BRCA1 downregulation leads to premature inactivation of spindle checkpoint and confers paclitaxel resistance

BRCA1 germline mutations predispose women to early onset, familial breast and ovarian cancer. BRCA1 has been recently implicated in the cellular response to agents that disrupt the mitotic spindle. In this report, we studied BRCA1 contribution to paclitaxel response in MCF-7 breast cancer cells. We show that MCF-7 cells transfected with BRCA1 siRNA display a significant increase in resistance to paclitaxel compared with the control cells. We next demonstrate that downregulation of BRCA1 reduces the mitotic index and triggers premature cyclin B1 degradation and decrease in Cdk1 activity following paclitaxel treatment, suggesting that BRCA1 downregulation results in precocious inactivation of the spindle checkpoint. These findings were confirmed by showing that BRCA1 downregulation induces premature sister-chromatids separation in MCF-7 cells following spindle damage. Furthermore, we show that BRCA1 up-regulates the expression of the protein kinase BubR1, essential component of the functional spindle checkpoint, whose downregulation is known to result in paclitaxel resistance in MCF-7 cells. Altogether, our findings support the notion that downregulation of BRCA1 expression mediates paclitaxel resistance through premature inactivation of spindle checkpoint in MCF-7 breast cancer cells. They link BRCA1 to the mitotic checkpoint that plays an essential role in the maintenance of chromosomal stability.

Transcription-coupled repair is inducible in hamster cells

In mammalian cells, the rate of nucleotide excision repair of UV dimers is heterogeneous throughout the genome, with repair occurring more rapidly in the transcribed strand of active genes than in the genome overall. This repair pathway is termed transcription-coupled repair (TCR) and is thought to permit the rapid resumption of RNA synthesis following UV irradiation. To evaluate the inducibility of the TCR process, we examined the repair of UV-induced cyclobutane pyrimidine dimers (CPDs) at the level of the gene following exposure of hamster cells to a sub-lethal UV fluence, 3 h prior to a higher dose. Repair was detected by a well-established technique allowing quantification of CPDs at the level of a specific strand by Southern blot hybridization. Here, we show that prior low-dose irradiation clearly enhanced the early rate of CPD removal in the transcribed strand of the active DHFR gene. Furthermore, the RNA synthesis recovery following UV exposure was stimulated by the priming UV dose. Thus, we provide evidence for an inducible TCR response to CPDs in hamster cells. This pathway is independent of the p53 activation, since the hamster cell line that we used expresses high levels of mutant p53 protein.

Involvement of apoptosis in mitomycin C hypersensitivity of Chinese hamster cell mutants

To elucidate the mechanisms of the mammalian cell defense against cross-linking agents, we studied previously cellular responses to mitomycin C (MMC) treatment in two MMC-hypersensitive hamster cell mutants' V-H4 and V-C8, as well as their parental cell line V79. In the present report, we investigated whether alterations in cell cycle checkpoints and induction of apoptosis could be responsible for the MMC hypersensitivity of the V-H4 and V-C8 mutant cell lines. First, we found that parental and mutant cells exhibited similar cell cycle responses to MMC concentrations of equivalent cytotoxicity, arguing against a defective cell cycle checkpoint in hypersensitive cell lines. In contrast, we showed that mutant cells underwent greater levels of apoptosis following MMC treatment than parental cells. These findings indicate that increased induction of apoptosis contributes to the hypersensitivity of V-H4 and V-C8 cells to the growth inhibitory effect of MMC. This differential apoptotic response was observed with both equimolar and equitoxic MMC doses and was specific to the cross-linking agent MMC, suggesting that control of the apoptotic process is altered in both MMC-hypersensitive mutants. The defective genes in V-H4 and V-C8 cells would then function in the regulation of an apoptotic pathway triggered by MMC-induced damage and independent of p53-mediated transcription.

