DNA DOUBLE-STRAND BREAK DISTRIBUTIONS IN X-RAY AND ALPHA-PARTICLE IRRADIATED V79 CELLS - EVIDENCE FOR NONRANDOM BREAKAGE

Many studies have shown that with increasing LET of ionizing radiation the RBE (relative biological effectiveness) for dsb (double strand br eaks) induction remains around 1.0 despite the increase in the RBE for cell killing. This has been attributed to an increase in the complexi ty of lesions, classified as dsb with current techniques, at multiply damaged sites. This study determines the molecular weight distribution s of DNA from Chinese hamster V79 cells irradiated with X-rays or 110 keV/mu m alpha-particles. Two running conditions for pulsed-field gel- electrophoresis were chosen to give optimal separation of fragments ei ther in the 225 kbp-5.7 Mbp range or the 0.3 kbp to 225 kbp range. Tak ing the total fraction of DNA migrating into the gel as a measure of f ragmentation, the RBE for dsb induction was less than 1.0 for both mol ecular weight regions studied. The total yields of dsb were 8.2 x 10(- 9) dsb/Gy/bp for X-rays and 7.8 x 10(-9) dsb/Gy/bp for a-particles, me asured using a random breakage model. Analysis of the RBE of alpha-par ticles versus molecular weight gave a different response. In the 0.4 M bp-57 Mbp region the RBE was less than 1.0; however, below 0.4 Mbp the RBE increased above 1.0. The frequency distributions of fragment size s were found to differ from those predicted by a model assuming random breakage along the length of the DNA and the differences were greater for alpha-particles than for X-rays. An excess of fragments induced b y a single-hit mechanism was found in the 8-300 kbp region and for X-r ays and alpha-particles these corresponded to an extra 0.8 x 10(-9) an d 3.4 x 10(-9) dsb/bp/Gy, respectively. Thus for every alpha-particle track that induces a dsb there is a 44% probability of inducing a seco nd break within 300 kbp and for electron tracks the probability is 10% . This study shows that the distribution of damage from a high LET alp ha-particle track is significantly different from that observed with l ow LET X-rays. In particular, it suggests that the fragmentation patte rns of irradiated DNA may be related to the higher-order chromatin rep ealing structures found in intact cells.