Molecular dosimetry for sister-chromatid exchange induction and cytotoxicity by monofunctional and bifunctional alkylating agents

Role of homologous recombination in DNA interstrand crosslink repair

Homologous recombination repair (HRR) encompasses mechanisms that employ homologous DNA sequences as templates for repair or tolerance of a wide range of DNA lesions that inhibit DNA replication in S phase. Arguably the most imposing of these DNA lesions is that of the interstrand crosslink (ICL), consisting of a covalently attached chemical bridge between opposing DNA strands. ICL repair requires the coordinated activities of HRR and a number of proteins from other DNA repair and damage response systems, including nucleotide excision repair, base excision repair, mismatch repair, and translesion DNA synthesis (TLS). Interestingly, different organisms favor alternative methods of HRR in the ICL repair process. E. coli perform ICL repair using a homology-driven damage bypass mechanism analogous to daughter strand gap repair. Eukaryotes from yeast to humans initiate ICL repair primarily during DNA replication, relying on HRR activity to restart broken replication forks associated with double-strand break intermediates induced by nucleolytic activities of other excision repair factors. Higher eukaryotes also employ several additional factors, including members of the Fanconi anemia damage-response network, which further promote replication-associated ICL repair through the activation and coordination of various DNA excision repair, TLS, and HRR proteins. This review focuses on the proteins and general mechanisms of HRR associated with ICL repair in different model organisms.

Formation of DNA adducts and tumor growth delay following intratumoral administration of DTI-015

Intratumoral (IT) administration of DTI-015 (BCNU in 100% ethanol) utilizes solvent facilitated perfusion for the treatment of tumors. RIF-1 tumors were treated by IT injection of either ethanol alone or 0.05-1.0 mg of DTI-015 or by i.v. injection of 0.5 mg of BCNU. Treatment with ethanol alone or i.v. injection of 0.5 mg of BCNU did not produce a significant growth delay. In contrast, IT administration of DTI-015 produced a significant growth delay at each of the treatment doses (p < 0.05 to p < 0.001). We have quantified the levels of N7-(2-hydroxyethyl) guanine (N7-HOEtG) in RIF-1 tumors 24h following either IT treatment with 0.5 mg DTI-015 or i.p. administration of 0.5 mg BCNU. Levels of N7-HOEtG (micromol/mol DNA) were < or = 0.08 for both untreated controls and following i.p. treatment with BCNU and 13.1 +/- 5.6 following IT administration of DTI-015. The levels of N7-HOEtG detected in RIF-1 tumors following IT administration of DTI-015 were 164-fold higher than the level(s) of N7-HOEtG in the i.p. BCNU treated tumor samples. These studies demonstrate that IT administration of DTI-015 produces high levels of DNA adducts in the tumor which correspond to a significant increase in tumor growth delay compared to the same dose of BCNU administered systemically.

Repair of DNA alkylation products formed in 9L cell lines treated with 1-(2-chloroethyl)-1-nitrosourea

The purpose of this study has been to measure the formation and repair of individual DNA alkylation products in 9L, 9L-2 and BTRC-19 cell lines after treatment with 1-(2-chloroethyl)-1-nitrosourea (CNU). The levels of seven DNA adducts N7-(2-hydroxyethyl)-guanine, N7-(2-chloroethyl)-guanine; 1,2-(diguan-7-yl)-ethane, N1-(2-hydroxyethyl)-2-deoxyguanosine, 1-(N1-2-deoxyguanosinyl), 2-(N3-2-deoxycytidyl)-ethane, O(6)-(2-hydroxyethyl)-2-deoxyguanosine and phosphotriesters were separated by HPLC and quantified by liquid scintillation counting. The levels of N7-(2-hydroxyethyl)-guanine, N7-(2-chloroethyl)-guanine; O(6)-(2-hydroxyethyl)-2-deoxyguanosine and phosphotriesters were not significantly different in the three glioma lines. Furthermore, comparison of the levels of these products in treated cells with the levels formed in purified DNA suggest that they were not actively repaired over the 6h interval. The levels of 1,2-(diguan-7-yl)-ethane and N1-(2-hydroxyethyl)-2-deoxyguanosine were reduced in 9L-2 and significantly reduced in BTRC-19 (P = 0.003) compared to 9L. Analysis of the data suggests that the reduction in the level of N1-(2-hydroxyethyl)-2-deoxyguanosine was due to repair of its precursor O(6)-ClEtdG by O(6)-alkylguanine-DNA-alkyltransferase (AGT). The level of the crosslinked product 1-(N1-2-deoxyguanosinyl), 2-(N3-2-deoxycytidyl)-ethane was significantly reduced (P < 0.001) in both 9L-2 and BTRC-19 as compared to 9L. Reduction in the level of 1-(N1-2-deoxyguanosinyl), 2-(N3-2-deoxycytidyl)-ethane in 9L-2 and BTRC-19 are consistent with repair of the precursor alkylation product O(6)-ClEtdG by AGT. This study demonstrates that there are very significant differences in the rates of removal of individual DNA adducts formed by CNU treatment of the glioma cell lines.

