Genistein is a bioflavonoid enriched in soy products. However, high levels of maternal soy consumption have been linked to the development of infant leukemia ALL and AML The majority of infant leukemia is linked to mixed lineage leukemia gene (MLL) translocations. Previous studies have implicated topoisomerase II (Top2) in genistein-induced infant leukemia. In order to understand the roles of the two Top2 isozymes in and the molecular mechanism for genistein-induced infant leukemia, we carried out studies in vitro using purified recombinant human Top2 isozymes, as well as studies in cultured mouse myeloid progenitor cells (32Dc13) and Top2 beta knockout mouse embryonic fibroblasts (MEFs). First, we showed that genistein efficiently induced both Top2 alpha and Top2 beta cleavage complexes in the purified system as well as in cultured mouse cells. Second, genistein induced proteasomal degradation of Top2 beta in 32Dc13 cells. Third, the genistein-induced DNA double-strand break (DSB) signal, gamma-H2AX, was dependent on the Top2 beta isozyme and proteasome activity. Fourth, the requirement for Top2 beta and proteasome activity was mirrored in genistein-induced DNA sequence rearrangements, as monitored by a DNA integration assay. Together, our results suggest a model in which genistein-induced Top2 beta cleavage complexes are processed by proteasome, leading to the exposure of otherwise Top2 beta-concealed DSBs and subsequent chromosome rearrangements, and implicate a major role of Top2 beta and proteasome in genistein-induced infant leukemia. (C) 2010 Elsevier Inc. All rights reserved.