Lecture 4: Stem cells, cancer and cancer stem cells

Question 1

What is a stem cell?

Answer 1

Stem cells are undifferentiated cells that can divide asymmetrically (to give rise to a stem cell (for self-renewal) and a progenitor cell (that goes on to become a more differentiated cell)) or symmetrically (gives rise to either two stem cells or two progenitor cells) in response to mitogen signalling
- this is important for tissue maintenance and regeneration as well as maintaining the stem cell pool.

Question 2

True or false: stem cells only divide asymmetrically?

Answer 2

False: they can divide symmetrically as well

Question 3

Describe the stem cell hierarchy with reference to stem cell potency

Answer 3

zygotes are formed of totipotent stem cells which have the capability of giving rise to all cell types of the body and extra-embryonic tissue

Blastocysts contain pluripotent embryonic stem cells which are capable of giving rise to all cell types of the body
(induced pluripotent stem cells have the same potency as embryonic stem cells)

Adult multipotent stem cells (progenitor cells) are capable of giving rise to all cells of a particular organ or tissue

These multipotent stem cells give rise to lineage committed cells (terminally differentiated cells) of a particular tissue type, these cells are not usually capable of giving rise to other cell types

Question 4

How does stem cell potency correlate with stage of differentiation?

Answer 4

As cells become more differentiated they have reduced potency (less open fates available).

Question 5

Which stem cells carry out the bulk of proliferation?

Answer 5

Committed transit amplifying cell (aka progenitor cell) - cells that undergo more division to give rise to terminally differentiated cells.

Question 6

Describe how a multipotent stem cell gives rise to differentiated blood cells

Answer 6

A multipotent haematopoietic stem cell will divide to give rise to a multipotent haematopoietic progenitor cell. This progenitor cell divides to give rise to the common myeloid progenitor and common lymphoid progenitor.
The common myeloid progenitor gives rise to blast cells (myeloblasts) that give rise to differentiated blood cells of the myeloid lineage.
The common lymphoid progenitor gives rise to blast cells (lymphoblasts) that give rise to the differentiated blood cells of the lymphoid lineage.

Question 7

What is leukaemia often characterised by?

Answer 7

The presence of large amounts of poorly differentiated blast-like cells in the blood (defined based on their cell of origin -lymphoblastic, myeloblastic or erythroblastic)

Question 8

How is differentiation and proliferation linked?

Answer 8

As cells mature, differentiation programmes are switched on and proliferation programmes are switched off (i.e. more differentiated cells are less proliferative)

Question 9

True or false: loss of differentiation is often tightly linked to the hyperproliferative state?

Answer 9

True: one cell becomes mutated in a way that blocks differentiation programmes but maintain or increase proliferation

Question 10

Using BCR-ABL oncogene mutation, explain why the cell of origin can be important in determining cancer type

Answer 10

Depending on the cell of origin, this can give rise to divergent cancer subtypes:
- If the BCR-ABL oncogene mutation occurs in a haematopoietic stem cell, this results in chronic myeloid leukaemia
- However, if the BCR-ABL oncogene mutation occurs in a progenitor cell, this results in B-cell acute lymphoblastic leukaemia

–> in this case, the cell of origin is more important than the mutation in determining the cancer type

Question 11

Using PTC1 gene mutation, explain why the cell of origin can be less important in determining cancer type

Answer 11

Loss of PTC1 tumour suppressor gene causes medulloblastoma whether it occurs in a neural stem cell or progenitor cell

–> in this case, the mutation is more important than the cell of origin in determining the cancer type

Question 12

What is a stem cell niche?

Answer 12

the local environment for stem cells
- provides growth stimulatory and inhibitory signals
- often located in a region of tissue that is protected from external damage

Question 13

Where is the stem cell niche in the intestines?

Answer 13

Intestinal crypts

Question 14

How is intestinal stem cell renewal and differentiation is governed in a wild-type intestinal crypt?

Answer 14

Stem cells in crypts receive growth signals in the form of wnt signalling from stromal cells.
This results in the production of beta-catenin, which translocates to the nucleus where it acts as a transcription factor promoting the proliferation and differentiation into progenitor cells
As the cells move towards the intestinal lumen they arrest in the cell cycle as fully differentiated cells with beta-catenin turned off (beta-catenin degradation is regulated by APC tumour suppressor gene).

Question 15

How is intestinal stem cell renewal and differentiation is governed in an APC mutated intestinal crypt?

Answer 15

APC is an antagonist of Wnt signalling and phosphorylates beta-catenin targeting it for the ubiquitin/proteasomal degradation.
This means that as the cells move towards the intestinal lumen, although they’re no longer receiving Wnt signalling, APC deletion means beta-catenin stays on as it is not targeted for degradation. These cells continue to proliferate and become a site for future polyp formation

Question 16

In normal colonic cancer cells, how does loss of APC result in uncontrolled proliferation?

Answer 16

No phosphorylation of beta-catenin, meaning it does not undergo ubiquitylation and proteasomal degradation.
This results in translocation to the nucleus and transcriptional activation of cyclin D1 and c-myc that drive cell proliferation and result in uncontrolled cell growth.

