Genomic Imprinting Flashcards
How was genomic imprinting discovered?
Make an:
• Gynogenetic embryo → take an oocyte, remove a pronuclei and insert an additional pronuclei from another oocyte and fuse. Gives an embryo derived entirely from maternal DNA
• Androgenetic embryo → take an oocyte, remove both female pronuclei and insert instead two sperm heads. Gives an embryo derived entirely from paternal DNA
What are the consequences?
• Gynogenote → get an embryo with normal anatomy (just smaller), however you get a very diminished trophectoderm, so you cant develop a placenta. Embryo wont survive past 10 days gestation.
• Androgenote → embryo is very underdeveloped and doesn’t have any normal anatomy. The trophectoderm has formed normallu.
Summary:
• You need both a sperm head and egg to give the normal development
• Maternal genome contributes significantly to the development of the fetus
• Paternal genome contributes significantly to the development of extra-embryonic tissues (placenta)
So, Parental genomes are therefore not functionally equivalent
• Mouse embryos with only maternal or paternal chromosomes wont develop normally
• Suggests existence of genes that are only expressed from one parental genome or the other → genomic imprinting.
What is the mechanism of imprinting?
• Most chromosomes are autosomes and most genes are biallelically expressed genes
• In case of imprinted genes:
− Paternally imprinted → if you inherit the allele from the father, it will be expressed. If you inherit the allele from the mother, it wont be.
− Materally imprinted → if you inherit the allele from the mother, if will be expressed. If you inherit the allele from the father, it wont be.
The mechanism is by epigenetics:
• Methylation and deacatylation keeps DNA tightly wrapped around histones
• Demethylation and acetylation decondenses DNA to allow it to be transcribed
• Epigenetic markers will be put on histones (often H3K9) , these mark for acetylation or methylation
What is the fate of imprinted genes in somatic cells vs gametes?
- a are maternally expressed alleles
- b are paternally expressed alleles
➢ In embryo somatic cells → expression depends on parental origin.
− Embryo expressed a1, because this was inherited form the father
− Embro will express b2, because this was inherited from the mother
− Wont express a2, because a is a paternally imprinted allele, and this embryo received a2 from the mother, so wont be expressed.
− Wont express b2, because b is a maternally imprinted allele, and this embryo received b2 from the father, so wont be expressed.
➢ In the embryo gametes → parental imprint is erased, and reset according to the sex of the developing embryo
− If the embryo is male, the a copies of the gene are expressed (so both a1 and a2)
− If the embryo is female, the b copies of the gene are expressed (both b1 and b2)
What is meant by the 150M year war between the sexes and what is the relationship to Mendels first law?
- Genomic imprinting sometimes called the 150M year war because it is only observed in the mammalian evolutionary lineage
- Imprinted genes are rare → total to date is just over 100
- They tend to be on clusters of maternal/paternal genes on chromsomes
- Many found clustered on chromosome 7
- They fall easily into specific functional groups
Paternal Genes Growth enhanced Muscle differentiated Hypothalamic development Hyperkinetic behavior
Maternal Genes Growth decreased Epidermis differentiated Neocortical development Hypokinetic behaviour
• Genomic imprinting violates Mendel’s first law
− The Law of Segregation states that every individual possesses a pair of alleles for a particular trait, and that each parent passes a randomly selected copy of only one of these to its offspring. The offspring thus has its own pair of alleles for that trait, and whichever of the two alleles is dominant determines how the offpsing expresses that trait.
Describe the Haig Hypothesis (the Parent/Offspring conflict theory of genomic imprinting) and how it lead to imprinting in mammals.
• Parents and offspring are only ‘interested’ in their own propagation
• Genes expressed on the offspring have only a 50% relatedness to the maternal genome
• Maternal genes in different fetuses are more likely to be related to each other than the paternal genomes due to polyandry → offspring in a litter may have different fathers
• Selection thus favours a more selfish approach by paternally derived genes
➢ Prediction → genes which extract resources from the mother are more likely to be imprinted.
This relates to Richard Dawkins ‘The Selfish Gene’
➢ A single gene is just trying to get more numerous in the gene pool
➢ It does this by helping to program the bodies in which it finds itself to survive and reproduce
David Haig suggested imprinting may exist because of genetic conflict:
• Paternal genes enhance resource acquisition by the offspring in the interests of fitness (fits in with them contributing to placental development)
− In a polyandrous species, embryos sired by this male will grow more rapidly than embryos sired by other males.
• Maternal genes act to restrain and divide resources to benefit the limited reproductive fetness of the mother.
Genomic Imprinting in Mammals
• Imprinted genes from the male genome are strongly expressed in the placenta → the placenta extracts resources from the female.
