Imprinted genes in mammals

Question 1

What was the earliest indication that maternal and paternal genes are not equivalent?

Answer 1

The mule and hinny.
Horse (mare) and Donkey (jack) = Mule
Donkey (jenny) and Horse (stallion) = Hinny

Hinny has bad temperment and less well suited to domestication than a reliable mule

Question 2

Give another example of maternal and paternal genes not eqivalent.

Answer 2

Liger = Male lion and female tiger.
Tigon = Male tiger and female lion.

Males are sterile and on ocasion females can produce offspring.

Question 3

Why might ligers be larger and heavier?

Answer 3

Ligers tend to be larger and heavier than members of their parentspecies. Biologists suggest that the liger’s large size, or “growth dysplasia,” results from the absence of certain growth-limitinggenes.

Question 4

Whats the hypothesis that ligers are large?

Answer 4

Female lions mate with several male lions throughout their lives, so the genes of a male lion are adapted to maximize the growth of his offspring, since his offspring may be required to compete with those of other males produced by the same lioness.

The genes of female lions, however, are adapted to cancel or dampen the effects of the growth-maximizing genes of male lions, so lions remain within a given size range.

Tigers, on the other hand, have no such competitive mating strategy, and many biologists argue that tigresses do not possess the growth-limitingadaptationsof their lioness counterparts.

As a result, the influence of the growth-maximizing adaptations provided by male lions is greater, which allows ligers to become larger than their parents.

Question 5

Why are tigons only slighter larger than their parents?

Answer 5

For tigons, growth-limiting genes are found in both male tigers and female lions, so their offspring possess an abundance of these genes, which accounts for their smaller size.

Question 6

What do vast majority of genes in parental genomes do?

Answer 6

Cooperate to make a healthy human.

Question 7

What is essential for embryo development?

Answer 7

Both paternal and maternal genes.

Question 8

Where did the first evidence of genomic imprinting come from?

Answer 8

Two studies in mice. (see slide diagram)

Surani,M.A et. al. Nature 308, 548–550 (1984).
McGrath and Solter (1984)

Question 9

Describe the experiment which led to the finding of genomic imprinting

Question 10

Gynogenetic diploids

Answer 10

Infection of two female (haploid) pronuclei into a mouse egg

Question 11

Androgenetic diploids

Answer 11

Injection of two male (haploid) pronuclei into a mouse egg

Question 12

What was the outcome of this experiment which discovered genomic imprinting?

Answer 12

Even though both egg cells now contain a ‘diploid’ level of DNA, a normal embryo did not develop in either case.

As even the XX individuals did not survive, the authors concluded that this could not be due to the sex chromosomes.

Control embryos from a paternally-derived pronucleus and maternally-derived pronucleus developed normally when transferred into a foster mother.

It was down to genomic imprinting

Question 13

What is imprinting essential for?

Answer 13

Normal development in mammals.

Question 14

Whats an classic example of genomic imprinting

Answer 14

Maternal imprinting of IGF-II in mice.
DeChiara et al Cell (1992)

Question 15

DeChiara et al Cell (1992)

Answer 15

Targeted knockout of the insulin-like growth factor II gene results in growth-deficient (small) mice.

Heterozygotes of both sexes* are small
*This is not a sex-linked trait (IGF II is on chromosome 7)

Heterozygotes of both sexes* normal size

Question 16

What is the explanation of results of this experiment testing imprinting of IGF-II in mice. ?

Answer 16

The maternal copy is silenced.

So if you have a mini-mum and a regular-sized Dad, you are a regular sized mouse. Mini-Dad; Regular mum you are a mini mouse. This is because the phenotype is controlled only by the paternal copy.

Question 17

(partial_ pedigree if IGF2 mutant animals

Question 18

What was in situ hybridisation used to show?

Answer 18

Went on to show that only the paternal gene is expressed in the developing embryo.

Question 19

Conclusions from IGF-II imprinting paper

Answer 19

Transmission of the IGF-II mutation through the male germline results in heterozygous progeny that are growth deficient.
When the disrupted gene is transmitted maternally, the heterozygous off-spring are phenotypically normal.
The difference in growth phenotypes depends on the type of gamete contributing the mutated allele.
Homozyous mutants are indistinguishable in appearance from growth-deficient heterozygous siblings
Only the paternal allele is expressed in embryos, while the maternal allele is silent

Question 20

Key findings from the imprinting paper

Answer 20

In mice the paternal and maternal members of some autosomal gene pairs are functionally non-equivalent.
The IGF-II locus has since been studied in great detail and a lot of our current knowledge comes from manipulation of this system

Question 21

What do nuclease protection and in situ hybridisation analyses of the transcripts from the wild-type and mutated alleles indicate?

Answer 21

That only the paternal allele is expressed in embryos, while the maternal allele is silent.

Question 22

WHat is this now a model of?

Answer 22

This is now a model for studying imprinting – most of the molecular details of imprinting in mammals is based on mouse studies
mice that carry a targeted disruption of the gene encoding insulin-like growth factor II (IGF- II).

Question 23

What do chromatin states govern?

Answer 23

Gene activity.
Euchromatin and heterochromatin.

Question 24

Activate state of chromatin

Answer 24

Histone acetylation by HAT (p55)

Histone acetyl-transferase (HAT) targets H3 tail.
H3K14ac is a docking site for Bromo-domain proteins
Stimulates nucleosome accessibility and transcriptional activity.

Question 25

What is HDAC (Rpd3) reversed by?

Answer 25

histone de-acetylases - cause transcriptional repression.

Question 26

Histone repressed state.

Answer 26

Histone methylation by histone lysine methytransferase (KMT) Su(var)3-9 homologue 1 (SUV39H1)

Histone lysine methyltransferases (KMT) methylates H3 N-terminal tail.
H3K9me3 provides docking site for (chromo)-containing heterochomatin protein 1 (HP1)
Impairs nucleosome accessibility and induces gene repsression.

Question 27

Modifying enzymes and associated mechanisms in chromatin

Answer 27

alter chromatin in response to physiological or pathological signals have transformed our knowledge of epigenetics from a collection of curious biological phenomena to a functionally dissected research field.

Question 28

Name chief mechanisms that alter chromatin structure and function in an inter-dependent fashion.

Answer 28

Histone modifications (Mod)
DNA methylation (Me)
Histone variants and remodelling (yellow nucleosome).
Non-coding RNA (ncRNA; wavy blue lines),

Question 29

What adaptations of chromatin template have been associated with various functions of epigenome?

Answer 29

DNA methylation (DNMT and TET).
Imprinting (Air).
Epigenomic signatures (All instructive chromatin alterations).

Question 30

What is the molecular basis of epigenetic silencing: DNA methyaltion

Answer 30

Demethylation of CpG islands at promoter sequences is associated with active genes (transcription)
Methylation at CpG islands is associated with silencing of that locus.
At a maternally imprinted locus would the promoter be methylated or demethylated on the maternal allele?

Question 31

Maternal imprinting

Answer 31

Allele of a particular gene inherited from the mother is transcriptionally silent (not expressed!)
Direct observation of the phenotype governed by the paternal allele

Question 32

Paternal imprinting

Answer 32

Allele of a particular gene inherited from the father is transcriptionally silent (not expressed)
Direct observation of the phenotype governed by the maternal allele

Question 33

State genes that are imprinted in mice

Answer 33

Paternally - IG-DMR, H19-igf2, Rasgrf1
Maternally - Nespas, Peg3, Snrpn, Kcnq1, Grb10, Igf2r and Gnas.