Oncogenes and tumour suppressor genes

Question 1

What are the four major functional changes in cancer?

Answer 1

1. Increased growth (loss of growth regulation, stimulation of environment promoting growth e.g angiogenesis
2. Failure to undergo programmed cell death (apoptosis) or senescence
3. Loss of differentiation (including alterations in cell migration and adhesion)
4. Failure to repair DNA damage (including chromosomal instability) , those damaged cells are not repaired and not removed by apoptosis

Question 2

What is an oncogene?

Answer 2

Gain of function, ‘accelerator’ of a car, if activated it speeds up cell division

An altered gene whose product can act in a dominant fashion to help make a cell cancerous

Oncogene is a mutant form of a normal gene (a proto oncogene) involved in the control of cell growth or division

Question 3

What is a tumour suppressor gene?

Answer 3

Loss of function

the ‘brakes’, counteracts oncogene.

A gene whose normal activity prevents formation of a cancer

Both genes for the tumour suppressor must be mutated to knock out tumour suppressor function.

Loss of this function by mutation enhances the likelihood that a cell can become cancerous (a normal process to maintain control of cell division is lost)

Question 4

How does the retrovirus capture c-src (cellular oncogene)?

Answer 4

During evolution virus acquires fragments of genes from host at integration sites process results in creation of oncogenes

oncogene is 60kDa intracellular tyrosine kinase, phosphorylate cellular proteins and effect growth

go from RNA to DNA (normally DNA to RNA)

pro-virus is accidentally integrated next to host c-src sequence, fusion and packaged into capsid

end up with Rous Sarcoma Virus carrying src sequence

Question 5

What did the oncogene hypothesis show?

Answer 5

Identified v-src (protooncogene altered form transduced by retrovrius) oncogene as responsible for causing cancer

Used hybridisation experiments, found that the c-src gene was present in the genome of many species and that host cell c-src gene is involved in the positive regulation of cell growth and division

Following infection v-src oncogene was expressed at high levels in the host cell, leading to uncontrolled host cell growth, unrestricted host cell division and cancer

Proto-oncogenes are normal genes that can control growth

Various agents, including radiation, chemical carcinogens and perhaps exogenously added viruses may transform cells by switching on the endogenous oncogenic information 

Question 6

15-20% of human cancers caused by onocviruses, what can viral oncogenes be transmitted by?

Answer 6

DNA viruses

• Encode various proteins along with environmental factors can initiate and maintain tumours

RNA viruses

• Integrate DNA copies of their genomes into the genome of the host cell and as these contain transforming oncogenes they induce cancerous transformation of the host

Question 7

What activates oncogenes?

Answer 7

Mutations, amplification/duplication, translocations

These alter structure and function of protein or increase synthesis of the protein.

Question 8

Proto-oncogenes encode components of growth factor signal transduction pathway

What 4 types of proteins are involved in transduction of growth signal?

Answer 8

Growth factors

Growth factor receptors

Intracellular signal transducers

Nuclear transcription factors

Question 9

Ras Oncogene family

What are Ras genes?

Answer 9

Identified from 2 cancer causing viruses, Harvey sarcoma virus and Kristen sarcoma virus

Ras proteins are small GTPases, normally bound to GDP in a neutral state

oncogenic acitvation of ras seen in 30% of human cancer

glycine to valine - bladder carcinoma - mutation at codon 12

glycine to cysteine - lung cancer - codon 13

Question 10

What normally happens with Ras?

Answer 10

1. Binding of the extracellular growth factor signal
2. Promotes recruitment of RAS proteins to the receptor complex
3. Recruitment promotes Ras to exchange GDP (inactive ras) with GTP (activate ras)
4. Activated Ras then initiated the remainder of the signalling cascade (mitogen activated protein kinase)
5. These kinases ultimately phosphorylate targets such as transcription factor to promote expression of genes important for growth and survival

Question 11

What happens to ras pathway when you have mutations in codons?

Answer 11

hyperactive ras

point mutations in codons 12, 13, 61

Consequence of these mutations is loss of GTPase activity of the RAS protein normally required to return active RAS to the inactive RAS GDP

Causes constitutive activation, always switched on

Cells are continually dividing, can’t stop, leading to tumour.

Question 12

What does the myc oncogene family consist of?

Answer 12

C-MYC, MYCN, MYCL which encode c-Myc, N-myc and L-myc

family of transcription factors that regulate transcription of 15% of the entire genome

encodes helix-loop-helix leucine zipper transcription factor which dimerises with max to transactivate gene expression

Question 13

What is Myc activated by?

What does over expression cause?

Answer 13

Chromosomal translocation, Myc activated when it comes under the control of foreign transcriptional promoters

leads to deregulation of oncogene which drives relentless proliferation

contributes to cause of 40% of tumours

Question 14

What is Burkitt’s lymphoma?

How is c-myc expression deregulated?

what are the 3 chromosomal translocations that can occur in BL?

Answer 14

1. A high grade lymphoma, effects children from 2-16.

in central africa kids with chronic malaria infections have reduced resistance to virus - endemic BL

2. All BL cases carry 1 of 3 chracteresitc chromosomal translocations, places MYC gene under regulation of Ig heavy chain

Therefore c-myc deregulated

3. In chromosomes 2, 14, 22, places Myc in control of Ig chain 8, lost control of MYC - lots of proliferation - aggressive tumour

in all 3 translocations, a region from one of the chromosomes is fused to section of chr8

Question 15

What is the phildaelphia chromosome and which patients carry it?

What therapeutic strategy is used for CML?

