Protein Xeroderma Pigmentosum Research Articles

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

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

Decoding the Link Between XPC and Lung Cancer Susceptibility: A Study of Cigarette Smoke-Induced DNA Damage in the Setting of XPC Deficiency

Background/Objective:&#x0D; Despite the known mutagenicity of cigarette smoke, only 10-15% of smokers will develop lung cancer in their lifetimes. What determines a smoker’s susceptibility to lung cancer is poorly understood. We identify the nucleotide excision repair (NER) protein Xeroderma pigmentosum Complementation Group C (XPC) as a tumor suppressor that may contribute to lungtumorigenesis when mutated and combined with cigarette smoke. Micronuclei, which are chromosome fragments and/or lagging chromosomes separated from the main nucleus, occur in many cancers, and indicate genomic instability. We hypothesize that cigarette smoke and XPC knockdown will cause genomic instability that will activate the DNA Damage Response (DDR) and increase the frequency of micronuclei and nuclear aberrancies.&#x0D; Methods:&#x0D; A human bronchial epithelial cell line (Beas-2B), and two lung adenocarcinoma cell lines (H1299 and A549) with stable XPC lentiviral knock-down (shXPC) or control shRNA (shCtrl) were treated with cigarette smoke extract (CSE) or air control (AC). DNA Damage Response (DDR) proteins were analyzed via immunoblot (Western). Micronuclei and nuclear aberrancies were quantified through cytokinesis-block micronucleus assay (CBMT) using immunofluorescence microscopy (DAPI).&#x0D; Results:&#x0D; Both CSE and XPC knockdown independently amplify expression of phospho-ATM (pATM), a DDR protein, in H1299 cells. Nuclear aberrancies increased significantly (p&lt;0.05) with CSE in all three cell lines. Micronucleus frequency increased significantly with CSE in H1299 and Beas-2B cells (p&lt;0.05) and with XPC knockdown in Beas-2B cells compared to shCtrl (p&lt;0.001).&#x0D; Scientific Impact and Implications:&#x0D; We identified a previously uncharacterized role of XPC deficiency in augmenting cigarette smoke induced chromosomal breaks manifesting as micronuclei, particularly in non-cancerous Beas-2B cells. These findings offer insight into tumorigenesis in cigarette smoking and shed light on mechanisms of continued DNA damage in cancer cells. Future research should clarify the mechanisms of micronucleus formation in human translational specimens and pinpoint additional functions of XPC beyond NER, including in replication repair.

Two interaction surfaces between XPA and RPA organize the preincision complex in nucleotide excision repair

The xeroderma pigmentosum protein A (XPA) and replication protein A (RPA) proteins fulfill essential roles in the assembly of the preincision complex in the nucleotide excision repair (NER) pathway. We have previously characterized the two interaction sites, one between the XPA N-terminal (XPA-N) disordered domain and the RPA32 C-terminal domain (RPA32C), and the other with the XPA DNA binding domain (DBD) and the RPA70AB DBDs. Here, we show that XPA mutations that inhibit the physical interaction in either site reduce NER activity in biochemical and cellular systems. Combining mutations in the two sites leads to an additive inhibition of NER, implying that they fulfill distinct roles. Our data suggest a model in which the interaction between XPA-N and RPA32C is important for the initial association of XPA with NER complexes, while the interaction between XPA DBD and RPA70AB is needed for structural organization of the complex to license the dual incision reaction. Integrative structural models of complexes of XPA and RPA bound to single-stranded/double-stranded DNA (ss/dsDNA) junction substrates that mimic the NER bubble reveal key features of the architecture of XPA and RPA in the preincision complex. Most critical among these is that the shape of the NER bubble is far from colinear as depicted in current models, but rather the two strands of unwound DNA must assume a U-shape with the two ss/dsDNA junctions localized in close proximity. Our data suggest that the interaction between XPA and RPA70 is key for the organization of the NER preincision complex.

An Active Learning Framework Improves Tumor Variant Interpretation.

