Investigation Of Mechanism Of DNA Or Organism Damage Induced By High LET Radiation And Corresponding Radioprotection

It is widely accepted that DNA is the crucial sensitive target of radiation. Numerous lesions including sugar alteration, base damage, single- and double-strand breaks together with cross-links have been identified in irradiated DNA. The DNA double strand breaks (DSBs) are considered as the most critical type of lesions. Correct and complete repair of DSBs guarantees cellular survival, while incorrect repair and residual DNA damage may lead to cell killing, mutation or transformation.Heavy ions and protons with high linear energy transfer (LET) values can depose more energies and have different ionizing track structure while through the matter comparing with the low-LET radiation. Their complex ionizing tracks lead to multiple ionization events within nanometer regions of the target, resulting in the induction of complex damage and higher relative biological effectiveness (RBE). The use of high LET particles in radiobiological experiments provides a fundamental and powerful tool to study the relationship between different ionization densities and biological system as well as to prove the physical basis of models and theories. The knowledge of the interaction of high LET radiation with biological matter is of great importance for the application of heavier ions and protons therapy. It is also essential in radioprotection for estimation of risk to crews and patients in case of exposure to high LET radiation, such as space or nuclear radiation environment.The goal of this study is to investigate the physical and chemical mechanism of DNA strand breaks induced by direct and indirect effect of heavy ions radiation with high LET and corresponding radioprotection action of free radical scavenger by means of atomic force microscopy (AFM) and gel electrophoresis at the molecular level. In addition, the biological effects induced by protons are observed using the optical microscopy at the cellular level.Choosing ⁷Li and ¹²C heavy ions with different LET accelerated by HI-13 tandem accelerator respectively, the dry and aqueous pUC19 plasmid DNA with or without free radical scavenger are irradiated with different doses in air. AFM is used to directly visualize the DNA fragments and the change of DNA form resulting from exposure to heavy ions radiation at the nanometer scale. The distributions of DNA fragment lengths and the changes of number of DSBs as the dose increase are obtained. The radioprotective capability of scavenger, mannitol and vitamin C is estimated. At the same time, the irradiated DNA samples are also analyzed by gel electrophoresis technique in order to prove and supplement experimental results obtained by AFM. The results are as follows,There is a maximum biological effectiveness when the pUC19 plasmid DNA in aqueous solution is irradiated by ⁷Li ions at a LET around 110keV/μm. Comparing with the low LET electron and high LET neutron irradiation, the higher LET ⁷Li ions induce the cluster DSB lesions that is formation of much larger and much shorter small DNA fragments, and then the distribution of DSB more locally and more densely at comparable dose. The influence of dry DNA, the concentration of DNA solution and free radical scavenger on yielding DNA strand breaks suggest that the DNA strand breaks induced by ⁷Li ions as the corporate result of direct effect and indirect effect of free radical, and the free radical effect may be the main factor.In ⁷Li and ¹²C ions radiation, the scavenger, mannitol and vitamin C can compete free radical with DNA molecule and scavenge the free radical generated during radiolysis, then reduce the yields of DNA strand breaks induced by heavy ions efficiently, which suggest that they have stronger radioprotective capability against heavy ions radiation. The scavenging capacity of scavenger for heavy ions radiation induces DSB is lower than that one induced byγ-rays, namely the heavy ions radiation may induce a much number of DSBs that can not be scavenged, those DSBs mainly are produced by mechanism of direct effect and are induced by the locally multiply damaged sites (LMDS).The protons radiation induces the ICR mouse' tissue, such as skin, cardiac muscle, lung and liver appear pathologic change, which results bring forward a challenge to further study of organism damage induced by protons radiation.In conclusion, combined traditional nuclear physical experimental technique with advanced AFM and modern molecular biological technology successfully, an important and powerful means is provided for study of the mechanism of biological damage induced by radiation and corresponding radioprotection. Using of those means, the more delicate information and more abundant experimental data of DNA damage and its protection induced by high LET radiation can be obtained at molecular level.