Topology of double minutes (dmins) and homogeneously staining regions (HSRs) in nuclei of human neuroblastoma cell lines

Amplification of the MYCN gene is a characteristic feature of many neurobla stomas and is correlated with aggressive tumor growth, Amplicons containing this gene form either double minutes (dmins) or homogeneously staining reg ions (HSRs). To study the nuclear topology of these tumor-specific and tran scriptionally active chromatin structures in comparison to chromosome terri tories, we performed fluorescence in situ hybridization with a MYCN probe a nd various chromosome paint probes, confocal laser scanning microscopy, and quantitative three-dimensional image analysis. The dmins formed dot-like s tructures in interphase nuclei and were typically located at the periphery of complexly folded chromosome territories; dmins noted in the chromosome t erritory interior were often detected within an invagination of the territo ry surface. Interphase HSRs typically formed extremely expanded structures, which we have never observed for chromosome territories of normal and tumo r cell nuclei. Stretches of HSR-chromatin often extended throughout a large part of the cell nucleus, but appeared well separated from neighboring chr omosome territories. We hypothesize that dmins are located within the inter chromosomal domain (ICD) space and that stretches of HSR-chromatin align al ong this space. Such a topology could facilitate access of amplified genes to transcription and splicing complexes that are assumed to localize in the ICD space. (C) 2000 Wiley-Liss, Inc.