A novel machine learning approach predicts the impact of tumor mutations on cellular phenotypes, overcomes limited training data, minimizes costly functional validation, and advances efforts to implement cancer precision medicine.

Cover Image

AbstractOver the past years, significant advances have been made regarding the proteins intimately associated with the main coordinators of the nucleotide excision repair pathway, the xeroderma pigmentosum proteins. The image depicts their closest partners: RAD23B, CETN2, DDB1, ERCC1, RPA70, RPA32, RPA14, and the TFIIH complex. See article by Feltes, pp. 2187–2205. image

HSSB2 (NABP1) is required for the recruitment of RPA during the cellular response to DNA UV damage

Maintenance of genomic stability is critical to prevent diseases such as cancer. As such, eukaryotic cells have multiple pathways to efficiently detect, signal and repair DNA damage. One common form of exogenous DNA damage comes from ultraviolet B (UVB) radiation. UVB generates cyclobutane pyrimidine dimers (CPD) that must be rapidly detected and repaired to maintain the genetic code. The nucleotide excision repair (NER) pathway is the main repair system for this type of DNA damage. Here, we determined the role of the human Single-Stranded DNA Binding protein 2, hSSB2, in the response to UVB exposure. We demonstrate that hSSB2 levels increase in vitro and in vivo after UVB irradiation and that hSSB2 rapidly binds to chromatin. Depletion of hSSB2 results in significantly decreased Replication Protein A (RPA32) phosphorylation and impaired RPA32 localisation to the site of UV-induced DNA damage. Delayed recruitment of NER protein Xeroderma Pigmentosum group C (XPC) was also observed, leading to increased cellular sensitivity to UVB. Finally, hSSB2 was shown to have affinity for single-strand DNA containing a single CPD and for duplex DNA with a two-base mismatch mimicking a CPD moiety. Altogether our data demonstrate that hSSB2 is involved in the cellular response to UV exposure.

Every protagonist has a sidekick: Structural aspects of human xeroderma pigmentosum-binding proteins in nucleotide excision repair.

The seven xeroderma pigmentosum proteins (XPps), XPA-XPG, coordinate the nucleotide excision repair (NER) pathway, promoting the excision of DNA lesions caused by exposition to ionizing radiation, majorly from ultraviolet light. Significant efforts are made to investigate NER since mutations in any of the seven XPps may cause the xeroderma pigmentosum and trichothiodystrophy diseases. However, these proteins collaborate with other pivotal players in all known NER steps to accurately exert their purposes. Therefore, in the old and ever-evolving field of DNA repair, it is imperative to reexamine and describe their structures to understand NER properly. This work provides an up-to-date review of the protein structural aspects of the closest partners that directly interact and influence XPps: RAD23B, CETN2, DDB1, RPA (RPA70, 32, and 14), p8 (GTF2H5), and ERCC1. Structurally and functionally vital domains, regions, and critical residues are reexamined, providing structural lessons and perspectives about these indispensable proteins in the NER and other DNA repair pathways. By gathering all data related to the major human xeroderma pigmentosum-interacting proteins, this review will aid newcomers on the subject and guide structural and functional future studies.

Isoconazole and Clemizole Hydrochloride Partially Reverse the Xeroderma Pigmentosum C Phenotype.

Xeroderma Pigmentosum protein C (XPC) is involved in recognition and repair of bulky DNA damage such as lesions induced by Ultra Violet (UV) radiation. XPC-mutated cells are, therefore, photosensitive and accumulate UVB-induced pyrimidine dimers leading to increased cancer incidence. Here, we performed a high-throughput screen to identify chemicals capable of normalizing the XP-C phenotype (hyper-photosensitivity and accumulation of photoproducts). Fibroblasts from XP-C patients were treated with a library of approved chemical drugs. Out of 1280 tested chemicals, 16 showed ≥25% photo-resistance with RZscore above 2.6 and two drugs were able to favor repair of 6-4 pyrimidine pyrimidone photoproducts (6-4PP). Among these two compounds, Isoconazole could partially inhibit apoptosis of the irradiated cells especially when cells were post-treated directly after UV irradiation while Clemizole Hydrochloride-mediated increase in viability was dependent on both pre and post treatment. No synergistic effect was recorded following combined drug treatment and the compounds exerted no effect on the proliferative capacity of the cells post UV exposure. Amelioration of XP-C phenotype is a pave way towards understanding the accelerated skin cancer initiation in XP-C patients. Further examination is required to decipher the molecular mechanisms targeted by these two chemicals.

