A role for the base excision repair enzyme NEIL3 in replication-dependent repair of interstrand DNA cross-links derived from psoralen and abasic sites

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

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

Transcriptional factor MAZ promotes cisplatin-induced DNA damage repair in lung adenocarcinoma by regulating NEIL3

BACKGROUND

Cisplatin remains a common chemotherapy drug for lung adenocarcinoma (LUAD) in clinical treatment. Long-term use of cisplatin in patients may lead to acquired drug resistance, resulting in poor prognoses of patients. NEIL3 was a glycosylase-encoding gene highly expressed in LUAD. NEIL3 can repair telomerase DNA damage in the S phase. Nevertheless, there are few reports on whether NEIL3 is involved in cisplatin resistance and its related mechanisms in LUAD.

METHODS

The expression of NEIL3 in LUAD patients was analyzed by bioinformatics. The regulator upstream of NEIL3 was predicted via hTFtarget. The possibly involved pathways of NEIL3 were obtained by performing Gene Set Enrichment Analysis. qRT-PCR and western blot were applied to test the expression level of genes and protein LUAD cells. Dual-luciferase assay and chromatin immunoprecipitation (ChIP) assay were conducted to validate the binding relationship between MAZ and NEIL3. Cell function assays were performed to test the DNA damage, cell viability, cell migration and invasion, and cell cycle of LUAD cells in the treatment group.

RESULTS

NEIL3 and its upstream regulatory factor MAZ were highly expressed in LUAD tissue, and NEIL3 was enriched in cell cycle and mismatch repair pathways. Dual-luciferase assay and ChIP assay proved that MAZ could target NEIL3. Cell experiments identified that MAZ/NEIL3 axis could repress DNA damage to advance cisplatin resistance of cancer cells, and foster cell migration and invasion in LUAD.

CONCLUSION

MAZ-activated NEIL3 could propel the cisplatin resistance in LUAD by repressing DNA damage.

Nucleic acid adductomics - The next generation of adductomics towards assessing environmental health risks

This Discussion article aims to explore the potential for a new generation of assay to emerge from cellular and urinary DNA adductomics which brings together DNA-RNA- and, to some extent, protein adductomics, to better understand the role of the exposome in environmental health. Components of the exposome have been linked to an increased risk of various, major diseases, and to identify the precise nature, and size, of risk, in this complex mixture of exposures, powerful tools are needed. Modification of nucleic acids (NA) is a key consequence of environmental exposures, and a goal of cellular DNA adductomics is to evaluate the totality of DNA modifications in the genome, on the basis that this will be most informative. Consequently, an approach which encompasses modifications of all nucleic acids (NA) would be potentially yet more informative. This article focuses on NA adductomics, which brings together the assessment of both DNA and RNA modifications, including modified (2'-deoxy)ribonucleosides (2'-dN/rN), modified nucleobases (nB), plus: DNA-DNA, RNA-RNA, DNA-RNA, DNA-protein, and RNA-protein crosslinks (DDCL, RRCL, DRCL, DPCL, and RPCL, respectively). We discuss the need for NA adductomics, plus the pros and cons of cellular vs. urinary NA adductomics, and present some evidence for the feasibility of this approach. We propose that NA adductomics provides a more comprehensive approach to the study of nucleic acid modifications, which will facilitate a range of advances, including the identification of novel, unexpected modifications e.g., RNA-RNA, and DNA-RNA crosslinks; key modifications associated with mutagenesis; agent-specific mechanisms; and adductome signatures of key environmental agents, leading to the dissection of the exposome, and its role in human health/disease, across the life course.

Biological Functions of the DNA Glycosylase NEIL3 and Its Role in Disease Progression Including Cancer

The accumulation of oxidative DNA base damage can severely disrupt the integrity of the genome and is strongly associated with the development of cancer. DNA glycosylase is the critical enzyme that initiates the base excision repair (BER) pathway, recognizing and excising damaged bases. The Nei endonuclease VIII-like 3 (NEIL3) is an emerging DNA glycosylase essential in maintaining genome stability. With an in-depth study of the structure and function of NEIL3, we found that it has properties related to the process of base damage repair. For example, it not only prefers the base damage of single-stranded DNA (ssDNA), G-quadruplex and DNA interstrand crosslinks (ICLs), but also participates in the maintenance of replication fork stability and telomere integrity. In addition, NEIL3 is strongly associated with the progression of cancers and cardiovascular and neurological diseases, is incredibly significantly overexpressed in cancers, and may become an independent prognostic marker for cancer patients. Interestingly, circNEIL3, a circular RNA of exon-encoded origin by NEIL3, also promotes the development of multiple cancers. In this review, we have summarized the structure and the characteristics of NEIL3 to repair base damage. We have focused on NEIL3 and circNEIL3 in cancer development, progression and prognosis.

