Chromosomal aberration induction in CHO cells by combined exposure to restriction enzymes and X-rays

Modelling the Formation of Polycentric Chromosome Aberrations

Exchange-type chromosome aberrations produced by ionizing radiation or restriction enzymes are believed to result from pairwise interaction of DNA double-strand breaks (dsb). In addition to dicentrics, such aberrations may include higher-order polycentrics (tricentrics, tetracentrics, etc.). We have developed computer programs that calculate the probability of the various polycentrics for a given average number of pairwise interactions. Two models are used. Model I incorporates kinetic competition between restitution, complete exchanges (illegitimate recombination events), and incomplete exchanges. Model II allows unrestituted breaks even if there is no recombination. The models were applied to experimental observations of aberrations produced in G1 Chinese hamster ovary cells after electroporation with the restriction enzyme PvuII, which produces blunt-end dsb. We found, experimentally and theoretically, that there was a maximum in the number and multiplicity of polycentrics per cell: beyond a certain PvuII concentration no additional or higher-order polycentrics were produced. Computer-generated relationships, which were remarkably similar for both models and for all values of the adjustable parameters, were found between dicentrics per cell and higher-order polycentrics per cell. Excellent agreement was found between the experimental observations and the consensus theoretical curve relating tricentrics per cell to dicentrics per cell. The observed number of higher polycentrics per cell for a given number of dicentrics per cell was somewhat larger than the consensus theoretical prediction. The observed number of centric rings per cell was markedly larger than the consensus theoretical value, presumably owing to intrachromosomal localization ('proximity effects'). The computer models also provided estimates for the adjustable parameters; for example, in model I the fraction of incomplete exchanges was found to be about 35%.

Transduction of adenovirus-mediated wild-type p53 after radiotherapy in human cervical cancer cells

OBJECTIVE

Radiotherapy is the mainstay of treatment for advanced cervical cancer and most cervical cancers have evidence of HPV (human papilloma virus) infection, which inactivates the p53 gene. The goal of this study was to determine the effect of combining radiotherapy and Adp53 infection on the growth of cervical carcinoma cells.

METHODS

The silta cervical carcinoma cell line contains wild-type p53 and HPV 16 infection. The C33A cell line has a p53 mutation. The Adp53 recombinant adenovirus contains the cytomegalovirus promoter, wild-type p53 cDNA, and a polyadenylation signal in a minigene cassette inserted into the El-deleted region of a modified adenovirus 5. Transduction efficiency was assessed by using an adenovirus containing the Escherichia coli beta-galactosidase gene and expression of wild-type p53 in infected cells was evaluated by Western blot analysis. One group of cells was irradiated prior to infection, the other group received no irradiation, but were either infected with virus or mock-infected. Cells were analyzed for viability 1 to 7 days after infection by using the sulforhodamine B assay. The percentage of cells undergoing apoptosis was determined by using a TUNEL assay.

RESULTS

Fifty percent of C33A cells were transduced with 5 muthplicities of infection of virus whereas SiHa cells required 25 multiplicities of infection. Adp53 expression was found 48 h after infection. In the cells treated with both radiation and Adp53 infection growth inhibition was increased compared with inhibition resulting with either treatment alone. The combination treatment also increased the percentage of cells undergoing apoptosis.

CONCLUSION

Combining radiotherapy with Adp53 infection increases the inhibition of growth of cervical cancer cells in vitro. This combination treatment has the potential of increasing efficacy and of therapy.

Frequencies of polycentric chromosomes following combinational treatments of Chinese hamster ovary cells with restriction endonucleases and X-rays, bleomycin or DNase I

Chinese hamster ovary cells were treated with combinations of the restriction endonucleases AluI, EcoRI or PstI and bleomycin, DNase I or X-rays. With few exceptions the frequencies of polycentric chromosomes calculated as dicentric chromosomes (DIC) were additive or less than additive after combinational treatments when compared with the effects of treatments with the single compounds. Less than additivity was found when the frequencies of DIC were higher than 150 DIC per 100 cells indicating a saturation effect. The data suggest that DNA double-strand breaks (DSB) with different end-structures produce polycentric chromosomes to the same extent as DSB with the same end-structures.

