Cytogenetic evidence of gene amplification as a mechanism for tumor cell invasion.

In order to study the process by which human melanoma cells achieve invasion of basement membranes, a modification of the Membrane Invasion Culture System was developed to allow the in vitro collection of human melanoma cell populations that had invaded acellular human amniotic membranes. A significant increase in the number of double-minute chromosomes (DMs) was observed in metaphase nuclei of A375P human melanoma cells which had passed through two amniotic membranes (A375P-2) over that of control cells. Eighteen percent of the first monolayer of A375P-2 cells contained 1-89 DMs/cell, whereas 3-8.3% of the control A375P cells contained 1-10 DMs/cell. There was a rapid loss of DMs in A375P-2 cells as a function of passage number. After 25 days in tissue culture, the incidence of DMs had essentially dropped below the control range. These data indicate that an unstable gene amplification event may be part of the process by which melanoma cells execute invasion through basement membranes.