Revision and analysis of the chromosome variability in the speciose genus Akodon (Rodentia, Sigmodontinae), including new data from Argentina.

Rodentia has a high species number and chromosomal variability. The South American genus Akodon is one of the most speciose muroids, with more than 40 species included in several species groups. Here, we characterize cytogenetically specimens of Akodon from central-western Argentina. Subsequently, we reviewed and analyzed the cytogenetic data for this genus, build a phylogeny and mapped chromosome changes to interpret its evolution. Specimens of A. dolores from central-western Argentina have 2n=42-44/FNa=44 (46, 48) due to a Robertsonian rearrangement. Our data expand the distribution range known for this polymorphism and confirm its geographic structure. Other specimens had 2n=40/FNa=40, representing populations of A. oenos, A. polopi, and A. spegazzinii. All karyotypes have a low amount of heterochromatin, concentrated in centromeres and sex chromosomes, as in other rodents. The complement with 2n=40/FNa=40 is the most frequent in Akodon and is shared by most species in some groups. Chromosome numbers are very diverse. The FNa shows less variability; FNa=42 was recovered as ancestral, excluding A. mimus, which was connected at the base of the Akodon tree and has FNa=44. This indicates a complex chromosome evolution in Akodon, and suggests that reductions and increases in the 2n and FNa evolved independently in some lineages.

Mutagen-induced telomere instability in human cells.

Telomere instability is one of the main sources of genome instability and may result from chromosome end loss (due to chromosome breakage at one or both ends) or, more frequently, telomere dysfunction. Dysfunctional telomeres arise when they lose their end-capping function or become critically short, which causes chromosomal termini to behave like a DNA double-strand break. Telomere instability may occur at the chromosomal or at the molecular level, giving rise, respectively, to telomere-related chromosomal aberrations or the loss or modification of any of the components of the telomere (telomere DNA, telomere-associated proteins, or telomere RNA). Since telomeres play a fundamental role in maintaining genome stability, the study of telomere instability in cells exposed to mutagens is of great importance to understand the telomere-driven genomic instability present in those cells. In the present review, we will focus on the current knowledge about telomere instability induced by physical, chemical, and biological mutagens in human cells.

Using telomeric chromosomal aberrations to evaluate clastogen-induced genomic instability in mammalian cells.

Telomeres, the specialized nucleoproteic complexes localized at the physical ends of linear eukaryotic chromosomes, play a fundamental role in maintaining chromosomal stability and integrity, being one of the leading guardians of genome stability. In recent years, the identification and analysis of chromosomal aberrations involving telomeres has proven to be a unique tool to evaluate misrepaired and unrepaired chromosome damage in mammalian cells. Telomere instability constitutes an important source of genomic instability, a phenomenon characteristic of cancer cells, and also common in cells exposed to chemical or physical mutagens which induce chromosomal aberrations by producing chromosome breakage (clastogens). In the present review, we will focus on the chromosomal aberrations involving telomeres and their importance to determine the clastogen-induced genomic instability present in mammalian cells.

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.

Interstitial telomeric sequences in vertebrate chromosomes: Origin, function, instability and evolution.

By definition, telomeric sequences are located at the very ends or terminal regions of chromosomes. However, several vertebrate species show blocks of (TTAGGG)n repeats present in non-terminal regions of chromosomes, the so-called interstitial telomeric sequences (ITSs), interstitial telomeric repeats or interstitial telomeric bands, which include those intrachromosomal telomeric-like repeats located near (pericentromeric ITSs) or within the centromere (centromeric ITSs) and those telomeric repeats located between the centromere and the telomere (i.e., truly interstitial telomeric sequences) of eukaryotic chromosomes. According with their sequence organization, localization and flanking sequences, ITSs can be classified into four types: 1) short ITSs, 2) subtelomeric ITSs, 3) fusion ITSs, and 4) heterochromatic ITSs. The first three types have been described mainly in the human genome, whereas heterochromatic ITSs have been found in several vertebrate species but not in humans. Several lines of evidence suggest that ITSs play a significant role in genome instability and evolution. This review aims to summarize our current knowledge about the origin, function, instability and evolution of these telomeric-like repeats in vertebrate chromosomes.

The methylating agent streptozotocin induces persistent telomere dysfunction in mammalian cells.

