TCDD Affects DNA Double Strand-Break Repair

Clara Y. Y. Chan, Perry M. Kim and Louise M. Winn1

Department of Pharmacology and Toxicology and School of Environmental Studies, Queen's University, Kingston, Ontario, Canada 27L3N6

Received February 13, 2004; accepted June 10, 2004


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), an environmental toxicant, elicits a spectrum of deleterious biological responses including carcinogenesis. We hypothesize that TCDD exposure exerts its carcinogenicity, in part, by affecting the repair of DNA double strand breaks (DSBs) through homologous recombination (HR), mediated by the AhR signaling pathway. To investigate this hypothesis we used a Chinese hamster ovary (CHO) cell line (CHO 33) containing a neo direct repeat recombination reporter substrate to determine whether TCDD affects DNA DSB repair. The Saccharomyces cerevisiae mitochondrial endonuclease I-SceI was used to induce a site specific DSB within the upstream neo recombination substrate in the CHO 33 cells. The cells were then exposed to 500 pM of TCDD in the presence or absence of the AhR antagonist {alpha}-naphthoflavone (0.1 µM) for 24 h. Two weeks later HR frequencies were determined by counting the number of functional neo expressing, G418-resistant colonies per live cells plated. TCDD significantly increased HR frequency, demonstrating that it does in fact modulate the repair of DNA DSBs. Southern blot analysis of G418-resistant colonies using a cDNA neo probe determined that both gene conversion and gene deletion HR events occurred as a result of DNA DSB repair and TCDD exposure. Exposure of cells to {alpha}–naphthoflavone resulted in a significant decrease in TCDD-induced HR frequency. These results demonstrate that TCDD, potentially acting via the AhR, can modulate HR repair of DNA DSBs in CHO 33 cells.

Key Words: TCDD; homologous recombination; AhR; DNA repair.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Evidence from both animal studies (Dragan and Schrenk, 2000Go) and human epidemiological data indicate that the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a potent carcinogen (Kogevinas, 2000Go); however, the molecular mechanism(s) by which this occurs remain unclear. It is known that many of the toxicities induced by TCDD are mediated through the aryl hydrocarbon receptor (AhR) signaling pathway. Studies have shown that AhR-knockout mice are resistant to a number of TCDD-induced acute toxicities and also exhibit negligible induction of metabolic enzymes after exposure to TCDD (Fernandez-Salguero et al., 1996Go; Gonzalez and Fernandez-Salguero, 1998Go). This signaling pathway has been reviewed extensively elsewhere (Mimura et al., 1997Go; Poellinger, 2000Go; Safe, 2001Go); however, the end result of TCDD binding to the AhR is alterations in the transcription of AhR-controlled genes (Gonzalez and Fernandez-Salguero, 1998Go; Okey et al., 1994Go).

Damage to DNA is well known to contribute to the development of cancer. Although the cell has developed an array of mechanisms to repair damaged DNA, inherited defects in DNA repair mechanism such as those seen in patients with xeroderma pigmentosum, ataxia telangiectasia, and Nijmegen breakage syndrome, enhance the susceptibility of these patients to certain types of cancers (Rotman and Shiloh, 1998Go). One particular form of DNA damage, DNA double strand breaks (DSBs), which can occur "spontaneously" in the genome, or as a consequence of ionizing radiation or chemical exposure, are thought to be particularly detrimental to the cell (Jackson, 2002Go).

