Differential role of transcription-coupled repair in UVB-induced G2 arrest and apoptosis in mouse epidermis

Ubiquitin signaling and the proteasome drive human DNA-protein crosslink repair

DNA-protein crosslinks (DPCs) are large cytotoxic DNA lesions that form following exposure to chemotherapeutic drugs and environmental chemicals. Nucleotide excision repair (NER) and homologous recombination (HR) promote survival following exposure to DPC-inducing agents. However, it is not known how cells recognize DPC lesions, or what mechanisms selectively target DPC lesions to these respective repair pathways. To address these questions, we examined DPC recognition and repair by transfecting a synthetic DPC lesion comprised of the human oxoguanine glycosylase (OGG1) protein crosslinked to double-stranded M13MP18 into human cells. In wild-type cells, this lesion is efficiently repaired, whereas cells deficient in NER can only repair this lesion if an un-damaged homologous donor is co-transfected. Transfected DPC is subject to rapid K63 polyubiquitination. In NER proficient cells, the DPC is subject to K48 polyubiquitination, and is removed via a proteasome-dependent mechanism. In NER-deficient cells, the DNA-conjugated protein is not subject to K48 polyubiquitination. Instead, the K63 tag remains attached, and is only lost when a homologous donor molecule is present. Taken together, these results support a model in which selective addition of polyubiquitin chains to DNA-crosslinked protein leads to selective recruitment of the proteasome and the cellular NER and recombinational DNA repair machinery.

Post-transcriptional polyadenylation site cleavage maintains 3'-end processing upon DNA damage

The recognition of polyadenylation signals (PAS) in eukaryotic pre-mRNAs is usually coupled to transcription termination, occurring while pre-mRNA is chromatin-bound. However, for some pre-mRNAs, this 3'-end processing occurs post-transcriptionally, i.e., through a co-transcriptional cleavage (CoTC) event downstream of the PAS, leading to chromatin release and subsequent PAS cleavage in the nucleoplasm. While DNA-damaging agents trigger the shutdown of co-transcriptional chromatin-associated 3'-end processing, specific compensatory mechanisms exist to ensure efficient 3'-end processing for certain pre-mRNAs, including those that encode proteins involved in the DNA damage response, such as the tumor suppressor p53. We show that cleavage at the p53 polyadenylation site occurs in part post-transcriptionally following a co-transcriptional cleavage event. Cells with an engineered deletion of the p53 CoTC site exhibit impaired p53 3'-end processing, decreased mRNA and protein levels of p53 and its transcriptional target p21, and altered cell cycle progression upon UV-induced DNA damage. Using a transcriptome-wide analysis of PAS cleavage, we identify additional pre-mRNAs whose PAS cleavage is maintained in response to UV irradiation and occurring post-transcriptionally. These findings indicate that CoTC-type cleavage of pre-mRNAs, followed by PAS cleavage in the nucleoplasm, allows certain pre-mRNAs to escape 3'-end processing inhibition in response to UV-induced DNA damage.

Multiple interaction nodes define the postreplication repair response to UV-induced DNA damage that is defective in melanomas and correlated with UV signature mutation load

Ultraviolet radiation‐induced DNA mutations are a primary environmental driver of melanoma. The reason for this very high level of unrepaired DNA lesions leading to these mutations is still poorly understood. The primary DNA repair mechanism for UV‐induced lesions, that is, the nucleotide excision repair pathway, appears intact in most melanomas. We have previously reported a postreplication repair mechanism that is commonly defective in melanoma cell lines. Here we have used a genome‐wide approach to identify the components of this postreplication repair mechanism. We have used differential transcript polysome loading to identify transcripts that are associated with UV response, and then functionally assessed these to identify novel components of this repair and cell cycle checkpoint network. We have identified multiple interaction nodes, including global genomic nucleotide excision repair and homologous recombination repair, and previously unexpected MASTL pathway, as components of the response. Finally, we have used bioinformatics to assess the contribution of dysregulated expression of these pathways to the UV signature mutation load of a large melanoma cohort. We show that dysregulation of the pathway, especially the DNA damage repair components, are significant contributors to UV mutation load, and that dysregulation of the MASTL pathway appears to be a significant contributor to high UV signature mutation load.

Loss of Epidermal HIF-1α Blocks UVB-Induced Tumorigenesis by Affecting DNA Repair Capacity and Oxidative Stress

HIF-1α is constitutively expressed in mouse and human epidermis. It plays a crucial role in skin physiology, including the response of keratinocytes to UVR. However, little information is available about its role in photocarcinogenesis. Using a multistage model of UVB radiation-induced skin cancer, we show that the knockout of Hif-1α in the epidermis prevents tumorigenesis but at the same time triggers the formation of hyperkeratotic plaques. Our results indicate that the absence of oncogenic transformation in Hif-1α-ablated mice is related to increased DNA repair in keratinocytes, whereas the formation of hyperkeratotic plaques is caused by an increase in the levels of reactive oxygen species. Indeed, impairing the DNA repair machinery by ablating xeroderma pigmentosum C restored the UVB-induced neoplastic transformation of Hif-1α-ablated keratinocytes, whereas the development of hyperkeratotic plaques was blocked by chronic antioxidant treatment. We conclude that HIF-1α plays a procarcinogenic role in UVB-induced tumorigenesis.

Solar UV damage to cellular DNA: from mechanisms to biological effects

Solar ultraviolet (UV) radiation generates bulky photodimers at di-pyrimidine sites that pose stress to cells and organisms by hindering DNA replication and transcription. In addition, solar UV also induces various types of oxidative DNA lesions and single strand DNA breaks. Relieving toxicity and maintenance of genomic integrity are of clinical importance in relation to erythema/edema and diseases such as cancer, neurodegeneration and premature ageing, respectively. Following solar UV radiation, a network of DNA damage response mechanisms triggers a signal transduction cascade to regulate various genome-protection pathways including DNA damage repair, cell cycle control, apoptosis, transcription and chromatin remodeling. The effects of UVC and UVB radiation on cellular DNA are predominantly accounted for by the formation of photodimers at di-pyrimidine sites. These photodimers are mutagenic: UVC, UVB and also UVA radiation induce a broadly similar pattern of transition mutations at di-pyrimidine sites. The mutagenic potency of solar UV is counteracted by efficient repair of photodimers involving global genome nucleotide excision repair (GG-NER) and transcription-coupled nucleotide excision repair (TC-NER); the latter is a specialized repair pathway to remove transcription-blocking photodimers and restore UV-inhibited transcription. On the molecular level these processes are facilitated and regulated by various post-translational modifications of NER factors and the chromatin substrate. Inherited defects in NER are manifested in different diseases including xeroderma pigmentosum (XP), Cockayne syndrome (CS), UV sensitive syndrome (UVsS) and the photosensitive form of trichothiodystrophy (TTD). XP patients are prone to sunlight-induced skin cancer. UVB irradiated XP and CS knockout mouse models unveiled that only TC-NER counteracts erythema/edema, whereas both GG-NER and TC-NER protect against UVB-induced cancer. Additionally, UVA radiation induces mutations characterized by oxidation-linked signature at non-di-pyrimidine sites. The biological relevance of oxidation damage is demonstrated by the cancer susceptibility of UVB-irradiated mice deficient in repair of oxidation damage, i.e., 8-oxoguanine.

