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M Bignami

Publications and source records attributed to M Bignami.

At least 37 records · Page 2Linked to original sources
PubMed1993

Defective mismatch binding and a mutator phenotype in cells tolerant to DNA damage.

Acquired resistance to alkylating agents such as N-methyl-N-nitrosourea or N-methyl-N'-nitro-N-nitrosoguanidine results from the ability to tolerate the potentially cytotoxic methylated base O6-methylguanine (m6-G) in DNA. In the absence of repair by demethylation in situ, m6-G is probably lethal through its inappropriate processing by the cell. DNA mismatch correction is an attractive candidate for the processing function because although it is replicated, m6-G has no perfect complementary base. Thus, m6-G in DNA might provoke abortive mismatch repair and tolerance could subsequently arise through loss of a mismatch repair pathway. Mismatch correction helps maintain genomic fidelity by removing misincorporated bases and deaminated 5-methylcytosine from DNA, and its loss by mutation confers a mutator phenotype on Escherichia coli. Here we describe human and hamster cell lines that are tolerant to N-methyl-N-nitrosourea and are defective in a DNA mismatch binding activity. The loss of this activity, which acts on G.T mispairs, confers a mutator phenotype.

Animals↗
PubMed1993

Mutagenic processing of ethylation damage in mammalian cells: the use of methoxyamine to study apurinic/apyrimidinic site-induced mutagenesis.

The aldehyde reagent methoxyamine is able to interact with apurinic/apyrimidinic sites formed in vivo within cells and displays both an anti-cytotoxic and an antimutagenic activity on N-ethyl-N'-nitro-N-nitrosoguanidine-induced DNA damage in Chinese hamster ovary cells. To clarify the underlying mechanism we have examined the mutational spectra induced by N-ethyl-N'-nitro-N-nitrosoguanidine alone and in the presence of methoxyamine in the hypoxanthine-guanine phosphoribosyltransferase gene of Chinese hamster ovary cells. In both cases all mutations were base pair substitutions, and their distribution among various classes did not differ significantly. Almost 60% were transitions, predominantly GC to AT, and the remaining 40% were transversions, mainly at AT base pairs. The analysis of the proportion of the different types of mutations showed that in the presence of methoxyamine, GC to AT transitions decreased by a factor of 1.8, and AT to CG transversions were reduced by a factor of 13. These data indicate that in mammalian cells the fixation of ethylation damage into mutations occurs by both (a) direct mutagenesis likely driven by O6-ethylguanine adducts and to a minor extent by O4-ethylthymine and (b) apurinic/apyrimidinic site-mediated mutagenesis. These apurinic/apyrimidinic sites are formed during the processing of ethylation at critical sites and are likely to involve O6-ethylguanine and O2-ethylthymine adducts.

Alkylation↗
PubMed1993

Quantitative relationship between ethylated DNA bases and gene mutation at two loci in CHO cells.

In a previous study we showed that the formation of O6-ethylguanine (O6-EtGua) in the DNA of CHO cells in culture correlated with mutations induced by ethylnitrosourea (ENU) and diethylsulfate (DES) at the hypoxanthine-guanine-phosphoribosyltransferase (hprt) locus but not at the Na, K-ATPase locus. This study was extended to another ethylating agent, ethyl methanesulfonate (EMS). DNA adduct formation and induction of mutation at the two gene loci were determined simultaneously in CHO cells after EMS exposure. The extent of ethylation at the N7 and O6 positions of guanine and at the N3 site of adenine were measured and the possible correlations with 6-thioguanine resistance (6-TGr) and ouabain resistance (ouar) mutations were investigated. A good correlation between the levels of ethylation at O6 guanine and mutation frequency at hprt gene by all three ethylating agents was observed. In the case of the ouar locus, the frequency of O6-EtGua adducts correlated with mutation induction by EMS and ENU but not by DES. Although both EMS and DES have similar reaction mechanisms, these results highlight differences in their mutational specificity. The comparison of this type of analysis with mutational spectra revealed that correlation studies may be inadequate to analyse multicomponent phenomena like mutation formation.

Adenine↗
PubMed1993

Genetic consequences of tolerance to methylation DNA damage in mammalian cells.