Question 17

What type of mutation is associated with familial adenomatous polyposis and what is the penetrance?

Answer 17

germline mutations in APC with 100% penetrance (i.e. all people with this mutation will develop cancer)

Question 18

True or false: tumours are homogenous?

Answer 18

False: tumours are heterogenous
- intertumour heterogeneity = different patients with same cancer will have different genetic profiles with different subtypes and different metastases.
- intratumour heterogeneity = various cells within the tumour microenvironment that differ in genome, epigenome, transcriptome and proteome

Question 19

How does the clonal evolution model explain the heterogenicity of tumours?

Answer 19

A single population of tumour cells develop into different clones that have different mutations that result in expression of different genes.
(from PowerPoint: all cells in the tumour have equal tumorigenic potential. The tumour cells may undergo genetic and/or epigenetic alterations, generating different tumour clones with the same ability to initiate tumour growth)

Question 20

How does the hierarchical/cancer stem cell model explain the heterogenicity of tumours?

Answer 20

Different populations of cells to start with (including tumour cells and cancer stem cells)
- it is only the cancer stem cells (CSCs) that can initiate and sustain tumour growth. CSCs can generate heterogeneous tumour cell progeny via asymmetric division and maintain the CSC component via symmetric division
(from Paper: only CSCs can generate the bulk of the tumour via symmetric division (for self-renewal) or asymmetric division (to generate differentiated cells) this is unidirectional and doesn’t account for the concept of cell fate reversibility from the progenitor and differentiated tumour cells)

Question 21

How does the cancer stem cell plasticity model explain the heterogenicity of tumours?

Answer 21

The model is more bidirectional and suggests that, similarly to the CSC model, CSC’s can self-renew and generate differentiated cells but also includes the concept that progenitor and differentiated tumour cells have the potential to acquire cancer stem cell properties by intrinsic (genetic and epigenetic modifications) and extrinsic signals (tumour microenvironment).
“For instance, mutation in a differentiated cell can endow self-renewal capacity and establish a new hierarchical CSC clone, adding the functional diversity within a tumour”.

Question 22

How is Fluorescence-Activated Cell Sorting used to identify cancer stem cells?

Answer 22

• Tag different cell with fluorescent antibodies against different CD proteins to separate them (cell sorting using markers expressed non-uniformly throughout the tumour)
• Samples of each fraction expressing different markers taken and injected into mice.
• the progenitor and differentiated tumour cells
• cell types expressing stem cell associated surface glycoproteins had the ability of self-renewal and tumour formation when grafted in immunocompromised (NOD/SCID) mouse model (more tumorigenic) = these cells are the cancer stem cells

Question 23

What is cancer cell plasticity?

Answer 23

The concept that non cancer stem cells (including progenitor and differentiated tumour cells) can undergo dedifferentiation in response to microenvironment (including inflammation, injury, cancer-associated fibroblasts, growth factors/cytokines) into a cancer stem cell, which can facilitate proliferation and EMT/MET.

Question 24

True or false: the tumour hierarchy is driven by CDCs, which make up the bulk of the tumour?

Answer 24

False:
The hierarchies are driven by rare, self-renewing CSCs, but the bulk of the tumour is composed of non-CSCs

Question 25

Name 2 of the surface markers expressed (glycoproteins) by CSCs

Answer 25

CD24
CD44
CD133
EpCAM

Question 26

What are 3 similarities between normal stem cells and cancer stem cells?

Answer 26

1. Capable of self-renewal
2. Express cell surface markers
3. Affected by niche/microenvironment

Question 27

What are three differences between normal stem cells and cancer stem cells?

Answer 27

1. CSCs have abnormal proliferation
2. CSCs have a failure to differentiate
3. CSCs have unreliable DNA repair mechanisms

Question 28

What is MSI (Musashi homologue) and how is it implicated in cancer?

Answer 28

MSI is an RNA-binding protein putatively expressed in CNS stem cells and neural progenitor cells. This protein normally represses the expression of NUMB (which promotes differentiation)

In cancer, MSI can become oncogenic by genetic and epigenetic modifications, which can lead to overexpression and an aggressive undifferentiated state.

Question 29

How can cancer stem cells enable treatment resistance?

Answer 29

They have elevated drug efflux due to overexpression of ABC transporters meaning chemotherapy drugs unable to cause cell death.

They also have commitment to certain DNA repair mechanisms meaning that radiation therapy that damages DNA is repaired and permits cell survival.

Question 30

What is the significance of cancer stem cells in the occurrence of cancer relapse?

Answer 30

If a cancer treatment does not eliminate the cancer stem cells, these can rapidly repopulate the tumour

Question 31

Give two other properties of cancer stem cells and how these are important in tumorigenesis.

Answer 31

1. survival in the perivascular niche by stimulating angiogenesis to ensure adequate blood supply. This is in response to signalling from endothelial cells (like in a normal tissue)
2. Adaption to hypoxic niche - activation of hypoxia-inducible factors (HIFs) that allow CSCs to adapt to survive and even thrive in hypoxic environments when other non-CSCs cannot