DSBs can be repaired by two distinct repair pathways: non-homologous end-joining and homologous recombination (HR). Non-homologous end-joining does not require extensive homology and involves the direct rejoining of the two ends of the broken DNA, which is accomplished with the cooperation of a number of different proteins involved in the recognition and targeting of the damaged DNA followed by the removal or addition of a few base pairs and finally ligation (van Gent et al., 2001Go). On the other hand, repair of DSBs via HR requires homologous donor duplex DNA, which serves as a template for repair. The donor DNA can be an undamaged allele, sister chromatid, or an ectopic DNA region that shares significant homology with the damaged site. Consequently, if the donor DNA is completely homologous, this type of repair will result in a normal functional gene, indistinguishable from the gene prior to the DSB. However, DSBs can induce HR between DNA that are not completely homologous and thus result in genetic changes. HR can also involve gene deletion events that can lead to the loss of critical genetic information (Bishop and Schiestl, 2000Go, 2001Go; Nickoloff and Brenneman, 2001Go). Thus, DSB repair via HR can lead to various deleterious events, including the loss of heterozygosity, translocations, and gene deletions or amplifications (Jackson and Loeb, 2001Go; Pierce et al., 2001Go; Van den et al., 2002Go).

Since TCDD does not directly damage DNA and is not considered a potent genotoxin, unlike many known carcinogens, the molecular events leading to the development of cancer as a result of TCDD exposure still need to be elucidated. Several studies have demonstrated that while TCDD lacks the ability to initiate carcinogenesis, it can act as a potent tumor promoter (reviewed in Dragan and Schrenk, 2000Go). It is well known that cells are continuously exposed to DNA damaging agents including ultraviolet light, xenobiotics and endogenously produced reactive oxygen species. While the cell does have mechanisms to repair this damage, these repair mechanisms are not error free. Similarly, as discussed above DSB repair via HR may result in detrimental genetic changes. Therefore, we hypothesize that TCDD exerts its carcinogenicity, in part, by affecting the repair of DNA DSBs caused by either endogenous and exogenous DNA damaging agents. Furthermore, given that the specific binding of TCDD to the AhR and the subsequent downstream signaling pathway, appear to mediate a variety of the toxic effects seen upon exposure to TCDD (Fernandez-Salguero et al., 1996Go; Gonzalez and Fernandez-Salguero, 1998Go), we hypothesize that the AhR plays a role in TCDD's effects on HR repair of DSBs.

In order to investigate our hypothesis, we used a previously characterized model (Taghian and Nickoloff, 1997Go) that utilizes the Saccharomyces cerevisiae mitochondrial endonuclease, I-SceI, to initiate a site specific DSB in a reporter tandem repeat neomycin (neo) recombination substrate which is stably integrated in the Chinese hamster ovary (CHO) cell line, strain 33. Previous studies have already shown that this yeast endonuclease efficiently induces a DSB in this model and induces a high rate of recombination (Choulika et al., 1995Go; Taghian and Nickoloff, 1997Go). Therefore, we were able to use this model to investigate the effect of TCDD exposure on the repair of an artificially created DSB in mammalian cells and evaluate the role of the AhR in this process.


    MATERIALS AND METHODS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Cell culture. Chinese Hamster Ovary cells, strain 33 (CHO 33), were obtained from J. A. Nickoloff (Department of Molecular Genetics and Microbiology, University of New Mexico). These cells have a single, stably integrated tandem repeat neo recombination substrate (Fig. 1, Taghian and Nickoloff, 1997Go), which confers resistance to the antibiotic Geneticin (G418; Gibco Life Technologies, Burlington, ON) upon HR. The neo recombination substrate is flanked with EcoRI restriction enzyme sites. The neo gene located on the 5' end of the substrate is inactive due to the insertion of the Saccharomyces cerevisiae mitochondrial endonuclease I-SceI recognition sequence (~18 bp long), causing a frame shift mutation. This yeast endonuclease I-SceI sequence is found only in the 5' neo, therefore the intracellular expression of I-SceI will cause a DSB at that specific site within the neo recombination substrate. On the 3' end of the substrate, the wildtype neo gene is silent due to the lack of a promoter. Its role is to serve as the DNA donor during HR repair of the I-SceI site in the 5' neo. Upon HR repair the 5' neo is converted to its wildtype sequence and therefore the cells become G418 resistant. This cell line therefore, allows for the selection of the occurrence of HR since only cells that have undergone recombination will be resistant to G418 due to the expression of a functional neo gene. Cells were grown in 15 cm culture dishes (Corning Incorporated, Corning, NY), maintained in {alpha}-minimum essential medium supplemented with 10% fetal bovine serum and 1% penicillin/streptomycin (all from Gibco Life Technologies, Burlington, ON), and incubated at 37°C in 5% CO2.



