abl and bcr genes play an important role in the diagnostics of chronic myelogenous leukemia (CML). The translocation of these genes results in an abnormal chromosome 22 called the Philadelphia chromosome (Ph). The chimeric bcr-abl gene is a fundamental phenomenon in the pathogenesis of CML. Malignant transformation of hematopoietic cells is also accompanied by the c-myc gene changes (translocation, amplification). Nuclear topology of the abl, bcr and c-myc genes was determined in differentiated as well as in irradiated HL-60 cells using dual-colour fluorescence in situ hybridisation and image analysis by means of a high resolution cytometer. After the induction of the granulocytic differentiation of HL-60 cells with all trans retinoic acid (ATRA) or dimethylsulfoxide (DMSO), the abl and bcr homologous genes were repositioned closer to the nuclear periphery and the average distances between homologous abl-abl and bcr-bcr genes as well as between heterologous abl-bcr genes were elongated as compared with untreated human leukemic promyelocytic HL-60 cells. Elongated gene-to-gene and centre-to-gene distances were also found for the c-myc gene during granulocytic differentiation. In the case of the monocytic maturation of HL-60 cells treated with phorbol esters (PMA), the abl and bcr homologous genes were repositioned closer to each other and closer to the nuclear centre. The position of the c-myc gene did not change significantly after the PMA stimulus. The proximity of the abl and bcr genes was also found after gamma irradiation using 60Co (5 Gy). Immediately after the gamma irradiation c-myc was repositioned closer to the nuclear centre, but 24 h after radiation exposure the c-myc position returned back to the pretreatment level. The c-myc gene topology after gamma irradiation (when the cells are blocked in G2 phase) was different from that detected in the G2 sorted control population. We suggest that changes in the abl, bcr and c-myc topology in the case of gamma irradiation are not the effects of the cell cycle. It is possible, that differences in the cell cycle of hematopoietic cells after the gamma irradiation and concurrent proximity of the abl, bcr and c-myc genes could be important from the point of view of contingent gene translocations, that are responsible for malignant transformation of cells.

Institute of Biophysics Academy of Sciences of Czech Republic Brno

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