Misrejoining of DNA double-strand breaks in primary and transformed human and rodent cells: a comparison between the HPRT region and other genomic locations

Many studies of radiation response and mutagenesis have been carried out wi th transformed human or rodent cell lines. To study whether the transfer of results between different cellular systems is justified with regard to the : repair of radiation-induced DNA double-strand breaks (DSBs), two assays t hat measure the joining of correct DSB ends and total rejoining in specific regions of the genome were applied to primary and cancer-derived human cel ls and a Chinese hamster cell Line. The experimental procedure involves Sou thern hybridization of pulsed-field gel electrophoresis blots and quantitat ive analysis of specific restriction fragments detected by a single-copy pr obe. The yield of X-ray-induced DSBs was comparable in all cell lines analy zed, amounting to about 1x10(-2) breaks/Mbp/Gy. For joining correct DSB end s following an 80 Gy X-ray exposure all cell lines showed similar kinetics and the same final level of correctly rejoined breaks of about 50%. Analysi s of all rejoining events revealed a considerable fraction of unrejoined DS Bs (15-20%) after 24 h repair incubation in the tumor cell Line, 5-10% unre joined breaks in CHO cells and complete DSB rejoining in primary human fibr oblasts, To study intragenomic heterogeneity of DSB repair, we analyzed the joining of correct and incorrect break ends in regions of different gene d ensity and activity in human cells. A comparison of the region Xq26 spannin g the hypoxanthine guanine phosphoribosyl transferase locus with the region 21q21 revealed identical characteristics for the induction and repair of D SBs, suggesting that there are no large variations between Giemsa-light and Giemsa-dark chromosomal bands. (C) 1999 Elsevier Science B.V. All rights r eserved.