Stem Cells and Cancer - Differentiation

Question 1

What is the purpose of the PcG of proteins?

Answer 1

The polycomb group (PcG) of proteins represses the transcription of specific sets of genes by epigenetic modifications.

Question 2

Do PcG have the ability to bind to specific DNA motifs?

Answer 2

No, they are thought to be recruited to genes by transcription factors and/or long non-coding RNAs (HOTAIR) which can bind to genomic DNA in a sequence -specific manner.

Question 3

p53 is nicknamed “the guardian of the genome”

PcG is nicknamed “the guardian of _________”

Answer 3

Stemness

Question 4

Examples of transcription factors that are repressed by PcG include

Answer 4

Dlx
Pax
Fox
Sox

Question 5

As well as suppressing differentiation promoters such as Dlx, Pax, Fox and Sox, PcG of proteins also repress what?

Answer 5

Tumour suppressor pathways such as CDKI IK4a (p16) and ARF (p14)

Question 6

What is the mechanism of epigenetic regulation that PcG are involved with?

Answer 6

Involves the formation of two PcG repressive complexes, PRC2 and PRC1.

PRC2 consists of proteins EED, EZH1, EZH2 and SUZ12.

This complex contains histone methyl-transferase activity and targets lysine 27 of histone H3. The trimethylated histone H3 may serve as an achor for PCR1.

It is proposed that repression involves direct inhibition of the transcriptional material, recruitment of methyltransferases, and chromatin compaction.

Question 7

The PcG complex of PRC1 and PRC2 targets Lysine __ of Histone H_.

Answer 7

Involves the formation of two PcG repressive complexes, PRC2 and PRC1.

PRC2 consists of proteins EED, EZH1, EZH2 and SUZ12.

This complex contains histone methyl-transferase activity and targets lysine 27 of histone H3. The trimethylated histone H3 may serve as an achor for PCR1.

It is proposed that repression involves direct inhibition of the transcriptional material, recruitment of methyltransferases, and chromatin compaction.

Question 8

Why does the ability of polycomb group proteins to suppress differentiation of stem cells implicate them in oncogenesis?

Answer 8

Abnormal regulation of differentiation can lead to cancer.

In acute myeloid leukemia, most leukemic cells have a limited capacity for proliferation but are replenshied by rare leukemic stem cells.

Therefore, the ability of stem cells to self-renew is important in the maintenance of this disease.

A PcG protein repressor, Bmi-1, has been demonstrated to be essential for the control of self-renewal in hematopoietic stem cells and in leukemic stem cells.

Question 9

What is Bmi-1?

Answer 9

A PcG protein repressor. Bmi-1 has been demonstrated to be essential for the control of self-renewal in hematopoietic stem cells and in leukemic stem cells.

Question 10

Leukemic cells that lack Bmi-1 (Bmi-1 gene knock out) show growth arrest in which phase?

Answer 10

G1 phase of the cell-cycle and then begin to differentiate.

Question 11

Bmi-1 normally exerts its effects partially by repressing the expression of two cdk inhibitors ___ and ___ via chromatin remodelling.

Answer 11

p14 and p16

Question 12

How may PcG proteins contribute to carcinogenesis?

Answer 12

Both the silencing of tumour-suppressing pathways AND maintenance of stem cell state.

Question 13

Bmi-1 is a PcG protein repressor that normally exerts its effects how?

Answer 13

By repressing the expression of two cdk inhibitors p14 and p16 via chromatin remodelling.

Question 14

AML is a disease that is characterised by what?

Answer 14

A block on the differentiation of the granulocyte or monocyte lineage.

Question 15

Acute promyelocytic leukemia, a subtype of AML, is most often characterised by the chromosomal translocation t(15;17) results in the fusion of the ____ gene with the _________ ________ ________ ______ (RARa) gene to create a ________ _______.

Answer 15

PML gene with the retinoic acid receptor alpha to create a hybrid fusion protein.

Question 16

The hybrid fusion protein created by fusion of RARa and PML, PML-RAR, has what effect?

Answer 16

It maintains both the DNA binding domain and the ligand binding domain of the RARa receptor. IT has a higher affinity for HDAC and does not dissociate in the presence of physiological concentrations of RA.

Question 17

The wild-type RARa receptors bind to the _______ _____ response element in target genes such as RAR-RXR hetero-dimers. In the absence of retinoic acid (RA), the receptors associate with ____-co-repressor complexes that silence target genes by histone deacetylation and subsequent chromatin compaction.

Answer 17

Retinoic acid response element.

HDAC-co-repressor complexes.

Question 18

What happens to RARa when RA binds to it?

Answer 18

It undergoes a change in shape that causes the receptor to dissociate from the HDAC-co-repressor complex and allows the receptor to interact with co-activators in order to transcriptionally induce its target genes and promote differentiation.

Question 19

What impact does the ability of the PML-RAR fusion protein to form homodimers have?

Answer 19

The ability of the PML-RAR fusion protein to form homodimers plays an important role in the development of the disease, suggesting that it acts in a dominant negative manner by blocking wild-type RAR-RXR heterodimers or by recruiting novel co-repressors.

Question 20

What is the normal role of PML protein that the fusion protein PML-RAR may disrupt?

Answer 20

Normally the PML protein is found in nuclear organelles called nuclear bodies and, as a co-activator of p53, acts as a pro-apoptotic protein. Thus, although altered gene expression of retinoic acid target genes is most likely the predominant mechanism of the oncogenic effect of PML-RAR, additional possibilities, such as affecting PML function, exist.

Question 21

What suggests that cancer stem cells have inherent drug resistance?

Answer 21

They express high levels of ATP-binding casette (ABC) transporters (eg. P-glycoprotein), members of the multi-drug resistance gene family.

Question 22

Why is the interaction between B-catenin and Tcf transcription factors an attractive target for cancer treatment?

Answer 22

This interaction occurs downstream of the APC degradation complex and is the end point effect of Wnt signalling.

This would counter both inactivating mutations in APC, axin, and GSK3B, and activating mutations in B-catenin that cause inappropriate formation of B-catenin -Tcf complexes.

Question 23

What is vismodegib?

Answer 23

Vismodegib is a drug for the treatment of basal-cell carcinoma. The approval of vismodegib on January 30, 2012, represents the first Hedgehog signaling pathway targeting agent to gain U.S. Food and Drug Administration approval.

Question 24

Hh signalling exerts its effects via

Answer 24

Gli zinc finger tfs