High LET radiation affects non-homologous end joining repair in human cells

Heavy ion particles are one of the critical components in space radiation. These are classified as high LET (linear energy transfer) radiation and also used in cancer treatment as a powerful alternative to the conventional radiotherapy. Therefore, it is critical to study the basic mechanism underlying the biological effects induced by high LET radiation. We examined cell survival and DNA double strand break (DSB) and chromosome repair in human cells irradiated with high LET radiation, and the results were compared with those with X-rays (low LET radiation). Normal human fibroblasts (HFLIII) and non-homologous end joining (NHEJ) deficient (180BR) cells were irradiated with various doses of high LET carbon ions (290 MeV,70 keV per micro m), high LET iron ions (500 MeV, 200keV per micro m) as well as X-rays, and the colony forming abilities of these cells were measured. Among the different sources of radiation, iron ions gave the lowest cell survival (relative biological effectiveness (RBE:3.0), followed by carbon ions(RBE:2.0), and then X-rays when normal cells were used. In contrast, 180BR cells gave almost identical cell survival for X-rays and carbon ions (RBE:1.0) with a lower survival with iron ions (RBE:1.5). Rejoining kinetics of G1 chromosomes were studied by the cell-fusion based premature chromosome condensation (PCC) technique. The PCC studies with irradiated normal cells seem to reflect the cell survival data; specifically iron ions showed significantly slower repair kinetics with a higher number of remaining chromosome breaks when compared with X-rays and carbon ions. With NHEJ deficient 180BR cells, the remaining number of chromosome breaks at 24 h post-irradiation point reflected the cell survival with a high number of breakage left with iron ions. We also studied the auto-phosphorylation status of an NHEJ repair protein, DNA-PKcs, using a phosphospecific antibody in irradiated cells. We found a significant difference in the kinetics of DNA-PKcs phosphorylation between X-irradiated and heavy ion (high LET) irradiated cells. These results indicate that high LET radiation disturbs the NHEJ type DSB repair, and also suggest a possibility that there are more than one type in the NHEJ repair pathway.96ｔｈ Annual Meetin