templating base, thereby competing against the mispairing with the templating base.

Translesion synthesis past acrolein-derived DNA adducts by human mitochondrial DNA polymerase γ

Acrolein, a mutagenic aldehyde, is produced endogenously by lipid peroxidation and exogenously by combustion of organic materials, including tobacco products. Acrolein reacts with DNA bases forming exocyclic DNA adducts, such as γ-hydroxy-1,N(2)-propano-2'-deoxyguanosine (γ-HOPdG) and γ-hydroxy-1,N(6)-propano-2'-deoxyadenosine (γ-HOPdA). The bulky γ-HOPdG adduct blocks DNA synthesis by replicative polymerases but can be bypassed by translesion synthesis polymerases in the nucleus. Although acrolein-induced adducts are likely to be formed and persist in mitochondrial DNA, animal cell mitochondria lack specialized translesion DNA synthesis polymerases to tolerate these lesions. Thus, it is important to understand how pol γ, the sole mitochondrial DNA polymerase in human cells, acts on acrolein-adducted DNA. To address this question, we investigated the ability of pol γ to bypass the minor groove γ-HOPdG and major groove γ-HOPdA adducts using single nucleotide incorporation and primer extension analyses. The efficiency of pol γ-catalyzed bypass of γ-HOPdG was low, and surprisingly, pol γ preferred to incorporate purine nucleotides opposite the adduct. Pol γ also exhibited ∼2-fold lower rates of excision of the misincorporated purine nucleotides opposite γ-HOPdG compared with the corresponding nucleotides opposite dG. Extension of primers from the termini opposite γ-HOPdG was accomplished only following error-prone purine nucleotide incorporation. However, pol γ preferentially incorporated dT opposite the γ-HOPdA adduct and efficiently extended primers from the correctly paired terminus, indicating that γ-HOPdA is probably nonmutagenic. In summary, our data suggest that acrolein-induced exocyclic DNA lesions can be bypassed by mitochondrial DNA polymerase but, in the case of the minor groove γ-HOPdG adduct, at the cost of unprecedented high mutation rates.

Multiple strategies for translesion synthesis in bacteria

Damage to DNA is common and can arise from numerous environmental and endogenous sources. In response to ubiquitous DNA damage, Y-family DNA polymerases are induced by the SOS response and are capable of bypassing DNA lesions. In Escherichia coli, these Y-family polymerases are DinB and UmuC, whose activities are modulated by their interaction with the polymerase manager protein UmuD. Many, but not all, bacteria utilize DinB and UmuC homologs. Recently, a C-family polymerase named ImuC, which is similar in primary structure to the replicative DNA polymerase DnaE, was found to be able to copy damaged DNA and either carry out or suppress mutagenesis. ImuC is often found with proteins ImuA and ImuB, the latter of which is similar to Y‑family polymerases, but seems to lack the catalytic residues necessary for polymerase activity. This imuAimuBimuC mutagenesis cassette represents a widespread alternative strategy for translesion synthesis and mutagenesis in bacteria. Bacterial Y‑family and ImuC DNA polymerases contribute to replication past DNA damage and the acquisition of antibiotic resistance.

A comprehensive strategy to discover inhibitors of the translesion synthesis DNA polymerase κ

Human DNA polymerase kappa (pol κ) is a translesion synthesis (TLS) polymerase that catalyzes TLS past various minor groove lesions including N(2)-dG linked acrolein- and polycyclic aromatic hydrocarbon-derived adducts, as well as N(2)-dG DNA-DNA interstrand cross-links introduced by the chemotherapeutic agent mitomycin C. It also processes ultraviolet light-induced DNA lesions. Since pol κ TLS activity can reduce the cellular toxicity of chemotherapeutic agents and since gliomas overexpress pol κ, small molecule library screens targeting pol κ were conducted to initiate the first step in the development of new adjunct cancer therapeutics. A high-throughput, fluorescence-based DNA strand displacement assay was utilized to screen ∼16,000 bioactive compounds, and the 60 top hits were validated by primer extension assays using non-damaged DNAs. Candesartan cilexetil, manoalide, and MK-886 were selected as proof-of-principle compounds and further characterized for their specificity toward pol κ by primer extension assays using DNAs containing a site-specific acrolein-derived, ring-opened reduced form of γ-HOPdG. Furthermore, candesartan cilexetil could enhance ultraviolet light-induced cytotoxicity in xeroderma pigmentosum variant cells, suggesting its inhibitory effect against intracellular pol κ. In summary, this investigation represents the first high-throughput screening designed to identify inhibitors of pol κ, with the characterization of biochemical and biologically relevant endpoints as a consequence of pol κ inhibition. These approaches lay the foundation for the future discovery of compounds that can be applied to combination chemotherapy.