View larger version (14K):
[in this window]
[in a new window]
 
FIG. 1. Structure of the neomycin direct repeat recombination substrate. CHO 33 cells contain a stably integrated tandem repeat neomycin recombination reporter plasmid that consists of a dexamethasone inducible 5' mouse mammary tumor virus (MMTV) neo which is inactivated by an I-SceI endonuclease recognition sequence, a central simian virus (SV) 40 promoter-driven E.coli gpt (guanine phosphoribosyl transferase) gene which confers resistance to mycophenolic acid, and a wild-type 3' neo which acts as the DNA donor for the repair of the 5' neo. This 3' neo is inactive because it lacks a promoter (Taghian and Nickoloff, 1997Go). Pathway 1 demonstrates a gene conversion event and pathway 2 demonstrates a gene deletion event.

 
Double-strand break-induced recombination assay. CHO 33 cells were plated in six-well tissues culture plates (Corning Incorporated, Corning, NY) at a density of 3 x 105. Cells in each well were transiently transfected with 1 µg of plasmid cDNA expressing the I-SceI restriction enzyme or a control plasmid (pGem; an empty expression vector) 24 h after plating using Lipofectamine according to the manufacturer's instructions. Twenty-four h after transfection, 5 x 104 cells from each well were transferred to 10 cm culture dishes containing fresh media. The cells were allowed to adhere for 3 h after plating and then treated with 500 pM of TCDD (AccuStandard, Inc., New Haven, CT) or the vehicle control (DMSO; Sigma Chemical Co., St. Louis, MO). After 24 h of drug exposure, the media was removed and the cells were washed with PBS and then fresh cell culture media containing 250 µg/ml of G418 was added. The cells were grown for two weeks and G418-resistant colonies consisting of 20 or more cells were then stained with 1% crystal violet dye (Sigma Chemical Co., St. Louis, MO) in methanol. HR frequency was determined by counting the number of G418-resistant colonies per live cells plated.

Plating efficiency experiments (i.e., cell death experiments) were conducted in a similar manner as the recombination assay except that CHO 33 cells were plated at a density of 300 cells per 10 cm dish, cells were grown only in fresh cell culture media without G418, and the colonies were scored after one week.

Southern blot analysis. To determine the types of HR events (gene conversion or gene deletion) induced by TCDD, G418-resistant colonies were identified and isolated using a light microscope and colonies were expanded in one well of a six-well plate in culture media containing G418. Once confluent, genomic DNA was then isolated using a commercially available Qiagen DNeasy Tissue Kit (Qiagen Incorporated, CA). The isolated genomic DNA was digested with EcoRI and I-SceI (New England BioLabs Inc., Mississauga, ON). Digested DNA was run on a 1.5% agarose gel and transferred to a nylon membrane and a neo cDNA was used as the probe (Taghian and Nickoloff, 1997Go). A single 10.3 KB band represents the product of a gene conversion event while a deletion event results in a single 5.2 KB band. The I-SceI site is lost in both types of HR event and thus digestion with I-SceI should not alter the size of the band.

Aryl hydrocarbon receptor–mediated DSB repair studies. In order to evaluate the effects of the AhR on TCDD-mediated alterations in DSB repair, transfection assays were performed as described above and cells were exposed to TCDD (500 pM) alone or in the presence of the AhR antagonist, {alpha}-naphthoflavone ({alpha}-NF), at a final concentration of 0.1 µM.

Statistical analysis. Results were analyzed using a standard, computerized statistical program (GraphPad Prism 3.0). Groups were compared using a one factor analysis of variance (ANOVA). The minimum level of significance used throughout was p < 0.05.


