HJURP knockdown disrupts clonogenic capacity and increases radiation-induced cell death of glioblastoma cells.

The Holliday Junction-Recognition Protein (HJURP) was reported as overexpressed in several cancers and also strongly correlated with poor prognosis of patients, especially in glioblastoma (GBM), the most common and deadly type of primary brain tumor. HJURP is responsible for loading the histone H3 variant-the Centromeric Protein A (CENP-A)-at the centromeres in a cell cycle-regulated manner, being required for proper chromosome segregation. Here we investigated HJURP association with survival and radioresistance of different GBM cell lines. HJURP knockdown compromised the clonogenic capacity and severely impaired survival of five distinct GBM cells, while nontumor astrocytes were not affected. U251MG cells showed a robust cell cycle arrest in G2/M phases followed by a drastic increment in cell death after HJURP silencing, while U138MG and U343MG cell lines presented augmented senescence with a comparable increase in cell death. Importantly, we verified that the impact on cell cycle dynamics and clonogenic survival were associated with loss CENP-A at the centromeres. Moreover, radiation resistance was also impacted by HJURP modulation in several GBM cell lines. U87MG, T98G, U138MG, and U343MG cells were all sensitized to ionizing radiation after HJURP reduction. These data reinforce the requirement of HJURP for proliferative capacity and radioresistance of tumor cells, underlining its potential as a promising therapeutic target for GBM.

An alternative approach for the induction of premature chromosome condensation in human peripheral blood lymphocytes using mitotic Akodon cells.

Purpose: The premature chromosome condensation (PCC) technique is used to study exposure to external radiation through the determination of chromosome fragments observed in interphase cells. The presence of large telomeric signals in CHO cells interferes with the detection of PCC fragments and the identification of dicentric chromosomes. We present an improved method for the fusion of G0-lymphocytes with mitotic Akodon cells (few chromosomes and weakly-staining telomeric sequences) to induce PCC in combination with rapid quantification of dicentric chromosomes and centric rings as an alternative to the classical CHO cell fusion technique.Materials and methods: Whole blood from three healthy volunteers was γ-irradiated with 0, 2, or 4 Gy. Following a 24 h incubation post-exposure at 37 °C, chromosome spreads of isolated lymphocytes were prepared by standard PCC procedures using mitotic Akodon cells.Results: The percentage of scorable fusions, measured by telomere/centromere (T/C) staining, for Akodon-induced PCC was higher than that for CHO-induced PCC, irrespective of radiation exposure. Importantly, both techniques gave the same result for biodosimetry evaluation.Conclusion: The mitotic Akodon cell-induced PCC fusion assay, in combination with the scoring of dicentric chromosomes and rings by T/C staining of G0-lymphocytes is a suitable alternative for fast and reliable dose estimation after accidental radiation exposure.

Use of human lymphocyte G0 PCCs to detect intra- and inter-chromosomal aberrations for early radiation biodosimetry and retrospective assessment of radiation-induced effects.

A sensitive biodosimetry tool is required for rapid individualized dose estimation and risk assessment in the case of radiological or nuclear mass casualty scenarios to prioritize exposed humans for immediate medical countermeasures to reduce radiation related injuries or morbidity risks. Unlike the conventional Dicentric Chromosome Assay (DCA), which takes about 3-4 days for radiation dose estimation, cell fusion mediated Premature Chromosome Condensation (PCC) technique in G0 lymphocytes can be rapidly performed for radiation dose assessment within 6-8 hrs of sample receipt by alleviating the need for ex vivo lymphocyte proliferation for 48 hrs. Despite this advantage, the PCC technique has not yet been fully exploited for radiation biodosimetry. Realizing the advantage of G0 PCC technique that can be instantaneously applied to unstimulated lymphocytes, we evaluated the utility of G0 PCC technique in detecting ionizing radiation (IR) induced stable and unstable chromosomal aberrations for biodosimetry purposes. Our study demonstrates that PCC coupled with mFISH and mBAND techniques can efficiently detect both numerical and structural chromosome aberrations at the intra- and inter-chromosomal levels in unstimulated T- and B-lymphocytes. Collectively, we demonstrate that the G0 PCC technique has the potential for development as a biodosimetry tool for detecting unstable chromosome aberrations (chromosome fragments and dicentric chromosomes) for early radiation dose estimation and stable chromosome exchange events (translocations) for retrospective monitoring of individualized health risks in unstimulated lymphocytes.