Conflict theory predictions:
• Paternally expressed genes enhance fetal growth
− Selected to extract resources from the mother
• Maternally expressed genes suppress fetal growth (to preserve the mother)
− Selected to inhibit the transfer of resources to the fetus
eg) Conflict for nutrient resources
In polyandrous mammals:
• 2 pairs of paternal alleles present (ie, 2 fathers)
• 1 pair of maternal alleles present
• The degree of relatedness of paternal and maternal genome is low
• This gives rise to genetic conflict to extract resources from the mother
− When paternally derived genomes are in competition with those of other males, you want the offspring to grow more rapidly than those of other males, so selection will favour genes that extract resources from the mother.
In monogamous mammals:
• 1 pair of maternal allels and 1 pair of paternal alleles
• There isn’t competition for any of the litter to grow rapidly, so don’t need to extract resources from the mother
→ Evolution of genomic imprinting therefore likely in species where polygamy is common.
Describe the imprinting features of IGF2
- First imprinting gene to be discovered
- Essential for life
IGF2 is paternally imprinted
• IGF2 is not expressed if it comes through the female germline, but it is if it comes through the male
• If you mate a mate with a mutated gene with a female with a normal gene, offspring is abnormal
• If you mate a female with a mutated gene and a male with a normal gene, offspring are normal → genomic imprinting has silenced the mutation.
however!
The soluble IGF2 receptor is maternally imprinted
• If you only have the paternally imprinted IGF2, you get overgrowth
• A maternal imprint makes a soluble receptor which binds IGF-2 and renders it inactive
→ Both copies therefore required for normal growth!!
So: • Paternal imprint codes for IGF2 − Extracts resources from the mother − Promotes growth − Key in placental development • Maternal imprint codes for soluble IGF2 receptor − Acts to modify the action of IGF2
• Imprinting has only been observed so far in the vertebrates → doesn’t happen in monotremes (and these do not produce a placenta)
What do transgenic markers tell us about the functions of maternal and paternal derived genes?
- Take a gene expressed in every cell
- Put a lacZ cDNA gene infront of the promoter
- Make a transgenic mouse with this construct
- All cells should stain blue when exposed to beta-galactosidase.
- Take the transgenic mouse and use this to culture a gynogenetic/androgenetic emnryo
- Dissaggregate the embryos and mix the cells in with a normal mouse embryo
- The embryos will grow normally as long as the ratio of uniparantel cells is
Describe the imprinting features of Peg1
Lefevre et al, 1998
• In the adult, Peg1 is strongly expressed in the brain, in structures we know are sexually dimorphic: − Olfactory bulb − Hippocampus − Amydala − Hypothalamus
What effect does Peg1 deletion have?
• Serious effect on maternal behavior:
− They investigate pups normally and can find hidden food (so olfaction is okay)
− But they do not retrieve pups well and do not build nests for them.
• Mothers with deleted paternal allele crossed with males with a deleted allele produce pups that don’t survive.
Summary of Peg1 deletion:
• If the paternal allele is knocked out in the pup, they grow badly because of poor placental development
• If the paternal allele is knocked out in the mother, and she mates with a wild-type male, the pups are normal because they get their active allele from the father
− However, they survive poorly because the knock out mother has defective maternal behavior
− If the pups are cross-fostered, they will survive well
Peg1 deletion confirms the Haig hypothesis:
• A paternally expressed gene influences fetal growth and development
• The same gene affects maternality in the female, and therefore pup survival
Describe the imprinting features of peg3
Li et al, 1999
Peg3:
• Also paternally imprinted
• Also expressed in many brain structures involved in maternal behavior
What effect does Peg3 deletion have>
• Defective maternal behavior
− Increased latency of pup retrieval
− Increased time spent to retrieve all pups
− Increased latency of nest building
− Increased latency of crouching over pups
− Pups have decreased weight gain before weaning
Peg3 acts on oxytocin pathways
• Peg3 mutants have less oxytocin in the PVN and SON
• We already know oxytocin has a critical role in maternal bonding, so we now know it is under the control of an imprinted gene.
What is the placental/brain cross talk in terms of imprinting?
- The bias of behavioural phenotype in the developing brain is likely to be female-based since it is developing under the influence of imprinted genes that are developing the placenta, which produces the hormones that regulate the hypothalamus for maternalism.
- Masculine behavior traits are likely to be determined postnatally as a consequence of homone secretions from the male gonad
- The placental trophoblast produces endocrine secretions that enable the fetus to regulate its destiny, and they often function by acting on maternal receptors. Here, the maternal and fetal genomes seem to function as part of a single phenotype.
Do peg genes influence male reproductive strategy?
- Male rodent sexual behavior and interest in estrous female urine increases after sexual behavior, but not in Peg3 mutant males
- These defects have been traced to the male VNO pathway → olfaction plays a significant role in all aspects of male sexual behavior
How has the whole system of olfaction, maternal bonding etc… transitioned to larger brains?
• In rodent, olfaction plays a key role
• In higher primates, olfaction has been replaced with development of trichromatic vision
• Primates have increased social organization and increased cognitive skills required to act on social information
• This has lead to increasing encephalisation
− This hasn’t been symmetrical
− Those parts of the brain concerted with higher order cognitive capacities get bigger
− Those parts of the brain involved in regulating hormonal control of primary motivated behaviours, such as sex and maternal care, get relatively smaller.