Sublethal UV irradiation induces squamous differentiation via a p53-independent, DNA damage-mitosis checkpoint

The epidermis is a self-renewal epithelium continuously exposed to the genotoxic effects of ultraviolet (UV) light, the main cause of skin cancer. Therefore, it needs robust self-protective mechanisms facing genomic damage. p53 has been shown to mediate apoptosis in sunburn cells of the epidermis. However, epidermal cells daily receive sublethal mutagenic doses of UV and massive apoptosis would be deleterious. We have recently unravelled an anti-oncogenic keratinocyte DNA damage-differentiation response to cell cycle stress. We now have studied this response to high or moderate single doses of UV irradiation. Whereas, as expected, high levels of UV induced p53-dependent apoptosis, moderate levels triggered squamous differentiation. UV-induced differentiation was not mediated by endogenous p53. Overexpression of the mitosis global regulator FOXM1 alleviated the proliferative loss caused by UV. Conversely, knocking-down the mitotic checkpoint protein Wee1 drove UV-induced differentiation into apoptosis. Therefore, the results indicate that mitosis checkpoints determine the response to UV irradiation. The differentiation response was also found in cells of head and neck epithelia thus uncovering a common regulation in squamous tissues upon chronic exposure to mutagens, with implications into homeostasis and disease.

Cbl-b deficiency provides protection against UVB-induced skin damage by modulating inflammatory gene signature

Exposure of skin to ultraviolet (UV) radiation induces DNA damage, inflammation, and immune suppression that ultimately lead to skin cancer. However, some of the pathways that regulate these events are poorly understood. We exposed mice to UVB to study its early effects in the absence of Cbl-b, a known suppressor of antitumor immune response in the skin. Cbl-b^-/- mice were protected from UV-induced cell damage as shown by the lower number of cyclobutane pyrimidine dimers and sunburn cells in exposed skin compared to wild-type mice. Microarray data revealed that deficiency of Cbl-b resulted in differential expression of genes involved in apoptosis evasion, tumor suppression and cell survival in UV-exposed skin. After UVB, Cbl-b^-/- mice upregulated gene expression pattern associated with regulation of epidermal cell proliferation linked to Wnt signaling mediators and enzymes that relate to cell removal and tissue remodeling like MMP12. Additionally, the skin of Cbl-b^-/- mice was protected from chronic inflammatory responses and epidermal hyperplasia in a 4-weeks UVB treatment protocol. Overall, our results suggest a novel role for Cbl-b in regulating inflammation and physiologic clearance of damaged cells in response to UVB by modulating inflammatory gene signature.

NADPH Oxidase-1 Plays a Key Role in Keratinocyte Responses to UV Radiation and UVB-Induced Skin Carcinogenesis

The nicotinamide adenine dinucleotide phosphate oxidase (NOX) family enzymes are involved in several physiological functions. However, their roles in keratinocyte responses to UV radiation have not been clearly elucidated. This study shows that, among other NOX family members, UVB irradiation results in a biphasic activation of NOX1 that plays a critical role in defining keratinocyte fate through the modulation of the DNA damage response network. Indeed, suppression of both bursts of UVB-induced NOX1 activation by using a specific peptide inhibitor of NOX1 (InhNOX1) is associated with increased nucleotide excision repair efficiency and reduction of apoptosis, which is finally translated into decreased photocarcinogenesis. On the contrary, when only the second peak of UVB-induced NOX1 activation is blocked, both nucleotide excision repair efficiency and apoptosis are decreased. Our results show that inhibition of NOX1 activation could be a promising target for the prevention and treatment of UVB-induced skin cancer in nucleotide excision repair-proficient and -deficient patients.

Nucleotide excision repair of oxidised genomic DNA is not a source of urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine

Urinary 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) is a widely measured biomarker of oxidative stress. It has been commonly assumed to be a product of DNA repair, and therefore reflective of DNA oxidation. However, the source of urinary 8-oxodGuo is not understood, although potential confounding contributions from cell turnover and diet have been ruled out. Clearly it is critical to understand the precise biological origins of this important biomarker, so that the target molecule that is oxidised can be identified, and the significance of its excretion can be interpreted fully. In the present study we aimed to assess the contributions of nucleotide excision repair (NER), by both the global genome NER (GG-NER) and transcription-coupled NER (TC-NER) pathways, and sanitisation of the dGTP pool (e.g. via the activity of the MTH1 protein), on the production of 8-oxodGuo, using selected genetically-modified mice. In xeroderma pigmentosum A (XPA) mice, in which GG-NER and TC-NER are both defective, the urinary 8-oxodGuo data were unequivocal in ruling out a contribution from NER. In line with the XPA data, the production of urinary 8-oxodGuo was not affected in the xeroderma pigmentosum C mice, specifically excluding a role of the GG-NER pathway. The bulk of the literature supports the mechanism that the NER proteins are responsible for removing damage to the transcribed strand of DNA via TC-NER, and on this basis we also examined Cockayne Syndrome mice, which have a functional loss of TC-NER. These mice showed no difference in urinary 8-oxodGuo excretion, compared to wild type, demonstrating that TC-NER does not contribute to urinary 8-oxodGuo levels. These findings call into question whether genomic DNA is the primary source of urinary 8-oxodGuo, which would largely exclude it as a biomarker of DNA oxidation. The urinary 8-oxodGuo levels from the MTH1 mice (both knock-out and hMTH1-Tg) were not significantly different to the wild-type mice. We suggest that these findings are due to redundancy in the process, and that other enzymes substitute for the lack of MTH1, however the present study cannot determine whether or not the 2'-deoxyribonucleotide pool is the source of urinary 8-oxodGuo. On the basis of the above, urinary 8-oxodGuo is most accurately defined as a non-invasive biomarker of oxidative stress, derived from oxidatively generated damage to 2'-deoxyguanosine.

Review of narrowband ultraviolet B radiation in vitiligo

Vitiligo is a common, acquired pigmentary disorder of unknown etiology with great impact on patient’s appearance and quality of life. It presents a therapeutic challenge to many dermatologists. Photochemotherapy using psoralen and ultraviolet A (UVA) therapy, topical and oral immunosuppresants, as well as cosmetic camouflage are also commonly employed with varying clinical efficacy. Phototherapy is a popular treatment option, which includes both of the generalized ultraviolet B (UVB) therapies, broadband UVB and narrowband UVB (NB-UVB). It has been used favorably, both alone as well as in combination with other agents like topical calcineurin inhibitors, vitamin-D analogs. Combination therapies are useful and may provide quicker regimentation and treat vitiligo with an additive mechanism of action than UVB phototherapy. Advances in technology may lead to the continuing use of UVB phototherapy as a treatment for vitiligo through the development of sophisticated devices and delivery systems as well as innovative application methods. These will provide increased therapeutic options for all vitiligo patients, particularly those with refractory disease. In this article, I have reviewed the available data pertaining to efficacy and safety issues for NB-UVB as monotherapy, its comparison with psoralen plus UVA and other modes of phototherapy, combination regimens that have been tried and future prospects of NB-UVB in vitiligo.

XPC deficiency is related to APE1 and OGG1 expression and function

Oxidative DNA damage is considered to be a major cause of neurodegeneration and internal tumors observed in syndromes that result from nucleotide excision repair (NER) deficiencies, such as Xeroderma Pigmentosum (XP) and Cockayne Syndrome (CS). Recent evidence has shown that NER aids in removing oxidized DNA damage and may interact with base excision repair (BER) enzymes. Here, we investigated APE1 and OGG1 expression, localization and activity after oxidative stress in XPC-deficient cells. The endogenous APE1 and OGG1 mRNA levels were lower in XPC-deficient fibroblasts. However, XPC-deficient cells did not show hypersensitivity to oxidative stress compared with NER-proficient cells. To confirm the impact of an XPC deficiency in regulating APE1 and OGG1 expression and activity, we established an XPC-complemented cell line. Although the XPC complementation was only partial and transient, the transfected cells exhibited greater OGG1 expression and activity compared with XPC-deficient cells. However, the APE1 expression and activity did not significantly change. Furthermore, we observed a physical interaction between the XPC and APE1 proteins. Together, the results indicate that the responses of XPC-deficient cells under oxidative stress may not only be associated with NER deficiency per se but may also include new XPC functions in regulating BER proteins.

