Drug-induced apoptosis is delayed and reduced in XPD lymphoblastoid cell lines: possible role of TFIIH in p53-mediated apoptotic cell death

Role of Xeroderma pigmentosum D (XPD) protein in genome maintenance in human cells under oxidative stress

Xeroderma pigmentosum D (XPD) protein plays a pivotal role in the nucleotide excision repair pathway. XPD unwinds the local area of the damaged DNA by virtue of constituting transcription factor II H (TFIIH) and is important not only for repair but also for basal transcription. Although cells deficient in XPD have shown to be defective in oxidative base-lesion repair, the effects of the oxidative assault on primary fibroblasts from patients suffering from Xeroderma Pigmentosum D have not been fully explored. Therefore, we sought to investigate the role of XPD in oxidative DNA damage-repair by treating primary fibroblasts derived from a patient suffering from Xeroderma Pigmentosum D, with hydrogen peroxide. Our results show dose-dependent increase in genotoxicity with minimal effect on cytotoxicity with H₂O₂ in XPD deficient cells compared to control cells. XPD deficient cells displayed increased susceptibility and reduced repair capacity when subjected to DNA damage induced by oxidative stress. XPD deficient fibroblasts exhibited increased telomeric loss after H₂O₂ treatment. In addition, we demonstrated that chronic oxidative stress induced accelerated premature senescence characteristics. Gene expression profiling revealed alterations in genes involved in transcription and nucleotide metabolisms, as well as in cellular and cell cycle processes in a more significant way than in other pathways. This study highlights the role of XPD in the repair of oxidative stress and telomere maintenance. Lack of functional XPD seems to increase the susceptibility of oxidative stress-induced genotoxicity while retaining cell viability posing as a potential cancer risk factor of Xeroderma Pigmentosum D patients.

Research progress in strategies to improve the efficacy and safety of doxorubicin for cancer chemotherapy

INTRODUCTION

DOX exerts strong anticancer activity and is commonly used to treat different cancers, including bone sarcomas, soft tissues, bladder, ovary, stomach, thyroid, breast, acute lymphoblastic leukemia, Hodgkin lymphoma, lung cancer, and myeloblastic leukemia. However, the cumulative doses of DOX above 550mg/m2 cause irreversible cardiotoxicity and other severe adverse effects. In this context, concerning DOX, several patents have been published in the last two decades. This activity highlights various aspects of DOX, such as registered patent analysis, pharmacological action, toxicityminimization, formulation development such as those approved by FDA, under clinical trials, and newly developed nano-delivery systems.

AREAS COVERED

This review analyzes the different aspects of DOX-based chemotherapeutics and the development of drug delivery systems in theliterature published from 2000 to early 2020.

EXPERT OPINION

DOX-based chemotherapy is still few steps away from being "perfect and safe" therapy. Certain severe systemic side effects are associated with DOX therapy. It is expected that, in the near future, DOX therapy can be much effective by selecting an ideal nanocarrier system, DOX conjugates, proper structural modifications, DOX-immunotherapy, and combination therapy. The advanced formulationsof DOX from the registered patents and recent research articles need clinical trials to bring safe treatment for cancer patients.

Nanoformulations of doxorubicin: how far have we come and where do we go from here?

Nanotechnology, focused on discovery and development of new pharmaceutical products is known as nanopharmacology, and one research area this branch is engaged in are nanopharmaceuticals. The importance of being nano has been particularly emphasized in scientific areas dealing with nanomedicine and nanopharmaceuticals. Nanopharmaceuticals, their routes of administration, obstacles and solutions concerning their improved application and enhanced efficacy have been briefly yet comprehensively described. Cancer is one of the leading causes of death worldwide and evergrowing number of scientific research on the topic only confirms that the needs have not been completed yet and that there is a wide platform for improvement. This is undoubtedly true for nanoformulations of an anticancer drug doxorubicin, where various nanocarrriers were given an important role to reduce the drug toxicity, while the efficacy of the drug was supposed to be retained or preferably enhanced. Therefore, we present an interdisciplinary comprehensive overview of interdisciplinary nature on nanopharmaceuticals based on doxorubicin and its nanoformulations with valuable information concerning trends, obstacles and prospective of nanopharmaceuticals development, mode of activity of sole drug doxorubicin and its nanoformulations based on different nanocarriers, their brief descriptions of biological activity through assessing in vitro and in vivo behavior.

Interactions of chromatin context, binding site sequence content, and sequence evolution in stress-induced p53 occupancy and transactivation

Cellular stresses activate the tumor suppressor p53 protein leading to selective binding to DNA response elements (REs) and gene transactivation from a large pool of potential p53 REs (p53REs). To elucidate how p53RE sequences and local chromatin context interact to affect p53 binding and gene transactivation, we mapped genome-wide binding localizations of p53 and H3K4me3 in untreated and doxorubicin (DXR)-treated human lymphoblastoid cells. We examined the relationships among p53 occupancy, gene expression, H3K4me3, chromatin accessibility (DNase 1 hypersensitivity, DHS), ENCODE chromatin states, p53RE sequence, and evolutionary conservation. We observed that the inducible expression of p53-regulated genes was associated with the steady-state chromatin status of the cell. Most highly inducible p53-regulated genes were suppressed at baseline and marked by repressive histone modifications or displayed CTCF binding. Comparison of p53RE sequences residing in different chromatin contexts demonstrated that weaker p53REs resided in open promoters, while stronger p53REs were located within enhancers and repressed chromatin. p53 occupancy was strongly correlated with similarity of the target DNA sequences to the p53RE consensus, but surprisingly, inversely correlated with pre-existing nucleosome accessibility (DHS) and evolutionary conservation at the p53RE. Occupancy by p53 of REs that overlapped transposable element (TE) repeats was significantly higher (p<10⁻⁷) and correlated with stronger p53RE sequences (p<10⁻¹¹⁰) relative to nonTE-associated p53REs, particularly for MLT1H, LTR10B, and Mer61 TEs. However, binding at these elements was generally not associated with transactivation of adjacent genes. Occupied p53REs located in L2-like TEs were unique in displaying highly negative PhyloP scores (predicted fast-evolving) and being associated with altered H3K4me3 and DHS levels. These results underscore the systematic interaction between chromatin status and p53RE context in the induced transactivation response. This p53 regulated response appears to have been tuned via evolutionary processes that may have led to repression and/or utilization of p53REs originating from primate-specific transposon elements.

