3.2. oncogenes and tumour suppressor genes intro

Question 1

what does ‘the Hallmarks of cancer’ mean?

Answer 1

Various characteristics cancer cells develop necessary for malignant tumour development

Question 2

list the hallmarks of cancer

Answer 2

•Autonomous proliferative signalling (activated oncogenes)
•Inhibition of growth inhibitory signals (loss of tumour suppressor genes)
•Evasion of programmed cell death
•Immortalisation of malignant cell (telomerase)
•Tumour angiogenesis
•Cancer energy metabolism
•Evasion of immune system
•Acquisition of ability to invade surrounding tissue and metastasise
**NB can occur in any order except last point

Question 3

RSV

Answer 3

Rous sarcoma virus

Question 4

Proto-oncogenes or cellular oncogenes:

Answer 4

• Normal gene which encodes for protein regulating proliferation or survival
• genes that encode cellular proteins that regulate normal cell proliferation and survival

Question 5

Oncogenes

Answer 5

• mutated forms of proto-oncogenes that encode proteins that stimulate uncontrolled proliferation or promote cell survival by either being constitutively active or over-expressed.
  •Dominant
• Normal gene “kidnapped” from genome of organism which it infects. This gene encodes for proteins regulating proliferation or survival.

Question 6

what is the function of proto-oncogenes?

Answer 6

to encode key regulatory proteins

Question 7

what key regulatory proteins do proto-oncogenes encode

Answer 7

1. proliferative signalling pathways
2. survival signalling pathways
3. cell cycle
   * Please note that many people refer to the genes and their encoded proteins as proto-oncogenes

Question 8

The functions of proto-oncoPROTEINS:

1) Growth Factors/ proliferative pathway ligands:Autocrine signalling:

Answer 8

Tumour makes own growth factor

PDGF, HGF, VEGF, IL 8, WNT,

Question 9

The functions of proto-oncoPROTEINS:

2)Growth Factor Receptor (RTK)

Answer 9

EGF receptor, KIT, Flt3

Question 10

The functions of proto-oncoPROTEINS:
Signal Transducers
3)Small monomeric guanine nucleotide-binding proteins

Answer 10

RAS

Question 11

The functions of proto-oncoPROTEINS:

4)Non receptor kinases

Answer 11

Tyrosine kinases:SRC
Serine/threonine kinases:AKT, CDK2
Lipid kinase:PI3K

Question 12

The functions of proto-oncoPROTEINS:

5)Transcription factors

Answer 12

MYC, FOS, JUN, βCATENIN

Question 13

The functions of proto-oncoPROTEINS:

6)Apoptosis associated proteins

Answer 13

BCL2

Question 14

what makes BCL2 a proto-oncoprotein?

Answer 14

BCL2 inhibits apoptosis.
They bind BAX –> prevent pore formation in mitochondrion resulting in increased survival and hence are proto-oncoproteins

Question 15

what is the role of proto-oncogene/proto-oncoproteins in the cell cycle?

Answer 15

cyclins and CDKs are regulatory proteins in the cell cycle and regulate cell cycle progression and hence proliferation.
They would thus be considered proto-oncoproteins encoded by proto-oncogenes.

Question 16

what are the mechanisms of oncogene activation/ Genetic mechanisms of oncogene conversion

Answer 16

There are five different genetic ways i.e. the sequence of the DNA changes, in which proto-oncogenes are converted to oncogenes (encoded proteins are overactive or over-expressed)

1. Gain of function mutation
2. amplification of the gene
3. viral insertion
4. translocation:
5. 1. fusion gene formation
6. 2. transcriptional activation
7. retroviral activation

Question 17

describe gain of function mutation and give an example

Answer 17

Gain of function mutation in a proto-oncogene and now an oncogene-encoded protein would have a different sequence which activates the protein usually an enzyme and it is constitutively active e.g. RAS, MAPK

Question 18

describe amplification of the gene and give an example

Answer 18

Amplification of the gene-Here multiple copies of the proto-oncogene gene are found converting it to an oncogene. This results in more protein being made when all the genes are transcribed –overexpression e.g. HER 2 or EGFR

Question 19

describe viral insertion

Answer 19

Here a retrovirus inserts upstream (5’) of a proto-oncogene and due to its LTRs results in the gene being over transcribed. Thus converting it to an oncogene which is thus overexpressed and we get overexpressed protein.

