The Sister Chromatid Exchange (SCE) Assay.

A fully optimized staining method for detecting sister chromatid exchanges in cultured cells is presented. The method gives reproducibly robust quantitative results. Sister chromatid exchange is a classic toxicology assay for genotoxicity and for detecting alterations to the biochemistry underlying cellular homologous recombination. Growth of cells in the presence of 5'-bromo-deoxyuridine for two rounds of DNA replication followed by collecting metaphase spreads on glass slides, treatment with the UV-sensitive dye Hoechst 33258, long-wave UV light exposure, and Giemsa staining gives a permanent record of the exchanges.

Quantifying biological effects of radiation from high-energy linear accelerators on lymphocytes.

The aim of this study was to investigate the radiobiological effects of flattening filter (FF) and flattening filter-free (FFF) modes of linear electron accelerators and to understand whether there is any difference between the effects of these modes. We evaluated the number of chromosome aberrations following irradiation of lymphocytes from healthy volunteers with X-ray photons at two energy levels, 6 and 10 MV; the dose rate ranged between 5.50 and 23.08 Gy/min and absorbed doses ranged between 0.5 and 8 Gy. A 60Co curve was employed for comparison. Metaphases from the lymphocyte cultures were prepared using standard cytogenetic techniques and chromosome analysis was performed. Our results allow the performance of biodosimetry at higher energies and doses than the currently used reference dosimetry. We observed significant differences in aberration frequencies when different irradiation techniques were used. FFF mode has a higher radiobiological effect than the FF mode. Linear-quadratic dose response calibration curves were constructed and relative biological effectiveness (RBE) values were calculated. Average RBE values using 6 MV (5.50 Gy/min) as a reference radiation were 1.28 for 60Co γ irradiation, 1.11 for 6 FFF and 0.79-0.92 for 10 FFF. Since there are compelling differences between radiation modalities in cases of hypofractionation, these results may be even more important in a therapeutic situation. In case of an accidental overdose of a patient, use of the appropriate calibration curves for biodosimetry are also essential for quantifying the overdose.

18F-FDG-induced DNA damage, chromosomal aberrations, and toxicity in V79 lung fibroblast cells.

18F-FDG PET/CT imaging is used in the diagnosis of diseases, including cancers. The principal photons used for imaging are 511 ke V gamma photons resulting from positron annihilation. The absorbed dose varies among body organs, depending on administered radioactivity and biological clearance. We have attempted to evaluate DNA double-strand breaks (DSB) and toxicity induced in V79 lung fibroblast cells in vitro by 18F-FDG, at doses which might result from PET procedures. Cells were irradiated by 18F-FDG at doses (14.51 and 26.86 mGy), comparable to absorbed doses received by critical organs during PET procedures. The biological endpoints measured were formation of γ-H2AX foci, mitochondrial stress, chromosomal aberrations, and cell cycle perturbation. Irradiation induced DSB (γH2AX assay), mitochondrial depolarization, and both chromosome and chromatid types of aberrations. At higher radiation doses, increased aneuploidy and reduced mitotic activity were also seen. Thus, significant biological effects were observed at the doses delivered by the 18F-FDG exposure and the effects increased with dose.

Radiation-Induced Chromosomal Breaks may be DNA Repair Fragile Sites with Larger-scale Correlations to Eight Double-Strand-Break Related Data Sets over the Human Genome.

In this work, we compared the genomic distribution of common radiation-induced chromosomal breaks to eight different data sets covering the whole human genome. Sites with a high probability of chromatid breakage after exposure to low and high ionization density radiations were often located inside common and rare fragile sites, indicating that they may be a new and more local type of DNA repair-related fragility. Breaks in specific chromosome bands after acute exposure to oil and benzene also showed strong correlation with these sites and fragile sites. In addition, close correlation was found with cytologically detected chiasma and MLH1 immunofluorescence sites and with the HapMap recombination density distributions. Also, of interest, copy number changes occurred predominantly at radiation-induced breaks and fragile sites, at least for breast cancers with poor prognosis, and they decreased weakly but significantly in regions with increasing recombination and CpG density. An increased CpG density is linked to regions of high gene density to secure high-fidelity reproduction and survival. To minimize cancer induction, cancer-related genes are often located in regions of decreased recombination density and/or higher-than-average CpG density. It is compelling that all these data sets were influenced by the cells' handling of double-strand breaks and, more generally, DNA damage on its genome. In fact, the DNA repair genes systematically avoid regions with a high recombination density, as they need to be intact to accurately handle repairable DNA lesions.

