Recognition and repair of the cyclobutane thymine dimer, a major cause of skin cancers, by the human excision nuclease

An Integrated Approach for Analysis of the DNA Damage Response in Mammalian Cells: NUCLEOTIDE EXCISION REPAIR, DNA DAMAGE CHECKPOINT, AND APOPTOSIS

DNA damage by UV and UV-mimetic agents elicits a set of inter-related responses in mammalian cells, including DNA repair, DNA damage checkpoints, and apoptosis. Conventionally, these responses are analyzed separately using different methodologies. Here we describe a unified approach that is capable of quantifying all three responses in parallel using lysates from the same population of cells. We show that a highly sensitive in vivo excision repair assay is capable of detecting nucleotide excision repair of a wide spectrum of DNA lesions (UV damage, chemical carcinogens, and chemotherapeutic drugs) within minutes of damage induction. This method therefore allows for a real-time measure of nucleotide excision repair activity that can be monitored in conjunction with other components of the DNA damage response, including DNA damage checkpoint and apoptotic signaling. This approach therefore provides a convenient and reliable platform for simultaneously examining multiple aspects of the DNA damage response in a single population of cells that can be applied for a diverse array of carcinogenic and chemotherapeutic agents.

Genome-wide analysis of human global and transcription-coupled excision repair of UV damage at single-nucleotide resolution

Hu et al. developed a method for genome-wide mapping of DNA excision repair named XR-seq (excision repair sequencing) and used it to produce stranded, nucleotide-resolution maps of repair of two UV-induced DNA damages in human cells. XR-seq and the resulting repair maps will facilitate studies of the effects of genomic location, chromatin context, transcription, and replication on DNA repair in human cells.

We developed a method for genome-wide mapping of DNA excision repair named XR-seq (excision repair sequencing). Human nucleotide excision repair generates two incisions surrounding the site of damage, creating an ∼30-mer. In XR-seq, this fragment is isolated and subjected to high-throughput sequencing. We used XR-seq to produce stranded, nucleotide-resolution maps of repair of two UV-induced DNA damages in human cells: cyclobutane pyrimidine dimers (CPDs) and (6-4) pyrimidine–pyrimidone photoproducts [(6-4)PPs]. In wild-type cells, CPD repair was highly associated with transcription, specifically with the template strand. Experiments in cells defective in either transcription-coupled excision repair or general excision repair isolated the contribution of each pathway to the overall repair pattern and showed that transcription-coupled repair of both photoproducts occurs exclusively on the template strand. XR-seq maps capture transcription-coupled repair at sites of divergent gene promoters and bidirectional enhancer RNA (eRNA) production at enhancers. XR-seq data also uncovered the repair characteristics and novel sequence preferences of CPDs and (6-4)PPs. XR-seq and the resulting repair maps will facilitate studies of the effects of genomic location, chromatin context, transcription, and replication on DNA repair in human cells.

Effect of point substitutions within the minimal DNA-binding domain of xeroderma pigmentosum group A protein on interaction with DNA intermediates of nucleotide excision repair

Xeroderma pigmentosum factor A (XPA) is one of the key proteins in the nucleotide excision repair (NER) process. The effects of point substitutions in the DNA-binding domain of XPA (positively charged lysine residues replaced by negatively charged glutamate residues: XPA K204E, K179E, K141E, and tandem mutant K141E/K179E) on the interaction of the protein with DNA structures modeling intermediates of the damage recognition and pre-incision stages in NER were analyzed. All these mutations decreased the affinity of the protein to DNA, the effect depending on the substitution and the DNA structure. The mutant as well as wild-type proteins bind with highest efficiency partly open damaged DNA duplex, and the affinity of the mutants to this DNA is reduced in the order: K204E > K179E >> K141E = K141/179E. For all the mutants, decrease in DNA binding efficiency was more pronounced in the case of full duplex and single-stranded DNA than with bubble-DNA structure, the difference between protein affinities to different DNA structures increasing as DNA binding activity of the mutant decreased. No effect of the studied XPA mutations on the location of the protein on the partially open DNA duplex was observed using photoinduced crosslinking with 5-I-dUMP in different positions of the damaged DNA strand. These results combined with earlier published data suggest no direct correlation between DNA binding and activity in NER for these XPA mutants.

Understanding nucleotide excision repair and its roles in cancer and ageing

Nucleotide excision repair (NER) eliminates various structurally unrelated DNA lesions by a multiwise 'cut and patch'-type reaction. The global genome NER (GG-NER) subpathway prevents mutagenesis by probing the genome for helix-distorting lesions, whereas transcription-coupled NER (TC-NER) removes transcription-blocking lesions to permit unperturbed gene expression, thereby preventing cell death. Consequently, defects in GG-NER result in cancer predisposition, whereas defects in TC-NER cause a variety of diseases ranging from ultraviolet radiation-sensitive syndrome to severe premature ageing conditions such as Cockayne syndrome. Recent studies have uncovered new aspects of DNA-damage detection by NER, how NER is regulated by extensive post-translational modifications, and the dynamic chromatin interactions that control its efficiency. Based on these findings, a mechanistic model is proposed that explains the complex genotype-phenotype correlations of transcription-coupled repair disorders.

A quantum chemical perspective on (6-4) photolesion repair by photolyases

(6-4)-Photolyases are fascinating enzymes which repair (6-4)-DNA photolesions utilizing light themselves. It is well known that upon initial photo-excitation of an antenna pigment an electron is transferred from an adjacent FADH(-) cofactor to the photolesion initiating repair, i.e. restoration of the original undamaged DNA bases. Concerning the molecular details of this amazing repair mechanism, the early steps of energy transfer and catalytic electron generation are well understood, the terminal repair mechanism, however, is still a matter of ongoing debate. In this perspective article, recent results of quantum chemical investigations are presented, and their meaning for the repair mechanism under natural conditions is outlined. Consequences of natural light conditions, temperature and thermal equilibration are highlighted when issues like the initial protonation state of the relevant histidines and the lesion, or the direction of electron transfer are discussed.

Molecular mechanism of global genome nucleotide excision repair

Nucleotide excision repair (NER) is a multistep process that recognizes and eliminates a wide spectrum of damage causing significant distortions in the DNA structure, such as UV-induced damage and bulky chemical adducts. The consequences of defective NER are apparent in the clinical symptoms of individuals affected by three disorders associated with reduced NER capacities: xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD). These disorders have in common increased sensitivity to UV irradiation, greatly elevated cancer incidence (XP), and multi-system immunological and neurological disorders. The eucaryotic NER system eliminates DNA damage by the excision of 24-32 nt single-strand oligonucleotides from a damaged strand, followed by restoration of an intact double helix by DNA repair synthesis and DNA ligation. About 30 core polypeptides are involved in the entire repair process. NER consists of two pathways distinct in initial damage sensor proteins: transcription-coupled repair (TC-NER) and global genome repair (GG-NER). The article reviews current knowledge on the molecular mechanisms underlying damage recognition and its elimination from mammalian DNA.

