The lysine-specific methyltransferase KMT2C/MLL3 regulates DNA repair components in cancer.

Genome-wide studies in tumor cells have indicated that chromatin-modifying proteins are commonly mutated in human cancers. The lysine-specific methyltransferase 2C (KMT2C/MLL3) is a putative tumor suppressor in several epithelia and in myeloid cells. Here, we show that downregulation of KMT2C in bladder cancer cells leads to extensive changes in the epigenetic status and the expression of DNA damage response and DNA repair genes. More specifically, cells with low KMT2C activity are deficient in homologous recombination-mediated double-strand break DNA repair. Consequently, these cells suffer from substantially higher endogenous DNA damage and genomic instability. Finally, these cells seem to rely heavily on PARP1/2 for DNA repair, and treatment with the PARP1/2 inhibitor olaparib leads to synthetic lethality, suggesting that cancer cells with low KMT2C expression are attractive targets for therapies with PARP1/2 inhibitors.

Requirement for Parp-1 and DNA ligases 1 or 3 but not of Xrcc1 in chromosomal translocation formation by backup end joining.

In mammalian cells, ionizing radiation (IR)-induced DNA double-strand breaks (DSBs) are repaired in all phases of the cell cycle predominantly by classical, DNA-PK-dependent nonhomologous end joining (D-NHEJ). Homologous recombination repair (HRR) is functional during the S- and G2-phases, when a sister chromatid becomes available. An error-prone, alternative form of end joining, operating as backup (B-NHEJ) functions robustly throughout the cell cycle and particularly in the G2-phase and is thought to backup predominantly D-NHEJ. Parp-1, DNA-ligases 1 (Lig1) and 3 (Lig3), and Xrcc1 are implicated in B-NHEJ. Chromosome and chromatid translocations are manifestations of erroneous DSB repair and are crucial culprits in malignant transformation and IR-induced cell lethality. We analyzed shifts in translocation formation deriving from defects in D-NHEJ or HRR in cells irradiated in the G2-phase and identify B-NHEJ as the main DSB repair pathway backing up both of these defects at the cost of a large increase in translocation formation. Our results identify Parp-1 and Lig1 and 3 as factors involved in translocation formation and show that Xrcc1 reinforces the function of Lig3 in the process without being required for it. Finally, we demonstrate intriguing connections between B-NHEJ and DNA end resection in translocation formation and show that, as for D-NHEJ and HRR, the function of B-NHEJ facilitates the recovery from the G2-checkpoint. These observations advance our understanding of chromosome aberration formation and have implications for the mechanism of action of Parp inhibitors.

UGT1A1*28 polymorphism in chronic lymphocytic leukemia: the first investigation of the polymorphism in disease susceptibility and its specific cytogenetic abnormalities.

Chronic lymphocytic leukemia (CLL) has been recently attributed to a combination of genetic predisposition and exposure to environmental factors. UDP-glucuronosyltransferase (UGT)1A1*28 is an inborn polymorphism that results in significant downregulation of uridine diphosphate glucuronyltransferase 1-1 (UGT1A1) activity, one of the most critical metabolizing enzymes involved in the detoxification of toxic substances, some of which contribute to CLL pathogenesis. Here, for the first time, we investigated the putative impact of UGT1A1*28 on CLL incidence and on the formation of the most common chromosomal abnormalities of CLL. UGT1A1*28 was investigated in 109 CLL patients and 108 healthy controls, and was associated with karyotypic and fluorescence in situ hybridization (FISH) results. A significant high frequency of the mutant genotype was observed in patients carrying abnormal FISH patterns, especially del(11q) and +12, which are CLL-specific abnormalities. We also observed a significant association between UGT1A1*28 and the intermediate to unfavorable cytogenetic CLL risk groups. No difference, though, was observed in genotypes between patients and controls. Therefore, we could suggest that UGT-deficient individuals may be at a greater risk for developing CLL-specific abnormalities. Our study might serve as a starting point to consider UGT1A1*28 polymorphism as one of the possible predisposing factors of CLL pathogenesis.

The radiosensitizing potential of glutaraldehyde on MCF7 breast cancer cells as quantified by means of the G2-chromosomal radiosensitivity assay.

