Cyclobutane pyrimidine dimers are responsible for the vast majority of mutations induced by UVB irradiation in mammalian cells

Infliximab inhibits DNA repair in ultraviolet B-irradiated premalignant keratinocytes

Anti-tumor necrosis factor-alpha (TNFalpha) approaches are increasingly used in the therapy of autoimmune diseases. One of the safety concerns is the potential enhancement of skin carcinogenesis. The aim of this study was to investigate if the TNFalpha neutralizing antibody, infliximab, directly affects the cell cycle and DNA repair in premalignant human keratinocytes after ultraviolet-B (UVB) irradiation. We found that infliximab-treated cells exhibited an enhanced G2/M cell cycle arrest and increased apoptosis after 10-20 mJ/cm(2) UVB. In spite of this, the level of cyclobutane pyrimidine dimers (CPD) in infliximab-treated cells was significantly increased at both 24 and 48 h after irradiation with 10 mJ/cm(2) UVB. As we have recently shown that protein kinase B/Akt is involved in the TNFalpha signalling pathway and promotes cell survival and skin carcinogenesis, we measured activatory phosphorylations of Akt (Ser-473 and Thr-308) and the signalling via related pathways Erk 1/2, p38 and p70-S6K. Infliximab inhibited Akt and its downstream targets p70-S6K and Erk 1/2, and stimulated p38 both in sham-irradiated and UVB-irradiated cells. In conclusion, despite the fact that infliximab blocks Akt and stimulates the G2/M checkpoint and apoptosis in UVB-irradiated keratinocytes, the repair of CPD is impaired. It is conceivable that anti-TNFalpha treatments may contribute to the accumulation of mutagenic lesions in the epidermis and enhance the early stages of skin carcinogenesis.

Differential repair of UVB-induced cyclobutane pyrimidine dimers in cultured human skin cells and whole human skin

Cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are the two main classes of mutagenic DNA damages induced by UVB radiation. Numerous studies have been devoted so far to their formation and repair in human cells and skin. However, the biochemical methods used often lack the specificity that would allow the individual study of each of the four CPDs and 6-4PPs produced at TT, TC, CT and CC dinucleotides. In the present work, we applied an HPLC-mass spectrometry assay to study the formation and repair of CPDs and 6-4PPs photoproducts in primary cultures of human keratinocytes and fibroblasts as well as in whole human skin. We first observed that the yield of dimeric lesions was slightly higher in fibroblasts than in keratinocytes. In contrast, the rate of global repair was higher in the last cell type. Moreover, removal of DNA photoproducts in skin biopsies was found to be slower than in both cultured skin cells. In agreement with previous works, the repair of 6-4PPs was found to be more efficient than that of CPDs in the three types of samples, with no observed difference between the removal of the TT and TC derivatives. In contrast, a significant influence of the nature of the two modified pyrimidines was observed on the repair rate of CPDs. The decreasing order of removal efficiency was the following: C<>T>C<>C>T<>C>T<>T. These data, together with the known intrinsic mutational properties of the lesions, would support the reported UV mutation spectra. A noticeable exception concerns CC dinucleotides that are mutational hotspots with an UV-specific CC to TT tandem mutation, although related bipyrimidine photoproducts are produced in low yields and efficiently repaired.

Polydeoxyribonucleotide promotes cyclobutane pyrimidine dimer repair in UVB-exposed dermal fibroblasts

BACKGROUND

DNA is the main cellular chromophore for ultraviolet B (UVB). Its absorption leads to the generation of typical photoproducts. The most frequent types (about 80%) are cyclobutane pyrimidine dimers (CPDs). Several studies have suggested that treatment with deoxyribonucleosides can protect some cell types from DNA damage. The aim of this work was to evaluate the ability of the polydeoxyribonucleotide (PDRN) to protect human dermal fibroblasts from UVB-induced DNA damage.

METHODS

Human dermal fibroblasts were irradiated with 600 mJ/cm(2) of UVB radiation. Cells were analyzed at increasing time points from irradiation to study the recovery from UVB-induced DNA photodamage. Damage repair was subsequently assessed by immunocytochemical analysis of CPDs levels and by measurement of p53 protein expression.

RESULTS

The extracellular addition of 100 microg/ml PDRN immediately after irradiation caused a strong activation of p53 protein in the first 24 h. This signal was accompanied by an increase in CPDs repair rates at early time points of recovery.

CONCLUSIONS

The addition of PDRN to the culture medium supports CPDs repair probably providing a faster supply of precursors for the deoxyribonucleotide triphosphates pool necessary to UVB-damaged cells. This condition could promote the action of the salvage pathway, thereby accelerating DNA repair, but other inducible responses linked to increased p53 expression could be involved.

Ultraviolet B regulation of transcription factor families: roles of nuclear factor-kappa B (NF-kappaB) and activator protein-1 (AP-1) in UVB-induced skin carcinogenesis

Prolonged and repeated exposure of the skin to ultraviolet light (UV) leads not only to aging of the skin but also increases the incidence of non-melanoma skin cancer (NMSC). Damage of cells induced by ultraviolet B (UVB) light both at the DNA level and molecular level initiates the activation of transcription factor pathways, which in turn regulate the expression of a number of genes termed the "UV response genes". Two such transcription factor families that are activated in this way are those of the nuclear factor-kappaB (NF-kappaB) and activator protein-1 (AP-1) families. These two transcription factor families have been identified to be involved in the processes of cell proliferation, cell differentiation and cell survival and therefore play important roles in tumorigenesis. The study of these two transcription factor pathways and the cross-talk between them in response to UVB exposure may help with the development of new chemopreventive strategies for the prevention of UVB-induced skin carcinogenesis.

High frequency induction of CC to TT tandem mutations in DNA repair-proficient mammalian cells

The CC to TT tandem mutation is induced by UV radiation exposure, though at relatively low frequencies when compared with the more commonly induced C to T mutation. Induction of the tandem mutation by UV is enhanced in mammalian cells with certain genetic deficiencies; however, conditions have not been described in which the frequency of this mutation is enhanced in DNA repair-proficient mammalian cells. For this study, an integrated construct that detects C to T and CC to TT mutations at a single codon in mouse Aprt was used to examine UVB mutagenesis under various conditions. Oxidative stress, in the form of intracellular hydrogen peroxide, increased the frequency of UVB-induced CC to TT mutations. Surprisingly, exposure of the cells to two antioxidants (N-acetylcysteine and trolox), either alone or in combination, also enhanced UVB induction of CC to TT tandem mutations. These results demonstrate, for the first time, that the frequency of UVB-induced CC to TT tandem mutations can be enhanced dramatically in DNA repair-proficient mammalian cells, and suggest that the enhancing effect does not require direct damage to DNA.

CPDs and 6-4PPs play different roles in UV-induced cell death in normal and NER-deficient human cells

Ultraviolet (UV) light generates two major DNA lesions: cyclobutane pyrimidine dimers (CPDs) and pyrimidine-(6-4)-pyrimidone photoproducts (6-4PPs), but the specific participation of these two lesions in the deleterious effects of UV is a longstanding question. In order to discriminate the precise role of unrepaired CPDs and 6-4PPs in UV-induced responses triggering cell death, human fibroblasts were transduced by recombinant adenoviruses carrying the CPD-photolyase or 6-4PP-photolyase cDNAs. Both photolyases were able to prevent UV-induced apoptosis in cells deficient for nucleotide excision repair (NER) to a similar extent, while in NER-proficient cells UV-induced apoptosis was prevented only by CPD-photolyase, with no effects observed when 6-4PPs were removed by the specific photolyase. These results strongly suggest that both CPDs and 6-4PPs contribute to UV-induced apoptosis in NER-deficient cells, while in NER-proficient cells, CPDs are the only lesions responsible for UV-killing, probably due to the rapid repair of 6-4PPs by NER. As a consequence, the difference in skin photosensitivity, including carcinogenesis, of most of the xeroderma pigmentosum patients and of normal people is probably not only a quantitative aspect, but depends on the type of DNA damage induced by sunlight and its rate of repair.

