Alterations of the p53 gene in nasopharyngeal carcinoma

Tumourigenesis associated with the p53 tumour suppressor gene

The p53 gene is contained within 16-20 kb of cellular DNA located on the short arm of human chromosome 17 at position 17p13.1. This gene encodes a 393-amino-acid nuclear phosphoprotein involved in the regulation of cell proliferation. Current evidence suggests that loss of normal p53 function is associated with cell transformation in vitro and development of neoplasms in vivo. More than 50% of human malignancies of epithelial, mesenchymal, haematopoietic, lymphoid, and central nervous system origin analysed thus far, were shown to contain an altered p53 gene. The oncoproteins derived from several tumour viruses, including the SV40 large T antigen, the adenovirus E1B protein and papillomavirus E6 protein, as well as specific cellular gene products, e.g. murine double minute-2 (MDM2), were found to bind to the wild-type p53 protein and presumably lead to inactivation of this gene product. Therefore, the inactivation of p53 tumour suppressor gene is currently regarded as an almost universal step in the development of human cancers. The current data on p53-associated tumourigenesis are briefly discussed in this minireview.

P53 overexpression and Epstein-Barr virus infection in undifferentiated and squamous cell nasopharyngeal carcinomas

We have analysed 22 nasopharyngeal carcinomas (NPCs) for expression of the small nuclear Epstein-Barr virus (EBV)-encoded RNAs (EBERs) and for immunohistologically detectable overexpression of p53. In situ hybridization demonstrated expression of the EBERs in 13 undifferentiated NPCs while nine squamous cell NPCs were EBER-negative. These results therefore confirm our previous DNA-DNA in situ hybridization studies and demonstrate that in the nasopharynx EBV is exclusively associated with undifferentiated but not with squamous cell carcinomas. p53 overexpression was demonstrated by immunohistology in 5 of 9 squamous cell NPCs and in 9 of 13 undifferentiated NPCs. Thus, there appears to be no correlation of p53 overexpression with EBV infection. These results are unexpected in the light of previous studies demonstrating that the p53 gene in primary undifferentiated NPC is consistently in the wild-type configuration. By contrast, analyses of squamous cell carcinomas of the head and neck have demonstrated that p53 overexpression in these cases is the result of p53 gene mutation. Whilst more detailed genetic analysis is required, our results suggest that mechanisms other than mutation of the p53 gene may be responsible for the stabilization of the protein in cases of undifferentiated NPC. It is tempting to speculate that an EBV-encoded protein may be involved.

p53 gene alterations in human prostate carcinoma

Alterations of the p53 gene are among the most frequent genetic lesions in a variety of human malignancies. Their role in prostate cancer is, however, unclear. We have analyzed 10 primary and two metastatic human prostate carcinomas as well as 3 prostate cancer cell lines for mutations of the p53-gene. Using single strand conformational polymorphism analysis (SSCP) and direct sequencing of the polymerase chain reaction (PCR) products, p53 mutations were detected in 1 of 10 primary prostate cancers. By contrast, 1 of 2 metastatic tumors and all 3 prostate cancer cell lines (derived from metastases) were found to contain a mutant p53 gene. Thus, our data suggest that alterations of the p53 gene at the mutational "hot spots" appear to occur infrequently in primary human prostate cancer, but may emerge in later stages of tumor progression. Furthermore, we confirm that loss of heterozygosity at a locus telomeric to p53, but not including p53, is associated with metastatic progression in 1 case.

Cytogenetic studies in three xenografted nasopharyngeal carcinomas

Cytogenetics results of three xenografted nasopharyngeal carcinomas (NPC) are reported. One (C15) was almost diploid and had only an isochromosome 1q, trisomy 2, and loss of chromosome X. The two other tumors, C17 and C19, were hypodiploid and had complex karyotypes with some variations. Nonrandom structural abnormalities of chromosomes 1, 3, 8, and 17 were observed. A correlation between a del(17)(p11-12) observed in C17 and loss of both alleles of p53 recently shown in this tumor is emphasized.

Overexpression of p53 in Hodgkin's disease: lack of correlation with Epstein-Barr virus infection

Recent work has shown that p53 gene mutations are frequently found in Epstein-Barr virus (EBV)-positive and EBV-negative cases of Burkitt's lymphoma but not in EBV-associated undifferentiated nasopharyngeal carcinomas (NPCs). Similar viral gene expression patterns are observed in undifferentiated NPCs and in EBV-positive cases of Hodgkin's disease (HD), suggesting that the contribution of the virus to the pathogenesis of these malignancies may also be similar. We have analysed 116 cases of HD for EBV association and for immunohistologically detectable overexpression of p53. p53 overexpression was detected in the tumour cell population of 37 (32 per cent) of the cases. Fifteen cases showed p53-specific labelling of more than 40 per cent of tumour cells; in six of these, virtually all tumour cells were stained. In eight cases, between 5 and 40 per cent of tumour cells were labelled, and in another 14 cases, less than 5 per cent of tumour cells expressed detectable amounts of p53. EBV-positive HD cases were found in all groups with different levels of p53 overexpression as well as amongst p53-negative cases. While a more detailed analysis of the p53 gene in HD is required, these data show that overexpression of p53 in HD is heterogeneous and that there is no simple correlation between EBV infection and p53 overexpression.

Altered cell cycle arrest and gene amplification potential accompany loss of wild-type p53

Gene amplification occurs at high frequency in transformed cells (10(-3)-10(-5)), but is undetectable in normal diploid fibroblasts (less than 10(-9)). This study examines whether alterations of one or both p53 alleles were sufficient to allow gene amplification to occur. Cells retaining one wild-type p53 allele mimicked the behavior of primary diploid cells: they arrested growth in the presence of drug and failed to demonstrate amplification. Cells losing the second p53 allele failed to arrest when placed in drug and displayed the ability to amplify at a high frequency. Thus, loss of wild-type p53 may lead to amplification, possibly caused by changes in cell cycle progression. Other determinants can by-pass this p53 function, however, since tumor cells with wild-type p53 have the ability to amplify genes.