p53, also known as tumor protein 53 (TP53), is a transcription factor that regulates the cell cycle and hence functions as a tumor suppressor. It is very important for cells in multicellular organisms to suppress cancer. p53 has been described as "the guardian of the genome", referring to its role in conserving stability by preventing genome mutation [6]. The name is due to its molecular mass: it runs as a 53 kilodalton (kDa) protein on SDS-PAGE.
The human gene that codes for p53 is TP53. The gene is named TP53 after the protein it codes for (TP53 is another name for p53). Italics are used to distinguish the TP53 gene name from the TP53 protein name. The gene is located on the human chromosome 17 (17p13.1).
The location has also been mapped on other model animals:
A central DNA-binding core domain (DBD). Contains zinc molecules and Arginine Amino Acid Residues.
A C-terminal homo-oligomerisation domain (OD). Tetramerization greatly increases the activity of p53 in vivo.
Most of the mutations that deactivate p53 in cancer usually occur in the DBD. The mutations destroy the ability of the protein to bind to its target DNA sequences, and thus prevents transcriptional activation of these genes. As such, mutations in the DBD are recessive loss-of-function mutations. Molecules of p53 with mutations in the OD dimerise with wild-type p53, and prevent them from activating transcription. Therefore OD mutations have a dominant negative effect on the function of p53.
Wild-type p53 is a labile protein, comprising folded and unstructured regions which function in a synergistic manner [2].
Functional Significance
p53 has many anti-cancer mechanisms:
It can activate DNA repair proteins when DNA has sustained damage.
It can also hold the cell cycle at the G1/S regulation point on DNA damage recognition.
It can initiate apoptosis, the programmed cell death, if the DNA damage proves to be irrepairable.
p53 is central to many of the cell's anti-cancer mechanisms. It can induce growth arrest, apoptosis and cell senescence. In normal cells p53 is usually inactive, bound to the protein MDM2, which prevents its action and promotes its degradation by acting as ubiquitinligase. Active p53 is induced after the effects of various cancer-causing agents such as UV radiation, oncogenes and some DNA-damaging drugs. DNA damage is sensed by 'checkpoints' in a cell's cycle, and causes proteins such as ATM, CHK1 and CHK2 to phosphorylate p53 at sites that are close to or within the MDM2-binding region of the protein. Oncogenes also stimulate p53 activation, mediated by the protein p14ARF. Some oncogenes can also stimulate the transcription of proteins which bind to MDM2 and inhibit its activity. Once activated p53 transcribes several genes including one for p21. p21 binds to the G1-S/CDK and S/CDK complexes (molecules important for the G1/S transition in the cell cycle) inhibiting their activity. p53 has many anticancer mechanisms, and plays a role in apoptosis, genetic stability, and inhibition of angiogenesis[7].
Recent research has also linked the p53 and RB1 pathways, via p14ARF, raising the possibility that the pathways may regulate each other [1].
Role in disease
If the TP53 gene is damaged, tumor suppression is severely reduced. People who inherit only one functional copy of the TP53 gene will most likely develop tumors in early adulthood, a disease known as Li-Fraumeni syndrome. The TP53 gene can also be damaged in cells by mutagens (chemicals, radiation or viruses), increasing the likelihood that the cell will begin uncontrolled division. More than 50 percent of humantumors contain a mutation or deletion of the TP53 gene.
Certain pathogens can also affect the p53 protein that the TP53 gene expresses. One such example, the Human papillomavirus (HPV), encodes for a protein, E6, which binds the p53 protein and inactivates it. This, in synergy with the inactivation of another cell cycle regulator, p105RB, allows for repeated cell division manifestested in the clinical disease of warts.
In healthy humans, the p53 protein is continually produced and degraded in the cell. The degradation of the p53 protein is, as mentioned, associated with MDM2 binding. In a negative feedback loop MDM2 is itself induced by the p53 protein. However mutant p53 proteins often don't induce MDM2, and are thus able to accumulate at very high concentrations. Worse, mutant p53 protein itself can inhibit normal p53 protein levels[4].
Potential therapeutic use
In-vitro introduction of the p53 protein to p53-deficient cells has been shown to cause rapid death of cancer cells or prevention of further division. It is these more acute effects which hopes rest upon therapeutically [5]. The rationale for developing therapeutics targeting the p53 protein is that "the most effective way of destroying a network is to attack its most connected nodes". The p53 protein is extremely well connected (in network terminology it is a hub) and knocking it out cripples the normal functioning of the cell. This can be seen as 50% of cancers have missense point mutations in the TP53 gene, these mutations impair its anti-cancer gene inducing effects. Restoring its function would be a major step in curing many cancers [7].
Various strategies have been proposed to restore p53 protein function in cancer cells [4]. A number of groups have found molecules which appear to restore proper tumour suppressor activity of the p53 protein in vitro. These work by altering the conformation of the mutant conformation of the p53 protein back to an active form. So far, no molecules have shown to induce biological responses, but some may be lead compounds for more biologically active agents. A promising target for anti-cancer drugs is the molecular chaperone Hsp90, which interacts with the p53 protein in vivo.
Adenoviruses have been used to study the functions of the p53 protein by scientists for years, but in a twist it is now modified adenoviruses which are being used as new cancer therapy tools. ONYX-015 (dl1520, H101, CI-1042) is a modified adenovirus which was initially proposed selectively replicate in TP53-deficient cancer cells, but not normal cells [3]. The wild form of the virus expresses the early region protein, E1B55kDa, which binds to and inactivates the p53 protein - this inactivation is necessary for the virus to replicate and kill, or lyse, a cell. In ONYX-015, E1B55kDa has been deleted. It was hoped that in cells with the normal p53 protein, ONYX-015 would be disabled by the p53 protein's activity, yet in cells with the dysfunctional p53 protein, ONYX-015 would selectively replicate in and lyse the tumour cells. The virus produced from this replication cycle could then spread to other surrounding malignant tissue and, over many cycles of infection, replication and lysis, eventually clear the tumour cells from the patient.
Preclinical trials using the ONYX-015 virus on mice were promising. However, the clinical trials that followed were less so. Furthermore, many other scientists have since found that the virus is able replicate in cells with wild-type p53 protein as effectively as in cells with the dysfunctional p53 protein. Nevertheless, when the virus was used in combination with chemotherapy the results looked encouraging [5]. Following on from this the virus has now been licensed for therapeutic use in China. Without a complete understanding of exactly how the virus is selective for cancer cells the virus is unlikely to be used as a therapeutic in western countries.
Importantly, the normal p53 protein is exploited in radiation therapy. Cells with healthy expression of the p53 protein will apoptose (undergo programmed cell death) in the presence of irreparable damage to the DNA. By inducing double-strand DNA damage using therapeutic radiation, p53-mediated apoptosis can be elicited. In cells properly expressing the p53 protein, this pathway can be a powerful tool in the battle with neoplastic disease.
History
p53 was identified in 1979 by Arnold Levine, David Lane, and Lloyd Old, working at Princeton University, Imperial Cancer Research Fund (UK), and Sloan-Kettering Memorial Hospital, respectively. It had been hypothesized to exist before as the target of the SV40 virus, a strain that induced development of tumors. The TP53 gene was first cloned in 1983 by Moshe Oren (Weizmann Institute).
It was initially presumed to be an oncogene due to the use of mutated cDNA following purification of tumour cell mRNA. Its character as a tumor suppressor gene was finally revealed in 1989.
In 1993, p53 was voted molecule of the year by Science magazine.
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