KARYOTYPIC CHANGES ASSOCIATED WITH SPONTANEOUS ACQUISITION AND LOSS OF TUMORIGENICITY IN A HUMAN TRANSFORMED BRONCHIAL EPITHELIAL-CELL LINE- EVIDENCE FOR IN-VIVO SELECTION OF TRANSFORMED CLONES

In this study, we describe the karyotypic changes associated with the spontaneous acquisition of tumorigenicity in an immortalized tumor bro nchial cell line. Neoplastic transformation of the NL20 human bronchia l epithelial cell line occurred after 3 yr in culture, and was associa ted with loss of chromosome 18 together with acquisition of multiple c opies of 9q21.2-->34. The nontumorigenic NL20 cell line had been estab lished by transfection of human bronchial epithelial cells with the SV 40 T antigen, and had retained a relatively stable karyotype after the first 32 passages in vitro. However, when cells from p184 were inocul ated into nude mice, a transplantable tumor was obtained that was deri ved from a minor clone present in this otherwise stable line. Subseque nt passaging of the NL20 cells in vitro did not yield further tumors, and the minor clone from which the tumorigenic NL20T cell line derived was no longer evident in NL20 cells by Passage 205. Furthermore, the original tumorigenic NL20T cells lost the neoplastic phenotype after 2 5 passages in vitro and reverted to the nontumorigenic karyotype obser ved at p189. In contrast to the loss of the tumorigenic phenotype and karyotype, which occurred with in vitro passaging of the original tumo r, when the NL20T cells were passaged in other nude mice, they continu ed to give rise to tumors with sevenfold amplifications of 9q sequence s and loss of chromosome 18, and cells from the secondary tumors (NL20 T-A cells) have maintained a stable karyotype and remain tumorigenic e ven after 64 passages in vitro. A mixture of 10% tumorigenic NL20T-A a nd 90% nontumorigenic NL20 cells formed tumors in athymic nude mice wh en cultured in vitro on fibronectin, but not on plastic; cytogenetic a nalysis demonstrated that the tumors and cell cultures were composed o f tumorigenic NL20T-A cells, whereas cytogenetic analysis of cells cul tured on plastic were identical to the nontumorigenic NL20 cells. Thes e data support the hypothesis that neoplastic transformation in our or iginal cell line arose from in vivo selection of a small mutant clone, which had arisen in culture and was subsequently selected in vivo but was lost with in vitro culture.