22- Oncogenes and Tumour Suppressor Genes

Question 1

define cancer

Answer 1

a group of diseases characterized by the uncontrolled and abnormal growth of cells, often forming tumours

these cells have the potential to invade surrounding tissues and metastasize to other parts of the body

Question 2

what are the 4 major functional changes in cancer?

Answer 2

increased growth, from loss of growth regulation

failure to undergo apoptosis or senescence

loss of differentiation

failure to repair DNA damage (including chromosomal instability)

Question 3

3 ways oncogenes contribute to the hallmarks of cancer

Answer 3

sustain proliferative signalling

contribute to genome instability and mutation

enable replicative immortality - increase rate of cell division

Question 4

3 ways tumour suppressor genes contribute to the hallmarks of cancer

Answer 4

contribute to genome instability and mutation

allow cancer cells to resist cell death

evade growth suppressors

Question 5

describe the role of oncogenes in cancer

Answer 5

normally, oncogenes regulate cell growth, division, and death (act as the go signals for growth)

gain of function mutations in oncogenes contribute to cancer - they become permanently active

Question 6

describe the role of TS genes in cancer

Answer 6

normally, TS genes prevent the development of cancer, encoding proteins that inhibit cell cycle progression, promote DNA repair, and induce apoptosis (act as the brakes/ stop signals for growth)

TS genes can usually counteract mutated oncogenes and prevent cancer formation

loss of function mutations in TS genes prevent their ‘stop’ signals for uncontrolled growth, allowing cancer progression

Question 7

define a proto-oncogene

Answer 7

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

have a normal function in the body

Question 8

define an oncogene

Answer 8

a mutated proto-oncogene, with the ability to contribute to cancer development

not all mutated genes seen in cancer are oncogenes

Question 9

how is a proto-oncogene activated into an oncogene?

Answer 9

proto-oncogene can be transformed into an oncogene through a gain-of-function mutation

• enhances a protein’s actions and promotes signalling pathways which contributes to uncontrolled cell proliferation
• types of mutations: deletions, duplications, translocations, subtle/ point mutations

Question 10

what is a deletion mutation? how can it activate a proto-oncogene into an oncogene?

Answer 10

deletion = loss of genetic information

can alter/reduce the function of the encoded protein

Question 11

what is a duplication mutation? how can it activate a proto-oncogene into an oncogene?

Answer 11

duplication = gene amplification; replication of the same DNA region

can increase the amount of a normal growth signalling protein produced in excess

too much growth protein = too much uncontrolled growth

Question 12

what is a translocation mutation? how can it activate a proto-oncogene into an oncogene?

Answer 12

translocation = a segment of DNA breaks off from one chromosome and becomes attached to another non-homologous chromosome

relocation of proto-oncogenic sequences can create a fusion oncogene/oncoprotein that alters gene regulation = excessive production of normal growth protein

Question 13

what is a point mutation? how can it activate a proto-oncogene into an oncogene?

Answer 13

point mutation = a single base/nucleotide change in a DNA sequence

a point mutation within a control element can cause excess production of a normal growth stimulating protein

within a gene = can produce an oncogene

Question 14

describe the discovery of the Src oncogene involving RSV

Answer 14

RSV = Rous’ sarcoma virus

sarcoma was induced in chickens through transducing the Src gene into chickens via the RSV virus

c-src is a normal cellular gene involved in regular cell functions

retroviral transduction of the Src gene into other chickens via RSV resulted in creating proto-oncogene v-scr

an extra copy of the Src gene deposited in a cell by the RSV virus contributed to abnormal cell growth and sarcoma development

Question 15

what is the oncogene hypothesis?

Answer 15

normal cellular genes or proto-oncogenes when altered through exposure to carcinogens can contribute to endogenous uncontrolled cell growth and cancer development

carcinogens can be chemical, physical, (exogenous) viruses - can transform and ‘switch on’ endogenous oncogenic potential of genes

Question 16

what are the 4 main proteins/components involved in transducing growth signals for growth factor pathways? give an example of each

Answer 16

growth factors - e.g. EGF

growth factor receptors - ErbB

intracellular signal transducers - e.g. Ras and Raf

nuclear transcription factors

Question 17

three specific examples of different mutations that can contribute to cancer development?

