Spatial distribution of selected genetic loci in nuclei of human leukemia cells after irradiation

Fluorescence in situ hybridization (FISH) combined with high-resolution cyt ometry was used to determine the topographic characteristics of the centrom eric heterochromatin (of the chromosomes 6, 8, 9, 17) and the tumor suppres sor gene TP53 (which is located on chromosome 17) in cells of the human leu kemia cell lines ML-1 and U937. Analysis was performed on cells that were e ither untreated or irradiated with gamma rays and incubated for different i ntervals after exposure. Compared to untreated cells, homologous centromere s and the TP53 genes were found closer to each other and also closer to the nuclear center 2 h after irradiation. The spatial relationship between gen etic elements returned to that of the unirradiated controls during the next 2-3 h. Statistical evaluation of our experimental results shows that homol ogous centromeres and the homologous genes are positioned closer to each ot her 2 h after irradiation because they are localized closer to the center o f the nucleus (probably due to more pronounced decondensation of the chroma tin related to repair), This radial movement of genetic loci, however, is n ot connected,vith repair of DSBs by processes involving homologous recombin ation, because the angular distribution of homologous sequences remains ran dom after irradiation. (C) 2001 by Radiation Research Society.