Lecture 14: Re-wiring the genome – mechanisms of imprinting (I) Flashcards
discovery of imprinting
Lorenz 1973
bonds with first moving stimulus it sees.
occurs during critical periods during very early life.
“Imprinting” describes a fundamental process of life occurring during circumscribed time windows of prenatal and postnatal ontogenesis and having lasting effects.
genomic imprinting
as gamete-of-origin dependent modification of genotype.
experiments in 80s with genomic imprinting?
mice.
- nuclear transplantation made embryos with 1 of 2 sets of parental chromosomes, uni parental embryos.
- embryos that inherited specific chromosome born. one parents only - unifparental disomy.
mammalian genes could function differently depending on whether they come from the mother/father.
first imprinted genes
expressed differently on maternal/paternal.
imprinting has a substantial effect on human genetic disorders.
DNA methylation found to be a key molecular mechanism of imprinting.
mechanism of imprinting?
methylation marks genes differently in egg/sperm, leads to differential gene expression.
key early disoveries
Completion of mouse embryogenesis requires both maternal and paternal genomes:
insert additional pronucleus and fuse into oocyte instead of sperm, creating a parthenogenetic embryo (PG). they contain 2 copies of the maternal genome and no paternal
Development of reconstituted mouse eggs suggests imprinting of the genome during gametogenesis:
removed both female pronuclei frmo the oocyte + replaced with 2 sperm heads, then fuse to form diploid cell. forms an androgenetic embryo (AG), contains 2 copies of the paternal genome.
what happens on day 10 of gestation for AG and PG embryos
PG fairly normal, placental development shit.
AG developement fucked but placenta fairly normal.
Therefore maternal genome contributes significantly to development of the foetus.
paternal genome contributes to development of extra embryonic tissues (ie placenta).
some genes are selectively switched off in germ cells in the testis/ovaries.
Other genes are selectively switched off in germ cells of the ovary.
functional equivalence of parental chromosomes
not functionally equivalent.
Both maternal and paternal genomes are required for normal development.
Mouse embryos with only maternal or paternal chromosomes cannot develop normally, despite being diploid.
This suggests the existence of genes that are only expressed from one parental genome or the other: “genomic imprinting”
imprinted gene expression
depends upon parental opigin.
imprinted genes are only expressed from a single allele, other genes are usually biallelicaly, therefore cannot follow mendellian inheritence.
compare mendelian and imprinted inheritance
mendelian:
In the second generation, both the heterozygous father (+/-) and heterozygous mother (+/-) can parent offspring displaying the “A” trait
imprinted:
Maternally expressed genes are transmitted solely through the matriline.
In the second generation, only the heterozygous mother (+/-) can parent offspring displaying the “A” trait.
The *heterozygous female (-/+) does not express the “A” trait but will transmit it to half of her offspring, unlike her heterozygous brother (-/+)
example of imprinting disease
turner syndrome.
imprinting genes found where
eutherian mammals, marsupials and flowering plants.
selfish gene
it is ALL REPLICAS of a particular bit of DNA distributed throughout the world”
it is trying to get more numerous in the gene pool. Basically it does this by helping to program the bodies in which it finds itself to survive and to reproduce…”
imprinted genes work how?
Imprinted genes are thought to influence the transfer of nutrients to the foetus and the new born from the mother.
paternally expressed genes are selected to
extract more resources from the mother to benefit offspring fitness
maternally expressed genes tend to
conserve resources
divide resources among more offspring
maximize reproductive performance of the female
parental conflict hypothesis
“Parental conflict” hypothesis assumes that imprinting evolved as a result of conflicting interests between paternal and maternal genes.
applies to polygmous mammals, multiple fathers.
Paternally-derived genomes are in competition with those of other males, selection will thus favour those that extract most resources from the mother to give to the foetus
Therefore, imprinted genes from the male genome are strongly expressed in the placenta, which extracts resources from the female.
