ИСТИНА

Today we know that treating cells with DNA topoisomerase II poisons (such as etoposide) leads to formation of double-stranded DNA breaks in breakpoint cluster region (BCR) of some proto-oncogenes such as MLL or AML1. Erroneous repair of such breaks in its turn may lead to chromosome rearrangements that provokes oncogenesis, namely so-called “treatment-related” leukaemias. Our experimental data support the “breakage-first” theory supposing that DNA breaks formed at distant locations might be brought together and produce inter-chromosomal translocations. We have treated cultured human T-lymphocytes (Jurkat) with etoposide and then used 3D-FISH method to visualize chromosomal territory and proto-oncogene parts upstream and downstream of corresponding BCR. In order to increase statistical  significance of results, we have analyzed hundreds of cells in each experiment. To facilitate the task we have wrote a computer software in order to analyze three-dimensional confocal images automatically. The first step of analysis is the detection of cells by positioning of uniform cylinders overlapping the cells in optimal places within image. Next step is the detection of signals as groups of the brightest pixels within each cell. Visual control of random subsample of images confirmed the correctness of the automatic detection of both cells and signals. Final step is the measuring the distances between determined signals. New software allowed us to obtain number of data and reliable statistics. It shows that after etoposide treatment parts of some MLL or AML1 alleles are distanced from each other in the nuclear space. We obtained that separated parts of proto-oncogenes are located outside their chromosome territory more frequently than non-broken proto-oncogenes. It shows that DNA ends acquire additional mobility, which allows the meeting and incorrect joining of translocation partners. Acknowledgments: This research was partially supported by the Russian Foundation of Basic Research, project #14-04-93105.