TRANSACTIVATIONAL AND DNA-BINDING ABILITIES OF ENDOGENOUS P53 IN P53 MUTANT-CELL LINES

Cells with divergent mutant alleles of the p53 gene have different biological and biochemical properties in vitro. Increasing evidence indicates that p53 is a transcriptional activator, and recently, high affinity DNA binding sites for p53 have been identified. The purpose of this study was to determine in vivo, the effect that various mutant p53 proteins have on their ability to mediate transactivation and to bind specifically to DNA. Either a p53 responsive or control reporter gene was transfected into 18 human carcinoma cell lines, having various p53 mutations, either with or without a wild-type p53 expression vector, The CAT activity and DNA gel retardation were studied to measure transactivation and DNA binding by these endogenous p53s. As expected, the endogenously produced wild-type p53 binds to DNA binding sequences and can transactivate a reporter construct containing a p53 high affinity DNA binding site. Four of five cell lines with homozygous p53 mutations at codon 273 (273(His)), contained p53 which had the ability to bind to p53 DNA binding sequences and transactivate. In contrast, all the homozygous, non-codon 273 mutant p53s (156(Pro), 175(His), 223(Leu), 248(Gln), 248(Trp), 280(Lys)) present in the other cell lines had no transactivating ability. These findings suggest that the biology of cancers with mutations at codon 273 may be different than those with p53 mutations at other sites. The p53 from WRO, a thyroid carcinoma cell line with p53 mutation at codon 223 (223(Leu)), was able to bind p53 DNA recognition sequences, but was unable to transactivate. Interestingly, in a vulvar carcinoma cell line (A431) with a p53 mutation at codon 273 (273(His)), the p53 was unable to transactivate and gave an aberrant band on gel retardation. Both CEM and SK-UT-1, which have compound heterozygous mutations at codons 175/248 (175(His)/248(His)), produced p53 which can complex with DNA, as well as transactivate. In contrast, the p53 in cell lines with either homozygous 175(His) or 248(His) p53 mutations, were unable either to transactivate or bind to the p53 response element. A cell line (NPA) heterozygous for 266(Glu) p53 mutation, was able to efficiently transactivate a reporter containing a p53 DNA binding site, therefore showing no evidence of a dominant negative effect of the endogenous p53 mutant allele. In summary, this in vivo study further supports the idea that different p53 mutant alleles have various properties which may affect their function.