Impacts of teratogenic, carcinogenic and endocrine disrupting chemicals on the epigenome?

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  • Created by: Sarah
  • Created on: 24-05-19 21:03
what is a teratogen?
a chemical or biological agent that when an embryo or fetus is exposed to during development can cause non-heritable developmental malformations and other birth defects
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what is a carcinogen?
a chemical or biological agent that can cause cancer
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what is an endocrine disruptor?
an exogenous subastances that interferes with any aspect of hormonal action- by disrupting, modifying or mimicking endocrine signals in the body
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what are 3 example of a teratogen?
1) cyclopamine 2) alcohol 3) thalidomide
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what does cyclopamine do?
causes developmental defects by impacting on shh signalling by binding Smo
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how was cyclopamine discovered to be a teratogen?
sheep were eating corn lilies and that was causing babies to have a cyclopic eye
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how is alcohol a teratogen what does it do to fetuses?
shows dose dependent malformations of craniofacial and brain development in rodents [linked to similar abnormalities in humans]
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what happened with thalidomide?
was developmed as a sedative and prescribed to pregnant mother with devastating consequences
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what did thalidomide due to development?
is a synthetic compound that affects organ systems and the development of the limbs
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what examples of endocrine disruptors are there?
1) DDT 2) Oestrogen mimics
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what is DTT used for?
used to kill agricultural pests sprayed on fields
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why can DTT have devastating ecological impacts?
it causes bird eggs shells to be too thin so it cannot contain the developing embryo- fragile eggs lots of bird die
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what does DTT sprayed on fields increase the risk of? by how much?
breast cancer , 5-10 fold
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how did they find out DTT increases the risk of breast cancer 5-10 fold?
retrospective study of women who were over 50 at the time some had had breast cancer others hadn't
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what did they find?
the likelihood of having breast cancer was profoundly increased if at age 4 or less they were exposed to moderate- high doses of DTT (lived in a place lot of DTT sprayed in fields)
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when did the risk of breast cancer diminish somewhat?
if the time they had encountered exposure was later in life so teenage or older- still had a risk in early teenage years but dropped to baseline levels after puberty
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what are oestrogen mimics? where are they found?
many synthetic small molecules we encounter in our daily lives that have the ability to mimic natural oestrogen
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what is one oestrogen mimic?
bisphenol A
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why does bisphenol A's structure allow it to be a oestrogen mimic?
it has a pair of cyclic aromatic rings that allows it to behave as oestrogen (estradiol) by binding to the oestrogen receptor- transligand regulated TF and affect target gene expression
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what drug acts as an oestrogen mimic?
diethylstilbestrol
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what was diethylstilbestrol developed for?
therapeuric applications and later was found to have endocrine disrupting effects
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3 oestrogen mimics?
polychlorinated biphenols, diethylstilbestrol and bisphenol A
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what is tobacco smoke
a teratogen and carcinogen
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how many compoinds in tobacco are known to cause defects and disease causing activities?
70 compounds
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why is the evidence for epiegentic and epigenomic impacts of many of the compounds limited?
studies have been limited
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what is the evidence for epigenetic effects of which environmental chemical most strong for?
tobacco smoke
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what has been proved lately about tobacco smoke being a teratogen?
recent paper established beyond doubt that many different types of birth defects are caused by exposure of developing embryos or fetuses to tobacco smoke during development and elevates the risk of birth defects
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what is one of the compounds in smoke known for disease-causing activities
polyaromatic compounds- including benzo[a]pyrene
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what are polyaromatic compounds- including benzo[a]pyrene?
known carcinogens, mutagens AND modifiers of the epigenome
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what kind of family fo benzopyrenes belong to?
polyaromatics compounds = aryl hydrocarbons
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what detects the polyaromatic like benozpyrene?
aryl hydrocarbon receptor family of proteins
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what does aryl mean?
has an aromatic ring on it
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what does aromatic mean?
the chemicals are smelly so they're known as aromatic or aryl hydrocarbons
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whats another aryl hydrocarbon other than benzopyrene?
dioxin- a very powerful poison
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what specifically binds to these aryl hydrocarbons?
aryl hydrocarbon receptors (AHRs)
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why do we have the AHR?
because endogenous hydrocarbons bind to these receptors
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the benzopyrenes we're talking about are then endogenous?
no they're exogenous ligands
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what is aryl hydrocarbon receptors ?
