Ultra-soft X-ray system for imaging the early cellular responses to X-ray induced DNA damage.

The majority of the proteins involved in processing of DNA double-strand breaks (DSBs) accumulate at the damage sites. Real-time imaging and analysis of these processes, triggered by the so-called microirradiation using UV lasers or heavy particle beams, yielded valuable insights into the underlying DSB repair mechanisms. To study the temporal organization of DSB repair responses triggered by a more clinically-relevant DNA damaging agent, we developed a system coined X-ray multi-microbeam microscope (XM3), capable of simultaneous high dose-rate (micro)irradiation of large numbers of cells with ultra-soft X-rays and imaging of the ensuing cellular responses. Using this setup, we analyzed the changes in real-time kinetics of MRE11, MDC1, RNF8, RNF168 and 53BP1-proteins involved in the signaling axis of mammalian DSB repair-in response to X-ray and UV laser-induced DNA damage, in non-cancerous and cancer cells and in the presence or absence of a photosensitizer. Our results reveal, for the first time, the kinetics of DSB signaling triggered by X-ray microirradiation and establish XM3 as a powerful platform for real-time analysis of cellular DSB repair responses.

Inhibition of homologous recombination by hyperthermia shunts early double strand break repair to non-homologous end-joining.

In S and G2 phase mammalian cells DNA double strand breaks (DSBs) can potentially be repaired by homologous recombination (HR) or non-homologous end-joining (NHEJ). Results of several studies suggest that these two mechanistically distinct repair pathways can compete for DNA ends. Because HR and NHEJ differ with respect to error susceptibility, generation of chromosome rearrangements, which are potentially carcinogenic products of DSB repair, may depend on the pathway choice. To investigate this hypothesis, the influence of HR and NHEJ inhibition on the frequencies of chromosome aberrations in G2 phase cells was investigated. SW-1573 and RKO cells were treated with mild (41 °C) hyperthermia in order to disable HR and/or NU7441/cisplatin to inactivate NHEJ and frequencies of chromosomal fragments (resulting from unrepaired DSBs) and translocations (products of erroneous DSB rejoining) were studied using premature chromosome condensation (PCC) combined with fluorescence in situ hybridization (FISH). It is shown here that temporary inhibition of HR by hyperthermia results in increased frequency of ionizing-radiation (IR)-induced chromosomal translocations and that this effect is abrogated by NU7441- or cisplatin-mediated inhibition of NHEJ. The results suggest that in the absence of HR, DSB repair is shifted to the error-prone NHEJ pathway resulting in increased frequencies of chromosomal rearrangements. These results might be of consequence for clinical cancer treatment approaches that aim at inhibition of one or more DSB repair pathways.

Mild hyperthermia inhibits homologous recombination, induces BRCA2 degradation, and sensitizes cancer cells to poly (ADP-ribose) polymerase-1 inhibition.

Defective homologous recombination (HR) DNA repair imposed by BRCA1 or BRCA2 deficiency sensitizes cells to poly (ADP-ribose) polymerase (PARP)-1 inhibition and is currently exploited in clinical treatment of HR-deficient tumors. Here we show that mild hyperthermia (41-42.5 °C) induces degradation of BRCA2 and inhibits HR. We demonstrate that hyperthermia can be used to sensitize innately HR-proficient tumor cells to PARP-1 inhibitors and that this effect can be enhanced by heat shock protein inhibition. Our results, obtained from cell lines and in vivo tumor models, enable the design of unique therapeutic strategies involving localized on-demand induction of HR deficiency, an approach that we term induced synthetic lethality.

In silico analysis of kinase expression identifies WEE1 as a gatekeeper against mitotic catastrophe in glioblastoma.

Kinases execute pivotal cellular functions and are therefore widely investigated as potential targets in anticancer treatment. Here we analyze the kinase gene expression profiles of various tumor types and reveal the wee1 kinase to be overexpressed in glioblastomas. We demonstrate that WEE1 is a major regulator of the G(2) checkpoint in glioblastoma cells. Inhibition of WEE1 by siRNA or small molecular compound in cells exposed to DNA damaging agents results in abrogation of the G(2) arrest, premature termination of DNA repair, and cell death. Importantly, we show that the small-molecule inhibitor of WEE1 sensitizes glioblastoma to ionizing radiation in vivo. Our results suggest that inhibition of WEE1 kinase holds potential as a therapeutic approach in treatment of glioblastoma.

