Analysis of baseline and BrdU-dependent SCEs at different BrdU concentrations

Evaluating the Genotoxic and Cytotoxic Effects of Thymidine Analogs, 5-Ethynyl-2'-Deoxyuridine and 5-Bromo-2'-Deoxyurdine to Mammalian Cells

BrdU (bromodeoxyuridine) and EdU (ethynyldeoxyuridine) have been largely utilized as the means of monitoring DNA replication and cellular division. Although BrdU induces gene and chromosomal mutations and induces sensitization to photons, EdU's effects have not been extensively studied yet. Therefore, we investigated EdU's potential cytotoxic and mutagenic effects and its related underlying mechanisms when administered to Chinese hamster ovary (CHO) wild type and DNA repair-deficient cells. EdU treatment displayed a higher cytotoxicity and genotoxicity than BrdU treatment. Cells with defective homologous recombination repair displayed a greater growth delay and severe inhibition of clonogenicity with EdU compared to wild type and other DNA repair-deficient cells. Inductions of sister chromatid exchange and hypoxanthine phosphorybosyl transferase (HPRT) mutation were observed in EdU-incorporated cells as well. Interestingly, on the other hand, EdU did not induce sensitization to photons to the same degree as BrdU. Our results demonstrate that elevated concentrations (similar to manufacturers suggested concentration; >5-10 μM) of EdU treatment were toxic to the cell cultures, particularly in cells with a defect in homologous recombination repair. Therefore, EdU should be administered with additional precautions.

Sister Chromatid Exchange as a Genotoxic Stress Marker

Sister chromatid exchange (SCE) is the phenomenon of partial DNA exchange during DNA replication. SCE detection has been developed through eliciting DNA's semiconservative replicative nature. Thymidine analogues such as 5'-bromodeoxyuridine (BrdU) and ethynyldeoxyuridine (EdU) are incorporated into the newly synthesized DNA for two cell cycles. The addition of Colcemid to the culture blocks and synchronizes cells at mitosis, and conventional cytogenetic preparations are made. Differential staining methods with Hoechst dye and Giemsa (Fluorescence Plus Giemsa staining), antibody detection against BrdU, or highly specific Click reaction to EdU, allow the newly synthesized DNA within a chromatid to be recognized. SCEs represent a point of DNA template exchange during DNA synthesis, visualized by differential chromatid staining or harlequin chromosomes. We will introduce three basic protocols in this chapter including non-fluorescence and fluorescence methods for SCE microscopic analysis. SCE is a very sensitive marker of genotoxic stress during replication.

Cell death and renewal during prey capture and digestion in the carnivorous sponge Asbestopluma hypogea (Porifera: Poecilosclerida)

The sponge Asbestopluma hypogea is unusual among sponges due to its peculiar carnivorous feeding habit. During various stages of its nutrition cycle, the sponge is subjected to spectacular morphological modifications. Starved animals are characterized by many elongated filaments which are crucial for the sponge to capture prey. After capture, and during the digestion process, these filaments actively regress before being regenerated during a subsequent period of starvation. Here, we demonstrate that these morphological events repose on a highly dynamic cellular turnover implying a coordinated sequence of programmed cell death (apoptosis and autophagy), cell proliferation and cell migration. A candidate niche for cell renewal by stem cell proliferation and differentiation was identified at the base of the sponge peduncle, characterized by surpassing levels of BrdU/EdU incorporation. Therefore, BrdU/EdU positive-cells of the peduncle base are candidate motile cells responsible for the regeneration of the prey-capturing main sponge body, i.e. the dynamic filaments. Altogether, our results demonstrate that dynamic of cell renewal in sponge appears to be regulated by cellular mechanisms as multiple and complex as those already identified in bilaterian metazoans.

