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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

DNA double-strand break movement in heterochromatin depends on the histone acetyltransferase dGcn5

Abstract Cells employ diverse strategies to repair double-strand breaks (DSBs), a dangerous form of DNA damage that threatens genome integrity. Eukaryotic nuclei consist of different chromatin environments, each displaying distinct molecular and biophysical properties that can significantly influence the DSB-repair process. DSBs arising in the compact and silenced heterochromatin domains have been found to move to the heterochromatin periphery in mouse and Drosophila to prevent aberrant recombination events. However, it is poorly understood how chromatin components, such as histone post-translational modifications, contribute to these DSB movements within heterochromatin. Using irradiation as well as locus-specific DSB induction in Drosophila tissues and cultured cells, we find enrichment of histone H3 lysine 9 acetylation (H3K9ac) at DSBs in heterochromatin but not euchromatin. We find this increase is mediated by the histone acetyltransferase dGcn5, which rapidly localizes to heterochromatic DSBs. Moreover, we demonstrate that in the absence of dGcn5, heterochromatic DSBs display impaired recruitment of the SUMO E3 ligase Nse2/Qjt and fail to relocate to the heterochromatin periphery to complete repair. In summary, our results reveal a previously unidentified role for dGcn5 and H3K9ac in heterochromatic DSB repair and underscore the importance of differential chromatin responses at heterochromatic and euchromatic DSBs to promote safe repair.

Biochemistry & Molecular Biology↗
DOE OSTI2024

The lowdown on breakdown: Open questions in plant proteolysis

Abstract Proteolysis, including post-translational proteolytic processing as well as protein degradation and amino acid recycling, is an essential component of the growth and development of living organisms. In this article, experts in plant proteolysis pose and discuss compelling open questions in their areas of research. Topics covered include the role of proteolysis in the cell cycle, DNA damage response, mitochondrial function, the generation of N-terminal signals (degrons) that mark many proteins for degradation (N-terminal acetylation, the Arg/N-degron pathway, and the chloroplast N-degron pathway), developmental and metabolic signaling (photomorphogenesis, abscisic acid and strigolactone signaling, sugar metabolism, and postharvest regulation), plant responses to environmental signals (endoplasmic-reticulum-associated degradation, chloroplast-associated degradation, drought tolerance, and the growth-defense trade-off), and the functional diversification of peptidases. We hope these thought-provoking discussions help to stimulate further research.

Eckardt, Nancy A. (ORCID:0000000316581412)↗
DOE OSTI2026

Photonuclear cross sections for the 197 Au ⁢(𝛾, 𝑝⁢𝑛)⁢ 195⁢𝑚 Pt reaction near threshold

Platinum radioisotopes are of growing interest for targeted cancer therapy and diagnostic imaging because their decay delivers highly localized radiation doses in tissue, herewith enabling precise DNA damage through Auger-electron emission. Developing production technologies that provide platinum isotopes with high specific activity is essential in radioisotope therapy. Photonuclear reactions on stable nuclei offer a viable accelerator-based route for isotope production when supported by reliable cross-section data. We report photonuclear cross-section measurements for the 197 Au(γ, pn) 195m Pt reaction at incident γ-ray energies of 27, 29, and 31 MeV using the activation method. The measurements were performed by irradiating a stack of concentric-ring gold targets with a quasi-monoenergetic γ-ray beam provided by the High Intensity Gamma-ray Source (HI γS). The induced 195m Pt activity was quantified using off-line γ-ray spectroscopy. These data provide the first experimental constraints on the 197 Au(γ, pn) 195m Pt cross section in the near-threshold region. Furthermore, the comparison of the measured excitation function to PHITS and TALYS calculations indicates that the reaction becomes measurable only near 30 MeV and that substantially higher bremsstrahlung end-point energies are required for practically meaningful production.