Patients with xeroderma pigmentosum complementation groups C, E and V do not have abnormal sunburn reactions

BACKGROUND

Xeroderma pigmentosum (XP) is a rare autosomal recessive disorder of DNA repair. It is divided into eight complementation groups: XP-A to XP-G (classical XP) and XP variant (XP-V). Severe and prolonged sunburn reactions on minimal sun exposure have been considered a cardinal feature of classical XP. However, it has recently become clear that not all patients have abnormal sunburn reactions.

OBJECTIVES

To examine sunburn reactions in a cohort of patients with XP and correlate this to the complementation group.

METHODS

Sixty patients with XP attending the U.K. National XP Service from 2010 to 2012 were studied. Their history of burning after minimal sun exposure was assessed using a newly developed sunburn severity score. The age at which the first skin cancer was histologically diagnosed in each patient, and the presence of any neurological abnormality, was also recorded.

RESULTS

Sunburn severity scores were abnormally high in patients with XP-A, XP-D, XP-F and XP-G compared with non-XP controls. There was no significant difference in sunburn score of patients with XP-C, XP-E and XP-V compared with controls (P > 0·05). Patients with XP-C, XP-E and XP-V were more likely to have skin cancer diagnosed at an earlier age than those with severe sunburn on minimal sun exposure. In addition, patients with XP with severe sunburn had an increased frequency of neurological abnormalities.

CONCLUSIONS

Not all patients with XP have a history of severe and prolonged sunburn on minimal sun exposure. The normal sunburn response of patients with XP-C, XP-E and XP-V may relate to the preservation of transcription-coupled DNA repair in these groups. Those with a history of severe sunburn on minimal sun exposure developed their first skin cancer at an older age compared with patients with XP-C, XP-E and XP-V, but they had an increased frequency of neurological abnormalities. Physicians need to be aware that about half of all patients with XP will present without a history of abnormal sunburn.

Dual role for mammalian DNA polymerase ζ in maintaining genome stability and proliferative responses

DNA polymerase ζ (polζ) is critical for bypass of DNA damage and the associated mutagenesis, but also has unique functions in mammals. It is required for embryonic development and for viability of hematopoietic cells, but, paradoxically, skin epithelia appear to survive polζ deletion. We wished to determine whether polζ functions in a tissue-specific manner and how polζ status influences skin tumorigenesis. Mice were produced in which Rev3L (the catalytic subunit of polζ) was deleted in tissues expressing keratin 5. Efficient epidermal deletion of Rev3L was tolerated but led to skin and hair abnormalities, accompanied by evidence of DNA breaks. Unchallenged mice developed tumors in keratin 5-expressing tissues with age, consistent with the chromosomal instability accompanying a polζ defect. Unexpectedly, mice with the Rev3L deletion were much more sensitive to UVB radiation than mice defective in other DNA repair genes. Following irradiation, polζ-defective mice failed to mount skin-regenerative responses and responded to stress by mobilizing melanocytes to the epidermis. However, they did not develop skin tumors after chronic UVB irradiation. To determine the proliferative potential of polζ-deficient skin epithelia, keratinocytes were isolated and examined. These keratinocytes harbored chromosomal gaps and breaks and exhibited a striking proliferation defect. These results can be unified by a model in which slowly dividing cells accumulate replication-associated DNA breaks but otherwise survive Rev3L deletion, but functional polζ is essential for responses requiring rapid proliferation, both in cell culture and in vivo. The results reveal a biological role for mammalian polζ in tolerating DNA damage and enabling proliferative responses in vivo.

The role of altered nucleotide excision repair and UVB-induced DNA damage in melanomagenesis

UVB radiation is the most mutagenic component of the UV spectrum that reaches the earth's surface and causes the development of DNA damage in the form of cyclobutane pyrimidine dimers and 6-4 photoproducts. UV radiation usually results in cellular death, but if left unchecked, it can affect DNA integrity, cell and tissue homeostasis and cause mutations in oncogenes and tumour-suppressor genes. These mutations, if unrepaired, can lead to abnormal cell growth, increasing the risk of cancer development. Epidemiological data strongly associates UV exposure as a major factor in melanoma development, but the exact biological mechanisms involved in this process are yet to be fully elucidated. The nucleotide excision repair (NER) pathway is responsible for the repair of UV-induced lesions. Patients with the genetic disorder Xeroderma Pigmentosum have a mutation in one of eight NER genes associated with the XP complementation groups XP-A to XP-G and XP variant (XP-V). XP is characterized by diminished repair capacity, as well as a 1000-fold increase in the incidence of skin cancers, including melanoma. This has suggested a significant role for NER in melanoma development as a result of UVB exposure. This review discusses the current research surrounding UVB radiation and NER capacity and how further investigation of NER could elucidate the role of NER in avoiding UV-induced cellular death resulting in melanomagenesis.

Transcription-coupled repair and apoptosis provide specific protection against transcription-associated mutagenesis by ultraviolet light

Recent data reveal that gene transcription affects genome stability in mammalian cells. For example, transcription of DNA that is damaged by the most prevalent exogenous genotoxin, UV light, induces nucleotide substitutions and chromosomal instability, collectively called UV-induced transcription-associated mutations (UV-TAM). An important class of UV-TAM consists of nucleotide transitions that are caused by deamination of cytosine-containing photolesions to uracil, presumably occurring at stalled transcription complexes. Transcription-associated deletions and recombinational events after UV exposure may be triggered by collisions of replication forks with stalled transcription complexes. In this Point-of-View we propose that mammalian cells possess two tailored mechanisms to prevent UV-TAM in dermal stem cells. First, the transcription-coupled nucleotide excision repair (TCR) pathway removes lesions at transcribed DNA strands, forming the primary barrier against the mutagenic consequences of transcription at a damaged template. Second, when TCR is absent or when the capacity of TCR is exceeded, persistently stalled transcription complexes induce apoptosis, averting the generation of mutant cells following replication. We hypothesize that TCR and the apoptotic response in conjunction reduce the risk of skin carcinogenesis.

A UVR-induced G2-phase checkpoint response to ssDNA gaps produced by replication fork bypass of unrepaired lesions is defective in melanoma

UVR is a major environmental risk factor for the development of melanoma. Here we describe a coupled DNA-damage tolerance (DDT) mechanism and G2-phase cell cycle checkpoint induced in response to suberythemal doses of UVR that is commonly defective in melanomas. This coupled response is triggered by a small number of UVR-induced DNA lesions incurred during G1 phase that are not repaired by nucleotide excision repair (NER). These lesions are detected during S phase, but rather than stalling replication, they trigger the DDT-dependent formation of single-stranded DNA (ssDNA) gaps. The ssDNA attracts replication protein A (RPA), which initiates ATR-Chk1 (ataxia telangiectasia and Rad3-related/checkpoint kinase 1) G2-phase checkpoint signaling, and colocalizes with components of the RAD18 and RAD51 postreplication repair pathways. We demonstrate that depletion of RAD18 delays both the resolution of RPA foci and exit from the G2-phase arrest, indicating the involvement of RAD18-dependent postreplication repair in ssDNA gap repair during G2 phase. Moreover, the presence of RAD51 and BRCA1 suggests that an error-free mechanism may also contribute to repair. Loss of the UVR-induced G2-phase checkpoint results in increased UVR signature mutations after exposure to suberythemal UVR. We propose that defects in the UVR-induced G2-phase checkpoint and repair mechanism are likely to contribute to melanoma development.