Radiosensitizing effect of the histone acetyltransferase inhibitor anacardic acid on various mammalian cell lines.

Agents that enhance the effectiveness of ionizing radiation have been investigated over many decades. A relatively new group of potential radiosensitizers consists of agents that inhibit histone acetyltransferases (HATs). This study evaluated the radiosensitizing properties of the HAT inhibitor anacardic acid (AA), used at a low-toxic concentration of 100 µM in V79, SW1573 and U2OS cells. Radiation survival curves were analyzed according to the linear quadratic model. Significant radiosensitization by AA was only obtained in U2OS cells. AA significantly increased the value of the linear parameter α, but not of the quadratic parameter ß, indicating fixation of potentially lethal damage and an intact repair function of sublethal damage. The increase of the α value was also observed in SW1573 cells, but was not accompanied by a significant radiosensitization. A likely explanation for the enhancement of the α value may be an increase in the amount of lethal lesions due to the compacted chromatin structure. Despite the conflicting results of the radiosensitizing effect of AA in the three cell lines tested, the ability of AA to increase the α value suggests potential advantages for clinical application.

An ultrasoft X-ray multi-microbeam irradiation system for studies of DNA damage responses by fixed- and live-cell fluorescence microscopy.

Localized induction of DNA damage is a valuable tool for studying cellular DNA damage responses. In recent decades, methods have been developed to generate DNA damage using radiation of various types, including photons and charged particles. Here we describe a simple ultrasoft X-ray multi-microbeam system for high dose-rate, localized induction of DNA strand breaks in cells at spatially and geometrically adjustable sites. Our system can be combined with fixed- and live-cell microscopy to study responses of cells to DNA damage.

Heterochromatin protein 1 is recruited to various types of DNA damage.

Heterochromatin protein 1 (HP1) family members are chromatin-associated proteins involved in transcription, replication, and chromatin organization. We show that HP1 isoforms HP1-alpha, HP1-beta, and HP1-gamma are recruited to ultraviolet (UV)-induced DNA damage and double-strand breaks (DSBs) in human cells. This response to DNA damage requires the chromo shadow domain of HP1 and is independent of H3K9 trimethylation and proteins that detect UV damage and DSBs. Loss of HP1 results in high sensitivity to UV light and ionizing radiation in the nematode Caenorhabditis elegans, indicating that HP1 proteins are essential components of DNA damage response (DDR) systems. Analysis of single and double HP1 mutants in nematodes suggests that HP1 homologues have both unique and overlapping functions in the DDR. Our results show that HP1 proteins are important for DNA repair and may function to reorganize chromatin in response to damage.

Analysis of the mobility of DNA double-strand break-containing chromosome domains in living mammalian cells.

DNA double-strand breaks (DSBs) are among the most dangerous types of DNA damage. Unrepaired, DSBs may lead to cell death, and when misrejoined, they can result in potentially carcinogenic chromosome rearrangements. The induction of DSBs and their repair take place in a chromatin microenvironment. Therefore, understanding and describing the dynamics of DSB-containing chromatin is of crucial importance for understanding interactions among DSBs and their repair. Recent developments have made it possible to study ionizing radiation-induced foci of DSB repair proteins in vivo. In this chapter, we describe techniques that can be applied to visualize and analyze the spatio-temporal dynamics of DSB-containing chromatin domains in mammalian cell nuclei. Analogous procedures may also be applied to the analysis of mobility of other intranuclear structures in living cells.

Induction of linear tracks of DNA double-strand breaks by alpha-particle irradiation of cells.