Glutaraldehyde (GA) is a high production volume chemical that is very reactive with a wide spectrum of medical, scientific and industrial applications. Concerning the genotoxic and carcinogenic effect of GA, controversial results have been reported, while in humans no studies with positive carcinogenic results for GA have been published. However, our previous study concerning the combined effects of exposure to both GA and ionising radiation (IR) in peripheral blood lymphocytes of healthy donors has shown that non-genotoxic doses of the chemical induces a statistically significant increase in chromosomal radiosensitivity. The lack of information concerning the radiosensitizing potential of GA on cancerous cells triggered us to test the radiosensitizing effect of GA on breast cancer cells (MCF7). For this purpose the G2-chromosomal radiosensitivity assay (G2-assay) was used. The assay involves G2-phase irradiation and quantitation of the chromosomal fragility in the subsequent metaphase. The experimental data show that 48 h exposure to GA, at doses that are not clastogenic to MCF7 breast cancer cells enhances G2-chromosomal radiosensitivity of this cell line. In an effort to evaluate whether the observed increase in GAs-induced G2-chromosomal radiosensitization is linked to GA-induced alterations in the cell cycle and feedback control mechanism, Mitotic Index analysis was performed. The results have shown that such a mechanism cannot be directly related to the observed GA-induced increase in G2-chromosomal radiosensitivity. Since increased G2-chromosomal radiosensitivity has been linked with cancer proneness, the radiosensitizing effect of GA at non-clastogenic doses highlights its potential carcinogenic profile.

Chromatin dynamics during cell cycle mediate conversion of DNA damage into chromatid breaks and affect formation of chromosomal aberrations: biological and clinical significance.

The formation of diverse chromosomal aberrations following irradiation and the variability in radiosensitivity at different cell-cycle stages remain a long standing controversy, probably because most of the studies have focused on elucidating the enzymatic mechanisms involved using simple DNA substrates. Yet, recognition, processing and repair of DNA damage occur within the nucleoprotein complex of chromatin which is dynamic in nature, capable of rapid unfolding, disassembling, assembling and refolding. The present work reviews experimental work designed to investigate the impact of chromatin dynamics and chromosome conformation changes during cell-cycle in the formation of chromosomal aberrations. Using conventional cytogenetics and premature chromosome condensation to visualize interphase chromatin, the data presented support the hypothesis that chromatin dynamic changes during cell-cycle are important determinants in the conversion of sub-microscopic DNA lesions into chromatid breaks. Consequently, the type and yield of radiation-induced chromosomal aberrations at a given cell-cycle-stage depends on the combined effect of DNA repair processes and chromatin dynamics, which is cell-cycle-regulated and subject to up- or down-regulation following radiation exposure or genetic alterations. This new hypothesis is used to explain the variability in radiosensitivity observed at various cell-cycle-stages, among mutant cells and cells of different origin, or among different individuals, and to revisit unresolved issues and unanswered questions. In addition, it is used to better understand hypersensitivity of AT cells and to provide an improved predictive G2-assay for evaluating radiosensitivity at individual level. Finally, experimental data at single cell level obtained using hybrid cells suggest that the proposed hypothesis applies only to the irradiated component of the hybrid.

G2/M Checkpoint Abrogation With Selective Inhibitors Results in Increased Chromatid Breaks and Radiosensitization of 82-6 hTERT and RPE Human Cells.

While technological advances in radiation oncology have led to a more precise delivery of radiation dose and a decreased risk of side effects, there is still a need to better understand the mechanisms underlying DNA damage response (DDR) at the DNA and cytogenetic levels, and to overcome tumor resistance. To maintain genomic stability, cells have developed sophisticated signaling pathways enabling cell cycle arrest to facilitate DNA repair via the DDR-related kinases and their downstream targets, so that DNA damage or DNA replication stress induced by genotoxic therapies can be resolved. ATM, ATR, and Chk1 kinases are key mediators in DDR activation and crucial factors in treatment resistance. It is of importance, therefore, as an alternative to the conventional clonogenic assay, to establish a cytogenetic assay enabling reliable and time-efficient results in evaluating the potency of DDR inhibitors for radiosensitization. Toward this goal, the present study aims at the development and optimization of a chromosomal radiosensitivity assay using the DDR and G2-checkpoint inhibitors as a novel modification compared to the classical G2-assay. Also, it aims at investigating the strengths of this assay for rapid radiosensitivity assessments in cultured cells, and potentially, in tumor cells obtained from biopsies. Specifically, exponentially growing RPE and 82-6 hTERT human cells are irradiated during the G2/M-phase transition in the presence or absence of Caffeine, VE-821, and UCN-1 inhibitors of ATM/ATR, ATR, and Chk1, respectively, and the induced chromatid breaks are used to evaluate cell radiosensitivity and their potency for radiosensitization. The increased yield of chromatid breaks in the presence of DDR inhibitors, which underpins radiosensitization, is similar to that observed in cells from highly radiosensitive AT-patients, and is considered here as 100% radiosensitive internal control. The results highlight the potential of our modified G2-assay using VE-821 to evaluate cell radiosensitivity, the efficacy of DDR inhibitors in radiosensitization, and reinforce the concept that ATM, ATR, and Chk1 represent attractive anticancer drug targets in radiation oncology.

