Correlation between the clonogenic initial slope and the response of polykaryon-forming units: The behavior of strains defective in XRCC5 and ATM andthe heritability of small variations in radioresponse

The polykaryon-forming unit (PFU) assay measures the survival of multiple c ycles of DNA synthesis after exposure to ionizing radiation, and it is know n that there is a strong correlation between the slope of the PFU dose-resp onse curve and the clonogenic initial slope. This suggests that DNA lesions expressed in clonogens are also important in PFU. Cells having a mutation in XRCC5 (also known as Ku80; strain xrs-6) and ATM(strain AT5BIVA) were hy persensitive in the PFU assay and in clonogens, while a strain of xrs-6 cel ls transfected with hamster wild-type XRCC5 cDNA displayed wild-type resist ance in both assays. These data suggest that the DNA double-strand break (D SB) is an important lesion in PFU, although the relative radioresistance of PFU compared to clonogens indicates differential DSB toxicity. We propose that this results from the absence of cytokinesis-related loss of DNA fragm ents. Small variations in the radioresponse of PFU were observed between CH O K1 cell substrains, such that the xrs parental substrain RR-CHOK1 (carryi ng wild-type XRCC5) was more sensitive than an independent K1 substrain (E- CHOK1). Somatic hybridization showed that this variation is heritable and t hat the resistant E phenotype is dominant. In RR-CHOK1 cells there was a bi phasic PFU radioresponse, which suggests that there may be transient expres sion at a locus selectively affecting PFU sensitivity. (C) 2000 by Radiatio n Research Society.