Epigenetics and Abnormal Gene Expression

Question 1

Describe the central dogma of information flow

Answer 1

DNA
(undergoes transcription to become)
RNA
(undergoes translation to become)
protein

Question 2

How can chromosomes be distinguished ?

Answer 2

By size and G-banding

Question 3

What is higher DNA structure in the form of ?

Answer 3

Chromosomes

Question 4

What overlays chromosome structure ?

Answer 4

Epigenetic tags

Question 5

What is epigenetic information ?

Answer 5

An extra layer of contextual information above just the DNA sequence.

Question 6

What happens to epigenetic features over time ?

Answer 6

They diverge and change

(seen in identical twin study - older twins have more divergent patterns of epigenetics)

Still identical DNA

Question 7

What does epigenetic gene splicing involve ?

mechanisms

Answer 7

Multiple mechanisms including:

• DNA methylation
• Nucleosome remodelling
• Histone modifications

Question 8

What is epigenetics ?

Answer 8

Heritable modifications of DNA that do not alter the primary sequence (e.g. me-C at CpG)

Question 9

What is the result of epigenetics ?

Answer 9

Altered gene expression

Question 10

Describe DNA methylation

Answer 10

Covalent modification of a methyl group to cytosine at position C5 to make 5-methylcytosine.

Question 11

Where does DNA methylation occur ?

Answer 11

CpG islands
- regions of genes that control gene expression

Question 12

What are CpG islands ?

Answer 12

Most genes have GC rich areas of DNA in their promoter regions.

These areas are called CpG islands.

Question 13

What causes gene silencing ?

Answer 13

Methylation of the C residues within the CpG islands leads to gene slicing.

Question 14

DNA methylation in yeast and Drosophila

Answer 14

Little or NO detectable DNA methylation

Question 15

Key feature of DNA methylation

Answer 15

DNA methylation is heritable

Question 16

Mechanism of action of DNA methylation

Answer 16

Inhibits gene transcription

Prevents the binding of transcription factors to the promoter

AND

Inhibits transcription by converting chromatin from an open to closed formation.

Question 17

How does DNA methylation inhibit gene transcription ?

Answer 17

Converts chromatin from an open to a closed formation.

Question 18

Function of Methyl CPG binding proteins

Answer 18

Methyl CPG binding proteins contain a methyl binding domain that specifically recognises methylated CpGs.

Question 19

What does DNA methylation recruit ?

Answer 19

Recruits other proteins such as histone deacetylases which remove acetyl groups, favouring compact chromatin.

Question 20

Causes of disease in epigenetics

Answer 20

Defects in DNA methylation machinery

• X linked syndromes with variable phenotypes

Question 21

Rett Syndrome

Answer 21

Dominant X linked, neurodegenerative disorder

Caused by a mutation in gene encoding MeCP2, which in turn leads to a loss of gene silencing at many loci.

Question 22

Who does rett syndrome affect ?

Answer 22

Females only
- males do not survive

Question 23

Prader Willi syndrome signs

Answer 23

PWS
- mental retardation
- obesity

Question 24

Angelman Syndrome signs

Answer 24

• mental retardation
• ‘happy puppet’ syndrome
• jerky movements + inappropriate laughter

Question 25

Causes of Angelman syndrome and Prada Willi syndrome

Answer 25

Chromosome 15 abnormality AS - maternal deficiency PWS - paternal deficiency Occurs due to defects in imprinted genes: defective expression in brain tissue

Question 26

Heterochromatin

Answer 26

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

Question 27

Euchromatin

Answer 27

Organised in 30nm fibre during interphase Transcriptionally active Replicates early in S phase

Question 28

Position effect

Answer 28

Spreading of heterochromatin into euchromatic regions causes cell to cell variability in gene expression.

Question 29

X chromosome inactivation

Answer 29

LIONISATION Females need to silence one X chromosome. Mechanism of silencing is initiated by Xist

Question 30

Why does X chromosome inactivation occur ?

Answer 30

Discrepancy of 1 X-chromosome in males (XY) but 2 X-chromosomes in females (XX)

Question 31

Mechanism of X chromosome inactivation

Answer 31

X chromosome is inactivated by transcripts that are made from that X chromosome. Function as RNA. No protein product and RNA remains in the nucleus. This is followed by DNA methylation.

Question 32

Feature of X chromosome inactivation

Answer 32

The X chromosome that gets inactivated is eventually inactivated by DNA methylation.

Question 33

When does X chromosome inactivation occur ?

Answer 33

Early in embryonic development Around the 64 cell stage embryo

Question 34

Klinefelter's syndrome

Answer 34

Male Genotype (47XXY) Number of Barr bodies (inactivated X chromosomes) would also be 1

Question 35

Barr body

Answer 35

Inactivated X chromosome

Question 36

Genomic imprinting

Answer 36

Silencing (around 200) DNA genes (individual genes) on autosomes. Imprinted genes only expressed from 1 allele Dependent on parental origin Imprinting resets on passage through germline.

Question 37

Genomic Imprinting - fertilisation

Answer 37

There is a wave of de-methylation in the fertilised egg, but imprinted genes escape this. During the production of haploid sperm and egg, the patterns of DNA methylation are re-set for imprinted genes In sperm, imprint is paternal In eggs, imprint is maternal

Question 38

Uniparental disomy

Answer 38

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)

Question 39

Chromosome 11 - uniparental disomy

Answer 39

Wilm's tumour

Question 40

Chromosome 15 - uniparental disomy

Answer 40

PWS AS

Question 41

What is angelman syndrome ?

Answer 41

Genetic disorder that affects the nervous system and causes severe physical and intellectual disability. Caused when the Angelman gene is either absent or malfunctions. - paternal uniparental disomy

Question 42

What is Prader Willi syndrome ?

Answer 42

Rare genetic condition that causes a wide range of physical symptoms, learning difficulties and behavioural problems. Fault on Chr15 leads to a number of problems and is thought to affect the hypothalamus, which produces hormones and regulates growth and appetite. - maternal uniparental disomy

Question 43

AS

Answer 43

Maternal deficiency

Question 44

PWS

Answer 44

Paternal deficiency

Question 45

Non-disjunction in Meiosis 2

Answer 45

Uniparental isodisomy

Question 46

Non-disjunction in Meiosis 1

Answer 46

Uniparental Heterodisomy

Question 47

Diseases associated with genetic imprinting

Answer 47

Beckwith-Wiedemann Syndrome Wilms' tumour Fragile X syndrome Myotonic dystrophy PWS AS