Structure and dynamics of poly(T) single-strand DNA: implications toward CPD formation

The formation of cyclobutane pyrimidine dimers between adjacent thymines by UV radiation is thought to be the first event in a cascade leading to skin cancer. Recent studies showed that thymine dimers are fully formed within 1 ps of UV irradiation, suggesting that the conformation at the moment of excitation is the determining factor in whether a given base pair dimerizes. MD simulations on the 50 ns time scale are used to study the populations of reactive conformers that exist at any given time in T18 single-strand DNA. Trajectory analysis shows that only a small percentage of the conformations fulfill distance and dihedral requirements for thymine dimerization, in line with the experimentally observed quantum yield of 3%. Plots of the pairwise interactions in the structures predict hot spots of DNA damage where dimerization in the ssT18 is predicted to be most favored. The importance of hairpin formation by intra-strand base pairing for distinguishing reactive and unreactive base pairs is discussed in detail. The data presented thus explain the structural origin of the results from the ultrafast studies of thymine dimer formation.

Exploring DNA damage responses in human cells with recombinant adenoviral vectors

Recombinant adenoviral vectors provide efficient means for gene transduction in mammalian cells in vitro and in vivo. We are currently using these vectors to transduce DNA repair genes into repair deficient cells, derived from xeroderma pigmentosum (XP) patients. XP is an autosomal syndrome characterized by a high frequency of skin tumors, especially in areas exposed to sunlight, and, occasionally, developmental and neurological abnormalities. XP cells are deficient in nucleotide excision repair (affecting one of the seven known XP genes, xpa to xpg) or in DNA replication of DNA lesions (affecting DNA polymerase eta, xpv). The adenovirus approach allows the investigation of different consequences of DNA lesions in cell genomes. Adenoviral vectors carrying several xp and photolyases genes have been constructed and successfully tested in cell culture systems and in vivo directly in the skin of knockout model mice. This review summarizes these recent data and proposes the use of recombinant adenoviruses as tools to investigate the mechanisms that provide protection against DNA damage in human cells, as well as to better understand the higher predisposition of XP patients to cancer.

p53 and the pathogenesis of skin cancer

The p53 tumor suppressor gene and gene product are among the most diverse and complex molecules involved in cellular functions. Genetic alterations within the p53 gene have been shown to have a direct correlation with cancer development and have been shown to occur in nearly 50% of all cancers. p53 mutations are particularly common in skin cancers and UV irradiation has been shown to be a primary cause of specific 'signature' mutations that can result in oncogenic transformation. There are certain 'hot-spots' in the p53 gene where mutations are commonly found that result in a mutated dipyrimidine site. This review discusses the role of p53 from normal function and its dysfunction in pre-cancerous lesions and non-melanoma skin cancers. Additionally, special situations are explored, such as Li-Fraumeni syndrome in which there is an inherited p53 mutation, and the consequences of immune suppression on p53 mutations and the resulting increase in non-melanoma skin cancer in these patients.

Growth stimulation of UV-induced DNA damage retaining epidermal basal cells gives rise to clusters of p53 overexpressing cells

Ultraviolet (UV) radiation induces cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts ((6-4)PPs) in DNA, which may give rise to clusters of cells expressing mutant p53 ('p53 patches') and eventually to skin carcinomas. We have previously reported that some basal cells in murine skin accumulate CPDs upon chronic low-level UV exposure and that these CPD-retaining basal cells (CRBCs) encompass epidermal stem and progenitor cells. Through replication of their damaged DNA CRBCs may become mutagenic foci from which tumors might form. We therefore investigated whether CRBCs may give rise to p53 patches after forced proliferation by repeated applications of 12-O-tetradecanoylphorbol-13-acetate (TPA). CRBCs, induced in SKH-1 hairless mice by chronic low-level UV exposure (70 J/m(2) daily for 40 days), disappeared in the TPA-induced epidermal hyperplasia within 2 weeks and numerous clusters of epidermal cells with overexpressed p53 appeared after 4 weeks. Neither mutant p53 patches nor any foci of pErk1/2-overexpressing cells that could have caused reactive wild type p53 expression were found. In skin exposed to a single high UV dose (2.8 kJ/m(2)) no CRBCs occurred, and no p53 clusters were observed after TPA treatment. These experiments suggest that CRBCs are a prerequisite for the formation of clusters of p53-overexpressing cells. The high frequency of these clusters (about 1 for every 3 CRBCs) precludes mutations in p53 as a likely cause. We surmise that forced proliferation of CRBCs gives rise to genomic instability that is propagated in daughter cells and evokes wild type p53 overexpression, signifying a potentially oncogenic process different from classic UV carcinogenesis involving mutant p53.

A model for triplet mutation formation based on error-prone translesional DNA synthesis opposite UV photolesions

A triplet mutation is defined as multiple base substitutions or frameshifts within a three-nucleotide sequence which includes a dipyrimidine sequence. Triplet mutations have recently been identified as a new type of UV-specific mutation, although the mechanism of their formation is unknown. A total of 163 triplet mutations were identified through an extensive search of previously published data on UV-induced mutations, including mutations from skin, skin cancer, and cultured mammalian cells. Seven common patterns of sequence changes were found: Type I, NTC-->TTT; Type IIa, NCC-->PyTT or PyCT (Py, pyrimidine); Type IIb, TCC-->PuTT or PuCT (Pu, purine); Type III, NCC-->NAT or NTA; Type IV, NTT-->AAT; Type Va, NCT-->NTX; and Type Vb, PuCT-->XTT (N and X, independent anonymous bases). Furthermore, it is suggested that the type of UV lesion responsible for each of these triplet mutation classes are (a) pyrimidine(6-4)pyrimidone photoproducts for Types I, IIb, III, IV and Vb, (b) cyclobutane pyrimidine dimers for Type Va, and (c) Dewar valence isomers for Types IIa and IIb. These estimations are based primarily on results from previous studies using photolyases specific for each type of UV lesion. A model is proposed to explain the formation of each type of triplet mutation, based on error-prone translesional DNA synthesis opposite UV-specific photolesions. The model is largely consistent with the 'A-rule', and predicts error-prone insertions not only opposite photolesions but also opposite the undamaged template base one-nucleotide downstream from the lesions.

The human Tim/Tipin complex coordinates an Intra-S checkpoint response to UV that slows replication fork displacement

UV-induced DNA damage stalls DNA replication forks and activates the intra-S checkpoint to inhibit replicon initiation. In response to stalled replication forks, ATR phosphorylates and activates the transducer kinase Chk1 through interactions with the mediator proteins TopBP1, Claspin, and Timeless (Tim). Murine Tim recently was shown to form a complex with Tim-interacting protein (Tipin), and a similar complex was shown to exist in human cells. Knockdown of Tipin using small interfering RNA reduced the expression of Tim and reversed the intra-S checkpoint response to UVC. Tipin interacted with replication protein A (RPA) and RPA-coated DNA, and RPA promoted the loading of Tipin onto RPA-free DNA. Immunofluorescence analysis of spread DNA fibers showed that treating HeLa cells with 2.5 J/m(2) UVC not only inhibited the initiation of new replicons but also reduced the rate of chain elongation at active replication forks. The depletion of Tim and Tipin reversed the UV-induced inhibition of replicon initiation but affected the rate of DNA synthesis at replication forks in different ways. In undamaged cells depleted of Tim, the apparent rate of replication fork progression was 52% of the control. In contrast, Tipin depletion had little or no effect on fork progression in unirradiated cells but significantly attenuated the UV-induced inhibition of DNA chain elongation. Together, these findings indicate that the Tim-Tipin complex mediates the UV-induced intra-S checkpoint, Tim is needed to maintain DNA replication fork movement in the absence of damage, Tipin interacts with RPA on DNA and, in UV-damaged cells, Tipin slows DNA chain elongation in active replicons.

