Santiago Mateos - Academia.edu (original) (raw)

Papers by Santiago Mateos

Mutation Research Letters, 1987

We have investigated the influence of pH on the induction of chromatid-type aberrations and siste... more We have investigated the influence of pH on the induction of chromatid-type aberrations and sisterchromatid exchanges (SCEs) by maleic hydrazide (MH) in root-tip cells of Allium cepa. For both cytogenetic endpoints, the lower the pH of the treatment solution, the higher were the frequencies of chromosome alterations detected at metaphase. We have further studied the persistence of lesions giving rise to SCEs during successive cell cycles, as well as the influence of BrdU concentration in the posttreatment medium on the yield of MH-induced SCEs. Our results suggest that the cytogenetic action of MH in many respects resembles that of bifunctional alkylating agents. Maleic hydrazide (1,2-dihydro-3,6-pyridazinedione) (MH), a structural isomer of uracil, is a major commercial herbicide and plant-growth regulator which has been reported as a strong clastogenic agent in plant cells (Darlington and

Experientia, 1987

In Allium cepa root tips treated with acetaldehyde, metaphase cells showing diplochromosomes are ... more In Allium cepa root tips treated with acetaldehyde, metaphase cells showing diplochromosomes are occasionally observed. The short treatment time excludes the possibility that the endoreduplication has been induced by the chemical. Instead, it seems that acetaldehyde is able to stimulate, directly or indirectly, the division of mature cells previously endoreduplicated.

International Journal of Radiation Biology, 1996

Pulsed-field electrophoresis is being used extensively in the gene mapping studies and in the ana... more Pulsed-field electrophoresis is being used extensively in the gene mapping studies and in the analysis of DNA strand breakage by ionizing radiation. We have evaluated the relationship between the fraction of S phase DNA in a cell population and its ability to modify the migration of DNA in pulsed-field gel electrophoresis. We have shown that increasing the proportion of S phase DNA reduced the effective rate of migration of MGH-U1 cellular DNA. This effect was observed after treatment with ionizing radiation or the restriction enzyme Not I. However, when radiation-induced damage was studied using intact cells, only the DNA with 70 percent S phase showed apparent differences in damage induction. These studies therefore provide data to indicate the percentage of S phase cells at which overall DNA migration might be affected significantly.

Mutation research, 2003

DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the... more DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the DNA molecule in a very precise and unique fashion. Taking into account their fundamental roles in many events during DNA metabolism such as replication, transcription, recombination, condensation or segregation, it is no wonder that the last decade has witnessed an exponential interest on topoisomerases, mainly after the discovery of their potential role as targets in novel antitumor therapy. The difficulty of the lack of topoisomerase II mutants in higher eukaryotes has been partly overcome by the availability of drugs that act as either poisons or true catalytic inhibitors of the enzyme. These chemical tools have provided strong evidence that accurate performance of topoisomerase II is essential for chromosome segregation before anaphase, and this in turn constitutes a prerequisite for the development of normal mitosis. In the absence of cytokinesis, cells become polyploid or endoredu...

DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the... more DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the DNA molecule in a very precise and unique fashion. Taking into account their fundamental roles in many events during DNA metabolism such as replication, transcription, recombination, condensation or segregation, it is no wonder that the last decade has witnessed an exponential interest on topoisomerases, mainly after the discovery of their potential role as targets in novel antitumor therapy. The difficulty of the lack of topoisomerase II mutants in higher eukaryotes has been partly overcome by the availability of drugs that act as either poisons or true catalytic inhibitors of the enzyme. These chemical tools have provided strong evidence that accurate performance of topoisomerase II is essential for chromosome segregation before anaphase, and this in turn constitutes a prerequisite for the development of normal mitosis. In the absence of cytokinesis, cells become polyploid or endoredu...