G2 Chromosomal Radiosensitivity Assay for Testing Individual Radiation Sensitivity.

The G2 chromosomal radiosensitivity assay or, simply G2 assay, measures the number of chromatid type aberrations induced by radiation in G2 phase. Typically, asynchronous growing cells are irradiated with less than 1 Gy and allowed 0.5-1 h for cells in mitosis, at the time of irradiation, to transit into G1. Later, the G2 phase cells, at the time irradiation, are blocked by colcemid for 1-4 h at metaphase. Cells are collected by standard hypotonic solution and Carnoy solution fixation or directly fixed onto the culture vessels. The G2 assay can detect severe radiosensitivity in ATM homozygous mutated cells and relatively small differences among cellular radiosensitivity such as heterozygous mutation carriers of ATM and BRCA1/2 mutation carriers. The G2 assay also has the capability to detect cancer prone individuals. This assay only requires a conventional cell culture facility and the standard microscopic observation.

A Roberts Syndrome Individual With Differential Genotoxin Sensitivity and a DNA Damage Response Defect.

PURPOSE: Roberts syndrome (RBS) is a rare, recessively transmitted developmental disorder characterized by growth retardation, craniofacial abnormalities, and truncation of limbs. All affected individuals to date have mutations in the ESCO2 (establishment of cohesion 2) gene, a key regulator of the cohesin complex, which is involved in sister chromatid cohesion and DNA double-strand break (DSB) repair. Here we characterize DNA damage responses (DDRs) for the first time in an RBS-affected family. METHODS AND MATERIALS: Lymphoblastoid cell lines were established from an RBS family, including the proband and parents carrying ESCO2 mutations. Various DDR assays were performed on these cells, including cell survival, chromosome break, and apoptosis assays; checkpoint activation indicators; and measures of DNA breakage and repair. RESULTS: Cells derived from the RBS-affected individual showed sensitivity to ionizing radiation (IR) and mitomycin C-induced DNA damage. In this ESCO2 compound heterozygote, other DDRs were also defective, including enhanced IR-induced clastogenicity and apoptosis; increased DNA DSB induction; and a reduced capacity for repairing IR-induced DNA DSBs, as measured by γ-H2AX foci and the comet assay. CONCLUSIONS: In addition to its developmental features, RBS can be, like ataxia telangiectasia, considered a DDR-defective syndrome, which contributes to its cellular, molecular, and clinical phenotype.

Comparative analysis the frequency and spectrum of chromosome aberrations in irradiated in vitro peripheral blood lymphocytes of the elderly and centenarians from Kyiv. / PORIVNIaL'NYY̆ ANALIZ RIVNIa TA SPEKTRU ABERATsIY̆ KhROMOSOM V OPROMINENYKh IN VITRO LIMFOTsYTAKh PERYFERYChNOÏ KROVI OSIB LITN'OGO VIKU TA DOVGOZhYTELIV m. KYIeVA.

OBJECTIVE: To establish and compare the frequency and spectrum of chromosome aberrations under X radiation exposure in vitro in dose 0.25 Gy peripheral blood lymphocytes of the elderly and centenarians. MATERIAL AND METHODS: Material of cytogenetic research were peripheral blood lymphocytes from 11 elderly and 10 centenarians, which were irradiated in vitro in dose 0.25 Gy and cultured by generally accepted semi micromethod; slides of metaphase chromosomes were GTG stained and analyzed under the microscope with magnification x 1000. RESULTS: Under irradiation of blood in vitro the mean group frequencies of chromosome aberrations exceeded such without irradiation (р < 0.001) and were 11.60 ± 0.95 аnd 6.82 ± 0.63 per 100 cells in the elderly and the centenar ians, accordingly. Radiation induced increase in the frequency of chromosomal injuries occurred due to chromo some type aberrations which are markers of radiation exposure. In the elderly the elevated frequency of chromatid type aberrations also was registered what is considered a sign of chromosome instability. CONCLUSIONS: The results indicate increased sensitivity the blood lymphocytes from the elderly to radiation expo sure in low doses and allow to assume the advantage of persons with hereditary determined chromosomal stability in achieving longevity.

The Use of Laser Microirradiation to Investigate the Roles of Cohesins in DNA Repair.

