Topic 5 Flashcards
Cancers arise when critical ____ that regulate ____ are _____.
critical genes that regulate cell cycle are mutated
What do mutations cause in the cell?
Cell proliferation - multiply
Impaired cell death - survive
Cancer mutations deregulate:
- cell signalling (damage sensor)
- cell cycle control (proliferation)
- gene expression
- cell death (apoptosis)
START checkpoint
-decision on whether to divide or not in the cell cycle (divide or apoptosis)
What are the key regulators of the cell cycle?
CDK - cyclin dependent kinases.
-very tight control between transitions to different phases
4 phases of cell cycle
G1, S, G2, M
Cyclin
regulatory protein that regulates CDK
Cyclin abundance (in order of cell cycle)
G1: Cyclin D Cyclin E S: Cyclin A G2: Cyclin B
CDK activity (in order of cell cycle)
G1: CDK4 CDK2 S: CDK2 G2: CDK1
CDKI
cyclin dependent inhibitor that can restrain cell cycle progress
When CDKs are activated by cyclins, what do they do?
Phosphorylate specific proteins that control cell cycle progression in temporal and cyclical pattern
How do CDKI (inhibitors work)?
-CDKI bind to the CDK to inactivate
or
-bind to the CDK/cyclin complex
Cell cycle checkpoints (definition)
control mechanisms that ensure cell cycle progression occurs appropriately
4 Cell Cycle Checkpoints
G1: G1/S Checkpoint G2: DNA replication checkpoint M: Spindle assembly checkpoint Chromosome segregation checkpoint G1: DNA damage checkpoint
Purpose of checkpoints:
to guard the genome from
- loss of genetic info (unreplicated DNA, DNA damage, chromosome breakage)
- missing or extra chromosomes (unattached chromosomes)
G1/S transition checkpoint
- requires (dependent) nutrient/growth factors (–> cyclin D)
- independent synthesis of cyclin E (late G1)
- when G1/S checkpoint activated, CDK(2/4) - which are in in G1 - are inhibited (to prevent forward progression to S phase)
Spindle assembly checkpoint
- abnormal spindle
- cell cycle arrested
- signal apoptosis machinery
DNA replication and DNA damage
- excessive ssDNA (arrest or pertubation/change of replication forks)
- dsDNA breakage
- unfinished DNA repair
activation allows for:
- arrest cell, repair damage, resume
- nonrepairable, apoptosis
Apoptosis
molecular and morphological process to controlled cellular self destruction
3 steps of apoptosis
- reduction in nuclear size, condensation of chromatin at nuclear periphery, detachment from surrounding cells
- cell shrinkage, blebbing of cell membrane = apoptotic bodies
- phagocytosis by macrophage of apoptotic bodies
Excess apoptosis diseases
- huntington’s
- alzheimer’s
- parkinson’s
- traumatic brain injury
- stroke
Inadequate apoptosis diseases
- cancer
- multiple sclerosis
- diabetes
- arthritis
- systemic lupu erythematosus
Caspase
- cysteinyl aspartate specific proteinase
- cysteine proteases (active site cysteine) that cleave after aspartate
- -present in inactive pro-form in healthy cells
- degrade specific proteins that lead to cell death
Initator caspase
effector
-caspase 9 = stress
caspase 8 = death receptor
Effector caspase
executioner
- cleave specific cellular proteins = inactivation
- caspase 3, 6, 7
Adaptor proteins
bind to initiator caspases to activate
Apaf-1 & FADD
Caspase activation regulated by _____ proteins
BCL-2 family proteins
Pro-apoptotic Bax/Bak
induce apoptosis by producing channels that release cytochrome c from mitochondrial outer membrane
Cytochrome C
binds to adapter protein (Apaf-1) which oligomerizes apoptosome
-activates initiator caspase-9
Activated caspase-9
cleaves effector caspase 3
Caspase 3
cleaves proteins leading to apoptotic body
BCL-2
proSURVIVAL BCL-2 inhibits apoptosis
via preventing BAX/BAK oligomerization
BH3-only proteins
proAPOPTOTIC inhibit proSURVIVAL BCL-2 and may directly activate BAX/BAK
Oncogene
-normal function: promote cell survival or proliferation (proto-oncogene)
-hyperactive activator - no longer growth factor dependent
-over expression/mutation: cause cancer
due to point mutation, chromosomal translocation, amplification
-requires ONE mutation to enable oncogene to stimulate cell proliferation
Proto-oncogene:
the normal counterpart to an oncogene – proper regulation
-promote cell division and cell survival
Tumor suppressor genes
-normal function: inhibit cell survival or proliferation (division)
-loss of function mutation: cause cancer
due to deletion, point mutation, promoter methylation (deactivates)
-require TWO inactivating mutations to eliminate tumor supressor gene and stimulate cell proliferation
Proto-oncogenes and tumor suppressor genes are ____ that regulate _____. Only when ___ do they ____
normal genes that regulate cell proliferation.
mutate induce cancer or fail to prevent progression.
Examples of oncogenes
HER2
ras
c-myc
Bcr/Abl
Examples of tumor suppressor genes
Rb
p53
Overexpression/hyperactivity of oncogenes induce signalling pathways that are ______________
- no longer growth factor dependent.
- oncogenes directly controls expression of cyclin D
- therefore GF independent
c-ras
inactive ras / GDP
active ras / GTP
-ras is an oncogene
Normal regulation of Ras protein
- in the cytoplasm, extracellular signalling influences the state of ras (inactive/active)
- activated by phosphorylation of GDP to GTP
- active Ras transduces signal to nucleus to regulate transcription of genes involved in cell division
Mutant Ras protein is _______
unregulated.
