Human cells compromised for p53 function exhibit defective global and transcription-coupled nucleotide excision repair, whereas cells compromised for pRb function are defective only in global repair

ATM Pathway Is Essential for HPV-Positive Human Cervical Cancer-Derived Cell Lines Viability and Proliferation

Infection with some mucosal human papillomavirus (HPV) types is the etiological cause of cervical cancer and of a significant fraction of vaginal, vulvar, anal, penile, and head and neck carcinomas. DNA repair machinery is essential for both HPV replication and tumor cells survival suggesting that cellular DNA repair machinery may play a dual role in HPV biology and pathogenesis. Here, we silenced genes involved in DNA Repair pathways to identify genes that are essential for the survival of HPV-transformed cells. We identified that inhibition of the ATM/CHK2/BRCA1 axis selectively affects the proliferation of cervical cancer-derived cell lines, without altering normal primary human keratinocytes (PHK) growth. Silencing or chemical inhibition of ATM/CHK2 reduced the clonogenic and proliferative capacity of cervical cancer-derived cells. Using PHK transduced with HPV16 oncogenes we observed that the effect of ATM/CHK2 silencing depends on the expression of the oncogene E6 and on its ability to induce p53 degradation. Our results show that inhibition of components of the ATM/CHK2 signaling axis reduces p53-deficient cells proliferation potential, suggesting the existence of a synthetic lethal association between CHK2 and p53. Altogether, we present evidence that synthetic lethality using ATM/CHK2 inhibitors can be exploited to treat cervical cancer and other HPV-associated tumors.

Three Prime Repair Exonuclease 1 (TREX1) expression correlates with cervical cancer cells growth in vitro and disease progression in vivo

Alterations in specific DNA damage repair mechanisms in the presence of human papillomavirus (HPV) infection have been described in different experimental models. However, the global effect of HPV on the expression of genes involved in these pathways has not been analyzed in detail. In the present study, we compared the expression profile of 135 genes involved in DNA damage repair among primary human keratinocytes (PHK), HPV-positive (SiHa and HeLa) and HPV-negative (C33A) cervical cancer derived cell lines. We identified 9 genes which expression pattern distinguishes HPV-positive tumor cell lines from C33A. Moreover, we observed that Three Prime Repair Exonuclease 1 (TREX1) expression is upregulated exclusively in HPV-transformed cell lines and PHK expressing HPV16 E6 and E7 oncogenes. We demonstrated that TREX1 silencing greatly affects tumor cells clonogenic and anchorage independent growth potential. We showed that this effect is associated with p53 upregulation, accumulation of subG1 cells, and requires the expression of E7 from high-risk HPV types. Finally, we observed an increase in TREX1 levels in precancerous lesions, squamous carcinomas and adenocarcinomas clinical samples. Altogether, our results indicate that TREX1 upregulation is important for cervical tumor cells growth and may contribute with tumor establishment and progression.

Human papillomavirus and genome instability: from productive infection to cancer

Infection with high oncogenic risk human papillomavirus types is the etiological factor of cervical cancer and a major cause of other epithelial malignancies, including vulvar, vaginal, anal, penile and head and neck carcinomas. These agents affect epithelial homeostasis through the expression of specific proteins that deregulate important cellular signaling pathways to achieve efficient viral replication. Among the major targets of viral proteins are components of the DNA damage detection and repair machinery. The activation of many of these cellular factors is critical to process viral genome replication intermediates and, consequently, to sustain faithful viral progeny production. In addition to the important role of cellular DNA repair machinery in the infective human papillomavirus cycle, alterations in the expression and activity of many of its components are observed in human papillomavirus-related tumors. Several studies from different laboratories have reported the impact of the expression of human papillomavirus oncogenes, mainly E6 and E7, on proteins in almost all the main cellular DNA repair mechanisms. This has direct consequences on cellular transformation since it causes the accumulation of point mutations, insertions and deletions of short nucleotide stretches, as well as numerical and structural chromosomal alterations characteristic of tumor cells. On the other hand, it is clear that human papillomavirus-transformed cells depend on the preservation of a basal cellular DNA repair activity level to maintain tumor cell viability. In this review, we summarize the data concerning the effect of human papillomavirus infection on DNA repair mechanisms. In addition, we discuss the potential of exploiting human papillomavirus-transformed cell dependency on DNA repair pathways as effective antitumoral therapies.

Enhancement of UVB-induced DNA damage repair after a chronic low-dose UVB pre-stimulation

Absorption of solar ultraviolet (UV) radiation by DNA leads to the formation of the highly mutagenic cyclobutane pyrimidine dimer (CPD). The mutagenicity of CPD is caused, in part, by the fact that their recognition and repair by the nucleotide excision repair (NER) pathway is challenging and slow. It has been previously shown that a pre-stimulation with genotoxic agents improve NER efficiency of CPD, indicating a potential adaptive response of this repair pathway. We have pre-treated human dermal fibroblasts with repeated subletal low doses of UVB (chronic low-dose of UVB; CLUV) to determine whether it could enhance NER capacity to repair CPD. Our results show that CLUV pre-treatment greatly enhances CPD repair but have little effect on the repair of another UV-induced bypirimidine photoproduct, the pyrimidine (6-4) pyrimidone photoproducts (6-4 PP). We have determined that the CLUV treatment activates p53 and we found an increase of DDB2 and XPC gene expression. This is consistent with an increasing level of NER recognition proteins, DDB2 and XPC, we found concentrated at the chromatin. This study represents the first demonstration that chronic UVB exposure can stimulate NER pathway. Altogether, these results shed light on the potential adaptability of the NER by chronic UVB irradiation and the mechanisms involved.

HMGB1 Protein: A Therapeutic Target Inside and Outside the Cell

High-mobility group box 1 protein (HMGB1) is a nonhistone chromosomal protein discovered more than 30 years ago. It is an abundant nuclear protein that has a dual function-in the nucleus, it binds DNA and participates in practically all DNA-dependent processes serving as an architectural factor. Outside the cell, HMGB1 plays a different role-it acts as an alarmine that activates a large number of HMGB1-"competent" cells and mediates a broad range of physiological and pathological responses. This universality makes it an attractive target for innovative therapeutic strategies in the treatment of various diseases. Here we present an overview of the major nuclear and extracellular properties of HMGB1 and describe its interaction with different molecular partners as specific receptors or inhibitors, which are important for its role as a target in multiple diseases. We highlight its pivotal role as a target for cancer treatment at two aspects: first in terms of its substantial impact on the repair capacity of cancer cells, thus affecting the effectiveness of chemotherapy with the antitumor drug cis-platinum and, second, the possibility to be targeted by microRNAs influencing different pathways of human diseases, thus making it a promising candidate for a new strategy for therapeutic interventions against various pathological conditions but mainly cancer.

The repair capacity of lung cancer cell lines A549 and H1299 depends on HMGB1 expression level and the p53 status

Elucidation of the cellular components responsive to chemotherapeutic agents as cisplatin rationalizes the strategy for anticancer chemotherapy. The removal of the cisplatin/DNA lesions gives the chance to the cancer cells to survive and compromises the chemotherapeutical treatment. Therefore, the cell repair efficiency is substantial for the clinical outcome. High mobility group box 1 (HMGB1) protein is considered to be involved in the removal of the lesions as it binds with high affinity to cisplatin/DNA adducts. We demonstrated that overexpression of HMGB1 protein inhibited cis-platinated DNA repair in vivo and the effect strongly depended on its C-terminus. We registered increased levels of DNA repair after HMGB1 silencing only in p53 defective H1299 lung cancer cells. Next, introduction of functional p53 resulted in DNA repair inhibition. H1299 cells overexpressing HMGB1 were significantly sensitized to treatment with cisplatin demonstrating the close relation between the role of HMGB1 in repair of cis-platinated DNA and the efficiency of the anticancer drug, the process being modulated by the C-terminus. In A549 cells with functional p53, the repair of cisplatin/DNA adducts is determined by а complex action of HMGB1 and p53 as an increase of DNA repair capacity was registered only after silencing of both proteins.