We previously characterized a clone of CHO cells, clone B, that displayed tolerance to the cytotoxic effects of N-methylnitrosourea (MNU) and 6-thioguanine (6-TG). To determine whether this phenotype affected the mutagenic response of the cells, MNU-induced mutation to 8-azaadenine resistance (8-AAr) was measured in the parental and clone B cells. Comparable mutation frequencies were found in the two cell lines up to 0.5 mM MNU, while at higher MNU concentrations mutations could be reproducibly measured only in clone B cells. Similar amounts of DNA methylated bases were found in the two cell lines after a 30 min treatment with different concentrations of [3H]MNU and the same linear relationship was observed when mutation induction by MNU was plotted as a function of the amount of O6-methylguanine (O6-MeGua) in DNA, indicating that mutation induction in both cell lines was related to the presence of this methylated base. Fifteen MNU-induced 8-AAr mutants were isolated from each cell line and the sequences of the adenine phosphoribosyltransferase (aprt) mutations determined. The type (in 90% of the cases, GC to AT transitions), the sequence context and the strand localization of the mutations indicated that all mutations were targeted at O6-MeGua in DNA and no difference was found between the two lines. These results are consistent with a mechanism of tolerance of O6-MeGua that does not alter the processing of this methylated base into a mutation. Growth in 6-TG induced point mutations in clone B but not in the parental cells. A model is proposed in which the alkylation tolerant variant is altered in a mismatch correction pathway responsible for the cytotoxicity of the methylated base.

Adenine↗
PubMed1993

Association of viral oncogene-induced changes in gap junctional intercellular communication and morphological transformation in BALB/c3T3 cells.

In order to study the relationship between altered gap junctional intercellular communication (GJIC) and induction of cell transformation by oncogenes, we transfected six viral oncogenes into BALB/c3T3 A31-1-1 cells. BALB/c3T3 cells with v-src, v-ras or polyoma middle T (PyMT) genes grew in soft agar and formed distinct transformed foci in the absence or presence of a vast excess of non-transfected cells. On the other hand, those with v-myc, v-fos or polyoma large T (PyLT) genes expressed less distinctly transformed phenotypes (less transformed morphology, higher saturation density than non-transfected counterparts and less growth in soft agar), and did not form distinct foci in coculture with non-transformed cells. When their homologous GJIC capacities were examined by the microinjection/dye transfer assay, no decrease in GJIC was observed in any of the v-onc-transformed cells. Non-transformed and all v-onc-transformed cell lines expressed similar levels of connexin 43 mRNA. v-myc-, v-fos- and PyLT-transformed cells, but not v-ras-, v-src- and PyMT-transformed cells were able to communicate heterologously with non-transformed cells. Tumor promoting phorbol esters strongly inhibited GJIC of non-transformed and all v-onc-transformed BALB/c3T3 cell lines. In cocultures of v-myc-, v-fos- or PyLT-transformed cells with non-transformed BALB/c3T3 A31-1-1 cells, 12-O-tetradecanoylphorbol-13-acetate (TPA) increased the number of transformed foci. However, when these v-onc-transformed cells were co-cultured with non-transformed BALB/c3T3 A31-1-13 cells (which lose GJIC at growth confluence, as if TPA had been added), no morphologically transformed foci appeared. These results suggest that factors other than GJIC are involved in the suppression of oncogene-transformed cells by surrounding normal counterparts.

3T3 Cells↗
PubMed1992

Fidelity of replication of the leading and the lagging DNA strands opposite N-methyl-N-nitrosourea-induced DNA damage in human cells.

Semi-conservative replication of double-stranded DNA in eukaryotic cells is an asymmetric process involving leading and lagging strand synthesis and different DNA polymerases. We report a study to analyze the effect of these asymmetries when the replication machinery encounters alkylation-induced DNA adducts. The model system is an EBV-derived shuttle vector which replicates in synchrony with the host human cells and carries as marker gene the bacterial gpt gene. A preferential distribution of N-methyl-N-nitrosourea (MNU)-induced mutations in the non transcribed DNA strand of the shuttle vector pF1-EBV was previously reported. The hypermutated strand was the leading strand. To test whether the different fidelity of DNA polymerases synthesizing the leading and the lagging strands might contribute to MNU-induced mutation distribution the mutagenesis study was repeated on the shuttle vector pTF-EBV which contains the gpt gene in the inverted orientation. We show that the base substitution error rates on an alkylated substrate are similar for the replication of the leading and lagging strands. Moreover, we present evidence that the fidelity of replication opposite O6-methylguanine adducts of both the leading and lagging strands is not affected by the 3' flanking base. The preferential targeting of mutations after replication of alkylated DNA is mainly driven by the base at the 5' side of the G residues.

Animals↗
PubMed1992

Expression of the endogenous O6-methylguanine-DNA-methyltransferase protects Chinese hamster ovary cells from spontaneous G:C to A:T transitions.