a ligand regulated TF (so when the aryl hydrocarbon binds it can move into nucleus and act as a TF)
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what domain allows it to act as a TF
DNA binding domain, basic helix-loop-helix domain, dimerisation domain and ligand binding domain,
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why do aryl hydrocarbon receptors have dimerisation domain?
allows it to interact with this other TF = ARNT
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what does the heterodimer of aryl hydrocarbon receptors and ARNT do?
assemble on specific DNA sequences within target genes and promote transcription of those genes
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what does the AHR receptor TF promote?
the expression of cytochrome P450 1a class of enzymes = CYP1a and CYP1B
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what does cytochrome P450 enzymes do?
metabolises polyaromatic hydrocarbons (including aryl hydrocarbons) to mutagens
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what happens when ARH detects a compound in the cytoplasm?
it moves to the nucleus and dimerises with ARNT, find target genes and turn them on
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what is the 3rd member of the Aryl hydrocarbon receptor family?
AhRR
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what is AhRR?
a repressor- interferes an inhibits AHR and ARNT
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What happens when AhRR is expressed?
it interferes with the ability of ARH to bind its target genes in a ligand dependent manner
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what will AHR not be able to turn on if AhRR inhibits it?
CYP1A1
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what does does CYP1A1 + cytochrome P450s convert?
benzopyrene into a powerful mutant
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what is the outcome of being exposed to an exogenous aryl hydrocarbon?
binds to ARH and turns on cytochrome genes that turn it into a powerful mutagen
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what prevents turning it into a mutagen?
AhRR
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what are the powerful mutagens that cytochrome P450 enzymes CYP1A1 and CYP1A2 convert polyaromatic hydrocabons into?
epoxides
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what do epoxides do?
damage DNA
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how do epoxides damage DNA?
they intefere with double stranded DNA, covalently attach themselves to purine rings like guanine so physically covalently modify DNA- change its coding properties = mutate DNA
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What does AhRR stand for?
aryl hydrocarbon receptor repressor
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what does AhRR do?
binds to AHR and inhibits its function as its partner ARNT can't bind
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what does inhibiting AHR reduce the production of?
cytochrome P450 enxyme CYP1A1
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how might AhRR be good?
puts a limiting amount on the amount of epoxides mutagens that are produced from aryl hydrocarbons- guardian or surveillance mech to limit mutagenic power of external compounds we're exposed to
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when an epigenome wide association study was done what was looked at?
epigenetic changes to the DNA as DNA samples are readily avalibale and chromatin samples aren't- characterised the DNA methylation patterns of those samples using bisulfite sequencing
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what did this mapping of methylation patterns for many thousands of DNA samples was to answer what question?
are there any specific methylation patterns that seem to be more common in individuals that have been exposed to a particular substance or who have a particular disease (or not been exposed)
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what were they looking for association between?
methylation patterns and phenotypes/exposure
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what was found to have a very very high association of methylation on the AHRR locus?
smoking
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what was found?
AHRR is super highly associated, 3 CpGs in the genome are associated with the AHRR gene that show differential methylation in smokers versus non smokers
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what happened to the methylation of CpGs within the AHRR gene that were associated with smoking?
hypomethylation of CpGs
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where else did they prove that there was extensive hypomethylation of CpGs within the AHRR gene?
in lung alveolar macrophages and blood monocytes of smokers compared to non-smokers
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how many CpG sites did they find within the AHRR gene in both alveolar macs and total WBCs have hypomethylation in smokers vs non-smokers?
49 CpGs sites within the AHRR gene
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for the vast majority of these CpGs what was there?
a lower methylation in smokers versus non-smokers = this AHRR gene is super hypomethylated after exposure to tobacco smoke
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what changes in the AHRR gene are very robustly associated with smoking?
hypomethylation of CpGs within the AHRR gene and increased AHRR expression
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where was the expression of the AHRR gene looked at?
in human lung tissue
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how did they show further that AHRR is hypomethylated in smokers?
looked at the total level of methylation of the AHRR gene in current smokers, former smokers and people who have never smoked
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what differences did they find between people who had never smoked, current smokers and former smokers?
