Etoposide-induced treatment-related acute myelogenous leukemia (t-AML) is characterized by rearrangements of the mixed lineage leukemia (MLL) gene with one of its >50 partner genes, most probably as a consequence of etoposide-induced DNA double-strand breaks (DSBs). Recent studies have shown that etoposide-induced DSBs occur predominantly within the breakpoint cluster region (bcr) of the MLL gene. However, bcr-specific DSBs induced by etoposide are not topoisomerase II-linked but the result of apoptotic nuclease-mediated DNA cleavage. Here, we test the involvement of caspase-activated DNase (CAD) and other apoptotic components in etoposide-induced gene rearrangements using two methods. First, we measured the effect of etoposide on the integration frequency of a transfected plasmid. Etoposide strongly stimulated plasmid integration in CAD cDNA- complemented mouse embryonic fibroblasts (MEFs) but not in CAD knockout (KO) MEFs. Consistently, down-regulation of ICAD (inhibitor of CAD, also required for proper folding of CAD) in an HT29-derived cell line, which leads to decreased CAD activity, significantly reduced etoposide-induced plasmid integration. Second, we used long-template inverse PCR to focus on gene rearrangements at the MLL locus. Etoposide stimulated MLL fusion product formation in CAD cDNA-complemented MEFs but not in CAD KO MEFs. Together, these results suggest that CAD and other apoptotic components may play an important role in etoposide-induced t-AML.
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