Transcription Blockage Leads to New Beginnings

Environmental agents are constantly challenging cells by damaging DNA, leading to the blockage of transcription elongation. How do cells deal with transcription-blockage and how is transcription restarted after the blocking lesions are removed? Here we review the processes responsible for the removal of transcription-blocking lesions, as well as mechanisms of transcription restart. We also discuss recent data suggesting that blocked RNA polymerases may not resume transcription from the site of the lesion following its removal but, rather, are forced to start over from the beginning of genes.

The role of DNA damage responses in p53 biology

The tumour suppressor p53 is a central player in cellular DNA damage responses. P53 is upregulated and activated by genotoxic stress and induces a transcriptional programme with effectors promoting apoptosis, cell cycle arrest, senescence and DNA repair. For the best part of the last three decades, these DNA damage-related programmes triggered by p53 were unequivocally regarded as the major if not sole mechanism by which p53 exerts its tumour suppressor function. However, this interpretation has been challenged by a number of recent in vivo studies, demonstrating that mice which are defective in inducing p53-dependent apoptosis, cell cycle arrest and senescence suppress thymic lymphoma as well as wild-type p53 expressing animals. Consequently, the importance of DNA damage responses for p53-mediated tumour suppression has been questioned. In this review, I summarize current knowledge on p53-controlled DNA damage responses and argue that these activities, while their role has certainly changed, remain an important feature of p53 biology with relevance for cancer therapy and tumour suppression.

Interaction between the Cockayne syndrome B and p53 proteins: implications for aging

The CSB protein plays a role in the transcription coupled repair (TCR) branch of the nucleotide excision repair pathway. CSB is very often found mutated in Cockayne syndrome, a segmental progeroid genetic disease characterized by organ degeneration and growth failure. The tumor suppressor p53 plays a pivotal role in triggering senescence and apoptosis and suppressing tumorigenesis. Although p53 is very important to avoid cancer, its excessive activity can be detrimental for the lifespan of the organism. This is why a network of positive and negative feedback loops, which most likely evolved to fine-tune the activity of this tumor suppressor, modulate its induction and activation. Accordingly, an unbalanced p53 activity gives rise to premature aging or cancer. The physical interaction between CSB and p53 proteins has been known for more than a decade but, despite several hypotheses, nobody has been able to show the functional consequences of this interaction. In this review we resume recent advances towards a more comprehensive understanding of the critical role of this interaction in modulating p53’s levels and activity, therefore helping the system find a reasonable equilibrium between the beneficial and the detrimental effects of its activity. This crosstalk re-establishes the physiological balance towards cell proliferation and survival instead of towards cell death, after stressors of a broad nature. Accordingly, cells bearing mutations in the csb gene are unable to re-establish this physiological balance and to properly respond to some stress stimuli and undergo massive apoptosis.

Defective DNA repair and cell cycle arrest in cells expressing Merkel cell polyomavirus T antigen

The pathways by which Merkel cell polyomavirus (MCV) infection contributes to the formation of Merkel cell carcinomas are important for understanding the pathogenesis of these cancers. We hypothesized that MCV T antigen suppresses normal responses to ultraviolet radiation (UVR)-induced DNA damage. An MCV-infected cell line (MKL-1) exhibited UVR hypersensitivity, impaired repair of DNA lesions and cell cycle arrest after UVR, as well as reduced levels of the DNA damage recognition protein, XPC. When ectopically expressed in uninfected UISO cells, mutant but not wild-type T antigen resulted in loss of repair of UVR-induced cyclobutane pyrimidine dimers and reductions in XPC, p53 and p21 levels, whereas both wild-type and mutant T antigen inhibited cell cycle arrest after UVR. Similarly, only mutant T antigen in normal fibroblasts inhibited DNA repair and XPC expression, while both mutant and wild-type T antigens produced cell cycle dysregulation. Wild-type T antigen expression produced large T, 57 kT and small T antigens while mutant T antigen was only detectable as a truncated large T antigen protein. Expression of wild-type large T antigen but not small T antigen inhibited the G1 checkpoint in UISO cells, but neither wild-type large T nor small T antigens affected DNA repair, suggesting that large T antigen generates cell cycle defects, and when mutated may also impair DNA repair. These results indicate that T antigen expression by MCV can inhibit key responses to UVR-induced DNA damage and suggest that progressive MCV-mediated abrogation of genomic stability may be involved in Merkel cell carcinogenesis.

Reciprocally regulated chromatin association of Cockayne syndrome protein B and p53 protein

The Cockayne syndrome complementation group B (CSB) protein is an ATP-dependent chromatin remodeler with an essential function in transcription-coupled DNA repair, and mutations in the CSB gene are associated with Cockayne syndrome. The p53 tumor suppressor has been known to interact with CSB, and both proteins have been implicated in overlapping biological processes, such as DNA repair and aging. The significance of the interaction between CSB and p53 has remained unclear, however. Here, we show that the chromatin association of CSB and p53 is inversely related. Using in vitro binding and chromatin immunoprecipitation approaches, we demonstrate that CSB facilitates the sequence-independent association of p53 with chromatin when p53 concentrations are low and that this is achieved by the interaction of CSB with the C-terminal region of p53. Remarkably, p53 prevents CSB from binding to nucleosomes when p53 concentrations are elevated. Examining the enzymatic properties of CSB revealed that p53 excludes CSB from nucleosomes by occluding a nucleosome interaction surface on CSB. Together, our results suggest that the reciprocal regulation of chromatin access by CSB and p53 could be part of a mechanism by which these two proteins coordinate their activities to regulate DNA repair, cell survival, and aging.

Hesperidin promotes cyclobutane pyrimidine dimer repair in UVB-exposed mice epidermis

PURPOSE

To investigate whether topical application of hesperin affords protection to Balb/C mice epidermis from UVB-induced cyclobutane pyrimidine dimers (CPDs).

METHODS

A DNA damage model of UVB irradiation-induced mice epidermis was established. The immunohistochemical staining and southwestern dot blotting were used for CPDs detection; western blotting was used for P53 detection.

RESULTS

Topical application of hesperidin on Balb/C mice skin significantly decreased the amount of epidermal CPDs 24 and 48 h after 180 mJ/cm(2) of UVB irradiation as compared to untreated mice. UVB-induced p53 expression was more pronounced in hesperidin-treated mice epidermis compared to that of untreated mice.

CONCLUSION

Taken together, these results suggest that topical hesperidin application promotes DNA photo-damage repair. Hesperidin is therefore a promising protective substance against UVB radiation.

[Mapping DNA damage to understand somatic mutagenesis]

Somatic mutation theory explains how DNA damage can lead to the malignant transformation of cells. It therefore elucidates the connection between genotoxic agents and cancers. Mutational spectra, which tend to be characteristic of a cancer type, are available for certain genes like p53 which is frequently mutated in tumors. A mutational spectrum could therefore be the signature of the genotoxic agent(s) at the origin of the malignant transformation. Ligation-mediated PCR (LMPCR) is a genomic sequencing method that can be used for the mapping of DNA damage at nucleotide resolution. Such a mapping can then be compared to a mutational spectrum to test the hypothesis that implies one agent can cause mutations into one cancer type. LMPCR has been used this way to map DNA damage generated by different UV wavelengths. The frequently damaged sites following UVB irradiation correlate with the mutational spectrum of p53 in skin cancer. Similarly, BPDE, the activated form of the benzo[a]pyrene present in tobacco smoke, generates frequent adducts at sites corresponding to mutation hotspots of p53 in lung cancers. Still, the correlation between BPDE damage sites and p53 mutations is not perfect and this suggests a role of other genotoxic substances that are also present in tobacco smoke, such as the nitrosamine NNK. Finally, and beyond this objective of better understanding somatic mutagenesis, LMPCR is commonly used whenever DNA damage frequency and/or repair is to be investigated.

