Molecular cloning and biological characterization of a human gene, ERCC2, that corrects the nucleotide excision repair defect in CHO UV5 cells

Role of Human N-Acetyltransferase 2 Genetic Polymorphism on Aromatic Amine Carcinogen-Induced DNA Damage and Mutagenicity in a Chinese Hamster Ovary Cell Mutation Assay

Carcinogenic aromatic amines such as 4-aminobiphenyl (ABP) and 2-aminofluorene (AF) require metabolic activation to form electrophilic intermediates that mutate DNA leading to carcinogenesis. Bioactivation of these carcinogens includes N-hydroxylation catalyzed by CYP1A2 followed by O-acetylation catalyzed by arylamine N-acetyltransferase 2 (NAT2). To better understand the role of NAT2 genetic polymorphism in ABP- and AF-induced mutagenesis and DNA damage, nucleotide excision repair-deficient (UV5) Chinese hamster ovary (CHO) cells were stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles. ABP and AF both caused significantly (P < 0.001) greater mutagenesis measured at the hypoxanthine phosphoribosyl transferase (hprt) locus in the UV5/CYP1A2/NAT2*4 acetylator cell line compared to the UV5, UV5/CYP1A2, and UV5/CYP1A2/NAT2*5B cell lines. ABP- and AF-induced hprt mutant cDNAs were sequenced and over 80% of the single-base substitutions were at G:C base pairs. DNA damage also was quantified by γH2AX in-cell western assays and by identification and quantification of the two predominant DNA adducts, N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP) and N-(deoxyguanosin-8-yl)-2-aminofluorene (dG-C8-AF) by liquid chromatography-mass spectrometry. DNA damage and adduct levels were dose-dependent, correlated highly with levels of hprt mutants, and were significantly (P < 0.0001) greater in the UV5/CYP1A2/NAT2*4 rapid acetylator cell line following treatment with ABP or AF as compared to all other cell lines. Our findings provide further clarity on the importance of O-acetylation in CHO mutagenesis assays for aromatic amines. They provide evidence that NAT2 genetic polymorphism modifies aromatic amine-induced DNA damage and mutagenesis that should be considered in human risk assessments following aromatic amine exposures. Environ. Mol. Mutagen. 61:235-245, 2020. © 2019 Wiley Periodicals, Inc.

Remodeling of Interstrand Crosslink Proximal Replisomes Is Dependent on ATR, FANCM, and FANCD2

Eukaryotic replisomes are driven by the mini chromosome maintenance (MCM [M]) helicase complex, an offset ring locked around the template for leading strand synthesis by CDC45 (C) and GINS (G) proteins. Although the CDC45 MCM GINS (CMG) structure implies that interstrand crosslinks (ICLs) are absolute blocks to replisomes, recent studies indicate that cells can restart DNA synthesis on the side of the ICL distal to the initial encounter. Here, we report that restart requires ATR and is promoted by FANCD2 and phosphorylated FANCM. Following introduction of genomic ICLs and dependent on ATR and FANCD2 but not on the Fanconi anemia core proteins or FAAP24, FANCM binds the replisome complex, with concomitant release of the GINS proteins. In situ analysis of replisomes proximal to ICLs confirms the ATR-dependent release of GINS proteins while CDC45 is retained on the remodeled replisome. The results demonstrate the plasticity of CMG composition in response to replication stress.

Uranyl acetate induced DNA single strand breaks and AP sites in Chinese hamster ovary cells

The aim of this study is to characterize the genotoxicity of depleted uranium (DU) in Chinese Hamster Ovary cells (CHO) with mutations in various DNA repair pathways. CHO cells were exposed to 0-300 μM of soluble DU as uranyl acetate (UA) for 0-48 h. Intracellular UA concentrations were measured via inductively coupled mass spectrometry (ICP-MS) and visualized by transmission electron microscopy (TEM). Cytotoxicity was assessed in vitro by clonogenic survival assay. DNA damage response was assessed via Fast Micromethod® to determine UA-induced DNA single strand breaks. Results indicate that UA is entering the CHO cells, with the highest concentration localizing in the nucleus. Clonogenic assays show that UA is cytotoxic in each cell line with the greatest cytotoxicity in the base excision repair deficient EM9 cells and the nuclear excision repair deficient UV5 cells compared to the non-homologous end joining deficient V3.3 cells and the parental AA8 cells after 48 h. This indicates that UA is producing single strand breaks and forming UA-DNA adducts rather than double strand breaks in CHO cells. Fast Micromethod® results indicate an increased amount of single strand breaks in the EM9 cells after 48 h UA exposure compared to the V3.3 and AA8 cells. These results indicate that DU induces DNA damage via strand breaks and uranium-DNA adducts in treated cells. These results suggest that: (1) DU is genotoxic in CHO cells, and (2) DU is inducing single strand breaks rather than double strand breaks in vitro.

Trichothiodystrophy: Current Concepts

Background:

Trichothiodystrophy (TTD) is a rare, autosomal recessive disorder in which the hair identifies a group of genetic disorders with deficient high-sulfur matrix proteins, and a defect in excision repair of ultraviolet damage in fibroblasts of most patients. TTD patients may be grouped as follows: (1) the major group with defects in the excision repair cross-complementing gene ERCC2, the gene for xeroderma pigmentosum group D (XP-D); (2) those with defects in ERCC3, the gene for XP-B; and (3) those with a repair defect distinct from those in XP-D and XP-B. In contrast to XP patients, TTD patients have no increased frequency of skin cancers.

Objective:

The article explains the relationship of TTD and XP and helps clarify why TTD patients with defects in the same gene(s) as those with XP do not have increased skin cancers.

Methods:

Methods include biochemical studies, mutational analysis, and genomic sequence analysis of cell lines from skin biopsies of TTD and XP patients.

Results:

The ERCC2 gene is a component of the TFIIH complex which controls two distinct DNA-metabolizing processes, transcription initiation and nucleotide excision repair.

Conclusion:

In TTD, the major defect is in transcription initiation, whereas in XP-D, DNA repair is primarily altered.

XRCC1 Arg194Trp and Arg399Gln polymorphisms and arsenic methylation capacity are associated with urothelial carcinoma

The association between DNA repair gene polymorphisms and bladder cancer has been widely studied. However, few studies have examined the correlation between urothelial carcinoma (UC) and arsenic or its metabolites. The aim of this study was to examine the association between polymorphisms of the DNA repair genes, XRCC1 Arg194Trp, XRCC1 Arg399Gln, XRCC3 Thr241Met, and XPD Lys751Gln, with urinary arsenic profiles and UC. To this end, we conducted a hospital-based case-control study with 324 UC patients and 647 age- and gender-matched non-cancer controls. Genomic DNA was used to examine the genotype of XRCC1 Arg194Trp, XRCC1 Arg399Gln, XRCC3 Thr241Met, and XPD Lys751Gln by PCR-restriction fragment length polymorphism analysis (PCR-RFLP). Urinary arsenic profiles were measured by high performance liquid chromatography (HPLC) linked with hydride generator and atomic absorption spectrometry. The XRCC1 399 Gln/Gln and 194 Arg/Trp and Trp/Trp genotypes were significantly related to UC, and the odds ratio (OR) and 95% confidence interval (95%CI) were 1.68 (1.03-2.75) and 0.66 (0.48-0.90), respectively. Participants with higher total urinary arsenic levels, a higher percentage of inorganic arsenic (InAs%) and a lower percentage of dimethylarsinic acid (DMA%) had a higher OR of UC. Participants carrying XRCC1 risk diplotypes G-C/G-C, A-C/A-C, and A-T/G-T, and who had higher total arsenic levels, higher InAs%, or lower DMA% compared to those with other XRCC1 diplotypes had a higher OR of UC. Our results suggest that the XRCC1 399 Gln/Gln and 194 Arg/Arg DNA repair genes play an important role in poor arsenic methylation capacity, thereby increasing the risk of UC in non-obvious arsenic exposure areas.

