DNA damage of exposed tumour tissue leading to cell death is one of the detrimental effects of ionising radiation that is exploited, with beneficial consequences, for radiotherapy. The pattern of the discrete energy depositions during passage of the ionising track of radiation defines the spatial distribution of lesions induced in DNA with a fraction of the DNA damage sites containing clusters of lesions, formed over a few nanometres, against a background of endogenously induced individual lesions. These clustered DNA damage sites, which may be considered as a signature of ionising radiation, underlie the deleterious biological consequences of ionising radiation. The concepts developed rely in part on the fact that ionising radiation creates significant levels of clustered DNA damage, including complex double-strand breaks (DSB), to kill tumour cells as clustered damage sites are difficult to repair. This reduced repairability of clustered DNA damage using specific repair pathways is exploitable in radiotherapy for the treatment of cancer. We discuss some potential strategies to enhance radiosensitivity by targeting the repair pathways of radiation-induced clustered damage and complex DNA DSB, through inhibition of specific proteins that are not required in the repair pathways for endogenous damage. The variety and severity of DNA damage from ionising radiation is also influenced by the tumour microenvironment, being especially sensitive to the oxygen status of the cells. For instance, nitric oxide is known to influence the types of damage induced by radiation under hypoxic conditions. A potential strategy based on bioreductive activation of pro-drugs to release nitric oxide is discussed as an approach to deliver nitric oxide to hypoxic tumours during radiotherapy. The ultimate aim of this review is to stimulate thinking on how knowledge of the complexity of radiation-induced DNA damage may contribute to the development of adjuncts to radiotherapy.

To study the characteristics of molecular damage induced by ionizing radiation at the DNA level, Monte Carlo track simulation of energetic electrons and ions in liquid water, a canonical model of B-DNA, and a comprehensive classification of DNA damage in terms of the origin and complexity of damage were used to calculate the frequencies of simple and complex strand breaks. A threshold energy of 17.5 eV was used to model the damage by direct energy deposition, and a probability of 0.13 was applied to model the induction of a single-strand break produced in DNA by OH radical reactions. For preliminary estimates, base damage was assumed to be induced by the same direct energy threshold deposition or by the reaction of an OH radical with the base, with a probability of 0.8. Computational data are given on the complexity of damage, including base damage by electrons with energies of 100-4500 eV and ions with energies of 0.3-4.0 MeV/nucleon (59-9 keV microm(-1) protons and 170-55 keV microm(-1) alpha particles). Computational data are presented on the frequencies of single- and double-strand breaks induced as a function of the LET of the particles, and on the relative frequencies of complex single- and double-strand breaks for electrons. The modeling and calculations of strand breaks show that: (1) The yield of strand breaks per unit absorbed dose is nearly constant over a wide range of LET. (2) The majority of DNA damage is of a simple type, but the majority of the simple single-strand breaks are accompanied by at least one base damage. (3) For low-energy electrons, nearly 20-30% of the double-strand breaks are of a complex type by virtue of additional breaks. The proportion of this locally clustered damage increases with LET, reaching about 70% for the highest-LET alpha particles modeled, with the complexity of damage increasing further, to about 90%, when base damage is considered. (4) The extent of damage in the local hit region of the DNA duplex is mostly limited to a length of a few base pairs. (5) The frequency of base damage when no strand breaks are present in the hit segment of DNA varies between 20-40% as a function of LET for protons and alpha particles.

The accumulated evidence in the literature indicates that a cluster of two or more lesions within one or two helical turns of the DNA is more challenging to repair than individual, widely dispersed lesions. The biological importance of clustered DNA lesions, especially complex double-strand breaks (DSB) and some types of non-DSB clusters (e.g., opposed bases that are oxidized), are now well known within the radiation research community. Still, many details of the induction and biological processing of complex clusters remain to be elucidated, especially in human cells. In this mini-review, we discuss recent advances in our understanding of the pathway(s) used by the mammalian cells to process and efficiently repair complex clusters other than the DSB. The effects of radiation quality and hypoxia on cluster induction and complexity are also briefly reviewed and discussed. Additional research is needed to better understand and quantify the multi-scale physiochemical and biological processes ultimately responsible for radiation-induced mutagenesis and genomic instability. New information and models to better quantify intermediate events (outcomes) related to the biological processing of non-DSB clusters are also important for ongoing efforts to assess the human health risks of terrestrial and space radiation environments and to guide the radiation therapy treatment planning process, especially for protons and carbon ions.