MTA1 coregulator regulates p53 stability and function

Although metastasis-associated protein 1 (MTA1) has recently been shown as a DNA damage responsive protein, the underlying mechanism for its role in DNA double-strand break (DSB) repair remains unknown. Here, we show that MTA1 controls p53 stability through inhibiting its ubiquitination by E3 ubiquitin ligases mouse double minute 2 (Mdm2) and constitutive photomorphogenic protein 1 (COP1). The underlying mechanisms involve the ability of MTA1 to compete with COP1 to bind to p53 and/or to destabilize COP1 and Mdm2. Consequently, MTA1 regulates the p53-dependent transcription of p53R2, a direct p53 target gene for supplying nucleotides to repair damaged DNA. Depletion of MTA1 impairs p53-dependent p53R2 transcription and compromises DNA repair. Interestingly, these events could be reversed by MTA1 reintroduction, indicating that MTA1 interjects into the p53-dependent DNA repair. Given the fact that MTA1 is widely up-regulated in human cancers, these findings in conjunction with our earlier finding of a crucial role of MTA1 in DSB repair suggest an inherent role of the MTA1-p53-p53R2 pathway in DNA damage response in cancer cells.

Rapid repair of UVA-induced oxidized purines and persistence of UVB-induced dipyrimidine lesions determine the mutagenicity of sunlight in mouse cells

Despite the predominance of ultraviolet A (UVA) relative to UVB in terrestrial sunlight, solar mutagenesis in humans and rodents is characterized by mutations specific for UVB. We have investigated the kinetics of repair of UVA- and UVB-induced DNA lesions in relation to mutagenicity in transgenic mouse fibroblasts irradiated with equilethal doses of UVA and UVB in comparison to simulated-sunlight UV (SSL). We have also analyzed mutagenesis-derived carcinogenesis in sunlight-associated human skin cancers by compiling the published data on mutation types found in crucial genes in nonmelanoma and melanoma skin cancers. Here, we demonstrate a resistance to repair of UVB-induced cis-syn cyclobutane pyrimidine-dimers (CPDs) together with rapid removal of UVA-induced oxidized purines in the genome overall and in the cII transgene of SSL-irradiated cells. The spectra of mutation induced by both UVB and SSL irradiation in this experimental system are characterized by significant increases in relative frequency of C-->T transitions at dipyrimidines, which are the established signature mutation of CPDs. This type of mutation is also the predominant mutation found in human nonmelanoma and melanoma tumor samples in the TP53, CDKN2, PTCH, and protein kinase genes. The prevailing role of UVB over UVA in solar mutagenesis in our test system can be ascribed to different kinetics of repair for lesions induced by the respective UV irradiation.

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.

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.

A sensitive flow cytometry-based nucleotide excision repair assay unexpectedly reveals that mitogen-activated protein kinase signaling does not regulate the removal of UV-induced DNA damage in human cells

In response to diverse genotoxic stimuli (e.g. UV and cisplatin), the mitogen-activated protein kinases ERK1/2, JNK1/2, and p38alpha/beta become rapidly phosphorylated and in turn activate multiple downstream effectors that modulate apoptosis and/or growth arrest. Furthermore, previous lines of evidence have strongly suggested that ERK1/2 and JNK1/2 participate in global-genomic nucleotide excision repair, a critical antineoplastic pathway that removes helix-distorting DNA adducts induced by a variety of mutagenic agents, including UV. To rigorously evaluate the potential role of mitogen-activated protein kinases in global-genomic nucleotide excision repair, various human cell strains (primary skin fibroblasts, primary lung fibroblasts, and HCT116 colon carcinoma cells) were treated with highly specific chemical inhibitors, which, following UV exposure, (i) abrogated the capacities of ERK1/2, JNK1/2, or p38alpha/beta to phosphorylate specific downstream effectors and (ii) characteristically modulated cellular proliferation, clonogenic survival, and/or apoptosis. A highly sensitive flow cytometry-based nucleotide excision repair assay recently optimized and validated in our laboratory was then employed to directly demonstrate that the kinetics of UV DNA photoadduct repair are highly similar in mock-treated versus mitogen-activated protein kinase inhibitor-treated cells. These data on primary and tumor cells treated with pharmacological inhibitors were fully corroborated by repair studies using (i) short hairpin RNA-mediated knockdown of ERK1/2 or JNK1/2 in human U2OS osteosarcoma cells and (ii) expression of a dominant negative p38alpha mutant in human primary lung fibroblasts. Our results provide solid evidence for the first time, in disaccord with a burgeoning perception, that mitogen-activated protein kinase signaling does not influence the efficiency of human global-genomic nucleotide excision repair.

Impaired repair of cyclobutane pyrimidine dimers in human keratinocytes deficient in p53 and p63

While many p53-deficient cell types are impaired in global genomic nucleotide excision repair of cyclobutane pyrimidine dimers (CPDs), human epidermal keratinocytes expressing human papillomavirus type 16 E6 and E7 are p53 deficient and yet maintain repair of CPD. We hypothesized that the p53 homolog, p63, may participate in governing global repair instead of p53 in keratinocytes. Following ultraviolet radiation (UVR) of E6/E7 keratinocytes, depletion of p63 but not of p73 impaired global genomic repair of CPD relative to control cells. In all cases, repair of pyrimidine(6-4)pyrimidone photoproducts, the other major UVR-induced DNA lesions, was unaffected. In E6/E7 keratinocytes treated with p63 small interfering RNA, reduced global repair of CPD was associated not with reduced levels of messenger RNA-encoding DNA damage recognition proteins but rather with decreased levels of DDB2 and XPC proteins, suggesting that p63 posttranscriptionally regulates levels of these proteins. These results indicate that global repair may be regulated at multiple levels and suggest a novel role for p63 in modulating repair of DNA damage in human keratinocytes. The results may provide insight into mechanisms of genomic stability in epithelia infected with oncogenic human papilloma viruses and may further explain the lack of increased skin cancer incidence in Li-Fraumeni syndrome.

Repair kinetics of specific types of nitroso-induced DNA damage using the comet assay in human cells

The comet assay is sensitive and can detect DNA damage frequencies less than 1 in 10(7) bases. We have previously shown that several types of DNA damage associated with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), a tobacco-specific pro-mutagen, can be investigated with some specificity using this technique. Little is known about their repair. We verified the ability of the comet assay to quantify the repair kinetics of specific types of damage in normal fibroblasts, e.g., dimethylsulfate-induced 7-methylguanines (7-mG) and UVB-induced cyclobutane pyrimidine dimers. The time course, formation and repair, of DNA damage after acute doses of NNK reactive metabolites, were then compared in normal human cells (fibroblasts and lymphocytes) and in cells proficient for activating NNK (U937 and NCI-H23). NNK can be activated in cells into reactive metabolites that can either methylate or pyridyloxobutylate DNA. The 7-mG generated by methylation gave post-treatment patterns that were sufficiently different between cell types to conclude that repair of 7-mG in U937 cells was fast, repair in lymphocytes was slow, and repair in NCI-H23 cells and fibroblasts displayed intermediate rates. Pyridyloxobutylation generated formamidopyrimidine (fapy) glycosylase (fpg)-sensitive sites that could be the fapy form of 7-pyridyloxobutylguanines produced in cells. For this type of adducts, the post-treatment patterns of adduct frequency as a function of time depended even more clearly on the cell type: fibroblasts and NCI-H23 cells showed an initial rapid increase in fpg-sensitive damage frequency that did not occur in lymphocytes and U937 cells. This increase seemed associated with p53 proficiency in fibroblasts. Our results show that repair kinetics can be investigated with the comet assay and that differences between cell types can be observed with that technique. But it seems that pro-mutagen activation and/or the way a type of adducts is formed can affect the quantification of the repair.