Answer 17

Philadelphia chromosome 22 - chromosomal translocation

HER2 gene amplification

Ras point mutation

Question 18

Philadelphia chromosome - what is it? how is it produced? how can it contribute to cancer development?

Answer 18

Philadelphia chromosome is the product of inappropriate non-homologous end joining following a ds break, causing a chromosomal translocation between chromosomes 9 and 22

chromosome 9 carries the ABL gene - its tyrosine kinase activity makes it constitutively active

chromosome 22 carries the BCR gene

BCR and ABL on the same derivative chromosome 22 creates fusion oncoprotein BCR-ABL = constitutive active state of ABL leads to abnormal proliferation and uncontrolled growth

common cause of chronic myeloid leukaemia

Question 19

HER2 gene amplification - define? how can it contribute to cancer development? how does it link to cancer treatment?

Answer 19

HER2 protein is encoded by ERBB2 gene which is important in growth factor signalling pathways

ERBB2 gene amplification causes overexpression of HER2 receptor on cell surfaces = makes cells hypersensitive to growth factor signals = promotes uncontrolled cell growth and survival

common cause of HER2 positive breast cancer development

HER2 growth factor receptor blocker Trastuzumab used for treatment = improves survival

Question 20

RAS point mutation - define? how can it contribute to cancer development?

Answer 20

Ras is a GTP binding protein that cycled between active GTP-bound and inactive GDP-bound in response to cellular signals

a point mutation in Ras from G to T, glycine to valine causes structural changes inhibiting GTP hydrolysis to deactivate Ras

Rs in a constantly active GTP-bound state = constant downstream signalling for growth = uncontrolled growth

Question 21

what is trastuzumab/ herceptin?

Answer 21

a HER2 growth factor receptor blocker - used in the treatment of HER2 positive breast cancers

improves overall survival, prevents/ decreases uncontrolled growth

Question 22

what is the MYC oncogene family?

Answer 22

master oncogenes consisting of three oncogenes - C-MYC, MYCN, MYCL which encode for oncoproteins c-MYC, N-Myc and L-Myc respectively

these oncoproteins act as transcription factors - a single TF influences multiple genes which amplifies the effects of any mutations

Question 23

what are the 4 downstream effects of MYC oncoproteins activity?

Answer 23

ribosome biogenesis
protein translation
cell cycle progression
metabolism

affect cell proliferation, differentiation and immune surveillance

Question 24

what is type of transcription factor/ oncoproteins the MYC oncogenes encode for?

Answer 24

helix-loop-helix leucine zipper TF

Question 25

mechanism behind how MYC oncogene mutations contribute to cancer development?

Answer 25

MYC oncoprotein TF forms dimer with its partner protein Max - activated once placed under the control of foreign transcriptional promoters and transactivate gene expression together

deregulation of MYC leads to MYC-Max being placed under the control of inappropriate promoters = abnormal activation of gene expression = uncontrolled proliferation

Question 26

why are mutations within the MYC oncogene family significant?

Answer 26

MYC oncoproteins act as TFs - a single TF influences multiple genes. any mutations will have amplified effects

mutations in the MYC oncogene are family implicated in many types of cancers - cause overexpression, influence cellular activities

Question 27

what are the 3 primary roles of TS genes in cellular function?

Answer 27

stop uncontrolled growth by negatively regulating proliferation

promote differentiation

trigger apoptosis

Question 28

name three specific functions performed by TS genes, with examples

Answer 28

cell cycle checkpoints (e.g., RB1)

cell differentiation (e.g., APC)

DNA repair (e.g., BRCA1)

other functions = negatively regulate proliferation, regulate cell numbers, stop uncontrolled growth, promote differentiation and trigger apoptosis

Question 29

what is the ‘two-hit’ concept related to TS genes?

Answer 29

individuals with a single inherited mutation in a TS gene may be predisposed to cancer, requiring a second mutation in the other functional allele for cancer development

the recessive nature of TS genes requires mutations in both alleles of a TS gene for functional loss

Question 30

why are TS genes called ‘anti-oncogenes’ in the context of cancer?

Answer 30

TS genes counteract the effects of oncogenes and maintain the balance necessary for normal cell growth

loss of TS genes disrupts this balance, contributing to cancer development

Question 31

what is the significance/role of the RB1 gene?

