9- Carcinogenesis/ DNA Damage & Repair Flashcards
Why do most human cancers develop over many decades of life?
- mutations accumulate over time > ↑ cancer with age
- cells accumulate genetic changes as tumor progression proceeds
How does cancer development follow the rules of Darwinian evolution?
Succession of Clonal Expansion
- random mutations in a cell providing survival/ proliferation advantages are passed on to descendants
What is the Philadelphia chromosome?
- chromosome translocation of tip of chromosome 9 with larger portion of chromosome 22
> in 95% of all CML (chronic myeloid leukemia)
What is a small-scale mutation?
- change in genotype that alters the nucleotide sequence of a DNA segment
What is a carcinogen?
- anything capable of causing cancer
- does NOT mean that it will always cause cancer
What are the groups of carcinogens?
(broken down based on risk)
Group 1- Carcinogenic to humans > ethanol
Group 2A- Probably carcinogenic to humans > red meat
Group 2B- Possibly carcinogenic to humans > gasoline
Group 3- Not classifiable carcinogenicity in humans
How are carcinogenic hazards to humans identified?
- groups not based on risk
- Epidemiological studies > humans exposed to agent
- Experimental studies > lab animals treated with agent
- Response to agent > evidence of mechanisms
What is the relationship between mutagenicity/ carcinogenicity?
- link between ability of chemical to mutate bacteria (salmonella) and ability to induce tumors in rodents
> all mutagenic compounds likely to also be carcinogenic BUT
NOT all carcinogenic compounds likely to also be mutagenic
What is important to know about mutations?
- not all mutations have negative consequences (codon redundancy)
- mutations can alter biochemical properties like polarity
What are point mutations?
- single base pair changes > substitutions/ insertions/ deletions
Silent- 1 changed/ same AA > no effect (codon redundancy)
Nonsense- STOP codon > no longer functional protein (truncated)
Missense- Conservative > lose AA but not polarity
Missense- Non-conservative > lose AA and polarity
What can point mutations cause?
- loss of function > inactivate tumor suppressors
- gain of function > activate oncogenes
- gain of novel function > neomorphic mutations
What are some endogenous mechanisms of DNA damage?
- breakage at replication fork
- depurination/ depyrimidination
- deamination
- ROS
What are the purines/ pyrimidines?
Purines- Adenine/ Guanine (2 ring bases)
Pyrimidines- Thymine/ Cytosine/ Uracil (1 ring bases)
How is breakage at the replication fork an endogenous mechanism of DNA damage?
- during replication, DNA unwinds/ forms 2 single strands through the action of helicase/ replication fork
- single-stranded DNA is vulnerable to break
- altered bases cause DNA polymerase to stall as it recognizes abnormal base > puts strain on the strands
- consequence = loss of DNA
How are depurination/ depyrimidination endogenous mechanisms of DNA damage?
> base pairs lost
Depurination > G-A
- hydrogen in environment (H20) causes a spontaneous break between purine/ deoxyribose
Depyrimidination > C-T
- deletion of a bp causes a frameshift in sequence (read wrong)
> frameshift mutation
What are transition/ transversion mutations?
Transition- purine > purine/ pyrimidine > pyrimidine
Transversion- purine > pyrimidine/ pyrimidine > purine
What is the consequence of deamination?
Transition mutation
How are ROS an endogenous mechanism of DNA damage?
- react and form covalent bonds with bases of DNA > strand breaks/ change base pairing
- consequence = transversion mutation
What are some exogenous mechanisms of DNA damage?
- ionizing radiation/ UV radiation
- Aflatoxin (mould) > xenobiotic mutagen
- Heterocyclic amines
How is UV radiation an exogenous mechanism of DNA damage?
- cause pyrimidine dimers (photoproducts) = covalently bonded/ adjacent pyrimidines
- very stable/ remain for long periods of time > skin cancer
- causes CC > TT substitution
How is aflatoxin (mould) an exogenous mechanism of DNA damage?
- potent exogenous carcinogen
- attacks guanine forming DNA adducts
- causes G > T transversion mutation
How are heterocyclic amines an exogenous mechanism of DNA damage?
- formed when meats cooked at high temps
- processes oxidize rings/ anime groups from the rings
- creates reactive compounds that form covalent bonds with DNA
> bulky lesions > DNA breaks
What are some protective mechanisms of DNA damage?
- physical protection (skin)
> melanin delivered to keratinocytes in epidermis/ form supranuclear caps/ cover keratinocytes from UV radiation - free radical scavengers (vitamin E/C)
> donate electrons to turn ROS into H20 - detoxification
> Glutathione S Transferase
> links carcinogens to glutathione/ allows inactivation/ excretion
What are DNA repair mechanisms when there are normal bases?
- Proofreading (DNA polymerase is a fast/ sloppy enzyme)
- MMR (Mismatch Repair) > catch DNA polymerase overlooked mistakes
What is the consequence of no mismatch repair/ MMR?
- in colon cancer > TGF-B has growth inhibitory effects, inactivated due to lack of MMR genes > causing microsatellite instability (shortening/ elongating of microsatellites)
- deletion results in nonsense mutation/ early termination of translation
What are DNA repair mechanisms when there are abnormal bases?
- Enzyme-Catalyzed Reversal of Damage
> MGMT removes methyl/ethyl adducts from guanine
> AlkB enzyme oxidizes methyl/ethyl attached to bases - Base Excision Repair (BER)
> fixes lesions made by endogenous sources - recognize abnormal bases that do NOT distort DNA structure
> DNA glycosylases are specific to abnormal bases > cleave them
Nucleotide Excision Repair (NER)
> fixes lesions made by exogenous sources
- recognize abnormal bases that DO distort DNA structure
> cleave upstream/ downstream (25-30 nucleotides)
How does DNA repair relate to cell cycle phase?
give an example…
- repair mechanism depends on lesion type/ where in cell cycle
- if dsDNA break occurs in G1, there’s no homologous chromatid to use as a template for repair so it has to do NHEJ (non-homologous end joining)
- but if it happens in S/G2 when there are then it can use it as a template and do HDR (homology-directed repair)
What is a cell-cycle-specific DNA-repair event?
G1- bulky lesion repair > NER (nucleotide excision repair)
What is the difference between a tumor initiator/ promoter?
Initiator- causes permanent/ stable mutations in cells
ex) DMBA (tar)
Promoter- causes ↑ proliferation but is reversible
ex) TPA (skin irritant)
How can tumor cells avoid crisis/ become immortal?
- Telomerase maintains telomeres in tumor cells
- Alt Pathway > telomerase independent (exchange sequences)