BCH80001 Biochemistry | The Role of P53 in Genome Integrity and Cancer

The role of p53 in genome integrity and cancer

p53 is a tumour suppressor gene. It is the hub of a plethora of various signalling pathways that controls and co-ordinates cell cycle while maintaining the proper integrity of the human genome. The following assignment aims of highlight different functional and structural aspects of p53 gene.

Structure of p53 protein

p53 contains negatively a natively unfolded amino-terminal transactivation domain (TAD) that can be folded further into two subdomains TAD1 and TAD2 followed by PRR (protein rich region). The ordered DNA-binding domains along with tetramerization domain are associated with each other via flexible linker region. The structure of the DNA binding domain which is made up of 94 to 292 residues is made up of central beta sandwich scaffold like immunoglobulin molecules along with additional structural elements that form the surface of the DNA binding. This surface of DNA binding includes loop-sheet-helix motif along with two large loops (L2 and L3). The stability of L1 and L2 is attained under the influence of the zinc ions which is again co-ordinated tetrahedrally via Cys176, Cys 238, Cys 242 and His179 (Joerger and Fersht 2010). According to Joerger and Fersht (2010) the p53 protein of human is relatively of low intrinsic thermodynamic stability which instantly unfolds at the normal body temperature with a half life of 9 minutes.

Figure: Structure of p53 protein

(Source: Joerger and Fersht 2010)

The p53 forms a tetramer under the actions of the teramerization domains. The DNA binding domains which remains associated with the tetramerization domain binds with the target double stranded DNA molecule with decameric motifs (sequence of DNA binding domains is RRRCWWGYYY and this remains separated by 0 to 13 bp). Apart from forming a tetramer, the tertramerization domain also plays an important role behind the oligimerization of p53 gene which further promotes the structural evolution of the p53 genes among different species (Joerger and Fersht 2010)

Function of p53

The genetic analysis of the p53 gene in cultured cells of mice has indicated that p53 genes have antiproliferative functions (Zilfou and Lowe 2009). In cell-cycle checkpoints, p53 plays an important role in prevention of cell cycle proliferation under the action of DNA damage. According to Zilfou and Lowe (2009), murine embryonic fibroblasts (MEFs) are exposed in relation to DNA damage. This activates ATM/ATR pathways leading to the downstream activation of p53 gene and subsequent arrest in the G1 phase of the cell cycle. P53 mainly induce G1 arrest via transactivation of a cyclin-dependent kinase inhibitor,  p21 ^Waf1/Cip1. Moreover, upstream regulation of cells from G2 phase to mitosis is driven via the maturation promotion factor (MPF) which is made of cyclin B1 and cdc complex. P53 is found to induce cell cycle arrest at G2/M phase via disrupting the function of cyclinB1/cdc complex. More specifically it can be said that p53 represses cdc25c, a phosphatise that activates mitosis after DNA damage. Importance of cell cycle check points and its association with p53 genes is, cell cycle checkpoints are crucial control domains that ensure the fidelity of process of cell division via cross-checking whether the steps of cell division is completed accurately. Any damage in cell cycle (in this case DNA damage in relation of p53) promotes the generation of faulty cells which in turn have susceptibility of developing into cancerous cell lines (Zilfou and Lowe 2009).

Autophagy is a catabolic process which involves the degradation of cell’s self components via lysosomal machinery. Importance of autophagy is, it has both pro and antioncogenic functions which get reflected in their actions either as a prosurvival or through pro-death mechanism. p53 also promotes autophagy via transcriptionally activating the damage regulated autophagy modulator (DRAM) gene which produce lysosomal proteins and thereby inducing the process of autophagy in the DRAM-dependent pathway (Zilfou and Lowe 2009).  

