Cell-cycle-dependent repair of damage in alpha and bulk DNA of monkey cells

Multiple subphases of DNA repair and poly (ADP-ribose) synthesis in Chinese hamster ovary (CHO-K1) cells

The two types of DNA synthesis as well as poly(ADP-ribose) biosynthesis were measured simultaneously in synchronized intact populations of CHO cells throughout the duration of S phase. Naturally occurring DNA fragmentation was detected by random primed oligonucleotide synthesis (ROPS assay). Fractions of synchronous cell populations were obtained by counterflow centrifugal elutriation. By gradually increasing the resolution of centrifugal elutriation multiple non-overlapping repair and replication peaks were obtained. The elutriation profile of DNA repair peaks corresponded to the DNA fragmentation pattern measured by ROPS assay. The number and position of poly(ADP-ribose) peaks during S phase resembled those seen in the DNA replication profile. Our results indicate that PAR synthesis is coupled to DNA replication serving the purpose of genomic stability.

The in vitro effect of Viscum album (VA) extract on DNA repair of peripheral blood mononuclear cells (PBMC) in cancer patients

Viscum album (VA) extract as an immunomodulator was tested in an in vitro model to investigate DNA repair in damaged peripheral blood mononuclear cells (PBMC) of ten breast cancer patients. The cells were exposed by gamma rays or 4-hydroxycyclophosphamide (4-HCy). Two hours after exposure the following were measured, without or with VA extract (1) DNA repair using the alkaline sucrose gradient for the sedimentation of DNA strand breaks, (2) DNA-gamma-production in the supernatant of the cultured cells. The VA extract led to an improvement of DNA repair in gamma-ray or 4-HCy damaged PBMC and to a significant increase of the IFN-gamma-production both in undamaged and in damaged cells. The results indicate that the VA extract affects positively DNA repair in PBMC damaged by two different agents and suggest that an increased IFN-gamma-production may play an important role in the DNA repair process.

Cell cycle-independent removal of UV-induced pyrimidine dimers from the promoter and the transcription initiation domain of the human CDC2 gene

To assess whether removal of UV-induced cyclobutane pyrimidine dimers (CPDs) occurs with equal efficiency at different stages of the cell cycle in a cell cycle-regulated gene, we have analyzed repair of CPDs, following a single dose of UV, in normal human fibroblasts that were synchronized in either G(0) or S phase. Based on a single nucleotide resolution analysis, we established a detailed map of DNA repair rates along the promoter region and the transcription initiation area of the human CDC2 gene. The promoter of this gene is covered by an array of sequence-specific transcription factors located between nt -280 and -9 relative to the major transcription start site. In both quiescent and S phase-synchronized fibroblasts the majority of these sequences were poorly repaired even after 24 h, probably as a result of the constitutive binding of transcription factors throughout the cell cycle. A domain of fast repair was found at sequences surrounding the transcription initiation site and continuing downstream for approximately 80 nt. CPD removal from this domain was preferential in both quiescent and proliferating fibroblasts, despite lower levels of global genome repair and a lack of CDC2 transcription in quiescent cells. We suggest that sequences involved in transcription initiation may be book-marked for efficient repair throughout the cell cycle, even when the gene is temporarily not expressed.

Catalytic activity of poly(ADP-ribose) polymerase is necessary for repair of N-methylpurines in nontranscribed, but not in transcribed, nuclear DNA sequences

The role of poly(ADP-ribose) polymerase (PADPRP) in nuclear DNA repair and other nuclear processes has been intensely studied and debated for decades. Recent studies have begun to shed new light on these arguments with firm experimental data for its role, primarily, as a remodeler of chromatin structure. Those studies imply that PADPRP plays an indirect role in DNA repair, serving to expose DNA to repair enzymes through chromatin remodeling. Only DNA that is tightly packaged would require PADPRP activity for its repair; while DNA in an open conformation would be accessible to DNA repair enzymes and not require PADPRP activity. The purpose of the current studies was to address the above hypothesis directly. Using quantitative Southern blot analysis, we studied repair in transcribed and nontranscribed nuclear DNA sequences in ADPRT 351 cells 95% deficient in PADPRP activity. Cells were exposed to methylnitrosourea (MNU) for 1 h and allowed to repair for 8 or 24 h. Densitometric scans of autoradiographs revealed that, when compared to their parental V79 cell line, ADPRT 351 cells 95% deficient in PADPRP activity were equally as efficient in repair of N-methylpurines in the transcribed sequence containing the dihydrofolate reductase gene. However, the ADPRT 351 cells were deficient in the ability to repair these lesions in the nontranscribed sequence containing the IgE gene compared to repair of the same sequence in the parental V79 cells. Nucleoid sedimentation assays demonstrated that the ADPRT 351 cells are deficient in repair across the entire genome when compared to the parental V79 cells. These studies indicate that PADPRP activity is not required for repair of N-methylpurines in transcribed nuclear DNA sequences but is necessary for the repair of these lesions in nontranscribed nuclear DNA sequences as well as across the entire genome since the DNA in a given cell is predominantly nontranscribed.

