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 culture derived from the carcinogen-treated explants. Investigations were performed on the sequential changes occurring in the course of neoplastic progression of BP cells and in the tumor cells that arose in vivo from implanted BP cells. During the neoplastic progression a mutation 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 subpopulation. This mutational event was already detected at passage 14 but was probably not a direct consequence of a specific alteration caused by the carcinogen in the target cell. This mutation was retained through the subsequent progressional steps first as a heterozygous mutation, then converted to a homozygous state. From passage 18 on, it was possible in BP cell cultures to detect foci of larger morphologically distinct cells emerging on a background of cells maintaining the original morphology. These foci were shown to derive from a single cell carrying the p53 mutation in a homozygous state. During the neoplastic progression the mutant p53 allele frequency steadily increased and this mutant allele eventually came to predominate completely in the late stages of the neoplastic progression, including in the transplantation-induced tumors. The pattern of a directional selection for mutant p53 gene towards fixation is probably applicable to a wide range of human malignancies and may reflect the particular importance of this gene for tumorigenesis.