Increased expression of p53 enhances transcription-coupled repair and global genomic repair of a UVC-damaged reporter gene in human cells

Ultraviolet (UV) light-induced DNA damage is repaired by nucleotide excision repair, which is divided into two sub-pathways: global genome repair (GGR) and transcription-coupled repair (TCR). While it is well established that the GGR pathway is dependent on the p53 tumour suppressor protein in human cells, both p53-dependent and p53-independent pathways have been reported for TCR. In the present work, we investigated the role of p53 in both GGR and TCR of a UVC-damaged reporter gene in human fibroblasts. We employed a non-replicating recombinant human adenovirus, AdCA17lacZ, that can efficiently infect human fibroblasts and express the beta-galactosidase (beta-gal) reporter gene under the control of the human cytomegalovirus promoter. We examined host cell reactivation (HCR) of beta-gal expression for the UVC-treated reporter construct in normal fibroblasts and in xeroderma pigmentosum (XP) and Cockayne syndrome (CS) fibroblasts deficient in GGR, TCR, or both. HCR was examined in fibroblasts that had been pre-infected with Ad5p53wt, which expresses wild-type p53, or a control adenovirus, AdCA18luc, which expresses the luciferase gene. We show that increased expression of p53 results in enhanced HCR of the UVC-damaged reporter gene in both untreated and UVC-treated cells for normal, CS-B (TCR-deficient), and XP-C (GGR-deficient), but not XP-A (TCR- and GGR-deficient) fibroblasts. These results indicate an involvement of p53 in both TCR and GGR of the UV-damaged reporter gene in human cells.

DNA mismatch repair and p53 function are major determinants of the rate of development of cisplatin resistance

As opposed to factors that control sensitivity to the acute cytotoxic effect of cisplatin, little is known about the factors that determine the rate at which resistance develops. This study examined how loss of p53 or DNA mismatch repair (MMR) function affected the rate of development of resistance to cisplatin in human colon carcinoma cells during sequential cycles of cisplatin exposure that mimic the way the drug is used in the clinic. We used a panel of sublines molecularly engineered to express either the MMR- and p53-proficient phenotype or singly or doubly deficient phenotypes. Loss of either MMR or p53 alone increased the rate of development of resistance to cisplatin by 1.8- and 2.4-fold, respectively; however, loss of both MMR and p53 increased the rate by 4.8-fold. Inhibition of DNA polymerase zeta by suppression of the expression of its REV3 subunit eliminated the increased rate of development of resistance observed in the MMR-deficient cells. Loss of p53 or MMR increased the steady-state level of REV3 and of REV1 mRNA; loss of both functions increased these levels much further by a factor of 20.2-fold for REV3 and 10.3-fold for REV1. The basal level of homologous recombination measured using a reporter vector was 1.3- to 1.7-fold higher in cells that had lost either p53 or MMR function, and 2.6-fold higher in cells that had lost both. In the p53- and MMR-proficient cells, cisplatin induced a 17-fold increase in homologous recombination even when the recombining sequences that did not contain cisplatin adducts; the magnitude of induction was even greater in cells that had lost either one or both functions. We conclude that separate from effects on sensitivity to the acute cytotoxic effect of cisplatin, loss of MMR, especially when combined with loss of p53, results in rapid evolution of cisplatin resistance during sequential rounds of drug exposure that is likely mediated by enhanced mutagenic translesion synthesis. The DNA damage response activated by cisplatin is accompanied by a p53- and MMR-dependent increase in homologous recombination even between adduct-free sequences.

Effect of high-risk human papillomavirus oncoproteins on p53R2 gene expression after DNA damage

The p53R2 protein is a p53-inducible small subunit of ribonucleotide reductase. It plays a crucial role in p53-dependent cellular response to DNA damage and oxidative stress by providing deoxyribonucleotides (dNTPs) to the DNA repair machinery and by scavenging reactive oxygen species (ROS). To investigate the effects of high-risk human papillomavirus (HPV) oncoproteins on p53R2 expression after DNA damage, we analyzed the p53R2 protein levels in human cells ectopically expressing the HPV-16 E6 and E7 genes, and in the HPV-positive cancer cell lines SiHa, CaSki and HeLa, exposed to adriamycin or to H(2)O(2). We found that in normal cells, p53R2 expression is efficiently induced by both H(2)O(2) and adriamycin, supporting the role of p53R2 in cellular response to oxidative stress. Ectopic expression of E6 impaired p53 and p53R2 induction after DNA damage in human fibroblasts. Moreover, SiHa, CaSki and HeLa cells were unresponsive to H(2)O(2) exposure, and adriamycin induced p53R2 levels only in SiHa cells. Our results imply that high-risk HPV infection may suppress the p53R2-dependent dNTPs supply to the DNA repair system and the ROS scavenging activity; they also suggest that an altered p53R2 response to genotoxins and to oxidative stress may contribute to HPV-induced genetic instability and carcinogenesis.

Proficient global nucleotide excision repair in human keratinocytes but not in fibroblasts deficient in p53

The p53 tumor suppressor protein is important for many cellular responses to DNA damage in mammalian cells, but its role in regulating DNA repair in human keratinocytes is undefined. We compared the nucleotide excision repair (NER) response of human fibroblasts and keratinocytes deficient in p53. Fibroblasts expressing human papillomavirus 16 E6 oncoprotein had impaired repair of UV radiation-induced cyclobutane pyrimidine dimers in association with reduced levels of p53 and XPC, which is involved in DNA damage recognition. In contrast, keratinocytes expressing E6 alone or concurrently with the E7 oncoprotein, while possessing reduced levels of p53 but normal levels of XPC, continued to repair pyrimidine dimers as efficiently as control cells with normal p53 levels. Despite preservation of DNA repair, E6 and E6/E7 keratinocytes were hypersensitive to UV radiation. E6 fibroblasts exhibited markedly reduced basal and induced levels of mRNA encoding DDB2, another protein implicated in early events in global NER. In contrast, E6 or E6/E7 keratinocytes possessed basal DDB2 mRNA levels that were not significantly altered relative to control cells, although little induction occurred following UV radiation. Intact global NER was also confirmed in SCC25 cells possessing inactivating mutations in p53 as well as in cells treated with pifithrin-alpha, a chemical inhibitor of p53 that decreased sensitivity of cells to UV radiation. Collectively, these results indicate that human keratinocytes, unlike fibroblasts, do not require p53 to maintain basal global NER activity, but p53 may still be important in mediating inducible responses following DNA damage.

17β-Estradiol modulates UVB-induced cellular responses in estrogen receptors positive human breast cancer cells

Genotoxic agents produce numerous cellular responses that are principally dedicated to maintain or restore DNA integrity. In human cells, nucleotide excision repair (NER) is one of the major pathways for the repair of DNA damage such as ultraviolet (UV) radiation-induced lesions. Endocrine disrupting compounds are environmental contaminants that interfere with the function of the endocrine system. Among them, the natural estrogen 17beta-estradiol (E(2)) exhibits the most potent activity. Some proteins directly or indirectly involved in NER also fulfill other functions such as transcription, DNA damage checkpoints or cell cycle. Moreover, steroids such as E(2) are believed to interact with a large number of proteins including some involved in NER and DNA damage checkpoint control. We therefore investigated the potential modulation of genotoxic stress-cells responses by E(2) treatment. Estrogen receptor (ER)-positive human breast cancer cells were submitted to E(2) before and/or after UVB irradiation and thereafter the repair kinetics of UV-induced DNA damage were evaluated. We report here that the repair rate of UVB-induced DNA damage is enhanced when cells are submitted to an estrogenic stimulation. Moreover, our results suggest that this response could be mediated by cell cycle regulatory proteins in a p53-independent manner.

