Sister chromatid exchange induced by DNA topoisomerases poisons in late replicating heterochromatin: influence of inhibition of replication and transcription

Induction of homologous recombination following in utero exposure to DNA-damaging agents

Much of our understanding of homologous recombination, as well as the development of the working models for these processes, has been derived from extensive work in model organisms, such as yeast and fruit flies, and mammalian systems by studying the repair of induced double strand breaks or repair following exposure to genotoxic agents in vitro. We therefore set out to expand this in vitro work to ask whether DNA-damaging agents with varying modes of action could induce somatic change in an in vivo mouse model of homologous recombination. We exposed pregnant dams to DNA-damaging agents, conferring a variety of lesions at a specific time in embryo development. To monitor homologous recombination frequency, we used the well-established retinal pigment epithelium pink-eyed unstable assay. Homologous recombination resulting in the deletion of a duplicated 70 kb fragment in the coding region of the Oca2 gene renders this gene functional and can be visualized as a pigmented eyespot in the retinal pigment epithelium. We observed an increased frequency of pigmented eyespots in resultant litters following exposure to cisplatin, methyl methanesulfonate, ethyl methanesulfonate, 3-aminobenzamide, bleomycin, and etoposide with a contrasting decrease in the frequency of detectable reversion events following camptothecin and hydroxyurea exposure. The somatic genomic rearrangements that result from such a wide variety of differently acting damaging agents implies long-term potential effects from even short-term in utero exposures.

Genotoxic profile of inhibitors of topoisomerases I (camptothecin) and II (etoposide) in a mitotic recombination and sex-chromosome loss somatic eye assay of Drosophila melanogaster

Genotoxic carcinogens which interact with DNA may produce double-strand breaks as normal intermediates of homologous mitotic recombination, and may give rise to structural chromosome aberrations and inter-chromosomal deletion-recombination. The genotoxic profile of two inhibitors of DNA topoisomerases were evaluated using an in vivo somatic w/w+ eye assay of Drosophila melanogaster for the detection of loss of heterozygosity (LOH) by homologous mitotic recombination, intra-chromosomal recombination and structural chromosomal aberrations. We studied camptothecin (CPT) as a topoisomerase-I-interactive agent and etoposide (ETOP) as a topoisomerase II inhibitor. These drugs act by stabilizing a ternary complex consisting of topoisomerases covalently linked to DNA at single-strand or at double-strand breaks, thereby preventing the relegation step of the breakage/rejoining reaction mediated by the enzyme. The genotoxic profiles were determined from the appearance of eye tissue in adult flies, in which LOH and expression of the reporter gene white produced light clones. The results demonstrated that both compounds were significantly genotoxic, with CPT being more effective than ETOP. Inter-chromosomal mitotic recombination was the major mechanism responsible for the induction of light spots by both compounds in XX females. Loss of the ring X chromosome (rX), was significantly enhanced by CPT, and this topoisomerase blocker also produced intra-chromosomal recombination (XY males).

Repair of topoisomerase I-mediated DNA damage

Topoisomerase I (Top1) is an abundant and essential enzyme. Top1 is the selective target of camptothecins, which are effective anticancer agents. Top1-DNA cleavage complexes can also be trapped by various endogenous and exogenous DNA lesions including mismatches, abasic sites and carcinogenic adducts. Tyrosyl-DNA phosphodiesterase (Tdp1) is one of the repair enzymes for Top1-DNA covalent complexes. Tdp1 forms a multiprotein complex that includes poly(ADP) ribose polymerase (PARP). PARP-deficient cells are hypersensitive to camptothecins and functionally deficient for Tdp1. We will review recent developments in several pathways involved in the repair of Top1 cleavage complexes and the role of Chk1 and Chk2 checkpoint kinases in the cellular responses to Top1 inhibitors. The genes conferring camptothecin hypersensitivity are compiled for humans, budding yeast and fission yeast.