AA8 Chinese hamster ovary cells were treated with halogenated nucleosides analogues of thymidine,... more AA8 Chinese hamster ovary cells were treated with halogenated nucleosides analogues of thymidine, namely CldU, 5-iodo-2 ′-deoxyuridine (IdU), and 5-bromo-2 ′-deoxyuridine (BrdU), following different experimental protocols. The purpose was to see whether incorporation of exogenous pyrimidine analogues into DNA could interfere with normal chromosome segregation. The endpoint chosen was endoreduplication, that arises after aberrant mitosis when daughter chromatids segregation fails. Treatment with any of the halogenated nucleosides for two consecutive cell cycles resulted in endoreduplication, with a highest yield for CldU, intermediate for IdU, and lowest for BrdU. The frequency of endoreduplicated cells paralleled in all cases the level of analogue substitution into DNA. Our results seem to support that thymidine analogue substitution into DNA is responsible for the triggering of endoreduplication. Besides, the lack of any effect on endoreduplication when CldU was present for only on...

Epidemiological studies have found a positive association between coffee consumption and a lower ... more Epidemiological studies have found a positive association between coffee consumption and a lower risk of cardiovascular disorders, some cancers, diabetes, Parkinson and Alzheimer disease. Coffee consumption, however, has also been linked to an increased risk of developing some types of cancer, including bladder cancer in adults and leukemia in children of mothers who drink coffee during pregnancy. Since cancer is driven by the accumulation of DNA alterations, the ability of the coffee constituent caffeic acid to induce DNA damage in cells may play a role in the carcinogenic potential of this beverage. This carcinogenic potential may be exacerbated in cells with DNA repair defects. People with the genetic disease Fanconi Anemia have DNA repair deficiencies and are predisposed to several cancers, particularly acute myeloid leukemia. Defects in the DNA repair protein Fanconi Anemia D2 (FANCD2) also play an important role in the development of a variety of cancers (e.g., bladder cancer)...

Oncotarget

The Cockayne Syndrome Protein B (CSB) plays an essential role in Transcription-Coupled Nucleotide... more The Cockayne Syndrome Protein B (CSB) plays an essential role in Transcription-Coupled Nucleotide Excision Repair (TC-NER) by recruiting repair proteins once transcription is blocked with a DNA lesion. In fact, CSB-deficient cells are unable to recover from transcription-blocking DNA lesions. 5-Aza-2′-deoxycytidine (5-azadC) is a nucleoside analogue that covalently traps DNA methyltransferases (DNMTs) onto DNA. This anticancer drug has a double mechanism of action: it reverts aberrant hypermethylation in tumour-suppressor genes, and it induces DNA damage. We have recently reported that Homologous Recombination and XRCC1/PARP play an important role in the repair of 5-azadC-induced DNA damage. However, the mechanisms involved in the repair of the DNMT adducts induced by azadC remain poorly understood. In this paper, we show for the first time the importance of CSB in the repair of azadCinduced DNA lesions. We propose a model in which CSB initiates a signalling pathway to repair transcription blocks induced by incorporated 5-azadC. Indeed, CSB-deficient cells treated with 5-azadC show a delay in the repair of trapped DNMT1, increased levels of DNA damage and reduced survival.

We recently screened a series of new aziridines β-D-galactopyranoside derivatives for select... more We recently screened a series of new aziridines β-D-galactopyranoside derivatives for selective anticancer activity and identified 2-methyl-2,3-[N-(4-methylbenzenesulfonyl)imino]propyl 2,3-di-O-benzyl-4,6-O-(S)-benzylidene-β-D-galactopyranoside (AzGalp) as the most promising compound. In this article, we explore possible mechanisms involved in the cytotoxicity of this aziridine and evaluate its selective anticancer activity using cancer cells and normal cells from a variety of tissues. Our data show that AzGalp induces DNA damage (detected with the comet assay). Cells deficient in the DNA repair pathway nucleotide excision repair (NER) were hypersensitive to the cytotoxicity of this compound. These results suggest that AzGalp induces bulky DNA adducts, and that cancer cells lacking a functional NER pathway may be particularly vulnerable to the anticancer effects of this aziridine. Several experiments revealed that neither the generation of oxidative stress nor the inhibiti...

