Xeroderma pigmentosum and the role of UV-induced DNA damage in skin cancer

The Current Status of DNA-Repair-Directed Precision Oncology Strategies in Epithelial Ovarian Cancers

Survival outcomes for patients with advanced ovarian cancer remain poor despite advances in chemotherapy and surgery. Platinum-based systemic chemotherapy can result in a response rate of up to 80%, but most patients will have recurrence and die from the disease. Recently, the DNA-repair-directed precision oncology strategy has generated hope for patients. The clinical use of poly(ADP-ribose) polymerase (PARP) inhibitors in BRCA germ-line-deficient and/or platinum-sensitive epithelial ovarian cancers has improved survival. However, the emergence of resistance is an ongoing clinical challenge. Here, we review the current clinical state of PARP inhibitors and other clinically viable targeted approaches in epithelial ovarian cancers.

A matter of delicate balance: Loss and gain of Cockayne syndrome proteins in premature aging and cancer

DNA repair genes are critical for preserving genomic stability and it is well established that mutations in DNA repair genes give rise to progeroid diseases due to perturbations in different DNA metabolic activities. Cockayne Syndrome (CS) is an autosomal recessive inheritance caused by inactivating mutations in CSA and CSB genes. This review will primarily focus on the two Cockayne Syndrome proteins, CSA and CSB, primarily known to be involved in Transcription Coupled Repair (TCR). Curiously, dysregulated expression of CS proteins has been shown to exhibit differential health outcomes: lack of CS proteins due to gene mutations invariably leads to complex premature aging phenotypes, while excess of CS proteins is associated with carcinogenesis. Thus it appears that CS genes act as a double-edged sword whose loss or gain of expression leads to premature aging and cancer. Future mechanistic studies on cell and animal models of CS can lead to potential biological targets for interventions in both aging and cancer development processes. Some of these exciting possibilities will be discussed in this review in light of the current literature.

Clinical and Mutational Spectrum of Xeroderma Pigmentosum in Egypt: Identification of Six Novel Mutations and Implications for Ancestral Origins

Xeroderma pigmentosum is a rare autosomal recessive skin disorder characterized by freckle-like dry pigmented skin, photosensitivity, and photophobia. Skin and ocular symptoms are confined to sun exposed areas of the body. Patients have markedly increased risk for UV-induced skin, ocular, and oral cancers. Some patients develop neurodegenerative symptoms, including diminished tendon reflexes and microcephaly. In this study, we describe clinical and genetic findings of 36 XP patients from Egypt, a highly consanguineous population from North Africa. Thorough clinical evaluation followed by Sanger sequencing of XPA and XPC genes were done. Six novel and seven previously reported mutations were identified. Phenotype-genotype correlation was investigated. We report clinical and molecular findings consistent with previous reports of countries sharing common population structure, and geographical and historical backgrounds with implications on common ancestral origins and historical migration flows. Clinical and genetic profiling improves diagnosis, management, counselling, and implementation of future targeted therapies.

On the relevance of hydroxyl radical to purine DNA damage

Hydroxyl radical (HO^•) is the most reactive toward DNA among the reactive oxygen species (ROS) generated in aerobic organisms by cellular metabolisms. HO^• is generated also by exogenous sources such as ionizing radiations. In this review we focus on the purine DNA damage by HO^• radicals. In particular, emphasis is given on mechanistic aspects for the various lesion formation and their interconnections. Although the majority of the purine DNA lesions like 8-oxo-purine (8-oxo-Pu) are generated by various ROS (including HO^•), the formation of 5',8-cyclopurine (cPu) lesions in vitro and in vivo relies exclusively on the HO^• attack. Methodologies generally utilized for the purine lesions quantification in biological samples are reported and critically discussed. Recent results on cPu and 8-oxo-Pu lesions quantification in various types of biological specimens associated with the cellular repair efficiency as well as with distinct pathologies are presented, providing some insights on their biological significance.

Progressive length-dependent polyneuropathy in xeroderma pigmentosum group A

BACKGROUND

In this study, we aimed to investigate the progression of peripheral nervous system involvement in xeroderma pigmentosum group A (XP-A).

METHODS

We performed nerve conduction studies in 17 genetically confirmed XP-A patients and conducted follow-ups. Of these patients we also analyzed gray matter volume (GMV) using brain MRI and assessed the severity score of clinical and skin manifestation.

RESULTS

We found significant reduction in the motor and sensory nerve action potential amplitude and mild reduction in conduction velocity. These findings were predominant in sensory nerves and the lower limbs, were observed since early childhood, and gradually deteriorated with age.

CONCLUSIONS

The electrophysiological characteristics of XP-A patients are consistent with length-dependent axonal polyneuropathy and there is progressive deterioration from early childhood.

Whole-exome sequencing reveals the impact of UVA light mutagenesis in xeroderma pigmentosum variant human cells

UVA-induced mutagenesis was investigated in human pol eta-deficient (XP-V) cells through whole-exome sequencing. In UVA-irradiated cells, the increase in the mutation frequency in deficient cells included a remarkable contribution of C>T transitions, mainly at potential pyrimidine dimer sites. A strong contribution of C>A transversions, potentially due to oxidized bases, was also observed in non-irradiated XP-V cells, indicating that basal mutagenesis caused by oxidative stress may be related to internal tumours in XP-V patients. The low levels of mutations involving T induced by UVA indicate that pol eta is not responsible for correctly replicating T-containing pyrimidine dimers, a phenomenon known as the ‘A-rule’. Moreover, the mutation signature profile of UVA-irradiated XP-V cells is highly similar to the human skin cancer profile, revealing how studies involving cells deficient in DNA damage processing may be useful to understand the mechanisms of environmentally induced carcinogenesis.

Mitophagy and DNA damage signaling in human aging

Aging is associated with multiple human pathologies. In the past few years mitochondrial homeostasis has been well correlated with age-related disorders and longevity. Mitochondrial homeostasis involves generation, biogenesis and removal of dysfunctional mitochondria via mitophagy. Mitophagy is regulated by various mitochondrial and extra-mitochondrial factors including morphology, oxidative stress and DNA damage. For decades, DNA damage and inefficient DNA repair have been considered as major determinants for age-related disorders. Although defects in DNA damage recognition and repair and mitophagy are well documented to be major factors in age-associated diseases, interactivity between these is poorly understood. Mitophagy efficiency decreases with age leading to accumulation of dysfunctional mitochondria enhancing the severity of age-related disorders including neurodegenerative diseases, inflammatory diseases, cancer, diabetes and many more. Therefore, mitophagy is being targeted for intervention in age-associated disorders. NAD⁺ supplementation has emerged as one intervention to target both defective DNA repair and mitophagy. In this review, we discuss the molecular signaling pathways involved in regulation of DNA damage and repair and of mitophagy, and we highlight the opportunities for clinical interventions targeting these processes to improve the quality of life during aging.