Answer 31

it’s a cell cycle regulator, can block the G1-S checkpoint and prevent cell cycle progression

Question 32

how does RB protein work as a cell cycle regulator - mechanism?

Answer 32

RB protein in early G1 is active and binds to E2F transcription factors, preventing their transcriptional activity for DNA synthesis = this blocks G1 to S transition

growth signals are released for cell cycle progression which activate kinases such as CDKs

CDKs phosphorylate RB protein - causes a structural change that inactivates RB and it releases E2F

E2F can be transcriptionally active and promote gene expression for DNA replication and synthesis for G1 to S

Question 33

what is the role of the RB1 gene in retinoblastoma?

Answer 33

RB1 gene acts as a tumour suppressor, important in regulating the cell cycle

in retinoblastoma, an inherited defect in one copy of the RB1 gene is present in every cells. one more hit can lead to the development of retinoblastoma

RB1 blocks G1-S transition and prevents cell cycle progression

Question 34

define apoptosis

Answer 34

programmed cell death, a natural process for controlling cell proliferation and eliminating intracellular toxic substances in a regulated manner

Question 35

describe the key (cellular) features of apoptosis

Answer 35

cell shrinkage
membrane blebbing
increased permeability of mitochondria
nuclear fragmentation

Question 36

what is p53?

Answer 36

a tumour suppressor protein encoded by TP53 gene

it acts as a transcription factor by binding to approx. 300 different gene promoter regions

Question 37

list the domains within the structure of p53

Answer 37

amino transactivation domain
central DNA binding domain
tetramerization domain
carboxyl regulatory domain

Question 38

what is the role of p53 in cancer prevention?

Answer 38

prevents the formation of cancers by responding to DNA damage or abnormal cell proliferation = prevents the accumulation of mutations that could contribute to cancer development

with severe DNA damage, p53 stops cell division to allow for repair mechanisms to work

with irreparable DNA damage, p53 promotes apoptosis

Question 39

what is the effect of mutated TP53? what is the most common mutation?

Answer 39

mutational activation of TP53 allows damaged cells to survive and contribute to cancer - is the reason behind many cancers developing

most common mutation is a missense mutation in the DNA binding domain of p53 protein

Question 40

what is Li-Fraumeni syndrome? how is it related to TP53 mutations?

Answer 40

Li Fraumeni syndrome = a condition predisposing individuals to various tumour types

it’s linked to inheriting a defective copy of the TP53 gene = more susceptible to cancers form a young age as they have only one functional TP53 copy

Question 41

what are the two main sources of DNA damage, with examples?

Answer 41

external sources = x-rays, certain chemicals and heavy metals, environmental agents

internal sources = errors during DNA replication, metabolism, other cellular processes

Question 42

how do cells respond to DNA damage? mechanism involved?

Answer 42

cells respond to DNA damage through repair, cell cycle arrest, and apoptosis

mechanism: multiple proteins converge at the site of DNA breaks or damage, activating signalling pathways that coordinate these responses

Question 43

explain the role of the BRCA1 gene in DNA repair mechanisms

Answer 43

important in DNA repair mechanisms

dysfunction in the BRCA1 gene impairs the cell’s ability to repair DNA damage = mutations accumulate, and increases the risk for mutations in TS genes an/or oncogenes

Question 44

Q: what is the relationship between oncogenes and tumour suppressors in carcinogenesis and development?

Answer 44

proto-oncogenes are normal genes that promote regulated cell growth and proliferation

TS genes negatively regulate cell growth by promoting differentiation, getting involved in cell cycle checkpoints and differentiation, regulating cell numbers and triggering apoptosis

mutated proto-oncogenes become oncogenes = promote uncontrolled cell growth and differentiation by stopping apoptosis and differentiation, causing overexpression of growth factors and increased transduction of growth signalling pathways

balance between oncogenes and ts genes is required to prevent cancer formation

a one hit gain-of-function mutation in oncogenes can contribute to cancer development, however, functionality of a TS gene can overcome its oncogenic effects

however, a two-hit loss-of-function mutation in TS genes creates an imbalance = contributes to cancer development

Question 45

Q: what are the genetic mechanisms that lead to the development of oncogenes from proto-oncogenes?