- impaired control by growth factors
- constitutively active
- extracellular signal has NO influence on mutant Ras
- mutant Ras = active state
- signal to nucleus
- lots of cell division genes transcribed
- uncontrolled
Main contributors to cancer signalling pathways: (4)
- deregulation of growth factors
- hyperactive ras, myc, jun, fos, src (oncogenes)
- continuous grow/divide signal (constitutive)
- G1/S checkpoint compromised
Rb and p53 flow chart typically.
Typically: growth factors regulates cyclin D (G1 phase) which inhibits Rb (tumor supressor) inhibitting E2F (G1/S checkpoint also inhibits E2F) allowing for cyclin E to regulate S phase
(DNA damage checkpoint inhibits)
p53
which initiates transcription of protein
p21 (CDKI)
which inhibits
CDK2/cyclin E/A complex (regulating S phase)
Rb acts as a brake on ____ progression
G1/S phase progression.
E2F
transcription factor that is inactivated by binding Rb.
Otherwise, it transcribes genes needed for S phase.
What phosphorylates Rb? What does this do?
Growth factors stimulate cyclin D.
CDK4/cyclinD + CDK2/cyclinE phosphorylate Rb.
This prevents Rb to bind to E2F.
E2F can now transcribe genes for S phase.
For S phase to progress what does the Rb and E2F look like.
Rb and E2F are not bound.
Rb is phosphorylated by CDK4/cyclinD and CDK2/cyclin E
E2F is free and active as a transcription factor..
Tumor suppressor genes: ______- of mutant cell cycle inhibitor p53/Rb allows for cell proliferation
Loss of function or loss of expression. Without Rb E2F is free to follow through with transcription of S phase i.e. cyclin E
Mutation in which 4 molecules can lead to cancer. (regarding tumor suppressor genes)
CycD
CycE
Rb
E2F
p53 gene
mutation in __ of cancers
leads to___
- mutation occurs in 50% of cancers
- leads to genome instability
loss of p53 = genome instability
- cells escape apoptosis and divide with damaged DNA
- these cells can accumulate MORE mutations which increase probability of tumor formation (two hit)
what is p53?
- transcription factor that upregulates apoptotic genes
- i.e. BAX = caspase activation = cell death
- typically p53 is activated if there is unreparable DNA damage (increase in p53)
Name the processes that mutations in these genes affect in order: ras, Rb, p53/p21, p53/BAX
- signal transduction pathway
- checkpoint regulation (G1/S)
- response to checkpoint to DNA damage
- response or execution of apoptosis
3 ways to turn proto-oncogene into oncogene?
- deletion or point mutation in coding sequence (hyperactive protein)
- gene amplification (overproduction of the normal protein)
- chromosome rearrangement (nearby regulatory sequence causes overproduction or fusion to actively transcribed gene)
Gene amplification of proto-oncogene to oncogene. 2 specific genes.
c-myc or HER2
Abnormal expression at abnormally elevated levels.
C-myc
transcription factor that regulates G1/S transition
HER2
receptor that activates MAPK signalling (proliferation)
Chromosome rearrangement (proto-oncogene to oncogene). 2 specific chromosomes and disease associated with
Philadelphia chromosome. Fusion protein is hyperactive. Chronic myelogenous leukemia. Burkitt's lymphoma Nearby regulatory sequence causes overproduction of c-myc protein.
fusion protein bcr-abl
tyrosine kinase activity is now constituitively active
Philadelphia chromosome
- chromosome 9 and 22
- reciprocal translocation produce philadelphia
- fusion protein of bcr (from small chr22) and abl (from large chr9)
Burkitt’s lymphoma
- reciprocal translocation of chromosome 8 and 14
- IGH immunoglobulin gene enhancer now controls c-myc (next to each other)
- this causes the overproduction of c-myc
note: c-myc control G1/S phase transition
Knudson’s two hit hypothesis
appearance = single random mutation of familial tumors and need two random hits for sporadic tumors
Inherited bilateral retinoblastoma (two eyes childhood tumor) - high risk of 2 tumor Sporadic unilateral retinoblastoma -affects only one eye, no family history -no further risk of 2 tumor -rare somatic mutation of BOTH alleles
Li-Fraumeni syndrome is caused by?
inherited p53 mutation (DOMINANT)
-p53 is signal that realizes DNA is damaged and will express genes that repair DNA or suppress cell cycle p21, and apoptosis BAX
Cancers develop through accumulation of _______ in ____ and _____
accumulation of somatic mutations in proto-oncogenes and tumor supressor genes
Six essential alterations for tumorigenesis
- self-sufficiency in growth signals
- Insensitivity to antigrowth signals
- Evasion of apoptosis
- Limitless replicative potential
- Tissue invasion and metastasis
- Sustained angiogenesis
Self-sufficiency in growth signals
- synthesize own growth factors
- growth factor receptor over expressed
- alteration in signal transduction
- over expression of transcription factors
- express mutant transcription factors
Insensitivity to antigrowth signals
TGFb distruption
- downregulation of TGFb receptors
- express mutant receptors
- mutations in SMAD4
TGFb inhibits G1/S progression
- inhibits Rb phosphorylation
- inhibits c-myc expression
- increase CDKI
Limitless replicative potential
- immortalize
- loss of p53
- inhibit Rb
- telomerase expression is overexpressed in cancer via end to end fusion
Sustained angiogenesis
- encourage blood vessel growth to provide O2 and nutrients
- secrete endothelial growth factors
Tissue invasion and metastasis
-protease secretion
-altered expression of matrix binding proteins
-escape immune killing cells
tissue invasion, intravasation, survival in blood, extravasation, colonization
Evasion of apoptosis
-inactivate pro-apoptotic genes (FasR)
activate anti-apoptotic protein genes (Bcl-2)