Association between XPD Lys751Gln polymorphism and bladder cancer susceptibility: an updated and cumulative meta-analysis based on 6,836 cases and 8,251 controls

The association between xeroderma pigmentosum group D (XPD) Lys751Gln polymorphism and bladder cancer (BC) susceptibility was investigated by two meta-analyses, however, their results were contrary. We conjecture the reason might be the sample size, thus we performed this updated and cumulative meta-analysis using the Comprehensive Meta-Analysis software. We searched PubMed up to August 25th, 2013 and yielded 20 published articles with 21 case-control trails including 6,836 BC patients and 8,251 controls. The meta-analysis results showed that XPD Lys751Gln polymorphism was borderline significantly associated with BC susceptibility for overall population [Gln vs. Lys: OR 1.07, 95% CI 1.01-1.12, P = 0.01; Gln/Gln vs. Lys/Lys: OR 1.15, 95% CI 1.03-1.29, P = 0.01; Gln/Gln vs. (Lys/Gln + Lys/Lys): OR 1.13, 95% CI 1.02-1.26, P = 0.02]. The cumulative meta-analysis according to the publication year showed the CI became increasingly narrower and tended to have statistical significance for the studies incessantly accumulated. In the subgroup analysis according to ethnicity, there was a significant association in Asian population and no association in Caucasian population. There was no publication bias detected. However, due to the limitations and cumulative analysis result of this meta-analysis, more well-designed and larger studies with risk factors adjusted are suggested to be performed to obtain a conclusive result on this topic.

Xeroderma pigmentosum complementation group D (XPD) gene polymorphisms contribute to bladder cancer risk: a meta-analysis

Numerous epidemiological studies have been conducted to investigate the association between Xeroderma pigmentosum complementation group D (XPD) Asp312Asn (rs1799793 G > A) and Lys751Gln (rs13181 A > C) polymorphisms and bladder cancer risk; however, the conclusions remain controversial. With this in mind, we performed this meta-analysis with 11 studies including 3,797 cases and 5,094 controls for Asp312Asn and 21 studies including 6,360 cases and 7,894 controls for Lys751Gln polymorphism. We searched available literatures from PubMed, Embase, and CBM databases. Crude odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to assess the strength of the associations. Moreover, to validate biological plausibility of our findings, the effects of these two polymorphisms on XPD gene expression within three ethnicities was determine by gene expression analysis based on imputed genotypes from HapMap. Overall, the variant allele of Asp312Asn polymorphism was associated with an increased risk of bladder cancer (Asn/Asn vs. Asp/Asp: OR = 1.51, 95% CI = 1.19-1.91; Asp/Asn vs. Asp/Asp: OR = 1.23, 95% CI = 1.12-1.35; recessive model: OR = 1.33, 95% CI = 1.10-1.61; dominant model: OR = 1.32, 95% CI = 1.14-1.52; and allele comparing: OR = 1.26, 95% CI = 1.11-1.42). We found the Lys751Gln was associated with increased bladder cancer risk only under the recessive model (OR = 1.14, 95% CI = 1.01-1.29). Stratification analyses demonstrated an increased risk for Asians and hospital-based studies under all genetic models while only under the dominant model for Caucasians as to the Asp312Asn polymorphism and for Caucasians under the recessive model as to the Lys751Gln polymorphism. We also found the Asp312Asn polymorphism can significantly influence mRNA expression levels among Asians and Caucasians, and the Lys751Gln polymorphism has a similar effect for Caucasians. Despite some limitations, this meta-analysis suggests that polymorphisms in XPD gene may contribute to bladder cancer susceptibility. These findings need further validation by large well-designed prospective studies.

An interethnic variability and a functional prediction of DNA repair gene polymorphisms: the example of XRCC3 (p.Thr241>Met) and XPD (p.Lys751>Gln) in a healthy Tunisian population

Genetic polymorphisms in DNA repair genes might influence the repair activities of the enzymes predisposing individuals to cancer risk. Owing to the presence of these genetic variants, interethnic differences in DNA repair capacity have been observed in various populations. The present study was undertaken to determine the allele and genotype frequencies of two common non-synonymous SNPs, XRCC3 p.Thr241>Met (C > T, rs861539) and XPD p.Lys751>Gln (T > G, rs13181) in a healthy Tunisian population and to compare them with HapMap ( http://www.hapmap.org/ ) populations. Also, we predicted their eventual functional effect based on bioinformatics tools. The genotypes of 154 healthy and unrelated individuals were determined by PCR-RFLP procedure. Our findings showed a close relatedness with Caucasians from European ancestry which might be explained by the strategic geographic location of Tunisia in the Mediterranean, thus allowing exchanges with Europeans countries. The in silico predictions showed that p.Thr241>Met substitution in XRCC3 protein was predicted as possibly damaging, indicating that it is likely to have functional consequences as well. To the best of our knowledge, this is the first study in this regard in Tunisia. So, these data could provide baseline database and help us to explore the relationship of XRCC3 and XPD polymorphisms with both cancer risk and DNA repair variability in our population.

Premature aging and cancer in nucleotide excision repair-disorders

During the past decades, the major impact of DNA damage on cancer as 'disease of the genes' has become abundantly apparent. In addition to cancer, recent years have also uncovered a very strong association of DNA damage with many features of (premature) aging. The notion that DNA repair systems protect not only against cancer but also equally against to fast aging has become evident from a systematic, integral analysis of a variety of mouse mutants carrying defects in e.g. transcription-coupled repair with or without an additional impairment of global genome nucleotide excision repair and the corresponding segmental premature aging syndromes in human. A striking correlation between the degree of the DNA repair deficiency and the acceleration of specific progeroid symptoms has been discovered for those repair systems that primarily protect from the cytotoxic and cytostatic effects of DNA damage. These observations are explained from the perspective of nucleotide excision repair mouse mutant and human syndromes. However, similar principles likely apply to other DNA repair pathways including interstrand crosslink repair and double strand break repair and genome maintenance systems in general, supporting the notion that DNA damage constitutes an important intermediate in the process of aging.

Transcription inhibition by DRB potentiates recombinational repair of UV lesions in mammalian cells

Homologous recombination (HR) is intricately associated with replication, transcription and DNA repair in all organisms studied. However, the interplay between all these processes occurring simultaneously on the same DNA molecule is still poorly understood. Here, we study the interplay between transcription and HR during ultraviolet light (UV)-induced DNA damage in mammalian cells. Our results show that inhibition of transcription with 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB) increases the number of UV-induced DNA lesions (γH2AX, 53BP1 foci formation), which correlates with a decrease in the survival of wild type or nucleotide excision repair defective cells. Furthermore, we observe an increase in RAD51 foci formation, suggesting HR is triggered in response to an increase in UV-induced DSBs, while inhibiting transcription. Unexpectedly, we observe that DRB fails to sensitise HR defective cells to UV treatment. Thus, increased RAD51 foci formation correlates with increased cell death, suggesting the existence of a futile HR repair of UV-induced DSBs which is linked to transcription inhibition.

Monocyclic aromatic amines as potential human carcinogens: old is new again

Alkylanilines are a group of chemicals whose ubiquitous presence in the environment is a result of the multitude of sources from which they originate. Exposure assessments indicate that most individuals experience lifelong exposure to these compounds. Many alkylanilines have biological activity similar to that of the carcinogenic multi-ring aromatic amines. This review provides an overview of human exposure and biological effects. It also describes recent investigations into the biochemical mechanisms of action that lead to the assessment that they are most probably more complex than those of the more extensively investigated multi-ring aromatic amines. Not only is nitrenium ion chemistry implicated in DNA damage by alkylanilines but also reactions involving quinone imines and perhaps reactive oxygen species. Recent results described here indicate that alkylanilines can be potent genotoxins for cultured mammalian cells when activated by exogenous or endogenous phase I and phase II xenobiotic-metabolizing enzymes. The nature of specific DNA damage products responsible for mutagenicity remains to be identified but evidence to date supports mechanisms of activation through obligatory N-hydroxylation as well as subsequent conjugation by sulfation and/or acetylation. A fuller understanding of the mechanisms of alkylaniline genotoxicity is expected to provide important insights into the environmental and genetic origins of one or more human cancers and may reveal a substantial role for this group of compounds as potential human chemical carcinogens.