Functional cell-cycle chromatin conformation changes in the presence of DNA damage result into chromatid breaks: a new insight in the formation of radiation-induced chromosomal aberrations based on the direct observation of interphase chromatin.

Experiments were carried out to explore the correlation between chromatin conformation changes in the presence of DNA lesions and the formation of radiation-induced chromosomal aberrations. To modulate the onset and dynamics of chromatin conformation changes following irradiation, premature chromosome condensation (PCC) was induced by means of cell fusion. G2-check point abrogation by caffeine or elevated heat treatment was also applied. In addition, transfer of irradiated mitotic cells was employed either into depleted media to restrain them from proceeding through G1/S, or holding them further in colcemid to avoid M/G1 transition. To investigate the correlation between efficiency of chromosomal conformation changes and chromosomal breakage in irradiated G0 peripheral blood lymphocytes, cell fusion with different mitotic PCC-inducer cells was used. The experimental evidence supports the hypothesis that functional cell-cycle chromatin conformation changes in the presence of DNA damage are important determinants in the formation of radiation-induced chromosomal aberrations. Specifically, it is proposed here that following irradiation, chromatin structure may not be broken but instead it unfolds to a conformation that is more accessible to repair enzymes at the sites of DNA lesions. If subsequent chromosomal conformation changes occur while DNA is still being repaired, such changes will lead into an energetically unfavorable state, thus exerting mechanical stress on the unfolded chromatin at the damaged sites, which will in turn result into chromatid breaks that may not be able to restitute or mis-rejoin. Therefore, this biophysical conversion process of DNA damage into chromatid breaks as such is antagonistic to the DNA repair process. Alternatively, in the absence of chromosomal conformation changes, either DNA repair will take place efficiently or DNA misrepair will cause the formation of exchanges and chromosomal rearrangements. Consequently, the type and yield of radiation-induced chromosomal aberrations at a given cell cycle stage will be the combined effect of the interaction, at that particular stage, of the DNA repair process and the proposed conversion process of DNA lesions into chromatid breaks.

Chromosome breaks generated by low doses of ionizing radiation in G2-phase are processed exclusively by gene conversion.

Four repair pathways process DNA double-strand breaks (DSBs). Among these pathways the homologous recombination repair (HRR) subpathway of gene conversion (GC) affords error-free processing, but functions only in S- and G2-phases of the cell cycle. Classical non-homologous end-joining (c-NHEJ) operates throughout the cell cycle, but causes small deletions and translocations. Similar deficiencies in exaggerated form, combined with reduced efficiency, are associated with alternative end-joining (alt-EJ). Finally, single-strand annealing (SSA) causes large deletions and possibly translocations. Thus, processing of a DSB by any pathway, except GC, poses significant risks to the genome, making the mechanisms navigating pathway-engagement critical to genome stability. Logically, the cell ought to attempt engagement of the pathway ensuring preservation of the genome, while accommodating necessities generated by the types of DSBs induced. Thereby, inception of DNA end-resection will be key determinant for GC, SSA and alt-EJ engagement. We reported that during G2-phase, where all pathways are active, GC engages in the processing of almost 50 % of DSBs, at low DSB-loads in the genome, and that this contribution rapidly drops to nearly zero with increasing DSB-loads. At the transition between these two extremes, SSA and alt-EJ compensate, but at extremely high DSB-loads resection-dependent pathways are suppressed and c-NHEJ remains mainly active. We inquired whether in this processing framework all DSBs have similar fates. Here, we analyze in G2-phase the processing of a subset of DSBs defined by their ability to break chromosomes. Our results reveal an absolute requirement for GC in the processing of chromatid breaks at doses in the range of 1 Gy. Defects in c-NHEJ delay significantly the inception of processing by GC, but leave processing kinetics unchanged. These results delineate the essential role of GC in chromatid break repair before mitosis and classify DSBs that underpin this breakage as the exclusive substrate of GC.