We have investigated whether the presence of a DNA repair enzyme, O6-methylguanine-DNA-methyltransferase (MGMT), affects the nature of spontaneous mutations in a mammalian cell line. We compared spontaneous mutations in the adenine phosphoribosyl transferase gene of a Chinese hamster ovary (CHO) cell line that expressed 14,000 MGMT molecules/cell with those in the parental CHO cells lacking this DNA repair activity. The mutation rate/cell/generation of the two CHO cell lines did not differ significantly. However, DNA sequence analysis of spontaneous mutations in the MGMT-proficient CHO cell line revealed a complex picture. No significant difference from the parental CHO cells was found in the number or type of deletions, frameshifts, multiple substitutions, or insertions. The frequency of G:C to T:A transversions was elevated in MGMT-proficient CHO cells. Expression of the enzyme considerably reduced G:C to A:T transitions (25% versus 8.3%). This latter result is the first evidence that this protein is active on an endogenous source of O6-methylguanine that is normally responsible for spontaneous G:C to A:T transition mutations.

Adenine Phosphoribosyltransferase↗
PubMed1992

Non-phenotypic selection of N-methyl-N-nitrosourea-induced mutations in human cells.

The distribution of mutations in a particular gene as detected by a selective mutation assay could be affected by the structural properties of the target protein. To investigate this, we have analysed N-methyl-N-nitrosourea (MNU)-induced mutations in two restriction recognition sequences of a target gene for mutation analysis and compared these data with what previously observed in a phenotypic mutation assay. DNA base changes in the Ncil and EcoRV sites of the gpt gene maintained in human cells by a shuttle vector system were measured by restriction fragment length polymorphism/polymerase chain reaction (RFLP/PCR) technique. After MNU-treatment of human cells, mutations were detected in the Ncil recognition sequence but not in the EcoRV site. DNA sequencing analysis revealed that all Ncil-resistant mutations were GC to AT transitions located over four bases of the Ncil recognition sequence. Only one of these mutations drastically affected the functionality of the GPT protein. The Ncil-resistant mutations were randomly distributed in both DNA strands of the gpt gene and were preferentially targeted at guanine residues flanked 5' by a guanine. Our results indicate that the structure of the GPT protein is the main contributor to the strand-specificity of MNU-induced mutations previously reported by using a phenotypic mutation assay. The potential use of the RFLP/PCR technique as a general tool for mutation detection is also discussed.

Base Sequence↗
PubMed1992

Methoxyamine modification of abasic sites protects CHO cells from the cytotoxic and mutagenic effects of oxygen alkylation.

The biological effects of the interaction of methoxyamine (MX) with apurinic/apyrimidinic (AP) sites produced in CHO cells by treatment with alkylating agents were examined. A decrease in cytotoxicity was observed after a 10 min treatment with the SN1 alkylating agents ethylnitrosourea (ENU), N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG) and N-methyl-nitrosourea when MX was present in the culture medium. Furthermore MX reduced the number of mutations to 6-thioguanine resistance induced by ENU and ENNG and the number of sister chromatid exchanges induced by ENU. In contrast, no protective effect of MX on survival was observed after a 10 min treatment with the SN2 alkylating agents diethylsulfate (DES), ethyl methane sulfonate and methyl methane sulfonate. A 3 h exposure to MX abolished the protective effect of MX on ENU-induced cytotoxicity and increased the cytotoxicity of DES. In vitro studies with synthetic oligonucleotides containing a single AP site opposite a normal guanine or O6-methylguanine showed that MX inhibits the cleavage of AP sites by the CHO AP endonuclease(s). A model is proposed in which different DNA lesions are involved in AP site formation after treatment with SN2 or SN2 alkylating agents. The involvement of specific alkylation products in cytotoxicity and mutagenesis is also discussed.

Alkylating Agents↗
PubMed1991

Processing in vitro of an abasic site reacted with methoxyamine: a new assay for the detection of abasic sites formed in vivo.

In this study we demonstrate that the different substrate recognition properties of bacterial and human AP endonucleases might be used to quantify and localize apurinic (AP) sites formed in DNA in vivo. By using a model oligonucleotide containing a single AP site modified with methoxyamine (MX), we show that endonuclease III and IV of E. coli are able to cleave the alkoxyamine-adducted site whereas a partially purified HeLa AP endonuclease and crude cell-free extracts from HeLa cells are inhibited by this modification. In addition MX-modified AP sites in a DNA template retain their ability to block DNA synthesis in vitro. Since MX can efficiently react with AP sites formed in mammalian cells in vivo we propose that the MX modified abasic sites thus formed can be quantitated and localized at the level of the individual gene by subsequent site specific cleavage by either E. coli endonuclease III or IV in vitro.

