P53 ALTERATION IS A COMMON EVENT IN THE SPONTANEOUS IMMORTALIZATION OF PRIMARY BALB/C MURINE EMBRYO FIBROBLASTS

It has been shown previously that mutant p53 can act as an immortalizing gene when cotransfected into primary rat embryo fibroblasts along with a selectable marker. To determine whether a mutation at the p53 locus is a common event in the pathways leading to spontaneous cellular immortalization, 11 clonally derived BALB/c murine embryo fibroblast lines were established by passage on a 3T3 schedule and examined for p53 alterations. By the following criteria, all 11 independently established lines contain at least one mutant allele of p53. Seven of these lines have a PAb240-reactive p53 species and exhibit an extended p53 half-life as determined by pulse-chase analysis. The p53 protein species in a subset of these lines is also capable of complex formation with the constitutive heat shock protein hsc70. p53 cytoplasmic DNAs (cDNAs) from several of these lines have been cloned by reverse transcription of cytoplasmic RNA followed by PCR amplification, and the mutations have been mapped by DNA sequence analysis. Point mutation in conserved domains of p53 appears to be a common alteration in these lines, although one established line carries a 24-bp in-frame deletion of p53. The remaining four cell lines do not express detectable p53 protein. For each line there is a different molecular event underlying the lack of p53 expression: (1) deletion of at least the first 6 exons of both p53 alleles; (2) expression of a single p53 mRNA encoding a stop codon at amino acid position 173; (3) no detectable p53 mRNA; and (4) greatly diminished expression of p53 mRNA. These findings indicate that p53 alteration commonly occurs in spontaneously immortalized BALB/c mouse embryo fibroblasts passaged on a 3T3 schedule and, therefore, may be an important event for the immortalization process.