Colon Cancer Flashcards
What type of cancers make up 95% of colorectal cancer cases?
Adenocarcinomas
- Neoplasia of epithelial tissue that begins in gland forming cells that produce mucus
- The colon has numerous glands within the tissue
- When these glands undergo genetic change they move from benign to invasive, malignant colon cancer
- Colon cells lose the APC (Adenomatous Polyposis Coli ) tumour suppressor gene and become a small polyp
What is the most common gene mutation in colorectal cancer?
APC
(Adenomatous polyposis coli)
- The APC protein is a negative regulator that controls β-catenin concentrations and interacts with E-cadherin (involved in cell adhesion).
- For cancer to develop, both alleles of APC must be mutated.
- FAP - caused by inherited, inactivating mutation of APC gene, most of these mutations cause a truncated and nonfunctional APC.
- This shortened protein cannot suppress the cellular overgrowth that leads to the formation of polyps, which can become cancerous.
What condition is caused by an inherited, inactivating mutation in the APC gene?
Familial adenomatous polyposis (FAP)
- Inherited condition in which many adenomatous polyps form in the epithelium of the large intestine.
- These polyps begin as benign but undergo malignant transformation into colon cancer.
- A flawed APC gene inhibits the tumour suppressor properties of APC.
How many deaths does colorectal cancer cause per year in the UK?
15-20,000 deaths/year in the UK
What are the main roles of the small intestine?
Small Intestine
- Majority of digestion and absorption of food occurs.
- Three distinct regions: duodenum, jejunum and ileum.
- Villi project out into the lumen of the gut to maximise nutrient absorption.
What are the main roles of the large intestine?
Large Intestine
- Water is absobed
- Remaining waste material is stored as feces before being defecated
- No villi, but epithelial lining with crypts
Why are gut epithelial cells predisposed to become cancerous?
Intestine Epithelial Cells
- Cells are lost from the tips of SI villi and the lining of the S and L intestine.
- Groups of continually dividing cells at base of crypts.
- Stem cells produce undifferentiated cells.
- The transit amplifying compartment allows these non-differentiated cells to divide.
- Cells migrate upwards out of the crypt towards the epithelial lining surface.
What defence mechanism against cancer does the colorectal system possess?
Shedding/apoptosis of gut lining
- Almost all mutated cells will die soon after formation.
- This may be an escape mechanism from the development of a tumour.
- For a tumour to form, cells must escape apoptosis and avoid being shed.
How does a mutation within a colorectal cell lead to tumour formation?
- Mutation occurs in stem cell triggering APC loss.
- Cells no longer migrate out, instead remaining embedded in the crypt.
- A mass of continually dividing cells arises that are not shed and do not experience apoptosis.
- These cells divide and produce new stem cells in population.
- The adenoma grows and gives rise to more stem cells allowing for continuous growth.
What are the two types of inherited colorectal cancer?
- FAP (Familial Adenomatous Polyposis Coli)
- HNPCC (Hereditary Non-Polyposis Colorectal Cancer)
What are the key stages of colon tumour progression?
- The normal colorectal epithelium experiences a loss of APC and LOH (loss of heterozygosity) on Chr.5q
- This leads to a state of hyperplastic epithelium triggering DNA hypomethylation
- Masses of tissue develop and K-ras is activated. These adenomas (benign tumours) progress from Early → Intermediate → Late stage
- There is LOH Chr.18q and Loss of 18q TSG
- There is LOH Chr.17p and Loss of p53
- A carcinoma develops leading to invasion and metastasis
What is APC?
APC stands for Adenomatous Polyposis Coli
- APC is the ‘gatekeeper’ of cellular proliferation.
- APC is a large protein (> 300KDa)
- The protein can form oligos with itself, forming large complex structures.
- The middle third of the APC protein mediates β-catenin binding.
- The carboxy terminus contains a basic region that can bind microtubules and thus interact with cell scaffolding.
- Colon cancer cells with mutant APC have high levels of intracellular β-catenin.