    RESULTS
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Transfection-Induced Cell Death
Cell survival (plating efficiency) in the different treatment groups was determined and used to calculate the recombination frequency. As previously seen in this cell line, cell survival after transfection with DNA was low. However there were no significant differences between any of the treatment groups (Fig. 2).



View larger version (14K):
[in this window]
[in a new window]
 
FIG. 2. TCDD and transfection-induced cell death. Plating efficiency (cell survival) after transfection and 24 h exposure to 500 pM of TCDD or the vehicle control (DMSO). Cell survival was determined by calculating the number of colonies formed after one week, divided by the number of cells plated. Numbers in parentheses indicate the number of independent determinations per treatment.

 
TCDD-Induced DSB Repair
Exposure to TCDD (500 pM) alone for 24 h, without the specific I-SceI-induced DSB, did not increase HR in CHO 33 cells compared to the vehicle control (Fig. 3). Transfecting cells with I-SceI to create a DSB in the 5' neo did significantly increase the frequency of HR compared to cells transfected with the control plasmid (p < 0.05) (Fig. 3). In cells that were transfected with I-SceI, exposure to TCDD led to a significantly higher frequency of HR compared to cells that were exposed to the TCDD vehicle (Fig. 3) (p < 0.01).



View larger version (15K):
[in this window]
[in a new window]
 
FIG. 3. TCDD-induced DSB repair. HR frequencies for the various transfection and TCDD treatment groups after 24 h were determined by counting the number of G418-resistant colonies per live cells plated. Numbers in parentheses indicate the number of independent determinations per treatment. The asterisk indicates a significant difference from cells transfected with the control plasmid (pGem) and the dagger indicates a significant difference from cells treated with the vehicle (DMSO) (p < 0.01).

 
HR Events as a Result of TCDD Exposure
HR in CHO 33 cells yields two distinctive products, a 10.3 Kb gene conversion product and a 5.2 Kb gene deletion product. Southern blot analysis of genomic DNA samples isolated from 10 different G418-resistant colonies showed 8 out of 10 colonies had undergone gene conversion events as a result of 24 h exposure to TCDD (500 pM), while two colonies had undergone gene deletion events as indicated by their respective band sizes (Table 1). A previous study by Taghian and Nickloff (1997)Go, using the same cell line, found that 97% of the DSB-induced G418-resistant colonies had undergone gene conversion events while only 3% had undergone a gene deletion event (Table 1).


View this table:
[in this window]
[in a new window]
 
TABLE 1 Occurrence of HR Products in CHO 33 Recombination Cells as a Result of TCDD Exposure

 
HR Frequencies as a Result of Exposure to an AhR Antagonist
To determine whether the AhR plays a role in the modulation of HR repair of DNA DSBs by TCDD, CHO 33 cells were exposed to 500 pM of TCDD in the presence or absence of 0.1 µM of {alpha}-NF for 24 h. There were no significant differences in cell survival between any of the treatment groups (Fig. 4a). Exposure of cells to {alpha}-NF blocked the increase in DSB-induced HR frequency as a result of TCDD exposure (Fig. 4b) (p < 0.05).



View larger version (23K):
[in this window]
[in a new window]
 
FIG. 4. Effect of an AhR antagonist on TCDD's affect on DNA DSB repair. (a) Plating efficiency (cell survival) after transfection and exposure to 500 pM of TCDD in the presence or absence of 0.1 µM {alpha}-naphthoflavone for 24 h. Cell survival was determined by calculating the number of colonies formed without addition of G418, after one week divided by the number of cells plated. Numbers in parentheses indicate the number of independent determinations per treatment. (b) Frequency of HR in I-SceI transfected CHO 33 cells treated with 500 pM TCDD in the presence or absence of 0.1 µM {alpha}-naphthoflavone for 24 h. The asterisk indicates a significant difference from cells in all other treatment groups (n = 8, p < 0.01).