Induction of lacI mutations in Big Blue Rat-2 cells treated with 1-(2-hydroxyethyl)-1-nitrosourea: a model system for the analysis of mutagenic potential of the hydroxyethyl adducts produced by 1,3-bis (2-chloroethyl)-1-nitrosourea

We have investigated the genotoxic effects of 1-(2-hydroxyethyl)-1-nitrosourea (HENU). We have chosen this agent because of its demonstrated ability to produce N7-(2-hydroxyethyl) guanine (N7-HOEtG) and O(6)-(2-hydroxyethyl) 2'-deoxyguanosine (O(6)-HOEtdG); two of the DNA alkylation products produced by 1,3-bis (2-chloroethyl)-1-nitrosourea (BCNU). For these studies, we have used the Big Blue Rat-2 cell line that contains a lambda/lacI shuttle vector. Treatment of these cells with HENU produced a dose dependent increase in the levels of N7-HOEtG and O(6)-HOEtdG as quantified by HPLC with electrochemical detection. Treatment of Big Blue Rat-2 cells with either 0, 1 or 5mM HENU resulted in mutation frequencies of 7.2+/-2.2x10(-5), 45.2+/-2.9x10(-5) and 120.3+/-24.4x10(-5), respectively. Comparison of the mutation frequencies demonstrates that 1 and 5mM HENU treatments have increased the mutation frequency by 6- and 16-fold, respectively. This increase in mutation frequency was statistically significant (P<0.001). Sequence analysis of HENU-induced mutations have revealed primarily G:C-->A:T transitions (52%) and a significant number of A:T-->T:A transversions (16%). We propose that the observed G:C-->A:T transitions are produced by the DNA alkylation product O(6)-HOEtdG. These results suggest that the formation of O(6)-HOEtdG by BCNU treatment contributes to its observed mutagenic properties.

Repair of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

Involvement of nucleotide excision repair in a recombination-independent and error-prone pathway of DNA interstrand cross-link repair

DNA interstrand cross-links (ICLs) block the strand separation necessary for essential DNA functions such as transcription and replication and, hence, represent an important class of DNA lesion. Since both strands of the double helix are affected in cross-linked DNA, it is likely that conservative recombination using undamaged homologous regions as a donor may be required to repair ICLs in an error-free manner. However, in Escherichia coli and yeast, recombination-independent mechanisms of ICL repair have been identified in addition to recombinational repair pathways. To study the repair mechanisms of interstrand cross-links in mammalian cells, we developed an in vivo reactivation assay to examine the removal of interstrand cross-links in cultured cells. A site-specific psoralen cross-link was placed between the promoter and the coding region to inactivate the expression of green fluorescent protein or luciferase genes from reporter plasmids. By monitoring the reactivation of the reporter gene, we showed that a single defined psoralen cross-link was removed in repair-proficient cells in the absence of undamaged homologous sequences, suggesting the existence of an ICL repair pathway that is independent of homologous recombination. Mutant cell lines deficient in the nucleotide excision repair pathway were examined and found to be highly defective in the recombination-independent repair of ICLs, while mutants deficient in homologous recombination were found to be proficient. Mutation analysis of plasmids recovered from transfected cells showed frequent base substitutions at or near positions opposing a cross-linked thymidine residue. Based on these results, we suggest a distinct pathway for DNA interstrand cross-link repair involving nucleotide excision repair and a putative lesion bypass mechanism.