It is well established that p53 binds DNA elements near p53 target genes to regulate the response to cellular stress. To assess factors influencing binding to response elements and subsequent gene expression, we have analyzed 2932 p53-occupied response elements (p53REs) in the context of genome-wide chromatin state, DNA accessibility and dynamics, and considered roles for binding-sequence specificity and evolutionary conservation. While p53 occupancy level shows little apparent direct relationship to gene expression change, after grouping expressed genes by their chromatin status at baseline, a relationship between occupancy of p53REs and gene expression change emerged. Analysis of p53RE sequences demonstrated that p53 occupancy was strongly correlated with sequence similarity to p53RE consensus, but surprisingly, was inversely correlated with nucleosome accessibility (DHS) and evolutionary conservation. These data revealed a systematic interaction between p53RE content and chromatin context that affects both quantitative p53 occupancy and the induced transactivation response to exposure. Moreover, this interaction appears to have been tuned via evolutionary events involving transposable elements, which strongly bind p53, but in only a few instances affect gene expression levels. Models of p53-regulated gene expression response that consider both chromatin state and sequence context may prove useful in guiding strategies for cancer prevention or therapy.

Polymorphisms in the DNA repair gene ERCC2/XPD and breast cancer risk: a HapMap-based case-control study among Han Women in a Chinese less-developed area

AIMS

Genetic variations in DNA repair genes may impact repair functions, DNA damage, and breast cancer risk. This study is aimed to assess the associations of genetic polymorphisms in excision repair cross-complementing group 2 (ERCC2) with the risk of developing breast cancer.

MATERIALS AND METHODS

In total, 101 histopathologically confirmed breast cancer cases and 101 age/region-matched healthy controls were genotyped for rs 3916840, rs 1799793, and rs 238416 in ERCC2 by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

The rs 238416 heterozygous GA genotype combined with the rs 238416 genotypes (GA+AA) showed a significant association with breast cancer susceptibility (corrected p<0.01, odds ratio [OR]=0.29, 95% confidence interval [CI]=0.15-0.54; corrected p<0.01, OR=0.31, 95% CI=0.17-0.56, respectively). The rs 238416 GA genotype carriers had a decreased risk of breast cancer. However, we observed no significant association between the rs 3916840 and rs 1799793 polymorphisms in ERCC2 and breast cancer risk. Moreover, haplotype analysis showed that the ACG haplotype was associated with a significantly decreased risk of breast cancer, whereas the GCG haplotype was associated with a significantly increased risk of breast cancer (corrected p=0.004 and p=0.002, respectively). Multifactor dimensionality reduction analysis demonstrated that the interactions between rs 3916840 and rs 238416 were significantly synergistic.

CONCLUSION

To the best of our knowledge, this study is the first to demonstrate that the rs 238416 heterozygous genotype likely has a higher DNA repair capacity and, thus, can be protective against breast cancer in Chinese Han women.

Chemotherapeutic agents for the treatment of hepatocellular carcinoma: efficacy and mode of action

Hepatocellular carcinoma (HCC) is a dreaded malignancy that every year causes half a million deaths worldwide. Being an aggressive cancer, its incidence exceeds 700,000 new cases per year worldwide with a median survival of 6-8 months. Despite advances in prognosis and early detection, effective HCC chemoprevention or treatment strategies are still lacking, therefore its dismal survival rate remains largely unchanged. This review will characterize currently available chemotherapeutic drugs used in the treatment of HCC. The respective mode(s) of action, side effects and recommendations will be also described for each drug.

XPD-dependent activation of apoptosis in response to triplex-induced DNA damage

DNA sequences capable of forming triplexes are prevalent in the human genome and have been found to be intrinsically mutagenic. Consequently, a balance between DNA repair and apoptosis is critical to counteract their effect on genomic integrity. Using triplex-forming oligonucleotides to synthetically create altered helical distortions, we have determined that pro-apoptotic pathways are activated by the formation of triplex structures. Moreover, the TFIIH factor, XPD, occupies a central role in triggering apoptosis in response to triplex-induced DNA strand breaks. Here, we show that triplexes are capable of inducing XPD-independent double strand breaks, which result in the formation of γH2AX foci. XPD was subsequently recruited to the triplex-induced double strand breaks and co-localized with γH2AX at the damage site. Furthermore, phosphorylation of H2AX tyrosine 142 was found to stimulate the signaling pathway of XPD-dependent apoptosis. We suggest that this mechanism may play an active role in minimizing genomic instability induced by naturally occurring noncanonical structures, perhaps protecting against cancer initiation.

