Induction of a mutant phenotype in human repair proficient cells after overexpression of a mutated human DNA repair gene

Expression of DNA repair genes in lens cortex of age-related cortical cataract

The formation and development of age-related cataract (ARC) has been demonstrated to have the involvement of defective DNA repair in lens epithelial cells (LECs). This study aimed to investigate DNA repair genes expression in human lens cortex collected from age-related cortical cataract (ARCC) and controls during surgery. The expression levels of the genes were evaluated by xx genes microarray analysis. The results were further confirmed by Quantitative Real-Time PCR (qRT-PCR). The mRNA levels of 7 genes decreased and 4 genes out of 92 genes increased in lens cortex of ARCCs compared with controls with the fold change >1.5. Using Comet assay, we found the DNA breaks in the LECs of ARCCs were obviously severer than that of controls. The present data provide a global perspective on expression of DNA repair genes that may contribute to cataract pathogenesis. The DNA damage and repair pathway might be an effective target to delay the onset of ARC.

Mutational analysis of the human nucleotide excision repair gene ERCC1

The human DNA repair protein ERCC1 resides in a complex together with the ERCC4, ERCC11 and XP-F correcting activities, thought to perform the 5' strand incision during nucleotide excision repair (NER). Its yeast counterpart, RAD1-RAD10, has an additional engagement in a mitotic recombination pathway, probably required for repair of DNA cross-links. Mutational analysis revealed that the poorly conserved N-terminal 91 amino acids of ERCC1 are dispensable for both repair functions, in contrast to a deletion of only four residues from the C-terminus. A database search revealed a strongly conserved motif in this C-terminus sharing sequence homology with many DNA break processing proteins, indicating that this part is primarily required for the presumed structure-specific endonuclease activity of ERCC1. Most missense mutations in the central region give rise to an unstable protein (complex). Accordingly, we found that free ERCC1 is very rapidly degraded, suggesting that protein-protein interactions provide stability. Survival experiments show that the removal of cross-links requires less ERCC1 than UV repair. This suggests that the ERCC1-dependent step in cross-link repair occurs outside the context of NER and provides an explanation for the phenotype of the human repair syndrome xeroderma pigmentosum group F.

Induction of Fanconi anemia cellular phenotype in human 293 cells by overexpression of a mutant FAC allele

The polypeptide encoded by the Fanconi anemia (FA) complementation group C gene, FAC, binds to a group of cytoplasmic proteins in vitro and may form a multimeric complex. A known mutant allele of FAC resulting from the substitution of Pro for Leu at codon 554 fails to correct the sensitivity of FA group C cells to mitomycin C. We reasoned that overexpression of the mutant protein in a wild-type cellular background might induce the FA phenotype by competing with endogenous FAC for binding to the accessory proteins. After stable transfection of 293 cells with wild-type and a mutant FAC allele containing the L554P substitution, four independent clones that expressed four-to-fifteen fold higher levels of transcript from the mutant transgene relative to the endogenous FAC gene showed hypersensitivity to mitomycin C. By contrast, both parental and FAC-overexpressing cells maintained their relative resistance to mitomycin C. No differences in the biosynthesis, subcellular localization and protein interactions of the normal and mutant proteins were detected. The induction of the FA phenotype in this system is compatible with the competition hypothesis and provides support for a functional role of the FAC-binding proteins in vivo.

Enhanced host cell reactivation capacity and expression of DNA repair genes in human breast cancer cells resistant to bi-functional alkylating agents

Human breast carcinoma (MCF7-MLNr) cells resistant to the bifunctional drugs L-phenylalanine mustard (L-PAM, 5-fold resistance), mechlorethamine (9-fold), cisplatin (3-fold), and BCNU (3-fold) were used to investigate the role of DNA repair in the development of resistance to alkylating agents. We have previously shown that neither L-PAM transport and metabolism nor glutathione-associated enzymes were altered in MCF7-MLNr cells, compared to the sensitive cells MCF7-WT. This study shows that treatment of pRSV-CAT plasmid with L-PAM at concentrations up to 1 microM proportionally inhibit the expression of chloramphenicol acetyl transferase (CAT) activity, while higher concentrations abolished CAT activity. pRSV-CAT reactivation was significantly increased when plasmid was transfected into MCF7-MLNr cells, compared to MCF7-WT cells. This indicates that resistant cells have more efficient capacity to recognize and repair L-PAM induced DNA damage. The mRNA expression of DNA nucleotide excision repair genes ERCC1, XPD (ERCC2), XPB (ERCC3), and polymerase beta was found to be similar in both the MCF7-WT and MCF7-MLNr cells. Western blot analysis also reveals no difference in the expression of ERCC1, AP endonuclease, poly (ADP-ribose) polymerase, and alkyl-N-purine-DNA glycosylase proteins. The lack of correlation between enhanced host cell reactivation capacity in resistant cells, and the expression of these specific DNA repair genes suggests that proteins encoded by these genes are not rate limiting steps for resistance to bi-functional alkylating drugs in human breast cancer cells.

