Imprinting

Question 1

How is IGF2 imprinting regulated through epigenetics?

Answer 1

IGF2 is paternally expressed (growth factor), the maternal chromosome expresses H19 - a ncRNA. A DMR (differentially methylated region) is methylated on the paternal chromosome to allow the enhancer to act on the IGF2 gene promoter, whilst the maternal chromosome is non-methylated. This allows binding of the CTCF insulator protein, causing the enhancer to not be able to ‘reach’ the promoter of the IGF2 gene, instead activating transcription of H19.

Question 2

What is the genetic evidence from mChr2 for imprinting?

Answer 2

Mice with a paternal duplication have broad flat backs and are hyperkinetic
Mice with a maternal duplication have narrow bodies and fail to suckle and are hypo kinetic
Indicates imprinted genes on the disomic segment

Question 3

How were imprinted genes mapped?

Answer 3

By analysing uniparental disomies for most chromosomes

Question 4

How was Igf2 imprinting discovered?

Answer 4

Paternal knock out and double knock out had the same phenotype, whilst a maternal knock out had a normal birth weight. Suggested gene was expressed from paternal chromosome only.

Question 5

What is the lifecycle of an imprinted genes?

Answer 5

Imprinting is established in gamete formation, maintained through embryogenesis (survives epigenetic reprogramming) and then read in life. In the gonads, imprinting is erased then reestablished to start over again

Question 6

How is imprinting maintained through early development?

Answer 6

Paternal allele is protected against de novo methylation though H3K4me3 markers
Maternal allele maintains DNA methylation through imprint specific factor Zfp57 - a zinc finger protein that binds to imprint control regions and recruits Dnmt1 and other methylators.

Question 7

How can viable mice be generated from bi-maternal embryos?

Answer 7

Engineer one of the chromosomes to imprinting control regions to prevent repression.

Question 8

What molecular changes cause human imprinting disorders?

Answer 8

Uniparental disomy
Chromosomal rearrangements (deletions, duplications, translocations)
Intragenic mutations
Epimutations (aberrant methylation of a differentially methylated region without alteration of the genomic DNA sequence. Could be primary - no DNA sequence change or secondary - occurs as a result of a DNA mutation acting in cis or trans)

Question 9

What does the imprinting control region ICR1 do?

Answer 9

At the Igf2/H19 imprinting region. Hypomethylation of ICR1 on the maternal chromosome allows binding of CTCF which is a barrier to enhancer looping, thus preventing production of the Igf2 gene, instead inducing expression of the H19 ncRNA
Structure of the gene region:
Igf2 gene — ICR1 — H19 — enhancer x2

Question 10

What do disturbances in Igf2 imprinting result in?

Answer 10

Over expression of Igf2 - beckwith-Wiedemann syndrome. Over growth. ICR is methylated on maternal chromosome preventing CTCF binding and allowing enhancer looping
Under expression of Igf2 - Silver-Russel syndrome. ICR is hypomethylated on paternal chromosome, allowing CTCF binding and preventing enhancer looping

Question 11

What does the imprinting control region ICR2 do?

Answer 11

Is methylated on the maternal allele, preventing production of the ncRNA Kcnq1ot1.

Question 12

What is imprinting?

Answer 12

Genes are expressed in a parent-of-origin specific manner. Genes are expressed from the non-imprinted allele. Genes can be silenced through DNA methylation, histone modifications or regulatory RNAs

Question 13

What genes are imprinted and why?

Answer 13

Genes involving foetal growth, placental growth, suckling and nutrient metabolism.
A mechanism to balance parental resource allocation in the offspring

Question 14

What are imprinted DMRs?

Answer 14

Differentially methylated regions due to imprinting. As methylation of genes is reset during the initial divisions of a zygote, these regions escape the reset.
Establishes transgenerational epigenetic inheritance

Question 15

Describe imprinted DMRs

Answer 15

Cytosines are methylated.
Imprinted regions are normally found in clusters. Genes tend to have a CpG rich DMR (differentially methylated region) related to allele repression. Get allelic histone modifications. Often have a high number of tandem repeats and presence of CTCF and YY1 transcription factor binding sites and ncRNA transcriptional units

Question 16

How did imprinting evolve?

Answer 16

Mainly found in placental (eutherian) animals.
Kinship theory - paternal genes drive foetal growth by extracting maximum resources from the mother whilst maternal genes ensure the mothers survival and equal allocation of nutrients between offspring

Question 17

What is parthenogenesis?

Answer 17

Reproduction in which an organism develops from an unfertilised germ cell/development of a germ cell without fertilisation.
Komodo dragons can replicate without males

Question 18

What is gynecogenesis in humans?

Answer 18

Parthenogenesis in which ovarian teratomas (dermoid cysts) form. All chromosomes come from the mother

Question 19

What is androgenesis in humans?

Answer 19

Hydatidiform mole - a tumour in the uterus where all chromosomes come from the male partner. Usually an empty ovum is fertilised by sperm and the genome duplicates.

Question 20

What are the types of epigenetic disease?

Answer 20

Embryo/Placenta (ovarian teratoma, hydatidiform moles)
Brain - behaviour, psychiatric, neurodegenerative
Endocrine
Growth - cancer, nutrient metabolism and many others

Question 21

What is Prader-Willi syndrome?

Answer 21

An imprinting disorder due to abnormal imprinting at 15q11-13 (opposite is Angelman syndrome). Leads to weak muscle tone and floppiness at birth, poor suckling, obesity (overeating), hypogonadism (immature sexual characteristics), CNS and endocrine gland disfunction - varying learning disability

Question 22

What is Angelman syndrome?

Answer 22

An imprinting disorder due to abnormal impritining at 15q11-13 (opposite is Prader-Willi syndrome). Developmental delay with severe speech impairment, behavioural uniqueness (laughing, smiling, hand flapping), seizures, microcephaly, movement or balance disorder

Question 23

Describe the chromosome region associated with Prader-Willi/Angelman syndrome

Answer 23

Chromosome 15, q11-13. Single bipartite imprinting control region controls a whole gene cluster
Many are expressed only/preferentially on the paternal chromosome (including small nucleolar RNAs - snoRNAs). 2 genes are expressed on the maternal chromosome

Question 24

How does Prader-Willi/Angelman syndrome arise?

Answer 24

PWS: loss of the paternal chromosome expression of snoRNAs (and other) (through deletion or through inheritance of 2 maternal chromosomes and no paternal or rarely through a translocation). The maternal chromosome is silenced, so genes such as snoRNAs are not made
AS: loss of maternal chromosome expression of UBE3A (through deletion or mutation in the gene region, inheritance of 2 paternal chromosomes and no maternal or rarely through translocation). In most tissues of the body, both paternal and maternal UBE3A is expressed, but in certain areas of the brain only the maternal copy is expressed