New insights into p53 protein stabilisation in oral squamous cell carcinoma

p53 is a transcription factor which regulates cell proliferation and apoptosis to prevent division of potentially malignant cells. In many tumours mutation of the p53 gene leads to stabilisation of this protein which can be detected by immunohistochemistry (IHC). However, there are many reports describing detection of p53 by IHC in the absence of gene mutation, and in these cases other factors stabilise p53. To shed light on the mechanisms which permit detection of this protein in these mutation-negative cases we have examined 45 primary oral squamous cell carcinomas (SCCs) by IHC and gene sequencing for p53 (exons 4-8) and related the results to a FAL score (determined using microsatellite assay and expressing the number of loci showing allelic imbalance as a fraction of the total number of informative markers for each case). We also investigated the pattern of MDM2 expression in these tumours. High levels of p53 protein were detected in 24/45 cases and point mutations involving exons 4-9 were seen in 11 cases. A further four cases harboured deletions or a stop codon. For 6/48 cases there was concordance of AI within the p53 gene and mutation. However nine cases showed p53 mutation only and 5 AI without mutation, suggesting that oral tumours frequently retain one normal p53 allele. Detection of p53 by IHC correlated strongly with the FAL score. Thus whilst it is possible that some tumours harbour p53 mutations outside the open reading frames examined, or are missed due to sequencing a mixture of normal and tumour tissue, a subgroup of tumours may express high levels of wild-type p53 as a reflection of the high FAL score and ongoing genomic stress. Levels of MDM2 transcripts and protein were similar in all SCCs examined. However, MDM2 may be non-functional, or there may be defects affecting other important regulatory proteins in tumours which which express wild type p53 protein.