Several clinically useful anticancer drugs selectively kill cancer cells by inducing DNA damage; ... more Several clinically useful anticancer drugs selectively kill cancer cells by inducing DNA damage; the genomic instability and DNA repair defects of cancer cells make them more vulnerable than normal cells to the cytotoxicity of DNA-damaging agents. Because epoxide-containing compounds can induce DNA damage, we have used the MTT assay to evaluate the selective cytotoxicity of three epoxyalkyl galactopyranosides against A549 lung cancer cells and MRC-5 lung normal cells. Compound (2S,3S)-2,3-Epoxydecyl 4,6-O-(S)-benzylidene-β-D-galactopyranoside (EDBGP) showed the highest selective anticancer activity and was selected for mechanistic studies. After observing that EDBGP induced cellular DNA damage (comet assay), we found that cells deficient in nucleotide excision repair were hypersensitive to the cytotoxicity of this compound; this suggests that EDBGP may induce bulky DNA adducts. EDBGP did not inhibit glycolysis (glucose consumption and lactate production). Pre-treatment of lung ...

Mutation Research Letters

DNA Repair

Zebularine is a second-generation, highly stable hydrophilic inhibitor of DNA methylation with or... more Zebularine is a second-generation, highly stable hydrophilic inhibitor of DNA methylation with oral bioavailability that preferentially target cancer cells. It acts primarily as a trap for DNA methyl transferases (DNMTs) protein by forming covalent complexes between DNMT protein and zebularine-substrate DNA. It's well documented that replication-blocking DNA lesions can cause replication fork collapse and thereby to the formation of DNA double-strand breaks (DSB). DSB are dangerous lesions that can lead to potentially oncogenic genomic rearrangements or cell death. The two major pathways for repair of DSB are non-homologous end joining (NHEJ) and homologous recombination (HR). Recently, multiple functions for the HR machinery have been identified at arrested forks. Here we investigate in more detail the importance of the lesions induced by zebularine in terms of DNA damage and cytotoxicity as well as the role of HR in the repair of these lesions. When we examined the contribution of NHEJ and HR in the repair of DSB induced by zebularine we found that these breaks were preferentially repaired by HR. Also we show that the production of DSB is dependent on active replication. To test this, we determined chromosome damage by zebularine while transiently inhibiting DNA synthesis. Here we report that cells deficient in single-strand break (SSB) repair are hypersensitive to zebularine. We have observed more DSB induced by zebularine in XRCC1 deficient cells, likely to be the result of conversion of SSB into toxic DSB when encountered by a replication fork. Furthermore we demonstrate that HR is required for the repair of these breaks. Overall, our data suggest that zebularine induces replication-dependent DSB which are preferentially repaired by HR.

Cytobios, Feb 1, 1999

Cultured Chinese hamster ovary (CHO) cells were treated with the cytidine analogue 5-azacytidine ... more Cultured Chinese hamster ovary (CHO) cells were treated with the cytidine analogue 5-azacytidine (5-azaC) which, in good agreement with results previously described from studies carried out in other primary or established mammalian cell lines, resulted in extensive chromosome decondensation and a shift in the time of replication of normally late-replicating heterochromatin to earlier replication. DNA topoisomerases (mainly topo I) have been involved in transcription, and the hypomethylating effect of 5-azaC reportedly results in the expression of silenced genes. Whether such an increase in transcription is paralleled by increased levels of both topo I and topo II, as well as by an enhancement in the topoisomerase activities, has been investigated in this work. The results seem to suggest that both the relative amount of topoisomerases and their activities are enhanced after a protracted treatment with the cytidine analogue over those observed in untreated controls. These observations could be significant for antitumour therapy.

Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis, 1991

Abstract Poly-d-lysine has been reported to induce a triggering of mitosis in plant cells due to ... more Abstract Poly-d-lysine has been reported to induce a triggering of mitosis in plant cells due to a selective stimulatory effect on cells arrested in G 2. Root-tip cells of Allium cepa L. were first exposed to maleic hydrazide (MH) early in the cell cycle and posttreated with different ...

The radiosensitive xrs5 mutant cell line of CHO K1 shows an overall deficiency in DNA double-stra... more The radiosensitive xrs5 mutant cell line of CHO K1 shows an overall deficiency in DNA double-strand break (dsb) rejoining. However, xrs5 paradoxically shows an apparently normal rate of disappearance of chromatid breaks with time, the kinetics of which is thought to reflect the underlying rejoining of dsb. Nevertheless the yield of chromatid breaks is elevated by four-fold in xrs5. A possible explanation of the paradox might be that xrs5 is proficient in rejoining dsb in the G2 phase of the cell cycle but converts a higher number of dsb into chromatid breaks. In order to test this we have measured the rejoining Of dsb in partially synchronised G2 xrs5 cells and compared the kinetics with those of cells synchronised in the G1 phase. Synchronisation of cells was achieved in G2 by release of cells from an aphidicolin block, and in G1 by staurosporine block. Cell synchrony was monitored by cytofluorometry and showed typically a 67% synchronisation of G2 cells and a 91% synchronisation of G1 cells. Rejoining of dsb was measured using neutral filter elution at pH 9.6. G2 cells showed a two-component kinetic with tl/2 values of 9 min and 3.6 h for dsb rejoining. Corresponding tl/2 values for G1 cells were 15 min and approximately 8.8 h. The tl/2 value of 3.6 h found for dsb rejoining in G2 cells is similar to a previously published value for asynchronous parental CHO K1 cells of approximately 4 h. The kinetics of chromatid break rejoining was measured in both xrs5 and CHO K1 following a dose of 0.75 Gy. The kinetics were found to be similar (tl/2 = 2.4 h) in the two cell lines, as previously reported using an equiclastogenic dose.

Mutation Research Letters, 1987

We have investigated the influence of pH on the induction of chromatid-type aberrations and siste... more We have investigated the influence of pH on the induction of chromatid-type aberrations and sisterchromatid exchanges (SCEs) by maleic hydrazide (MH) in root-tip cells of Allium cepa. For both cytogenetic endpoints, the lower the pH of the treatment solution, the higher were the frequencies of chromosome alterations detected at metaphase. We have further studied the persistence of lesions giving rise to SCEs during successive cell cycles, as well as the influence of BrdU concentration in the posttreatment medium on the yield of MH-induced SCEs. Our results suggest that the cytogenetic action of MH in many respects resembles that of bifunctional alkylating agents. Maleic hydrazide (1,2-dihydro-3,6-pyridazinedione) (MH), a structural isomer of uracil, is a major commercial herbicide and plant-growth regulator which has been reported as a strong clastogenic agent in plant cells (Darlington and

Experientia, 1987

In Allium cepa root tips treated with acetaldehyde, metaphase cells showing diplochromosomes are ... more In Allium cepa root tips treated with acetaldehyde, metaphase cells showing diplochromosomes are occasionally observed. The short treatment time excludes the possibility that the endoreduplication has been induced by the chemical. Instead, it seems that acetaldehyde is able to stimulate, directly or indirectly, the division of mature cells previously endoreduplicated.

International Journal of Radiation Biology, 1996

Pulsed-field electrophoresis is being used extensively in the gene mapping studies and in the ana... more Pulsed-field electrophoresis is being used extensively in the gene mapping studies and in the analysis of DNA strand breakage by ionizing radiation. We have evaluated the relationship between the fraction of S phase DNA in a cell population and its ability to modify the migration of DNA in pulsed-field gel electrophoresis. We have shown that increasing the proportion of S phase DNA reduced the effective rate of migration of MGH-U1 cellular DNA. This effect was observed after treatment with ionizing radiation or the restriction enzyme Not I. However, when radiation-induced damage was studied using intact cells, only the DNA with 70 percent S phase showed apparent differences in damage induction. These studies therefore provide data to indicate the percentage of S phase cells at which overall DNA migration might be affected significantly.

