Primary information of p53 gene (2023)

Primaryinformationof p53gene

1.INTRODUCTION

p53, also known as TP53or tumor protein(EC :2.7.1.37) is a genethat codesfor a proteinthat regulates the cellcycle and hence functions as a tumorsuppression. It is very important for cells in multicellularorganisms to suppress cancer.P53 has been described as "the guardian of the genome",referring to its role in conserving stability by preventing genomemutation (Strachanand Read, 1999). The name is due to its molecularmass: it is in the 53 kilodaltonfraction of cell proteins.

2.HISTORY

p53 was identified in 1979by ArnoldLevine,DavidLane and WilliamOld,working at PrincetonUniversity, DundeeUniversity (UK) and Sloan-KetteringMemorial Hospital, respectively. It had been hypothesized to existbefore asthe target of the SV40virus, a strain that induced development of tumors.Although it wasinitiallypresumed to be an oncogene,its character as a tumor suppressor gene was revealed in 1989.In 1993,p53 proteinhas been voted moleculeof the year by the Sciencemagazine

3. GENE

The human p53 gene is located on the seventeenth chromosome(17p13.1).

4. STRUCTURE

The p53 protein is a phosphoprotein made of 393 amino acids. Itconsists of fourunits (or domains):

• A domain thatactivates transcription factors.

  (Video) NEOPLASIA 4: p53 gene: The Guardian of the genome. functions, regulation and inactivation

• A domain thatrecognizes specific DNA sequences (core domain).

• A domain that isresponsible for the tetramerization of the protein.

• A domain thatrecognized damaged DNA, such as misaligned base pairs orsingle-stranded DNA.

Wild-type p53 is a labile protein, comprising folded and unstructuredregionswhich function in a synergistic

manner (Bell et al. 2002).p53 proteinhas beenvoted molecule of the year.

5. MECHANISM

It plays an important role in cell cycle control andapoptosis.Defective p53 could allow abnormal cells to proliferate, resulting incancer.As many as 50% of all human tumors contain p53 mutants.

In normal cells, the p53 protein level is low. DNA damage andother stresssignals may trigger the increase of p53 proteins, which have threemajorfunctions: growth arrest, DNA repair and apoptosis (celldeath). The growth arrest stops the progression of cell cycle,preventingreplication of damaged DNA. During the growth arrest, p53 mayactivate thetranscription of proteins involved in DNA repair. Apoptosis isthe"last resort" to avoid proliferation of cells containing abnormal DNA.

The cellular concentration of p53 must be tightly regulated.While it cansuppress tumors, high level of p53 may accelerate the aging process byexcessiveapoptosis. The major regulator of p53 is Mdm2, which cantrigger thedegradation of p53 by the ubiquitin system.

Target Genes

p53 is a transcriptional activator, regulating the expression of Mdm2(for its own regulation) and the genes involved in growth arrest, DNArepair andapoptosis. Some important examples are listed below.

1. Growth arrest: p21, Gadd45, and 14-3-3s.
2. DNA repair:p53R2.
3. Apoptosis: Bax, Apaf-1, PUMA and NoxA.

Regulation of p53

As mentioned above, p53 is mainly regulated by Mdm2. Theregulationmechanism is illustrated in the following figure.

Figure 1.0.Regulation of p53.

(a)Expression of Mdm2 is activated by p53.

(b)Binding of p53 by Mdm2 can trigger the degradation of p53 via theubiquitinsystem.

(c)Phosphorylation of p53 at Ser15, Thr18 or Ser20 will disrupt itsbinding withMdm2. In normal cells, these three residues are notphosphorylated, andp53 is maintained at low level by Mdm2.

(d)DNA damage may activate protein kinase (such as ATM, DNA-PK, or CHK2)tophosphorylate p53 at one of these three residues, thereby increasingp53 level.Since Mdm2 expression is activated by p53, the increase of p53 alsoincreasesMdm2, but they have no effect while p53 is phosphorylated. Afterthe DNAdamage is repaired, the ATM kinase is no longer active. p53 willbequickly dephosphorylated and destroyed by the accumulatedMdm2.

Roles of p53

The roles of p53 in growth arrest and apoptosis are illustratedin Figure4-H-6. p53 is also directly involved in DNA repair. One of itstranscriptional target gene, p53R2, encodes ribonucleotide reductase,which isimportant for both DNA replication and repair. p53 also interactsdirectlywith AP endonuclease and DNA polymerase which are involved inbaseexcision repair.

Figure2.0. The roles of p53 in growtharrest andapoptosis.

(a)The cell cycle progression into the S phase requires the enzyme Cdk2,which canbe inhibited by p21. The progression into the M phase requiresCdc2 whichcan be inhibited by p21, GADD45 or 14-3-3s. p53regulates the expression of these inhibitory proteins to induce growtharrest.

