Flashcards in lecture 32 Deck (43):
- relate the role of important factors in normal vs cancer settings
- appreciate the various signals contributing to carciogenesis
- understand the role of oncogenes and tumour suppressor genes (TSGs) at the cellular lvel
- understand the basic genetics/heritability of major oncogenes or TSGs
- relate the various signalling pathways and genes in the context of complex networks
- know some of the major events that can trigger carcinogenesis
- learn some basic terminology specific to cancer causing genes
How have 5-year relative survival, incidence and mortality changed?
- 5 year: increasing
- incidence: increasing
- mortality: decreasing
What pathways are almost always altered in cancer?
- signalling pathways controlling cell survival, growth and differentiation and even metastatic potential are almost invariably altered in cancer
- it is components of these tumour 'specific' (a better term is 'enriched') signalling pathways which differ from 'normal' which need to be therapeutically targeted
What do we mean by signalling?
- transmission of information from one cell to another
- transmission of information from the 'environment' to a cell
- biological communication at the level of subcellular/molecular
What was the view of the hallmarks of cancer in 2000?
- self-sufficiency in growth signals
- insensitivity to antigrowth signals
- tissue invasion and metastasis
- limitless potential for replication
- sustained angiogenesis
- evading apoptosis
What further mechanisms have been added?
- deregulating cellular energetics
- avoiding immune destruction
- tumour-promoting inflammation
- genome instability and mutation
What are the molecular pathways in cancer?
- pathway circuitry dictates biological outcome and therapeutic response – especially important to understand this is cancer
- signalling pathways are very complex
- but can be divided into particular 'circuits'
→ cytostasis and differentiation circuits
→ motility circuits
→ proliferation circuits
→ viability circuits
- having drugs that target different circuits allows for more effective therapy
What are the major molecular events in cancer evolution?
environmental agents that damage DNA
normal cell ↔ DNA damage
↓ failed repair (← inherited mutations in genes affecting: DNA repair, cell growth, apoptosis [only about 15%])
mutations in somatic cells
1. activation of growth promoting oncogenes
2. impaired apoptosis
3. inactivation of tumour suppressor genes
altered gene products (proteins); abnormal structural and regulatory proteins
What are general cellular features typically seen in cancer?
- evolve slowly
- normal tissue has a very ordered structure of the different types of cells over different layers
- organisation in terms of each cell
- lose the organisation of the cells → dysplasia
- dysplasia also appears in benign tumours
- disorgisation/disordered signalling within cells becomes so problematic that the cells can break away, survive away from that tissue, metastasise to a different site
What genes are typically involved in cancer?
- four classes of normal regulatory genes are the prinicipal targets of genetic damage relevant in carcinogenesis:
- genes involved in DNA repair
- growth-promoting proto-oncogenes
- growth-inhibiting tumour suppressor genes
- genes that regulate programmed cell death (i.e. apoptosis)
→ remember that in almost all cases of carcinogenesis, all classes of genes are involved and the pathways of which they are part cooperate/interact
What are types of mutations in cancer?
- errors in DNA replication not repaired – DNA genes e.g. BRCA1 and BRCA2, leads to accumulation of erros – some genomic regions are more prone to this: mutations hotspots in oncogenes, tumour suppressor genes (TSGs), regulatory regions (of oncogenes, TSGs), controlling levels of expression
- point mutations – activating in oncogenes; inactivating in TSGs
- amplification of oncogenes (multiple copies)
- chromosomal rearrangements
What is the role of DNA repair genes in cancer?
- aberrant function of this gene class may be an early evenet or the event that allows the rapid accumulation of secondary etc mutations
- affecting genes encoding oncogenes and TSGs
BRCA1 and BRCA2 and homologous recombination proteins involved in repairing double-strand breaks
mutations in these genes → breast, ovarian and pancreatic cnacer
treated with PARP inhibitors, platinum salts
MSH2 and MLH1 are mismatch repair proteins that repair things such as base mismatches, insertions and deletions
What are types of DNA damage?
- single strand break
- double strand break
- bulky abducts
- base mismatches, insertions and deletions
- base alkylation
What is a mutation?
- a mutation is any change in a DNA sequence away from normal
- this implies there is a normal allele that is prevelent in the population and that the mutation changes this to a rare abormal variant
- wild-type proto-oncogene → mutated oncogene (e.g. via point mutation) → constitutively active protein
What are oncogene amplifications?
