Radiation-induced electron migration in nucleic acids

Influence of Chromatin Structure and Radical Scavengers on Yields of Radiation-Induced 8-oxo-dG and DNA Strand Breaks in Cellular Model Systems

Radiation-induced formation of 7,8-dihydro-8-oxo-2'-deoxyguanosine (8-oxo-dG) and DNA strand breaks was studied in cultured cells with normal or modified chromatin structure. Human fibroblasts were irradiated as cellular monolayers (intact cells), nuclear monolayers (permeabilized cells with intact chromatin structure), and nucleoid monolayers (permeabilized and salt-treated cells with histone-free DNA). 8-oxo-dG was assayed with reverse-phase HPLC coupled to an electrochemical detector and strand breaks with the alkali unwinding assay. Depletion of low-molecular-weight nuclear components increased the radiation-induced formation of 8-oxo-dG fivefold compared to twofold for the formation of strand breaks. Removal of both low-molecular-weight components and histones increased the yield of 8-oxo-dG 46-fold and the yield of strand breaks 43-fold. Removal of only the histones thus leads to a two times greater increase in the yield of strand breaks compared to 8-oxo-dG. Addition of radical scavengers to nuclear and nucleoid monolayers provided a significantly better protection against the formation of 8-oxo-dG relative to the formation of strand breaks. These results suggest that in intact cells, 8-oxo-dG is preferentially formed in histone-free structures of chromatin, indicating a larger role for the indirect effect of radiation in the formation of 8-oxo-dG than in the formation of strand breaks.

Radiation damage of biosystems mediated by secondary electrons: Resonant precursors for uracil molecules

Calculations are presented for the energy locations and spatial structures of low-energy resonant states describing transient negative ions (TNIs) of the uracil molecule in the gas phase. The resonant states are modeled using scattering calculations of low energy electrons interacting with isolated molecules in their equilibrium geometry. The interaction forces used in this model are described in detail. Examination of the spatial densities of the excess resonant electrons for the various TNIs found by the calculations allows one to associate the metastable anions with specific features of the experimentally observed fragmentation patterns.

Nanoscopic aspects of radiobiological damage: Fragmentation induced by secondary low-energy electrons

Low-energy electrons (LEEs) are produced in large quantities in any type of material irradiated by high-energy particles. In biological media, these electrons can fragment molecules and lead to the formation of highly reactive radicals and ions. The results of recent experiments performed on biomolecular films bombarded with LEEs under ultra-high vacuum conditions are reviewed in the present article. The major type of experiments, which measure fragments produced in such films as a function of incident electron energy (0.1-45 eV), are briefly described. Examples of the results obtained from DNA films are summarized along with those obtained from the fragmentation of elementary components of the DNA molecule (i.e., thin solid films of H(2)O, DNA bases, sugar analogs, and oligonucleotides) and proteins. By comparing the results of these different experiments, it is possible to determine fundamental mechanisms that are involved in the dissociation of biomolecules and the production of single- and double-strand breaks in DNA, and to show that base damage is dependent on the nature of the bases and on their sequence context. Below 15 eV, electron resonances (i.e., the formation of transient anions) play a dominant role in the fragmentation of all biomolecules investigated. These transient anions fragment molecules by decaying into dissociative electronically excited states or by dissociating into a stable anion and a neutral radical. These fragments usually initiate other reactions with nearby molecules, causing further chemical damage. The damage caused by transient anions is dependent on the molecular environment.

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.