Strand-specific recognition of DNA damages by XPD provides insights into nucleotide excision repair substrate versatility

Recognition and removal of DNA damages is essential for cellular and organismal viability. Nucleotide excision repair (NER) is the sole mechanism in humans for the repair of carcinogenic UV irradiation-induced photoproducts in the DNA, such as cyclobutane pyrimidine dimers. The broad substrate versatility of NER further includes, among others, various bulky DNA adducts. It has been proposed that the 5'-3' helicase XPD (xeroderma pigmentosum group D) protein plays a decisive role in damage verification. However, despite recent advances such as the identification of a DNA-binding channel and central pore in the protein, through which the DNA is threaded, as well as a dedicated lesion recognition pocket near the pore, the exact process of target site recognition and verification in eukaryotic NER still remained elusive. Our single molecule analysis by atomic force microscopy reveals for the first time that XPD utilizes different recognition strategies to verify structurally diverse lesions. Bulky fluorescein damage is preferentially detected on the translocated strand, whereas the opposite strand preference is observed for a cyclobutane pyrimidine dimer lesion. Both states, however, lead to similar conformational changes in the resulting specific complexes, indicating a merge to a "final" verification state, which may then trigger the recruitment of further NER proteins.

Highly specific and sensitive method for measuring nucleotide excision repair kinetics of ultraviolet photoproducts in human cells

The nucleotide excision repair pathway removes ultraviolet (UV) photoproducts from the human genome in the form of short oligonucleotides ∼30 nt in length. Because there are limitations to many of the currently available methods for investigating UV photoproduct repair in vivo, we developed a convenient non-radioisotopic method to directly detect DNA excision repair events in human cells. The approach involves extraction of oligonucleotides from UV-irradiated cells, DNA end-labeling with biotin and streptavidin-mediated chemiluminescent detection of the excised UV photoproduct-containing oligonucleotides that are released from the genome during excision repair. Our novel approach is robust, with essentially no signal in the absence of UV or a functional excision repair system. Furthermore, our non-radioisotopic methodology allows for the sensitive detection of excision products within minutes following UV irradiation and does not require additional enrichment steps such as immunoprecipitation. Finally, this technique allows for quantitative measurements of excision repair in human cells. We suggest that the new techniques presented here will be a useful and powerful approach for studying the mechanism of human nucleotide excision repair in vivo.

Viral interference with DNA repair by targeting of the single-stranded DNA binding protein RPA

Correct repair of damaged DNA is critical for genomic integrity. Deficiencies in DNA repair are linked with human cancer. Here we report a novel mechanism by which a virus manipulates DNA damage responses. Infection with murine polyomavirus sensitizes cells to DNA damage by UV and etoposide. Polyomavirus large T antigen (LT) alone is sufficient to sensitize cells 100 fold to UV and other kinds of DNA damage. This results in activated stress responses and apoptosis. Genetic analysis shows that LT sensitizes via the binding of its origin-binding domain (OBD) to the single-stranded DNA binding protein replication protein A (RPA). Overexpression of RPA protects cells expressing OBD from damage, and knockdown of RPA mimics the LT phenotype. LT prevents recruitment of RPA to nuclear foci after DNA damage. This leads to failure to recruit repair proteins such as Rad51 or Rad9, explaining why LT prevents repair of double strand DNA breaks by homologous recombination. A targeted intervention directed at RPA based on this viral mechanism could be useful in circumventing the resistance of cancer cells to therapy.

Nucleotide excision repair in eukaryotes

Nucleotide excision repair (NER) is the main pathway used by mammals to remove bulky DNA lesions such as those formed by UV light, environmental mutagens, and some cancer chemotherapeutic adducts from DNA. Deficiencies in NER are associated with the extremely skin cancer-prone inherited disorder xeroderma pigmentosum. Although the core NER reaction and the factors that execute it have been known for some years, recent studies have led to a much more detailed understanding of the NER mechanism, how NER operates in the context of chromatin, and how it is connected to other cellular processes such as DNA damage signaling and transcription. This review emphasizes biochemical, structural, cell biological, and genetic studies since 2005 that have shed light on many aspects of the NER pathway.

Combined QM/MM investigation on the light-driven electron-induced repair of the (6-4) thymine dimer catalyzed by DNA photolyase

The (6-4) photolyases are blue-light-activated enzymes that selectively bind to DNA and initiate splitting of mutagenic thymine (6-4) thymine photoproducts (T(6-4)T-PP) via photoinduced electron transfer from flavin adenine dinucleotide anion (FADH(-)) to the lesion triggering repair. In the present work, the repair mechanism after the initial electron transfer and the effect of the protein/DNA environment are investigated theoretically by means of hybrid quantum mechanical/molecular mechanical (QM/MM) simulations using X-ray structure of the enzyme-DNA complex. By comparison of three previously proposed repair mechanisms, we found that the lowest activation free energy is required for the pathway in which the key step governing the repair photocycle is electron transfer coupled with the proton transfer from the protonated histidine, His365, to the N3' nitrogen of the pyrimidone thymine. The transfer simultaneously occurs with concerted intramolecular OH transfer without formation of an oxetane or isolated water molecule intermediate. In contrast to previously suggested mechanisms, this newly identified pathway requires neither a subsequent two-photon process nor electronic excitation of the photolesion.

Quantum chemical study of the enzymatic repair of T(6-4)C/C(6-4)T UV-photolesions by DNA photolyases

Several strategies have evolved to repair one of the abundant UV radiation-induced damages caused to DNA, namely the mutagenic pyrimidine (6-4) pyrimidone photolesions. DNA (6-4)-photolyases are enzymes repairing these lesions by a photoinitiated electron transfer. An important aspect of a possible repair mechanism is its generality and transferability to different (6-4) lesions. Therefore, previously suggested mechanisms for the repair of the T(6-4)T lesion are here transferred to the T(6-4)C and C(6-4)T lesions and investigated theoretically using quantum chemical methods. Despite the different functional groups of the pyrimidine bases involved, a general valid molecular mechanism was identified, in which the initial step is an electron transfer coupled to a proton transfer from the protonated HIS365 to the N3(') nitrogen of the 3(') pyrimidine, followed by an intramolecular OH/NH2 transfer in one concerted step, which does not require an oxetane/azetidine or isolated water/ammonia intermediate.

Nucleotide excision repair in human cells: fate of the excised oligonucleotide carrying DNA damage in vivo

Nucleotide excision repair is the sole mechanism for removing the major UV photoproducts from genomic DNA in human cells. In vitro with human cell-free extract or purified excision repair factors, the damage is removed from naked DNA or nucleosomes in the form of 24- to 32-nucleotide-long oligomers (nominal 30-mer) by dual incisions. Whether the DNA damage is removed from chromatin in vivo in a similar manner and what the fate of the excised oligomer was has not been known previously. Here, we demonstrate that dual incisions occur in vivo identical to the in vitro reaction. Further, we show that transcription-coupled repair, which operates in the absence of the XPC protein, also generates the nominal 30-mer in UV-irradiated XP-C mutant cells. Finally, we report that the excised 30-mer is released from the chromatin in complex with the repair factors TFIIH and XPG. Taken together, our results show the congruence of in vivo and in vitro data on nucleotide excision repair in humans.