AHRR was highly methylated in people who'd never smoked, fomer smokers show some methylation but a lot less than smokers and current smokers show substantial hypomethylation
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what did the susbtantial hypomethylation of AHRR in current smokers correlate with?
increased expression of AHRR in human lung tissue samples taken from smokers+ non-smokers
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what is the hypomethylation and increased expression of AHRR possibly a response to?
the exposure to aryl hydrocarbons in tobacco smoke activating AHR receptor and activating CYP P450 enzymes to make epoxide mutagens, body may be preventing this so less conversion into mutagenic epoxide forms
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what study did they do looking at pregnant mothers and fetuses with smoking?
looked at cord blood of newborn babies that were or were not exposed to smoking whilst their mum was pregnant
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what did they find?
if their mums smoked whilst they were pregnant with them there was a high association of AHRR and hypomethylation in cord blood of newborns whose mums were smokers
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what did they find out when they looked at how long the pregnant mothers were smokers for during pregancy (eg just 1st or 1st+2nd trimester or whole trimesters)?
the longer the mums were smokers for during pregnancy the more hypomethylation of AHRR
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When they looked at the number of altered methylation sites in the genome over time following people who had stopped smoking?
CpG demethylation of AHRR persisted in smokers for many years after stopping smoking, but a lot of the genes adapted and went back to normal
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what does this tell us?
40 years after stopping smoking the body still has a memory of exposure to tobacco smoke that is registered at a small number of CpGs which include those in the AHRR gene
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what does the AHR TF promote?
expression of cytochrome P450 enzyme CYPA1 which metabolises aryl hydrocarbons to mutagens
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what hypothesis abotu the AHRR gene being hypomethylated did this propose?
AHRR may be de-repressed and expressed to counteract the production of epoxide mutagens made as a result of benzopyrenes in tobacco smoke
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what would this hypothesis suggest AHRR was?
a tumour supressing gene- could do that by inhibiting all of AHRs ability to transcribe CYP1A/B/A2 which metabolises these compounds
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why does the body have this system if it existed long before environment because full of synthetic compounds and before cigarettes
endogenous ligands can bind to AHR
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what is an endogenous ligand for AHR?
Tryptophan- a small aromatic compound
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what does tryptophan have a natural tendency to be converted into when exposed to daylight?
FICZ compound
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what is different about tryptophan and FICZ compound?
FICZ compound has a high affinity for the AHR and tryptophan doesn't
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what has the highest known affinity for the aryl hydrocarbon receptor?
FICZ
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What does FICZ induce by binding to AHR?
CYP1A1/1A2/1B1 transciption
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what do these cytochrome enzymes do to FICZ?
breakdown FICZ into products that have lower biological activity
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what does the FICZ do if its not broken down?
powerful immunomodulator- accumulation of FICZ has bad inflammatory effects
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what is the biological rationale for this system
to breakdown endogenous hydrocarbons that have bad inflammatory effects
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what was it not for?
to provide protection against exogenous substances that might be mutagenic in nature
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does the increased expression of AHRR in smokers protect smokers from lung cancer?
no
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what did they find with lung cancer and hypomethylation of AHRR?
being able to activate AHRR expression by hypmethylation doesn't reduce your risk of lung cancer at all
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what have follow on studies now shown about AHRR hypomethylation?
individuals with hypomethylated AHRR have an increased risk of lung cancer
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what does this tell us?
AHRR is not a tumour supressor gene
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what is AHR gene essential for?
protecting the liver against tumorigenesis
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how was it shown that the AHR gene is essential for protecting the liver against tumorigenesis?
did a mouse KO of AHR and they had a dramatically decreased survival because of liver tumors probably
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what did they see when they looked at the liver from AHR KO animals treated with a compound that causes tumorigenesis?
they were full of tumors and the weight of the liver was heavier in mutants because they had so many tumors
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what happened to livers in the KO ARH mutant and in normal mice when they were treated with a compound that encourages tumorigenesis?
in mutant they got liver tumors but in the WT they were protected from tumors by the ARH gene
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what does an agouti mouse look like?
hairs are black with a broad band of yellow so look like brownish
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why do you have the black and yellow fur colour in agouti mice
hair produces 2 types of pigment at slightly different times of development- black and then yellow production
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whats one example of agouti mice?
the mutant agouti viable yellow allele = Avy
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in animals with a dominant Avy mutation what do the mice look like?
some of them look yellow, some of them look agouti (pseudoagouti)
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what are the ones that look agouti actually pseudoagouti?
they're not genetically agouti they carry this dominant mutation which is an insertion of a retroviral like transposable element IAP transpospon upstream of the WT agouti locus
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what does the IAP retroviral like transposable element code for?
a protein that stimulates the production of yellow pigment
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where is the retrotransposon element inserted? what does it do?
inserted just upstream of the agouti gene there is a promoter within this element that is CA and it means whatever hair growth cycle its in it promotes strong expression of agouti protein that promotes the synthesis of yellow pigment and blocks black
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what is variable with this dominant Avy gene
the phenotype is variable it has variable expressivity so some animals form a litter of agouti viable mice
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what does the heritability of agouti viable (Avy) mean?