Base Sequence↗
PubMed1991

Site-dependent inhibition by single O6-methylguanine bases of SV40 T-antigen interactions with the viral origin of replication.

The effects of O6-methylguanine on the reactions involved in initiation of DNA replication were investigated by measuring the interactions of SV40 T antigen with oligonucleotides substituted with the methylated base. O6-Methylguanine residues were positioned in either binding site I or binding site II of the SV40 origin of replication. Binding of purified T antigen, measured by both nitrocellulose filter binding and delayed oligonucleotide migration, was unaffected by the presence of seven methylated bases in binding site II. Single substitutions within binding site I were sufficient to inhibit T-antigen binding, and the extent of inhibition was dependent on the position of O6-methylguanine in the DNA sequence. Unwinding by T antigen was analyzed by measuring displacement of a single-stranded oligonucleotide from similarly substituted, partially duplex substrates. The presence of three O6-methylguanine residues in binding site I facilitated the helicase activity of T antigen. In contrast, single O6-methylguanine bases inhibited unwinding. A correlation was observed between the position of the methylated base and the inhibition of both binding and unwinding by T antigen.

Animals↗
PubMed1990

Tolerance to O6-methylguanine and 6-thioguanine cytotoxic effects: a cross-resistant phenotype in N-methylnitrosourea-resistant Chinese hamster ovary cells.

The biochemical and genetic characteristics of a clone of Chinese hamster ovary cells displaying resistance to N-methyl-N-nitrosourea (MNU) and 6-thioguanine (6-TG) were analyzed. The initial level of 7-methylguanine, 3-methyladenine, and O6-methylguanine formation and the repair rates for these alkylated bases were the same in the resistant and in the parental cell line, indicating that the resistance to alkylation damage is not due to differences in DNA alkylation. After exposure for 24 or 48 h to 6-TG (0.6 micrograms/ml) in culture medium, the resistant clone in contrast to them, was able to replicate the DNA containing the base analogue during the following 24 h. These data are in agreement with the hypothesis that resistant cells tolerate both O6-methylguanine and 6-TG present in DNA. The tolerance to MNU and 6-TG also included chromosomal damage induced by these two agents, and MNU-resistant cells incurred less sister chromatid exchanges after treatment with either MNU or 6-TG. 6-TG-resistant cells, selected by growth in 6-TG, exhibited cross-resistance to MNU but not to methyl methanesulfonate, confirming that a common pathway of tolerance is responsible for resistance to 6-TG and O6-methylguanine.

Alkylation↗
PubMed1990

O6-methylguanine in the SV40 origin of replication inhibits binding but increases unwinding by viral large T antigen.

To study the effect of the potentially cytotoxic base O6-methylguanine (O6-meG) on the initiation of DNA replication, double-stranded oligonucleotides corresponding to the SV40 origin of replication were constructed in which O6-meG replaced guanine in one strand. Out of 14 methylated residues, 10 were present in the Binding sites for T antigen (3 in Binding Site 1 and 7 in Binding Site 2). Binding of purified T antigen to the substituted oligonucleotide was considerably reduced in comparison to the unsubstituted one, as measured by nitrocellulose filter binding. Both the ATP-dependent and ATP-independent binding of T antigen were affected by the presence of the methylated base. Band shift analysis revealed an altered pattern of delayed-migrating complexes of T antigen with the O6-meG-containing oligonucleotide. Competition experiments, in which unmodified oligonucleotides containing Binding Site 1 or 2 were included in the binding assays, indicated that the affinity of T antigen for the O6-meG modified sites was reduced. When partially duplex oligonucleotides containing either Binding Site 1 or Site 2 of the origin of replication were used as substrates for the helicase activity of T antigen, the presence of O6-meG increased the extent of T antigen catalysed displacement of single-stranded DNA fragments.

Adenosine Triphosphate↗
PubMed1990

Evidence for AP site formation related to DNA-oxygen alkylation in CHO cells treated with ethylating agents.