New synthetic substrates of mammalian nucleotide excision repair system

DNA probes for the studies of damaged strand excision during the nucleotide excision repair (NER) have been designed using the novel non-nucleosidic phosphoramidite reagents that contain N-[6-(9-antracenylcarbamoyl)hexanoyl]-3-amino-1,2-propandiol (nAnt) and N-[6-(5(6)-fluoresceinylcarbamoyl)hexanoyl]-3-amino-1,2-propandiol (nFlu) moieties. New lesion-imitating adducts being inserted into DNA show good substrate properties in NER process. Modified extended linear nFlu– and nAntr–DNA are suitable for estimation of specific excision activity catalysed with mammalian whole-cell extracts. The following substrate activity range was revealed for the model 137-bp linear double-stranded DNA: nAnt–DNA ≈ nFlu–DNA > Chol–DNA (Chol–DNA—legitimate NER substrate that contains non-nucleoside fragment bearing cholesterol residue). In vitro assay shows that modified DNA can be a useful tool to study NER activity in whole-cell extracts. The developed approach should be of general use for the incorporation of NER-sensitive distortions into model DNAs. The new synthetic extended linear DNA containing bulky non-nucleoside modifications will be useful for NER mechanism study and for applications.

Intermolecular Coulombic Decay in Biology: The Initial Electron Detachment from FADH(-) in DNA Photolyases

Intermolecular coulombic decay (ICD) is an efficient mechanism of low-energy electron generation in condensed phases and is discussed as their potential source in living cells, tissues, and materials. The first example of ICD as an operating mechanism in real biological systems, that is, in the DNA repair enzymes photolyases, is presented. Photolyase function involves light-induced electron detachment from a reduced flavin adenine dinucleotide (FADH(-)), followed by its transfer to the DNA-lesion triggering repair of covalently bound nucleobase dimers. Modern quantum chemical methods are employed to demonstrate that the transferred electron is efficiently generated via a resonant ICD process between the antenna pigment and the FADH(-) cofactors.

NER initiation factors, DDB2 and XPC, regulate UV radiation response by recruiting ATR and ATM kinases to DNA damage sites

ATR and ATM kinases are central to the checkpoint activation in response to DNA damage and replication stress. However, the nature of the signal, which initially activates these kinases in response to UV damage, is unclear. Here, we have shown that DDB2 and XPC, two early UV damage recognition factors, are required for the damage-specific ATR and ATM recruitment and phosphorylation. ATR and ATM physically interacted with XPC and promptly localized to the UV damage sites. ATR and ATM recruitment and their phosphorylation were negatively affected in cells defective in DDB2 or XPC functions. Consequently, the phosphorylation of ATR and ATM substrates, Chk1, Chk2, H2AX, and BRCA1 was significantly reduced or abrogated in mutant cells. Furthermore, UV exposure of cells defective in DDB2 or XPC resulted in a marked decrease in BRCA1 and Rad51 recruitment to the damage site. Conversely, ATR- and ATM-deficiency failed to affect the recruitment of DDB2 and XPC to the damage site, and therefore did not influence the NER efficiency. These findings demonstrate a novel function of DDB2 and XPC in maintaining a vital cross-talk with checkpoint proteins, and thereby coordinating subsequent repair and checkpoint activation.

Functional analysis of Rad14p, a DNA damage recognition factor in nucleotide excision repair, in regulation of transcription in vivo

Rad14p is a DNA damage recognition factor in nucleotide excision repair. Intriguingly, we show here that Rad14p associates with the promoter of a galactose-inducible GAL1 gene after transcriptional induction in the absence of DNA lesion. Such an association of Rad14p facilitates the recruitment of TBP, TFIIH, and RNA polymerase II to the GAL1 promoter. Furthermore, the association of RNA polymerase II with the GAL1 promoter is significantly decreased in the absence of Rad14p, when the coding sequence was deleted. These results support the role of Rad14p in transcriptional initiation. Consistently, the level of GAL1 mRNA is significantly decreased in the absence of Rad14p. Similar results are also obtained at other galactose-inducible GAL genes such as GAL7 and GAL10. Likewise, Rad14p promotes transcription of other non-GAL genes such as CUP1, CTT1, and STL1 after transcriptional induction. However, the effect of Rad14p on the steady-state levels of transcription of GAL genes or constitutively active genes such as ADH1, PGK1, PYK1, and RPS5 is not observed. Thus, Rad14p promotes initial transcription but does not appear to regulate the steady-state level. Collectively, our results unveil a new role of Rad14p in stimulating transcription in addition to its well-known function in nucleotide excision repair.

Thymine dimer-induced structural changes to the DNA duplex examined with reactive probes (†)

Despite significant progress in the past decade, questions still remain about the complete structural, dynamic, and thermodynamic effect of the cis-syn cyclobutane pyrimidine dimer lesion (hereafter called the thymine dimer) on double-stranded genomic DNA. We examined a 19-mer oligodeoxynucleotide duplex containing a thymine dimer lesion using several small, base-selective reactive chemical probes. These molecules probe whether the presence of the dimer causes the base pairs to be more accessible to the solution, either globally or adjacent to the dimer. Though all of the probes confirm that the overall structure of the dimer-containing duplex is conserved compared to that of the undamaged parent duplex, reactions with both diethyl pyrocarbonate and Rh(bpy)(2)(chrysi)(3+) indicate that the duplex is locally destabilized near the lesion. Reactions with potassium permanganate and DEPC hint that the dimer-containing duplex may also be globally more accessible to the solution through a subtle shift in the double-stranded DNA ↔ single-stranded DNA equilibrium. To begin to distinguish between kinetic and thermodynamic effects, we determined the helix melting thermodynamic parameters for the dimer-containing and undamaged parent duplexes by microcalorimetry and UV melting. The presence of the thymine dimer causes this DNA duplex to be slightly less stable enthalpically but slightly less unstable entropically at 298 K, causing the overall free energy of duplex melting to remain unchanged by the dimer lesion within the error of the experiment. Here we consider these results in the context of what has been learned about the thymine dimer lesion from NMR, X-ray crystallographic, and molecular biological methods.

Damaged DNA binding protein 2 in reactive oxygen species (ROS) regulation and premature senescence

Premature senescence induced by DNA damage or oncogene is a critical mechanism of tumor suppression. Reactive oxygen species (ROS) have been implicated in the induction of premature senescence response. Several pathological disorders such as cancer, aging and age related neurological abnormalities have been linked to ROS deregulation. Here, we discuss how Damaged DNA binding Protein-2 (DDB2), a nucleotide excision repair protein, plays an important role in ROS regulation by epigenetically repressing the antioxidant genes MnSOD and Catalase. We further revisit a model in which DDB2 plays an instrumental role in DNA damage induced ROS accumulation, ROS induced premature senescence and inhibition of skin tumorigenesis.