Question 20

describe translocation and name the two types

Answer 20

movement of part of a chromosome to a different chromosome. There are two types:

1. fusion gene formation
2. transcriptional activation

Question 21

describe fusion gene formation

Answer 21

Here the chromosomes each break in the middle of a gene and new fusion genes are formed on translocation. One of the pair is a proto-oncogene and the resultant fusion gene (oncogene) encodes a chimeric protein with overactive function. An example is BCR ABL where the ABL kinase’s (oncoprotein) regulatory domain portion of gene is replaced by the BCR gene and thus the kinase is constitutively active. The mainly BCR fusion gene has no oncogenic function

Question 22

describe transcriptional activation

Answer 22

Here the translocated proto-oncogene is placed downstream (3’) of an actively transcribed promoter or enhancer and as a result the gene is over transcribed thus converting it to an oncogene and more protein is made –overexpressed. EgBCL2

Question 23

give two examples of retroviral activation

Answer 23

MYC,SRC,

Question 24

describe oncogene activation by epigenetic mechanisms

Answer 24

In many cancers the genome is hypomethylated resulting in increased expression of genes.
Hypomethylated promoters result in increased transcription of genes.

Question 25

describe oncogene Oncogene conversion by miRNAs

Answer 25

• Loss of 3’UTR of transcribed proto-oncogene mRNA which is where the miRNA binds. Thus no degradation of mRNA and protein levels go up –gene is now an oncogene
  •Loss of miRNA which regulates levels of mRNA or translation of the mRNA, less is degraded or inhibited and protein levels go up.

Question 26

describe the basis of the Mechanisms of oncogene conversion

Answer 26

• Only one allele needs to be mutated –dominant
• The proteins encoded by oncogenes act to increase cell cycling and hence proliferation or cell survival like a stuck accelerator pedal increases the speed of a car.

Question 27

what are Tumour suppressor genes

Answer 27

encode proteins that normally:
*suppress cell proliferation and
*promote cell death;
-They slow down cell division, activate apoptosis and repair DNA
-That is they are like brakes to stop cells progressing down a road which could lead to cancer
•Inactivation/loss of tumour suppressor genes is associated with cancer development

Question 28

what is Tumour suppressor gene loss termed in genetics

Answer 28

recessive
–> both alleles need to be lost (and mutations are loss of function mutations) for the potential for cancer development to occur. This is not the same as the genetics definition of recessive inheritance.

Question 29

Explain Knudson’s two hit hypothesis (1971)

Answer 29

He suggested that two hits to DNA were necessary to cause cancer. In the children with inherited retinoblastoma, the first insult was inherited in the germline and every cell would already have mutated DNA, and any second hit in any cell would rapidly lead to cancer –hereditary retinoblastoma. In non-inherited retinoblastoma, two “hits” had to take place in the same somatic cell before a tumour could develop, explaining the later age of onset and fewer tumours.

Question 30

how does Knudson’s Two hit hypothesis pertain to tumour suppressor genes?

Answer 30

Knudson’s two hit hypothesis refers to the requirement of inactivation of both alleles of a tumour suppressor before cancer can potentially develop
*Humans carrying germlinemutations in a tumour suppressor gene should exhibit increased cancer susceptibility.

Question 31

explain Tumour suppressor silencing and miRNA

Answer 31

If miRNA controlling degradation of tumour suppressor mRNA or translation of mRNA is overexpressed –silencing of tumour suppressor will occur

Question 32

explain Epigenetic inactivation of tumour suppressor genes

Answer 32

Promoters of tumour suppressor genes are hyper-methylated resulting in silencing of genes e.g. RB, PTEN

Question 33

what is required for cancer to develop in a cell

Answer 33

one needs both tumour suppressor loss and oncogene activation in the same cell.

Question 34

draw a table to compare proto-oncogene and tumour suppressor, and oncogenes and tumour suppressor genes

Answer 34

refer to pg. 45 of notes