Signalling Pathways Flashcards
What is Familial Adenomatous Polyposis Coli?
condition with 100s of adenomatous polyps in the large intestine (caused by inheriting 1 mutant APC & 1 wild-type APC allele) epithelial cells in crypts of colon
What is the APC Protein?
Cytoplasmic, mutated in most (~80%) colonic carcinomas and adenomas
APC regulates the Wnt signaling pathway – a crucial pathway both in development and the maintenance of tissue organization. Critical in cell-to-cell signalling.
APC controls beta-catenin levels (signalling intermediate)
What is beta- catenin?
- at plasma membrane it forms a complex with E-cadherin in adherens junctions- act as transcription factor
- in the cytoplasm it’s a “free” protein that is phosphorylated (by GSK3B) and rapidly degraded by the proteasome
– APC promotes this phosphorylation & degradation
Mutant APC (truncated) allows accumulation of beta-catenin (reduced degradation)
What is the Wnt signalling pathway?
Wnt is a paracrine growth factor.
b-Catenin binds to TCF in the nucleus.
The activity of the b-Cat/TCF complex is a master switch in intestinal crypts controlling proliferation versus differentiation (upregulates c-myc, cyclinD1 & others).
Pathway deregulation occurs in many cancers, involving different players in individual cancers (e.g. inactivating APC mutation or activating b-Catenin mutation).
Describe the Wnt signalling pathway
Wnt ligand binds to receptors, intermediates inhibit GSK3B, unphosphorylated b-catenin accumulates, translocated to the nucleus and transcription of growth promoting genes occurs, via transcription factor TCF-4
Why don’t cancers die by apoptosis?
- p53 inactivation
- Telomerase activation
- Bcl2 overexpression
Examples of apoptotic pathways
Death Ligand Fas L binds to death receptor Fas, death inducing signalling complex causes activation of caspase cascade
Cellular stress/ DNA damage, mitochondrial release of cytochrome c, formation of apoptosome, activation of caspase cascade
How can apoptotic pathways be targeted in cancer treatment?
In tumour cells at least one death pathway remain intact and the apoptosis evasion mutations are targeted (not the caspases): inactivation of genes encoding the receptors (Fas, TNFR) or activation of anti-apoptotic proteins such as Bcl-2 or downregulation of pro-apoptotic proteins (Bax), as well as p53 inactivation (-> Bax, PUMA, NOXA).
Why is genetic instability important?
- Transition from normal growth regulated cell through to a malignant cancer cell requires several mutations- at least 6
- The probability of spontaneously acquiring >6 mutations in a single cell, basis of “background” mutation rates would require life span of several 100 years.
- Something has to speed up the mutation rate -the acquisition of genetic instability- mutator phenotype.
What is the evidence for the mutator phenotype?
- Most solid tumours are aneuploid – they have abnormal chromosome numbers and chromosome rearrangements
- Most cancers have deregulated the p53 pathway that arrests the cell cycle to allow DNA repair
- Mutations in genes encoding DNA repair proteins gives an increased susceptibility to cancer
Name the major known DNA repair activities
(a) DNA Mismatch Repair
(b) DNA Nucleotide Excision Repair
(c) DNA Strand Break Repair
(d) Fanconi Anaemia DNA Crosslink Repair
(e) Chromothripsis & (f) Kataegis
Describe DNA Mismatch Repair
•MMR corrects mismatched bases (C-T instead of C-G)
•MMR corrects insertion / deletion loops that most
commonly occur where short sequences are repeated
e.g. AAAAAAA or CACACACA – microsatellite sequences
Such repeats are common in the human genome
What happens in the absence of DNA mismatch repair?
mutation rate increases 100-1000x
If mutations occur in a coding region -> mutant protein
Mutations in mismatch repair genes (MLH1, MSH2) occur in Hereditary Non-Polyposis Colorectal Cancer (HNPCC) syndrome ~2-5% of all colorectal cancers (aka Lynch syndrome)
Sporadic: MLH1 silencing (promoter methylation) ~15% colorectal cancer
How does hypermutation lead to defective mismatch repair?
Proofreading domain, Poly enzyme
Complexes synthesise DNA with checking, has right shape
Cant recognise when it puts on wrong base
Describe Nucleotide Excision Repair
Adjacent thymine crosslinked by UV to make thymine dimer/ carcinogen attached to base
Excision of damaged or altered bases
Defects- Xeroderma Pigmentosum
