M ore than 15 years after obser- vations of an association between a distinct form of leukemia characterized by balanced chromosomal translocations involving band 11q23 and introduction of epipodophyllotoxins into clinical usage for anti-cancer treatment in the late1980s, the translocation mecha- nism is still debated. Epipodophyllo- toxins and other anticancer drugs that have been linked to leukemias with var- ious balanced translocations as a treat- ment complication share the property of being topoisomerase II poisons. In the early 1990s the MLL (ALL-1, HRX, hTRX1) gene was cloned by several groups at the genomic breakpoint junctions of translocations disrupting chromosome band 11q23. This paper will review evidence in favor of the hypothesis that topoisomerase II cleav- age complexes induced by poisons of this enzyme damage DNA directly and lead to translocations in secondary leukemias with MLL translocations. Topoisomerase II changes DNA topology from supercoiled to relaxed states by creating transient 4-base stag- gered double stranded breaks that enable passing of a separate intact dou- ble helix through the cleaved DNA. Covalent phosphodiester bonds are formed between active site tyrosyl residues (one per each subunit of the topoisomerase II homodimer) and the 5’ phosphate residue of the base 3’ to the cleavage (reviewed in (1)). The covalent enzyme-cleaved DNA com- plex is referred to as the cleavage com- plex. Formation of cleavage complexes is essential to normal cellular func- tions. The cleavage complex also is the target for several cytotoxic anticancer drugs (reviewed in (1)). Agents targeting the topoisomerase II cleavage complex that have been implicated in leukemia include the epipodophyllotoxins etoposide and teniposide, the anthracyclines daunorubicin, doxorubicin and 4-epi- doxorubicin, the anthracinedione mitoxantrone, and dactinomycin. These agents share the property of being topoisomerase II poisons, i.e. they convert topoisomerase II into a potent cellular toxin by increasing the concen- tration of cleavage complexes, either by decreasing the religation rate or increasing the forward rate of cleavage (reviewed in (1)). Their desired anti- neoplastic action is initiation of apop- totic pathways as the cellular response to irreversible DNA damage and illegit- imate DNA recombination from the increased cleavage complexes. The sta- bilization of cleavage complexes by anticancer drugs has led to the hypoth- esis that resultant chromosomal break- age from perturbation of the topoiomerase II cleavage-religation equilibrium can cause untoward DNA recombination in the form of translo- cations and lead to leukemogenesis. The action of topoisomerase II poisons is distinct from the action of catalytic inhibitors of the enzyme; however, cer- tain agents such as anthracyclines and dactinomycin behave as topoisomerase II poisons at low concentrations but as catalytic inhibitors at higher concentra- tions. In the clinical setting topoiso- merase II targeted drugs collectively have been called topoisomerase II inhibitors (reviewed in (1)). The balanced translocations in sec- ondary leukemias are many and include diverse translocations of the MLL gene at chromosome band 11q23, t(8;21) and variant AML1(CBFA2) translocations, inver- sions and translocations fusing CBFB and MYH11 at chromosome bands 16p13 and 16q22, t(15;17) fusing PML and RARA, t(8;16) fusing MOZ and CBP, t(9;22) fusing BCR and ABL, and diverse NUP98 translocations involving chromosome band 11p15. 1,2 MLL haematologica reports 2006; 2(issue 15):November 2006 | 30 | [haematologica reports] 2006;2(15):30-33 CAROLYN A.FELIX 1,2 BLAINE W.ROBINSON 1 GIUSEPPE GERMANO 1 CHRISTOS P.KOLARIS 1 LESLIE J.RAFFINI1* 2,3 LUCA LO NIGRO 1*,4 EMILY ROUMM 1* MAUREEN D.MEGONIGAL 1* DIANA J.SLATER 1* RYAN J.WHITMARSH 1* CHARLES SAGINARIO1* BRIAN D.LOVETT 1* JOLANTA LIBURA 5 LINDA D.PEGRAM 1* NAIYU ZHENG 6* SHAOKUN P ANG 6* XIAOLIANG ZHOU 6* ERIC F.RAPPAPORT7 CHRISTINE A.RICHARDSON 5*,8 NAI-KONG V.CHEUNG 9 IAN A.BLAIR 6 NEIL OSHEROFF 10 1 Division of Oncology,Childre- n's Hospital of Philadelphia, Philadelphia,PA; 2 Department of Pediatrics, University of Pennsylvania School of Medi- cine, Philadelphia,PA; 3 Divi- sion of Hematology,Childre- n's Hospital of Philadelphia, Philadelphia,PA; 4 University of Catania,Catania,IT; 5 Institute of Cancer Genetics, Depart- ment of Pathology,Columbia University College of Physi- cians and Surgeons; 6 Center for Cancer Pharmacology, University of Pennsylvania School of Medicine, Philadel- phia, PA; 7 NAPCORE, Childre- n's Hospital of Philadelphia, Philadelphia,PA; 8 Department of Biology and Bioinformatics Research Center,University of North Carolina, Charlotte, NC; 9 Department of Pediatrics, Memorial Sloan-Kettering Cancer Center,NewYork,NY; 10 Departments of Biochemistry and Medicine,Vanderbilt Uni- versity School of Medicine, Nashville,TN (*INDICATES AFFILIATION WHERE WORK COMPLETED, NOT CURRENT AFFILIATION) C.A.F.supported by NIH Grants R01CA77683,R01CA80175, R01CA85469,Leukemia & Lympho- ma Society SCOR, a Grant from Pennsylvania Department of Health, Joshua Kahan Founda- tion, Friends of the Joseph Claffey Fund. Pennsylvania Department of Health disclaims responsibility for any analyses, interpretations or conclusions. N.O. supported by NIH Grants R01GM33944 and R01GM53960. Translocation Mechanism in Secondary Leukemias Following Topoisomerase II Poisons Third International Symposium on Secondary Leukemias, Rome, 3-4 November 2006