Mouse Models for Xeroderma Pigmentosum Group A and Group C Show Divergent Cancer Phenotypes

XPC Protects against Carcinogen-Induced Histologic Progression to Lung Squamous Cell Carcinoma by Reduced Basal Epithelial Cell Proliferation

Lung squamous cell carcinoma (LUSC) is the second leading cause of lung cancer. Although characterized by high DNA mutational burdens and genomic complexity, the role of DNA repair in LUSC development is poorly understood. We sought to better understand the role of the DNA repair protein Xeroderma Pigmentosum Group C (XPC) in LUSC development. XPC knock-out (KO), heterozygous, and wild-type (WT) mice were exposed topically to N-nitroso-tris-chloroethylurea (NTCU), and lungs were evaluated for histology and pre-malignant progression in a blinded fashion at various time-points from 8-24 weeks. High-grade dysplasia and LUSC were increased in XPC KO compared with XPC WT NTCU mice (56% vs. 34%), associated with a higher mean LUSC lung involvement (p < 0.05). N-acetylcysteine pre-treatment decreased bronchoalveolar inflammation but did not prevent LUSC development. Proliferation, measured as %Ki67+ cells, increased with NTCU treatment, in high-grade dysplasia and LUSC, and in XPC deficiency (p < 0.01, ANOVA). Finally, pre-LUSC dysplasia developed earlier and progressed to higher histologic classification sooner in XPC KO compared with WT mice. Overall, this supports the protective role of XPC in squamous dysplasia progression to LUSC. Mouse models of early LUSC development are limited; this may provide a valuable model to study mechanisms of LUSC development and progression.

Mitochondria and oxygen homeostasis

Molecular oxygen possesses a dual nature due to its highly reactive free radical property: it is capable of oxidizing metabolic substrates to generate cellular energy, but can also serve as a substrate for genotoxic reactive oxygen species generation. As a labile substance upon which aerobic life depends, the mechanisms for handling cellular oxygen have been fine-tuned and orchestrated in evolution. Protection from atmospheric oxygen toxicity as originally posited by the Endosymbiotic Theory of the Mitochondrion is likely to be one basic principle underlying oxygen homeostasis. We briefly review the literature on oxygen homeostasis both in vitro and in vivo with a focus on the role of the mitochondrion where the majority of cellular oxygen is consumed. The insights gleaned from these basic mechanisms are likely to be important for understanding disease pathogenesis and developing strategies for maintaining health.

Xeroderma Pigmentosum Complementation Group C (XPC): Emerging Roles in Non-Dermatologic Malignancies

Xeroderma pigmentosum complementation group C (XPC) is a DNA damage recognition protein essential for initiation of global-genomic nucleotide excision repair (GG-NER). Humans carrying germline mutations in the XPC gene exhibit strong susceptibility to skin cancer due to defective removal via GG-NER of genotoxic, solar UV-induced dipyrimidine photoproducts. However, XPC is increasingly recognized as important for protection against non-dermatologic cancers, not only through its role in GG-NER, but also by participating in other DNA repair pathways, in the DNA damage response and in transcriptional regulation. Additionally, XPC expression levels and polymorphisms likely impact development and may serve as predictive and therapeutic biomarkers in a number of these non-dermatologic cancers. Here we review the existing literature, focusing on the role of XPC in non-dermatologic cancer development, progression, and treatment response, and highlight possible future applications of XPC as a prognostic and therapeutic biomarker.

Inflammation response, oxidative stress and DNA damage caused by urban air pollution exposure increase in the lack of DNA repair XPC protein

Air pollution represents a considerable threat to health worldwide. The São Paulo Metropolitan area, in Brazil, has a unique composition of atmospheric pollutants with a population of nearly 20 million people and 9 million passenger cars. It is long known that exposure to particulate matter less than 2.5 µm (PM_2.5) can cause various health effects such as DNA damage. One of the most versatile defense mechanisms against the accumulation of DNA damage is the nucleotide excision repair (NER), which includes XPC protein. However, the mechanisms by which NER protects against adverse health effects related to air pollution are largely unknown. We hypothesized that reduction of XPC activity may contribute to inflammation response, oxidative stress and DNA damage after PM_2.5 exposure. To address these important questions, XPC knockout and wild type mice were exposed to PM_2.5 using the Harvard Ambient Particle concentrator. Results from one-single exposure have shown a significant increase in the levels of anti-ICAM, IL-1β, and TNF-α in the polluted group when compared to the filtered air group. Continued chronic PM_2.5 exposure increased levels of carbonylated proteins, especially in the lung of XPC mice, probably as a consequence of oxidative stress. As a response to DNA damage, XPC mice lungs exhibit increased γ-H2AX, followed by severe atypical hyperplasia. Emissions from vehicles are composed of hazardous substances, with polycyclic aromatic hydrocarbons (PAHs) and metals being most frequently cited as the major contributors to negative health impacts. This analysis showed that benzo[b]fluoranthene, 2-nitrofluorene and 9,10-anthraquinone were the most abundant PAHs and derivatives. Taken together, these findings demonstrate the participation of XPC protein, and NER pathway, in the protection of mice against the carcinogenic potential of air pollution. This implicates that DNA is damaged directly (forming adducts) or indirectly (Reactive Oxygen Species) by the various compounds detected in urban PM_2.5.

Xeroderma Pigmentosum C (XPC) Mutations in Primary Fibroblasts Impair Base Excision Repair Pathway and Increase Oxidative DNA Damage

Xeroderma Pigmentosum C (XPC) is a multi-functional protein that is involved not only in the repair of bulky lesions, post-irradiation, via nucleotide excision repair (NER) per se but also in oxidative DNA damage mending. Since base excision repair (BER) is the primary regulator of oxidative DNA damage, we characterized, post-Ultraviolet B-rays (UVB)-irradiation, the detailed effect of three different XPC mutations in primary fibroblasts derived from XP-C patients on mRNA, protein expression and activity of different BER factors. We found that XP-C fibroblasts are characterized by downregulated expression of different BER factors including OGG1, MYH, APE1, LIG3, XRCC1, and Polβ. Such a downregulation was also observed at OGG1, MYH, and APE1 protein levels. This was accompanied with an increase in DNA oxidative lesions, as evidenced by 8-oxoguanine levels, immediately post-UVB-irradiation. Unlike in normal control cells, these oxidative lesions persisted over time in XP-C cells having lower excision repair capacities. Taken together, our results indicated that an impaired BER pathway in XP-C fibroblasts leads to longer persistence and delayed repair of oxidative DNA damage. This might explain the diverse clinical phenotypes in XP-C patients suffering from cancer in both photo-protected and photo-exposed areas. Therapeutic strategies based on reinforcement of BER pathway might therefore represent an innovative path for limiting the drawbacks of NER-based diseases, as in XP-C case.

XPC deficiency increases risk of hematologic malignancies through mutator phenotype and characteristic mutational signature

Recent studies demonstrated a dramatically increased risk of leukemia in patients with a rare genetic disorder, Xeroderma Pigmentosum group C (XP-C), characterized by constitutive deficiency of global genome nucleotide excision repair (GG-NER). The genetic mechanisms of non-skin cancers in XP-C patients remain unexplored. In this study, we analyze a unique collection of internal XP-C tumor genomes including 6 leukemias and 2 sarcomas. We observe a specific mutational pattern and an average of 25-fold increase of mutation rates in XP-C versus sporadic leukemia which we presume leads to its elevated incidence and early appearance. We describe a strong mutational asymmetry with respect to transcription and the direction of replication in XP-C tumors suggesting association of mutagenesis with bulky purine DNA lesions of probably endogenous origin. These findings suggest existence of a balance between formation and repair of bulky DNA lesions by GG-NER in human body cells which is disrupted in XP-C patients.

