Stem Cells and Cancer - Self-renewal

Question 1

What are two defining features of stem cells?

Answer 1

Ability to self-renew and their ability to give rise to committed progenitors of differentiated cell types of one or more cell lineages.

Question 2

Self-renewal provides an extended window of time for

Answer 2

Mutations.
Stem cells are long-lived targets for chance mutations compared with many differentiated cells that die within days of months. The accumulation of mutations necessary for carcinogenesis is more likely to occur in stem cells that self-renew over the lifetime of an individual rather than in mature cells that exit the cell cycle and/or undergo apoptosis after a brief period.

Question 3

Why does self-renewal allow for the accumulation of numerous mutations?

Answer 3

An individual mutation may be passed on to a daughter cell which itself is susceptible to mutations and can then pass these on to future progeny cells.

Question 4

Stem cells must maintain a balance between self-renewal and differentiation. How may this be of relevance to carcinogenesis?

Answer 4

Loss of the balance by stem cells can lead to upregulated self-renewal, a hallmark of cancer.

Therefore, tumour cells may arise from stem cells.

Or, differentiated cells may acquire a mutation that reactivates a self-renewal program.

Question 5

What other evidence is there for the role of cancer stem cells?

Answer 5

Several types of cancer tumours are maintained by only a small fraction of particular tumour cells.

These cells have surface markers indicative of the tissue stem cells.

For example, only about one in a million acute myeloid leukemia cells can develop into new leukemias when transferred in vivo.

Also these cells have the ability to give rise to cells with different characteristics or phenotypes that make up the bulk of the tumour.

Question 6

What is believed to be one of the reasons that an early first full-term pregnancy is protective against developing breast cancer in later life?

Answer 6

The depletion of mammary stem cells that occurs during pregnancy is suggested as the reason why pregnancy is protective against BC.

Question 7

What are Wnt proteins?

Answer 7

19 members.

Secreted intercellular signalling molecules that act as a ligand to trigger specific signal transduction pathways.

Question 8

In the absence of Wnt ligands what is the state of the cell?

Answer 8

Several proteins associate together in the cytoplasm to form a degradation complex.

Axin + Adenomatous polyposis coli (APC) protein, glycogen synthase kinase (GSK3B), and casein kinase I (CKI).

B-catenin (transcriptional co-activator) is modified by this complex via sequential phosphorylation followed by ubiquitination via a newly recruited ubiquitin ligase. Flags B-catenin for degradation by proteosomes.

Thus, B-catenin is not available to associate with the TcF/LEF family of transcription factors. In the absence of B-catenin, TcF associates with the repressor Groucho.

Question 9

When Wnt ligands are absent, what is the state of B-catenin?

Answer 9

Several proteins associate together in the cytoplasm to form a degradation complex.

Axin + Adenomatous polyposis coli (APC) protein, glycogen synthase kinase (GSK3B), and casein kinase I (CKI).

B-catenin (transcriptional co-activator) is modified by this complex via sequential phosphorylation followed by ubiquitination via a newly recruited ubiquitin ligase. Flags B-catenin for degradation by proteosomes.

Question 10

In the absence of Wnt ligands, why is TcF associated with the transcriptional repressor Groucho?

Answer 10

Several proteins associate together in the cytoplasm to form a degradation complex.

Axin + Adenomatous polyposis coli (APC) protein, glycogen synthase kinase (GSK3B), and casein kinase I (CKI).

B-catenin (transcriptional co-activator) is modified by this complex via sequential phosphorylation followed by ubiquitination via a newly recruited ubiquitin ligase. Flags B-catenin for degradation by proteosomes.

Thus, B-catenin is not available to associate with the TcF/LEF family of transcription factors. In the absence of B-catenin, TcF associates with the repressor Groucho.

Question 11

When Wnt is _______. Several proteins associate together in the cytoplasm to form a degradation complex.

They are: ____ + ___ + _____ + ___. This complex serves to modify _______ via sequential __________ followed by ___________ via newly recruited ubiquitin ligase. This has the effect of ________________________.

As a result, ________ is not available to associate with the ________ family of transcription factors. In the absence of ______, This family (___) associates with the repressor ________.

Answer 11

Several proteins associate together in the cytoplasm to form a degradation complex.

Axin + Adenomatous polyposis coli (APC) protein, glycogen synthase kinase (GSK3B), and casein kinase I (CKI).

B-catenin (transcriptional co-activator) is modified by this complex via sequential phosphorylation followed by ubiquitination via a newly recruited ubiquitin ligase. Flags B-catenin for degradation by proteosomes.

Thus, B-catenin is not available to associate with the TcF/LEF family of transcription factors. In the absence of B-catenin, TcF associates with the repressor Groucho.

Question 12

Wnt binds to what receptor?

Answer 12

The seven-pass transmembrane receptor, Frizzled, and the co-receptor LRP - this disrupts the assembly of the degradation complex which normally degrades B-catenin.

Thus, B-catenin can move into the nucleus and act as a co-activator of the Tcf/LEF family of transcription factors to regulate specific target genes such as c-myc, cyclin D and genes that code for adhesion molecules from the EPH family.

Activation of these target genes also depends on nuclear proteins Bcl9 and Pygopus.

Question 13

What happens when Wnt binds to its receptor?

Answer 13

The seven-pass transmembrane receptor, Frizzled, and the co-receptor LRP - this disrupts the assembly of the degradation complex which normally degrades B-catenin.

Thus, B-catenin can move into the nucleus and act as a co-activator of the Tcf/LEF family of transcription factors to regulate specific target genes such as c-myc, cyclin D and genes that code for adhesion molecules from the EPH family.

Activation of these target genes also depends on nuclear proteins Bcl9 and Pygopus.