Repair of UV lesions in nucleosomes--intrinsic properties and remodeling

Nucleotide excision repair and reversal of pyrimidine dimers by photolyase (photoreactivation) are two major pathways to remove UV-lesions from DNA. Here, it is discussed how lesions are recognized and removed when the DNA is condensed into nucleosomes. During the recent years it was shown that nucleosomes inhibit photolyase and excision repair in vitro and slow down repair in vivo. The correlation of DNA-repair rates with nucleosome positions in yeast suggests that intrinsic properties of nucleosomes such as mobility and transient unwrapping of nucleosomal DNA facilitate damage recognition. Moreover, it was shown that nucleosome remodeling activities can act on UV-damaged DNA in vitro and facilitate repair suggesting that random remodeling of chromatin might contribute to damage recognition in vivo. Recent work on nucleosome structure and mobility is included to evaluate how nucleosomes accommodate DNA lesions and how nucleosome mobility and remodeling can take place on damaged DNA.

New strategies of photoprotection

Adequate photoprotection is essential to control UV-related disorders, including sunburn, photoaging and photocarcinogenisis. Sun avoidance, protection of skin with clothing, and sunscreens are presently the best way of photoprotection, assuming that they are used properly. However, new strategies, which are based on or make use of the endogenous protective response to UV light, may further improve currently used photoprotective means. The addition of repair enzymes and/or antioxidants has a positive effect on skin's recovery from UV-induced DNA-damage. Several botanical agents, mainly vitamins and polyphenols, have shown to influence signal transduction pathways leading to photoprotective effects. Also stimulation of endogenous UV-response pathways via irradiation with a low UV dose or via simulation of UV-induced DNA-damage results in photoprotective effects. Future research in this field and combination of different photoprotective strategies will hopefully lead to improved photoprotection.

High-resolution crystal structure of the intramolecular d(TpA) thymine-adenine photoadduct and its mechanistic implications

A high-resolution crystal structure is reported for d(TpA)*, the intramolecular thymine–adenine photoadduct that is produced by direct ultraviolet excitation of the dinucleoside monophosphate d(TpA). It confirms the presence of a central 1,3-diazacyclooctatriene ring linking the remnants of the T and A bases, as previously deduced from heteronuclear NMR measurements by Zhao et al. (The structure of d(TpA)*, the major photoproduct of thymidylyl-(3′-5′)-deoxyadenosine. Nucleic Acids Res., 1996, 24, 1554–1560). Within the crystal, the d(TpA)* molecules exist as zwitterions with a protonated amidine fragment of the eight-membered ring neutralizing the charge of the internucleotide phosphate monoanion. The absolute configuration at the original thymine C5 and C6 atoms is determined as 5S,6R. This is consistent with d(TpA)* arising by valence isomerization of a precursor cyclobutane photoproduct with cis–syn stereochemistry that is generated by [2 + 2] photoaddition of the thymine 5,6-double bond across the C6 and C5 positions of adenine. This mode of photoaddition should be favoured by the stacked conformation of adjacent T and A bases in B-form DNA. It is probable that the primary photoreaction is mechanistically analogous to pyrimidine dimerization despite having a much lower quantum yield.

Genética molecular aplicada ao câncer cutâneo não melanoma

Os cânceres cutâneos não melanoma são as neoplasias malignas mais comuns em humanos. O carcinoma basocelular e o carcinoma espinocelular representam cerca de 95% dos cânceres cutâneos não melanoma, o que os torna um crescente problema para a saúde pública mundial devido a suas prevalências cada vez maiores. As alterações genéticas que ocorrem no desenvolvimento dessas malignidades cutâneas são apenas parcialmente compreendidas, havendo muito interesse no conhecimento e determinação das bases genéticas dos cânceres cutâneos não melanoma que expliquem seus fenótipos, comportamentos biológicos e potenciais metastáticos distintos. Apresenta-se uma revisão atualizada da genética molecular aplicada aos cânceres cutâneos não melanoma, em especial ao carcinoma basocelular e carcinoma espinocelular, enfatizando os mais freqüentes genes e os principais mecanismos de instabilidade genômica envolvidos no desenvolvimento dessas malignidades cutâneas.

UV-B radiation induces epithelial tumors in mice lacking DNA polymerase eta and mesenchymal tumors in mice deficient for DNA polymerase iota

DNA polymerase eta (Pol eta) is the product of the Polh gene, which is responsible for the group variant of xeroderma pigmentosum, a rare inherited recessive disease which is characterized by susceptibility to sunlight-induced skin cancer. We recently reported in a study of Polh mutant mice that Pol eta is involved in the somatic hypermutation of immunoglobulin genes, but the cancer predisposition of Polh-/- mice has not been examined until very recently. Another translesion synthesis polymerase, Pol iota, a Pol eta paralog encoded by the Poli gene, is naturally deficient in the 129 mouse strain, and the function of Pol iota is enigmatic. Here, we generated Polh Poli double-deficient mice and compared the tumor susceptibility of them with Polh- or Poli-deficient animals under the same genetic background. While Pol iota deficiency does not influence the UV sensitivity of mouse fibroblasts irrespective of Polh genotype, Polh Poli double-deficient mice show slightly earlier onset of skin tumor formation. Intriguingly, histological diagnosis after chronic treatment with UV light reveals that Pol iota deficiency leads to the formation of mesenchymal tumors, such as sarcomas, that are not observed in Polh(-/-) mice. These results suggest the involvement of the Pol eta and Pol iota proteins in UV-induced skin carcinogenesis.

Photolyases: capturing the light to battle skin cancer

Photolyases comprise efficient enzymes to remove the major UV-induced DNA lesions, cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). While photolyases are present in all three kingdoms of life (i.e., bacteria, prokaryotes and eukaryotes), placental mammals appear to have lost these enzymes when they diverted from marsupials during evolution. Consequently, man and mice have to rely solely on the more complex and, for these lesions, less efficient nucleotide excision repair (NER) system. To assess the relative contribution of CPDs and 6-4PPs to the cytotoxic and genotoxic effects of the UV component of sunlight, we have recently generated a comprehensive set of transgenic mice expressing CPD and/or 6-4PP photolyases. Here, we discuss the use of photolyase transgenic mice as effective tools to study the adverse effects of UV irradiation.