- e.g. N-MYC
- multiple copies
- break off of N-MYC genes forming mini-chromosomes called doube minutes
What are gene translocations and fusions?
- e.e. BCR-ABL
- geneation of oncogenic chimaeric molecules
- chronic myelogenous leukaemia
- ABL (chr 9 → chr 22)
→ tyrosine kinase
What are classic immunohistochemical markers for cancer?
- proliferation markers
- PCNA (proliferating cell nuclear antigen)
- Ki-67 (aka MIB-1) → the name reflects the city of origin (Kiel, Germany) and the number of the clone recognising a specific antigen in Hodgkin lymphoma
What is the rate of neoplastic growth?
- how long does it take for one transformed cell to produce a clinically detectable tumour containing 10^9 cells (1g)?
- if everyone of the daughter cells remained in cell cycle and no cells were shed or lost, to reach 1g tumour need 90 days (30 population doublings, with a cell cycle type of ~3 days)
- 10 more doublings would yield 10^12 (1kg)
in reality takes much longer: not all tumour cells divide; some die/differentiatiate
- slow initation phase, cells still sensitive to microenvironment, don't yet have a blood supply
- vascularised tumour: increased rate of growth, angiogenesis important step
- vascularised tumour with central necrosis : tumour is too big : plateau of growth
What is steps in normal proliferation?
- growth factor binds to its specific receptor
- transient, limited activation of the growth factor receptor with signal transduction
- transmission of signal across the cytosol to nucleus via second messengers or signal transduction cascade
- initiation of DNA transcription
- entry and progression into the cell cycle
transcription → translation ↓
function – cell proliferation
What are steps in tumour cell proliferation?
- many steps in the pathway can be mutated or changed
- e.g. mutant receptor that no longer needs a ligand/growth factor to signal
- mutant intracellular kinase: don't need receptor ?
- mutant TF
- three major point
What are proto-oncogenes?
- normal cellular genes whose products almost always promote ___ and/or supress ___ (e.g. differentiation)
- tumour cells typically repress differentiation
What are oncogenes?
- mutant versions of proto-oncogenes that function autonomously without a requirement for normal growth-promoting signals
What are oncoproteins?
- proteins encoded by oncogenes
What are oncogeneic factors?
- growth factors → over-expression
- growth factor receptors
→ over-expression or always active/"on"
- signal transduction proteins
→ intermediates in cascade, especially G-proteins, phosphorylases, kinases
- transcription factors
- cyclins and CDKs
→ uncontrolled cell cycle progression
What is the PI3 kinase pathway?
- growth factor binds receptors with intrinsice tyrosine kinase activity
→ PI3 kinase
→ PI3 kinase pathway ⊢Pten (TSG)
→ Akt (PKB)
→ transcription factor activation
loss of Pten and mutation in pathway → completely uncontrolled tumour growth
What are oncogenes and TSGs?
- opposing factors in cell proliferation and carcinogenesis
- oncogene mutations cause uncontrolled growth by accelerating growth
- TSG mutations cause uncontrolled growth by allowing continuous growth
- Oncogenes = Her2-neu, Ras, Myc
- TSGs: P53, Rb, APC, PTEN
What is needed to promote carcinogenesis?
- only 1 allele of oncogenes needs to be activated/mutated
- tumour suppressor function must be lost, so both alleles must be affected
gain of function vs loss of function
What was seen in combined PI3K pathway mutated mice?
- combined PI3K pathway oncogeneic mutation and tumour suppressor loss = cancer
- Pik3ca ~25%
- Pten-DEL ~50%
- tumour much bigger in double mutant rather than single gene mutants
- single gene mutants displayed little change in ovary size
What are TSGs?
- TSGs encode proteins that inhibit cellular proliferation by regulating the cell cycle directly (e.g. Rb, p53) or inhibit oncogenic pathways (e.g. Pten)
- unlike oncogenes, both copies of the gene must be lost for tumour development, leading to loss of heterozygosity (LOH) at the gene locus
- in cases with familial predisposition to develop tumours, the affected individuals inherity one defective (non-functional) copy of a tumour suppressor gene and lose the second one through somatic mutation
- in sporadic cases both copies are lost through somatic mutations
What is loss of heterozygosity (LOH)?