Mutation research, 2003

DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the... more DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the DNA molecule in a very precise and unique fashion. Taking into account their fundamental roles in many events during DNA metabolism such as replication, transcription, recombination, condensation or segregation, it is no wonder that the last decade has witnessed an exponential interest on topoisomerases, mainly after the discovery of their potential role as targets in novel antitumor therapy. The difficulty of the lack of topoisomerase II mutants in higher eukaryotes has been partly overcome by the availability of drugs that act as either poisons or true catalytic inhibitors of the enzyme. These chemical tools have provided strong evidence that accurate performance of topoisomerase II is essential for chromosome segregation before anaphase, and this in turn constitutes a prerequisite for the development of normal mitosis. In the absence of cytokinesis, cells become polyploid or endoredu...

DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the... more DNA topoisomerases are highly specialized nuclear enzymes that perform topological changes in the DNA molecule in a very precise and unique fashion. Taking into account their fundamental roles in many events during DNA metabolism such as replication, transcription, recombination, condensation or segregation, it is no wonder that the last decade has witnessed an exponential interest on topoisomerases, mainly after the discovery of their potential role as targets in novel antitumor therapy. The difficulty of the lack of topoisomerase II mutants in higher eukaryotes has been partly overcome by the availability of drugs that act as either poisons or true catalytic inhibitors of the enzyme. These chemical tools have provided strong evidence that accurate performance of topoisomerase II is essential for chromosome segregation before anaphase, and this in turn constitutes a prerequisite for the development of normal mitosis. In the absence of cytokinesis, cells become polyploid or endoredu...

AA8 Chinese hamster ovary cells were treated with halogenated nucleosides analogues of thymidine,... more AA8 Chinese hamster ovary cells were treated with halogenated nucleosides analogues of thymidine, namely CldU, 5-iodo-2 ′-deoxyuridine (IdU), and 5-bromo-2 ′-deoxyuridine (BrdU), following different experimental protocols. The purpose was to see whether incorporation of exogenous pyrimidine analogues into DNA could interfere with normal chromosome segregation. The endpoint chosen was endoreduplication, that arises after aberrant mitosis when daughter chromatids segregation fails. Treatment with any of the halogenated nucleosides for two consecutive cell cycles resulted in endoreduplication, with a highest yield for CldU, intermediate for IdU, and lowest for BrdU. The frequency of endoreduplicated cells paralleled in all cases the level of analogue substitution into DNA. Our results seem to support that thymidine analogue substitution into DNA is responsible for the triggering of endoreduplication. Besides, the lack of any effect on endoreduplication when CldU was present for only on...

Epidemiological studies have found a positive association between coffee consumption and a lower ... more Epidemiological studies have found a positive association between coffee consumption and a lower risk of cardiovascular disorders, some cancers, diabetes, Parkinson and Alzheimer disease. Coffee consumption, however, has also been linked to an increased risk of developing some types of cancer, including bladder cancer in adults and leukemia in children of mothers who drink coffee during pregnancy. Since cancer is driven by the accumulation of DNA alterations, the ability of the coffee constituent caffeic acid to induce DNA damage in cells may play a role in the carcinogenic potential of this beverage. This carcinogenic potential may be exacerbated in cells with DNA repair defects. People with the genetic disease Fanconi Anemia have DNA repair deficiencies and are predisposed to several cancers, particularly acute myeloid leukemia. Defects in the DNA repair protein Fanconi Anemia D2 (FANCD2) also play an important role in the development of a variety of cancers (e.g., bladder cancer)...