TFIIH: when transcription met DNA repair

The transcription initiation factor TFIIH is a remarkable protein complex that has a fundamental role in the transcription of protein-coding genes as well as during the DNA nucleotide excision repair pathway. The detailed understanding of how TFIIH functions to coordinate these two processes is also providing an explanation for the phenotypes observed in patients who bear mutations in some of the TFIIH subunits. In this way, studies of TFIIH have revealed tight molecular connections between transcription and DNA repair and have helped to define the concept of 'transcription diseases'.

Mechanism of release and fate of excised oligonucleotides during nucleotide excision repair

A wide range of environmental and carcinogenic agents form bulky lesions on DNA that are removed from the human genome in the form of short, ∼30-nucleotide oligonucleotides by the process of nucleotide excision repair. Although significant insights have been made regarding the mechanisms of damage recognition, dual incisions, and repair resynthesis during nucleotide excision repair, the fate of the dual incision/excision product is unknown. Using excision assays with both mammalian cell-free extract and purified proteins, we unexpectedly discovered that lesion-containing oligonucleotides are released from duplex DNA in complex with the general transcription and repair factor, Transcription Factor IIH (TFIIH). Release of excision products from TFIIH requires ATP but not ATP hydrolysis, and release occurs slowly, with a t(1/2) of 3.3 h. Excised oligonucleotides released from TFIIH then become bound by the single-stranded binding protein Replication Protein A or are targeted by cellular nucleases. These results provide a mechanism for release and an understanding of the initial fate of excised oligonucleotides during nucleotide excision repair.

p21 cooperates with DDB2 protein in suppression of ultraviolet ray-induced skin malignancies

Exposure to ultraviolet rays (UV) in sunlight is the main cause of skin cancer. Here, we show that the p53-induced genes DDB2 and p21 are down-regulated in skin cancer, and in the mouse model they functionally cooperate to prevent UV-induced skin cancer. Our previous studies demonstrated an antagonistic role of DDB2 and p21 in nucleotide excision repair and apoptosis. Surprisingly, we find that the loss of p21 restores nucleotide excision repair and apoptosis in Ddb2(-/-) mice, but it does not protect from UV-mediated skin carcinogenesis. In contrast, Ddb2(-/-)p21(-/-) mice are significantly more susceptible to UV-induced skin cancer than the Ddb2(-/-) or the p21(-/-) mice. We provide evidence that p21 deletion in the Ddb2(-/-) background causes a strong increase in cell proliferation. The increased proliferation in the Ddb2(-/-)p21(-/-) background is related to a severe deficiency in UV-induced premature senescence. Also, the oncogenic pro-proliferation transcription factor FOXM1 is overexpressed in the p21(-/-) background. Our results show that the anti-proliferative and the pro-senescence pathways of DDB2 and p21 are critical protection mechanisms against skin malignancies.

Regulation of nucleotide excision repair by UV-DDB: prioritization of damage recognition to internucleosomal DNA

This study reveals the molecular mechanism by which the nucleotide excision repair protein DDB2 prioritises excision of UV-induced DNA lesions in the nucleosome landscape.

How tightly packed chromatin is thoroughly inspected for DNA damage is one of the fundamental unanswered questions in biology. In particular, the effective excision of carcinogenic lesions caused by the ultraviolet (UV) radiation of sunlight depends on UV-damaged DNA-binding protein (UV-DDB), but the mechanism by which this DDB1-DDB2 heterodimer stimulates DNA repair remained enigmatic. We hypothesized that a distinctive function of this unique sensor is to coordinate damage recognition in the nucleosome repeat landscape of chromatin. Therefore, the nucleosomes of human cells have been dissected by micrococcal nuclease, thus revealing, to our knowledge for the first time, that UV-DDB associates preferentially with lesions in hypersensitive, hence, highly accessible internucleosomal sites joining the core particles. Surprisingly, the accompanying CUL4A ubiquitin ligase activity is necessary to retain the xeroderma pigmentosum group C (XPC) partner at such internucleosomal repair hotspots that undergo very fast excision kinetics. This CUL4A complex thereby counteracts an unexpected affinity of XPC for core particles that are less permissive than hypersensitive sites to downstream repair subunits. That UV-DDB also adopts a ubiquitin-independent function is evidenced by domain mapping and in situ protein dynamics studies, revealing direct but transient interactions that promote a thermodynamically unfavorable β-hairpin insertion of XPC into substrate DNA. We conclude that the evolutionary advent of UV-DDB correlates with the need for a spatiotemporal organizer of XPC positioning in higher eukaryotic chromatin.

Like all molecules in living organisms, DNA undergoes spontaneous decay and is constantly under attack by endogenous and environmental agents. Unlike other molecules, however, DNA—the blueprint of heredity—cannot be re-created de novo; it can only be copied. The original blueprint must therefore remain pristine. All kinds of DNA damage pose a health hazard. DNA lesions induced by the ultraviolet (UV) component of sunlight, for example, can lead to skin aging and skin cancer. A repair process known as nucleotide excision repair (NER) is dedicated to correcting this UV damage. Although the enzymatic steps of this repair process are known in detail, we still do not understand how it copes with the native situation in the cell, where the DNA is tightly wrapped around protein spools called nucleosomes. Our study has revealed the molecular mechanism by which an enigmatic component of NER called UV-DDB stimulates excision of UV-induced lesions in the landscape of nucleosome-packaged DNA in human skin cells. In particular, we describe how this accessory protein prioritizes, in space and time, which UV lesions in packaged DNA to target for repair by NER complexes, thus optimizing the repair process.

Nucleotide excision repair: DNA damage recognition and preincision complex assembly

Nucleotide excision repair (NER) is one of the major DNA repair pathways in eukaryotic cells counteracting genetic changes caused by DNA damage. NER removes a wide set of structurally diverse lesions such as pyrimidine dimers arising upon UV irradiation and bulky chemical adducts arising upon exposure to carcinogens or chemotherapeutic drugs. NER defects lead to severe diseases including some forms of cancer. In view of the broad substrate specificity of NER, it is of interest to understand how a certain set of proteins recognizes various DNA lesions in the context of a large excess of intact DNA. This review focuses on DNA damage recognition and following stages resulting in preincision complex assembly, the key and still most unclear steps of NER. The major models of primary damage recognition and preincision complex assembly are considered. The contribution of affinity labeling techniques in study of this process is discussed.

Role of interaction of XPF with RPA in nucleotide excision repair

Nucleotide excision repair (NER) is a very important defense system against various types of DNA damage, and it is necessary for maintaining genomic stability. The molecular mechanism of NER has been studied in considerable detail, and it has been shown that proper protein-protein interactions among NER factors are critical for efficient repair. A structure-specific endonuclease, XPF-ERCC1, which makes the 5' incision in NER, was shown to interact with a single-stranded DNA binding protein, RPA. However, the biological significance of this interaction was not studied in detail. We used the yeast two-hybrid assay to determine that XPF interacts with the p70 subunit of RPA. To further examine the role of this XPF-p70 interaction, we isolated a p70-interaction-deficient mutant form of XPF that contains a single amino acid substitution in the N-terminus of XPF by the reverse yeast two-hybrid assay using randomly mutagenized XPF. The biochemical properties of this RPA-interaction-deficient mutant XPF-ERCC1 are very similar to those of wild-type XPF-ERCC1 in vitro. Interestingly, expression of this mutated form of XPF in the XPF-deficient Chinese hamster ovary cell line, UV41, only partially restores NER activity and UV resistance in vivo compared to wild-type XPF. We discovered that the RPA-interaction-deficient XPF is not localized in nuclei and the mislocalization of XPF-ERCC1 prevents the complex from functioning in NER.

