tumour biology/signalling/genomic instability

Question 1

what does Rb control

Answer 1

the G1/S checkpoint.

must be phosphorylated by CDK4,6/cyclin D to release E2F which acts as a TF for S phase proteins

Question 2

what does E2F do

Answer 2

activates transcription of itself and cyclin E/cdk2 which further stimulates Rb phos and more E2F.
overall the E2F coordinates regulation of genes that together promote entry into S phase

Question 3

what happens when Rb/E2F binds DNA

Answer 3

recruits hisotne deacetylases and chromatin remodelling factors to the E2F responsive promoters to repress them

Question 4

how many cancers does Rb disregulation occur?

Answer 4

almost all. but different players in the pathway in different cancers.

Question 5

whats the pathway inc Rb and how can it be compromised at each stage

Answer 5

p16INK4a inhibits cyclinD+CDK4/6. it can be mutated, deleted or promoter hypermethylated (silences).

• cyclinD+CDK can be amplified
• Rb can be mutated or deleted

Question 6

how does p53 work

Answer 6

short half life so normally low concs.
- DNA damage stablises it so conc rises.
-p53 acts as a TF for the CDKI p21CIP which inhibits almost all cdk/cyclin complexes.
- leads to G1 and G2 arrest.
if DNA damage is severe it can activate pro-apoptotic genes eg BAX

Question 7

how many cancers have mutant p53

Answer 7

about half of all human cancers.

the bioactive form is a tetramer so mutating one allele sig reduces prob of a normal tetramer

Question 8

what does the INK4a gene encode

Answer 8

2 potent tumour suppressors
- p16INK4a
-p14INK4a
the exon combination is different.

Question 9

how is p53 activated in oncogene activation

Answer 9

ARF accumilates and activates p53 in response to abnormal proliferative signals such as oncogene activation.
- ARF accumilation sequesters mdm2 which would normally have targeted p53 for degredation.

Question 10

what activates p53 in DNA damage

Answer 10

AT protein kinase inhibiting mdm2 and activating p53.

Question 11

so what happens in general when p53 is mutated

Answer 11

cell can survive with oncogenes on and gene amplification.

Question 12

what is essential to overcoming replicative senescence

Answer 12

deregulation of both Tb and p53.

Question 13

phenotype of APC

Answer 13

100s of adenomatous polyps in the large intestine following inherited mutation of APC

Question 14

describe the APC protein

Answer 14

cytoplasmic. mutated in most colonic adenomas and carcinomas.
- regulates Wnt signalling pathway, crucial in development and maintenance of tissue organisation. cell to cell signalling.
- control beta catenin levels by promoting its phos and degredation

Question 15

what is beta catenin

Answer 15

a bi functional molecule.

• forms complex with e cadherin at plasma membrane in adherens junctions.
• in cytoplasm it is phosphorylated by GSK3beta and rapidly degraded by the proteasome.
• other regulatory proteins also act on beta catenin

Question 16

Wnt signalling pathway

Answer 16

Wnt ligand binds receptors. intermediates inhibit GSK3beta.
- unphos beta catenin is stabilised, translocated to the nucleus, promotes transcription of growth genes (eg c-myc) via the TF TCF-4 (which is complexes with)
TCF-1 inhibits TCF-4

Question 17

what is Wnt

Answer 17

a paracrine growth factor

Question 18

what is the beta catenin/TCF complex

Answer 18

a master switch in intestinal crypts controlling prolif vs differentiation via c-myc etc

Question 19

why do cells fail to die

Answer 19

• p53 inactivation

- telomerase activation

Question 20

how do cancer therapies, radiation and chemo work in general

Answer 20

by inducing apoptosis.

in tumour cells at least one death pathway remains intact.

Question 21

cancer mutation to block apoptosis targets what

Answer 21

• death receptors (fas, TNFR)
• activation of anti apoptotic protiens eg Bcl-2
• downregulation of proapoptotic proteins eg Bax and p53 (which activates Bax, PUMA, NOXA)
• doesnt target the caspases

Question 22

why is genetic instability important in oncogenesis

Answer 22

normal to cancer reuires probably at least 6 mutations in a single cell, several in tumor suppressor genes hence needing to be in both alleles. for this to happen within a lifespan the mutation rate has to be sped up by acquisition of genetic instability, called by some ‘the mutator phenotype’.

Question 23

what is the evidence for genetic instability is malignant cells

Answer 23

• most solid tumours are aneuploid. abnormal numbers and arrangements.
• most have deregulated p53 which normally arrests the cell cycle to allow DNA repair
• mutations in DNA repair genes gives an increased susceptability to cancer.

Question 24

the 4 known major DNA repair mechanisms

Answer 24

1 - DNA mismatch repair
2 - nucleotide excision repair
3 - DNA strand break repair
4 - fanconi anaemia repair

Question 25

what does mismatch repair correct

Answer 25

• mismatched bases like C-t instead of C-G.
• insertion/deletion loops, commonly occur where short sequences are repeated ie CACACACA. microsatellite sequences. these occur often in the human genome.

Question 26

what happpens when mismatch repair fails and when does it do so.

Answer 26

mutation rates rise 100-1000 times.

• genes: MLH1, MSH2. mutated in hereditary non-polyposis colorectal cancer (HNPCC). and 2-5% of all colorectal cancers such as lynch syndrome.
• also sporadic changes such as MLH1 silencing due to promoter methylation, found in about 15% of colorectal cancer.

Question 27

name 2 things that NER repairs

Answer 27

crosslinked adjacent thymines crosslinked by UV light. or a carcinogen attached to a base.

Question 28

what happens when NER fails

Answer 28

xeroderma pigmentosa

Question 29

what does DNA strand break repair do

Answer 29

fixes double and single strand breaks which can be induced by ionising radiation, chemical carcinogens and sometimes in virally infected cells.

Question 30

genes in DNA strand break repair

Answer 30

BRCA1 and BRCA2 in double strand break repair

Question 31

whats fanconi anaemia repair

Answer 31

used for the repair od DNA intrestrand crosslinks eg by acetaldehyde (ethanol metabolite) or formaldehyde (smoking) which would otherwise block both transcription and replication.
mutations in the mechanisms lead to a range of cancers and fanconi anaemia

Question 32

where do you find chromosomal rearrangements

Answer 32

in solid tumours, particularly extensive in carcinomas. the changes are not characteristic of a specific cancer except in the case of some leukaemias.
- rearrangements often involve tuour genes

Question 33

how do aneuploidies arise

Answer 33

poorly understood but the loss of a functional p53 pathway is usually critical

Question 34

what’s an aneuploidy

Answer 34

an abnormal number of chromosomes

Question 35

what are karyotypic rearrangements

Answer 35

structural rearrangements of the chromosomes.

Question 36

overview of possible damage that leads to genetic instability

Answer 36

1 - damage sensors
2 - cell cycle checkpoint controllers
3 - dna repair mechanisms
4 - apoptosis

cancer is a multistep process, reflecting the accumulation of multiple genetic changes to oncogenes and tumour suppressor genes.