Oncogenes, TSGs and unrestrained growth -L3

Question 1

What is a kinase ?

Answer 1

it is a protein that phosphorylates another protein target - addition of a phosphate group to other proteins can lead to activation

Question 2

What are some examples of kinases ?

Answer 2

Receptor tyrosine kinase- present in the cell membrane and assesses the local environment. They can activate signalling cascades through phosphorylation

Cyclin-dependent kinase- they phosphorylate proteins when they are in a complex with a cyclin

Question 3

What are growth factors ?

Answer 3

Poorly defined, larger heterogeneous group of molecules
Soluble ligands produced by one cell to influence another
Act at cell, tissue and systemic levels
They are not necessarily to do with growth

Question 4

What are the 3 main functions of growth factors in relation to homeostasis?

Answer 4

HOMEOSTASIS- the balance between these factors tells the tissue it is ok
Survival Factors= nerve growth factor, they control cell numbers via a limited supply. If a cell doesn’t receive enough of these responses then it will initiate apoptosis
Growth Factors= TGF-alpha, stimulate growth when its necessary
Differentiation Factors= TGF-beta, reduce growth and stimulate terminal differentiation

Question 5

In normal cells, cell number is tightly controlled by what processes?

Answer 5

Stem cell differentiation= asymmetrical division into a cell with a specific function and another stem cell
Proliferation
Programmed cell death
it is the balance between these 3 processes that maintains a consistent number of cells within a tissue

Question 6

What are the different signalling methods that allow tissues to maintain normal cell numbers?

Answer 6

Regulated by local and circulating factors
Paracine signalling
- cell to cell contact= directly contacting cells will have proteins between them that can relate info
-growth factor capture= secretion of factors to bind to receptors upon cells
Endocrine signalling= hormonal signalling

Question 7

How does paracrine signalling work ?

Answer 7

1) cells is either stimulated or inhibited by growth factors for cell division
2) once the grwoth factor has bound to the receptor the signal is transduced into the cell and relayed to the nucleus
3) then transcription factors are activated to either produce mRNA that promotes cell division or mRNA that inhibits cell division

Question 8

What are the different processes for sustaining proliferative signalling ?

Answer 8

1) Autocrine regulation= cell releases a ligand that acts on itself
2) Disturbed Paracrine signalling= stroma/epithelial cell interactions
3) receptor dysregulation= overexertion of receptors leading to hypersensitivity. e.g. overexertion of receptor tyrosine kinase
4) ligand independence= mutation in the receptor so it doesn’t need a ligand to activate it because it is always active
5) Constitutive activation of downstream pathways- individual branches are activated= AKt

Question 9

In abnormal cells what can happen between tumour cells and fibroblasts for example ?

Answer 9

paracrine signalling can occur between the tumour cell and the fibroblast because the tumour cell tells the fibroblast to produce GFs or stimulate energy requirements

Question 10

What are the steps in overexpression of receptors ?

Answer 10

there is an increase in gene copy number
there is an increase in mRNA transcription
there is an increase in cell surface receptor protein expression - leads to hypersensitivity from the ligand

Question 11

When mutations occur in receptors what happens ?

Answer 11

mutations can cause the receptor monomers to come together without activation by the ligand and therefore they are active all the time
e.g. EGFR in 25% of neuroblastoma

Question 12

What can happen when a ligand activates EGF-R?

Answer 12

it recruits Sos Guanine exchange factor to RAS

• in normal tissue recruitment of Sod leads to activation by converting GDP to GTP
• in abnormal tissue the RAS can be mutated and it prevents the GTP being hydrolysed and this means it is active all the time

Question 13

What makes the EGF-R even more complicated ?

Answer 13

the fact it is actually part of a family of 4 proteins that can form homo or heterodimers therefore there are multiple different forms
it can also be activated by many different molecules

Question 14

What does EGF-R2/Her2 contribute to ?

Answer 14

overexpression of the receptors contributes to about 30% of breast cancers

Question 15

How else can EGF-R also activate p13K?

Answer 15

it can activate it through RAS but it can also activate it directly

Question 16

What are some examples of Ras genes and what are they the 1st signalling molecules of?

Answer 16

H-ras, K-ras and N-ras

they are the 1st signalling molecule downstream of receptor tyrosine kinase

Question 17

What is the Ras oncogene?

Answer 17

it is a mutant form of the G protein Ras- very common in tumours

Question 18

What does the Ras oncogene encode?