Testing excision models for responses of mismatch-repair systems to UV photoproducts in DNA

Mismatch-repair (MMR) systems correct DNA replication errors and respond to a variety of DNA lesions. Previous observations that MMR antagonizes UV mutagenesis, and that the mismatch-recognition protein heterodimer MSH2*MSH6 (MutSalpha) selectively binds DNA containing "mismatched" photoproducts (T[CPD]T/AG, T[6-4]T/AG) but not "matched" photoproducts (T[CPD]T/AA, T[6-4]T/AA), suggested that mismatched photoproducts would provoke MMR excision similar to mismatched bases. Excision of incorrect nucleotides inserted opposite template photoproducts might then prevent UV-induced mutation. We tested T[CPD]T/AG DNA, in a sequence context in which it is bound substantially by hMutSalpha and in three other contexts, for stimulation of 3' MMR excision in mammalian nuclear extracts. T[CPD]T/AG was inactive in HeLa extracts, or in extracts deficient in the photoproduct-binding proteins DDB or XPC* hHR23B, arguing against interference from the nucleotide excision repair pathway. Prior incubation with hMutSalpha and MLH2.PMS2 (hMutLalpha) did not increase excision relative to homoduplex controls. T[6-4]T/AG also failed to provoke excision. T/G, C/A, and T/T substrates, even though bound by hMutSalpha no better than T[CPD]T/AG substrates, efficiently provoked excision. Even a substrate containing three T[CPD]T/AG photoproducts (in different contexts) did not significantly provoke excision. Thus, MMR may suppress UV mutagenesis by non-excisive mechanisms.

Mutagenesis at methylated CpG sequences

5-Methylcytosine in DNA is genetically unstable. Methylated CpG (mCpG) sequences frequently undergo mutation resulting in a general depletion of this dinucleotide sequence in mammalian genomes. In human genetic disease- and cancer-relevant genes, mCpG sequences are mutational hotspots. It is an almost universally accepted dogma that these mutations are caused by random deamination of 5-methylcytosines. However, it is plausible that mCpG transitions are not caused simply by spontaneous deamination of 5-methylcytosine in double-stranded DNA but by other processes including, for example, mCpG-specific base modification by endogenous or exogenous mutagens or, alternatively, by secondary factors operating at mCpG sequences and promoting deamination. We also discuss that mCpG sequences are favored targets for specific exogenous mutagens and carcinogens. When adjacent to another pyrimidine, 5-methylcytosine preferentially undergoes sunlight-induced pyrimidine dimer formation. Certain polycyclic aromatic hydrocarbons form guanine adducts and induce G to T transversion mutations with high selectivity at mCpG sequences.

The role of DNA polymerase iota in UV mutational spectra

UVB (280-320 nm) and UVC (200-280 nm) irradiation generate predominantly cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA. CPDs are thought to be responsible for most of the UV-induced mutations. Thymine-thymine CPDs, and probably also CPDs containing cytosine, are replicated in vivo in a largely accurate manner by a DNA polymerase eta (Pol eta) dependent process. Pol eta is a DNA damage-tolerant and error-prone DNA polymerase encoded by the POLH (XPV) gene in humans. Another member of the Y family of error-prone DNA polymerases is POLI encoding DNA polymerase iota (Pol iota). In order to clarify the specific role of Pol iota in UV mutagenesis, we have used an siRNA knockdown approach in combination with a supF shuttle vector which replicates in mammalian cells, similar as we have previously done for Pol eta. Synthetic RNA duplexes were used to efficiently inhibit Pol iota expression in 293 T cells. The supF shuttle vector was irradiated with 254 nm UVC and replicated in 293 T cells in presence of anti-Pol iota siRNA. Surprisingly, there was a consistent reduction of recovered plasmid from cells with Pol iota knockdown and this was independent of UV irradiation of the plasmid. The supF mutant frequency was unchanged in the siRNA knockdown cells relative to control cells confirming that Pol iota does not play an important role in UV mutagenesis. UV-induced supF mutants were sequenced from siRNA-treated cells and controls. Neither the type of mutations nor their distribution along the supF gene were significantly different between controls and siRNA knockdown cells and were predominantly C to T and CC to TT transitions at dipyrimidine sites. These results show that Pol iota has no significant role in UV lesion bypass and mutagenesis in vivo and provides some initial data suggesting that this polymerase may be involved in replication of extrachromosomal DNA.

Actions of ultraviolet light on cellular structures

Solar radiation is the primary source of human exposure to ultraviolet (UV) radiation. Overexposure without suitable protection (i.e., sunscreen and clothing) has been implicated in mutagenesis and the onset of skin cancer. These effects are believed to be initiated by UV-mediated cellular damage, with proteins and DNA as primary targets due to a combination of their UV absorption characteristics and their abundance in cells. UV radiation can mediate damage via two different mechanisms: (a) direct absorption of the incident light by the cellular components, resulting in excited state formation and subsequent chemical reaction, and (b) photosensitization mechanisms, where the light is absorbed by endogenous (or exogenous) sensitizers that are excited to their triplet states. The excited photosensitizers can induce cellular damage by two mechanisms: (a) electron transfer and hydrogen abstraction processes to yield free radicals (Type I); or (b) energy transfer with O2 to yield the reactive excited state, singlet oxygen (Type II). Direct UV absorption by DNA leads to dimers of nucleic acid bases including cyclobutane pyrimidine species and pyrimidine (6-4) pyrimidone compounds, together with their Dewar isomers. These three classes of dimers are implicated in the mutagenicity of UV radiation, which is typified by a high level of CC-->TT and C-->T transversions. Single base modifications can also occur via sensitized reactions including Type 1 and Type II processes. The main DNA product generated by (1)O2 is 8-oxo-Gua; this is a common lesion in DNA and is formed by a range of other oxidants in addition to UV. The majority of UV-induced protein damage appears to be mediated by (1)O2, which reacts preferentially with Trp, His, Tyr, Met, Cys and cystine side chains. Direct photo-oxidation reactions (particularly with short-wavelength UV) and radicals can also be formed via triplet excited states of some of these side chains. The initial products of (1)O2-mediated reactions are endoperoxides with the aromatic residues, and zwitterions with the sulfur-containing residues. These intermediates undergo a variety of further reactions, which can result in radical formation and ring-opening reactions; these result in significant yields of protein cross-links and aggregates, but little protein fragmentation. This review discusses the formation of these UV-induced modifications and their downstream consequences with particular reference to mutagenesis and alterations in protein structure and function.

Differential biologic effects of CPD and 6-4PP UV-induced DNA damage on the induction of apoptosis and cell-cycle arrest

Background

UV-induced damage can induce apoptosis or trigger DNA repair mechanisms. Minor DNA damage is thought to halt the cell cycle to allow effective repair, while more severe damage can induce an apoptotic program. Of the two major types of UV-induced DNA lesions, it has been reported that repair of CPD, but not 6-4PP, abrogates mutation. To address whether the two major forms of UV-induced DNA damage, can induce differential biological effects, NER-deficient cells containing either CPD photolyase or 6-4 PP photolyase were exposed to UV and examined for alterations in cell cycle and apoptosis. In addition, pTpT, a molecular mimic of CPD was tested in vitro and in vivo for the ability to induce cell death and cell cycle alterations.

Methods

NER-deficient XPA cells were stably transfected with CPD-photolyase or 6-4PP photolyase to specifically repair only CPD or only 6-4PP. After 300 J/m² UVB exposure photoreactivation light (PR, UVA 60 kJ/m²) was provided for photolyase activation and DNA repair. Apoptosis was monitored 24 hours later by flow cytometric analysis of DNA content, using sub-G1 staining to indicate apoptotic cells. To confirm the effects observed with CPD lesions, the molecular mimic of CPD, pTpT, was also tested in vitro and in vivo for its effect on cell cycle and apoptosis.

