L4, Stem cells, cancer and cancer stem cells

Question 1

Potency; basic definition

Answer 1

Potency = Number of open cell fates

Question 2

Types of division in stem cells

What signals this to occur?

Answer 2

• Asymmetric; one progenitor, one self renewal
• Symmetric
• In response to mitogens
• Terminally differentiated cells are post-mitotic and do not divide (short-lived)

Question 3

Stem cell hierarchy…

Answer 3

• Totipotent: Zygote
• Pluripotent: Blastocyst
• -> iPS
• Multipotent: Progenitors (Ectodermal, Mesodermal, Endodermal)
• -> Lineage committed
• Loss of open fates down hierarchy

Question 4

Lifespan of cells in blood/skin vs CNS

Answer 4

• Red blood cells and skin: matter of days
• CNS cells: up to 30 years

Question 5

Transit amplifying cells

Answer 5

• Carry out bulk of proliferation
• a.k.a progenitor cells
• Switch in differentiation/transcription programmes

Question 6

Hematopoietic system: How do progenitor cells work?

Answer 6

Multipotent hemopoietic stem cells -> multipotent hemopoietic progenitor -> common lymphoid progenitor OR common myeloid progenitor

• Unidirectional process
• End result is differentiated cells of the two different lineages

Question 7

Example: Loss of hierarchy in Leukamia

Answer 7

• Leukaemia is often characterised by the presence of large amounts of poorly differentiated blast-like cells in the blood
• Leukaemia are defined based on cell of origin (e.g. lymphoblastic, myeloblastic, erythroblastic)
• As blast-like cells become more prevalent, tissue loses functionality; crisis point at 20%

Question 8

Why do blast-like cells form in leukaemia?

Answer 8

• Loss of differentiation is often tightly linked to the hyperproliferative state

Question 9

Cancer subtypes in BCR-ABL vs PTC1

Answer 9

• In the hematopoietic system, the BCR-ABL oncogene causes chronic myeloid leukaemia if it arises in a stem cell but B cell acute lymphocytic leukaemia if it arises in a progenitor cells
• In contrast, loss of PTC1 causes medulloblastoma whether it occurs in a neural stem cell or progenitor cell

Question 10

What is the stem cell niche?

Answer 10

• Refers to the microenvironment, within the specific anatomic location where stem cells are found, which interacts with stem cells to regulate cell fate
• The niche provides a balance of growth stimulatory and inhibitory signals
• The niche is often located in a region of tissue that is protected from external damage (e.g. intestinal crypt, limbus, basal layer of skin)

Question 11

Stem cells in the intestinal crypt

Answer 11

• At base of crypt, stromal cells in the lining signal to stem cells to proliferate (via Wnt pathway using Beta-catenin as the effector)
• Beta-catenin translocated to nucleus to activate TFs
• Fail to proliferate without signalling, antagonised by APC (type of TSG) -> phosph. beta-catenin -> targeting degradation

Question 12

APC in colonic crypt

Effectors involved in the mechanism

Answer 12

• Beta catenin stabilised instead of degraded
• Translocates to nucleus and, with Tcf/Lef + CBP upregulates c-myc and cyclin D1
• c-myc role: Oncogenic (see other lectures)

Question 13

APC in FAP

Answer 13

• Familial Adenomatous Polyposis
• Germline mutations in APC assoiated with FAP (autosomal dominant)
• 100% penetrance i.e. all develop cancer
• FAP is relatively rare (<1% of colon cancer)

Question 14

Cancer cell origin and differentiation review:

Answer 14

• Initiating cancer cells are thought to often derive from either stem cells or progenitor cells that have mutated
• Cancer cells exist in poorly differentiated state due to the regulation of differentiation programmes, often linked to the hyperproliferative state

Question 15

Tumour heterogeneity

Give the two types

Answer 15

Intertumor…

• Among different patients
• Among primary breast cancer and different metasasis

Intratumor…

• Different cell types within one tumour -> Clinical relevance

Question 16

Cancer cell hypotheses:

Answer 16

• Clonal Evolution Model (all cells within tumor have equal tumorigenic potential)
• Hierarchical/CSC model: Only CSCs can initiate and sustain tumour growth
• CSC plasticity model: Tumor hierarchy is very dynamic; plastic cancer cells can constantly shift between non-CSC and CSC states depending on various intrinsic and extrinsic stimuli -> heterogenous tumour population

Question 17

Cancer cell plasticity

Answer 17

• Explains the CSC hierarchy as much more flexible
• Dedifferentiated population maintained within strict parameters
• Need to switch off state of migration to allow proliferation

Question 18

Key concepts in CSC model (Battle and Clevers, 2017)

Answer 18

1. The cellular heterogeneity observed in tumours can result from their hierarchial organisation
2. These hierarchies are driven by rare, self-renewing CSCs, whereas the bulk of the tumour is composed of non-CSCs
3. CSC identity is hardwired, as illustrated by the fact that non-CSCs seldom initiate tumours in xenograft assays

Question 19

What do stem cells and CSCs have in common?

Answer 19

• Self renewal
• Identifiable by cell surface markers
• Affected by niche

Question 20

Why are CSCs resistant to treatments?

Answer 20

• Elevated drug efflux (overexpression of ABC transporters)
• Elevated DNA damage surveillance/repair (stemness marker) -> these cells are addicted to DNA repair mechanisms

Question 21

+ GSCs - what are they? Relevance to cancer treatment.

Answer 21

• GSCs = Gliomal stem cells (brain cancer)
• Putatively responsible for the high relapse rates in brain cancer (90% of patients will reportedly face relapse within 2 years of initial diagnosis).
• Highly heterogenous population.
• Proteasomal inhibiting drugs shown to selectively inhibit GSCs over their non-differentiated counterparts

Question 22

What two properties of CSCs given them selective advantages?

Answer 22

• CSC signalling promotes angiogenesis
• Normal bulk cells can’t survive in deoxygenated environment whereas CSCs thrive in low oxygen