Cellular UV damage responses--functions of tumor suppressor p53

DNA damage, provoked by ultraviolet (UV) radiation, evokes a cellular damage response composed of activation of stress signaling and DNA checkpoint functions. These are translated to responses of replicative arrest, damage repair, and apoptosis aimed at cellular recovery from the damage. p53 tumor suppressor is a central stress response protein, activated by multiple endogenous and environmental insults, including UV radiation. The significance of p53 in the DNA damage responses has frequently been reviewed in the context of ionizing radiation or other double strand break (DSB)-inducing agents. Despite partly similar patterns, the molecular events following UV radiation are, however, distinct from the responses induced by DSBs and are profoundly coupled with transcriptional stress. These are illustrated, e.g., by the UV damage-specific translocations of Mdm2, promyelocytic leukemia protein, and nucleophosmin and their interactions with p53. In this review, we discuss UV damage-provoked cellular responses and the functions of p53 in damage recovery and cell death.

Pre-UV-Treatment of Cells Results in Enhanced Host Cell Reactivation of a UV Damaged Reporter Gene in CHO-AA8 Chinese Hamster Ovary Cells but Not in Transcription-Coupled Repair Deficient CHO-UV61 Cells

We have used a non-replicating recombinant adenovirus, Ad5MCMVlacZ, which expresses the β-galactosidase reporter gene, to examine both constitutive and inducible repair of UV-damaged DNA in repair proficient CHO-AA8 Chinese hamster ovary cells and in mutant CHO-UV61 cells which are deficient in the transcription-coupled repair (TCR) pathway of nucleotide excision repair. Host cell reactivation (HCR) of β-galactosidase activity for UV-irradiated Ad5MCMVlacZ was significantly reduced in non-irradiated CHO-UV61 cells compared to that in non-irradiated CHO-AA8 cells suggesting that repair in the transcribed strand of the UV-damaged reporter gene in untreated cells utilizes TCR. Prior UV-irradiation of cells with low UV fluences resulted in a transient enhancement of HCR for expression of the UV-damaged reporter gene in CHO-AA8 cells but not in TCR deficient CHO-UV61 cells. These results suggest the presence of an inducible DNA pathway in CHO cells that results from an enhancement of TCR or a mechanism that involves the TCR pathway.

SW480, a p53 Double-mutant Cell Line Retains Proficiency for Some p53 Functions

During certain types of cellular stress, the p53 tumor suppressor protein binds to DNA and transactivates a variety of genes that regulate critical responses including apoptosis, cell cycle checkpoints, differentiation, and angiogenesis. In addition, functional p53 is known to be required for efficient nucleotide excision repair (NER) of bulky DNA adducts generated through exposure to environmental mutagens such as UV light. Nonetheless, we previously showed that the model p53-mutated human adenocarcinoma strain SW480 is proficient in the removal of UV-induced cyclobutane pyrimidine dimers (CPD) via NER. We undertook the present study to begin probing the molecular basis for this unexpected repair phenotype. Cytogenetic analysis indicated that SW480 is stable at the chromosomal level, i.e. manifests a karyotypic profile very similar to that revealed for this line as far back as 14 years ago. After fluorescence in situ hybridization (FISH), using a probe complementary to the p53 gene, we found that 98% of the SW480 interphase nuclei contains three copies of the gene, later revealed to be localized on intact short arms of three chromosomes 17. DNA sequence analysis further showed that all three p53 copies in SW480 carry two point mutations (R273H and P309S), and levels of the corresponding mutated p53 protein are about 20-fold higher than in the closely related p53 wild-type strain LoVo. Using an electrophoretic mobility shift assay (EMSA), we demonstrated that R273H/P309S p53 is able to bind with wild-type affinity to its consensus DNA sequence in vitro. Analysis of p21(Cip1/WAF1) expression and in vivo footprinting by ligation-mediated PCR (LMPCR) showed that, in wild-type LoVo cells, an exposure to cellular stress (e.g. UV or ionizing radiation) is necessary for p53 activation of the p21(Cip1/WAF1) promoter. In contrast, the R273H/P309S-mutated p53 protein in SW480 constitutively activates p21(Cip1/WAF1) in the absence of stress through an unknown mechanism. A similar phenomenon whereby mutated p53 in SW480 is able to induce NER-related proteins might explain the normal DNA repair phenotype previously observed in this strain. For now we conclude that, in general, results obtained using SW480 as a p53-deficient cell line should be interpreted very cautiously.

Enhanced host cell reactivation of a UV-damaged reporter gene in pre-UV-treated cells is delayed in Cockayne syndrome cells

We have used a non-replicating recombinant adenovirus, Ad5HCMVlacZ, which expresses the beta-galactosidase (beta-gal) reporter gene, to examine the time course of UV-inducible repair of UV-damaged DNA in human fibroblasts. Host cell reactivation (HCR) of beta-gal activity for UV-irradiated Ad5HCMVlacZ was examined in non-irradiated and UV-irradiated nucleotide excision repair (NER) proficient normal human fibroblasts, xeroderma pigmentosum (XP) group C fibroblasts which are defective in the global genomic repair (GGR) pathway of NER and Cockayne syndrome (CS) fibroblasts which are defective in the transcription coupled repair (TCR) pathway of NER. HCR was deficient in untreated XP-C and CS cells indicating that both TCR and GGR are involved in removal of photolesions from the transcribed strand of the reporter gene in unirradiated human cells as reported previously. Prior UV-irradiation of cells with low UV fluences resulted in a transient enhancement of HCR in normal and XP-C fibroblasts that reached a maximum when cells were infected at 25-35 h after UV. In contrast, UV-enhanced HCR was delayed in CS-B cells, reaching levels similar to that in normal cells only when cells were infected between 40 and 60 h after UV exposure. These results are consistent with a UV-induced up-regulation of GGR through a TCR dependent pathway in CS cells.

Loss of p21WAF1/Cip1 in Gadd45-deficient keratinocytes restores DNA repair capacity

Ultraviolet light (UV)-induced DNA damage is repaired primarily by the nucleotide excision repair (NER) pathway. Gadd45 is a multifunctional protein that regulates NER. Gadd45-deficient keratinocytes fail to repair UV-induced DNA damage, but the mechanism by which Gadd45 stimulates repair of UV-induced DNA damage is unknown. p21WAF1/Cip1 (p21) is a well-characterized downstream target of p53 that binds to Gadd45 and proliferating cell nuclear antigen (PCNA). The role of p21 in NER is somewhat controversial, however, recent studies appear to suggest that it inhibits DNA repair by inhibiting PCNA activity. Since a physical interplay exists between p21, Gadd45 and PCNA, we hypothesized that Gadd45 promoted DNA repair via p21. Initially, we examined p21 protein expression in Gadd45-deficient and proficient mice and found a higher base level of p21 protein in Gadd45-deficient keratinocytes and in most other tissues. With these results, we next speculated on the role played by p21 in Gadd45 regulated NER, by exposing keratinocytes from wild-type, single and double knockout (Gadd45 and p21) mice to UV, and measuring the responses. We confirmed that Gadd45-deficient keratinocytes were defective in UV-induced NER, but interestingly Gadd45/p21-null keratinocytes had normal NER in response to UV. Furthermore, Gadd45/p21-null keratinocytes were more resistant to UV-induced cell death than Gadd45-deficient keratinocytes. These results support the hypothesis that Gadd45 enhances NER by negatively regulating basal p21 expression in keratinocytes.

p53: traffic cop at the crossroads of DNA repair and recombination

p53 mutants that lack DNA-binding activities, and therefore, transcriptional activities, are among the most common mutations in human cancer. Recently, a new role for p53 has come to light, as the tumour suppressor also functions in DNA repair and recombination. In cooperation with its function in transcription, the transcription-independent roles of p53 contribute to the control and efficiency of DNA repair and recombination.