Specific inhibition of DNA polymerase beta by its 14 kDa domain: role of single- and double-stranded DNA binding and 5'-phosphate recognition

DNA polymerase beta (beta-polymerase) has been implicated in short-patch DNA synthesis in the DNA repair pathway known as base excision repair. The native 39 kDa enzyme is organized into four structurally and functionally distinct domains. In an effort to examine this enzyme as a potential therapeutic target, we analyzed the effect of various beta-polymerase domains on the activity of the enzyme in vitro. We show that the 14 kDa N-terminal segment of beta-polymerase, which binds to both single- and double-stranded DNA, but lacks DNA polymerase activity, inhibits beta-polymerase activity in vitro. Most importantly, the 8, 27 and 31 kDa domains of beta-polymerase do not inhibit beta-polymerase activity, demonstrating that the inhibition by the 14 kDa domain is specific. The inhibition of beta-polymerase activity in vitro is abolished by increasing the concentrations of both of the substrates (template-primer and deoxynucleoside triphosphate). In contrast, an in vitro base excision repair assay is inhibited in a domain specific manner by the 14 kDa domain even in the presence of saturating substrates. The inhibition of beta-polymerase activity by the 14 kDa domain appears specific to beta-polymerase as this domain does not inhibit either mammalian DNA polymerase alpha or Escherichia coli polymerase I (Klenow fragment). These data suggest that the 14 kDa domain could be used as a potential inhibitor of intracellular beta-polymerase and that it may provide a means for sensitizing cells to therapeutically relevant DNA damaging agents.

Studies on phenotypic complementation of ataxia-telangiectasia cells by chromosome transfer

Cells derived from patients with the cancer-prone inherited disorder ataxia-telangiectasia (A-T) show an abnormal response to ionizing radiation-induced DNA damage, such as an increased cell killing and a diminished inhibition of DNA synthesis. The enhanced killing of A-T (group D) cells by X-rays can be corrected by multiple cDNAs, mapping to different chromosomes (6, 11, 17, and 18). In order to examine whether genes located on these chromosomes complement AT-D cells, normal neo-tagged chromosomes 6, 11, 17, and 18 were introduced into AT-D cells by microcell-mediated chromosome transfer. However, correction of the enhanced killing of AT-D cells by X-rays could only be achieved by chromosome 11 and by none of the other chromosomes tested. The enhanced killing of A-T (complementation group C) cells was also corrected by chromosome 11. Usually, but not in all microcell hybrid clones, chromosome 11 also corrected the radioresistant DNA synthesis (RDS) phenotype of AT-D and AT-C cells. These results (i) confirm findings by others suggesting assignment of the ATD and ATC genes to chromosome 11, (ii) demonstrate that several genes can modify the cellular radiation response when they are taken out of their normal genomic context and/or control, and (iii) indicate that the RDS phenotype and the enhanced cell killing in A-T are independent pleiotropic features resulting from the primary mutations in A-T. Also, our findings underscore that, in establishing cDNAs as candidate genes for A-T, microcell-mediated chromosome transfer studies are needed to exclude nonspecific correcting effects of these candidate cDNA genes.

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.

Messenger RNA levels of XPAC and ERCC1 in ovarian cancer tissue correlate with response to platinum-based chemotherapy

Nucleotide excision repair is a DNA repair pathway that is highly conserved in nature, with analogous repair systems described in Escherichia coli, yeast, and mammalian cells. The rate-limiting step, DNA damage recognition and excision, is effected by the protein products of the genes ERCC1 and XPAC. We therefore assessed mRNA levels of ERCC1 and XPAC in malignant ovarian cancer tissues from 28 patients that were harvested before the administration of platinum-based chemotherapy. Cancer tissues from patients whose tumors were clinically resistant to therapy (n = 13) showed greater levels of total ERCC1 mRNA (P = 0.059), full length transcript of ERCC1 mRNA (P = 0.026), and XPAC mRNA (P = 0.011), as compared with tumor tissues from those individuals clinically sensitive to therapy (n = 15). In 19 of these tissues, the percentage of alternative splicing of ERCC1 mRNA was assessed. ERCC1 splicing was highly variable, with no difference observed between responders and nonresponders. The alternatively spliced species constituted 2-58% of the total ERCC1 mRNA in responders (median = 18%) and 4-71% in nonresponders (median = 13%). These data suggest greater activity of the DNA excision repair genes ERCC1 and XPAC in ovarian cancer tissues of patients clinically resistant to platinum compounds. These data also indicate highly variable splicing of ERCC1 mRNA in ovarian cancer tissues in vivo, whether or not such tissues are sensitive to platinum-based therapy.

