Single-step selection of unique human melanoma variants displaying unusually aggressive metastatic behavior in nude athymic mice.

Using two different approaches, variants from a pigmented human melanoma cell line called MeWo were selected in a single step which displayed an unusually aggressive ability to metastasize in adult athymic nude mice. The first set of variants was obtained by recovery, and establishment in culture, of 'spontaneous' lung metastases obtained 5-6 months after subcutaneous inoculation of the parent MeWo line. That these metastases arose by a nonrandom process, and were authentic variants, was shown by the fact that they were always highly aneuploid, having predominantly near-triploid or near-tetraploid chromosome numbers. In contrast, the parent MeWo cells had a predominant hypodiploid chromosome mode with a minor population (less than 10%) of near-tetraploid cells. A second set of variants was obtained through in vitro selection of cloned lectin-resistant (Lecr) aneuploid 'membrane mutants' from MeWo, using wheat germ agglutinin (WGA) as the selective agent. Some of these mutants manifested an extraordinary ability to disseminate widely and extensively to many extrapulmonary sites after intravenous inoculation of the cells; furthermore, the metastases, even those less than 0.5 mm in diameter ('pinhead' sized), were easily visible because of the remarkably intense pigmented nature of the mutant cells. These results provide a promising direction to take for the derivation of heterogenous sublines of human tumors which not only metastasize aggressively in nude mice, but which do so in a manner not unlike what is actually observed in their natural host.