XAB2 TagSNP Is Associated with the Risk of Gastric Cancer in Chinese Population: A Case-Control Study.

XAB2 protein (xeroderma pigmentosum group A-binding protein 2) plays a significant role in the nucleotide excision repair pathway. Polymorphisms in the XAB2 gene may have an effect on the capability of DNA repair and further contribute to the risk of developing various cancers. In order to investigate the relationship between XAB2 genetic variants and the risk of gastric cancer, we performed a hospital-based case–control study. XAB2 tagSNPs were selected and then genotyped by iPlex Gold Genotyping Assay and Sequenom MassArray. By performing logistic regression analysis, odds ratio (OR) and 95% confidence interval (CI) were used to estimate the association of XAB2 tagSNPs with the risk of gastric cancer. Our results showed that XAB2 rs794078AA genotype was associated with a significantly lower risk of gastric cancer compared with GG genotype with OR (95% CI) of 0.33 (0.12–0.91). Stratified analysis indicated a significantly decreased risk for gastric cancer among smokers with rs794078AA genotype compared with nonsmokers with GG genotype (OR = 0.11, 95% CI = 0.01–0.91, p = 0.040). The gene–gene interactions by multifactor dimensionality reduction (MDR) showed that tagSNP rs794078 was the best predictive element for gastric cancers (Testing Bal. Acc = 51.68%, p = 0.055, cross-validation consistency = 9). Therefore, the XAB2 tagSNP rs794078 may play an important role in the development of gastric cancer.

DNA base excision repair and nucleotide excision repair proteins in malignant salivary gland tumors

ObjectiveTo analyze the immunohistochemical expression of the base excision repair (BER) proteins apurinic/apyrimidinic endonuclease 1 (APE1) and X-ray repair cross-complementing protein 1 (XRCC1) and nucleotide excision repair (NER) protein xeroderma pigmentosum group F (XPF) in malignant salivary gland tumors (MSGTs). DesignSixty-two cases of MSGTs were selected, including 14 acinic cell carcinomas (AcCC), 15 polymorphous adenocarcinomas (PAC), 16 adenoid cystic carcinomas (ACC), and 17 mucoepidermoid carcinomas (MEC). The specimens were submitted to quantitative immunohistochemical analysis. ResultsAll MSGTs exhibited nuclear or nucleo-cytoplasmic immunostaining of APE1, XRCC1 and XPF, with a high percentage of positive cells (median = 78.31, 70.48 and 75.46, respectively). XRCC1 expression was higher in PAC compared to MEC (p = 0.032). Nuclear APE1 immunostaining was significantly higher than nucleo-cytoplasmic expression in the selected MSGTs (p < 0.0001). APE1 expression was significantly associated with T1-T2 tumors in ACC (p = 0.006). Increased expression of XPF was associated with age older than 60 years in MEC (p = 0.015) and with ACC involving the minor salivary gland (p = 0.012), while a lower expression was found in AcCC and ACC patients treated by surgery combined with adjuvant therapy (p = 0.036 and p = 0.020, respectively). Low expression of XRCC1 in the nucleus (p = 0.028) and concomitant expression of this protein in the nucleus/cytoplasm were associated with a lower overall 5-year survival rate (p = 0.017). ConclusionsThis study showed that BER and NER proteins evaluated are highly expressed in the MSGTs studied, indicating mechanisms of genotoxic control in these tumors. In addition, the dysregulation of XRCC1 expression was a prognostic predictor in MSGTs analyzed.