We analyzed chromosomal aberrations involving telomeres in the progeny of mammalian cells exposed to the methylating agent and antineoplastic/diabetogenic drug streptozotocin (STZ), to test whether it induces long-term telomere instability (by chromosome end loss and/or telomere dysfunction). Rat cells (ADIPO-P2 cell line, derived from Sprague-Dawley rat adipose cells) were treated with a single concentration of STZ (2mM). Chromosomal aberrations were analyzed 18h, 10 days, and 15 days after treatment, using PNA-FISH with a pan-telomeric probe [Cy3-(CCCTAA)3] to detect (TTAGGG)n repeats. Cytogenetic analysis revealed a higher frequency of chromosomal aberrations in STZ-exposed cultures vs. untreated cultures at each time point analyzed. The yield of induced aberrations was very similar at each time point. Induction of aberrations not involving telomere dysfunction was only observed 18h and 15 days after treatment, whereas induction of telomere dysfunction-related aberrations by STZ (mainly in the form of telomere FISH signal loss and duplications, most of them chromatid-type aberrations) was observed at each time point. Our results show that STZ induces persistent telomere instability in mammalian cells, cytogenetically manifested as telomere dysfunction-related chromosomal aberrations. Neither telomere length nor telomerase activity is related to the telomere dysfunction.

Characterization of smart auto-degradative hydrogel matrix containing alginate lyase to enhance levofloxacin delivery against bacterial biofilms.

The aim of the present work is the characterization of smart auto-degradable microspheres composed of calcium alginate/high methoxylated pectin containing an alginate lyase (AL) from Sphingobacterium multivorum and levofloxacin. Microspheres were prepared by ionotropic gelation containing AL in its inactive form at pH 4.0. Incubation of microspheres in Tris-HCl and PBS buffers at pH 7.40 allowed to establish the effect of ion-chelating phosphate on matrix erodability and suggested an intrinsically activation of AL by turning the pH close to neutrality. Scanning electron and optical microscopies revealed the presence of holes and surface changes in AL containing microspheres. Furthermore, texturometric parameters, DSC profiles and swelling properties were showing strong changes in microspheres properties. Encapsulation of levofloxacin into microspheres containing AL showed 70% efficiency and 35% enhancement of antimicrobial activity against Pseudomonas aeruginosa biofilm. Levofloxacin release from microspheres was not changed at acidic pH, but was modified at neutral pH in presence of AL. Advantageously, only gel matrix debris were detectable after overnight incubation, indicating an autodegradative gel process activated by the pH. Absence of matrix cytotoxicity and a reduction of the levofloxacin toxicity after encapsulation were observed in mammalian CHO-K1 cell cultures. These properties make the system a potent and versatile tool for antibiotic oral delivery targeted to intestine, enhancing the drug bioavailability to eradicate bacterial biofilm and avoiding possible intestinal obstructions.

Distribution of Telomeric Sequences (TTAGGG)n in Rearranged Chromosomes of Phyllotine Rodents (Cricetidae, Sigmodontinae).

Phyllotines are sigmodontine rodents endemic to South America with broad genetic variability, Robertsonian polymorphisms being the most frequent. Moreover, this taxon includes a species with multiple sex chromosomes, which is infrequent in mammals. However, molecular cytogenetic techniques have never been applied to phyllotines to elucidate their karyotypic evolution. We studied the chromosomes of 4 phyllotine species using FISH with a pantelomeric probe (TTAGGG)n. Graomys griseoflavus, Eligmodontia puerulus, and E. morgani are polymorphic for Robertsonian translocations, whereas Salinomys delicatus possesses XX/ XY1Y2 sex chromosomes. Telomeric signals were detected at both ends of all chromosomes of the studied species. In S. delicatus interstitial telomeric sequences (ITS) were observed in the 3 major chromosome pairs, which are equidistant from one of the telomeres in these chromosomes. These results suggest that ITS are important in the reshuffling of the highly derived karyotype of S. delicatus. Considering the phylogeny of phyllotines, the Robertsonian rearrangements of G. griseoflavus, E. puerulus, and E. morgani possibly represent chromosome fusions which have occurred independently. The pericentromeric regions of the biarmed chromosomes of these species do not contain telomeric sequences characteristic for strict fusions of recent origin, suggesting a common pattern of telomeric repeat loss during chromosomal evolution of these rodents.

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.

Effect of thiol compounds on bleomycin-induced DNA and chromosome damage in human cells.

Non-protein thiols are considered radioprotectors, preventing DNA damage by ionizing radiation. As bleomycin (BLM) is a radiomimetic agent it was proposed that thiols may prevent DNA damage produced by this antibiotic. However, results obtained with thiols and BLM-combined treatments in living cells are contradictory. The goal of this work was to assess the influence of five non-protein thiols of different electrical charge and chemical composition, on the DNA damage, DNA repair, chromosomal aberrations and cell killing induced by BLM. We found that, at the chromosomal level and cell killing, Glutathione, ß-Mercaptoethanol and cysteine showed a protective effect, while ditiothreitol and cysteamine increased them, whereas at the DNA level all thiols potentiated the DNA damage induced by BLM, most probably due to a reactivation of the BLM complex.