Foundations of identifying individual chromosomes by imaging flow cytometry with applications in radiation biodosimetry.

Biodosimetry is an important tool for triage in the case of large-scale radiological or nuclear emergencies, but traditional microscope-based methods can be tedious and prone to scorer fatigue. While the dicentric chromosome assay (DCA) has been adapted for use in triage situations, it is still time-consuming to create and score slides. Recent adaptations of traditional biodosimetry assays to imaging flow cytometry (IFC) methods have dramatically increased throughput. Additionally, recent improvements in image analysis algorithms in the IFC software have resulted in improved specificity for spot counting of small events. In the IFC method for the dicentric chromosome analysis (FDCA), lymphocytes isolated from whole blood samples are cultured with PHA and Colcemid. After incubation, lymphocytes are treated with a hypotonic solution and chromosomes are isolated in suspension, labelled with a centromere marker and stained for DNA content with DRAQ5. Stained individual chromosomes are analyzed on the ImageStream®X (EMD-Millipore, Billerica, MA) and mono- and dicentric chromosome populations are identified and enumerated using advanced image processing techniques. Both the preparation of the isolated chromosome suspensions as well as the image analysis methods were fine-tuned in order to optimize the FDCA. In this paper we describe the method to identify and score centromeres in individual chromosomes by IFC and show that the FDCA method may further improve throughput for triage biodosimetry in the case of large-scale radiological or nuclear emergencies.

Loss of lamin B receptor is necessary to induce cellular senescence.

Cellular transition to senescence is associated with extensive chromatin reorganization and changes in gene expression. Recent studies appear to imply an association of lamin B1 (LB1) reduction with chromatin rearrangement in human fibroblasts promoted to senescence, while the mechanisms and structural features of these relationships have not yet been clarified. In this work, we examined the functions of LB1 and the lamin B receptor (LBR) in human cancer cells. We found that both LB1 and LBR tend to deplete during cancer cell transfer to senescence by γ-irradiation. A functional study employing silencing of LBR by small hairpin ribonucleic acid (shRNA) constructs revealed reduced LB1 levels suggesting that the regulation of both proteins is interrelated. The reduced expression of LBR resulted in the relocation of centromeric heterochromatin (CSH) from the inner nuclear membrane (INM) to the nucleoplasm and is associated with its unfolding. This indicates that LBR tethers heterochromatin to INM in cycling cancer cells and that LB1 is an integral part of this tethering. Down-regulation of LBR and LB1 at the onset of senescence are thus necessary for the release of heterochromatin binding to lamina, resulting in changes in chromatin architecture and gene expression. However, the senescence phenotype was not manifested in cell lines with reduced LBR and LB1 expression suggesting that other factors, such as deoxyribonucleic acid (DNA) damage, are needed to trigger senescence. We conclude that the primary response of cells to various stresses leading to senescence consists of the down-regulation of LBR and LB1 to attain reversal of the chromatin architecture.

A cytogenetic study of hospital workers occupationally exposed to radionuclides in Serbia: premature centromere division as novel biomarker of exposure?

PURPOSE: The health risk of chronic exposure to radionuclides includes changes in the genome (e.g., chromosomal aberrations and micronuclei) that increase chromosomal instability. There are also other phenomena, which seem to appear more frequently in metaphases of exposed persons (such as premature centromere division). The aim of this study was to discover whether or not there is correlation between incidence of named cytogenetic changes in persons occupationally exposed to radionuclides in comparison with unexposed control group, and if significant correlation is determined, can premature centromere division be consider as a biomarker of radiation exposure? METHODS: The exposed group comprised 50 individuals occupationally exposed to radionuclides. The reference control group consisted of 40 unexposed individuals. Chromosomal aberrations, micronuclei and premature centromere division were analyzed according to a standard International Atomic Energy Agency protocol. Statistical analyses were performed using SPSS 17.0 statistics. RESULTS: The means for analyzed cytogenetic changes were significantly higher in the exposed group. Positive correlation between them was found in exposed group. Premature centromere division parameter PCD5-10 was selected as particularly suitable for separating groups (exposed/unexposed). CONCLUSIONS: Identification of other phenomena related to radionuclide exposure, beside well known, may clarify recent problems in radiobiology concerning the biological response to low doses of ionizing radiation and its consequences.

Delayed Numerical Chromosome Aberrations in Human Fibroblasts by Low Dose of Radiation.

