Tumor suppressors as targets for anti-cancer therapies

Question 1

What are oncogenes?

Answer 1

Genes that promote the growth of cells or protect them from cell death

Question 2

What are tumor suppressors?

Answer 2

Genes that halt the growth of cells or induce cell death

Question 3

Are tumor suppressor genes recessive or dominant?

Answer 3

They are dominant

Both alleles need to be inactivated/mutated in order to get cancer phenotype

Question 4

What is “loss of heterozygosity” (LOH) and what can it result from?

Answer 4

LOH: a way to eliminate wild-type copies of tumor suppressor genes
Result from:
- mitotic recombination
- gene conversion
- loss of the whole locus by deletion 
- methylation

Question 5

Generally speaking, what are the two types of tumor suppressors and what are their function?

Answer 5

1. Caretakers: DNA repair

2. Gatekeepers: cellular responses (cell death and cell cycle arrest)

Question 6

What is pRb?

Answer 6

Enzyme regulating cell cycle passage from G1 to S phase (represses transcriptional activity of E2F)
DNA tumor viruses can inactivate it and p53 (like HPV)

Question 7

What is p53, what activates it and what does it induce?

Answer 7

Transcriptional factor
Potent tumor suppressor activated by stress (DNA damage, oncogenes, hypoxia, telomere shortening)
Growth arrest/senescence, prevention of metastasis/angiogenesis and APOPTOSIS

Question 8

What does inhibition of apoptosis and increased cell survival induce?

Answer 8

Cancer, autoimmune diseases, inflammatory diseases and viral infections

Question 9

What does excess of apoptosis and enhanced cell death induce?

Answer 9

AIDS, neurodegenerative diseases, haematological diseases and tissue damage

Question 10

What happens when crossing an apoptotic threshold?

Answer 10

Apoptosis occurs when the pro-apoptotic load of the cell exceeds its anti-apoptotic buffering capacity

Question 11

What does Bax/Bak induce?

Answer 11

It activates caspases and induces cell death

Question 12

What does Bcl-2 do?

Answer 12

It suppresses Bax/Bak, it is pro-survival

Question 13

Describe the intrinsic pathway of apoptosis

Answer 13

Pro-apoptotic signals (like Bax) open a channel in the outer membrane of mitochondria and release cytochrome C which binds to Apaf-1 and procaspase 9 (in the middle of the complex), this becomes the apoptosome (the wheel of death) and induces apoptosis

Question 14

Describe the extrinsic pathway of apoptosis

Answer 14

Death receptors are activated and activate the DISC complex which induces a caspase cascade starting with caspase 8 leading to apoptosis

Question 15

What does p53-mediated apoptosis involve?

Answer 15

p53-mediated apoptosis involves both extrinsic and intrinsic pathways

Question 16

What are p53 -/- mice prone to?

Answer 16

They are prone to tumors (decreased life expectancy)

Question 17

What does Mdm2 do and what happens if it is amplified?

Answer 17

In healthy cells Mdm2 suppresses p53

If amplified, it suppresses it too much and p53 losses control on tumor growth

Question 18

How is p53 restoration possible for tumor regression?

Answer 18

Most cancers eliminate p53 function, but it seems that its pathway remains intact so resurrecting it might provide a cancer therapy

Question 19

What are the 2 strategies to reactivate p53?

Answer 19

1. Mutant p53: 50% of tumors express mutant p53
   Solution => small molecules which stabilize the folding of mutant p53
2. Wild type p53: in 50% of tumors WT-p53 is rendered non-functional by deregulated p53 inhibitors
   Solution => small molecules which release p53 from its inhibitor (Mdm2)

Question 20

What can be said on p53 mutations?

Answer 20

• Occur in 50% of human tumors
• The majority of p53 mutations are point missence
  mutations
• Cluster in the core domain (unable to bind to DNA)
• Result in PARTIAL unfolding of the core domain
• Mutant p53 proteins are over-expressed in tumors

Question 21

What is PRIMA-1, what does it do and how does it do it?

Answer 21

PRIMA-1 = p53 reactivation and induction of massive apoptosis:

• restores the DNA binding
• rescues p53 conformation and transcriptional transactivation function of p53
• induces apoptosis in tumor cells in a mutant p53-dependent manner

Question 22

What is the dual mechanism of action of APR-246 (structural analog of PRIMA-1)?

Answer 22

1. Induces oxidative stress leading to cell death

2. Restores mutant p53 conformation and transcriptional transactivation function (back to WT-p53) leading to cell death