Histone post-translational modifications in DNA damage response.

The fact that eukaryotic DNA is packed into chromatin constitutes a physical barrier to enzymes and regulatory factors to reach the DNA molecule for replication, transcription, recombination and repair. Although most studies in this field have concentrated on how chromatin regulates transcription, there is a recent emphasis on studying the role of chromatin in the response to DNA damage. Two main chromatin-remodeling mechanisms have been identified, namely, ATP-dependent chromatin-remodeling complexes and histone post-translational modifications (PTMs). PTMs constitute reversible covalent modifications in aminoacidic residues, such as serine and threonine phosphorylation, lysine acetylation, lysine and arginine methylation and lysine ubiquitylation, among others. Moreover, nucleosome composition can be modified by the incorporation of histone variants, which are assembled into nucleosomes independently of DNA replication. The phosphorylation of the histone variant H2AX (gammaH2AX) is one of the best examples of histone PTMs in response to DNA damage induction, but many others have recently been revealed. In this review, we focus on and summarize the best-known histone PTMs observed in excision repair (base excision and nucleotide excision) and double-strand break (non-homologous end-joining and homologous recombination) repair pathways. In brief, the interplay between chromatin remodelers and DNA repair factors is discussed in relation to DNA damage response mechanisms.

Distribution of UVC-induced chromosome aberrations along the X chromosome of TCR deficient and proficient Chinese hamster cell lines.

Cells with a transcription coupled repair (TCR) deficiency are characterized by a higher sensitivity to UVC irradiation and by an increase in apoptosis and chromosomal aberration frequencies. It has been claimed that the higher frequency of chromosomal aberrations results from the transcription blockage caused by UVC-lesions located in the transcribed strands of the genome. The distribution of chromosome breakpoints in euchromatic and heterochromatic regions of the X chromosome from TCR deficient and proficient Chinese hamster cell lines was studied. Most UVC-induced breakpoints occurred in euchromatic regions of the X chromosome in both cell lines. No increase of UVC-induced breakpoints in the euchromatic region of the UV61 X chromosome was observed, indicating that TCR failure alone cannot be responsible for the increased frequency of chromosomal aberrations. Differential chromatin remodeling in the TCR defective cell line is proposed as a possible mechanism involved in the distribution of UVC-induced breakpoints along the Chinese hamster X chromosome. A similar explanation for the increase of UVC-induced chromosomal aberrations in TCR defective cells is given.

Distribution of breakpoints induced by etoposide and X-rays along the CHO X chromosome.

SORB (selected observed residual breakpoints) induced by ionizing radiation or endonucleases are often non-randomly distributed in mammalian chromosomes. However, the role played by chromatin structure in the localization of chromosome SORB is not well understood. Anti-topoisomerase drugs such as etoposide are potent clastogens and unlike endonucleases or ionizing radiation, induce DNA double-strand breaks (DSB) by an indirect mechanism. Topoisomerase II (Topo II) is a main component of the nuclear matrix and the chromosome scaffold. Since etoposide leads to DSB by influencing the activity of Topo II, this compound may be a useful tool to study the influence of the chromatin organization on the distribution of induced SORB in mammalian chromosomes. In the present work, we compared the distribution of SORB induced during S-phase by etoposide or X-rays in the short euchromatic and long heterochromatic arms of the CHO9 X chromosome. The S-phase stage (early, mid or late) at which CHO9 cells were exposed to etoposide or X-rays was marked by incorporation of BrdU during treatments and later determined by immunolabeling of metaphase chromosomes with an anti-BrdU FITC-coupled antibody. The majority of treated cells were in late S-phase during treatment either with etoposide or X-rays. SORB induced by etoposide mapped preferentially to Xq but random localization was observed for SORB produced by X-rays. Possible explanations for the uneven distribution of etoposide-induced breakpoints along Xq are discussed.

Interlaboratory comparison of dicentric chromosome assay using electronically transmitted images.

