lecture 32 Flashcards
Objectives?
- 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
or - 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?
emerging hallmarks
- deregulating cellular energetics
- avoiding immune destruction
enabling characteristics
- 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 - chemicals - radiation - virsuses ↓ 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 ↓ malignant tumour
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
e.g.
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
→ colorectal
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
GF ligand ↓ GF receptor ↓ intracellular kinase ↓ 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
why
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
epigenetic control
- 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
medical significance:
- highly regulated by many factors - oncogenes and TSGs are the factors most often mutated in cancers
What is p53?
- TSG
- ‘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?
- reduced CD95 level (Fas death receptor)
- inactivation of death-induced signalling complex by FLICE protein
- up-regulation of BCL2 (anti-apoptotic)
- reduced levels of proapoptotic BAX resulting from loss of p53
- Loss of APAF-1 (??)
- up-regulation of inhibitors of apoptosis
extrinsic and intrinisc pathways of apoptosis
both pathways affected in cancer
What are telomeres?
- immortality
- 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?
- detachment of tumour cells from each other
- degradation of ECM
- attachment to novel ECM components
- 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?
- no
- 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?
tumour-initiating 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)
What drugs target specific ‘hallmarks of cancer’?
- aerobic glycolysis inhibitors → deregulating cellular energetics
- EGFR inhibitiors → sustaining proliferative signalling
- CDK inhibitors → evading growth suppressors
- immune activating anti-CTLA4 mAb → avoiding immune destruction
- telomerase inhibitors → enabling replicative immortality
- selective anti-inflammatory drugs → tumour promoting inflammation
- Inhibitors of HGD/c-Met → activating invasion and metastasis
- inhibitors of VEGF signalling → inducing angiogenesis
- PARP inhibitors → genome instability and mutation
- proapoptotic BH3 mimetics → resisting cell death