Xeroderma Pigmentosum group C (XP-C) is a rare genetic disorder characterised by deficient DNA repair leading to skin and internal cancer, but the latter is not well understood molecularly. Here the authors sequence genomes of non-skin cancers from XP-C patients to unravel its mutational patterns.

Deficiency of nucleotide excision repair is associated with mutational signature observed in cancer

Nucleotide excision repair (NER) is one of the main DNA repair pathways that protect cells against genomic damage. Disruption of this pathway can contribute to the development of cancer and accelerate aging. Mutational characteristics of NER-deficiency may reveal important diagnostic opportunities, as tumors deficient in NER are more sensitive to certain treatments. Here, we analyzed the genome-wide somatic mutational profiles of adult stem cells (ASCs) from NER-deficient Ercc1 ^-/Δ mice. Our results indicate that NER-deficiency increases the base substitution load twofold in liver but not in small intestinal ASCs, which coincides with the tissue-specific aging pathology observed in these mice. Moreover, NER-deficient ASCs of both tissues show an increased contribution of Signature 8 mutations, which is a mutational pattern with unknown etiology that is recurrently observed in various cancer types. The scattered genomic distribution of the base substitutions indicates that deficiency of global-genome NER (GG-NER) underlies the observed mutational consequences. In line with this, we observe increased Signature 8 mutations in a GG-NER-deficient human organoid culture, in which XPC was deleted using CRISPR-Cas9 gene-editing. Furthermore, genomes of NER-deficient breast tumors show an increased contribution of Signature 8 mutations compared with NER-proficient tumors. Elevated levels of Signature 8 mutations could therefore contribute to a predictor of NER-deficiency based on a patient's mutational profile.

XPC protects against smoking- and carcinogen-induced lung adenocarcinoma

Cigarette smoke (CS) contains hundreds of carcinogens and is a potent inducer of oxidative and bulky DNA damage, which when insufficiently repaired leads to activation of DNA damage response and possibly mutations. The DNA repair protein xeroderma pigmentosum group C (XPC) is primed to play an important role in CS-induced DNA damage because of its function in initiating repair of both bulky oxidative DNA damage. We hypothesized that loss of XPC function will increase susceptibility to developing CS- and carcinogen-induced lung cancer through impaired repair of oxidative DNA damage. Mice deficient in XPC (XPC-/-) exposed to chronic CS developed lung tumors whereas their wild-type littermates (XPC+/+) did not. XPC-/- mice treated with the CS-carcinogen urethane developed lung adenocarcinomas representing progressive stages of tumor development, with lung tumor number increased 17-fold compared with XPC+/+ mice. Mice heterozygous for XPC (XPC+/-) demonstrated a gene-dose effect, developing an intermediate number of lung tumors with urethane treatment. Treatment of XPC-/- mice with the carcinogen 3-methylcholanthrene followed by the proliferative agent butylated hydroxytoluene resulted in a 2-fold increase in lung adenocarcinoma development. Finally, tumor number decreased 7-fold in the lungs of XPC-/- mice by concurrent treatment with the antioxidant, N-acetylcysteine. Altogether, this supports a mechanism by which decreased XPC expression promotes lung adenocarcinoma development in response to CS-carcinogen exposure, due in part to impaired oxidative DNA damage repair.

DNA repair as an emerging target for COPD-lung cancer overlap

Cigarette smoking is the leading cause of lung cancer and chronic obstructive pulmonary disease (COPD). Many of the detrimental effects of cigarette smoke have been attributed to the development of DNA damage, either directly from chemicals contained in cigarette smoke or as a product of cigarette smoke-induced inflammation and oxidative stress. In this review, we discuss the environmental, epidemiological, and physiological links between COPD and lung cancer and the likely role of DNA damage and repair in COPD and lung cancer development. We explore alterations in DNA damage repair by DNA repair proteins and pathways. We discuss emerging data supporting a key role for the DNA repair protein, xeroderma pigmentosum group C (XPC), in cigarette smoke-induced COPD and early lung cancer development. Understanding the interplay between cigarette smoke, DNA damage repair, COPD, and lung cancer may lead to prognostic tools and new, potentially targetable, pathways for lung cancer prevention and treatment.

Hallmarks of progeroid syndromes: lessons from mice and reprogrammed cells

Ageing is a process that inevitably affects most living organisms and involves the accumulation of macromolecular damage, genomic instability and loss of heterochromatin. Together, these alterations lead to a decline in stem cell function and to a reduced capability to regenerate tissue. In recent years, several genetic pathways and biochemical mechanisms that contribute to physiological ageing have been described, but further research is needed to better characterize this complex biological process. Because premature ageing (progeroid) syndromes, including progeria, mimic many of the characteristics of human ageing, research into these conditions has proven to be very useful not only to identify the underlying causal mechanisms and identify treatments for these pathologies, but also for the study of physiological ageing. In this Review, we summarize the main cellular and animal models used in progeria research, with an emphasis on patient-derived induced pluripotent stem cell models, and define a series of molecular and cellular hallmarks that characterize progeroid syndromes and parallel physiological ageing. Finally, we describe the therapeutic strategies being investigated for the treatment of progeroid syndromes, and their main limitations.

Summary: This Review defines the molecular and cellular hallmarks of progeroid syndromes according to the main cellular and animal models, and discusses the therapeutic strategies developed to date.

Regulation of XPC deubiquitination by USP11 in repair of UV-induced DNA damage

Nucleotide excision repair (NER) is the most versatile DNA repair pathway for removing DNA damage caused by UV radiation and many environmental carcinogens. NER is essential for suppressing tumorigenesis in the skin, lungs and brain. Although the core NER proteins have been identified and characterized, molecular regulation of NER remains poorly understood. Here we show that ubiquitin-specific peptidase 11 (USP11) positively regulates NER by deubiquitinating xeroderma pigmentosum complementation group C (XPC) and promoting its retention at the DNA damage sites. In addition, UV irradiation induces both USP11 recruitment to the chromatin and USP11 interaction with XPC in an XPC-ubiquitination-dependent manner. Furthermore, we found that USP11 is down-regulated in chronically UV-exposed mouse skin and in skin tumors from mice and humans. Our findings indicate that USP11 plays an important role in maintaining NER capacity, and suggest that USP11 acts as a tumor suppressor via its role in DNA repair.

Deconstructing networks of p53-mediated tumor suppression in vivo

The transcription factor p53 is a vital tumor suppressor. Upon activation by diverse stresses including oncogene activation, DNA damage, hypoxia and nutrient deprivation, p53 activates a panoply of target genes and orchestrates numerous downstream responses that suppress tumorigenesis. Although early studies of p53 suggested that its ability to induce cell cycle arrest, senescence and apoptosis programs accounted for its tumor-suppressor activity, more recent studies have challenged this notion. Moreover, p53 regulates a suite of additional processes, such as metabolism, stem cell function, invasion and metastasis. The processes p53 coordinately regulates to enact tumor suppression, and how such regulation occurs, thus remain elusive. In this review, we will summarize our current knowledge of p53-mediated tumor-suppressive mechanisms gleaned from in vivo studies in mouse models.