All the offspring from one female are equally related to their mother.
BUT each father is only related to a subset of these offspring.
THEREFORE paternal genes enhance resource acquisition by offspring in the interests of their fitness
BY CONTRAST, maternal genes act to restrain and divide resources to benefit lifetime reproductive fitness of the mother
examples of imprinted genes
on slides
coadaptation hypothesis
keverne and curley 2008.
contrasts the parental conflict hypothesis.
imprinting may aid coordination between mother and offspring, the imprinted genes act to optimise foetal development and maternal provisioning and nurturing.
seems to only be relevant to a subset of mainly paternally expressed genes, ie peg3, that are expressed in the placenta and hypothalamus.
first imprinted genes discovered in mice
1991
Igf2 (insulin-like growth factor 2)
Igf2r (IGF2 receptor)
H19 (encodes an imprinted maternally expressed non-coding transcript)
IGF2 plays an important role in development and brain function (regulation of brain morphology, food intake, memory consolidation and memory enhancement)
Igf2
Paternal imprint codes for Igf2:
Extracts resources from the mother.
Promotes growth.
Key in placenta development.
Maternal imprint codes for Igf2r:
When IGF2 binds to IGF2R, it is targeted for destruction.
Reduces effects of IGF2 on growth.
number of imprinted genes
100-200 but varies depending on source
imprinted gene characteristics
> 80% of the known imprinted genes are clustered together.
Each cluster contains 2-15 genes and vary in size from <100 kb to several megabases.
All clusters contain both maternally and paternally expressed genes, protein-coding and non-coding RNAs.
Parent-specific expression of multiple genes within a cluster is under the overall control of an imprinting control region (ICR)
ICR
imprinting control region ICRs
show parental allele-specific DNA methylation and chromatin modifications
An ICR is active when unmethylated and inactive when methylated.
ICR structure
differentially methylated regions DMR
methylation of DMR or ICR will inactivate the gene
IGF2-H19 cluster
on maternal chromosome, unmethylated DMD/ICR binds to CTCF protein and forms insulator.
preventing common enhancers from activating IGF2, instead activating H19 promotor.
paternal IGF2 expressed normally.
how does inactivation of imprinted genes occur?
nucleosomal condensation via deacetlation and methylation.
gene condensed, transcription factors cannot access DNA, cannot be transcribed.
how does activation of imprinted genes occur?
acetylation and demethylation.
chromatin is opened, DNA can be accessed and transcribed.
mechanisms of methylation
DNA methylation maintenance -
In the presence of Dnmt1(DNAmethyltransferase 1), hemi-methylated DNA becomes fully methylated and so DNA methylation patterns are maintained.
maintenance of methylation - Dnmt1
Dnmt3a and Dnmt3b (enzymes) are required for de novo methylation in post implantation embryos.
When DNA is replicated, methyl group on template strand is recognised and one isintroduced on opposite strand by Dnmt1.
when are imprints established
read up on paper cos she dont make sense
mouse chimaera experiments
mouse chimaeras created by adding normal cells to PG/AG embryos at early development stages.
PG-N and AG-N.
demonstrate the influence of imprinted genes on growth.
survived as long as they had below 40% contribution of PG/AG.
PG-N had large brain, small body, increased brain/body weight ratio, increased aggressive behaviour in male PG-N mice.
AG-N small brain, large body, reduced brain/body weight ratio.
suggests that maternal/paternal genomes have different interests regarding allocation of rescources to brain/body development.
mouse chimera phenotypes
PG higher cognitive function, selectively excluded from hypothalamic and pre optic areas, accumulate in frontal cortex, striatum, hippocampus.
AG opposite, emotional or autonomic function.
hypothalamic or pre optic areas, excluded from cortex, striatum.
therefore:
imprinted genes are strongly implicated in neurodevelopment.
maternal/paternal genomes could have dissociable effects on brain function.