 

    DISCUSSION
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The molecular mechanism by which TCDD exerts its carcinogenic effects still remains to be elucidated. Given that tumor cells often contain genomic rearrangements such as deletions, and patients diagnosed with cancer-prone diseases, such as Fanconi's anemia and Bloom's syndrome, often exhibit higher than normal levels of genomic recombination, it is likely that genomic instability can mediate tumorigenesis (Schiestl et al., 1997Go). We and others have previously demonstrated that TCDD can induce HR in both mammalian cells and mouse embryos leading to genomic instability (Chan et al., in press; Schiestl et al., 1997Go). Specifically using a similar CHO recombination cell line (CHO 3–6), we demonstrated that TCDD-induced recombination involved both gene conversion and gene deletion events, both of which have been associated with increased cancer risk (Chan et al., in press). However, since CHO 3–6 cells have a spontaneous level of HR, the initiating factor for recombination in these cells still remains unknown. One potential molecular pathway that could lead to genomic instability is DNA DSB repair via HR (reviewed in Pfeiffer et al., 2000Go). Therefore, in this study we investigated whether exposure to TCDD altered the repair of DSBs in CHO 33 cells, which intrinsically have little or no baseline levels of HR. Since we previously showed that TCDD-induced HR could be mediated through the AhR (Chan et al., in press), we also investigated the role of this signaling pathway in TCDD's effect on DSB repair.

DNA DSBs are generated when both strands of the DNA double helix are broken in close proximity. Sources of DNA DSBs include ionizing radiation and genotoxic xenobiotics (van Gent et al., 2001Go). HR is one mechanism by which DNA DSBs can be repaired using an undamaged sister chromatid or a homologous chromosome as a template for repair (van den et al., 2002Go). While HR is a necessary repair mechanism, erroneous repair via HR can produce detrimental genetic changes, such as loss of heterozygosity and gene deletions or duplications, which can lead to genome instability and carcinogenesis. In this study we used the Saccharomyces cerevisiae mitochondrial endonuclease I-SceI to induce HR by initiating a site specific DSB in the neo tandem repeat recombination substrate located in the genome of the CHO 33 cell line (Taghian and Nickoloff, 1997Go). The yeast endonuclease I-SceI recognition sequence is rare in the mammalian genome, making this model a very useful tool to study DSB repair by HR (Choulika et al., 1995Go; Taghian and Nickoloff, 1997Go; Theirry et al., 1991Go). Our results support the fact that DSBs are potent inducers of HR since I-SceI transfected cells exhibited a significantly higher HR frequency than cells that were transfected with the control plasmid (Fig. 3). More importantly, here we demonstrate that exposure to TCDD can lead to an increase in the frequency of DNA DSB repair, since TCDD exposure alone did not lead to an increase in HR in CHO 33 cells when a DSB was not initiated while exposure of I-SceI transfected cells to TCDD resulted in a significant increase in HR compared to I-SceI transfected cells that were exposed to the vehicle (Fig. 3). As previously mentioned, TCDD can act as a tumor promoter and we hypothesize that TCDD can act by increasing the repair of DSBs created either endogenously or via exposure to DNA damaging agents. This increased frequency of HR can then lead to a greater possibility of repair mistakes and ultimately genomic instability.

In this study we also investigated the type of HR events that occurred as a result of the repair of a DSB followed by exposure to TCDD by Southern blot analysis. Similar to our previous studies using CHO 3–6 cells, our results using CHO 33 demonstrate that TCDD changes the proportion of gene conversion (80%) versus gene deletion (20%) events. This is based on previous studies (Kim et al., 2001Go; Taghian and Nickoloff, 1997Go) which showed that CHO 33 cells underwent mainly gene conversion events (97%). While gene conversion events are considered a conservative process, which involves the direct copying of the genetic information from the donor template, this repair pathway can be detrimental if the donor DNA is not completely homologous. Gene conversion events involving heterologous DNA can lead to the loss of heterozygosity of key genes (e.g., tumor suppressor gene), which can promote tumorigenesis (Johnson and Jasin, 2000Go; Pfeiffer et al., 2000Go). Nevertheless, TCDD's ability to increase the frequency of gene deletion events may be particularly detrimental since there is a direct loss of genetic material and therefore a greater likelihood of important genes being deleted.