Biological effects of a bifunctional DNA crosslinker. I. Generation of triradial and quadriradial chromosomes

Interduplex crosslinks by a bifunctional anthramycin DNA crosslinker produced triradial and quadriradial chromosomes. The crosslinker alkylates guanine at N-2. Bovine chromosomes contain GC-rich density satellite DNAs at the centromeric heterochromatin and is the basis for the formation of triradial and quadriradial chromosomes at the centromeres. The in situ crosslinking of interphase chromosomes indicates that the distance between centromeres is 17.5 A. We conclude that the nuclear matrix associated DNA in the centromeric heterochromatin of interphase chromosomes are positioned close enough for crosslinking to occur. We propose a model for the generation of triradial and quadriradial chromosomes based upon the number of interduplex crosslinks between two chromosomes.

Detection of N7-(2-hydroxyethyl)guanine adducts in DNA and 9L cells treated with 1-(2-chloroethyl)-1-nitrosourea

A sensitive analytical method, HPLC-ED, was developed for the measurement of N7-(2-hydroxyethyl)guanine (N7-HOEtG). A detection limit of 3.2 N7-HOEtG/10(8) nucleotides was obtained with this method. Linear dose response curves for the formation of N7-HOEtG were obtained following treatment of either calf thymus DNA or 9L cells with 1-(2-chloroethyl)-1-nitrosourea (CNU). Using HPLC-ED a significant increase in the level of N7-HOEtG could be detected in 9L cells following treatment with 5 microM CNU. Our study suggests that with this analytical method the formation of N7-HOEtG in the white blood cells of patients treated with chloroethylnitrosoureas may be determined.

DNA damage and repair in somatic and germ cells in vivo

Alkylation-induced germ cell mutagenesis in the mouse versus Drosophila is compared based on data from forward mutation assays (specific-locus tests in the mouse and in Drosophila and multiple-locus assays in the latter species) but not including assays for structural chromosome aberrations. To facilitate comparisons between mouse and Drosophila, forward mutation test results have been grouped into three categories. Representatives of the first category are MMS (methyl methanesulfonate) and EO (ethylene oxide), alkylating agents with a high s value which predominantly react with ring nitrogens in DNA. ENU (N-ethyl-N-nitrosourea), MNU (N-methyl-N-nitrosourea), PRC (procarbazine), DEN (N-nitrosodiethylamine), and DMN (N-nitrosodimethylamine) belong to the second category. These agents have in common a considerable ability for modification at oxygens in DNA. Cross-linking agents (melphalan, chlorambucil, hexamethylphosphoramide) form the third category. The most unexpected, but encouraging outcome of this study is the identification of common features for three vastly different experimental indicators of genotoxicity: hereditary damage in Drosophila males, genetic damage in male mice, and tumors (TD50 estimates) in rodents. Based on the above three category classification scheme the following tentative conclusions are drawn. Monofunctional agents belonging to category 1, typified by MMS and EO, display genotoxic effects in male germ cell stages that have passed meiotic division. This phenomenon seems to be the consequence of a repair deficiency during spermiogenesis for a period of 3-4 days in Drosophila and 14 days in the mouse. We suggest that the reason for the high resistance of premeiotic stages, and the generally high TD50 estimates observed for this class in rodents, is the efficient error-free repair of N-alkylation damage. If we accept this hypothesis, then the increased carcinogenic potential in rodents, seen when comparing category 2 (ENU-type mutagens) to category 1 (MMS-type mutagens), along with the ability of category 2 genotoxins to induce genetic damage in premeiotic stages, must presumably be due to their enhanced ability for alkylations at oxygens in DNA; it is this property that actually distinguishes the two groups from each other. In contrast to category 1, examination of class 2 genotoxins (ENU and DEN) in premeiotic cells of Drosophila gave no indication for a significant role of germinal selection, and also removal by DNA repair was less dramatic compared to MMS. Thus category 2 mutagens are expected to display activity in a wide range of both post- and premeiotic germ cell stages. A number of these agents have been demonstrated to be among the most potent carcinogens in rodents.(ABSTRACT TRUNCATED AT 400 WORDS)

Cytogenetic mechanisms in the selective toxicity of cyclophosphamide analogs and metabolites towards avian embryonic B lymphocytes in vivo

