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Although it is clear that p53 plays a pivotal role in G1/G2 checkpoints to conserve genomic integrity, its role in S phase checkpoint is less well understood. Recently, it has been reported that p53 is transcriptionally impaired even though it is stabilized during replication blockade. However, the mechanisms underlying this phenomenon are not known. In the present study, it has been shown that p53 accumulates and transactivates its target genes such as p21, gadd45 and bax in response to replication blockade in normal and cancer cells. Lack of transcriptional activation under similar conditions in cells lacking p53 shows that p53-target gene activation during replication blockade is indeed p53-dependent. Further, transactivation of p21 in response to replication blockade by hydroxyurea and aphidicolin is similar to that in response to ionizing radiation except that the latter is more immediate compared to the response to replication blockade. These findings suggest that impairment of transcriptionally active p53 in response to replication blockade is not a general phenomenon.

The frequency of p53 mutations is low and there is evidence of p53 protein overexpression even without p53 mutations in cervical cancers. This suggests that alternative mechanisms other than p53 mutation could be responsible for tumourigenesis of the uterine cervix. Therefore, an attempt has been made in the present investigation to analyze mutation and rearrangement of p53 gene in primary cervical cancers. The results indicated absence of mutation and presence of rearrangement in about 35% of cervical cancer patients. However, p53 overexpression in 50% of patients was demonstrated by immunohistochemistry and Western blot analysis. Further, rearrangement of p53 has been correlated with p53 mRNA and p53 protein status. The results indicated presence of overexpressed p53 protein in the samples with rearranged p53 gene. Thus, it is presumed that rearrangement of p53 might lead to production of defective p53 protein by affecting the level of p53 protein and this might have a role in the process of tumourigenesis. This study reports for the first time rearrangement of p53 in cervical cancers.

Different transcription factors activate and repress the p53 gene expression. Recently, a tissue specific binding of NF1/YY1 to p53 promoter has been reported and further, it has been demonstrated that NF1/YY1 activates p53 promoter activity. The deregulated expression of p53 appears to be a central feature of malignant transformation and the basis of this deregulation is not well defined. Hence, an attempt has been made to know the binding of NF1/YY1 to p53 promoter taking breast tumour as a model system. Results have indicated a differential binding of NF1 to p53 promoter and a depletion or low level of NF1 in majority of breast tumour samples. Further, a correlation between NF1 and p53 has indicated the presence of p53 RNA even without NF1. Hence it is assumed that p53 expression is not NF1-dependent in breast tumours. However, the results clearly demonstrate a deregulation of NF1 transcription factor in breast tumours.

Few reports are available on mutations in the promoter of tumour suppressor genes like p16, WT1 and Rb in cancers. However, the involvement of p53 promoter in cancers is not clearly known. Further, methylation of CpG sites is a major contributor of mutations in several genes. So an attempt has been made to determine the mutation and methylation status of p53 promoter in breast tumours. Results have demonstrated absence of mutations and deletions in p53 promoter, leading us to conclude that mutation of p53 promoter is probably not a significant factor in breast tumorigenesis. Methylation analysis has shown that the CCGG sites in the p53 promoter are unmethylated unlike that of its exons. Further, it has been shown that there is no change in the methylation profile of the CCGG sites in breast tumours. However, such studies are to be conducted in different types of tumours to define the role of p53 promoter mutation and methylation in the process of tumorigenesis., (Copyright 1999 S. Karger AG, Basel)

Different postreplicative, transcriptional, and translational mechanisms responsible for p53 gene inactivation are slowly unfolding. Rearrangement of the p53 gene is a very rare event in human solid tumors and has been reported only in osteosarcomas. From our laboratory we have recently reported rearrangement only in the coding region of the p53 gene in breast tumors. In this report we have undertaken a systemic investigation of p53 in oral tumors. Results have shown rearrangement in the coding region of p53 in 20% of cases and in the 5' region of p53 gene in approximately 8% of cases. No allelic loss and amplification of p53 gene was observed in these tumor samples. In our earlier studies on breast tumors, we found no abnormality in the 5' region of p53. However, in the present study we report for the first time rearrangement in the 5' region of p53 in oral tumors. Correlation of p53 gene rearrangement with p53 expression of RNA and protein indicates that rearrangement in the 5' region of p53 might not have a role in p53 expression. However, rearrangement in the coding region of p53 might play a critical role in controlling p53 gene activity in the process of tumorigenesis.

Rearrangement of the p53 gene is frequent in virus transformed cell lines and in chronic myelogenous leukemia. It is a rare event in solid tumours and has been reported only in osteosarcomas. In this study we have examined rearrangement of the p53 gene in human breast tumours. We found rearrangement in 35% of the patients (7 of 20 tumours examined). Normal tissue from these patients had an unrearranged gene, indicating that the genetic abnormality in the tumour is acquired during the natural process of tumorigenesis. No intronic rearrangement or allelic loss of the p53 gene was found in the breast tumour samples studied. Further, rearrangement of the p53 gene has been correlated with the p53 transcriptional status. Only two patients with rearranged p53 showed a high level of p53 RNA as well as protein expression. Thus, for the first time we report the rearrangement of the p53 gene in breast tumours, which may play a role in the process of tumorigenesis.

It is well established that TP53 regulates the expression of many genes, but the regulation of expression of TP53 itself is poorly understood. Recently, it has been shown that there is a tissue-specific binding of nuclear proteins in the TP53 gene promoter. The aim of this study was to determine the nuclear proteins that bind to the TP53 promoter elements (between -104 and -458) in male breast cancer. The results of our study, using the electrophoretic mobility shift assay (EMSA) and Southwestern analysis, have showed: (1) nuclear proteins or factors other than p53 bind to the TP53 promoter; (2) the levels of at least four nuclear proteins vary between normal and tumour breast tissue; and (3) two newly discovered nuclear proteins bind to the TP53 promoter in tumour tissue but are absent in normal tissue. This differential binding of nuclear proteins to the TP53 gene promoter might play a critical role in TP53 transcription and cancer progression in male breast tumours.

The aim of this investigation was to study the prevalence of p53 gene mutations in male and female breast cancers and to find out the relationship between this event and p53 protein expression. Genomic p53 was amplified by polymerase chain reaction (PCR). Exons 5-8 were screened for mutations using single stranded conformation polymorphism (SSCP) analysis. P53 protein expression was detected by immunohistochemistry (IHC) with the monoclonal antibody DO-1. In female breast cancer, p53 gene mutation was detected in 33% cases in either exon 5 or 6. However, in male, mutation was detected only in exon 6 in 90% cases. On the other hand, p53 protein expression was observed in all of these cases. Moreover, p53 protein immunostaining was observed in some of those cancer tissues, where no mutation was detected in exons 5-8. P53 dysfunction, as indicated by mutation or increased protein expression, common in both male and female breast cancer, but rate of occurrence or site of mutation differ from each other. Our results in male breast tumors indicate a positive correlation between p53 mutation and p53 protein overexpression, whereas the results in female breast tumors indicate an overexpression of p53 protein even without p53 gene mutation. Therefore, it may be presumed that p53 protein accumulation can result primarily from mutation. In addition, stabilization of p53 through binding to other proteins is another possible reason of p53 overexpression.