Oncogenes and Tumour Suppressor Genes

Question 1

Which distinct genetic changes are involved in tumour development?

Answer 1

Activation of oncogenes (from proto-oncogenes) and inactivation of tumour suppressor genes

Question 2

What is a proto-oncogene?

Answer 2

A normal gene that becomes an oncogene after being mutated - usually promotes cell growth.

Question 3

What is an oncogene?

Answer 3

A protein coding gene, that when activated/overexpressed/mutated has the potential to promote oncogenesis. Accelerator of a car that gets stuck.

Question 4

What is oncogenesis?

Answer 4

The process by which healthy cells become transformed into cancer cells

Question 5

What is an oncovirus?

Answer 5

A cancer causing virus

Question 6

What is a retrovirus?

Answer 6

An RNA virus which produces dsDNA copies of their genome that integrate into the host cell chromosome.

Question 7

How are retroviruses grouped?

Answer 7

On their structure and replication method within the host

Question 8

What is a tumour suppressor gene?

Answer 8

A protein coding gene that can slow cell growth/division, repair DNA mistakes and promote appropriate apoptosis. Absence/repression/inactivation of the gene can promote oncogenesis. Brakes of a car that don’t work.

Question 9

What type of mutations occurs in tumour suppressor genes?

Answer 9

Loss of function mutations (inactivation) - usually “reccessive” ie. two copies of the mutation needed to cause cancer.

Question 10

Describe the two hit hypothesis

Answer 10

As loss of function mutations are “reccessive”, two hits are needed to cause cancer. A child with an inherited mutation is more likely to get cancer as they only need one extra “hit” - patients usually younger and have multiple tumours. A child with no inherited mutations is less likely to get two hits in the same gene, as a result the patients are older and only have one tumour usually.

Question 11

How common is inheritance of tumour suppressor gene mutations?

Answer 11

They’ve been found in some familial cancers, causing them to run in the family. APC gene mutations are associated with increased colon cancer risk in people with familial adenomatous polyposis. Most tumour suppressor gene mutations aren’t inherited but acquired - acquired TP53 mutations been found in >50% of human cancers

Question 12

What are the two main cellular functions of tumour suppressor genes?

Answer 12

Caretakers and gatekeepers

Question 13

Describe the caretaker function of tumour suppressor genes

Answer 13

They maintain chromosomal integrity (genome stability). Includes DNA repair genes eg. TP53 -> p53 which also has gatekeeper properties

Question 14

Describe the gatekeeper function of tumour suppressor genes

Answer 14

Negative regulation of cell cycle. Inhibition of proliferation, induction of apoptosis, inhibition of angiogenesis. eg. Retinoblastoma protein

Question 15

What is retinoblastoma?

Answer 15

A rare cancer usually affecting children under 5. Starts in the light sensitive lining in the retina and can occur in one or both eyes. Affects 50-60 kids in the UK per year. If caught early, 95% treatment success but metastasis can be fatal.

Question 16

What are the causes of retinoblastoma?

Answer 16

40% of cases - mutations in RB1 gene, usually presenting as bilateral retinoblastoma. Mutations can be inherited or acquired in the womb during early development. One functional copy of RB1 gene produces sufficient RB protein.
60% of cases - Unknown causes, no mutant gene. Usually unilateral

Question 17

How does RB protein function?

Answer 17

When a cell reaches the G1 checkpoint, E2F (a TF) lets S phase proceed. RB protein inactivates E2F by binding to it and not allowing the cell cycle to proceed (stops replication. RB protein is inactivated by phosphorylation, so S phase proceeds. Mutant RB protein cannot bind to E2F regardless of phosphorylation state, so S phase always proceeds - uncontrolled replication.

Question 18

How can tumour suppressor genes also act in a dominant manner?

