Genomic Imprinting Disorders Flashcards
What is androgenesis?
Development of an embryo containing only paternal chromosomes.
What is parthenogenesis?
Reproduction by development of an unfertilized female gamete.
-e.g. gecko
What is a hydatidiform mole?
Mass of tissue in womb that won’t develop into a baby.
- androgenetic (usually 46, XX)
- false pregnancy
What is a potential complication of a hydatidiform mole?
Can develop into malignant trophoblastic tumour.
What can parthenogenesis cause?
Benign ovarian teratomas.
- from oocytes than have complete 1st/both meiotic divisions
- predominantly epithelial tissue
Are androgenesis and parthenogenesis viable?
Not in humans.
-even though there’s a normal chromosome number
Why do pathenogenesis embryos die?
Due to failure of development of extraembryonic structures.
-e.g. trophoblast, yolk sac
Why do androgenetic embryos die?
Poor embryo development.
-well developed extra-embryonic membranes
What stage do androgenetic embryos die at?
6 somite stage.
What is genomic imprinting?
Certain genes are expressed in a parent-of-origin specific manner.
-if a gene is imprinted, it’s silenced (epigenetic)
What are the main features of angelman syndrome?
- Facial dysmorphism (smiling)
- Mental disability
- Seizures
What are the main features of Prader-Willi syndrome?
- Hypotonia
- Mental disability
- Hypogenitalism
- Hyperphagia (obesity)
How are Angelman syndrome and Prader-Willi syndrome linked genetically?
Both due to deletion on chromosome 15 (q11-13).
- always de novo
- UBE3A gene
What is the genetic difference between Angelman syndrome and Prader-Willi syndrome?
Angelman is due to maternal chromosome deletion.
Prader-Willi is due to paternal chromosome deletion.
What is DNA methylation?
Epigenetic mechanism to control gene expression.
- CH3 added to cytosine
- doesn’t normally alter DNA sequence
What is the effect of CpG island methylation?
The gene is silenced.
What enzyme is involved in DNA methylation?
DNA methyltransferases.
What are possible reasons for genomic imprinting?
PATERNAL»_space; increased size and foetal fitness (increased foetal survival).
MATERNAL»_space; decreased size (increased maternal survival).
What are the main features of Beckwith-Wiedemann syndrome?
Foetal overgrowth
Organomegaly
Hypoglycaemia
Tumour risk
What are the main features of Russell-Silver syndrome?
Growth retardation
Triangular face
Asymmetry
How are Beckwith-Wiedemann syndrome and Russell-Silver syndrome linked?
Both involved insulin-like growth factor (IGF2).
- BWS: hypermethylation - increased IGF2
- RSR: hypomethylation - decreased IGF2
What is imprint switching?
Establishment of new parental imprint during gametogenesis.
-erasure of grandparental imprint
What has more genes; chromosome X or Y?
Chromosome X.
What process occurs in females involving the X chromosome?
X inactivation / Lyonisation.
-epigenetic silencing of certain genes
What cells remember / reverse X inactivation?
- Somatic cells remember X inactivation
- Germ cells reverse it
How does X inactivation differ from imprinting?
- Whole X chromosome is silenced
- Random choice of parental chromosome
- Occurs early in embryogenesis (blastocyst)
What causes hypohidrotic ectodermal dysplasia, and what are the main features?
X-linked mutation.
- patches of skin with / without sweat
- starch / iodine test used
When does x-inactivation occur?
When there are a small number of precursor cells.
-random skewing
