p53, also known as TP53 or tumor protein (EC: 188.8.131.52) is a gene that encodes a protein that regulates the cell cycle and therefore functions as a tumor suppressor. It is very important that the cells of multicellular organisms suppress cancer. P53 has been described as “the guardian of the genome”, referring to its role in maintaining stability by preventing genome mutation (Strachan and Read, 1999). The name is due to its molecular mass: it is found in the 53 kilodalton fraction of cellular proteins.
p53 was identified in 1979 by Arnold Levine, David Lane, and William Old, who worked at Princeton University, Dundee University (UK), and Sloan-Kettering Memorial Hospital, respectively. It had been hypothesized that the SV40 virus, a strain that induced tumor development, existed as the target before. Although initially presumed to be an oncogene, its status as a tumor suppressor gene was revealed in 1989. In 1993, the p53 protein was voted molecule of the year by the journal Science
The human p53 gene is located on the seventeenth chromosome (17p13.1).
The p53 protein is a phosphoprotein made up of 393 amino acids. It consists of four units (or domains):
- A domain that activates transcription factors.
- A domain that recognizes specific DNA sequences (core domain).
- A domain that is responsible for the tetramerization of the protein.
- A domain that recognizes damaged DNA, such as misaligned base pairs or single-stranded DNA.
It plays an important role in the control of the cell cycle and apoptosis. Defective p53 could allow abnormal cells to proliferate and lead to cancer. Up to 50% of all human tumors contain mutants of p53.
In normal cells, the level of p53 protein is low. DNA damage and other stress signals can trigger an increase in p53 proteins, which have three main functions: growth arrest, DNA repair, and apoptosis (cell death). Growth arrest stops cell cycle progression, preventing damaged DNA from replicating. During growth arrest, p53 can activate the transcription of proteins involved in DNA repair. Apoptosis is the “last resort” to prevent the proliferation of cells that contain abnormal DNA.
The cellular concentration of p53 must be strictly regulated. While it can suppress tumors, a high level of p53 can accelerate the aging process from excessive apoptosis. The main regulator of p53 is Mdm2, which can trigger the degradation of p53 by the ubiquitin system.
p53 is a transcriptional activator, which regulates the expression of Mdm2 (by its own regulation) and the genes involved in growth arrest, DNA repair, and apoptosis. Some important examples are listed below.
- Growth arrest: p21, Gadd45, and 14-3-3s.
- DNA repair: p53R2.
- Apoptosis: Bax, Apaf-1, PUMA, and NoxA.
6. Role In The Disease
If the p53 gene is damaged, tumor suppression is dramatically reduced. People who inherit only one functional copy of p53 will likely develop tumors in early adulthood, a condition known as Li-Fraumeni syndrome. p53 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 the p53 gene.
In health, p53 is continuously produced and degraded in the cell. The degradation of p53 is, as mentioned, associated with the binding of MDM-2. In a negative feedback loop, MDM-2 is induced by p53. However, p53 mutants often do not induce MDM-2 and therefore can accumulate in very high concentrations. Worse still, the mutant p53 protein itself can inhibit normal p53 (Blagosklonny, 2002).
7. Potential Therapeutic Use
In vitro introduction of p53 into p53-deficient cells has been shown to cause rapid death of cancer cells or prevention of further division. It is more these acute effects that are expected from the therapeutic point of view (McCormick F, 2001). The rationale for developing therapies targeting p53 is that “the most effective way to destroy a network is to attack its most connected nodes.” The P53 is very well connected (in-network terminology it is a hub) and knocking it out paralyzes the normal operation of the cell.
This can be seen as 50% of cancers have nonsense point mutations in the p53 gene, these mutations alter their inducing effects of the anti-cancer gene. Restoring its function would be an important step in the cure of many cancers (Vogelstein et al 2000). Several strategies have been proposed to restore p53 function in cancer cells (Blagosklonny, 2002). Several groups have found molecules that appear to restore adequate tumor suppressor activity of p53 in vitro.
These work by altering the conformation of the mutant p53 conformation back to an active form. So far, no molecules have been shown to induce biological responses, but some may be lead compounds of more biologically active agents. A promising target for cancer drugs is the molecular chaperone Hsp90, which interacts with p53 in vivo. Adenoviruses depend on host cells to replicate, they do so by secreting proteins that force the host to replicate viral DNA. Adenoviruses have been implicated in cancer-causing diseases, but now modified viruses are used in cancer therapy.
ONYX-015 (dl1520, CI-1042) is a modified adenovirus that selectively replicates in cancer cells deficient in p53 but not in normal cells (Bischoff, 1996). It is modified from a virus that expresses the early region protein, E1B, which binds to p53 and inactivates. The deletion of P53 is necessary for the virus to replicate. In the modified version the E1B virus has been removed. The viruses were expected to select tumor cells, replicate, and spread to other surrounding malignant tissues, thus increasing distribution and efficacy. The cells in which the adenovirus replicates are lysed and the tumor dies.
Preclinical trials using the ONYX-015 virus in mice were promising, however, clinical trials have not been so promising. No objective responses have been observed except when the virus was used in combination with chemotherapy (McCormick, 2001). This may be due to the discovery that E1B has been found to have other functions vital to the virus. Furthermore, its specificity has been undermined by findings showing that the virus is capable of replicating in some cells with wild-type p53.