XPC inhibits NSCLC cell proliferation and migration by enhancing E-Cadherin expression

Xeroderma pigmentosum complementation group C (XPC) protein is an important DNA damage recognition factor in nucleotide excision repair. Deletion of XPC is associated with early stages of human lung carcinogenesis, and reduced XPC mRNA levels predict poor patient outcome for non-small cell lung cancer (NSCLC). However, the mechanisms linking loss of XPC expression and poor prognosis in lung cancer are still unclear. Here, we report evidence that XPC silencing drives proliferation and migration of NSCLC cells by down-regulating E-Cadherin. XPC knockdown enhanced proliferation and migration while decreasing E-Cadherin expression in NSCLC cells with an epithelial phenotype. Restoration of E-Cadherin in these cells suppressed XPC knockdown-induced cell growth both in vitro and in vivo. Mechanistic studies showed that the loss of XPC repressed E-Cadherin expression by activating the ERK pathway and upregulating Snail expression. Our findings indicate that XPC silencing-induced reduction of E-Cadherin expression contributes, at least in part, to the poor outcome of NSCLC patients with low XPC expression.

Polymorphisms within base and nucleotide excision repair pathways and risk of differentiated thyroid carcinoma

The thyrocytes are exposed to high levels of oxidative stress which could induce DNA damages. Base excision repair (BER) is one of the principal mechanisms of defense against oxidative DNA damage, however recent evidences suggest that also nucleotide excision repair (NER) could be involved. The aim of present work was to identify novel differentiated thyroid cancer (DTC) risk variants in BER and NER genes. For this purpose, the most strongly associated SNPs within NER and BER genes found in our previous GWAS on DTC were selected and replicated in an independent series of samples for a new case-control study. Although a positive signal was detected at the nominal level of 0.05 for rs7689099 (encoding for an aminoacid change proline to arginine at codon 117 within NEIL3), none of the considered SNPs (i.e. rs7990340 and rs690860 within RFC3, rs3744767 and rs1131636 within RPA1, rs16962916 and rs3136166 in ERCC4, and rs17739370 and rs7689099 in NEIL3) was associated with the risk of DTC when the correction of multiple testing was applied. In conclusion, a role of NER and BER pathways was evoked in the susceptibility to DTC. However, this seemed to be limited to few polymorphic genes and the overall effect size appeared weak.

Genetic Evidence for XPC-KRAS Interactions During Lung Cancer Development

Lung cancer causes more deaths than breast, colorectal and prostate cancers combined. Despite major advances in targeted therapy in a subset of lung adenocarcinomas, the overall 5-year survival rate for lung cancer worldwide has not significantly changed for the last few decades. DNA repair deficiency is known to contribute to lung cancer development. In fact, human polymorphisms in DNA repair genes such as xeroderma pigmentosum group C (XPC) are highly associated with lung cancer incidence. However, the direct genetic evidence for the role of XPC for lung cancer development is still lacking. Mutations of the Kirsten rat sarcoma viral oncogene homolog (Kras) or its downstream effector genes occur in almost all lung cancer cells, and there are a number of mouse models for lung cancer with these mutations. Using activated Kras, Kras(LA1), as a driver for lung cancer development in mice, we showed for the first time that mice with Kras(LA1) and Xpc knockout had worst outcomes in lung cancer development, and this phenotype was associated with accumulated DNA damage. Using cultured cells, we demonstrated that induced expression of oncogenic KRAS(G12V) led to increased levels of reactive oxygen species (ROS) as well as DNA damage, and both can be suppressed by anti-oxidants. Our results suggest that XPC may help repair DNA damage caused by KRAS-mediated production of ROS.

XAB2 tagSNPs contribute to non-small cell lung cancer susceptibility in Chinese population

Background

XPA-binding protein 2 (XAB2) interacts with Cockayne syndrome complementation group A (CSA), group B (CSB) and RNA polymerase II to initiate nucleotide excision repair. This study aims to evaluate the association of XAB2 genetic variants with the risk of non-small cell lung cancer (NSCLC) using a tagging approach.

Methods

A hospital-based case-control study was conducted in 470 patients with NSCLC and 470 controls in Chinese population. Totally, 5 tag single nucleotide polymorphisms (SNPs) in XAB2 gene were selected by Haploview software using Hapmap database. Genotyping was performed using iPlex Gold Genotyping Asssy and Sequenom MassArray. Unconditional logistic regression was conducted to estimate odd ratios (ORs) and 95 % confidence intervals (95 % CI).

Results

Unconditional logistic regression analysis showed that the XAB2 genotype with rs794078 AA or at least one rs4134816 C allele were associated with the decreased risk of NSCLC with OR (95 % CI) of 0.12 (0.03–0.54) and 0.46 (0.26–0.84). When stratified by gender, we found that the subjects carrying rs4134816 CC or CT genotype had a decreased risk for developing NSCLC among males with OR (95 % CI) of 0.39 (0.18–0.82), but not among females. In age stratification analysis, we found that younger subjects (age ≤ 60) with at least one C allele had a decreased risk of NSCLC with OR (95 % CI) of 0.35 (0.17–0.74), but older subjects didn’t. We didn’t find that XAB2 4134816 C > T variant effect on the risk of NSCLC when stratified by smoking status. The environmental factors, such as age, sex and smoking had no effect on the risk of NSCLC related to XAB2 genotypes at other polymorphic sites.

Conclusions

The XAB2 tagSNPs (rs794078 and rs4134816) were significantly associated with the risk of NSCLC in Chinese population, which supports the XAB2 plays a significant role in the development of NSCLC.

Oxidative stress, bone marrow failure, and genome instability in hematopoietic stem cells

Reactive oxygen species (ROS) can be generated by defective endogenous reduction of oxygen by cellular enzymes or in the mitochondrial respiratory pathway, as well as by exogenous exposure to UV or environmental damaging agents. Regulation of intracellular ROS levels is critical since increases above normal concentrations lead to oxidative stress and DNA damage. A growing body of evidence indicates that the inability to regulate high levels of ROS leading to alteration of cellular homeostasis or defective repair of ROS-induced damage lies at the root of diseases characterized by both neurodegeneration and bone marrow failure as well as cancer. That these diseases may be reflective of the dynamic ability of cells to respond to ROS through developmental stages and aging lies in the similarities between phenotypes at the cellular level. This review summarizes work linking the ability to regulate intracellular ROS to the hematopoietic stem cell phenotype, aging, and disease.

Effects of benzo[a]pyrene on mouse germ cells: heritable DNA mutation, testicular cell hypomethylation and their interaction with nucleotide excision repair

Benzo[a]pyrene induces heritable mutations in male germ cells.

Premature skin aging features rescued by inhibition of NADPH oxidase activity in XPC-deficient mice

Xeroderma pigmentosum type C (XP-C) is characterized mostly by a predisposition to skin cancers and accelerated photoaging, but little is known about premature skin aging in this disease. By comparing young and old mice, we found that the level of progerin and p16(INK4a) expression, β-galactosidase activity, and reactive oxygen species, which increase with age, were higher in young Xpc(-/-) mice than in young Xpc(+/+) ones. The expression level of mitochondrial complexes and mitochondrial functions in the skin of young Xpc(-/-) was as low as in control aged Xpc(+/+)animals. Furthermore, the metabolic profile in young Xpc(-/-) mice resembled that found in aged Xpc(+/+) mice. Furthermore, premature skin aging features in young Xpc(-/-) mice were mostly rescued by inhibition of nicotinamide adenine dinucleotide phosphate oxidase 1 (NOX1) activity by using a NOX1 peptide inhibitor, suggesting that the continuous oxidative stress due to overactivation of NOX1 has a causative role in the underlying pathophysiology.