Answer 18

it encodes a protein with normal GTP binding but without GTPase activity so therefore it is always active
- this can lead to unregulated growth

Question 19

What % of lung, colon and pancreatic cancers are associated with Ras mutations ?

Answer 19

lung and colon = 30-50%
pancreatic = > 90%
therefore Ras targetted drugs would be beneficial to the treatment of cancers, particularly for pancreatic

Question 20

Summary of growth factors in cancer:

Answer 20

1) pathway is overactivated by several potential mechanisms
2) extracellular signalling and cell fate are developed by mutations in key genes
3) result is always the same; enhancement of proliferation

Question 21

What are Cyclin dependent kinases (CDKs) required for ?

Answer 21

They are necessary for progression through mitotic checkpoints

Question 22

What does elevated levels of GFs cause?

Answer 22

it results in abnormal progression through cell cycle checkpoints

Question 23

What do mitogenic GFs do ?

Answer 23

They increase cyclin D expression levels

promote mitosis

Question 24

What do negative GFs do ?

Answer 24

increases expression of key CDK inhibitors

e.g. P21- disrupts activity between cyclin dependent kinases and their kinases

Question 25

How are different phases of the cell cycle regulated ?

Answer 25

different kinases bind to different cyclins to regulate the different phases
the levels of cyclins change

Question 26

What is pRb and what does it do ?

Answer 26

retino blastoma protein - commonly mutated TSG in cancer

it binds to E2F and hides it preventing its entry to the nucleus

Question 27

What is the E2F family?

Answer 27

They are a family of transcription factors - some activate transcription while others repress it

Question 28

What are the pocket proteins of pRb?

Answer 28

p107 and p130- they look similar to pRb

Question 29

What are nuclear receptors ?

Answer 29

they are a superfamily of ligand activated transcription factors e.g. oestrogen receptor and vit D receptor

Question 30

What do nuclear receptors respond to ?

Answer 30

xenobiotics, metabolites, dietary factors and hormones

Question 31

What effects upon proliferation do nuclear receptors induce ?

Answer 31

they can be either pro or anti-proliferative and their roles can change in a cell dependent complex

Question 32

What do nuclear receptors do and what regulates them ?

Answer 32

they interact and modulate other growth signals
regulated by cofactors and by each other
can be unregulated in cancer

Question 33

What are examples of nuclear receptors ?

Answer 33

Vitamin D receptor VDR
vitamin A receptor RAR
cholesterol/bile acids/oxysterols (LXR)
hormones AR, ER, PR

Question 34

What do TSGs encode?

Answer 34

they encode proteins that normally restrain cell division and if mutations occur in these genes these restraints are removed

Question 35

What are mutations in TSGs and what does it mean ?

Answer 35

they are genetically recessive
- this means that you need mutations on both alleles
- if you inherit one correct copy and one defective copy you will not be diseased
-

Question 36

What does loss of heterozygosity mean ?

Answer 36

its when you develop a mutation in your good copy causing mutations in both alleles
- loss of the copy that was protecting you

Question 37

What is p53 gene known as and why ?

Answer 37

known as the “guardian of the genome”

it halts cell division

Question 38

How many cancers involve p53 mutations and what do they cause ?

Answer 38

half of all known cancers involve mutations in p53
- most other cancers will have mutations upstream or downstream of p53

mutations lead to half life extension

Question 39

What actually is p53?

Answer 39

it is a transcription factor with more than 100 gene targets

Question 40

What down regulates p53 at the protein level ?

Answer 40

down regulated at the protein level by Mdm2

Question 41

What does Mdm2 do to p53?

Answer 41

as soon as Mdm2 binds p53 it degrades it - it regulates stability
in the absence of Mdm2 p53 is unregulated

Question 42

What do many of the mutations in p53 mean ?

Answer 42

they mean that Mdm2 cant degrade it

Question 43

What happens when p53 induces p21?

Answer 43

it prevents hyperphosphorylation of Rb which induces G1/S growth arrest- if this becomes irreversible (SENESCENCE)- the cell cant go through the checkpoints

Question 44

When does senscence occur?

Answer 44

if the cell is maintained in arrest for too long

Question 45

What can high levels of p53 induce ?

Answer 45

can induce apoptosis (via bax) in response to excessive levels of DNA damage

Question 46

What happens when p53 is lost ?

Answer 46

it allows replication of damaged DNA, loss of apoptotic responses and lack of senescence

Question 47

What happens in gain of function and loss of function mutations ?

Answer 47

GOF= mutation of a proto-oncogene into an oncogene
LOF= mutation in tumour suppressor gene
LOF are more common than GOF but there must mutations in both alleles