Answer 18

In terms of causing cancer, tumour suppressor genes are recessive in that two mutations are required. But in terms of causing predisposition to cancer, they are dominant, as an inherited mutation makes cancer more likely to occur

Question 19

Describe p53

Answer 19

It’s a TF that acts in many pathways to prevent mutations and stabilise the genome. It’s encoded by TP53 gene on chr 17. Expressed in nearly all cells

Question 20

What is the role of p53?

Answer 20

It responds to stresses such as hypoxia, DNA damage, inhibition of transcription, oncogene signalling, radiation. Causes apoptosis, senescence, autophagy, cell cycle arrest etc. that lead to tumour suppression.

Question 21

What are the two mechanisms by which p53 stops tumours?

Answer 21

Response to DNA damage/stress is activation of a gene that stops the cell cycle.
Triggers apoptosis in damaged cells

Question 22

Describe the relation between p53 mutations and cancer.

Answer 22

50% of cancers have TP53 mutations. Rarely, the mutations are inherited, which leads to Li Fraumeni syndrome, where 50% of patients get cancer by age 30.

Question 23

How does p53 result in cell cycle arrest?

Answer 23

p53 induction activates cell cycle checkpoints - the highest level being at G1/S checkpoint, so damaged genomes don’t replicate. Here, p53 upregulates p21 which binds to CDK1 or CDK2 complex, preventing entry into S phase. This is either reversible or not (senescence).

Question 24

How does p53 induce DNA repair?

Answer 24

p53 regulates NER. It’s role is best characterised in reponse to UV, which upregulates p53 expression (also other carcinogens like H202 and radiation). p53 then promotoes expression of genes in the repair pathway. p53R2 codes for ribonucleotide reductase and Gadd45 which recognise and bind DNA damage. p53 may also interact with BER proteins like AP endonuclease and DNA Polymerase

Question 25

How is p53 involved in angiogenesis?

Answer 25

p53’s activity as a transcription factor allows it to regulate angiogenesis genes. Loss of p53 -> increased tumour vascularisation.

Question 26

How is p53 involved in apoptosis?

Answer 26

p53 switches on BAX in healthy cells, which initiated drilling of the mitochondrial membrane and release of cytochrome C -> apoptosis. p53 loss results in evasion of apoptosis

Question 27

How is p53 regulated in healthy cells?

Answer 27

Mdm2 E3-ubiquit keeps cellular p53 levels low by binding to the transactivation domain, preventing its TF ability. This also promotes movement of p53 to the cytosol where it’s bound by ubiquitin and degraded by proteasomes.

Question 28

How is p53 regulation different in stressed cells?

Answer 28

DNA damage triggers kinases to phosphorylate p53 at various sites. This results in Mdm2 not being able to bind to p53, so it isn’t degraded.

Question 29

How else is p53 stabilised, apart from phosphorylation?

Answer 29

ARF protein can act as a decoy, binding to Mdm2, making it unable to bind to p53 and cause its degredation, leading to p53 build-up in the nucleus. Also, mutations in p53 don’t allow p53 to bind to Mdm2

Question 30

Describe the discovery of the viral oncogene

Answer 30

Rous showed that a filtered cell extract from a chicken’s spontaneous spindle cell sarcoma causes tumours in healthy chickens. The fine filter proves that the cancer doesn’t come from the cell transplant but from a small infectious agent - Rous Sarcoma Virus. Cells infected in vivo with RSV develop characteristics commonly found in cancer cell cultures, and the cause was found to be v-src gene. Retroviral oncogenes help us to understand cancer as a genetic disese

Question 31

How do oncogenes arise?

Answer 31

By the mutation, or overexpression of a proto-oncogene (usually promotes cell growth). This can lead to constitutive activation of the proto-oncogene, turning it into an oncogene, leading to uncontrolled growth -> cancer. Mutations can be inherited, usually acquired.

Question 32

In which ways are proto-oncogenes turned into oncogenes?

