Chapter 6: Growth inhibition and tumor suppressor genes (lecture 1) Flashcards
When looking at this figure, please explain the role of tumor suppression and oncogenes, and how it affects cell proliferation
PTEN and PI3K are kinases. PTEN works as a tumor suppressor gene, and if it’s function is lost this will mean there is no ‘brake’ in the cell. However, this is not a very big issue, since the cell is only stimulated upon binding of EGF. If there is a mutation in the EGFR-gene, it might become activated too much, causing fast proliferation of the cell (the ‘gas’ in the cell is higher, especially if PTEN is lost)
Explain how the two-hit hypothesis of Knudson’s works (1971, it is still applicable today)
He explains that two hits (=mutations) need to happen for cancer to occur. In healthy individuals with two wild-types, two mutations are needed for sporadic cancer to occur. However, in individuals with one inherited gene, only one hit is needed for (familial) cancer to occur.
At the moment there are many Tumor Suppressor Genes known, however, we will only discuss/need to know RB1, p53, APC and BRCA. Can you name for each gene what syndrome/cancer occurs because of a mutation in this gene?
- RB1, retinoblastoma
- p53, Li-Fraumeni
- APC, colorectal cancer
- BRCA, breast/ovarian cancer
Which tumor suppressor gene was first discovered?
RB1
What mutations can occur in the RB pathway that leads to cancer?
- Loss of pRB (deletion, Fram shift, nonsense mutation)
- Missense mutation pocket domain pRB (no binding of E2F)
- Hyperphosphorylation through upstream mutation
- Sequestration pRB by DNA tumor virus protein (e.g. HPV E7)
Explain which upstream activators of p53, and which downstream cell effects of p53 there are.
Upstream activators of p53 (-> deregulation)
- DNA Damage
- Aberrant grot signals oncogene activation
- Cell stress
- hypoxia
- nucleotide depletion
Downstream cell effects of p53 (-> restoration or suicide)
- Cell cycle arrest or senescence
- DNA repair
- Apoptosis
- Inhibition of angiogenesis
What are the four domains of p53?
- Transactivation domain and MDM2 binding site
- DNA binding domain
- Tetramerization domain
- Regulatory domain
Can you explain how all the domains together form the (active) p53 protein?
It all starts with the tetramerization domain, that forms the active site together with MDM2 on a specific DNA binding sequence. In this state, p53 is inactive. When MDM2 is not present, transcription starts. The amount of transcription is regulated by post-translational modifications to p53 and by binding of co-factors.
Is p53 regulated at pre- or post-transcriptional level?
Post-transcriptional level
Besides the binding and inactivation of p53, what other functions does MDM2 have?
It can transport p53 from the nucleus to the cytoplasm and can activate ubiquitination (so p53 is broken down). The three functions are also depicted in this figure (green blocks)
P53 is always present, but in a low amount. Why is that?
Because of the P53-MDM2 feedback loop. This can be seen on the left side of the figure. If MDM2 is low, p53 is increased and vise versa.
What happens to 53 when there is a stress signal?
P53 is activated and the amount of p53 increases. This is a very strong and fast effect. The feedback mechanism makes sure that p53 is stopped when the response is successful by producing more MDM2
By which two upstream pathways is p53, via MDM2, regulated?
- The transactivation domain and MDM2 binding site can be phosphorylated, hereby blocking MDM2 from binding. This is done after cell stress or DNA Damage by kinases.
- MDM2 can also be blocked by an inhibitor called p14, that is activated upon oncogene activation.
Note how the kinases phosphorylate the binding site, and how the oncogene activation directly bind and inhibit MDM2
P53 can be regulated by sensors (that deregulate) or by effectors (for restoration/suicide). Are sensors part of upstream or downstream factors?
upstream (so e.g. the kinases that phosphorylate the transactivation domain or p14 that inactivates MDM2)
P53 can be regulated by sensors (that deregulate) or by effectors (for restoration/suicide). Are effectors part of upstream or downstream factors?
downstream
What are examples of downstream effectors by p53 (that do restoration/suicide)?
- Cell cycle arrest or senescence
- DNA repair
- Apoptosis
- Inhibition of angiogenesis
The bold subjects will be discussed here (the others are/were discussed in other chapters)
What two steps need to occur for E2F to be removed from RB
The cyclinD/Cdk4/6 complex phosphorylates RB, and as a second step cyclinE/Cdk2 complex phosphorylates RB as well, resulting in the release of E2F (leading to cell proliferation)
P53 induces the transcription of p21. What does p21 do?
P21 inhibits the cyclin complexes, which cannot phosphorylate the RB, so the E2F says attached to the RB, so no proliferation (aka cell cycle arrest/senescence)
Recent studies have shown that p53 does not only regulate via proteins. How else does it regulate?
Via miRNA. Remember that miRNAs usually inhibit genes. The process for this is depicted in the following figure but you do not have to study this by heart