Protection from UV-induced skin carcinogenesis by genetic inhibition of the ataxia telangiectasia and Rad3-related (ATR) kinase

Multiple human epidemiologic studies link caffeinated (but not decaffeinated) beverage intake with significant decreases in several types of cancer, including highly prevalent UV-associated skin carcinomas. The mechanism by which caffeine protects against skin cancer is unknown. Ataxia telangiectasia and Rad3-related (ATR) is a replication checkpoint kinase activated by DNA stresses and is one of several targets of caffeine. Suppression of ATR, or its downstream target checkpoint kinase 1 (Chk1), selectively sensitizes DNA-damaged and malignant cells to apoptosis. Agents that target this pathway are currently in clinical trials. Conversely, inhibition of other DNA damage response pathways, such as ataxia telangiectasia mutated (ATM) and BRCA1, promotes cancer. To determine the effect of replication checkpoint inhibition on carcinogenesis, we generated transgenic mice with diminished ATR function in skin and crossed them into a UV-sensitive background, Xpc(-/-). Unlike caffeine, this genetic approach was selective and had no effect on ATM activation. These transgenic mice were viable and showed no histological abnormalities in skin. Primary keratinocytes from these mice had diminished UV-induced Chk1 phosphorylation and twofold augmentation of apoptosis after UV exposure (P = 0.006). With chronic UV treatment, transgenic mice remained tumor-free for significantly longer (P = 0.003) and had 69% fewer tumors at the end of observation of the full cohort (P = 0.019), compared with littermate controls with the same genetic background. This study suggests that inhibition of replication checkpoint function can suppress skin carcinogenesis and supports ATR inhibition as the relevant mechanism for the protective effect of caffeinated beverage intake in human epidemiologic studies.

DNA damage response and transcription

A network of DNA damage surveillance systems is triggered by sensing of DNA lesions and the initiation of a signal transduction cascade that activates genome-protection pathways including nucleotide excision repair (NER). NER operates through coordinated assembly of repair factors into pre- and post-incision complexes. Recent work identifies RPA as a key regulator of the transition from dual incision to repair-synthesis in UV-irradiated non-cycling cells, thereby averting the generation of unprocessed repair intermediates. These intermediates could lead to recombinogenic events and trigger a persistent ATR-dependent checkpoint signaling. It is now evident that DNA damage signaling is not limited to NER proficient cells. ATR-dependent checkpoint activation also occurs in UV-exposed non-cycling repair deficient cells coinciding with the formation of endonuclease APE1-mediated DNA strand breaks. In addition, the encounter of elongating RNA polymerase II (RNAPIIo) with DNA damage lesions and its persistent stalling provides a strong DNA damage signaling leading to cell cycle arrest, apoptosis and increased mutagenesis. The mechanism underlying the strong and strand specific induction of UV-induced mutations in NER deficient cells has been recently resolved by the finding that gene transcription itself increases UV-induced mutagenesis in a strand specific manner via increased deamination of cytosines. The cell removes the RNAPIIo-blocking DNA lesions by transcription-coupled repair (TC-NER) without displacement of the DNA damage stalled RNAPIIo. Deficiency in TC-NER associates with mutations in the CSA and CSB genes giving rise to the rare human disorder Cockayne syndrome (CS). CSB functions as a repair coupling factor to attract NER proteins, chromatin remodelers and the CSA-E3-ubiquitin ligase complex to the stalled RNAPIIo; CSA is dispensable for attraction of NER proteins, yet in cooperation with CSB is required to recruit XAB2, the nucleosomal binding protein HMGN1 and TFIIS. The molecular mechanisms by which these proteins bring about efficient TC-NER and trigger signaling after transcription arrest remain elusive; particularly the role of chromatin remodeling in TC-NER needs to be clarified in the context of anticipated structural changes that allow repair and transcription restart.

Matched cultures of keratinocytes and fibroblasts derived from normal and NER-deficient mouse models

The uppermost layer of our skin, the epidermis, is formed largely of keratinocytes which constitute the skin's major barrier function and the first line of defence against environmental physical, chemical and biological agents. The subsequent layer, the dermis, which is mainly formed by fibroblasts, has a more supportive function, containing large amounts of collagen, blood vessels and nerve endings and is less directly affected by external insults. Hence it is likely that keratinocytes and fibroblasts have evolved different strategies to cope with the dangers of the environment. Mouse models with various genetic backgrounds in genome care-taking systems, such as DNA repair processes, are well suited to study differences between these two cell types and their implications for cancer and aging. In this chapter we describe a simple procedure to establish long-term keratinocyte and fibroblast cultures from, respectively, the epidermis and dermis of normal or NER-deficient newborn mice. The importance of the external O(2) pressure during the establishment and maintenance of these matched cultures is discussed.

Stochastic fate of p53-mutant epidermal progenitor cells is tilted toward proliferation by UV B during preneoplasia

UV B (UVB) radiation induces clones of cells mutant for the p53 tumor suppressor gene in human and murine epidermis. Here we reanalyze large datasets that report the fate of clones in mice subjected to a course of UVB radiation, to uncover how p53 mutation affects epidermal progenitor cell behavior. We show that p53 mutation leads to exponential growth of clones in UV-irradiated epidermis; this finding is also consistent with the size distribution of p53 mutant clones in human epidermis. Analysis of the tail of the size distribution further reveals that the fate of individual mutant cells is stochastic. Finally, the data suggest that ending UVB exposure results in the p53 mutant cells adopting the balanced fate of wild-type cells: the loss of mutant cells is balanced by proliferation so that the population of preneoplastic cells remains constant. We conclude that preneoplastic clones do not derive from long-lived, self-renewing mutant stem cells but rather from mutant progenitors with random cell fate. It follows that ongoing, low-intensity UVB radiation will increase the number of precancerous cells dramatically compared with sporadic, higher-intensity exposure at the same cumulative dose, which may explain why nonmelanoma skin cancer incidence depends more strongly on age than on radiation dosage. Our approach may be applied to determine cell growth rates in clonally labeled material from a wide range of tissues including human samples.

The acute cutaneous inflammatory response is attenuated in Slug-knockout mice

We previously reported ultraviolet radiation (UVR) induction of Slug, a Snail family zinc-finger transcription factor, in the epidermis of mice; we now report that Slug-knockout mice are, unexpectedly, more resistant to sunburn than wild-type mice. There was a marked difference between the cutaneous inflammatory response in the skin of Slug-knockout and wild-type mice from 12 h to 1 week following a single exposure to 3 minimal erythemal doses of UVR. Slug-knockout mice showed a much reduced immediate increase in skin thickness and neutrophil infiltration compared to wild-type mice. However, there were as many or more intraepidermal T cells, dermal mast cells, and dermal blood vessels in the UVR-exposed skin of Slug-knockout mice as in the skin of wild-type mice. Differences in cytokine and chemokine expression following UVR appeared to account for at least some differences between the genotypes in cutaneous inflammatory response. Despite the reported antiapoptotic and antiproliferative role for Slug in some cell types, we observed little difference between the genotypes in UVR-induced keratinocyte apoptosis or proliferation. Our findings indicate an unexpected but important role for Slug in the acute cutaneous inflammatory response to UVR.

Transcription-coupled nucleotide excision repair in mammalian cells: molecular mechanisms and biological effects

The encounter of elongating RNA polymerase II (RNAPIIo) with DNA lesions has severe consequences for the cell as this event provides a strong signal for P53-dependent apoptosis and cell cycle arrest. To counteract prolonged blockage of transcription, the cell removes the RNAPIIo-blocking DNA lesions by transcription-coupled repair (TC-NER), a specialized subpathway of nucleotide excision repair (NER). Exposure of mice to UVB light or chemicals has elucidated that TC-NER is a critical survival pathway protecting against acute toxic and long-term effects (cancer) of genotoxic exposure. Deficiency in TC-NER is associated with mutations in the CSA and CSB genes giving rise to the rare human disorder Cockayne syndrome (CS). Recent data suggest that CSA and CSB play differential roles in mammalian TC-NER: CSB as a repair coupling factor to attract NER proteins, chromatin remodellers and the CSA- E3-ubiquitin ligase complex to the stalled RNAPIIo. CSA is dispensable for attraction of NER proteins, yet in cooperation with CSB is required to recruit XAB2, the nucleosomal binding protein HMGN1 and TFIIS. The emerging picture of TC-NER is complex: repair of transcription-blocking lesions occurs without displacement of the DNA damage-stalled RNAPIIo, and requires at least two essential assembly factors (CSA and CSB), the core NER factors (except for XPC-RAD23B), and TC-NER specific factors. These and yet unidentified proteins will accomplish not only efficient repair of transcription-blocking lesions, but are also likely to contribute to DNA damage signalling events.