Effect of rapid human N-acetyltransferase 2 haplotype on DNA damage and mutagenesis induced by 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx)

Heterocyclic amines such as 2-amino-3-methylimidazo-[4,5-f]quinoline (IQ) and 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MeIQx) are dietary carcinogens generated when meats are cooked well-done. Bioactivation includes N-hydroxylation catalyzed by cytochrome P4501A2 (CYP1A2) followed by O-acetylation catalyzed by N-acetyltransferase 2 (NAT2). Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human CYP1A2 and either NAT2*4 (rapid acetylator) or NAT2*5B (slow acetylator) alleles were treated with IQ or MeIQx to examine the effect of NAT2 genetic polymorphism on IQ- or MeIQx-induced DNA adducts and mutagenesis. MeIQx and IQ both induced decreases in cell survival and significantly (p<0.001) greater number of endogenous hypoxanthine phosphoribosyl transferase (hprt) mutants in the CYP1A2/NAT2*4 than the CYP1A2/NAT2*5B cell line. IQ- and MeIQx-induced hprt mutant cDNAs were sequenced and over 85% of the mutations were single-base substitutions with the remainder exon deletions likely caused by splice-site mutations. For the single-base substitutions, over 85% were at G:C base pairs. Deoxyguanosine (dG)-C8-IQ and dG-C8-MeIQx adducts were significantly (p<0.001) greater in the CYP1A2/NAT2*4 than the CYP1A2/NAT2*5B cell line. DNA adduct levels correlated very highly with hprt mutants for both IQ and MeIQx. These results suggest substantially increased risk for IQ- and MeIQx-induced DNA damage and mutagenesis in rapid NAT2 acetylators.

Differences between human slow N-acetyltransferase 2 alleles in levels of 4-aminobiphenyl-induced DNA adducts and mutations

Aromatic amines such as 4-aminobiphenyl (ABP) require biotransformation to exert their carcinogenic effects. Genetic polymorphisms in biotransformation enzymes such as N-acetyltransferase 2 (NAT2) may modify cancer risk following exposure. Nucleotide excision repair-deficient Chinese hamster ovary (CHO) cells stably transfected with human cytochrome P4501A1 (CYP1A1) and a single copy of either NAT2*4 (rapid acetylator), NAT2*5B (common Caucasian slow acetylator), or NAT2*7B (common Asian slow acetylator) alleles (haplotypes) were treated with ABP to test the effect of NAT2 polymorphisms on DNA adduct formation and mutagenesis. ABP N-acetyltransferase catalytic activities were detectable only in cell lines transfected with NAT2 and were highest in cells transfected with NAT2*4, lower in cells transfected with NAT2*7B, and lowest in cells transfected with NAT2*5B. Following ABP treatment, N-(deoxyguanosin-8-yl)-4-aminobiphenyl (dG-C8-ABP) was the primary adduct formed. Cells transfected with both CYP1A1 and NAT2*4 showed the highest concentration-dependent cytotoxicity, hypoxanthine phosphoribosyl transferase (hprt) mutants, and dG-C8-ABP adducts. Cells transfected with CYP1A1 and NAT2*7B showed lower levels of cytotoxicity, hprt mutagenesis, and dG-C8-ABP adducts. Cells transfected with CYP1A1 only or cells transfected with both CYP1A1 and NAT2*5B did not induce cytotoxicity, hprt mutagenesis or dG-C8-ABP adducts. ABP-DNA adduct levels correlated very highly (r>0.96) with ABP-induced hprt mutant levels following each treatment. The results of the present study suggest that investigations of NAT2 genotype or phenotype associations with disease or toxicity could be more precise and reproducible if heterogeneity within the "slow" NAT2 acetylator phenotype is considered and incorporated into the study design.

XPD Lys(751)Gln and Asp (312)Asn polymorphisms and bladder cancer risk: a meta-analysis

Studies on the polymorphisms of Xeroderma Pigmentosum Group D (XPD) have shown inconclusive trends in the risk of bladder cancer. The purpose of this study is to evaluate the role of XPD single nucleotide polymorphisms in bladder cancer susceptibility. We performed a meta-analysis on all available studies, which included 5,368 and 6,683 XPD Lys(751)Gln cases and controls and 3,220 and 4,391 Asp(312)Asn cases and controls, respectively. Overall, Significant risk effects of Lys(751)Gln genotype was found under recessive model contrast [Gln/Gln vs. (Gln/Lys + Lys/Lys)] [P = 0.04, OR = 1.12; 95% CI (1.01, 1.26)], and subtle but insignificantly increased risks between Lys(751)Gln and bladder cancer were observed under allele contrast (Gln vs. Lys) and homologous contrast (Gln/Gln vs. Lys/Lys) in all subjects. The (751)Gln allele had no significant effect on bladder cancer in all subgroups (Asian, Caucasian and USA). Significant risk effects of Asp(312)Asn polymorphism on bladder susceptibility were observed in all subjects under all genetic contrasts, however, stratified analyses showed that the (312)Asn allele showed different risk effects in USA and Caucasian. The Gln/Gln genotype acts as a risk factor in its association with bladder cancer, and the effect of Lys(751)Gln polymorphism on bladder susceptibility should be studied with larger, stratified population; the (312)Asn allele has an important role in the etiology of bladder cancer whereas the ethnic background should be carefully concerned in further studies.

Relationship between DNA lesions, DNA repair and chromosomal damage induced by acetaldehyde

Acetaldehyde (AA) was tested along with two other crosslinking agents: formaldehyde (FA), an inducer of DNA-protein crosslinks (DPCs) and mitomycin C (MMC), an inducer of interstrand crosslinks (ICLs), to find out whether the mechanism of action of AA resembles more MMC or FA. Using a modification of the standard protocol for comet assay we demonstrate that AA induces crosslinks. Using a combination of alkaline comet version and proteinase-K, a clear abrogation of AA-induced reduction in DNA migration, like after FA treatment, was observed demonstrating that both agents induce DPCs, whereas MMC induces predominantly ICLs. A possible correlation between the types of induced crosslink and the induction chromosome damage in different repair deficient mutant Chinese hamster ovary cell lines treated with AA, MMC and FA was investigated. TCR/NER pathways are involved in repairing FA induced DPCs, but less in AA-induced DPCs. Our preliminary data suggest that DPCs are weaker inducers of SCEs in comparison with ICLs.

Associations betweenXRCC1andERCC2polymorphisms and DNA damage in peripheral blood lymphocyte among coke oven workers

A wide variety of base damages and single-strand breaks formed by reactive oxygen species during metabolic activation of polycyclic aromatic hydrocarbons (PAHs) have been recognized to be involved in PAH carcinogenesis. In this study, alkaline comet assay was used to detect the DNA damage in peripheral blood lymphocytes among 143 coke-oven workers and 50 non-coke-oven workers, and the effects of genetic polymorphisms of XRCC1 and ERCC2 genes on DNA damage were evaluated. The olive tail moment was significantly higher in coke-oven workers than in non-coke-oven workers (2.6, 95% CI=2.1-3.3 versus 1.0, 95% CI=0.8-1.2, p<0.01), and significant correlation between ln-transformed urinary 1-OHP and ln-transformed olive tail moment was found in total population (n=193, Pearson's r=0.393, p<0.001) and in coke-oven workers (n=143, Pearson's r=0.224, p=0.007). The olive tail moment was significantly higher in coke-oven workers with GA genotype of G27466A polymorphism of XRCC1 than those with GG genotype (4.6, 95% CI=2.5-8.7 versus 2.4, 95% CI=1.9-2.9, p<0.01 with adjustment for covariates). No significant associations between C26304T, G28152A and G36189A polymorphisms of XRCC1 and G23591A and A35931C polymorphisms of ERCC2 and olive tail moment were found in both groups. The study showed that the alkaline comet assay is a suitable biomarker in the detection of DNA damage among coke-oven workers and it suggested that the A allele of G27466A polymorphism of XRCC1 may be associated with decreased DNA repair capacity toward PAH-induced base damage and strand breaks.