New discoveries linking transcription to DNA repair and damage tolerance pathways

In Escherichia coli, the transcription elongation factor NusA is associated with all elongating RNA polymerases where it functions in transcription termination and antitermination. Here, we review our recent results implicating NusA in the recruitment of DNA repair and damage tolerance mechanisms to sites of stalled transcription complexes.

Role of high-fidelity Escherichia coli DNA polymerase I in replication bypass of a deoxyadenosine DNA-peptide cross-link

Reaction of bifunctional electrophiles with DNA in the presence of peptides can result in DNA-peptide cross-links. In particular, the linkage can be formed in the major groove of DNA via the exocyclic amino group of adenine (N⁶-dA). We previously demonstrated that an A family human polymerase, Pol ν, can efficiently and accurately synthesize DNA past N⁶-dA-linked peptides. Based on these results, we hypothesized that another member of that family, Escherichia coli polymerase I (Pol I), may also be able to bypass these large major groove DNA lesions. To test this, oligodeoxynucleotides containing a site-specific N⁶-dA dodecylpeptide cross-link were created and utilized for in vitro DNA replication assays using E. coli DNA polymerases. The results showed that Pol I and Pol II could efficiently and accurately bypass this adduct, while Pol III replicase, Pol IV, and Pol V were strongly inhibited. In addition, cellular studies were conducted using E. coli strains that were either wild type or deficient in all three DNA damage-inducible polymerases, i.e., Pol II, Pol IV, and Pol V. When single-stranded DNA vectors containing a site-specific N⁶-dA dodecylpeptide cross-link were replicated in these strains, the efficiencies of replication were comparable, and in both strains, intracellular bypass of the lesion occurred in an error-free manner. Collectively, these findings demonstrate that despite its constrained active site, Pol I can catalyze DNA synthesis past N⁶-dA-linked peptide cross-links and is likely to play an essential role in cellular bypass of large major groove DNA lesions.

γ-Hydroxy-1,N2-propano-2'-deoxyguanosine DNA adduct conjugates the N-terminal amine of the KWKK peptide via a carbinolamine linkage

The γ-hydroxy-1,N(2)-propano-2'-deoxyguanosine adduct (γ-OH-PdG) was introduced into 5'-d(GCTAGCXAGTCC)-3'·5'-d(GGACTCGCTAGC)-3' (X = γ-OH-PdG). In the presence of excess peptide KWKK, (13)C isotope-edited NMR revealed the formation of two spectroscopically distinct DNA-KWKK conjugates. These involved the reaction of the KWKK N-terminal amino group with the N(2)-dG propylaldehyde tautomer of the γ-OH-PdG lesion. The guanine N1 base imino resonance at the site of conjugation was observed in isotope-edited (15)N NMR experiments, suggesting that the conjugated guanine was inserted into the duplex and that the guanine imino proton was protected from exchange with water. The conjugates could be reduced in the presence of NaCNBH(3), suggesting that they existed, in part, as imine (Schiff base) linkages. However, (13)C isotope-edited NMR failed to detect the imine linkages, suggesting that these KWKK conjugates existed predominantly as diastereomeric carbinolamines, in equilibrium with trace amounts of the imines. The structures of the diastereomeric DNA-KWKK conjugates were predicted from potential energy minimization of model structures derived from the refined structure of the fully reduced cross-link [ Huang, H., Kozekov, I. D., Kozekova, A., Rizzo, C. J., McCullough, A., Lloyd, R. S., and Stone, M. P. ( 2010 ) Biochemistry , 49 , 6155 -6164 ]. Molecular dynamics calculations carried out in explicit solvent suggested that the conjugate bearing the S-carbinolamine linkage was the major species due to its potential for intramolecular hydrogen bonding. These carbinolamine DNA-KWKK conjugates thermally stabilized duplex DNA. However, the DNA-KWKK conjugates were chemically reversible and dissociated when the DNA was denatured. In this 5'-CpX-3' sequence, the DNA-KWKK conjugates slowly converted to interstrand N(2)-dG:N(2)-dG DNA cross-links and ring-opened γ-OH-PdG derivatives over a period of weeks.