Interphase Cytogenetic Analysis of G0 Lymphocytes Exposed to α-Particles, C-Ions, and Protons Reveals their Enhanced Effectiveness for Localized Chromosome Shattering-A Critical Risk for Chromothripsis.

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.

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.

Premature chromosome condensation reveals DNA-PK independent pathways of chromosome break repair.

Cells of higher eukaryotes process double strand breaks (DSBs) in their genome using a non-homologous end joining apparatus that utilizes DNA-PK and other well characterized factors (D-NHEJ). Cells with defects in D-NHEJ, repair the majority of DSBs using a slow-repair pathway which is independent of genes of the RAD52 epistasis group and functions as a backup (B-NHEJ). Recent studies implicate DNA ligase III, PARP-1 and histone H1 in this pathway of NHEJ. The present study investigates the operation of B-NHEJ in the repair of interphase chromosome breaks visualized in irradiated G0 human lymphocytes by premature chromosome condensation (PCC). Chromosome breaks are effectively repaired in human lymphocytes, but repair is significantly compromised after treatment with wortmannin, a DNA-PK inhibitor. Despite slower kinetics, cells exposed to wortmannin rejoin the majority of IR induced chromosome breaks suggesting that B-NHEJ is also functional at the chromosome level. Complementation of D-NHEJ defect in wortmannin-treated lymphocytes by newly made DNA-PK is only possible under conditions of nuclear envelope break down and premature chromosome condensation, suggesting that in interphase cells the shunting of chromosome breaks from D-NHEJ to B-NHEJ is irreversible. The understanding of chromosomal aberration formation allows mechanistic explanations for the carcinogenic potential of D-NHEJ defects.

Pre-irradiation exposure of peripheral blood lymphocytes to glutaraldehyde induces radiosensitization by increasing the initial yield of radiation-induced chromosomal aberrations.

Glutaraldehyde (GA) is a high production volume chemical that is very reactive with a wide spectrum of medical, scientific and industrial applications. Since human exposure in anthropogenic and occupational environment occurs frequently, GA has been extensively tested for genotoxic activity in vitro and in vivo. However, there are conflicting results in the literature and there is a lack of information concerning the combined effects of exposure to both GA and ionizing radiation in human cells. In the present study, the results obtained using conventional cytogenetic analysis do not suggest a statistically significant clastogenic or genotoxic activity of GA when concentrations in the range of 10(-6) to 10(-2) mM were applied. However, a 24-h pre-irradiation exposure of human peripheral blood lymphocytes (PBLs) to non-genotoxic doses of GA showed a statistically significant (P > 0.05) increase in chromosomal radiosensitivity. The observed increase may be an effect of GA-induced alterations in the cell-cycle and feedback control mechanisms during the cell-cycle transition points or it may be a consequence of an effect of GA either on the DNA repair capacity of the cells after irradiation or on the initial induction of radiation-induced chromosomal damage. To elucidate the mechanism underlying the obtained radiosensitization, conventional cytogenetics, the G2 chromosomal radiosensitivity assay and premature chromosome condensation methodologies were applied. The results support the hypothesis that pre-irradiation exposure of PBLs to GA induces radiosensitization by increasing the initial yield of chromosomal aberrations following irradiation.

Disruption of the ETV6 gene as a consequence of a rare translocation (12;12)(p13;q13) in treatment-induced acute myeloid leukemia after breast cancer.

We describe a case of treatment-induced acute myeloid leukemia M2 after breast cancer with a rare reciprocal t(12;12)(p13;q13) as a secondary cytogenetic abnormality in addition to the t(11;19)(q23;p13.1). Fluorescence in situ hybridization analysis revealed that both ETV6 genes (previously TEL) were located on the same der(12)t(12;12) as a result of t(12;12). Interestingly, the translocated ETV6 gene was disrupted, indicating the breakpoint on the large der(12)t(12;12) to be within the ETV6 gene and thus the possible formation of a new fusion gene. CHOP gene at 12q13, was found to be translocated intact to the other homologue chromosome 12, indicating that the breakpoint on the small der(12) is proximal to CHOP. To the best of our knowledge, our patient represents the first report of the rare t(12;12)(p13;q13) described in treatment-induced leukemia and the possible formation of a new fusion gene.

