INFLUENCE OF TOPOISOMERASE-II ON THE FORMATION OF OXYGEN-DEPENDENT RADIATION-INDUCED DNA-DAMAGE

Several laboratories have recently demonstrated the feasibility of usi ng radiation-induced DNA strand breaks (SBs) and DNA-protein cross-lin ks (DPCs) to detect and quantify hypoxic cells in tumours and normal t issues. However, if radiation-induced SBs and DPCs are going to provid e reasonable estimates of the hypoxic fraction or fractional hypoxic v olume of tumours and normal tissues, their formation as a function of the oxygen concentration must be relatively independent of biological factors such as cell type, proliferative status or the composition and properties of proteins that are intimately associated with the DNA. I n the present study, the shape of the oxygen dependence curves and the K-m values for radiation-induced SBs and DPCs were measured by alkali ne elution for two human leukaemia cell lines, CEM and CEM/VM-1, whose nuclear matrix-associated topoisomerase II varied substantially in qu antity, activity and binding properties. The sigmoidal shape of the ox ygen dependence curves, the K-m for SB formation (approximate to 0.027 mM), and the K-m for DPC formation (approximate to 0.064 mM) were ide ntical for both of these human leukaemia cell lines. Consequently, the quantity and properties of topoisomerase II had no measurable influen ce on the oxygen-dependent formation of radiation-induced SBs and DPCs . These data suggest that varying levels of nuclear matrix-associated proteins and DNA binding proteins will not be a complicating factor wh en using radiation-induced SBs and DPCs for estimating the hypoxic fra ction or fractional hypoxic volume of tumours and normal tissues.