Radiation-induced genomic instability refers to a type of damage transmitted over many generations following irradiation. This delayed impact of radiation exposure may pose a high risk to human health and increases concern over the dose limit of radiation exposure for both the public and radiation workers. Therefore, the development of additional biomarkers is still needed for the detection of delayed responses following low doses of radiation exposure. In this study, we examined the effect of X-irradiation on delayed induction of numerical chromosomal aberrations in normal human fibroblasts irradiated with 20, 50 and 100 cGy of X-rays using the micronucleus-centromere assay. Frequencies of centromere negative- and positive-micronuclei, and aneuploidy of chromosome 1 and 4 were analyzed in the surviving cells at 28, 88 and 240 h after X-irradiation. X-irradiation increased the frequency of micronuclei (MN) in a dose-dependent manner in the cells at all measured time-points, but no significant differences in MN frequency among cell passages were observed. Aneuploid frequency of chromosomes 1 and 4 increased with radiation doses, and a significantly higher frequency of aneuploidy was observed in the surviving cells analyzed at 240 h compared to 28 h. These results indicate that low-dose of X-irradiation can induce delayed aneuploidy of chromosomes 1 and 4 in normal fibroblasts.

Biological effects of in vitro THz radiation exposure in human foetal fibroblasts.

In recent years, terahertz (THz) radiation has been widely used in a variety of applications: medical, security, telecommunications and military areas. However, few data are available on the biological effects of this type of electromagnetic radiation and the reported results, using different genetic or cellular assays, are quite discordant. This multidisciplinary study focuses on potential genotoxic and cytotoxic effects, evaluated by several end-points, associated with THz radiation. For this purpose, in vitro exposure of human foetal fibroblasts to low frequency THz radiation (0.1-0.15THz) was performed using a Compact Free Electron Laser. We did not observe an induction of DNA damage evaluated by Comet assay, phosphorylation of H2AX histone or telomere length modulation. In addiction, no induction of apoptosis or changes in pro-survival signalling proteins were detected. Moreover, our results indicated an increase in the total number of micronuclei and centromere positive micronuclei induction evaluated by CREST analysis, indicating that THz radiation could induce aneugenic rather than clastogenic effects, probably leading to chromosome loss. Furthermore, an increase of actin polymerization observed by ultrastructural analysis after THz irradiation, supports the hypothesis that an abnormal assembly of spindle proteins could lead to the observed chromosomal malsegregation.

Dicentric chromosome aberration analysis using giemsa and centromere specific fluorescence in-situ hybridization for biological dosimetry: An inter- and intra-laboratory comparison in Indian laboratories.

To facilitate efficient handling of large samples, an attempt towards networking of laboratories in India for biological dosimetry was carried out. Human peripheral blood samples were exposed to (60)Co γ-radiation for ten different doses (0-5Gy) at a dose rate of 0.7 and 2Gy/min. The chromosomal aberrations (CA) were scored in Giemsa-stained and fluorescence in-situ hybridization with centromere-specific probes. No significant difference (p>0.05) was observed in the CA yield for given doses except 4 and 5Gy, between the laboratories, among the scorers and also staining methods adapted suggest the reliability and validates the inter-lab comparisons exercise for triage applications.

New tool for biological dosimetry: reevaluation and automation of the gold standard method following telomere and centromere staining.

PURPOSE: The dicentric chromosome (dicentric) assay is the international gold-standard method for biological dosimetry and classification of genotoxic agents. The introduction of telomere and centromere (TC) staining offers the potential to render dicentric scoring more efficient and robust. In this study, we improved the detection of dicentrics and all unstable chromosomal aberrations (CA) leading to a significant reevaluation of the dose-effect curve and developed an automated approach following TC staining. MATERIAL AND METHODS: Blood samples from 16 healthy donors were exposed to (137)Cs at 8 doses from 0.1 to 6Gy. CA were manually and automatically scored following uniform (Giemsa) or TC staining. The detection of centromeric regions and telomeric sequences using PNA probes allowed the detection of all unstable CA: dicentrics, centric and acentric rings, and all acentric fragments (with 2, 4 or no telomeres) leading to the precise quantification of estimated double strand breaks (DSB). RESULTS: Manual scoring following TC staining revealed a significantly higher frequency of dicentrics (p<10(-3)) (up to 30%) and estimated DSB (p<10(-4)) compared to uniform staining due to improved detection of dicentrics with centromeres juxtaposed with other centromeres or telomeres. This improvement permitted the development of the software, TCScore, that detected 95% of manually scored dicentrics compared to 50% for the best currently available software (DCScore™). CONCLUSION: The use of TC staining has permitted a reevaluation of the dose-response curve and the highly efficient automation of the scoring process, marking a new step in the management and follow-up of populations exposed to genotoxic agents including ionizing radiation.