Impaired processing of DNA photoproducts and ultraviolet hypermutability with loss of p16INK4a or p19ARF

Reduced DNA repair has been linked to an increased risk of cutaneous malignant melanoma, but insights into the molecular mechanisms of that link are scarce. The INK4a/ARF (CDKN2a) locus, which codes for the p16(INK4a) and p19ARF proteins, is often mutated in sporadic and familial malignant melanoma, but it has not been directly associated with reduced DNA repair. We transfected unirradiated mouse fibroblast cells with UV-treated DNA to measure DNA repair in normal, p16INK4a mutant, p19ARF mutant, or double mutant mouse host cells. Loss of either p16(INK4a) or p19ARF reduced the ability of the cells to process UV-induced DNA damage, independent of cell cycle effects incurred by the loss. These results may further explain why INK4a/ARF mutations predispose to malignant melanoma, a UV-induced tumor.

Expression of hepatitis B virus X oncoprotein inhibits transcription-coupled nucleotide excision repair in human cells

The hepatitis B virus X protein (HBx) is implicated in liver cancer development, and this presumably involves its ability to bind and functionally inactivate the p53 tumour suppressor. For example expression of HBx in cultured cells has been shown to inhibit global nucleotide excision repair, a p53-dependent subpathway of nucleotide excision repair (NER) which eliminates helix-distorting DNA adducts, e.g., UV-induced cyclobutane pyrimidine dimers (CPDs), from the genome overall. However it remains undetermined whether HBx also interferes with transcription-coupled NER (TCNER), another NER subpathway which removes DNA adducts uniquely from the transcribed strand (TS) of active genes. To address this, we employed the model human lymphoblastoid strain TK6 and its isogenic p53-null counterpart NH32, in conjunction with derivatives of these strains constitutively expressing HBx (TK6-HBx and NH32-HBx). Relative to TK6, following exposure to either UVB (290-320 nm) or UVC (254 nm), TK6-HBx, NH32 and NH32-HBx manifested significantly reduced apoptotic capacity to varying degrees, although no striking differences in clonogenic survival between the four strains were observed. As previously documented in our laboratory [Proc. Natl. Acad. Sci. 100 (2003) 7219-7224], ligation-mediated PCR analysis revealed NH32 to be deficient compared with TK6 in CPD removal along the TS strand of the chromosomal c-jun locus following UVB exposure, but to be proficient in this respect following UVC exposure, i.e., the requirement for p53 in TCNER exhibits wavelength dependence in human cells. Remarkably however, in contrast to the situation for NH32, TK6-HBx and NH32-HBx manifested defective repair along the TS of c-jun after irradiation with either UVB or UVC. The data demonstrate that HBx expression can reduce the efficiency of TCNER in addition to GNER in human cells via p53-independent as well as p53-dependent pathways.

Correlation between p53 mutations and HPV in bilharzial bladder cancer

Alterations of the p53 tumor suppressor gene are the most common genetic changes detected in human cancers as well as in papillary and invasive bladder cancer. Several studies have demonstrated an association between HPV infection and urological malignancies. In the present work, the p53 gene status was studied together with the frequency of HPV in 99 cases of Bilharzial bladder cancer [BBC] in Egypt and both were correlated to the clinicopathological features of the patients. SSCP and sequencing were used to screen the p53 gene for mutations at exons 4-10 and IHC was performed to detect protein overexpression. PCR was used for detection and typing of HPV-DNA in tumor samples. p53 mutations were detected in 33.3% of the studied cases whereas protein overexpression was detected in 35.6% of the cases. The highest concordance rate was observed in cases harboring mutations at exon 4 [87.5%]. Bilharzial infestation was obvious in 72.2% of the cases that showed mutations. Exon 8 showed the highest rate of mutation [32%] followed by exons 4 and 5 [22% each]. The commonest mutational event was G:C transversion [15/50] especially at CpG dinucleotides. A mutational hot spot was detected at exon 4, codons 72-73. HPV-DNA was detected in 48.97% of the cases the majority of which [64.6%] were of type 16. Significant correlation was found between p53 mutation and the pathological stage as well as p53 overexpression and tumor grade. Our results demonstrate that the mutational spectrum in BBC is different from that of bladder cancer in Western countries in many aspects and suggest an etiological role of HPV in this type of neoplasm. However, both HPV infection and p53 gene abnormalities may contribute to Bilharzial bladder carcinogenesis in an independent way.

Repair of UV-induced thymine dimers is compromised in cells expressing the E6 protein from human papillomaviruses types 5 and 18

Ultraviolet (UV) irradiation is a major mutagenic environmental agent, causing the appearance of DNA adducts that, if unrepaired, may give rise to mutations. Ultraviolet radiation has been indicated as a major risk factor in the development of nonmelanoma skin cancers; however, recent reports have suggested that infections with human papillomaviruses, a widespread family of epitheliotropic DNA viruses, may also contribute to the tumorigenic process. Here, we investigated whether expression of the E6 protein from different HPV types interfere with the repair of thymine dimers caused by UV-B radiation. Results show that unrepaired DNA damage can be observed in UV-B-irradiated cells expressing the E6 protein of HPV types found in cervical and epithelial cancers. Moreover, such cells have the ability to overcome the G(1) cell cycle checkpoint induced as a result of unrepaired DNA.

p53 and regulation of DNA damage recognition during nucleotide excision repair

In response to a variety of types of DNA damage, the p53 tumor suppressor gene product is activated and regulates a number of downstream cellular processes such as cell cycle arrest, apoptosis and DNA repair. Recent discoveries concerning the regulation of DNA repair processes by p53, such as nucleotide excision repair (NER) and base excision repair (BER) have paved the way for studies to understand the mechanisms governing p53-dependent DNA repair. Although several theories have been proposed, accumulating evidence points to a transcriptional regulatory role for p53 in NER, mediating expression of the global genomic repair (GGR)-specific damage recognition genes, DDB2 and XPC. In BER, a more direct role for p53 has been proposed, potentially acting through protein-protein interactions with BER specific factors. These advances have greatly enhanced our understanding of the role of p53 in DNA repair and this review comprehensively summarizes current opinions on the mechanisms of p53-dependent DNA repair.

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.

Synchronous Occurrence of Glioblastoma Multiforme and Esophageal Adenocarcinoma

Synchronous malignancies are rare occurrences for which there may be a genetic link between two cancers or which may be simply coincidental. Although glioblastoma multiforme and esophageal adenocarcinoma have few clinical similarities there are no known biochemical or genetic links between the two malignancies. This case discussion details the synchronous occurrences of these two lesions and highlights possible clinical, biochemical, and genetic commonalities.

Subpathways of nucleotide excision repair and their regulation

Nucleotide excision repair provides an important cellular defense against a large variety of structurally unrelated DNA alterations. Most of these alterations, if unrepaired, may contribute to mutagenesis, oncogenesis, and developmental abnormalities, as well as cellular lethality. There are two subpathways of nucleotide excision repair; global genomic repair (GGR) and transcription coupled repair (TCR), that is selective for the transcribed DNA strand in expressed genes. Some of the proteins involved in the recognition of DNA damage (including RNA polymerase) are also responsive to natural variations in the secondary structural features of DNA. Gratuitous repair events in undamaged DNA might then contribute to genomic instability. However, damage recognition enzymes for GGR are normally maintained at very low levels unless the cells are genomically stressed. GGR is controlled through the SOS stress response in E. coli and through the activated p53 tumor suppressor in human cells. These inducible responses in human cells are important, as they have been shown to operate upon chemical carcinogen DNA damage at levels to which humans are environmentally exposed. Interestingly, most rodent tissues are deficient in the p53-dependent GGR pathway. Since rodents are used as surrogates for environmental cancer risk assessment, it is essential that we understand how they differ from humans with respect to DNA repair and oncogenic responses to environmental genotoxins. In the case of terminally differentiated mammalian cells, a new paradigm has appeared in which GGR is attenuated but both strands of expressed genes are repaired efficiently.