The genetic depletion or the triptolide inhibition of TFIIH in p53 deficient cells induce a JNK-dependent cell death in Drosophila

TFIIH participates in transcription, nucleotide excision repair and the control of the cell cycle. In this work, we demonstrate that the Dmp52 subunit of TFIIH in Drosophila physically interacts with the fly p53 homologue, Dp53. The depletion of Dmp52 in the wing disc generates chromosome fragility, increases apoptosis and produces wings with a reduced number of cells; cellular proliferation, however, is not affected. Interestingly, instead of suppressing the apoptotic phenotype, the depletion of Dp53 in Dmp52-depleted wing disc cells increases apoptosis and the number of cells that suffer from chromosome fragility. The apoptosis induced by the depletion of Dmp52 alone is partially dependent on the JNK pathway. In contrast, the enhanced apoptosis caused by the simultaneous depletion of Dp53 and Dmp52 is absolutely JNK-dependent. In this study, we also show that the anti-proliferative drug triptolide, which inhibits the ATPase activity of the XPB subunit of TFIIH, phenocopies the JNK-dependent massive apoptotic phenotype of Dp53-depleted wing disc cells; this observation suggests that the mechanism by which triptolide induces apoptosis in p53-deficient cancer cells involves the activation of the JNK death pathway.

XPD could suppress growth of HepG2.2.15 and down-regulate the expression of hepatitis B virus x protein through P53 pathway

OBJECTIVES

We investigated the effects of xeroderma pigmentosum D (XPD) on the growth of hepatoma cells and the expressions of P21, Bax, Bcl-2 and Hepatitis B virus X protein (HBx). In addition, we examined whether XPD affected the aforementioned genes via the P53 pathway.

METHODS

Human hepatoma cells (HepG2.2.15) were transfected with XPD expression vector, followed by incubation with Pifithrin-α (P53 inhibitor). By using RT-PCR and Western blotting, the expression levels of XPD, P53, phospho-P53 (ser-15), P21, Bax, Bcl-2 and HBx were detected. The cell cycle and the apoptosis rate were examined with flow cytometry, and the cell viability was detected by MTT.

RESULTS

Over-expression of XPD up-regulated the expressions of P53, phospho-P53 (ser-15), P21 and Bax but down-regulated the expressions of Bcl-2 and HBx. XPD inhibited the viability of HepG2.2.15 and exacerbated the apoptosis. However, the inhibition of P53 by Pifithrin-α abolished the above-mentioned effects of XPD.

CONCLUSION

XPD could suppress growth of hepatoma cells, up-regulate the expressions of P21 and Bax, and down-regulate the expressions of Bcl-2 and HBx through the P53 pathway. There may be mutual influences among XPD, P53 and HBx that co-regulate hepatocarcinogenesis.

SNP-SNP interactions between DNA repair genes were associated with breast cancer risk in a Korean population

BACKGROUND

Single nucleotide polymorphisms (SNPs) in nucleotide excision repair (NER) pathway genes may modulate DNA repair capacity and increase susceptibility to breast cancer (BC). A case-control study was conducted by evaluating genes involved in DNA repair to identify polymorphisms associated with BC.

METHODS

The 384 SNPs of 38 candidate genes were genotyped using the Illumina GoldenGate method. Genotypes were determined in a case-control study that consisted of 346 BC patients and 361 controls. Odds ratios and 95% confidence intervals were computed using logistic regression models. Multiple logistic regression models adjusted for age, family history of BC, and body mass index were used.

RESULTS

Gene-gene interaction analysis among the DNA repair pathway genes showed significant effects on BC risk. ERCC2 rs50872 (TC genotype) in combination with XPA rs2808668 (TC genotype) and rs1800975 (AG genotype) was strongly associated with an increased risk of BC (P = .0004 and .0002, P(Bonferroni) = .023 and .014, respectively). Moreover, the T-G (including rs2808668 and rs1800975) haplotype in XPA combined with the ERCC2 T allele in rs50872 carriers was also associated with additive risk effect of BC (odds ratios: 2.58, 2.62, and 3.49, respectively).

CONCLUSION

Genetic variation in DNA repair genes involved in NER mechanisms increased the risk of BC development. These results suggested that a stronger combined effect of SNPs via gene-gene interaction may help to predict BC risk.

Genetic variants in DNA repair predicts the survival of patients with esophageal cancer

OBJECTIVE

To investigate the association of the genetic variants in excision repair cross-complementation group 2 (ERCC2) R156R and ERCC4 rs3136038 with survival duration for patients with esophageal cancer.

BACKGROUND

ERCC2 and ERCC4 are important molecules participating nucleotide excision repair system. The clinical relevance of the genetic variants of these genes is largely unknown currently.

PATIENTS AND METHODS

A total of 400 patients with a diagnosis of esophageal cancer were included. The genetic variants in the promoter regions of ERCC2 on R156R and ERCC4 on rs3136038 were analyzed with the TaqMan assay from leukocyte DNA collected before treatment and correlated to survival of the patients.

RESULTS

Presence with ERCC2 R156R C/C or ERCC4 rs3136038 C/T genotype of the patients could additively increase risk of death and disease progression. Under multivariate analysis, T, N staging and simultaneous presentation of these unfavorable genotypes were found significant for prognosis (P < 0.05). Accumulation of each unfavorable genotype would associate with adjusted HRs [95% CI] of 1.35 [1.10-1.65] and 1.37 [1.12-1.68] (P ≤ 0.05) for death and disease progression respectively. The prognostic impact of these genotypes were more evident in the subgroup of patients with early disease status including T staging (II or less), free from lymph node metastasis or being able to undergo surgical resection (P < 0.05 for both overall and disease progression-free survival duration, respectively).

CONCLUSION

Genetic variants in ERCC2 and ERCC4 may provide further survival prediction in addition to TNM staging system of esophageal cancer, which is more evident in the patients with early disease status.