Cell-autonomous progeroid changes in conditional mouse models for repair endonuclease XPG deficiency

As part of the Nucleotide Excision Repair (NER) process, the endonuclease XPG is involved in repair of helix-distorting DNA lesions, but the protein has also been implicated in several other DNA repair systems, complicating genotype-phenotype relationship in XPG patients. Defects in XPG can cause either the cancer-prone condition xeroderma pigmentosum (XP) alone, or XP combined with the severe neurodevelopmental disorder Cockayne Syndrome (CS), or the infantile lethal cerebro-oculo-facio-skeletal (COFS) syndrome, characterized by dramatic growth failure, progressive neurodevelopmental abnormalities and greatly reduced life expectancy. Here, we present a novel (conditional) Xpg^−/− mouse model which -in a C57BL6/FVB F1 hybrid genetic background- displays many progeroid features, including cessation of growth, loss of subcutaneous fat, kyphosis, osteoporosis, retinal photoreceptor loss, liver aging, extensive neurodegeneration, and a short lifespan of 4–5 months. We show that deletion of XPG specifically in the liver reproduces the progeroid features in the liver, yet abolishes the effect on growth or lifespan. In addition, specific XPG deletion in neurons and glia of the forebrain creates a progressive neurodegenerative phenotype that shows many characteristics of human XPG deficiency. Our findings therefore exclude that both the liver as well as the neurological phenotype are a secondary consequence of derailment in other cell types, organs or tissues (e.g. vascular abnormalities) and support a cell-autonomous origin caused by the DNA repair defect itself. In addition they allow the dissection of the complex aging process in tissue- and cell-type-specific components. Moreover, our data highlight the critical importance of genetic background in mouse aging studies, establish the Xpg^−/− mouse as a valid model for the severe form of human XPG patients and segmental accelerated aging, and strengthen the link between DNA damage and aging.

Accumulation of DNA damage has been implicated in aging. Many premature aging syndromes are due to defective DNA repair systems. The endonuclease XPG is involved in repair of helix-distorting DNA lesions, and XPG defects cause the cancer-prone condition xeroderma pigmentosum (XP) alone or combined with the severe neurodevelopmental progeroid disorder Cockayne syndrome (CS). Here, we present a novel (conditional) Xpg^−/− mouse model which -in a C57BL6/FVB F1 hybrid background- displays many progressive progeroid features, including early cessation of growth, cachexia, kyphosis, osteoporosis, neurodegeneration, liver aging, retinal degeneration, and reduced lifespan. In a constitutive mutant with a complex phenotype it is difficult to dissect cause and consequence. We have therefore generated liver- and forebrain-specific Xpg mutants and demonstrate that they exhibit progressive anisokaryosis and neurodegeneration, respectively, indicating that a cell-intrinsic repair defect in neurons can account for neuronal degeneration. These findings strengthen the link between DNA damage and the complex process of aging.

Aging on a different scale--chronological versus pathology-related aging

In the next decades the elderly population will increase dramatically, demanding appropriate solutions in health care and aging research focusing on healthy aging to prevent high burdens and costs in health care. For this, research targeting tissue-specific and individual aging is paramount to make the necessary progression in aging research. In a recently published study we have attempted to make a step interpreting aging data on chronological as well as pathological scale. For this, we sampled five major tissues at regular time intervals during the entire C57BL/6J murine lifespan from a controlled in vivo aging study, measured the whole transcriptome and incorporated temporal as well as physical health aspects into the analyses. In total, we used 18 different age-related pathological parameters and transcriptomic profiles of liver, kidney, spleen, lung and brain and created a database that can now be used for a broad systems biology approach. In our study, we focused on the dynamics of biological processes during chronological aging and the comparison between chronological and pathology-related aging.

Unravelling mechanisms of p53-mediated tumour suppression

p53 is a crucial tumour suppressor that responds to diverse stress signals by orchestrating specific cellular responses, including transient cell cycle arrest, cellular senescence and apoptosis, which are all processes associated with tumour suppression. However, recent studies have challenged the relative importance of these canonical cellular responses for p53-mediated tumour suppression and have highlighted roles for p53 in modulating other cellular processes, including metabolism, stem cell maintenance, invasion and metastasis, as well as communication within the tumour microenvironment. In this Opinion article, we discuss the roles of classical p53 functions, as well as emerging p53-regulated processes, in tumour suppression.

Life spanning murine gene expression profiles in relation to chronological and pathological aging in multiple organs

Aging and age-related pathology is a result of a still incompletely understood intricate web of molecular and cellular processes. We present a C57BL/6J female mice in vivo aging study of five organs (liver, kidney, spleen, lung, and brain), in which we compare genome-wide gene expression profiles during chronological aging with pathological changes throughout the entire murine life span (13, 26, 52, 78, 104, and 130 weeks). Relating gene expression changes to chronological aging revealed many differentially expressed genes (DEGs), and altered gene sets (AGSs) were found in most organs, indicative of intraorgan generic aging processes. However, only ≤ 1% of these DEGs are found in all organs. For each organ, at least one of 18 tested pathological parameters showed a good age-predictive value, albeit with much inter- and intraindividual (organ) variation. Relating gene expression changes to pathology-related aging revealed correlated genes and gene sets, which made it possible to characterize the difference between biological and chronological aging. In liver, kidney, and brain, a limited number of overlapping pathology-related AGSs were found. Immune responses appeared to be common, yet the changes were specific in most organs. Furthermore, changes were observed in energy homeostasis, reactive oxygen species, cell cycle, cell motility, and DNA damage. Comparison of chronological and pathology-related AGSs revealed substantial overlap and interesting differences. For example, the presence of immune processes in liver pathology-related AGSs that were not detected in chronological aging. The many cellular processes that are only found employing aging-related pathology could provide important new insights into the progress of aging.

Slow accumulation of mutations in Xpc-/- mice upon induction of oxidative stress

XPC is one of the key DNA damage recognition proteins in the global genome repair route of the nucleotide excision repair (NER) pathway. Previously, we demonstrated that NER-deficient mouse models Xpa(-/-) and Xpc(-/-) exhibit a divergent spontaneous tumor spectrum and proposed that XPC might be functionally involved in the defense against oxidative DNA damage. Others have mechanistically dissected several functionalities of XPC to oxidative DNA damage sensitivity using in vitro studies. XPC has been linked to regulation of base excision repair (BER) activity, redox homeostasis and recruitment of ATM and ATR to damage sites, thereby possibly regulating cell cycle checkpoints and apoptosis. XPC has additionally been implicated in recognition of bulky (e.g. cyclopurines) and non-bulky DNA damage (8-oxodG). However, the ultimate contribution of the XPC functionality in vivo in the oxidative DNA damage response and subsequent mutagenesis process remains unclear. Our study indicates that Xpc(-/-) mice, in contrary to Xpa(-/-) and wild type mice, have an increased mutational load upon induction of oxidative stress and that mutations arise in a slowly accumulative fashion. The effect of non-functional XPC in vivo upon oxidative stress exposure appears to have implications in mutagenesis, which can contribute to the carcinogenesis process. The levels and rate of mutagenesis upon oxidative stress correlate with previous findings that lung tumors in Xpc(-/-) mice overall arise late in the lifespan and that the incidence of internal tumors in XP-C patients is relatively low in comparison to skin cancer incidence.

Oxidative DNA damage and nucleotide excision repair

SIGNIFICANCE

Oxidative DNA damage is repaired by multiple, overlapping DNA repair pathways. Accumulating evidence supports the hypothesis that nucleotide excision repair (NER), besides base excision repair (BER), is also involved in neutralizing oxidative DNA damage.