Mice with DNA repair gene (ERCC-1) deficiency have elevated levels of p53, liver nuclear abnormalities and die before weaning

Defects in nucleotide excision repair are associated with the human condition xeroderma pigmentosum which predisposes to skin cancer. Mice with defective DNA repair were generated by targeting the excision repair cross complementing gene (ERCC-1) in the embryonic stem cell line, HM-1. Homozygous ERCC-1 mutants were runted at birth and died before weaning with liver failure. Examination of organs revealed polyploidy in perinatal liver, progressing to severe aneuploidy by 3 weeks of age. Elevated p53 levels were detected in liver, brain and kidney, supporting the hypothesised role for p53 as a monitor of DNA damage.

Nucleotide excision repair

Nucleotide excision repair is the major DNA repair mechanism in all species tested. This repair system is the sole mechanism for removing bulky adducts from DNA, but it repairs essentially all DNA lesions, and thus, in addition to its main function, it plays a back-up role for other repair systems. In both pro- and eukaryotes nucleotide excision is accomplished by a multisubunit ATP-dependent nuclease. The excision nuclease of prokaryotes incises the eighth phosphodiester bond 5' and the fourth or fifth phosphodiester bond 3' to the modified nucleotide and thus excises a 12-13-mer. The excision nuclease of eukaryotes incises the 22nd, 23rd, or 24th phosphodiester bond 5' and the fifth phosphodiester bond 3' to the lesion and thus removes the adduct in a 27-29-mer. A transcription repair coupling factor encoded by the mfd gene in Escherichia coli and the ERCC6 gene in humans directs the excision nuclease to RNA polymerase stalled at a lesion in the transcribed strand and thus ensures preferential repair of this strand compared to the nontranscribed strand.

Increased gene-specific repair of cisplatin interstrand cross-links in cisplatin-resistant human ovarian cancer cell lines

We have studied several aspects of DNA damage formation and repair in human ovarian cancer cell lines which have become resistant to cisplatin through continued exposure to the anticancer drug. The resistant cell lines A2780/cp70 and 2008/c13*5.25 were compared with their respective parental cell lines, A2780 and 2008. Cells in culture were treated with cisplatin, and the two main DNA lesions formed, intrastrand adducts and interstrand cross-links, were quantitated before and after repair incubation. This quantitation was done for total genomic lesions and at the level of individual genes. In the overall genome, the initial frequency of both cisplatin lesions assayed was higher in the parental than in the derivative resistant cell lines. Nonetheless, the total genomic repair of each of these lesions was not increased in the resistant cells. These differences in initial lesion frequency between parental and resistant cell lines were not observed at the gene level. Resistant and parental cells had similar initial frequencies of intrastrand adducts and interstrand cross-links in the dihydrofolate reductase (DHFR) gene and in several other genes after cisplatin treatment of the cells. There was no increase in the repair efficiency of intrastrand adducts in the DHFR gene in resistant cell lines compared with the parental partners. However, a marked and consistent repair difference between parental and resistant cells was observed for the gene-specific repair of cisplatin interstrand cross-links. DNA interstrand cross-links were removed from three genes, the DHFR, multidrug resistance (MDR1), and delta-globin genes, much more efficiently in the resistant cell lines than in the parental cell lines. Our findings suggest that acquired cellular resistance to cisplatin may be associated with increased gene-specific DNA repair efficiency of a specific lesion, the interstrand cross-link.

Increased gene-specific repair of cisplatin interstrand cross-links in cisplatin-resistant human ovarian cancer cell lines

We have studied several aspects of DNA damage formation and repair in human ovarian cancer cell lines which have become resistant to cisplatin through continued exposure to the anticancer drug. The resistant cell lines A2780/cp70 and 2008/c13*5.25 were compared with their respective parental cell lines, A2780 and 2008. Cells in culture were treated with cisplatin, and the two main DNA lesions formed, intrastrand adducts and interstrand cross-links, were quantitated before and after repair incubation. This quantitation was done for total genomic lesions and at the level of individual genes. In the overall genome, the initial frequency of both cisplatin lesions assayed was higher in the parental than in the derivative resistant cell lines. Nonetheless, the total genomic repair of each of these lesions was not increased in the resistant cells. These differences in initial lesion frequency between parental and resistant cell lines were not observed at the gene level. Resistant and parental cells had similar initial frequencies of intrastrand adducts and interstrand cross-links in the dihydrofolate reductase (DHFR) gene and in several other genes after cisplatin treatment of the cells. There was no increase in the repair efficiency of intrastrand adducts in the DHFR gene in resistant cell lines compared with the parental partners. However, a marked and consistent repair difference between parental and resistant cells was observed for the gene-specific repair of cisplatin interstrand cross-links. DNA interstrand cross-links were removed from three genes, the DHFR, multidrug resistance (MDR1), and delta-globin genes, much more efficiently in the resistant cell lines than in the parental cell lines. Our findings suggest that acquired cellular resistance to cisplatin may be associated with increased gene-specific DNA repair efficiency of a specific lesion, the interstrand cross-link.