Polycomb proteins Flashcards
1
Q
Why is gene expression important?
A
- a delicate balance between proliferation and differentiation during development
- cellular phenotype is the product of its expressed genes
2
Q
How does gene expression change during development?
A
- in early developmental stages there are high expression of pluripotency genes
- developmental genes increase to take their place
- in germ cells the pluripotency genes are switched back on and then also give rise to developmental ones again
3
Q
How can gene expression be controlled?
A
- transacting factors such as transcription factors, activators or repressors
- epigenetic modifications such as DNA methylation (harder to reverse) or histone modification (easier to reverse)
4
Q
What are polycomb group proteins?
A
- chromatin-associated proteins localised to loci containing developmental genes
- suppress inappropriate gene expression during development by targeting cis-elements in the genome known as Pcg response elements
- mutants can disrupt expression of development-specific genes
5
Q
How were polycomb proteins discovered?
A
- in drosophila
- found that mutations in certain genes disrupt the expression of development-specific genes such as the Hox genes
- found to be largely repressive but some groups can be activating
6
Q
What can be seen in PcG KOs?
A
- identified in almost all multi-cellular organisms
- KOs in mice can lead to anterior-posterior defects and impaired differentiation and proliferation
- in plants can lead to homeotic transformations of flower organs
7
Q
How are mammalian polycomb proteins expressed?
A
- in heterocomplexes the composition of which is dependent on cell lineage and stage of life
- PRC1 + PRC2
8
Q
Describe PRC1
A
- no methyltransferase activity
- able to bind H3K27
- uses its E3 ubiquitin ligase activity to ubiquitinate histone proteins
- contains co-factors for E3 ubiquitin ligase
9
Q
Describe PRC2
A
- EZH1+2 are H3K27 histone methyltransferases
- also requires co-factors for methylatransferase activity and histone binding proteins
10
Q
How can histone 3 modifications affect chromatin structure?
A
- H3K4me3 is always activating - open chromatin structure
- H3K9me3 is inactivating
- H3K27me3 is inactivating - close chromatin structure
11
Q
Give an example of how polycomb proteins mark target genes for transcriptional silencing
A
- PRC2 can trimethylate H3K27
- H3K27me3 attracts PRC1 which brings in further epigenetic marks such as ubiquitination and hypoacetylation
- closes chromatin structure
12
Q
How do we know about PcG target genes in ESCs?
A
- chromatin immunoprecipitation has been used to identify genome-wide H3K27 methylation
- most developmental genes suppressed by PcG in ESCs are also marked by active histone modifications - bivalent domains
13
Q
What affects on gene expression do H3K4me and H3K27me3 have and in which genes?
A
14
Q
How are H3K4/H3K27 bivalent domains remodelled during development?
A
- in ESCs bivalent domain has both H3S methylated and low expression levels of transcriptional machinery etc
- developmental stimuli causes Pcg-mediated histone remodelling pushes a change to differentiating cell phenotype and the repressive marks are removed to form a monovalent domain with just H3K4me3 with high expression
15
Q
Name two important points to remember about polycomb groups in ESCs
A
- PcG stably and inheritably suppress developmental genes within bivalent domains in ESCs thus maintaining their pluripotency
- bivalent domains are primed for rapid expression changes upon differentiation stimuli as the machinery is already present to get a head start
16
Q
How are PcG proteins involved with long non-coding RNAs?
A
- involved in X inactivation
- Xist binds to chromatin on the X to be inactivated
- brings in histone deacetylase and PcG
- these shut down the X and bring in further repressive marks such as DNA and histone methylation
- inactivity becomes independent of Xist and is continued in division
- PcGs can also silence alleles in genomic imprinting wuch as the silencing of paternal alleles of the mouse Kcnq1
17
Q
Discuss polycomb expression in cancer
A
- their functions in gene regulation and maintenance of stem cell phenotype make it a good candidate for cancer mutations
- EZH2 overexpression is seen in many cancers such as breast and prostate where expression levels are inversely associated with survival
- some polycombS regulate TSGs
- ezh2 OR OTHER POLYCOMB COMPONENTS COULD BE USED AS DIAGNOSTIC MARKERS
18
Q
What are 3 examples of PcG mutations seen in cancer?
A
- EZH2 amplification in prostate
- other EZH2 mutations leading to increased action seen in lymphomas
- UTX (H3K27 demethylase) mutations seen in colon and lung
- both up and down regulation of H3K27me3 can contribute to malignancy and the effects are cancer specific
19
Q
How can PcG modifications be linked to DNA methylation and cancer?
A
- many genes that are DNA methylated in cancer are polycomb marked in ESCs
- may suggest an epigenetic stem cell signature in cancer whereby DNA methylation is instructing polycomb marks