Epistatic roles for Pseudomonas aeruginosa MutS and DinB (DNA Pol IV) in coping with reactive oxygen species-induced DNA damage

Pseudomonas aeruginosa is especially adept at colonizing the airways of individuals afflicted with the autosomal recessive disease cystic fibrosis (CF). CF patients suffer from chronic airway inflammation, which contributes to lung deterioration. Once established in the airways, P. aeruginosa continuously adapts to the changing environment, in part through acquisition of beneficial mutations via a process termed pathoadaptation. MutS and DinB are proposed to play opposing roles in P. aeruginosa pathoadaptation: MutS acts in replication-coupled mismatch repair, which acts to limit spontaneous mutations; in contrast, DinB (DNA polymerase IV) catalyzes error-prone bypass of DNA lesions, contributing to mutations. As part of an ongoing effort to understand mechanisms underlying P. aeruginosa pathoadaptation, we characterized hydrogen peroxide (H₂O₂)-induced phenotypes of isogenic P. aeruginosa strains bearing different combinations of mutS and dinB alleles. Our results demonstrate an unexpected epistatic relationship between mutS and dinB with respect to H₂O₂-induced cell killing involving error-prone repair and/or tolerance of oxidized DNA lesions. In striking contrast to these error-prone roles, both MutS and DinB played largely accurate roles in coping with DNA lesions induced by ultraviolet light, mitomycin C, or 4-nitroquinilone 1-oxide. Models discussing roles for MutS and DinB functionality in DNA damage-induced mutagenesis, particularly during CF airway colonization and subsequent P. aeruginosa pathoadaptation are discussed.

DNA polymerases and cancer

There are 15 different DNA polymerases encoded in mammalian genomes, which are specialized for replication, repair or the tolerance of DNA damage. New evidence is emerging for lesion-specific and tissue-specific functions of DNA polymerases. Many point mutations that occur in cancer cells arise from the error-generating activities of DNA polymerases. However, the ability of some of these enzymes to bypass DNA damage may actually defend against chromosome instability in cells, and at least one DNA polymerase, Pol ζ, is a suppressor of spontaneous tumorigenesis. Because DNA polymerases can help cancer cells tolerate DNA damage, some of these enzymes might be viable targets for therapeutic strategies.

Minor groove orientation of the KWKK peptide tethered via the N-terminal amine to the acrolein-derived 1,N2-gamma-hydroxypropanodeoxyguanosine lesion with a trimethylene linkage

DNA−protein conjugates are potentially repaired via proteolytic digestion to DNA−peptide conjugates. The latter have been modeled with the amino-terminal lysine of the peptide KWKK conjugated via a trimethylene linkage to the N²-dG amine positioned in 5′-d(GCTAGCXAGTCC)-3′·5′-d(GGACTCGCTAGC)-3′ (X = N²-dG−trimethylene link−KWKK). This linkage is a surrogate for the reversible linkage formed by the γ-OH-1,N²-propanodeoxyguanosine (γ-OH-PdG) adduct. This conjugated KWKK stabilizes the DNA. Amino acids K²⁶, W²⁷, K²⁸, and K²⁹ are in the minor groove. The W²⁷ indolyl group does not intercalate into the DNA. The G⁷N² amine and the K²⁶ N-terminal amine nitrogens are in the trans configuration with respect to the C_α or C_γ of the trimethylene tether, respectively. The structure of this DNA−KWKK conjugate is discussed in the context of its biological processing.

Roles for the transcription elongation factor NusA in both DNA repair and damage tolerance pathways in Escherichia coli

We report observations suggesting that the transcription elongation factor NusA promotes a previously unrecognized class of transcription-coupled repair (TCR) in addition to its previously proposed role in recruiting translesion synthesis (TLS) DNA polymerases to gaps encountered during transcription. Earlier, we reported that NusA physically and genetically interacts with the TLS DNA polymerase DinB (DNA pol IV). We find that Escherichia coli nusA11(ts) mutant strains, at the permissive temperature, are highly sensitive to nitrofurazone (NFZ) and 4-nitroquinolone-1-oxide but not to UV radiation. Gene expression profiling suggests that this sensitivity is unlikely to be due to an indirect effect on gene expression affecting a known DNA repair or damage tolerance pathway. We demonstrate that an N(2)-furfuryl-dG (N(2)-f-dG) lesion, a structural analog of the principal lesion generated by NFZ, blocks transcription by E. coli RNA polymerase (RNAP) when present in the transcribed strand, but not when present in the nontranscribed strand. Our genetic analysis suggests that NusA participates in a nucleotide excision repair (NER)-dependent process to promote NFZ resistance. We provide evidence that transcription plays a role in the repair of NFZ-induced lesions through the isolation of RNAP mutants that display altered ability to survive NFZ exposure. We propose that NusA participates in an alternative class of TCR involved in the identification and removal of a class of lesion, such as the N(2)-f-dG lesion, which are accurately and efficiently bypassed by DinB in addition to recruiting DinB for TLS at gaps encountered by RNAP.

