Comparative study of the repair kinetics of chromosomal aberrations and DNA strand breaks in proliferating and quiescent

Abstract

Repair kinetics observable at the level of exchange-type chromosomal aberrations (dicentric chromosomes), using fractionation and delayed-plating techniques, have been compared with repair kinetics of radiation-induced DNA double-strand breaks, measured with PFGE, and with repair kinetics of all strand breaks, measured with the alkali-unwinding technique. Only data from quiescent or proliferating CHO K1 cells obtained in the same laboratory were used. We determined repair kinetics in terms of the time constant tau (equal to half-time/log(e)2). The repair kinetics (tau approximately 11-14 min) observed in the split-dose formation of dicentric chromosomes agrees with fast repair kinetics of double-strand breaks (tau approximately 11-13 min), thus permitting us to identify the latter as the 'primary lesions' whose pairwise interaction leads to the beta D2 yield term of the aberrations. The repair kinetics observed for dicentric chromosomes formed under delayed-plating conditions (tau approximately 75 min), which mainly affects the alpha D yield term, is attributed to an intermediate interchromosomal product temporarily existing in the course of aberration formation; it is suggested that this product is mechanistically correlated with the slow repair kinetics of 'clustered damage' to DNA seen with the applied molecular methods (tau approximately 90 min).

A contribution to the linear no-threshold discussion

“…Studies of the kinetics of double-strand break repair show a rapid initial reduction with a tail at longer times which invariably reveals a proportion of breaks which are unrepaired. This repair has been analysed in terms of exponential functions representing fast and slow repair processes (Frankenberg-Schwager 1990, Frankenberg-Schwager et al 1990, Jenner et al 1993, Kysela et al 1993, Sternlow et al 1994, 2000, Dahm-Daphi and Dikomey 1996, Greinert et al 1996, Belli et al 2000, Ahnstrom et al 2000 but has also been analysed in terms of a spectrum of repair probabilities (Foray et al 1998) as well as a bimolecular reaction rate, which is probably closer to the actual repair processes going on in the cell (Fowler 1999, Cucinotta et al 2000. It must be noted that the measurements of repair of DNA double-strand breaks are made of necessity at high radiation doses as the measurement techniques are not sensitive enough to measure the few remaining unrepaired breaks if experiments are done at doses of direct significance for radiological protection.…”

Review: Proximity effects in the production of chromosome aberrations by ionizing radiation

“…Particularly relevant on the experimental side are PCC data (recent surveys in Cornforth and Bedford 1993, Goodwin et al 1994, Gray et al 1994, Loucas and Geard 1994, Durante et al 1996, Evans et al 1996, low dose-rate studies (survey in Lloyd and Edwards 1983) and split-dose experiments (e.g. Greinert et al 1996). Theoretical studies show that the details of how aberrations develop in time can in¯uence the shape of dose response curves (Appendix 3).…”

Increased complexity of radiation-induced chromosome aberrations consistent with a mechanism of sequential formation

“…The mechanism for induction remains to be fully understood, but it is generally accepted that formation conforms to a model of "breakage-first" followed by "repair/ misrepair" of the broken "ends" (Sax, 1938;Brown et al, 1993). DNA double strand breaks (dsb) of varying complexity are an important class of damage induced after exposure to ionising radiation and they are considered to be the critical lesion in the formation of radiation-induced chromosome aberrations (Bender et al, 1974;Natarajan and Zwanenburg, 1982;Cornforth and Bedford, 1983;Bryant, 1984;Greinert et al, 1996). In effect, radiation-induced dsb represent "breakage" and chromosome aberration formation is one consequence of the cells attempts to "repair" this damage.…”