DNA single-strand breaks (ssb) induced by N-ethyl-N-nitrosourea (ENU) in CHO cells are quickly resealed within 10 min after treatment. This rapid repair kinetics is not explained by the rate of base excision repair which removes the main ethyl products with a half-life in the order of hours. We have explored the potential use of methoxyamine (MX), a chemical that reacts at neutral pH with AP sites in DNA in vitro, to clarify the origin of ENU-induced ssb. The presence of 50 mM MX during cell treatment with diethyl sulfate (DES) caused selective inhibition of the repair of AP sites generated during base excision repair and inhibited alkaline cleavage at these sites. The treatment of CHO cells with ENU in the presence of MX clearly showed that the burst of ssb observed immediately after treatment was due to AP site formation. Plasmid DNA treated in vitro with ENU did not present AP endonuclease-sensitive sites; therefore, the AP sites produced in CHO cells by ENU treatment are not due to the chemical hydrolysis of a very unstable ethyl adduct but rather are intermediates of an as yet undefined enzymatic pathway. This process occurs specifically after treatment with SN1-type ethylating agents (ENU and N-ethyl-N'-nitro-N-nitrosoguanidine) suggesting an association between this phenomenon and DNA-oxygen alkylation. We suggest that these breaks are generated by a mechanism of O6-ethylguanine processing without removal of the modified base.

Animals↗
PubMed1989

Tolerance to methylnitrosourea-induced DNA damage is associated with 6-thioguanine resistance in CHO cells.

Clones (13 and B) of O6-methylguanine-DNA-methyl-transferase-proficient (MT+) CHO cells showing different levels of resistance to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) but similar MT activity, were found to be sensitive to methyl methanesulphonate and resistant to N-methyl-N-nitrosourea (MNU). A 2.8-fold increase in resistance to MNU-induced cytotoxicity was observed in clone 13 and a 16-fold increase in clone B. A slight increase in survival (1.5-fold) after N-ethyl-N-nitrosourea treatment was observed in clone B. These data indicate that the resistant phenotype is specific for agents that preferentially methylate O atoms in DNA. The survival of MNNG- and MNU-resistant clones as well as of the parental CHO cell line was analysed after exposure to purine analogues substituted in different positions, 8-azaguanine (8-AG), 8-azaadenine (8-AA) and 6-thioguanine (6-TG). A 6-fold increase in resistance to 6-TG was found in clone B, although the hypoxanthine guanine phosphoribosyltransferase gene is functional in these cells. The same cytotoxicity was found in all the lines after treatment with 8-AG and 8-AA. These data are in agreement with the previous observation that clone 13 and clone B belong to two different classes of resistance, clone 13 resistance being explained by MT levels. The finding that clone B is cross-resistant to 6-TG is discussed in the light of a mechanism of tolerance to modifications at specific positions of guanine.

Animals↗
PubMed1989

O6-methyltransferase-deficient and -proficient CHO cells differ in their responses to ethyl- and methyl-nitrosourea-induced DNA alkylation.

The mutagenic and cytotoxic effects of N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) were compared in two isogenic Chinese hamster ovary (CHO) cell lines differing for the expression of the repair function for O6-methylguanine (O6-meGua), the O6-methyl-DNA-methyltransferase (MT). Survival and ouabain resistance (ouar) mutation frequency were similar in the two cell lines after treatment with ENU while both effects were strongly reduced in the MT-proficient (MT+) CHO cells after exposure to MNU. The slow repair kinetics of O6-ethylguanine (O6-etGua) when compared to O6-meGua, i.e. 25% versus 88% removal at 20 h after treatment, could still account for the similar mutational curves reported in the two cell lines after ENU treatment. The number of ENU-induced sister chromatid exchanges (SCE) was slightly reduced in the MT+ as compared to MT-deficient CHO cells suggesting a role for O6-etGua in SCE formation. Comparison of survival after exposure to ENU and MNU showed that, at similar levels of O6-alkylguanine on DNA, the ethyl- is more tolerated than the methyl-adduct. These data focus the attention on the importance of DNA damage processing in the cytotoxic response to alkylating agents.

Alkylation↗
PubMed1989

Quantitative evaluation of genotoxic effects by molecular dosimetry.

The covalent binding of alkylating agents with DNA is considered a critical event in the mutagenic and carcinogenic effect of these chemicals. Several studies have indicated that only certain DNA alkylation products contribute to the mutagenic or carcinogenic activity of these agents. In this paper we summarize data obtained in Chinese hamster ovary (CHO) cells after treatment with two ethylating agents, N-ethylnitrosourea and diethylsulfate, which are known to alkylate DNA sites with different efficiency. A correlation study between DNA adduct formation and induction of mutations at two gene loci, i.e. hypoxanthine-guanine-phosphoribosyltransferase (HPRT) and Na,K-ATPase, has been performed. The influence of DNA repair processes on the final yield of mutation is specifically discussed.

Alkylating Agents↗