Relative tumorigenicities of hybrid cells with and without hsr‐bearing chromosomes from a human melanoma cell line

Some cell types within the human melanoma cell line MeWo contain homogeneously staining regions (HSRs) consisting of repetitive DNA sequences and ribosomal RNA (rRNA) genes derived from chromosome 15. To further examine the association between enhanced tumorigenicity and the presence of HSR-bearing chromosomes, hybrid cell lines were constructed by fusing X-HSR-containing MeWo cells with ouabain-resistant, HPRT-deficient Chinese hamster ovary cells and culturing in HAT medium containing ouabain. A hybrid containing the X-HSR chromosome and several MeWo chromosomes was more tumorigenic in BALB/c nude mice than derivative cells lacking the X-HSR and human chromosome 18. However, since this enhanced tumorigenicity could be due to sequences on either the X-HSR or chromosome 18, a second series of hybrids was constructed by micro-cell fusion. In this case, the tumorigenicity of hybrid cells containing 2 copies of the X-HSR as the only MeWo chromosome was similar to that of derivative cells lacking these chromosomes. Cytogenetic analysis revealed that the nucleolar organizer regions (NORs) on the HSR were inactive in the hybrid cells. Our data indicate that DNA sequences amplified on MeWo HSRs do not enhance tumorigenicity under experimental conditions in which rRNA genes are not expressed. As the only active NORs in MeWo HSR-containing cells are on the HSRs, we suggest that expression of these amplified rRNA genes is responsible for the selective growth advantage of these cell types in nude mice. Our data also indicate that the enhanced tumorigenicity of MeWo HSR-containing cells is not due to co-amplification of a dominant oncogene.