Reductively activated cleavage of DNA mediated by o, o'-diphenylenehalonium compounds

o,o'-Diphenylenehalonium (DPH) cations represent a novel class of DNA-affinic compounds characterized by binding constants within the range of 10(5)-10(6) M(-1). The maximum binding capacity of 2-2.5 base pairs per DPH cation and about 30% hypochromic reduction in the optical absorption of DPH cations upon binding to DNA suggest intercalation as a likely binding mode. In a DNA-bound form, DPH cations induce strand breaks upon reduction by radiation-produced electrons in aqueous solutions. In keeping with this mechanism, the cleavage is strongly inhibited by oxygen and is not affected by OH radical scavengers in the bulk. The yields of DPH-mediated base release significantly exceed the yield of base release caused by hydroxyl radical (in the absence of scavenger) in anoxic solutions. The yields are weakly dependent on DNA loading within the range from 5 to 50 base pairs per intercalator, which indicates the ability of excess electrons in DNA to react with a scavenger separated by tens of base pairs from the electron attachment site. The question regarding the mechanism by which the distant reactants reach each other in DNA remains unanswered, although it most likely involves electron hopping rather than a single-step long-distance tunneling. The latter conclusion is based on our finding that the electron affinity of DPH cations does not affect their properties as electron scavengers in DNA as would be expected if the direct long-distance tunneling is involved.

Ultraviolet-induced detection of halogenated pyrimidines: simultaneous analysis of DNA replication and cellular markers

BACKGROUND

We describe a new nonenzymatic methodology that allows the simultaneous detection of DNA replication and other cellular markers such as immunophenotyping. DNA replicating cells are identified by their incorporation of halogenated thymidine analogs, e.g., 5-bromo-deoxyuridine (BrdUrd).

METHODS

Irradiation with ultraviolet (UV)-B or UV-A light in the presence of Hoechst 33258 and subsequent treatment with a hypotonic buffer makes BrdUrd accessible to monoclonal antibodies (mAb), thus allowing its sensitive detection.

RESULTS

The photolysis of BrdUrd in DNA with UV light is sequence dependent and results in DNA damage, allowing the detection of remaining BrdUrd using hypotonic conditions. However, treatment with other inducers of single or double- strand breaks of DNA such as gamma irradiation or hydrogen peroxide did not allow BrdUrd detection. The new methodology is compatible with both mild crosslinking fixation, i.e., aldehydes, or coagulative fixation, i.e., alcohols. The successful identification of CD34+, CD138+, or CD19+ cells out of heterogeneous cell suspensions and their cell-cycle analysis are described. Results correlated very well with acid denaturation (r = 0.972). The average coefficient of variation (CV) of G(1) in the DNA histogram was smaller than 5%, resulting in good preservation of DNA distribution. Also, the signal-to-noise ratio was almost twice as high as for 2N acid denaturation, facilitating convenient discrimination of BrdUrd-positive cells.

CONCLUSIONS

In contrast to previous approaches, this methodology eliminates the need for any additional enzymatic treatment such as DNA digestion or strand-break labeling after UV irradiation. The method is fast, convenient, and inexpensive and should be able to promote the use of halogenated pyrimidines in basic and clinical research of cancer, immunology, and pharmacology.

DNA strand breakage by 125I-decay in a synthetic oligodeoxynucleotide--2. Quantitative analysis of fragment distribution

The DNA breakage produced by decay of 125I in a double-stranded 41 bp oligodeoxynucleotide was investigated by DNA sequencing gel analysis. Use of both 5'- and 3'-end 32P labelling of the 125I containing strand provided the single-stranded breakage pattern at either side of the 125I-dC. The asymmetric pattern of breakage relative to the 125I-labelled nucleotide enabled deconvolution of two components of breakage. One of them declines very quickly with nucleotide number from 125I-dC and dominates within 4-5 nucleotides. We assume this component to be associated with neutralisation of charged Te atom resulted from decay, or/and with radiation damage to an initial target other than the deoxyribosyl moiety--probably the bases. The second component depends on the geometrical distance between the 125I atom and deoxyribosyl atoms. These two components are responsible for breakage under conditions limiting radical mediated damage, namely in the presence of 2M dimethylsulphoxide. The third component, associated with radical-mediated damage, contributes to the total breakage during incubation in 20 mM phosphate buffer alone, and under these incubation conditions dominates the breakage beyond 8-9 nucleotides from 125I-dC. The estimated average numbers of single-stranded breaks produced in the 125I-containing strand by each mechanism in 41-mer oligodeoxynucleotide with 125I-dC at 21st position are respectively 2.33, 0.98 and 0.63.