Cancer Biology Flashcards
How many chromosomes need to be mutated to have activation of oncogenes?
One of the two chromosomes in a pair
How many chromosomes need to be mutated to have inactivation of Tumor Supressor Gene?
Both chromosomes in the pair
What are the 2 phases of the cell cycle?
- Interphase
- Mitosis
Interphase
- longest part
- state of preparation
- cell performs basic functions
- grows and replicates DNA
What are the 3 sub-phases of Interphase?
(1) G1
(2) S
(3) G2
G1 Phase
- longest sub-phase
- cell grows
- organelles perform normal functions like synthesis of proteins and energy production
- chromosomes are made-up of a single chromatid
- control point at G1 called G1 checkpoint
G1/S Checkpoint
- RESTRICTION POINT
- checks for damaged DNA
- mediated by cyclin E/CDK2 complex and Rb protein and E2F
- when cyclin E/CDK2 complex forms, it phosphorylates Rb which releases E2F; E2F then binds to promoter region and upregulates proteins required for DNA replication (i.e. DNA polymerase)
S Phase
- DNA is replicated
- each daughter cell receives identical copies of genetic material
- number of chromosomes does not change – still 46 chromosomes
- amount of DNA doubles
- 2 identical chromatids makeup chromosome
G2 Phase
- cell must duplicate organelles
- G2 checkpoint
Mitosis
- occurs after/during G2 phase
- cell divides into two daughter cells
- steps:
(1) Prophase: nuclear membrane disintegrates, chromosomes condense, centrosomes migrate to opposite ends of cell
(2) Metaphase: chromosomes move toward center of cell on line called metaphase plate, centrosome sends out spindle fibers to attach to centromere of each chromosome
(3) Anaphase: centrosomes pull sister chromatids apart
(4) Telophase: nuclear membrane forms around each new set of 46 single chromatid chromosomes
(5) Cytokinesis: pinching of cell membrane until daughter cells separate
G0 Phase
- cells can enter here after G2/M
- non-dividing phase (not actively dividing or preparing to divide)
- phase outside the cell cycle
- Examples: Hepatocytes, Neurons
Proto-Oncogenes
stimulate cell proliferation
Cyclin bound to CDK ___ the progression of the cell cycle/proliferation
activates
Rb bound to E2F __ the progression of the cell cycle/proliferation
inhibits
Rb bound to E2F __ the progression of the cell cycle/proliferation
inhibits
ATM
- gene that produces different types of proteins/enzymes that sense DNA damage
- gene responsible for initiating proofreading of DNA
P53
- most important tumor suppressor gene
- gene that is activated when ATM derived proteins/enzymes sense damage/mistake in DNA; responds by inducing P21 which is the protein that inhibits formation of all cyclin/CDK complexes (cell unable to move thru cell cycle b/c of this)
- ensures only cells free of DNA errors can undergo cell division
- if cell contains DNA errors then P53 will arrest cell division until repair is made or it will trigger apoptosis of cell
- loss of P53 leads to accumulation of DNA mutations and eventually cancer
What cell phase in the cell cycle is Cyclin E at its highest concentration?
Checkpoint of G1/S
What cell phase in the cell cycle is Cyclin A at its highest concentration?
G2 Phase
What cell phase in the cell cycle is Cyclin B at its highest concentration?
G2/Mitosis
Rb
- present at restriction point in G1
- function: prevent excessive cell division by inhibiting cell cycle progression
- normally bound to transcription factor E2F (inactivates it) when cell cycle functioning normally/not actively dividing
- inactivated when phosphorylated by Cyclin E/Cdk2 complex
- if mutated, unable to bind E2F which leads to continuous activation of proteins required for DNA replication/cell division
G2/M Checkpoint
- occurs after replication and before mitosis
- ensures any potential DNA errors that occurred during replication are repaired before entering mitosis
Retinoblastoma
- tumor suppressor
- result of homozygous loss of function mutations in Rb gene (located on chromosome 13) – loss of function of Rb
- malignant tumor of retinal cells
- occurs exclusively in children
- presents with leukocoria (white pupil)
What 3 proteins inhibit the formation of Cdk4/6 / cyclin D complex?
- P16 (only works in G1)
- P21
- P27
Defects in cancer cells occur mostly in what phase?
