Detection of incomplete chromosome elements and interstitial fragments induced by bleomycin in hamster cells using a telomeric PNA probe

Bleomycin-induced chromosomal aberrations in Epstein-Barr virus-transformed human lymphoblastoid cells

We have evaluated the induction of complete (i.e., without open ends) and incomplete (i.e., with non-rejoined or open ends) chromosomal aberrations by the radiomimetic antibiotic bleomycin (BLM) in human lymphoblastoid cells immortalized with the Epstein-Barr virus (EBV). An EBV-induced lymphoblastoid cell line (T-37) was exposed to BLM (10-200 µg/mL) for 2 h at 37ºC, and chromosomal aberrations were analyzed 24 h after treatment, using PNA-FISH with pan-telomeric and pan-centromeric probes. Both complete (multicentrics, rings, compound acentric fragments, and interstitial deletions) and incomplete (incomplete chromosomes or IC, and terminal acentric fragments or TAF) chromosomal aberrations increased significantly in BLM-exposed cells, although the concentration-response relationship was non-linear. Of the acentric fragments (ace) induced by BLM, 40 % were compound fragments (CF, ace +/+). TAF (ace, +/-) and interstitial fragments (IAF, ace -/-) were induced at similar frequencies (30 %). 230 ICE were induced by BLM, of which 52 % were IC and 48 % TAF. The average ratio between total incomplete chromosome elements (ICE) and multicentrics was 1.52. These findings suggest that human lymphoblastoid cells exhibit less repair capacity than human lymphocytes, with respect to BLM-induced ICE, and that chromosomal incompleteness is a common event following exposure of these cells to BLM.

DNA and chromosome damage induced by bleomycin in mammalian cells: An update

Bleomycin (BLM) is an antibiotic isolated from Streptomyces verticillus. It has radiomimetic actions on DNA thus it has been widely used in clinical chemotherapy for the treatment of different types of cancer, including head and neck tumors, lymphomas, squamous-cell carcinomas and germ-cell tumors. Because of this, the study of BLM genotoxicity is of practical interest. This antibiotic is an S-independent clastogen and an agent that generates free radicals and induces single- and double-strand breaks in DNA. In the present review, we will summarize our current knowledge concerning the DNA and chromosome damage induced by BLM in mammalian cells, with emphasis on new developments published since 1991.

Dual roles of TRF1 in tethering telomeres to the nuclear envelope and protecting them from fusion during meiosis

Telomeres integrity is indispensable for chromosomal stability by preventing chromosome erosion and end-to-end fusions. During meiosis, telomeres attach to the inner nuclear envelope and cluster into a highly crowded microenvironment at the bouquet stage, which requires specific mechanisms to protect the telomeres from fusion. Here, we demonstrate that germ cell-specific knockout of a shelterin complex subunit, Trf1, results in arrest of spermatocytes at two different stages. The obliterated telomere-nuclear envelope attachment in Trf1-deficient spermatocytes impairs homologue synapsis and recombination, resulting in a pachytene-like arrest, while the meiotic division arrest might stem from chromosome end-to-end fusion due to the failure of recruiting meiosis specific telomere associated proteins. Further investigations uncovered that TRF1 could directly interact with Speedy A, and Speedy A might work as a scaffold protein to further recruit Cdk2, thus protecting telomeres from fusion at this stage. Together, our results reveal a novel mechanism of TRF1, Speedy A, and Cdk2 in protecting telomere from fusion in a highly crowded microenvironment during meiosis.

