Chapter 8: Stem cells and differentiation (Book, main)

Question 1

True/false: Stem cells show differentiation plasticity (=the ability of cells to change their phenotypes)

Answer 1

True! For example, hematopoietic stem cells (HSCs) can give rise to non-hematopoietic cells

Question 2

Why is a block in differentiation a mechanism for tumor formation in some cancers such as leukemia?

Answer 2

Because the block in differentiation results in a higher net number of cells (=mechanism for tumor formation)

Question 3

What are the features of cancer stem cells (CSCs) (name at least 3)?

Answer 3

These are rare cells within a tumor that have the ability to self-renew and to give rise to phenotypically diverse cancer cells with limited proliferative potential that make up the rest of the tumor. CSCs have the ability to iniate new tumors when transplanted into host animals

Question 4

Two proposals for the relevance of stem cell biology to carcinogenesis have been suggested. What are these two?

Answer 4

1. Self-renewal provides increased opportunities for carcinogenesic changes to occur
2. Altered regulation of self-renewal directly underlies carcinogenesis

Question 5

Earlier, we stated that self-renewal provides increased opportunities for carcinogenic changes to occur. Can you further explain what this means?

Answer 5

Stem cells are long-lived targets for chance mutations, compared with many differentiated cells that die within days or 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 6

The skin is layered with different layers. Epithelial cells of the skin have a turnover rate of 60 days, and malignant tranformation takes 18+ months, so the lifespan of a differentiating cell is not long enough for sufficient amount of mutations to accumulate. Self-renewal is a quiality of stem cells that allows for the accumulation of transforming mutations, as an individual mutation may be passed on to daughter cells (which are susceptible to additional mutations). What does this rationale suggest?

Answer 6

That the accumulation of mutation is required for the initiation of skin cancer is likely to occur in the normal stem cell or early progenitor cell compartment

Question 7

Which different type of cells exist that are a target for transformation (differentiation)? What does this mean?

Answer 7

• Stem cells
• Progenitor cells
• Terminally differentiated cells

This means that cancer can initiaite either (1) in a stem cell that has lost regulation of self-renewal or (2) in a differentiated cell that has obtained the ability to self-renew

Question 8

Fill in: The Wnt pathway is responsible for 90% of ___ cancer

Answer 8

The Wnt pathway is responsible for 90% of colorectal cancer

Note: Most of the mutations inactivate the function of APC or activate ß-catenin, but rarely the ligand Wnt!

Question 9

There are two forms of colorectal cancer: familial and sporadic forms. What inherited cancer predisposition syndrome do the familial patients have and what does this lead to?

Answer 9

Familial adenomatous polyposis coli (FAP that carry a germline mutation in the APC-gene and develop high numbers of polyps in the colon in early adulthood, having an increased risk for colorectal cancer

Question 10

Is the APC-gene a tumor suppressor gene or a proto-oncogene?

Answer 10

Tumor suppressor gene

Question 11

Which target gene of the Wnt pathway is restricted to stem cells in the intestine (by binding tot he ligand R-spondin and physically interacting with Frizzled/LRP complex to enhance the Wnt/ß-catenin signaling) and therefore an important marker of adult stem cells?

Answer 11

Lgr5

Question 12

What important roles do the Hedghog (Hh) signaling pathways play a role in?

Answer 12

Embryonic development, tissue self-renewal and carcinogenesis

Question 13

Is Patched (transmembrane protein that is responsible for signal transduction by Hh) a tumor suppressor- or proto-oncogene?

Answer 13

Tumor suppressor gene

Question 14

Which syndrome do patients have that carry a germline mutation in one copy of Patched? And which predispositions do they have to certein cancers?

Answer 14

Gorlin synndrome, and they have a predisposition to develop skin, cerebellar and muscle tumors (BCC, medulloblastomas and rhabdomyosarcomas respectively)

Question 15

What additional properties do tumors have that may be due to cancer stem cells (CSCs)?

Answer 15

Tumors display hetereogeneity, plasticity and an ability to migrate, these properties may be due to CSCs.

Question 16

Progeny of both normal and cancer stem cells also exhibit developmental plasticity whereby cells may switch between two developmental pathways. How is this development program called (and explain)?