Effects of Local Sequence, Reaction Conditions, and Various Additives on the Formation and Stability of Interstrand Cross-Links Derived from the Reaction of an Abasic Site with an Adenine Residue in Duplex DNA

The experiments described here examined the effects of reaction conditions, various additives, and local sequence on the formation and stability interstrand cross-links (ICLs) derived from the reaction of an apurinic/apyrimidinic (AP) site with the exocyclic amino group of an adenine residue on the opposing strand in duplex DNA. Cross-link formation was observed in a range of different buffers, with faster formation rates observed at pH 5. Inclusion of the base excision repair enzyme alkyladenine DNA glycosylase (hAAG) which binds tightly to AP-containing duplexes decreased, but did not completely prevent, formation of the dA-AP ICL. Formation of the dA-AP ICL was not altered by the presence of the biological metal ion Mg²⁺ or the biological thiol, glutathione. Several organocatalysts of imine formation did not enhance the rate of dA-AP ICL formation. Duplex length did not have a large effect on dA-AP yield, so long as the melting temperature of the duplex was not significantly below the reaction temperature (the duplex must remain hybridized for efficient ICL formation). Formation of the dA-AP ICL was examined in over 40 different sequences that varied the neighboring and opposing bases at the cross-linking site. The results indicate that ICL formation can occur in a wide variety of sequence contexts under physiological conditions. Formation of the dA-AP ICL was strongly inhibited by the aldehyde-trapping agents methoxyamine and hydralazine, by NaBH₃CN, by the intercalator ethidium bromide, and by the minor groove-binding agent netropsin. ICL formation was inhibited to some extent in bicarbonate and Tris buffers. The dA-AP ICL showed substantial inherent stability under a variety of conditions and was not a substrate for AP-processing enzymes APE1 or Endo IV. Finally, we characterized cross-link formation in a small (11 bp) stem-loop (hairpin) structure and in DNA-RNA hybrid duplexes.

TRIM26 Maintains Cell Survival in Response to Oxidative Stress through Regulating DNA Glycosylase Stability

Oxidative DNA base lesions in DNA are repaired through the base excision repair (BER) pathway, which consequently plays a vital role in the maintenance of genome integrity and in suppressing mutagenesis. 8-oxoguanine DNA glycosylase (OGG1), endonuclease III-like protein 1 (NTH1), and the endonuclease VIII-like proteins 1-3 (NEIL1-3) are the key enzymes that initiate repair through the excision of the oxidized base. We have previously identified that the E3 ubiquitin ligase tripartite motif 26 (TRIM26) controls the cellular response to oxidative stress through regulating both NEIL1 and NTH1, although its potential, broader role in BER is unclear. We now show that TRIM26 is a central player in determining the response to different forms of oxidative stress. Using siRNA-mediated knockdowns, we demonstrate that the resistance of cells to X-ray radiation and hydrogen peroxide generated as a consequence of trim26 depletion can be reversed through suppression of selective DNA glycosylases. In particular, a knockdown of neil1 or ogg1 can enhance sensitivity and DNA repair rates in response to X-rays, whereas a knockdown of neil1 or neil3 can produce the same effect in response to hydrogen peroxide. Our study, therefore, highlights the importance of TRIM26 in balancing cellular DNA glycosylase levels required for an efficient BER response.

Synthesis of DNA Duplexes Containing Site-Specific Interstrand Cross-Links via Sequential Reductive Amination Reactions Involving Diamine Linkers and Abasic Sites on Complementary Oligodeoxynucleotides

Interstrand DNA cross-links are important in biology, medicinal chemistry, and materials science. Accordingly, methods for the targeted installation of interstrand cross-links in DNA duplexes may be useful in diverse fields. Here, a simple procedure is reported for the preparation of DNA duplexes containing site-specific, chemically defined interstrand cross-links. The approach involves sequential reductive amination reactions between diamine linkers and two abasic (apurinic/apyrimidinic, AP) sites on complementary oligodeoxynucleotides. Use of the symmetrical triamine, tris(2-aminoethyl)amine, in this reaction sequence enabled the preparation of a cross-linked DNA duplex bearing a derivatizable aminoethyl group.