- Mutations in APC, a tumour suppressor gene, can lead to the formation of benign polyps on the epithelial surface of the intestinal tract.
Where do the majority of APC mutations occur?
Most APC mutations occur within the mutator cluster region (MCR)
- The majority of these mutations lead to truncated proteins.
- Truncated proteins contain ASEF and β-catenin binding sites in the armadillo repeat domain.
- They lose β-catenin regulator activity (located in 20-aa repeat domain).
How many new cells do the crypts of the gut produce each day?
200-300 new cells/day
What is the purpose of the stem cells in the gut?
The stem cells produce replacement cells near the base of the crypt.
Why is the lining of the gut single cell layered?
The single layer allows for maximisation of absorption.
In a mouse how many cells does a single crypt contain?
~250 cells, one of which is a stem cell.
150 of these are proliferating.
How long does it take to replace the lining of the gut?
~3-4 days
What mechanisms must a mutated cell avoid in order to develop a tumour?
- Escape apoptosis
- Escape out-migration and shedding
If these cannot be avoided, the mutated cell is likely to be shed, digested and destroyed.
How many deaths does colorectal cancer cause in the UK a year?
15-20,000
What are colorectal polyps?
They are essential clusters of cells on the gut wall. These progress to form an adenoma, composed of abnormal glandular tissue.
What percentage of colorectal cancers are associated with dominant family history?
10%
What are hyperplastic polyps?
Hyperplastic polyps are those that will remain benign forever.
What is the difference between hyperplastic polyps and adenomas?
Hyperplastic polyps will always remain benign.
Adenomas have the potential to become malignant.
When a mutation occurs in the gut stem cells, what then occurs?
The mutation triggers APC loss. The cells, instead of migrating out, stay embedded in the crypt. This leads to a mass of continually dividing cells which are not shed and do not undergo apoptosis.
The cells divide and produce new stem cells in the population. As the adenoma grows, new stem cells are formed that are able to continually increase the number of cells.
What are the driving force behind colorectal cancer?
Polyps are the driving force behind CRC.
What are the 6 main processes that must be achieved in order for tumourigenesis to occur?
- Evading apoptosis
- Self-sufficiency in growth signals
- Insensitivity to anti-growth signals
- Tissue invasion and metastasis
- Limitless replicative potential
- Sustained angiogenesis
What does a loss of heterozygosity (LOH) at Chr.5q correspond with?
Loss of APC
What event triggers the transition in the gut, from Normal epithelium to Hyperplastic epithelium?
A LOH at Chr.5q, corresponds with the loss of APC and leads to the development of hyperplastic epithelium.
What LOH event triggers the transition of normal colon epithelium to hyperplastic epithelium?
LOH at Chr.5q, corresponding with the loss of APC.
What triggers hyperplastic epithelium to develop into an early adenoma?
The hypomethylation of DNA.
What does the LOH at Chr.17p correspond with?
The loss of p53.
What are the two types of inherited colorectal cancer?
FAP (Familial Adenomatous Polyposis Coli)
HNPCC (Hereditary Non-Polyposis Colorcetal Cancer)
What is FAP?
FAP = Familial Adenomatous Polyposis Coli
- Arises through mutations in the APC gene
Where do the majority of mutations occur within APC?
The majority of mutations in APC occur in the mutator cluster region (MCR)
The majority of these mutations lead to truncated proteins
How can APC become truncated?
Mutations within the mutator cluster region (MCR) of APC can lead to truncated proteins. These truncated proteins contain ASEF and β-catenin binding sites in the armadillo-repeat domain.
How does Wnt interact with DSH? (Drosophila)
Wnt activates Dishevelled (Dsh) through binding with Frizzled (Fz). Fz transduces signal into the cell where it activates Dsh. Dsh negatively regulates ZW3.
Normally ZW3 makes the armadillo molecule unstable. It then gets degraded.
If ZW3 is functioning, armadillio is stabilised, it can then enter the nucleus and activate gene expression.
In the absence of Wnt signalling how does the Wnt pathway operate?