5'RARA submicroscopic deletion from new variant translocation involving chromosomes 15, 17, and 18, in a case of acute promyelocytic leukemia.

Submicroscopic deletions of the PML-RARA fusion genes constitute rare rearrangements in acute promyelocytic leukemia (APL). We describe a rare case of APL carrying a novel complex translocation involving chromosomes 15, 17, and 18 associated with a submicroscopic deletion of the 5' part of the RARA gene, as evidenced by fluorescence in situ hybridization (FISH). A PML/RARA dual-fusion probe did not reveal the RARA-PML fusion signal on the der(17q), usually detected in the typical t(15;17). The RARA break-apart probe showed a deletion hybridization pattern with loss of the signal corresponding to the 5' portion of the RARA gene. Reverse transcriptase-polymerase chain reaction confirmed the absence of the fusion RARA-PML transcript. The patient achieved complete remission, but died during consolidation therapy, 2 months after diagnosis. To our knowledge, this is the first reported case of APL with a complex variant t(15;17) involving chromosome 18 at band q12 and one of the very rare described cases displaying a submicroscopic deletion of the RARA 5' region. Further cases are needed to delineate the incidence of submicroscopic deletions in APL and elucidate their prognostic impact.

Switch in X-inactivation in a JAK2 V617F-negative case of polycythemia vera with two acquired X-autosome translocations.

We report a JAK2 V617F-negative case of polycythemia vera with two acquired balanced X-autosome translocations and no history of previous exposure to chemo/radiotherapy. The patient's first clone carried a novel translocation t(X;15)(q24;q13) as a sole abnormality. The second clone exhibited an additional translocation, t(X;20)(q13;q13.3), which is a rare recurrent abnormality in myeloid malignancies. This is the first report of a hematological disorder with both X chromosomes being translocated. Late replication studies revealed a switch in X-inactivation from the X chromosome involved in t(X;15) (first clone) to the X chromosome involved in the t(X;20)(q13;q13.3) (second clone). The inactivation of the translocated X chromosomes could provide potential for the inactivation of the adjacent autosomal regions, resulting in epigenetic gene silencing.

The benzene metabolite hydroquinone enhances G2-chromosomal radiosensitivity by inducing a less-efficient G2-M-checkpoint in irradiated lymphocytes.

The hypothesis tested is that a 24-h pre-irradiation-exposure of peripheral blood lymphocytes (PBL) to the benzene metabolite hydroquinone (HQ), at doses that are non-acutely toxic (5 microM), induces a less efficient G2-M-checkpoint and enhances the G2-chromosomal radiosensitivity in a statistically significant manner (p<0.01). A less efficient G2-M-checkpoint may allow the transition of damaged cells from G2- to M-phase and experimental data in the present work support this hypothesis. In fact HQ sensitizes lymphocytes obtained from healthy donors, as they exhibit increased G2-chromosomal radiosensitivity which interestingly is similar to that observed in cases of radiosensitive cancer-prone individuals. This finding is important since a deficiency in cell cycle checkpoints and an increase in G2-chromosomal radiosensitivity are linked to chromosomal instability, cancer proneness and the development of leukemia. The observed chromosome radiosensitization may be a consequence either of an effect of HQ on the initial induction of radiation-induced chromosomal aberrations, or on the DNA repair capacity of the cells, or it may be linked to HQ-induced alterations in the cell cycle and feedback control mechanism during the G2- to M-phase transition. In order to elucidate which is the mechanism involved, conventional cytogenetics and premature chromosome condensation (PCC) methodologies were applied. The experimental data obtained support the hypothesis that HQ increases G2-chromosomal radiosensitivity in human peripheral blood lymphocytes by inducing a less efficient G2-M-checkpoint, facilitating thus the transition of damaged cells from G2- to M-phase.

Translocation (X;12)(p11;p13) as a sole abnormality in biphenotypic acute leukemia.

A reciprocal t(X;12)(p11;p13) was found as the sole clonal abnormality in biphenotypic leukemia with myeloid and B-lymphoid differentiation. With fluorescence in situ hybridization analysis, the ETV6 gene (previously TEL) was found to be translocated intact to the derivative X chromosome; no MLL and BCR/ABL rearrangements were found. The patient achieved complete remission after induction chemotherapy. To our knowledge, this cytogenetic aberration has not been reported previously as a sole abnormality in hematological malignancies. Its presence may suggest an important role in the pathogenesis of biphenotypic leukemia.