if the mum carries it because its dominant if mated to a WT male the offspring will be heterozygous for that agouti viable yellow mutation- but degree they show phenotype will vary
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what does the configuration of this regulatory element (Avy) mean?
sometimes the animals have CA active agouti expression and other times they have much lower levels- so get normal agouti or peusodagouti phenotype where you just have a yellow band
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what kind of allele is Avy (agouti variable yellow)?
a metastatic epialle
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what kind of mutation is agouti viable yellow?
dominant mutation
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why are some mice with agouti viable yellow look agouti (but are genetically not) or yellow
retroviral like trasnposable element (IAP) is upstream of the WT agouti locus. Agouti viable yellow codes for protein that stimulates production of yellow pigment- IAP CA that promotes yellow pigment but can be swithced on and off is instability of a
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why do some Avy mutant animals look agouti?
because the retrotransposon is having very little impact on the regulation of this gene- IAP is in heterochromatin?
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what happens in all yellow mice?
IAP is expressed fully because its in euchromatin
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what happens in a yellow mouse with brown patches?
in a mixture of euchromatic and heterochromatic state
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what does the Avy locus show variation in to correlate with the Avy coat phenotype?
DNA methylation (of 0 closely CpG sites in the IAP promoter)
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what happens to the DNA methylation of IAP sites in Avy mutants that are all yellow?
they are hypomethylated so express IAP and become yellow
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what happens to the DNA methylation of IAP sites in Avy mutants that all look agouti (are pseudoagouti)?
IAP is hypermethylated so IAP cannot be expressed
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what happens to the mottled (spotty) ones?
methylation is in between that of yellow and pseudo-agouti so has some methylation of sites but not all so some yellow is still expressed
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how stable are these methylation patterns in these mice?
very stable methylation patterns- the methylation is present in the testes, brain, kidneys and lungs
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what happened to Avy methylation when Bisphenol A was added to the maternal diet during pregnancy?
Bisphenol A in the maternal diet in pregnancy promotes Avy hypOmethylation
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what did this mean for the coat colour of the progeny from bisphenol A fed mothers?
more activity of the Avy allele means more progeny have coats with more yellow pigment
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where can bisphenol A be found?
in the environment all around for example in plastics- that come into contact with food and drink
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what is bisphenol A?
an oestrogen mimic
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bisphenol A fed mothers progeny?
more yellow and slightly mottled animals
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what does this tell us bisphenol A is doing to methylation of these CpG sites in the promoter region?
methylation is reduced, hypomethylation so more Avy expressed
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what is bisphenol A doing to the Avy methylation and activity?
bisphenol A promotes AVy hypomethylation and increases its activity and more progeny have coats with yellow pigment
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what experiment was done so we know bisphenol A has much wider impacts on the epigenome that just modulating the activity of the agouti viable yellow locus?
genome wide study and asked what is the effect of Bisphenol A in the diet on the methylation patterns across the genome and looked at the liver
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what did they do?
had control animals (pregnant mums fed no Bisphenol A) 50 micrograms, a low dose or high dose of bisphenol A 50mg (1000 times more)
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what did they find happened to methylation with bisphenol A?
methylation pattern decreases as a function of exposure to low or high doses of bisphenol A. Some genes don't change at all some genes show modest differences, some heavily demethylated with bisp A
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what was there a whole group of genes doing?
that were unmethylated in the controls and then acquire methylation as a result of exposure to both high and low doses of bisphenol A
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so what happened overall?
had genes that were losing methylation and some that were acquiring methylation with bisphenol A - most of the genes lose methylation
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when genomotology was done to analsye the relationship of methylation to known functions of genes whose methylation patterns were being changed what was the effect of BPA treatment or loss of methylation as a result of BPA changing genes involved in?
metabolic genes
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what did this tell us about bisphenol A?
the effect of this endocrine disrupting agent oestrogen mimic can affect agouti viable yellow expression is widespread in the epigenome and affects hundreds of genes methylation
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what cell does smoking induce long term hypomethylation to?
WBCs
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what gene in particular is increased expression and hypomethylation with smoking
AHRR- aryl hydrocarbon receptor, hypomethylation of AHRR in smokers is accompanied by increased transcription of RNA in WBCs
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what are smokers that have a hypomethyalted AHRR gene at higher risk of?
lung cancer
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the hypomethylation of what persists for many decades after the cessation of smoking?
AHRR
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