A basic question of cell biology is how DNA folds to chromosome. A number of recently accumulated evidences have suggested that folding of chromosome proceeds tightly coupled with DNA replication progresses. Drug-induced PCC is a useful tool for visualization of the interphase nuclei, in particular, S-phase, as S-phase prematurely condensed chromosomes (S-phase PCC). Active replicating DNA is labeled directly with Cy3-dUTP by bead loading method, and then S-phase nuclei is immediately condensed prematurely by calyculin A to obtain S-phase PCC. Active replicating regions on S-PCC are observed under a scanning confocal microscope. Cy3-dUTP-labeled S-phase PCCs clearly reveal the drastic transitional change of chromosome formation through S-phase, starting from a \"cloudy nebula\" to numerous numbers of \"beads on a string\" and finally to \"striped arrays of banding structured chromosome\" known as G- or R-banding pattern. The number, distribution, and shape of replication foci were also measured in individual subphase of S-phase; maximally ~1400 foci of 0.35 μm average radius size were scored at the beginning of S-phase, and the number is reduced to ~100 at the end of S-phase. Drug-induced PCC clearly provided the new insight that eukaryote DNA replication is tightly coupled with the chromosome condensation/compaction for construction of eukaryote higher-ordered chromosome structure.

Chromosome analysis is one of most fundamental techniques for cytogenetic studies. Chromosomes are conventionally prepared from mitotic cells arrested by colcemid block protocol. Premature chromosome condensation (PCC) technique is an alternative to obtain chromosomes. It was more than half century ago that the first observation of PCC phenomena reported. Since then, cell-fusion-mediated PCC method has been developed and introduced in many fields of chromosome analysis. More than quarter century ago, novel PCC technique using chemical drug has been developed. Afterwards, this simple and efficient drug-induced PCC technique becomes a standard protocol for preparing chromosomes. Thus, it seems to be the good time to introduce PCC technique protocol for the artisans in the field of cytogenetic studies.

We recently reported that when low doses of ionizing radiation induce low numbers of DNA double-strand breaks (DSBs) in G2-phase cells, about 50 % of them are repaired by homologous recombination (HR) and the remaining by classical non-homologous end-joining (c-NHEJ). However, with increasing DSB-load, the contribution of HR drops to undetectable (at ∼10 Gy) as c-NHEJ dominates. It remains unknown whether the approximately equal shunting of DSBs between HR and c-NHEJ at low radiation doses and the predominant shunting to c-NHEJ at high doses, applies to every DSB, or whether the individual characteristics of each DSB generate processing preferences. When G2-phase cells are irradiated, only about 10 % of the induced DSBs break the chromatids. This breakage allows analysis of the processing of this specific subset of DSBs using cytogenetic methods. Notably, at low radiation doses, these DSBs are almost exclusively processed by HR, suggesting that chromatin characteristics awaiting characterization underpin chromatid breakage and determine the preferential engagement of HR. Strikingly, we also discovered that with increasing radiation dose, a pathway switch to c-NHEJ occurs in the processing of this subset of DSBs. Here, we confirm and substantially extend our initial observations using additional methodologies. Wild-type cells, as well as HR and c-NHEJ mutants, are exposed to a broad spectrum of radiation doses and their response analyzed specifically in G2 phase. Our results further consolidate the observation that at doses <2 Gy, HR is the main option in the processing of the subset of DSBs generating chromatid breaks and that a pathway switch at doses between 4-6 Gy allows the progressive engagement of c-NHEJ. PARP1 inhibition, irrespective of radiation dose, leaves chromatid break repair unaffected suggesting that the contribution of alternative end-joining is undetectable under these experimental conditions.

For precision cancer radiotherapy, high linear energy transfer (LET) particle irradiation offers a substantial advantage over photon-based irradiation. In contrast to the sparse deposition of low-density energy by χ- or γ-rays, particle irradiation causes focal DNA damage through high-density energy deposition along the particle tracks. This is characterized by the formation of multiple damage sites, comprising localized clustered patterns of DNA single- and double-strand breaks as well as base damage. These clustered DNA lesions are key determinants of the enhanced relative biological effectiveness (RBE) of energetic nuclei. However, the search for a fingerprint of particle exposure remains open, while the mechanisms underlying the induction of chromothripsis-like chromosomal rearrangements by high-LET radiation (resembling chromothripsis in tumors) await to be elucidated. In this work, we investigate the transformation of clustered DNA lesions into chromosome fragmentation, as indicated by the induction and post-irradiation repair of chromosomal damage under the dynamics of premature chromosome condensation in G0 human lymphocytes. Specifically, this study provides, for the first time, experimental evidence that particle irradiation induces localized shattering of targeted chromosome domains. Yields of chromosome fragments and shattered domains are compared with those generated by γ-rays; and the RBE values obtained are up to 28.6 for α-particles (92 keV/μm), 10.5 for C-ions (295 keV/μm), and 4.9 for protons (28.5 keV/μm). Furthermore, we test the hypothesis that particle radiation-induced persistent clustered DNA lesions and chromatin decompaction at damage sites evolve into localized chromosome shattering by subsequent chromatin condensation in a single catastrophic event-posing a critical risk for random rejoining, chromothripsis, and carcinogenesis. Consistent with this hypothesis, our results highlight the potential use of shattered chromosome domains as a fingerprint of high-LET exposure, while conforming to the new model we propose for the mechanistic origin of chromothripsis-like rearrangements.