190 ≤ A ≤ 219↗
NASA NTRS1986

In vitro cytogenetic studies of organic chemicals found as contaminants in spacecraft cabin atmospheres

Astronauts can be exposed during spaceflight to organic chemical contaminants in the spacecraft cabin atmosphere. Toxic exposures may cause lesions in the cellular DNA which are subsequently expressed as sister-chromatid exchanges (SCE). Analysis of SCE is a sensitive short-term assay technique to detect and quantitate exposures to DNA-damaging (mutagenic) substances. The increase in SCE incidence over baseline (control) levels is generally proportional to the concentration of the mutagen and to the duration of exposure. Dichloromethane (methylene chloride) was chosen for this study since it occurred as an atmospheric contaminant in ten of the first 12 STS flights, and has been reported to have toxic and mutagenic effects in various test systems. Glutaraldehyde was chosen because relatively few data are available on the toxicity or mutagenicity of this common biological fixative, which is carried on STS flights for use in biological experiments. The BHK-21 baby hamster kidney cell line was the in vitro test system used in this study. Neither dichloromethane (10 ppm to 500 ppm) nor glutaraldehyde (1 ppm to 10 ppm) increased SCE levels following 20-hour exposure of BHK-21 cells to the test chemicals.

Torres, J.↗
NASA NTRS1987

Genetic toxicity studies of organic chemicals found as contaminants in spacecraft cabin atmospheres

Astronauts can be exposed during spaceflight to organic chemical contaminants in the spacecraft cabin atmosphere. Toxic exposures may cause lesions in the cellular DNA which are subsequently expressed as sister-chromatid exchanges (SCE). Analysis of SCE is a sensitive short term assay techinque to detect and quantitate exposures to DNA damaging (mutagenic) substances. The increase in SCE incidence over baseline (control) levels is generally proportional to the concentration of the mutagen and to the duration of exposure. The BHK-21 baby hamster kidney cell line was the in vitro test system used. Test organics were added to the culture media for 18 hrs, in concentrations ranging from one to 20 ppm. Acetaldehyde and carbon disulfide were chosen for this study since they have occurred as atmospheric contaminants in many of the STS flights, and have been reported to have toxic and mutagenic effects in various test systems. Glutaraldehyde was chosen because few data are available on the mutagenicity of this common fixative, which is carried on STS flights for use in biological experiments. Acetaldehyde was a very strong inducer of SCE at concentrations of 2 ppm and above. Glutaraldehyde and carbon disulfide failed to induce SCE.

Torres, Joseph, Jr.↗
NASA NTRS1992

Microgravitational effects on chromosome behavior (7-IML-1)

The effects of the two major space-related conditions, microgravity and radiation, on the maintenance and transmission of genetic information have been partially documented in many organisms. Specifically, microgravity acts at the chromosomal level, primarily on the structure and segregation of chromosomes, in producing major abberations such as deletions, breaks, nondisjunction, and chromosome loss, and to a lesser degree, cosmic radiation appears to affect the genic level, producing point mutations and DNA damage. To distinguish between the effects from microgravity and from radiation, it is necessary to monitor both mitotic and meiotic genetic damage in the same organism. The yeast Saccharomyces cerevisiae is used to monitor at high resolution the frequency of chromosome loss, nondisjunction, intergenic recombination, and gene mutation in mitotic and meiotic cells, to a degree impossible in other organisms. Because the yeast chromosomes are small, sensitive measurements can be made that can be extrapolated to higher organisms and man. The objectives of the research are: (1) to quantitate the effects of microgravity and its synergism with cosmic radiation on chromosomal integrity and transmission during mitosis and meiosis; (2) to discriminate between chromosomal processes sensitive to microgravity and/or radiation during mitosis and meiosis; and (3) to relate these findings to anomalous mitotic mating type switching and ascosporogenesis following meiosis.

Bruschi, Carlo↗
NASA NTRS1993

Potential health effects of space radiation

Crewmembers on missions to the Moon or Mars will be exposed to radiation belts, galactic cosmic rays, and possibly solar particle events. The potential health hazards due to these space radiations must be considered carefully to ensure the success of space exploration. Because there is no human radioepidemiological data for acute and late effects of high-LET (Linear-Energy-Transfer) radiation, the biological risks of energetic charged particles have to be estimated from experimental results on animals and cultured cells. Experimental data obtained to date indicate that charged particle radiation can be much more effective than photons in causing chromosome aberrations, cell killing, mutation, and tumor induction. The relative biological effectiveness (RBE) varies with biological endpoints and depends on the LET of heavy ions. Most lesions induced by low-LET radiation can be repaired in mammalian cells. Energetic heavy ions, however, can produce large complex DNA damages, which may lead to large deletions and are irreparable. For high-LET radiation, therefore, there are less or no dose rate effects. Physical shielding may not be effective in minimizing the biological effects on energetic heavy ions, since fragments of the primary particles can be effective in causing biological effects. At present the uncertainty of biological effects of heavy particles is still very large. With further understanding of the biological effects of space radiation, the career doses can be kept at acceptable levels so that the space radiation environment need not be a barrier to the exploitation of the promise of space.

