9- Carcinogenesis/ DNA Damage & Repair

Question 1

Why do most human cancers develop over many decades of life?

Answer 1

• mutations accumulate over time > ↑ cancer with age
• cells accumulate genetic changes as tumor progression proceeds

Question 2

How does cancer development follow the rules of Darwinian evolution?

Answer 2

Succession of Clonal Expansion
- random mutations in a cell providing survival/ proliferation advantages are passed on to descendants

Question 3

What is the Philadelphia chromosome?

Answer 3

• chromosome translocation of tip of chromosome 9 with larger portion of chromosome 22
  > in 95% of all CML (chronic myeloid leukemia)

Question 4

What is a small-scale mutation?

Answer 4

• change in genotype that alters the nucleotide sequence of a DNA segment

Question 5

What is a carcinogen?

Answer 5

• anything capable of causing cancer
• does NOT mean that it will always cause cancer

Question 6

What are the groups of carcinogens?
(broken down based on risk)

Answer 6

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

Question 7

How are carcinogenic hazards to humans identified?
- groups not based on risk

Answer 7

1. Epidemiological studies > humans exposed to agent
2. Experimental studies > lab animals treated with agent
3. Response to agent > evidence of mechanisms

Question 8

What is the relationship between mutagenicity/ carcinogenicity?

Answer 8

• 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

Question 9

What is important to know about mutations?

Answer 9

• not all mutations have negative consequences (codon redundancy)
• mutations can alter biochemical properties like polarity

Question 10

What are point mutations?

Answer 10

• 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

Question 11

What can point mutations cause?

Answer 11

• loss of function > inactivate tumor suppressors
• gain of function > activate oncogenes
• gain of novel function > neomorphic mutations

Question 12

What are some endogenous mechanisms of DNA damage?

Answer 12

• breakage at replication fork
• depurination/ depyrimidination
• deamination
• ROS

Question 13

What are the purines/ pyrimidines?

Answer 13

Purines- Adenine/ Guanine (2 ring bases)
Pyrimidines- Thymine/ Cytosine/ Uracil (1 ring bases)

Question 14

How is breakage at the replication fork an endogenous mechanism of DNA damage?

Answer 14

• 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

Question 15

How are depurination/ depyrimidination endogenous mechanisms of DNA damage?

> base pairs lost

Answer 15

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

Question 16

What are transition/ transversion mutations?

Answer 16

Transition- purine > purine/ pyrimidine > pyrimidine
Transversion- purine > pyrimidine/ pyrimidine > purine

Question 17

What is the consequence of deamination?

Answer 17

Transition mutation

Question 18

How are ROS an endogenous mechanism of DNA damage?

Answer 18

• react and form covalent bonds with bases of DNA > strand breaks/ change base pairing
• consequence = transversion mutation

Question 19

What are some exogenous mechanisms of DNA damage?

Answer 19

• ionizing radiation/ UV radiation
• Aflatoxin (mould) > xenobiotic mutagen
• Heterocyclic amines

Question 20

How is UV radiation an exogenous mechanism of DNA damage?

Answer 20

• cause pyrimidine dimers (photoproducts) = covalently bonded/ adjacent pyrimidines
• very stable/ remain for long periods of time > skin cancer
• causes CC > TT substitution

Question 21

How is aflatoxin (mould) an exogenous mechanism of DNA damage?

Answer 21

• potent exogenous carcinogen
• attacks guanine forming DNA adducts
• causes G > T transversion mutation

Question 22

How are heterocyclic amines an exogenous mechanism of DNA damage?

Answer 22

• 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

Question 23

What are some protective mechanisms of DNA damage?

Answer 23

• 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

Question 24

What are DNA repair mechanisms when there are normal bases?

Answer 24

• Proofreading (DNA polymerase is a fast/ sloppy enzyme)
• MMR (Mismatch Repair) > catch DNA polymerase overlooked mistakes

Question 25

What is the consequence of no mismatch repair/ MMR?

Answer 25

• 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

Question 26

What are DNA repair mechanisms when there are abnormal bases?

Answer 26

• 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)

Question 27

How does DNA repair relate to cell cycle phase?

give an example…

Answer 27

• 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)

Question 27

What is a cell-cycle-specific DNA-repair event?

Answer 27

G1- bulky lesion repair > NER (nucleotide excision repair)

Question 28

What is the difference between a tumor initiator/ promoter?

Answer 28

Initiator- causes permanent/ stable mutations in cells
ex) DMBA (tar)

Promoter- causes ↑ proliferation but is reversible
ex) TPA (skin irritant)

Question 29

How can tumor cells avoid crisis/ become immortal?

Answer 29

• Telomerase maintains telomeres in tumor cells
• Alt Pathway > telomerase independent (exchange sequences)