Comparative analysis of sister-chromatid exchanges in plant and human chromosomes

Species specificity concerning the two main SCE characteristics, namely frequency and localization, of "spontaneous" SCEs in plant cells (root tips of Crepis capillaris) and in human lymphocytes was investigated under comparable conditions. The FPG technique was used for detection of SCEs after bifilar incorporation of BrdU into DNA (TB-BB). Data of parallel experiments showed that the frequency of SCEs in plant cells was considerably higher than that observed in human lymphocytes--13.2 and 7.3 SCEs per cell respectively. The difference was even more pronounced when the SCE frequency was estimated on the basis of DNA content/cell (pg). Analysis of SCE distribution was limited to SCEs localized in the centromere, since contradictory results most frequently concerned this chromosome region. The data of the present experiments showed that the frequency of SCEs localized in the centromere regions of plant chromosomes was considerably lower than that observed in human chromosomes. Compared with the relative sizes of the centromere regions in the two genomes, however, these frequencies proved to be quite similar. In both systems the centromeres were involved in SCE, as could be expected for random distribution, along the length of the chromosomes.

The involvement of discrete genome regions in post-mitotic chromosome decondensation and in G1 timing in Allium cepa L. meristematic cells

The role of DNA regions replicated at different times during the S period in the process of chromatin decondensation that takes place in the next G1 period, as well as in the timing of the G1 to S transition, was analyzed in synchronous populations of cells in Allium cepa L. root meristems. For this analysis, DNA bromosubstitution (10-7 M 5-bromo-2′-deoxyuridine feeding) was carried out at similar times corresponding to the first, middle and last thirds of the S period prior to telophase when anoxic 313 nm irradiation was carried out. Evaluation, after Feulgen staining, of the chromatin pattern corresponding to G0 and G1 phenotypes in these synchronous cells showed that chromatin decondensation after mitosis appeared to be under the control of a double negative/positive regulatory mechanism related, respectively, to early and late replicating sequences. Thus, such decondensation was favoured when bromosubstitution took place during the first part of the S period, but delayed when bromosubstitution took place during the third part or the whole of the S period. Nevertheless, bromosubstitution at both the early and late stages of the S period delayed the timing of the G1 to S transition. In this way, the initiation of replication appears to be positively controlled by the function of early replicating DNA. Finally, the DNA sequences replicated in the middle third of the S period did not modify the decondensing capacity of the chromatin in G1 or the timing of the G1 to S transition.

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.

Analysis of spontaneous sister-chromatid exchanges in vivo by three-way differentiation

By applying an adaptation of the method of three-way differentiation to murine bone marrow cells in vivo, the basal frequency of sister-chromatid exchange (SCE) per cell was evaluated. An SCE frequency directly proportional to the estimated relative incorporation of 5-bromodeoxyuridine (BrdU) to the chromosomes was observed for the 3 consecutive cell cycles, implying that the majority, if not all, of the SCEs in vivo were produced by the incorporated BrdU. This conclusion was supported by the finding that in the first cycle of division, a very high frequency of cells without SCE was observed. From these data, a spontaneous frequency of SCE as low as 0.15 SCE/cell/cell cycle was inferred.

Caffeine induces sister-chromatid exchanges during the whole S-phase of the cell cycle

The capacity of caffeine to induce sister chromatid exchanges (SCEs) in different cell cycle stages and the proliferation kinetics were studied. Continuous treatment with this xanthine during the whole second cycle significantly increased the baseline SCE frequency. Pulse-treatment experiments showed that the induction of SCEs by caffeine, which was dose-dependent, was restricted to the S-phase of the cell cycle without effect on G1 or G2 cells. Moreover, unlike other SCE-inducing agents, such as DNA-synthesis inhibitors and DNA-damaging agents, caffeine produced similar SCE increases in cells treated at different times throughout the S-phase. In the light of Painter's model for SCE formation and the known effects of caffeine on the DNA replication pattern, the most likely mechanism of SCE induction by caffeine is an increase in the number of DNA-replication sites.