Interstitial telomeric sequences are not preferentially involved in the chromosome damage induced by the methylating compound streptozotocin in Chinese hamster cells.

The effect of the methylating compound streptozotocin (STZ) on interstitial telomeric sequences (ITSs) was investigated in Chinese hamster ovary (CHO) cells by using peptide nucleic acid-fluorescence in situ hybridization with a pantelomeric probe. Cells were exposed to increasing concentrations of STZ, and chromosomal aberrations were analyzed at the first mitosis after treatment. The frequency of chromosomal aberrations directly involving ITSs increased in STZ-treated cells by a factor of 2.6 (2 mM) and 3.6 (4 mM) when compared with the frequency of these aberrations in control cells (P < 0.05). However, no significant differences were found between control and exposed cells in the percentage of aberrations directly involving ITSs, demonstrating that these repeat regions were not preferentially involved in the chromosome damage induced by STZ. In addition, STZ did not alter telomerase activity, suggesting that this enzyme may not be involved in the induction of chromosomal aberrations by this compound.

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.

Streptonigrin induces delayed chromosomal instability involving interstitial telomeric sequences in Chinese hamster ovary cells.

We analyzed the induction of chromosomal aberrations in Chinese hamster ovary (CHO) cells exposed to the radiomimetic compound streptonigrin (SN), in order to determine whether interstitial telomeric sequences (ITSs) are involved in the long-term clastogenic effect of this antibiotic. CHO cells were treated with a single concentration of SN (100ng/ml), and the frequency of unstable chromosomal aberrations was determined at three times after treatment (18h, and 6 and 15 days) by using PNA-FISH with a pan-telomeric probe. Cytogenetic analysis revealed a higher frequency of aberrations at 18h and 6 days after treatment in SN-exposed cultures vs. untreated cultures. The percentage of damaged cells and the yield of SN-induced aberrations at 6 days after treatment increased on average twofold compared with the ones at 18h after treatment. Moreover, a significant decrease in the frequency of aberrations was observed in SN-exposed cells at 15 days after treatment, resulting in a frequency of aberrations significantly lower than the frequency of aberrations observed in the corresponding control cultures. These data indicate that SN induces delayed chromosomal instability in CHO cells, and that the in vitro clastogenic effect of this compound persists for at least 6 days but less than 15 days after treatment. In addition, we found that SN induces delayed ITSs instability, cytogenetically detectable as additional FISH signals and centromeric breaks involving dissociation of the telomeric signal 6 days after treatment. We propose that the delayed effect of SN on ITSs results from breakage of heterochromatic centromeric ITSs blocks and further insertion of these sequences at the sites of mono- or isochromatid breaks occurring at G2 or G1-S phases of the cell cycle, respectively, since most of the additional FISH signals were present as single or double dots, and located at interstitial sites of the involved chromosomes.

Bleomycin induces delayed instability of interstitial telomeric sequences in Chinese hamster ovary cells.

We analyzed the behavior of interstitial telomeric sequences (ITSs) in the progeny of Chinese Hamster Ovary (CHO) cells exposed to the radiomimetic compound bleomycin (BLM) in order to determine if ITSs play some role in the long-term clastogenic effect of this antibiotic. To this end, CHO cells were treated with a single concentration of BLM (2.5µg/ml), and the frequency of unstable chromosomal aberrations was determined at several times after treatment (18h, and 6, 15 and 34/36 days) by using PNA-FISH with a pan-telomeric probe [(TTAGGG)n repeats]. Cytogenetic analysis revealed a higher frequency of aberrations at 18h and 6 days after treatment in BLM-exposed cultures vs. untreated cultures, although the yield of BLM-induced aberrations decreased on average five times 6 days after treatment compared with the one induced 18h after treatment. Moreover, no significant differences in the frequency of aberrations were observed between untreated and BLM-exposed cells at 15 or 34/36 days after treatment. These data indicate that, in terms of unstable aberrations, the in vitro clastogenic effect of BLM on CHO cells persists for at least 6 days but less than 15 days after exposure. In addition, we found that BLM induces ITSs instability, cytogenetically detectable as acentric fragments (18h after treatment) or additional (new) FISH signals (6 days after treatment). We propose that the delayed effect of BLM on ITSs mainly results from breakage of heterochromatic ITSs blocks and further insertion of these sequences at the sites of monochromatid breaks occurring at G2 phase of the cell cycle, since most of the additional FISH signals were present as single dots and located at interstitial sites of the involved chromosomes.

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.

Relationship between heterochromatic interstitial telomeric sequences and chromosome damage induced by the radiomimetic compound streptonigrin in Chinese hamster ovary cells.