DNA repair during organogenesis

DNA damage caused by genotoxic agents can impact on virtually any cellular process due to its ability to affect gene expression and subsequent gene products. The importance of repairing damaged DNA is evidenced by the variety of DNA repair pathways that have evolved in all living organisms, and the human syndromes caused by a lack of this repair ability. This review focuses on the expression and activity of DNA repair pathways during mammalian organogenesis, and the role of these pathways in ensuring the stability of the conceptal genome. DNA repair capacity may play a role also in the response of the conceptus to genotoxic agents that may induce malformations; the consequences of exposure to a genotoxic agent during organogenesis depend on the extent of the damage and on the ability of the embryo to respond by repairing DNA or arresting cell division. The four main repair pathways (nucleotide excision repair, base excision repair, mismatch repair, and recombination repair) are expressed to various degrees during organogenesis, as are members of the genotoxic stress-activated cell cycle checkpoint pathways. Developmental-stage-specific alterations in transcript levels, protein levels, as well as activity, indicate that the regulation of DNA repair pathways during development is complex. The importance of DNA repair pathways in endogenous damage control is illustrated by the sensitivity of development to their disruption if some of these genes are mutated. Furthermore, the conceptus has a limited capacity to alter DNA repair responses following exposure to genotoxic agents.

Human cells bearing homozygous mutations in the DNA mismatch repair genes hMLH1 or hMSH2 are fully proficient in transcription-coupled nucleotide excision repair

The transcription-coupled nucleotide excision repair (TCNER) pathway maintains genomic stability by rapidly eliminating helix-distorting DNA adducts, such as UV-induced cyclobutane pyrimidine dimers (CPDs), specifically from the transcribed strands of active genes. DNA mismatch repair (MMR) constitutes yet another critical antimutagenic pathway that removes mispaired bases generated during semiconservative replication. It was previously reported that the human colon adenocarcinoma strains HCT116 and LoVo (bearing homozygous mutations in the MMR genes hMLH1 and hMSH2, respectively), besides manifesting hallmark phenotypes associated with defective DNA mismatch correction, are also completely deficient in TCNER of UV-induced CPDs. This revealed a direct mechanistic link between MMR and TCNER in human cells, although subsequent studies have either supported, or argued against, the validity of this important notion. Here, the ligation-mediated polymerase chain reaction was used to show at nucleotide resolution that MMR-deficient HCT116 and LoVo retain the ability to excise UV-induced CPDs much more rapidly from the transcribed vs the nontranscribed strands of active genes. Moreover, relative to DNA repair-proficient counterparts, MMR-deficient cells were not more sensitive to the cytotoxic effects of UV, and displayed equal ability to recover mRNA synthesis following UV challenge. These results conclusively demonstrate that hMLH1- and hMSH2-deficient human colon adenocarcinoma cells are fully proficient in TCNER.

Lack of functional pRb results in attenuated recovery of mRNA synthesis and increased apoptosis following UV radiation in human breast cancer cells

Lack of functional pRb results in attenuated recovery of mRNA synthesis and increased apoptosis following UV radiation in human breast cancer cells. We have previously demonstrated that a human breast cancer cell line, MDA-MB-468, which lacks the retinoblastoma protein (pRb), is particularly sensitive to low doses of ultraviolet (UV) radiation. These cells are 15-20-fold more sensitive to UV radiation than cells with wild-type pRb. In order to understand the mechanisms of the high apoptotic response of MDA-MB-468 cells to UV radiation, we examined the effects of UV on these cells with regards to both membrane-mediated events and DNA damage. We found that MDA-MB-468 cells were resistant to all ligand-induced death receptor signaling. In addition, although UV activated caspase 8 in MDA-MB-468 cells, a peptide inhibitor of caspase 8 failed to inhibit UV-induced apoptosis. We then tested the possibility that nuclear events mediated the enhanced sensitivity to UV-induced apoptosis in these cells. Unlike UV-resistant cells, MDA-MB-468 cells were unable to recover mRNA synthesis after 5 J/m2 UVC. We also found that the pRb-null DU-145 cells similarly had attenuated recovery of mRNA synthesis after UV radiation. In UV-resistant cells with wild-type pRb, the inactivation of pRb with HPV-16 E7 resulted in significant inhibition in their ability to recover mRNA synthesis and increased levels of apoptosis following UV radiation. Furthermore, pRb-null cells were deficient in repair of UV radiation-induced DNA damage. These data suggest that the sensitivity of MDA-MB-468 cells to UV radiation is due to defects in repair of DNA damage and recovery of mRNA synthesis rather than to membrane death receptor pathways. Inactivation of pRb may contribute to an increased sensitivity to UV radiation by attenuating repair of DNA lesions and recovery of mRNA synthesis following UV radiation.

Physical and functional interactions of the tumor suppressor protein p53 and DNA polymerase alpha-primase

The wild-type form of p53 contains an intrinsic 3'-5'-exonuclease activity. As p53 forms a complex with DNA polymerase alpha-primase (pol-prim) in vivo this finding suggests that p53 might cooperate with pol-prim to stabilize the genetic information of living cells. To test this hypothesis, exonuclease-free DNA pol-prim was expressed alone or together with p53 for purification. Pol-prim formed a complex with p53, which was purified by ion exchange and immunoaffinity chromatography from baculovirus-infected insect cells. The p53-containing pol-prim fractions removed a 3'-unpaired nucleotide with a 1.5-2-fold higher rate than a paired nucleotide, whereas the four subunit pol-prim did not have any exonuclase activity. Therefore, only p53/pol-prim was able to elongate a primer-template that contained a 3'-unpaired primer end in vitro. To achieve this, the 3'-5'-exonuclease activity of p53 excised the unpaired nucleotide at the 3'-end of the primer and created a paired 3'-end, which pol-prim was able to elongate. The exonuclease activity of p53 as well as the elongation of a primer with a mispaired 3'-end was inhibited specifically by the anti-p53 monoclonal antibodies PAb240 and PAb421.

A role for p53 in the frequency and mechanism of mutation

The tumor suppressor protein, p53, is often referred to as the guardian of the genome. When p53 function is impaired, its ability to preserve genomic integrity is compromised. This may result in an increase in mutation on both a molecular and chromosomal level and contribute to the progression to a malignant phenotype. In order to study the effect of p53 function on the acquisition of mutation, in vitro and in vivo models have been developed in which both the frequency and mechanism of mutation can be analyzed. In human lymphoblastoid cells in which p53 function was impaired, both the spontaneous and induced mutant frequency increased at the autosomal thymidine kinase (TK) locus. The mutant frequency increased to a greater extent in cell lines in which p53 harbored a point mutation than in those lines in which a "null" mutation had been introduced by molecular targeting or by viral degradation indicating a possible "gain-of-function" associated with the mutant protein. Further, molecular analysis revealed that the loss of p53 function was associated with a greater tendency towards loss-of-heterozygosity (LOH) within the TK gene that was due to non-homologous recombination than that found in wild-type cells. Most data obtained from the in vivo models uses the LacI reporter gene that does not efficiently detect mutation that results in LOH. However, studies that have examined the effect of p53 status on mutation in the adenine phosphoribosyl transferase (APRT) gene in transgenic mice also suggest that loss of p53 function results in an increase in mutation resulting from non-homologous recombination. The results of these studies provide clear and convincing evidence that p53 plays a role in modulating the mutant frequency and the mechanism of mutation. In addition, the types of mutation that occur within the p53 gene are also of importance in determining the mutant frequency and the pathways leading to mutation.

