Retinoblastoma Flashcards
What is retinoblastoma?
- eye tumour
- 40% hereditary with early onset with multiple tumours in both eyes
- 60% sporadic with one tumour in one eye
- Rb protein name comes from its involvement in restinoblastoma but it is found all over the body
What checkpoints through the cell cycle are there?
- G1 checkpoints check for extracellular environment and DNA damage and is most important in cancer
- S + G2 checkpoints check for incompletely replicated DNA
- M checkpoint checks for correct chromosome attachment to mitotic spindles
- once past G a cell is commited to the cell cycle unless killed
What can cause Rb to hault the cell cycle?
- stresses such as hypoxia, DNA damage and destruction of the mitotic spindle
- activates phosphatases which return Rb to a hyperphosphorylated state and halt the cell cycle
What 4 ways are cyclin dependent kinases regulated?
- activated by cyclin binding
complexes are then: - deactivated by phosphorylation
- activated by dephosphorylation
- inactivated by CDK inhibitors that bind cyclin CDK complexes and inhibit their kinase actvivity
How was the exact importance of Rb shown in mice?
- Rb1 gene mutations cause embyronal death due to developmental issues
- replication still occurred in early stages showing that Rb is not essential until differentiation occurs
-Checkpoint proteins are therefore important for pause and exit of the cycle but not for the cycle itself
Name some molecular effectors of Rb activity
those involved in:
- development, mitochondrial biogenesis and chromatin modulation
- E2Fs are involved in cell cycle, cell death and cancer progrsesion
How does Rb change throughout the cell cycle
- unphosphorylated at G0
- monophosphorylated at G1
- hyperphosphorylated at the R checkpoint and beyond
- after exit from the M phase, phosphates are stripped off by protein phosphatase type 1 PP1
What was some evidence for Rb’s involvement in cancer?
- Rb is mutated or deleted in many cancers
- DNA tumour viruses recruit oncoproteins that disrupt growth by forming complexes with hypophosphorylated Rb
What are the pocket proteins?
Rb famiyl proteins
- Rb, p107 and p130
What is the structure of the pocket proteins?
- N terminus
- Large pocket contains A + B regions + the C-terminus
- small pocket describes just A and B
- E2F binds at the large pocket with most Rb interactions occuring at B
- p107 and 130 also have a kinase inhibitor site at their N terminus and a cyclin binding site between A and B
Where do other proteins bind to and interact with pocket proteins?
- in many different regions
- mDm2 binds at the large pocket
- cyclin D at the small pocket
- DNMT1 at B
Describe p107 and p130
- more similar to eachother than to Rb
- more extensive sequence homology
- shared cyclin binding and CDK inhibitory domains
Describe the phosphorylation that occurs to Rb during the cell cycle in more detail.
- cyclin D CDK6 leads to Rb monophosphorylation in early G1and it is still active
- Cyclin E CDK2 lead to Rb hypoerphosphorylation in late G1 to inactive Rb and allow cell cycle to continue
- hyperphosphorylaton can only occur after mono
How can anti-mitogenic signals affect Rb and the cell cycle?
- can return Rb to hypo or dephosphorylated active state through:
- attenuation of cyclin expression
- induction of CDK inhibitors
- direct modification of pRb by phosphatases
- cell cycle halted
Once through the restriction point of G1, how is Rb regulated?
- kept inactive by cyclins A, B and E containing complexes
- these are not sensitive to extracellular signals
- guarantees the execution of transition from S to G2 to M
What about phosphorylation of p107 and p130 pocket proteins during the cell cycle?
- p017 phosphorylated by CDK 2 from G1-G2
- p130 + CDK2 are important in G0 when cells are withdrawn from the cell cycle
- some overlap with Rb function but neither can replace Rb entirely
How does expression of pocket proteins change throughout the cell cycle?
- p130 high at G0 and decreases during active cycling
- pRb levels are constant
- p170 levels are induce at entry to G1