Results

The specific repair of 6-4PP lesions after UVB exposure resulted in a dramatic reduction in apoptosis. These findings suggested that 6-4PP lesions may be the primary inducer of UVB-induced apoptosis. Repair of CPD lesions (despite their relative abundance in the UV-damaged cell) had little effect on the induction of apoptosis. Supporting these findings, the molecular mimic of CPD, (dinucleotide pTpT) could mimic the effects of UVB on cell cycle arrest, but were ineffective to induce apoptosis.

Conclusion

The primary response of the cell to UV-induced 6-4PP lesions is to trigger an apoptotic program whereas the response of the cell to CPD lesions appears to principally involve cell cycle arrest. These findings suggest that CPD and 6-4 PP may induce differential biological effects in the UV-damaged cell.

Binding of MutS mismatch repair protein to DNA containing UV photoproducts, "mismatched" opposite Watson--Crick and novel nucleotides, in different DNA sequence contexts

Mismatch-repair (MMR) systems suppress mutation via correction of DNA replication errors (base-mispairs) and responses to mutagenic DNA lesions. Selective binding of mismatched or damaged DNA by MutS-homolog proteins-bacterial MutS, eukaryotic MSH2.MSH6 (MutSalpha) and MSH2.MSH3-initiates mismatch-correction pathways and responses to lesions, and may cumulatively increase discrimination at downstream steps. MutS-homolog binding selectivity and the well-known but poorly understood effects of DNA-sequence contexts on recognition may thus be primary determinants of MMR specificity and efficiency. MMR processes that modulate UV mutagenesis might begin with selective binding by MutS homologs of "mismatched" T[CPD]T/AG and T[6--4]T/AG photoproducts, reported previously for hMutSalpha and described here for E. coli MutS protein. If MMR suppresses UV mutagenesis by acting directly on pre-mutagenic products of replicative bypass, mismatched photoproducts should be recognized in most DNA-sequence contexts. In three of four contexts tested here (three substantially different), T[CPD]T/AG was bound only slightly better by MutS than was T[CPD]T/AA or homoduplex DNA; only one of two contexts tested promoted selective binding of T[6--4]T/AG. Although the T:G pairs in T[CPD]T/AG and T/G both adopt wobble conformations, MutS bound T/G well in all contexts (K(1/2) 2.1--2.9 nM). Thus, MutS appears to select the two mismatches by different mechanisms. NMR analyses elsewhere suggest that in the (highly distorted) T[6--4]T/AG a forked H-bond between O2 of the 3' thymine and the ring 1-imino and exocyclic 2-amino guanine protons stabilizes a novel planar structure not possible in T[6--4]T/AA. Replacement of G by purines lacking one (inosine, 2-aminopurine) or both (nebularine) protons markedly reduced or eliminated selective MutS binding, as predicted. Previous studies and the work here, taken together, suggest that in only about half of DNA sequence contexts could MutS (and presumably MutSalpha) selectively bind mismatched UV photoproducts and directly suppress UV mutagenesis.

DNA lesions induced by UV A1 and B radiation in human cells: comparative analyses in the overall genome and in the p53 tumor suppressor gene

The UV components of sunlight (UVA and UVB) are implicated in the etiology of human skin cancer. The underlying mechanism of action for UVB carcinogenicity is well defined; however, the mechanistic involvement of UVA in carcinogenesis is not fully delineated. We investigated the genotoxicity of UVA1 versus UVB in the overall genome and in the p53 tumor suppressor gene in normal human skin fibroblasts. Immuno-dot blot analysis identified the cis-syn cyclobutane pyrimidine-dimer (CPD) as a distinctive UVB-induced lesion and confirmed its formation in the genomic DNA of UVA1-irradiated cells dependent on radiation dose. HPLC/tandem MS analysis showed an induction of 8-oxo-7,8-dihydro-2'-deoxyguanosine in the genomic DNA of UVA1-irradiated cells only. Mapping of DNA damages by terminal transferase-dependent PCR revealed preferential, but not identical, formation of polymerase-blocking lesions and/or strand breaks along exons 5-8 of the p53 gene in UVB- and UVA1-irradiated cells. The UVB-induced lesions detected by terminal transferase-PCR were almost exclusively mapped to pyrimidine-rich sequences; however, the UVA1-induced lesions were mapped to purine- and pyrimidine-containing sequences along the p53 gene. Cleavage assays with lesion-specific DNA repair enzymes coupled to ligation-mediated PCR showed preferential, but not identical, formation of CPDs along the p53 gene in UVB- and UVA1-irradiated cells. Additionally, dose-dependent formation of oxidized and ring-opened purines and abasic sites was established in the p53 gene in only UVA1-irradiated cells. We conclude that UVA1 induces promutagenic CPDs and oxidative DNA damage at both the genomic and nucleotide resolution level in normal human skin fibroblasts.

Effect of administration of caffeine or green tea on the mutation profile in the p53 gene in early mutant p53-positive patches of epidermal cells induced by chronic UVB-irradiation of hairless SKH-1 mice

Irradiation of SKH-1 mice with UVB light for 20 weeks resulted in a large number of patches of epidermal cells, which was visualized with an antibody that recognizes mutated p53 protein. Oral treatment of mice with caffeine (0.4 mg/ml) or green tea (6 mg tea solids/ml) as the drinking fluid during UVB irradiation decreased the number of patches by approximately 40%. Sequencing analysis of the p53 gene (exons 3 to 9) detected 88, 82 or 39 point mutations in 67, 70 or 29 patches from water, caffeine or tea treated mice, respectively. A major hotspot at codon 270 (Arg-->Cys) accounted for 47.7% (water), 70.7% (caffeine) or 46.2% (tea) of all mutations. Patches from caffeine treated mice had fewer types of mutations than patches from mice treated with water or tea. Administration of caffeine or tea during 20 weeks of UVB irradiation eliminated mutations at codons 149 (Pro-->Ser) and 210 (Arg-->Cys) but increased the frequency of mutations at codon 238 (Ser-->Phe). Topical applications of caffeine (1.2 mg in 100 microl acetone) once a day, five times a week for 6 weeks after stopping UVB decreased the number of patches by 63% when compared with mice treated with acetone. DNA sequencing analysis detected 63 and 68 mutations in 48 and 57 patches from acetone or caffeine treated mice, respectively. Although no differences in the frequency, position or types of mutations were observed, the caffeine group harbored less homozygous mutations (12.3% of the total) than the acetone group (31.3% of the total, P = 0.029). In summary, oral treatment of mice with caffeine or green tea during chronic UVB irradiation changed the mutation profile of the p53 gene in early mutant p53 positive epidermal patches, and topical applications of caffeine after discontinuation of chronic UVB irradiation specifically eliminated patches harboring homozygous p53 mutations.

E6* oncoprotein expression of human papillomavirus type-16 determines different ultraviolet sensitivity related to glutathione and glutathione peroxidase antioxidant defence

Clinical observations of non-melanoma skin cancer in immunocompromised patients, such as organ transplant recipients, suggest co-operative effects of human papillomavirus (HPV) and ultraviolet (UV) radiation. The aim of the present study is to evaluate UV sensitivity and DNA damage formation according to antioxidant status in HPV16-infected keratinocytes. We used SKv cell lines, infected with HPV16 and well characterized for their proliferative and tumorigenic capacities. We showed that SKv cell lines presented various E6* (a truncated form of E6) RNA levels. We demonstrated that the higher oncoprotein RNA expression level was associated with a higher resistance to solar-simulated radiation, more specifically to UVB radiation and to hydrogen peroxide. Moreover, this high resistance was associated with a low oxidative DNA damage formation after UV radiation and was related to high glutathione content and glutathione peroxidase activities. Therefore, the results of our study suggest that E6* levels could modulate the glutathione/glutathione peroxidase pathway providing a mechanism to protect HPV-infected keratinocytes against an environmental oxidative stress, such as UV radiation.