Human cells deficient in p53 regulated p21(waf1/cip1) expression exhibit normal nucleotide excision repair of UV-induced DNA damage

Cancer development requires the accumulation of numerous genetic changes, which are believed to initiate through the presence of unrepaired lesions in the genome. In the absence of proficient repair, genotoxic agents can lead to crucial mutations of vital cellular genes via replication of damaged DNA. Many cell cycle regulatory proteins are known to modulate the repair capacity and consequently the fate of cells. We and others have recently shown that p53 tumor suppressor gene product is required for efficient global genomic repair (GGR) but not the transcription coupled repair (TCR) of the nucleotide excision repair (NER) sub-pathways. In order to discern the nature of the p53 modulation to be direct or indirect through a downstream mediator, we have investigated the processing of UV radiation induced lesions in human colon carcinoma, HCT116 cells expressing wild-type p53 but having different p21(waf1cip1) (hereafter p21) genotypes (p21+/+, p21+/-, p21-/-). Following 20 J/m(2) UV, all the three cell lines showed rapid increase in p53 protein but the accompanying increase in the expression of its downstream target protein p21 could only be seen in p21+/+ and p21+/- cells and not in p21-/- cells. Nevertheless, an absence of detectable p21 protein in deficient cells had no demonstrable effect on DNA repair response to UV irradiation, as measured by an immunoassay to detect removal of UV photoproducts from genomic DNA (GGR) and by individual strand specific removal of endonuclease-sensitive CPD from a target gene fragment (TCR). Introduction of cytomegalovirus (CMV)-driven luciferase reporter plasmid, UV damaged in vitro, into the un-irradiated cells of varying p21 background, revealed a relatively small but statistically significant decrease in the reporter expression in the host p21-/- as compared with p21+/+ and p21+/- HCT116 cells. Super-expression of p21 protein upon reintroduction of p21 expression construct, showed an enhanced recovery of UV damaged reporter activity that was not greatly different from a similar enhancement observed with undamaged plasmid reporter DNA. Taken together, the results indicate that (i) the p21 protein does not have a significant role in the repair of genomic DNA at chromosomal level; (ii) the well-established p53 dependent modulation of NER is distinct and independent of its cell cycle checkpoint function; and (iii) the reproducible enhancing effect of p21 expression observed through host cell reactivation (HCR) of extrachromosomal DNA is mainly attributable to an effect exerted on transcription rather than repair.

Human urinary bladder epithelial cells lacking wild-type p53 function are deficient in the repair of 4-aminobiphenyl-DNA adducts in genomic DNA

The effect of the tumor suppressor gene TP53 on repair of genomic DNA damage was examined in human urinary bladder transitional cell carcinoma (TCC) cell lines. Utilizing TCC10 containing wild-type p53 (wt-p53) as the parental line, an isogenic set of cell lines was derived by retroviral infection that expressed a transdominant mutant p53 (Arg --> His at codon 273, TDM273-TCC10), or the human papilloma virus 16-E6 oncoprotein (E6-TCC10). 32P-postlabeling analyses were performed on DNA from TCC cultures obtained after treatment with N-hydroxy-4-aminobiphenyl (N-OH-ABP), N-hydroxy-4-acetylaminobiphenyl (N-OH-AABP) and N-acetoxy-4-acetylaminobiphenyl (N-OAc-AABP). The major adduct was identified as N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP) with all three chemicals. The amount of adducts in urothelial DNA ranged between 0.1 and 20 per 10(6) nucleotides, N-OAc-AABP yielding the highest levels, followed by N-OH-ABP and N-OH-AABP. To determine, if the functional status of p53 affects the rate of repair of dG-C8-ABP in genomic DNA, TCC10 and the TDM273-TCC10 and E6-TCC10 isotypes were exposed to N-OH-AABP for 12h and the DNA damage was allowed to repair up to 24h. The adduct levels were quantified and compared between the TCC10 isotypes. The amounts of dG-C8-ABP that remained in genomic DNA from E6-TCC10 and TDM273-TCC10 were approximately two-fold higher, as compared to the parental TCC10. At the dose used for DNA repair studies, N-OH-AABP or N-OAc-AABP did not induce apoptosis in TCC10. However, N-OAc-AABP at high doses (>5 microM) induced apoptosis, as evidenced by DNA fragmentation analyses. Furthermore, N-OAc-AABP-mediated apoptosis was independent of the functional status of wt-p53, since both E6-TCC10 and the parental TCC10 exhibited DNA fragmentation following treatment. These results suggest that p53 might modulate the repair of DNA adducts generated from the human bladder carcinogen ABP in its target human uroepithelial cells. This implies that in p53 null cells the unrepaired DNA damage could cause accumulation of mutation, which might contribute to increased genomic instability and neoplastic progression.

Molecular characteristics of non-small cell lung cancer

We used hierarchical clustering to examine gene expression profiles generated by serial analysis of gene expression (SAGE) in a total of nine normal lung epithelial cells and non-small cell lung cancers. Separation of normal and tumor, as well as histopathological subtypes, was evident by using the 3,921 most abundant transcript tags. This distinction remained when only 115 highly differentially expressed tags were used. Furthermore, these 115 transcript tags clustered into groups suggestive of the unique biological and pathological features of the different tissues examined. Adenocarcinomas were characterized by high-level expression of small airway-associated or immunologically related proteins, whereas squamous cell carcinomas overexpressed genes involved in cellular detoxification or antioxidation. The messages of two p53-regulated genes, p21(WAF1/CIP1) and 14-3-3final sigma, were consistently underexpressed in the adenocarcinomas, suggesting that the p53 pathway itself might be compromised in this cancer type. Gene expression patterns observed by SAGE were consistent with results obtained by quantitative real-time PCR or cDNA array analyses by using a total of 43 lung tumor and normal samples. Thus, although derived from only a few tissue libraries, gene expression profiles obtained by using SAGE most likely represent an unbiased yet distinctive molecular signature for the most common forms of human lung cancer.

UV-induced DNA incision and proliferating cell nuclear antigen recruitment to repair sites occur independently of p53-replication protein A interaction in p53 wild type and mutant ovarian carcinoma cells

The tumour suppressor gene TP53 plays an important role in the regulation of DNA repair, and particularly of nucleotide excision repair. The influence of p53 status on the efficiency of the principal steps of this repair pathway was investigated after UV-C irradiation in the human ovarian carcinoma cell line IGROV-1 (expressing wild-type p53) and in the derived clone IGROV-1/Pt1 (with p53 mutations at codons 270 and 282). Clonogenic survival after UV-C irradiation showed that IGROV-1/Pt1 cells were approximately 2-fold more resistant to DNA damage than parental cells. Modulation of p53 protein levels, cell cycle arrest and apoptosis were induced in UV-irradiated IGROV-1 cells, but not in the p53-mutant cell line. Exposure to UV or cisplatin induced down-regulation of p53-replication protein A (RPA) interaction in parental, but not in IGROV-1/Pt1 cells. However, persistent binding of p53 to RPA did not affect the early steps of DNA repair. In fact, both UV-induced DNA incision and the recruitment of proliferating cell nuclear antigen (PCNA) to DNA repair sites occurred to a comparable extent in p53-wild type and -mutant cell lines, although PCNA remained associated with chromatin for a longer period of time in IGROV-1/Pt1 cells. Global genome repair, as detected by immunoblot analysis of cyclobutane pyrimidine dimers, was not significantly different in the two cell lines at 3 h after UV irradiation. In contrast, lesion removal at 24 h was markedly reduced in IGROV-1/Pt1 cells, being approximately 25% of the initial amount of damage, as compared with approximately 50% repair in parental cells. These results indicate that the presence of mutant p53 protein and its persistent interaction with RPA do not affect the early steps of nucleotide excision repair in IGROV-1/Pt1 cells. Thus, repair defects in p53-mutant ovarian carcinoma cells may be attributed to late events, possibly related to a reduced removal/recycling of PCNA at repair sites.