Selective loss of endogenous p21waf1/cip1 induction underlies the G1 checkpoint defect of monomeric p53 proteins

Wild-type p53 protein displays a spectrum of activities including the ability to suppress transformed cell growth to direct apoptotic cell death and to mediate G1 checkpoint in response to cellular DNA damage. Earlier work showed that a self-association defective p53 protein retained transformation suppressor activity in rat embryo fibroblast based assays, but that monomerisation of tumour mutant p53 proteins resulted in loss of dominant transforming activity. In order to acquire a more detailed understanding of the biological consequences attendant on disruption of p53:p53 association we have carried out a study of the wild-type-like activities that are retained by monomeric p53 proteins and which are associated with the suppression of transformation. Here we show that monomeric p53 proteins are G1 checkpoint defective. Although able to stimulate transcription via a p53 DNA binding motif from the p21waf1/cip1 gene promoter in episome based assays these p53 proteins are unable to transactive the chromosomal p21waf1/cip1 gene and are sensitive both to degeneracy of consensus binding site and to half site spacing. Monomeric p53 proteins fail to trigger apoptosis in a BRK cell line transformed with E7 and ras. However, they retain wild type transformation suppressor activity in BRK cell based transformation assays. Our results indicate that p21waf1/cip1 induction and all related p53 dependent G1 checkpoint activities are dispensable for the p53 directed suppression of transformed cell growth, and that such transformation suppression by monomeric p53 proteins may occur in the absence of an apoptotic response.

The relationship between serum p53 autoantibodies and characteristics of human breast cancer

Sera from 182 newly diagnosed breast cancer patients were assayed for antibodies to p53 using an enzyme-linked immunosorbent assay (ELISA) method, and antibodies were detected in 48 (26%) compared with 1 out of 76 (1.3%) normal control volunteers (P = 0.0001). In breast cancer patients, autoantibodies were found in all stages of disease progression: carcinoma in situ, primary invasive breast cancer and in metastatic disease. In the subset of patients in whom sequential sera were assessed over a 6 month period, changes in the p53 antibody titres were observed. The presence of antibodies to p53 correlated positively with high histological grade (P = 0.0012) and a history of second primary cancer (six positive out of eight cases). The incidence of autoantibodies was lower in those patients with a first-degree relative with breast cancer (P = 0.046). Out of 68 patients, there was a significant correlation between positive p53 autoantibody status and the detection of p53 protein in the tissue sections by immunocytochemistry (P = 0.002). In the seronegative patients, positive p53 tumour staining was strongly associated with a family history of breast cancer (P = 0.009). The p53 protein overexpressed in heritable breast cancers may therefore be less immunogenic. The presence of p53 autoantibodies provides important additional information to immunochemistry and may identify patients with aggressive histological types of breast cancer.

Serum p53 auto-antibodies: incidence in familial breast cancer

Inactivation of the p53 gene, which codes for a tumour suppressor protein, is known to occur in the majority of human malignancies. An ELISA technique has been developed which has detected auto-antibodies to p53 in the serum of 25.6% of 176 women with breast cancer, considerably higher than previously reported with an immunoblotting technique. The incidence of auto-antibodies in those cases with a family history of breast cancer was 9.1%, compared to 29.4% in those with no family history (P = 0.029). In women without clinical breast cancer, 4 out of 36 (11.1%) of those with a positive family history were seropositive, compared to 1 out of 73 control women. Auto-antibodies were more frequently seen in the serum of breast cancer patients whose biopsies demonstrated overexpression of p53 protein. We conclude that auto-antibodies to p53 may have a role in the molecular characterisation of familial breast cancer.

Human p53 binds DNA as a protein homodimer but monomeric variants retain full transcription transactivation activity

Wild-type human p53 protein is able to self-associate and consists predominantly of homotetramers in solution. In earlier work we identified the protein sequence motifs involved in p53 quaternary structure and showed that while monomeric p53 protein retains tumour suppressor function, monomeric tumour mutant p53 lacks dominant transforming activity. In this report we use point mutated and truncated cDNA genes encoding self-association defective human p53 proteins to investigate the relationship between p53 protein quaternary structure and the associated activities of transcription transactivation and target specific DNA binding. We show that p53 binds to a target oligonucleotide as a protein homodimer and that p53 dimerisation is required for detectable DNA binding. We found no evidence for p53 tetramer: DNA complexes and we suggest that the quaternary structure status of p53 may regulate a DNA binding associated activity. Monomeric p53 proteins failed to bind DNA in these assays but exhibited increased transactivating activity. Thus, both transcription transactivation and tumour suppressor functions act independently of p53 protein self-association and DNA binding. We propose that our results validate the p53 dimerisation motif as a target for rational anticancer drug design. We predict that compounds able to block p53 dimer assembly would inhibit the dominant transforming activities of mutant p53 in tumours retaining expression of a mutant allele, while leaving intact the wild-type p53 associated activities of transcription transactivation and transformation suppression in unaffected tissue.