Oncogenes and Tumour Suppressors

Question 1

What are the major functional changes in cancer?

Answer 1

• Increased growth (loss of growth regulation, stimulation of environment
  promoting growth e.g. angiogenesis)
• Failure to undergo programmed cell death ( apoptosis) or senescence
• Loss of differentiation (including alterations in cell migration and adhesion)
• Failure to repair DNA damage (including chromosomal instability)

Question 2

What are the four classes of cancer genes

Answer 2

• Oncogenes
• Tumour suppressor genes
• Apoptosis genes
• DNA repair genes

Question 3

Describe regulated cell growth

Answer 3

Normal cells are signalled to increase in number with an accelerator signal

Once the cell number has been reached the cells stop proliferating with a brake signal

Question 4

describe deregulated cell growth

Answer 4

Cancer cell receives an increased accelerator signal to proliferate quickly

There is an issue with the brake signal and cell division isn’t suppressed

Question 5

Which hallmarks of cancers relates to oncogenes

Answer 5

• sustaining proliferative signal
• Enabling replicative immortality
• Genome instability and mutation

Question 6

Which hallmarks of cancer relate to Tumour suppressor genes

Answer 6

• Evading growth suppressors
• Genome instability and mutation
• Resisting cell death

Question 7

How do oncogenes lead to cancer

Answer 7

Their normal job is to make cells divide, driving cell division forward

In cancer, pick up mutations that mean they are permanently active

Oncogene: “Gain of function”

Question 8

How to tumour suppressor genes lead to cancer

Answer 8

Even if you have a mutation in an oncogene that pushes cell division forward, if your tumour suppressor genes are strong enough, they should still be able to counteract the oncogene

In cancer, pick up mutations that switch the gene off.

Tumour Suppressor gene: “Loss of function”

Question 9

Define proto-oncogene

Answer 9

A “normal” gene that has the potential to become an oncogene when mutated

Question 10

Describe how oncogenes were discovered

Answer 10

Studies of retroviruses essential in understanding oncogenes

Landmark experiments:

• Frances Peyton Rous began his work in 1910 that lead to the discovery of Rous sarcoma virus (RSV).
• 50 years later he received the Nobel Prize in Medicine in 1986
• In 1911 when a farmer brought Rous a prized Plymouth Rock hen that had a large tumour growing in the chest muscle,

He used the cell free filtrate from the chicken sarcoma and was able to induce sarcomas in healthy chickens

Question 11

Describe the Rous’ protocol

Answer 11

• Tumours developed weeks later
• Taking the new sarcoma, filtrates produced could also induce tumours in other chickens
• The cycles could be repeated indefinitely. Also the carcinogenic agent was small enough to pass through
  a filter
• Although the filter used excluded bacteria it was not small enough to exclude viruses
• Rous concluded that a virus must be responsible for the induction of tumour formation
• Discovery that this sarcoma was transmissible through viruses- Rous Sarcoma Virus

Question 12

How can viruses transmit the src Oncogene

Answer 12

• RSV retrovirus integrates into the genome - oncogenic virus
• Fibroblast which was infected with RSV proliferate abnormally and transforms into cancer cells
• RSV stole genes from chicken cells and incorporated it into their genome, cellular origin

SRC genes exist in human cells (c-src/ v-src)

Question 13

Why are proto-oncogenes in our cells

Answer 13

They are involved in normal proliferation

Test for possible oncogenes:

• bits of DNA transfected into mouse fibroblasts.
• used immortalized mouse cells (not transformed / not tumorigenic)
• occasionally a piece of DNA caused cell transformation and cells were also tumorigenic.
• then have to isolate the human gene that had caused the transformation of the mouse cells
• discovery of ras oncogene

Question 14

Describe the capture of c-src by retroviruses

Answer 14

• During evolution, the virus can acquire fragments of genes from the host at integration sites and this process results in the creation of oncogenes
• The oncogene product was characterised as a 60kDa intracellular tyrosine kinase
• Can phosphorylate cellular proteins and effect growth

Question 15

How can DNA viruses lead to cancer

Answer 15

DNA viruses can cause lytic infection leading to the death of the cellular host or can replicate their DNA along with that of the host and promote neoplastic transformation

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

Question 16

How can RNA viruses lead to cancer

Answer 16

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 17

What is the oncogene hypothesis

Answer 17

Normal genes are switched to oncogenes via carcinogens such as:

• Chemicals
• physical
• Viruses

Question 18

what are the 4 proteins involved in NORMAL transduction of growth signals

Answer 18

• Growth factors
• Growth factor receptors
• Intracellular signal transducers
• Nuclear transcription factors

Question 19

What are the mutations that turn proto-oncogenes into oncogenes

Answer 19

• Deletion
• Duplication - too much proteins made
• Inversion
• Translocation
• Subtle mutations - point mutations

GAIN OF FUNCTION MUTUATIONS

Question 20

Describe the formation of the Philadelphia chromosome - Translocation

Answer 20

• Chromosomal translocation between chromosomes 9 and 22
• genes coding for BCR and ABL are brought closer together
• Creates a fusion protein: BCR-ABL
• As a result of this translocation the tyrosine kinase activity of the oncogene ABL is
  constitutive leading to abnormal proliferation
• Seen in 95% of Chronic Myelogenous Leukaemia (CML) cases

Question 21

Describe HER2 mutation in oncogene formation - Gene Amplification

Answer 21

• Coded by the ERBB2 gene – amplified in cancer
• Human epidermal growth factor receptor 2
• Growth factor signalling
• Important in subsets of breast cancer
• There are now drugs that target HER2 – eg Trastuzumab (Herceptin)

Question 22

Describe Ras mutation in oncogene formation - Point Mutations

Answer 22

G- T change - Glycine to Valine

Ras signaling pathway:

• Ras GTP to GDP cycling
• Ras – GTP binding protein.
• GTP – active
• GDP – inactive

Mutated Ras - GTP hydrolysis is Inhibited. Protein stuck in active state - CONSTITUTIVELY ACTIVE

Question 23

Describe the MYC Oncogene family

Answer 23

• Consists of 3 members: C-MYC, MYCN and MYCL which encodes c-myc, n-myc and l-myc
• Originally identified in avian myelocytomatosis virus (AMV)
• It is over-expressed in majority of human cancers and contributes to the cause of at least 40% of tumours
• Encodes a helix-loop-helix leucine zipper transcription factor that dimerises with its partner protein, Max, to transactivate gene expression
• Sub activation is a result of chromosomal translocation

Question 24

How were tumour suppressor genes identified and discovered

Answer 24

• in 1969 Henry Harris performed somatic cell hybridisation
• Fusion of normal cells with tumour cells made cells with chromosomes from both parents that weren’t capable of forming tumours.
• Genes derived from normal parents acceded to suppress tumour development.
  The first tumour suppressor gene was identifies by studies of retinoblastoma

Question 25

What are tumour suppressor genes

Answer 25

• products from these genes act as stop signs for uncontrolled growth, promote differentiation or trigger apoptosis.
• Usually regulators of cell cycle checkpoints
• Loss of tumour suppressor gene functions requires inactivation of both alleles of the gene
• Defined as recessive genes and referred to as anti-oncogenes

Question 26

Describe the effects of sporadic mutations in retinoblastoma

Answer 26

Both mutation hits occur in the retina.
Late onset and often unilateral (one eye)

Question 27

Describe the effects of inherited mutations in retinoblastoma

Answer 27

The first mutation is already present in the germline and the second occurs in the retina.
early onset and often bilateral (both eyes)

Question 28

What is the function of the retinoblastoma protein

Answer 28

Normal function:

• active RB blocks progression form G1 to S phase by no releasing bound E2F
• When RB is phosphorylated it releases E2F which progresses the cell cycle

Mutated function:

• E2F is not bound by RB so the cell cycle is always progressed uncontrollably

Question 29

What is the function of p53

Answer 29

Guardian of the genome/transcription factor; mutated in 50% of cancers

• detects abnormal proliferation and DNA damage
• Causes cell cycle arrest and apoptosis

Question 30

Describe the p53 signalling network

Answer 30

• Protein has an amino transactivation
  domain, a central DNA binding
  domain, a tetramerization domain
  and a carboxyl regulatory domain
• Can bind to around 300 different
  gene promoter regions-main role as
  a transcription factor

Question 31

How are elephants resistant to cancer

Answer 31

They have 20 copies of the tp53 gene where as humans only have 2.
Individuals with Li-Fraumeni syndrome only have 1 functioning tp53 gene

Question 32

Give some examples of TSGs

Answer 32

Different functions associated with each:

• regulators of cell cycle checkpoints (e.g. RB1),
• differentiation (e.g. APC)
• DNA repair (e.g. BRCA1)