10. Cancer genetics 2: tumour suppressor genes

Question 1

What are the 2 main classes of tumours?

Answer 1

1. Inherited
2. Sporadic

Question 2

What are the characteristics of hereditary tumours?

Answer 2

1. Early onset
2. multiple tumours
3. requires 1 hit

Question 3

What are the characteristics of sporadic tumours?

Answer 3

1. Late onset
2. usually contained to 1 tumour
3. requires 2 hits

Question 4

What is Knudson’s 2-hit theory?

Answer 4

1. For hereditary tumours the first “hit” is the germline mutations so they only require 1 somatic “hit” to initiate the tumour
2. For sporadic tumours 2 somatic mutations or “hits” are needed to imitate a tumour

Question 5

How does Knudson’s 2-hit theory work?

Answer 5

1. The 1st hit mutates one allele of the gene. This can be a germline or somatic mutation.
2. The 1st hit doesn’t really cause cancer due to the other allele still providing the function.
3. The 2nd hit mutates the other allele and the function is lost often leading to cancer.

Question 6

Why are hereditary cancer early onset?

Answer 6

Due to the germline mutation, it requires only 1 somatic hit to knock out its function and increase risk of cancer.

Question 7

What evidence is there for Knudson’s 2-hit theory? - Retinoblastoma

Answer 7

1. Chromosome deletions in 13q14
2. Germline defects
3. 1 allele with a lack of function
4. The other allele is hit by a sporadic mutation
5. The risk of retinoblastoma then massively increases

Question 8

What evidence is there for Knudson’s 2-hit theory? - Loss of heterozygosity

Answer 8

1. This is when 1 allele becomes the same as the other
2. A mutation or loss of 1 allele resulting in a loss of phenotype

Question 9

How can you lose heterozygosity?

Answer 9

1. Losing a whole copy of a chromosome
2. Losing a chunk of a chromosome
3. Chromosomal translocations causing the loss of the original genes
4. Loss of the original allele that is then replaced by copying the other allele so that they are identical
5. Homologous recombination in meiosis

Question 10

How do we detect the loss of heterozygosity? - RFLP

Answer 10

1. Use restriction enzymes to cut the locus at a specific location
2. This produces fragments of a particular size
3. In diseased cases deletions and additions mean the enzymes cannot cut properly resulting in different sized fragments and bands
4. Then identify the pattern for that patient

Question 11

What is RFLP?

Answer 11

Restriction fragment length polymorphism

Question 12

How do we detect the loss of heterozygosity? - PCR

Answer 12

1. Use PCR to amplify microsatellite regions
2. It works the same way as RFLP
3. It is more sensitive, accurate and faster
4. Shows the difference in sequence

Question 13

What evidence is there for Knudson’s 2-hit theory? - Cell fusion

Answer 13

1. Inject a mouse with a specific antigen to induce antibody production.
2. Harvest the antibody producing B lymphocytes and fuse them with a cancer cell.
3. when they fuse you get a cell that can make antibodies like B lymphocytes but also have the cancerous ability to grow
4. This is often used in antibody producing companies
5. The fusion of the cell causes the normalisation of the cancer cells and the tumorigenic properties are suppressed.
6. This is due to the TSG function returning from the normal cell

Question 14

What are the properties of oncogenes?

Answer 14

1. Active in cancer through a gain of function mutation
2. caused by dominant mutations that arise from point mutations, gene amplification, chromosomal translocations
3. arise in somatic cells
4. not inherited

Question 15

What are the properties of tumour suppressor genes?

Answer 15

1. inactive in cancer through loss of function mutations
2. caused by recessive mutations that arise from point mutations, deletions and epigenetic silencing
3. Present in somatic or germ cells
4. can be inherited

Question 16

What is WAGR syndrome caused by?

Answer 16

A deletion of a cluster of genes on chromosome 11 (Pax6 to WT1)

Question 17

What will a patient with WAGR syndrome have?

Answer 17

1. Wilms’ tumour which is a rare kidney cancer with a WT1 mutation
2. Aniridia which is a lack of the iris in the eye which is caused by a lack of Pax6
3. Genitourinary abnormalities like ovarian defects
4. Mental retardation

Question 18

What does WT1 encode?

Answer 18

1. A transcriptional repressor that binds DNA.
2. targets insulin-like growth factor 2
3. when WT1 is lost this is over expressed
4. causes rapid cell growth

Question 19

What is PTEN?

Answer 19

1. A lipid phosphatase
2. converts PIP3 to PIP2 to inactivate it
3. PIP 3 activates the oncogene Akt which causes lots of proliferation
4. no PTEN = constant Akt signalling = proliferation

Question 20

What is PIP3 kinase?

Answer 20

1. an oncogene that converts PIP2 to PIP3
2. Over expression of this causes activation of Akt and cell proliferation

Question 21

What does Akt do?

Answer 21

It regulates a huge number of signalling pathways to do with growth and proliferation

Question 22

What is RB1?

Answer 22

1. It was the 1st biological proof of the existence of TSGs
2. mutated in all retinoblastomas
3. Predisposes carrier to cancers like pinealoma, osteosarcoma and melanoma
4. involved in the cell cycle

Question 23

How does Rb1 control cell proliferation?

Answer 23

1. Rb1 inhibits E2F
2. E2F drives the cell into S phase by activating proliferation promoting genes
3. When Rb1 is bound to E2F proliferation is inhibited and confined to the appropriate time of the cell cycle
4. When Rb1 is phosphorylated it releases E2F and the cell cycle can continue
5. Mutation in RB1 means E2F is always active so the proliferation is always active and causes cancer

Question 24

What cancer is Rb1 present in?

Answer 24

1. childhood eye tumour
2. 1 in 14,000-18,000 in USA and Europe
3. autosomal dominant inheritance
4. 50% chance of passing it on

Question 25

What is TP53 or P53?

Answer 25

1. discovered as a protein that binds viral antigens
2. P53 is expressed when there is DNA damage
3. P53 causes cell cycle arrest in G1 to allow time to repair the damage
4. If the damage cannot be repaired it can induce apoptosis

Question 26

What happens when p53 is mutated?

Answer 26

Mutations in the cell will accumulate and be passed on as there is nothing to prevent the cell replicating and cancer develops

Question 27

What is Li-Fraumeni syndrome?

Answer 27

1. a hereditary p53 mutation that predisposes the carrier to multiple cancers
2. this syndrome was first described 20 years before p53 was discovered

Question 28

What pathways can p53 activate?

Answer 28

1. Regulation of the cell cycle
2. Activate P21 to trigger cell cycle arrest
3. BAX to trigger apoptosis
4. Mdm2 is a negative regulator to keep p53 under control by ubiquitination

Question 29

How are p53 and RB1 connected?

Answer 29

they both are important in cell cycle control

Question 30

What other type of gene can be a TSG?

Answer 30

A DNA repair gene