Oncotarget

The Cockayne Syndrome Protein B (CSB) plays an essential role in Transcription-Coupled Nucleotide... more The Cockayne Syndrome Protein B (CSB) plays an essential role in Transcription-Coupled Nucleotide Excision Repair (TC-NER) by recruiting repair proteins once transcription is blocked with a DNA lesion. In fact, CSB-deficient cells are unable to recover from transcription-blocking DNA lesions. 5-Aza-2′-deoxycytidine (5-azadC) is a nucleoside analogue that covalently traps DNA methyltransferases (DNMTs) onto DNA. This anticancer drug has a double mechanism of action: it reverts aberrant hypermethylation in tumour-suppressor genes, and it induces DNA damage. We have recently reported that Homologous Recombination and XRCC1/PARP play an important role in the repair of 5-azadC-induced DNA damage. However, the mechanisms involved in the repair of the DNMT adducts induced by azadC remain poorly understood. In this paper, we show for the first time the importance of CSB in the repair of azadCinduced DNA lesions. We propose a model in which CSB initiates a signalling pathway to repair transcription blocks induced by incorporated 5-azadC. Indeed, CSB-deficient cells treated with 5-azadC show a delay in the repair of trapped DNMT1, increased levels of DNA damage and reduced survival.

We recently screened a series of new aziridines β-D-galactopyranoside derivatives for select... more We recently screened a series of new aziridines β-D-galactopyranoside derivatives for selective anticancer activity and identified 2-methyl-2,3-[N-(4-methylbenzenesulfonyl)imino]propyl 2,3-di-O-benzyl-4,6-O-(S)-benzylidene-β-D-galactopyranoside (AzGalp) as the most promising compound. In this article, we explore possible mechanisms involved in the cytotoxicity of this aziridine and evaluate its selective anticancer activity using cancer cells and normal cells from a variety of tissues. Our data show that AzGalp induces DNA damage (detected with the comet assay). Cells deficient in the DNA repair pathway nucleotide excision repair (NER) were hypersensitive to the cytotoxicity of this compound. These results suggest that AzGalp induces bulky DNA adducts, and that cancer cells lacking a functional NER pathway may be particularly vulnerable to the anticancer effects of this aziridine. Several experiments revealed that neither the generation of oxidative stress nor the inhibiti...

Several clinically useful anticancer drugs selectively kill cancer cells by inducing DNA damage; ... more Several clinically useful anticancer drugs selectively kill cancer cells by inducing DNA damage; the genomic instability and DNA repair defects of cancer cells make them more vulnerable than normal cells to the cytotoxicity of DNA-damaging agents. Because epoxide-containing compounds can induce DNA damage, we have used the MTT assay to evaluate the selective cytotoxicity of three epoxyalkyl galactopyranosides against A549 lung cancer cells and MRC-5 lung normal cells. Compound (2S,3S)-2,3-Epoxydecyl 4,6-O-(S)-benzylidene-β-D-galactopyranoside (EDBGP) showed the highest selective anticancer activity and was selected for mechanistic studies. After observing that EDBGP induced cellular DNA damage (comet assay), we found that cells deficient in nucleotide excision repair were hypersensitive to the cytotoxicity of this compound; this suggests that EDBGP may induce bulky DNA adducts. EDBGP did not inhibit glycolysis (glucose consumption and lactate production). Pre-treatment of lung ...

