Several oxidative DNA-damaging agents, including ionizing radiation, can generate multiply damaged sites in DNA. Among the postulated lesions are those with abasic sites located in close proximity on opposite strands. The repair of an abasic site requires strand scission by a repair endonuclease such as human apurinic/apyrimidinic endonuclease (Ape) or exonuclease III in Escherichia coli. Therefore, a potential consequence of the "repair" of bistranded abasic sites is the formation of double-strand breaks. To test this possibility and to investigate the influence of the relative distance between the two abasic sites and their orientation to each other, we prepared a series of oligonucleotide duplexes containing abasic sites at defined positions either directly opposite each other or separated by 1, 3, or 5 base pairs in the 5-or 3-direction. Analysis following Ape and exonuclease III treatment of these substrates indicated a variety of responses. In general, cleavage at abasic sites was slower in duplexes with paired lesions than in control duplexes with single lesions. Double-strand breaks were, however, readily generated in duplexes with abasic sites positioned 3 to each other. With the duplex containing abasic sites set 1 base pair apart, 5 to each other, both Ape and exonuclease III slowly cleaved the abasic site on one strand only and were unable to incise the other strand. With the duplex containing abasic sites set 3 base pairs apart, 5 to each other, Ape protein was unable to cleave either strand. These data suggest that closely positioned abasic sites could have several deleterious consequences in the cell. In addition, this approach has allowed us to map bases that make significant contact with the enzymes when acting on an abasic site on the opposite strand.The cytotoxic effects of many agents are believed to be the result of damage to DNA. In addition to producing isolated DNA base and deoxyribose lesions, several important antineoplastic agents, including ionizing radiation and drugs such as bleomycin and neocarzinostatin, can generate clustered lesions or locally multiply damaged sites (LMDS) 1 located within one or two helical turns (1-5). Such LMDS may be critical lesions in the DNA as they present an additional challenge to the repair machinery of the cell. One type of LMDS, the DNA double-strand break, has been the subject of considerable study and is considered to contribute significantly to cytotoxicity (6, 7). However, the biological consequences of other types of LMDS, such as 2 modified bases or two abasic sites close to each other, require further investigation. A great deal will depend on if and how these lesions are recognized by the initial damage recognition enzymes. In the case of abasic sites, the enzymes that recognize these lesions and cleave the DNA have been classified according to mechanism. The major apurinic/apyrimidinic (AP) DNA repair endonuclease in human cells is Ape (8) or HAP1 (9). The major AP endonuclease of Escherichia coli is the multifunctional enzyme exonuclease I...

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Human glioblastomas often develop resistance to radiation therapy. The molecular details of this phenomenon are not completely understood. Recent studies have suggested that deficiency in DNA repair pathways may alter the resistance to ionizing radiation in gliobastomas. The human glioma cell line M059J is deficient in DNA-dependent protein kinase (DNA-PK), whereas cell line M059K, isolated from the same malignant tumor, has normal DNA-PK activity. DNA-PK plays a central role in the repair of ionizing-radiation-induced double-strand break repair, and its deficiency has been correlated with ionizing radiation sensitivity in these glioblastoma cells. We argued that other cellular pathways could also play a role in the resistance to radiation therapy in gliomas. We hypothesized that micro-RNAs (miRNAs) are differentially modulated in M059J and M059K cells exposed to ionizing radiation and that the miRNA modulation contributes to the resistance to ionizing radiation. miRNAs are small nonprotein coding single-stranded RNA molecules, which are crucial posttranscriptional regulators of gene expression. Numerous studies have documented the participation of miRNAs in a wide range of biological processes. The contribution of miRNAs in mediating resistance of glioblastoma cell to ionizing radiation treatment has not been elucidated. To test this hypothesis, we examined the expression patterns of a number of miRNAs involved in carcinogenesis in irradiated M059J and M059K cells. The relative expression level as determined by real-time quantitative PCR for miRNAs belonging to the let-7 family indicated an upregulation in irradiated M059K cells. On the contrary, the analysis of irradiated M059J cells for the modulation of let-7 family of miRNAs revealed an overall downregulation. The miR-17-3p, miR-17-5p, miR-19a, miR-19b, miR-142-3p, and miR-142-5p were upregulated in both M059K and M059J cells. The miR-15a, miR-16, miR-143, miR-155, and miR-21 were upregulated in M059K, and the modulation of these miRNAs fluctuated in M059J cells in a time-dependent manner. These results indicate the involvement of miRNAs in the differential response of glioblastoma cells to ionizing radiation treatment.

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Gene regulation in cells exposed to ionizing radiation (IR) occurs at the transcriptional and post-transcriptional levels. Recent studies have suggested that micro-RNA (miRNA) play a significant role in post-transcriptional gene regulation in irradiated cells. miRNA are RNA molecules 18–24 nucleotides in length that are involved in negatively regulating the stability or translation of target messenger RNA. Previous studies from our laboratory have shown that the expression of various miRNA is altered in IR-treated cells. In the present study we monitored genome-wide expression changes of miRNA transcriptome by massively parallel sequencing of human cells irradiated with X-rays. The baseline expression of 402 miRNA indicated a wide range of modulation without exposure to IR. Differences in the expression of many miRNA were observed in a time-dependent fashion following radiation treatment. The Short Time-series Expression Miner (STEM) clustering tool was used to characterize 190 miRNA to six statistically significant temporal expression profiles. miR-19b and miR-93 were induced and miR-222, miR-92a, and miR-941 were repressed after radiation treatment. miR-142-3p, miR-142-5p, miR-107, miR-106b, miR-191, miR-21, miR-26a, miR-182, miR-16, miR-146a, miR-22 and miR-30e exhibited two peaks of induction: one at 8 h and the other at 24 h post-irradiation. miR-378, miR-let-7a, miR-let-7g, miR-let-7f, miR-103b, miR-486-3p, miR-423-5p, miR-4448, miR-3607-5p, miR-20b, miR-130b, miR-155, miR-181, miR-30d and miR-378c were induced only at the 8-h time-point. This catalogue of the inventory of miRNA that are modulated as a response to radiation exposure will be useful for explaining the mechanisms of gene regulation under conditions of stress.