The role of DNA polymerase eta in UV mutational spectra

UV irradiation generates predominantly cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts in DNA. CPDs are thought to be responsible for most of the UV-induced mutations. Thymine-thymine CPDs, and probably also CPDs containing cytosine, are replicated in vivo in a largely accurate manner by a DNA polymerase eta (Pol eta) dependent process. Pol eta is encoded by the POLH (XPV) gene in humans. In order to clarify the specific role of Pol eta in UV mutagenesis, we have used an siRNA knockdown approach in combination with a supF shuttle vector which replicates in mammalian cells. This strategy provides an advantage over studying mutagenesis in cell lines derived from normal individuals and XP-V patients, since the genetic background of the cells is identical. Synthetic RNA duplexes were used to inhibit Pol eta expression in 293T cells. The reduction of Pol eta mRNA and protein was greater than 90%. The supF shuttle vector was irradiated with UVC and replicated in 293T cells in presence of anti-Pol eta siRNA. The supF mutant frequency was increased by up to 3.6-fold in the siRNA knockdown cells relative to control cells confirming that Pol eta plays an important role in mutation avoidance and that the pol eta knockdown was efficient. UV-induced supF mutants were sequenced from siRNA-treated cells and controls. Surprisingly, neither the type of mutations nor their distribution along the supF gene were substantially different between controls and siRNA knockdown cells and were predominantly C to T and CC to TT transitions at dipyrimidine sites. The data are compatible with two models. (i) Incorrect replication of cytosine-containing photoproducts by a polymerase other than Pol eta produces similar mutations as when Pol eta is present but at a higher frequency. (ii) Due to lack of Pol eta or low levels of remaining Pol eta, lesion replication is delayed allowing more time for cytosine deamination within CPDs to occur. We provide proof of principle that siRNA technology can be used to dissect the in vivo roles of lesion bypass DNA polymerases in DNA damage-induced mutagenesis.

Light-driven enzymatic catalysis of DNA repair: a review of recent biophysical studies on photolyase

More than 50 years ago, initial experiments on enzymatic photorepair of ultraviolet (UV)-damaged DNA were reported [Proc. Natl. Acad. Sci. U. S. A. 35 (1949) 73]. Soon after this discovery, it was recognized that one enzyme, photolyase, is able to repair UV-induced DNA lesions by effectively reversing their formation using blue light. The enzymatic process named DNA photoreactivation depends on a non-covalently bound cofactor, flavin adenine dinucleotide (FAD). Flavins are ubiquitous redox-active catalysts in one- and two-electron transfer reactions of numerous biological processes. However, in the case of photolyase, not only the ground-state redox properties of the FAD cofactor are exploited but also, and perhaps more importantly, its excited-state properties. In the catalytically active, fully reduced redox form, the FAD absorbs in the blue and near-UV ranges of visible light. Although there is no direct experimental evidence, it appears generally accepted that starting from the excited singlet state, the chromophore initiates a reductive cleavage of the two major DNA photodamages, cyclobutane pyrimidine dimers and (6-4) photoproducts, by short-distance electron transfer to the DNA lesion. Back electron transfer from the repaired DNA segment is believed to eventually restore the initial redox states of the cofactor and the DNA nucleobases, resulting in an overall reaction with net-zero exchanged electrons. Thus, the entire process represents a true catalytic cycle. Many biochemical and biophysical studies have been carried out to unravel the fundamentals of this unique mode of action. The work has culminated in the elucidation of the three-dimensional structure of the enzyme in 1995 that revealed remarkable details, such as the FAD-cofactor arrangement in an unusual U-shaped configuration. With the crystal structure of the enzyme at hand, research on photolyases did not come to an end but, for good reason, intensified: the geometrical structure of the enzyme alone is not sufficient to fully understand the enzyme's action on UV-damaged DNA. Much effort has therefore been invested to learn more about, for example, the geometry of the enzyme-substrate complex, and the mechanism and pathways of intra-enzyme and enzyme <-->DNA electron transfer. Many of the key results from biochemical and molecular biology characterizations of the enzyme or the enzyme-substrate complex have been summarized in a number of reviews. Complementary to these articles, this review focuses on recent biophysical studies of photoreactivation comprising work performed from the early 1990s until the present.

A role for Pms2 in the prevention of tandem CC --> TT substitutions induced by ultraviolet radiation and oxidative stress

DNA mismatch repair (MMR) is important for preventing base-pair substitutions caused by spontaneous or damage-related DNA polymerase errors. We have used a reversion assay based on mouse Aprt to investigate the role of MMR in preventing ultraviolet radiation (UV) and oxidative stress induced tandem CC --> TT base pair substitutions in cultured mammalian cells. The reversion construct used for this assay can detect both C --> T and CC --> TT mutational events. Most spontaneous mutations in Pms2-deficient cells were single C --> T substitutions (88%), with the remainder being tandem CC --> TT substitutions (12%). The percentage of tandem CC --> TT substitutions rose to 64% and 94% for Pms2-deficient cells exposed to UV and a mixture of hydrogen peroxide and metals (Cu/Fe), respectively. Exposure to hydrogen peroxide alone or metals alone did not induce the tandem substitutions, nor did treatment of the cells with the alkylating agent ethylmethane sulfonate, which induces G --> A substitutions on the opposite strand. Tandem CC --> TT substitutions were also induced by UV irradiation and the hydrogen peroxide/metal mixture in Pms2-proficient cells, but at frequencies significantly lower than those observed in the Pms2-deficient cells. We conclude that mismatch repair plays an important role in preventing tandem CC --> TT substitutions induced by certain genotoxin exposures.

Skin cancer: lights on genome lesions

Sunlight generates skin damage mainly by inducing DNA lesions in epidermal cells. The recent development of transgenic mice expressing specific photolyases has identified cyclobutane pyrimidine dimers as the major player in ultraviolet-induced damage, including skin cancer.

Powerful skin cancer protection by a CPD-photolyase transgene

BACKGROUND

The high and steadily increasing incidence of ultraviolet-B (UV-B)-induced skin cancer is a problem recognized worldwide. UV introduces different types of damage into the DNA, notably cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs). If unrepaired, these photolesions can give rise to cell death, mutation induction, and onset of carcinogenic events, but the relative contribution of CPDs and 6-4PPs to these biological consequences of UV exposure is hardly known. Because placental mammals have undergone an evolutionary loss of photolyases, repair enzymes that directly split CPDs and 6-4PPs into the respective monomers in a light-dependent and lesion-specific manner, they can only repair UV-induced DNA damage by the elaborate nucleotide excision repair pathway.

RESULTS

To assess the relative contribution of CPDs and 6-4PPs to the detrimental effects of UV light, we generated transgenic mice that ubiquitously express CPD-photolyase, 6-4PP-photolyase, or both, thereby allowing rapid light-dependent repair of CPDs and/or 6-4PPs in the skin. We show that the vast majority of (semi)acute responses in the UV-exposed skin (i.e., sunburn, apoptosis, hyperplasia, and mutation induction) can be ascribed to CPDs. Moreover, CPD-photolyase mice, in contrast to 6-4PP-photolyase mice, exhibit superior resistance to sunlight-induced tumorigenesis.

CONCLUSIONS

Our data unequivocally identify CPDs as the principal cause of nonmelanoma skin cancer and provide genetic evidence that CPD-photolyase enzymes can be employed as effective tools to combat skin cancer.