Multiple ATR-Chk1 pathway proteins preferentially associate with checkpoint-inducing DNA substrates

The ATR-Chk1 DNA damage checkpoint pathway is a critical regulator of the cellular response to DNA damage and replication stress in human cells. The variety of environmental, chemotherapeutic, and carcinogenic agents that activate this signal transduction pathway do so primarily through the formation of bulky adducts in DNA and subsequent effects on DNA replication fork progression. Because there are many protein-protein and protein-DNA interactions proposed to be involved in activation and/or maintenance of ATR-Chk1 signaling in vivo, we systematically analyzed the association of a number of ATR-Chk1 pathway proteins with relevant checkpoint-inducing DNA structures in vitro. These DNA substrates included single-stranded DNA, branched DNA, and bulky adduct-containing DNA. We found that many checkpoint proteins show a preference for single-stranded, branched, and bulky adduct-containing DNA in comparison to undamaged, double-stranded DNA. We additionally found that the association of checkpoint proteins with bulky DNA damage relative to undamaged DNA was strongly influenced by the ionic strength of the binding reaction. Interestingly, among the checkpoint proteins analyzed the checkpoint mediator proteins Tipin and Claspin showed the greatest differential affinity for checkpoint-inducing DNA structures. We conclude that the association and accumulation of multiple checkpoint proteins with DNA structures indicative of DNA damage and replication stress likely contribute to optimal ATR-Chk1 DNA damage checkpoint responses.

Regulation of nucleotide excision repair activity by transcriptional and post-transcriptional control of the XPA protein

The XPA (Xeroderma pigmentosum A) protein is one of the six core factors of the human nucleotide excision repair system. In this study we show that XPA is a rate-limiting factor in all human cell lines tested, including a normal human fibroblast cell line. The level of XPA is controlled at the transcriptional level by the molecular circadian clock and at the post-translational level by a HECT domain family E3 ubiquitin ligase called HERC2. Stabilization of XPA by downregulation of HERC2 moderately enhances excision repair activity. Conversely, downregulation of XPA by siRNA reduces excision repair activity in proportion to the level of XPA. Ubiquitination and proteolysis of XPA are inhibited by DNA damage that promotes tight association of the protein with chromatin and its dissociation from the HERC2 E3 ligase. Finally, in agreement with a recent report, we find that XPA is post-translationally modified by acetylation. However, contrary to the previous claim, we find that in mouse liver only a small fraction of XPA is acetylated and that downregulation of SIRT1 deacetylase in two human cell lines does not affect the overall repair rate. Collectively, the data reveal that XPA is a limiting factor in excision repair and that its level is coordinately regulated by the circadian clock, the ubiquitin–proteasome system and DNA damage.

GCN5 and E2F1 stimulate nucleotide excision repair by promoting H3K9 acetylation at sites of damage

Chromatin structure is known to be a barrier to DNA repair and a large number of studies have now identified various factors that modify histones and remodel nucleosomes to facilitate repair. In response to ultraviolet (UV) radiation several histones are acetylated and this enhances the repair of DNA photoproducts by the nucleotide excision repair (NER) pathway. However, the molecular mechanism by which UV radiation induces histone acetylation to allow for efficient NER is not completely understood. We recently discovered that the E2F1 transcription factor accumulates at sites of UV-induced DNA damage and directly stimulates NER through a non-transcriptional mechanism. Here we demonstrate that E2F1 associates with the GCN5 acetyltransferase in response to UV radiation and recruits GCN5 to sites of damage. UV radiation induces the acetylation of histone H3 lysine 9 (H3K9) and this requires both GCN5 and E2F1. Moreover, as previously observed for E2F1, knock down of GCN5 results in impaired recruitment of NER factors to sites of damage and inefficient DNA repair. These findings demonstrate a direct role for GCN5 and E2F1 in NER involving H3K9 acetylation and increased accessibility to the NER machinery.

Strand- and site-specific DNA lesion demarcation by the xeroderma pigmentosum group D helicase

The most detrimental responses of the UV-exposed skin are triggered by cyclobutane pyrimidine dimers (CPDs). Although placental mammals rely solely on nucleotide excision repair (NER) to eliminate CPDs, none of the core NER factors are apparently able to distinguish this hazardous lesion from native DNA, raising the question of how CPDs are circumscribed to define correct excision boundaries. A key NER intermediate involves unwinding of the damaged duplex by transcription factor TFIIH, a reaction that requires xeroderma pigmentosum group D (XPD) protein. This study was prompted by the observation that the ATPase/helicase activity of XPD is necessary for an effective anchoring of this subunit to UV lesions in mammalian nuclei. The underlying mechanism by which XPD impinges on damaged DNA has been probed with a monomeric archaeal homolog, thus revealing that the collision with a single CPD inhibits the helicase but stimulates its ATPase activity. Restriction and glycosylase protection assays show that the XPD helicase remains firmly bound to a CPD situated in the translocated strand along which the enzyme moves with 5'-3' polarity. Competition assays confirm that a stable complex is formed when the XPD helicase encounters a CPD in the translocated strand. Instead, the enzyme dissociates from the substrate after running into a CPD in the complementary 3'-5' strand. These results disclose a damage verification and demarcation process that takes place by strand-selective immobilization of the XPD helicase and its conversion to a site-specific ATPase at DNA lesions.

Reaction mechanisms of DNA photolyase

DNA photolyase uses visible light and a fully reduced flavin cofactor FADH(-) to repair major UV-induced lesions in DNA, the cyclobutane pyrimidine dimers (CPDs). Electron transfer from photoexcited FADH(-) to CPD, splitting of the two intradimer bonds, and back electron transfer to the transiently formed flavin radical FADH° occur in overall 1ns. Whereas the kinetics of FADH° was resolved, the DNA-based intermediates escaped unambiguous detection yet. Another light reaction, named photoactivation, reduces catalytically inactive FADH° to FADH(-) without implication of DNA. It involves electron hopping along a chain of three tryptophan residues in 30ps, as elucidated in detail by transient absorption spectroscopy. The same triple tryptophan chain is found in cryptochrome blue-light photoreceptors and may be involved in their primary photoreaction.

DDB2, an essential mediator of premature senescence

Reactive oxygen species (ROS) is critical for premature senescence, a process significant in tumor suppression and cancer therapy. Here, we reveal a novel function of the nucleotide excision repair protein DDB2 in the accumulation of ROS in a manner that is essential for premature senescence. DDB2-deficient cells fail to undergo premature senescence induced by culture shock, exogenous oxidative stress, oncogenic stress, or DNA damage. These cells do not accumulate ROS following DNA damage. The lack of ROS accumulation in DDB2 deficiency results from high-level expression of the antioxidant genes in vitro and in vivo. DDB2 represses antioxidant genes by recruiting Cul4A and Suv39h and by increasing histone-H3K9 trimethylation. Moreover, expression of DDB2 also is induced by ROS. Together, our results show that, upon oxidative stress, DDB2 functions in a positive feedback loop by repressing the antioxidant genes to cause persistent accumulation of ROS and induce premature senescence.