Modulation of UvrD helicase activity by covalent DNA-protein cross-links

UvrD (DNA helicase II) has been implicated in DNA replication, DNA recombination, nucleotide excision repair, and methyl-directed mismatch repair. The enzymatic function of UvrD is to translocate along a DNA strand in a 3' to 5' direction and unwind duplex DNA utilizing a DNA-dependent ATPase activity. In addition, UvrD interacts with many other proteins involved in the above processes and is hypothesized to facilitate protein turnover, thus promoting further DNA processing. Although UvrD interactions with proteins bound to DNA have significant biological implications, the effects of covalent DNA-protein cross-links on UvrD helicase activity have not been characterized. Herein, we demonstrate that UvrD-catalyzed strand separation was inhibited on a DNA strand to which a 16-kDa protein was covalently bound. Our sequestration studies suggest that the inhibition of UvrD activity is most likely due to a translocation block and not helicase sequestration on the cross-link-containing DNA substrate. In contrast, no inhibition of UvrD-catalyzed strand separation was apparent when the protein was linked to the complementary strand. The latter result is surprising given the earlier observations that the DNA in this covalent complex is severely bent ( approximately 70 degrees ), with both DNA strands making multiple contacts with the cross-linked protein. In addition, UvrD was shown to be required for replication of plasmid DNAs containing covalent DNA-protein complexes. Combined, these data suggest a critical role for UvrD in the processing of DNA-protein cross-links.

Mutagenicity of acrolein and acrolein-induced DNA adducts

Acrolein mutagenicity relies on DNA adduct formation. Reaction of acrolein with deoxyguanosine generates alpha-hydroxy-1, N(2)-propano-2'-deoxyguanosine (alpha-HOPdG) and gamma-hydroxy-1, N(2)-propano-2'-deoxyguanosine (gamma-HOPdG) adducts. These two DNA adducts behave differently in mutagenicity. gamma-HOPdG is the major DNA adduct and it can lead to interstrand DNA-DNA and DNA-peptide/protein cross-links, which may induce strong mutagenicity; however, gamma-HOPdG can be repaired by some DNA polymerases complex and lessen its mutagenic effects. alpha-HOPdG is formed much less than gamma-HOPdG, but difficult to be repaired, which contributes to accumulation in vivo. Results of acrolein mutagenicity studies haven't been confirmed, which is mainly due to the conflicting mutagenicity data of the major acrolein adduct (gamma-HOPdG). The minor alpha-HOPdG is mutagenic in both in vitro and in vivo test systems. The role of alpha-HOPdG in acrolein mutagenicity needs further investigation. The inconsistent result of acrolein mutagenicity can be attributed, at least partially, to a variety of acrolein-DNA adducts formation and their repair in diverse detection systems. Recent results of detection of acrolein-DNA adduct in human lung tissues and analysis of P53 mutation spectra in acrolein-treated cells may shed some light on mechanisms of acrolein mutagenicity. These aspects are covered in this mini review.

Role of the DinB homologs Rv1537 and Rv3056 in Mycobacterium tuberculosis

The environment encountered by Mycobacterium tuberculosis during infection is genotoxic. Most bacteria tolerate DNA damage by engaging specialized DNA polymerases that catalyze translesion synthesis (TLS) across sites of damage. M. tuberculosis possesses two putative members of the DinB class of Y-family DNA polymerases, DinB1 (Rv1537) and DinB2 (Rv3056); however, their role in damage tolerance, mutagenesis, and survival is unknown. Here, both dinB1 and dinB2 are shown to be expressed in vitro in a growth phase-dependent manner, with dinB2 levels 12- to 40-fold higher than those of dinB1. Yeast two-hybrid analyses revealed that DinB1, but not DinB2, interacts with the beta-clamp, consistent with its canonical C-terminal beta-binding motif. However, knockout of dinB1, dinB2, or both had no effect on the susceptibility of M. tuberculosis to compounds that form N(2)-dG adducts and alkylating agents. Similarly, deletion of these genes individually or in combination did not affect the rate of spontaneous mutation to rifampin resistance or the spectrum of resistance-conferring rpoB mutations and had no impact on growth or survival in human or mouse macrophages or in mice. Moreover, neither gene conferred a mutator phenotype when expressed ectopically in Mycobacterium smegmatis. The lack of the effect of altering the complements or expression levels of dinB1 and/or dinB2 under conditions predicted to be phenotypically revealing suggests that the DinB homologs from M. tuberculosis do not behave like their counterparts from other organisms.