Cancer stem cells

Question 1

What is a stem cell

Answer 1

• self-renewal

- differentiation

Question 2

What is cell differentiation?

Answer 2

• less specialised cells becomes more specialised

- stable, complex change in gene expression with change in cell type

Question 3

What is the relationship between stem cell-renewal and differentiation?

Answer 3

2 types

• symmetrical SC renewal = 2 of same cells
• asymmetrical cell division = 2 diff cells (1 SC and 1 committed progenitor cell)

Question 4

Describe SC differentiation

Answer 4

• long term SC –> short term SC –> early progenitors –> late progenitors –> differentiating cells –> differentiated cells

Question 5

What is a cancer stem cell?

Answer 5

• rare immortal cells within a tumour that can both self-renew by dividing and give rise to many cell types that constitute the tumour - forming tumours

Question 6

What are the key points of cancer stem cells (CSC)?

Answer 6

• not al cells are equally capable of regenerating the tumour
• CSC are only cells within the tumour with the capacity to maintain and regenerate the tumour
• tumours are stem-cell maintained tissue
• CSC-specific therapies should complement current anticancer treatments - mainly aimed at the reduction of the tumour mass

Question 7

When were CSCs first discovered?

Answer 7

• 1994 by Dick - used cell-surface protein markers to identify a relatively rare population of stem-like cells in AML
• present in peripheral blood of persons with leukaemia
• able to initiate human AML when transplanted into mice with compromised immune systems
• relatively immature in terms of differentiation - stem like

Question 8

What are the properties shared by normal stem cells and CSCs?

Answer 8

• self-renewal - tissue-specific SC must self-renew throughout lifetime = maintain organs
• differentiation - give rise to a heterogenous population of cells that compose the organ
• asymmetric cell division - all SC are capable of symmetric and symmetric cell division
• cellular signalling - pathways that regulate self-renewal

Question 9

What signalling pathways are related to CSCs?

Answer 9

• hedgehog
• notch
• Wnt
• NK-kb
• c-myc

Question 10

How is the epithelial-mesenchymal transition and CSCs linked?

Answer 10

• epithelial cells are more proliferative while mesenchymal cells are more migratory
• cancer-associates EMTs enhance stemness
• EMT and stemness has been observed in pancreatic, prostate, breast cancer
• mesenchymal properties = breast cancer cell lines with high CD44+ cells show high stem/progenitor properties

Question 11

Describe tumour cell heterogeneity?

Answer 11

clonal evolution model

• non-hierarchical model where mutations arising in tumour cells confer a selective growth advantage
• cell acquires a series of mutations and produced a dominant clone
• tumour cells arising from this clone have similar tumourigenic capacity
• other cells may lack tumourigenicity due to stochastic events
• tumour heterogeneity results from the diversity of cells present within the tumour

Question 12

Describe the cancer stem cell model

Answer 12

• hierarchical organisation of cells where a small subset of cells has the ability to sustain tumourigenesis and generated heterogeneity
• mutations in a progenitor cell endows the tumour cells with stem cell like properties
• these cells have self-renewing capability and give rise to a range of tumour cells- accounting for tumour heterogeneity

Question 13

What was the method used to show CSCs in vivo?

Answer 13

• genetic lineage tracing approaches

Question 14

How do CSCs arise?

Answer 14

3 types

1. cancer cells arise from stem cells
2. cancer cells arise from progenitor cells - partly differentiated cells retain a partial capacity for self-renewal, divide to produce mature cells
3. cancer cells arise from differentiated cells = dedifferentiation

Question 15

Describe reciprocal interactions between the CSC and its niche

Answer 15

• normal niche contains a SC, progenitor cell and supporting cells
• genetic or epigenetic changes in SC = generation of CSC - expansion of cells in niche
• genetic epigenetic changes in a cell within the niche = inappropriate production of a growth factor = generation of CSC
• niche adapts to presence of CSCs with cells changing properties and recruitment of cells that would be normally present

