The spatial distribution of radiation-induced DNA breaks within the cell nucleus depends on radiation quality in terms of energy deposition pattern. It is generally assumed that the higher the radiation linear energy transfer (LET), the greater the DNA damage complexity. Using a combined experimental and theoretical approach, we examined the phosphorylation-dephosphorylation kinetics of radiation-induced γ-H2AX foci, size distribution and 3D focus morphology, and the relationship between DNA damage and cellular end points (i.e., cell killing and lethal mutations) after exposure to gamma rays, protons, carbon ions and alpha particles. Our results showed that the maximum number of foci are reached 30 min postirradiation for all radiation types. However, the number of foci after 0.5 Gy of each radiation type was different with gamma rays, protons, carbon ions and alpha particles inducing 12.64 ± 0.25, 10.11 ± 0.40, 8.84 ± 0.56 and 4.80 ± 0.35 foci, respectively, which indicated a clear influence of the track structure and fluence on the numbers of foci induced after a dose of 0.5 Gy for each radiation type. The γ-H2AX foci persistence was also dependent on radiation quality, i.e., the higher the LET, the longer the foci persisted in the cell nucleus. The γ-H2AX time course was compared with cell killing and lethal mutation and the results highlighted a correlation between cellular end points and the duration of γ-H2AX foci persistence. A model was developed to evaluate the probability that multiple DSBs reside in the same gamma-ray focus and such probability was found to be negligible for doses lower than 1 Gy. Our model provides evidence that the DSBs inside complex foci, such as those induced by alpha particles, are not processed independently or with the same time constant. The combination of experimental, theoretical and simulation data supports the hypothesis of an interdependent processing of closely associated DSBs, possibly associated with a diminished correct repair capability, which affects cell killing and lethal mutation.

Purpose:To quantify the role played by radiation track structure and background fragments in modulating DNA fragmentation in human cells exposed to γ-rays and light ions. Materials and methods: Human fibroblasts were exposed in vitro to different doses (in the range from 40 – 200 Gy) of ⁶⁰Co γ-rays and 0.84 MeV protons (Linear Energy Transfer, LET, in tissue 28.5 keV/μm). The resulting DNA fragments were scored under two electrophoretic conditions, in order to optimize separation in the size ranges 0.023 – 1.0 Mbp and 1.0 – 5.7 Mbp. In parallel, DNA fragmentation was simulated both with a phenomenological approach based on the “generalized broken-stick” model, and with a mechanistic approach based on the PARTRAC (acronym of PARticle TRACk) Monte Carlo code (1.32 MeV photons were used for the simulation of ⁶⁰Co γ-rays). Results: For both γ-rays and protons, the experimental dose response in the range 0.023 – 5.7 Mbp could be approximated as a straight line, the slope of which provided a yield of (5.3 ± 0.4) • 10⁻⁹ Gy⁻¹ bp⁻¹ for γ-rays and (7.1 ± 0.6) • 10⁻⁹ Gy⁻¹ bp⁻¹ for protons, leading to a Relative Biological Effectiveness (RBE) of 1.3 ± 0.2. From both theoretical analyses it appeared that, while γ-ray data were consistent with double-strand breaks (DSB) random induction, protons at low doses showed significant deviation from randomness, implying enhanced production of small fragments in the low molecular weight part of the experimental range. The theoretical analysis of fragment production was then extended to ranges where data were not available, i.e. to fragments larger than 5.7 Mbp and smaller than 23 kbp. The main outcome was that small fragments (<23 kbp) are produced almost exclusively via non-random processes, since their number is considerably higher than that produced by a random insertion of DSB. Furthermore, for protons the number of these small fragments is a significant fraction (about 20%) of the total number of fragments; these fragments remain undetected in these experiments. Calculations for 3.3 MeV alpha particle irradiation (for which no experimental data were available) were performed to further investigate the role of fragments smaller than 23 kbp; in this case, besides the non-random character of their production, their number resulted to be at least as much as half of the total number of fragments. Conclusion: Comparison between experimental data and two different theoretical approaches provided further support to the hypothesis of an important role of track structure in modulating DNA damage. According to the theoretical approaches, non-randomness of fragment production was found for proton irradiation for the smaller fragments in the experimental size range and, in a significantly larger extent, for fragments of size less than 23 kbp, both for protons and alpha particles.

In the last decade, many innovative nanodrugs have been developed, as well as many nanoradiocompounds that show amazing features in nuclear imaging and/or radiometabolic therapy. Their potential uses offer a wide range of possibilities. It can be possible to develop nondimensional systems of existing radiopharmaceuticals or build engineered systems that combine a nanoparticle with the radiopharmaceutical, a tracer, and a target molecule, and still develop selective nanodetection systems. This review focuses on recent advances regarding the use of gold nanoparticles and nanorods in nuclear medicine. The up-to-date advancements will be shown concerning preparations with special attention on the dimensions and functionalizations that are most used to attain an enhanced performance of gold engineered nanomaterials. Many ideas are offered regarding recent in vitro and in vivo studies. Finally, the recent clinical trials and applications are discussed.

Purpose : To compare the results on DNA fragmentation induced in Chinese hamster V79 cells by various doses of γ-rays and low-energy protons and helium-4 ions. Materials and methods : V79 cells were irradiated as monolayers with monoenergetic protons and helium-4 ions; γ-rays were used as the reference radiation. DNA double-strand breaks were evaluated by calibrated pulsed-field gel electrophoresis using conditions covering the range 5.7 Mbp-23.1 kbp. Results : The fragment-counting method gave double-strand breaks yields and the relative biological effectiveness higher than those obtained by the fraction of activity released method. The frequency distribution of fragments showed that protons and helium ions induced more fragments below the Mbp region than did γ-rays at the same dose. The distributions for both the irradiated and non-irradiated samples clearly appeared to be non-random. Conclusion : Differences were observed in the yield and spatial correlation, at a molecular size scale characteristic of loop dimensions, of the double-strand breaks induced by γ-rays and by light ions. These effects may have a role in the observed different cell response to these radiations.

Antonelli, F., Belli, M., Cuttone, G., Dini, V., Esposito, G., Simone, G., Sorrentino, E. and Tabocchini, M. A. Induction and Repair of DNA Double-Strand Breaks in Human Cells: Dephosphorylation of Histone H2AX and its Inhibition by Calyculin A. Radiat. Res. 164, 514–517 (2005).Phosphorylation of histone H2AX at serine 139 (γ-H2AX) represents one of the earliest steps in DNA DSB signaling and repair, but the mechanisms of coupling this histone modification to DSB processing remain to be established. In this work, H2AX phosphorylation-dephosphorylation kinetics induced by low doses of γ rays in MRC-5 human fibroblasts was studied. The number of γ-H2AX foci increased rapidly, with the maximum reached 20 min after irradiation. Using calyculin A, a protein phosphatase inhibitor, no significant dephosphorylation was found in this time. At longer times, no further induction of γ-H2AX foci occurred. This indicates that the number of γ-H2AX foci scored at 20 min can be used as representative of the initial number of DSBs. Pulsed-field gel electrophoresis (PFGE) was also used to determine whether calyculin A-mediated inhibition of γ-H2AX dephosphorylation and DSB rejoining are independent phenomena. We found that the maintenance of the phosphate group at Ser 139 in γ-H2AX does not represent an obstacle for DSB rejoining. Preliminary experiments performed with 62 MeV/nucleon carbon ions have shown a longer persistence of γ-H2AX foci with respect to γ rays, consistent with the induction of damage that is more severe and difficult to repair.