21st Feb - Senescence

Question 1

What is cellular senescence?

Answer 1

A terminal differentiation state in which metabolically active cells are permanently and irreversibly arrested with distinct morphological changes

Question 2

What is the hayflick limit?

Answer 2

Normal human cells will divide a set number of times before they stop permanently

Question 3

What is telomere shortening?

Answer 3

With each division telomeres shorten because duplication is not entirely complete

Question 4

What are telomeres?

Answer 4

repetitive DNA sequences (TTAGGG) that cap the chromosomes and protect their ends from erosion

Question 5

What is the end replication problem?

Answer 5

The inability of DNAP to work on 3’ ends leading to telomere shortening

Question 6

How does telomere shortening cause cell cycle arrest?

Answer 6

When the telomeres become too short (<12.8 repeats of repetitive clusters) it leads to cell cycle arrest, senescence and apoptosis

Question 7

What is the function of telomerase?

Answer 7

It can elongate the telomeres, refreshing the cell

Question 8

What is SIPS?

Answer 8

Stress induced premature senescence

Question 9

What triggers SIPS?

Answer 9

Detection of DNA damage signals

Question 10

What are the main pathways for the activation of senescence?

Answer 10

Senescence signals –> p16 –| CDKs –| RB –| E2F –> senescence
Senescence signals –> p16 –| CDKs –| RB –> senescence
Senescence signals –> ARF –| HDM2 –| p53 –> p21 -> senescence
Senescence signals –> p53 –> p21 –> senescence

Question 11

What are the markers of senescence?

Answer 11

Growth Arrest
SA-Bgal
DNA Damage Foci
Heterochromatin foci
p16INK4A
Senescence associated secretory phenotype

Question 12

How does senescence associated-Beta galactosidase show senescence?

Answer 12

Hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides only in senescent cells, thus turning senescent cells blue

Question 13

How are DNA damage foci created?

Answer 13

DNA damage response proteins (e.g. ATM) accumulate in the vicinity of a chromosomal ds DNA break creating subnuclear foci

Question 14

What are heterochromatin foci?

Answer 14

Specialized domains of facultative heterochromatin that contribute to silencing of proliferation promoting genes e.g. E2AF

Question 15

How can heterochromatin foci be visualized?

Answer 15

immunoflouresence staining

Question 16

What are the characteristic chromatin marks associated with heterochromatin foci?

Answer 16

H3K9me3
H4K20me3
macroH2A
gamma-H2AX
HP1
Hypo-acetylation
DNA methylation

Question 17

What form of cancer is p16INK4a used as a marker for?

Answer 17

Cervical cancer

Question 18

Give some examples of molecules that senescent cells secrete to the environment

Answer 18

Promote senescence
IGFBPs
ILs
PAI1
IFNs
TGFbata

Inhibit senescence
IGFs
WNT2

Question 19

Give an example of a human tumour showing cell senescence and their oncogenic event

Answer 19

Dermal neurofibromas –NF1 inactivation

Nevi – BRAFv600E mutation

Question 20

Describe an experiment performed to test whether ageing was linked to cancer suppression and its conclusion

Answer 20

Test by KO studies

p53 -/- = lethal
p53 OE - expected accelerated ageing but received mixed results, super p53 led to cancer protection but normal ageing therefore indicates the two can be seperated, hypomorphic MDM2 knock in elevated p53 and led to normal ageing and cancer prevention, p53 and ARF knock in had an anti-ageing effect and prevented cancer, suggeting the two are linked

–> The relationship between cancer and ageing is still unclear

Question 21

Describe the SASP

Answer 21

The senescence associated secretory phenotype
Is an autocrine method of regulating senescence acting as a positive feedback loop, secreted from the senescent cell. It is not required for senescence to occur

Question 22

What is antagonistic pleiotropy?

Answer 22

SASP is believed to have a paracrine function causing neighbouring cells to transform into proinflammatory cells.

As in early life senescent cells are cleared away quickly –> no effect

In later life senescent cells aren’t cleared –> inflammation –> to more senescence
thus don’t need many cells ina tissue to affect it
this could explain why senescence only has tumour suppressive effects in youth

Question 23

Outline an experiment which delayed ageing by Baker (2011)

Answer 23

Crossed drug sensitive mouse with BubRI insufficient mouse (premature ageing phenotype)

–> Hybrid offspring in which senescent cells were cleared from the mouse therefore delayed ageing, cleared p16INK4a expressing cells