RECENT ADVANCES

NER includes two distinct sub-pathways: transcription-coupled NER (TC-NER) and global genome repair (GG-NER). The CSA and CSB proteins initiate the onset of TC-NER. Recent findings show that not only CSB, but also CSA is involved in the repair of oxidative DNA lesions, in the nucleus as well as in mitochondria. The XPG protein is also of importance for the removal of oxidative DNA lesions, as it may enhance the initial step of BER. Substantial evidence exists that support a role for XPC in NER and BER. XPC deficiency not only results in decreased repair of oxidative lesions, but has also been linked to disturbed redox homeostasis.

CRITICAL ISSUES

The role of NER proteins in the regulation of the cellular response to oxidative (mitochondrial and nuclear) DNA damage may be the underlying mechanism of the pathology of accelerated aging in Cockayne syndrome patients, a driving force for internal cancer development in XP-A and XP-C patients, and a contributor to the mixed exhibited phenotypes of XP-G patients.

FUTURE DIRECTIONS

Accumulating evidence indicates that DNA repair factors can be involved in multiple DNA repair pathways. However, the distinct detailed mechanism and consequences of these additional functions remain to be elucidated and can possibly shine a light on clinically related issues.

Nucleotide excision repair-initiating proteins bind to oxidative DNA lesions in vivo

In vivo imaging reveals that CSB and XPC promote the repair of oxidative DNA lesions independent of the canonical nucleotide excision repair process.

Base excision repair (BER) is the main repair pathway to eliminate abundant oxidative DNA lesions such as 8-oxo-7,8-dihydroguanine. Recent data suggest that the key transcription-coupled nucleotide excision repair factor (TC-NER) Cockayne syndrome group B (CSB) and the global genome NER-initiating factor XPC are implicated in the protection of cells against oxidative DNA damages. Our novel live-cell imaging approach revealed a strong and very rapid recruitment of XPC and CSB to sites of oxidative DNA lesions in living cells. The absence of detectable accumulation of downstream NER factors at the site of local oxidative DNA damage provide the first in vivo indication of the involvement of CSB and XPC in the repair of oxidative DNA lesions independent of the remainder of the NER reaction. Interestingly, CSB exhibited different and transcription-dependent kinetics in the two compartments studied (nucleolus and nucleoplasm), suggesting a direct transcription-dependent involvement of CSB in the repair of oxidative lesions associated with different RNA polymerases but not involving other NER proteins.

The cross talk between pathways in the repair of 8-oxo-7,8-dihydroguanine in mouse and human cells

Although oxidatively damaged DNA is repaired primarily via the base excision repair (BER) pathway, it is now evident that multiple subpathways are needed. Yet, their relative contributions and coordination are still unclear. Here, mouse embryo fibroblasts (MEFs) from selected nucleotide excision repair (NER) and/or BER mouse mutants with severe (Csb(m/m)/Xpa(-/-) and Csb(m/m)/Xpc(-/-)), mild (Csb(m/m)), or no progeria (Xpa(-/-), Xpc(-/-), Ogg1(-/-), Csb(m/m)/Ogg1(-/-)) or wild-type phenotype were exposed to an oxidizing agent, potassium bromate, and genomic 8-oxo-7,8-dihydroguanine (8-oxoGua) levels were measured by HPLC-ED. The same oxidized DNA base was measured in NER/BER-defective human cell lines obtained after transfection with replicative plasmids encoding siRNA targeting DNA repair genes. We show that both BER and NER factors contribute to the repair of 8-oxoGua, although to different extents, and that the repair profiles are similar in human compared to mouse cells. The BER DNA glycosylase OGG1 dominates 8-oxoGua repair, whereas NER (XPC, XPA) and transcription-coupled repair proteins (CSB and CSA) are similar, but minor contributors. The comparison of DNA oxidation levels in double versus single defective MEFs indicates increased oxidatively damaged DNA only when both CSB and XPC/XPA are defective, indicating that these proteins operate in different pathways. Moreover, we provide the first evidence of an involvement of XPA in the control of oxidatively damaged DNA in human primary cells.

Benzo[a]pyrene (BP) DNA adduct formation in DNA repair-deficient p53 haploinsufficient [Xpa(-/-)p53(+/-)] and wild-type mice fed BP and BP plus chlorophyllin for 28 days

We have evaluated DNA damage (DNA adduct formation) after feeding benzo[a]pyrene (BP) to wild-type (WT) and cancer-susceptible Xpa(-/-)p53(+/-) mice deficient in nucleotide excision repair and haploinsufficient for the tumor suppressor p53. DNA damage was evaluated by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry (HPLC/ES-MS/MS), which measures r7,t8,t9-trihydroxy-c-10-(N (2)-deoxyguanosyl)-7,8,9,10-tetrahydrobenzo[a]pyrene (BPdG), and a chemiluminescence immunoassay (CIA), using anti-r7,t8-dihydroxy-t-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE)-DNA antiserum, which measures both BPdG and the other stable BP-DNA adducts. When mice were fed 100 ppm BP for 28 days, BP-induced DNA damage measured in esophagus, liver and lung was typically higher in Xpa(-/-)p53(+/-) mice, compared with WT mice. This result is consistent with the previously observed tumor susceptibility of Xpa(-/-)p53(+/-) mice. BPdG, the major DNA adduct associated with tumorigenicity, was the primary DNA adduct formed in esophagus (a target tissue in the mouse), whereas total BP-DNA adducts predominated in higher levels in the liver (a non-target tissue in the mouse). In an attempt to lower BP-induced DNA damage, we fed the WT and Xpa(-/-)p53(+/-) mice 0.3% chlorophyllin (CHL) in the BP-containing diet for 28 days. The addition of CHL resulted in an increase of BP-DNA adducts in esophagus, liver and lung of WT mice, a lowering of BPdG in esophagi of WT mice and livers of Xpa(-/-)p53(+/-) mice and an increase of BPdG in livers of WT mice. Therefore, the addition of CHL to a BP-containing diet showed a lack of consistent chemoprotective effect, indicating that oral CHL administration may not reduce PAH-DNA adduct levels consistently in human organs.

The structure of the XPF-ssDNA complex underscores the distinct roles of the XPF and ERCC1 helix- hairpin-helix domains in ss/ds DNA recognition

Human XPF/ERCC1 is a structure-specific DNA endonuclease that nicks the damaged DNA strand at the 5' end during nucleotide excision repair. We determined the structure of the complex of the C-terminal domain of XPF with 10 nt ssDNA. A positively charged region within the second helix of the first HhH motif contacts the ssDNA phosphate backbone. One guanine base is flipped out of register and positioned in a pocket contacting residues from both HhH motifs of XPF. Comparison to other HhH-containing proteins indicates a one-residue deletion in the second HhH motif of XPF that has altered the hairpin conformation, thereby permitting ssDNA interactions. Previous nuclear magnetic resonance studies showed that ERCC1 in the XPF-ERCC1 heterodimer can bind dsDNA. Combining the two observations gives a model that underscores the asymmetry of the human XPF/ERCC1 heterodimer in binding at an ss/ds DNA junction.

