The proposed experiments aim to gain more insight into the biological efficiency of lighter ions at different biological levels for the induction and repair of DNA strand breaks in both chromosomal and plasmid DNA.
A method has been established to detect double strand breaks in sequences of several kbp, using a hybrid hamster human cell line and hybridization techniques on the human part of the genome. The influence of radiation on chromosome aberrations have been analysed in V79 Chinese hamster cells and simian virus (SV40) after exposure to heavy ions.
The effect of gamma rays on cultured mammalian cells was analyzed using inverse gas chromatography and there was found to be a dramatic difference in Gibbs free energy values (attributed to hydrogen bond availability) between irradiated and non irradiated deoxyribonucleic acid (DNA). The higher the dose the higher the elimination of available hydrogen bonds. Gibbs free energy values rapidly decreased up to 50 Gy followed by a saturation level. Similar results were observed when the affect of gamma rays was studied by thermal transition spectrophotometry. There was a dramatic drop in melting temperature (Tm) values up to 50 Gy, followed by a saturation level. A theoretical model for transgression of the DNA molecules from the double strand stage to the single strand stage was based on a quasi 1-dimensional lattice and attempted to correlate the melting of the DNA molecule with development of single and double strand breaks on exposure to gamma rays. The derived function was used to define the DNA Tm point as well as the flexibility of the DNA molecule. The dramatic changes on the hydrogen bonding observed from exposure to gamma rays were not present on exposure to alpha particles.
Description of research work
In the last ten years, heavy charged particles have been used in radiobiological experiments more extensively than before. This development has basically two reasons: the increasing use of these particles in radiotherapy and radioprotection problems of manned space flights.
In radiotherapy, approximately ten thousand patients have been treated with charged particles (mostly protons) with extraordinary success. Because of the better dose distribution and the increased relative biological efficiency at the end of the particle range, a strong trend is visible toward a treatment with heavier ions (eg carbon or neon ions).
In manned space flights outside the shielding of the magnetosphere of the earth which are proposed by NASA and ESA, the heavy component of cosmic radiation pose a major risk for the health of the astronauts. In the case of the solar flare, lethal doses of protons can be reached even in short excursions outside the space craft. For long term space flights the risk of cancer induction is also important because the highly energetic heavy ions cannot be shielded very efficiently by the spacecraft and the radiation risk accumulates with time (ie over the duration of the flight).
In both cases, radiotherapy and radioprotection in space, more information is needed on the inactivation process caused by the particle radiation where the data for lighter ions are scarce. But almost no information exists on the genetic risk caused by heavy charged particles. In addition, no theoretical approach exists which allows calculation of the biological effects with sufficient accuracy. Also the molecular nature of the very slowly restoring breaks has not been explored.
In order to gain more molecular information, DNA damage of genetically well known plasmid sequences inserted in mammalian cells should be studied in greater detail, and new methods in gene technology should be used to analyse induced DNA damage. In the proposed experiments both approaches will be started and used to analyze the complexity of particle induced DNA damage.
In summary, the radiobiological effects of charged particles like protons or heavier ions are of great importance for the development of heavy particle radiotherapy as well as for the estimation of the radiation risk in manned space flights. Because a unique theory of the RBE does not exist up to now, the radiobiological effects of the particle radiation have to be measured in detail.
Funding SchemeCSC - Cost-sharing contracts
2311 EZ Leiden
OX11 0RQ Didcot,harwell,chilton