7. DNA damage and repair Flashcards
With which type of cancer is ethanol particularly associated?
Breast
Recall an example of an occupational carcinogen
Tar smoke in laying roads
Recall 2 medical examples of carcinogens
X Rays
Chemotherapy
Recall the 4 main mechanisms of DNA damage
- DNA adducts
- Base dimers + chemical crosslinks
- Strand breaks
- Base hydroxylation
What is the most common class of environmental pollutants? Recall how they are produced
Polycyclic aromatic hydrocarbons - produced by burning fossil fuels + tobacco
Summarise the normal metabolism of B[a]P
2 step oxidation:
- CYP450 –> epoxide
- Epoxide hydrolase –> 2 unreactive alcohol molecules
Recall where the metabolism of B[a]P most frequently is dysfunctional
Production of diol epoxide = hugely reactive with DNA to produce adduct
Why are unstable products of metabolism attracted to DNA?
Since it is nitrogenous = good source of electrons to stabilise molecule by binding with it
Where is faulty B[a]P metabolism most likely to cause cancer?
Anywhere in body
Recall three examples of polycyclic aromatic hydrocarbons
- Benzo[a]pyrene
- Aflatoxin B1
- 2-napthylamine
How is aflatoxin B1 formed and where is it most commonly found?
Aspergillus flavus mould
Poorly stored grains and peanuts
Where is a DNA adduct caused by aflatoxin B1 most likely to cause cancer?
Liver
Where is a DNA adduct caused by 2-napthylamine most likely to cause cancer?
Bladder
Recall the normal metabolism of 2-napthylamine
- CYP1A2 –> reactive metabolite
2. Detoxified by glucuronidation (sugar addition)
Why is 2-napthylamine particularly associated with bladder cancer?
Acidic urine pH removes the sugar from the metabolite of 2-napthylamine, thus toxifying it
How does solar radiation cause damage?
UV light reacts with thymine to produce a crosslink, when the cell tries to repair this it introduces a mutation
How does ionising radiation lead to DNA damage?
Generates superoxide/ hydroxyl free radicals –> very reactive with DNA
What are the consequences of oxygen radicals?
- Double and single strand breaks
- Apurinic and apyrimidinic sites
- Base modification
What is the normal function of p53
TSG that detects and responds to DNA damage:
Mild damage –> repair, overwhelming damage –> apoptosis
How is P53 activated?
Released from MDM2
Recall the 4 main mechanisms for DNA repair
- Direct reversal of DNA damage
- Base excision repair
- Nucleotide excision repair
- During or post-replication repair
Give an example of direct reversal of DNA damage
Pyrimidine dimers produced by UVB exposure can be fixed by a family of enzymes called photolyases
What are the main enzymes involved in identifying sites for base-excision repair
DNA glycoylases
Apurinic/ apyrimidinic endonucleases
Summarise the mechanism of nucleotide excision repair
XP proteins assemble at damage site and stretch of nucleotides either side of adduct is cut out, repair polymerases fill gap
How might a mutation be fixed in the DNA?
Wrong base re-inserted during DNA repair
Recall the 5 steps of testing for potential damage of a chemcial/ drug
- Structural alerts
- In vitro bacterial
- In vitro mammalian
- In vivo mammalian
- Investigation of in vivo mammalian
How is in vitro bacterial testing of a chemical for potential to damage DNA carried out?
Ames test:
GM bacteria to not be able to produce histidine
Incubate with rat liver enzymes
Mix with active chemical
How many colonies produced? Cannot grow without histidine so all growth is due to mutation that permits histidine production
What is the purpose of in vitro mammalian testing of a chemical for potential to damage DNA?
Adds complexity of chromosome: may protect of potentiate the chemical’s ability to damage DNA
What is the most common way to carry out in vivo mammalian testing of a chemical for potential to damage DNA?
Bone marrow micronucleus test in mice
How is in vivo mammalian testing of a chemical for potential to damage DNA carried out?
Lifetime bioassay of a mammal - looking for epigenetic effects