A new frontier in Fanconi anemia: From DNA repair to ribosome biogenesis

Described by Guido Fanconi almost 100 years ago, Fanconi anemia (FA) is a rare genetic disease characterized by developmental abnormalities, bone marrow failure (BMF) and cancer predisposition. The proteins encoded by FA-mutated genes (FANC proteins) and assembled in the so-called FANC/BRCA pathway have key functions in DNA repair and replication safeguarding, which loss leads to chromosome structural aberrancies. Therefore, since the 1980s, FA has been considered a genomic instability and chromosome fragility syndrome. However, recent findings have demonstrated new and unexpected roles of FANC proteins in nucleolar homeostasis and ribosome biogenesis, the alteration of which impacts cellular proteostasis. Here, we review the different cellular, biochemical and molecular anomalies associated with the loss of function of FANC proteins and discuss how these anomalies contribute to BMF by comparing FA to other major inherited BMF syndromes. Our aim is to determine the extent to which alterations in the DNA damage response in FA contribute to BMF compared to the consequences of the loss of function of the FANC/BRCA pathway on the other roles of the pathway.

Formation and Repair of an Interstrand DNA Cross-Link Arising from a Common Endogenous Lesion

Interstrand DNA cross-links (ICLs) are cytotoxic because they block the strand separation required for read-out and replication of the genetic information in duplex DNA. The unavoidable formation of ICLs in cellular DNA may contribute to aging, neurodegeneration, and cancer. Here, we describe the formation and properties of a structurally complex ICL derived from an apurinic/apyrimidinic (AP) site, which is one of the most common endogenous lesions in cellular DNA. The results characterize a cross-link arising from aza-Michael addition of the N²-amino group of a guanine residue to the electrophilic sugar remnant generated by spermine-mediated strand cleavage at an AP site in duplex DNA. An α,β-unsaturated iminium ion is the critical intermediate involved in ICL formation. Studies employing the bacteriophage φ29 polymerase provided evidence that this ICL can block critical DNA transactions that require strand separation. The results of biochemical studies suggest that this complex strand break/ICL might be repaired by a simple mechanism in which the 3'-exonuclease action of the enzyme apurinic/apyrimidinic endonuclease (APE1) unhooks the cross-link to initiate repair via the single-strand break repair pathway.

NEIL3 Prevents Senescence in Hepatocellular Carcinoma by Repairing Oxidative Lesions at Telomeres during Mitosis

Patients with hepatocellular carcinoma (HCC) suffer from few treatment options and poor survival rates. Here we report that endonuclease VIII-like protein 3 (NEIL3) is overexpressed in HCC and correlates with poor survival. All six HCC cell lines investigated were dependent on NEIL3 catalytic activity for survival and prevention of senescence, while NEIL3 was dispensable for nontransformed cells. NEIL3-depleted HCC cell lines accumulated oxidative DNA lesions specifically at telomeres, resulting in telomere dysfunctional foci and 53BP1 foci formation. Following oxidative DNA damage during mitosis, NEIL3 relocated to telomeres and recruited apurinic endonuclease 1 (APE1), indicating activation of base excision repair. META-FISH revealed that NEIL3, but not NEIL1 or NEIL2, is required to initiate APE1 and polymerase beta (POLB)-dependent base excision repair at oxidized telomeres. Repeated exposure of NEIL3-depleted cells to oxidizing damage induced chromatin bridges and damaged telomeres. These results demonstrate a novel function for NEIL3 in repair of oxidative DNA damage at telomeres in mitosis, which is important to prevent senescence of HCC cells. Furthermore, these data suggest that NEIL3 could be a target for therapeutic intervention for HCC. SIGNIFICANCE: This study describes compartmentalization of base excision repair during mitosis that is dependent on NEIL3, APE1, and POLB to repair oxidative damage accumulating at telomeres in hepatocellular carcinoma.

Consequences and repair of radiation-induced DNA damage: fifty years of fun questions and answers

PURPOSE

To summarize succinctly the 50 years of research undertaken in my laboratory and to provide an overview of my career in science. It is certainly a privilege to have been asked by Carmel Mothersill and Penny Jeggo to contribute to this special issue of the International Journal of Radiation Biology focusing on the work of women in the radiation sciences.

CONCLUSION

My students, post-docs and I identified and characterized a number of the enzymes that recognize and remove radiation-damaged DNA bases, the DNA glycosylases, which are the first enzymes in the Base Excision Repair (BER) pathway. Although this pathway actually evolved to repair oxidative and other endogenous DNA damages, it is also responsible for removing the vast majority of radiation-induced DNA damages including base damages, alkali-labile lesions and single strand breaks. However, because of its high efficiency, attempted BER of clustered lesions produced by ionizing radiation, can have disastrous effects on cellular DNA. We also evaluated the potential biological consequences of many of the radiation-induced DNA lesions. In addition, with collaborators, we employed computational techniques, x-ray crystallography and single molecule approaches to answer many questions at the molecular level.

