Block II: Tumor Suppressor Genes

What are some anti-proliferation signals that are used?

-cell to cell contact (contact inhibition) - specific soluble factors (TGF-b inhibits epithelial cells)

What does p53 do?

1. encodes proteins and miRNA 2. blocks cells from acquiring tumor cell properties including: stem cell like behavior, enhanced cell survival, unregulated proliferation, and abnormal metabolism 3. induces a large number of proapoptotic genes, including *bax* and genes involved in DNA repair

describe a typical proliferation signal

1. growth factors 2. induce cyclin D 3. phosphorylation of Rb 4. E2F is released from Rb 5. DNA synthesis

What are four mechanisms that result in inactivation of tumor suppressor genes?

1. mutations that inactivate both copies of a TSG 2. mutations that result in TSG haploinsufficiency 3. sequestration of the TS protein 4. epigenetic silencing of TSG transcription

describe anti-proliferation signals

1. oncogenes, TGF-b, cell to cell contact 2. activate cyclin dependent kinase inhibitor p16 3. blocks phosphorylation of Rb 4. E2F is not released from Rb 5. No DNA synthesis

Alterations in Rb pathway gene that have been identified in various cancers include:

1. reduced expression or inactivation of Rb (tumor suppressor) 2. overexpression of cyclin D or cyclin E (oncogenes) 3. Point mutations in cdk4 that prevent CKI binding (oncogene) 4. reduced expression or inactivation of CKI's, usually p16 (tumor suppressor)

What are some general functions of Tumor Suppressor Genes?

1. regulate a large number of cellular processes including cell proliferation 2. mediate the action of anti-growth signals and inhibit tumorigenesis by promoting growth arrest and apoptosis

What is the p53 pathway?

1. some sort of stress (DNA damage, hypoxia, oncogenes) 2. p53 3a. can use BAX to get to apoptosis 3b. can act on CKIs

Somatic cell hybridization experiments

An experiment that showed evidence that tumor suppressor genes exist When a normal cell is fused with a cancer cell, the hybrid daughter cells are unable to form tumors in the appropriate host. Occasionally, hybrid cells do produce tumors because these tumor cells have lost specific chromosomes that were from normal cell parent. These results indicate that normal cells contain genetic information that suppresses the cancer phenotype

What are mutations that result in TSG haploinsufficiency?

Applies to a small number of TSG. Inactivation of one copy of TSG is sufficient enough to reduce the level fo the TS protein below that is needed to function

What is meant by sequestration of the TS protein?

Certain DNA viruses express oncoproteins that bind to and inactivate specific TS proteins. Examples are: simian virus 40 T antigen, adenovirus E1A and E1B, and papilloma E7 and E6 oncoproteins

What are mutations that inactivate both copies of a TSG?

Common mechanism by which Rb and p53 are inactivated. the mutations that inactivate both TSG can occur independently. This can be described as loss of heterozygosity

What does Rb regulate?

G1 checkpoint and by doing that it controls many cellular processes including growth arrest, differentiation, and apoptosis. *Rb inactivates E2F, which results in growth arrest*

What is the nickname for p53?

Guardian of the genome

What happens to p53 in response to stress?

In response to stress, p53 is phosphorylated. This stabilizes the protein and allows it to accumulate in the cell and function.

TRUE OR FALSE Rb is a critical cell cycle regulator that is inactivated in a high percentage of cancers.

TRUE

TRUE OR FALSE inheriting an inactivated tumor suppressor gene not only increases an individual's risk of developing cancer, but tumors usually develop at an earlier age compared to the general population

TRUE

TRUE OR FALSE most tumors contain a mutation in only ONE of these Rb pathway gene because any of these mutations is sufficient to inactivate Rb and allow tumor cells to bypass the G1 checkpoint and proliferate.

TRUE

How is inherited retinoblastoma manifested?

