Epigenetics and Abnormal Gene Expression Flashcards

1
Q

What is the central dogma of information flow in epigenetics?

A
DNA - loop back via replication 
I    
I    transcription
I
V 
RNA - loop back via replication - can ^ return to DNA via reverse transcription 
I
I    translation
I
V
protein
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2
Q

What are the two structures of note found on the chromosome?

A

Centromere

Telomere

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3
Q

What overlays chromosome structure that can be used for identifying twins, etc?

A

Epigenetic tags

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4
Q

What does epigenetic gene silencing involve?

A

Multiple mechanisms,
DNA methylation
Histone modifications
Nucleosome remodelling

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5
Q

What are epigenetics?

A

The study of:
• Heritable modifications of DNA that do not alter the primary sequence (e.g. me-C at CpG)
• Result: Altered gene expression

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6
Q

What is DNA methylation chemically?

A
  • Covalent modification of a methyl group to cytosine at position C5 to make 5-methylcytosine.
  • Most genes have GC rich areas of DNA in their promoter regions - CpG islands.
  • Methylation of the C residues within the CpG islands leads to gene silencing
  • Little or no detectable DNA methylation in yeast and Drosophila
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7
Q

Is DNA methylation heritable?

A

Yeah

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8
Q

Provide a broad overview of the effects of DNA methylation?

A
  • DNA methylation inhibits gene transcription
  • Methylation prevents the binding of transcription factors to the promoter and inhibits transcription by converting chromatin from an open to a closed conformation
  • Methyl CpG binding proteins contain a methyl binding domain that specifically recognizes methylated CpGs
  • Recruits other proteins such as histone deacetylases which remove acetyl groups, favouring compact chromatin
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9
Q

What diseases may be linked to defects in DNA methylation machinery?

A

X-linked syndromes with variable phenotypes

e.g. Rett Syndrome:
dominant X-linked, neurodegenerative disorder
affects 1:10,000-15,000 (females only)
Caused by a mutation in the gene encoding Methyl-CpG-binding protein 2 (MeCP2),
which in turn leads to loss of gene silencing at many loci.

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10
Q

What is Prader-Willi syndrome?

A

– mental retardation

– obesity

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11
Q

What is Angelman syndrome?

A

– mental retardation
– “happy puppet” syndrome
– jerky movements + inappropriate laughter

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12
Q

How are Prader-Willi and Angelman syndromes related?

A

They are both chromosome 15 abnormalities, however AS is a maternal deficiency and PWS is a paternal deficiency
Both due to defects in imprinted genes: defective expression in brain tissue

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13
Q

Compare heterochromatin and euchromatin

A
Heterochromatin
Highly condensed in interphase
Transcriptionally inactive
(contains few genes)
Replicates late in S phase

Euchromatin
Organized in 30nm fiber during interphase
Transcriptionally active
Replicates early in S phase

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14
Q

Does DNA higher order influence gene expression?

A

Yeah

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15
Q

What can DNA higher order influence?

A

Gene expression

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16
Q

What is the position effect?

A

The spreading of heterochromatin
into euchromatic regions causes
cell to cell variability
in gene expression

17
Q

What is the spreading of heterochromatin into euchromatic regions causing
cell to cell variability
in gene expression called?

A

position effect

18
Q

What does the spreading of heterochromatin into euchromatic regions cause?

A

Cell to cell variability in gene expression

19
Q

At what stage does the ratio heterochromatin in euchromatin regions become stable?

A

Early in the developing embryo, heterochromatin forms and spreads into neighbouring euchromatin to different extends in different cells.
These expression states are stably inherited in the daughter cells

20
Q

Describe X-chromosome inactivation

A
  • Discrepancy of 1 X-chromosome in males (XY) but 2 X-chromosomes in females (XX)
  • Solution - Females need to silence one X-chromosome - X-chromosome inactivation (Mary Lyon, 1961)

• Mechanism of silencing is initiated by Xist
- X-inactive-specific-transcript ‘marks’ inactive X:
only expressed from inactive X-chromosome and codes for an RNA (~17kb in humans). No protein product and RNA remains in the nucleus. Followed by DNA methylation.

21
Q

When is DNA methylation reset?

A

DNA methylation completely reset upon fertilisation

22
Q

What are Barr bodies?

A

A small, densely staining structure in the cell nucleus of females, consisting of condensed, inactive X chromosome

23
Q

What unusual inactivation scenarios can occur in X chromosome inactivation?

A

• Only 1 X chromosome should remain active:
so in rare examples such as a male with Klinefelter’s syndrome (where the genotype is 47XXY), the number of Barr bodies would also be 1.

24
Q

What is the classic cat based example of X chromosome inactivation?

A

• Calico (tortoiseshell) cats are female
• Random X-chr inactivation manifested
in coat colour (The paternal or the maternal X-chromosome express either orange or black coat-colour gene; white is autosomal)

25
Q

Describe genomic imprinting

A

• ~200 imprinted genes on autosomes
• Imprinted genes only expressed from one allele
• Dependent on parental origin
• Imprinting resets on passage through germline
• Patterns reset in gametes
that escape wave of de-methylation just after fertilisation
• In sperm, imprint is “paternal”
• In eggs, imprint is “maternal”

26
Q

What is uniparental disomy

A

• Both copies of a chromosome are inherited from the same parent
• Individual is missing the chromosome from one of the parents
• Expression altered of imprinted genes on affected chromosome (non-imprinted genes are not affected)
• Result: chr 15, PWS/ AS
chr 11, Wilms’ tumour

27
Q

Go into detail on the angelman syndrome

A

• genetic disorder that affects the nervous system and causes severe physical and intellectual disability
• caused when the Angelman gene, known as UBE3A, is either absent or malfunctions
• In most cases of Angelman syndrome (about 70%),
the child’s maternal copy of the UBE3A gene is missing (deleted), which means there’s no active copy of the UBE3A gene in the child’s brain. In a small number of cases, Angelman syndrome occurs when a child inherits two copies of chromosome 15
from the father, rather than inheriting one from each parent.
This is known as (paternal) uniparental disomy.

28
Q

Go into detail on Praeder-Willi Syndrome

A

• rare genetic condition that causes a wide range of physical symptoms, learning difficulties and behavioural problems
• caused by a fault in a group of genes on chr15
• fault leads to a number of problems and is thought to affect the hypothalamus, which produces hormones and regulates growth and appetite.
• Most cases of Prader-Willi syndrome (about 70%) occur when a segment of
the paternal chromosome 15 is deleted genes which means there’s no active copy of chr 15 in the child’s brain. In another 25%, a person has two copies of chr 15 inherited from the mother. This is known as (maternal) uniparental disomy.

29
Q

What results in uniparental isodisomy or uniparental heterodisomy?

A
  • Non-disjunction in meiosis II -> uniparental isodisomy

* Non-disjunction in meiosis I -> uniparental heterodisomy

30
Q

What are some diseases that are associated with genomic imprinting?

A

Beckwith–Wiedemann (BWS) syndrome
Wilms’ Tumour
Fragile X syndrome
Myotonic dystrophy (congenital) Prader–Willi syndrome Angelman syndrome