Unexpected functional similarities between gatekeeper tumour suppressor genes and proto-oncogenes revealed by systems biology

Familial tumor suppressor genes comprise two subgroups: caretaker genes (CTs) that repair DNA, and gatekeeper genes (GKs) that trigger cell death. Since GKs may also induce cell cycle delay and thus enhance cell survival by facilitating DNA repair, we hypothesized that the prosurvival phenotype of GKs could be selected during cancer progression, and we used a multivariable systems biology approach to test this. We performed multidimensional data analysis, non-negative matrix factorization and logistic regression to compare the features of GKs with those of their putative antagonists, the proto-oncogenes (POs), as well as with control groups of CTs and functionally unrelated congenital heart disease genes (HDs). GKs and POs closely resemble each other, but not CTs or HDs, in terms of gene structure (P<0.001), expression level and breadth (P<0.01), DNA methylation signature (P<0.001) and evolutionary rate (P<0.001). The similar selection pressures and epigenetic trajectories of GKs and POs so implied suggest a common functional attribute that is strongly negatively selected-that is, a shared phenotype that enhances cell survival. The counterintuitive finding of similar evolutionary pressures affecting GKs and POs raises an intriguing possibility: namely, that cancer microevolution is accelerated by an epistatic cascade in which upstream suppressor gene defects subvert the normal bifunctionality of wild-type GKs by constitutively shifting the phenotype away from apoptosis towards survival. If correct, this interpretation would explain the hitherto unexplained phenomenon of frequent wild-type GK (for example, p53) overexpression in tumors.

The role of XPD in cell apoptosis and viability and its relationship with p53 and cdk2 in hepatoma cells

We investigated the role of XPD in cell apoptosis of hepatoma and its relationship with p53 during the regulation of hepatoma bio-behavior. RT-PCR and Western blot were used to detect the expression levels of XPD, p53, c-myc, and cdk2. The cell apoptosis and cell cycle were analyzed with flow cytometry. Compared with the control cells, XPD-transfected cells displayed a lower viability and higher apoptosis rate. A decreased expression of p53 gene was detected in XPD-transfected cells. In contrast, both c-myc and cdk2 showed increased expressions of mRNAs and proteins in the transfected cells. Our results indicate that XPD may play an important role in cell apoptosis of hepatoma by inducing an over-expression of p53, but suppressing expressions of c-myc and cdk2.

UV-induced histone H2AX phosphorylation and DNA damage related proteins accumulate and persist in nucleotide excision repair-deficient XP-B cells

DNA double strand breaks (DSB) may be caused by ionizing radiation. In contrast, UV exposure forms dipyrimidine photoproducts and is not considered an inducer of DSB. We found that uniform or localized UV treatment induced phosphorylation of the DNA damage related (DDR) proteins H2AX, ATM and NBS1 and co-localization of γ-H2AX with the DDR proteins p-ATM, p-NBS1, Rad51 and FANCD2 that persisted for about 6h in normal human fibroblasts. This post-UV phosphorylation was observed in the absence of nucleotide excision repair (NER), since NER deficient XP-B cells (lacking functional XPB DNA repair helicase) and global genome repair-deficient rodent cells also showed phosphorylation and localization of these DDR proteins. Resolution of the DDR proteins was dependent on NER, since they persisted for 24h in the XP-B cells. In the normal and XP-B cells p53 and p21 was detected at 6h and 24h but Mdm2 was not induced in the XP-B cells. Post-UV induction of Wip1 phosphatase was detected in the normal cells but not in the XP-B cells. DNA DSB were detected with a neutral comet assay at 6h and 24h post-UV in the normal and XP-B cells. These results indicate that UV damage can activate the DDR pathway in the absence of NER. However, a later step in DNA damage processing involving induction of Wip1 and resolution of DDR proteins was not observed in the absence of NER.

Impact of ERCC2 gene polymorphism on HIV-1 disease progression to AIDS among North Indian HIV patients

HIV/AIDS remains to be one of the killing diseases of mankind. Host genetic response is one of the factor which determine susceptibility to HIV and disease progression to AIDS. The aim of the present study was to evaluate the impact of ERCC2 Lyc ( 751 ) Gln (excision repair cross complementing rodent repair deficiency, complementation group 2) polymorphism on HIV-1 susceptibility and disease progression to AIDS, as this gene has been reported to intervene in degrading retroviral cDNA before it integrates with the host DNA. This case control study included 300 HIV seropositive cases and an equal number of HIV seronegative controls. DNA was isolated from the blood samples of study subjects and genotyping of ERCC2 was conducted by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) method. The Gln/Gln genotype showed a significant variation between cases and controls (P = 0.047, OR 1.71, 95% CI 1.00-2.93), indicating a possible role of susceptibility in reference to controls and disease progression when compared within cases.

DNA damage induced by the anthracycline cosmomycin D in DNA repair-deficient cells

PURPOSE

Anthracyclines have been widely used as antitumor agents, playing a crucial role in the successful treatment of many types of cancer, despite some side effects related to cardiotoxicity. New anthracyclines have been designed and tested, but the first ones discovered, doxorubicin and daunorubicin, continue to be the drugs of choice. Despite their extensive use in chemotherapy, little is known about the DNA repair mechanisms involved in the removal of lesions caused by anthracyclines. The anthracycline cosmomycin D is the main product isolated from Streptomyces olindensis, characterized by a peculiar pattern of glycosylation with two trisaccharide rings attached to the A ring of the tetrahydrotetracene.

METHODS

We assessed the induction of apoptosis (Sub-G1) by cosmomycin D in nucleotide excision repair-deficient fibroblasts (XP-A and XP-C) as well as the levels of DNA damage (alkaline comet assay).