Electronic structure of (6-4) DNA photoproduct repair involving a non-oxetane pathway

Mutagenic pyrimidine-pyrimidone (6-4) photoproducts are one of the main DNA lesions induced by solar UV radiation. These lesions can be photoreversed by (6-4) photolyases. The originally published repair mechanism involves rearrangement of the lesion into an oxetane intermediate upon binding to the (6-4) photolyase, followed by light-induced electron transfer from the reduced flavin cofactor. In a recent crystallographic study on a (6-4) photoproduct complexed with (6-4) photolyase from Drosophila melanogaster no oxetane was observed, raising the possibility of a non-oxetane repair mechanism. Using quantum-chemical calculations we find that in addition to repair via an oxetane, a direct transfer of the hydroxyl group results in reversal of the radical anion (6-4) photoproduct. In both mechanisms, the transition states have high energies and correspond to avoided crossings of the ground and excited electronic states. To study whether the repair can proceed via these state crossings, the excited-state potential energy curves were computed. The radical excitation energies and accessibility of the nonadiabatic repair path were found to depend on hydrogen bonds and the protonation state of the lesion. On the basis of the energy calculations, a nonadiabatic repair of the excited (6-4) lesion radical anion via hydroxyl transfer is probable. This repair mechanism is in line with the recent structural data on the (6-4) photolyase from D. melanogaster .

Nucleotide excision repair in higher eukaryotes: mechanism of primary damage recognition in global genome repair

Nucleotide excision repair (NER) is one of the major DNA repair pathways in eukaryotic cells that counteract the formation of genetic damage. NER removes structurally diverse lesions such as pyrimidine dimers, arising upon UV irradiation, and bulky chemical adducts, arising upon exposure to carcinogens and some chemotherapeutic drugs. NER defects lead to severe diseases, including some forms of cancer. In view of the broad substrate specificity of NER, it is of interest to understand how a certain set of proteins recognizes various DNA lesions in the contest of a large excess of intact DNA. This review focuses on DNA damage recognition, the key and, as yet, most questionable step of NER. Understanding of mechanism of this step of NER may give a key contribution to study of similar processes of DNA damage recognition (base excision repair, mismatch repair) and regulation of assembly of various DNA repair machines. The major models of primary damage recognition and pre-incision complex assembly are considered. The model of a sequential loading of repair proteins on damaged DNA seems most reasonable in the light of the available data. The possible contribution of affinity labeling technique in study of this process is discussed.

An alternative form of replication protein a expressed in normal human tissues supports DNA repair

Replication protein A (RPA) is a heterotrimeric protein complex required for a large number of DNA metabolic processes, including DNA replication and repair. An alternative form of RPA (aRPA) has been described in which the RPA2 subunit (the 32-kDa subunit of RPA and product of the RPA2 gene) of canonical RPA is replaced by a homologous subunit, RPA4. The normal function of aRPA is not known; however, previous studies have shown that it does not support DNA replication in vitro or S-phase progression in vivo. In this work, we show that the RPA4 gene is expressed in normal human tissues and that its expression is decreased in cancerous tissues. To determine whether aRPA plays a role in cellular physiology, we investigated its role in DNA repair. aRPA interacted with both Rad52 and Rad51 and stimulated Rad51 strand exchange. We also showed that, by using a reconstituted reaction, aRPA can support the dual incision/excision reaction of nucleotide excision repair. aRPA is less efficient in nucleotide excision repair than canonical RPA, showing reduced interactions with the repair factor XPA and no stimulation of XPF-ERCC1 endonuclease activity. In contrast, aRPA exhibits higher affinity for damaged DNA than canonical RPA, which may explain its ability to substitute for RPA in the excision step of nucleotide excision repair. Our findings provide the first direct evidence for the function of aRPA in human DNA metabolism and support a model for aRPA functioning in chromosome maintenance functions in nonproliferating cells.

The human intra-S checkpoint response to UVC-induced DNA damage

The intra-S checkpoint response to 254 nm light (UVC)-induced DNA damage appears to have dual functions to slow the rate of DNA synthesis and stabilize replication forks that become stalled at sites of UVC-induced photoproducts in DNA. These functions should provide more time for repair of damaged DNA before its replication and thereby reduce the frequencies of mutations and chromosomal aberrations in surviving cells. This review tries to summarize the history of discovery of the checkpoint, the current state of understanding of the biological features of intra-S checkpoint signaling and its mechanisms of action with a focus primarily on intra-S checkpoint responses in human cells. The differences in the intra-S checkpoint responses to UVC and ionizing radiation-induced DNA damage are emphasized. Evidence that [6-4]pyrimidine-pyrimidone photoproducts in DNA trigger the response is discussed and the relationships between cellular responses to UVC and the molecular dose of UVC-induced DNA damage are briefly summarized. The role of the intra-S checkpoint response in protecting against solar radiation carcinogenesis remains to be determined.

Role of DNA repair in the protection against genotoxic stress

The genome of all organisms is constantly attacked by a variety of environmental and endogenous mutagens that cause cell death, apoptosis, senescence, genetic diseases and cancer. To mitigate these deleterious endpoints of genotoxic reactions, living organisms have evolved one or more mechanisms for repairing every type of naturally occurring DNA lesion. For example, double-strand breaks are rapidly religated by non-homologous end-joining. Homologous recombination is used for the high-fidelity repair of interstrand cross-links, double-strand breaks and other DNA injuries that disrupt the replication fork. Some genotoxic lesions inflicted by alkylating agents can be repaired by direct reversal of DNA damage. The base excision repair pathway takes advantage of multiple DNA glycosylases to remove modified or incorrect bases. Finally, the nucleotide excision repair machinery provides a versatile strategy to monitor DNA quality and eliminate all forms of helix-distorting DNA lesions, including a wide diversity of carcinogen adducts. The efficiency of DNA repair responses is enhanced by their coupling to transcription and coordination with the cell cycle circuit.