[HSM6 gene is identical to PSY4 gene in Saccharomyces cerevisiae yeasts].

Previously, we isolated mutant yeasts Saccharomyces cerevisiae with an increased rate of spontaneous mutagenesis. Here, we studied the properties of HSM6 gene, the hsm6-1 mutation of which increased the frequency of UV-induced mutagenesis and decreased the level of UV-induced mitotic crossover at the centromere gene region, ADE2. HSM6 gene was mapped on the left arm of chromosome 11 in the region where the PSY4 gene is located. The epistatic analysis has shown that the hsm6-1 mutation represents an allele of PSY4 gene. Sequencing of hsm6-1 mutant allele has revealed a frameshift mutation, which caused the substitution of Lys218Glu and the generation of a stop codon in the next position. The interactions of hsm6-1 and rad52 mutations were epistatic. Our data show that the PSY4 gene plays a key role in the regulation of cell withdrawal from checkpoint induced by DNA disturbances.

Micronuclei in lymphocytes from radon spa personnel in the Czech Republic.

PURPOSE: To assess the radiation exposure of radon spa personnel in Jáchymov, Czech Republic. METHODS: The frequency of micronucleus-containing cells and the percentage of centromere-free micronuclei (micronuclei containing only acentric fragments) was determined in peripheral blood lymphocytes of 42 individuals working at the Jáchymov spa and 42 control individuals. RESULTS: There was a highly significant increase in the frequency of micronucleus-containing cells as well as the percentage of centromere-free micronuclei in the lymphocytes of spa personnel versus controls. No individual dosimetry data were available. A comparison with results from currently active uranium miners suggests that the individuals examined at the Jáchymov spa had accumulated effective doses in the order of several tens of mSv, very similar to those of the miners. CONCLUSION: The spa personnel in Jáchymov needs to be monitored on an individual level.

Micronuclei in lymphocytes from currently active uranium miners.

Micronuclei can be used as markers of past radiation exposure, but only few studies have dealt with uranium miners. In this paper, we report on micronuclei in lymphocytes from individuals currently working at Rozná, Czech Republic, the last functioning uranium mine in the European Union. A modified micronucleus-centromere test was applied to assess the occurrence of micronuclei in stimulated lymphocytes, as well as their content in terms of whole chromosomes or fragments. Compared with unexposed individuals, the miners had higher frequencies of micronucleus-containing lymphocytes and higher percentages of micronuclei without centromeres, and the differences were significant for both parameters (0.74 ± 0.60 vs. 0.50 ± 0.42, p = 0.017 and 49 ± 44 vs. 12 ± 21, p = 0.0002; means ± standard deviations). There were also significant correlations between one or other of these parameters on the one hand and various dose values on the other, in particular with a 'retrievable' dose, that is, a dose whose effect should still be recognisable in lymphocytes assuming a half-life of 3 years. The 'retrievable' dose at which a doubling of the micronucleus frequency was observed was around 35 mSv, corresponding to a total dose of 90 mSv received while working in the mines. Altogether, our data show that the micronucleus-centromere test is a valuable tool for the assessment of past radiation exposure in uranium miners. The scatter in the data is of course far too great to allow individual dosimetry, but for groups of a few dozen exposed individuals, the method can be used to monitor doses clearly below 100 mSv.

The use of caffeine to assess high dose exposures to ionising radiation by dicentric analysis.

Dicentric analysis is considered as a 'gold standard' method for biological dosimetry. However, due to the radiation-induced mitotic delay or inability to reach mitosis of heavily damaged cells, the analysis of dicentrics is restricted to doses up to 4-5 Gy. For higher doses, the analysis by premature chromosome condensation technique has been proposed. Here, it is presented a preliminary study is presented in which an alternative method to analyse dicentrics after high dose exposures to ionising radiation (IR) is evaluated. The method is based on the effect of caffeine in preventing the G2/M checkpoint allowing damaged cells to reach mitosis. The results obtained indicate that the co-treatment with Colcemid and caffeine increases significantly increases the mitotic index, and hence allows a more feasible analysis of dicentrics. Moreover in the dose range analysed, from 0 to 15 Gy, the dicentric cell distribution followed the Poisson distribution, and a simulated partial-body exposure has been clearly detected. Overall, the results presented here suggest that caffeine has a great potential to be used for dose-assessment after high dose exposure to IR.