Mechanisms of DNA damage recognition and strand discrimination in human nucleotide excision repair

Using only a limited repertoire of recognition subunits, the nucleotide excision repair (NER) system is able to detect a nearly infinite variety of bulky DNA lesions. This extraordinary substrate versatility has generally been ascribed to an indirect readout mechanism, whereby particular distortions of the double helix, induced by a damaged nucleotide, provide the molecular determinants not only for lesion recognition but also for subsequent verification or demarcation processes. Here, we discuss the evidence in support of a bipartite mechanism of substrate discrimination that is initiated by the detection of thermodynamically unstable base pairs followed by direct localization of the lesion through an enzymatic proofreading activity. This bipartite discrimination mechanism is part of a dynamic reaction cycle that confers high levels of selectivity to avoid futile repair events on undamaged DNA and also protect the intact complementary strand from inappropriate cleavage.

Mutation spectrum in sunlight-exposed mouse skin epidermis: small but appreciable contribution of oxidative stress-mediated mutagenesis

We studied the mutations induced in skin by sunlight using transgenic Muta mice. Noon sunlight during summer at Sendai, Japan induced mutations efficiently in both epidermis and dermis. The mutant frequency (MF) in epidermis reached nearly 0.5% during the first 40 min irradiation but became saturated at this level with the appearance of skin inflammation after further irradiation. At the equivalent inflammatory dose, sunlight was twice as genotoxic as 313 nm-peak UVB. The 81 mutations detected in 80 lacZ transgene mutants isolated from the sunlight-exposed epidermis were dominated by C --> T transitions (89%), occurring exclusively at dipyrimidine sites, and also included a CC --> TT tandem substitution. Thus, the sunlight-induced mutation spectrum is highly UV-specific, quite similar to that induced by UVB but significantly different from that induced by UVA. Although oxidative damage-related C --> A transversions were detected only in five mutants (6%), their frequency was elevated to at least 15 times the background level, suggesting that the contribution of UVA-mediated oxidative stress is comparatively small but considerable. An analysis of bases adjacent to the mutated cytosines revealed that the sunlight-induced mutations prefer 5'-TC-3' dipyrimidine sites to 5'-CC-3' and 5'-CT-3'. The distribution of the frequent C --> T transition sites in the transgene was well associated with the CpG motif, which is known to be completely methylated in the gene, and quite similar to that induced by UVB rather than that by UVA. Thus, the UVB component contributes to the sunlight-induced mutations in the mammalian skin much more than the UVA component, whose influence through reactive oxygen species (ROS)-mediated mutagenesis is still appreciable.

Unrepaired cyclobutane pyrimidine dimers do not prevent proliferation of UV-B-irradiated cultured human fibroblasts

Mutagenic and carcinogenic UV-B radiation is known to damage DNA mostly through the formation of bipyrimidine photoproducts, including cyclobutane dimers (CPD) and (6-4) photoproducts ((6-4) PP). Using high-performance liquid chromatography coupled to tandem mass spectrometry, we investigated the formation and repair of thymine-thymine (TT) and thymine-cytosine (TC) CPD and (6-4) PP in the DNA of cultured human dermal fibroblasts. A major observation was that the rate of repair of the photoproducts did not depend on the identity of the modified pyrimidines. In addition, removal of CPD was found to significantly decrease with increasing applied UV-B dose, whereas (6-4) PP were efficiently repaired within less than 24 h, irrespective of the dose. As a result, a relatively large amount of CPD remained in the genome 48 h after the irradiation. Because the overall applied doses (<500 J m(-2)) were chosen to induce moderate cytotoxicity, fibroblasts could recover their proliferation capacities after transitory cell cycle arrest, as shown by 5-bromo-2'-deoxyuridine (BrdUrd) incorporation and flow cytometry analysis. It could thus be concluded that UV-B-irradiated cultured primary human fibroblasts normally proliferate 48 h after irradiation despite the presence of high levels of CPD in their genome. These observations emphasize the role of CPD in the mutagenic effects of UV-B.

Low concentrations of formaldehyde induce DNA damage and delay DNA repair after UV irradiation in human skin cells

Long-term occupational exposure to formaldehyde (FA) increases the risk for nasopharyngeal squamous cell carcinoma. As the skin is also in contact with FA by environmental exposure, we tested the genotoxic properties of appropriate low concentrations (<100 microM) of FA on cultured keratinocytes and fibroblasts of human skin. The initial DNA damage was assessed by comet assay. The induction of DNA protein crosslinks was measured by the ability of FA to reduce DNA migration induced by methyl-methane-sulfonate. Upon 4 h of exposure to FA, significant (P < 0.05) crosslink formations were observed in fibroblasts (50 microM FA) and in keratinocytes (25 microM FA). Upon 8 h of exposure to FA (25 microM FA), significant crosslink formations were observed in both the cell types. FA is known to inhibit different DNA repair pathways. Therefore, we studied the effect of FA on UV-induced repair. Human keratinocytes and fibroblasts exposed to 10 microM FA prior to UV irradiation showed disturbed repair kinetics after UVC and UVB, but not after UVA irradiation. Single-strand breaks (SSBs) derived from nucleotide excision repair disappeared 6 h after solely UVC (3 mJ/cm2) or 3 h solely UVB (30 mJ/cm2) exposure in both the cell types. In the presence of FA, SSBs were still present at these time points containing a reference to a delay in DNA resynthesis/ligation. FA at a concentration not inducing micronuclei (12.5 microM) caused significant increase of UVC-induced (4 mJ/cm2) chromosomal damage. Proliferation of keratinocytes and fibroblasts was in parallel to observed DNA damages. In conclusion, our data suggest that environmental exposure to FA may contribute to UV-induced skin carcinogenesis.

Topical DNA oligonucleotide therapy reduces UV-induced mutations and photocarcinogenesis in hairless mice

UV-induced DNA damage gives rise to mutations and skin cancer. We have previously reported that treatment of skin cells in vitro with thymidine dinucleotide (pTT) activates p53 and increases the ability of cells to repair subsequent UV-induced DNA damage by enhancing endogenous DNA repair capacity. Here we show that topical pTT pretreatment enhances the rate of DNA photoproduct removal, decreases UV-induced mutations, and reduces photocarcinogenesis in UV-irradiated hairless WT repair-proficient and Xpc(+/-) heterozygous partially repair-deficient mice, both transgenic for the lacZ/pUR288 mutation-indicator gene. These data support the existence of inducible mammalian DNA damage responses that increase DNA repair capacity after DNA damage and hence reduce the impact of future exposures to environmental carcinogens. The ability of topically applied pTT to induce protective physiologic responses that normally result from DNA damage suggests a previously undescribed means of reducing skin cancer in high-risk individuals.

Pyrimidine dimer formation and oxidative damage in M13 bacteriophage inactivation by ultraviolet C irradiation

The mechanism by which UV-C irradiation inactivates M13 bacteriophage was studied by analyzing the M13 genome using agarose gel electrophoresis and South-Western blotting for pyrimidine dimers. The involvement of singlet oxygen (1O2) was also investigated using azide and deuterium oxide and under deoxygenated conditions. With a decrease in M13 infectivity on irradiation, single-stranded circular genomic DNA (sc-DNA) was converted to Form I and Form II, which had an electrophoretic mobility between that of sc-DNA and linear-form DNA. However, the amount of sc-DNA remaining was not correlated with the survival of M13. The formation of cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidone photoproducts ((6-4)PP) increased as a function of irradiation dose. The decrease in M13 infectivity was highly correlated with the increase in CPD and (6-4)PP, whereas no change was seen in M13 coat protein on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. 8-Oxo-7,8-dihydro-2'-deoxyguanosine did not form in the M13 genome after UV-C irradiation. Inactivation of M13 was neither enhanced by deuterium oxide nor inhibited by azide. Deoxygenation of the M13 suspension did not affect the inactivation, indicating that 1O2 did not participate in the inactivation of M13 by UV-C irradiation under these conditions. These results indicated that UV-C irradiation induced not only CPD and (6-4)PP formation but also additional tertiary structural change in DNA inside the M13 virions, resulting in primary damage and a loss of infectivity. The indirect effect of UV-C irradiation such as 1O2 production followed by oxidative damage to nucleic acids and proteins might have contributed less, if at all, to the inactivation of M13 than the direct effect of UV-C.