Checkpoint abrogation in G2 compromises repair of chromosomal breaks in ataxia telangiectasia cells.

Checkpoint abrogation in G(2) compromises repair of DNA double-strand breaks (DSB) and confers enhanced G(2) chromosomal radiosensitivity in ataxia telangiectasia (AT) cells. To directly test this hypothesis, we combined calyculin A-induced premature chromosome condensation with conventional cytogenetics to evaluate chromosome damage before and after the G(2) checkpoint in irradiated primary AT and normal human lymphocytes and their lymphoblastoid derivatives. Direct analysis of radiation damage in G(2) by premature chromosome condensation reveals practically indistinguishable levels of chromosomal breaks in AT and normal cells. Yet a 4-fold increase in metaphase chromosome damage is observed in AT cells as compared with normal cells which, in contrast to AT cells, exhibit a strong G(2) arrest manifest as an abrupt reduction in the mitotic index. Thus, an active checkpoint facilitates repair of chromosomal breaks in normal cells. Treatment with caffeine that abrogates the G(2) checkpoint without significantly affecting DSB rejoining increases metaphase chromosome damage of normal cells to the AT level but leaves unchanged interphase chromosome damage in G(2). Caffeine has no effect on any of these end points in AT cells. These observations represent the first direct evidence that the G(2) checkpoint facilitates repair of chromosome damage, presumably by supporting repair of DNA DSBs. Failure to arrest will lead to chromatin condensation and conversion of unrepaired DNA DSBs to chromosomal breaks during G(2)-to-M phase transition.

The harmonization process to set up and maintain an operational biological and physical retrospective dosimetry network: QA QM applied to the RENEB network.

PURPOSE: The European Network of Biological and Physical Retrospective Dosimetry 'RENEB' has contributed to European radiation emergency preparedness. To give homogeneous dose estimation results, RENEB partners must harmonize their processes. MATERIALS AND METHODS: A first inter-comparison focused on biological and physical dosimetry was used to detect the outliers in terms of dose estimation. Subsequently, trainings were organized to improve both tools dose estimation. A second inter-comparison was performed to validate training efficiency. Simultaneously, based on ISO standards, a QA&QM manual on all dosimetry assays was produced which states a common basis and harmonized procedures for each assay. The evaluation of the agreement of RENEB partners to follow the QA&QM manual was performed through a questionnaire. The integration of new members into the network was carried out in the same way, whatever the assays. RESULTS: The training courses on biological and physical dosimetry were judged to be successful because most of the RENEB members' dose estimates improved in the second inter-comparison. The QA&QM manual describes the consensus for the minimum requirements and the performance criteria for both dosimetry assays. The questionnaire revealed that the whole network capacity currently can manage between 15 and 3800 samples once. CONCLUSION: The methodology used to harmonize all dosimetry practice within the network RENEB was highly successful. The network is operational to manage a mass casualty radiation accident for immediate dose assessment.

Dose assessment intercomparisons within the RENEB network using G0-lymphocyte prematurely condensed chromosomes (PCC assay).

PURPOSE: Dose assessment intercomparisons within the RENEB network were performed for triage biodosimetry analyzing G0-lymphocyte PCC for harmonization, standardization and optimization of the PCC assay. MATERIALS AND METHODS: Comparative analysis among different partners for dose assessment included shipment of PCC-slides and captured images to construct dose-response curves for up to 6 Gy γ-rays. Accident simulation exercises were performed to assess the suitability of the PCC assay by detecting speed of analysis and minimum number of cells required for categorization of potentially exposed individuals. RESULTS: Calibration data based on Giemsa-stained fragments in excess of 46 PCC were obtained by different partners using galleries of PCC images for each dose-point. Mean values derived from all scores yielded a linear dose-response with approximately 4 excess-fragments/cell/Gy. To unify scoring criteria, exercises were carried out using coded PCC-slides and/or coded irradiated blood samples. Analysis of samples received 24 h post-exposure was successfully performed using Giemsa staining (1 excess-fragment/cell/Gy) or centromere/telomere FISH-staining for dicentrics. CONCLUSIONS: Dose assessments by RENEB partners using appropriate calibration curves were mostly in good agreement. The PCC assay is quick and reliable for whole- or partial-body triage biodosimetry by scoring excess-fragments or dicentrics in G0-lymphocytes. Particularly, analysis of Giemsa-stained excess PCC-fragments is simple, inexpensive and its automation could increase throughput and scoring objectivity of the PCC assay.