Ipsilateral presentation of ocular surface squamous neoplasia and conjunctival melanoma in xeroderma pigmentosum: A rare occurrence

A case of a 16-year-old male with xeroderma pigmentosum (XP) who presented with multiple pigmented and nonpigmented conjunctival lesions in both eyes is reported. He had a keratinized lesion at the limbus and a pigmented lesion in bulbar conjunctiva in the left eye and multiple pigmented bulbar conjunctival lesions and a keratinized limbal nodule in the right eye. Excision biopsy confirmed the diagnosis of ocular surface squamous neoplasia (OSSN) and conjunctival intraepithelial melanocytic neoplasia-2 (CIMN-2) in the right eye and OSSN and conjunctival melanoma in situ (CIMN-5) in the left eye. Two malignant conjunctival lesions occurring simultaneously in the same eye of a patient with XP have not been reported earlier.

Unusual intraoral cancer with unexpected outcome in a patient with xeroderma pigmentosum: An alert for antineoplastic treatment

Xeroderma pigmentosum (XP) is a rare autosomal disorder characterized by extreme sensitivity to ultraviolet radiation. DNA repair mechanisms are impaired, and minimal sun exposure can lead to the development of cutaneous neoplasms in very young patients. Intraoral carcinomas are uncommon and, when present, are located mainly at the tongue tip. We report an unprecedented case of squamous cell carcinoma (SCC) in the floor of mouth of a 23-year-old woman with XP. The patient was referred to the oncologist, and 2 months after surgical resection, she underwent a single session of chemotherapy plus radiotherapy. However, she died 73 hours after undergoing her first chemotherapy session. Considering the unexpected outcome of this case, we also investigated possible exacerbated adverse effects of antineoplastic treatments (especially cisplatin-based chemotherapy) in patients with XP and reviewed the main characteristics of the disease, especially cases with oral manifestations reported in the literature.

High levels of oxidatively generated DNA damage 8,5'-cyclo-2'-deoxyadenosine accumulate in the brain tissues of xeroderma pigmentosum group A gene-knockout mice

Xeroderma pigmentosum (XP) is a genetic disorder associated with defects in nucleotide excision repair, a pathway that eliminates a wide variety of helix-distorting DNA lesions, including ultraviolet-induced pyrimidine dimers. In addition to skin diseases in sun-exposed areas, approximately 25% of XP patients develop progressive neurological disease, which has been hypothesized to be associated with the accumulation of an oxidatively generated type of DNA damage called purine 8,5'-cyclo-2'-deoxynucleoside (cyclopurine). However, that hypothesis has not been verified. In this study, we tested that hypothesis by using the XP group A gene-knockout (Xpa^-/-) mouse model. To quantify cyclopurine lesions in this model, we previously established an enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (CdA-1) that specifically recognizes 8,5'-cyclo-2'-deoxyadenosine (cyclo-dA). By optimizing conditions, we increased the ELISA sensitivity to a detection limit of ˜one cyclo-dA lesion/10⁶ nucleosides. The improved ELISA revealed that cyclo-dA lesions accumulate with age in the brain tissues of Xpa^-/- and of wild-type (wt) mice, but there were significantly more cyclo-dA lesions in Xpa^-/- mice than in wt mice at 6, 24 and 29 months of age. These findings are consistent with the long-standing hypothesis that the age-dependent accumulation of endogenous cyclopurine lesions in the brain may be critical for XP neurological abnormalities.

An Error-Prone Polymerase in the Fight against Cancer

Error-prone polymerases are alleged to induce mutations while replicating damaged DNA and to increase the risk of cancer. Using in vitro studies and mice models, Yoon et al. (2019) provide evidence that the error-prone Pol θ polymerase protects against ultraviolet light-induced skin cancer despite its mutagenic potential.

TFIIH: A multi-subunit complex at the cross-roads of transcription and DNA repair

Transcription factor IIH (TFIIH) is a multiprotein complex involved in both eukaryotic transcription and DNA repair, revealing a tight connection between these two processes. Composed of 10 subunits, it can be resolved into a 7-subunits core complex with the XPB translocase and the XPD helicase, and the 3-subunits kinase complex CAK, which also exists as a free complex with a distinct function. Initially identified as basal transcription factor, TFIIH also participates in transcription regulation and plays a key role in nucleotide excision repair (NER) for opening DNA at damaged sites, lesion verification and recruitment of additional repair factors. Our understanding of TFIIH function in eukaryotic cells has greatly benefited from studies of the genetic rare diseases xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD), that are not only characterized by cancer and aging predispositions but also by neurological and developmental defects. Although much remains unknown about TFIIH function, significant progresses have been done regarding the structure of the complex, the functions of its catalytic subunits and the multiple roles of the regulatory core-TFIIH subunits. This review provides a non-exhaustive survey of key discoveries on the structure and function of this pivotal factor, which can be considered as a promising target for therapeutic strategies.

Formation of UV-induced DNA damage contributing to skin cancer development

UV-induced DNA damage plays a key role in the initiation phase of skin cancer. When left unrepaired or when damaged cells are not eliminated by apoptosis, DNA lesions express their mutagneic properties, leading to the activation of proto-oncogene or the inactivation of tumor suppression genes. The chemical nature and the amount of DNA damage strongly depend on the wavelength of the incident photons. The most energetic part of the solar spectrum at the Earth's surface (UVB, 280-320 nm) leads to the formation of cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (64PPs). Less energetic but 20-times more intense UVA (320-400 nm) also induces the formation of CPDs together with a wide variety of oxidatively generated lesions such as single strand breaks and oxidized bases. Among those, 8-oxo-7,8-dihydroguanine (8-oxoGua) is the most frequent since it can be produced by several mechanisms. Data available on the respective yield of DNA photoproducts in cells and skin show that exposure to sunlight mostly induces pyrimidine dimers, which explains the mutational signature found in skin tumors, with lower amounts of 8-oxoGua and strand breaks. The present review aims at describing the basic photochemistry of DNA and discussing the quantitative formation of the different UV-induced DNA lesions reported in the literature. Additional information on mutagenesis, repair and photoprotection is briefly provided.

