NEUROBLASTOMA-CELLS CAN ACTIVELY ELIMINATE SUPERNUMERARY MYCN GENE COPIES BY MICRONUCLEUS FORMATION - SIGN OF TUMOR-CELL REVERTANCE

Human neuroblastoma cell lines frequently exhibit MYCN amplification a nd many are characterised by the presence of morphologically distinct cell types. The neuronal cells (N-cells) and the so-called flat cells (F-cells) are thought to represent manifestations of different neural crest cell lineages and are considered to be the consequence of neurob lastoma cell pluripotency. In this study, various neuroblastoma cell l ines were examined for micronuclei. In F-cells of neuroblastoma cell L ines with extrachromosomally amplified MYCN, we observed the frequent occurrence of micronuclei. Using fluorescence in situ hybridisation (F ISH) with a MYCN specific probe, we demonstrated that these micronucle i were packed with MYCN hybridisation signals. In addition, in a minor percentage of cells, MYCN signals occurred in clusters, adhered to th e nuclear membrane and aggregated in nuclear protrusions. In F-cells, a substantial reduction or lack of amplified MYCN copies was observed. These observations let us conclude that extrachromosomally amplified genes can be actively eliminated from the nucleus resulting in a drama tic loss of amplified sequences in the F-cells. Moreover, reduction or loss of amplified sequences in F-cells was shown to be accompanied by downregulation of MYCN expression, by a decrease in proliferative act ivity and by upregulation of molecules of the major histocompatibility complex class I (MHC I). Interestingly, F-cells are not restricted to neuroblastoma cell cultures, but also occur in cell Lines of other ti ssue origin. All F-cells share important biological features, interpre ted as cell revertance, i.e. loss of the malignant phenotype and prope rties. This fact, together with the demonstration that neuroblastoma c ells do not differentiate into Schwann cells in vivo [1] Ambros et al. NEJM 1996, 334, 1505-1511, do not support the hypothesis that F-cells represent Schwannian/glial differentiation in vitro. We therefore pos tulate that the elimination of amplified MYCN gene copies in cultivate d neuroblastoma cells is in line with the phenomenon of tumour cell re vertance. (C) 1997 Elsevier Science Ltd.