DNA repair deficiency and BPDE-induced chromosomal alterations in CHO cells

The induction of chromosomal aberrations and sister chromatid exchanges by BPDE was evaluated in parental and different DNA repair deficient Chinese hamster ovary cell lines in order to elucidate the mechanisms involved in their induction. These included the parental line (AA8), nucleotide excision repair (UV4, UV5, UV61), base excision repair (EM9), homologous recombination repair (Irs1SF) and non-homologous end joining (V3-3) deficient ones. The ranking of different cell lines for BPDE-induced chromosome aberrations was: UV4, Irs1SF, UV5, UV 61, EM9, V3-3, and AA8 in a descending order. Cells deficient in NER and HRR were found to be very sensitive, indicating the importance of these pathways in the repair of lesions induced by BPDE. For induction of SCEs, HRR and BER deficient cells were refractory, whereas the other cell lines responded with a dose-dependent increase. The possible mechanisms involved in BPDE-induced chromosomal alterations are discussed.

Polymorphisms of the DNA gene XPD and risk of bladder cancer in a Southeastern Chinese population

The incidence rate for bladder cancer has been increasing in many countries, and bladder cancer is the most common urinary cancer in China. We explored the association of single-nucleotide polymorphisms in DNA repair genes with bladder cancer. The hypothesis is that the xeroderma pigmentosum complementary group D (XPD) 156-22541C-->A and 751-35931A-->C polymorphisms are associated with the risk of bladder cancer. In a population-based case-control study, 215 patients with newly diagnosed bladder transitional cell carcinoma and 245 cancer-free controls/healthy subjects (frequency-matched by the age and sex) were genotyped. These two polymorphisms were studied using the polymerase chain reaction restriction fragment length polymorphism method. We found that the A allele of XPD Arg156Arg (C22541A) and the C allele of XPD Lys751Gln (A35931C) is associated with increased risk of bladder cancer (adjusted odds ratio = 1.54 and 95% confidence interval = 1.19-2.01, 1.65, and 1.12-2.73, respectively). Smoking is also a risk factor in the etiology of bladder cancer, but alcohol intake is a protective factor during the development of bladder cancer. These two XPD polymorphisms may play an important role in the etiology of bladder cancer in the southeastern Chinese population.

The effect of UDP-glucuronosyltransferase 1A1 expression on the mutagenicity and metabolism of the cooked-food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine in CHO cells

UDP-glucuronosyltransferase proteins (UGT) catalyze the glucuronidation of both endogenous and xenobiotic compounds. In previous studies, UGT1A1 has been implicated in the detoxification of certain food-borne carcinogenic-heterocyclic amines. To determine the importance of UDP-glucuronosyltransferase 1A1 (UGT1A1) in the biotransformation of the cooked-food carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), genetically modified CHO cells that are nucleotide excision repair-deficient, and express cytochrome P4501A2 (UV5P3 cell line) were transfected with a cDNA plasmid of human UGT1A1 to establish the UDP-glucuronosyltransferase 1A1 expressing 5P3hUGT1A1 cell line. Expression of the UGT1A1 gene was verified by screening neo gene expressing clonal isolates (G-418 resistant) for their sensitivity to cell killing from PhIP exposure. Five of 11 clones were chosen for further analysis due to their resistance to cell killing. Western blot analysis was used to confirm the presence of the UGT1A1 and CYP1A2 proteins. All five clones displayed a 52-kDa protein band, which corresponded to a UGT1A1 control protein. Only four of the clones had a protein band that corresponded to the CYP1A2 control protein. Correct fragment size of the cDNAs in the remaining four clones was confirmed by RT-PCR and quantification of the mRNA product was accomplished by real-time RT-PCR. Expression of UGT1A1 in the transfected cells was 10(4)-10(5)-fold higher relative to the UV5P3 parental cells. One clone (#14) had a 10-fold higher increase in expression at 1.47 x 10(5) over the other three clones. This clone was also the most active in converting N-hydroxy-PhIP to N-hydroxy-PhIP glucuronide conjugates in microsomal metabolism assays. Based on the D50 values, the cytotoxic effect of PhIP was decreased approximately 350-fold in the 5P3hUGT1A1 cells compared to the UV5P3 control cells. In addition, no significant increase in mutation frequency was observed in the transfected cells. These results clearly indicate that UGT1A1 plays a critical role in PhIP biotransformation, providing protection against PhIP-mediated cytotoxicity and mutagenicity.

Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients

Mutations in the XPD gene result in xeroderma pigmentosum (XP) and trichothiodystrophy (TTD), the phenotypes of which are often intricate. To understand the genotype/phenotype relationship, we engineered recombinant TFIIHs in which XPD subunits carry amino acid changes found in XPD patients. We demonstrate that all the XPD mutations are detrimental for XPD helicase activity, thus explaining the NER defect. We also show that TFIIH from TTD patients, but not from XP patients, exhibits a significant in vitro basal transcription defect in addition to a reduced intracellular concentration. Moreover, when XPD mutations prevent interaction with the p44 subunit of TFIIH, transactivation directed by certain nuclear receptors is inhibited, regardless of TTD versus XP phenotype, thus explaining the overlapping symptoms. The implications of these mutations are discussed using a structural model of the XPD protein. Our study provides explanations for the nature and the severity of the various clinical features.

Development and characterization of CHO repair-proficient cell lines for comparative mutagenicity and metabolism of heterocyclic amines from cooked food

In order to understand the role of repair and metabolism in the mutagenicity of heterocyclic amines from cooked foods, we previously developed the nucleotide excision repair-deficient CHO 5P3NAT2 cell line engineered to coexpress the mouse CYP1A2 and human N-acetyltransferase genes. In the present study, we have made a matched repair-competent cell line by mutagenizing 5P3NAT2 cells with ethyl methanesulfonate and selecting for resistance to cytotoxicity by 2-amino-3-methylimidazo[4,5-f]quinoline (IQ). In the differential cytotoxicity (DC) assay, 4 out of 15 clones showed no cytotoxic effect with IQ at the highest dose (30 microg/ml) tested, in contrast to repair-deficient 5P3NAT2 cells, which showed approximately 100% cytotoxicity at 0.3 microg/ml. Subsequently, these IQ-resistant clones were examined for resistance to killing by UV irradiation. All four IQ-resistant clones, which show resistance to UV similar to that of repair-proficient AA8 cells, still express both the CYP1A2 and N-acetyltransferase genes. Sequence analysis of CXPD cDNA from the 5P3NAT2R9 clone revealed an A:T-->G:C reversion event at the site of the UV5 mutation. This base change results in reversion of the codon 116 tyrosine in UV5 cells back to the original cysteine in AA8 cells, thereby restoring wild-type CXPD activity and repair function. In contrast to 5P3NAT2 cells, the repair-proficient 5P3NAT2R9 revertant cell line shows little IQ-induced cell killing, and dramatically lower levels of induced mutation at the adenine phosphoribosyltransferase (Aprt) gene locus over the range of 2-40 microg/ml IQ. This matched pair of repair-proficient/deficient cell lines can provide insight not only into the genotoxicity of heterocyclic amine dietary carcinogens such as IQ and PhIP, but also into the effects of nucleotide excision repair on the ultimate mutagenicity of these compounds.