The discovery of chromothripsis in cancer genomes challenges the long-standing concept of carcinogenesis as the result of progressive genetic events. Despite recent advances in describing chromothripsis, its mechanistic origin remains elusive. The prevailing conception is that it arises from a massive accumulation of fragmented DNA inside micronuclei (MN), whose defective nuclear envelope ruptures or leads to aberrant DNA replication, before main nuclei enter mitosis. An alternative hypothesis is that the premature chromosome condensation (PCC) dynamics in asynchronous micronucleated cells underlie chromosome shattering in a single catastrophic event, a hallmark of chromothripsis. Specifically, when main nuclei enter mitosis, premature chromatin condensation provokes the shattering of chromosomes entrapped inside MN, if they are still undergoing DNA replication. To test this hypothesis, the agent RO-3306, a selective ATP-competitive inhibitor of CDK1 that promotes cell cycle arrest at the G2/M boundary, was used in this study to control the degree of cell cycle asynchrony between main nuclei and MN. By delaying the entrance of main nuclei into mitosis, additional time was allowed for the completion of DNA replication and duplication of chromosomes inside MN. We performed interphase cytogenetic analysis using asynchronous micronucleated cells generated by exposure of human lymphocytes to γ-rays, and heterophasic multinucleated Chinese hamster ovary (CHO) cells generated by cell fusion procedures. Our results demonstrate that the PCC dynamics during asynchronous mitosis in micronucleated or multinucleated cells are an important determinant of chromosome shattering and may underlie the mechanistic origin of chromothripsis.

Chromosome analysis is a fundamental technique for a wide range of cytogenetic studies. Chromosome aberrations are easily introduced by many kinds of clastogenic agents such as ionizing irradiation, UV, or alkylating agents, and damaged chromosomes may be prone to cancer. Chromosomes are conventionally prepared from mitotic cells arrested by the colcemid block method. However, obtaining of mitotic chromosomes is sometimes hampered under several circumstances, for example after high-dose (over several Gys of γ-rays) ionizing irradiation exposure accident. As a result, cytogenetic analysis will be often difficult or even impossible in such cases. Premature chromosome condensation (PCC) is an alternative technique that has proved to be a unique and useful way in chromosome analysis. Previously, PCC has been achieved following cell fusion mediated either by fusogenic viruses (for example Sendai virus) or by polyethylene glycol (PEG) (cell-fusion PCC), but the cell-fusion PCC has several drawbacks. The novel drug-induced PCC use of specific inhibitors for serine/threonine protein phosphatase was introduced about 20 years ago. This method is much simple and easy even than the conventional mitotic chromosome preparation using colcemid block protocol and the obtained PCC index (equivalent to mitotic index for metaphase chromosome) is much higher. Furthermore, this method allows the interphase chromatin to be condensed and visualized like mitotic chromosomes, and thus has been opening the way for chromosome analysis not only in metaphase chromosomes but also in interphase chromatin. The drug-induced PCC has therefore proven the usefulness in cytogenetics and other many cell biology fields. Since the first version of drug-induced PCC protocol has been published in 2009 (Gotoh, Methods in molecular biology. Humana Press, New York, 2009), many newer applications of drug-induced PCC in radiation biology and chromosome science fields in a wide range of species from animal to plant have been reported (Gotoh et al., Biomed Res 16:63-68, 1995; Lamadrid Boada et al., Mutat Res 757:45-51, 2013; Ravi et al., Biochimie 95:124-33, 2013; Ono et al., J Cell Biol 200:429-41, 2013; Vagnarelli, Exp Cell Res 318:1435-41, 2012; Roukos et al., Nat Protoc 9:2476-92, 2014; Miura and Blakely, Cytometry A 79:1016-22, 2013; Zabka et al., J Plant Physiol 174:62-70, 2015; Samaniego et al., Planta 215:195-204, 2002; Rybaczek et al., Folia Histochem Cytobiol 40:51-9, 2002; Gotoh and Durante J Cell Physiol 209:297-304, 2006). Therefore as a new edition, I will write in this chapter the drug-induced PCC technique with newer findings, in particular focused drug-induced PCC protocols in radiation biology with referring updated articles published recently.

Premature chromosome condensation (PCC) is a sensitive and unique way to detect interphase chromosome damage and its recovery in mammalian cells irradiated with ionizing radiation. In this chapter, we describe G1 PCC assay with which one can measure immediate chromosome breaks in G1 type chromosomes and their repair/rejoining. In order to induce G1 PCC, one needs to fuse mitotic cells with G1 cells to be tested. There are two methods to fuse cells; one is to use Sendai virus or its equivalent, and another method needs polyethylene glycol (PEG) as a fusing agent. The date obtained with PCC assay can bridge the gap between radiation-induced DNA damage (mainly double strand breaks) and chromosome aberrations observable at metaphase stage.