Since many of the toxicities induced by TCDD are believed to be mediated via the AhR, we investigated the role of the AhR in TCDD's ability to increase DSB repair. Our results suggest that the AhR does play a role in this increase since the AhR antagonist {alpha}-NF significantly reduced TCDD-induced HR frequency (Fig. 4). The binding of TCDD to the AhR results in the induction or repression of many AhR-controlled genes, some of which are involved in DNA repair (Puga et al., 2000Go). HR involves a number of proteins including members of the RAD50 group and the breast cancer susceptibility genes, BRCA1 and BRCA2, and it is likely that TCDD affects the transcription of these genes. While the function of BRCA1 remains unknown, many studies implicate this gene in cell-cycle control and DNA repair (Gowen et al., 1998Go; MacLachlan et al., 2002Go). Recently, Rattenborg et al. (2002)Go demonstrated in vitro that TCDD down regulated the basal as well as the estradiol-inducible BRCA1 promoter activity. These studies are in agreement with earlier studies showing inhibition of BRCA1 expression by the polycyclic aromatic hydrocarbon benzo[a]pyrene (Jeffy et al., 1999Go). Given that TCDD appears to increase the frequency of DSB repair in our studies, we suspect that while TCDD can repress BRCA1 activity, it can induce other proteins involved in HR.

While our results suggest an involvement of the AhR in TCDD's modulation of DSB repair, these results are somewhat in contrast to those demonstrated by Schiestl et al. (1997)Go where TCDD-induced deletion events did not correlate with induction of AH hydroxylase activity in embryos and in a lymphoblastoma cell line. A number of potential reasons may explain our conflicting results: (1) Our study investigated the role of TCDD in DSB repair induced by a specific insult (i.e., I-SceI-induced DSB) whereas the assumption in the Schiestl paper is that TCDD is either directly initiating the deletion events or that TCDD modulates the deletion event process initiated by an unknown mechanism. Our study uses DSBs as the initiating factor, which we and others have shown are strong inducers of HR in CHO 33 cells and as shown in Figure 3 TCDD does not increase HR in these cells but rather modulates the frequency of DSB-induced HR. (2) While TCDD alone did not effect HR in CHO 33 cells (Fig. 3) and our {alpha}-NF studies suggest a role for the AhR in TCDD's effects on DSB-induced HR, we cannot dismiss the effects of TCDD and {alpha}-NF on the glucocorticoid receptor which has been shown to bind with these two compounds and also bind to the MMTV promoter which drives the 5'-neo gene in the reporter construct in the CHO 33 cells (Fig. 1). (3) And furthermore, while our results using {alpha}-NF support a role for the AhR, it is important to remember that the use of all chemical probes have associated problems. While the concentration of {alpha}-NF used in our study was carefully chosen after a considerable literature review, studies have demonstrated that even at this low concentration {alpha}-NF also has partial agonist properties. However, we did not find that {alpha}-NF had a similar effect to that of TCDD but rather antagonzied the effects of TCDD. It is clear that additional studies are necessary to further implicate the AhR in TCDD's effects on DNA DSB repair.

In summary, we demonstrate that exposure to TCDD can affect the frequency and types of HR repair of DNA DSBs potentially through the AhR and propose that this may be a potential molecular mechanism mediating the carcinogenicity of this highly toxic environmental contaminant.


    ACKNOWLEDGMENTS
 
These studies were supported by a grant from the Canadian Institutes of Health Research.


    NOTES
 
Preliminary reports of this research were presented at the 35th annual meeting of the Society of Toxicology of Canada, 2002, Montreal, Canada.