Association of XPD Asp312Asn polymorphism and response to oxaliplatin-based first-line chemotherapy and survival in patients with metastatic colorectal cancer.

Identification of biomarkers predicting a response to chemotherapeutic drugs would greatly ease the selection of personalized therapy. The protein xeroderma pigmentosum group D (XPD) functions in nucleotide excision repair (NER) to remove DNA cross-links and in the regulation of transcription. The potential role of the Asp312Asn polymorphism in predicting the response to chemotherapy has not been established. This prospective study was designed to determine the role of the XPD Asp312Asn polymorphism in predicting the response to oxaliplatin-based first-line chemotherapy and survival in patients with metastatic colorectal cancer. A total of 106 patients treated with 2 cycles of either FOLFOX4 (n = 72) or XELOX (n = 34) regimen as the chemotherapy were enrolled. The genotype of XPD Asp312Asn polymorphism was analyzed using TaqMan probe-based real-time polymerase chain reaction (PCR). Logistic regression was applied to predict the response to treatment protocols. Cox regression models were applied to predict overall survival. The overall response to chemotherapy was 57.6% (61/106). FOLFOX4 and XELOX regimens demonstrated comparable efficacy. The XPD Asp312Asn polymorphism was not associated with the response to either FOLFOX4 or XELOX regimen in univariate and in multivariate logistic regression analyses. Levels of carcinoembryonic antigen (CEA) ≥5 ng/mL and female gender were associated with a lack of response to FOLFOX4, but not to XELOX regimen. In a multivariate survival analysis, XPD Asp312Asn AA genotype, lack of response to chemotherapy, CEA ≥ 5 ng/mL, and age ≥65 were significantly associated with worse overall survival. The XPD Asp312Asn polymorphism is associated with overall survival, but it is not a biomarker in predicting the response to oxaliplatin-based first-line chemotherapy in patients with metastatic colorectal cancer.

XPC-Related Protection Against Carcinogen-Induced Lung Adenocarcinoma is Independent of Lung Inflammation

Background and Hypothesis: Cigarette smoke (CS) exposure causes lung cancer, with both DNA damage and local inflammation playing a critical role in development. Our previous research links the DNA repair protein Xeroderma Pigmentosum Group C (XPC) with protection against lung cancer in CS- and carcinogen-exposed mouse models. In mice (XPC-deficient and wild-type [WT] littermates) exposed to continuous CS for 9 months, neither XPC-deficient nor WT mice develop lung cancer. XPC-deficient but not WT mice exposed to 5 months CS + 4 months air control (AC) (recovery model) develop lung cancer. In a direct carcinogen model, XPC-deficiency accelerates NTCU lung squamous cell development. Our hypothesis is that lung cancers in XPC-deficient mice are independent of treatment alterations in BAL inflammation.&#x0D; Experimental Design or Project Methods: Acellular bronchoalveolar lavage (BAL) samples from the three different mouse CS and carcinogen models were analyzed for the inflammatory cytokines IL-6, IL10, IL-12p70, IL-17A, IFN-γ, MCP-1, and TNF-α by a cytometric bead array (BD Biosciences) using a flow cytometer (FACScan) according to the manufacturer’s instructions. Raw data (mean fluorescence intensity) was analyzed by BD CBA Software. Statistical comparisons were by ANOVA, with p&lt;0.05 considered significant.&#x0D; Results: All measured cytokine levels were low or undetectable in BAL from AC and CS mice, with no significant XPC genotype or treatment-related changes in the measured cytokines. Differences were observed in TNF-α and IL-6 between continuous and recovery CS models independent of treatment or genotype. NTCU caused a significant increase in BAL IL-6 independent of genotype. IL-17a was elevated in NTCU-treated mice that developed lung squamous cell carcinoma.&#x0D; Conclusion and Potential Impact: The XPC genotypic variation seen in lung carcinogenesis appears to be independent of BAL cytokines, suggesting that the variation is due to DNA damage rather than differences in local inflammation. Further mechanistic investigations will focus on DNA damage and repair as drivers of XPC-deficient lung cancers.&#x0D;

Polynuclear ruthenium organometallic compounds induce DNA damage in human cells identified by the nucleotide excision repair factor XPC.