Age-related neuronal degeneration: complementary roles of nucleotide excision repair and transcription-coupled repair in preventing neuropathology

Neuronal degeneration is a hallmark of many DNA repair syndromes. Yet, how DNA damage causes neuronal degeneration and whether defects in different repair systems affect the brain differently is largely unknown. Here, we performed a systematic detailed analysis of neurodegenerative changes in mouse models deficient in nucleotide excision repair (NER) and transcription-coupled repair (TCR), two partially overlapping DNA repair systems that remove helix-distorting and transcription-blocking lesions, respectively, and that are associated with the UV-sensitive syndromes xeroderma pigmentosum (XP) and Cockayne syndrome (CS). TCR–deficient Csa^−/− and Csb^−/− CS mice showed activated microglia cells surrounding oligodendrocytes in regions with myelinated axons throughout the nervous system. This white matter microglia activation was not observed in NER–deficient Xpa^−/− and Xpc^−/− XP mice, but also occurred in Xpd^XPCS mice carrying a point mutation (G602D) in the Xpd gene that is associated with a combined XPCS disorder and causes a partial NER and TCR defect. The white matter abnormalities in TCR–deficient mice are compatible with focal dysmyelination in CS patients. Both TCR–deficient and NER–deficient mice showed no evidence for neuronal degeneration apart from p53 activation in sporadic (Csa^−/−, Csb^−/−) or highly sporadic (Xpa^−/−, Xpc^−/−) neurons and astrocytes. To examine to what extent overlap occurs between both repair systems, we generated TCR–deficient mice with selective inactivation of NER in postnatal neurons. These mice develop dramatic age-related cumulative neuronal loss indicating DNA damage substrate overlap and synergism between TCR and NER pathways in neurons, and they uncover the occurrence of spontaneous DNA injury that may trigger neuronal degeneration. We propose that, while Csa^−/− and Csb^−/− TCR–deficient mice represent powerful animal models to study the mechanisms underlying myelin abnormalities in CS, neuron-specific inactivation of NER in TCR–deficient mice represents a valuable model for the role of NER in neuronal maintenance and survival.

Metabolism produces reactive oxygen species that damage our DNA and other cellular components, and as such it contributes to the aging process, including neuronal degeneration. Accordingly, genetic disorders associated with impaired DNA damage repair are frequently associated with premature onset of aging pathology in a variety of tissues, including the brain. This is well-illustrated by the progeroid DNA repair syndromes xeroderma pigmentosum (XP) and Cockayne syndrome (CS), in which patients suffer from defects in nucleotide excision repair (NER) and transcription-coupled repair (TCR), two partially overlapping DNA repair systems that remove helix-distorting and transcription-blocking lesions, respectively. We have used a panel of XP and CS mice (including conditional double-mutant animals) to systematically investigate the impact of NER and TCR defects on neuronal degeneration. We have shown that, whereas a TCR defect causes white matter pathology, a NER defect can result in age related cumulative loss of neurons. These findings well match the neuropathology observed in CS and XP patients, underscoring the impact of spontaneous DNA damage in the onset of neuronal aging. Therefore, the XP and CS mouse models serve as valuable tools to delineate intervention strategies that combat age-associated pathology of the brain.

Role of nucleotide excision repair proteins in oxidative DNA damage repair: an updating

DNA repair is a crucial factor in maintaining a low steady-state level of oxidative DNA damage. Base excision repair (BER) has an important role in preventing the deleterious effects of oxidative DNA damage, but recent evidence points to the involvement of several repair pathways in this process. Oxidative damage may arise from endogenous and exogenous sources and may target nuclear and mitochondrial DNA as well as RNA and proteins. The importance of preventing mutations associated with oxidative damage is shown by a direct association between defects in BER (i.e. MYH DNA glycosylase) and colorectal cancer, but it is becoming increasingly evident that damage by highly reactive oxygen species plays also central roles in aging and neurodegeneration. Mutations in genes of the nucleotide excision repair (NER) pathway are associated with diseases, such as xeroderma pigmentosum and Cockayne syndrome, that involve increased skin cancer and/or developmental and neurological symptoms. In this review we will provide an updating of the current evidence on the involvement of NER factors in the control of oxidative DNA damage and will attempt to address the issue of whether this unexpected role may unlock the difficult puzzle of the pathogenesis of these syndromes.

The role of XPC: implications in cancer and oxidative DNA damage

The accumulation of DNA damage is a slow but hazardous phenomenon that may lead to cell death, accelerated aging features and cancer. One of the most versatile and important defense mechanisms against the accumulation of DNA damage is nucleotide excision repair (NER), in which the Xeroderma pigmentosum group C (XPC) protein plays a prominent role. NER can be divided into global genome repair (GG-NER) and transcription coupled repair (TC-NER). XPC is a key factor in GG-NER where it functions in DNA damage recognition and after which the repair machinery is recruited to eliminate the DNA damage. Defective XPC functioning has been shown to result in a cancer prone phenotype, in human as well as in mice. Mutation accumulation in XPC deficient mice is accelerated and increased, resulting in an increased tumor incidence. More recently XPC has also been linked to functions outside of NER since XPC deficient mice show a divergent tumor spectrum compared to other NER deficient mouse models. Multiple in vivo and in vitro experiments indicate that XPC appears to be involved in the initiation of several DNA damage-induced cellular responses. XPC seems to function in the removal of oxidative DNA damage, redox homeostasis and cell cycle control. We hypothesize that this combination of increased oxidative DNA damage sensitivity, disturbed redox homeostasis together with inefficient cell cycle control mechanisms are causes of the observed increased cancer susceptibility in oxygen exposed tissues. Such a phenotype is absent in other NER-deficient mice, including Xpa.

The neuro-glial-vascular interrelations in genomic instability symptoms

A hallmark of neurodegenerative diseases is impairment of certain aspects of "brain functionality", which is defined as the total input and output of the brain's neural circuits and networks. A given neurodegenerative disorder is characterized by affected network organization and topology, cell numbers, cellular functionality, and the interactions between neural circuits. Neuroscientists generally view neurodegenerative disorders as diseases of neuronal cells; however, recent advances suggest a role for glial cells and an impaired vascular system in the etiology of certain neurodegenerative diseases. It is now clear that brain pathology is, to a very great extent, pathology of neurons, glia and the vascular system as these determine the degree of neuronal death as well as the outcome and scale of the neurological deficit. This review article is focused on the intricate interrelations among neurons, glia, the vascular system, neuronal cells, and the DNA damage response. Here I describe various aspects of neural and glial cell fate and the vascular system in genomic instability disorders including ataxia telangiectasia (A-T) and Nijmegen breakage syndrome.

Broad segmental progeroid changes in short-lived Ercc1(-/Δ7) mice

Genome maintenance is considered a prime longevity assurance mechanism as apparent from many progeroid human syndromes that are caused by genome maintenance defects. The ERCC1 protein is involved in three genome maintenance systems: nucleotide excision repair, interstrand cross-link repair, and homologous recombination. Here we describe in-life and post-mortem observations for a hypomorphic Ercc1 variant, Ercc1(-/Δ7), which is hemizygous for a single truncated Ercc1 allele, encoding a protein lacking the last seven amino acids. Ercc1(-/Δ7) mice were much smaller and median life span was markedly reduced compared to wild-type siblings: 20 and 118 weeks, respectively. Multiple signs and symptoms of aging were found to occur at an accelerated rate in the Ercc1(-/Δ7) mice as compared to wild-type controls, including a decline in weight of both whole body and various organs, numerous histopathological lesions, and immune parameters. Together they define a segmental progeroid phenotype of the Ercc1(-/Δ7) mouse model.

