VISUALIZATION OF THE DNA REPAIR PROCESS IN MAMMALIAN CELLS TRANSFECTED WITH EGFP-EXPRESSING PLASMID DNA AFTER EXPOSURE TO X-RAYS IN VITRO

Core level ionization or excitation and Auger relaxation induce clustered DNA damage

Ionizing radiation causes various types of DNA damage, such as single- (SSBs) and double-strand breaks (DSBs), nucleobase lesions, abasic sites (AP sites), and cross-linking between complementary strands of DNA or DNA and proteins. DSBs are among the most harmful type of DNA damage, inducing serious genetic effects such as cell lethality and mutation. Nucleobase lesions and AP sites, on the other hand, may be less deleterious and are promptly repaired by base excision repair (BER) pathways. Recently, biochemical approaches to quantify nucleobase lesions and AP sites have revealed certain types of non-strand break lesions as harmful DNA damage, called clustered DNA damage. Such clusters can retard nucleobase excision repair enzymes, and can sometimes be converted to DSBs by BER catalysis. This unique character of clustered DNA damage strongly depends on the spatial density of ionization or excitation events occurring at the track end of initial radiation or low energy secondary electrons. In particular, the photoelectric effect of elements comprising biological molecules, followed by emission of Auger electrons, are key factors in determining the future fate of each clustered damage site. This chapter describes biological studies of clustered nucleobase lesions with SSBs or AP sites, and mechanistical studies on core level excitation and Auger relaxation giving rise to clustered DNA damage.

Expression of an X-Ray Irradiated EGFP-Expressing Plasmid Transfected into Nonirradiated Human Cells

To investigate the repairability of X-ray induced DNA damage, particularly non-double-strand breaks in living cells, enhanced green fluorescent protein (EGFP)-expressing plasmids X-ray irradiated and then transfected into nonirradiated human cells, MCF7 and MCF10A. Live-cell imaging of EGFP fluorescence was performed to measure the efficiency of plasmid repair in cells. The number of EGFP-expressing cells significantly decreased with increasing X-ray dose for both cell lines. The obtained kinetic curves of EGFP expression indicating plasmid repair were quantitatively compared against algebraically calculated ones based on the values of the transfected plasmids that had been treated with nicking or restriction enzymes. Then, assuming a Poisson distribution of single-strand breaks (SSBs), the number of cells carrying these nicked plasmids that could express EGFP were estimated. Our experimental results revealed considerably fewer cells expressing EGFP compared to the expected values we had calculated. These results suggest that the lower proportion of cells expressing EGFP as a measure of plasmid repair was due not only to the complex chemical structures of termini created by SSBs compared to those created by enzyme treatments, but also that base lesions or AP sites proximately arising at the strand-break termini might compromise EGFP expression. These results emphasize that radiation-induced DNA breaks are less repairable than enzymatically induced DNA breaks, which is not apparent when using conventional gel electrophoresis assays of plasmid DNA.