The inherited form results in multiple tumors in both eyes. an individual inherits an inactivated copy of Rb gene from one of the parents. All the cells of the body are heterozygous for Rb, containing one wildtype and one mutated copy of the gene. A mutation that inactivates the remaining wildtype copy of Rb in the retinoblasts is required for tumor formation. In the absence of a functional Rb protein, the cells proliferate and acquire additional mutations, resulting in retinoblastoma.

Viral oncogene action

The same cellular proteins (tumor suppressor proteins) are inactivated by oncoproteins from different DNA tumor viruses. Examples include SV40 T antigen (TAg) - inactivates the TSG's retinoblastoma (Rb) and p53 adenovirus proteins - E1A and E1B - inactivate Rb and P53 inactivation of these tumor suppressor genes is required in order for these viruses to transform cells and produce tumors in the appropriate host

Tumor Suppressor Genes

Tumor Suppressor Genes

how does epigenetic silencing of TSG transcription work?

Tumors often misregulate the expression of DNA methylases and hypermethylate TSG promoters; and histone modifying enzymes that alter the histone code. In both instances, TSG transcription is repressed.

What is retinoblastoma?

childhood cancer affecting the retinal precursor cells in the eye

What p53 protein-coding target gene is associated with growth arrest?

cyclin-dependent kinase inhibitor p21, which functions to keep Rb active.

When Rb inactivates E2F, what does that result in?

growth arrest

What happen if Rb is absent?

in the absence of the Rb protein, high levels of active E2F stimulate the transcription of pro-apoptotic genes, resulting in massive (live) cell apoptosis. note: example was about the liver

What are the two forms of retinoblastoma?

inherited and sporadic

Why is p53 nicknamed the Guardian of the Genome?

it functions to prevent inappropriate DNA replication and mitosis, either by blocking proliferation or by inducing apoptosis. In normal cells, p53 can temporarily block proliferation until the stress has been alleviated. In transformed cells expressing oncogenes that allow the cells to bypass the G1 checkpoint (Rb), p53 promotes *permanent growth arrest* or more often *apoptosis*

What are the domains of Rb?

it has two pocket domains through which Rb interacts with many important cellular proteins. Some of the proteins that Rb interacts with are: viral oncoproteins, transcription factor E2F, and histone deacetylases. The Rb protein also contains many phosphorylation sites that regulate the activity of Rb.

What are some roles of p53?

maintaining the genome preventing tumorigenesis

Compare the following property between oncogenes and tumor suppressor genes: function of the mutant allele

oncogenes: gain of function, dominant tumor suppressor genes: loss of function, recessive

Compare the following property between oncogenes and tumor suppressor genes: number of events require to contribute to cancer

oncogenes: one tumor suppressor genes: two

Compare the following property between oncogenes and tumor suppressor genes: tissue specificity

oncogenes: some, but can act in many tissues tumor suppressor genes: the inherited form frequently has a tissue preference

Compare the following property between oncogenes and tumor suppressor genes: mutant allele may be inherited through the germ line

oncogenes: very few examples tumor suppressor genes: frequently has an inherited form

Compare the following property between oncogenes and tumor suppressor genes: somatic mutation contributes to cancer

oncogenes: yes tumor suppressor genes: yes

Occurrence of characteristic chromosomal deletions

recurrent chromosomal deletions are associated with specific cancers. Deleted genes were identified as tumor suppressor genes that were frequently inactivated in specific tumor types. Examples include BRCA1 and 2 in breast cancers and APC in colon cancers.

How is the activity of Rb regulated?

the activity of Rb is regulated by phosphorylation during the cell cycle.

What does the p53 protein contain?

the p53 protein contains a nuclear localization signal and a DNA sequence-specific binding domain, and functions primarily as a *transcription factor*

how is sporadic retinoblastoma manifested?

the sporadic form is rare and is associated with a single tumor in one eye. The sporadic form is rare because mutations that inactivate both copies of the Rb gene must occur in a single retinoblast

What is Rb?

tumor suppressor gene protein

What's the concentration of p53 under normal conditions?

very low because it has a very short half life

Why are tumor suppressor genes important?

viruses have to be able to inactivate the tumor suppressor genes in order to transform the cell and produce tumors