Identification of uracil as a major lesion in E. coli DNA following the incorporation of 5-bromouracil, and some of the accompanying effects

A mammalian-like DNA damage response of fission yeast to nucleoside analogs

Nucleoside analogs are frequently used to label newly synthesized DNA. These analogs are toxic in many cells, with the exception of the budding yeast. We show that Schizosaccharomyces pombe behaves similarly to metazoans in response to analogs 5-bromo-2'-deoxyuridine (BrdU) and 5-ethynyl-2'-deoxyuridine (EdU). Incorporation causes DNA damage that activates the damage checkpoint kinase Chk1 and sensitizes cells to UV light and other DNA-damaging drugs. Replication checkpoint mutant cds1Δ shows increased DNA damage response after exposure. Finally, we demonstrate that the response to BrdU is influenced by the ribonucleotide reductase inhibitor, Spd1, suggesting that BrdU causes dNTP pool imbalance in fission yeast, as in metazoans. Consistent with this, we show that excess thymidine induces G1 arrest in wild-type fission yeast expressing thymidine kinase. Thus, fission yeast responds to nucleoside analogs similarly to mammalian cells, which has implications for their use in replication and damage research, as well as for dNTP metabolism.

5-Bromodeoxyuridine suppresses position effect variegation of transgenes in HeLa cells

An ectopic gene integrated in the host genome is occasionally silenced due to a position effect of its adjacent chromatin structure. We found that 5-bromodeoxyuridine clearly activated such a transgene in HeLa cells. The transgene was also activated to various degrees by inhibitors of histone deacetylase, DNA topoisomerases, or DNA methyltransferase. The peptide antibiotic distamycin A potentiated markedly the effect of 5-bromodeoxyuridine. Transient expression of an artificial AT-hook protein termed MATH20 also potentiated its effect although significantly activated the transgene alone. Since distamycin A and MATH20 are able to displace histone H1 and other DNA-binding proteins bound to specific AT-rich sequences by a dominant, mutually exclusive fashion, these results suggest that 5-bromodeoxyuridine targets such an AT-rich sequence located adjacent to the silenced transgene, resulting in chromatin accessibility.

Roles of uracil-DNA glycosylase and apyrimidinic endonucleases in the molecular 5-bromo-2'-deoxyuridine photosensitization in Escherichia coli K-12

The molecular mechanism for 5-bromo-2'-deoxyuridine (BrdU) photosensitization was studied in thymine-requiring wild-type and uracil-DNA glycosylase (UDG)-deficient ung mutant cells of Escherichia coli K-12. Wild-type cells were more sensitive to BrdU photosensitization than ung mutant cells. UV induced the identical numbers of alkaline sucrose single-strand breaks (SSB) in 5-bromouracil-DNA (BrU-DNA) of both the wild type and ung mutant. The ung mutant cells repaired SSB almost completely, whereas the wild-type cells with UDG produced more adverse SSB by 90 min after UV. Neutral agarose gel electrophoresis of minipreps indicated that UV induced (1) more smears of host BrU-DNA possibly by more double-strand breaks (DSB) and (2) a greater decline of pBR322 Form I BrU-DNA in the wild-type cells than the ung cells. These results indicated a greater induction of SSB by apyrimidinic (AP) endonucleases in wild-type cells. The ung/wild ratios (= 1.7-1.9) for cellular and plasmid BrdU sensitizations after growth in 50% BrdU were similar. The extents of UDG-dependent and UDG-independent sensitizations in wild-type cells were approximately 40 and approximately 60%, respectively. The xth nfo double mutant defective in both exonuclease III and endonuclease IV was more sensitive to BrdU photosensitization than the wild type, indicating that an excess of AP sites remaining after uracil excision in the xth nfo mutant causes a greater BrdU photosensitization than SSB by AP endonucleases in wild-type cells.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetic toxicology of 5-bromodeoxyuridine in mammalian cells

The thymidine analog, BrdUrd, induces many biological responses which are of importance to the field of genetic toxicology and related disciplines. These include the induction of SCE, specific-locus mutations, and toxicity, inhibition of cell proliferation, and the expression of fragile sites in the human genome. In early models which addressed the mechanisms of the biological effects of BrdUrd exposure, two pathways were proposed to account for the induction of the biological responses. Incorporation of the enol form of BrdUrd into the nascent DNA strand after pairing with deoxyguanosine was proposed as one pathway, whereas the incorporation of BrdUrd opposite adenosine in place of thymidine was proposed as the second pathway. Many novel and sophisticated techniques have been applied to the study of the mechanism of the induction of biological effects by BrdUrd leading to a substantial increase in our understanding of these mechanisms. However, the experimental evidence clearly supports the contention that BrdUrd exerts its effects on eukaryotic cells through mechanisms similar to those originally proposed to explain the genotoxicity of BrdUrd.

Bromodeoxyuridine: a comparison of its photosensitizing and radiosensitizing properties

The photo- and radiosensitizing properties of bromodeoxyuridine (BUdR) were assessed in vitro using the 9L rat brain tumor cell line. Pretreatment of 9L cells with 10 microM BUdR for 24 hours followed by irradiation with ultraviolet (UV) light resulted in a dose-enhancement ratio of 3.8:1 compared with UV radiation alone. X-radiation of BUdR-pretreated cells produced a dose-enhancement ratio of 1.7:1. Alkaline elution analysis of deoxyribonucleic acid (DNA) from cells treated with BUdR and UV irradiation showed the presence of DNA single-strand breaks and DNA-protein cross-links. Analysis of DNA from cells treated with BUdR and then x-irradiated showed no increase in DNA single-strand breaks compared with cells treated with x-radiation alone; no DNA-protein cross-links could be detected. The possible clinical relevance of these findings is discussed.

