Rearrangements involving the MLL gene on chromosome band 11q23 are a hallmark of therapy-related acute myeloid leukemias following treatment with topoisomerase II poisons including etoposide. Therapy-related and de novo genomic translocation breakpoints cluster within a well-characterized 8.3-kb fragment of MLL. Repair of etoposidestabilized DNA topoisomerase II covalent complexes may initiate MLL rearrangements observed in patients. We used a culture system of primary human hematopoietic CD34 ؉ cells and inverse polymerase chain reaction to characterize the spectrum of stable genomic rearrangements promoted by etoposide exposure originating within an MLL translocation hotspot in therapy-related leukemia. Alterations to the region were observed at a readily detectable frequency in etoposidetreated cells. Illegitimate repair events after minimal repair included MLL tandem duplications and translocations, with minor populations of deletions or insertions. In stably repaired cells that proliferated for 10 to 14 days, the significant majority of illegitimate events were MLL IntroductionGenome integrity is controlled by multiple damage surveillance pathways, cell-cycle checkpoints, and repair mechanisms. [1][2][3] Despite these safeguards, illegitimate repair of chromosomal doublestrand breaks (DSBs), such as those produced by chemotherapy agents that target topoisomerase II (topo II), can promote chromosomal rearrangements and, ultimately, tumorigenesis. Topo II is an essential cellular enzyme that catalyzes changes in DNA topology via its cleavage-religation equilibrium. Topo II-targeted drugs that are poisons of this enzyme stabilize topo II-DNA covalent complexes, most often by decreasing the rate of religation in a dose-dependent manner. Disruption of the cleavage-religation reaction results in accumulation of DSBs, p53 activation, and induction of apoptosis or repair. 4,5 Chromosomal DSBs are potent inducers of recombination, stimulating the exchange of homologous sequences between 2 DNA duplexes 1000-fold 6-8 not only between sister chromatids, but also between homologs, and sequence repeats on heterologous chromosomes. 9-12 DSBs may be repaired by homologous recombination or nonhomologous end joining (NHEJ), and both repair mechanisms have been associated with chromosomal translocations, a hallmark of leukemias, lymphomas, and soft-tissue sarcomas. [13][14][15] There is clear evidence that exposure to chemotherapy or irradiation can result in subsequent development of therapy-related leukemias, which occur in 1% to 15% of patients exposed to DNA-damaging agents in anticancer regimens. 16,17 Exposure to topo II poisons such as etoposide is predominantly associated with therapy-related leukemias characterized by rearrangements of the MLL gene on chromosome band 11q23. 16 MLL spans 100 kb, encodes a 430-kDa protein homologous to the Drosophila trithorax gene, and has important functions in embryogenesis and hematopoiesis. 18,19 Mixed-lineage leukemia (MLL) is a critical transcriptional regulator and numerous tr...

An inactivating polymorphism at position 609 in the NAD(P)H:quinone oxidoreductase 1 gene (NQO1 C609T) is associated with an increased risk of adult leukemia. A small British study suggested that NQO1 C609T was associated with an increased risk of infant leukemias with MLL translocations, especially infant acute lymphoblastic leukemia (ALL) with t(4; 11