ATR/Chk1 Pathway is Activated by Oxidative Stress in Response to UVA Light in Human Xeroderma Pigmentosum Variant Cells

The crucial role of DNA polymerase eta in protecting against sunlight-induced tumors is evidenced in Xeroderma Pigmentosum Variant (XP-V) patients, who carry mutations in this protein and present increased frequency of skin cancer. XP-V cellular phenotypes may be aggravated if proteins of DNA damage response (DDR) pathway are blocked, as widely demonstrated by experiments with UVC light and caffeine. However, little is known about the participation of DDR in XP-V cells exposed to UVA light, the wavelengths patients are mostly exposed. Here, we demonstrate the participation of ATR kinase in protecting XP-V cells after receiving low UVA doses using a specific inhibitor, with a remarkable increase in sensitivity and γH2AX signaling. Corroborating ATR participation in UVA-DDR, a significant increase in Chk1 protein phosphorylation, as well as S-phase cell cycle arrest, is also observed. Moreover, the participation of oxidative stress is supported by the antioxidant action of N-acetylcysteine (NAC), which significantly protects XP-V cells from UVA light, even in the presence of the ATR inhibitor. These findings indicate that the ATR/Chk1 pathway is activated to control UVA-induced oxidatively generated DNA damage and emphasizes the role of ATR kinase as a mediator of genomic stability in pol eta defective cells.

The Morbid Anatomy of the Dermatologic Genome: An Update for the Third Millennium

Background:

Much progress has been made in recent years in the identification of genes underlying many hereditary skin diseases.

Objective:

To provide an update on the status of the identification of genes involved in hereditary skin disorders and to compare the current standing with that in the last decade.

Methods:

A review of the literature is presented here in a series of lists describing the chromosomal location, specific gene, clinical relevance, and availability of molecular-based genetic tests for each genodermatosis.

Results:

Progress has been made in identifying the genes underlying many disorders of cornification, genodermatoses with malignant potential, bullous disorders, pigmentary disorders, disorders affecting the epidermal appendages and the dermis, and other miscellaneous genodermatoses.

Conclusion:

The great progress made toward the completion of the human gene sequence and the continued efforts of many clinical and molecular scientists to identify disease genes will make diagnosis of hereditary dermatological disorders more precise and allow accurate family counseling as well as possibly leading to more targeted therapies during this millennium.

Human polyomaviruses and incidence of cutaneous squamous cell carcinoma in the New Hampshire skin cancer study

Squamous cell carcinoma (SCC) of the skin is a malignancy arising from epithelial keratinocytes. Experimental and epidemiologic evidence raise the possibility that human polyomaviruses (PyV) may be associated with the occurrence of SCC. To investigate whether the risk for SCC was associated with PyV infection, seropositivity to 10 PyV types was assessed following diagnosis in a population-based case-control study conducted in the United States. A total of 253 SCC cases and 460 age group and gender-matched controls were included. Antibody response against each PyV was measured using a multiplex serology-based glutathione S-transferase capture assay of recombinantly expressed VP1 capsid proteins. Odds ratios (OR) for SCC associated with seropositivity to each PyV type were estimated using logistic regression, with adjustment for potentially confounding factors. SCC cases were seropositive for a greater number of PyVs than controls (P = 0.049). Those who were JC seropositive had increased odds of SCC when compared to those who were JC seronegative (OR = 1.37, 95% CI: 0.98-1.90), with an increasing trend in SCC risk with increasing quartiles of seroreactivity (P for trend = 0.04). There were no clear associations between SCC risk and serostatus for other PyV types. This study provides limited evidence that infection with certain PyVs may be related to the occurrence of SCC in the general population of the United States.

Role of Oxidative RNA Damage in Chronic-Degenerative Diseases

Normal cellular metabolism and exposure to ionizing and ultraviolet radiations and exogenous agents produce reactive oxygen species (ROS). Due to their reactivity, they can interact with many critical biomolecules and induce cell damage. The reaction of ROS with free nucleobases, nucleosides, nucleotides, or oligonucleotides can generate numerous distinct modifications in nucleic acids. Oxidative damage to DNA has been widely investigated and is strongly implicated in the development of many chronic-degenerative diseases. In contrast, RNA damage is a poorly examined field in biomedical research. In this review, I discuss the importance of RNA as a target of oxidative damage and the role of oxidative damage to RNA in the pathogenesis of some chronic-degenerative diseases, such as neurological disorders, atherosclerosis, and cancer. Furthermore, I review recent evidence suggesting that RNA may be the target for toxic agents and indicating RNA degradation as a powerful tool to treat any pathology in which there is an aberrant expression of mRNA and/or its gene products.

Regulation and disregulation of mammalian nucleotide excision repair: a pathway to nongermline breast carcinogenesis

Nucleotide excision repair (NER) is an important modulator of disease, especially in constitutive deficiencies such as the cancer predisposition syndrome Xeroderma pigmentosum. We have found profound variation in NER capacity among normal individuals, between cell-types and during carcinogenesis. NER is a repair system for many types of DNA damage, and therefore many types of genotoxic carcinogenic exposures, including ultraviolet light, products of organic combustion, metals and oxidative stress. Because NER is intimately related to cellular metabolism, requiring components of both the DNA replicative and transcription machinery, it has a narrow range of functional viability. Thus, genes in the NER pathway are expressed at the low levels manifested by, for example, nuclear transcription factors. As NER activity and gene expression vary by cell-type, it is inherently epigenetically regulated. Furthermore, this epigenetic modulation is disregulated during sporadic breast carcinogenesis. Loss of NER is one basis of genomic instability, a required element in cellular transformation, and one that potentially influences response to therapy. In this study, we demonstrate differences in NER capacity in eight adult mouse tissues, and place this result into the context of our previous work on mouse extraembryonic tissues, normal human tissues and sporadic early stage human breast cancer.

