The role of cyclin D1 in response to long-term exposure to ionizing radiation

The health-related hazards resulting from long-term exposure to radiation remain unknown. Thus, an appropriate molecular marker is needed to clarify these effects. Cyclin D1 regulates the cell cycle transition from the G 1 phase to the S phase. Cyclin D1 is degraded as a G1/S checkpoint after 10 Gy of single acute radiation exposure, whereas conversely, cyclin D1 is stabilized when human tumor cells are exposed to fractionated radiation (FR) with 0.5 Gy of x-rays for 31 d. In this article, we review new findings regarding cyclin D1 overexpression in response to long-term exposure to FR. Cyclin D1 overexpression is associated with induction of genomic instability in irradiated cells. Therefore, repression of cyclin D1 expression is likely to cancel the harmful effects of long-term exposure to FR. Thus cyclin D1 may be a marker of long-term exposure to radiation and is a putative molecular radioprotection target for radiation safety.

Detection of γ-H2AX, a Biomarker for DNA Double-strand Breaks, in Urinary Bladders of N -Butyl- N -(4-Hydroxybutyl)-Nitrosamine-Treated Rats

To evaluate the potential role of DNA repair in bladder carcinogenesis, we performed an immunohistochemical analysis of expression of various DNA repair enzymes and γ-H2AX, a high-sensitivity marker of DNA double-strand breaks, in the urothelium of male F344 rats treated with N-butyl-N-(4-hydroxybutyl)-nitrosamine (BBN), a bladder-specific carcinogen. Our results clearly demonstrated that γ-H2AX aggregation was specifically generated in nuclei of bladder epithelial cells of BBN-treated rats, which was not found in untreated controls or mesenchymal cells. γ-H2AX-positive cells were detected not only in hyperplastic and neoplastic areas but also in the normal-like urothelium after BBN treatment. These data indicate that γ-H2AX has potential as a useful biomarker for early detection of genotoxicity in the rat urinary bladder. To the best of our knowledge, this is the first report demonstrating expression of γ-H2AX during bladder carcinogenesis.

To trim or not to trim: progression and control of DSB end resection

DNA double-strand breaks (DSBs) are the most cytotoxic form of DNA damage, since they can lead to genome instability and chromosome rearrangements, which are hallmarks of cancer cells. To face this kind of lesion, eukaryotic cells developed two alternative repair pathways, homologous recombination (HR) and non-homologous end joining (NHEJ). Repair pathway choice is influenced by the cell cycle phase and depends upon the 5'-3' nucleolytic processing of the break ends, since the generation of ssDNA tails strongly stimulates HR and inhibits NHEJ. A large amount of work has elucidated the key components of the DSBs repair machinery and how this crucial process is finely regulated. The emerging view suggests that besides endo/exonucleases and helicases activities required for end resection, molecular barrier factors are specifically loaded in the proximity of the break, where they physically or functionally limit DNA degradation, preventing excessive accumulation of ssDNA, which could be threatening for cell survival.

Activation-Induced Cytidine Deaminase Does Not Impact Murine Meiotic Recombination

Activation-induced cytidine deaminase (AID) was first described as the triggering enzyme of the B-cell-specific reactions that edit the immunoglobulin genes, namely somatic hypermutation, gene conversion, and class switch recombination. Over the years, AID was also detected in cells other than lymphocytes, and it has been assigned additional roles in the innate defense against transforming retroviruses, in retrotransposition restriction and in DNA demethylation. Notably, AID expression was found in germline tissues, and in heterologous systems it can induce the double-strand breaks required for the initiation of meiotic recombination and proper gamete formation. However, because AID-deficient mice are fully fertile, the molecule is not essential for meiosis. Thus, the remaining question that we addressed here is whether AID influences the frequency of meiotic recombination in mice. We measured the recombination events in the meiosis of male and female mice F1 hybrids of C57BL/6J and BALB/c, in Aicda^+/+ and Aicda^-/- background by using a panel of single-nucleotide polymorphisms that distinguishes C57BL/6J from BALB/c genome across the 19 autosomes. In agreement with the literature, we found that the frequency of recombination in the female germline was greater than in male germline, both in the Aicda^+/+ and Aicda^-/- backgrounds. No statistical difference was found in the average recombination events between Aicda^+/+ and Aidca^-/- animals, either in females or males. In addition, the recombination frequencies between single-nucleotide polymorphisms flanking the immunoglobulin heavy and immunoglobulin kappa loci was also not different. We conclude that AID has a minor impact, if any, on the overall frequency of meiotic recombination.