Lack of correlation between repair of DNA interstrand cross-links and hypersensitivity of hamster cells towards mitomycin C and cisplatin

The ability to repair DNA interstrand cross-links may be an important factor contributing to mitomycin C (MMC) and cisplatin cytotoxicities. We have assessed the repair of interstrand cross-links induced by MMC in two MMC-hypersensitive hamster cell mutants and their resistant parental cell line. Using a gene-specific repair assay, we found no evidence for repair of MMC cross-links in either parental or mutant cells, suggesting that persistence of DNA interstrand cross-links is not responsible for the differential toxicity of MMC towards hypersensitive cells. Repair of cisplatin-induced interstrand cross-links was efficient in resistant as well as in mutant cells. Therefore we concluded that a defect in excision repair of interstrand cross-links was not responsible for the cytotoxic effects of MMC and cisplatin in these hypersensitive mutants.

Gene-specific nuclear and mitochondrial repair of formamidopyrimidine DNA glycosylase-sensitive sites in Chinese hamster ovary cells

This study examines the capacity of a mammalian cell to repair, at the gene level, DNA base lesions generated by photoactivation of acridine orange. Chinese hamster ovary fibroblasts were exposed to acridine orange and visible light, and gene-specific DNA repair was measured in the dihydrofolate reductase (DHFR) gene and in the mitochondrial genome. DNA lesions were recognized by Escherichia coli formamidepyrimidine-DNA glycosylase (FPG) which removes predominantly 8-oxodG and the corresponding formamidopyrimidine ring opened bases, and subsequently cleaves the DNA at the resulting apurinic site. FPG-recognized DNA lesions increased linearly with increasing photo-activation of AO, while cell survival was not affected by light alone and was negligibly affected by preincubation with AO in the dark. The frequency of induction of FPG-sensitive DNA damage by photoactivation of AO was similar in the transcribed and non-transcribed nuclear DNA as well as in the mitochondrial DNA. FPG-sensitive sites in the DHFR gene were repaired quickly, with 84% of adducts repaired within 4 h. The lesion frequency, kinetics and percent of repair of non-transcribed genomic DNA did not differ significantly from repair in the active DHFR gene up to 1 h postexposure. At late time points, transcribed DNA was repaired faster than the non-transcribed DNA. Mitochondrial DNA was efficiently repaired, at a rate similar to that in the active nuclear DNA.

Increased sensitivity to cisplatin by nm23-transfected tumor cell lines

We report a functional link between expression of the metastasis suppressor gene nm23 and cancer cell sensitivity to the alkylating agent cisplatin. Cisplatin was 2-15-fold more inhibitory to the growth in vitro of nm23 transfectants of the K-1735 TK murine melanoma, MDA-MB-435 human breast carcinoma, and OVCAR-3 human ovarian carcinoma cell lines as compared to matched control transfectants. Administration of a single dose of cisplatin i.v. after injection of control- or nm23-1-transfected K-1735 TK melanoma cells resulted in a more pronounced inhibition of pulmonary metastatic colonization by the nm23-1 transfectants. The mechanism of nm23-dependent sensitivity to cisplatin is unknown, but was correlated with increased formation of interstrand DNA cross-links in nm23-H1 transfected breast carcinoma cells. These data suggest that elevation of tumor cell nm23 expression may be considered as a potential therapeutic strategy in combination with cisplatin treatment.

DNA repair in the endogenous and episomal amplified c-myc oncogene loci in human tumor cells

We have studied the repair of u.v.-induced cyclobutane pyrimidine dimers (CPDs) in amplified c-myc oncogene loci in human colon cancer cells to better understand the relationship between chromatin structure, transcription and DNA repair. To assess the variation in DNA repair in the same gene whether located in a chromosomal site or in a extra-chromosomal site, we have quantitated the efficiency of excision repair after u.v. exposure in the endogenous and episomal c-myc genes isolated from COLO320HSR and DM cells. In the HSR cells, c-myc is localized in a homogeneously staining region (HSR), and in the DM cells, the gene is localized in double minute chromosomes (DM). Our results indicate that the repair is less efficient in c-myc amplicons organized as double minute chromosomes than in the endogenous c-myc amplicons. The episomal gene is not repaired with the same efficiency as when it is intrachromosomal. This may reflect differences in chromatin structure. An advantage of this biological system is that the cells possess two different alleles of the c-myc gene, one that is active and another which is inactive. We have studied the relationship between DNA repair and transcriptional activity in the c-myc locus by measuring the efficiency of excision repair after u.v. exposure in the normal and rearranged alleles of the c-myc gene. Surprisingly, the c-myc gene is repaired with similar efficiency in the highly transcribed allele as in the poorly expressed allele. However, u.v. damage is selectively removed from the transcribed strand of the active c-myc allele, but DNA repair is not strand specific in the non-expressed c-myc allele.