Frequent recovery of triplet mutations in UVB-exposed skin epidermis of Xpc-knockout mice

Mutations of the Xpc gene cause a deficiency in global genome repair, a subpathway of nucleotide excision repair (NER), in mammalian cells. We used transgenic mice harboring the lambda-phage-based lacZ mutational reporter gene to study the effect of an Xpc null mutation (Xpc-/-) on damage induction, repair and mutagenesis in mouse skin epidermis after UVB irradiation. UVB induced equal amounts of cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts (64PPs) in mouse skin epidermis of Xpc-/- and wild-type mice. CPDs were not significantly removed in either of the mouse genotypes by 12h after irradiation, whereas removal of 64PPs was observed in the wild-type. Irradiation with 300 and 400J/m2 UVB increased the lacZ mutant frequency in the Xpc-/- epidermis to at least twice as high as in the wild-type. Ninety-nine lacZ mutants isolated from the UVB-exposed epidermis of Xpc(-/-)mice were analyzed and compared with mutant sequences from irradiated wild-type mice. The spectra of the mutations in the two genotypes were both highly UV-specific and similar in the dominance of C-->T transitions at dipyrimidine sites; however, Xpc-/- mice had a higher frequency of two-base tandem substitutions, including CC-->TT mutations, three-base tandem substitutions and double base substitutions that were separated by one unchanged base in a three-base sequence (alternating mutations). These tandem/alternating mutations included a remarkably large number of triplet mutations, a recently reported, novel type of UV-specific mutation, characterized by multiple base substitutions or frameshifts within a three-nucleotide sequence containing a dipyrimidine. We concluded that the triplet mutation is a UV-specific mutation that preferably occurs in NER deficient genetic backgrounds.

Mutation spectrum in UVB-exposed skin epidermis of Xpa-knockout mice: frequent recovery of triplet mutations

Knockout mutations in both alleles of the Xpa gene give rise to a complete deficiency in nucleotide excision repair (NER) in mammalian cells. We used transgenic mice harboring the lambda-phage-based lacZ mutational reporter gene to study the effect of Xpa null mutation (Xpa(-/-)) on damage induction, repair, and mutagenesis in mouse skin epidermis after UVB irradiation. UVB induced equal amounts of cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts (64PPs) in mouse skin epidermis of Xpa(-/-) and wild-type mice. Neither photolesion was removed in the Xpa(-/-) epidermis by 12 hr after irradiation whereas removal of 64PPs was observed in the epidermis of wild-type mice. Irradiation with 200 and 300 J/m(2) UVB increased the lacZ mutant frequency in the epidermis of Xpa(-/-) mice, but the induced mutant frequencies were not significantly different from those previously determined for wild-type mice. One-hundred lacZ mutants isolated from the UVB-exposed epidermis of Xpa(-/-) mice were analyzed and compared with mutant sequences previously determined for irradiated wild-type mice. The distribution of the mutations along the lacZ transgene and the preferred dipyrimidine context of the UV-specific mutations were similar in mutants from the Xpa(-/-) and wild-type mice. The spectra of the mutations in the two genotypes were both highly UV-specific and similar in a dominance of C --> T transitions at dipyrimidine sites; however, Xpa(-/-) mice had a higher frequency than wild-type mice of two-base tandem substitutions, including CC --> TT mutations, three-base tandem mutations and double base substitutions that were separated by one unchanged base in a three-base sequence (alternating mutations). These tandem/alternating mutations included a remarkably large number of triplet mutations, a recently reported, novel type of UV-specific mutation, characterized by multiple base substitutions or frameshifts within a three-nucleotide sequence containing a dipyrimidine. We conclude that the triplet mutation is a UV-specific mutation that preferably occurs in NER-deficient genetic backgrounds.

Nucleotide excision repair in differentiated cells

Nucleotide excision repair (NER) is the principal pathway for the removal of a wide range of DNA helix-distorting lesions and operates via two NER subpathways, i.e. global genome repair (GGR) and transcription-coupled repair (TCR). Although detailed information is available on expression and efficiency of NER in established mammalian cell lines, little is known about the expression of NER pathways in (terminally) differentiated cells. The majority of studies in differentiated cells have focused on repair of UV-induced cyclobutane pyrimidine dimers (CPD) and 6-4-photoproducts (6-4PP) because of the high frequency of photolesions at low level of toxicity and availability of sensitive technologies to determine photolesions in defined regions of the genome. The picture that emerges from these studies is blurred and rather complex. Fibroblasts and terminally differentiated myocytes of the rat heart display equally efficient GGR of 6-4PP but poor repair of CPD due to the absence of p48 expression. This repair phenotype is clearly different from human terminal differentiated neurons. Furthermore, both cell types were found to carry out TCR of CPD, thus mimicking the repair phenotype of established rodent cell lines. In contrast, in intact rat spermatogenic cells repair was very inefficient at the genome overall level and in transcriptionally active genes indicating that GGR and TCR are non-functional. Also, non-differentiated mouse embryonic stem (ES) cells exhibit low levels of NER after UV irradiation. However, the mechanisms that lead to low NER activity are clearly different: in differentiated spermatogenic cells differences in chromatin compaction and sequestering of NER proteins may underlie the lack of NER activity in pre-meiotic cells, whereas in non-differentiated ES cells NER is impaired by a strong apoptotic response.

UV-induced ubiquitylation of XPC complex, the UV-DDB-ubiquitin ligase complex, and DNA repair

The DNA nucleotide excision repair (NER) system is our major defense against carcinogenesis. Defects in NER are associated with several human genetic disorders including xeroderma pigmentosum (XP), which is characterized by a marked predisposition to skin cancer. For initiation of the repair reaction at the genome-wide level, a complex containing one of the gene products involved in XP, the XPC protein, must bind to the damaged DNA site. The UV-damaged DNA-binding protein (UV-DDB), which is impaired in XP group E patients, has also been implicated in damage recognition in global genomic NER, but its precise functions and its relationship to the XPC complex have not been elucidated. However, the recent discovery of the association of UV-DDB with a cullin-based ubiquitin ligase has functionally linked the two damage recognition factors and shed light on novel mechanistic and regulatory aspects of global genomic NER. This article summarizes our current knowledge of the properties of the XPC complex and UV-DDB and discusses possible roles for ubiquitylation in the molecular mechanisms that underlie the efficient recognition and repair of DNA damage, particularly that induced by ultraviolet light irradiation, in preventing damage-induced mutagenesis as well as carcinogenesis.

Enhanced photocarcinogenesis in interleukin-12-deficient mice

UV-induced DNA damage is the basis for the development of UV-mediated skin cancer because reduction of DNA damage lowers the risk for photocarcinogenesis. The cytokine interleukin (IL)-12 was shown to exhibit the capacity to reduce UV-induced DNA damage presumably via induction of nucleotide excision repair. Because IL-12 is also produced in the skin, we wondered whether endogenous IL-12 protects from photocarcinogenesis. Therefore, we used knockout mice that lack the IL-12p40 chain and thus do not secrete biologically active IL-12. IL-12p40 knockout (IL-12p40-/-) and wild-type (wt) mice were exposed thrice weekly to UV. Skin biopsies obtained after 6 weeks revealed significantly increased numbers of sunburn cells in IL-12p40-/- mice. Additionally, a higher load of UV-induced pyrimidine dimers could be detected in the skin of UV-exposed IL-12p40-/- mice. Staining of epidermal sheets with an antibody against the tumor suppressor gene p53 revealed a higher number of p53 patches in the skin of IL-12p40-/- mice. After approximately 200 days, first skin tumors developed. Kaplan-Meier analysis indicated a significantly increased probability of tumor development in the IL-12p40-/- mice. In addition, the number of tumors developing in the individual mice was significantly higher in IL-12p40-/- mice than in wt mice. Tumors obtained in IL-12p40-/- mice grew faster than those obtained from wt mice on inoculation into nu/nu mice. This was confirmed in an electrophysiologic assay evaluating the intrinsic invasive potency of tumor cells. Together, these data indicate that IL-12 deficiency is associated with an increased risk to develop UV-induced skin cancer, implying that endogenous IL-12 may protect from photocarcinogenesis.