The effects of DNA double-strand breaks on mouse oocyte meiotic maturation

Both endogenous and exogenous factors can induce DNA double-strand breaks (DSBs) in oocytes, which is a potential risk for human-assisted reproductive technology as well as animal nuclear transfer. Here we used bleomycin (BLM) and laser micro-beam dissection (LMD) to induce DNA DSBs in germinal vesicle (GV) stage oocytes and compared the germinal vesicle breakdown (GVBD) rates and first polar body extrusion (PBE) rates between DNA DSB oocytes and untreated oocytes. Employing live cell imaging and immunofluorescence labeling, we observed the dynamics of DNA fragments during oocyte maturation. We also determined the cyclin B1 expression pattern in oocytes to analyze spindle assembly checkpoint (SAC) activity in DNA DSB oocytes. We used parthenogenetic activation to determine if the DNA DSB oocytes could be activated. As a result, we found that the BLM- or LMD-induced DSB oocytes showed lower GVBD rates and took a longer time to undergo GVBD compared with untreated oocytes. PBE was also delayed in DSB oocytes, but once GVBD had occurred, PBE was not affected, even in oocytes with severe DSBs. Compared with control oocytes, the DSB oocytes showed higher SAC activity, as indicated by less Ccnb1-GFP degradation during metaphase I to anaphase I transition. Parthenogenetic activation could activate the metaphase to interphase transition in the DNA DSB mature oocytes, but many oocytes contained multiple pronuclei or numerous micronuclei. These data suggest that DNA damage inhibits or delays the G2/M transition, but once GVBD occurs, DNA-damaged oocytes can complete chromosome separation and polar body extrusion even under a higher SAC activity, causing the formation of numerous micronuclei in early embryos.

Common variants of Drosophila melanogaster Cyp6d2 cause camptothecin sensitivity and synergize with loss of Brca2

Many chemotherapeutic agents selectively target rapidly dividing cells, including cancer cells, by causing DNA damage that leads to genome instability and cell death. We used Drosophila melanogaster to study how mutations in key DNA repair genes affect an organism's response to chemotherapeutic drugs. In this study, we focused on camptothecin and its derivatives, topotecan and irinotecan, which are type I topoisomerase inhibitors that create DNA double-strand breaks in rapidly dividing cells. Here, we describe two polymorphisms in Drosophila Cyp6d2 that result in extreme sensitivity to camptothecin but not topotecan or irinotecan. We confirmed that the sensitivity was due to mutations in Cyp6d2 by rescuing the defect with a wild-type copy of Cyp6d2. In addition, we showed that combining a cyp6d2 mutation with mutations in Drosophila brca2 results in extreme sensitivity to camptothecin. Given the frequency of the Cyp6d2 polymorphisms in publcly available Drosophila stocks, our study demonstrates the need for caution when interpreting results from drug sensitivity screens in Drosophila and other model organisms. Furthermore, our findings illustrate how genetic background effects can be important when determining the efficacy of chemotherapeutic agents in various DNA repair mutants.