Ruthenium organometallic compounds represent an attractive avenue in developing alternatives to platinum-based chemotherapeutic agents. While evidence has been presented indicating ruthenium-based compounds interact with isolated DNA in vitro, it is unclear what effect these compounds exert in cells. Moreover, the antibiotic efficacy of polynuclear ruthenium organometallic compounds remains uncertain. In the present study, we report that exposure to polynuclear ruthenium organometallic compounds induces recruitment of damaged DNA sensing protein Xeroderma pigmentosum Group C into chromatin-immobilized foci. Additionally, we observed one of the tested polynuclear ruthenium organometallic compounds displayed increased cytotoxicity against human cells deficient in nucleotide excision repair (NER). Taken together, these results suggest that polynuclear ruthenium organometallic compounds induce DNA damage in cells, and that cellular resistance to these compounds may be influenced by the NER DNA repair phenotype of the cells.

Green Fluorescent Probe for Imaging His6-Tagged Proteins Inside Living Cells.

Small molecule-based fluorescent probes offer great opportunities for specifically tracking proteins in living systems with minimal perturbation on the protein function and localization. Herein, we report a small green fluorescent probe (Ni2+- NTA-AF) consisting of a Ni2+-NTA moiety, a fluorescein, and an arylazide group, that binds specifically to His6-tagged proteins with fluorescence enhancement in vitro upon photoactivation of the arylazide group. Importantly, the probe can cross the cell membranes and stoichiometrically label His6-tagged proteins rapidly (∼15 min) in living prokaryotic and eukaryotic cells exemplified by a DNA repair protein Xeroderma pigmentosum group A (XPA). Using the probe, we successfully visualized Sirtuin 5, which is localized to the mitochondria. This probe exhibits high quantum yields and improved solubility, offering a new opportunity for imaging intracellular His6-tagged proteins inside living cells with better contrast.

XPC protects against smoking- and carcinogen-induced lung adenocarcinoma.

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

Alternative excision repair of topoisomerase inhibitor-induced DNA damage

Topoisomerase 1 (Top1) and 2 (Top2) are common targets of anti-cancer drugs, which induce specific DNA lesions that are thought to lead to cell death in tumour cells. Camptothecin blocks DNA religation in the Top1–DNA complex and induces Top1-attached nick DNA lesions (Top1–DNA damage). We propose that nucleotide excision repair (NER) factors, excision repair cross complementing 1 protein-xeroderma pigmentosum group F (ERCC1–XPF) endonuclease, and replication protein A (RPA), participate in the repair of Top1-attached nick DNA lesions. ERCC1–XPF exhibits nuclease activity in the presence of RPA on a 3′-phosphotyrosyl bond nick-containing DNA (3′-Tyr-nick DNA) substrate, which mimics a Top1-attached nick DNA lesion. Moreover, repair of the 3′-Tyr-nick DNA occurred in the presence of NER factors, including ERCC1–XPF, RPA, DNA polymerase delta, flap endonuclease 1 (FEN1) and DNA ligase 1 (LIG1). We also illustrate a repair model for DNA lesions induced by Top2 inhibitor etoposide (VP-16), Top2-attached nick DNA (Top2-DNA damage). FEN1 participates in the cleavage of the 5′-phosphotyrosyl bond nick-containing DNA (5′-Tyr-nick DNA) substrates, which mimic Top2-attached nick DNA lesions. Under DNA repair synthesis conditions, FEN1 efficiently repaired the 5′-Tyr-nick DNA substrates in the presence of other base excision repair (BER) factors, LIG1 and DNA polymerase beta (POLB). Taken together, some of the NER and BER machinery might represent an alternative repair pathway for Top1- and Top2-DNA damage.

