S10 DNA Repair And Cancer Flashcards
What happens if a cell recognises damaged DNA?
DNA repair mechanisms work to fix the DNA
However if these mechanisms don’t work properly, mutation occurs
What happens if a cell doesn’t recognise damaged DNA?
Mutation occurs
What are 5 exogenous sources of DNA damage?
- Ionising radiation
- Alkylating agents
- Mutagenic chemicals
- Anti-cancer drugs
- Free radicals
What are two endogenous sources of DNA damage?
- Replication errors
2. Free radicals
How can DNA be damaged?
- Apurinic site (base missing)
- Deamination (amino group removed)
- Mismatches
- Pyrimidine dimer (bases on same strand bind)
- Double strand breaks
- Intercalating agent (chemicals fixing between strands)
- Interstrand crosslink (prevents unwinding of DNA)
- Bulky adduct (cancer causing chemical bound to DNA)
What is DNA replication stress?
Inefficient replication leads to replication fork slowing, stalling and breakage
What 3 things can lead to replication stress?
- Replication machinery defects
- Replication fork progression hindrance
- Defects in response pathways
Give some examples of replication machinery defects.
Things could go wrong with:
- DNA polymerase
- DNA helicase
- Topoisomerase (breaks DNA to remove unwinding tension and then repairs)
What happens if the wrong base is incorporated into a strand?
The mismatch is removed by 3’ to 5’ exonucleases an then DNA polymerase continues as normal
What replication fork progression hindrance issues could arise?
- Ribonucleotide incorporation
- Transcription/DNA-RNA hybrids
- DNA lesions
- Fragile sites/oncogene-induced stress
- DNA secondary structure
- Repetitive DNA
What can repetitive DNA lead to?
Fork slippage
What are the two fork slippage scenarios?
- New strand loops out, an extra nucleotide is added to new stand (due to slow DNA polymerase) - backward slippage
- Template strand loops out, new strand is missing a nucleotide (due to fast DNA polymerase) - forward slippage
Can fork slippage be slippage of a base triplet? What is it called?
Yes - trinucleotide repeat disorder/expansion
What is an example of a trinucleotide repeat disorder?
Huntington’s disease
What is Huntington’s disease?
Occurs due to extra CAG repeats (35-121 rather than 6-39) in HTT gene, meaning the protein doesn’t fold properly so aggregates in the neurones and leads to neurone degeneration.
The normal HTT gene protein function is unknown.
What defects in response pathways can occur?
Each of the replication fork hindrance issues have response pathways however things can go wrong with these.
What is the DNA damage response pathway?
- DNA damage occurs due to replication stress
- Sensors pick up signals
- Transducers send signals to effectors
- Effectors: senescence (permanent cell cycle arrest), cell apoptosis, transcription, DNA repair or cell cycle transitions occur
- Outcomes are: proliferation (repair DNA and maintain function) apoptosis or senescence (the last two if DNA damage levels are very high/persist)
Where are the cell cycle checkpoints?
G1 checkpoint - as enter S phase
G2 checkpoint - as enter mitosis
Mitosis checkpoint - around metaphase
What does the G1 checkpoint check?
Is the environment favourable?
What does the G2 checkpoint check?
Is all the DNA replicated? Is all the DNA damage repaired?
What does the mitosis checkpoint check?
Are all chromosomes attached to the mitotic spindles?
What is base excision repair?
Detection of incorrect base, incorrect base is removed and the hole is filled with the correct base by DNA polymerase and sealed with ligase.
What is an example of when base excision repair is needed?
When deamination occurs converting cytosine to uracil
What is nucleotide excision repair?
Detection of incorrect/affected nucleotide, surrounding DNA and affected part opens forming a ‘bubble’ which is cut out by enzymes and DNA polymerase then replaces the cut out DNA and ligase seals the backbone.
What is an example of when nucleotide excision repair is needed?
When UV radiation produces a thymine dimer (two adjacent thymine’s chemically bonded)
What is mismatch repair?
A mismatch is detected, the new DNA strand is cut and the misplaced nucleotide and neighbours are removed, the DNA is replaced with correct nucleotides by DNA polymerase and DNA ligase seals the backbone.
How is a double strand break repaired?
- Homologous-directed repair - use info of the sister chromatid to repair (preferred way - less chance of mutations)
- Non-homologous end joining - rejoined with ligases
What stimulates carcinogenesis?
What prevents carcinogenesis?
Replication stress
DNA repair mechanisms
What is intracellular-tumour heterogeneity?
What does this mean for cancer treatment?
There are many sub clones within a tumour (different cell types).
Need to target with multiple drugs to have an effect on all the different sub clones.
What are synthetic lethality strategies?
You can block a specific pathway that leads to cell death