The sequence dependence of human nucleotide excision repair efficiencies of benzo[a]pyrene-derived DNA lesions: insights into the structural factors that favor dual incisions

Nucleotide excision repair (NER) is a vital cellular defense system against carcinogen-DNA adducts, which, if not repaired, can initiate cancer development. The structural features of bulky DNA lesions that account for differences in NER efficiencies in mammalian cells are not well understood. In vivo, the predominant DNA adduct derived from metabolically activated benzo[a]pyrene (BP), a prominent environmental carcinogen, is the 10S (+)-trans-anti-[BP]-N(2)-dG adduct (G*), which resides in the B-DNA minor groove 5'-oriented along the modified strand. We have compared the structural distortions in double-stranded DNA, imposed by this adduct, in the different sequence contexts 5'-...CGG*C..., 5'-...CG*GC..., 5'-...CIG*C... (I is 2'-deoxyinosine), and 5'-...CG*C.... On the basis of electrophoretic mobilities, all duplexes manifest moderate bends, except the 5'-...CGG*C...duplex, which exhibits an anomalous, slow mobility attributed to a pronounced flexible kink at the site of the lesion. This kink, resulting from steric hindrance between the 5'-flanking guanine amino group and the BP aromatic rings, both positioned in the minor groove, is abolished in the 5'-...CIG*C...duplex (the 2'-deoxyinosine group, I, lacks this amino group). In contrast, the sequence-isomeric 5'-...CG*GC...duplex exhibits only a moderate bend, but displays a remarkably increased opening rate at the 5'-flanking base pair of G*, indicating a significant destabilization of Watson-Crick hydrogen bonding. The NER dual incision product yields were compared for these different sequences embedded in otherwise identical 135-mer duplexes in cell-free human HeLa extracts. The yields of excision products varied by a factor of as much as approximately 4 in the order 5'-...CG*GC...>5'...CGG*C...>or=5'...CIG*C...>or=5'-...CG*C.... Overall, destabilized Watson-Crick hydrogen bonding, manifested in the 5'-...CG*GC...duplex, elicits the most significant NER response, while the flexible kink displayed in the sequence-isomeric 5'-...CGG*C...duplex represents a less significant signal in this series of substrates. These results demonstrate that the identical lesion can be repaired with markedly variable efficiency in different local sequence contexts that differentially alter the structural features of the DNA duplex around the lesion site.

Physical and functional interaction between DDB and XPA in nucleotide excision repair

Damaged DNA-binding protein (DDB), consisting of DDB1 and DDB2 subunits recognizes a wide spectrum of DNA lesions. DDB is dispensable for in vitro nucleotide excision repair (NER) reaction, but stimulates this reaction especially for cyclobutane pyrimidine dimer (CPD). Here we show that DDB directly interacts with XPA, one of core NER factors, mainly through DDB2 subunit and the amino-acid residues between 185 and 226 in XPA are important for the interaction. Interestingly, the point mutation causing the substitution from Arg-207 to Gly, which was previously identified in a XP-A revertant cell-line XP129, diminished the interaction with DDB in vitro and in vivo. In a defined system containing R207G mutant XPA and other core NER factors, DDB failed to stimulate the excision of CPD, although the mutant XPA was competent for the basal NER reaction. Moreover, in vivo experiments revealed that the mutant XPA is recruited to damaged DNA sites with much less efficiency compared with wild-type XPA and fails to support the enhancement of CPD repair by ectopic expression of DDB2 in SV40-transformed human cells. These results suggest that the physical interaction between DDB and XPA plays an important role in the DDB-mediated NER reaction.

XPA gene, its product and biological roles

The 31 kDa XPA protein is part of the core incision complex of the mammalian nucleotide excision repair (NER) system and interacts with DNA as well as with many other NER subunits. In the absence of XPA, no incision complex can form and no excision of damaged DNA damage occurs. A comparative analysis of the DNA-binding properties in the presence of different substrate conformations indicated that XPA protein interacts preferentially with kinked DNA backbones. The DNA-binding domain of XPA protein displays a positively charged deft that is involved in an indirect readout mechanism, presumably by detecting the increased negative potential encountered at sharp DNA bends. We propose that this indirect recognition function contributes to damage verification by probing the susceptibility of the DNA substrate to be kinked during the assembly of NER complexes.

Interaction of nucleotide excision repair factors XPC-HR23B, XPA, and RPA with damaged DNA

The interaction of nucleotide excision repair factors--xeroderma pigmentosum complementation group C protein in complex with human homolog of yeast Rad23 protein (XPC-HR23B), replication protein A (RPA), and xeroderma pigmentosum complementation group A protein (XPA)--with 48-mer DNA duplexes imitating damaged DNA structures was investigated. All studied proteins demonstrated low specificity in binding to damaged DNA compared with undamaged DNA duplexes. RPA stimulates formation of XPC-HR23B complex with DNA, and when XPA and XPC-HR23B are simultaneously present in the reaction mixture a synergistic effect in binding of these proteins to DNA is observed. RPA crosslinks to DNA bearing photoreactive 5I-dUMP residue on one strand and fluorescein-substituted dUMP analog as a lesion in the opposite strand of DNA duplex and also stimulates cross-linking with XPC-HR23B. Therefore, RPA might be one of the main regulation factors at various stages of nucleotide excision repair. The data are in agreement with the cooperative binding model of nucleotide excision repair factors participating in pre-incision complex formation with DNA duplexes bearing damages.

RPA repair recognition of DNA containing pyrimidines bearing bulky adducts

Recognition of new DNA nucleotide excision repair (NER) substrate analogs, 48-mer ddsDNA (damaged double-stranded DNA), by human replication protein A (hRPA) has been analyzed using fluorescence spectroscopy and photoaffinity modification. The aim of the present work was to find quantitative characteristics of RPA-ddsDNA interaction and RPA subunits role in this process. The designed DNA structures bear bulky substituted pyrimidine nitrogen bases at the inner positions of duplex forming DNA chains. The photoreactive 4-azido-2,5-difluoro-3- pyridin-6-yl (FAP) and fluorescent antracenyl, pyrenyl (Antr, Pyr) groups were introduced via different linker fragments into exo-4N of deoxycytidine or 5C of deoxyuridine. J-dU-containing DNA was used as a photoactive model of undamaged DNA strands. The reporter group was a fluorescein residue, introduced into the 5'-phosphate end of one duplex-forming DNA strand. RPA-dsDNA association constants and the molar RPA/dsDNA ratio have been calculated based on fluorescence anisotropy measurements under conditions of a 1:1 RPA/dsDNA molar ratio in complexes. The evident preference for RPA binding to ddsDNA over undamaged dsDNA distinctly depends on the adduct type and varies in the following way: undamaged dsDNA < Antr-dC-ddsDNA < mmdsDNA < FAPdU-, Pyr-dU-ddsDNA < FAP-dC-ddsDNA (K(D) = 68 +/- 1; 25 +/- 6; 13 +/- 1; 8 +/- 2, and 3.5 +/- 0.5 nM correspondingly) but weakly depends on the chain integrity. Interestingly the bulkier lesions not in all cases have a greater effect on RPA affinity to ddsDNA. The experiments on photoaffinity modification demonstrated only p70 of compactly arranged RPA directly interacting with dsDNA. The formation of RPA-ddsDNA covalent adducts was drastically reduced when both strands of DNA duplex contained virtually opposite located FAP-dC and Antr-dC. Thus RPA requires undamaged DNA strand presence for the effective interaction with dsDNA bearing bulky damages and demonstrates the early NER factors characteristic features underlying strand discrimination capacity and poor activity of the NER system toward double damaged DNA.