RESULTS

Treatment of XP-A and XP-C cells with cosmomycin D resulted in apoptosis in a time-dependent manner, with highest apoptosis levels observed 96 h after treatment. The effects of cosmomycin D were equivalent to those obtained with doxorubicin. The broad caspase inhibitor Z-VAD-FMK strongly inhibited apoptosis in these cells, and DNA damage induced by cosmomycin D was confirmed by alkaline comet assay.

CONCLUSIONS

Cosmomycin D induced time-dependent apoptosis in nucleotide excision repair-deficient fibroblasts. Despite similar apoptosis levels, cosmomycin D caused considerably lower levels of DNA damage compared to doxorubicin. This may be related to differences in structure between cosmomycin D and doxorubicin.

Genetic Polymorphisms in the DNA Repair Enzyme ERCC2 and Breast Tumour Risk in a Chinese Population

DNA repair systems enable our cells to respond to carcinogens and mutagens. The ERCC2 gene encodes a DNA repair protein that has multiple regulatory cellular functions including nucleotide excision repair, basal transcription, cell cycle control, and apoptosis. The ERCC2 polymorphism Lys751Gln may alter the capacity for DNA repair, which could affect the risk of certain cancers. We examined whether the Lys751Gln polymorphism was associated with the risk of breast cancer in Chinese women by analysing the genotype frequencies in 486 patients with breast cancer and 479 cancer-free control subjects. At least one variant allele (Lys/Gln or Gln/Gln) was associated with a 44% decreased risk of breast cancer. Patients with breast cancer with the 751Lys/Gln or 751Gln/Gln genotypes were less likely to be hormone receptor positive. In conclusion, our results demonstrated that ERCC2 polymorphisms might be potential risk markers for breast cancer in the Chinese population.

p53 in recombination and repair

Convergent studies demonstrated that p53 regulates homologous recombination (HR) independently of its classic tumour-suppressor functions in transcriptionally transactivating cellular target genes that are implicated in growth control and apoptosis. In this review, we summarise the analyses of the involvement of p53 in spontaneous and double-strand break (DSB)-triggered HR and in alternative DSB repair routes. Molecular characterisation indicated that p53 controls the fidelity of Rad51-dependent HR and represses aberrant processing of replication forks after stalling at unrepaired DNA lesions. These findings established a genome stabilising role of p53 in counteracting error-prone DSB repair. However, recent work has also unveiled a stimulatory role for p53 in topoisomerase I-induced recombinative repair events that may have implications for a gain-of-function phenotype of cancer-related p53 mutants. Additional evidence will be discussed which suggests that p53 and/or p53-regulated gene products also contribute to nucleotide excision, base excision, and mismatch repair.

A common genetic variant in XPD associates with risk of 5q- and 7q-deleted acute myeloid leukemia

Numerous structural genetic abnormalities observed in acute myeloid leukemia (AML) illustrate the heterogeneity of this disease, which likely has contributed to difficulty in identifying susceptibility alleles for AML. We previously reported that carriers of the glutamine-encoding allele at codon 751 of the xeroderma pigmentosum group D (XPD) DNA repair gene were significantly more likely to have a karyotype associated with a less favorable prognosis, and hypothesized that this observation was driven by an association between the codon 751 variant and risk of developing AML with specific structural abnormalities. Using a case series of 927 patients with AML, we show here that the XPD codon 751 glutamine-encoding variant significantly associates with risk of developing AML with a chromosome 5q deletion (odds ratio [OR] 2.09; 95% confidence interval [CI] 1.14-3.81; n=69; P=.02) or a chromosome 7q deletion (OR 2.27; 95% CI 1.09-4.71; n=47; P=.03), but not with any other commonly recurring cytogenetic lesion.

hHR23B is required for genotoxic-specific activation of p53 and apoptosis

Rad23 proteins function in both DNA repair and protein stability regulation. As ubiquitinated forms of p53 are stabilized after DNA damage in concert with p53 functional activation, and human Rad23 proteins (hHR23A and B) regulate p53 stability in unstressed cells, the role of hHR23B in post-genotoxin regulation of p53 was investigated. Depletion of hHR23B by specific short interfering RNA before genotoxic exposure attenuated p53, p21 and bax induction, abrogated the accumulation of ubiquitinated p53 and suppressed apoptosis. Expression of ubiquitin derivatives with all lysines mutated except K48 or K63 demonstrated that K48-linked p53-ubiquitin conjugates were specifically induced after DNA damage. hHR23B, along with native and ubiquitinated p53, accumulated in chromatin after genotoxic exposure, and the accumulation of ubiquitinated p53 in chromatin was prevented by hHR23B depletion. Chromatin immunoprecipitation analysis demonstrated that hHR23B and p53 both localized to the p21 promoter shortly after DNA damage. hHR23B thus plays a critical role in the activation and function of p53 after specific genotoxic exposures.

XPD Lys751Gln polymorphism in the etiology and outcome of childhood acute myeloid leukemia: a Children's Oncology Group report

Genetic polymorphisms result in interindividual variation in DNA repair capacity and may, in part, account for susceptibility of a cell to genotoxic agents and to malignancy. Polymorphisms in XPD, a member of the nucleotide excision repair pathway, have been associated with development of treatment-related acute myeloid leukemia (AML) and with poor outcome of AML in elderly patients. We hypothesized that XPD Lys751Gln polymorphism may play a role in causation of AML in children and, as shown in adults, may affect the outcome of childhood AML therapy. Genotyping of 456 children treated for de novo AML was performed at XPD exon 23. Genotype frequencies in patients were compared with healthy control subject frequencies, and patient outcomes were analyzed according to genotype. Gene frequencies in AML patients and healthy controls were similar. There were no significant differences in overall survival (P = .82), event-free survival (P = .78), treatment-related mortality (P = .43), or relapse rate (RR) (P = .92) between patients with XPD751AA versus 751AC versus 751CC genotypes, in contrast to reports in adult AML. These data, representing the only data in pediatric AML, suggest that XPD genotype does not affect the etiology or outcome of childhood AML.

