DNA CLEAVAGE WITHIN THE MLL BREAKPOINT CLUSTER REGION IS A SPECIFIC EVENT WHICH OCCURS AS PART OF HIGHER-ORDER CHROMATIN FRAGMENTATION DURING THE INITIAL-STAGES OF APOPTOSIS

A distinct population of therapy-related acute myeloid leukemia (t-AML ) is strongly associated with prior administration of topoisomerase Il (topo II) inhibitors, These t-AMLs display distinct cytogenetic alter ations, most often disrupting the MLL gene on chromosome 11q23 within a breakpoint cluster region (bcr) of 8.3 kb, We recently identified a unique site within the MLL bcr that is highly susceptible to DNA doubl e-strand cleavage By classic topo ii inhibitors (e.g., etoposide and d oxorubicin). Here, we report that site-specific cleavage within the ML L bcr can be induced by either catalytic topo II inhibitors, genotoxic chemotherapeutic agent which do not target topo TI, or nongenotoxic s timuli of apoptotic cell death, suggesting that this site-specific cle avage is part of a generalized cellular response to an apoptotic stimu lus, We also show that site-specific cleavage within the MLL bcr can b e linked to the higher-order chromatin fragmentation that occurs durin g the initial stages of apoptosis, possibly through cleavage of DNA lo ops at their anchorage sites to the nuclear matrix. In addition, tsf s how that site-specific cleavage is conserved between species, as speci fic DNA cleavage can also be demonstrated within the murine MLL locus, Lastly, site-specific cleavage during apoptosis can also be identifie d at the AML1 locus, a locus which is also frequently involved in chro mosomal rearrangements present in t-AML patients, In conclusion, these results suggest the potential involvement of higher-order chromatin f ragmentation which occurs as a part of a generalized apoptotic respons e in a mechanism leading to chromosomal translocation of the MLL and A IML1 genes and subsequent t-AML.