DNA double-strand breaks (DSBs) are biologically one of the most important cellular lesions and possess varying degrees of chemical complexity. The notion that the repairability of more chemically complex DSBs is inefficient led to the concept that the extent of DSB complexity underlies the severity of the biological consequences. The repair of DSBs by non-homologous end joining (NHEJ) has been extensively studied but it remains unknown whether more complex DSBs require a different sub-set of NHEJ protein for their repair compared with simple DSBs. To address this, we have induced DSBs in fluorescently tagged mammalian cells (Ku80-EGFP, DNA-PKcs-YFP or XRCC4-GFP, key proteins in NHEJ) using ultra-soft X-rays (USX) or multi-photon near infrared (NIR) laser irradiation. We have shown in real-time that simple DSBs, induced by USX or NIR microbeam irradiation, are repaired rapidly involving Ku70/80 and XRCC4/Ligase IV/XLF. In contrast, DSBs with greater chemical complexity are repaired slowly involving not only Ku70/80 and XRCC4/Ligase IV/XLF but also DNA-PKcs. Ataxia telangiectasia-mutated inhibition only retards repair of the more chemically complex DSBs which require DNA-PKcs. In summary, the repair of DSBs by NHEJ is highly regulated with pathway choice and kinetics of repair dependent on the chemical complexity of the DSB.

A signature of ionizing radiation exposure is the induction of DNA clustered damaged sites, defined as two or more lesions within one to two helical turns of DNA by passage of a single radiation track. Clustered damage is made up of double strand breaks (DSB) with associated base lesions or abasic (AP) sites, and non-DSB clusters comprised of base lesions, AP sites and single strand breaks. This review will concentrate on the experimental findings of the processing of non-DSB clustered damaged sites. It has been shown that non-DSB clustered damaged sites compromise the base excision repair pathway leading to the lifetime extension of the lesions within the cluster, compared to isolated lesions, thus the likelihood that the lesions persist to replication and induce mutation is increased. In addition certain non-DSB clustered damaged sites are processed within the cell to form additional DSB. The use of E. coli to demonstrate that clustering of DNA lesions is the major cause of the detrimental consequences of ionizing radiation is also discussed. The delayed repair of non-DSB clustered damaged sites in humans can be seen as a “friend”, leading to cell killing in tumour cells or as a “foe”, resulting in the formation of mutations and genetic instability in normal tissue.

An important stage in tumorigenesis is the ability of a precancerous cell to escape natural anticancer signals imposed on it by neighboring cells and its microenvironment. We have previously characterized a system of intercellular induction of apoptosis whereby nontransformed cells selectively remove transformed cells from coculture via cytokine and reactive oxygen/nitrogen species (ROS/RNS) signaling. We report that irradiation of nontransformed cells with low doses of either high linear energy transfer (LET) A-particles or low-LET ;-rays leads to stimulation of intercellular induction of apoptosis. The use of scavengers and inhibitors confirms the involvement of ROS/RNS signaling and of the importance of transformed cell NADPH oxidase in the selectivity of the system. Doses as low as 2-mGy ;-rays and 0.29-mGy Aparticles were sufficient to produce an observable increase in transformed cell apoptosis. This radiation-stimulated effect saturates at very low doses (50 mGy for ;-rays and 25 mGy for A-particles).

This paper presents data on modelling of DNA damage induced by electrons, protons and alpha-particles to provide an insight into factors which determine the biological effectiveness of radiations of high and low linear energy transfer (LET). These data include the yield of single- and double-strand breaks (ssb, dsb) and base damage in a cellular environment. We obtain a ratio of 4-15 for ssb:dsb for solid and cellular DNA and a preliminary ratio of about 2 for base damage to strand breakage. Data are also given on specific characteristics of damage at the DNA level in the form of clustered damage of varying complexity, that challenge the repair processes and if not processed adequately could lead to the observed biological effects. It is shown that nearly 30% of dsb are of complex form for low-LET radiation, solely by virtue of additional breaks, rising to about 70% for high-LET radiation. Inclusion of base damage increases the complex proportion to about 60% and 90% for low- and high-LET radiation, respectively. The data show a twofold increase in frequencies of complex dsb from low-LET radiation when base damage is taken into account. It is shown that most ssb induced by high-LET radiation have associated base damages, and also a substantial proportion is induced by low-energy electrons.

mantle (8). The water decreases the melting temperature, resulting in partial melting. Some high-pressure partitioning experiments suggest that, when partial melting occurs in subducted crustal materials, hollandite can preferentially incorporate several incompatible elements (K, Pb, Sr, light rare earth elements, and so forth) but is not likely to be a host for uranium and heavy rare earth elements, relative to the coexisting melt (9). Therefore, the stability of hollandite will strongly influence trace element geochemistry of magmas produced in the deep mantle as well as alkali transport processes in the transition zone and the lower mantle.Detailed studies of shocked meteorites may provide further evidence for dense minerals stable in the deep mantle. Other alkali-host minerals such as calcium ferrite-type NaA1Si04 and a related structural phase (1, 10) may be found in shocked meteorites. Together with comprehensive experimental studies on the melting relations and trace element partitioning between the alkali-host minerals, silicate melt, and fluid at the pressuretemperature conditions of the transition zone and the lower mantle, they will shed light on the behavior of alkali elements in the deep mantle and on crust formation processes. A Sting in the Tail ofing counterparts of ' OH (principally the hydrated electron, e,;;) are relatively ineffective, especially at inducmg DNA strand breaks.