Genetic variation in XPD, sun exposure, and risk of skin cancer

The XPD gene is involved in the nucleotide excision repair pathway removing DNA photoproducts induced by UV radiation. Genetic variation in XPD may exert a subtle effect on DNA repair capacity. We assessed the associations between two common nonsynonymous polymorphisms (Asp312Asn and Lys751Gln) with skin cancer risk in a nested case-control study within the Nurses' Health Study (219 melanoma, 286 squamous cell carcinoma, 300 basal cell carcinoma, and 874 controls) along with exploratory analysis on the haplotype structure of the XPD gene. There were inverse associations between the Lys751Gln and Asp312Asn polymorphisms and the risks of melanoma and squamous cell carcinoma. No association was observed between these two polymorphisms and basal cell carcinoma risk. We also observed that the association of the 751Gln allele with melanoma risk was modified by lifetime severe sunburns, cumulative sun exposure with a bathing suit, and constitutional susceptibility score (P for interaction = 0.03, 0.04, and 0.02 respectively). Similar interactions were also observed for the Asp312Asn. Our data suggest these two XPD nonsynonymous polymorphisms may be associated with skin cancer risk, especially for melanoma.

p53 in neuronal apoptosis

The tumor suppressor and transcription factor p53 is a key modulator of cellular stress responses, and activation of p53 can trigger apoptosis in many cell types including neurons. Apoptosis is a form of programmed cell death that occurs in neurons during development of the nervous system and may also be responsible for neuronal deaths that occur in neurological disorders such as stroke, and Alzheimer's and Parkinson's diseases. p53 production is rapidly increased in neurons in response to a range of insults including DNA damage, oxidative stress, metabolic compromise, and cellular calcium overload. Target genes induced by p53 in neurons include those encoding the pro-apoptotic proteins Bax and the BH3-only proteins PUMA and Noxa. In addition to such transcriptional control of the cell death machinery, p53 may more directly trigger apoptosis by acting at the level of mitochondria, a process that can occur in synapses (synaptic apoptosis). Preclinical data suggest that agents that inhibit p53 may be effective therapeutics for several neurodegenerative conditions.

ERCC2 Genotypes and a Corresponding Haplotype Are Linked with Breast Cancer Risk in a German Population

The polygenic concept of breast cancer susceptibility calls for the identification of genetic variants that contribute to breast cancer risk. Reduced DNA repair proficiencies in women with breast cancer pointed to a possible role of DNA repair enzymes in the risk to develop the disease. The nucleotide excision repair enzyme encoded by the excision repair cross-complementing group 2 gene ERCC2 (formerly XPD) known to cause skin cancer by germ line mutations has multiple regulatory cellular functions, including nucleotide excision repair, basal transcription, cell cycle control, and apoptosis. ERCC2 polymorphisms ERCC2_6540_G&gt;A (Asp312Asn) and ERCC2_18880_A&gt;C (Lys751Gln) within the coding region of this evolutionarily highly conserved gene have been of functional relevance and therefore are potential candidates to confer breast cancer susceptibility. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, we analyzed genotype frequencies in constitutional DNA of study participants of a German case-control study that included 688 cases of incident breast cancer and 724 population-based, age-matched controls. We identified ERCC2_6540_GG (Asp312Asp) as an at-risk genotype [odds ratio (OR), 2.06; 95% confidence interval (95% CI), 1.39-3.07]. The ERCC2_6540_GG-associated breast cancer risk was even higher in women who were also carriers of the ERCC2_18880_CC (Gln751Gln) genotype (OR, 3.69; 95% CI, 1.76-7.74). We identified ERCC2_6540_G/ERCC2_18880_C (Asp312/Gln751) as the most potent risk-conferring haplotype (OR, 3.49; 95% CI, 2.30-5.28). To our knowledge, this is the first study assigning breast cancer risk to both the ERCC2 genotype encoding Asp312Asp and the haplotype encoding Asp312/Gln751.

Polymorphisms in the DNA repair enzyme XPD are associated with increased levels of PAH-DNA adducts in a case-control study of breast cancer

We present findings on the associations between DNA adduct levels in breast tissue, risk of breast cancer, and polymorphisms in the DNA repair enzyme XPD. Breast cancer cases, benign breast disease (BBD) controls, and healthy controls were enrolled. Polycyclic aromatic hydrocarbons (PAH)-DNA adduct levels were measured by immunohistochemistry in breast tissue samples from cases and BBD controls. XPD polymorphisms at codons 312 and 751 was determined by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) analysis using white blood cell DNA. Neither of the polymorphisms were associated with case-control status, both in comparisons of cases and BBD controls, and cases and healthy controls. XPD polymorphisms at codons 312 and 751 were associated with higher levels of PAH-DNA in tumor tissue from breast cancer cases. Subjects with an Asp/Asn or Asn/Asn polymorphic genotype in codon 312 of XPD had elevated levels of PAH-DNA adducts compared to subjects with the Asp/Asp genotype (0.55 optical density (OD) v.s. 0.33 OD, p < 0.01). PAH-DNA adducts were associated with increasing copy number of the Gln allele for the codon 751 polymorphism (p for trend <0.01). Among subjects with the Asp/Asn or Asn/Asn genotype at codon 312, adduct levels were higher in tumor tissue compared to tissue from BBD controls (0.55 OD v.s. 0.36 OD, p = 0.003). Among subjects with the Gln/Gln genotype at codon 751 adduct levels were higher in tumor tissue compared to tissue from BBD controls (0.68 OD v.s. 0.40 OD, p = 0.01). The trend of increasing PAH-DNA adduct levels with either the Asn/Asn or Gln/Gln genotype was greater in tumor tissue than the trend in BBD control tissue.

