Recombination-dependent deletion formation in mammalian cells deficient in the nucleotide excision repair gene ERCC1

UvrA and UvrB suppress illegitimate recombination: synergistic action with RecQ helicase

Illegitimate recombination is a major cause of genetic instability in prokaryotes as well as in eukaryotes. This recombination usually occurs at a low frequency, but it is greatly enhanced by UV irradiation or other environmental stresses. DNA damages produced by these environmental stresses are thought to induce DNA double-strand breaks, leading to illegitimate recombination. In this paper we show that UV-induced illegitimate recombination is enhanced by mutations of nucleotide excision repair genes, uvrA or uvrB, and partially by uvrC mutation, but not by uvrD mutation. Unexpectedly, the recombination was enhanced by the uvrA uvrB double mutation even without UV irradiation, but the uvrB uvrC double mutation has not shown this effect, suggesting that illegitimate recombination is mostly suppressed by UvrA and UvrB. Moreover, illegitimate recombination was synergistically enhanced by the recQ uvrA double mutation. In addition, overproduction of the UvrA protein suppressed the hyperrecombination phenotype of the recQ or uvrB mutant, but it did not affect the UV-sensitive phenotype of the uvrB mutant. We concluded that the UvrAB complex suppresses illegitimate recombination in a pathway shared with RecQ helicase. In addition, UvrA protein alone can suppress illegitimate recombination in the pathway, in which RecQ helicase and UvrAB complex work. Possible functions of the proteins involved in these pathways are also discussed.

Absence of evidence for allelic loss or allelic gain for ERCC1 or for XPD in human ovarian cancer cells and tissues

We have previously reported on mRNA expression of ERCC1, XPA and XPD in human ovarian cancer cells and tissues. Several factors can influence mRNA expression for any given gene. Alterations in gene copy number for ERCC1 and/or XPD have been reported to occur in malignant glioma specimens. Human ovarian cancer cell lines and tissues were therefore examined for evidence of altered gene copy number in selected genes within the nucleotide excision repair (NER) pathway. Six ovarian cancer cell lines were studied: A2780, A2780/CP70, SKOV3, MCAS, QvCar3 and Caov4. Cellular sensitivity to cisplatin varies by more than 1 log between some of these cells. In each of these cell lines, the genes examined included ERCC1, XPA, XPB, XPD, XPG, CSB and p53. Genomic DNA was also extracted from ovarian cancer specimens taken from 22 patients and assessed for evidence of allelic loss and/or allelic gain for ERCC1 and XPD. Twelve of the clinical specimens were from patients with platinum-sensitive tumors and ten were from patients with platinum-resistant tumors. In no case could we demonstrate a reproducible variation in gene copy number in any cell line. Among the human tissues studied, there was one case of allelic gain out of 22 specimens. We therefore conclude that alterations in gene copy number is not a common event in human ovarian cancer. Other mechanisms must be invoked to explain differences in mRNA expression for these genes.

Characterization of the mouse Xpf DNA repair gene and differential expression during spermatogenesis

The human XPF protein, an endonuclease subunit essential for DNA excision repair, may also function in homologous recombination. To investigate a possible link between mammalian XPF and recombination that occurs during meiosis, we isolated, characterized, and determined an expression profile for the mouse Xpf gene. The predicted mouse XPF protein, encoded by a 3.4-kb cDNA, contains 917 amino acids and is 86% identical to human XPF. Appreciable similarity also exists between mouse XPF and homologous proteins in budding yeast (Rad1), fission yeast (Rad16), and fruit fly (Mei-9), all of which have dual functions in excision repair and recombination. Sequence analysis of the 38.3-kb Xpf gene, localized to a region in proximal mouse chromosome 16, revealed greater than 72% identity to human XPF in 16 regions. Of these conserved elements, 11 were exons and 5 were noncoding sequence within introns. Xpf transcript and protein levels were specifically elevated in adult mouse testis. Moreover, increased levels of Xpf and Ercc1 mRNAs correlated with meiotic and early postmeiotic spermatogenic cells. These results support a distinct role for the XPF/ERCC1 junction-specific endonuclease during meiosis, most likely in the resolution of heteroduplex intermediates that arise during recombination.

Quality control by DNA repair

Faithful maintenance of the genome is crucial to the individual and to species. DNA damage arises from both endogenous sources such as water and oxygen and exogenous sources such as sunlight and tobacco smoke. In human cells, base alterations are generally removed by excision repair pathways that counteract the mutagenic effects of DNA lesions. This serves to maintain the integrity of the genetic information, although not all of the pathways are absolutely error-free. In some cases, DNA damage is not repaired but is instead bypassed by specialized DNA polymerases.

The role of nucleotide excision repair in protecting embryonic stem cells from genotoxic effects of UV-induced DNA damage

In this study the role of nucleotide excision repair (NER) in protecting mouse embryonic stem (ES) cells against the genotoxic effects of UV-photolesions was analysed. Repair of cyclobutane pyrimidine dimers (CPD) in transcribed genes could not be detected whereas the removal of (6-4) photoproducts (6-4PP) was incomplete, already reaching its maximum (30%) 4 h after irradiation. Measurements of repair replication revealed a saturation of NER activity at UV doses >5 J/m2 while at a lower dose (2.5 J/m2) the repair kinetics were similar to those in murine embryonic fibroblasts (MEFs). Cytotoxic and mutagenic effects of photolesions were determined in ES cells differing in NER activity. ERCC1-deficient ES cells were hypermutable (10-fold) compared to wild-type cells, indicating that at physiologically relevant doses ES cells efficiently remove photolesions. The effect of the NER deficiency on cytoxicity was only 2-fold. Exposure to high UV doses (10 J/m2) resulted in a rapid and massive induction of apoptosis. Possibly, to avoid the accumulation of mutated cells, ES cells rely on the induction of a strong apoptotic response with a simultaneous shutting down of NER activity.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Physical interaction between components of DNA mismatch repair and nucleotide excision repair

Nucleotide excision repair (NER) and DNA mismatch repair are required for some common processes although the biochemical basis for this requirement is unknown. Saccharomyces cerevisiae RAD14 was identified in a two-hybrid screen using MSH2 as "bait," and pairwise interactions between MSH2 and RAD1, RAD2, RAD3, RAD10, RAD14, and RAD25 subsequently were demonstrated by two-hybrid analysis. MSH2 coimmunoprecipitated specifically with epitope-tagged versions of RAD2, RAD10, RAD14, and RAD25. MSH2 and RAD10 were found to interact in msh3 msh6 and mlh1 pms1 double mutants, suggesting a direct interaction with MSH2. Mutations in MSH2 increased the UV sensitivity of NER-deficient yeast strains, and msh2 mutations were epistatic to the mutator phenotype observed in NER-deficient strains. These data suggest that MSH2 and possibly other components of DNA mismatch repair exist in a complex with NER proteins, providing a biochemical and genetical basis for these proteins to function in common processes.