6. DNA damage and repair Flashcards
(40 cards)
What can cause DNA damage?
Chemicals (carcinogens) • dietary • lifestyle • environmental • occupational • medical • endogenous
Radiation
• ionising
• solar
• cosmic
What percentage of cancer is associated with diet?
40-45%
Do most chemicals damage DNA in their initial form or metabolically converted forms?
Metabolically converted
Give an example of an endogenous cause of cancer
Mitochondria produce reactive oxygen species that may damage DNA
Name 6 ways DNA can be damaged
- Base dimers and chemical-cross-links
- Base hydroxylations
- Abasic sites
- Single strand breaks
- Double strand breaks
- DNA adducts and alkylation
What do base hydroxylations involve?
- Oxidative reaction occurring on one of the DNA bases
- DNA has to be repaired
- Mutation can occur during the repair process
What do abasic sites involve?
- Entire DNA base accidentally removed during the repair process
- Sugar backbone still maintained, but missing base causes problems during replication
What do single strand breaks involve?
• Very common (can be very useful)
• Physiological enzymes usually involved
• Topoisomerase relaxes and unwinds the DNA
- done by chopping the strand of DNA so it can unwind, and gain access as it is re-annealed
• These breaks can therefore be dealt with
What do double strand breaks involve?
- After a double strand breaks, there is a tendency for the 2 bits of DNA to drift apart
- This is intolerable
- Number of DNA repair mechanisms, but sometimes this can go wrong
What do DNA adducts and alkylation involve?
- General type of damage caused by chemicals
- Some chemicals are metabolically activated into electrophiles
- DNA is very rich in electrons (because of nitrogen in bases)
- Electrophiles bind to DNA and form a covalent bond
- DNA polymerase can’t recognise base and work during replication due to this bulk
What is phase I in mammalian metabolism?
- Addition of functional groups (introduce or unmask functional groups
- e.g. oxidations, reductions, hydrolysis
- Mainly cytochrome p450-mediated (oxidation)
What is phase II in mammalian metabolism?
- Conjugation of phase I functional groups
- e.g. sulphation, glucuronidation, acetylation, methylation, amino acid and glutathione conjugation
- generates polar (water soluble) metabolites by adding a polar endogenous group
- easier to excrete
What are polycyclic aromatic hydrocarbons?
- Common environmental pollutants
- Formed from the combustion of fossil fuels and tobacco
- Poisonous and carcinogenic
What is one of the most common polycyclic aromatic hydrocarbons, how is it metabolised and how does it cause damage?
• Benzo[a]pyrene
• Oxidised by CYP450 to produce an epoxide/oxide
- this is reactive and unstable (electrophile)
• Epoxide hydrolase metabolises this into a dihydrodiol
- this is harmless
• Second CYP450 oxidises this to form another oxide (diol epoxide)
- incredibly reactive
- rapidly forms positively derived material (electrophile)
- best source of electrons is DNA
- DNA adducts formed, usually at guanine => mutation
What is aflatoxin B1 and where does it come from?
- Potent human liver carcinogen
- Formed by Aspergillus flavus mould
- Common on poorly stored grains/peanuts
- Especially in Africa and Far-East
Outline the metabolism of Aflatoxin B1 and the way it causes DNA damage
- Oxidised by P450 into B1-2,3-epoxide (very reactive)
- This product reacts with the N7-position of guanine to form bulky DNA adducts
- DNA is now read as damaged
- It’s fixed inappropriately and a mutation has been introduced into the DNA
How can benzo[a]pyrene cause tumours all over the body?
P450 is in cells throughout the body
What is 2-napthylamine and benzidine and where was it common?
- Potent bladder carcinogens
* Used in the German dye industry
Outline the metabolism of 2-napthylamine and how it causes damage
• Substrate for CYP450
- converts the amino group to form a hydroxylamine
• Hydroxylamines are reactive
• They are glucuronidated (detoxified) in the liver by glucuronyl transferase
• Inactive metabolite is excreted and mixes with the urine
- Acidic urine hydrolyses the glucuronides
- Hydroxylamine derivative is released
- Molecule rearranges to form a positively charged nitrogen (nitrenium ion) (electrophile)
- Nitrenium binds to the DNA and forms adducts
- Bladder can’t detoxify the hydroxylamine derivative as the liver
How does solar radiation cause skin cancer?
• UV light leads to the formation of pyrimidine dimers (T, C, U)
- if there are 2 pyrimidines next to each other, UV radiation can cause them to covalently link
• Cell tries to repair this, but introduces a mutation in the process
How does ionising radiation cause cancer?
- All ionising radiation generates free radicals in cells
- This includes oxygen free radicals e.g. superoxide and hydroxyl - very reactive
- Free radicals possess unpaired electrons - seek out electron-rich DNA
Compare the superoxide radical to the hydroxyl radical
- Superoxide radical - molecule of oxygen with an extra electron
- Hydroxyl radical - hydroxyl group that has grabbed an extra electron, even more reactive than superoxide (very electrophilic and DNA is electron-rich)
How do oxygen free radicals damage DNA? (3 ways)
• Single and double strand breaks
- double strand breaks have to be reannealed, can introduce mutatations during the process
- Base can be oxidised by free radical and DNA repair enzymes cut out the base
- Abasic site left
• Base modifications are also introduced:
- e.g. 8-hydroxyadenine + 8-hydroxyguanine (mutagenic)
- while replicating, the machinery has to guess where it is a guanine present, so makes mistakes
Which protein releases p53 for activity during stress on the cell, and what happens to p53 once released?
- Mdm2
- Suppresses activity and keeps it in check
- p53 then forms a dimer that activates many pathways