Yang, Chui-Hsu↗
NASA NTRS1993

Space radiation health research, 1991-1992

The present volume is a collection of 227 abstracts of radiation research sponsored by the NASA Space Radiation Health Program for the period 1991-1992. Each abstract has been categorized within one of three discipline areas: Physics, Biology and Risk Assessment. Topic areas within each discipline have been assigned as follows: Physics - Atomic Physics, Theory, Cosmic Ray and Astrophysics, Experimental, Environments and Environmental Models, Solar Activity and Prediction, Experiments, Radiation Transport and Shielding, Theory and Model Development, Experimental Studies, and Instrumentation. Biology - Biology, Molecular Biology, Cellular Radiation Biology, Transformation, Mutation, Lethality, Survival, DNA Damage and Repair, Tissue, Organs, and Organisms, In Vivo/In Vitro Systems, Carcinogenesis and Life Shortening, Cataractogenesis, Genetics/Developmental, Radioprotectants, Plants, and Other Effects. Risk Assessment - Risk Assessment, Radiation Health and Epidemiology, Space Flight Radiation Health Physics, Inter- and Intraspecies Extrapolation and Radiation Limits and Standards. Section I contains refereed journals; Section II contains reports/meetings. Keywords and author indices are provided. A collection of abstracts spanning the period 1986-1990 was previously issued as NASA Technical Memorandum 4270.

Jablin, M. H.↗
NASA NTRS2003

Characterization of a novel epigenetic effect of ionizing radiation: the death-inducing effect

The detrimental effects associated with exposure to ionizing radiation have long been thought to result from the direct targeting of the nucleus leading to DNA damage; however, the emergence of concepts such as radiation-induced genomic instability and bystander effects have challenged this dogma. After cellular exposure to ionizing radiation, we have isolated a number of clones of Chinese hamster-human hybrid GM10115 cells that demonstrate genomic instability as measured by chromosomal destabilization. These clones show dynamic and persistent generation of chromosomal rearrangements multiple generations after the original insult. We hypothesize that these unstable clones maintain this delayed instability phenotype by secreting factors into the culture medium. To test this hypothesis we transferred filtered medium from unstable cells to unirradiated GM10115 cells. No GM10115 cells were able to survive this medium. This phenomenon by which GM10115 cells die when cultured in medium from chromosomally unstable GM10115 clones is the death-inducing effect. Medium transfer experiments indicate that a factor or factors is/are secreted by unstable cells within 8 h of growth in fresh medium and result in cell killing within 24 h. These factors are stable at ambient temperature but do not survive heating or freezing, and are biologically active when diluted with fresh medium. We present the initial description and characterization of the death-inducing effect. This novel epigenetic effect of radiation has implications for radiation risk assessment and for health risks associated with radiation exposure.

Non-NASA Center↗
NASA NTRS2002

Relative biological effectiveness of light ions in human tumoural cell lines: role of protein p53

Protons and alpha particles of high linear energy transfer (LET) have shown an increased relative biological effectiveness (RBE) with respect to X/gamma rays for several cellular and molecular endpoints in different in vitro cell systems. To contribute to understanding the biochemical mechanisms involved in the increased effectiveness of high LET radiation, an extensive study has been designed. The present work reports the preliminary result of this study on two human tumoural cell lines, DLD1 and HCT116, (with different p53 status), which indicate that for these cell lines, p53 does not appear to take a part in the response to radiation induced DNA damage, suggesting an alternative p53-independent pathway and a cell biochemical mechanism dependent on the cell type.

