20. Epigenetics / DNA methylation across the genome

Question 1

What is meant by the term epigenetics?

Answer 1

Heritable and transient changes in DNA expression

Primary sequence not altered

Effects persist through life and passed onto subsequent generations

Imprinting and X-inactivation result in monoallelic gene expression

Question 2

What is the role of DNA methylation?

Answer 2

Suppresses gene expression - maintains genome stability and prevents illegitimate recombination

Question 3

Describe the process of DNA methylation

Answer 3

Addition of CH3 group to C5 of cytosine –> 5-methylcytosine

Restricted to cytosines of CpG dinucleotides concentrated at promoters and pericentric heterochromatin

Carried out by DNA methyltransferase enzymes

Question 4

What are the roles of the different DNA methyltransferases?

Answer 4

DNMT1 - maintains methylation pattern - copies from hemimethylated DNA to new partner strand after replication

DNMT2 & DNMT3 - de novo methylation - adds pattern in early embryo

Question 5

How do methylation patterns change after fertilisation?

Answer 5

Maternal and paternal genomes demethylated (at different rates) after fertilsiation

Then tissue and stage specific methylation is established

Disruption of establishment, maintenance and erasure of imprints can cause disease

Question 6

What is the role of MECP2?

Answer 6

Binds to methylated CpGs and recruits other proteins (e.g. histone deacetylases) –> represses expression of other genes by regulating chromatin structure

Question 7

How does histone modification affect gene expression?

Answer 7

Modification to N-terminal tails determine chromatin conformation and therefore transcription

Condensed = no transcription
Relaxed = transcription

Question 8

What is the role of histone acetylation?

Answer 8

Adds/removes acetyl group (COCH3) to lysine/arginine

Catalysed by histone acetyltransferases (HATs) and histone deacetylases (HDACs)

Acetylation = relaxed chromatin = transcription

Question 9

What is the role of histone methylation?

Answer 9

Adds/removes CH3 to lysine/arginine

Catalysed by histone methyltransferases (HMTs) and histone demethlases (HDMs)

Variable effect

Question 10

How do non-coding RNAs effect gene expression?

Answer 10

1. microRNAs interfere at translation - bind to 3’-UTR of mRNA –> enzymatic degradation

Multiple genes can be targeted by single miRNA, multiple miRNAs can target a single gene

2. lncRNAs - form ribonucleoprotein complexes that interact with chromatin and regulate histone modification

Question 11

Describe the role of epigenetics in cancer

Answer 11

Characterised by global loss of normal methylation –> high gene activation

Causes genomic instability - demethylation favours mitotic recombination –> chr rearrangements

CpG islands become excessively methylated - silence TSGs and DNA repair genes –> microsatellite instability