Question 16

Describe cancer stem cell assays

Answer 16

• limiting-dilution transplant are used to determine the freq of CSCs
• ideally tumours that form in primary hosts are again tested for their content of cells with CSC activity to conform initial CSC have self-replicating ability
• most sensitive assays are those in which there is no immunological difference between host and tumour

Question 17

Xenograft assays to measure CSCs

Answer 17

• human tumour containing CSC and other cancer cells is dissociated then injected into mouse
• causes tumour growth
• tumour dissociates again
• results in a regenerated human CSC population
• injected into secondary mouse
• leads to tumour growth

Question 18

Do multiple CSC pools exist in individual tumours?

Answer 18

• yes
• can have distinct leukemic SC populations defined by CD34, CD38
• heterogenous CSC compartments have been describes in solid tumours where distinct CSC populations regenerate the phenotypic and functional heterogeneity of parental tumour
• phenotypic conversion among distinct CSC subsets of tumour

Question 19

Describe models of tumour propagation by CSCs

Answer 19

• one CSC subset may be present in tumour, non-CSCs are incapable of regenerating tumour
• multiple distinct CSC pools each capable
• long-lived dormant CSCs may produce local and or distant tumour recurrence after activation many years after therapy
• as tumour progresses, second distinct CSC may arise as a result of clonal evolution = acquisition of additional mutation or epigenetic modification
• CSC phenotype may be unstable - phenotypic reversion of cell surface markers = switching of CSC phenotype - response to cell-intrinsic or micro-environment

Question 20

Describe tumour propagation - metastatic cancer stem cells

Answer 20

• metastatic CSCs may be the same or distinct from the primary CSC
• CSC is responsible for both local and disseminated tumour progression
• CSC enters the vasculature and metastasises to distant organ
• a metastatic CSC is responsible for disseminated tumour propagation
• genetic or epigenetic mechanisms acting in the primary CSC could lead to the emergence of a self-renewing metastatic CSC = distinct markers from original CSC –> secondary tumour

Question 21

What is the CSC concept?

Answer 21

1. CSC is a functional definition - should be defined in assays by their ability to generate serially transplantable tumours. simple marker expression and in vitro assays are not sufficient to define any cancer cells as CSCs
2. CSCs may or may not originate from normal SC
3. Metastatic CSCs may the same or distinct from the primary CSCs
4. CSCs may or may not be rare, and their relative abundance likely varies between tumours
5. CSCs are transformed cells with complex genetic mutations and epigenetic alterations - should not be equated to normal SCs
6. like normal SC, CSCs are heterogenous and their progeny may also possess plasticity

Question 22

Evolution of CSCs and tumour cell plasticity

Answer 22

• CSC and clonal evolution concepts are not mutually exclusive
• tumour cell plasticity may contribute to phenotypic and functional heterogeneity
• CSC function may be induced by specific micro-environmental cues from growth factors or stress-related contexts

Question 23

What is the implication of CSC in cancer therapy?

Answer 23

• therapy is used for primary tumour
• cancer cells that are resistant to therapy survive
• can have : tumour relapse driven by non-CSCs, additional mutations or tumour relapse driven by CSCs
• therefore require - alternative therapeutic strategies, specific strategies against resistant cells

Question 24

What therapies are used to target CSCs?

Answer 24

• targeting surface markers = targeting CD44, CD90, CD33
• targeting ABC cassette = verapamil, MS-209, VX-710
• targeting microenvironment = CXCL12/CXCR4, VEGF
• targeting signal cascades = Notch, Wnt

Question 25

What are some drugs that target CSCs?

Answer 25

• WZB117 targets GLUT1 in pancreatic CSC = regulates metabolism
• Rituximab targets CD20 in melanomas by inhibiting self-renewal
• Transtuzumab targets HER2 in breast cancer by inhibiting self-renewal