Understanding the origins of UV-induced recombination through manipulation of sister chromatid cohesion

Ultraviolet light (UV) can provoke genome instability, partly through its ability to induce homologous recombination (HR). However, the mechanism(s) of UV-induced recombination is poorly understood. Although double-strand breaks (DSBs) have been invoked, there is little evidence for their generation by UV. Alternatively, single-strand DNA lesions that stall replication forks could provoke recombination. Recent findings suggest efficient initiation of UV-induced recombination in G1 through processing of closely spaced single-strand lesions to DSBs. However, other scenarios are possible, since the recombination initiated in G1 can be completed in the following stages of the cell cycle. We developed a system that could address UV-induced recombination events that start and finish in G2 by manipulating the activity of the sister chromatid cohesion complex. Here we show that sister-chromatid cohesion suppresses UV-induced recombination events that are initiated and resolved in G2. By comparing recombination frequencies and survival between UV and ionizing radiation, we conclude that a substantial portion of UV-induced recombination occurs through DSBs. This notion is supported by a direct physical observation of UV-induced DSBs that are dependent on nucleotide excision repair. However, a significant role of nonDSB intermediates in UV-induced recombination cannot be excluded.

Radiation-induced alterations in histone modification patterns and their potential impact on short-term radiation effects

Detection and repair of radiation-induced DNA damage occur in the context of chromatin. An intricate network of mechanisms defines chromatin structure, including DNA methylation, incorporation of histone variants, histone modifications, and chromatin remodeling. In the last years it became clear that the cellular response to radiation-induced DNA damage involves all of these mechanisms. Here we focus on the current knowledge on radiation-induced alterations in post-translational histone modification patterns and their effect on the chromatin accessibility, transcriptional regulation and chromosomal stability.

DNA damage and repair kinetics after microbeam radiation therapy emulation in living cells using monoenergetic synchrotron X-ray microbeams

A novel synchrotron-based approach, known as microbeam radiation therapy (MRT), currently shows considerable promise in increased tumour control and reduced normal tissue damage compared with conventional radiotherapy. Different microbeam widths and separations were investigated using a controlled cell culture system and monoenergetic (5.35 keV) synchrotron X-rays in order to gain further insight into the underlying cellular response to MRT. DNA damage and repair was measured using fluorescent antibodies against phosphorylated histone H2AX, which also allowed us to verify the exact location of the microbeam path. Beam dimensions that reproduced promising MRT strategies were used to identify useful methods to study the underpinnings of MRT. These studies include the investigation of different spatial configurations on bystander effects. γH2AX foci number were robustly induced in directly hit cells and considerable DNA double-strand break repair occurred by 12 h post-10 Gy irradiation; however, many cells had some γH2AX foci at the 12 h time point. γH2AX foci at later time points did not directly correspond with the targeted regions suggesting cell movement or bystander effects as a potential mechanism for MRT effectiveness. Partial irradiation of single nuclei was also investigated and in most cases γH2AX foci were not observed outside the field of irradiation within 1 h after irradiation indicating very little chromatin movement in this time frame. These studies contribute to the understanding of the fundamental radiation biology relating to the MRT response, a potential new therapy for cancer patients.

Mutagenesis dependent upon the combination of activation-induced deaminase expression and a double-strand break

We explored DNA metabolic events potentially relevant to somatic hypermutation (SHM) of immunoglobulin genes using a yeast model system. Double-strand break (DSB) formation has been discussed as a possible component of the SHM process during immunoglobulin gene maturation. Yet, possible mechanisms linking DSB formation with mutagenesis have not been well understood. In the present study, a linkage between mutagenesis in a reporter gene and a double-strand break at a distal site was examined as a function of activation-induced deaminase (AID) expression. Induction of the DSB was found to be associated with mutagenesis in a genomic marker gene located 7 kb upstream of the break site: mutagenesis was strongest with the combination of AID expression and DSB induction. The mutation spectrum of this DSB and AID-mediated mutagenesis was characteristic of replicative bypass of uracil in one strand and was dependent on expression of DNA polymerase delta (Polδ). These results in a yeast model system illustrate that the combination of DSB induction and AID expression could be associated with mutagenesis observed in SHM. Implications of these findings for SHM of immunoglobulin genes in human B cells are discussed.

