Radiation-induced chromosomal breakage and rejoining in interphase-metaphase chromosomes of human lymphocytes

The premature chromosome condensation (PCC) technique and conventional chromosome analysis were applied to examine the kinetics of radiation-induced primary chromosome breaks, their rejoining and formation of dicentrics in human peripheral blood lymphocytes (PBLs). Numbers of chromosomal elements and dicentrics per cell were analyzed for each dose. Dose-dependent increases were observed for chromosome fragments (linear) and dicentrics (linear/quadratic). For an assessment of repair kinetics, numbers of breaks and dicentrics were estimated immediately and at several recovery periods after irradiation, using the PCC technique. It was found that chromosome fragments restitute with time, whereas the dicentrics are formed very quickly and their frequency remains the same, despite the decline in the number of chromosome breaks at later recovery times. Fractionation experiments were conducted to study the time-dependent interaction of primary breaks in the formation of dicentric chromosomes. PBLs were irradiated with 3 Gy X-rays split into 2 equal fractions separated by different intervals up to 5 h. No marked difference was observed in the yield of dicentrics following the different fractionation protocols, except that the mean levels of dicentrics declined when the fraction interval was 4 h or more. It appears that most of the dicentrics are formed by misrepair of strand breaks, produced directly by radiation and not resulting from rejoining of existing breaks during the slow repair process which follows. We also studied the role of the chromatin configuration at the time of irradiation on the yield of chromosome fragments and dicentrics. Highly condensed chromatin due to pretreatment with 0.3 M NaCl was found to reduce the frequency of radiation-induced chromosomal aberrations.