REPOSITIONING OF HUMAN INTERPHASE CHROMOSOMES BY NUCLEOLAR DYNAMICS IN THE REVERSE TRANSFORMATION OF HT1080 FIBROSARCOMA CELLS

An experimental system which should be valuable for studying the role of spatial positioning of the nuclear genome in human cell function ha s been developed., Reverse transformation of the malignant HT1080 fibr osarcoma cell line upon treatment with 8-chloro-cAMP results in growth inhibition, cytoskeletal reorganization, changes in nuclear shape and chromatin accessibility, and formation of prominent nucleoli. Fluores cent in situ hybridization was used to study DNA positioning during nu clear remodelling. Morphometric analysis of the hybridization sites fo r both repetitive sequences and ''painting probes'' for whole chromoso mes indicated dispersal of acrocentric chromosomes in untreated cells and a highly organized central location of these ribosome gene-contain ing chromosomes in association with one or a few large nucleoli in non dividing treated cells. The results suggest that there was a directed movement of interphase chromosomes during a response which normalized a malignant cell line. These large-scale repositionings may serve two functions in restoring a normal transcriptional setup to the nucleus. First, ribosome genes are placed in the nucleolus, their transcription al sub-organelle. Second, nucleolar anchorings together with additiona l perinucleolar centromeric associations orient the domain shapes of e ntire chromosomes, installing gene-rich chromosomal regions into pocke ts of (accessible) DNAse I-sensitive chromatin populated by spliceosom es. (C) 1998 Academic Press.