Xeroderma pigmentosum: clinicopathological review of the multiple oculocutaneous malignancies and complications

Multiple oculo-cutaneous malignancies are a common manifestation on sun-exposed facial areas in patients with Xeroderma pigmentosum (XP). Commonly seen are the basal cell carcinoma and the squamous cell carcinomas which manifest in the early first decade in contrast to fifth and sixth decade in the general population. XP manifests as photosensitivity, hyperpigmentation, premature skin aging and malignant changes like squamous cell carcinoma, basal cell carcinoma, fibrosarcoma and rarely malignant melanoma as well as internal malignancies. We report 11 cases of Xeroderma pigmentosa managed in our institute which included sex males and five females. All had photosensitivity, hyperpigmentation and consanguinity with facial malignant lesions like SCC and BCC. Ocular signs of photophobia and excessive lacrimation was seen in all the cases while blurring of vision due to corneal clouding, corneal injection, pterygium and limbal SCC were seen in 5 cases. SCC of the lids were seen in 7 cases while BCC seen in 8 cases and limbal and conjunctival SCC seen in one case. All were managed with excision while one case of melanoma with neck secondaries needed radical neck dissection while the other orbital exenteration. Oculo-cutaneous malignancies occur in the sun exposed areas so patients are advised regular follow up with speciality care. Awareness about the rare condition and importance of early detection and prevention of UV rays induced skin damage should be propagated. The disease is ultimately fatal, life can be prolonged by simple preventive measures to minimize sun exposure and early detection of the skin lesions and management.

DNA repair diseases: What do they tell us about cancer and aging?

The discovery of DNA repair defects in human syndromes, initially in xeroderma pigmentosum (XP) but later in many others, led to striking observations on the association of molecular defects and patients' clinical phenotypes. For example, patients with syndromes resulting from defective nucleotide excision repair (NER) or translesion synthesis (TLS) present high levels of skin cancer in areas exposed to sunlight. However, some defects in NER also lead to more severe symptoms, such as developmental and neurological impairment and signs of premature aging. Skin cancer in XP patients is clearly associated with increased mutagenesis and genomic instability, reflecting the defective repair of DNA lesions. By analogy, more severe symptoms observed in NER-defective patients have also been associated with defective repair, likely involving cell death after transcription blockage of damaged templates. Endogenously induced DNA lesions, particularly through oxidative stress, have been identified as responsible for these severe pathologies. However, this association is not that clear and alternative explanations have been proposed. Despite high levels of exposure to intense sunlight, patients from tropical countries receive little attention or care, which likely also reflects the lack of understanding of how DNA damage causes cancer and premature aging.

Acetaldehyde and the genome: beyond nuclear DNA adducts and carcinogenesis

The designation of acetaldehyde associated with the consumption of alcoholic beverages as "carcinogenic to humans" (Group 1) by the International Agency for Research on Cancer (IARC) has brought renewed attention to the biological effects of acetaldehyde, as the primary oxidative metabolite of alcohol. Therefore, the overall focus of this review is on acetaldehyde and its direct and indirect effects on the nuclear and mitochondrial genome. We first consider different acetaldehyde-DNA adducts, including a critical assessment of the evidence supporting a role for acetaldehyde-DNA adducts in alcohol related carcinogenesis, and consideration of additional data needed to make a conclusion. We also review recent data on the role of the Fanconi anemia DNA repair pathway in protecting against acetaldehyde genotoxicity and carcinogenicity, as well as teratogenicity. We also review evidence from the older literature that acetaldehyde may impact the genome indirectly, via the formation of adducts with proteins that are themselves critically involved in the maintenance of genetic and epigenetic stability. Finally, we note the lack of information regarding acetaldehyde effects on the mitochondrial genome, which is notable since aldehyde dehydrogenase 2 (ALDH2), the primary acetaldehyde metabolic enzyme, is located in the mitochondrion, and roughly 30% of East Asian individuals are deficient in ALDH2 activity due to a genetic variant in the ALDH2 gene. In summary, a comprehensive understanding of all of the mechanisms by which acetaldehyde impacts the function of the genome has implications not only for alcohol and cancer, but types of alcohol related pathologies as well.

Quantitative and in situ detection of oxidatively generated DNA damage 8,5'-cyclo-2'-deoxyadenosine using an immunoassay with a novel monoclonal antibody

Xeroderma pigmentosum (XP) is a genetic disorder associated with defects in nucleotide excision repair, which eliminates a wide variety of helix-distorting types of DNA damage including sunlight-induced pyrimidine dimers. In addition to skin disease, approximately 30% of XP patients develop progressive neurological disease, which has been hypothesized to be associated with the accumulation of a particular type of oxidatively generated DNA damage called purine 8,5'-cyclo-2'-deoxynucleosides (purine cyclonucleosides). However, there are no currently available methods to detect purine cyclonucleosides in DNA without the need for DNA hydrolysis. In this study, we generated a novel monoclonal antibody (CdA-1) specific for purine cyclonucleosides in single-stranded DNA that recognizes 8,5'-cyclo-2'-deoxyadenosine (cyclo-dA). An immunoassay using CdA-1 revealed a linear dose response between known amounts of cyclo-dA in oligonucleotides and the antibody binding to them. The quantitative immunoassay revealed that treatment with Fenton-type reagents (CuCl(2)/H(2)O(2)/ascorbate) efficiently produces cyclo-dA in DNA in a dose-dependent manner. Moreover, immunofluorescent analysis using CdA-1 enabled the visualization of cyclo-dA in human osteosarcoma cells, which had been transfected with oligonucleotides containing cyclo-dA. Thus, the CdA-1 antibody is a valuable tool for the detection and quantification of cyclo-dA in DNA, and may be useful for characterizing the mechanism(s) underlying the development of XP neurological disease.

Post-transcriptional regulation of DNA damage-responsive gene expression

SIGNIFICANCE

Production of proteins requires the synthesis, maturation, and export of mRNAs before their translation in the cytoplasm. Endogenous and exogenous sources of DNA damage pose a challenge to the co-ordinated regulation of gene expression, because the integrity of the DNA template can be compromised by DNA lesions. Cells recognize and respond to this DNA damage through a variety of DNA damage responses (DDRs). Failure to deal with DNA damage appropriately can lead to genomic instability and cancer.

RECENT ADVANCES

The p53 tumor suppressor plays a dominant role in DDR-dependent changes in gene expression, but this transcription factor is not solely responsible for all changes. Recent evidence indicates that RNA metabolism is integral to DDRs as well. In particular, post-transcriptional processes are emerging as important contributors to these complex responses.

CRITICAL ISSUES

Transcriptional, post-transcriptional, and translational regulation of gene expression is subject to changes in response to DNA damage. How these processes are intertwined in the unfolding of DDR is not fully understood.

