Abstract
The mixed lineage leukaemia (MLL) gene is frequently rearranged in secondary leukaemias, in which it could fuse to a variety of different partners. Breakage in the MLL gene preferentially occurs within a ~8 kb region that possesses a strong DNA topoisomerase II cleavage site. It has been proposed that DNA topoisomerase II-mediated DNA cleavage within this and other regions triggers translocations that occur due to incorrect joining of broken DNA ends. To further clarify a possible mechanism for MLL rearrangements, we analysed the frequency of MLL cleavage in cells exposed to etoposide, a DNA topoisomerase II poison commonly used as an anticancer drug, and positioning of the broken 3’-end of the MLL gene in respect to inherent chromosomal territories. It was demonstrated that exposure of human Jurkat cells to etoposide resulted in frequent cleavage of MLL genes. Using MLL-specific break-apart probes we visualised cleaved MLL genes in ~17% of nuclei. Using confocal microscopy and 3D modelling, we demonstrated that in cells treated with etoposide and cultivated for 1 h under normal conditions, ~9% of the broken MLL alleles were present outside the chromosome 11 territory, whereas in both control cells and cells inspected immediately after etoposide treatment, virtually all MLL alleles were present within the chromosomal territory. The data are discussed in the framework of the “breakage first” model of juxtaposing translocation partners. We propose that in the course of repairing DNA topoisomerase II-mediated DNA lesions (removal of stalled DNA topoisomerase II complexes and non-homologous end joining), DNA ends acquire additional mobility, which allows the meeting and incorrect joining of translocation partners.
Highlights
Chromosomal translocations are believed to cause different neoplasias, including leukaemias
It is known that treatment of Jurkat cells with topoisomerase II (topo II) poison etoposide causes cleavage of the mixed lineage leukaemia (MLL) gene within a relatively short breakpoint cluster region [26,27], it was not clear whether the broken DNA ends are separated or remain in close proximity
The presence of the so-called in vivo DNA topoisomerase II cleavage sites within the MLL breakpoint cluster region (BCR) [26,27] suggests that the MLL gene could be preferentially cleaved by DNA topoisomerase II under conditions of treatment with topo II poisons
Summary
Chromosomal translocations are believed to cause different neoplasias, including leukaemias (for a review, see [1,2]). The DSBs introduced by topo II can eventually be repaired [13,14], whereas in cycling cells, the inability to repair these lesions during the short period of time before mitosis can cause cell death These facts explain the selective toxic effect of topo II poisons on fast-proliferating cancer cells. The majority of all known MLL rearrangements initiate within a 8.3 kb breakpoint cluster region (BCR) containing a strong in vivo DNA topoisomerase II cleavage site [26,27] This suggests that preferential cleavage of the MLL gene by DNA topoisomerase II makes it a frequent partner in different translocation events
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