p53: a molecular marker for the detection of cancer

Abstract

Background: The p53 gene is the most frequently mutated gene in cancer and accordingly has been the subject of intensive investigation for almost 30 years. Loss of p53 function due to mutations has been unequivocally demonstrated to promote cancer in both humans and in model systems. As a consequence, there exists an enormous body of information regarding the function of normal p53 in biology and the pathobiological consequences of p53 mutation. It has long been recognised that analysis of p53 has considerable potential as a tool for use in both diagnostic and, to a greater extent, prognostic settings and some significant progress has been made in both of these arenas.

Objective: To provide an overview of the biology of p53, particularly in the context of uses of p53 as a diagnostic tool.

Methods: A literature review focused upon the methods and uses of p53 analysis in the diagnosis of sporadic cancers, rare genetic disorders and in detection of residual disease.

Conclusion: p53 is currently an essential diagnostic for the rare inherited cancer prone syndrome (Li-Fraumeni) and is an important diagnostic in only a limited number of settings in sporadic disease. Research in specific cancers indicates that the uses of increasingly well informed p53 mutational analysis are likely to expand to other cancers.

Figure 1. p53 is induced by a range of stresses and coordinates a wide range of cellular responses

p53 can be stabilised and subsequently activated following phosphorylation through direct modification by a range of kinases including ATM, ATR, DNA-dependent protein kinase and casein kinase II. In addition, p53 can be activated following oncogenic mutations by induction of p14^ARF which protects it from ubiquitylation by MDM2.

Figure 2. The p53–MDM2 auto-regulatory feedback loop

High levels of p53 and MDM2 can occur transiently following genotoxic stress, but stable high expression of both proteins is prevented by the feedback of MDM2 upon p53. MDM2 promotes degradation of p53 and this then reduces transcriptional activation of the MDM2 gene by p53, bringing MDM2 levels down. How stable levels of both proteins are maintained in same cancers is unclear but factors such as HSP90 may bind to both proteins preventing degradation of p53 by MDM2. Other possibilities include MDMX expression, which can inhibit p53 but does not directly promote its degradation (although it can interact with MDM2 to promote this). This would not explain the high levels of MDM2 that can arise and does not explain the strong association between p53 and MDM2 dual upregulation observed in some cancers such as renal cell carcinoma.

Figure 3. The p53 FASAY can be used to detect mutant p53 from a mixture of normal and cancer cells that contains as little as 1 – 5% of mutant p53-expressing cells

Red (p53 mutant) colonies on the left are detected in a sample containing 10% p53 mutant and 90% p53 wild-type cells. Typically 5 – 10% mutant cells can be detected in this way but as little as 1% can be detected through sequencing of multiple colonies to identify rarer mutants. The panel on the right shows the result of p53 FASAY using wild-type p53. Note that low levels of red colonies appear on these plates and are due to either recombination of the plasmid used for the FASAY and to spontaneous mutations introduced during PCR amplification. These are readily identified following sequencing of the p53 cDNA in the red colonies.