Cancer Bio (Dan Lecture 3 - DNA damage)

Question 1

Name come common types of DNA damage?

Answer 1

• Oxidation
• Methylation
• Strand breaks
• Bulky adducts
• Cross links

Question 2

Name some DNA damaging agents

Answer 2

• UV
• Cigarette smoke
• Dyes
• Ionising radiation
• Chemotherapy
• Foods?

Question 3

Name some types of endogenous DNA damage

Answer 3

• Deamination (e.g. cytosine to uracil)
• Oxidation
• Methylation
• Lipid peroxidation

Question 4

Name some types of exogenous DNA damage

Answer 4

• Pollutants
• UV
• Ionising radiation
• Ciggy smoke

Question 5

Name the repair processes responsible for the repair of the following:

1) Oxidative damage and SSBs
2) Bulky adducts, CPDs and 6-4PPS
3) Cross links and DSBs
4) Mismatches, insertions or deletions

Answer 5

1) Base-excision repair
2) Nucleotide excision repair
3) Recombinational repair
4) Mismatch repair

Question 6

What are the potential consequences if DNA damage is not repaired?

Answer 6

1) Cell cycle arrest
2) Apoptosis
- caused by inhibition of transcription, replication or chromosome segregation
3) Cancer, Ageing
- Caused by mutations and chromosome abrrations

Question 7

What are the 3 classifications of UV radiation and what is the wavelength of each?

Answer 7

UVA (320-400nm)
UVB (295-320nm)
UVC (100-295nm)

Question 8

Why does solar radiation contain 95% UVA with the rest UVB?

Answer 8

Because the ozone absorbs UV with wavelength below 300nm

Question 9

What year was UV classified as a human carcinogen and by which body?

Answer 9

Internation Agency for Research on Cancer 1992

Question 10

What are the 3 types of skin cancer and what are the characteristics of each?

Answer 10

1) Basal cell carcinoma
- Relatively common
- Can be invasive
- Rarely metastatic

2) Squamous cell carcinoma
- Less common
- Significant risk of metastasis to lymph nodes

3) Melanoma - affecting melanocytes
- Rare
- Highly invasive
- Common metastasis
- 75% of skin cancer death attributed to melanoma

Question 11

Describe the penetrance of different UV types

Answer 11

UVA is lower energy but has higher penetrance than UVB

Question 12

What countries have the highest rates of skin cancer?

Answer 12

Australia and New Zealand

Question 13

Why are skin cancer rates so high in Australia and NZ?

Answer 13

1) Thin layer of ozone
- allows more UV exposure

2) Culture
- Outdoor lifestyles
- Fashion of the tan
- Society of not applying sun cream
- Slip, slop, slap campaign in 1981 to try and introduce sun protection into Australian society

3) Skin tone
- Movement of fair skinned Europeans in 18th century

Question 14

What are the main 2 types of cancer relevant DNA damage induced by UV?

Answer 14

1) Cyclobutane pyrimidine dimers (CPD)

2) (6-4) pyrimidine-pyrimidone photoproducts (6-4PPS)

Question 15

What is the structure of CPDs and 6-4PPs?

Answer 15

CPDs:

• covalent link between adjacent pyrimidine bases
• due to saturation of 5,6 double bonds, predominantly in the cis-syn conformation

6-4PPs:
- linking the C6 of the 5’ pyrimidine with the C4 of the 3’ pyrimidine

Question 16

What other damage does UVA and B cause?

Answer 16

• Oxidative damage

- Protein distortion

Question 17

What can happen as a result of UV induced DNA damage?

Answer 17

• Cell cycle arrest pre S phase, reducing copying errors
• Extensive damage leading to apoptosis
• Damage repair by NER

IF THESE DO NOT HAPPEN:

• Cells enter S phase
• Copying errors introduced, leading to mutations

Question 18

What is the definition of a mutation?

Answer 18

Heritable change in the sequence of an organisms DNA

Question 19

What is the definition of a mutagen?

Answer 19

An agent that leads to an increase in the frequency of mutations

Question 20

What is the definition of mutagenesis?

Answer 20

The process by which mutations are introduced

Question 21

What is the most common UV induced DNA mutation?

Answer 21

Pyrimidine dimers cause CC to TT transitions

- Hallmarks of certain skin cancers, especially non-melanomas

Question 22

What process repairs pyrimidine dimers?

Answer 22

Nucleotide excision repair

Question 23

What are the 2 types of NER and what are the characteristics of each?

Answer 23

1) Global genomic NER
- deals with whole genome

2) Transcription-coupled NER
- Faster repair in highly transcribed genes

Question 24

Describe the process of lesion recognition in GGNER and TCNER

Answer 24

GG-NER:

• helix distortion recognised by XPC complex
• CPDs do not cause as much helix distortion so require assistance by XPE and the UV radiation-DNA damage-binding protein (UV-DDB)

TC-NER:

• Lesions causes RNA polymerase II to stall
• Acitvation of CSB protein

Question 25

Describe the rest of the NER process (conserved in both types)

Answer 25

DNA unwinding: - TFIIH binds XPC - TFIIH subunits XPB and XPD have helicase activity and unwind DNA Scaffold support: - XPA binds to damaged strand - RPA binds to undamaged strand Excision: - XPF-ERCC1 binds and cleaves 5' side of lesion - XPG binds and cleaves 3' side DNA synthesis and ligation: - Binding of RFC and sliding clamp PCNA - Facilitates activity of DNA polymerase δ or ε - Ligation by DNA ligase I

Question 26

What is Xeroderma Pigmentosum (XP) and what are the symptoms?

Answer 26

- Rare autosomal recessive disorder in GG-NER - Extreme sensitivity to sunlight - Freckle-like pigmentation in sen-exposed areas - 10,000 fold increase in non-melanoma skin cancer - 2,000 fold increase in melanoma

Question 27

What causes XP?

Answer 27

- Mutations in XPA-XPG genes | - Results in dysfunctional XPA-XPG proteins

Question 28

What are XP complementation groups?

Answer 28

Different types of XP dependent on the XP protein/s that are affected i.e Complementation group A = XPA affected

Question 29

Which complementation groups gives less severe phenotypes?

Answer 29

- Severity of disease depends on complementation group - Some give neurological problems - XPC and XPE groups give milder phenotype and no neurological disorders

Question 30

How is p53 involved in repair of CPDs?

Answer 30

- CPDs cause significantly less helix distortion - 6-4PPs recognised by XPC and repaired faster than CPDs - p53 regulates CPD repair by upregulation of XPE

Question 31

Cisplatin is commonly used to treat which cancers? What is its mechanism of action?

Answer 31

Testicular and ovarian Causes DNA adducts, primarily intrastrand crosslink adducts

Question 32

How is NER adapted to cause cisplatin resistance?

Answer 32

- Resistant cells enhance ERCC1 - Enhanced ERCC1 leads to enhanced NER - Greater reapir of cisplatin induced DNA adducts