FUTURE DIRECTIONS

Many complex regulatory responses combine to determine cell fate after DNA damage. Understanding how transcriptional, post-transcriptional, and translational processes interdigitate to create a web of regulatory interactions will be one of the key challenges to fully understand DDRs.

Toxicogenomics and cancer susceptibility: advances with next-generation sequencing

The aim of this review is to comprehensively summarize the recent achievements in the field of toxicogenomics and cancer research regarding genetic-environmental interactions in carcinogenesis and detection of genetic aberrations in cancer genomes by next-generation sequencing technology. Cancer is primarily a genetic disease in which genetic factors and environmental stimuli interact to cause genetic and epigenetic aberrations in human cells. Mutations in the germline act as either high-penetrance alleles that strongly increase the risk of cancer development, or as low-penetrance alleles that mildly change an individual's susceptibility to cancer. Somatic mutations, resulting from either DNA damage induced by exposure to environmental mutagens or from spontaneous errors in DNA replication or repair are involved in the development or progression of the cancer. Induced or spontaneous changes in the epigenome may also drive carcinogenesis. Advances in next-generation sequencing technology provide us opportunities to accurately, economically, and rapidly identify genetic variants, somatic mutations, gene expression profiles, and epigenetic alterations with single-base resolution. Whole genome sequencing, whole exome sequencing, and RNA sequencing of paired cancer and adjacent normal tissue present a comprehensive picture of the cancer genome. These new findings should benefit public health by providing insights in understanding cancer biology, and in improving cancer diagnosis and therapy.

Does melanoma begin in a melanocyte stem cell?

What is the cellular origin of melanoma? What role do melanocyte stem cells (MSC) and other melanocyte precursors play in the development of melanoma? Are MSCs and other latent melanocyte precursors more susceptible to solar radiation? These and many other questions can be very effectively addressed using the zebrafish model. Zebrafish have a robust regenerative capability, permitting the study of how MSCs are regulated and recruited at specific times and places to generate the pigment pattern following fin amputation or melanocyte ablation. They can be used to determine the effects of environmental radiation on the proliferation, survival, repair, and differentiation of MSCs. Our lab is using zebrafish to investigate how UVA- (320-400 nm) and UVB- (290-320 nm) induced damage to MSCs may contribute to the development of melanoma. A review is given of MSCs in zebrafish as well as experimental techniques and drugs for manipulating MSC populations. These techniques can be used to design experiments to help answer many questions regarding the role of MSCs or melanocyte precursors in the formation of melanoma stem cells and tumors following exposure to UVA/UVB radiation.

c.1643_1644delTG XPC mutation is more frequent in Moroccan patients with xeroderma pigmentosum

Xeroderma pigmentosum is a rare autosomal recessive disease characterized by hypersensitivity to UV light which is due to alterations of the nucleotide excision repair pathway. Eight genes (XPA to XPG and XPV) are responsible for the disease. Among them, the XPC gene is known to be the most mutated in Mediterranean patients. The aim of this study was to determine the frequency of the most common XPC mutation and describe the clinical features of Moroccan patients with xeroderma pigmentosum. Twenty four patients belonging to 21 unrelated Moroccan families and 58 healthy subjects were investigated. After clinical examination, the screening for the c.1643_1644delTG (p.Val548AlafsX25) mutation in the XPC gene was performed by PCR and automated sequencing of exon 9 in all patients and controls. The molecular analysis showed that among the 24 patients, 17 were homozygous for the c.1643_1644delTG mutation and all their tested parents were heterozygous, whereas the others (7 patients) did not carry the mutation. The frequency of this mutation was estimated to be 76.19 % (16/21 families). None of the 58 healthy individuals carried this mutation. In addition, clinical investigation showed that the majority of the patients bearing this mutation have the same clinical features. Our results revealed that the p.Val548AlafsX25 mutation is the major cause (76.19 %) of xeroderma pigmentosum in Moroccan families. This would have an important impact on improving management of patients and their relatives.

Association between single nucleotide polymorphisms in ERCC4 and risk of squamous cell carcinoma of the head and neck

BACKGROUND

Excision repair cross-complementation group 4 gene (ERCC4/XPF) plays an important role in nucleotide excision repair and participates in removal of DNA interstrand cross-links and DNA double-strand breaks. Single nucleotide polymorphisms (SNPs) in ERCC4 may impact repair capacity and affect cancer susceptibility.

METHODOLOGY/PRINCIPAL FINDINGS

In this case-control study, we evaluated associations of four selected potentially functional SNPs in ERCC4 with risk of squamous cell carcinoma of the head and neck (SCCHN) in 1,040 non-Hispanic white patients with SCCHN and 1,046 cancer-free matched controls. We found that the variant GG genotype of rs2276466 was significantly associated with a decreased risk of SCCHN (OR = 0.69, 95% CI 0.50-0.96), and that the variant TT genotype of rs3136038 showed a borderline significant decreased risk with SCCHN (OR = 0.76, 95% CI: 0.58-1.01) in the recessive model. Such protective effects were more evident in oropharyngeal cancer (OR = 0.61, 95% CI: 0.40-0.92 for rs2276466; OR = 0.69, 95% CI: 0.48-0.98 for rs3136038). No significant associations were found for the other two SNPs (rs1800067 and rs1799798). In addition, individuals with the rs2276466 GG or with the rs3136038 TT genotypes had higher levels of ERCC4 mRNA expression than those with the corresponding wild-type genotypes in 90 Epstein-Barr virus-transformed lymphoblastoid cell lines derived from Caucasians.

CONCLUSIONS

These results suggest that these two SNPs (rs2276466 and rs3136038) in ERCC4 may be functional and contribute to SCCHN susceptibility. However, our findings need to be replicated in further large epidemiological and functional studies.

Shining a light on xeroderma pigmentosum

Xeroderma pigmentosum (XP) is a rare, autosomal recessive disorder of DNA repair characterized by sun sensitivity and UV radiation-induced skin and mucous membrane cancers. Initially described in 1874 by Moriz Kaposi in Vienna, nearly 100 years later, James Cleaver in San Francisco reported defective DNA repair in XP cells. This eventually provided the basis for a mechanistic link between sun exposure, DNA damage, somatic mutations, and skin cancer. XP cells were found to have defects in seven of the proteins of the nucleotide excision repair pathway and in DNA polymerase η. XP cells are hypersensitive to killing by UV radiation, and XP cancers have characteristic "UV signature" mutations. Clinical studies at the National Institutes of Health found a nearly 10,000-fold increase in skin cancer in XP patients under the age of 20 years, demonstrating the substantial importance of DNA repair in cancer prevention in the general population. Approximately 25% of XP patients have progressive neurological degeneration with progressive loss of neurons, probably from DNA damage induced by oxidative metabolism, which kills nondividing cells in the nervous system. Interestingly, patients with another disorder, trichothiodystrophy, have defects in some of the same genes as XP, but they have primary developmental abnormalities without an increase in skin cancer.