Abstract B14: Characterization of p53-HDM2 inhibitor in nucleotide excision repair deficient cell line

Abstract The ubiquitin (Ub)/proteasome system has a special role in the control on the cellular concentration of p53 tumor suppressor, a master regulator of cell survival and death. Over the last years, many groups have elucidated the mechanisms of Ub ligase–mediated p53 ubiquitylation under various conditions. One pivotal regulatory mechanism is HDM2-mediated p53 degradation, but it can also export p53 from nucleus impairing its transcriptional activity or by binding p53 transactivation domain, that inhibits p53 transcriptional activity. Rad23 can directly bind Ub and the proteasome and has been shown to act as an adaptor that shuttles the ubiquitylated proteins to the proteasome. Rad23 is required for the degradation of p53 and one possible helper of Rad23 is its binding protein XPC, a key factor for nucleotide excision repair (NER). Mutations in XP proteins can cause xeroderma pigmentosum (XP), a genetic disease that often leads to high rates of skin cancer. Pathogenic mutations in p53 are found in more than 50% of all human malignancies, which makes p53 the most frequently mutated gene in transformed cells in humans. In cancers in which the p53 gene is not mutated, the function of the p53 pathway is often suppressed through overexpression or deregulation of HDM2. We aimed to study the impact of HDM2-P53 inhibition in XPC-deficient fibroblasts by treating these cells with the drug CGM9097, which is a potent inhibitor of HDM2-p53 protein-protein interaction used in preclinical trials. In this poster we present preliminary data. We firstly evaluated CGM9097 cytotoxicity in XPC deficient and corrected cells, combined with 5-fluorouracil (5FU) treatment. XPC deficient cells were more sensitive to 5FU+ CGM9097 than normal fibroblast (MRC5) and correct fibroblast (XPC+ XPC gene) (P&amp;lt;0,05). Protein analysis by Western blotting showed dose-response expression of p53 in both lineages when treated with 5FU, with the higher expression level within 6 h. When cells were treated with 5FU+ CGM9097, XPC-deficient cells presented high levels of p53 even in the untreated fraction compared to corrected cell line, suggesting an impairment of p53 degradation in the absence of XPC. These preliminary data suggest that XPC has an important role in the cytotoxicity of 5FU in cells with impairment of p53. P53 mutant cancers are sensitive to some chemotherapeutic drugs, but the reported sensitivity is generally modest (2-3 fold), and this can be explained by the fact that p53 affects the GGR NER sub pathway through GADD45. In addition, p53-mediated DNA repair may be counterbalanced by the opposing response of apoptosis, also regulated in part by p53. Thus, there appears to be much potential to exploit DNA repair defects in p53 mutant cancers. Citation Format: Douglas Felipe de Lima Silva, Ana Rafaela de Souza Timoteo, Lucymara Fassarela Agnez Lima, Tirzah Braz Petta Lajus. Characterization of p53-HDM2 inhibitor in nucleotide excision repair deficient cell line [abstract]. In: Proceedings of the AACR International Conference held in cooperation with the Latin American Cooperative Oncology Group (LACOG) on Translational Cancer Medicine; May 4-6, 2017; São Paulo, Brazil. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(1_Suppl):Abstract nr B14.

RAD4 and RAD23/HMR Contribute to Arabidopsis UV Tolerance.

