Errors In DNA replication

Question 1

Define DNA replication stress

Answer 1

DNA replication stress defined as ‘inefficient replication that leads to replication fork slowing, stalling &/or breakage’

Question 2

What are the 3 main causes of DNA replication stress?

Answer 2

1 - replication machinery factor has defect
2 - factors hinder replication fork progression
3 - defects in response pathways

Question 3

What is nucleotide misincorporation?

Answer 3

Sometimes wrong base added
Mismatch removed by 3’ to 5’ DNA exonuclease
Defects in 3’ to 5’ exonuclease capacity of DNA polymerases cause mutation rate increase. This could lead to cancer
Sometimes wrong base removed

Question 4

What is repetitive DNA?

Answer 4

Fork slippage
A base could be copied twice leading to an extra synthesized base - strand loops out
If template strand loops out a base could be lost - the effect of this depends on where the error is

Question 5

Give an example of a trinucleotide repeat disorder

Answer 5

Huntington’s

Question 6

How can trinucleotide repeat disorders occur?

Answer 6

Backward slippage - insertion mutation

Forward slippage - deletion mutation

Question 7

Explain the error in DNA replication that leads to Huntington’s disease

Answer 7

Backward slippage - insertion
Expansion due to replication errors
HTT gene identified in 1993
• Normal function still not clear
• Gene has CAG repeats (polyglutamine) - Normal gene, 6–39 repeats
Disease gene, 35–121 repeats - no. of repeats inversely correlated with age of onset
Triplet repeat expansion leads to neurodegeneration

Question 8

What is the Huntingtin protein?

Answer 8

Protein coded for by HTT gene
• Function of normal protein unclear • Mutant protein accumulates & aggregates in neurons & disrupts various cellular processes & functions, leading to cell dysfunction (e.g. reduced dopamine) & death

Question 9

What is Huntington’s disease?

Answer 9

Progressive brain disorder (death usually 15-20 years after symptoms begin)
• Basal ganglia most affected
• Basal ganglia has role in voluntary motor functions, procedural learning, routines/habits, cognition and emotions
• Symptoms can include irritability, depression, small involuntary movements, poor coordination, decision making, twitching, trouble speaking or swallowing

Question 10

Name some response pathways which can become defected

Answer 10

Repair,DNA damage tolderance
RNase H
Helicases - BLM, WRN etc

Question 11

What is Werner syndrome?

Answer 11

WRN gene mutations
• Werner protein is a helicasethat is important during DNA replication
• Defects in Werner lead to DNA replication defects & DNA
damage
• Leads to cell growth defects
• Werner syndrome inherited in an autosomal recessive pattern
Patients prematurely display features associated with normal
ageing
• Symptoms include cataracts, skin ulcers, type 2 diabetes, atherosclerosis, osteoporosis, cancer predisposition

Question 12

How can replication stress lead to cancer?

Answer 12

Accumulation of mutations turn cell premalignant then malignant
If also mutate programmed cell death pathway (apoptosis) in mutant mice that have very high levels of DNA damage & replication stress response defects = 100% get cancer - Medulloblastoma
- Lymphoma