Clonal drift and role of chromosome dosage in human melanoma metastatic cell lines: a statistical analysis

Karyotypic analyses were performed on twenty human melanoma clones and variants all deriving, following in vivo selections and/or in vitro cloning, from the parental M4Be cell line, but expressing different tumorigenicity (expressed as mean tumor weight) and metastatic ability (expressed as pulmonary metastasis frequency) after s.c. injection in antithymocyte immunosuppressed newborn rats. These cells were hypertriploid, showing quite comparable modal numbers (around 70). They all expressed a wide range of chromosome number per mitosis (28 to 198), as well as a large extent of karyotypic heterogeneity, showed by extensive clonal drifts within the different cell lines. Their common origin was ascertained by five clonal abnormal marker chromosomes deriving from chromosomes 6, 7, 8, 9, 11 and 14. Twenty-one additional marker chromosomes, most of them non clonal, were observed in the different cell lines. We developed a statistical analysis to search for putative relationship between the expression of tumorigenicity and metastatic ability and the evolution of specific subclones within the different cell lines. We showed that the expression of a high mean tumor weight and/or a high metastasis incidence was related to the modification of the ratio of different subclones within each cell line, and more especially to the emergence of subclones presenting partial losses of individual chromosomes or chromosomal fragments, which were encountered with a lower frequency in the low metastatic cell lines. The biological relevance of these findings in terms of clonal evolution and role of chromosome dosage in tumor progression is discussed.