Mutation Research Letters

DNA Repair

Zebularine is a second-generation, highly stable hydrophilic inhibitor of DNA methylation with or... more Zebularine is a second-generation, highly stable hydrophilic inhibitor of DNA methylation with oral bioavailability that preferentially target cancer cells. It acts primarily as a trap for DNA methyl transferases (DNMTs) protein by forming covalent complexes between DNMT protein and zebularine-substrate DNA. It's well documented that replication-blocking DNA lesions can cause replication fork collapse and thereby to the formation of DNA double-strand breaks (DSB). DSB are dangerous lesions that can lead to potentially oncogenic genomic rearrangements or cell death. The two major pathways for repair of DSB are non-homologous end joining (NHEJ) and homologous recombination (HR). Recently, multiple functions for the HR machinery have been identified at arrested forks. Here we investigate in more detail the importance of the lesions induced by zebularine in terms of DNA damage and cytotoxicity as well as the role of HR in the repair of these lesions. When we examined the contribution of NHEJ and HR in the repair of DSB induced by zebularine we found that these breaks were preferentially repaired by HR. Also we show that the production of DSB is dependent on active replication. To test this, we determined chromosome damage by zebularine while transiently inhibiting DNA synthesis. Here we report that cells deficient in single-strand break (SSB) repair are hypersensitive to zebularine. We have observed more DSB induced by zebularine in XRCC1 deficient cells, likely to be the result of conversion of SSB into toxic DSB when encountered by a replication fork. Furthermore we demonstrate that HR is required for the repair of these breaks. Overall, our data suggest that zebularine induces replication-dependent DSB which are preferentially repaired by HR.

Cytobios, Feb 1, 1999

Cultured Chinese hamster ovary (CHO) cells were treated with the cytidine analogue 5-azacytidine ... more Cultured Chinese hamster ovary (CHO) cells were treated with the cytidine analogue 5-azacytidine (5-azaC) which, in good agreement with results previously described from studies carried out in other primary or established mammalian cell lines, resulted in extensive chromosome decondensation and a shift in the time of replication of normally late-replicating heterochromatin to earlier replication. DNA topoisomerases (mainly topo I) have been involved in transcription, and the hypomethylating effect of 5-azaC reportedly results in the expression of silenced genes. Whether such an increase in transcription is paralleled by increased levels of both topo I and topo II, as well as by an enhancement in the topoisomerase activities, has been investigated in this work. The results seem to suggest that both the relative amount of topoisomerases and their activities are enhanced after a protracted treatment with the cytidine analogue over those observed in untreated controls. These observations could be significant for antitumour therapy.

Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis, 1991

Abstract Poly-d-lysine has been reported to induce a triggering of mitosis in plant cells due to ... more Abstract Poly-d-lysine has been reported to induce a triggering of mitosis in plant cells due to a selective stimulatory effect on cells arrested in G 2. Root-tip cells of Allium cepa L. were first exposed to maleic hydrazide (MH) early in the cell cycle and posttreated with different ...

The radiosensitive xrs5 mutant cell line of CHO K1 shows an overall deficiency in DNA double-stra... more The radiosensitive xrs5 mutant cell line of CHO K1 shows an overall deficiency in DNA double-strand break (dsb) rejoining. However, xrs5 paradoxically shows an apparently normal rate of disappearance of chromatid breaks with time, the kinetics of which is thought to reflect the underlying rejoining of dsb. Nevertheless the yield of chromatid breaks is elevated by four-fold in xrs5. A possible explanation of the paradox might be that xrs5 is proficient in rejoining dsb in the G2 phase of the cell cycle but converts a higher number of dsb into chromatid breaks. In order to test this we have measured the rejoining Of dsb in partially synchronised G2 xrs5 cells and compared the kinetics with those of cells synchronised in the G1 phase. Synchronisation of cells was achieved in G2 by release of cells from an aphidicolin block, and in G1 by staurosporine block. Cell synchrony was monitored by cytofluorometry and showed typically a 67% synchronisation of G2 cells and a 91% synchronisation of G1 cells. Rejoining of dsb was measured using neutral filter elution at pH 9.6. G2 cells showed a two-component kinetic with tl/2 values of 9 min and 3.6 h for dsb rejoining. Corresponding tl/2 values for G1 cells were 15 min and approximately 8.8 h. The tl/2 value of 3.6 h found for dsb rejoining in G2 cells is similar to a previously published value for asynchronous parental CHO K1 cells of approximately 4 h. The kinetics of chromatid break rejoining was measured in both xrs5 and CHO K1 following a dose of 0.75 Gy. The kinetics were found to be similar (tl/2 = 2.4 h) in the two cell lines, as previously reported using an equiclastogenic dose.