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. TP53 has been described as "the guardian of the genome", referring to its role in conserving stability by preventing genome mutation (). The name is due to its molecular mass: it runs as a 53 kilodalton (kDa) protein on SDS-PAGE.
Names of Protein
- Official protein name: Cellular tumor antigen p53
- Tumor suppressor p53
- Phosphoprotein p53
- Antigen NY-CO-13
The human TP53 gene, TP53, is located on the human chromosome 17 (17p13.1).
TP53 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 p53 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.
TP53 is 393 amino acids long and has three domains:
- An N-terminal transcription-activation domain (TAD), which activates transcription factors
- A central DNA-binding core domain (DBD). Contains zinc molecules.
- 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 TP53 is a labile protein, comprising folded and unstructured regions which function in a synergistic manner (}.
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 ubiquitin ligase. 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 ().
Recent research has also linked the p53 and RB1 pathways, via p14ARF, raising the possibility that the pathways may regulate each other ()
Role in disease
If TP53 is damaged, tumor suppression is severely reduced. People who inherit only one functional copy of TP53 will most likely develop tumors in early adulthood, a disease known as Li-Fraumeni syndrome. TP53 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 human tumors contain a mutation or deletion of TP53.
Certain pathogens can also affect p53. One such example, the Human papillomavirus (HPV), encodes for a protein, E6, which binds p53 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 health p53 is continually produced and degraded in the cell. The degradation of p53 is, as mentioned, associated with MDM2 binding. In a negative feedback loop MDM2 is itself induced by p53. However mutant p53s often don't induce MDM2, and are thus able to accumulate at very high concentrations. Worse, mutant p53 itself can inhibit normal p53 ().
Potential therapeutic use
In-vitro introduction of p53 in 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 (). The rationale for developing therapeutics targeting p53 is that "the most effective way of destroying a network is to attack its most connected nodes". p53 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 TP53, these mutations impair its anti-cancer gene inducing effects. Restoring its function would be a major step in curing many cancers.
Various strategies have been proposed to restore p53 function in cancer cells . A number of groups have found molecules which appear to restore proper tumour suppressor activity of p53 in vitro. These work by altering the conformation of mutant conformation of p53 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 p53 in vivo.
Adenoviruses have been used to study the functions of p53 by scientists for years, but in a twist it is now modified adenoviruses which are being used as new cancer therapy tools. ONYX-O15 (dl1520, H101, CI-1042) is a modified adenovirus which was initially proposed selectively replicate in TP53-deficient cancer cells, but not normal cells . The wild form of the virus expresses the early region protein, E1B55kDa, which binds to and inactivates p53 - this inactivation is necessary for the virus to replicate and kill, or lyse, a cell. In ONYX-O15, E1B55kDa has been deleted. It was hoped that in cells with a normal p53, ONYX-O15 would be disabled by p53's activity, yet in cells with a dysfunctional p53, ONYX-O15 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-O15 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 as effectively as in cells with dysfunctional p53. Nevertheless, when the virus was used in combination with chemotherapy the results looked encouraging. Following on from this the virus has now been liscenced 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, normal p53 is exploited in radiation therapy. Cells with healthy expression of p53 apoptose (undergo programmed cell death) in the presence of irreparable damage to the DNA. By inducing double-strand DNA damage using theraputic radiation, p53-mediated apoptosis can be elicited. In cells properly expressing p53, this pathway can be a powerful tool in the battle with neoplastic disease.
Diet, Helicobacter pylori, and p53 mutations in gastric cancer: a molecular epidemiology study in Italy
Cancer Epidemiol Biomarkers Prev. 1997 Dec;6(12):1065-9. Palli D, Caporaso NE, Shiao YH, Saieva C, Amorosi A, Masala G, Rice JM, Fraumeni JF Jr.
- A series of 105 gastric cancer (GC) cases with paraffin-embedded specimens interviewed in a previous population-based case-control study conducted in a high-risk area around Florence, Italy, was examined for the presence of p53 mutations. Overall, 33 of 105 cases had a mutation (p53+) identified by single-strand conformational polymorphism and confirmed by sequencing (Y-H. Shiao et al., submitted for publication). p53+ cases had a more traditional dietary pattern (i.e., corn meal mush, meat soup, and other homemade dishes) and reported less frequent consumption of raw vegetables (particularly lettuce and raw carrots). A positive association with a high nitrite intake and a negative association with raw vegetables and diffuse type histology persisted in a multivariate analysis. In addition, p53+ cases tended to be located in the upper portion of the stomach and to be associated with advanced age and blood group A. No relation was found between the presence of p53 mutations and histologically defined Helicobacter pylori infection, smoking history, family history of gastric cancer, education, and social class. Of the 33 p53+ cases, 19 had G:C-->A:T transitions at CpG sites. These tumors tended to occur in females and in association with H. pylori infection but not other risk factors. The remaining 14 cases with a p53 mutation had mainly transversions but also two deletions and two transitions at non-CpG sites. These tumors showed a strong positive association with a traditional dietary pattern and with the estimated intake of selected nutrients (nitrite, protein, and fat, particularly from animal sources). The findings of this case-case analysis suggest that p53 mutations at non-CpG sites are related to exposure to alkylating compounds from diet, whereas p53 mutations at CpG sites might be related to H. pylori infection.