Neutron and cobalt-60 gamma irradiation produce similar changes in DNA supercoiling.
The effects of 60Co gamma-ray and d(20 MeV)Be neutron irradiation on DNA supercoiling have been studied using a nucleoid rewinding technique. Irradiation of viable CHO AA8 cells on ice with from 4 to 25 Gy of either radiation produced a similar resistance to rewinding of nuclear supercoils after treatment with ethidium bromide. The restitution from the effects of 12 Gy of either radiation was also similar, leaving no detectable residual damage. The discrepancy between these data and the reduced ability of neutrons to produce DNA breaks, as defined by the alkaline elution assay, is explained by the discontinuous deposition of dose associated with neutron irradiation. It is suggested from a microdosimetric analysis that the neutron radiation interacts with DNA at sites on average 5-10 times further apart than the interactions with gamma rays. The long DNA sequences which results after neutron irradiation are consequently eluted inefficiently during alkaline elution, giving a reported RBE of approximately 0.3. Restrictions in the rewinding of individual supercoils are not dependent on the interionization distance and thus give rise to an RBE of approximately 1. Furthermore, the complete removal of DNA damage, as measured by this technique, supports the hypothesis that neutron toxicity is associated with incorrect, not incomplete, rejoining of the DNA molecule.
Vaughan
A T
AT
Department of Radiotherapy, Loyola-Hines University Medical Center, Illinois 60141.
Gordon
D J
DJ
Chettle
D R
DR
Green
S
S
D000818:Animals; D002470:Cell Survival / Q000528:radiation effects; D003037:Cobalt Radioisotopes; D004249:DNA Damage; D004278:DNA, Superhelical / Q000528:radiation effects*; D005214:Fast Neutrons; D005720:Gamma Rays; D066298:In Vitro Techniques; D012062:Relative Biological Effectiveness
D003037:Cobalt Radioisotopes; D004278:DNA, Superhelical