Answer 45

one-hit gain of function mutations in normal proto-oncogenes can lead to oncogenes - affect growth factors, growth factor receptors, intrac. signalling molecules and nuclear transc. factors

mutations such as:
- deletions
= can cause reduced/ altered function of a gene product - affect growth signalling pathways
- duplication/ gene amplification
= overexpression/ production of a gene and its product - increase growth signalling
- point mutations
= small base sequence change can affect production or activity of a gene product
- chromosomal amplifications
= two genes from different chromosomes translocated together can create a fusion oncoprotein and influence each others’ activity - lead to abnormal/ uncontrolled cell growth

Question 46

Q: What are the main differences between TSGs and oncogenes?

Answer 46

TS genes are recessive - contribute to cancer by loss-of-function mutations and need ‘two hits’ for TS gene inactivation

oncogenes are dominant - contribute to cancer through gain-of-function mutations and only need one hit for oncogene activation

Question 47

Q: can you outline the main mechanisms of action of different examples of oncogenes and TSGs? HER2, p53 etc?

Answer 47

oncogenes - HER2, BCR-ABL fusion oncogene, Ras point mutation

HER2/ ERBB2 gene amplification
= HER2 is encoded by ERBB2 gene - amplification of this gene causes overexpression = overproduction of HER2 on cell surfaces = oversensitivity to growth signals/ factors = abnormal and uncontrolled growth = contributed to development of HER2 positive breast cancer

BCR-ABL fusion oncogene
= inappropriate non homologous end joining between ABL gene on chrom 9 and BCR gene on chrom 22 = two genes translocated and fused together on Philadelphia chromosome 22 forming the BCR-ABL fusion oncogene
= ABL gene is constitutively active due to its tyrosine kinase activity - influences BCR gene activity, contributes to abnormal/ uncontrolled cell growth in cancer
= present in 95% of chronic myeloid leukaemia cases

Ras point mutation
= Ras-GTP bound is active, through hydrolysis becomes Ras-GDP bound and inactive - activity stimulated by growth signals
= point mutation in Ras from G to T/ glycine to valine causes a structural change where Ras can’t hydrolyse its bound GTP - ends up constantly active, no deactivation, constant stimulation of growth pathways, leads to abnormal cell growth

TS genes - p53, BRCA1, RB1

p53 protein, TP53 gene
= TP53 gene encodes p53 protein - maintains genomic stability by responding to DNA damage and preventing accumulation of mutations that could contribute to cancer
= with severe DNA damage it stops cell division and allows for repair mechs to work
= dysfunctional p53 is present in many cancers - often a missense mutation in the DNA binding region - allows damaged cell to survive, mutations accumulate, cancer!
= contributes to Li Fraumeni syndrome - condition where a defective copy of TP53 gene leaves you predisposed to certain cancers from a young age

BRCA1 gene
= important for DNA damage form external/ internal sources - causes cell cycle arrest, allows proteins to converge at site of DNA damage and repair it
= mutations causing BRCA1 dysfunction impair repair mechanisms, mutations accumulate, higher chance of TS/ oncogene mutations, contributes to cancer!

RB1 gene
= encoded RB protein - cell cycle regulator, blocks G1 to S checkpoint and prevents cell cycle progression
= RB protein is active in early G1 - binds E2F transc. factors prevents transcriptional activity and DNA synthesis = prevents G1 to S transition and cell cycle
= growth factors released for progression of cell cycle - activate kinases like CDKs which phosphorylate RB protein, cause a conformational change, E2F TFs are released and can be transc. active and promote DNA synthesis and cell cycle progression
= two hits in the RB1 gene (it’s a TS gene, recessive, loss of function mutations) = contribute to retinoblastoma as it allows uncontrolled cell cycle progression

Question 48

Q: why are oncogenes and TSGs important?

Answer 48

oncogenes are involved in cell growth, TS genes are involved in inhibiting cell growth

cancer tissues have an imbalance of cell growth and proliferation - affected by the imbalance of oncogenes and ts genes due to one-hit oncogenic mutations or two-hit ts gene mutations or an accumulation of mutations contributing to cancer development

mutated gene can be targeted during therapeutic treatments - e.g. TP53 as a TS gene is a therapeutic target as it’s commonly mutated in many cancers; it’s important in maintaining genomic instability, and re-introducing its wild-type in place of the mutated version could prevent cancer development