Cancer Stem Cells

Question 1

What is a stem cell?

Answer 1

A cell that is capable of both self replication and differentiation into other kinds of specialised, functional cells

Question 2

What is cell differentiation?

Answer 2

• process by which a less specialised cell becomes a more specialised cell type
• this is through a stable, complex change in gene expression
• occurs in steps
• at each step, an immature cell type (precursor/progenitor) changes into a more mature one

Question 3

What are the 3 types of renewal a stem cell can undergo?

Answer 3

1. Self renewal = divides into 2 identical cells of the same type as the original cell
2. Differentiation = division into 2 identical cells which are more mature than the original cell type
3. Asymmetric cell division (ACD) = division into 2 cells which are different to each other - one may be identical to the original cell, the other more mature

Question 4

What is a committed progenitor cell?

Answer 4

Descendants of stem cells which can be further differentiated themselves to produce more specialised cells

Question 5

What are the different cell stages of stem cells through differentiation?

Answer 5

• Long term stem cell (LT-SC)
• short term stem cell (ST-SC)
• early progenitor
• late progenitor
• differentiating cells
• differentiated cells

Question 6

Which type of stem cells can self renew?

Answer 6

long term and short term stem cells can self renewal before they are differentiated into progenitors

Question 7

Which type of stem cells have the best probability of differentiating?

Answer 7

• Long term and short term stem cells and early progenitors have the best probability
• after that, probability decreases the more differentiation occurs

Question 8

What is a cancer stem cell (CSC)?

Answer 8

• a rare immortal cell within a tumour that can both self renew and give rise to many cell types through differentiation
• these differentiated cells can constitute to the tumour therefore tumours are formed

Question 9

Why are there cancer stem cells?

Answer 9

• not all the cells within the tumour are equally capable of regenerating the tumour
• CSCs are the only cells within the tumour with the capacity to maintain and regenerate the rumour as well as being responsible for relapse
• tumours are therefore stem cell maintained tissues like many other tissues

Question 10

Should there be CSC-specific therapies?

Answer 10

• Current anti cancer treatments target reduction of tumour mass by targeting non self renewal cells
• this could eradicate all the non self renewing cells however any CSCs could be resistant and therefore can multiply themselves to relapse a tumour
• therefore CSC-specific therapy should be used to compliment other therapies to aim to reduce all types of cancer cells

Question 11

How were CSCs discovered?

Answer 11

• In 1994 , Dick et al. provided evidence of this hypothesis
• they used cell surface protein markers to identify a rare population of stem-like cells in acute myeloid leukaemia (AML)
• the cells were present in the peripheral blood of people with AML at approx 1:250,000 cells
• stem cell-like populations have since been thought to use cell-surface protein makers in breast, colon, pancreas and other cancers

Question 12

What are properties shared by normal stem cells and CSCs?

Answer 12

1. Self renewal = tissue specific cells must self renew to maintain specific organs, CSCs do the same to maintain tumour growth
2. Differentiation = normal SCs do this to give rise to a heterogeneous population and there is evidence to show CSCs do the same
3. Asymmetric cell division = all stem cells are capable of symmetric/asymmetric cell division
4. Cellular signalling = both have pathways that regulate self renewal

Question 13

What are some signalling pathways related to CSCs?

Answer 13

many CSC subsets and normal tissue SCs share core regulatory genes and development pathways such as:
- hedgehog (Hh)
- Notch
- Wnt
- b-catenin
- NF-kB
- c-MYC
- BMI-1

Question 14

How does epithelial-mesenchymal transition (EMT) aid CSCs?

Answer 14

• epithelial cells are more proliferative whereas mesenchymal cells are more migratory
• together they form EMT which enhances CSCs stemless
• this association has been observed in pancreatic, prostate, colorectal and breast cancer
• breast cancer cells with high CD44+/CD24- cell number show high stem/progenitor properties and exhibit stro Ng enhanced mesenchymal markers

Question 15

Do CSCs have selective growth advantage in a non hierarchical model?

Answer 15

• a cell can acquire a series of mutations and produce a dominant clone
• tumour cells arising from this clone have similar tumourigenic capacity
• however, other derivatives may lack tumourigenecity
• therefore tumour heterogeneity results from the diversity of cells present within the tumour

Question 16

What is tumour cell heterogeneity like in a hierarchical model?-

Answer 16

• a small subset of cells has the ability to sustain tumourigenesis and generate heterogeneity through differentiation
• a mutation in a progenitor cell endows the tumour with stem cell like properties
• these cells have self renewing capability and give rise to a range of tumour cells thereby accosting for tumour heterogeneity

Question 17

What is tumour heterogeneity?

Answer 17

The observation that different tumour cells can show distinct morphological and phenotypic profiles

Question 18

Where have CSCs been observed in humans?

Answer 18

• 3 studies (2012) identified specific cell subsets that acted as CSCs in the brain, skin and intestinal tumours

Question 18

Where have CSCs been observed in humans?

Answer 18

• 3 studies (2012) identified specific cell subsets that acted as CSCs in the brain, skin and intestinal tumours

Question 18

Where have CSCs been observed in humans?

Answer 18

• 3 studies (2012) identified specific cell subsets that acted as CSCs in brain, skin and intestinal tumours
• the reports indicated that targeting these specific cells may improve therapeutic outcome

Question 19

Where have CSCs been observed in vivo?

