Cancer 5: DNA damage Flashcards
How reactive are basepairs? What are the main reaction it can undergo?
Quite reactive actually
and quite easy to change (like T-> U (just remove 1 methyl)
1) Deamination-remove an NH2 group- (Cytosine -> Uracil is deamination)/ (guanine-> xanthine)/ (5-methyl-cytosine -> thymine)
2) Oxidation - open double bonds-thiamine -> thyamine glycol (2 extra OH) -can happen after radiation, ROS –> can also cause DNA to bind to other proteins through fromation of “adducts”
3) Photodamage-UV light absorbed by bases-> causes neighbouring T to bind together (Thyamine dimer)
What are 4 broad groups of DNA damage?
nick-split the phosphodiester bond between 2 pairs
base pair mismatch-not the right one-usually cause
thymine dimer
gap-loss of a large ssDNA gap or even dsDNA-
generally what can cause DNA damage?
Carcinogens (chemicals) (eg: diet, lifestyle, medical, endogenous) and Radiations (ionising, solar, cosmic)
this is used therapeutically for cancer therapy (radio/chemotherapy)
What kind of damage do carcinogen cause?
DNA adducts/alkylation, base dimers/cross links, double/single strand breaks, base hydroxylation/abasic sites (so much damage to the base its pretty much not there-most common one)
carcinogens tend to be polycyclic aromatic hydrocarbons-like pollutants/fossil fuels/tobacco
How/Why are polycyclic aromatic hydrocarbons carcinogenic-give 3 exemples?
they are all recognied as foreign and to be metabolised by drug metabolism-Phase 1 (Cyt p450) and 2 (conjugation)
Benzo[a]pyrene-common carcinogenic-BAP-itself not one-only causes damage after being metabolised by p450 -> form an oxide form-and that damages DNA
Aflatoxin B1-Apserigillius mould (fungi)-common on old grain/peanuts)-potent carcinogenic after being activates by P450 –epoxide group created binds with G at N7 position
2-naphthylamine-component of dye-stuff-potent bladder carcinogen-not carcinogenic before reacher bladder-just has glucoronide group, where urine pH causes glucoronide to leave and produce DNA reactive electrophile
How does ionising radioation cause DNA damage?
Not expected to be much-and flight crews seem to have more of these damages
Radioation generates free Radical in cels (ROS, RNS)-and sometimes can damage DNA directly
ROS-
How does ROS cause DNA damage? Is that common?
ROS is very common-its part of metabolism
ROS causes double and single strand breaks
Apurinic/apyrimidic sites
modify rings- make them more reactive
What is the role of p53? in DNA damage?
has many roles in the cell-but can react to DNA damage
Normally inactivated by MDM2-and that is lost with the stress-like DNA damage, mitosis issues, dsDNA breaks
p53 is a transcription factor that upregulates many protein for DNA repair
What are the 4 types of DNA repair?
Direct reversal
Base excistion
nucletotide exicione
post-replicatoon repair
How does Direct DNA repair work?
Direct repair reversal/remove simple damage
Photolyases (need light but not UV)-repair thymine dimers-removes them and lets DNA polymerase repair
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MGMTs-can directly remove bases that have been methylated (recognise base+methyl group)
How is DNA mismatch repaired?
DNA polymerase are not foolproof
But mismatch forms a buldge-recognosed by MLH and MSH–removes the offending base and a few on each side-then DNA polymerase re-adds them
=> can only happen during DNA replication
How does excision repair DNA?
base excision-If a single base is mutated-DNA glycosylases remove just the base (diester bond fine)-then AP endonucleases opens the DNA, DNA polymerase enters and re-adds pair-> then DNA ligase finishes bond
Nucleotide excision- if damage to the diester bond too-large adduct group -endonuclease cuts it out (a large gap+diester backbone)
helicase opens it up, then DNA polymerase readds bases, then ligase close up
very important
Xeradoma pigmentosa-extreme UV sensitivity-causes by lack of proper NER-not treatable
Trichothiodystrophy-sulfur deficicent
Cockwayne syndrome
How does DNA double strand break repair?
dsDNA break repair pathway-
dsDNA break occur under physiologcal recombinated VDJ/Homologous recombination. or can be result of ROS and ionising radiation
If the ends dont match up-start increasing overhangs to try and find something that matches-once it happens bind together and remove what doesnt fit-but still lose some DNA -bad
non-homologous end joining-desperate way to get some dsDNA back
What is the consequences of DNA damage?
Hopefully repair
Can get incorrect repair-change primary sequence-mutations can modify proteins activity-cell survival and more
If too much damage or specific recognised-apoptosis-cell death -> chemotherapy’s main goal
How do you test for carcinogens?
series of assays-
start with bacterial assays, then mammalian cells, then again in vivo, then full on rodents
bacterial assays-ames test-use bacteria that can make histadine + the substance tested+liver extract (with p450/phase II enzymes)-check if it can mutate histadine synthesis again-and if it happens more often then substance is carcinogen
then mammalian-check for chromosomal damage
in vitro micronucleus assay
then most important is the rodent-if fed to rats can you find effects –how often doyou see micronuclei in new ertyhtrocytes (shouldnt have any)-any if see them appear bad