The radiomimetic compound streptonigrin induces persistent telomere dysfunction in mammalian cells

We analyzed the chromosomal aberrations involving telomeres in the progeny of mammalian cells exposed to the radiomimetic compound streptonigrin (SN) in order to determine if this antineoplastic drug induces long-term telomere instability. To this end, rat cells (ADIPO-P2 cell line, derived from adipose cells from Sprague-Dawley rat) were treated with a single concentration of SN (100ng/ml), and chromosomal aberrations were analyzed 18h and 10 and 15 days after treatment by using PNA-FISH with a pan-telomeric probe [Cy3-(CCCTAA)3] to detect (TTAGGG)n repeats. Cytogenetic analysis revealed a higher frequency of telomere dysfunction-related aberrations (additional telomeric FISH signals, extra-chromosomal telomeric FISH signals, and telomere FISH signal loss and duplications) in SN-exposed cultures vs. untreated cultures at every time points analyzed. The yield of SN-induced aberrations remained very similar at 18h, 10 days as well as 15 days after treatment. Thus, our data demonstrate that SN induces persistent telomere dysfunction in mammalian cells. Moreover, we found that the level of telomerase activity in SN-treated cells was significantly lower (up to 77%) than that of untreated control cells at each time points analyzed. This fact suggests that telomerase could be involved in SN-induced telomere dysfunction.

Telomere instability is present in the progeny of mammalian cells exposed to bleomycin

We analyzed the chromosomal aberrations involving telomeres in the progeny of mammalian cells exposed to the radiomimetic compound bleomycin (BLM) in order to determine if this antineoplastic drug induces long-term telomere instability. To this end, rat cells (ADIPO-P2 cell line, derived from adipose cells from Sprague-Dawley rat) were treated with a single concentration of BLM (2.5 μg/ml), and chromosomal aberrations were analyzed 18 h and 10 days after treatment by using PNA-FISH with a pan-telomeric probe [(TTAGGG)n repeats]. Cytogenetic analysis revealed a higher frequency of aberrations at 18 h and 10 days after treatment in BLM-exposed cultures vs. untreated cultures, although the yield of BLM-induced aberrations 10 days after treatment decreased about 25% compared with the one at 18 h after treatment. Moreover, the level of telomerase activity in BLM-treated cells compared with that of untreated control cells was significantly higher at 10 days after treatment, but did not differ at 18 h after treatment. These data indicate that in terms of unstable aberrations, the in vitro clastogenic effect of BLM on ADIPO-P2 cells persists for at least 10 days after exposure. In addition, our data demonstrate, for the first time, that BLM-induced telomere instability in mammalian cells (cytogenetically detectable as incomplete chromosome elements and telomere FISH signal loss and duplication) persists for several generations after exposure. Moreover, the appearance of telomere fusions in BLM-exposed cells 10 days after treatment suggests that this compound can induce delayed telomere instability. The increase in telomerase activity in BLM-exposed cells 10 days after treatment is accompanied by the presence of aberrations directly related to telomere dysfunction. This fact suggests that telomerase is not directly involved in BLM-induced telomere instability.

Genotoxic and cytotoxic evaluation of the herbicide flurochloridone on Chinese hamster ovary (CHO-K1) cells

The in vitro effects of flurochloridone (FLC) and its formulations Twin Gold Pack® (25% a.i.) and Rainbow® (25% a.i.) were evaluated on Chinese hamster ovary (CHO-K1) cells by genotoxicity [sister chromatid exchange (SCE)] and cytotoxicity [cell-cycle progression, proliferative rate index (PRI), mitotic index (MI), MTT, and neutral red] end points. Cells were treated for 24h within the 0.25-15μg/ml concentration range. FLC and Twin Pack Gold® induced a significant and equivalent increase in SCEs regardless of the concentration. Rainbow®-induced SCEs at concentrations higher than 2.5μg/ml; however, the increases were always lower than those induced by FLC and Twin Pack Gold®. For all compounds, the PRI decreased as a function of the concentration titrated into cultures. Whereas only the highest FLC and Twin Pack Gold® concentrations induced a significant reduction of the MI, all tested Rainbow® concentrations induced MI inhibition. Overall, the results demonstrated that although all compounds were not able to reduce the lysosomal activity, the mitochondrial activity was diminished when the highest concentrations were employed. These observations represent the first study analyzing the genotoxic and cytotoxic effects exerted by FLC and two formulated products on mammalian cells in vitro, at least on CHO-K1 cells.