Answer 16

The epithelial-mesenchymal transition (EMT), it describes cell type changes that involve cells losing epithelial cell traits and gaining mesenchymal cell traits

Question 17

EMT (epithelial-mesenchymal transition) is observed in some tumor cells. What does it show (3 answers)?

Answer 17

That EMT in tumor cells is linked to metastasis and gives rise to stem-like characteristics. The reversible phenotype swtiching may also account for the development of drug resistance

Question 18

True/false: the succes of metastasis may be based on the number of CSCs in the primary tumor; non-cancer stem cells in a primary tumor may not have the ability to form new tumors at distant sites

Answer 18

True

Question 19

What is the nickname of polycomb group (PcG) of proteins? (e.g. like p53 is the “guardian of the genome”)

Answer 19

The “guardians of stemness”

Question 20

Why are polycomb group (PcG) of proteins called the guardians of stemness?

Answer 20

Because the target genes that they repress include a large number of developmental regulators that promote differentiation (e.g. Dix, Pax family, Fox and Sox familise)

Question 21

What do polycomb group (PcG) of proteins do?

Answer 21

They represse the transcription of specific sets of genes by epigenetic modifications (they can’t bind to specific DNA motifs but can recruit to genes by TF and/or IncRNAs.

In short, PcG proteins are implicated in stem cell maintannce and repress key tumor suppressor pathways

Question 22

The mechanism of epitgeneitc regulation by PcG prtoeins involves the formation of two repressive complexes. Explain what they are and how they work.

Answer 22

PRC2 and PRC1 are the two PcG repressive complexes. PRC2 contains histone methyltransferase activity and targets lysine 27 (and lysine 9) of histone H3, so that thetrimethylated histone H3 product may serve as an anchor for PRC1. PRC1, which contains Bmi-1 and chromobox protein subunits, causes the addition of a ubiquitin molecule to histone H2A at lysine 119.

Meaning: direct inhibition of the transciptional machinery, recruitment of methyltransferases and chromatin compaction

Question 23

The ability of PcG proteins to suppress the differentiation and promote self-renewal of stem cells implicates them in oncogenesis as abnormal regulation of differentiation can lead to cancer. Which two genes are identified to be over-expressed in a variety of different cancers?

Answer 23

Bmi-1 and EZH2

(BMI is part of the PRC1 complex, involved in the ubiquitination of histone H2A at lysine 119, EZH2 is a protein that is part of the PRC2 complex, involved in the histone methyltransferase activity)

Question 24

Which genes of the PcG proteins are shown to be linked to oncogenesis in which cancer types?

I don’t think you ought to know this by heart

Answer 24

Bmi-1 plays an essential role in the self-renewal in HSCs in AML. Bmi-1 gene amplification has been idientified in some lymphomas, SUZ12 is overexpressed in breas- and colon cancers and EZH2 is overexpressed in lymphoma-, breast- and prostate tumors

Question 25

What are some important transcription factors that are important in the development of hematopoietic lineages

Answer 25

• Runx-1, aka AML-1 (involved in almost all lineages)
• Pu.1
• CCAAT/enhancer-binding protein a (C/EBPa)

Question 26

What are the two functions of lineage-specific transciption factors?

Answer 26

They activate a particular ste of lineage-specific genes and/or inhibit the cell cycle for terminally differentiated cells

Question 27

It has been demonstrated in mice that deletion of PTEN (a tumor suppressor phosphatase protein) resulted in leukemia-initiating cells that could transfer disease upon transplantation to irradiated mice, but also caused initial proliferation and later depletion of normal HSCs. What does this mean?

Answer 27

That a distinciton was made between the CSCs and normal stem cells of the hematopoietic system: PTEN dletion promotes the generation of leukima-initiating cells but the depletion of normal stem cells

Question 28

Cyclopamine is a pathway inhibitor. For which pathway?

Answer 28

The Hh pathway

Question 29

How does cyclopamine suppress the Hh pathway

Answer 29

By inhibiting the activity of Smoothened (so transciption target genes are repressed)

Question 30

How can PcG proteins be inhibited (also share some results)?

Answer 30

To target self-renewal, a Bmi1-inhibitor on colerectal CSCs has been investigated. The data showed that the Bmi-1 inhibtior patched-209 decreases CSCs and irreversibly inhibited tumor grwoh in colorectal cancer xenographs