Understanding how cells maintain genome integrity when challenged with DNA double-strand breaks (DSBs) is of major importance, particularly since the discovery of multiple links of DSBs with genome instability and cancer-predisposition disorders. Ionizing radiation is the agent of choice to produce DSBs in cells; however, targeting DSBs and monitoring changes in their position over time can be difficult. Here we describe a procedure for induction of easily recognizable linear arrays of DSBs in nuclei of adherent eukaryotic cells by exposing the cells to alpha particles from a small Americium source (Box 1). Each alpha particle traversing the cell nucleus induces a linear array of DSBs, typically 10-20 DSBs per 10 mum track length. Because alpha particles cannot penetrate cell-culture plastic or coverslips, it is necessary to irradiate cells through a Mylar membrane. We describe setup and irradiation procedures for two types of experiments: immunodetection of DSB response proteins in fixed cells grown in Mylar-bottom culture dishes (Option A) and detection of fluorescently labeled DSB-response proteins in living cells irradiated through a Mylar membrane placed on top of the cells (Option B). Using immunodetection, recruitment of repair proteins to individual DSB sites as early as 30 s after irradiation can be detected. Furthermore, combined with fluorescence live-cell microscopy of fluorescently tagged DSB-response proteins, this technique allows spatiotemporal analysis of the DSB repair response in living cells. Although the procedures might seem a bit intimidating, in our experience, once the source and the setup are ready, it is easy to obtain results. Because the live-cell procedure requires more hands-on experience, we recommend starting with the fixed-cell application.

Polyglutamine expansion accelerates the dynamics of ataxin-1 and does not result in aggregate formation.

BACKGROUND: Polyglutamine expansion disorders are caused by an expansion of the polyglutamine (polyQ) tract in the disease related protein, leading to severe neurodegeneration. All polyQ disorders are hallmarked by the presence of intracellular aggregates containing the expanded protein in affected neurons. The polyQ disorder SpinoCerebellar Ataxia 1 (SCA1) is caused by a polyQ-expansion in the ataxin-1 protein, which is thought to lead to nuclear aggregates. METHODOLOGY/PRINCIPAL FINDINGS: Using advanced live cell fluorescence microscopy and a filter retardation assay we show that nuclear accumulations formed by polyQ-expanded ataxin-1 do not resemble aggregates of other polyQ-expanded proteins. Instead of being static, insoluble aggregates, nuclear accumulations formed by the polyQ-expanded ataxin-1 showed enhanced intracellular kinetics as compared to wild-type ataxin-1. During mitosis, ataxin-1 accumulations redistributed equally among daughter cells, in contrast to polyQ aggregates. Interestingly, polyQ expansion did not affect the nuclear-cytoplasmic shuttling of ataxin-1 as proposed before. CONCLUSIONS/SIGNIFICANCE: These results indicate that polyQ expansion does not necessarily lead to aggregate formation, and that the enhanced kinetics may affect the nuclear function of ataxin-1. The unexpected findings for a polyQ-expanded protein and their consequences for ongoing SCA1 research are discussed.

Mitochondrial PO2 measured by delayed fluorescence of endogenous protoporphyrin IX.

Molecular oxygen is the primary oxidant in biological systems. The ultimate destination of oxygen in vivo is the mitochondria where it is used in oxidative phosphorylation. The ability of this process to produce an amount of high-energy phosphates adequate to sustain life highly depends on the available amount of oxygen. Despite a vast array of techniques to measure oxygen, major (patho)physiological questions remain unanswered because of the unavailability of quantitative techniques to measure mitochondrial oxygen in situ. Here we demonstrate that mitochondrial PO(2) can be directly measured in living cells by harnessing the delayed fluorescence of endogenous protoporphyrin IX (PpIX), thereby providing a technique with the potential for a wide variety of applications. We applied this technique to different cell lines (V-79 Chinese hamster lung fibroblasts, HeLa cells and IMR 32-K1 neuroblastoma cells) and present the first direct measurements of the oxygen gradient between the mitochondria and the extracellular volume.

Dynamics of DNA double-strand breaks revealed by clustering of damaged chromosome domains.

Interactions between ends from different DNA double-strand breaks (DSBs) can produce tumorigenic chromosome translocations. Two theories for the juxta-position of DSBs in translocations, the static "contact-first" and the dynamic "breakage-first" theory, differ fundamentally in their requirement for DSB mobility. To determine whether or not DSB-containing chromosome domains are mobile and can interact, we introduced linear tracks of DSBs in nuclei. We observed changes in track morphology within minutes after DSB induction, indicating movement of the domains. In a subpopulation of cells, the domains clustered. Juxtaposition of different DSB-containing chromosome domains through clustering, which was most extensive in G1 phase cells, suggests an adhesion process in which we implicate the Mre11 complex. Our results support the breakage-first theory to explain the origin of chromosomal translocations.