Interstrand Cross-Link Formation Involving Reaction of a Mispaired Cytosine Residue with an Abasic Site in Duplex DNA

The formation of interstrand cross-links in duplex DNA is important in biology, medicine, and biotechnology. Interstrand cross-links arising from the reaction of the aldehyde residue of an abasic (apurinic or AP) site with the exocyclic amino groups of guanine or adenine residues on the opposing strand of duplex DNA have previously been characterized. The canonical nucleobase cytosine has an exocyclic amino group but its ability to form interstrand cross-links by reaction with an AP site has not been characterized before now. Here it is shown that substantial yields of interstrand cross-links are generated in sequences having a mispaired cytosine residue located one nucleotide to the 3'-side of the AP site on the opposing strand (e.g., 5'XA/5'CA, where X = AP). Formation of the dC-AP cross-link is pH-dependent, with significantly higher yields at pH 5 than pH 7. Once formed, the dC-AP cross-link is quite stable, showing less than 5% dissociation over the course of 96 h at pH 7 and 37 °C. No significant yields of cross-link are observed when the cytosine residue is paired with its Watson-Crick partner guanine. It was also shown that a single AP site can engage with multiple nucleobase cross-linking partners in some sequences. Specifically, the dG-AP and dC-AP cross-links coexist in dynamic equilibrium in the sequence 5'CXA/5'CAG (X = AP). In this sequence, the dC-AP cross-link dominates. However, in the presence of NaBH3CN, irreversible reduction of small amounts of the dG-AP cross-link present in the mixture shifts the equilibria away from the dC-AP cross-link toward good yields of the dG-APred cross-link.

Role of Base Excision Repair Pathway in the Processing of Complex DNA Damage Generated by Oxidative Stress and Anticancer Drugs

Chemical alterations in DNA induced by genotoxic factors can have a complex nature such as bulky DNA adducts, interstrand DNA cross-links (ICLs), and clustered DNA lesions (including double-strand breaks, DSB). Complex DNA damage (CDD) has a complex character/structure as compared to singular lesions like randomly distributed abasic sites, deaminated, alkylated, and oxidized DNA bases. CDD is thought to be critical since they are more challenging to repair than singular lesions. Although CDD naturally constitutes a relatively minor fraction of the overall DNA damage induced by free radicals, DNA cross-linking agents, and ionizing radiation, if left unrepaired, these lesions cause a number of serious consequences, such as gross chromosomal rearrangements and genome instability. If not tightly controlled, the repair of ICLs and clustered bi-stranded oxidized bases via DNA excision repair will either inhibit initial steps of repair or produce persistent chromosomal breaks and consequently be lethal for the cells. Biochemical and genetic evidences indicate that the removal of CDD requires concurrent involvement of a number of distinct DNA repair pathways including poly(ADP-ribose) polymerase (PARP)-mediated DNA strand break repair, base excision repair (BER), nucleotide incision repair (NIR), global genome and transcription coupled nucleotide excision repair (GG-NER and TC-NER, respectively), mismatch repair (MMR), homologous recombination (HR), non-homologous end joining (NHEJ), and translesion DNA synthesis (TLS) pathways. In this review, we describe the role of DNA glycosylase-mediated BER pathway in the removal of complex DNA lesions.

Structure of a Stable Interstrand DNA Cross-Link Involving a β-N-Glycosyl Linkage Between an N6-dA Amino Group and an Abasic Site

Abasic (AP) sites are one of the most common forms of DNA damage. The deoxyribose ring of AP sites undergoes anomerization between α and β configurations, via an electrophilic aldehyde intermediate. In sequences where an adenine residue is located on the opposing strand and offset 1 nt to the 3' side of the AP site, the nucleophilic N⁶-dA amino group can react with the AP aldehyde residue to form an interstrand cross-link (ICL). Here, we present an experimentally determined structure of the dA-AP ICL by NMR spectroscopy. The ICL was constructed in the oligodeoxynucleotide 5'-d(T¹A²T³G⁴T⁵C⁶T⁷A⁸A⁹G¹⁰T¹¹T¹²C¹³A¹⁴T¹⁵C¹⁶T¹⁷A¹⁸)-3':5'-d(T¹⁹A²⁰G²¹A²²T²³G²⁴A²⁵A²⁶C²⁷X²⁸T²⁹A³⁰G³¹A³²C³³A³⁴T³⁵A³⁶)-3' (X=AP site), with the dA-AP ICL forming between A⁸ and X²⁸. The NMR spectra indicated an ordered structure for the cross-linked DNA duplex and afforded detailed spectroscopic resonance assignments. Structural refinement, using molecular dynamics calculations restrained by NOE data (rMD), revealed the structure of the ICL. In the dA-AP ICL, the 2'-deoxyribosyl ring of the AP site was ring-closed and in the β configuration. Juxtapositioning the N⁶-dA amino group and the aldehydic C1 of the AP site within bonding distance while simultaneously maintaining two flanking unpaired A⁹ and T²⁹ bases stacked within the DNA is accomplished by the unwinding of the DNA at the ICL. The structural data is discussed in the context of recent studies describing the replication-dependent unhooking of the dA-AP ICL by the base excision repair glycosylase NEIL3.