G1
What are the 3 stages of Carcinogenesis?
(1) Initiation
(2) Promotion
(3) Progression
Initiation of Carcinogenesis
- irreversible
- no threshold – only need one mutation
- genotoxic event occurs that changes DNA sequence (ex. chemicals, radiation, ROS, viruses)
- can be result of activation of oncogenes or inactivation of TSGs
Promotion of Carcinogenesis
- occurs over long period of time
- reversible in early stages
- involves gene activation or repression such that the latent phenotype of the initiated cell becomes expressed through cellular selection and clonal expansion (2 types: one clone of cell goes uncontrolled, or multiclonal type occurs from multiple uncontrolled clones in same tissue)
- does have threshold
Progression of Carcinogenesis
- involves complex genetic changes (chromosomal translocations, deletions, gene amplifications)
- irreversible
- continuing change of the basically unstable karyotype
Hallmarks of Cancer
- do not require external stimulation to produce growth signals – self sufficient
- insensitive to growth inhibitory signals
- evade cell death
- sustain angiogenesis
- acquire capabilities to invade tissues and metastasize
- create genome instability
- promote inflammation
- avoid immune destruction
- reprogram energy metabolism
Oncogenes
- cellular genes whose expression stimulate cell division and/or growth
- result from “gain of function” mutation – remain constitutively active
- only need one of the two alleles to be activated for effects on cell growth to be felt
- Examples: CDK4/6, CDC25, Cyclin A, Ras, Myc
Tumor Suppressor Genes
- cellular genes that serve to check or inhibit cell division
- “brakes” on a car
- recessive
- in cancer: inactivating mutations, deletions, loss of expression
- germline inheritance involved in cancer development
- Examples: P53, Rb, P14ARF, P16, P27
Three forms of Oncogenes
- Cellular proto-oncogenes that have been captured by retro viruses
- Virus specific genes that behave like cellular proto-oncogenes that have been mutated
- Cellular proto-oncogenes that have been mutated
Oncogenes Signal Transduction
(1) Growth Factors
(2) Growth Factor Receptors
(3) GTP Binding Protein
(4) Transcription Factors
(5) Cell-Cycle Control Molecules
(6) DNA Repair Proteins
(7) Anti-Apoptosis Proteins
(1) PDGF, EGF, M-CSF
(2) PDGFR, EGFR, M-CSFR
(3) Ras
(4) myc, fos, jun, TR RAR
(5) Cyclins
(6) ATM
(7) Bcl-2, Bcl-x
Tumor Microenvironment (TME)
- Cellular components (non-cancerous cells): lymphocytes, B cells, marophages, fibroblasts, endothelial cells, epothelial-mesenchymal transitions
- Secretory Factors: signaling molecules, growth factor, inflammatory factors, enyzmes
- Extracellular Matrix: provide structure for multicellular environment
Role of TME
- barrier for therapy
- paracrine signaling
- desmoplastic reaction
- promoting tumor progression, therapy, resistance, and recurrence
Molecular Features of Breast Cancer
(1) Activation of human epidermal growth factor receptor 2 (HER2, encoded by ERBB2)
(2) Activation of hormone receptors (estrogen receptor, progesterone receptor)
(3) BRCA mutations
Estrogen Receptor Signaling Pathway
ERα and ERβ form hetero or homodimers upon binding to estrogen -> translocation of E with ER to cell nucleus -> ER dimers bind to ERE region of target genes and recruit co-regulators -> increased expression of genes that increase cellular proliferation
Breast cancer cells have high ER_ expression and low ER_ expression
- ERα
- ERβ
HER2 Signaling Pathway
ligands bind to HER2 (receptor tyrosine kinase) -> phosphorylation of tyrosine kinase -> follows AKT pathway or MAPK pathway -> cell cycle progression, survival, proliferation -> breast tumorigenesis
Increased head and neck cancers due to___
HPV
Pathway of Head and Neck Cancer
(1) HPV integrates into DNA
(2) production of E6 and E7 proteins
(3) (a) E6 binds to P53 which results in degradation of P53
(b) E7 binds to Rb, thus inactivating it
(4) (a) loss of tumor suppression by p53
(b) E7 stays bound to Rb but E2F is released and increases expression of genes that lead to proteins that activate cell cycle and proliferation
Pancreatic Cancer
- most common exocrine pancreatic neoplasm is pancreatic ductal adenocarcinoma
- k-ras mutation believed to be early genetic event, followed by loss of functional P16, P53, SMAD4 and many other changes
Cell Death can Result from Damages to Cellular Targets:
(1) cell membrane
(2) cellular organelles like mitochondria and ER
(3) metabolic alterations
What are the 3 types of cell death?
