ITERATIVE CHROMOSOME MUTATION AND SELECTION AS A MECHANISM OF COMPLETE TRANSFORMATION OF HUMAN-DIPLOID FIBROBLASTS BY SV40-T ANTIGEN

The acquisition of an extended lifespan and of neoplastic properties, including anchorage independence, ability to grow in low serum-contain ing media, morphological transformation, immortalization and tumorigen icity in nude mice were studied in 31 human fibroblast lineages transf ected with plasmids containing the SV40 early genes. Plasmids were use d that contained sequences for large T alone, or large T plus small t or large T plus small t plus the SV40 origin. Cells expressing large T antigen gradually acquired the ability to form colonies in low serum or to form anchorage-independent colonies. Large T antigen was suffici ent to cause complete transformation to tumorigenicity if multi-step l ineage evolution was obtained by prolonged serial passage and if in vi vo progression was assisted by means of a gelatin sponge implantation technique. Cells derived from progressive tumors initiated in sponges showed enhanced tumorigenicity as measured by ability to obtain tumors without using sponges and with reduced latent period, higher incidenc e and with fewer cells inoculated. Multiple lineages, of human fibrobl asts have been converted to tumorigenicity without additional treatmen ts such as transfection with activated oncogenes or exposure to carcin ogens. These data, taken in conjunction with earlier studies showing t hat T antigen causes chromosome mutation preceding and accompanying th e accumulation of the neoplastic phenotype, suggests that the T protei n drives the transformation process by acting as a mutagen and cells w ith growth advantages were selected for in vitro and in vivo. With the possible exception of morphological transformation, the presence or a bsence of genes for small t and the SV40 origin were not critical for the process.