In vivo recruitment of XPC to UV-induced cyclobutane pyrimidine dimers by the DDB2 gene product

The initial step in mammalian nucleotide excision repair (NER) of the major UV-induced photoproducts, cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs), requires lesion recognition. It is believed that the heterodimeric proteins XPC/hHR23B and UV-DDB (UV-damaged DNA binding factor, composed of the p48 and p127 subunits) perform this function in genomic DNA, but their requirement and lesion specificity in vivo remains unknown. Using repair-deficient xeroderma pigmentosum (XP)-A cells that stably express photoproduct-specific photolyases, we determined the binding characteristics of p48 and XPC to either CPDs or 6-4PPs in vivo. p48 localized to UV-irradiated sites that contained either CPDs or 6-4PPs. However, XPC localized only to UV-irradiated sites that contained 6-4PPs, suggesting that XPC does not efficiently recognize CPDs in vivo. XPC did localize to CPDs when p48 was overexpressed in the same cell, signifying that p48 activates the recruitment of XPC to CPDs and may be the initial recognition factor in the NER pathway.

Effect of cell confluence on ultraviolet light apoptotic responses in DNA repair deficient cells

One of the major critical factors for cancer proneness is the cell response to DNA damage. In this work, we used human DNA repair deficient cell lines to investigate the responses to ultraviolet irradiation that lead to apoptosis, and the influence of maintaining the cells resting in confluent state. UV-induced apoptosis is prevented in photolyase-proficient HeLa cells when cyclobutane pyrimidine dimers (CPDs) are removed by photorepair. At the same time, we show recovery of RNA synthesis, thus indicating that blockage of RNA transcription may trigger apoptosis in human cells. On the other hand, confluent primary XPC and trichothiodystrophy (TTD)/XPD cell lines, related to xeroderma pigmentosum and trichothiodystrophy repair syndromes, had a reduced and delayed apoptosis when compared to non-confluent cells. In contrast, XPA cells were similarly sensitive in both the confluent and non-confluent growing state. The effect of cellular confluence on UV-mediated apoptosis in CSB cells, related to Cockayne's syndrome, was unclear. Thus, these results indicate that the induction of apoptosis by UV light may also be affected by DNA replication. In addition, they argue for the use of confluent primary cells in studies of induction of apoptosis by UV, a condition close to skin cells in vivo.

Identification and characterization of an intermediate in the alkali degradation of (6-4) photoproduct-containing DNA

The (6-4) photoproduct formed by ultraviolet light is known as an alkali-labile DNA lesion. Strand breaks occur at (6-4) photoproducts when UV-irradiated DNA is treated with hot alkali. We have analyzed the degradation reaction of this photoproduct under alkaline conditions using synthetic oligonucleotides. A tetramer, d(GT(6-4)TC), was prepared, and its degradation in 50 mm KOH at 60 degrees C was monitored by high performance liquid chromatography. A single peak with a UV absorption spectrum similar to that of the starting material was detected after the reaction, and this compound was regarded as an intermediate before the strand break. The formation of this intermediate was compared with intermediates from the degradation of other alkali-labile lesions such as the abasic site, thymine glycol, and 5,6-dihydrothymine. The results strongly suggested that the first step of the alkali degradation of the (6-4) photoproduct was the hydrolysis between the N3 and C4 positions of the 5'-pyrimidine component. Analyses by NMR spectroscopy and mass spectrometry supported the chemical structure of this product. Assays of the complex formation with XPC.HR23B and the translesion synthesis by DNA polymerase eta revealed that the biochemical properties are indistinguishable between the intact and hydrolyzed photoproducts.

Efficient repair of cyclobutane pyrimidine dimers at mutational hot spots is restored in complemented Xeroderma pigmentosum group C and trichothiodystrophy/xeroderma pigmentosum group D cells

Xeroderma pigmentosum (XP) and trichothiodystrophy (TTD) are rare heritable diseases. Patients suffering from XP and 50% of TTD afflicted individuals are photosensitive and have a high susceptibility to develop skin tumors. One solution to alleviating symptoms of these diseases is to express the deficient cDNAs in patient cells as a form of gene therapy. XPC and TTD/XPD cell lines were complemented using retroviral transfer. Expressed wild-type XPC or XPD cDNAs in these cells restored the survival to UVC radiation to wild-type levels in the respective complementation groups. Although complemented XP cell lines have been studied for years, data on cyclobutane pyrimidine dimer (CPD) repair in these cells at different levels are sparse. We demonstrate that CPD repair is faster in the complemented lines at the global, gene, strand specific, and nucleotide specific levels than in the original lines. In both XPC and TTD/XPD complemented lines, CPD repair on the non-transcribed strand is faster than that for the MRC5SV line. However, global repair in the complemented cell lines and MRC5SV is still slower than in normal human fibroblasts. Despite the slower global repair rate, in the complemented XPC and TTD/XPD cells, almost all of the CPDs at "hotspots" for mutation in the P53 tumor database are repaired as rapidly as in normal human fibroblasts. Such evaluation of repair at nucleotide resolution in complemented nucleotide excision repair deficient cells presents a crucial way to determine the efficient re-establishment of function needed for successful gene therapy, even when full repair capacity is not restored.

UV wavelength-dependent regulation of transcription-coupled nucleotide excision repair in p53-deficient human cells

Nucleotide excision repair (NER) prevents skin cancer by eliminating highly genotoxic cyclobutane pyrimidine dimers (CPDs) induced in DNA by the UVB component of sunlight. NER consists of two distinct but overlapping subpathways, i.e., global NER, which removes CPD from the genome overall, and transcription-coupled NER (TCNER), which removes CPD uniquely from the transcribed strand of active genes. Previous investigations have clearly established that the p53 tumor suppressor plays a crucial role in the NER process. Here we used the ligation-mediated PCR technique to demonstrate, at nucleotide resolution along two chromosomal genes in human cells, that the requirement for functional p53 in TCNER, but not in global NER, depends on incident UV wavelength. Indeed, relative to an isogenic p53 wild-type counterpart, p53-deficient human lymphoblastoid strains were shown to remove CPD significantly less efficiently along both the transcribed and nontranscribed strands of the c-jun and hprt loci after exposure to polychromatic UVB (290-320 nm). However, in contrast, after irradiation with 254-nm UV, p53 deficiency engendered less efficient CPD repair only along the nontranscribed strands of these target genes. The revelation of this intriguing wavelength-dependent phenomenon reconciles an apparent conflict between previous studies which used either UVB or 254-nm UV to claim, respectively, that p53 is required for, or plays no role whatsoever in, TCNER of CPD. Furthermore, our finding highlights a major caveat in experimental photobiology by providing a prominent example where the extensively used "nonsolar" model mutagen 254-nm UV does not accurately replicate the effects of environmentally relevant UVB.