Preferential repair of the transcribed DNA strand in the dihydrofolate reductase gene throughout the cell cycle in UV-irradiated human cells

We examined repair of UV-induced cyclobutane pyrimidine dimers (CPD) in each strand of the expressed dihydrofolate reductase gene in human cells in different phases of the cell cycle: G1, early S, middle S, late S, and G2/M. After 4 h of incubation, repair of the transcribed strand was substantially more efficient than repair of the non-transcribed strand in all phases. Furthermore, we observed no remarkable cell cycle-dependent differences in either the initial lesion frequency or the efficiency of repair of the transcribed strand. We conclude that transcription coupled repair operates generally and with high efficiency throughout the cell cycle.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Developmental regulation of the base excision repair enzyme uracil DNA glycosylase in the rat

The developmental regulation of the mammalian DNA-repair enzyme uracil DNA glycosylase was examined in the rat at specific intervals ranging from -4 days before to 106 days after birth. Enzyme activity was quantitated by in vitro biochemical assay. In the adult animal, as measured in crude cell extracts, three organs (liver, kidney and spleen) had significant levels of activity. In contrast, three organs (brain, heart and lung) had low activity. Partial purification of this enzyme identified one major species of molecular weight 32,700 Da, demonstrating the quantitation of the nuclear glycosylase. During development, with the exception of the liver, the specific activity of the glycosylase paralleled the regulation of DNA synthesis. In these organs the highest levels of the glycosylase and the rate of DNA replication were observed around the time of birth. In the liver, DNA replication was similarly regulated. However, glycosylase activity was minimal at early stages of life. Instead, maximal levels were observed at 14-21 days after birth. At that time DNA replication was severely reduced. These results demonstrate that individual organs express this DNA-repair enzyme in a distinct and specific pattern during development. Accordingly, the regulation of the uracil DNA glycosylase during development may provide a model system to examine the differential regulation of DNA-repair genes.

Proliferation-dependent regulation of DNA glycosylases in progeroid cells

The regulation of the base excision repair enzymes uracil DNA glycosylase and hypoxanthine DNA glycosylase was examined in 2 different progeroid cell strains. The immunoreactivity of the uracil DNA glycosylase in progeroid cells was examined by enzyme linked immunosorbent assay (ELISA) and by immunoblot analysis. The enzyme was recognized in a quantitative manner by 2 different anti-human uracil DNA glycosylase monoclonal antibodies in the ELISA. Western blot analysis identified a glycosylase protein of Mr = 37,000. In randomly proliferating progeroid cells, the uracil DNA glycosylase was enhanced 3-fold during cell growth. In synchronous cells, uracil DNA glycosylase and hypoxanthine DNA glycosylase were induced with an extent of induction (5-6-fold) comparable to that observed for normal human cells. Further, the activity of each base excision repair enzyme was enhanced with a comparable temporal sequence prior to the induction of DNA synthesis and DNA polymerase activity. These results indicate a normal cell cycle regulation of base excision repair in progeroid cells.

DNA repair endonuclease activity during synchronous growth of diploid human fibroblasts

DNA-repair endonuclease activity in response to UV-induced DNA damage was quantified in diploid human fibroblasts after synchronizing cell cultures to selected stages of the cell cycle. Incubation of irradiated cells with aphidicolin, an inhibitor of DNA polymerases alpha and delta, delayed the sealing of repair patches and allowed estimation of rates of strand incision by the repair endonuclease. The apparent Vmax for endonucleolytic incision and Km for substrate utilization were determined by Lineweaver-Burk and Eadie-Hofstee analyses. For cells passing through G1, S or G2, Vmax for reparative incision was, respectively, 7.6, 8.4 and 8.4 breaks/10(10) Da per min, suggesting that there was little variation in incision activity during these cell-cycle phases. The Km values of 2.4-3.1 J/m2 for these cells indicate that the nucleotidyl DNA excision-repair pathway operates with maximal effectiveness after low fluences of UV that are in the shoulder region of survival curves. Fibroblasts in mitosis demonstrated a severe attenuation of reparative incision. Rates of incision were 11% of those seen in G2 cells. Disruption of nuclear structure during mitosis may reduce the effective concentration of endonuclease in the vicinity of damaged chromatin. The extreme condensation of chromatin during mitosis also may restrict the accessibility of reparative endonuclease to sites of DNA damage. Confluence-arrested fibroblasts in G0 expressed endonuclease activity with Vmax of 5.5 breaks/10(10) Da per min and a Km of 5.5 J/m2. The greater condensation of chromatin in quiescent cells may restrict the accessibility of endonuclease to dimers and so explain the elevated Km. When fibroblasts were synchronized by serum-deprivation, little variation in reparative endonuclease activity was discerned as released cells transited from early G1 through late G1 and early S. Proliferating fibroblasts in G1 were shown to express comparatively high numbers of reparative incision events in the absence of aphidicolin which was normally used to inhibit DNA polymerases and hold repair patches open. It was calculated that in G0, S and G2 phase cells, single-strand breaks at sites of repair remained open for 30, 19 and 14 sec, respectively. In G1 phase cells, repair sites remained open for 126 sec. Addition of deoxyribonucleosides to G1 cells reduced this time to 42 sec suggesting that the slower rate of synthesis and ligation of repair patches in G1 was due to a relative deficiency of deoxyribonucleotidyl precursors for DNA polymerase.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA base and strand damage in X-irradiated monkey CV-1 cells: influence of pretreatment using small doses of radiation