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.

DNA repair pathways and their implication in cancer treatment

Many cytotoxic agents used in cancer treatment exert their effects through their ability to directly or indirectly damage DNA and thus resulting in cell death. Major types of DNA damage induced by anticancer treatment include strand breaks (double or single strand), crosslinks (inter-strand, intra-strand, DNA-protein crosslinks), and interference with nucleotide metabolism and DNA synthesis. On the other hand, cancer cells activate various DNA repair pathways and repair DNA damages induced by cytotoxic drugs. The purpose of the current review is to present the major types of DNA damage induced by cytotoxic agents, DNA repair pathways, and their role as predictive agents, as well as evaluate the future perspectives of the novel DNA repair pathways inhibitors in cancer therapeutics.

Hydrogen peroxide induced genomic instability in nucleotide excision repair-deficient lymphoblastoid cells

BACKGROUND

The Nucleotide Excision Repair (NER) pathway specialises in UV-induced DNA damage repair. Inherited defects in the NER can predispose individuals to Xeroderma Pigmentosum (XP). UV-induced DNA damage cannot account for the manifestation of XP in organ systems not directly exposed to sunlight. While the NER has recently been implicated in the repair of oxidative DNA lesions, it is not well characterised. Therefore we sought to investigate the role of NER factors Xeroderma Pigmentosum A (XPA), XPB and XPD in oxidative DNA damage-repair by subjecting lymphoblastoid cells from patients suffering from XP-A, XP-D and XP-B with Cockayne Syndrome to hydrogen peroxide (H2O2).

RESULTS

Loss of functional XPB or XPD but not XPA led to enhanced sensitivity towards H2O2-induced cell death. XP-deficient lymphoblastoid cells exhibited increased susceptibility to H2O2-induced DNA damage with XPD showing the highest susceptibility and lowest repair capacity. Furthermore, XPB- and XPD-deficient lymphoblastoid cells displayed enhanced DNA damage at the telomeres. XPA- and XPB-deficient lymphoblastoid cells also showed differential regulation of XPD following H2O2 treatment.

CONCLUSIONS

Taken together, our data implicate a role for the NER in H2O2-induced oxidative stress management and further corroborates that oxidative stress is a significant contributing factor in XP symptoms. Resistance of XPA-deficient lymphoblastoid cells to H2O2-induced cell death while harbouring DNA damage poses a potential cancer risk factor for XPA patients. Our data implicate XPB and XPD in the protection against oxidative stress-induced DNA damage and telomere shortening, and thus premature senescence.

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.

Proteins of nucleotide and base excision repair pathways interact in mitochondria to protect from loss of subcutaneous fat, a hallmark of aging

Defects in the DNA repair mechanism nucleotide excision repair (NER) may lead to tumors in xeroderma pigmentosum (XP) or to premature aging with loss of subcutaneous fat in Cockayne syndrome (CS). Mutations of mitochondrial (mt)DNA play a role in aging, but a link between the NER-associated CS proteins and base excision repair (BER)-associated proteins in mitochondrial aging remains enigmatic. We show functional increase of CSA and CSB inside mt and complex formation with mtDNA, mt human 8-oxoguanine glycosylase (mtOGG)-1, and mt single-stranded DNA binding protein (mtSSBP)-1 upon oxidative stress. MtDNA mutations are highly increased in cells from CS patients and in subcutaneous fat of aged Csb(m/m) and Csa(-/-) mice. Thus, the NER-proteins CSA and CSB localize to mt and directly interact with BER-associated human mitochondrial 8-oxoguanine glycosylase-1 to protect from aging- and stress-induced mtDNA mutations and apoptosis-mediated loss of subcutaneous fat, a hallmark of aging found in animal models, human progeroid syndromes like CS and in normal human aging.

Both XPD alleles contribute to the phenotype of compound heterozygote xeroderma pigmentosum patients

Mutations in the XPD subunit of the DNA repair/transcription factor TFIIH result in the rare recessive genetic disorder xeroderma pigmentosum (XP). Many XP patients are compound heterozygotes with a “causative” XPD point mutation R683W and different second mutant alleles, considered “null alleles.” However, there is marked clinical heterogeneity (including presence or absence of skin cancers or neurological degeneration) in these XPD/R683W patients, thus suggesting a contribution of the second allele. Here, we report XP patients carrying XPD/R683W and a second XPD allele either XPD/Q452X, /I455del, or /199insPP. We performed a systematic study of the effect of these XPD mutations on several enzymatic functions of TFIIH and found that each mutation exhibited unique biochemical properties. Although all the mutations inhibited the nucleotide excision repair (NER) by disturbing the XPD helicase function, each of them disrupted specific molecular steps during transcription: XPD/Q452X hindered the transactivation process, XPD/I455del disturbed RNA polymerase II phosphorylation, and XPD/199insPP inhibited kinase activity of the cdk7 subunit of TFIIH. The broad range and severity of clinical features in XP patients arise from a broad set of deficiencies in NER and transcription that result from the combination of mutations found on both XPD alleles.

The nucleosome-binding protein HMGN2 modulates global genome repair

The HMGN family comprises nuclear proteins that bind to nucleosomes and alter the structure of chromatin. Here, we report that DT40 chicken cells lacking either HMGN2 or HMGN1a, or lacking both HMGN1a and HMGN2, are hypersensitive to killing by UV irradiation. Loss of both HMGN1a and HMGN2 or only HMGN2 increases the extent of UV-induced G(2)-M checkpoint arrest and the rate of apoptosis. HMGN null mutant cells showed slower removal of UV-induced DNA lesions from native chromatin, but the nucleotide excision repair remained intact, as measured by host cell reactivation assays. These results identify HMGN2 as a component of the global genome repair subpathway of the nucleotide excision repair pathway, and may indicate that HMGN2 facilitates the ability of the DNA repair proteins to access and repair UV-induced DNA lesions in chromatin. Our finding that HMGNs play a role in global DNA repair expands the role of these proteins in the maintenance of genome integrity.

