7- Cancer Stem Cells

Question 1

What is the historical overview of CSCs? (4x)
- evidence of tumor heterogeneity

Answer 1

• miscroscopic examination of solid/ liquid tumors
  > saw different cell types (looked heterogeneous)
• transplantation experiments
  > 10^3-10^7 cells needed to efficiently transplant tumor into new host
  > suggests not all cells can regenerate/ create a tumor
• teratocarcinoma analysis (benign/ derived from ESCs)
  > composed of both highly tumorigenic/ well-differentiated (non-tumorigenic) cells
• H-thymidine DNA labelling experiments in leukemia patients
  > most tumor cells = post-mitotic/ continuously replenished by a small fraction (5%) that cycle rapidly

Question 2

What did all the heterogeneity evidence suggest about tumors?

Answer 2

• tumors have hierarchical cell organization

Question 3

What are the levels of the hierarchical cell organization?

Answer 3

self-renewing stem cell
> transit-amplifying/ progenitor cells
> post-mitotic/ differentiated cells

Question 4

What are the implications of the theoretical CSC? (3x)

Answer 4

• generate a large # of cells from only 1 division
• do not divide constantly, only periodically (quiescent)
• do not represent the majority of proliferative cells in the tumor

Question 5

What is a major cause of relapse in patients?

Answer 5

• inability to eradicate CSCs
• since therapies target dividing cells/ only divide periodically

Question 6

What are the 2 models of tumor heterogeneity?

Answer 6

Stochastic Model
- tumors are biologically homogeneous/ functional heterogeneity caused by random influences at a given moment that affect the behaviour of individual cells within the tumor > intrinsic/ extrinsic

Hierarchy Model
- tumors are like normal tissue where cell hierarchies are maintained by stem cells
> heterogeneity caused by CSCs (biologically distinct/ self-renewing)

Question 7

What is another term for CSC?

Answer 7

TIC = tumor-initiating cell

Question 8

What are the random influences in the stochastic model?

Answer 8

Intrinsic- degree of cell signalling activation/ levels of TFs
Extrinsic- host factors > microenvironment/ immune response

Question 9

How is a CSC defined by the hierarchy model?

Answer 9

• ability to self-renew
• able to recapitulate tumor heterogeneity through differentiation

Question 10

What do the stochastic/ hierarchy model have in common?

Answer 10

• both predict that only a fraction of cells in tumor can initiate tumor growth/ neither predicts frequency of CSCs in a tumor

Question 11

What are the differences between the stochastic/ hierarchy model?

Answer 11

Stochastic Model
- all cells in tumor are biologically similar in oncogenic capacity
- any cell can become a TIC, given the right influences
> therapy should be designed to target all tumor cells

Hierarchy Model
- CSCs are biologically distinct from majority of cells in tumor
- CSCs responsible for generating other tumor cells
> therapy should be designed to target CSCs

Question 12

What cancer type were CSCs first discovered in?

Answer 12

AML (acute myeloid leukemia)

Question 13

What was needed to test the hierarchy/ CSC model? (2x)

Answer 13

• ability to purify subpopulations of tumors based on properties (surface antigen expression)
• functional transplantation assay to test ability of cell populations to generate tumors in vivo

Question 14

Why were CSCs first identified in AML? (4x)

Answer 14

• accumulated knowledge on hematopoiesis
• well-characterized hematopoietic cell surface antigens
• availability of xenotransplantation assays > SCID mice
• cell-sorting methods > FACS

Question 15

How can CSCs be identified?

Answer 15

FACS = Fluorescent-activated cell sorting
- fluorescent antibodies label cell surface antigens
- separate populations based on fluorescent intensity

Question 16

How are CSC/ TICs identified in many different cancers?

Answer 16

• CSC cell surface markers (different)

Question 17

What are the main CSC cell surface markers for breast cancer?

Answer 17

CD44 high/ CD24 low

Question 18

What are the 2 possibilities of CSC origin?

Answer 18

1. stem cell compartment (of normal tissue) undergoes transforming mutation (already has stem cell characteristics)
2. progenitor compartment (of normal tissue) > transforming mutation must be strong enough to induce stem cell features

Question 19

Are standard therapies targeting CSCs?

Answer 19

• no > why patients relapse
• CSCs evade standard therapies (chemo/ radiation)
• breast CSC population ↑/ higher mammosphere forming efficiency after treatment

Question 20

Is the CSC fraction defined/ constant within a tumor/ comparable among tumors of same type?

Answer 20

• no > CSC heterogeneity is a problem for treatment
• tumor evolution > ↑ heterogeneity (already to begin with)
  > different responses to therapy
• CSCs can be generated de novo > EMT (tumor cell plasticity)

Question 21

What is a source of stem cells?

Answer 21

EMT (epithelial-mesenchymal transition)
- new CSCs can be generated (promotes stem-like properties)

Question 22

How does tumor cell plasticity further complicate things?

Answer 22

• ↑ plasticity enables bidirectional interconvertibility between CSCs/ non-CSCs (transit-amplifying/ progenitor cells)
• CSCs = moving target

Question 23

Why do CSCs not respond to conventional therapy? (3x)

Answer 23

• quiescent (divide periodically, not constantly)
• have activated anti-apoptotic programs
• express drug resistance transporters (drug pumps)

Question 24

What are the 3 main challenges to target CSCs?

Answer 24

1. CSCs do not respond to conventional therapies
2. Hard to define CSC “unique” signalling pathways since they share many with normal stem cells
3. Moving target (replenished due to tumor cell plasticity/ EMT)

Question 25

What are 2 CSC-targeted therapy approaches?

Answer 25

1. Knowledge-based approaches
2. Blind approaches (screen to find selective inhibitors of CSCs)

Question 26

How do we now think of CSCs?

Answer 26

• functionally distinct population that is stochastically renewable
• both models must be accounted for in therapy design

Question 27

What is the best possible cancer therapy?

Answer 27

• kill proliferating tumor cells + CSCs > remission cure

Question 28

What is the problem with therapies that kill proliferating tumor cells but not CSCs?

Answer 28

• tumor shrinks > residual CSCs > tumor regenerates

Question 29

What is a potential problem with therapies that only kill CSCs?

Answer 29

• tumor degenerates BUT rapidly proliferating tumor cells could become new CSCS with new phenotypes
  (replenish CSCs even if we get rid of initial population)