In vitro functional effects of XPC gene rare variants from bladder cancer patients

The XPC gene is involved in repair of bulky DNA adducts formed by carcinogenic metabolites and oxidative DNA damage, both known bladder cancer risk factors. Single nucleotide polymorphisms (SNPs) in XPC have been associated with increased bladder cancer risk. Recently, rarer genetic variants have been identified but it is difficult to ascertain which are of functional importance. During a mutation screen of XPC in DNA from 33 bladder tumour samples and matched blood samples, we identified five novel variants in the patients' germ line DNA. In a case-control study of 771 bladder cancer cases and 800 controls, c.905T>C (Phe302Ser), c.1177C>T (Arg393Trp), c.*156G>A [3' untranslated region (UTR)] and c.2251-37C>A (in an intronic C>G SNP site) were found to be rare variants, with a combined odds ratio of 3.1 (95% confidence interval 1.0-9.8, P=0.048) for carriage of one variant. The fifth variant was a 2% minor allele frequency SNP not associated with bladder cancer. The two non-synonymous coding variants were predicted to have functional effects using analytical algorithms; a reduced recruitment of GFP-tagged XPC plasmids containing either c.905T>C or c.1177C>T to sites of 408 nm wavelength laser-induced oxidative DNA damage was found in vitro. c.*156G>A appeared to be associated with reduced messenger RNA stability in an in vitro plasmid-based assay. Although the laser microbeam assay is relevant to a range of DNA repair genes, our 3' UTR assay based on Green fluorescent protein(GFP) has widespread applicability and could be used to assess any gene. These assays may be useful in determining which rare variants are functional, prior to large genotyping efforts.

XPC silencing in normal human keratinocytes triggers metabolic alterations through NOX-1 activation-mediated reactive oxygen species

Cancer cells utilize complex mechanisms to remodel their bioenergetic properties. We exploited the intrinsic genomic stability of xeroderma pigmentosum C (XPC) to understand the inter-relationships between genomic instability, reactive oxygen species (ROS) generation, and metabolic alterations during neoplastic transformation. We showed that knockdown of XPC (XPC(KD)) in normal human keratinocytes results in metabolism remodeling through NADPH oxidase-1 (NOX-1) activation, which in turn leads to increased ROS levels. While enforcing antioxidant defenses by overexpressing catalase, CuZnSOD, or MnSOD could not block the metabolism remodeling, impaired NOX-1 activation abrogates both alteration in ROS levels and modifications of energy metabolism. As NOX-1 activation is observed in human squamous cell carcinomas (SCCs), the blockade of NOX-1 could be a target for the prevention and the treatment of skin cancers.

XPC silencing in normal human keratinocytes triggers metabolic alterations that drive the formation of squamous cell carcinomas

DNA damage is a well-known initiator of tumorigenesis. Studies have shown that most cancer cells rely on aerobic glycolysis for their bioenergetics. We sought to identify a molecular link between genomic mutations and metabolic alterations in neoplastic transformation. We took advantage of the intrinsic genomic instability arising in xeroderma pigmentosum C (XPC). The XPC protein plays a key role in recognizing DNA damage in nucleotide excision repair, and patients with XPC deficiency have increased incidence of skin cancer and other malignancies. In cultured human keratinocytes, we showed that lentivirus-mediated knockdown of XPC reduced mitochondrial oxidative phosphorylation and increased glycolysis, recapitulating cancer cell metabolism. Accumulation of unrepaired DNA following XPC silencing increased DNA-dependent protein kinase activity, which subsequently activated AKT1 and NADPH oxidase-1 (NOX1), resulting in ROS production and accumulation of specific deletions in mitochondrial DNA (mtDNA) over time. Subcutaneous injection of XPC-deficient keratinocytes into immunodeficient mice led to squamous cell carcinoma formation, demonstrating the tumorigenic potential of transduced cells. Conversely, simultaneous knockdown of either NOX1 or AKT1 blocked the neoplastic transformation induced by XPC silencing. Our results demonstrate that genomic instability resulting from XPC silencing results in activation of AKT1 and subsequently NOX1 to induce ROS generation, mtDNA deletions, and neoplastic transformation in human keratinocytes.

Oxygen as a friend and enemy: How to combat the mutational potential of 8-oxo-guanine

The maintenance of genetic stability is of crucial importance for any form of life. Prior to cell division in each mammalian cell, the process of DNA replication must faithfully duplicate the three billion bases with an absolute minimum of mistakes. Various environmental and endogenous agents, such as reactive oxygen species (ROS), can modify the structural properties of DNA bases and thus damage the DNA. Upon exposure of cells to oxidative stress, an often generated and highly mutagenic DNA damage is 7,8-dihydro-8-oxo-guanine (8-oxo-G). The estimated steady-state level of 8-oxo-G lesions is about 10(3) per cell/per day in normal tissues and up to 10(5) lesions per cell/per day in cancer tissues. The presence of 8-oxo-G on the replicating strand leads to frequent (10-75%) misincorporations of adenine opposite the lesion (formation of A:8-oxo-G mispairs), subsequently resulting in C:G to A:T transversion mutations. These mutations are among the most predominant somatic mutations in lung, breast, ovarian, gastric and colorectal cancers. Thus, in order to reduce the mutational burden of ROS, human cells have evolved base excision repair (BER) pathways ensuring (i) the correct and efficient repair of A:8-oxo-G mispairs and (ii) the removal of 8-oxo-G lesions from the genome. Very recently it was shown that MutY glycosylase homologue (MUTYH) and DNA polymerase lambda play a crucial role in the accurate repair of A:8-oxo-G mispairs. Here we review the importance of accurate BER of 8-oxo-G damage and its regulation in prevention of cancer.

Evaluation of benzo(a)pyrene-induced gene mutations in male germ cells

Polycyclic aromatic hydrocarbons (PAHs) are mutagenic in somatic cells, whereas it remains unclear whether PAHs induce mutations in male germ cells, subsequently increasing health risks in offspring. Although results from the classical specific locus test are negative or inconclusive, recent studies with environmentally exposed animals suggest that PAHs are mutagenic in sperm cells. Therefore, we studied whether benzo(a)pyrene (B[a]P) was able to induce gene mutations in testis and sperm cells of wild-type (Wt) and Xpc(-/-) mice containing the pUR288 lacZ reporter gene. Mice were exposed to B[a]P (13 mg/kg body weight, three times per week) during 1, 4, or 6 weeks and sacrificed 6 weeks after the final exposure to obtain mutations in sperm derived from B[a]P-exposed spermatogonial stem cells. The lacZ gene mutation assay was used to assess mutant frequencies in spleen, testis, and mature sperm, and (32)P-postlabeling was used for the detection of DNA adducts in testis. Successful exposure was confirmed by a dose-related higher mutant frequency in spleen of Xpc(-/-) mice as compared with Wt mice. Mutant frequencies were also increased in all ethyl nitrosourea-exposed samples, which were used as positive control. Although B[a]P-related DNA adducts were detected in testis, mutant frequencies were not increased. On the other hand, B[a]P increased mutant frequencies in sperm of Wt mice, but not in Xpc(-/-) mice, after 6 weeks exposure. Therefore, we conclude that B[a]P can induce gene mutations in spermatogonial cells of mice, but it remains to be elucidated whether these mutations can be transmitted to offspring.