Question 14

What happens when Wnt binds to its receptor?

Answer 14

The seven-pass transmembrane receptor, Frizzled, and the co-receptor LRP - this disrupts the assembly of the degradation complex which normally degrades B-catenin.

Thus, B-catenin can move into the nucleus and act as a co-activator of the Tcf/LEF family of transcription factors to regulate specific target genes such as c-myc, cyclin D and genes that code for adhesion molecules from the EPH family.

Activation of these target genes also depends on nuclear proteins Bcl9 and Pygopus.

Question 15

What are the target genes of B-catenin?

Answer 15

C-myc, Cyclin D, genes that code for adhesion molecules.

Question 16

What does the activation of the target genes of B-catenin depend on, other than the presence of Wnt ligand binding to frizzled receptor?

Answer 16

The seven-pass transmembrane receptor, Frizzled, and the co-receptor LRP - this disrupts the assembly of the degradation complex which normally degrades B-catenin.

Thus, B-catenin can move into the nucleus and act as a co-activator of the Tcf/LEF family of transcription factors to regulate specific target genes such as c-myc, cyclin D and genes that code for adhesion molecules from the EPH family.

Activation of these target genes also depends on nuclear proteins Bcl9 and Pygopus.

Question 17

How are B-catenin and E2F similar?

Answer 17

E2F is prevented from acting until pRB phosphorylated.

B-catenin is prevented from acting until Wnt pathway activation occurs.

Question 18

Mutations that result in constitutive activation of the Wnt pathway are responsible for __% of colorectal cancer.

Answer 18

90%

Question 19

Most of the mutations that result in constitutive activation of the Wnt pathway involve

Answer 19

1. Inactivation of the function of APC (degradation complex, familial: adenomatous polyposis)
2. activation of B-catenin.

Question 20

What is famailial adenomatous polyposis coli caused by?

Answer 20

Germline mutation in the APC gene.
Develop high numbers of polyps in the colon in early adulthood.
Some of these then develop into colorectal cancer.

The APC gene acts a true tumour suppressor gene in that both copies of the APC gene are inactivated in colorectal tumours.

Question 21

What is a well-studied model for examining the link between stem cells, the Wnt pathway and cancer?

Answer 21

The small intestine/colon.
Intestinal tissue is highly regenerative ; stem cells and epithelial progenitors, also called transit-amplifying cells that reside in the crypts give rise to more differentiated cells that migrate up along the villi.

Question 22

What is Lgr5?

Answer 22

A target gene, encoding a G protein-coupled receptor, of the Wnt pathway that is located only in the stem cells of the intestine.

Question 23

What is the ultimate molecular consequence of having inactivated mutations in APC?

Answer 23

Constitutive activation of TcF transcription factors.

Question 24

The mutation cluster region of APC is where?

Answer 24

Located between 1250-1500 codons.

Question 25

What evidence is with regard to APC gene activity that supports the cancer stem cell hypothesis?

Answer 25

Deletion of APC gene in long-lived intestinal stem cells led to adenomas.

Deletion of APC gene in progenitor transit-amplifying cells or differentiated cells does not trigger tumours.

Question 26

Mutations in the axin gene are found in ______________ carcinoma.

Answer 26

Hepatocellular

Question 27

Activating mutations of B-catenin that affect the regulatory sequences essential for its targeted degradation can lead to what?

Answer 27

Skin tumours.

Question 28

What is the HH pathway?

Answer 28

It is signalling pathway that plays an important role in embyronic development, tissue self-renewal, and carcinogenesis. HH ligands, indian, sonic, desert are secreted intercellular molecules that act as a ligand t trigger specific signal transduction pathways.

Question 29

In the absence of HH ligands, how do Patched and Smoothened interact?

Answer 29

No HH = Patched inhibits Smoothened and thus suppresses the pathway.

Question 30

In the presence of HH ligands, how do Patched and Smoothened interact?

Answer 30

Hh binds to Patched, releasing the inhibition of Smoothened.

Thus the signal is transduced into the cell and causes a large protein complex to dissociate and release the zinc finger transcription factor Gli, so that it can be translocated to the nucleus and regulate the expression of its target genes.

Hh target genes include those for Gli, amplifying the initial Hh signal at the transcriptional level.

Other target genes include: Cyclin Ds, Bcl2, VEGF and Snail.

VEGF: angiogenesis
Snail: metastasis

Question 31

What are the target genes for Hh?

Answer 31

Gli (amplifying the initial Hh signal at transcriptional level);
Cyclin Ds;
Bcl2
VEGF - angiogenesis
Smail - metastasis

Question 32

Patched is defined as what?

Answer 32

A tumour suppresor gene; patients with Gorlin’s syndrome carry a germline mutation in one copy of PAtched and have a predisposition to develop skin, cerebellar, and muscle tumours.

Question 33

Patients with Gorlins syndrome have a predisposition to develop skin, cerebellar and muscle tumours why?

Answer 33

They have a germline mutation in one copy of the Patched tumour suppressor gene which normally inhibits smoothened unless bound to HH ligand.

Question 34

Why is the Hh signalling pathway important in the development of drugs to treat Gleevec resistant cancers?

Answer 34

Inhibition of the Hh pathway inhibits the growth of Gleevec resistant mouse and human CML.

Question 35

Why is it thought that tumours which arise from a stem cell are more likely to metastasise?

Answer 35

Stem cells have an inherent ability to migrate and thus these cells tend to be aggressively metastatic if transformed.

Question 36

How does telomerase activity support the cancer stem cell hypothesis?

Answer 36

Telomerase activity is necessary for tumour proliferation and progression and is present in 90% of human cancers, is present in normal stem cells and proliferative cells.