Tumor suppressor gene mutations and deletions result in loss of growth inhibitory pathways.
A tumor suppressor gene, or anti-oncogene, is a gene that protects a cell from one step on the path to cancer. When this gene is mutated to cause a loss or reduction in its function, the cell can progress to cancer, usually in combination with other genetic changes. The p53–Mdm2 paradigmThe affair between Mdm2 and p53 is closely controlled by a complex array of post-translational modifications, which in turn dictates the stability and activity of p53 and Mdm2.
P53 (Tumor suppressor gene/TP53) is a gene that codes for a protein that regulates the cell cycle and hence functions as tumor suppression. It is very important for cells in humans to suppress cancer. Hence, p53 has been referred as “the guardian of the genome”, describing its role in conserving stability by preventing genome mutation.
P53 plays a pivotal role in controlling the mechanisms of cell cycle and apoptosis (programmed cell death). If the p53 gene is mutated, ie; defective p53 could allow abnormal cells to proliferate/divide make them double, resulting in cancer. It is estimated through the recent studies, that 50% of all human tumors contain p53 mutants.In normal cells, the p53 protein level is low. DNA damage and other stress signals may trigger the increase of p53 proteins, which have three major functions – growth arrest, DNA repair and apoptosis.
The growth arrest stops the progression of cell cycle, preventing replication of damaged DNA. During the growth arrest, p53 may activate the transcription of proteins involved in DNA repair. Apoptosis is the "last resort" to avoid proliferation of cells containing abnormal DNA. The cellular concentration of p53 must be tightly regulated. While it can suppress tumors, high level of p53 may accelerate the aging process by excessive apoptosis. The major regulator of p53 is Mdm2, which can trigger the degradation of p53. The murine double minute 2 (mdm2) gene encodes a negative regulator of the p53 tumor suppressor. Amplification of mdm2 or increased expression by unknown mechanisms occurs in many tumors. Thus, increased levels of MDM2 would inactivate the apoptotic and cell cycle arrest functions of p53, as do deletion or mutation of p53, common events in the genesis of many kinds of tumors.
p53 is a transcriptional activator, regulating the expression of Mdm2 (for 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.
Thus, it is with the expression of tumor suppressor gene that helps cell division, growth and other cellular activities necessary for the proper functioning of our body. If the gene doesn’t express properly, the body develop tumors.