Answer 19

• Parada et al. (2012) showed in a glioma mouse model that tumour cells genetically labelled with Nestin-GF (adult SC marker) have properties consistent with CSCs
• these cells were able to reinitiate tumour growth following chemotherapeutic inhibition of proliferation
• specific elimination of the CSCs significantly impaired tumour growth

Question 20

How do CSCs arise through self renewal?

Answer 20

• normal stem/progenitor/differentiated cells can all self renew to form an identical stem cell
• however this stem cell could undergo gene mutation which would result in the cell now producing both normal stem cells and mutated stem cells.
• The mutation could cause the self renewal gene to be permanently turned on which would result in rapid cell growth of CSCs

Question 21

Which cancers arise from different stem cells?

Answer 21

• Stem cells = AML originates from a hierarchy of leukaemic stem cell classes that differ in self renewal capacity
• Progenitor cells = a double maker approach showed that CSCs from gliomas were oligodendrocyte precursor cells
• Differentiatied cells = therapy-induced developmental reprogramming of prostate cancer has an acquired therapy resistance

Question 22

What happens to the niche when CSCs develop?

Answer 22

• normal niche contains a stem cell, a progenitor cell and supporting cells
• genetic or epigenetic changes in a stem cell leads to the production of a CSC and therefore an expansion of cells in the niche
• the niche adapts to the presence of CDCs with its cells changing their properties and the recruitment of cells that would not normally be present

Question 23

How do CSC assays work?

Answer 23

• limiting-dilution transplants or other clonal tracking strategies are used to determine the frequency of CSCs in the initial tumour-derived cell suspension
• ideally, the tumours that form in primary hosts are tested again for the content of their cells with CSC activity to confirm that the initial CSCs had self replicating abilities
• the most sensitive assays are those in which there is no immunological difference between the host and the tumour
• when this is not possible (eg human tumours), xenografts into highly immunodeficient mice are used

Question 24

What evidence is there to show the existence of multiple CSC pool within individual tumours?

Answer 24

Cancers can harbour heterogenous and biologically distinct populations of CSCs:
- distinct leukaemic stem cell populations defined by CD34, CD38 and IL3Ra expression
- heterogeneous CSC compartments have been described in solid tumours (ovarian/breast) where distinct populations regenerated the phenotypic and functional heterogeneity of the parental tumour
- phenotypic conversion among distinct CSC subsets within a tumour

Question 25

What are some models of tumour propagation?

Answer 25

• one CSC subset may be present within the tumour, non CSCs are incapable of generating a tumour
• multiple distinct CSC pools, each independently capable of tumour propagation, may exit with an individual tumour
• long-lived dormant CSCs may produce local and/or distant tumour recurrence after activation many years after anti cancer therapy
• as tumour progression occurs, a second distinct CSC may arise as a result of clonal evolution - this may result from the acquisition of an additional mutation or by epigenetic modification. The more aggressive CSCs will become dominant and drive tumour formation
• the CSC phenotype may be unstable, resulting in phenotype reversion of cell surface markers and switching of the CSC phenotype - this may occur in response to cell intrinsic or microenvironmental cues

Question 26

What are metastatic CSCs?

Answer 26

• may be the same or distinct from the primary CSC
• a CSC is responsible for both local and disseminated tumour propagation, a metastatic CSC is only responsible for disseminated
• genetic/epigenetic mechanisms acting in the primary CSC could lead to the emergence of a self renewing metastatic CSC which expresses distinct markers from the original CSC
• the metastatic CSC seeds secondary tumours into distant organs through a series of invasive processes
• in pancreatic cancer, only CD133+CXCR4+ cells show metastatic activity

Question 27

What is the concept of CSC?

Answer 27

1. CSC is a functional definition. They should be defined in functional assays by their ability to generate serially transplantable tumours. Simple marker expression and in vitro assays are not sufficient enough to define CSCs
2. CSCs may/may not originate from normal stem cells
3. Metastatic CSCs may be the same or distinct from primary CSCs
4. CSCs may/may not be rare, and their relative abundance likely varies with individual patient tumours, tumour type, grade and treatment status
5. Like normal stem cells, CSCs are heterogeneous and their progeny may also posses plasticity, especially accompanying tumour progression

Question 28

Do all cancers harbour CSCs?

Answer 28

• eg melanoma
• numerous groups have detected sub populations of melanoma cells that fulfil the criteria of CSCs
• Melanoma may use distinct cellular mechanisms from most other solid malignancies, given the highly migratory nature of neural crest cells

Question 29

How have CSCs and tumour cell plasticity gone through evolution?

Answer 29

• CSC and clonal evolution concepts are not mutually exclusive
• tumour cell plasticity may contribute to phenotypic and functional heterogeneity
• many cell surface markers on melanoma cells are reversibly expressed - phenotypically diverse melanoma cells can recapitulate tumour heterogeneity of the parent tumour, irrespective of whether they arose from marker positive cells

Question 30

What are some therapies that target CSCs?

Answer 30

1. Targeting ABC cassette:
   - verapamil
   - MS-209
2. Targeting surface markers:
   - target CD44/CD90/CD33
3. Targeting signal cascades:
   - target Notch/Hedgehog/Wnt
4. Targeting mircoenvironment:
   - CXCL12/CXCR4/VEGF

Question 31

What are some drugs that directly target CSCs

Answer 31

1. WZB117 = pancreatic, ovarian = regulates metabolism
2. PTC-209 = colorectal = inhibits self renewal
3. ABT-737 = breast = induces CSC apoptosis
4. Rituximab = melanoma = inhibits metastasis