Purpose: This study aimed to construct a calibration curve for high-dose exposure using cell fusion-induced premature chromosome condensation (PCC). Some of the associated practicalities and methodological details were also investigated. Materials and methods: Peripheral blood from two donors was used. PCC mediated by fusing mitotic CHO cells with interphase lymphocytes was carried out. Lymphocytes were irradiated with 60Co (0-20 Gy) and held at 37 °C for 24 h post exposure. Results: The protocol for PCC induction was effective at all doses and the number of rings increased with increasing dose. No significant difference was found between the donors (p = .896) and data were pooled. Ring aberration frequencies followed a Poisson distribution and the dose-response relationship favored a linear fitting: Y = 0.0007(±0.0004)+0.0186(±0.001)×D. Blind tests showed that the estimated doses were all within the 95% confidence limits of the delivered doses. This study has shown that it is valid to score only 100 cells per sample in a triage mode for doses above 5 Gy and that it is valid to score only hollow rings to reduce the scoring time. Conclusion: Scoring rings in cell fusion-induced PCC assay can be a feasible and fast approach for the analysis of high-dose exposures.

In radiation accidents and large-scale radiological emergencies, a fast and reliable triage of individuals according to their degree of exposure is important for accident management and identification of those who need medical assistance. In this work, the applicability of cell-fusion-mediated premature chromosome condensation (PCC) in G0-lymphocytes is examined for the development of a rapid, minimally invasive and automatable micro-PCC assay, which requires blood volumes of only 100 μl and can be performed in 96-well plates, towards risk assessments and categorization of individuals based on dose estimates. Chromosomal aberrations are visualized for dose-estimation analysis within two hours, without the need of blood culturing for two days, as required by conventional cytogenetics. The various steps of the standard-PCC procedure were adapted and, for the first time, lymphocytes in blood volumes of 100 μl were successfully fused with CHO-mitotics in 96-well plates of 2 ml/well. The plates are advantageous for high-throughput analysis since the various steps required are applied to all 96-wells simultaneously. Interestingly, the use of only 1.5 ml hypotonic and Carnoy\'s fixative per well offers high quality PCC-images, and the morphology of lymphocyte PCCs is identical to that obtained using the conventional PCC-assay, which requires much larger blood volumes and 15 ml tubes. For dose assessments, appropriate calibration curves were constructed and for PCC analysis specialized software (MetaSystems) was used. The micro-PCC assay can be combined with fluorescence in situ hybridization (FISH), using simultaneously centromeric/telomeric (C/T) peptide nucleic acid (PNA) probes. This allows dose assessments on the basis of accurate scoring of dicentric and centric ring chromosomes in G0-lymphocyte PCCs, which is particularly helpful when further evaluation into treatment-level categories of exposed individuals is needed. The micro-PCC assay has significant advantages for early triage biodosimetry when compared to other cytogenetic biodosimetry assays. It is rapid, cost-effective, and could pave the way to its subsequent automation.

We have studied the induction of chromosome aberrations in human fibroblasts exposed in G0/G1 to X-rays or heavy ions to study the influence of G1 cell cycle arrest. Confluent normal fibroblasts were exposed to X-rays or accelerated particles with different LET values and chromosome aberrations were investigated in the first G0/G1 and G2//M phase. The particles used here were 490MeV/nucleon Si, 500MeV/nucleon Fe, and 200MeV/nucleon Fe ions. Cells were subcultured 24h after exposure and premature chromosome condensation (PCC) was performed by fusion-induced method for analysis of G0/G1 cells, and chemically-induced method for analysis of G2 and metaphase cells. Chromosome damage was assessed in chromosomes 1 and 3 using whole chromosome fluorescence in situ hybridization (FISH). Cell cycle was analyzed by flow cytometry at different incubation times following subculture. After irradiation with 2Gy of high-LET particles, the yields of chromosome aberrations and fragments were significantly higher in G0/G1 phase than in G2/M phase, whereas similar yields of damage were measured in both phases after exposure to X-rays. In contrast, the yield of misrepair, assessed by the number of color junctions, was similar in the G0/G1 and G2/M phases after exposure to either X-rays or high-LET particles. The yields of chromosome aberrations, fragments, and color junctions in both the G0/G1 and the G2/M phases, increased with LET up to 200keV/μm, then decreased for 440keV/μm Fe particles. A good correlation was found between chromosome aberrations in both G0/G1 and G2/M cells and survival fractions after 2Gy of different LET radiations, although the slopes were steeper for the G0/G1 cells. Flow cytometry analysis indicated that high-LET particles induce more non cycling G0/G1 cells within 48h of subculture than X-rays, suggesting that chromosome aberrations scored at the G2/M phase may not accurately describe the true radiation effect.