Answer 15

1. Chronic myelogenous leukaemia accounts for 15-25% of leukaemias, 95% of CML patients carry philadelphia chromosome.

The product of chromosomal translocation t(9:22) (q34:q11), generates BCR-ABL fusion protein

as a result, tyrosine kinase activity of oncogene ABL is constitutive - abnormal proliferation

2. Imatinib (Gleevac), tyrosine kinase inhibitor - 96% remission in early stage patients

Question 16

How do tumour supressors and oncogenes work together?

Answer 16

Oncogenes:

• Target cell proliferation
• And cell survival

Tumour suppressors

• Repair mutation
• Cell cycle checkpoints
• Remove damaged cells by apoptosis

Question 17

How many tumour suppressor genes are there?

Answer 17

15, different functions associated with each

Regulators of cell cycle checkpoints (e.g RB1, retinoblastoma. )

Differentiation (e.g APC, adenomatous polyposis coli, colon carcinoma)

DNA repair (e.g BRCA1, breast ovary)

Question 18

What is retinoblastoma?

Is it hereditary or sporadic?

Answer 18

rare childhood cancer, 1 in 20,000

develops when immature retinoblasts continue to grow very fast and don’t turn into mature retinal cells

eye with tumour reflects light back in white colour - leukcoria

two forms of disease, familial (40%) and sporadic (60%)

hereditary mutation on chr13 (13q14), retinoblastoma 1 gene (Rb1)

Question 19

What is the two hit hypothesis for Rb gene?

Answer 19

Retinboblastoma requires two mutations

Loss of heterozygosity used to describe process that leads to inacitvation of second copy of tumour - heterozygous cell recieves second hit in remaining functional copy of tumour suppressor gene

becomes homozygous for mutated gene

mutation which inactivates tumour supressor genes = loss of function mutation

often point mutations or deletions

in sporadic tumour need 2 mutations ine ach of alleles, tumour developed later

Question 20

What are the three proteins in RB gene family

Answer 20

Rb/(p105/110)

P107

Rb2/p130

pocket proteins, has small and large pocket, through large pocket interacts with E2F

Question 21

What is the main function of Rb?

Answer 21

Regulate cell cycle by inhibiting G1 to S phase transition (growth to DNA synthesis)

2 important proteins involved - cyclins and their associated Cdks

Passage of a cell through the cell cycle is regulated by cyclins and CDKS

Cyclin D is the first cyclin to be synthesised and drives progression through G1 together with cdks1/6

G1 checkpoint leads to arrest of the cell cycle in response to DNA damage

Key substrate for cyclin D is RB protein

Cyclin D and E families and their cdks phosphorylate RB

Question 22

What function does RB have with E2F transcriptionf actor?

Answer 22

Regulates E2F, crucial for expression of genes needed for S phase

RB activity regulated by phosphorylation

When RB is dephosphorylated it is active, remains bound to E2F -

When RB tumour suppressor is active it inhibits cell proliferation

Blocks progression to S phase, encourages cell cycle arrest.

Extracellular signals hyperphosphorylate Rb as a result of cyclin - inactive

Upon phosphorylation, E2F released, migrates to nucleus, induces transcription, cell cycle from G1 to S occurs

Inactivating tumour suppressor cells = progression in cell cycle

Question 23

What can inactivate RB?

Answer 23

phosphorylation, mutation or viral oncoprotein binding

In retinoblastoma pRb is functionally inactivated by mutations or partial deletions

Viral inactivation found in small DNA tumour viruses mainly by disrupting E2F binding or destabilisation of Rb

• Adenovirus – E1A
• Papilloma – E7
• Polyoma – large T antigen

In cancer cells Rb phosphorylation is deregulated throughout cell cycle. As a direct consequence E2F transcription factors can induce the deregulation of the cell cycle

Without RB on watch, cells move through G1 into S and are not subjected to usual checks

Question 24

What is p53?

Answer 24

Tumour supressor gene

involved in sensing DNA damage and regulating cell death/apoptosis and other pathways

• oxidative stress, nutrient deprivation, oncogene supression, hypoxia, ribosomal dysfunction

P53 mutated in 30-50% of commonly occuring cancers, suggests tumour cells try to eliminate p53 function before they can thrive

Question 25

How does MDM2 regulate P53?

Answer 25

Normally levels of p53 proteins are low in cells

kept low by MDM2 protein, a ubiquitin ligase (also an oncogene).

Adds ubiquitin onto lysine residues of molecule and it gets targeted to proteosome for proteasomal degradation

In unstressed normal cells both p53 and MDM2 move between the nucleus and cytosol

MDM2 binds p53 to form a complex in the nucleus where MDM modifies the carboxyl terminus of P53 and targets it for degradation by the proteasome

Wt P53 has a short 20 min half life

Question 26

What activates P53 tumour suppressor?

Answer 26

stress signals

Signals sensed by kinases that then phosphorylate p53

Phosphorylation of p53 disrupts interactions between it and MDM2

P53 can thus regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest

Question 27

What causes dysfunction of P53?

Answer 27

Mutational inactivation.

More than half of cancers carry mutations of P53.

Therapeutic strategies aimed at correcting P53 mutations and restoring wild-type p53 function by targeting its regulators

Question 28

What are some therapeutic strategries for treating p53 mutations?

Answer 28

Gene therapy

Inhibitors, can refold mutant P53 back into wild type or can regulate regulators of P53

Primar-1 restores mutant P53 by modifying the thiol groups in the core domain of protein

Nutlin – potent MDM2 antagonist

RITA binds to p53 and can restore mutp53 activity

Inhibitors of CRM1 result in nuclear accumulation of p53