Crystal structure of the FeS cluster-containing nucleotide excision repair helicase XPD

DNA damage recognition by the nucleotide excision repair pathway requires an initial step identifying helical distortions in the DNA and a proofreading step verifying the presence of a lesion. This proofreading step is accomplished in eukaryotes by the TFIIH complex. The critical damage recognition component of TFIIH is the XPD protein, a DNA helicase that unwinds DNA and identifies the damage. Here, we describe the crystal structure of an archaeal XPD protein with high sequence identity to the human XPD protein that reveals how the structural helicase framework is combined with additional elements for strand separation and DNA scanning. Two RecA-like helicase domains are complemented by a 4Fe4S cluster domain, which has been implicated in damage recognition, and an α-helical domain. The first helicase domain together with the helical and 4Fe4S-cluster–containing domains form a central hole with a diameter sufficient in size to allow passage of a single stranded DNA. Based on our results, we suggest a model of how DNA is bound to the XPD protein, and can rationalize several of the mutations in the human XPD gene that lead to one of three severe diseases, xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy.

Preserving the structural integrity of DNA, and hence the genetic information stored in this molecule, is essential for cellular survival. It is estimated that the DNA in each human cell acquires about 10⁴ lesions per day. Consequently, efficient DNA repair mechanisms have evolved to protect the genome. One of these DNA repair mechanisms, nucleotide excision repair (NER), is present in all organisms and is unique in its ability to repair a broad range of damage. In humans, NER is the major repair mechanism protecting DNA from damage induced by ultraviolet light. Defects in the genes and proteins responsible for NER can lead to one of three severe diseases: xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy. The XPD protein is one of the key components of a ten-protein complex and is essential to initiate NER. In particular, the XPD protein verifies the presence of damage to the DNA and thereby allows DNA repair to proceed. We have solved the 3-dimensional structure of the XPD protein, and show how XPD has assembled several domains to form a donut-shaped molecule, which is able to separate two DNA strands and scan the DNA for damage. The structure also helps to explain why some of the mutations that have been identified in humans are associated with disease.

The structure of the DNA repair protein XPD provides insights into how the protein binds and recognizes damaged DNA and how mutations inXPD disrupt its function and lead to disease.

Fibroblasts from naked mole-rats are resistant to multiple forms of cell injury, but sensitive to peroxide, ultraviolet light, and endoplasmic reticulum stress

Fibroblasts from long-lived mutant mice are resistant to many forms of lethal injury as well as to the metabolic effects of rotenone and low-glucose medium. Here we evaluated fibroblasts from young adult naked mole-rats (NMR; Heterocephalus glaber), a rodent species in which maximal longevity exceeds 28 years. Compared to mouse cells, NMR cells were resistant to cadmium, methyl methanesulfonate, paraquat, heat, and low-glucose medium, consistent with the idea that cellular resistance to stress may contribute to disease resistance and longevity. Surprisingly, NMR cells were more sensitive than mouse cells to H(2)O(2), ultraviolet (UV) light, and rotenone. NMR cells, like cells from Snell dwarf mice, were more sensitive to tunicamycin and thapsigargin, which interfere with the function of the endoplasmic reticulum (ER stress). The sensitivity of both Snell dwarf and NMR cells to ER stress suggests that alterations in the unfolded protein response might modulate cell survival and aging rate.

Cells from long-lived mutant mice exhibit enhanced repair of ultraviolet lesions

Fibroblasts isolated from long-lived hypopituitary dwarf mice are resistant to many cell stresses, including ultraviolet (UV) light and methyl methane sulfonate (MMS), which induce cell death by producing DNA damage. Here we report that cells from Snell dwarf mice recover more rapidly than controls from the inhibition of RNA synthesis induced by UV damage. Recovery of messenger RNA (mRNA) synthesis in particular is more rapid in dwarf cells, suggesting enhanced repair of the actively transcribing genes in dwarf-derived cells. At early time points, there was no difference in the repair of cyclobutane pyrimidine dimers (CPD) or 6-4 photoproducts (6-4PP) in the whole genome, nor was there any significant difference in the repair of UV lesions in specific genes. However, at later time points we found that more lesions had been removed from the genome of dwarf-derived cells. We have also found that cells from dwarf mice express higher levels of the nucleotide excision repair proteins XPC and CSA, suggesting a causal link to enhanced DNA repair. Overall, these data suggest a mechanism for the UV resistance of Snell dwarf-derived fibroblasts that could contribute to the delay of aging and neoplasia in these mice.

Nucleotide excision repair impairment by nodularin in CHO cell lines due to ERCC1/XPF inactivation

The problem of toxicity of cyanobacterial toxins is of increasing concern, as the incidence of such blooms grows. Among the toxins, the most abundant in the environment are hepatotoxins known as nodularins and microcystins. These toxins are responsible for almost all known cases of fresh and brackish water intoxication and are responsible for recurrent episodes of human and animal illness and death. Moreover, they are believed to be potent tumor promoters and initiators. However, the mechanisms by which these toxins induce liver cancer are not well understood. The aim of the present study was to determine the effect of nodularin on the kinetics of nucleotide excision repair (NER) in Chinese hamster ovary (CHO) cells exposed to UV radiation. The first set of experiments was performed to define the optimal treatment conditions for nodularin to avoid the possibility of encountering false positive signals in the comet assay due to the apoptogenic activity of nodularin. Based on the analysis of apoptosis, the 6-h treatment time of cells with nodularin (1mug/ml, 10mug/ml and 20mug/ml) was chosen for the alkaline comet assay. The kinetics of NER was determined in CHO cell lines: AA8 (wild-type) and mutant cell lines: UV135 (XPG(-)), UV41 (XPF(-)) and UV20 (ERCC1(-)) exposed to 20J/m(2) UV radiation. The micronucleus assay was performed to determine a residual DNA damage in four cell lines treated with nodularin (10mug/ml) and exposed to equitoxic doses UV radiation. Radiation doses of UV producing 50% of survival for AA8, UV135, UV20 and UV41 cell lines were calculated from UV survival curves. The results show that nodularin impairs the incision/excision step of NER in CHO cells by the ERCC1/XPF inactivation and leads to an increased level of UV-induced cytogenetic DNA damage.

Versatile protection from mutagenic DNA lesions conferred by bipartite recognition in nucleotide excision repair

Nucleotide excision repair is a cut-and-patch pathway that eliminates potentially mutagenic DNA lesions caused by ultraviolet light, electrophilic chemicals, oxygen radicals and many other genetic insults. Unlike antigen recognition by the immune system, which employs billions of immunoglobulins and T-cell receptors, the nucleotide excision repair complex relies on just a few generic factors to detect an extremely wide range of DNA adducts. This molecular versatility is achieved by a bipartite strategy initiated by the detection of abnormal strand fluctuations, followed by the localization of injured residues through an enzymatic scanning process coupled to DNA unwinding. The early recognition subunits are able to probe the thermodynamic properties of nucleic acid substrates but avoid direct contacts with chemically altered bases. Only downstream subunits of the bipartite recognition process interact more closely with damaged bases to delineate the sites of DNA incision. Thus, consecutive factors expand the spectrum of deleterious genetic lesions conveyed to DNA repair by detecting distinct molecular features of target substrates.