DNA repair protein levels vis-à-vis anticancer drug resistance in the human tumor cell lines of the National Cancer Institute drug screening program

Nucleotide excision repair (NER) is a multi-enzyme DNA repair pathway in eukaryotes. Several NER genes in this pathway including XPB, XPD, XPA and ERCC-1 have been implicated in anticancer drug resistance in human tumor cells. In this study, we assessed the levels of the above-mentioned proteins in the NCI panel of 60 human tumor cell lines in relation to the cytotoxicity patterns of 170 compounds that constitute the standard agent (SA) database. The database consists of drugs used in the clinic for which a mechanism of action has been at least partially defined. The ERCC-1, XPD and XPB protein expression patterns yielded significant negative Pearson correlations with 13, 32 and 17 out of the 170 compounds, respectively (using p<0.05). XPA produced a random assortment of negative and positive correlations, and did not appear to confer an overall resistance or sensitivity to these drugs. Protein expression was also compared with a pre-defined categorization of the standard agents into six mechanism-of-action groups resulting in an inverse association between XPD and alkylating agent sensitivity. Our present data demonstrate that XPD protein levels correlate with resistance to alkylating agents in human tumor cell lines suggesting that XPD is implicated in the development of this resistance. NER activity, using the in vitro cell-free system repair assay, revealed no correlation between NER activity and the level of XPD protein in four cell lines with widely varying XPD protein levels. This lack of correlation may be due to the contribution of XPD to other functions including interactions with the Rad51 repair pathway.

Cell proliferation and DNA breaks are involved in ultraviolet light-induced apoptosis in nucleotide excision repair-deficient Chinese hamster cells

UV light targets both membrane receptors and nuclear DNA, thus evoking signals triggering apoptosis. Although receptor-mediated apoptosis has been extensively investigated, the role of DNA damage in apoptosis is less clear. To analyze the importance of DNA damage induced by UV-C light in apoptosis, we compared nucleotide excision repair (NER)-deficient Chinese hamster ovary cells (lines 27-1 and 43-3B mutated for the repair genes ERCC3 and ERCC1, respectively) with the corresponding DNA repair-proficient fibroblasts (CHO-9 and ERCC1 complemented 43-3B cells). NER-deficient cells were hypersensitive as to the induction of apoptosis, indicating that apoptosis induced by UV-C light is due to unrepaired DNA base damage. Unrepaired lesions, however, do not activate the apoptotic pathway directly because apoptosis upon UV-C irradiation requires DNA replication and cell proliferation. It is also shown that in NER-deficient cells unrepaired lesions are converted into DNA double-strand breaks (DSBs) and chromosomal aberrations by a replication-dependent process that precedes apoptosis. We therefore propose that DSBs arising from replication of DNA containing nonrepaired lesions act as an ultimate trigger of UV-C-induced apoptosis. Induction of apoptosis by UV-C light was related to decline in the expression level of Bcl-2 and activation of caspases. Decline of Bcl-2 and subsequent apoptosis might also be caused, at least in part, by UV-C-induced blockage of transcription, which was more pronounced in NER-deficient than in wild-type cells. This is in line with experiments with actinomycin D, which provoked Bcl-2 decline and apoptosis. UV-C-induced apoptosis due to nonrepaired DNA lesions, replication-dependent formation of DSBs, and activation of the mitochondrial damage pathway is independent of functional p53 for which the cells are mutated.

Codominance associated with overexpression of certain XPD mutations

Mutations in the XPD gene are associated with three complex clinical phenotypes, namely xeroderma pigmentosum (XP), XP in combination with Cockayne syndrome (XP-CS), and trichothiodystrophy (TTD). XP is caused by a deficiency in nucleotide excision repair (NER) that results in a high risk of skin cancer. TTD is characterized by severe developmental and neurological defects, with hallmark features of brittle hair and scaly skin, and sometimes has defective NER. We used CHO cells as a system to study how specific mutations alter the dominant/recessive behavior of XPD protein. Previously we identified the T46I and R75W mutations in two highly UV-sensitive hamster cell lines that were reported to have paradoxically high levels of unscheduled DNA synthesis. Here we report that these mutants have greatly reduced XPD helicase activity and fully defective NER in a cell-extract excision assay. We conclude that the unscheduled DNA synthesis seen in these mutants is caused by abortive "repair" that does not contribute to cell survival. These mutations, as well as the K48R canonical helicase-domain mutation, each produced codominant negative phenotypes when overexpressed in wild-type CHO cells. The common XP-specific R683W mutation also behaved in a codominant manner when overexpressed, which is consistent with the idea that this mutation may affect primarily the enzymatic activity of the protein rather than impairing protein interactions, which may underlie TTD. A C-terminal mutation uniquely found in TTD (R722W) was overexpressed but not to levels sufficiently high to rigorously test for a codominant phenotype. Overexpression of mutant XPD alleles may provide a simple means of producing NER deficiency in other cell lines.

Identification of ICR170-induced XPD mutations in UV-sensitive CHO cells

We highlight selected contributions of Dr. Richard Setlow that contributed to our earlier understanding of excision repair processes and set the stage for dissecting nucleotide excision repair (NER) in mammalian cells through molecular genetics. More than 20 years ago, large-scale screens for UV-sensitive mutants of hamster CHO cells isolated approximately 200 mutants, many of which were assigned to the XPD/ERCC2 complementation group, but the nature of the mutations was not determined. The XPD protein performs not only an essential viability function as a structural component of transcription initiation factor TFIIH, but also an NER function as a 5' to 3' DNA helicase within TFIIH that unwinds DNA on the 3' side of bulky lesions. Alterations in these XPD functions are responsible for three UV-sensitivity genetic disorders that have distinguishable clinical features. In this study, we sequenced six UV-sensitive ICR170-induced Chinese hamster ovary (CHO) cell mutants that previously were assigned to the XPD complementation group to determine whether they carry frameshift mutations. All six mutants show 3- to 5-fold increased hypersensitivity to UV irradiation, similar to the XPD mutant prototype UV5. Even though ICR170 is a strong frameshift mutagen, all six cell lines contain base substitution mutations, five of which are unique among all mutations identified so far in human and rodent cells. The sixth mutation was identical to the R75W mutation previously found in CHO UVL-1. The results presented here contribute to a mutation database that should prove useful in structure-function studies of this unique DNA-structure-specific helicase and its complex mutant phenotypes.

DNA repair characteristics and mutations in the ERCC2 DNA repair and transcription gene in a trichothiodystrophy patient

Patient TTD183ME is male and has typical trichothiodystrophy characteristics, including brittle hair, ichthyosis, characteristic face with receding chin and protruding ears, sun sensitivity, and mental and growth retardation. The relative amount of NER carried out by a TTD183ME fibroblast cell strain after ultraviolet (UV) exposure was approximately 65% of normal as determined by a method that converts repair patches into quantifiable DNA breaks. UV survival curves show a reduction in survival only at doses greater than 4 J/m2. Nucleotide sequence analysis of the ERCC2 (XPD) DNA repair and transcription gene cDNA revealed both a Leu461-to-Val substitution and a deletion of amino acids 716-730 in one allele and an Ala725-to-Pro substitution in the other allele. The first allele has also been reported in one xeroderma pigmentosum group D patient and two other trichothiodystrophy patients, while the second allele has not been previously reported. Comparisons suggest that the mutation of Ala725 to Pro correlates with TTD with intermediate UV sensitivity.