In addition to their mitotic and transcriptional functions, cohesin plays critical roles in DNA damage response (DDR) and repair. Specifically, cohesin promotes homologous recombination (HR) repair of DNA double-strand breaks (DSBs), which is conserved from yeast to humans, and is a critical effector of ATM/ATR DDR kinase-mediated checkpoint control in mammalian cells. Optical laser microirradiation has been instrumental in revealing the damage site-specific functions of cohesin and, more recently, uncovering the unique role of cohesin-SA2, one of the two cohesin complexes uniquely present in higher eukaryotes, in DNA repair in human cells. In this review, we briefly describe what we know about cohesin function and regulation in response to DNA damage, and discuss the optimized laser microirradiation conditions used to analyze cohesin responses to DNA damage in vivo.

Chromosomal Integrity after UV Irradiation Requires FANCD2-Mediated Repair of Double Strand Breaks.

Fanconi Anemia (FA) is a rare autosomal recessive disorder characterized by hypersensitivity to inter-strand crosslinks (ICLs). FANCD2, a central factor of the FA pathway, is essential for the repair of double strand breaks (DSBs) generated during fork collapse at ICLs. While lesions different from ICLs can also trigger fork collapse, the contribution of FANCD2 to the resolution of replication-coupled DSBs generated independently from ICLs is unknown. Intriguingly, FANCD2 is readily activated after UV irradiation, a DNA-damaging agent that generates predominantly intra-strand crosslinks but not ICLs. Hence, UV irradiation is an ideal tool to explore the contribution of FANCD2 to the DNA damage response triggered by DNA lesions other than ICL repair. Here we show that, in contrast to ICL-causing agents, UV radiation compromises cell survival independently from FANCD2. In agreement, FANCD2 depletion does not increase the amount of DSBs generated during the replication of UV-damaged DNA and is dispensable for UV-induced checkpoint activation. Remarkably however, FANCD2 protects UV-dependent, replication-coupled DSBs from aberrant processing by non-homologous end joining, preventing the accumulation of micronuclei and chromatid aberrations including non-homologous chromatid exchanges. Hence, while dispensable for cell survival, FANCD2 selectively safeguards chromosomal stability after UV-triggered replication stress.

Human DNA helicase B functions in cellular homologous recombination and stimulates Rad51-mediated 5'-3' heteroduplex extension in vitro.

Homologous recombination is involved in the repair of DNA damage and collapsed replication fork, and is critical for the maintenance of genomic stability. Its process involves a network of proteins with different enzymatic activities. Human DNA helicase B (HDHB) is a robust 5'-3' DNA helicase which accumulates on chromatin in cells exposed to DNA damage. HDHB facilitates cellular recovery from replication stress, but its role in DNA damage response remains unclear. Here we report that HDHB silencing results in reduced sister chromatid exchange, impaired homologous recombination repair, and delayed RPA late-stage foci formation induced by ionizing radiation. Ectopically expressed HDHB colocalizes with Rad51, Rad52, RPA, and ssDNA. In vitro, HDHB stimulates Rad51-mediated heteroduplex extension in 5'-3' direction. A helicase-defective mutant HDHB failed to promote this reaction. Our studies implicate HDHB promotes homologous recombination in vivo and stimulates 5'-3' heteroduplex extension during Rad51-mediated strand exchange in vitro.

[The frequency and spectrum of cytogenetic anomalies in employees of Siberian Group of Chemical Enterprises].

The results of the study of frequency and spectrum of cytogenetic anomalies in 657 healthy employees of the main facilities of the Siberian Group of Chemical Enterprises exposed to external, internal and combined irradiation are presented. No dependence between age and chromosome aberrations frequency was revealed. Chronic external exposure appeared to be the main factor of induction of chromosome aberrations. The frequency of aberrant cells, chromosome type aberrations, paired fragments and rings was statistically significantly higher in employees exposed to external irradiation as compared to persons exposed to combined irradiation. A nonlinear dependence the dose of irradiation and frequency of chromosome aberrations was revealed. A statistically significant decrease of prevalence of aberrant cells, aberration of chromatid and chromosome type was established in employees exposed to irradiation at a dose range of > 0-10 mSv compared to the control group. This agrees with the phenomenon of radiation hormesis. A significant increase of the frequency of chromosome aberrations was not observed at doses below > 40 mSv. In employees exposed to irradiation at a dose range > 40-100 mSv, a statistically significant increase of frequencies of aberrant metaphases, aberrations of chromatid and chromosome types was established. Same was found for dicentrics at dose range of >100-200 mSv. This supports a well known linear threshold model. Dose-effect curve has a plateau at doses ranged from 100 to 500 mSv.