A semi-automated micronucleus-centromere assay to assess low-dose radiation exposure in human lymphocytes.

PURPOSE: The in vitro micronucleus (MN) assay is a reliable method to assess radiation-induced chromosomal damage in human peripheral blood lymphocytes. It is used to evaluate in vivo radiation over-exposure and to assess in vitro chromosomal radiosensitivity. A limitation of the MN assay is the relatively high and variable spontaneous MN frequency that restricts low-dose estimation to doses of about 0.3 gray (Gy). As radiation-induced MN mainly contain acentric fragments and spontaneous MN originate from lagging chromosomes, both MN types can be distinguished from each other by using fluorescence in situ hybridisation (FISH) with a pan-centromeric probe. The aim of this study was to investigate if the sensitivity, reliability and processing time of the MN assay can be enhanced by combining the automated MN assay with pan-centromere scoring. MATERIALS AND METHODS: Blood samples from 10 healthy donors were irradiated in vitro with low doses of gamma-rays. Dose response curves were determined for fully-automated and semi-automated MN scoring and semi-automated scoring of centromere negative MN (MNCM-). RESULTS: A good correlation was obtained between fully-automated and semi-automated MN scoring (r(2) = 0.9973) and between fully automated MN scoring and semi-automated scoring of MNCM- (r(2) = 0.998). With the Wilcoxon test, a significant p value was obtained between 0 and 0.2 Gy for the fully-automated MN analysis, between 0 and 0.1 Gy for semi-automated MN analysis and between 0 and 0.05 Gy for semi-automated scoring of MNCM-. CONCLUSION: The semi-automated micronucleus-centromere assay combines high-speed MN analysis with a more accurate assessment in the low-dose range which makes it of special interest for large-scale radiation applications.

Gene expression following ionising radiation: identification of biomarkers for dose estimation and prediction of individual response.

PURPOSE: To establish a panel of highly radiation responsive genes suitable for biological dosimetry and to explore inter-individual variation in response to ionising radiation exposure. MATERIALS AND METHODS: Analysis of gene expression in response to radiation was carried out using three independent techniques (Microarray, Multiplex Quantitative Real-Time Polymerase Chain Reaction (MQRT- PCR) and nCounter® Analysis System) in human dividing lymphocytes in culture and peripheral blood leukocytes exposed ex vivo from the same donors. RESULTS: Variations in transcriptional response to exposure to ionising radiation analysed by microarray allowed the identification of genes which can be measured accurately using MQRT PCR and another technique allowing direct count of mRNA copies. We have identified genes which are consistently up-regulated following exposure to 2 or 4 Gy of X-rays at different time points, for all individuals in blood and cultured lymphocytes. Down-regulated genes including cyclins, centromeric and mitotic checkpoint genes, particularly those associated with chromosome instability and cancer could be detected in dividing lymphocytes only. CONCLUSIONS: The data provide evidence that there are a number of genes which seem suitable for biological dosimetry using peripheral blood, including sestrin 1 (SESN1), growth arrest and DNA damage inducible 45 alpha (GADD45A), cyclin-dependent kinase inhibitor 1A (CDKN1A), cyclin G1 (CCNG1), ferredoxin reductase (FDXR), p53 up-regulated mediator of apoptosis (BBC3) and Mdm2 p53 binding protein homolog (MDM2). These biomarkers could potentially be used for triage after large-scale radiological incidents and for monitoring radiation exposure during radiotherapy.

The micronucleus assay in radiation accidents.

The cytokinesis-block micronucleus assay in peripheral blood lymphocytes is a standardised and validated technique for biodosimetry. Automated scoring of micronuclei allows large scale applications as in population triage in case of radiation accidents or malevolent use of radioactive sources. The dose detection limit (95% confidence) of the micronucleus assay for individual dose assessment is restricted to 0.2 Gy but can be decreased to 0.1 Gy by scoring centromeres in micronuclei using fluorescence in situ hybridization (FISH). In the past the micronucleus assay was applied for a number of large scale biomonitoring studies of nuclear power plant workers and hospital workers. Baseline micronucleus frequencies depend strongly on age and gender. The assay was also already used for biodosimetry of radiation accidents. In a multiple endpoint biodosimetry study for dose assessment of a worker exposed accidentally in 2003 to X-rays, a good agreement was obtained between dose estimates resulting from the micronucleus assay, the scoring of dicentrics and translocations. Automated scoring of micronuclei in combination with centromere signals, allowing systematic biodosimetry of exposed populations, remains a challenge for the future.