Formation and repair of unavoidable, endogenous interstrand cross-links in cellular DNA

Genome integrity is essential for life and, as a result, DNA repair systems evolved to remove unavoidable DNA lesions from cellular DNA. Many forms of life possess the capacity to remove interstrand DNA cross-links (ICLs) from their genome but the identity of the naturally-occurring, endogenous substrates that drove the evolution and retention of these DNA repair systems across a wide range of life forms remains uncertain. In this review, we describe more than a dozen chemical processes by which endogenous ICLs plausibly can be introduced into cellular DNA. The majority involve DNA degradation processes that introduce aldehyde residues into the double helix or reactions of DNA with endogenous low molecular weight aldehyde metabolites. A smaller number of the cross-linking processes involve reactions of DNA radicals generated by oxidation.

Synthesis of Oligonucleotides containing the cis-Interstrand Crosslink Produced by Mitomycins in their Reaction with DNA

Mitomycin C (MC) an antitumor drug and decarbamoylmitomycin C (DMC), a derivative of MC lacking the carbamoyl moiety, are DNA alkylating agents which can form DNA interstrand crosslinks (ICLs) between deoxyguanosine residues located on opposing DNA strands. MC forms primarily deoxyguanosine adducts with a 1"-R stereochemistry at the guanine-mitosene bond (1"-α, trans) whereas DMC forms mainly adducts with a 1"-S stereochemistry (1"-β, cis). The crosslinking reaction is diastereospecific: trans-crosslinks are formed exclusively at CpG sequences, while cis-crosslinks are formed only at GpC sequences. Until now, oligonucleotides containing 1"-β-deoxyguanosine adducts or ICL at a specific site could not be synthesized, thus limiting the investigation of the role played by the stereochemical configuration at C1'' in the toxicity of these compounds. Here, a novel biomimetic synthesis to access these substrates is presented. Structural proof of the adducted oligonucleotides and ICL were provided by enzymatic digestion to nucleosides, high resolution mass spectral analysis, CD spectroscopy and UV melting temperature studies. Finally, a virtual model of the 25-mer 1"-β ICL synthesized was created to explore the conformational space and structural features of the crosslinked duplex.

A non-canonical role for the DNA glycosylase NEIL3 in suppressing APE1 endonuclease-mediated ssDNA damage

The DNA glycosylase NEIL3 has been implicated in DNA repair pathways including the base excision repair and the interstrand cross-link repair pathways via its DNA glycosylase and/or AP lyase activity, which are considered canonical roles of NEIL3 in genome integrity. Compared with the other DNA glycosylases NEIL1 and NEIL2, Xenopus laevis NEIL3 C terminus has two highly conserved zinc finger motifs containing GRXF residues (designated as Zf-GRF). It has been demonstrated that the minor AP endonuclease APE2 contains only one Zf-GRF motif mediating interaction with single-strand DNA (ssDNA), whereas the major AP endonuclease APE1 does not. It appears that the two NEIL3 Zf-GRF motifs (designated as Zf-GRF repeat) are dispensable for its DNA glycosylase and AP lyase activity; however, the potential function of the NEIL3 Zf-GRF repeat in genome integrity remains unknown. Here, we demonstrate evidence that the NEIL3 Zf-GRF repeat was associated with a higher affinity for shorter ssDNA than one single Zf-GRF motif. Notably, our protein-protein interaction assays show that the NEIL3 Zf-GRF repeat but not one Zf-GRF motif interacted with APE1 but not APE2. We further reveal that APE1 endonuclease activity on ssDNA but not on dsDNA is compromised by a NEIL3 Zf-GRF repeat, whereas one Zf-GRF motif within NEIL3 is not sufficient to prevent such activity of APE1. In addition, COMET assays show that excess NEIL3 Zf-GRF repeat reduces DNA damage in oxidative stress in Xenopus egg extracts. Together, our results suggest a noncanonical role of NEIL3 in genome integrity via its distinct Zf-GRF repeat in suppressing APE1 endonuclease-mediated ssDNA breakage.