1 To whom correspondence should be addressed. Fax: (613) 533-6412. E-mail: winnl{at}biology.queensu.ca.


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Bishop, A. J., and Schiestl, R. H. (2000). Homologous recombination as a mechanism for genome rearrangements: Environmental and genetic effects. Hum. Mol. Genet. 9, 2427–2434.[Abstract/Free Full Text]

Bishop, A. J., and Schiestl, R. H. (2001). Homologous recombination as a mechanism of carcinogenesis. Biochim. Biophys. Acta 1471, M109–M121.[CrossRef][ISI][Medline]

Chan, C. Y. Y., Kim, P. M., and Winn, L. M. (in press). TCDD-induced homologous recombination: The role of the Ah receptor versus oxidative DNA damage. Mutat. Res. Genet. Toxicol. Environ. Mutagen.

Choulika, A., Perrin, A., Dujon, B., and Nicolas, J. F. (1995). Induction of homologous recombination in mammalian chromosomes by using the I-SceI system of Saccharomyces cerevisiae. Mol. Cell Biol. 15, 1968–1973.[Abstract]

Dragan, Y. P., and Schrenk, D. (2000). Animal studies addressing the carcinogenicity of TCDD (or related compounds) with an emphasis on tumour promotion. Food Addit. Conta. 17, 289–302.[CrossRef][ISI]

Fernandez-Salguero, P. M., Hilbert, D. M., Rudikoff, S., Ward, J. M., and Gonzalez, F. J. (1996). Aryl-hydrocarbon receptor-deficient mice are resistant to 2,3,7,8- tetrachlorodibenzo-p-dioxin-induced toxicity. Toxicol. Appl. Pharmacol. 140, 173–179.[CrossRef][ISI][Medline]

Gonzalez, F. J., and Fernandez-Salguero, P. (1998). The aryl hydrocarbon receptor: Studies using the AHR-null mice. Drug Metab. Dispos. 26, 1194–1198.[Abstract/Free Full Text]

Gowen, L. C., Avrutskaya, A. V., Latour, A. M., Koller, B. H., and Leadon, S. A. (1998). BRCA1 required for transcription-coupled repair of oxidative DNA damage. Science 281, 1009–1012.[Abstract/Free Full Text]

Jackson, A. L., and Loeb, L. A. (2001). The contribution of endogenous sources of DNA damage to the multiple mutations in cancer. Mutat. Res. 477, 7–21.[ISI][Medline]

Jackson, S. P. (2002). Sensing and repairing DNA double-strand breaks. Carcinogenesis 23, 687–696.[Abstract/Free Full Text]

Jeffy, B. D., Schultz, E. U., Selmin, O., Gudas, J. M., Bowden, G. T., and Romagnolo, D. (1999). Inhibition of BRCA-1 expression by benzo[a]pyrene and its diol epoxide. Mol. Carcinog. 26, 100–118.[CrossRef][ISI][Medline]

Johnson, R. D., and Jasin, M. (2000). Sister chromatid gene conversion is a prominent double-strand break repair pathway in mammalian cells. EMBO J. 19, 3398–3407.[Abstract/Free Full Text]

Kim, P. M., Allen, C., Wagener, B. M., Shen, Z., and Nickoloff, J. A. (2001) Overexpression of human RAD51 and RAD52 reduces double-strand break-induced homologous recombination in mammalian cells. Nucleic Acids Res. 29, 4352–4360.[Abstract/Free Full Text]

Kogevinas, M. (2000). Studies of cancer in humans. Food Addit. Conta. 17, 317–324.[CrossRef]

MacLachlan, T. K., Somasundaram, K., Sgagias, M., Shifman, Y., Muschel, R. J., Cowan, K. H., and El-Deiry, W. S. (2002). BRCA1 effects on the cell cycle and the DNA damage response are linked to altered gene expression. J. Biol. Chem. 275, 2777–2785.[CrossRef]

Mimura, J., Yamashita, K., Nakamura, K., Morita, M., Takagi, T. N., Nakao, K., Ema, M., Sogawa, K., Yasuda, M., Katsuki, M., and Fujii-Kuriyama, Y. (1997). Loss of teratogenic response to 2,3,7,8-tetrachlorodibenzo-p- dioxin (TCDD) in mice lacking the Ah (dioxin) receptor. Genes Cells 2, 645–654.[Abstract/Free Full Text]

Nickoloff, J. A., and Brenneman, M. A. (2001). Recombination. In The Encyclopedia of Molecular Medicine (T. E. Creighton, Ed.), pp. 2736–2741. Wiley, New York.