DNA condensation by protamine and arginine-rich peptides: analysis of toroid stability using single DNA molecules

Both somatic cells and sperm have been shown to take up exogenous DNA, but the frequency of its integration is usually low. Scanning probe microscopy studies of sperm chromatin and synthetic DNA-protamine complexes indicate that the coiling of DNA into toroidal subunits, a process initiated in the maturing spermatid to prepare its genome for delivery into the egg, can be mimicked by simply adding protamine to DNA in vitro. The increased resistance of DNA-protamine complexes to nuclease digestion and their structural similarity to native sperm chromatin suggest that the packaging of DNA by protamine might offer a new approach for improving the efficiency of DNA uptake by sperm. Decondensation experiments performed with individual DNA molecules have provided a direct measure of the stability of toroids produced using salmon protamine and smaller arginine-rich peptides. These experiments show that the arginine content of protamine-related sequences can have a dramatic effect on their rate of dissociation from DNA. This technique and the information it provides can be used to identify protamine analogs that can be bound to DNA to increase the efficiency of its uptake by sperm and other cells.

The DNA helicases acting in nucleotide excision repair, XPD, CSB and XPB, are not required for PCNA-dependent repair of abasic sites

DNA repair of abasic sites is accomplished in mammalian cells by two distinct base excision repair (BER) pathways: a single nucleotide insertion pathway and a proliferating cell nuclear antigen (PCNA)-dependent pathway involving a resynthesis patch of 2-10 nucleotides 3' to the lesion. The latter pathway shares some enzymatic components with the nucleotide excision repair (NER) pathway acting on damage induced by ultraviolet light: both pathways are strictly dependent on PCNA and several observations suggest that the polymerization and ligation phases may be carried out by common enzymatic activities (DNA polymerase delta/epsilon and DNA ligase I). Furthermore, it has been postulated that the transcription-NER coupling factor Cockayne syndrome B has a role in BER. We have investigated whether three NER proteins endowed with DNA helicase activities (the xeroderma pigmentosum D and B gene products and the Cockayne syndrome B gene product) may also be involved in repair of natural abasic sites, by using the Chinese hamster ovary mutant cell lines UV5, UV61 and 27-1. No defect of either the PCNA-dependent or the single nucleotide insertion pathways could be observed in UV5, UV61 or 27-1 mutant cell extracts, thus showing that the partial enzymatic overlap between PCNA-dependent BER and NER does not extend to DNA helicase activities.

Efficient PCNA complex formation is dependent upon both transcription coupled repair and genome overall repair

The protein proliferating cell nuclear antigen (PCNA) is an auxiliary factor for DNA polymerase delta and is involved in the resynthesis step of nucleotide excision repair (NER). After UV irradiation of quiescent cells, PCNA forms an insoluble complex with nuclear substructures. We have investigated associations between NER and its subcomponent pathway, transcription coupled repair (TCR) on PCNA complex formation using genetically related hamster cell lines with different repair characteristics. In DNA repair proficient cells, the PCNA complex was readily detectable within 30 min after UV irradiation by both immunofluorescence and western blot analyses. This complex formation after UV occurs efficiently in quiescent cells. In UV5 (human XP-D homolog) and UV 24 (human XP-B homolog) cells, which are totally deficient in NER, the PCNA complex was not detectable at 30 min after UV. The PCNA complex formation is restored to normal levels in UV5 cells after transfection with the human XPD gene, encoding a subunit of the basal transcription factor, TFIIH. In UV61 (Human CS-B homolog) cells, that are defective only in transcription coupled repair (TCR) of cyclobutane pyrimidine dimers (CPDs), the rate of PCNA complex formation was 2-fold slower than in repair proficient cells. This defect was complemented by transfection of the CSB gene into the UV61 cells. We thus conclude that efficient PCNA complex formation after UV is dependent upon both the NER and TCR pathways in hamster cells. The association of several other DNA repair proteins including XPA, RPA, TFIIH and p53 with the insoluble PCNA complex in UV treated cells suggests a central role for PCNA in different steps of NER.

Analysis of mutations in the XPD gene in Italian patients with trichothiodystrophy: site of mutation correlates with repair deficiency, but gene dosage appears to determine clinical severity

Xeroderma pigmentosum (XP) complementation group D is a heterogeneous group, containing patients with XP alone, rare cases with both XP and Cockayne syndrome, and patients with trichothiodystrophy (TTD). TTD is a rare autosomal recessive multisystem disorder associated, in many patients, with a defect in nucleotide-excision repair; but in contrast to XP patients, TTD patients are not cancer prone. In most of the repair-deficient TTD patients, the defect has been assigned to the XPD gene. The XPD gene product is a subunit of transcription factor TFIIH, which is involved in both DNA repair and transcription. We have determined the mutations and the pattern of inheritance of the XPD alleles in the 11 cases identified in Italy so far, in which the hair abnormalities diagnostic for TTD are associated with different disease severity but similar cellular photosensitivity. We have identified eight causative mutations, of which four have not been described before, either in TTD or XP cases, supporting the hypothesis that the mutations responsible for TTD are different from those found in other pathological phenotypes. Arg112his was the most common alteration in the Italian patients, of whom five were homozygotes and two were heterozygotes, for this mutation. The presence of a specifically mutated XPD allele, irrespective of its homozygous, hemizygous, or heterozygous condition, was always associated with the same degree of cellular UV hypersensitivity. Surprisingly, however, the severity of the clinical symptoms did not correlate with the magnitude of the DNA-repair defect. The most severe clinical features were found in patients who appear to be functionally hemizygous for the mutated allele.

Excision repair cross-complementing rodent repair deficiency gene 2 expression and chloroethylnitrosourea resistance in human glioma cell lines

OBJECTIVE

Nitrosoureas are the standard chemotherapeutic agents for malignant brain tumors. However, their anticancer effects are limited because many tumors are resistant to these agents. Nucleotide excision repair can repair bulky deoxyribonucleic acid adducts, including deoxyribonucleic acid damage induced by ultraviolet light and some chemotherapeutic agents, and may be implicated in nitrosoureas resistance. In this study, we compared excision repair cross-complementing rodent repair deficiency Gene 2 (ERCC2), an important component of the nucleotide excision repair system, with 1 ,3-bis-(2-chloroethyl)-1-nitrosourea or (2-chloroethyl)-3-sarcosinamide-1-nitrosourea resistance in human glioma cell lines.

METHODS

ERCC2 expression was evaluated by using established quantitative reverse-transcription polymerase chain reaction. 1,3-Bis-(2-chloroethyl)-1-nitrosourea and (2-chloroethyl)-3-sarcosinamide-1-nitrosourea cytotoxicity were determined by a modification of the sulforhodamine B colorimetric anticancer drug screening assay.

RESULTS

A significant correlation between ERCC2 expression and 1 ,3-bis-(2-chloroethyl)-1-nitrosourea or (2-chloroethyl)-3-sarcosinamide-1-nitrosourea cytotoxicity was determined (r=0.737, P=0.0226 and r=0.789, P=0.0113, respectively).

CONCLUSION

Our results suggest that nucleotide excision repair, specifically ERCC2, may play an important role in nitrosoureas drug resistance in human gliomas.

Evidence for nucleotide excision repair as a modifying factor of O6-methylguanine-DNA methyltransferase-mediated innate chloroethylnitrosourea resistance in human tumor cell lines

We examined the O6-methylguanine-DNA methyltransferase (MGMT) protein as well as MGMT activity levels and the excision repair cross-complementing rodent repair deficiency gene, ERCC2 (XPD), protein levels in 14 human tumor cell lines not selected for chloroethylnitrosourea (CENU) resistance. These results were compared with 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU) cytotoxicity and UV light sensitivity. MGMT protein correlated significantly with MGMT activity (r = 0.9497, p = 0.0001). There was no significant linear correlation between BCNU cytotoxicity and MGMT content as determined by both Western analysis (r = 0.139, p = 0. 6348) and activity assay (r = 0.131, p = 0.6515). However, MGMT-rich cell lines were found to be more resistant than MGMT-poor cell lines to BCNU (t = 2.2375, p = 0.0225) but not to UV (t = 1.1734, p = 0.1317). Furthermore, the most BCNU-sensitive cell lines were all MGMT-poor. UV sensitivity was significantly correlated to BCNU cytotoxicity (r = 0.858, p = 0.0001). Significant correlations were found between ERCC2 protein levels and BCNU cytotoxicity (r = 0.786, p = 0.0009) or UV sensitivity (r = 0.874, p = 0.0001). Our results confirm that MGMT plays an important role in CENU resistance, but not in UV resistance. The correlation of UV sensitivity with BCNU cytotoxicity suggests that nucleotide excision repair is an important modifying factor of MGMT-mediated innate CENU resistance in human tumor cell lines, especially in highly resistant cell lines. ERCC2 may be implicated in this process.