Methyl angolensate from callus of Indian redwood induces cytotoxicity in human breast cancer cells

AIM

Natural products discovered from medicinal plants have played an important role in the treatment of cancer. Methyl angolensate (MA), a tetranortriterpenoid obtained from the root callus of Indian Redwood tree, Soymida febrifuga Roxb. (A.Juss) was tested for its anticancer properties on breast cancer cells.

METHODS

Cell viability was tested using trypan blue, MTT and LDH assays. Tritiated thymidine assay and flowcytometry were used to study effect of MA on cell proliferation. The activation of apoptosis was checked by annexin V and JC-1 staining followed by FACS analysis. Immunoblotting analysis was used for studying expression of apoptotic and DNA double strand break repair proteins.

RESULTS

We find that MA inhibited the growth of breast cancer cell line, T47D in a time- and dose-dependent manner. MA treatment led to the inhibition of cell proliferation as detected by tritiated thymidine assay and flowcytometry. Further, MA treated cells exhibited typical apoptotic morphological changes and led to the accumulation of subG1 peak in cell cycle distribution. The induction of apoptosis was further confirmed both by annexin V staining and JC1 staining. We also find that MA activates MAP kinase pathway to induce apoptosis. Besides, we find a time dependent activation followed by degradation of DNA double-strand break repair proteins upon treatment with MA.

CONCLUSION

These results suggest that MA induces cytotoxicity in breast cancer cells. Further, the altered expression of DSB repair proteins in MA treated cells may control the induction of apoptosis in these cancer cells.

The cellular response to DNA damage: a focus on MDC1 and its interacting proteins

The DNA damage response (DDR) is comprised of a network of proteins that respond to DNA damage. Mediator of DNA Damage Checkpoint 1 (MDC1) plays an early and important role in the DDR. Recent data show that MDC1 binds multiple proteins that participate in various aspects of the DDR, positioning it at the core of the DDR. Furthermore, interactions with non-DDR proteins were also revealed, suggesting novel roles for MDC1.In this review we provide a comprehensive overview of all known MDC1-binding proteins and discuss their role. We present these binding partners according to their function, thereby providing the reader with a detailed and updated overview of the cellular response to DNA damage. We discuss more recent findings in detail and conclude by presenting the challenges the field faces in the future.

Telomere-dependent and telomere-independent origins of endogenous DNA damage in tumor cells

Human tumors and cultured cells contain elevated levels of endogenous DNA damage resulting from telomere dysfunction, replication and transcription errors, reactive oxygen species, and genome instability. However, the contribution of telomere-associated versus telomere-independent endogenous DNA lesions to this damage has never been examined. In this study, we characterized the relative amounts of these two types of DNA damage in five tumor cell lines by noting whether γ-H2AX foci, generally considered to mark DNA double-strand breaks (DSBs), were on chromosome arms or at chromosome ends. We found that while the numbers of non-telomeric DSBs were remarkably similar in these cultures, considerable variation was detected in the level of telomeric damage. The distinct heterogeneity in the numbers of γ-H2AX foci in these tumor cell lines was found to be due to foci associated with uncapped telomeres, and the amount of total telomeric damage also appeared to inversely correlate with the telomerase activity present in these cells. These results indicate that damaged telomeres are the major factor accounting for the variability in the amount of DNA DSB damage in tumor cells. This characterization of DNA damage in tumor cells helps clarify the contribution of non-telomeric DSBs and damaged telomeres to major genomic alterations.