Chromosomal aberrations involving telomeres and interstitial telomeric sequences

Telomeres are specialised nucleoproteic complexes localised at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. In vertebrate chromosomes, the DNA component of telomeres is constituted by (TTAGGG)n repeats, which can be localised at the terminal regions of chromosomes (true telomeres) or at intrachromosomal sites (interstitial telomeric sequences or ITSs, located at the centromeric region or between the centromere and the telomere). In the past two decades, the use of molecular cytogenetic techniques has led to a new spectrum of spontaneous and clastogen-induced chromosomal aberrations being identified, involving telomeres and ITSs. Some aberrations involve the chromosome ends and, indirectly, the telomeric repeats located at the terminal regions of chromosomes (true telomeres). A second type of aberrations directly involves the telomeric sequences located at the chromosome ends. Finally, there is a third class of aberrations that specifically involves the ITSs. The aims of this review are to provide a detailed description of these aberrations and to summarise the available data regarding their induction by physical and chemical mutagens.

Human telomeric DNA sequences are a major target for the antitumour drug bleomycin

The DNA sequence specificity of the cancer chemotherapeutic agent bleomycin was examined in a human telomeric DNA sequence and compared with that of non-telomeric sequences. The target DNA sequence contained 17 repeats of the human telomeric sequence and other primary sites of bleomycin cleavage. The 377-base-pair target DNA was fluorescently labelled at the 3'-end, damaged with bleomycin and electrophoresed in an ABI 3730 automated capillary sequencer to determine the intensity and sequence specificity of bleomycin damage. The results revealed that bleomycin cleaved primarily at 5'-GT in the telomeric sequence 5'-GGGTTA. Maxam-Gilbert chemical sequencing reactions were utilised as DNA size markers to determine the precise sites of bleomycin cleavage. The telomeric region contained strong sites of bleomycin cleavage and constituted 57% of the 30 most intense bleomycin damage sites in the DNA sequence examined. These data indicated that telomeric DNA sequences are a major site for bleomycin damage.

Sister chromatid exchanges and chromosomal aberrations in Chinese hamster ovary (CHO-K1) cells treated with the insecticide pirimicarb

Pirimicarb and its formulation Aficida (50% pirimicarb) effects were studied on CHO-K1 cells employing sister chromatid exchange (SCE), chromosomal aberrations (CA), cell-cycle progression and mitotic index analyses. Continuous treatments were performed within 10-300 microg/ml concentration-range. Pirimicarb, but not Aficida, induced a concentration-dependent increase of abnormal cells. Pirimicarb induced a greater frequency of chromatid/isochromatid breaks than Aficida did. Regression analyses showed a concentration-dependent increase in the frequency of chromatid-type breaks for both compounds whereas only the frequency of isochromatid-type breaks did in those pirimicarb-treated cultures. SCEs in pirimicarb- or Aficida-treated cultures were significantly higher than control values with concentrations of 100-200 microg/ml. Both test compounds induced equivalent frequency of SCEs. A delay in cell-cycle kinetics was observed for pirimicarb and Aficida within 100-300 and 200-300 microg/ml concentration-range, respectively. An inhibition of MI was observed for both chemicals regardless of tested concentrations. Finally, the CAs appears to be a higher sensitive bioassay to detect DNA damage at lower concentrations of pirimicarb than SCEs does. The results demonstrated that pirimicarb and Aficida exert geno-cytotoxicity, at least in CHO-K1 cells.

Effect of bleomycin on interstitial telomeric sequences of immortalized Chinese hamster ovary cells