Cooperation of the NEIL3 and Fanconi anemia/BRCA pathways in interstrand crosslink repair

The NEIL3 DNA glycosylase is a base excision repair enzyme that excises bulky base lesions from DNA. Although NEIL3 has been shown to unhook interstrand crosslinks (ICL) in Xenopus extracts, how NEIL3 participants in ICL repair in human cells and its corporation with the canonical Fanconi anemia (FA)/BRCA pathway remain unclear. Here we show that the NEIL3 and the FA/BRCA pathways are non-epistatic in psoralen-ICL repair. The NEIL3 pathway is the major pathway for repairing psoralen-ICL, and the FA/BRCA pathway is only activated when NEIL3 is not present. Mechanistically, NEIL3 is recruited to psoralen-ICL in a rapid, PARP-dependent manner. Importantly, the NEIL3 pathway repairs psoralen-ICLs without generating double-strand breaks (DSBs), unlike the FA/BRCA pathway. In addition, we found that the RUVBL1/2 complex physically interact with NEIL3 and function within the NEIL3 pathway in psoralen-ICL repair. Moreover, TRAIP is important for the recruitment of NEIL3 but not FANCD2, and knockdown of TRAIP promotes FA/BRCA pathway activation. Interestingly, TRAIP is non-epistatic with both NEIL3 and FA pathways in psoralen-ICL repair, suggesting that TRAIP may function upstream of the two pathways. Taken together, the NEIL3 pathway is the major pathway to repair psoralen-ICL through a unique DSB-free mechanism in human cells.

Unhooking of an interstrand cross-link at DNA fork structures by the DNA glycosylase NEIL3

Interstrand DNA-DNA cross-links (ICLs) are generated by endogenous processes, drugs, and environmental toxins. Understanding the cellular pathways by which various ICLs are repaired is critical to understanding their biological effects. Recent studies showed that replication-dependent repair of an ICL derived from the reaction of an abasic (AP) site with an adenine residue (dA) on the opposing strand of duplex DNA proceeds via a novel mechanism in which the DNA glycosylase NEIL3 unhooks the ICL. Here we examined the ability of the glycosylase domain of murine NEIL3 (MmuNEIL3-GD) to unhook dA-AP ICLs. The enzyme selectively unhooks the dA-AP ICL located at the duplex/single-strand junction of splayed duplexes that model the strand-separated DNA at the leading edge of a replication fork. We show that the ability to unhook the dA-AP ICL is a specialized function of NEIL3 as this activity is not observed in other BER enzymes. Importantly, NEIL3 only unhooks the dA-AP ICL when the AP residue is located on what would be the leading template strand of a model replication fork. The same specificity for the leading template strand was observed with a 5,6-dihydrothymine monoadduct, demonstrating that this preference is a general feature of the glycosylase and independent of the type of DNA damage. Overall, the results show that the glycosylase domain of NEIL3, lacking the C-terminal NPL4 and GRF zinc finger motifs, is competent to unhook the dA-AP ICL in splayed substrates and independently enforces important substrate preferences on the repair process.

Preparation and Purification of Oligodeoxynucleotide Duplexes Containing a Site-Specific, Reduced, Chemically Stable Covalent Interstrand Cross-Link Between a Guanine Residue and an Abasic Site

Methods for the preparation of DNA duplexes containing interstrand covalent cross-links may facilitate research in the fields of biochemistry, molecular biology, nanotechnology, and materials science. Here we report methods for the synthesis and isolation of DNA duplexes containing a site-specific, chemically stable, reduced covalent interstrand cross-link between a guanine residue and an abasic site. The method uses experimental techniques and equipment that are common in most biochemical laboratories and inexpensive, commercially available oligonucleotides and reagents.

Human NEIL3 Gene Expression Regulated by Epigenetic-Like Oxidative DNA Modification

The NEIL3 DNA repair gene is induced in cells or animal models experiencing oxidative or inflammatory stress along with oxidation of guanine (G) to 8-oxo-7,8-dihydroguanine (OG) in the genome. We hypothesize that a G-rich promoter element that is a potential G-quadruplex-forming sequence (PQS) in NEIL3 is a site for introduction of OG with epigenetic-like potential for gene regulation. Activation occurs when OG is formed in the NEIL3 PQS located near the transcription start site. Oxidative stress either introduced by TNFα or synthetically incorporated into precise locations focuses the base excision repair process to read and catalyze removal of OG via OG-glycosylase I (OGG1), yielding an abasic site (AP). Thermodynamic studies showed that AP destabilizes the duplex, enabling a structural transition of the sequence to a G-quadruplex (G4) fold that positions the AP in a loop facilitated by the NEIL3 PQS having five G runs in which the four unmodified runs adopt a stable G4. This presents AP to apurinic/apyrimidinic endonuclease 1 (APE1) that poorly cleaves the AP backbone in this context according to in vitro studies, allowing the protein to function as a trans activator of transcription. The proposal is supported by chemical studies in cellulo and in vitro. Activation of NEIL3 expression via the proposed mechanism allows cells to respond to mutagenic DNA damage removed by NEIL3 associated with oxidative or inflammatory stress. Lastly, inspection of many mammalian genomes identified conservation of the NEIL3 PQS, suggesting this sequence was favorably selected to function as a redox switch with OG as the epigenetic-like regulatory modification.