UV-C-induced DNA damage leads to p53-dependent nuclear trafficking of PML

The promyelocytic leukemia protein (PML) is a nuclear phosphoprotein that localizes to distinct domains in the nucleus, described as PML nuclear bodies (PML-NBs). Recent findings indicate that PML regulates the p53 response to oncogenic signals. Here, we define a p53-dependent role for PML in response to DNA damage. We exposed cells to ultraviolet light (UV-C) and imaged the nuclear distribution of PML, p53, and the BLM helicase by confocal microscopy. After DNA damage, PML partially relocated out of the PML-NBs, and colocalized with BLM and p53 at sites of DNA repair. In addition, using the isogenic HCT116 cell lines (p53+/+ and -/-), we show that the redistribution of PML was dependent on functional p53. Western analysis revealed that the level of PML protein remained unaltered after UV-C treatment. These results are consistent with the hypothesis that PML, in conjunction with p53 and BLM, contributes to the cellular response to UV-C-induced DNA damage and its repair.

DNA repair and transcriptional effects of mutations in TFIIH in Drosophila development

Mutations in XPB and XPD TFIIH helicases have been related with three hereditary human disorders: xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy. The dual role of TFIIH in DNA repair and transcription makes it difficult to discern which of the mutant TFIIH phenotypes is due to defects in any of these different processes. We used haywire (hay), the Drosophila XPB homolog, to dissect this problem. Our results show that when hay dosage is affected, the fly shows defects in structures that require high levels of transcription. We found a genetic interaction between hay and cdk7, and we propose that some of these phenotypes are due to transcriptional deficiencies. We also found more apoptotic cells in imaginal discs and in the CNS of hay mutant flies than in wild-type flies. Because this abnormal level of apoptosis was not detected in cdk7 flies, this phenotype could be related to defects in DNA repair. In addition the apoptosis induced by p53 Drosophila homolog (Dmp53) is suppressed in heterozygous hay flies.

Roles of the Werner syndrome protein in pathways required for maintenance of genome stability

Werners syndrome is a disease of premature aging where the patients appear much older than their chronological age. The gene codes for a protein that is a helicase and an exonuclease, and recently we have learned about some of its protein interactions. These interactions are being discussed as they shed light on the molecular pathways in which Werner protein participates. Insight into these pathways brings insight into the aging process.

DNA helicases, genomic instability, and human genetic disease

DNA helicases are a highly conserved group of enzymes that unwind DNA. They function in all processes in which access to single-stranded DNA is required, including DNA replication, DNA repair and recombination, and transcription of RNA. Defects in helicases functioning in one or more of these processes can result in characteristic human genetic disorders in which genomic instability and predisposition to cancer are common features. So far, different helicase genes have been found mutated in six such disorders. Mutations in XPB and XPD can result in xeroderma pigmentosum, Cockayne syndrome, or trichothiodystrophy. Mutations in the RecQ-like genes BLM, WRN, and RECQL4 can result in Bloom syndrome, Werner syndrome, and Rothmund-Thomson syndrome, respectively. Because XPB and XPD function in both nucleotide excision repair and transcription initiation, the cellular phenotypes associated with a deficiency of each one of them include failure to repair mutagenic DNA lesions and defects in the recovery of RNA transcription after UV irradiation. The functions of the RecQ-like genes are unknown; however, a growing body of evidence points to a function in restarting DNA replication after the replication fork has become stalled. The genomic instability associated with mutations in the RecQ-like genes includes spontaneous chromosome instability and elevated mutation rates. Mouse models for nearly all of these entities have been developed, and these should help explain the widely different clinical features that are associated with helicase mutations.

p53 Modulates the exonuclease activity of Werner syndrome protein

Werner syndrome (WS) is characterized by the early onset of symptoms of premature aging, cancer, and genomic instability. The molecular basis of the defects is not understood but presumably relates to the DNA helicase and exonuclease activities of the protein encoded by the WRN gene that is mutated in the disease. The attenuation of p53-mediated apoptosis in WS cells and reported physical interaction between WRN and the tumor suppressor p53 suggest that p53 and WRN functionally interact in a pathway necessary for the normal cellular response. In this study, we have demonstrated that p53 inhibits the exonuclease activity of the purified full-length recombinant WRN protein. p53 did not have an effect on a truncated amino-terminal WRN fragment that retains exonuclease activity but lacks the physical interaction domain for p53 located in the carboxyl terminus. Two naturally occurring p53 mutants found in human cancer displayed a reduced ability to inhibit WRN exonuclease activity. In cells arrested in S phase with hydroxyurea, WRN exits the nucleolus and colocalizes with p53 in the nucleoplasm. The regulation of WRN function by p53 is likely to play an important role in the maintenance of genomic integrity and prevention of cancer and other clinical symptoms associated with WS.