NASA Center JSC↗
NASA NTRS2002

Gene conversion is strongly induced in human cells by double-strand breaks and is modulated by the expression of BCL-x(L)

Homology-directed repair (HDR) of DNA double-strand breaks (DSBs) contributes to the maintenance of genomic stability in rodent cells, and it has been assumed that HDR is of similar importance in DSB repair in human cells. However, some outcomes of homologous recombination can be deleterious, suggesting that factors exist to regulate HDR. We demonstrated previously that overexpression of BCL-2 or BCL-x(L) enhanced the frequency of X-ray-induced TK1 mutations, including loss of heterozygosity events presumed to arise by mitotic recombination. The present study was designed to test whether HDR is a prominent DSB repair pathway in human cells and to determine whether ectopic expression of BCL-x(L) affects HDR. Using TK6-neo cells, we find that a single DSB in an integrated HDR reporter stimulates gene conversion 40-50-fold, demonstrating efficient DSB repair by gene conversion in human cells. Significantly, DSB-induced gene conversion events are 3-4-fold more frequent in TK6 cells that stably overexpress the antiapoptotic protein BCL-X(L). Thus, HDR plays an important role in maintaining genomic integrity in human cells, and ectopic expression of BCL-x(L) enhances HDR of DSBs. This is the first study to highlight a function for BCL-x(L) in modulating DSB repair in human cells.

NASA Discipline Radiation Health↗
NASA NTRS2001

Elevated breast cancer risk in irradiated BALB/c mice associates with unique functional polymorphism of the Prkdc (DNA-dependent protein kinase catalytic subunit) gene

Female BALB/c mice are unusually radiosensitive and more susceptible than C57BL/6 and other tested inbred mice to ionizing radiation (IR)-induced mammary tumors. This breast cancer susceptibility is correlated with elevated susceptibility for mammary cell transformation and genomic instability following irradiation. In this study, we report the identification of two BALB/c strain-specific polymorphisms in the coding region of Prkdc, the gene encoding the DNA-dependent protein kinase catalytic subunit, which is known to be involved in DNA double-stranded break repair and post-IR signal transduction. First, we identified an A --> G transition at base 11530 resulting in a Met --> Val conversion at codon 3844 (M3844V) in the phosphatidylinositol 3-kinase domain upstream of the scid mutation (Y4046X). Second, we identified a C --> T transition at base 6418 resulting in an Arg --> Cys conversion at codon 2140 (R2140C) downstream of the putative leucine zipper domain. This unique PrkdcBALB variant gene is shown to be associated with decreased DNA-dependent protein kinase catalytic subunit activity and with increased susceptibility to IR-induced genomic instability in primary mammary epithelial cells. The data provide the first evidence that naturally arising allelic variation in a mouse DNA damage response gene may associate with IR response and breast cancer risk.

Non-NASA Center↗
NASA NTRS1999

Monte Carlo mixture model of lifetime cancer incidence risk from radiation exposure on shuttle and international space station

Estimating uncertainty in lifetime cancer risk for human exposure to space radiation is a unique challenge. Conventional risk assessment with low-linear-energy-transfer (LET)-based risk from Japanese atomic bomb survivor studies may be inappropriate for relativistic protons and nuclei in space due to track structure effects. This paper develops a Monte Carlo mixture model (MCMM) for transferring additive, National Institutes of Health multiplicative, and multiplicative excess cancer incidence risks based on Japanese atomic bomb survivor data to determine excess incidence risk for various US astronaut exposure profiles. The MCMM serves as an anchor point for future risk projection methods involving biophysical models of DNA damage from space radiation. Lifetime incidence risks of radiation-induced cancer for the MCMM based on low-LET Japanese data for nonleukemia (all cancers except leukemia) were 2.77 (90% confidence limit, 0.75-11.34) for males exposed to 1 Sv at age 45 and 2.20 (90% confidence limit, 0.59-10.12) for males exposed at age 55. For females, mixture model risks for nonleukemia exposed separately to 1 Sv at ages of 45 and 55 were 2.98 (90% confidence limit, 0.90-11.70) and 2.44 (90% confidence limit, 0.70-10.30), respectively. Risks for high-LET 200 MeV protons (LET=0.45 keV/micrometer), 1 MeV alpha-particles (LET=100 keV/micrometer), and 600 MeV iron particles (LET=180 keV/micrometer) were scored on a per particle basis by determining the particle fluence required for an average of one particle per cell nucleus of area 100 micrometer(2). Lifetime risk per proton was 2.68x10(-2)% (90% confidence limit, 0.79x10(-3)%-0. 514x10(-2)%). For alpha-particles, lifetime risk was 14.2% (90% confidence limit, 2.5%-31.2%). Conversely, lifetime risk per iron particle was 23.7% (90% confidence limit, 4.5%-53.0%). Uncertainty in the DDREF for high-LET particles may be less than that for low-LET radiation because typically there is very little dose-rate dependence. Probability density functions for high-LET radiation quality and dose-rate may be preferable to conventional risk assessment approaches. Nuclear reactions and track structure effects in tissue may not be properly estimated by existing data using in vitro models for estimating RBEs. The method used here is being extended to estimate uncertainty in spacecraft shielding effectiveness in various space radiation environments.