The effect of the radiomimetic compound bleomycin (BLM) on interstitial telomeric sequences (ITSs) was investigated in Chinese hamster ovary (CHO) cells by using PNA-FISH with a pantelomeric probe. CHO cells were exposed to increasing concentrations of BLM and chromosomal aberrations were analyzed in the first mitosis after treatment. Cytogenetic analysis revealed that 18.1% and 9.5% of the total aberrations observed in cells exposed to BLM and harvested 18h and 3h after treatment, respectively, exhibited one or more FISH-detectable telomeric signals. Most of the chromosome breaks exhibiting telomeric signals observed in BLM-treated cells occurred in the centromeric regions of chromosomes. This observation, along with the finding of entirely labeled acentric fragments in BLM-exposed cells but not in untreated cells, shows that this antibiotic induces breakage at chromosomal sites containing ITSs. In addition, our results show that heterochromatic ITSs are involved more than expected in the formation of chromosome/chromatid breaks - and perhaps chromatid exchanges - induced by BLM, taking into account the percentage of the genome covered by telomeric sequences. On the contrary, our data strongly suggest that ITSs are not preferentially involved in the formation of dicentrics, multicentrics, centric rings, acentric fragments or chromatid deletions induced by BLM. Moreover, our results show that BLM is capable of inducing amplification and translocation of telomeric repeats, and suggest that this antibiotic produces breakage within centromeric ITSs, although chromosome regions containing these sequences are not the preferential target for BLM clastogenic action. On the other hand, our results show that BLM treatment increases the size of ITSs and that this effect is not related to the chromosomal sensitivity of the exposed cells to this compound.

Characterization of chromosomes and chromosomal fragments in human lymphocyte micronuclei by telomeric and centromeric FISH

Micronuclei (MN), used as a biomarker of effect in exposure to genotoxic carcinogens, derive from chromosomes and chromosomal fragments lagging behind in anaphase. The two types of MN are usually distinguished from each other by centromeric fluorescence in situ hybridization (FISH), centromere-positive (C(+)) MN representing entire chromosomes and centromere-negative (C(-)) MN chromosomal fragments. The incorporation of various types of chromosomal fragments and chromosomes and chromatids to MN is still poorly understood. We used directly labelled pancentromeric and pantelomeric DNA probes to examine the contents of MN in cultured binucleate lymphocytes of four unexposed, healthy subjects (two men and two women) 35-56 years of age. The presence and number of telomeric and centromeric signals was evaluated in 200 MN (50 MN per subject). These data were used to estimate the proportion of MN harbouring terminal/interstitial fragments, acentric/centric fragments, chromatid-type/chromosome-type fragments and entire chromatids/chromosomes. The majority of the C(+) MN (96% in men and 86% in women) found contained telomeric (T(+)) sequences. Most of the C(+) T(+) MN had one centromere and two or one telomere signals, suggesting that single chromatids were more frequently involved in MN than both sister chromatids. Among the C(-) MN, telomere signals were found in 91% (men) and 79% (women), showing that fragments in MN were mostly terminal. Most C(-) T(+) MN had one telomere signal, indicating higher prevalence for chromatid-type than chromosome-type terminal fragments. Combined centromeric and telomeric FISH is expected to increase the sensitivity of detecting exposure-related effects, when the exposure induces specific types of MN and its effect is low. This approach could particularly have use in discriminating between MN harbouring chromatid- and chromosome-type fragments in studies of human exposure to chemical clastogens and ionizing radiation.

Induction of complete and incomplete chromosome aberrations by bleomycin in human lymphocytes

Bleomycin (BLM) is a clastogenic compound, which due to the overdispersion in the cell distribution of induced dicentrics has been compared to the effect of high-LET radiation. Recently, it has been described that in fibroblast derived cell lines BLM induces incomplete chromosome elements more efficiently than any type of ionizing radiation. The objective of the present study was to evaluate in human lymphocytes the induction of dicentrics and incomplete chromosome elements by BLM. Peripheral blood samples have been treated with different concentrations of BLM. Two cytogenetic techniques were applied, fluorescence plus Giemsa (FPG) and FISH using pan-centromeric and pan-telomeric probes. The observed frequency of dicentric equivalents increases linearly with the BLM concentration, and for all BLM concentrations the distribution of dicentric equivalents was overdispersed. In the FISH study the ratio between total incomplete elements and multicentrics was 0.27. The overdispersion in the dicentric cell distribution, and the linear BLM-concentration dependence of dicentrics can be compared to the effect of high-LET radiation, on the contrary the ratio of incomplete elements and multicentrics is similar to the one induced by low-LET radiation (~0.40). The elevated proportion of interstitial deletions in relation to total acentric fragments, higher than any type of ionizing radiation could be a characteristic signature of the clastogenic effect of BLM.

