Analysis of unrejoined chromosome breakage in human fibrobrast cells exposed to low- and high-LET radiation

Reported studies of radiation induced DNA breakage have generally shown higher fractions of residual unrejoined breaks after high-LET exposures compared with low-LET exposures. This observation is supported by the argument that DNA breaks produced by high-LET radiation are more complex and, thus, less likely to be repaired. However, the dose used in these studies was generally very high. We studied unrejoined chromosome breakage by analyzing chromosome aberrations using a fluorescence in situ hybridization (FISH) technique with a combination of whole chromosome specific probes and probes for the telomere regions of all chromosomes. Confluent human fibroblast cells (AG1522) were irradiated with g rays, 490 MeV/nucleon Si, 200 MeV/nucleon Fe, or 500 MeV/nucleon Fe ions, and were incubated at 37 0C for 24 hours after exposure. A chemically induced premature chromosome condensation (PCC) technique was used to condense chromosomes mostly in the G2 phase of the cell cycle. Results showed that the frequency of unrejoined chromosome breaks was LET dependant, but the ratio of unrejoined to misrejoined breaks remained low for LET values up to 440 keV/mm. Therefore, for both low- and high-LET most chromosome breaks were either rejoined or misrejoined, when observed in the G2/M phase of the cell cycle. Unrejoined chromosome breaks associated with incomplete exchanges were also analyzed using an m-FISH technique where human chromosomes were painted in 24 different colors. As with the telomere analysis, the m-FISH analysis showed in a higher fraction of unrejoined chromosome breaks. It was also deduced that the ratio of unrejoined to misrejoined chromosome breaks, or of incomplete to complete interchromosomal exchanges is a potential biomarker for radiation quality for a wide range of LET values.The 2nd International Workshop on Space Radiation Researc