Micronucleus-centromere assay and DNA repair gene polymorphism in lymphocytes of industrial radiographers.

The micronucleus-centromere assay using a pan-centromeric probe was used to assess chromosomal damage in lymphocytes of 47 industrial radiographers occupationally exposed to low dose ionizing radiation and 47 controls. The influence of genotype of DNA repair genes (XRCC1(399), XRCC3(241) and XPD(751)) on micronuclei (MN) frequency was also investigated. Centromere negative micronuclei (MNC-) frequency was significantly higher in radiographers than in controls, whereas similar centromere positive micronuclei (MNC+) frequency was observed in both groups. Poisson regression analyses revealed that the MNC- frequency was significantly associated with radiation occupational exposure and with cumulative-radiation doses in radiographers, after adjusting for confounding variables such as age, smoking, alcohol intake and genotypes. Compared to homozygous wild-type subjects, MNC- frequency in radiographers with variant XRCC3 genotype was significantly higher using univariate analysis. There were no differences in MNC- or MNC+ frequencies by genotype in controls. In conclusion, scoring of MNC- is a useful cytogenetic biomonitoring method for radiographers. Polymorphisms in XRCC3 might contribute to the increased genetic damage in individuals occupationally exposed to chronic ionizing radiation.

Chromosome shattering: a mitotic catastrophe due to chromosome condensation failure.

Chromosome shattering has been described as a special form of mitotic catastrophe, which occurs in cells with unrepaired DNA damage. The shattered chromosome phenotype was detected after application of a methanol/acetic acid (MAA) fixation protocol routinely used for the preparation of metaphase spreads. The corresponding phenotype in the living cell and the mechanism leading to this mitotic catastrophe have remained speculative so far. In the present study, we used V79 Chinese hamster cells, stably transfected with histone H2BmRFP for live-cell observations, and induced generalized chromosome shattering (GCS) by the synergistic effect of UV irradiation and caffeine posttreatment. We demonstrate that GCS can be derived from abnormal mitotic cells with a parachute-like chromatin configuration (PALCC) consisting of a bulky chromatin mass and extended chromatin fibers that tether centromeres at a remote, yet normally shaped spindle apparatus. This result hints at a chromosome condensation failure, yielding a "shattered" chromosome complement after MAA fixation. Live mitotic cells with PALCCs proceeded to interphase within a period similar to normal mitotic cells but did not divide. Instead they formed cells with highly abnormal nuclear configurations subject to apoptosis after several hours. We propose a factor depletion model where a limited pool of proteins is involved both in DNA repair and chromatin condensation. Chromosome condensation failure occurs when this pool becomes depleted.

Tetrahymena meiotic nuclear reorganization is induced by a checkpoint kinase-dependent response to DNA damage.

In the ciliate Tetrahymena, meiotic micronuclei (MICs) undergo extreme elongation, and meiotic pairing and recombination take place within these elongated nuclei (the "crescents"). We have previously shown that elongation does not occur in the absence of Spo11p-induced DNA double-strand breaks (DSBs). Here we show that elongation is restored in spo11Delta mutants by various DNA-damaging agents including ones that may not cause DSBs to a notable extent. MIC elongation following Spo11p-induced DSBs or artificially induced DNA lesions is probably a DNA-damage response mediated by a phosphokinase signal transduction pathway, since it is suppressed by the ATM/ATR kinase inhibitors caffeine and wortmannin and by knocking out Tetrahymena's ATR orthologue. MIC elongation occurs concomitantly with the movement of centromeres away from the telomeric pole of the MIC. This DNA damage-dependent reorganization of the MIC helps to arrange homologous chromosomes alongside each other but is not sufficient for exact pairing. Thus, Spo11p contributes to bivalent formation in two ways: by creating a favorable spatial disposition of homologues and by stabilizing pairing by crossovers. The polarized chromosome orientation inside the crescent resembles the conserved meiotic bouquet, and crescent and bouquet also share the putative function of aiding meiotic pairing. However, they are regulated differently because in Tetrahymena, DSBs are required for entering rather than exiting this stage.