Maintaining genetic integrity in aging: a zero sum game

Aging of somatic cells can be defined as the gradual loss of the information embedded in the global and local properties of complex macromolecular networks. This loss of information may reflect the dynamic interplay between stochastic factors, such as the accumulation of unrepaired somatic damage, and gene-encoded programmatic responses. This would ultimately result in loss of function, impaired response to environmental challenge, and a progressively increased incidence of disease. Here the authors present the case for aging as a continuous battle between maintaining genomic integrity and ensuring sufficient cell functional mass. Focusing on aging of the liver in rodents, evidence is presented that normal aging is associated with a gradual accumulation of random alterations in the DNA of the genome as a consequence of imperfect DNA repair and a decrease in the rate of DNA damage-induced apoptosis. Apoptosis is the cell's genome maintenance mechanism of last resort and an imbalance towards apoptosis can contribute to manifestations of aging-related phenotypes, as exemplified by mouse models of premature aging due to genetic defects in genome maintenance. Prospects to reset the clock in this zero sum game between survival and the maintenance of phenotypic integrity will be discussed.

IL-18 Reduces Ultraviolet Radiation-Induced DNA Damage and Thereby Affects Photoimmunosuppression

UV-induced DNA damage has been recognized as the major molecular trigger for photoimmunosuppression. IL-12 prevents UV-induced immunosuppression via its recently discovered capacity to reduce DNA damage presumably via induction of DNA repair. Because IL-18 shares some biological activities with IL-12 we studied the effect of IL-18 on UV-induced DNA damage and immunosuppression. IL-18 reduced UV-induced apoptosis of keratinocytes and supported long-term cell survival on UV exposure. Injection of IL-18 into mice that were exposed to UV radiation significantly lowered the number of apoptotic keratinocytes. Accordingly, radiation immunohistochemistry revealed reduced amounts of DNA damage in epidermal cells upon injection of IL-18. These effects were not observed in DNA repair-deficient (XpaKO) mice, indicating that IL-18 like IL-12 reduces DNA damage via DNA repair. UV-mediated suppression of the induction of contact hypersensitivity, which is known to be primarily triggered by DNA damage, was prevented upon injection of IL-18 before UV exposure in wild-type but not in XpaKO mice. In contrast to IL-12, IL-18 was not able either in wild-type or in XpaKO mice to break UV-induced immunotolerance that is mediated via regulatory T cells rather than in a DNA damage-dependent fashion. This result indicates that IL-12 is still unique in its capacity to restore immune responses because of its effect on regulatory T cells. Together, these data identify IL-18 as a further cytokine that exhibits the capacity to affect DNA repair. Though being primarily a proinflammatory cytokine through this capacity, IL-18 can also foster an immune response that is suppressed by UV radiation.

Phototherapy: how does UV work?

Phototherapy has been largely empirical. Thus, despite the long term use its mechanisms of action are quite unclear. Because of the recent achievements in photoimmunology and molecular photobiology we are now beginning to understand some of the mechanisms. Many of the effects are certainly mediated via induction of apoptotic cell death. Another major mechanism is the induction of immunosuppression. This review primarily focuses on recent advances in photoimmunology which will contribute to the further understanding how phototherapy acts.

Enhanced DDB2 expression protects mice from carcinogenic effects of chronic UV-B irradiation

UV-damaged DNA-binding protein (UV-DDB) is essential for global genome repair (GGR) of UV-induced cyclobutane pyrimidine dimers (CPD). Unlike human cells, rodent epidermal cells are deficient in GGR of CPDs and express a subunit of UV-DDB, DDB2, at a low level. In this study, we generated mice (K14-DDB2) ectopically expressing mouse DDB2 at elevated levels. Enhanced expression of DDB2 both delayed the onset of squamous cell carcinoma and decreased the number of tumors per mouse in chronically UV-B light-exposed hairless mice. Enhanced expression of DDB2 improved repair of both CPDs and pyrimidine(6-4)pyrimidone photoproducts (6-4PP) in dermal fibroblasts. However, GGR of CPDs in K14-DDB2 mice did not reach the level of efficiency of human cells, suggesting that another repair protein may become rate limiting when DDB2 is abundantly present. To complement these studies, we generated mice in which the DDB2 gene was disrupted. DDB2-/- and DDB2+/- mice were found to be hypersensitive to UV-induced skin carcinogenesis. On the cellular level, we detected a delay in the repair of 6-4PPs in DDB2-/- dermal fibroblasts. Neither the absence nor the enhanced expression of DDB2 affected the levels of UV-induced apoptosis in epidermal keratinocytes or cultured dermal fibroblasts. Our results show an important role for DDB2 in the protection against UV-induced cancer and indicate that this protection is most likely mediated by accelerating the repair of photolesions.

Epidermal transit of replication-arrested, undifferentiated keratinocytes in UV-exposed XPC mice: an alternative to in situ apoptosis

The interplay among nucleotide excision repair, cell-cycle regulation, and apoptosis in the UV-exposed epidermis is extremely important to avoid mutations and malignant transformation. In Xpc(-/-) mice deficient in global genome nucleotide excision repair (GGR), a cell-cycle arrest of epidermal cells in late S-phase [with near-double normal diploid (4N) DNA content] was observed 48-72 h after UV exposure. This arrest resolved without apoptosis (96-168 h). We surmised that these arrested keratinocytes with persistent DNA damage were removed by epidermal turnover. In vivo BrdUrd pulse-chase labeling (>17 h after UV exposure) showed that DNA replication after UV exposure was resumed in Xpc(-/-) mice, but it did not reveal any evidence of retained BrdUrd-labeled S-phase cells in the basal layer of the epidermis at 72 h. Interestingly, by this time a maximum number of cytokeratin 10-negative and cytokeratin 5-positive cells had appeared in the suprabasal epidermal cell layers of UV-exposed Xpc(-/-) mice. Accumulation of these "basal cell"-like keratinocytes in the suprabasal layers was clearly aberrant and was not observed in WT and heterozygous mice. Flow cytometric analyses of single-cell suspensions from UV-exposed Xpc(-/-) epidermis further showed that the "near-4N" arrested cells retained cytokeratin 5 and lacked cytokeratin 10. Hence, we conclude that the arrested near-4N cells became detached from the basal layer without entering a proper differentiation program and were indeed subsequently lost through the epidermal turnover. This expulsion apparently constitutes an alternative route, different from in situ apoptosis, to eliminate DNA-damaged arrested cells from the epidermis.

Repair of the three main types of bipyrimidine DNA photoproducts in human keratinocytes exposed to UVB and UVA radiations

Induction of DNA damage by solar UV radiation is a key event in the development of skin cancers. Bipyrimidine photoproducts, including cyclobutane pyrimidine dimers (CPDs), (6-4) photoproducts (64 PPs) and their Dewar valence isomers, have been identified as major UV-induced DNA lesions. In order to identify the predominant and most persistent lesions, we studied the repair of the three types of photolesions in primary cultures of human keratinocytes. Specific and quantitative data were obtained using HPLC associated with tandem mass spectrometry. As shown in other cell types, 64 PPs are removed from UVB-irradiated keratinocytes much more efficiently than CPDs. In contrast, CPDs are still present in high amounts when cells recover their proliferation capacities after cell cycle arrest and elimination of a part of the population by apoptosis. The predominance of CPDs is still maintained when keratinocytes are exposed to a combination of UVB and UVA. Under these conditions, 64 PPs are converted into their Dewar valence isomers that are as efficiently repaired as their (6-4) precursors. Exposure of cells to pure UVA radiation generates thymine cyclobutane dimers that are slightly less efficiently repaired than CPDs produced upon UVB irradiation. Altogether, our results show that CPDs are the most frequent and the less efficiently repaired bipyrimidine photoproducts irrespectively of the applied UV treatment.