Emerging Roles of DNA Glycosylases and the Base Excision Repair Pathway

The base excision repair (BER) pathway historically has been associated with maintaining genome integrity by eliminating nucleobases with small chemical modifications. In the past several years, however, BER was found to play additional roles in genome maintenance and metabolism, including sequence-specific restriction modification and repair of bulky adducts and interstrand crosslinks. Central to this expanded biological utility are specialized DNA glycosylases - enzymes that selectively excise damaged, modified, or mismatched nucleobases. In this review we discuss the newly identified roles of the BER pathway and examine the structural and mechanistic features of the DNA glycosylases that enable these functions.

Remodeling of Interstrand Crosslink Proximal Replisomes Is Dependent on ATR, FANCM, and FANCD2

Eukaryotic replisomes are driven by the mini chromosome maintenance (MCM [M]) helicase complex, an offset ring locked around the template for leading strand synthesis by CDC45 (C) and GINS (G) proteins. Although the CDC45 MCM GINS (CMG) structure implies that interstrand crosslinks (ICLs) are absolute blocks to replisomes, recent studies indicate that cells can restart DNA synthesis on the side of the ICL distal to the initial encounter. Here, we report that restart requires ATR and is promoted by FANCD2 and phosphorylated FANCM. Following introduction of genomic ICLs and dependent on ATR and FANCD2 but not on the Fanconi anemia core proteins or FAAP24, FANCM binds the replisome complex, with concomitant release of the GINS proteins. In situ analysis of replisomes proximal to ICLs confirms the ATR-dependent release of GINS proteins while CDC45 is retained on the remodeled replisome. The results demonstrate the plasticity of CMG composition in response to replication stress.

Ex vivo assays to predict enhanced chemosensitization by hyperthermia in urothelial cancer of the bladder

INTRODUCTION

Bladder cancer (urothelial carcinoma) is a common malignancy characterized by high recurrence rates and intense clinical follow-up, indicating the necessity for more effective therapies. Current treatment regimens include intra-vesical administration of mitomycin C (MMC) for non-muscle invasive disease and systemic cisplatin for muscle-invasive or metastatic disease. Hyperthermia, heating a tumor to 40-44°C, enhances the efficacy of these chemotherapeutics by various modes of action, one of which is inhibition of DNA repair via homologous recombination. Here, we explore whether ex vivo assays on freshly obtained bladder tumors can be applied to predict the response towards hyperthermia.

MATERIAL AND METHODS

The cytochrome C release assay (apoptosis) and the RAD51 focus formation assay (DNA repair) were first established in the bladder cancer cell lines RT112 and T24 as measurements for hyperthermia efficiency, and subsequently tested in freshly obtained bladder tumors (n = 59).

RESULTS

Hyperthermia significantly increased the fraction of apoptotic cells after cisplatin or MMC treatment in both RT112 and T24 cells and in most of the bladder tumors (8/10). The RAD51 focus formation assay detected both morphological and numerical changes of RAD51 foci upon hyperthermia in the RT112 and T24 cell lines. In 64% of 37 analyzed primary bladder tumor samples, hyperthermia induced similar morphological changes in RAD51 foci.

CONCLUSION

The cytochrome C assay and the RAD51 focus formation assay are both feasible on freshly obtained bladder tumors, and could serve to predict the efficacy of hyperthermia together with cytotoxic agents, such as MMC or cisplatin.

Targeting BER enzymes in cancer therapy

Base excision repair (BER) repairs mutagenic or genotoxic DNA base lesions, thought to be important for both the etiology and treatment of cancer. Cancer phenotypic stress induces oxidative lesions, and deamination products are responsible for one of the most prevalent mutational signatures in cancer. Chemotherapeutic agents induce genotoxic DNA base damage that are substrates for BER, while synthetic lethal approaches targeting BER-related factors are making their way into the clinic. Thus, there are three strategies by which BER is envisioned to be relevant in cancer chemotherapy: (i) to maintain cellular growth in the presence of endogenous DNA damage in stressed cancer cells, (ii) to maintain viability after exogenous DNA damage is introduced by therapeutic intervention, or (iii) to confer synthetic lethality in cancer cells that have lost one or more additional DNA repair pathways. Here, we discuss the potential treatment strategies, and briefly summarize the progress that has been made in developing inhibitors to core BER-proteins and related factors.

