Influence of topoisomerase II on the formation of oxygen-dependent radiation-induced DNA damage.

Several laboratories have recently demonstrated the feasibility of using radiation-induced DNA strand breaks (SBs) and DNA-protein cross-links (DPCs) to detect and quantify hypoxic cells in tumours and normal tissues. However, if radiation-induced SBs and DPCs are going to provide reasonable estimates of the hypoxic fraction or fractional hypoxic volume 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. In the present study, the shape of the oxygen dependence curves and the K(m) values for radiation-induced SBs and DPCs were measured by alkaline elution for two human leukaemia cell lines, CEM and CEM/VM-1, whose nuclear matrix-associated topoisomerase II varied substantially in quantity, activity and binding properties. The sigmoidal shape of the oxygen dependence curves, the K(m) for sB formation (approximately 0.027 mM), and the K(m) for DPC formation (approximately 0.064 mM) were identical for both of these human leukaemia cell lines. Consequently, the quantity and properties of topoisomerase II had no measurable influence 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 when using radiation-induced SBs and DPCs for estimating the hypoxic fraction or fractional hypoxic volume of tumours and normal tissues.