Cyclophosphamide (CP) is selectively toxic to avian and mammalian B lymphocytes, but the mechanisms of action are incompletely understood. We used a structure-activity approach to determine the cytogenetic mechanisms underlying the selective lymphoid toxicity in chicken embryos at 18-19 days of incubation. Two doses of 5-bromo-2'-deoxyuridine (BrdU; 3 mg/200 microliters x 2) were pipetted onto the inner shell membrane to label lymphocyte DNA over 20 h. A single dose of the CP analogs or metabolites was given 1 h after the initial BrdU application. After a terminal 3-h exposure to demecolcine to block cells in metaphase, the embryos were sacrificed at hour 20, and their bursae and thymi were removed for cytogenetic processing. Microscope slide preparations of metaphases were stained by the fluorescence-plus-Giemsa technique to differentiate the sister chromatids for an assessment of sister-chromatid exchange (SCE) induction and cell cycle progression based on replication cycle-specific staining patterns. Isophosphamide (1.25-40 mg/kg), phosphoramide mustard (0.7-45.7 mg/kg), and 4-methylcyclophosphamide (1.3-42.1 mg/kg) selectively damaged B cells as shown by dose-related reductions in the mitotic activity, inhibition of cell cycle kinetics, and approximately 9-15-fold increases in the SCE frequency above control. B cells were up to 392 times more susceptible to the toxicity of these three bifunctional alkylating agents compared to T cells based on reductions in the mitotic activity. At most of the drug doses tested, the T-cell mitotic index was not depressed significantly and was usually higher than the control value by as much as 50-60%. Importantly, monochloroethylcyclophosphamide (70-245 mg/kg; monofunctional alkylation) did not induce differential lymphoid toxicity, although a 9-fold increase in the SCE frequency of B cells was observed at the highest dose. Didechlorocyclophosphamide (181-422 mg/kg; acrolein generation only) was a weak SCE inducer (approximately 1.8-fold increase) and was not selectively toxic to B cells. Our data show that selective toxicity to B lymphocytes is strongly associated with bifunctional alkylation via the chloroethyl groups rather than with monofunctional alkylation and acrolein-mediated damage. In addition, the results with phosphoramide mustard and 4-methylcyclophosphamide emphasize that aldehyde dehydrogenase activity is not the primary determinant in the relative sparing of T lymphocytes in vivo.

In vivo depletion of O6-alkylguanine-DNA-alkyltransferase in lymphocytes and melanoma of patients treated with CB 10-277, a new DTIC analogue

There is increasing evidence to suggest that alkylation of guanine residues in DNA at the O6 position is the critical cytotoxic event following treatment with dacarbazine (DTIC) and related drugs and that endogenous O6-alkylguanine-DNA alkyltransferase (ATase) gene expression may be a major factor in resistance to such agents. 1-p-Carboxyl-3,3-dimethylphenyltriazene (CB10-277) was recently selected for clinical evaluation as a DTIC analogue with improved solubility, stability and (possibly) metabolic activation. Serial ATase levels were measured in peripheral blood lymphocytes of nine patients and in biopsied melanoma samples of two patients undergoing treatment with 24-h continuous infusion of CB10-277 (12 g/m2). Wide individual variations in pre-treatment levels as well as in the post-treatment depletion of lymphocyte ATase were seen. Progressive depletion of lymphocyte ATase was seen during continuous infusion of CB10-277 in all patients. Complete suppression of lymphocyte ATase activity occurred in two patients whose pre-treatment ATase levels were low. Immediately following completion of the CB10-277 infusion, the median ATase activity was 17% of pre-treatment levels (range, 0-67%). At 24 h after the end of the infusion, no recovery of lymphocyte ATase activity was observed in six patients, but significant recovery to 50%, 100% and 102% of pre-treatment activity occurred in the other three. In three patients who returned for subsequent cycles of chemotherapy at 4 weeks after the first dose, pre-treatment ATase levels showed a 3- to 4-fold increase relative to the original pre-treatment values. A significant correlation was found between the extent of ATase depletion and the initial lymphocyte ATase levels (r = 0.725, P < 0.05). Haematological toxicity developed in two patients and was associated with low pre-treatment ATase activity. Depletion of tumour ATase activity was noted in these patients, with residual activity amounting to 8% and 11% of pre-treatment levels, respectively, in the biopsies melanoma tissues. These results indicate extensive metabolism of CB10-277 to a methylating agent capable of mediating alkylation of DNA and subsequent depletion of lymphocyte and tumour ATase levels and further indicate that the effects on lymphocytes may reflect effects on the target tumour.