Answer 32

Chromosomal rearrangements - Change in chromosome structure: deletion, inversion, translocation
Gene reduplication - Additional copies of a gene -> too much protein
Point mutation - Increasing protein activity of affecting normal regulation

Question 33

How can a chromosomal rearrangement of a proto-oncogene result in an oncogene?

Answer 33

Translocation of the proto-oncogene to control under a new promoter leads to overproduction of a normal protein

Question 34

How can gene amplification of a proto oncogene result in an oncogene?

Answer 34

Gene amplification results in overproduction of a normal protein

Question 35

How can point mutations result in the transformation of a proto-oncogene to an oncogene?

Answer 35

A point mutation within the gene can result in a hyperactive/degredation resistant protein. Mutation within a control element can result excessive amounts of normal protein.

Question 36

How are Ras proteins regulated?

Answer 36

Ras bound with GDP is inactive. A GTP exchange factor activates Ras by swapping GDP for GTP. Ras then activates a number of TFs that activate genes associated with promoting cell cycle activity and cell division. Ras’ intrinsic GTPase activity changes GTP to GDP, inactivating itself. This is aided by GTPase activity accelerating protein.

Question 37

How can mutations in Ras proteins lead to constitutive activation?

Answer 37

If the GTPase part of Ras is mutated, it won’t be able to inactivate itself.

Question 38

How do Ras mutations lead to cancer?

Answer 38

If Ras cannot inactivate itself, there will be uncontrolled cell division/growth. This is a gain of function mutation, so only one copy is needed.

Question 39

What are Ras proteins and list the main different types

Answer 39

GTPases that have roles in transmission of cellular signals via their molecular switch action. HRAS, KRAS, NRAS most important types and mutants are found in about 20-30% of tumours. The genes code for HRAS, NRAS, KRAS4A and KRAS4B - highly homologous

Question 40

Describe the role of the different Ras proteins

Answer 40

They have identical effector binding domains and can interact with the same set of downstream effectors, but differences in their post translational modifications mean that they are part of different signalling pathways

Question 41

What is Burkitt’s lymphoma?

Answer 41

Cancer resulting from a chromosomal rearrangement of most commonly a region of 8 to 14 and less commonly a region of 8 to 2 and 22.

Question 42

Describe the genetic changes that occur to lead to Burkitt’s lymphoma

Answer 42

At the chr 8 breakpoint there is a gene called MYC (the cellular equivalent of an RNA virus that causes sarcomas in chickens). Chromosome 14, 2 and 22 breakpoints contain Ig heavy chain, kappa light chain, and lambda light chains respectively. The translocation puts Myc (codes a TF usually under strict regulation) under the Ig promoters, which causes continuous Myc expression, leading to TF activating patheways that shouldn’t be permanently activated, causing cancer. Translocations frequently found in leukaemias and lymphomas.

Question 43

What is the Myc family of oncogenes?

Answer 43

A TF that regulates many genes involved in functions such as cell cycle, protein biogenesis, cell adhesion, metabolism, signal transduction, transcription, translation. In >50% of cancers, MYC is deregulated - poor prognosis and unfavourable patient survival

Question 44

What are double agent genes?

Answer 44

Proto-oncogenes with tumour suppressor function. Have been identified in 12 major cancer types. Their mutations types are consistent with those of oncogenes, but their incidence is similar to tumour suppressor genes. A single mutation event could release oncogenic function and eliminate tumour suppressive function - one hit cancer

Question 45

Give an example of a double agent gene and explain its effects

Answer 45

NOTCH receptors. Have an ongogenic role in T-lineage acute lymphoblastic leukaemia, and a tumour suppressor function in squamous epithelia. Also TF TP53, kinase BCR, and FAS are all double agents too.

Question 46

Give an example of some oncogenic gain of function mutations in p53

Answer 46

Some p53 mutants exert a tumorigenic gain of function effect by downregulating AMPK (a usually tumour suppressive energy sensor). Also they can stimulate the Warburg effect in cancer cells.