Complex aberrations in lymphocytes exposed to mixed beams of (241)Am alpha particles and X-rays.

Modern radiotherapy treatment modalities are associated with undesired out-of-field exposure to complex mixed beams of high and low energy transfer (LET) radiation that can give rise to secondary cancers. The biological effectiveness of mixed beams is not known. The aim of the investigation was the analysis of chromosomal damage in human peripheral blood lymphocytes (PBL) exposed to a mixed beam of X-rays and alpha particles. Using a dedicated exposure facility PBL were exposed to increasing doses of alpha particles (from (241)Am), X-rays and a mixture of both. Chromosomal aberrations were analysed in chromosomes 2, 8 and 14 using fluorescence in situ hybridisation. The found and expected frequencies of simple and complex aberrations were compared. Simple aberrations showed linear dose-response relationships with doses. A higher than expected frequency of simple aberrations was only observed after the highest mixed beam dose. A linear-quadratic dose response curve for complex aberrations was observed after mixed-beam exposure. Higher than expected frequencies of complex aberrations were observed for the two highest doses. Both the linear-quadratic dose-response relationship and the calculation of expected frequencies show that exposure of PBL to mixed beams of high and low LET radiation leads to a higher than expected frequency of complex-type aberrations. Because chromosomal changes are associated with cancer induction this result may imply that the cancer risk of exposure to mixed beams in radiation oncology may be higher than expected based on the additive action of the individual dose components.

Formation peculiarities of radiation-induced aberrations of chromosomes in human cells under the modifying influence of chemical agents (comparative aspects).

OBJECTIVE: The study objective was to determine and provide a comparative analysis of frequency and spectrum of the induced aberrations of chromosomes in culture of the human peripheral blood lymphocytes under the combined impact of radiation, co-mutagen, and chemical mutagen. METHODS: Culture of human peripheral blood lymphocytes and cytogenetic methods have been used. RESULTS: A co-mutagenic effect of the drug verapamil was established under the testing γ-irradiation of human peripheral blood lymphocytes in the dose range of 0.3-2.0 Gy at the expense of increased frequency of chromosomal aberrations (dicentrics). The combined effect of γ-irradiation and S-Nitrosoglutathione is directed on the induction and storage of chemical markers of exposure - the chromatid-type aberrations. CONCLUSION: A co-mutagenic effect of verapamil under the low-dose γ-irradiation as a 2-fold increase of the chromosome-type aberrations (radiation markers) incidence was revealed at a chromosomal level in human peripheral blood lymphocytes. Phenomenon of synergism of low-dose γ-irradiation and mutagen S-Nitrosoglutathione as a ~3-fold increased frequency of chromatid-type aberrations (chemical markers) was detected compared to the sole radiation effect.

Molecular characterisation of murine acute myeloid leukaemia induced by 56Fe ion and 137Cs gamma ray irradiation.

Exposure to sparsely ionising gamma- or X-ray irradiation is known to increase the risk of leukaemia in humans. However, heavy ion radiotherapy and extended space exploration will expose humans to densely ionising high linear energy transfer (LET) radiation for which there is currently no understanding of leukaemia risk. Murine models have implicated chromosomal deletion that includes the hematopoietic transcription factor gene, PU.1 (Sfpi1), and point mutation of the second PU.1 allele as the primary cause of low-LET radiation-induced murine acute myeloid leukaemia (rAML). Using array comparative genomic hybridisation, fluorescence in situ hybridisation and high resolution melt analysis, we have confirmed that biallelic PU.1 mutations are common in low-LET rAML, occurring in 88% of samples. Biallelic PU.1 mutations were also detected in the majority of high-LET rAML samples. Microsatellite instability was identified in 42% of all rAML samples, and 89% of samples carried increased microsatellite mutant frequencies at the single-cell level, indicative of ongoing instability. Instability was also observed cytogenetically as a 2-fold increase in chromatid-type aberrations. These data highlight the similarities in molecular characteristics of high-LET and low-LET rAML and confirm the presence of ongoing chromosomal and microsatellite instability in murine rAML.

High γ-radiation sensitivity is associated with increased gastric cancer risk in a Chinese Han population: a case-control analysis.