Unexpected occurrence of xeroderma pigmentosum in an uncle and nephew

BACKGROUND

Xeroderma pigmentosum (XP) is a rare autosomal recessive disorder characterized by a decreased ability to repair DNA damaged by UV radiation and the early development of cutaneous and ocular malignant neoplasms. Approximately 20% of patients with XP also develop progressive neurologic degeneration.

OBSERVATIONS

We describe a boy who was found to have XP after a severe burn following minimal sun exposure. His maternal uncle, now age 20 years, had been diagnosed with XP after a similar sunburn in infancy. The uncle has the typical skin pigmentary findings of XP along with severe progressive neurologic involvement. Although the infant's parents were not known to be blood relatives, the infant and his affected uncle proved to be compound heterozygotes for the same 2 frameshift mutations in the XPA DNA repair gene (c.288delT and c.349_353del). After the diagnosis of XP in the infant, genealogic investigation identified a common Dutch ancestor for both of his grandfathers 5 generations back.

CONCLUSIONS

Counseling families at risk for a rare inherited disease is not always straightforward. The sociocultural and demographic backgrounds of the families must be considered for evaluation of risk assessment.

Telomere attrition and genomic instability in xeroderma pigmentosum type-b deficient fibroblasts under oxidative stress

Xeroderma pigmentosum B (XPB/ERCC3/p89) is an ATP-dependent 3′→5′ directed DNA helicase involved in basal RNA transcription and the nucleotide excision repair (NER) pathway. While the role of NER in alleviating oxidative DNA damage has been acknowledged it remains poorly understood. To study the involvement of XPB in repair of oxidative DNA damage, we utilized primary fibroblasts from a patient suffering from XP with Cockayne syndrome and hydrogen peroxide (H₂O₂) to induce oxidative stress. Mutant cells retained higher viability and cell cycle dysfunction after H₂O₂ exposure. Cytokinesis blocked micronucleus assay revealed increased genome instability induced by H₂O₂. Single cell gel electrophoresis (comet) assay showed that the missense mutation caused a reduced repair capacity for oxidative DNA damage. Mutant fibroblasts also displayed decreased population doubling rate, increased telomere attrition rate and early emergence of senescent characteristics under chronic low dose exposure to H₂O₂. Fibroblasts from a heterozygous individual displayed intermediate traits in some assays and normal traits in others, indicating possible copy number dependence. The results show that a deficiency in functional XPB paradoxically renders cells more sensitive to the genotoxic effects of oxidative stress while reducing the cytotoxic effects. These findings have implications in the mechanisms of DNA repair, mutagenesis and carcinogenesis and ageing in normal physiological systems.

Agents that reverse UV-Induced immune suppression and photocarcinogenesis affect DNA repair

UV exposure induces skin cancer, in part, by inducing immune suppression. Repairing DNA damage, neutralizing the activity of cis-urocanic acid, and reversing oxidative stress abrogate UV-induced immune suppression and skin cancer induction, suggesting that DNA, UCA, and lipid photo-oxidation serve as UV photoreceptors. What is not clear is whether signaling through each of these different photoreceptors activates independent pathways to induce biological effects or whether there is a common checkpoint where these pathways converge. Here, we show that agents known to reverse photocarcinogenesis and photoimmune suppression, such as platelet-activating factor (PAF) and serotonin (5-HT) receptor antagonists, regulate DNA repair. Pyrimidine dimer repair was accelerated in UV-irradiated mice injected with PAF and 5-HT receptor antagonists. Nucleotide excision repair (NER), as measured by unscheduled DNA synthesis, was accelerated by PAF and 5-HT receptor antagonists. Injecting PAF and 5-HT receptor antagonists into UV-irradiated Xeroderma pigmentosum complementation group A-deficient mice, which lack the enzymes responsible for NER, did not accelerate photoproduct repair. Similarly, UV-induced formation of 8-oxo-deoxyguanosine was reduced by PAF and 5-HT receptor antagonists. We conclude that PAF and 5-HT receptor antagonists accelerate DNA repair caused by UV radiation, which prevents immune suppression and interferes with photocarcinogenesis.

The effects of topically applied glycolic acid and salicylic acid on ultraviolet radiation-induced erythema, DNA damage and sunburn cell formation in human skin

BACKGROUND

alpha-Hydroxy acids (alphaHAs) are reported to reduce signs of aging in the skin and are widely used cosmetic ingredients. Several studies suggest that alphaHA can increase the sensitivity of skin to ultraviolet radiation. More recently, beta-hydroxy acids (betaHAs), or combinations of alphaHA and betaHA have also been incorporated into antiaging skin care products. Concerns have also arisen about increased sensitivity to ultraviolet radiation following use of skin care products containing beta-HA.

OBJECTIVE

To determine whether topical treatment with glycolic acid, a representative alphaHA, or with salicylic acid, a betaHA, modifies the short-term effects of solar simulated radiation (SSR) in human skin.

METHODS

Fourteen subjects participated in this study. Three of the four test sites on the mid-back of each subject were treated daily Monday-Friday, for a total of 3.5 weeks, with glycolic acid (10%), salicylic acid (2%), or vehicle (control). The fourth site received no treatment. After the last treatment, each site was exposed to SSR, and shave biopsies from all four sites were obtained. The endpoints evaluated in this study were erythema (assessed visually and instrumentally), DNA damage and sunburn cell formation.

RESULTS

Treatment with glycolic acid resulted in increased sensitivity of human skin to SSR, measured as an increase in erythema, DNA damage and sunburn cell formation. Salicylic acid did not produce significant changes in any of these biomarkers.

CONCLUSIONS

Short-term topical application of glycolic acid in a cosmetic formulation increased the sensitivity of human skin to SSR, while a comparable treatment with salicylic acid did not.