(1) Type I: Apoptosis – programmed cell death
(2) Type II: Autophagy – self destruction/digestion
(3) Type III: Necrosis – explosive disaster
What two types of cell death are specific to cancer cells?
- Mitochondria Catastrophe
- Senescence: irreversible growth arrest, reproductive cell death
Necrosis
- uncontrolled form of cell death
- characterized by swelling and rupture of cell membrane, organelle degradation, mitochondrial swelling, clumping and random degradation of nuclear chromatin and DNA (karyolysis)
- all cell types involved
- inflammation: yes
- occurs when there is extensive failure in maintaining cellular homeostasis
- two players involved in mechanism that both target mitochondria: Receptor interacting protein 1 (RIP1), ADP-ribose polymerase 1 (PARP-1)
- results in Ca2+ overload, mitochondrial uncoupling, increased O2 consumption, excessive ROS
Apoptosis
- programmed cell death
- characterized by cell membrane blebbing and fragmenting into membrane bound apoptotic bodies, fragmentation and shrinkage of cytoplasm, chromatin condensation and degradation
- cells involved: hematopoietic cells and their malignant counterparts
- inflammation: no
- triggers:
(1) DNA damage: activation of ATM, P53
(2) Death receptor signaling (CD95, Fas receptor, TNF receptor, caspase-8)
(3) Cell membranes: activation sphingomyelinase and leading to hydrolysis of sphingomyelin to ceramide
(4) Mitochondrial damage: ceramide-mediated process
Extrinsic Pathway of Apoptosis
(1) Fas ligand binds to Fas receptor
(2) Dissociation of Casp-8 from FADD
(3) Activation of Casp-3
(4) Casp-3 cuts up proteins
(5) cell death
Intrinsic Pathway of Apoptosis
(1) DNA damage
(2) Activation of ATM
(3) Activation of P53
(4) P53 drives expression of BH3 proteins
(5) Bax and Bak proteins effect mitochondria to release cyt. c
(6) Cyt c bind to and activates effector Casp-7
(7) Casp-3 cuts up proteins
(8) Cell death
Autophagy
- self-eating and recycling
- process responsible for degrading long-lived proteins and cytoplasm organelles, products recycled to generate macromolecules and ATP to maintain cellular homeostasis
- survival mechanism in response to stresses like DNA damage, mitochondria, protein aggregation, pathogens, nutrient starvation
- characterized by membrane blebbing, accumulation of two-membrane autophagic vacuoles in cytoplasm, partial chromatid condensation
- all cell types involved
- no inflammation
- caspase-independent process
- mechanism:
(1) Beclin initiates formation of nucleation complex
(2) LC3 initiates conjugation process
(3) Conjugation continues until there is production of autophagosome
Mitotic Catastrophe
- type of cell death caused by aberrant mitosis
- associated with deficiencies in cell cycle checkpoints
- characterized by no change in cell membrane, larger cytoplasm with formation of giant cell, micronucleation and multinucleation, premature chromosome condensation
- occurs in most dividing cells
- no inflammation
- abnormal CDK1/Cyclin B activation
Fate of cells that undergo mitotic catastrophe
- die without exiting mitosis – mitotic death
- proceed to G1 and continue division for many cycles then undergo cell death – delayed cell death
- exit mitosis and undergoes permanent G1 arrest – senescence
Senescence
- permanent cell cycle arrest, reproductive death
- can be due to telomere shortening
- anti-transformation mechanism due to cellular damage
- characterized by no change to cell membrane, flattening and increased granularity of cytoplasm, distinct heterochromatic structure
- all cell types
- inflammation occurs due to induced secretory factors from the senescent cell itself
- two pathways: P53 - P21, or P16 - Rb
