Abnormal expression of the nuclear-associated enzyme DNA topoisomerase II 
(topoisomerase II) has been implicated in the in vitro phenotype of radiation 
hypersensitive ataxia-telangiectasia (A-T) cells and in modifying sensitivity of 
eukaryotic cells to topoisomerase II-inhibitor drugs [e.g., the DNA intercalator 
amsacrine (mAMSA)]. To study such relationships, various SV40- and Epstein-Barr 
Virus-transformed human cell lines derived from normal, A-T, or UV-sensitive 
xeroderma pigmentosum donors have been assayed for their sensitivity to mAMSA 
together with direct and indirect measurements of topoisomerase II expression. 
We report on the identification of an SV40-transformed A-T fibroblast cell line 
with abnormally high levels of topoisomerase II in nuclear protein extracts as 
determined by immunoblotting, measurement of kinetoplast DNA decatenation 
activity, and mAMSA-dependent DNA-protein cross-linking activity in a filter 
binding assay. Using a flow cytometric assay for the analysis of reactivity of 
nuclei with a polyclonal antitopoisomerase II antibody, overproduction was found 
to occur in all phases of the cell cycle. High levels of topoisomerase II were 
associated with hypersensitivity (5-10-fold) to mAMSA-induced cell cycle delay, 
cell kill, and DNA strand breakage (assayed under protein-denaturing 
conditions). Xeroderma pigmentosum (group A) cells demonstrated normal responses 
to mAMSA. The results provide evidence that cellular potential for the 
generation of topoisomerase II-dependent DNA damage is a major factor in 
governing the sensitivity to mAMSA. Furthermore, underexpression of 
topoisomerase II does not appear to be a primary factor in describing the in 
vitro A-T phenotype. The findings also relate to how changes in chromatin 
structure and function may either reflect or dictate the expression of 
topoisomerase II in human cells.