Molecular analysis of ERCC2 mutations in the repair deficient hamster mutants UVL-1 and V-H1

The cDNA sequence of the Chinese hamster ERCC2 nucleotide excision repair and transcription gene from the UVL-1 Chinese hamster ovary (CHO) mutant cell line and the V-H1 Chinese hamster V79 mutant line was analyzed. ERCC2 encodes a presumed ATP-dependent DNA helicase and is single copy in CHO lines due to the structural hemizygosity of chromosome 9. Both UVL-1 and V-H1 have intermediate levels of (6-4) photoproduct repair but are as highly UV sensitive as the group 2 mutants that have no detectable repair. Deficiency in cyclobutane dimer removal has also been shown for V-H1. In UVL-1, a single base substitution resulting in an Arg75-->Trp substitution in helicase domain Ia was identified. The equivalent amino acid position is also Arg in the human, mouse, Xiphophorus maculatus, Saccharomyces cerevisiae, and Schizosaccharomyces pombe homologs. In V-H1, a single base substitution resulting in a Thr46-->Ile substitution in helicase domain I (the ATP-binding domain) was identified in both alleles. The equivalent amino acid position is also Thr in the five homologs. Analysis of three V-H1 partial revertants revealed that they still have the original V-H1 mutation in both alleles, indicating that these are second site reversion events. Site-specific mutagenesis was used to introduce the Thr46-->Ile, Arg75-->Trp, and Lys48-->Arg (helicase domain I) mutations into a hamster ERCC2 expression plasmid. These plasmids each failed to confer UV resistance to group 2 mutant cells, further demonstrating that the changes identified are the causative mutations in V-H1 and UVL-1. Correlations between specific mutations, biochemical activities, and repair phenotype are discussed.

Differential effect of acetyltransferase expression on the genotoxicity of heterocyclic amines in CHO cells

We earlier developed the Chinese hamster ovary UV5P3 cell line that expresses cytochrome P4501A2 and lacks nucleotide excision repair for studying metabolism and mutagenicity of heterocyclic amines. The Chinese hamster ovary UV5P3 cells are approximately 50-fold more sensitive to the cooked food mutagen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) than 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), another genotoxic compound found in cooked food, with respect to cytotoxicity and mutation induction at the adenine phosphoribosyltransferase (aprt) locus. To test the hypothesis that the important missing activity in our CHO system for IQ genotoxicity was acetyltransferase, we transfected the UV5P3 cells with cDNA plasmids of either the human NAT2 N-acetyltransferase gene or a bacterial O-acetyltransferase gene. Functionally transformed clones were determined by the differential cytotoxicity assay using IQ, and confirmed by measuring the enzyme activity with isoniazid as substrate. Two clones designated 5P3NAT2 and 5P3YG (expressing human and bacterial transferases, respectively) were characterized. Both cell lines were sensitive to killing by IQ at concentrations as low as 4 ng/ml. Based on the D37 value, the dose that reduced the survival to 37% relative to untreated controls, the acetyltransferase expressing lines showed approximately 1000-fold increase in sensitivity to the killing effect of IQ over the parental UV5P3 cell line. The same dramatic change in sensitivity was also seen in mutation response at the aprt locus and with chromosomal aberrations and sister chromatid exchanges. In contrast, these cell lines showed cytotoxicity to PhIP similar to that of the parental line UV5P3. These results suggest that PhIP does not require acetyltransferase for metabolic activation leading to genotoxicity in these cells. These new cell lines constitute a sensitive cell system for assessing genotoxicity of compounds requiring metabolic activation by both P450IA2 and acetyltransferase, as well as for studying the molecular processes by which DNA damage can lead to mutation and cancer.

Transcription and DNA damage: a link to a kink

Living organisms are constantly exposed to a variety of naturally occurring and man-made chemical and physical agents that pose threats to health by causing cancer and other illnesses, as well as cell death. One mechanism by which these moieties can exert their toxic effects is by inducing modifications to the genome. Such changes in DNA often result in the formation of nucleotides not normally found in the double helix, bases containing covalent chemical alterations, single- and double-strand breaks, and interstrand and intrastrand cross-links. When these lesions are present during replication, mutations often result in the newly synthesized DNA. Likewise, when such damage occurs in a gene, transcription elongation, and hence expression, can be adversely affected because of pausing or arresting of the RNA polymerase at or near the altered site; this could result in the synthesis of a defective RNA molecule. It has become increasingly clear that transcription and DNA damage are intimately linked, since the removal of certain adducts from the genome is highly dependent on their location. When such lesions are present on the transcribed strand of actively expressed genetic loci, they are better cleared from that strand when compared to the complementary DNA or other quiescent regions. This process is called transcription-coupled DNA repair, and it modulates the mutagenic spectrum of many DNA-damaging agents. Furthermore, based upon evidence from systems in which it is absent, this process has a profound effect on ameliorating the adverse consequences of exposure to many environmentally relevant genotoxins. The precise cellular pathway that mediates the preferential clearance of DNA damage from active genetic loci has not yet been established, but it appears to be effected by a repertoire of proteins that are also involved in other DNA repair pathways and transcription as well as some factors that might be unique to it. Because a cellular process as indispensable as gene expression can be thwarted by the presence of DNA damage, an understanding of the mechanism underlying transcription-coupled DNA repair is relevant to the continued discernment of how environmental genotoxins endanger human health.

UV-induced signal transduction

Irradiation of cells with wavelength ultraviolet (UVA, B and C) induces the transcription of many genes. The program overlaps with that induced by oxidants and alkylating agents and has both protective and other functions. Genes transcribed in response to UV irradiation include genes encoding transcription factors, proteases and viral proteins. While the transcription factor encoding genes is initiated in minutes after UV irradiation (immediate response genes) and depends exclusively on performed proteins, the transcription of protease encoding occurs only many hours after UV irradiation. Transcription factors controlling the activity of immediate response genes are activated by protein kinases belonging to the group of proline directed protein kinases immediately after UV irradiation. Experimental evidence suggests that these kinases are activated in UV irradiated cells through pathways which are used by growth factors. In fact, the first cellular reaction detectable in UV irradiated cells is the phosphorylation of several growth factor receptors at tyrosine residues. This phosphorylation does not depend on UV induced DNA damage, but is due to an inhibition of the activity of tyrosine phosphatases. In contrast, for late cellular reactions to UV, an obligatory role of DNA damage in transcribed regions of the genome can be demonstrated. Thus, UV is absorbed by several target molecules relevant for cellular signaling, and it appears that numerous signal transduction pathways are stimulated. The combined action of these pathways establishes the genetic program that determines the fate of UV irradiated cells.

Construction of a functional cDNA clone of the hamster ERCC2 DNA repair and transcription gene

The complete hamster ERCC2 cDNA was constructed in a plasmid vector from clones of three overlapping reverse transcribed/polymerase chain reaction amplified fragments using unique restriction enzyme recognition sites within the regions of overlap. This complete cDNA insert was then cloned into a mammalian expression vector, pcD2E, and tested for function by the ability to confer UV resistance to the ERCC2 mutant CHO cell line UV5. Site-specific mutagenesis was used to introduce the G347-->A and G1844-->A changes resulting in the Cys116-->Tyr and Gly615-->Glu mutations previously identified in UV5 and UVL-13 (also an ERCC2 mutant CHO cell line), respectively. The 116Tyr and 615Glu plasmids each failed to confer UV resistance to UV5 or UVL-13 cells, respectively, demonstrating that the changes identified are indeed the causative mutations in UV5 and UVL-13.