The relationship between (heterochromatic) interstitial telomeric sequences (ITSs) and the chromosome damage induced by the radiomimetic compound streptonigrin (SN) was investigated in Chinese hamster ovary (CHO) cells by using PNA- and Q-FISH techniques with a pantelomeric probe. CHO cells were exposed to increasing concentrations of SN and chromosomal aberrations were analyzed in the first mitosis after treatment. Cytogenetic analysis revealed that 16.9% and 11.7% of the total aberrations induced by SN in cells harvested 18 h and 3 h after treatment, respectively, exhibited one or more FISH-detectable telomeric signals. Although there was a significant induction by SN of chromosome breaks at centromeric regions containing ITSs, about 70% of the chromosome breaks exhibiting telomeric signals observed in SN-treated cells occurred outside the centromeric regions of chromosomes. This observation, along with the finding of entirely labeled acentric fragments in both untreated and SN-treated cells show that, although this antibiotic induces breakage at centromeric regions containing ITSs, these chromosome regions are not the preferential target for the clastogenic action of SN. In addition, our results show that heterochromatic ITSs are involved more than expected in the formation of chromatid breaks and exchanges induced by SN, and that these sequences are not preferentially involved in the formation of dicentrics, multicentrics, centric rings, chromosome breaks, acentric fragments and chromatid deletions induced by this antibiotic. These findings indicate that the involvement of heterochromatic ITSs in the chromosome damage induced by SN is not random. Moreover, our results show that SN induces telomeric repeats translocations, although this effect depends on the concentration of the drug, and that this antibiotic increases the size of ITSs, this latter effect not being related to the chromosomal sensitivity of the exposed cells to this compound. The mechanism by which SN induces amplification of heterochromatic ITSs remains to be elucidated.

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.

Karyotypic characterization of Trachemys dorbigni (Testudines: Emydidae) and Chelonoidis (Geochelone) donosobarrosi (Testudines: Testudinidae), two species of Cryptodiran turtles from Argentina.

We describe for the first time the karyotypes of two species of Cryptodiran turtles from Argentina, namely, Trachemys dorbigni (Emydidae) and Chelonoidis (Geochelone) donosobarrosi (Testudinidae). The karyotype of T. dorbigni (2n = 50) consists of 13 pairs of macrochromosomes and 12 pairs of microchromosomes, whereas the karyotype of C. donosobarrosi (2n = 52) consists of 11 pairs of macrochromosomes and 15 pairs of microchromosomes. Fluorescence in situ hybridization (FISH) with a (TTAGGG)n telomeric probe showed that the chromosomes of these species have four telomeric signals, two at each end, indicating that none of the chromosomes of T. dorbigni and C. donosobarrosi are telocentric. The fact that no interstitial telomeric signals were observed after FISH, suggests that interstitial telomeric sequences did not have a major role in the chromosomal evolution of these species. Additional data will be needed to elucidate if interstitial telomeric sequences have a major role in the karyotypic evolution of Testudines.

Analysis of spontaneous and bleomycin-induced chromosome damage in peripheral lymphocytes of long-haul aircrew members from Argentina.

Spontaneous and bleomycin (BLM)-induced chromosomal aberrations in G0 and G2 stages of the cell cycle have been analyzed in peripheral lymphocytes of 21 long-haul aircrew members from Argentina in order to assess BLM-induced clastogenesis as a first approach to determine the DNA repair capacity and thereby the susceptibility to environmental cancers in aircrew. The possibility that occupational exposure of flight personnel to cosmic radiation can induce an adaptive response in their peripheral lymphocytes that can be detected by a subsequent in vitro treatment with BLM was also investigated. For comparison, aberrations were also scored in the lymphocytes of 15 healthy volunteers matched by age, health, sex, drinking and smoking habits to the flight personnel group. Aircrew exhibited a higher frequency of spontaneous dicentrics and ring chromosomes than the control population (p<0.05). BLM sensitivity test showed that aircrew and controls are equally sensitive to BLM G2 clastogenic effects, since both groups exhibited a similar frequency of chromatid breaks per cell (p>0.05). However, the aircrew sampled population was almost two times more sensitive to BLM G0 clastogenic effects than controls (p<0.05). Therefore, our data suggest that chronic exposure of aircrew to cosmic radiation increases the in vitro chromosomal sensitivity of their peripheral lymphocytes to BLM (at least in the G0 stage of the cell cycle), and that occupational exposure of flight personnel to cosmic radiation does not induce an adaptive response to this radiomimetic compound. Our results justify further studies aimed at determine if those aircrew members hypersensitive to BLM are more prone to develop environmental cancer than BLM-insensitive individuals.

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.