NASA Center LaRC↗
NASA NTRS1998

Joining of correct and incorrect DNA ends at double-strand breaks produced by high-linear energy transfer radiation in human fibroblasts

DNA double-strand breaks (DSBs) were measured within a 3.2-Mbp NotI fragment on chromosome 21 of cells of a normal human fibroblast cell line. Correct rejoining of DSBs was followed by measuring reconstitution of the original-size NotI fragment, and this was compared to total rejoining as measured by a conventional pulsed-field gel electrophoresis technique (FAR assay). After 80 Gy of particle irradiations with LETs in the range of 7-150 keV/microm, it was found that the repair kinetics was generally slower after irradiation with high-LET particles compared to X irradiation and that a larger proportion of the breaks remained unrepaired after 24 h. On the other hand, the misrejoining frequency as measured by the difference between correct and total rejoining after 24 h did not change with LET, but was approximately the same for all radiations at this dose, equal to 25-30% of the initial breaks. This result is discussed in relation to formation of chromosomal aberrations, deletion mutations and other biological end points.

NASA Discipline Radiation Health↗
NASA NTRS1996

Rejoining and misrejoining of radiation-induced chromatin breaks. II. Biophysical Model

A biophysical model for the kinetics of the formation of radiation-induced chromosome aberrations is developed to account for the recent experimental results obtained with a combination of the premature chromosome condensation (PCC) and fluorescence in situ hybridization (FISH) techniques. In this model, we consider the broken ends of DNA double-strand breaks (DSBs) to be reactant and make use of the interaction distance hypothesis. The repair/misrepair process between broken ends is suggested to consist of two steps; the first step represents the two break ends approaching each other, and the second step represents the enzymatic processes leading to DNA end-to-end rejoining. Only the second step is reflected in the kinetics observed in experiments using PCC. The model appears to be able to fit existing data for human cells. It is shown that the kinetics of the formation of chromosome aberrations can be explained by a single rate that characterizes both rejoining and misrejoining of DSBs, suggesting that repair and misrepair share the same mechanism. Fast repair (completed in minutes) in a subset of DSBs is suggested as an explanation of the complete exchanges observed with PCC in human lymphocytes immediately after irradiation. The fast repair component seems to be absent in human fibroblasts.

NASA Center JSC↗
NASA NTRS2005

NBS1 knockdown by small interfering RNA increases ionizing radiation mutagenesis and telomere association in human cells

Hypomorphic mutations which lead to decreased function of the NBS1 gene are responsible for Nijmegen breakage syndrome, a rare autosomal recessive hereditary disorder that imparts an increased predisposition to development of malignancy. The NBS1 protein is a component of the MRE11/RAD50/NBS1 complex that plays a critical role in cellular responses to DNA damage and the maintenance of chromosomal integrity. Using small interfering RNA transfection, we have knocked down NBS1 protein levels and analyzed relevant phenotypes in two closely related human lymphoblastoid cell lines with different p53 status, namely wild-type TK6 and mutated WTK1. Both TK6 and WTK1 cells showed an increased level of ionizing radiation-induced mutation at the TK and HPRT loci, impaired phosphorylation of H2AX (gamma-H2AX), and impaired activation of the cell cycle checkpoint regulating kinase, Chk2. In TK6 cells, ionizing radiation-induced accumulation of p53/p21 and apoptosis were reduced. There was a differential response to ionizing radiation-induced cell killing between TK6 and WTK1 cells after NBS1 knockdown; TK6 cells were more resistant to killing, whereas WTK1 cells were more sensitive. NBS1 deficiency also resulted in a significant increase in telomere association that was independent of radiation exposure and p53 status. Our results provide the first experimental evidence that NBS1 deficiency in human cells leads to hypermutability and telomere associations, phenotypes that may contribute to the cancer predisposition seen among patients with this disease.

Non-NASA Center↗