Origin of nuclear buds and micronuclei in normal and folate-deprived human lymphocytes

Micronuclei are formed from chromosomes and chromosomal fragments that lag behind in anaphase and are left outside daughter nuclei in telophase. They may also be derived from broken anaphase bridges. Nuclear buds, micronucleus-like bodies attached to the nucleus by a thin nucleoplasmic connection, have been proposed to be generated similarly to micronuclei during nuclear division or in S-phase as a stage in the extrusion of extra DNA, possibly giving rise to micronuclei. To better understand these phenomena, we have characterized the contents of 894 nuclear buds and 1392 micronuclei in normal and folate-deprived 9-day cultures of human lymphocytes using fluorescence in situ hybridization with pancentromeric and pantelomeric DNA probes. Such information has not earlier been available for human primary cells. Surprisingly, there appears to be no previous data on the occurrence of telomeres in micronuclei (or buds) of normal human cells in general. Our results suggest that nuclear buds and micronuclei have partly different mechanistic origin. Interstitial DNA without centromere or telomere label was clearly more prevalent in nuclear buds (43%) than in micronuclei (13%). DNA with only telomere label or with both centromere and telomere label was more frequent in micronuclei (62% and 22%, respectively) than in nuclear buds (44% and 10%, respectively). Folate deprivation especially increased the frequency of nuclear buds and micronuclei harboring telomeric DNA and nuclear buds harboring interstitial DNA but also buds and micronuclei with both centromeric and telomeric DNA. According to the model we propose, that micronuclei in binucleate lymphocytes primarily derive from lagging chromosomes and terminal acentric fragments during mitosis. Most nuclear buds, however, are suggested to originate from interstitial or terminal acentric fragments, possibly representing nuclear membrane entrapment of DNA that has been left in cytoplasm after nuclear division or excess DNA that is being extruded from the nucleus.

A comparative analysis of bleomycin-induced incomplete chromosome elements in two mammalian cell lines using a telomeric PNA probe

Fluorescence in situ hybridization (FISH) with a telomeric peptide nucleic acid probe was employed to analyze the induction of incomplete chromosome elements (ICE; i.e., incomplete chromosomes and terminal fragments) by bleomycin (BLM) in two mammalian cell lines. Chinese hamster embryo cells (CHE cell line, average 2n = 23) and domestic rabbit cells (CPC cell line, average 2n = 44) were treated with 2.5 micro g/ml BLM; after 18 hr of incubation, first-division metaphases were stained with the telomeric probe, and ICE and other unstable chromosomal aberrations were scored. BLM induced ICE, dicentrics, and interstitial acentric fragments in CHE cells, but only ICE in CPC cells. About 50% of the metaphases in BLM-treated CHE cells contained one or more pairs of ICE, while only 20% of treated CPC cells contained ICE. Almost 100% of the BLM-induced ICE in both cell lines consisted of pairs formed by an incomplete chromosome and a terminal fragment. Our results confirm that ICE are the most frequent type of unstable chromosomal aberration induced by BLM in mammalian cells. Moreover, the present study shows that an increase in the chromosome number does not necessarily result in an increase in the frequency of BLM-induced ICE. The results also show that the difference in the chromosomal sensitivity to BLM in CHE and CPC cells is due to differences in the absolute frequency but not in the pattern (i.e., type and proportion) of ICE.

