Correlation between the clonogenic initial slope and the response of polykaryon-forming units: the behavior of strains defective in XRCC5 and ATM and the heritability of small variations in radioresponse.
The polykaryon-forming unit (PFU) assay measures the survival of multiple cycles of DNA synthesis after exposure to ionizing radiation, and it is known that there is a strong correlation between the slope of the PFU dose-response 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 hypersensitive in the PFU assay and in clonogens, while a strain of xrs-6 cells transfected with hamster wild-type XRCC5 cDNA displayed wild-type resistance in both assays. These data suggest that the DNA double-strand break (DSB) 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 fragments. Small variations in the radioresponse of PFU were observed between CHO K1 cell substrains, such that the xrs parental substrain RR-CHOK1 (carrying wild-type XRCC5) was more sensitive than an independent K1 substrain (E-CHOK1). Somatic hybridization showed that this variation is heritable and that the resistant E phenotype is dominant. In RR-CHOK1 cells there was a biphasic PFU radioresponse, which suggests that there may be transient expression at a locus selectively affecting PFU sensitivity.
Manti
L
L
Department of Radiation Biology, Cancer Research Wales Laboratory, Velindre NHS Trust, Whitchurch, Cardiff, UK.
Davies
H E
HE
Venables
S
S
Bowen
I D
ID
Court
J B
JB
D000818:Animals; D034961:Antigens, Nuclear; D064007:Ataxia Telangiectasia Mutated Proteins; D016466:CHO Cells; D018797:Cell Cycle Proteins; D002460:Cell Line; D002470:Cell Survival / Q000528:radiation effects; D003114:Colony-Forming Units Assay; D006224:Cricetinae; D003571:Cytochalasin B / Q000494:pharmacology; D004265:DNA Helicases; D004268:DNA-Binding Proteins / Q000172:deficiency* / Q000235:genetics; D004307:Dose-Response Relationship, Radiation; D005260:Female; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects / Q000528:radiation effects*; D015726:Giant Cells / Q000473:pathology / Q000528:radiation effects*; D006801:Humans; D006822:Hybrid Cells / Q000528:radiation effects; D000072200:Ku Autoantigen; D009154:Mutation; D009687:Nuclear Proteins / Q000172:deficiency* / Q000235:genetics; D010053:Ovary / Q000166:cytology / Q000187:drug effects / Q000528:radiation effects*; D011123:Polyploidy; D017346:Protein Serine-Threonine Kinases / Q000172:deficiency* / Q000235:genetics; D011836:Radiation Tolerance / Q000235:genetics*; D025521:Tumor Suppressor Proteins
D034961:Antigens, Nuclear; D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D025521:Tumor Suppressor Proteins; D003571:Cytochalasin B; C576324:ATM protein, human; D064007:Ataxia Telangiectasia Mutated Proteins; D017346:Protein Serine-Threonine Kinases; D004265:DNA Helicases; C106893:XRCC5 protein, human; C000606509:Xrcc6 protein, human; D000072200:Ku Autoantigen