Homologous recombination in DNA repair and DNA damage tolerance

Homologous recombination (HR) comprises a series of interrelated pathways that function in the repair of DNA double-stranded breaks (DSBs) and interstrand crosslinks (ICLs). In addition, recombination provides critical support for DNA replication in the recovery of stalled or broken replication forks, contributing to tolerance of DNA damage. A central core of proteins, most critically the RecA homolog Rad51, catalyzes the key reactions that typify HR: homology search and DNA strand invasion. The diverse functions of recombination are reflected in the need for context-specific factors that perform supplemental functions in conjunction with the core proteins. The inability to properly repair complex DNA damage and resolve DNA replication stress leads to genomic instability and contributes to cancer etiology. Mutations in the BRCA2 recombination gene cause predisposition to breast and ovarian cancer as well as Fanconi anemia, a cancer predisposition syndrome characterized by a defect in the repair of DNA interstrand crosslinks. The cellular functions of recombination are also germane to DNA-based treatment modalities of cancer, which target replicating cells by the direct or indirect induction of DNA lesions that are substrates for recombination pathways. This review focuses on mechanistic aspects of HR relating to DSB and ICL repair as well as replication fork support.

Non-linear effects in the formation of DNA damage in medaka fish fibroblast cells caused by combined action of cadmium and ionizing radiation

Ionizing radiation-induced formation of genomic DNA damage can be modulated by nearby chemical species such as heavy metal ions, which can lead to non-linear dose response. To investigate this phenomenon, we studied cell survival and formation of 8-hydroxyguanine (8-OHG) base modifications and double strand breaks (DSB) caused by combined action of cadmium (Cd) and gamma radiation in cultured medaka fish (Oryzias latipes) fibroblast cells. Our data show that the introduction of Cd leads to a significant decrease in the fraction of surviving cells and to increased sensitivity of cells to ionizing radiation (IR). Cd also appears to cause non-linear increases in radiation-induced yields of 8-OHG and DSB as dose-yield plots of these lesions exhibit non-linear S-shaped curves with a sharp increase in the yields of lesions in the 10-20 microM range of Cd concentrations. The combined action of ionizing radiation and Cd leads to increased DNA damage formation compared to the effects of the individual stressors. These results are consistent with a hypothesis that the presence of Cd modulates the efficiency of DNA repair systems thus causing increases in radiation-induced DNA damage formation and decreases in cell survival.

Telomeres and DNA double-strand breaks: ever the twain shall meet?

Telomeres were first recognized as a bona fide constituent of the chromosome based on their inability to rejoin with broken chromosome ends produced by radiation. Today, we recognize two essential and interrelated properties of telomeres. They circumvent the so-called end-replication problem faced by genomes composed of linear chromosomes, which erode from their termini with each successive cell division. Equally vital is the end-capping function that telomeres provide, which is necessary to deter chromosome ends from illicit recombination. This latter property is critical in facilitating the distinction between the naturally occurring DNA double-strand breaks (DSBs) found at chromosome ends (i.e., telomeres) and DSBs produced by exogenous agents. Here we discuss, in a brief historical narrative, key discoveries that led investigators to appreciate the unique properties of telomeres in protecting chromosome ends, and the consequences of telomere dysfunction, particularly as related to recombination involving radiation-induced DSBs.

Atomic structure and nonhomologous end-joining function of the polymerase component of bacterial DNA ligase D

DNA ligase D (LigD) is a large polyfunctional protein that participates in a recently discovered pathway of nonhomologous end-joining in bacteria. LigD consists of an ATP-dependent ligase domain fused to a polymerase domain (Pol) and a phosphoesterase module. The Pol activity is remarkable for its dependence on manganese, its ability to perform templated and nontemplated primer extension reactions, and its preference for adding ribonucleotides to blunt DNA ends. Here we report the 1.5-A crystal structure of the Pol domain of Pseudomonas LigD and its complexes with manganese and ATP/dATP substrates, which reveal a minimized polymerase with a two-metal mechanism and a fold similar to that of archaeal DNA primase. Mutational analysis highlights the functionally relevant atomic contacts in the active site. Although distinct nucleoside conformations and contacts for ATP versus dATP are observed in the cocrystals, the functional analysis suggests that the ATP-binding mode is the productive conformation for dNMP and rNMP incorporation. We find that a mutation of Mycobacterium LigD that uniquely ablates the polymerase activity results in increased fidelity of blunt-end double-strand break repair in vivo by virtue of eliminating nucleotide insertions at the recombination junctions. Thus, LigD Pol is a direct catalyst of mutagenic nonhomologous end-joining in vivo. Our studies underscore a previously uncharacterized role for the primase-like polymerase family in DNA repair.