Hypersensitivity to radiation exposure has been suggested to be a risk factor for the development of several malignancies, but not including gastric cancer. In this case-control study, radiation sensitivity as measured by chromatid breaks per cell (b/c) was examined in cultured peripheral blood lymphocytes (PBLs) from 517 patients with gastric cancer and 525 healthy controls. Our results showed that b/c values were significantly higher in cases than in controls (Mean [SD], 0.47 [0.20] vs. 0.34 [0.17]; P<0.001). Using the 50(th) percentile value for controls (0.34 b/c) as the cutoff point, unconditional logistic regression analysis revealed that γ-radiation-sensitive individuals were at significantly higher risk for gastric cancer (adjusted odds ratio [OR] 2.01, 95% confidence interval [CI] 1.49-3.13). Quartile stratification analysis indicated a dose-response relationship between γ-radiation sensitivity and gastric cancer risk (P for trend <0.001). When using the subjects in first quartile of b/c values as reference, the adjusted ORs and corresponding CIs for the subjects in second, third, and fourth quartiles were 1.48 (0.91-2.17), 2.42 (1.76-3.64), and 3.40 (2.11-5.29), respectively. The γ-radiation sensitivity was related to age and smoking status. In addition, a clear joint effect on cancer risk was found between γ-Radiation sensitivity and smoking status. The risk for ever smokers with high sensitivity was higher than those for never smokers with high sensitivity and ever smokers with low sensitivity (OR [CI], 4.67 [2.31-6.07] vs. 2.14 [1.40-3.06] vs. 2.42 [1.57-3.95], respectively). No significant interaction was found between both factors (P for interaction= 0.42). We conclude that chromatid radiosensitivity is associated with gastric cancer susceptibility in a Chinese population.

[The low dose and low dose rate cytogenetic effects induced by gamma-radiation in human blood lymphocytes in vitro. II. the results of cytogenetic study].

The yield of chromosome aberrations induced by gamma-radiation of 60Co in human blood lymphocytes in vitro at low doses (30 divided by 600 mGy) and low dose rates (0.70, 5.05, 59.2 mGy/min) was investigated. It was found that the observed level of chromosomal aberrations induced by gamma-irradiation was unaffected by the value of the dose rate when using constant dose rate and obtaining different doses by altering the exposure time. However, a relatively enhanced level of chromatid aberrations was found at 5.05 and 59.2 mGy/min dose rates in the dose range less than 250 mGy. We have found that the observed level of the sum of chromosomal aberrations induced by gamma-irradiation at doses less than 250 mGy and a dose rate of 59.2 mGy/min was essentially larger compared with the level extrapolated from high doses (above 300 mGy) using a linear-quadratic dose curve. This complied with our previous finding in 1976, 1977 when the enhanced level of dicentrics was only found at a high dose rate approximately 500 mGy/min. Such a non-linear cytogenetic effect does not manifest itself statistically significantly at dose rates of 0.70 and 5.05 mGy/min for the sum of chromosomal aberrations and does not manifest itself at all for dicentrics at all the examined dose rates.

In vivo versus in vitro individual radiosensitivity analysed in healthy donors and in prostate cancer patients with and without severe side effects after radiotherapy.

BACKGROUND: A high cellular radiosensitivity may be connected with a risk for development of severe side effects after radiotherapy and indicate cancer susceptibility. Hence, a fast and robust in vitro test is desirable to identify radiosensitive individuals. MATERIALS AND METHODS: The study included 25 prostate cancer patients with severe side effects (S) and 25 patients without severe side effects (0) after radiotherapy as well as 23 male healthy age-matched donors. Blood samples were exposed to 0.5 Gy or 1 Gy of γ-rays. The initial level of double-strand breaks (dsb) and repair kinetics measured by phosphorylation of histone H2A (γ-H2AX-assay), apoptosis (Annexin V-assay) and the induction of chromatid aberrations after irradiation in the G2-phase of the cell cycle (G2-assay) were analysed. RESULTS: A significant higher chromatid aberration yield was found in lymphocytes from prostate cancer patients when compared to healthy donors. We found no significant differences between patients S and patients 0. CONCLUSIONS: There is no obvious correlation between clinical and cellular radiosensitivity in lymphocytes of prostate cancer patients when all chosen in vitro assays are considered. Although 25% of the patients showed both severe side effects and increased radiation-induced chromosomal sensitivity, predictive value of G2-assay is doubtful.