- LOH in a cell represents loss of normal function of one allele of a gene in which the other allele was already inactivated
- this is a general genetic feature involving tumour suppressors in the 'evolution' of cancer development
What is Knudson's model?
- two hit hypothesis for the generation of RB → LOH
- occasional deletion of one of the two RB genes
→ occasional inactivation of other functional RB gene copy
→ excessive cell proliferation, leading to retinoblastoma
→ tumour formation
What mechanisms regulate TSG expression?
an array of mechanisms
miRNA mediated control
- miRNAs are non-coding single-stranded RNAs approximately 22 nucleotides in length, that function as negative regulators of genes
- dna methylation of promoter region preventing expression of TSGs
What happens to the cell cycle in cancer?
- cancer involves uncontrolled cell division
- cell division is controlled by a mechanism = cell cycle
- mutations in certain types of genes may lead to cancer because they directly/indirectly affect the cell cycle
ergo cancer is a disease of the cell cycle
- cell cycle checkpoints pertubed in cancer and their regulation by opposing factors: oncogenes, TSGs
- pRB at G1 checkpoint
- p53 and S phase checkpoint
- Ras and Myc (oncogenes) at G1 and G2 checkpoints
What is the role of the cell cycle?
- regulate the growth and mitotic phases
- ensuring faithful replication and segregation of the genetic material
- highly regulated by many factors - oncogenes and TSGs are the factors most often mutated in cancers
What is p53?
- 'the guardian of the genome'
- p53 is a transcription factor which can regulate the expression of cell cycle factors
- p53 targets → apoptisis, DNA repair, cell-cycle arrest, differentiation
- defective when p53 mutated or deleted - 30-80%
How can the evasion of apoptosis occur?
1. reduced CD95 level (Fas death receptor)
2. inactivation of death-induced signalling complex by FLICE protein
3. up-regulation of BCL2 (anti-apoptotic)
4. reduced levels of proapoptotic BAX resulting from loss of p53
5. Loss of APAF-1 (??)
6. up-regulation of inhibitors of apoptosis
extrinsic and intrinisc pathways of apoptosis
both pathways affected in cancer
What are telomeres?
- link between ageing, limitless replicative potential and cancer
- in normal somatic cells, which have low telomerase activity, the shortened telomeres generated by cell division eventually activate cell cycle checkpoints, leading to senescence and placing a limit on the number of divisions a cell may undergo
- in cells that have disabled checkpints, DNA repair pathways are inappropriately activated by shortened telomeres, leading to massive chromosal instability and mitotic crisis
- tumour cells reactivate telomerase, thus staving off mitotic catastrophe and achieving immortality
What is metastasis?
1. detachment of tumour cells from each other
2. degradation of ECM
3. attachment to novel ECM components
4. migration of tumour cells
- there are certain tissues that are more prone to this
What are the molecular mechanisms of metastasis?
adherence molecules linked to cancer signalling pathways
- defective signalling pathways in their interactions with other cells
- e.g. E-cadherin
- E-cadherins normally involved in maintaining very tight junctions between cells - completely lost in cancer cells or dysregulated
- allow escape from primary site
What is angiogenesis?
- development of new blood vessels
- cancer cells produce lots of VEGF/VEGF-R (involved in recruiting endothelial precursor cells)
- significant target of new generation drugs
→ sorafenib (Nexavar)
→ sunitinib (Sutent)
→ paxopanib (Votrient)
→ everolimus (Afinitor)
- resistance to these drugs often occurs
Are tumours homogenous?
- tumour cell heterogeneity
- a galapagis isle of disease
- what's the cellular basis?
- genomic evolution
- cancer stem cells
- real challenge in targeting cancers
What are cancer cell lineages and cancer stem cells?
→ allow human tumour growth when transplanted into an immunodeficient mouse
→ 0.1% to 2% of the total cellularity but could be as high as 25%
cancer stem cells arise from:
- normal adult tissue stem cells → not likely ??
- transit amplifing/precursor cells
cancer stem cells have a high intrinsic resistance to convential therapies
changed the paradigm of how to target tumours
- good at designing drugs that target the mass not the stem cells (i.e. cellular basis of tumour)