Difference in biological effectiveness due to the endpoints and radiation quality

This paper introduces the effects of radiation quality with different ion species and various linear energy transfer on variable cell killing, remaining or initially measured breaks of G0/G1- prematurely condensed chromosome, mutation induction and deletion spectra in normal human fibroblasts. These results demonstrate that the LET-dependent structure is reflected in biological consequence of the repair process. Also, we indicate the possibility that the biological effects for different ion species are seen in not only a quantitative difference but also in a qualitative difference, in term of mutation induction. However, the importance of the biological effects caused by primary ions is still unclear in our irradiation systems, due to the use of polymethyl methacrylate absorbers of different thicknesses to change the energies of the ions. When considering the influence of galactic cosmic rays consists of high energy protons and heavy ions, it is important to further clarify the relationship between the different types of radiation and the effects

Heavy charged particles produce a bystander effct via cell-cell junctions

Radiation-induced damage to living cells results from either a direct hit to cellular DNA, or from indirect action which leads to DNA damage from radiation produced radicals. However, in recent years there is evidence that biological effects such as cell killing, mutation induction, chromosomal damage and modification of gene expression can occur in a cell population exposed to low doses of alpha particles. In fact these doses are so low that not all cells in the population will be hit directly by the radiation. Using a precision alpha-particle microbeam, it has been recently demonstrated that irradiated target cells can induce a bystander mutagenic response in neighboring "bystander" cells which were not directly hit by alpha particles. Furthermore, these results suggest that gap-junction mediated cell-to-cell communication plays a critical role in this bystander phenomenon. The purpose of this section is to describe recent studies on bystander biological effects. The recent work described here utilized heavy charged particles for irradiation, and investigated the role of gap-junction mediated cell-cell communication in this phenomenon

Difference in bystander cell-killing effect between normal and tumor cells irradiated with carbon-ion beams.

Since 1994, a Phase I/II clinical study and cancer radiotherapy have been begun using carbon-ion beams generated with the Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences. In the field of fundamental biological studies for high-LET radiations, there are many reports regarding bystander cellular effects after exposure to alpha particles derived from 238Pu or He-ion microbeams. However, only limited sets of studies have examined bystander effects after exposure to different ion species heavier than helium, such as carbon ions. In this study we has been investigating bystander cellular responses in both normal human fibroblasts and a human tumor cell line irradiated with low energy (6MeV/n) carbon ions generated with the Medium Energy Beam Course at the HIMAC. Cells were inoculated onto the Mylar film in the dishes and then irradiated with 6MeV/n carbon ions at a fluence of 1.5 x 105 particles/cm2. Cells were exposed using 4 different protocols; (1) Cells were irradiated with all of the cells on the Mylar film; (2) Cells were irradiated with half of the cell population on the Mylar film; (3) Irradiated cells and un-irradiated control cells were pooled in a 1:1 ratio and plated as a single culture; and (4) Half of the cells were irradiated and treated with a specific inhibitor of gap-junction mediated cell-cell communication (40µM lindane). Cell killing in normal human cells, which was assayed with a colony formation assay, showed that the surviving fraction of the irradiated group (2) was almost the same as for group (1). In addition, a similar level of survival was seen between in the group (4) and in the group (3). However, no bystander cell-killing effect was observed in a human tumor cell line. There is evidence that the different bystander cellular effects induced by carbon ions occurred in between normal and tumor cells.第67回日本癌学会学術総

LET and Ion-species Dependence for Mutation Induction and Mutation Spectrum on hprt Locus in Normal Human Fibroblasts.

We have been studying LET and ion species dependence of RBE in mutation frequency and mutation spectrum of deletion pattern of exons in hprt locus. Normal human skin fibroblasts were irradiated with heavy-ion beams, such as carbon- (290 MeV/u and 135 MeV/u), neon- (230 MeV/u and 400 MeV/u), silicon- (490 MeV/u) and iron- (500 MeV/u) ion beams, generated by Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS). Mutation induction in hprt locus was detected to measure 6-thioguanine resistant colonies and deletion spectrum of exons was analyzed by multiplex PCR. The LET-RBE curves of mutation induction for carbon- and neon-ion beams showed a peak around 75 keV/microm and 155 keV/microm, respectively. On the other hand, there observed no clear peak for silicon-ion beams. The deletion spectrum of exons was different in induced mutants among different ion species. These results suggested that quantitative and qualitative difference in mutation occurred when using different ion species even if similar LET values

LET and ion-species dependence for cell killing and mutation induction in normal human fibroblasts

We have been studying LET and ion species dependence of RBE values in cell killing and mutation induction. Normal human skin fibroblasts were irradiated with heavy-ion beams such as carbon (290MeV/u and 135MeV/u), neon (230MeV/u and 400MeV/u), silicon (490MeV/u) and iron (500MeV/u) ion beams, generated by Heavy Ion Medical Accelerator in Chiba (HIMAC) at National Institute of Radiological Sciences (NIRS). Cell killing effect was detected as reproductive cell death using a colony formation assay. Mutation induction in hprt locus was detected to measure 6-thioguanine resistant colonies. The RBE-LET curves of cell killing and mutation induction were different each ion beam. So, we plotted RBE for cell killing and mutation induction as function of Z*2/B2 instead of LET. RBE-Z*2/B2 curves of cell killing indicated that the discrepancy of RBE-LET curves was reconciled each ion species. But RBE-Z*2/B2 curves of mutation induction didn\u27t corresponded between carbon- and silicon-ion beams. These results suggested that different biological endpoints may be suitable for different physical parameter, which represent the track structure of energy deposition of ion beams

Radio-adaptive response in normal human cells induced by protons

The radio-adaptive response refers to the phenomenon by which cells irradiated with a sub-lethal dose of ionizing radiation (priming dose) become less sensitive to subsequent irradiation with high doses (challenging dose). Recently we reported that the mutation frequency at hprt locus in normal human cells induced by the X-ray challenging dose was reduced at 0.15 times in cells pre-treated with low-dose-rate neutrons (1mSv/8h) as a priming dose compared to unpre-treated cells, but the reduced mutation frequency was returned to the control level, when using a specific inhibitor of gap-junction mediated cell-cell communication. We set up a hypothesis that recoiled protons emitted by the interaction between neutrons and cells / water induce radio-adaptive response in irradiated cell population via gap-junction mediated bystander effect. To examine the hypothesis around 1.5% of cells were irradiated with one proton before irradiating the X-ray challenging dose using proton microbeams accelerated with the Single Particle Irradiation system to Cell (SPICE) in National Institute of Radiological Sciences. The result clearly showed that the X-ray induced mutation frequency was suppressed in cells pre-treated with proton microbeams and returned to the control level, when using a specific inhibitor of gap-junction mediated cell-cell communication. The result suggests that neutron-induced adaptive response is caused by recoiled protons and gap-junction mediated bystander effect plays an important role to induce such cellular response.日本宇宙生物科学会第23回大