Role of the human ERCC-1 gene in gene-specific repair of cisplatin-induced DNA damage

The human excision repair gene ERCC-1 gene restores normal resistance to UV and mitomycin C in excision repair deficient chinese hamster ovary cells of complementation group 1. To investigate the involvement of the ERCC-1 gene in gene-specific repair of bulky lesions, we have studied the removal of damage induced by the antitumor agent cisplatin in CHO mutant 43-3B cells of group 1, with or without transfection with the ERCC-1 gene. Firstly, we determined the contribution of the ERCC-1 gene to the repair of interstrand crosslinks (ICL) induced by cisplatin and found efficient removal of ICLs from the dihydrofolate reductase (DHFR) gene in the ERCC-1 transfected 43-3B cells. We then assessed the contribution of ERCC-1 to the repair of intrastrand adducts (IA) induced by cisplatin. Compared to the wild-type parental cell line, the ERCC-1 transfected 43-3B cells repaired the IAs in the DHFR gene inefficiently. Thus, our data suggest that the ERCC-1 gene is more involved in the repair of interstrand crosslinks than in the removal of intrastrand adducts.

Purification and biochemical characterization of Escherichia coli RecA proteins mutated in the putative DNA binding site

Escherichia coli RecA protein plays a central role both in DNA repair and in recombination. We report biochemical properties of three new RecA proteins mutated at positions 199 (RecA694), 207 (RecA659), and 211 (RecA611) in the putative DNA binding site. RecA694 had a wild-type phenotype, whereas RecA611 and RecA659 were deficient in promoting both the self-cleavage of LexA repressor and the DNA-strand exchange reaction. In order to determine the origin of this inhibition, we examined the capacity of wild-type and mutant proteins to bind to single-stranded DNA (with and without single-stranded binding protein, SSB), double-stranded DNA, and ATP. DNA strand exchange defects were correlated with the inability of mutant proteins to displace SSB from DNA. For the recA659 mutation this inhibition was reversed by equimolar wild-type protein. In contrast, mixtures of either wild-type/RecA659 or wild-type/RecA611 proteins remained deficient in LexA cleavage, suggesting that the dominant negative phenotype of the mutant proteins may be a consequence of the formation heterologous RecA complexes. Various mutations in the putative DNA binding site of RecA protein altered ATP binding, ATPase activity, displacement of SSB from single-stranded DNA, and protein-protein interactions. These results are consistent with the hypothesis that DNA binding to this site of RecA relays allosteric effects to several functional domains throughout the protein.

Repair of ribosomal RNA genes in hamster cells after UV irradiation, or treatment with cisplatin or alkylating agents