Chromosome damage induced by combined treatments with restriction endonucleases introduced into CHO cells by single or double electroporation

The possible recombination between non-homologous termini produced by restriction enzymes (REs) introduced in CHO cells by electroporation was studied. For this purpose, different combinations of REs that produced blunt or 5' overhanging DNA double-strand breaks were electroporated into cells either at the same time or separately by double electroporation experiments. Prior to double electroporation, it was confirmed that, once the cells have been electroporated, they resist a second electroporation, as assessed by cell viability analysis. Besides, the efficient and homogeneous introduction of labelled, non-permeable molecules was assessed by fluorescence microscopy. Our results showed interaction for most of the conditions, mainly when the REs were introduced separately. Differences found in the degree of interaction between the combinations studied are discussed.

Modulatory effect of sodium butyrate on AluI-induced chromosomal aberrations in CHO cells

Exponentially growing CHO cells exposed to millimolar concentrations of sodium butyrate (SB) for 24 h were treated with AluI using two methods of cell poration, i.e., electroporation and streptolysin O (SLO). Under both conditions, SB was found to induce a 2-4-fold increase in AluI-induced chromosomal aberrations. When cells in monolayer were treated with AluI/SLO, lower concentrations of SB (2.5 mM) and AluI (1-4 U/ml) were required to produce a similar effect as that observed for electroporated cells, demonstrating the differential sensitivity of the two methods. Furthermore, in AluI/SLO-treated cells, a higher percentage of cells was found to show increased frequencies of aberrations per cell, compared to AluI/electroporated cells. The mechanism by which SB modulates the cell response to AluI treatment might involve changes in chromatin configuration thereby increasing the accessibility of AluI to different parts of chromatin.

DNA strand breaks: the DNA template alterations that trigger p53-dependent DNA damage response pathways

The tumor suppressor protein p53 serves as a critical regulator of a G1 cell cycle checkpoint and of apoptosis following exposure of cells to DNA-damaging agents. The mechanism by which DNA-damaging agents elevate p53 protein levels to trigger G1/S arrest or cell death remains to be elucidated. In fact, whether damage to the DNA template itself participates in transducing the signal leading to p53 induction has not yet been demonstrated. We exposed human cell lines containing wild-type p53 alleles to several different DNA-damaging agents and found that agents which rapidly induce DNA strand breaks, such as ionizing radiation, bleomycin, and DNA topoisomerase-targeted drugs, rapidly triggered p53 protein elevations. In addition, we determined that camptothecin-stimulated trapping of topoisomerase I-DNA complexes was not sufficient to elevate p53 protein levels; rather, replication-associated DNA strand breaks were required. Furthermore, treatment of cells with the antimetabolite N(phosphonoacetyl)-L-aspartate (PALA) did not cause rapid p53 protein increases but resulted in delayed increases in p53 protein levels temporally correlated with the appearance of DNA strand breaks. Finally, we concluded that DNA strand breaks were sufficient for initiating p53-dependent signal transduction after finding that introduction of nucleases into cells by electroporation stimulated rapid p53 protein elevations. While DNA strand breaks appeared to be capable of triggering p53 induction, DNA lesions other than strand breaks did not. Exposure of normal cells and excision repair-deficient xeroderma pigmentosum cells to low doses of UV light, under conditions in which thymine dimers appear but DNA replication-associated strand breaks were prevented, resulted in p53 induction attributable to DNA strand breaks associated with excision repair. Our data indicate that DNA strand breaks are sufficient and probably necessary for p53 induction in cells with wild-type p53 alleles exposed to DNA-damaging agents.

DNA strand breaks: the DNA template alterations that trigger p53-dependent DNA damage response pathways

The tumor suppressor protein p53 serves as a critical regulator of a G1 cell cycle checkpoint and of apoptosis following exposure of cells to DNA-damaging agents. The mechanism by which DNA-damaging agents elevate p53 protein levels to trigger G1/S arrest or cell death remains to be elucidated. In fact, whether damage to the DNA template itself participates in transducing the signal leading to p53 induction has not yet been demonstrated. We exposed human cell lines containing wild-type p53 alleles to several different DNA-damaging agents and found that agents which rapidly induce DNA strand breaks, such as ionizing radiation, bleomycin, and DNA topoisomerase-targeted drugs, rapidly triggered p53 protein elevations. In addition, we determined that camptothecin-stimulated trapping of topoisomerase I-DNA complexes was not sufficient to elevate p53 protein levels; rather, replication-associated DNA strand breaks were required. Furthermore, treatment of cells with the antimetabolite N(phosphonoacetyl)-L-aspartate (PALA) did not cause rapid p53 protein increases but resulted in delayed increases in p53 protein levels temporally correlated with the appearance of DNA strand breaks. Finally, we concluded that DNA strand breaks were sufficient for initiating p53-dependent signal transduction after finding that introduction of nucleases into cells by electroporation stimulated rapid p53 protein elevations. While DNA strand breaks appeared to be capable of triggering p53 induction, DNA lesions other than strand breaks did not. Exposure of normal cells and excision repair-deficient xeroderma pigmentosum cells to low doses of UV light, under conditions in which thymine dimers appear but DNA replication-associated strand breaks were prevented, resulted in p53 induction attributable to DNA strand breaks associated with excision repair. Our data indicate that DNA strand breaks are sufficient and probably necessary for p53 induction in cells with wild-type p53 alleles exposed to DNA-damaging agents.