Telomeres, interstitial telomeric repeat sequences, and chromosomal aberrations

Telomeres are specialized nucleoproteic complexes localized at the physical ends of linear eukaryotic chromosomes that maintain their stability and integrity. The DNA component of telomeres is characterized by being a G-rich double stranded DNA composed by short fragments tandemly repeated with different sequences depending on the species considered. At the chromosome level, telomeres or, more properly, telomeric repeats--the DNA component of telomeres--can be detected either by using the fluorescence in situ hybridization (FISH) technique with a DNA or a peptide nucleic acid (PNA) (pan)telomeric probe, i.e., which identifies simultaneously all of the telomeres in a metaphase cell, or by the primed in situ labeling (PRINS) reaction using an oligonucleotide primer complementary to the telomeric DNA repeated sequence. Using these techniques, incomplete chromosome elements, acentric fragments, amplification and translocation of telomeric repeat sequences, telomeric associations and telomeric fusions can be identified. In addition, chromosome orientation (CO)-FISH allows to discriminate between the different types of telomeric fusions, namely telomere-telomere and telomere-DNA double strand break fusions and to detect recombination events at the telomere, i.e., telomeric sister-chromatid exchanges (T-SCE). In this review, we summarize our current knowledge of chromosomal aberrations involving telomeres and interstitial telomeric repeat sequences and their induction by physical and chemical mutagens. Since all of the studies on the induction of these types of aberrations were conducted in mammalian cells, the review will be focused on the chromosomal aberrations involving the TTAGGG sequence, i.e., the telomeric repeat sequence that "caps" the chromosomes of all vertebrate species.

Analysis of streptozotocin-induced incomplete chromosome elements and excess acentric fragments in Chinese hamster cells using a telomeric PNA probe

Fluorescence in situ hybridization (FISH) with a telomeric peptide nucleic acid (PNA) probe was employed to analyze the induction of incomplete chromosome elements (ICE, i.e., unjoined or "open" chromosome elements with telomeric signal at only one end) and excess acentric fragments (i.e., in excess of fragments resulting from the formation of dicentric and ring chromosomes) by the methylating agent streptozotocin (STZ) in a Chinese hamster embryo (CHE) cell line. CHE cells were treated with 0-4 mM STZ and chromosomal aberrations were analyzed in the first mitosis after treatment using the telomeric probe. Centric (incomplete chromosomes) and acentric (terminal fragments) ICE were the only unstable chromosome-type aberrations induced by STZ in CHE cells. The induction of these aberrations exhibited a curvilinear concentration-response relationship. About 40% of the metaphases present in cell cultures treated with STZ contained one or more pairs of ICE. In STZ-treated cells, ICE were always observed as pairs consisting of an incomplete chromosome and a terminal fragment. Moreover, all of the excess acentric fragments induced by STZ were of terminal type. These results indicate that chromosomal incompleteness is a very common event following exposure to STZ and suggest that all of the excess acentric fragments induced by STZ originate from terminal deletions.

Analysis of streptonigrin-induced incomplete chromosome elements and interstitial fragments in Chinese hamster cells using a telomeric PNA probe

We investigated the induction of incomplete chromosome elements (ICEs; i.e., elements with a telomeric signal at only one terminal end) and interstitial fragments induced by the antibiotic streptonigrin (SN) in a Chinese hamster embryo (CHE) cell line using FISH with a telomeric peptide nucleic acid probe. CHE cells were treated with 0-250 ng/ml SN and chromosomal aberrations were analyzed in the first mitosis after treatment using the telomeric probe. Exposure of CHE cells to SN resulted in a linear concentration-related increase in all of the aberration types analyzed (P < 0.05) except ring chromosomes. Depending on the SN concentration employed, 33-68% of the metaphases contained one or more pairs of ICEs (an incomplete chromosome accompanied by a terminal fragment or two incomplete chromosomes accompanied by a compound fragment). Pooled data from all SN concentrations revealed that 77.8% of the acentric fragments were terminal fragments, 18.8% interstitial fragments, and 3.4% compound fragments. Furthermore, it was estimated that about 80% of excess acentric fragments induced by SN originated from incomplete exchanges or terminal deletions and 20% from complete exchanges (interstitial deletions). These results show that incomplete chromosomes and terminal fragments are the most frequent asymmetrical chromosomal aberrations induced by SN and indicate that true incompleteness is a very common event following exposure to SN.