− This is not to say hormones aren’t important, but that higher order cognitive functioning has enabled social interactions to occur outside the context of hormonal priming
Pattern of Primate Evolution
• 50% human olfactory genes pseudogenes
• Decrease in reliance olfactory cues to engage social bonds → has lead to the reorganisaiton of the primary brain detailed above.
• The similarities in gene number across phylogenies, from mouse to human, suggests it is not the evolution of new genes, but the reorganization in expression of conserved genes.
How can imprinting lead to abnormalities in behaviour in humans?
Abnormalities of Behaviour in Humans
Turners Syndrome (XO)
• Ovaries or streak gonads (no germ cells, no steroid)
• Mullerian ducts present (no testis present, so no MIH)
• Wolffian ducts absent (no testosterone)
• No penis (no testosterone)
• No breast development (no ovary, so no estrogen)
• Thickened neck
• Behavioural abnormalities ←
- Normally you get random X chromosome inactivation in somatic cells
- In Turners, you only have one copy of that X, so whatever that X is, it is expressed.
Phsycological profiling of Turner’s syndrome females:
• DNA studies show both X45m and X45P subtypes – depending on whether they had maternal or paternal X present.
• Clinical studies show no differences in physical phenotype of hormone replacement needs
• 40% of X45m were statemented as in needs of special education
• Only 16% of X45p had special education needs
• 75% X45m had clinically significant social difficulties, only 28% X45p
• The specific neuropsychological defects include attention problems, memory impairments, deficits in fear recognition, anxiety, impairments in social cognition.
Skuse et al, 1997:
• Parents asked to rate children 0 (not at all true) – 2 (often true) on a range of behavioural aspects including awareness of peoples feelings, disruptive behavior, awareness of acceptable behavior, ability to follow commands…
• Im those girls with the maternal X chromosome, they scored higher
• Interestingly, all normal males carry a single X chromosome, and they all scored less well on the social cognition test.
A further test looked at IQ:
• X45p subjects had a higher IQ than X45m
→ Data suggests that genes which affect social adjustment are expressed only on the paternal X chromosome.
Links to Autism?
• Studies of Turner;s girls have shown a greater than 400 fold increased chance of autism
• In normal females, occurs at
What are some of the other theories for Genomic imprinting?
Prevention of Gynogenesis
• Proposed through noting of the absence of naturally occurring parthenogenesis in mammals
• Absence implies that both maternal and paternal contributions are essential to the developing mammalian zygote, which would obtain if one or more essential genes were expressed from only the maternal or paternal allele.
Problem:
• The selection pressure evoked is at the level of the group
• An individual that could reproduce gynogenetically would likely have a strong selective advantage
Ovarian Time-Bomb
• Proposed that imprinting evolved to prevent ovarian trophoblastic disease arising from a gynogentically developing (unfertilized) egg in the ovary of a female mammal
• If the maternal copy of an essential gene were inactivated, and hence a paternal contribution necessary for correct development, this ‘ovarian time bomb’ could be defused
• More over, the sorts of genes subject to maternal inactivation would be growth enhancing.
Problem:
• Doesn’t explain why imprinted expression persists long after fertilization
• Ovarian trophoblastic disease less of a problem in taxa with non-invasive placentas, yet imprinting occurs in these.
Dominance Modification
• imprinting is effectively parental specific dominance, and could have evolved in a similar way to genetic dominance
Problem:
• In order to explain this evolution, we need to say there is a selection pressure for imprinting, so there must also be some other model.
Variance Minimisation
• May have evolved to regulate the expression level of a gene
• Might be “easier to keep one faucet dripping at a slow rate, than two faucets dripping at half that rate, the risk being that one or both of the latter faucets may not drip at all”
Problem:
• Such selection is likely to be insufficient for the origin of imprinting, but could maintain it
• This hypothesis only requires inactivation of genes inherited from one parent (which would more liely be the father for the reasons in the parent/conflict hypothesis), so maternal inactivation would be improbable.
Genome Defence
• Imprinting evolved as a by-product of the defence of the genome against transposable elements and retroviruses
• Inspired by the observation that foreign DNA inserts are often silenced by methylation, the same mechanism underlying imprinting
• A number of imprinted genes contain, or are near, such insertions
Problem:
• Would presumably work best if both copies of the foreign genes were silenced, so why only one?
Selection for Parental Similarity
• If there were selection for similarity with the parent of one sex (ie, one sex has higher fitness for some trait), imprinting would be favoured
Problem:
• Doesn’t predict the phylogenetic pattern of observed imprinting cases
Cytonuclear Interactions
• Cytoplasmic organelles are transmitted uniparentally
• This may lead to imprinting thorugh the co-adaptation of the organelle and nuclear genes, which would favour inactivation of the nuclear copy inherited from the parent not transmitting the organelle
• Thus, in mammals which inherit mitochondria maternally, selection should favour maternal imprinting
Problem:
• Doesn’t explain maternal imprinting
