Epigenetics and cancer Flashcards
What are epigenetic traits?
Stable, heritable phenotypes resulting from changes to chromatin structure without alterations to the DNA sequence
What alters the chromatin?
Chromatin remodelling proteins-
they compress or open up the chromatin
How do epigenetics modifications result in a different phenotype?
Alterations of chromatin structure results in a change in gene expression
this results in an altered phenotype
How are chromatin remodelling proteins regulated?
In a signal regulated way
What are the 2 main impacts of epigenetic modifications?
- They control the accessibility of target genes to incoming transcription machinery e.g RNA polymerase 2
- They directly control the biochemical activity of transcriptional machinery itself
What are the 2 main histone modifications- what do they cause?
Methylation and acetylation
either:
conversion of genes into active domains of transcribable chromatin
or
inactive genes of transcriptionally silent chromatin
What 3 things must occur for a proliferating progenitor cell to become a differentiated cell?
- Suppression of cell renewal
- Lineage commitment
- One programme of differentiation pursued
Where are modifications made on histones?
The N terminal tails of each of the 4 substituent histone components
What are the 4 components of a histone?
H2A
H2B
H3
H4
What is a nucleosome?
DNA wound around 8 histone protein cores-
building block of chromatin
What are the covalent modifications made to core histones?
Acetylation- of lysine tails
Methylation- of lysine and arginine tails
Phosphorylation- of serine 10 in histone H3
What is the relationship between acetylation and methylation?
They are competing reactions
both can occur on lysine tails
What are HATs and HMTs? What is their main difference?
HATs= histone acetyltransferases
HMTs= histone methyltransferases
HATs can modify many different lysine residues whereas HMTs are much more site specific
Which histone methylation modifications are associated with activation and repression?
Dependent on the modification and where it was made
Histone lysine 4 methylation= activity
Histone lysine 27/9= inactivity- repression
What is acetylation predominantly associated with?
Transcriptionally active chromatin
What is the significance of histone modifications?
They act as recruitment signals- they recruit parts of the transcription machinery
What does acetylation recruit?
Proteins that contain an acetyl-lysine binding domain called a bromodomain
What does methylation recruit?
Proteins that contain either a chromodomain (REPRESSORS) or a PHD zinc finger domain (ACTIVATION)
What happens when genes in the ‘polycomb group gene enhancer of zeste’ are mutated?
Extra sex combs develop on the drosophila
normally just the forelegs
What does enhancer of zeste code for?
A HMT of lysine 27- involved in converting a homeotic complex to inactive- transcriptionally repressed
when its mutated= lose the inhibition and thus develop more sex combs
The polycomb group of proteins code for various proteins- what are the main ones?
Code writers- components of a complex in which EZH2 methyltransferase lies
Code readers- proteins of a complex that a chromodomain containing protein binds
The group codes for a chromatin regulatory system- which marks chromatin for silencing and makes it transcriptionally inert
What happened when they isolated the polycomb protein genes in vertebrates? What is the relevance with prostate cancer?
The polycomb family of proteins are frequently and highly expressed in cancers
In normal prostate and advanced prostate cancer-
immunostaining showed that there were increased levels of EZH2 in the advanced prostate biopsies
How can you study metastasis in cells?
Use a matragel membrane- punch small holes into it
invasive cells will try and squeeze through the holes
What happens in you KO EZH2 in cells and study their ability to invade? What can be concluded?
Much less invasion
EZH2 is required for the metastatic phenotype
What is EZH2 also required for?
Cell proliferation
What is the mutant EZH2 protein like? Where are mutations in EZH2 common?
Hyper-active protein which makes it constitutively active- hyper-methylating through the genome
non-Hodgkin’s lymphoma- EZH2 is the most frequently modified chromatin modifying gene
What else is characteristic of mutant EZH2 proteins? How is this being as a result?
The proteins are hypersensitive to a class of small molecule
can make specific inhibitors which target mutant EZH2 and have relatively no effect on WT EZH2
kills mutant lymphoma cells
How does EPZ affect mutant EZH2?
It prevents EZH2 from methylating histone lysine 27
(in a concentration dependent manner)
Inhibits proliferation of EZH2 mutated cells- cancer specific inhibitor (conc dependent)
What are the 2 point mutations associated with mutant EZH2?
Tyrosine to a phenylalanine
Alanine to a glycine
What can aberrant crossovers due to chromosomal translocations (between non homologous chromosomes) cause?
The creation of hybrid genes which code for hybrid proteins- which are characteristic of many lyphomas and leukaemias
What are the 2 cross overs associated with acute myeloid leukaemia and acute lymphoblastic leukaemia?
Cross over between chromosomes 8 and 16- between 2 HATs- AML
Cross over between chromosomes 11 and 16- between a HMT and HAT- ALL
Both of these translocations create fusion proteins that make a population of immortalised haemopoietic stem cells
What are the 3 main domains of the MLL protein?
Catalytic domain- allows it to make methylation lysine 4 marks
Interaction domain- can bind to HATs
PHD finger domain- can bind to histone lysine 4 when its methylated
Whats the main role of MLL?
To be able to recognise methylated histone lysine 4 and propagate new methylation marks on adjacent nucleosomes
=make a transcriptionally active DNA domain
Following a leukemogenic translocation- what is lost (and retained) in the MLL protein?
Lost- domains involved in histone lysine 4 methylation and recognition
Retains its transcriptional repressor domain
What can be suggested about the fusion MLL proteins new function?
MLL exerts its leukemogenic function by acting as a dominant negative inhibitor of its WT counterpart
It compromises the ability of target genes to be methylated- and thus activated