Homologous recombination and nonhomologous end-joining repair pathways regulate fragile site stability

Common fragile sites are specific loci that form gaps and constrictions on metaphase chromosomes exposed to replication stress, which slows DNA replication. These sites have a role in chromosomal rearrangements in tumors; however, the molecular mechanism of their expression is unclear. Here we show that replication stress leads to focus formation of Rad51 and phosphorylated DNA-PKcs, key components of the homologous recombination (HR) and nonhomologous end-joining (NHEJ), double-strand break (DSB) repair pathways, respectively. Down-regulation of Rad51, DNA-PKcs, or Ligase IV, an additional component of the NHEJ repair pathway, leads to a significant increase in fragile site expression under replication stress. Replication stress also results in focus formation of the DSB markers, MDC1 and gammaH2AX. These foci colocalized with those of Rad51 and phospho-DNA-PKcs. Furthermore, gammaH2AX and phospho-DNA-PKcs foci were localized at expressed fragile sites on metaphase chromosomes. These findings suggest that DSBs are formed at common fragile sites as a result of replication perturbation. The repair of these breaks by both HR and NHEJ pathways is essential for chromosomal stability at these sites.

Set1 is required for meiotic S-phase onset, double-strand break formation and middle gene expression

The Set1 protein of Saccharomyces cerevisiae is a histone methyltransferase (HMTase) acting on lysine 4 of histone H3. Inactivation of the SET1 gene in a diploid leads to a sporulation defect. We have studied various processes that take place during meiotic differentiation in set1delta diploid cells. The absence of Set1 leads to a delay of meiotic S-phase onset, which reflects a defect in DNA replication initiation. The timely induction of meiotic DNA replication does not require the Set1 HMTase activity, but depends on the SET domain. In addition, set1delta displays a severe impairment of the DNA double-strand break formation, which is not only the consequence of the replication delay. Transcriptional profiling experiments show that the induction of middle meiotic genes, but not of early meiotic genes, is affected by the loss of Set1. In contrast to meiotic replication, the transcriptional induction of the middle meiotic genes appears to depend on the methylation of H3-K4. Our results unveil multiple roles of Set1 in meiotic differentiation and distinguish between HMTase-dependent and -independent Set1 functions.

Role of human Pso4 in mammalian DNA repair and association with terminal deoxynucleotidyl transferase

Terminal deoxynucleotidyl transferase (TdT; EC 2.7.7.31) adds nucleotides to DNA ends generated during V(D)J recombination that are subsequently processed by proteins involved in general double-strand break (DSB) repair pathways. We report an association between TdT and a 55-kDa protein in lymphoid cells. This protein, identified as hPso4, is a homolog of the protein encoded by the PS04/PRP19 gene in Saccharomyces cerevisiae that has pleiotropic functions in DNA recombination and error-prone repair. Purified hPso4 binds double-stranded DNA in a sequence-nonspecific manner but does not bind single-stranded DNA. hPso4 protein is induced 15- to 30-fold in cells by gamma radiation and chemical mutagens but not by UV treatment. Loss of hPso4 expression induced by siRNA results in accumulation of DSBs, apoptosis, and decreased cell survival after DNA damage. We conclude that hPso4 plays a major and previously undefined role in mammalian DNA DSB repair.