(b)Apoptosis can be induced by the binding of Caspase 9 to cytochrome cand Apaf1.p53 may activate the expression of Apaf1 and Bax. The latter canthenstimulate the release of cytochrome c from mitochondria (seeMitochondria,Apoptosis and Aging).

6. ROLE IN DISEASE

If the p53 gene is damaged, tumor suppression is severely reduced.People whoinherit only one functional copy of p53 will most likely develop tumorsin earlyadulthood, a disease known as Li-Fraumeni syndrome. p53 can also bedamaged incells by mutagens (chemicals, radiation or viruses), increasing thelikelihoodthat the cell will begin uncontrolled division. More than 50 percent ofhumantumors contain a mutation or deletion of the p53 gene.

In health p53 is continually produced and degraded in the cell. Thedegradationof p53 is, as mentioned, associated with MDM-2 binding. In a negativefeedbackloop MDM-2 is itself induced by p53. However mutant p53s often don'tinduceMDM-2, and are thus able to accumulate at very high concentrations.Worse,mutant p53 protein itself can inhibit normal p53 (Blagosklonny, 2002).

7. POTENTIAL THERAPEUTIC USE

In-vitro introduction of p53 in to p53-deficient cells has been shownto causerapid death of cancer cells or prevention of further division. It ismore theseacute effects which hopes rest upon therapeutically (McCormick F,2001). Therationale for developing therapeutics targeting p53 is that "the mosteffective way of destroying a network is to attack its most connectednodes". P53 is extremely well connected (in network terminology it is ahub) and knocking it out cripples the normal functioning of the cell.This canbe seen as 50% of cancers have missense point mutations in the p53gene, thesemutations impair its anti-cancer gene inducing effects. Restoring itsfunctionwould be a major step in curing many cancers (Vogelstein et al 2000).

Variousstrategies have been proposed to restore p53 function in cancercells (Blagosklonny,2002).A number of groups have found molecules whichappear to restorepropertumour suppressor activity of p53 in vitro. These work by altering theconformation of mutant conformation of p53 back to an active form. Sofar, nomolecules have shown to induce biological responses, but some may beleadcompounds for more biologically active agents. A promising target foranti-cancer drugs is the molecular chaperone Hsp90, which interactswith p53 invivo.

Adenovirusesrely on their host cells to replicate, they do this bysecretingproteins which compel the host to replicate the viral DNA. Adenoviruseshavebeen implicated in cancer-causing diseases, but in a twist it is nowmodifiedviruses which are being used in cancer therapy. ONYX-015 (dl1520,CI-1042) is amodified adenovirus which selectively replicates in p53-deficientcancer cellsbut not normal cells (Bischoff, 1996). It is modified from a virus thatexpresses the early region protein, E1B, which binds to and inactivatesp53. P53suppression is necessary for the virus to replicate. In the modifiedversion ofthe virus E1B has been deleted. It was hoped that the viruses wouldselecttumour cells, replicate and spread to other surrounding malignanttissue thusincreasing distribution and efficacy. The cells which the adenovirusreplicatesin are lysed and so the tumour dies.

Preclinical trials using the ONYX-015 virus on mice were promisinghoweverclinical trials have been less so. No objective responses have beenseen exceptwhen the virus was used in combination with chemotherapy (McCormick,2001). Thismay be due to the discovery that E1B has been found to have otherfunctionsvital to the virus. Additionally its specificity has been undermined byfindingsshowing that the virus is able replicate in some cells with wild-typep53. Thefailure of the virus to produce clinical benefits may in large part bedue toextensive fibrotic tissue hindering virus distribution around thetumour(McCormick, 2001).

8. REFERENCES

• Bates S, PhillipsAC, Clark PA, Stott F, Peters G, Ludwig RL, Vousden KH. (1998) p14ARFlinks the tumour suppressors RB and p53. Nature 395:124-125

• Bell S, Klein C,Muller L, Hansen S, Buchner J. (2002). p53 contains large unstructuredregions in its native state. J Mol Biol, 322:917-927

• Bischoff JR, KirnDH, Williams A, Heise C, Horn S, Muna M, Ng L, Nye JA, Sampson-JohannesA, Fattaey A, McCormick F. (1996). An adenovirus mutant that replicatesselectively in p53-deficient human tumor cells. Science, 274:373-376

• Blagosklonny, MV.(2002). P53: An ubiquitous target of anticancer drugs. InternationalJournal of Cancer, 98:161-166

• McCormick F.(2001). Cancer gene therapy: fringe or cutting edge? Nat Rev Cancer,1:130-141

• Strachan T, ReadAP. (1999). Human Molecular Genetics 2. Ch. 18, Cancer Genetics

• Vogelstein B,Lane D, Levine AJ. (2000). Surfing the p53 network. Nature, 408:307-310

  (Video) Cancer Biology - Tumor Suppressor gene p53