Okey, A. B., Riddick, D. S., and Harper, P. A. (1994). The Ah receptor: Mediator of the toxicity of 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and related compounds. Toxicol. Lett. 70, 1–22.[CrossRef][ISI][Medline]

Pfeiffer, P., Goedecke, W., and Obe, G. (2000). Mechanisms of DNA double-strand break repair and their potential to induce chromosomal aberrations. Mutagenesis 15, 289–302.[Abstract/Free Full Text]

Pierce, A. J., Stark, J. M., Araujo, F. D., Moynahan, M. E., Berwick, M., and Jasin, M. (2001). Double-strand breaks and tumorigenesis. Trends Cell Biol. 11, S52–S59.[CrossRef][ISI][Medline]

Poellinger, L. (2000). Mechanistic aspects–the dioxin (aryl hydrocarbon) receptor. Food Addit. Contam. 17, 261–266.[CrossRef][ISI][Medline]

Puga, A., Maier, A., and Medvedovic, M. (2000). The transcriptional signature of dioxin in human hepatoma HepG2 cells. Biochem. Pharmacol. 60, 1129–1142.[CrossRef][ISI][Medline]

Rattenborg, T., Gjermandsen, I., and Bonefeld-Jorgensen, E. C. (2002). Inhibition of E2-induced expression of BRCA1 by persistent organochlorines. Breast Cancer Res. 4, R12.[CrossRef][Medline]

Rotman, G., and Shiloh, Y. (1998). ATM: From gene to function. Hum Mol. Genet. 7, 1555–1563.[Abstract/Free Full Text]

Safe, S. (2001). Molecular biology of the Ah receptor and its role in carcinogenesis. Toxicol. Lett. 120, 1–7.[CrossRef][ISI][Medline]

Schiestl, R. H., Aubrecht, J., Yap, W. Y., Kandikonda, S., and Sidhom, S. (1997). Polychlorinated biphenyls and 2,3,7,8-tetrachlorodibenzo-p-dioxin induce intrachromosomal recombination in vitro and in vivo. Cancer Res. 57, 4378–4383.[Abstract]

Taghian, D. G., and Nickoloff, J. A. (1997). Chromosomal double-strand breaks induce gene conversion at high frequency in mammalian cells. Mol. Cell Biol. 17, 6386–6393.[Abstract]

Theirry, A., Perrin, A., Boyer, J., Fairhead, C., Dujon, B., Frey, B., and Schmitz, G. (1991). Cleavage of yeast and bacteriophage T7 genomes at a single site using the rare cutter endonuclease I-Sce I. Nucleic Acids Res. 19, 189–190.[ISI][Medline]

van den, B. M., Lohman, P. H., and Pastink, A. (2002). DNA double-strand break repair by homologous recombination. Biol. Chem. 383, 873–892.[ISI][Medline]

van Gent, D. C., Hoeijmakers, J. H., and Kanaar, R. (2001). Chromosomal stability and the DNA double-stranded break connection. Nat. Rev. Genet. 2, 196–206.[CrossRef][ISI][Medline]





This Article
Abstract
FREE Full Text (PDF)
All Versions of this Article:
81/1/133    most recent
kfh200v1
Alert me when this article is cited
Alert me if a correction is posted
Services
Email this article to a friend
Similar articles in this journal
Similar articles in ISI Web of Science
Similar articles in PubMed
Alert me to new issues of the journal
Add to My Personal Archive
Download to citation manager
Disclaimer
Request Permissions
Google Scholar
Articles by Chan, C. Y. Y.
Articles by Winn, L. M.
PubMed
PubMed Citation
Articles by Chan, C. Y. Y.
Articles by Winn, L. M.