Repair characteristics and differentiation propensity of long-term cultures of epidermal keratinocytes derived from normal and NER-deficient mice

Epidermal keratinocytes constitute the most relevant cellular system in terms of DNA damage because of their continuous exposure to UV light and genotoxic chemicals from the environment. Here, we describe the establishment of long-term keratinocyte cultures from the skin of wild-type and nucleotide excision repair (NER) deficient mouse mutants. The use of media with a lowered calcium concentration and the inclusion of keratinocyte growth factor (KGF) permitted repeated passaging of the cultures and resulted in the generation of stable cell lines that proliferated efficiently. The cells retained their normal ability to engage into terminal differentiation when triggered with high calcium concentrations or after suspension in semi-solid medium. The cultures reflected the cellular characteristics (i.e. repair and transcription profiles) of the Xpa(-/-), Xpc(-/-), Csb(-/-) and Xpd(TTD) mouse models from which they were derived. For instance, in line with earlier in vivo results, Xpd(TTD) keratinocytes were disturbed in their ability to terminally differentiate in vitro. This was concluded from a delay in calcium-induced stratification and by reduced transcription of both early (keratin 10) and late (loricrin) terminal differentiation marker genes. UDS measurements in wild-type cells committed to terminal differentiation did not reveal any reduction in global DNA repair that could be indicative of differentiation associated repair (DAR) as found in neurons. UV sensitivity data revealed that in keratinocytes global genome repair contributes more to cell survival than previously concluded from fibroblast studies. It is inferred that these fully controllable in vitro cultures will be a valuable tool to assess critical parameters of genome care-taking systems in cell proliferation and differentiation.

Selective DNA damage responses in murine Xpa-/-, Xpc-/- and Csb-/- keratinocyte cultures

Cellular DNA damage responses (DDRs) are induced by unrepaired DNA lesions and constitute a protective back-up system that prevents the expansion of damaged cells. These cellular signaling pathways trigger either growth arrest or cell death and are believed to be major components of an early anti-cancer barrier. Cultures of C57BL/6J keratinocytes with various defects in NER sub-pathways allowed us to follow the kinetics of DDRs in an isogenic background and in the proper (physiologically relevant) target cells, supplementing earlier studies in heterogenic human fibroblasts. In a series of well-controlled parallel experiments we have shown that, depending on the NER deficiency, murine keratinocytes elicited highly selective DDRs. After a dose of UV-B that did not affect wild-type keratinocytes, Xpa(-/-) keratinocytes (complete NER deficiency) showed a rapid depletion of DNA replicating S-phase cells, a transient increase in quiescent S-phase cells (not replicating DNA), followed by massive apoptosis. Csb(-/-) keratinocytes (TC-NER deficient) responded by a more sustained increase in QS-phase cells and appeared more resistant to UV-B induced apoptosis than Xpa(-/-). In irradiated Xpc(-/-) keratinocytes (GG-NER deficient) the loss of replicating S-phase cells was associated with a gradual build-up of both QS-phase cells and cells arrested in late-S phase, in complete absence of apoptosis. Our analysis complements and extends previous in vivo investigations and highlights both similarities and differences with earlier fibroblast studies. In vitro cultures of murine keratinocytes provide a new tool to unravel the molecular mechanisms of UV-induced cellular stress responses in great detail and in a physiologically relevant background. This will be essential to fully appreciate the implications of DDRs in tumor suppression and cancer prevention.

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.

Relationship between UV-induced mutant p53 patches and skin tumours, analysed by mutation spectra and by induction kinetics in various DNA-repair-deficient mice

Clusters of p53 immunopositive epidermal keratinocytes (so-called p53 patches, clones or foci) are found in sun or ultraviolet (UV) light-exposed skin. We investigated to what extent these p53 patches are genuine precursors of skin carcinomas in chronically irradiated hairless (SKH1) mice. The mutation spectra of exons 5-8 of the p53 gene of laser-micro-dissected mutant p53 patches and carcinomas were therefore compared. The mutations we found were mainly UV-signature mutations (C-->T and CC-->TT at dipyrimidine sites) located at known hotspots. No significant differences were found between both spectra, indicating that all p53 patches harbour mutations with which they could progress to carcinomas. To examine whether these p53 patches can be used as tumour risk indicators, we made an extensive comparison of the induction kinetics of these patches and carcinomas in genetically modified mice with various defects in nucleotide excision repair (NER), i.e. xeroderma pigmentosum A (Xpa), Xpc and Cockayne syndrome B (Csb) and wild-type mice. In this aforementioned order, the mouse strains developed both p53 patches and carcinomas in the course of daily exposure to 40 J/m(2) UV. Hence, the order in which the NER-deficient mice developed patches was predictive of the order in which they developed tumours. The induction kinetics of the patches in Xpc-deficient mice differed notably from the others: there was a stationary phase (days 13-41) where the numbers were limited to 5-10 patches per mouse before an explosive increase which ran parallel to the other groups. The chance that a p53 patch progresses to carcinoma is relatively small (estimated at 1 out of 8300-40,000/individual when the first tumour appears), but our results are strongly indicative of a causal relationship between p53 patches and carcinomas.

DNA damage in transcribed genes induces apoptosis via the JNK pathway and the JNK-phosphatase MKP-1

The nucleotide excision repair (NER) system consists of two sub-pathways, global genome repair (GGR) and transcription-coupled repair (TCR), which exhibit distinct functions in the cellular response to genotoxic stress. Defects in TCR result in prolonged UV light-induced stalling of RNA polymerase II and hypersensitivity to apoptosis induced by UV and certain chemotherapeutic drugs. Here, we show that low doses of UV trigger delayed activation of the stress-induced MAPkinase JNK and its proapoptotic targets c-Jun and ATF-3 in TCR-deficient primary human fibroblasts from Xeroderma Pigmentosum (XP) and Cockayne syndrome (CS) patients. This delayed activation of the JNK pathway is not observed in GGR-deficient TCR-proficient XP cells, is independent of functional p53, and is established through repression of the JNK-phosphatase MKP-1 rather than by activation of the JNK kinases MKK4 and 7. Enzymatic reversal of UV-induced cyclobutane pyrimidine dimers (CPDs) by CPD photolyase abrogated JNK activation, MKP-1 repression, and apoptosis in TCR-deficient XPA cells. Ectopic expression of MKP-1 inhibited DNA-damage-induced JNK activity and apoptosis. These results identify both MKP-1 and JNK as sensors and downstream effectors of persistent DNA damage in transcribed genes and suggest a link between the JNK pathway and UV-induced stalling of RNApol II.

Nucleotide excision repair- and p53-deficient mouse models in cancer research

Cancer is caused by the loss of controlled cell growth due to mutational (in)activation of critical genes known to be involved in cell cycle regulation. Three main mechanisms are known to be involved in the prevention of cells from becoming cancerous; DNA repair and cell cycle control, important to remove DNA damage before it will be fixed into mutations and apoptosis, resulting in the elimination of cells containing severe DNA damage. Several human syndromes are known to have (partially) deficiencies in these pathways, and are therefore highly cancer prone. Examples are xeroderma pigmentosum (XP) caused by an inborn defect in the nucleotide excision repair (NER) pathway and the Li-Fraumeni syndrome, which is the result of a germ line mutation in the p53 gene. XP patients develop skin cancer on sun exposed areas at a relatively early age, whereas Li-Fraumeni patients spontaneously develop a wide variety of early onset tumors, including sarcomas, leukemia's and mammary gland carcinomas. Several mouse models have been generated to mimic these human syndromes, providing us information about the role of these particular gene defects in the tumorigenesis process. In this review, spontaneous phenotypes of mice deficient for nucleotide excision repair and/or the p53 gene will be described, together with their responses upon exposure to either chemical carcinogens or radiation. Furthermore, possible applications of these and newly generated mouse models for cancer will be given.

