Lecture 2 REVISION Flashcards
Why do cancer cells divide uncontrollably
Circumvent normal regulation of cell cycle
What are oncogenes
Gene whose presence results in cancerous phenotype
Dominant at cellular genetic level
Derive from cells normally involved in regulating proliferation (proto-oncogene) through mutation or misregulation
History of oncogenes
Early 1900s:
- Presumed cancer was infectious through a virus
- Later discovered this was not the case as epidemilogy didn’t match and no viruses found in tumours
Only certain cancers e.g. cervical and liver carcinomas known to be caused by viruses
1970s:
test created to test cancer is genetic hypothesis:
- Chemical carcinogens added to mouse/human cells
- DNA isolated from transformed cells and transfected back in human cells using calcium phosphate co-precipitation
- Cells became cancerous after a few weeks, and started growing abormally
- Cells injected into mice and tumours began to form, proving genetic origins
How are oncogenes activated?
Deletion or point mutation in coding sequences - hyperactive protein made in normal amounts
Regulatory mutation - overproduction of protein
Gene amplification - overproduction of protein
Chromosomal rearrangements - nearby DNA sequences cause protein overproduction OR fusion to actively transcribed gene produces hyperactive fusion protein
Types of oncogenes
Growth factors
Growth factor receptors e.g. receptor tyrosine kinase
G proteins
Intracellular serine/threonine kinase
Intracellular tyrosine kinase
Transcription factors
Negative regulators of apoptosis
Growth factor signalling
Growth factors bind to growth factor receptors
Enzymes activated e.g. kinases
Signalling cascades activated
Transcription factors activated which transcribe DNA
TGF-a signalling
Normal cells - paracine or endocrine signalling
Cancer cells autocrine signalling
Normal cells:
TGF-a is secreted from a mesenchymal cell, where it binds to a receptor tyrosine kinase called an epidermal growth factor receptor to stimulate downwards signalling in the epithelial cell, where the receptors dimerize. At the same time, platelet derived GFs are secreted from an epithelial cell which is bind to PDGR-R located on the surface of a mesenchymal cell, stimulating downwards signalling in the mesenchymal cell to allow for cell proliferation and growth
Cancer cells:
Transforming growth factor-a is secreted from epithelial cells and bind to EGF-a on the surface of the same epithelial cell, leading to uncontrolled cell growth
What cancers does TGFa cause?
Produced by lung, prostate, pancreatic, mesothelioma and breast cancers
Other examples of cells that undergo autocrine signalling in cancer
SCF (Kit), VEGF-A, (VEGF-R) HGF (Met), NRG (HER2/HER3) etc
Normal vs cancer growth factor receptors
Deregulation of receptor firing in cancer
Normal cells:
Growth factor binds tyrosine kinase receptors causing them to dimerize, forming a ligand-dependent firing receptor after dimerization
Cancer cells:
Caused by mutations affecting structures or overexpression of receptors:
Mutations affecting structure- ligand not required for dimerization and signalling leading to excessive cell proliferation
Overexpression of receptors means more growth factors bind to more receptors - increasing the quantity of signalling of the receptors to stimulate increased cell proliferation
G-proteins
- large family of proteins activated by binding GTP.
- Prominent subfamily of G-proteins are monomeric small GTPases - Ras prototypical member
-Bind GTP/GDP - GTP in ‘on’ form, GDP for ‘off’ form
- regulated by Guanine nucleotide exchange factors (GEFs, activate)
- or GTPase activating proteins, inactivating
Ras in cell signalling
Three genes encoding 4 21kDa ras proteins:
HRAS, NRAS, KRAS4A, and KRAS4B
Inactive Ras-GDP ->
Guanine exchange nucleotide factors e.g. SOS activate Ras and upstream stimulatory signal
GDP->GTP->
Active Ras-GTP undergoing downstream signalling->
GTP hydrolysis and Ras inactivation induced by GTPase activating proteins e.g. NF1, Pi released->
Inactive Ras-GDP
In cancer, oncogene blocks GTP hydrolysis and Ras inactivation by GAP
How do adaptor proteins link RTK to RAS
First way:
phosphate group on RTK binds SH2 which is bound to Grb2 and two SH3
SH3 binds the guanine nucleotide exhange factor SOS
Causes GDP -> GTP on Ras to allow downstream signalling
Second way:
- Phosphate group on receptor tyrosine kinase bind SH2-Shc
- Phosphate group on SH2-Shc binds SH2-Grb2 bound to 2 SH3 groups
- SH3 groups bind SOS
- Causes GDP -> GTP on Ras to allow downstream signalling
RAS hotspots
9787 tumours on amino acid 12
Ras mutation frequency
HRAS - 9% of cervical cancers, 15% of head/neck cancers
NRAS - 18% skin cancers, 10% haemopoietic cancers, 8% thyroid cancers
KRAS - 57% pancreatic cancers, 33% colorectal, 31% biliary tract, 20% small intestine, 17% lung, 14% ovary, 14% endometrium
B-raf V600E and V600K mutations in melanoma
In G0->G1, RAS stimulates the production of RAF
In cancer, due to Ras being stuck in the active phase, more downwards signalling stimulatyes the production of more Raf
This causes Raf to produce more cyclin D1-CDK4 or CDK6, which then causes more Phosphorylated Rb production
This causes more cells to later enter mitosis, increasing cell proliferation and division
bcr-abl oncogene
Translocation between chromosomes 9 and 22 causes ‘Philadelphia’ chromosome and bcr’abl oncogene
This encodes for the Bcr-Abl fusion protein, a constitutively active tyrosine kinase
Found in >95% of chronic myelogenous leukaemia
bcr-abl pathway
Kinase domain on Abl on BCR-ABL dimer transactivate
Cytoskeletal proteins produced, leading to altered adhesion/motility
Phosphorylated STAT5 causes survival and proliferation
Degradation of phosphorylated p27 causes proliferation
BCL-XL causes survival
Myc transcription factor
- Regulates expression of genes involved in promoting cell proliferation and survival
Becomes oncogene by:
- Myc locus amplified frequently in various laukaemias and carcinomas (10-50%)
- Expression of myc deregulated in Kurkitt’s lymphoma by chromosomal translocation between IgH gene on chromosome 14 and myc proto-oncopgene on chromosome 8.
Myc overexpression
- Over 70% of cancers overexpress Myc or one of its homologues (N/L-Myc)
> 10 copies of N-myc severly decreases event free survival of neuroblastoma patients