Response of radiation-sensitive human cells to defined DNA breaks

We have investigated the response of four human cell lines, representing a range of sensitivities to ionizing radiation, to enzymes which induce defined DNA double-strand breaks (dsbs). Cell lines were derived from a normal individual, from the cancer-prone disorders ataxia-telangiectasia (AT) and Bloom's syndrome (BS), and from an immunodeficient individual (46BR). The molecular defects in AT and BS are unknown, while 46BR is known to be DNA ligase I deficient. We assayed the clonogenic survival of the cell lines following in vivo scission of the DNA by the restriction endonucleases PvuII and BanI. These two enzymes differ in their action; PvuII gives rise to dsbs with blunt termini, while BanI generates staggered ends with a 4 bp overhang. We found a correlation between the sensitivity of the cell lines to X-rays and to the blunt-end cutter PvuII, but not to the cohesive-end cutter BanI.

Restriction-endonuclease-induced DNA double-strand breaks and chromosomal aberrations in mammalian cells

Restriction endonucleases (RE) can be used to mimic and model the clastogenic effects of ionising radiation. With the development of improved techniques for cell poration: electroporation and recently streptolysin O (SLO), it has become possible more confidently to study the relationships between DNA double-strand breaks (dsb) of various types (e.g. blunt or cohesive-ended) and the frequencies of induced metaphase chromosomal aberrations or micronuclei in cytokinesis-blocked cells. Although RE-induced dsb do not mimic the chemical end-structure of radiation-induced dsb (i.e. the 'dirty' ends of radiation-induced dsb), it has become clear that cohesive-ended dsb, which are thought to be the major type of dsb induced by radiation, are much less clastogenic than blunt-ended dsb. It has also been possible, with the aid of electroporation or SLO to measure the kinetics of dsb in cells as a function of time after treatment. These experiments have shown that some RE (e.g. Pvu II) are extremely stable inside CHO cells and at high concentrations persist and induce dsb over a period of many hours following treatment. Cutting of DNA by RE is thought to be at specific recognition sequences (as in free DNA) although the frequencies of sites in native chromatin available to RE is not yet known. DNA condensation and methylation are both factors limiting the numbers of available cutting sites. Relatively little is known about the kinetics of incision or repair of RE-induced dsb in cells. Direct ligation may be a method used by cells to rejoin the bulk of RE-induced dsb, since inhibitors such as araA, araC and aphidicolin appear not prevent rejoining, although these inhibitors have been found to lead to enhanced frequencies of chromosomal aberrations. 3-Aminobenzimide, the poly-ADP ribose polymerase inhibitor is the only agent that has so far been shown to inhibit rejoining of RE-induced dsb. Data from the radiosensitive xrs5 cell line, where chromosomal aberration frequencies are higher after RE treatments than in their normal parental CHO line, indicates that the xrs dsb repair pathway is involved in the repair of these dsb. We found that cells treated simultaneous with Pvu II and T4 ligase yielded lower levels of chromosomal damage than in the WT parental line indicating that Pvu II induced dsb retain their ability to be blunt-end ligated inside the cell.

Nonhomologous DNA end rejoining in chromosomal aberration formation

The role of recombination of nonhomologous DNA ends in chromosomal aberration formation was investigated in Chinese hamster ovary cells. Restriction enzymes that produce blunt, 3' overhanging, or 5' overhanging DNA double-strand breaks were electroporated into cells in various combinations, and chromosomal aberrations were analyzed at metaphase. For all enzyme combinations tested, there was a significant increase in the frequency of aberrations whose formation requires two breaks in the DNA over the sum obtained when each of the enzymes was tested separately and the aberration frequencies were totaled. No such pattern existed for terminal deletions, which presumably require only one DNA break. The extent of interaction did not depend on the homology in the overhanging sequences or on the combination of ends used, although the largest effect was seen with a combination of two blunt ends. This study shows that nonhomologous DNA double-strand breaks can interact to increase chromosomal aberration formation significantly.