The evolving role of DNA inter-strand crosslinks in chemotherapy

DNA crosslinking agents make up a broad class of chemotherapy agents that target rapidly dividing cancer cells by disrupting DNA synthesis. These drugs differ widely in both chemical structure and biological effect. In cells, crosslinking agents can form multiple types of DNA lesions with varying efficiencies. Inter-strand crosslinks (ICLs) are considered to be the most cytotoxic lesion, creating a covalent roadblock to replication and transcription. Despite over 50 years in the clinic, the use of crosslinking agents that specialize in the formation of ICLs remains limited, largely due to high toxicity in patients. Current ICL-based therapeutics have focused on late-stage and drug-resistant tumors, or localized treatments that limit exposure. In this article, we review the development of clinical crosslinking agents, our understanding of how cells respond to different lesions, and the potential to improve ICL-based chemotherapeutics in the future.

Frankenstein or a Submarine Alkaline Vent: Who Is Responsible for Abiogenesis?: Part 1: What is life-that it might create itself?

Origin of life models based on "energized assemblages of building blocks" are untenable in principle. This is fundamentally a consequence of the fact that any living system is in a physical state that is extremely far from equilibrium, a condition it must itself build and sustain. This in turn requires that it carries out all of its molecular transformations-obligatorily those that convert, and thereby create, disequilibria-using case-specific mechanochemical macromolecular machines. Mass-action solution chemistry is quite unable to do this. We argue in Part 2 of this series that this inherent dependence of life on disequilibria-converting macromolecular machines is also an obligatory requirement for life at its emergence. Therefore, life must have been launched by the operation of abiotic macromolecular machines driven by abiotic, but specifically "life-like", disequilibria, coopted from mineral precipitates that are chemically and physically active. Models grounded in "chemistry-in-a-bag" ideas, however energized, should not be considered.

Integration of Emerging Biomedical Technologies in Meat Processing to Improve Meat Safety and Quality

Modern-day processing of meat products involves a series of complex procedures designed to ensure the quality and safety of the meat for consumers. As the size of abattoirs increases, the logistical problems associated with large-capacity animal processing can affect the sanitation of the facility and the meat products, potentially increasing transmission of infectious diseases. Additionally, spoilage of food from improper processing and storage increases the global economic and ecological burden of meat production. Advances in biomedical and materials science have allowed for the development of innovative new antibacterial technologies that have broad applications in the medical industry. Additionally, new approaches in tissue engineering and nondestructive cooling of biological specimens could significantly improve organ transplantation and tissue grafting. These same strategies may be even more effective in the preservation and protection of meat as animal carcasses are easier to manipulate and do not have the same stringent requirements of care as living patients. This review presents potential applications of emerging biomedical technologies in the food industry to improve meat safety and quality. Future research directions investigating these new technologies and their usefulness in the meat processing chain along with regulatory, logistical, and consumer perception issues will also be discussed.

The Human DNA glycosylases NEIL1 and NEIL3 Excise Psoralen-Induced DNA-DNA Cross-Links in a Four-Stranded DNA Structure

Interstrand cross-links (ICLs) are highly cytotoxic DNA lesions that block DNA replication and transcription by preventing strand separation. Previously, we demonstrated that the bacterial and human DNA glycosylases Nei and NEIL1 excise unhooked psoralen-derived ICLs in three-stranded DNA via hydrolysis of the glycosidic bond between the crosslinked base and deoxyribose sugar. Furthermore, NEIL3 from Xenopus laevis has been shown to cleave psoralen- and abasic site-induced ICLs in Xenopus egg extracts. Here we report that human NEIL3 cleaves psoralen-induced DNA-DNA cross-links in three-stranded and four-stranded DNA substrates to generate unhooked DNA fragments containing either an abasic site or a psoralen-thymine monoadduct. Furthermore, while Nei and NEIL1 also cleave a psoralen-induced four-stranded DNA substrate to generate two unhooked DNA duplexes with a nick, NEIL3 targets both DNA strands in the ICL without generating single-strand breaks. The DNA substrate specificities of these Nei-like enzymes imply the occurrence of long uninterrupted three- and four-stranded crosslinked DNA-DNA structures that may originate in vivo from DNA replication fork bypass of an ICL. In conclusion, the Nei-like DNA glycosylases unhook psoralen-derived ICLs in various DNA structures via a genuine repair mechanism in which complex DNA lesions can be removed without generation of highly toxic double-strand breaks.