The role of the retinoblastoma/E2F1 tumor suppressor pathway in the lesion recognition step of nucleotide excision repair

The retinoblastoma Rb/E2F tumor suppressor pathway plays a major role in the regulation of mammalian cell cycle progression. The pRb protein, along with closely related proteins p107 and p130, exerts its anti-proliferative effects by binding to the E2F family of transcription factors known to regulate essential genes throughout the cell cycle. We sought to investigate the role of the Rb/E2F1 pathway in the lesion recognition step of nucleotide excision repair (NER) in mouse embryonic fibroblasts (MEFs). Rb-/-, p107-/-, p130-/- MEFs repaired both cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs) at higher efficiency than did wildtype cells following UV-C irradiation. The expression of damaged DNA binding gene DDB2 involved in the DNA lesion recognition step was elevated in the Rb family-deficient MEFs. To determine if the enhanced DNA repair in the absence of the Rb gene family is due to the derepression of E2F1, we assayed the ability of E2F1-deficient cells to repair damaged DNA and demonstrated that E2F1-/- MEFs are impaired for the removal of both CPDs and 6-4PPs. Furthermore, wildtype cells induced a higher expression of DDB2 and xeroderma pigmentosum gene XPC transcript levels than did E2F1-/- cells following UV-C irradiation. Using an E2F SiteScan algorithm, we uncovered a putative E2F-responsive element in the XPC promoter upstream of the transcription start site. We showed with chromatin immunoprecipitation assays the binding of E2F1 to the XPC promoter in a UV-dependent manner, suggesting that E2F1 is a transcriptional regulator of XPC. Our study identifies a novel E2F1 gene target and further supports the growing body of evidence that the Rb/E2F1 tumor suppressor pathway is involved in the regulation of the DNA lesion recognition step of nucleotide excision repair.

DNA repair phenotype and cancer susceptibility--a mini review

DNA repair is a complicated biological process, consisting of several distinct pathways, that plays a fundamental role in the maintenance of genomic integrity. The very important field of DNA repair and cancer risk has developed rapidly in the past decades. In this review of selected published data from our laboratory, we describe mostly our work on the study of phenotypic markers of nucleotide excision repair (NER), as measured by the benzo(a)pyrene diol epoxide (BPDE)/ultraviolet (UV)-induced mutagen sensitivity assays, BPDE-induced adduct assay, host cell reactivation (HCR)-DNA repair capacity (DRC) assay, reverse transcription-polymerase chain reaction (RT-PCR) assay and reverse-phase protein lysate microarray (RPP) assay, by using peripheral blood lymphocytes in a series of molecular epidemiological studies. Results of our studies suggest that individuals with reduced DRC have an elevated cancer risk. This finding needs additional validation by other investigators, and we also discussed issues in conducting this kind of research in the future.

Comparative study of nucleotide excision repair defects between XPD-mutated fibroblasts derived from trichothiodystrophy and xeroderma pigmentosum patients

To get a clue to understand how mutations in the XPD gene result in different skin cancer susceptibilities in patients with xeroderma pigmentosum (XP) or trichothiodystrophy (TTD), a thorough understanding of their nucleotide excision repair (NER) defects is essential. Here, we extensively characterize the possible causes of NER defects in XP-D and in TTD fibroblasts. The 3 XP-D cell strains examined were similarly deficient in repairing UV-induced cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs) from genomic DNA. The severity of NER defects correlated with their UV sensitivities. Possible alterations of TFIIH (which consists of 10 subunits including XPD) were then examined. All XP-D cell strains were normal in their concentrations of TFIIH, and displayed normal abilities to recruit TFIIH to sites of UV-induced DNA damage. However, replication protein A (RPA; single-stranded DNA binding protein) accumulation at DNA damage sites, which probably reflects the in vivo XPD helicase activity of TFIIH, is similarly impaired in all XP-D cell strains. Meanwhile, all 3 TTD cell strains had approximately 50% decreases in cellular TFIIH content. Importantly, 2 of the 3 TTD cell strains, which carry the major XPD mutations found in TTD patients, showed defective recruitment of TFIIH to DNA damage sites. Moreover, RPA accumulation at damage sites was impaired in all TTD cell strains to different degrees, which correlated with the severity of their NER defects. These results demonstrate that XP-D and TTD cells are both deficient in the repair of CPDs and 6-4PPs, but TTD cells have more multiple causes for their NER defects than do XP-D cells. Since TFIIH is a repair/transcription factor, TTD-specific alterations of TFIIH possibly result in transcriptional defects, which might be implication for the lack of increased incidence of skin cancers in TTD patients.

Mutation frequencies and spectra in DNA polymerase eta-deficient mice

The low-fidelity polymerase eta (poleta) is required for bypass of UV-induced pyrimidine dimers inserting adenine nucleotides opposite these lesions. Mutations in the poleta gene are responsible for the genetic defect in xeroderma pigmentosum variant patients. To study if the lack of poleta significantly elevates spontaneous mutation frequency in various organs and tissues of the mouse, we crossed poleta-deficient mice with transgenic mice harboring a chromosomally integrated lacZ-plasmid reporter construct. In cultured embryonic fibroblasts from the lacZ-poleta(-/-) mice, 2.5 J/m(2) UV irradiation induced approximately 5-fold more mutations than in cells from lacZ control mice, in which an approximately 3-fold increase in mutation frequency was found compared with the normal level. Whereas untreated cells harbored mainly 1-bp deletions, UV induced both transitions and transversions, with the latter type more highly represented in the poleta-null cells than in the controls. No difference in mutation induction between the poleta-null cells and the wild-type cells was observed after treatment with N-ethyl-N-nitrosourea. Having shown the validity of the lacZ model to accurately identify poleta-associated mutagenesis, we then determined the mutant frequency at the lacZ locus in liver, spleen, and small intestine of 12-month-old animals. No differences were found between poleta-null, heterozygous, or littermate control mice. We conclude that the poleta defect is specific for UV damage and has no effect on in vivo mutagenesis in mice.

Xeroderma pigmentosum-variant patients from America, Europe, and Asia

Xeroderma pigmentosum-variant (XP-V) patients have sun sensitivity and increased skin cancer risk. Their cells have normal nucleotide excision repair, but have defects in the POLH gene encoding an error-prone polymerase, DNA polymerase eta (pol eta). To survey the molecular basis of XP-V worldwide, we measured pol eta protein in skin fibroblasts from putative XP-V patients (aged 8-66 years) from 10 families in North America, Turkey, Israel, Germany, and Korea. Pol eta was undetectable in cells from patients in eight families, whereas two showed faint bands. DNA sequencing identified 10 different POLH mutations. There were two splicing, one nonsense, five frameshift (3 deletion and 2 insertion), and two missense mutations. Nine of these mutations involved the catalytic domain. Although affected siblings had similar clinical features, the relation between the clinical features and the mutations was not clear. POLH mRNA levels were normal or reduced by 50% in three cell strains with undetectable levels of pol eta protein, indicating that nonsense-mediated message decay was limited. We found a wide spectrum of mutations in the POLH gene among XP-V patients in different countries, suggesting that many of these mutations arose independently.