DIRECTIONAL SELECTION ASSOCIATED WITH CLONAL EXPANSION OF P53 MUTANT-CELLS DURING NEOPLASTIC DEVELOPMENT OF CARCINOGEN-TREATED RAT EMBRYO LUNG EPITHELIAL-CELLS

In this study, rat embryo lung organ cultures were exposed to benzo[a] pyrene (B[a]P). After carcinogen-treatment the cells were dissociated and an epithelial cell line (BP) was developed from the primary cell c ulture derived from the carcinogen-treated explants. Investigations we re performed on the sequential changes occurring in the course of neop lastic progression of BP cells and in the tumor cells that arose in vi vo from implanted BP cells. During the neoplastic progression a mutati on was shown to occur in p53 gene at codon 130 (AAG > AGG; Lys > Arg) in a single cell which expanded and gave rise to a predominant subpopu lation. This mutational event was already detected at passage 14 but w as probably not a direct consequence of a specific alteration caused b y the carcinogen in the target cell. This mutation was retained throug h the subsequent progressional steps first as a heterozygous mutation, then converted to a homozygous state. From passage 18 on, it was poss ible in BP cell. cultures to detect foci of larger morphologically dis tinct cells emerging on a background of cells maintaining the original morphology. These foci were shown to derive from a single cell carryi ng the p53 mutation in a homozygous state. During the neoplastic progr ession the mutant p53 allele frequency steadily increased and this mut ant allele eventually came to predominate completely in the late stage s of the neoplastic progression, including in the transplantation-indu ced tumors. The pattern of a directional selection for mutant p53 gene towards fixation is probably applicable to a wide range of human mali gnancies and may reflect the particular importance of this gene for tu morigenesis.