Wnt Flashcards
What is the epidemiology of colorectal cancer?
- affects more males
- usually 60+
- incidence in the younger population is increasing
What is the structure of the coloretcum?
- 3 layers with crypts
- singular layer of epithelial cells
- stem cells in the niche at the botton
- stem cells are pushed up and out and differentiate as they move upwards
- express LGF5 at the bottom to maintain their stem phenotype
Describe colorectal tumorgenesis
- ACF - adenoma - carcinoma
- multistage disease
- a rise in stem cells or dedifferentiation
- stage 1 has just pushed through the basement membrane
- stage 4 is deep into the bowel and involves other organs through metastasis or local spread
What are the genetics behind colorectal cancer?
- loss of APC is required - both alleles
- loss of one forms aberrant crypt foci where a second loss will form early tumours
- APC loss means cells become more long-lived and develop further mutations and epigenetic changes to help them outcompete their WT neighbours
What is MMR?
- mistmatch repair
- loss of this through mutations leads to a hypermutated phenotype as repair can’t stop accumulation of mutations
Does order of mutations affect colon cancer?
- previously thought not to
- ut when mutations are added in different orders into colon cells it affects the cancer phenotype produced
- APC first leads to higher proliferation and faster tumour formation
Describe the basic steps of tumour formation in the colon.
- loss of APC or B-catenin leads to transformation of normal crypts into ACFs
- accumulation of mutations such as ras, p53 increase genetic instability and progression towards invasive carcinoma
What is crypt fission?
- ACF split as they get bigger
- propagates mutated epithelial cells into other crypts
Why is it important to know that most carcinomas are derived from adenomas?
- early detection and screening
- foci to carcinoma can take 10-15 years
- can remove foci before carcinoma
What is FAP?-
- famililal adenomatous polyposis
- inherited one defective copy of APC
- 15% of colorectal cancers are as a result of inherited predispositions such as FAP
- many polyps by 20, cancer by 30-40
- can help with constant screening or removal of the large bowel
How is APC mutated?
- 95% of APC mutations create stop codons that give rise to inactivated, truncated APC protein
- both APC alleles are disrupted in cancer formation - loss of heterozygosity
What occurs in the cell when Wnt signaling is off? (no Wnt ligand)
- B-catenin is constantly produced and degraded
- binds the destruction complex (APC scaffold, GSK-3 and more)
- gets phosphorylated by GSK-3 and allows BtrcP to recognise it
- tags B-catenin for proteasomal degradation by ubiquitin
What happens in cells when Wnt signalling is turned on?
- FZD binds CRP5/6 and brings DVL to the membrane causing a conformational change in the destruction complex
- it can no longer bind B-catenin and it builds up, moves to the nucleus and interacts with TFs to drive gene expression
What kinds of genes does B-catenin activate in Wnt signalling?
- those involved in cell growth and proliferation
- c myc
- also genes involved in Wnt negative feedback
How does Wnt signalling negative feedback occur?
- wnt target genes include E3 ligase and RNF43 and others
- these ubiquitinate FZD receptors and lead to their degradation by stopping wnt signal transmission
- RNF43 moves the destruction complex back to its original location and form where it can degrade B-catenin again
What is LGR5?
- protein overexpressed in stem cells
- helps to maintain the stem cell phenotype
How is Wnt signalling different in stem cells in the intestinal crypt?
- R-spondins bind LRG5 and RNF43 promoting its degradation
- prevents FZD promoter clearance and increases B-catenin and gene expression
- only occurs in stem cells with lots of LGR5
Why is Wnt signalling important in colorectal cancer?
- > 90% of colorectal cancers have Wnt mutations
- 50-80% of those also have APC mutations
How do APC mutations (or other destruction complex parts) affect Wnt signalling?
- inactivation of APC - loss of B-catenin degradation APC not there for form a scaffold
- other components can also be mutated but these are mutually exclusive suggesting they have functional equivalence
What is the serrated pathway to colorectal cancer?
- mutations in RNF4 can prevent ubiquitination of the FZD receptor
- leads to persistent signalling and B-catenin accumulation
- often occurs in combination with B-catenin mutations
- produces a different cancer morphology
What kinds of processes and genes do B-catenin affect? specific genes
- proliferation cmyc
- differentiation decreased E-cadherin
- migration and invasion, survival
- stemmness LGR5
How could the Wnt signalling pathway be targeted to treat colorectal cancer?
- blocking cells reliant on single pathways such as Wnt
- R-spondin inhibitiors, GSK-inhibitors,Wnt mimetics, B-catenin inhibitors and many more
- can have side effects as Wnt is important in all stem cell niches