GSK-3 phosphorylates β-catenin, APC and Axin
This complex targets β-catenin for ubiquitination (targeted destruction) and it is broken down by the proteosome.
All the genes that β-catenin activates are not activated. β-catenin is degraded so it does not build up in the cytoplasm. LEF/TCF transcription factors with co-repressors, Groucho and CtBP repress gene transcription.
When Wnt signalling is activated how does the pathway operate?
Wnt binds to the Frizzled receptor
This triggers the activation of Dishevelled (likely by phosphorylation)
Dishevelled breaks apart APC-Axin-β-catenin-GSK3 complex
Repression of GSK-3 by Dsh involved GBP action and disassociation of GSK-3 from the complex.
β-catenin is not targeted for degradation, it accumulates in the cell, enters the nucleus, binds TCF and activates various different genes for transcription. The arrival of a ligand triggers gene expression.
In the presence of Wnt what breaks apart the APC-Axin-β-catenin-GSK-3 complex?
Dishevelled (Dsh), triggered by Wnt binding to Frizzled, breaks apart the complex.
What must TCF-4 interact with in order to trigger transcription?
β-catenin
If APC is functioning normally how is cell growth regulated?
β-catenin is degraded by the destruction complex through ubiquitination. β-catenin cannot activate transcription.
Are wnt signals active at the base of colon crypts?
Yes, β-catenin: Tcf/Lef target genes are induced by Wnts.
The stromal cells at the base of the crypts are the source of Wnt signalling, which drives cells to proliferate and migrate away. They continute to proliferate whilst receiving some Wnt signal. As they migrate away from the signal and toward the epithelial guy lining they stop dividing and start differentiating into normal, healthy, epithelial cells.
How does Wnt function differ in the crypts when APC mutations or other mutations interfere with the Wnt signals?
Stromal cells at the base of the crypt are the source of Wny signals. They act to drive cells to proliferate away and migrate to the epithelial gut lining.
Usually, cell cycle is arrested as cells migrate up from the crypts, β-catenin and Tcf/Lef are switched off.
In a damaged Wnt pathway these β-catenin and Tcf-Lef signals remain on. This leads to a build up of dividing cells that cannot leave the crypt - Polyp formation.
What is E-cadherin?
E-cadherin is responsible for cell-cell adhesion in epithelial cells. It requires a cytoplasmic catenin binding domain (either α,β or γ).
APC and ASEF act to maintain the actin cytoskeletal network.
In APC mutants how is E-cadherin affected?
In an APC mutant, ASEF interacts with E-cadherin resulting in reduced cell-cell adhesion. Thus there is no tight linkage between cells, migration is altered and occurs incorrectly.
How does mutated APC affect mitosis?
Cells lacking functional APC undergo aberrant mitosis. Normally, APC facilitates the interaction between microtubules and the kinetochore. In mutants this cannot occur so chromosomes with spindle abnormalities occur
In healthy cells, how does APC affect mitosis?
APC facilitates the interaction between microtubules and kinetochore.
APC binds and stabilises microtubules, it localises to clusters at the ends of the microtubules and is embedded within the kinetochore. It forms complexes with Bub1 and 3 and is an in vitro substrate for Bub kinases.
Why do mutations in TGFβ occur?
TGFβ contains a run of As
This run of identical bases is difficult to replicate correctly if there are problems with mismatch repair.
Deletions are very common - leads to a mutant form which has a premature STOP. This results in the formation of a truncate receptor, the TGFβ signal tends to shut cells down and stop division.
If the TGFβ receptor is mutated there will not be a stop growing signal. It will continue dividing.
90% of colorectal cancers with MSI (microsatellite instability) have truncated TGFβ receptor.
What is the function of TGFβ?
TGFβ signalling inhibits the growth of normal epithelial cells.
If a mutation occurs the TGFβ receptor is truncated and thus cannot respond to antigrowth signals.
How can curcumin (turmeric) affect cells with mutated APC/B-catenin?
Curcumin activates caspase-3 in the cells which disrupts wnt signalling and targets the cell for degradation.
