SATURATED AND UNSATURATED REPAIR OF DNA STRAND BREAKS IN CHO CELLS AFTER X-IRRADIATION WITH DOSES RANGING FROM 3 TO 90 GY

Citation
E. Dikomey et J. Lorenzen, SATURATED AND UNSATURATED REPAIR OF DNA STRAND BREAKS IN CHO CELLS AFTER X-IRRADIATION WITH DOSES RANGING FROM 3 TO 90 GY, International journal of radiation biology, 64(6), 1993, pp. 659-667
Citations number
38
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Radiology,Nuclear Medicine & Medical Imaging","Nuclear Sciences & Tecnology
ISSN journal
0955-3002
Volume
64
Issue
6
Year of publication
1993
Pages
659 - 667
Database
ISI
SICI code
0955-3002(1993)64:6<659:SAUROD>2.0.ZU;2-4
Abstract
The kinetics of DNA strand break repair was studied in exponentially-g rowing CHO cells after X-irradiation with doses of 3, 9, 30, 60 and 90 Gy. DNA strand breaks were measured using the alkaline unwinding tech nique. For all X-ray doses applied the kinetics of DNA strand break re pair consisted of fast, intermediate and slow phases. The latter, whic h was interpreted as the repair kinetic of DNA double-strand breaks, w as best described by an exponential decline. The actual repair half-ti me of double-strand break repair, tau(dsb), was obtained from the slop e of the slow component after subtracting the number of non-reparable breaks measured 24 h after irradiation. This half-time was found to be independent of the dose applied with a mean value of tau(dsb) = 168 /- 10 min. This result indicated that the repair of double-strand brea ks was unsaturated for doses up to 90 Gy. The repair kinetics of the b reaks of the fast and intermediate phases were found to be dependent o n the dose applied. These kinetics were associated with the induction and repair of primary and secondary single-strand breaks, the latter p ossibly generated by enzymatic incision at damaged bases. Analysis of these curves using the Michaelis-Menten equation showed that the half- time of enzymatic incision, tau(in), and the half-time at which both p rimary and secondary single-strand breaks were rejoined, tau(rep), var ied with the amount of damage present in the Cell. Tau(in) Increased f rom a minimum value tau(in,min) = 13 +/- 2 min proportionally to the n umber of base damage with a rate of a(in) = 0.138 +/- 0.015 min Gy-1, and tau(rep) from a minimum value tau(rep,min) = 1.4 +/- 0.2 proportio nally to the number of single-strand breaks with a rate of a(rep) = 0. 038 +/- 0.001 min Gy-1. The enzymatic incision was unsaturated for dos es up to about 30 Gy, whereas the repair of single-strand breaks was u nsaturated only for doses up to about 10 Gy. Up to these doses the inc rease in the half-time tau(in) and tau(rep) was so small that, within the range of experimental errors, the parameters may be approximated b y constant values. From the results it is concluded that for CHO cells the continuously-bending dose-response curve obtained for radiation-i nduced killing cannot be attributed to a saturated repair.