Absence of UV-induced non-homologous recombination in repair-deficient CHO cell lines transfected with ERCC genes

The nucleotide excision repair pathway removes a broad spectrum of DNA lesions, including UV-induced damage. To ascertain whether the repair of the latter has a causative role in the enhancement of non-homologous recombination, Chinese hamster CHO cell lines proficient and deficient in the ability to repair UV-induced damage were transfected with a plasmid containing the bacterial neoR gene. Following UV-treatment an enhancement of non-homologous recombination above the spontaneous level was observed in repair-proficient cells, whereas no increase was observed in repair-deficient cell lines. Hence, the latter were transfected with the corresponding excision repair cross complementing human genes and the resulting repair-proficient transfectants were tested for UV-induced non-homologous recombination. In both untreated and UV-treated transfectants, the frequencies of the event were not significantly different. Cumulatively, the results suggest that non-homologous recombination induced by UV-irradiation is not restored by the correction of the excision repair defect.

Rodent complementation group 8 (ERCC8) corresponds to Cockayne syndrome complementation group A

US31 is a UV-sensitive mutant cell line (rodent complementation group 8) derived from a mouse T cell line L5178Y. We analyzed removal kinetics for UV-induced cyclobutane pyrimidine dimers and (6-4) photoproducts in US31 cells using monoclonal antibodies against these photoproducts. While nearly all (6-4) photoproducts were repaired within 6 h after UV-irradiation, more than 70% of cyclobutane pyrimidine dimers remained unrepaired even 24 h after UV-irradiation. These kinetics resembled those of Cockayne syndrome (CS) cells. Since US31 cells had a low efficiency of cell fusion and transfection, which hampered both complementation tests and gene cloning, we constructed fibroblastic complementation group 8 cell line 6L1030 by fusion of US31 cells with X-irradiated normal mouse fibroblastic LTA cells. Complementation tests by cell fusion and transfection using 6L1030 cells revealed that rodent complementation group 8 corresponded to CS complementation group A.

Defects in the DNA repair and transcription gene ERCC2(XPD) in trichothiodystrophy

Trichothiodystrophy (TTD) is a rare autosomal recessive disorder characterized by brittle hair with reduced sulfur content, ichthyosis, peculiar face, and mental and growth retardation. Clinical photosensitivity is present in approximately 50% of TTD patients but is not associated with an elevated frequency of cancers. Previous complementation studies show that the photosensitivity in nearly all of the studied patients is due to a defect in the same genetic locus that underlies the cancer-prone genetic disorder xeroderma pigmentosum group D (XP-D). Nucleotide-sequence analysis of the ERCC2 cDNA from three TTD cell strains (TTD1V1, TTD3VI, and TTD1RO) revealed mutations within the region from amino acid 713-730 and within previously identified helicase functional domains. The various clinical presentations and DNA repair characteristics of the cell strains can be correlated with the particular mutations found in the ERCC2 locus. Mutations of Arg658 to either His or Cys correlate with TTD cell strains with intermediate UV-sensitivity, mutation of Arg722 to Trp correlates with highly UV-sensitive TTD cell strains, and mutation of Arg683 to Trp correlates with XP-D. Alleles with mutation of Arg616 to Pro or with the combined mutation of Leu461 to Val and deletion of 716-730 are found in both XP-D and TTD cell strains.

Genetically modified Chinese hamster ovary (CHO) cells for studying the genotoxicity of heterocyclic amines from cooked foods

We have developed metabolically competent Chinese hamster ovary (CHO) cells to evaluate the genotoxicity associated with heterocyclic amines, such as those that are present in cooked foods. Into repair-deficient UV5 cells we introduced cDNAs for expressing cytochrome P450IA2 and acetyltransferases. We then genetically reverted these transformed lines to obtain matched metabolically competent repair-deficient/proficient lines. For a high mutagenic response, we find a requirement for acetyltransferase with 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) but not with 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). This system allows for both quantifying mutagenesis and analyzing the mutational spectra produced by heterocyclic amines.

Effect of alkyl-N-purine DNA glycosylase overexpression on cellular resistance to bifunctional alkylating agents

Increased activity of alkyl-N-purine DNA glycosylase (ANPG; a.k.a. N3-methyladenine DNA glycosylase) has been correlated with resistance to both chloroethylnitrosoureas and nitrogen mustards. Also, overexpression of the human glycosylase in Escherichia coli results in resistance to alkylating agents. To determine how overexpression of the protein affects resistance to these bifunctional alkylating agents in mammalian cells, wild-type CHO-AA8 cells were transfected with an expression construct containing the human ANPG cDNA. Several clonally isolated lines that expressed increasing levels of glycosylase activity were selected. None of these lines displayed increased resistance to either bis-chloroethylnitrosourea or melphalan. To determine how overexpression of this protein affects cells in the absence of nucleotide excision repair, the mutant CHO-UV20 cell line was transfected with the same expression construct. This cell line lacks functional ERCC-1 protein and displays extreme hypersensitivity to bifunctional alkylating agents. Again, none of the UV20 transfectants displayed increased resistance. The results of these experiments indicate that unlike E. coli, overexpression of the glycosylase alone is not sufficient to confer resistance to bifunctional alkylating agents in this system. Structural differences between mammalian cells and E. coli may explain the interesting result that a mammalian gene can confer drug resistance in E. coli but not in mammalian cells.

Characterization of a mammalian cell line that exhibits spontaneous and ultraviolet light-induced hypermutability while retaining resistance to cell killing by ultraviolet light

Chronic exposure of V79 cells to 80 daily doses of 150 J/M2, 290-330-nm ultraviolet light (UVB) produced a mixed cell population that was found to be generally more resistant to cell killing by both UVB and UVC (254 nm) than the wild-type cells. Several subclones from this population were studied for their survival and mutation responses and then one was chosen for further characterization based on this data. The studies carried out on this subclone, designated N806, show that its spontaneous HPRT mutation rate is approximately 10 times higher than that of wild-type V79 cells and it is almost three times more mutable than the wild-type cells when both are induced by UVB or UVC. The mutation responses of N806 and MI2G cells to 50-kVp X-rays are different, but the N806 cells do not appear to be hypermutable as they are with UV. N806 cells are also moderately more resistant to the cytotoxic effects of UV radiation but are more sensitive than MI2G cells when exposed to X-rays. Assays to measure the removal of cyclobutane pyrimidine dimers (CPDs) and the incision step of nucleotide excision repair have revealed no detectable difference in the repair capacities of N806 and parental V79 cells. These results suggest that chronic, protracted UV irradiation may be able to induce a 'mutator phenotype' in a subpopulation of the progenitor cells.

Characterization of the Xiphophorus fish (Teleostei: Poeciliidae) ERCC2/XPD locus

We have cloned and sequenced the ERCC2/XPD locus of Xiphophorus maculatus. The human ERCC2/XPD gene is a nucleotide excision repair gene presumed to encode an ATP-dependent DNA helicase. The fish ERCC2/XPD gene is represented on 14.5 kb of genomic DNA and is composed of 23 exons. Within the coding regions, the overall nucleotide identity is 74% compared to the human cDNA. Of 760 amino acids compared between human and fish sequences, 127 differences are observed. Of these differences, 48 residues (38%) represent nonconservative amino acid changes, while 79 (62%) are conservative. The majority (73%) of nonconservative differences between the human and the fish amino acid sequences occur in eight distinct groups comprising only about 10% of the total protein. Overall, the fish and human sequences show 83% amino acid identity and 94% similarity when conservative amino acid substitutions are allowed.