Functional interaction of p53 and BLM DNA helicase in apoptosis

The Bloom syndrome (BS) protein, BLM, is a member of the RecQ DNA helicase family that also includes the Werner syndrome protein, WRN. Inherited mutations in these proteins are associated with cancer predisposition of these patients. We recently discovered that cells from Werner syndrome patients displayed a deficiency in p53-mediated apoptosis and WRN binds to p53. Here, we report that analogous to WRN, BLM also binds to p53 in vivo and in vitro, and the C-terminal domain of p53 is responsible for the interaction. p53-mediated apoptosis is defective in BS fibroblasts and can be rescued by expression of the normal BLM gene. Moreover, lymphoblastoid cell lines (LCLs) derived from BS donors are resistant to both gamma-radiation and doxorubicin-induced cell killing, and sensitivity can be restored by the stable expression of normal BLM. In contrast, BS cells have a normal Fas-mediated apoptosis, and in response to DNA damage normal accumulation of p53, normal induction of p53 responsive genes, and normal G(1)-S and G(2)-M cell cycle arrest. BLM localizes to nuclear foci referred to as PML nuclear bodies (NBs). Cells from Li-Fraumeni syndrome patients carrying p53 germline mutations and LCLs lacking a functional p53 have a decreased accumulation of BLM in NBs, whereas isogenic lines with functional p53 exhibit normal accumulation. Certain BLM mutants (C1055S or Delta133-237) that have a reduced ability to localize to the NBs when expressed in normal cells can impair the localization of wild type BLM to NBs and block p53-mediated apoptosis, suggesting a dominant-negative effect. Taken together, our results indicate both a novel mechanism of p53 function by which p53 mediates nuclear trafficking of BLM to NBs and the cooperation of p53 and BLM to induce apoptosis.

Transcriptional regulation of the TFIIH transcription repair components XPB and XPD by the hepatitis B virus x protein in liver cells and transgenic liver tissue

Human hepatitis B virus is a risk factor for the development of hepatocellular carcinoma. The hepatitis B virus x protein (HBx) has been shown to inactivate the p53 tumor suppressor protein and impair DNA repair, cell cycle, and apoptosis mechanisms. Herein we report that HBx represses two components of the transcription-repair factor TFIIH, XPB (p89), and XPD (p80), both in p53-proficient and p53-deficient liver cells. This inhibition is observed while HBx maintains its transactivation function. Expression of HBx in liver cells results in down-regulation of endogenous XPB and XPD mRNAs and proteins; this inhibition is not observed with other TFIIH subunits, XPA or PCNA. In liver tissue from HBx transgenics, XPB and XPD proteins are down-regulated in comparison to matched normal liver tissue. HBx has been shown to interact with Sp1 transcription factor and affects its DNA binding activity. Sp1 is essential for the basal promoter activity of XPB in liver cells and Drosophila SL2 cells. In the Sp1-deficient SL2 cells, HBx-induced XPB and XPD inhibition is Sp1-dependent. In summary, our results provide evidence that HBx represses the expression of key TFIIH proteins at least in part through Sp1 elements; this repression may impair TFIIH function in DNA repair mechanisms.

Efficiency alleles of the Pctr1 modifier locus for plasmacytoma susceptibility

The susceptibility of BALB/c mice to pristane-induced plasmacytomas is a complex genetic trait involving multiple loci, while DBA/2 and C57BL/6 strains are genetically resistant to the plasmacytomagenic effects of pristane. In this model system for human B-cell neoplasia, one of the BALB/c susceptibility and modifier loci, Pctr1, was mapped to a 5.7-centimorgan (cM) chromosomal region that included Cdkn2a, which encodes p16(INK4a) and p19(ARF), and the coding sequences for the BALB/c p16(INK4a) and p19(ARF) alleles were found to be polymorphic with respect to their resistant Pctr1 counterparts in DBA/2 and C57BL/6 mice (45). In the present study, alleles of Pctr1, Cdkn2a, and D4Mit15 from a resistant strain (BALB/cDAG) carrying DBA/2 chromatin were introgressively backcrossed to the susceptible BALB/c strain. The resultant C.DAG-Pctr1 Cdkn2a D4Mit15 congenic was more resistant to plasmacytomagenesis than BALB/c, thus narrowing Pctr1 to a 1.5-cM interval. Concomitantly, resistant C57BL/6 mice, from which both gene products of the Cdkn2a gene have been eliminated, developed pristane-induced plasma cell tumors over a shorter latency period than the traditionally susceptible BALB/cAn strain. Biological assays of the p16(INK4a) and p19(ARF) alleles from BALB/c and DBA/2 indicated that the BALB/c p16(INK4a) allele was less active than its DBA/2 counterpart in inducing growth arrest of mouse plasmacytoma cell lines and preventing ras-induced transformation of NIH 3T3 cells, while the two p19(ARF) alleles displayed similar potencies in both assays. We propose that the BALB/c susceptibility/modifier locus, Pctr1, is an "efficiency" allele of the p16(INK4a) gene.

P53 and IGFBP-3: apoptosis and cancer protection

p53, perhaps the single most important human tumor suppressor, is commonly mutated in human cancers. Normally genotoxic stress and hypoxia activate p53, which, through DNA-specific transcription activation, transcriptional repression, and protein-protein interactions, triggers cell cycle arrest and apoptosis. One of the genes induced by p53 was identified as that encoding the insulin-like growth factor binding protein (IGFBP)-3. IGFBP-3 was originally defined by the somatomedin hypothesis as the principal carrier of IGF-I in the circulation and the primary regulator of the amount of free IGF-I available to interact with the IGF-1 receptor. However, there is accumulating evidence that IGFBP-3 can also cause apoptosis in an IGF-independent manner. Thus, IGFBP-3 induction by p53 constitutes a new means of cross-talk between the p53 and IGF axes, and suggests that the ultimate function of IGFBP-3 may be to serve a protective role against the potentially carcinogenic effects of growth hormone and IGF-I.