In plants, exposure to solar ultraviolet (UV) light is unavoidable, resulting in DNA damage. Damaged DNA causes mutations, replication arrest, and cell death, thus efficient repair of the damaged DNA is essential. A light-independent DNA repair pathway called nucleotide excision repair (NER) is conserved throughout evolution. For example, the damaged DNA-binding protein Radiation sensitive 4 (Rad4) in Saccharomyces cerevisiae is homologous to the mammalian NER protein Xeroderma Pigmentosum complementation group C (XPC). In this study, we examined the role of the Arabidopsis thaliana Rad4/XPC homologue (AtRAD4) in plant UV tolerance by generating overexpression lines. AtRAD4 overexpression, both with and without an N-terminal yellow fluorescent protein (YFP) tag, resulted in increased UV tolerance. YFP-RAD4 localized to the nucleus, and UV treatment did not alter this localization. We also used yeast two-hybrid analysis to examine the interaction of AtRAD4 with Arabidopsis RAD23 and found that RAD4 interacted with RAD23B as well as with the structurally similar protein HEMERA (HMR). In addition, we found that hmr and rad23 mutants exhibited increased UV sensitivity. Thus, our analysis suggests a role for RAD4 and RAD23/HMR in plant UV tolerance.

Differential sensitivities of cellular XPA and PARP-1 to arsenite inhibition and zinc rescue

Arsenite directly binds to the zinc finger domains of the DNA repair protein poly (ADP ribose) polymerase (PARP)-1, and inhibits PARP-1 activity in the base excision repair (BER) pathway. PARP inhibition by arsenite enhances ultraviolet radiation (UVR)-induced DNA damage in keratinocytes, and the increase in DNA damage is reduced by zinc supplementation. However, little is known about the effects of arsenite and zinc on the zinc finger nucleotide excision repair (NER) protein xeroderma pigmentosum group A (XPA). In this study, we investigated the difference in response to arsenite exposure between XPA and PARP-1, and the differential effectiveness of zinc supplementation in restoring protein DNA binding and DNA damage repair. Arsenite targeted both XPA and PARP-1 in human keratinocytes, resulting in zinc loss from each protein and a pronounced decrease in XPA and PARP-1 binding to chromatin as demonstrated by Chip-on-Western assays. Zinc effectively restored DNA binding of PARP-1 and XPA to chromatin when zinc concentrations were equal to those of arsenite. In contrast, zinc was more effective in rescuing arsenite-augmented direct UVR-induced DNA damage than oxidative DNA damage. Taken together, our findings indicate that arsenite interferes with PARP-1 and XPA binding to chromatin, and that zinc supplementation fully restores DNA binding activity to both proteins in the cellular context. Interestingly, rescue of arsenite-inhibited DNA damage repair by supplemental zinc was more sensitive for DNA damage repaired by the XPA-associated NER pathway than for the PARP-1-dependent BER pathway. This study expands our understanding of arsenite's role in DNA repair inhibition and co-carcinogenesis.

Role of DNA Repair Factor Xeroderma Pigmentosum Protein Group C in Response to Replication Stress As Revealed by DNA Fragile Site Affinity Chromatography and Quantitative Proteomics.

Replication stress (RS) fuels genomic instability and cancer development and may contribute to aging, raising the need to identify factors involved in cellular responses to such stress. Here, we present a strategy for identification of factors affecting the maintenance of common fragile sites (CFSs), which are genomic loci that are particularly sensitive to RS and suffer from increased breakage and rearrangements in tumors. A DNA probe designed to match the high flexibility island sequence typical for the commonly expressed CFS (FRA16D) was used as specific DNA affinity bait. Proteins significantly enriched at the FRA16D fragment under normal and replication stress conditions were identified using stable isotope labeling of amino acids in cell culture-based quantitative mass spectrometry. The identified proteins interacting with the FRA16D fragment included some known CFS stabilizers, thereby validating this screening approach. Among the hits from our screen so far not implicated in CFS maintenance, we chose Xeroderma pigmentosum protein group C (XPC) for further characterization. XPC is a key factor in the DNA repair pathway known as global genomic nucleotide excision repair (GG-NER), a mechanism whose several components were enriched at the FRA16D fragment in our screen. Functional experiments revealed defective checkpoint signaling and escape of DNA replication intermediates into mitosis and the next generation of XPC-depleted cells exposed to RS. Overall, our results provide insights into an unexpected biological role of XPC in response to replication stress and document the power of proteomics-based screening strategies to elucidate mechanisms of pathophysiological significance.