Micronucleus evaluation in mitogen-stimulated lymphocytes of PUVA treated patients

PUVA describes the treatment of patients with psoralens plus an exposure to a source of UV light of 320-400 nm (UVA). Contradictory results have been reported on the chromosomal damage of PUVA when assayed by sister chromatid exchange (SCE) method. Micronucleus (MN) test is used to detect both clastogenic (breaking) and aneugenic (abnormal segregation) effect of physical/chemical agents on the chromosomes. No data have been found on the MN formation in the cells of PUVA treated patients. Frequency of micronuclei in 72 hours cultivated/mitogen-stimulated lymphocytes of patients have been evaluated at zero time and after 20, 40, 60 sessions of PUVA treatment. While the beginning MN frequency was approximately 0.22% (n=23), it raised to approximately 0.32 (n=23), approximately 0.42 (n=14) and approximately 0.53% (n=10) corresponding respectively to 20, 40 and 60 sessions. These sessions correspond reciprocally to 54+/-23, 172+/-48, 300+/-61 joules/cm2 of UVA and 13, 26, 39 mg/kg of 8-metoxypsoralen (8-MOP). While large interindividual variances were apparent, highly significant differences have been observed between initial MN frequency and after that of the 20, 40 and 60 sessions, (p = 0.000, p = 0.004, p = 0.005, reciprocally, Wilcoxon two-related samples test). The coefficient of correlation between MN frequency and UVA doses starting from zero to 60 sessions of treatment has been found as r = 0.61. This indicates a significant relationship between UVA doses and MN frequencies. However, MN inducibility and synergistic property of 8-MOP with UVA should be taken into account. Gradual MN increase during different sessions of PUVA treatment shows that--once appeared--a part of MN at least persist in the cells of patients from a few days to a few weeks. Smoking as a confounding factor seems to increase MN frequency (p = 0.053, Mann-Whitney U-test) in the beginning population, taken as the control population. This is the first report on the kinetics of MN formation during different sessions of PUVA treatment. Based on our results, we concluded that PUVA treatment causes a detectable chromosome damaging effect on the relatively profound cells/tissues of its human users. Therapists should be careful with its use, especially on the patients who may be more susceptible to carcinogenesis (e.g. immunosuppressed and/or elderly subjects).

Disulfiram is a potent in vitro inhibitor of DNA topoisomerases

The drug disulfiram is a thiol-reacting drug that is relatively nontoxic when used alone and has been used in the therapy of alcohol abuse for more than 40 years. Several effects of this drug have been reported for DNA synthesis and cell proliferation. In this study, the inhibitory effect of disulfiram on topoisomerase I and II activity was investigated by measuring the relaxation of superhelical plasmid pBR322 DNA. Disulfiram (1-100 microM) inhibited topoisomerase I and II in a concentration-dependent manner (IC(50) congruent with 42 +/- 8 and 30 +/- 9 microM, respectively). Consistent with the assumption that a thiol residue is involved, dithiothreitol (1 mM) markedly prevented the inhibitory effect of disulfiram on the activity of both classes of topoisomerases. These findings might explain certain aspects of disulfiram toxicity and encourage new studies to determine the usefulness of this drug and its analogues as antineoplastic agent.

Topologically constrained domains of supercoiled DNA in eukaryotic cells

The size of supercoiled, topologically constrained DNA domains within the squamous carcinoma cell line SQ-20B were determined by direct comparison with a panel of irradiated supercoiled plasmid DNAs. Loss of supercoiling in plasmids was determined by gel electrophoresis and in cells by nucleoid flow cytometry. Comparison of dose-response data for plasmid relaxation with that obtained from SQ-20B cells enabled a direct estimation of supercoil target size in these cells. Plasmids pUCD9P (3.9 kbp), pXT-1 (10.1 kbp), pdBPV-MMT-neo (14.6 kbp), pRK290 (20.0 kbp), and R6K (38 kbp) were used and analyzed under the same exposure conditions as nucleoid DNA. Two sizes of topologically closed domains were found in nucleoids of 0.51+/-0.17Mbp and 1.34+/-0.3 Mbp. In an attempt to relate these large-scale organizations of DNA with function, cells were exposed to the DNA topoisomerase II inhibitor, VP16 and the G1/S cell cycle blocking agent mimosine. A 1 h exposure to VP16 was effective in reducing DNA synthesis which was associated with a parallel increase in nucleoid supercoiling. Addition of the G1 > S inhibitor mimosine enhanced both responses. It is concluded that chromosomes and interphase nuclei are organized into at least two sizes of topologically constrained domains of DNA which may have functional relevance to the control and execution of DNA synthesis.