Thioguanine substitution alters DNA cleavage mediated by topoisomerase II

Thiopurines and topoisomerase II-targeted drugs (e.g., etoposide) are widel y used anticancer drugs. However, topoisomerase Ii-targeted drugs can cause acute myeloid leukemia, with the risk of this secondary leukemia linked to a genetic defect in thiopurine catabolism. Chronic thiopurines result in t hioguanine substitution in DNA. The effect of these substitutions on DNA to poisomerase II activity is not known. Our goal was to determine whether deo xthio-guanosine substitution alters DNA cleavage stabilized by human topois omerase If. We studied four variations of a 40 mer oligonucleotide with a t opoisomerase II cleavage site, each with a single deoxythioguanosine in a d ifferent position relative to the cleavage site (-1 or +2 in the top and +2 or +4 in the bottom strand). Deoxythioguanosine substitution caused positi on-dependent quantitative effects on cleavage. With the -1 or +2 top and +2 or +4 bottom substitutions, mean topoisomerase II-induced cleavage was 0.6 -, 2.0-, 1.1-, and 3.3-fold that with the wild-type substrate (P=0.011, < 0 .008, 0.51, and < 0.001, respectively). In the presence of 100 muM etoposid e, cleavage was enhanced for wild-type and all thioguanosin-modified substr ates relative to no etoposide, with the +4 bottom substitution showing grea ter etoposide-induced cleavage than the wild-type substrate (P=0.015). We c onclude that thioguanine incorporation alters the DNA cleavage induced by t opoisomerase II in the presence and absence of etoposide, providing new ins ights to the mechanism of thiopurine effect and on the leukemogenesis of th iopurines, with or without topoisomerase inhibitors.-Krynetskaia, N. F., Ca i, X., Nitiss, J. L., Krynetski, E. Y., Relling, M. V. Thioguanine substitu tion alters DNA cleavage mediated by topoisomerase II.