Status of the p53 tumor suppressor gene in human squamous carcinoma cell lines

Dominant-negative and/or loss-of-function mutations of the p53 tumor suppressor gene are frequently found in squamous cell carcinomas of the skin and of the head-and-neck region. In order to identify the precise mechanisms of inactivation of p53 in tumors of this class, we examined the status of p53 RNA, protein and DNA in a panel of eight human squamous carcinoma cell lines (head-and-neck, 3; esophagus, 1; lung, 1; uterine cervix, 2; vulva, 1). Three lines (A253, CaLu-1, SqCC/Y1) failed to express any p53 mRNA. A253 cells contained a single p53 allele without mutations in exons 2-9, suggesting that the lack of transcription was the result of mutations in the regulatory region of the gene. Both p53 alleles were deleted in CaLu-1 cells, whereas the single allele present in SqCC/Y1 cells was rearranged and carried two missense mutations in exon 5. Two cell lines (A431, FaDu) expressed only 50% of the normal level of p53 mRNA, either because only one allele was present (A431), or because only one of the two alleles was transcribed (FaDu). The two cervical carcinoma lines (CaSki, C4-1) expressed normal levels of p53 mRNA, but no wild type protein, presumably as a result of accelerated degradation by the human papillomavirus 16 or -18 E6 oncoprotein present in these cells as previously described (Scheffner et al., Proc. Natl. Acad. Sci. USA 88:5523-5527; 1991). Three of the lines expressed only mutant p53 protein (A431, FaDu, CE-48) resulting from missense mutations in codons 248 and 273. In summary, wild type p53 protein was expressed at a strongly reduced level or not at all in each of the eight SqCC lines, whereas only three of these lines expressed a mutant protein. The mechanisms responsible for the lack of expression of wild type protein were highly diverse, and included deletions of the gene, gene rearrangement, presumed mutations in the regulatory region of the gene, and accelerated rates of degradation of the protein. Our findings suggest that loss of expression of wild type p53 may be more important for malignant transformation of squamous epithelium than the acquisition of activating mutations.