Tumor Suppressor Genes

Characteristics of tumor-suppressor genes include: 1) demonstrated function related to negative regulation of the cell cycle, 2) homozygous loss or inactivation in a large proportion of human tumors and 3) ability to suppress tumorigenicity in cells that do not have normal alleles of the gene in question.

Tumor suppressor genes can be inactivated in various ways such as by point mutations, deletion, mitotic recombination. Loss-of-function mutations of tumor suppressor genes relieve cells of inhibitions that normally help to hold their numbers in check. Different combinations of mutations that activate specific oncogenes and inactivate specific tumor suppressor genes are found in different forms of cancer. For example, in colorectal cancer it is thought that mutations inactivating the APC (adhesion related protein) occur first, then a member of the Ras family and later mutations in p53.

Gene silencing that arises from methylation is also an important epigenetic mechanisms of tumor-suppressor inactivation. Methylated CpGs are recognized by proteins that recruit histone deacetylases, leading to stable transcriptional repression. Thus even when there is no mutation in tumor-suppressor genes, a cell can become cancerous as by silencing beneficial genes.

Mutations in tumor suppressor genes are generally recessive in their effects in that there is no loss of control until both gene copies are put out of action. A key insight that led to the discovery of the first tumor suppressor gene can from studies of the rare human cancer, retinoblastoma. Using the known location of the chromosomal deletion associated with retinoblastoma, it was possible to clone and sequence the Rb gene whose loss was critical for development of the cancer.

Known Tumor Supressor Proteins

retinoblastoma protein (pRb): The retinoblastoma susceptibiltiy gene RB-1 is a tumor suppressor gene that encodes a protin (Rb) that regulates the transition from the G1 phase to the S phase of the cell cycle. Rb is initially made in the cytoplasm and is then transported into the nucleus, where it exerts its effects.

The Rb protein inhibits entry into the cell-division cycle when it is unphosphorylated. When it is phosphorylated, it is removed from the transcription factor E2F and there is expression of S-phase genes. The complex of Cdk4 and cyclin D1 phosphorylates Rb, thereby encouraging cell proliferation. Thus hyperphosphorylated form of pRb predominate during the S, G₂, and M phases of the cell cycle, whereas hypophosphorylated pRb is present in cells during G₀/G₁.

Serum stimulation increases pRb phosphorylation in many different cell types, wheres non-proliferative agents such as TGF-B, cAMP, and interferon-gamma (IFN-α) decrease pRb phosphorylation.

pRb interacts with several cell-cycle regulatory proteins, including the cellular proteins c-myc, E2f, cdc2, cdk2, and ATF-2, as well as the DNA virus transforming oncoproteins SV40 large T antigen, HPV E7, and adenovirus E1A.

The Rb protein is encoded by the Rb1 gene. Inactivation of the Rb1 gene by mutation encourages cell division. In those people who suffer from the hereditary form of the disease, a deletion mutation occurs in one copy of the Rb gene in every cell of the body. Thus these cells become cancerous when a somatic mutation occurs which eliminates the remaining good copy.

p16: When a cell is stressed, p16 inhibits the formation of an active Cdk4/cyclin D1 complex, preventing proliferation. Inactivation of p16 by mutation encourages cell division and thus can be considered a tumor suppressor gene.

p53: p53 is involved in many cellular functions, including the cell cycle, apoptosis, and DNA repair. Its product is a 375 amino acid nuclear phosphoprotein that controls transcriptional activation of a seris of other genes, that cumulatively lead to cell cycle arrest and apoptosis. Mutations in the p53 gene are believed to be crucial for transition of cells from the normal to the malignant phenotype. The mutant protein is overexpressed by 5 to 100 fold in transformed cells and tumor cell lines.

The p53 damage response pathway interacts with several distinct cellular pathways, including the Rb pathway. The current model for cell-cycle arrest by p53 in response to irreparable DNA damage involves transactivation of key cell-cycle regulatory genes, most notably the cyclin-dependent kinase inhibitor p21^WAF1/CIP1. p25 inhibitor downregulates the activities of several cyclin-dependent kinases (cylin E/CDK2, cyclin A/CDK2, and cyclin D/CDK4) which are essential for cell cycle progression and thereby contributes to the DNA damage-induced G1 phase arrest.

In some cell types, p53 induces apoptosis when overexpressed and is required for apoptosis in response to severe DNA damage, chemotherapeutic drugs, or MYC or E1A overexpression. This apoptotic program does not depend on p21, and p53 may directly activate death genes, such as BAX, or down-regulated survival genes, such as BCL-2.

In many human tumors, loss of p53 function occurs through mutations within the p53 gene itself, although there are alternative ways which p53 function can be circumvented. For example, p53 mutations are rarely found in cervical carcinomas which are strongly associated with infection by a group of genital human papilomavirus (HPV) types. In these tumors, wild-type p53 protein is targeted for rapid proteolytic degradation by the virally encoded  human papillomavirus E6.

Apaf-9: can be an essential downstream effector of p53 during apoptosis, such that inactivation of either Apaf-1 or Casp9 substitutes for the loss of p53 in enhancing the transformation of primary murin fibroblasts. hence, both Apaf-1 and Casp9 are potential tumour suppressors.