The bottleneck in data acquisition during biological dosimetry based on a dicentric assay is the need to score dicentrics in a large number of lymphocytes. One way to increase the capacity of a given laboratory is to use the ability of skilled operators from other laboratories. This can be done using image analysis systems and distributing images all around the world. Two exercises were conducted to test the efficiency of such an approach involving 10 laboratories. During the first exercise (E1), the participant laboratories analysed the same images derived from cells exposed to 0.5 and 3 Gy; 100 images were sent to all participants for both doses. Whatever the dose, only about half of the cells were complete with well-spread metaphases suitable for analysis. A coefficient of variation (CV) on the standard deviation of ∼15 % was obtained for both doses. The trueness was better for 3 Gy (0.6 %) than for 0.5 Gy (37.8 %). The number of estimated doses classified as satisfactory according to the z-score was 3 at 0.5 Gy and 8 at 3 Gy for 10 dose estimations. In the second exercise, an emergency situation was tested, each laboratory was required to score a different set of 50 images in 2 d extracted from 500 downloaded images derived from cells exposed to 0.5 Gy. Then the remaining 450 images had to be scored within a week. Using 50 different images, the CV on the estimated doses (79.2 %) was not as good as in E1, probably associated to a lower number of cells analysed (50 vs. 100) or from the fact that laboratories analysed a different set of images. The trueness for the dose was better after scoring 500 cells (22.5 %) than after 50 cells (26.8 %). For the 10 dose estimations, the number of doses classified as satisfactory according to the z-score was 9, for both 50 and 500 cells. Overall, the results obtained support the feasibility of networking using electronically transmitted images. However, before its implementation some issues should be elucidated, such as the number and resolution of the images to be sent, and the harmonisation of the scoring criteria. Additionally, a global website able to be used for the different regional networks, like Share Points, will be desirable to facilitate worldwide communication.

Biological dosimetry intercomparison exercise: an evaluation of triage and routine mode results by robust methods.

Well-defined protocols and quality management standards are indispensable for biological dosimetry laboratories. Participation in periodic proficiency testing by interlaboratory comparisons is also required. This harmonization is essential if a cooperative network is used to respond to a mass casualty event. Here we present an international intercomparison based on dicentric chromosome analysis for dose assessment performed in the framework of the IAEA Regional Latin American RLA/9/054 Project. The exercise involved 14 laboratories, 8 from Latin America and 6 from Europe. The performance of each laboratory and the reproducibility of the exercise were evaluated using robust methods described in ISO standards. The study was based on the analysis of slides from samples irradiated with 0.75 (DI) and 2.5 Gy (DII). Laboratories were required to score the frequency of dicentrics and convert them to estimated doses, using their own dose-effect curves, after the analysis of 50 or 100 cells (triage mode) and after conventional scoring of 500 cells or 100 dicentrics. In the conntional scoring, at both doses, all reported frequencies were considered as satisfactory, and two reported doses were considered as questionable. The analysis of the data dispersion among the dicentric frequencies and among doses indicated a better reproducibility for estimated doses (15.6% for DI and 8.8% for DII) than for frequencies (24.4% for DI and 11.4% for DII), expressed by the coefficient of variation. In the two triage modes, although robust analysis classified some reported frequencies or doses as unsatisfactory or questionable, all estimated doses were in agreement with the accepted error of ±0.5 Gy. However, at the DI dose and for 50 scored cells, 5 out of the 14 reported confidence intervals that included zero dose and could be interpreted as false negatives. This improved with 100 cells, where only one confidence interval included zero dose. At the DII dose, all estimations fell within ±0.5 Gy of the reference dose interval. The results obtained in this triage exercise indicated that it is better to report doses than frequencies. Overall, in both triage and conventional scoring modes, the laboratory performances were satisfactory for mutual cooperation purposes. These data reinforce the view that collaborative networking in the case of a mass casualty event can be successful.