We have measured the DNA damage formation and repair in the ribosomal and the dihydrofolate reductase (DHFR) genes after treatment of hamster cells with different types of DNA damaging agents. In mammalian cells, the ribosomal DNA (rDNA) is transcribed by RNA polymerase I, whereas the DHFR is transcribed by RNA polymerase II, whereas the DHFR is transcribed by RNA polymerase II. Cells were treated with agents that induce different types of lesions, and that are known to be repaired via different pathways. We used UV (254 nm) irradiation, treatment with cisplatin and treatment with the alkylating agents nitrogen mustard (HN2) and methyl methanesulphonate (MMS). UV induced pyrimidine dimers were detected with the enzyme T4 endonuclease V, which creates nicks at the dimer sites; the breaks are then resolved and identified by denaturing electrophoresis and Southern blot. Intrastrand adducts formed by the alkylating agents HN2 and MMS were quantitated by generating strand breaks at abasic sites after neutral depurination. Interstrand crosslinks (ICL) formed by HN2 and cisplatin were detected by a denaturation-reannealing reaction before neutral agarose gel-electrophoresis. We find that the repair of the pyrimidine dimers is significantly less efficient in the RNA polymerase I transcribed rDNA genes than in RNA polymerase II transcribed DHFR gene at 8 and 24 h after irradiation. ICL and intrastrand adducts induced by HN2 are also removed more slowly from the rDNA than from the DHFR gene. In contrast, MMS induced intrastrand adducts and cisplatin induced ICL are repaired equally efficiently in the RNA polymerase I and RNA polymerase II transcribed genes. We conclude that for some types of DNA damage, there is less repair in the ribosomal genes than in the DHFR; but for other DNA lesions there is no difference. The difference in repair efficiency between the rDNA and the DHFR genes may reflect the different RNA polymerase involved in their transcription. It may, however, alternatively, reflect the different nuclear localization of these genes.

Gene-specific DNA repair of interstrand cross-links induced by chemotherapeutic agents can be preferential

The gene-specific formation and repair of interstrand cross-links (ICL) were measured in the dihydrofolate reductase (DHFR) gene in hamster cells. Cells were treated with two different chemotherapeutic agents, nitrogen mustard and cisplatin, and the frequency of cross-links was quantified in the active gene and in a downstream, inactive region. About 5% of total lesions induced by these agents were ICL. Whereas the frequencies of cross-links formed were similar in the gene and in the noncoding region after cisplatin treatment, there were more nitrogen mustard-induced cross-links in the inactive region than in the active gene. At low levels of cross-linking, we found preferential DNA repair in the active gene as compared to the inactive region. At higher levels of cross-linking, there was no difference in repair rates between the gene and the noncoding region due to an increase in the repair efficiency in the inactive DNA. Implications of fine structural organization of cross-link repair are discussed.

New mutations in and around the L2 disordered loop of the RecA protein modulate recombination and/or coprotease activity

The RecA protein plays a key role in Escherichia coli recombination and DNA repair. We have created new recA mutants with mutations in the vicinity of the recA430 mutation (Gly-204----Ser) which is known to affect RecA coprotease activity. Mutants carrying recA659 or recA611, located 3 and 7 amino acids downstream of residue 204, respectively, lose all RecA activities, while the mutant carrying recA616, which is located at 12 amino acids from this residue, keeps the coprotease activity but is unable to promote recombination. Complementation experiments show that both mutations recA611 and recA659 are dominant over the wild-type or recA430 allele while recA616 seems to be recessive to recA+ and dominant over recA430. It is suggested that these mutations are located in RecA domains which direct conformational modifications.

Site-directed mutagenesis in the Escherichia coli recA gene

Escherichia coli RecA protein plays a fundamental role in genetic recombination and in regulation and expression of the SOS response. We have constructed 6 mutants in the recA gene by site-directed mutagenesis, 5 of which were located in the vicinity of the recA430 mutation responsible for a coprotease deficient phenotype and one which was at the Tyr 264 site. We have analysed the capacity of these mutants to accomplish recombination and to express SOS functions. Our results suggest that the region including amino acid 204 and at least 7 amino acids downstream interacts not only with LexA protein but also with ATP. In addition, the mutation at Tyr 264 shows that this amino acid is essential for RecA activities in vivo, probably because of its involvement in an ATP binding site, as previously shown in vitro.

Modulation of the SOS response by truncated RecA proteins

RecA protein plays several key roles in the SOS response. We have constructed truncated proteins and examined their capacity to accomplish Weigle reactivation and mutagenesis of bacteriophage lambda and recombination in Escherichia coli. Our data indicate that the 17 carboxyl terminal amino acids are not essential to RecA function. However in the presence of wild-type RecA protein, the truncated protein reduces the efficiency of recombination without affecting either mutagenesis or induction of an SOS gene or Weigle reactivation. The data presented here suggest that activation of RecA protein does not involve mixed multimers or is not affected by their presence.