Gamma-ray-induced mutagen sensitivity and risk of sporadic breast cancer in young women: a case-control study.

Hypersensitivity to radiation exposure has been suggested to be a risk factor for the development of breast cancer. In this case-control study of 515 young women (≤ 55 years) with newly diagnosed sporadic breast cancer and 402 cancer-free controls, we examined the radiosensitivity as measured by the frequency of chromatid breaks induced by gamma-radiation exposure in the G2 phase of phytohemagglutinin-stimulated and short-term cultured fresh lymphocytes. We found that the average chromatid breaks per cell from 50 well-spread metaphases were statistically significantly higher in 403 non-Hispanic White breast cancer patients (0.52 ± 0.22) than that in 281 non-Hispanic White controls (0.44 ± 0.16) (P value < 0.001), and in 60 Mexican American breast cancer patients (0.52 ± 0.19) than that in 65 Mexican American controls (0.44 ± 0.16) (P value = 0.021), but the difference was not significant in African Americans (52 cases [0.45 ± 0.16] versus 56 controls [0.47 ± 0.16], P = 0.651). The frequency of chromatid breaks per cell above the median of control subjects was associated with two-fold increased risk for breast cancer in non-Hispanic Whites and Mexican Americans. A dose-response relationship was evident between radiosensitivity and risk for breast cancer (P (trend) < 0.001) in these two ethnic groups. We concluded that gamma-ray-induced mutagen sensitivity may play a role in susceptibility to breast cancer in young non-Hispanic White and Mexican American women.

High-resolution genome-wide analysis of irradiated (UV and γ-rays) diploid yeast cells reveals a high frequency of genomic loss of heterozygosity (LOH) events.

In diploid eukaryotes, repair of double-stranded DNA breaks by homologous recombination often leads to loss of heterozygosity (LOH). Most previous studies of mitotic recombination in Saccharomyces cerevisiae have focused on a single chromosome or a single region of one chromosome at which LOH events can be selected. In this study, we used two techniques (single-nucleotide polymorphism microarrays and high-throughput DNA sequencing) to examine genome-wide LOH in a diploid yeast strain at a resolution averaging 1 kb. We examined both selected LOH events on chromosome V and unselected events throughout the genome in untreated cells and in cells treated with either γ-radiation or ultraviolet (UV) radiation. Our analysis shows the following: (1) spontaneous and damage-induced mitotic gene conversion tracts are more than three times larger than meiotic conversion tracts, and conversion tracts associated with crossovers are usually longer and more complex than those unassociated with crossovers; (2) most of the crossovers and conversions reflect the repair of two sister chromatids broken at the same position; and (3) both UV and γ-radiation efficiently induce LOH at doses of radiation that cause no significant loss of viability. Using high-throughput DNA sequencing, we also detected new mutations induced by γ-rays and UV. To our knowledge, our study represents the first high-resolution genome-wide analysis of DNA damage-induced LOH events performed in any eukaryote.

Error-prone nonhomologous end joining repair operates in human pluripotent stem cells during late G2.

Genome stability of human embryonic stem cells (hESC) is an important issue because even minor genetic alterations can negatively impact cell functionality and safety. The incorrect repair of DNA double-stranded breaks (DSBs) is the ultimate cause of the formation of chromosomal aberrations. Using G2 radiosensitivity assay, we analyzed chromosomal aberrations in pluripotent stem cells and somatic cells. The chromatid exchange aberration rates in hESCs increased manifold 2 hours after irradiation as compared with their differentiated derivatives, but the frequency of radiation-induced chromatid breaks was similar. The rate of radiation-induced chromatid exchanges in hESCs and differentiated cells exhibited a quadratic dose response, revealing two-hit mechanism of exchange formation suggesting that a non-homologous end joining (NHEJ) repair may contribute to their formation. Inhibition of DNA-PK, a key NHEJ component, by NU7026 resulted in a significant decrease in radiation-induced chromatid exchanges in hESCs but not in somatic cells. In contrast, NU7026 treatment increased the frequency of radiation-induced breaks to a similar extent in pluripotent and somatic cells. Thus, DNA-PK dependent NHEJ efficiently participates in the elimination of radiation-induced chromatid breaks during the late G2 in both cell types and DNA-PK activity leads to a high level of misrejoining specifically in pluripotent cells.