Mismatch repair protein Msh2 contributes to UVB-induced cell cycle arrest in epidermal and cultured mouse keratinocytes

Nucleotide excision repair (NER), cell cycle regulation and apoptosis are major defence mechanisms against the carcinogenic effects of UVB radiation. NER eliminates UVB-induced DNA photolesions via two subpathways: global genome repair (GGR) and transcription-coupled repair (TCR). In a previous study, we found UVB-induced accumulation of tetraploid (4N) keratinocytes in the epidermis of Xpc(-/-) mice (no GGR), but not in Xpa(-/-) (no TCR and no GGR) or in wild-type (WT) mice. We inferred that this arrest in Xpc(-/-) mice is caused by erroneous replication past photolesions, leading to 'compound lesions' known to be recognised by mismatch repair (MMR). MMR-induced futile cycles of breakage and resynthesis at sites of compound lesions may then sustain a cell cycle arrest. The present experiments with Xpc(-/-)Msh2(-/-) mice and derived keratinocytes show that the MMR protein Msh2 indeed plays a role in the generation of the UVB-induced arrested cells: a Msh2-deficiency lowered significantly the percentage of arrested cells in vivo (40-50%) and in vitro (30-40%). Analysis of calyculin A (CA)-induced premature chromosome condensation (PCC) of cultured Xpc(-/-) keratinocytes showed that the delayed arrest occurred in late S phase rather than in G(2)-phase. Taken together, the results indicate that in mouse epidermis and cultured keratinocytes, the MMR protein Msh2 plays a role in the UVB-induced S-phase arrest. This indicates that MMR plays a role in the UVB-induced S-phase arrest. Alternatively, Msh2 may have a more direct signalling function.

2-AAF-induced tumor development in nucleotide excision repair-deficient mice is associated with a defect in global genome repair but not with transcription coupled repair

The nucleotide excision repair (NER) pathway comprises two sub-pathways, transcription coupled repair (TCR) and global genome repair (GGR). To establish the importance of these separate sub-pathways in tumor suppression, we exposed mice deficient for either TCR (Csb), GGR (Xpc) or both (Xpa) to 300 ppm 2-acetylaminofluorene (in feed, ad libitum) in a unique comparative exposure experiment. We found that cancer proneness was directly linked to a defect in the GGR pathway of NER as both Xpa and Xpc mice developed significantly more liver tumors upon 2-AAF exposure than wild type or Csb mice. In contrast, a defect in TCR appeared to act tumor suppressive, leading to a lower hepatocellular tumor response in Xpa mice (tumor incidence of 25%) as compared to Xpc mice (53% tumor-bearing mice). The link between deficient GGR and tumor proneness was most pronounced in the liver, but this phenomenon was also found in the urinary bladder. As tumor induction by 2-AAF appeared almost exclusively dependent on a defect in GGR, we examined whether gene mutation induction in the non-transcribed lacZ locus could reliably predict tumor risk. Interestingly, however, short-term 2-AAF exposure induced lacZ mutant levels in Csb mice almost as high as those found in Xpa or Xpc mice. This indicates that lacZ mutant frequencies are not correlated with a specific DNA repair defect and eventual tumor outcome, at least not in the experimental design presented here.

Prevention of UV radiation-induced immunosuppression by IL-12 is dependent on DNA repair

The immunostimulatory cytokine IL-12 is able to antagonize immunosuppression induced by solar/ultraviolet (UV) radiation via yet unknown mechanisms. IL-12 was recently found to induce deoxyribonucleic acid (DNA) repair. UV-induced DNA damage is an important molecular trigger for UV-mediated immunosuppression. Thus, we initiated studies into immune restoration by IL-12 to discern whether its effects are linked to DNA repair. IL-12 prevented both UV-induced suppression of the induction of contact hypersensitivity and the depletion of Langerhans cells, the primary APC of the skin, in wild-type but not in DNA repair-deficient mice. IL-12 did not prevent the development of UV-induced regulatory T cells in DNA repair-deficient mice. In contrast, IL-12 was able to break established UV-induced tolerance and inhibited the activity of regulatory T cells independent of DNA repair. These data identify a new mechanism by which IL-12 can restore immune responses and also demonstrate a link between DNA repair and the prevention of UV-induced immunosuppression by IL-12.

Xeroderma pigmentosum: from symptoms and genetics to gene-based skin therapy

Xeroderma pigmentosum (XP) is a rare, recessively inherited genodermatosis prone to ultraviolet (UV)-induced skin neoplasms from keratinocyte origin, i.e. basal and squamous cell carcinoma. Cells from classic XP patients fail to properly eliminate UV-induced DNA lesions by the nucleotide excision repair (NER) mechanism. A variant form of XP, called XP-V suffers from faulty translesion synthesis. We review here recent data on XP gene products whose alterations affect NER and result in one of the 7 complementation groups of XP. Encouraging results of retrovirus-based genetic correction of XP keratinocytes are summarized and support realistic prospects of gene therapy for the XP-C complementation group.

Regulation of epidermal apoptosis and DNA repair by E2F1 in response to ultraviolet B radiation

The E2F1 transcription factor regulates the expression of genes involved in cell proliferation, apoptosis and DNA repair. Following DNA damage, E2F1 is phosphorylated and stabilized, but the physiological role of E2F1 in the response to DNA damage is unclear. We find that mice lacking E2F1 have increased levels of epidermal apoptosis compared to wild-type mice following exposure to ultraviolet B (UVB) radiation. Moreover, transgenic overexpression of E2F1 in basal layer keratinocytes suppresses apoptosis induced by UVB. Inhibition of UVB-induced apoptosis by E2F1 is unexpected given that most studies have demonstrated a proapoptotic function for E2F1. E2F1-mediated suppression of apoptosis does not involve alterations in mitogen-activated protein kinase activation or Bcl-2 downregulation in response to UVB and is independent of p53. Instead, inhibition of UVB-induced apoptosis by E2F1 correlates with a stimulation of DNA repair. Mice lacking E2F1 are impaired for the removal of DNA photoproducts, while E2F1 transgenic mice repair UVB-induced DNA damage at an accelerated rate compared to wild-type mice. These findings suggest that E2F1 participates in the response to UVB by promoting DNA repair and suppressing apoptosis.

Differential Role of Transcription-Coupled Repair in UVB–Induced Response of Human Fibroblasts and Keratinocytes

Most solar radiation–induced skin cancers arise in keratinocytes. In the human epidermis, protection against cancer is thought to be mediated mainly by nucleotide excision repair (NER) of UVB-induced cyclobutane pyrimidine dimers, and by elimination of the damaged cells by apoptosis. NER consists of two subpathways: global genome repair (GGR) and transcription-coupled repair (TCR). Here, we investigate the impact of defects in NER subpathways on the cellular response to UVB-induced damage by comparing primary human keratinocytes and fibroblasts from normal, XP-C (GGR-defective), and CS-A (TCR-defective) individuals. We show that human keratinocytes are more resistant to UVB killing than fibroblasts and present higher levels of UVB-induced DNA repair synthesis due to a more efficient GGR. The CS-A defect is associated with a strong apoptotic response in fibroblasts but not in keratinocytes. Following an UVB dose of 1,000 J/m2, no p53-mediated transactivation of mdm2 is observed in CS-A fibroblasts, whereas the p53-mdm2 circuit is fully activated in CS-A keratinocytes. Thus, in fibroblasts, the signal for apoptosis originates from DNA photoproducts in the transcribed strand of active genes, whereas in keratinocytes, it is largely TCR-independent. This study shows that the response to UVB radiation is cell type–specific in humans and provides the first evidence that a deficiency in TCR has a different impact depending on the cell type. These findings have important implications for the mechanism of skin cancer protection after UVB damage and may explain the lack of skin cancer in patients with Cockayne syndrome.