Rejoining of double-stranded DNA-fragments studied in different size-intervals.
OBJECTIVE
To measure rejoining of radiation-induced doublestranded DNA-fragments of different sizes and to evaluate the effects of size-resolution in the analysis of rejoining.


METHODS
Normal human fibroblasts (GM5758) were irradiated with photons or accelerated nitrogen ions (linear energy transfer, LET = 125 keV microm(-1)) and incubated for repair for 0-22 h. Double-stranded DNA-fragments from the irradiated cells were separated by pulsed-field gel electrophoresis in the range approximately 5 kbp to 5.7 Mbp.


RESULTS
For cells irradiated with high LET nitrogen ions, there was an increase in the fast half-time from approximately 5 min for fragments < 400 kbp to 10 min when all fragments < 5.7 Mbp were measured. Further, the fraction of fragments rejoined by the slow-rejoining phase increased significantly for increased threshold sizes. The fraction of unrejoined fragments after 22 h and the half-time for the slow-rejoining phase remained constant for all threshold sizes. For cells irradiated with lower doses of low LET radiation the rejoining was shifted towards a slower kinetics when fragments up to 10 Mbp were excluded in the analysis.


CONCLUSIONS
DNA exclusion-size and resolution may affect the estimates of DNA double-strand break rejoining. Using a low-resolution technique that does not detect small fragments will result in an underestimation, or even disappearance of the fast-rejoining phase. This is due to substantial rejoining of fragments taking place before the fragments are of sufficient size to be monitored.
Stenerlöw
Bo
B
Division of Biomedical Radiation Sciences, Rudbeck Laboratory, Uppsala University, S-751 85 Uppsala, Sweden. Bo.Stenerlow@bms.uu.se
Höglund
E
E
D002460:Cell Line; D003037:Cobalt Radioisotopes; D004247:DNA / Q000528:radiation effects*; D004249:DNA Damage; D053938:DNA Fragmentation; D004260:DNA Repair; D004587:Electrophoresis, Agar Gel; D005347:Fibroblasts / Q000528:radiation effects*; D006801:Humans; D007477:Ions; D009584:Nitrogen; D017785:Photons; D013997:Time Factors
D003037:Cobalt Radioisotopes; D007477:Ions; D004247:DNA; D009584:Nitrogen