Base damage in alpha DNA from irradiated monkey CV-1 cells was determined by measuring release of 5'-32P-end labelled DNA fragments after digestion with endonuclease III of E. coli. The frequency and base sequence locations of the enzyme-sensitive sites were determined. Fragments were released from irradiated DNA at sequence sites of pyrimidines and guanines. The time for repair of half the single strand breaks was approximately 1.5 h. Repair of base damage as judged from loss of enzyme-sensitive sites in DNA was slower, with more than half of the damaged bases still detectable after 4 h of repair. Two important changes in the pattern of fragment release from DNA were produced when small radiation doses preceded the large ones needed to produce measurable DNA strand breaks and base damage. 5 Gy to cells incubated several hours before 320 Gy increased by five-fold the abundance of small DNA fragments with 3'-phosphoryl termini detected in high-resolution denaturing gels. These increases were detectable with doses as small as 0.2 Gy and were accompanied by the appearance of new species of DNA fragments of intermediate mobility at specific locations in the base sequence. The patterns resemble those produced by digesting DNA from heavily irradiated cells with endonuclease III.

Repair of N-methylpurines in specific DNA sequences in Chinese hamster ovary cells: absence of strand specificity in the dihydrofolate reductase gene

We have developed a quantitative method for examining the removal of N-methylpurines from specific genes to investigate their possible differential repair throughout the genome. Chinese hamster ovary cells were exposed to dimethyl sulfate, and the isolated DNA was treated with an appropriate restriction endonuclease. The DNA was heated to convert remaining N-methylpurines to apurinic sites to render them alkaline-labile. Duplicate samples heated in the presence of methoxyamine to protect the apurinic sites from alkaline hydrolysis provided controls to assess total DNA. After alkaline hydrolysis, agarose gel electrophoresis, Southern transfer, and probing for the fragment of interest, the ratios of band intensities of the test DNA sample to its methoxyamine-treated control counterpart were calculated to yield the percentage of fragments containing no alkaline-labile sites. The frequency of N-methylpurines was measured at different times after dimethyl sulfate treatment to study repair. We found no differences between the rates of repair of N-methylpurines in the active dihydrofolate reductase gene and a nontranscribed region located downstream from it in treated cells. Also, similar rates of repair were observed in the transcribed and nontranscribed strands of the gene, in contrast to previous results for the removal of cyclobutane pyrimidine dimers. Thus, there does not appear to be a coupling of N-methylpurine repair to transcription in Chinese hamster ovary cells. However, the repair in the dihydrofolate reductase domain appears to be somewhat more efficient than that in the genome overall. Our method permits the quantifying at the defined gene level of abasic sites or of any DNA adduct that can be converted to them.

Processing of psoralen adducts in an active human gene: repair and replication of DNA containing monoadducts and interstrand cross-links

We have examined DNA repair in the dihydrofolate reductase (DHFR) gene in cultured human cells treated with 4'-hydroxymethyl-4,5',8-trimethylpsoralen (HMT) using a newly developed assay for interstrand DNA cross-linking in defined genomic sequences. Within 24 hr, 80% of the cross-links, but only 45% of the monoadducts, were removed from a 32 kb transcribed sequence, demonstrating that repair efficiency in an active human gene varies with the nature of the damage. HMT monoadducts were also detected in the replicated DHFR sequence at frequencies indicating little interference with replication. The existence of cross-linkable monoadduct sites in the replicated DNA implies strand continuity opposite those sites and a relatively error-free mechanism of bypass. Translesion replication could circumvent transcription blockage in a damaged gene. These findings have important implications for mechanisms of mutagenesis and DNA lesion tolerance in human cells.