Genomic imprinting Flashcards
What is genomic imprinting? (2)
- Parent of origin specific gene expression in mammals
- Some mutant phenotypes are only expressed in progeny when the mutation is transmitted from a particular parent
What is an example of genomic imprinting? (5)
- Thp mutation leads to viable progeny when transmitted from father
- Thp mutation leads to inviability when transmitted from mother, causes increased body size
- These progeny are genotypically identical but one is normal and one is inviable
- Not linked to progeny sex because Thp mutation is autosomal (chromosome 17) and progeny can be male or female
- Implies that mammalian male and female gametes have distinct developmental potential
How are embryos with 2 male/2 female pronuclei constructed? (5)
- Maternal pronucleus is smaller than paternal pronucleus
- Remove paternal pronucleus from host mouse zygote and replace with a second maternal pronucleus from a second donor zygote to create a diploid gynogenetic embryo
- Do the same with paternal pronuclei to create a diploid androgenetic embryo
- Embryos with 2 male/2 female pronuclei are lethal
- Male AND female pronuclei are required for normal embryonic development
Why do diploid androgenetic embryos fail to develop to term?
Development of embryo and yolk sac is very impaired but trophoblast develops to normal size
Why do diploid gynogenetic embryos fail to develop to term?
Development of embryo is relatively normal but yolk sac and trophoblast development are both severely impaired
What are the paternally expressed imprinted genes? (2)
- Embryonic growth-promoting genes
- Igf2, Peg1, Peg3, Rasgfr1, Dlk1
What are the maternally expressed imprinted genes? (2)
- Embryonic growth-repressing genes
- Igf2r, Gnas, Cdkkn1c, Grb10
What is the parental conflict hypothesis? (3)
- Embryos of placental mammals depend on the maternal environment for nutrition during gestation
- The paternal genome optimises its fitness by maximising the embryo’s use of maternal nutritional resources i.e. by paternal expression of growth promoters
- The maternal genome optimises its fitness by limiting exploitation of maternal nutritional resources by the embryo i.e. by maternal expression of growth repressors
What are the key characteristics of genomic imprinting in mammals? (4)
- DNA methylation is the only modification that functions as the molecular imprint, histone modification plays a minor role
- The 7 clusters each have a differentially methylated region (DMR), a DNA sequence with a parent-of-origin specific DNA methylation imprint
- Many clusters of imprinted genes are controlled by gametic DMRs that function as regulatory elements for the cluster overall called imprint control elements (ICEs)
- Most of the 7 clusters contain a combination of protein coding and non-coding genes
How many of the 7 clusters have a maternal methylation imprint laid down in the oocyte?
5/7 - Igf2r, Kcnq1, Gnas, Grb10
How many of the 7 clusters have a paternal methylation imprint laid down in the sperm?
2/7 - Igf2, Dlk1
What are gametic DMRs?
Methylation imprints laid down in the oocyte/sperm
How many imprinted genes are there? (3)
- Around 150 imprinted genes identified
- 80% of these are clustered in 16 genomic regions containing 2 or more genes
- 7 clusters are well characterised and contain 3-12 imprinted genes
How is genomic imprinting affected by the genome wide DNA demethylation after fertilisation? (2)
- Imprinted DMRs are a special subset of DMRs which persist in SOMATIC tissues on paternal/maternal chromosomes throughout embryogenesis and into adult life (don’t lose the imprint)
- Genomic imprints ARE erased in the primordial GERM cells of the early embryo before sex determination and a new imprint is created in gametes according to the chromosomal sex of the embryo
What machinery is involved with erasure and creation of new imprints in gametes? (3)
- Erasure: TET demethylases and/or passive demethylation
- Acquisition: DNMT3A, DNMT3B de novo methyltransferases
- Maintenance: DNMT1 maintenance methyltransferase
What is the T hairpin (Thp) mutation? (3)
- Deletion (i.e. complete loss of function) on chromosome 17 (autosomal)
- Imprinted gene is Igf2r
- DNA methylation imprint is located in the second intron of Igf2r
Which combination of Thp mutation/wt results in normal progeny? (2)
- Thp mutation on paternal chromosome 17
- Wildtype on maternal chromosome 17
Which combination of Thp mutation/wt results in inviable progeny? (2)
- Thp mutation on maternal chromosome 17
- Wildtype on paternal chromosome 17
What is the function of IGF2R? (3)
- IGF2R (receptor) is a scavenges IGF2 and targets IGF2 to lysosomes
- Prevents IGF2 binding to IGF1 receptor and causing growth-promoting effects of IGF2 signalling
- Therefore IGF2R is a growth repressor
Why does embryo with maternally-derived Thp mutation show increased body size? (3)
- Loss of function of IGF2R so overactivation of IGF2 growth-promoting signalling
- Paternal copy isn’t doing growth suppression (silenced)
- When Thp mutation is paternally-derived, the maternal IGF2R copy is able to repress growth as normal
How does maternal/paternal differential methylation of Igf2r work? (5)
- Counterintuitive: maternal G-DMR is methylated and Igf2r is expressed, paternal G-DMR is unmethylated and Igf2r is silenced
- The methylation of the Igf2r G-DMR prevents transcription of the overlapping 108kb long ncRNA Airn RNA on the maternal chromosome
- Airn transcript attenuates Igf2r expression therefore silencing Airn de-represses Igf2r on the maternal chromosome
- No paternal methylation so Airn is expressed and silences Igf2r
- Igf2r G-DMR is an imprinting control region (ICE)
How are Igf2r and Igf imprinting linked? (3)
- Complementary imprinting
- Maternal Igf2r is methylated which causes Airn repression and Igf2r expression
- Paternal Igf2 is methylated which prevents CTCF binding so a downstream enhancer can activate Igf2 transcription
What is CTCF?
Insulator element across which transcription factors can’t function
Why isn’t Igf2 maternally expressed? (2)
- Maternal Igf2 ICE is unmethylated which allows it to bind CTCF
- CTCF prevents Igf2 being activated by a downstream enhancer which instead activates expression of H19 long ncRNA
