Cancer 5: DNA damage and repair

Question 1

What can cause base modification?

Answer 1

• deamination
• chemical modification
• photodamage

Question 2

How does deamination work with DNA?

Answer 2

-primary amino groups of nucleic acids are unstable and converted to ketogroups

Question 3

What are examples of deamination reactions?

Answer 3

adenine to hypoxanthine
guanine to xanthine
5-methyl cytosine to thymine

Question 4

Give an example of how chemical modification can occur

Answer 4

by hyper-reactive oxygen species which can be generated during normal oxidative metabolism or ionizing radiation

Question 5

What happen during photodamage?

Answer 5

UV light is absorbed by nucleic acid bases and the influx of energy can induce chemical changes

Question 6

List some types of DNA damage? (4)

Answer 6

• ruck
• gap
• thymine dimer
• base pair mismatch

Question 7

What can be a source of CARCINOGENS that contribute to DNA damage?

Answer 7

• dietary
• environmental
• lifestyle
• occupational
• medical
• endogenous

Question 8

What sources of RADIATION lead to DNA damage?

Answer 8

• ionizing
• solar
• cosmic

Question 9

List 4 types of DNA damage by carcinogens

Answer 9

• DNA adducts and alkylation
• base dimers and chemical cross-links
• base hydroxylation and abasic sites formed
• double and single strand breaks

Question 10

What are the 2 phases of mammalian metabolism?

Answer 10

phase I adds functional groups mainly via cytochrome p450

phase II conjugates the functional groups so it becomes water soluble

Question 11

What is cytochrome p450 responsible for?

Answer 11

oxidising chemicals

Question 12

How are polycyclic aromatic hydrocarbons formed?

Answer 12

• combustion of tobacco
• combustion of fossil fuels

they form carcinogens

Question 13

What can arise from the epoxidation of aflatoxin B1?

Answer 13

human liver carcinogen

Question 14

What can arise from the metabolism of 2-naphthylamine?

Answer 14

human bladder carcinogen

Question 15

What type of cancer can UV radiation lead to?

Answer 15

-skin

can lead to the formation of pyrimidine dimers

Question 16

How does ionising radiation cause damage?

Answer 16

-generates free radicals

super oxide radical and hydroxyl radical which then seek out electron-rich DNA

Question 17

What type of DNA damage does oxygen free radical attack cause?

Answer 17

• double and single strand breaks
• base modifications
• apurinic and apyramidic sites

Question 18

What is p53?

Answer 18

a CRUTIAL tumour suppressor gene

Question 19

How does p53 work with cellular stress?

Answer 19

it is usually paired with MDM2 which keeps it inactive
when it is released it forms an active dimer

• under MILD physiological stress it activates proteins that help repair the problem
• under HIGH physiological stress it triggers an apoptotic pathway

Question 20

List 4 types of DNA repair?

Answer 20

• direct reversal of DNA damage
• Base excision repair
• nucleotide excision repair
• during or post replication repair

Question 21

How is DNA mismatch repair done?

Answer 21

• it enlists the aid of enzymes involved in base-excision repair and nucleotide-excision repair
• recognition requires proteins including one encoded by MSH2
• cutting of the mismatch requires proteins, one encoded by MLH1

Question 22

How is direct DNA repair carried out?

Answer 22

• photolyases repair thymine dimers

- MGMT directly reverses simple alkylation adducts

Question 23

How does base excision repair work?

Answer 23

mainly targets apurines and apyrimidines

DNA-glycosylases and endonucleases repair this
DNA polymerase fills in the gap left by this base and then DNA ligase completes the repair

Question 24

How does nucleotide excision repair work?

Answer 24

for bulky DNA adducts

-Xeroderma pigmentosum proteins assemble at the site of damage

DNA polymerase fills int he gap and ligase completes the repair

Question 25

List 3 disease that involve NER

Answer 25

• xeroderma pigmentosum
• trichothiodystophy
• cockayne’s syndrome

Question 26

What mechanisms repair DNA double strand break repair?

Answer 26

• direct joining of the broken ends this type of joining is also called nonhomologous end-joining
• DNA-PK is required for this

Question 27

What are the options of carcinogen DNA damage?

Answer 27

• apoptosis of the cell and then cell death
• efficient repair leading to a normal cell
• altered DNA, altered primary sequence, DNA replication occurs with the mutations leading to aberrant proteins via transcription/translation or carcinogenesis if critical targets are mutated

Question 28

WHat is the first step of DNA testing?

Answer 28

• you look at the structure of the chemical to see if there are any groups that could cause issues
• then you introduce it to bacteria to see if a mutation occurs

Question 29

What is the second step of DNA damage testing?

Answer 29

-introduce it to mammalian cells

Question 30

What is the third step of DNA damage testing?

Answer 30

-introduce to mammal assay in VIVO and investigate these

Question 31

Summarise the steps in a bacterial test for mutagenicity

Answer 31

• the chemical is added to rat liver enzymes in preparation
• then you mix the genetically modified bacteria that does not contain histidine witht he chemical and spread it on a plate

-the more genetic mutations that occur the more colonies you will get

Question 32

How can you detect mammalian cell DNA damage?

Answer 32

-treat with a chemical in the presence of liver S9 and you can see the chromosomal damage

Question 33

How can you detect damage by invitro micronucleus assays

Answer 33

• assessed for presence of micronuclei

- can stain kinetochore to determine if the damage is caused by chromosomal break or chromosome loss

Question 34

How do you detect damage in a bone marrow micronucleus assay?

Answer 34

-treat animals with the chemical and examine bone marrow cells or peripheral blood erythrocytes for micronuclei