The DNA-repair enzyme uracil-DNA glycosylase in the human hematopoietic system

The expression of the DNA base-excision-repair enzyme uracil-DNA glycosylase in the human hematopoietic system followed a tightly regulated pattern: high enzyme activities were recorded in proliferating bone marrow progenitor cells and in peripheral blood T- and B-cells, both groups of cells requiring the integrity of their genetic information for their proper function. The blood quiescent immunocompetent cells retained their DNA-uracil exclusion capacity, even in the oldest age groups. Peripheral blood mature end cells, granulocytes, platelets and red cells had little activity, consistent with the fact that these cells are anuclear or short-lived, so that no template-primer functions of their DNA are required. Uracil-DNA glycosylase expression is high in all types of human leukemia, providing a selective advantage for survival of leukemic cells. Overall results show that a deficiency of this DNA base-excision-repair pathway is not likely to be an etiopathogenetic factor in the formation of non-random or other chromosomal abnormalities or in the leukemogenesis itself.

Deoxyuridine misincorporation causes site-specific mutational lesions in the lacI gene of Escherichia coli

Spontaneous forward mutation in lacI was analyzed by DNA sequencing in a Dut- strain of E. coli. Hyperuracil incorporation into DNA due to the defect in deoxyuridinetriphosphatase caused a 5-fold increase in mutation frequency. Deletion, duplication and base-substitution frequencies were all enhanced in the Dut- strain. However, the analysis of the specificity of mutation revealed a remarkable site- and class-specificity. For example, base substitutions at a single site, a G:C = greater than A:T transition (Ochre 34) accounted for 55% of the base substitutions recovered. The spontaneous A:T = greater than G:C hotspot at position +6 at the lac operator was also recovered at an enhanced frequency in the Dut- strain where it accounted for 25% of the base substitutions. Many of the deletion and duplication events were recovered more than once; most had endpoints in A/T rich regions. The spontaneous frameshift hotspot involving the gain or loss of 5'-CTGG-3' in a region where this tetramer is tandemly repeated 3 times, was also greatly enhanced. No frameshifts involving a single base pair nor IS1 insertions were identified among the 86 lacI mutants sequenced. The analysis of these events reveals them to be generally consistent with a mechanism involving AP sites generated by the removal of misincorporated uracil by uracil-N-glycosylase. Considering the number of potential AP sites (approximately 1 per 170 base pairs) E. coli is remarkably refractory to mutational consequences of deoxyuridine misincorporation in place of thymidine.

Involvement of DNA lesions and SOS functions in 5-bromouracil-induced mutagenesis

Mutagenesis resulting from incorporation of 5-bromouracil (BU) in the DNA of E. coli K12 proceeds largely (approximately 80%) via misrepair of the lesions resulting from incorporation of the analogue. The premutational lesions are due principally to dehalogenation of incorporated BU residues, leading to formation of uracil residues, and removal of these by uracil-DNA glycosylase with formation of apyrimidinic sites. In the xthA mutant, defective in AP endonuclease, there is a several-fold increase in the frequency of BU-induced mutations, underlining the importance of AP sites in BU-induced mutagenesis. Premutational lesions undergo mutation frequency decline (MFD), which is subject to delay in the xthA mutant, pointing to some role of AP endonuclease in MFD, and further supporting involvement of AP sites in BU-induced mutagenesis. Efficient BU mutagenesis is dependent on the functions of the genes recA and umuC and non-mutated lexA protein.

Induction of SOS functions in Escherichia coli by lesions resulting from incorporation of 5-bromouracil into DNA

Lesions induced by 5-bromouracil (BU), after its incorporation into DNA, led to effective induction of prophage lambda and W reactivation (or BU reactivation). Prophage induction due to incorporated BU occurred only with the wild-type prophage, and not for the lambda c1857 mutant with a thermosensitive repressor. Antipain, a protease inhibitor, inhibited wild-type prophage induction 70-90%. This indicates that BU-induced lesions may induce the SOS repair system. The finding that such lesions provoke BU reactivation permits the inference that BU-induced mutagenesis also proceeds via involvement of the error-prone repair system, and not directly as a result of base-pairing errors. Genetic evidence suggests that induction of the SOS repair system as a result of incorporation of BU into DNA is linked to the subsequent appearance of uracil residues and apyrimidinic sites, resulting from dehalogenation of incorporated BU. Apyrimidinic sites appear to be more effective than uracil residues in induction of the SOS system.

Involvement of the mismatch repair system in base analogue-induced mutagenesis

The influence of the mismatch repair system, and the role of mutS, mutR, and mutL genes, in mutagenesis induced by n2ome6Ade, n2oh6Ade and n2Pur have been investigated. From the frequency of reversion of Arg-Thr- and His- markers in AB2497, and its mut- derivatives, it was concluded that mismatches introduced by n2-ome6Ade and n2oh6Ade are better substrates for mismatch repair enzymes than that introduced by n2Pur. All these mut-gene products are more active in removing spontaneous or base analogue-induced mismatches which, when unexcised, lead to transversion of base repairs, than those which lead to transitions. Active engagements of mutL, mutR, or mutS gene products depend on the kind of mutation, the site of mutagenesis, and the inducing agent. Dam- cells are over-mutated by both n2ome6Ade and n2oh6Ade, but are hyper-sensitive to n2oh6Ade only. It is proposed that hyper-sensitivity of dam- cells is due not only to an increase in overlaping gap formation on both strands of DNA, but to a greater lability of the impaired cells. Results are presented which strongly suggest that n2ome6Ade in mut+ cells and n2oh6Ade in mut- only, can induce GC leads to TA transversions.