Lecture 14: Re-wiring the genome – mechanisms of imprinting (I)

Question 1

discovery of imprinting

Answer 1

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.

Question 2

genomic imprinting

Answer 2

as gamete-of-origin dependent modification of genotype.

Question 3

experiments in 80s with genomic imprinting?

Answer 3

mice.

1. nuclear transplantation made embryos with 1 of 2 sets of parental chromosomes, uni parental embryos.
2. 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.

Question 4

first imprinted genes

Answer 4

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.

Question 5

mechanism of imprinting?

Answer 5

methylation marks genes differently in egg/sperm, leads to differential gene expression.

Question 6

key early disoveries

Answer 6

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.

Question 7

what happens on day 10 of gestation for AG and PG embryos

Answer 7

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.

Question 8

functional equivalence of parental chromosomes

Answer 8

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”

Question 9

imprinted gene expression

Answer 9

depends upon parental opigin.
imprinted genes are only expressed from a single allele, other genes are usually biallelicaly, therefore cannot follow mendellian inheritence.

Question 10

compare mendelian and imprinted inheritance

Answer 10

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 (-/+)

Question 11

example of imprinting disease

Answer 11

turner syndrome.

Question 12

imprinting genes found where

Answer 12

eutherian mammals, marsupials and flowering plants.

Question 13

selfish gene

Answer 13

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…”

Question 14

imprinted genes work how?

Answer 14

Imprinted genes are thought to influence the transfer of nutrients to the foetus and the new born from the mother.

Question 15

paternally expressed genes are selected to

Answer 15

extract more resources from the mother to benefit offspring fitness

Question 16

maternally expressed genes tend to

Answer 16

conserve resources

divide resources among more offspring

maximize reproductive performance of the female

Question 17

parental conflict hypothesis

Answer 17

“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

Question 18

examples of imprinted genes

Answer 18

on slides

Question 19

coadaptation hypothesis

Answer 19

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.

Question 20

first imprinted genes discovered in mice

Answer 20

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)

Question 21

Igf2

Answer 21

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.

Question 22

number of imprinted genes

Answer 22

100-200 but varies depending on source

Question 23

imprinted gene characteristics

Answer 23

> 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)

Question 24

ICR

Answer 24

imprinting control region ICRs

show parental allele-specific DNA methylation and chromatin modifications

An ICR is active when unmethylated and inactive when methylated.

Question 25

ICR structure

Answer 25

differentially methylated regions DMR

methylation of DMR or ICR will inactivate the gene

Question 26

IGF2-H19 cluster

Answer 26

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.

Question 27

how does inactivation of imprinted genes occur?

Answer 27

nucleosomal condensation via deacetlation and methylation.

gene condensed, transcription factors cannot access DNA, cannot be transcribed.

Question 28

how does activation of imprinted genes occur?

Answer 28

acetylation and demethylation.

chromatin is opened, DNA can be accessed and transcribed.

Question 29

mechanisms of methylation

Answer 29

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.

Question 30

when are imprints established

Answer 30

read up on paper cos she dont make sense

Question 31

mouse chimaera experiments

Answer 31

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.

Question 32

mouse chimera phenotypes

Answer 32

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.