Age-related motor neuron degeneration in DNA repair-deficient Ercc1 mice

Degeneration of motor neurons contributes to senescence-associated loss of muscle function and underlies human neurodegenerative conditions such as amyotrophic lateral sclerosis and spinal muscular atrophy. The identification of genetic factors contributing to motor neuron vulnerability and degenerative phenotypes in vivo are therefore important for our understanding of the neuromuscular system in health and disease. Here, we analyzed neurodegenerative abnormalities in the spinal cord of progeroid Ercc1^Δ/− mice that are impaired in several DNA repair systems, i.e. nucleotide excision repair, interstrand crosslink repair, and double strand break repair. Ercc1^Δ/− mice develop age-dependent motor abnormalities, and have a shortened life span of 6–7 months. Pathologically, Ercc1^Δ/− mice develop widespread astrocytosis and microgliosis, and motor neuron loss and denervation of skeletal muscle fibers. Degenerating motor neurons in many occasions expressed genotoxic-responsive transcription factors p53 or ATF3, and in addition, displayed a range of Golgi apparatus abnormalities. Furthermore, Ercc1^Δ/− motor neurons developed perikaryal and axonal intermediate filament abnormalities reminiscent of cytoskeletal pathology observed in aging spinal cord. Our findings support the notion that accumulation of DNA damage and genotoxic stress may contribute to neuronal aging and motor neuron vulnerability in human neuromuscular disorders.

XPC epigenetic silence coupled with p53 alteration has a significant impact on bladder cancer outcome

PURPOSE

Varied XPC genetics are related to bladder cancer susceptibility. We determined whether decreased XPC expression influences bladder cancer malignancy and clinical outcome.

MATERIALS AND METHODS

Changes in XPC and p53 expression were detected by immunochemistry in 108 bladder cancers, including 29 papillary neoplasms of low malignant potential, and 48 low and 31 high grade lesions, of which 47 were stage Ta-T1 and 61 were stage T2-T3. XPC mRNA and methylation were evaluated in fresh tissue by real-time reverse transcriptase and methylation specific polymerase chain reaction. The clinical value of altered XPC and p53 expression was analyzed in 66 bladder cancers, including 6 papillary neoplasms of low malignant potential, and 41 low and 19 high stage lesions, of which 26 were stage Ta-T1 and 40 were stage T2-T3, by the Kaplan-Meier method and Cox proportional hazards regression.

RESULTS

The XPC defect was associated with bladder cancer higher pathological grade, metastasis and p53 mutation. Patients with XPC(-)/p53(+) had shorter survival than those with bladder cancer without XPC(-)/p53(+) (p = 0.0127). Cox regression analysis showed that XPC expression may be a potential predictive factor for bladder cancer (p = 0.043). In bladder cancer xpc gene hypermethylation was significantly higher than in normal mucosa (p = 0.0437).

CONCLUSIONS

Lower mRNA may be the result of XPC hypermethylation in bladder cancer. Epigenetic defects in the XPC gene impact bladder cancer malignant behavior and may also predict poor outcome in some bladder cancer cases, as characterized by p53 pathway alteration.

XPC silencing sensitizes glioma cells to arsenic trioxide via increased oxidative damage

Arsenic exerts its cytotoxicity via the generation of reactive oxygen species and inhibition of DNA repair. How arsenic disturbs oxidative DNA damage repair is, however, unclear. We found that arsenic trioxide (ATO), like ultraviolet (UV) irradiation, induced the expression of xeroderma pigmentosum group C (XPC) but not of xeroderma pigmentosum A in a human glioma cell line, U87. To explore the role of XPC in the toxic effects of ATO, small interfering RNA was used to silence XPC (siXPC) in U87 cells. siXPC cells were more susceptible to UV irradiation and ATO-induced cell death than control cells. Increased siXPC cell death induced by ATO was accompanied by increased senescence and autophagy. Because increased DNA strand breaks in siXPC cells were observed only when cells were concomitantly treated with ATO and DNA repair inhibitors, XPC silencing apparently did not interfere with repair of ATO-induced DNA damage. Although intracellular ROS levels were not significantly enhanced in siXPC cells, ATO treatment did result in increased 8-hydroxy-2'-deoxyguanosine and hyperoxidized peroxiredoxin. Enhanced superoxide production and autophagy by ATO in siXPC cells were suppressed by co-incubation with N-acetylcysteine (NAC). Furthermore, XPC silencing caused decreased glutathione levels and increased catalase and Mn-superoxide dismutase activities. Increased catalase activity in siXPC cells was suppressed by ATO treatment. XPC silencing also enhanced reporter activity of activator protein-1, whereas enhanced activity was suppressed by NAC. Taken together, our results indicate that XPC silencing causes increased ATO susceptibility by disturbing redox homeostasis rather than reducing DNA repair.

XPC is involved in genome maintenance through multiple pathways in different tissues

In an attempt to evaluate the role of the Xpc gene in maintaining genomic stability in vivo under normal conditions, the age-dependent accumulation of spontaneous mutations in different tissues was analyzed in Xpc-deficient lacZ-transgenic mice. Brain, testis, and small intestine revealed no effects from the Xpc-deficiency, whereas liver, spleen, heart, and lung showed an enhanced age-related accumulation of mutations in Xpc-deficient mice. In the spleen, the effect was not obvious at 2 and 12 months of age, but became apparent at 23 months. The magnitude of the observed effect at an advanced age was similar in the liver, spleen and heart, but was comparatively smaller in the lung. Haploinsufficiency was observed in liver and spleen but not in heart and lung. Analysis of DNA sequences in the mutants revealed that the frequency of G:C to T:A changes were elevated in the liver and heart of Xpc-deficient aged mice, supporting the possible involvement of XPC in base excision repair of oxidized guanine. The occurrence of two or more mutations within a single lacZ gene was termed a multiple mutation and was also elevated in old Xpc-deficient mice. Among the clones examined, two mutant clones showed as many as four mutations within a short stretch of DNA. This is the first demonstration to support suggestions for the existence of a role for XPC in the suppression of multiple mutations. These multiple mutations could conceivably be generated by error-prone trans-lesional DNA synthesis. Overall, these results indicate that there may be diverse roles or mechanisms through which XPC participates in genome maintenance in different tissues.

Disorders of nucleotide excision repair: the genetic and molecular basis of heterogeneity

Mutations in genes on the nucleotide excision repair pathway are associated with diseases, such as xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy, that involve skin cancer and developmental and neurological symptoms. These mutations cause the defective repair of damaged DNA and increased transcription arrest but, except for skin cancer, the links between repair and disease have not been obvious. Widely different clinical syndromes seem to result from mutations in the same gene, even when the mutations result in complete loss of function. The mapping of mutations in recently solved protein structures has begun to clarify the links between the molecular defects and phenotypes, but the identification of additional sources of clinical variability is still necessary.

Polymorphisms in DNA repair genes, smoking, and bladder cancer risk: findings from the international consortium of bladder cancer

Tobacco smoking is the most important and well-established bladder cancer risk factor and a rich source of chemical carcinogens and reactive oxygen species that can induce damage to DNA in urothelial cells. Therefore, common variation in DNA repair genes might modify bladder cancer risk. In this study, we present results from meta-analyses and pooled analyses conducted as part of the International Consortium of Bladder Cancer. We included data on 10 single nucleotide polymorphisms corresponding to seven DNA repair genes from 13 studies. Pooled analyses and meta-analyses included 5,282 cases and 5,954 controls of non-Latino white origin. We found evidence for weak but consistent associations with ERCC2 D312N [rs1799793; per-allele odds ratio (OR), 1.10; 95% confidence interval (95% CI), 1.01-1.19; P = 0.021], NBN E185Q (rs1805794; per-allele OR, 1.09; 95% CI, 1.01-1.18; P = 0.028), and XPC A499V (rs2228000; per-allele OR, 1.10; 95% CI, 1.00-1.21; P = 0.044). The association with NBN E185Q was limited to ever smokers (interaction P = 0.002) and was strongest for the highest levels of smoking dose and smoking duration. Overall, our study provides the strongest evidence to date for a role of common variants in DNA repair genes in bladder carcinogenesis.