P53: guardian of the genome

The main event which streamlines after the process of malignant transformation of cell is genetic damage along with oncogenic signalling. Both of these stress events are signalled to p53 gene via different genetic pathways. Based on this crucial function, it can be said that p53 is the guardian of genome. This is because, p53 mediates the regulation of the cell cycle check points via sensing the DNA damage. For example, p53 senses and reacts to the process of DNA damage via the action of ATM/ATR pathway and Chk1/Chk2 kinase pathways. Thus it can be said that p53 gene instructs the cell when to progress to cell cycle or when not via sensing the faults in the process of cell cycle. This cell cycle arrest under the action of p53 protein prevents the generation of faulty cell lines thereby preventing the generation of oncogenes. Alternatively, p53 is known as “policeman of the oncogenes” that consists of responding to the process of oncogenic signalling via p53 stabilizing protein, ARF. However, recent advancement in the domain of oncology has lead to the generation of few facts in p53 signalling (Efeyan and Serrano 2007). According to Efeyan and Serrano (2007) experiments conducted over mice highlighted that response of p53 in relation to DNA damage has negligible impact on cancer protection. On contrary, ARF-dependent activation of p53 is critical for the process of p53-mediated tumour suppression.

Loss of expression of p53

According to Rivlin et al. (2011), loss of fucntion of p53 gene leads to cancer progression. However, unike the other tumour suppressor gene like RB (retinoblastoma), APC or BRCA1 which are inactivated during the process of cancer progression via subsequent deletion or truncated mutations, p53 gene is human cancer often undergoes missense mutations. This hampers the thermodynamic stability of p53 gene leading to its subsequent inactivation. Mutation causes loss of p53's ability to bind with the DNA is a sequence specific manner. This lack of DNA binding leads to no scrutiny of the faulty DNA through the cell cycle checkpoints and thereby leading to the generation of faulty genes (cancerous).

Crosstalk between Wnt and p53 signalling pathways plays an important role in cancer development. The experiment conducted by Pe?ina?Šlaus et al. (2016) showed that meningiomas with loss of function of p53 gene has upregulated secretion of beta-catenin pathway which in turns activates Wnt signalling pathway and subsequent generation of meningeal brain tumours.

Benefits of p53 screening

Li-Fraumeni syndrome (LFS) is associated with germline mutation of p53 gene. The carriers of these mutations are at an increased risk of developing multiple primary cancers like breast cancer, lung cancer, leukaemia, melanoma and adrenocortical tumours. According to, Peterson et al. (2006), screening of the individuals for germline mutation of p53 gene is helpful in prevention necessary steps to prevent the development of primary cancers. However, Peterson et al. (2006) opined that video-based decision aid (DA) prior to clinical p53 counselling is helpful to curb ethical barriers.

References

Efeyan, A. and Serrano, M., 2007. p53: guardian of the genome and policeman of the oncogenes. Cell cycle, 6(9), pp.1006-1010.

Joerger, A.C. and Fersht, A.R., 2010. The tumor suppressor p53: from structures to drug discovery. Cold Spring Harbor perspectives in biology, 2(6), p.a000919.

Pe?ina?Šlaus, N., Kafka, A., Vladuši?, T., Tomas, D., Logara, M., Skoko, J. and Hraš?an, R., 2016. Loss of p53 expression is accompanied by upregulation of beta?catenin in meningiomas: a concomitant reciprocal expression. International journal of experimental pathology, 97(2), pp.159-169.

Peterson, S.K., Pentz, R.D., Blanco, A.M., Ward, P.A., Watts, B.G., Marani, S.K., James, L.C. and Strong, L.C., 2006. Evaluation of a decision aid for families considering p53 genetic counseling and testing. Genetics in Medicine, 8(4), p.226.

Rivlin, N., Brosh, R., Oren, M. and Rotter, V., 2011. Mutations in the p53 tumor suppressor gene: important milestones at the various steps of tumorigenesis. Genes & cancer, 2(4), pp.466-474.

Zilfou, J.T. and Lowe, S.W., 2009. Tumor suppressive functions of p53. Cold Spring Harbor perspectives in biology, 1(5), p.a001883.