Cancer Bio (Dan Lecture 3 - DNA damage) Flashcards
Name come common types of DNA damage?
- Oxidation
- Methylation
- Strand breaks
- Bulky adducts
- Cross links
Name some DNA damaging agents
- UV
- Cigarette smoke
- Dyes
- Ionising radiation
- Chemotherapy
- Foods?
Name some types of endogenous DNA damage
- Deamination (e.g. cytosine to uracil)
- Oxidation
- Methylation
- Lipid peroxidation
Name some types of exogenous DNA damage
- Pollutants
- UV
- Ionising radiation
- Ciggy smoke
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
1) Base-excision repair
2) Nucleotide excision repair
3) Recombinational repair
4) Mismatch repair
What are the potential consequences if DNA damage is not repaired?
1) Cell cycle arrest
2) Apoptosis
- caused by inhibition of transcription, replication or chromosome segregation
3) Cancer, Ageing
- Caused by mutations and chromosome abrrations
What are the 3 classifications of UV radiation and what is the wavelength of each?
UVA (320-400nm)
UVB (295-320nm)
UVC (100-295nm)
Why does solar radiation contain 95% UVA with the rest UVB?
Because the ozone absorbs UV with wavelength below 300nm
What year was UV classified as a human carcinogen and by which body?
Internation Agency for Research on Cancer 1992
What are the 3 types of skin cancer and what are the characteristics of each?
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
Describe the penetrance of different UV types
UVA is lower energy but has higher penetrance than UVB
What countries have the highest rates of skin cancer?
Australia and New Zealand
Why are skin cancer rates so high in Australia and NZ?
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
What are the main 2 types of cancer relevant DNA damage induced by UV?
1) Cyclobutane pyrimidine dimers (CPD)
2) (6-4) pyrimidine-pyrimidone photoproducts (6-4PPS)
What is the structure of CPDs and 6-4PPs?
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
What other damage does UVA and B cause?
- Oxidative damage
- Protein distortion
What can happen as a result of UV induced DNA damage?
- 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
What is the definition of a mutation?
Heritable change in the sequence of an organisms DNA
What is the definition of a mutagen?
An agent that leads to an increase in the frequency of mutations
What is the definition of mutagenesis?
The process by which mutations are introduced
What is the most common UV induced DNA mutation?
Pyrimidine dimers cause CC to TT transitions
- Hallmarks of certain skin cancers, especially non-melanomas
What process repairs pyrimidine dimers?
Nucleotide excision repair
What are the 2 types of NER and what are the characteristics of each?
1) Global genomic NER
- deals with whole genome
2) Transcription-coupled NER
- Faster repair in highly transcribed genes
Describe the process of lesion recognition in GGNER and TCNER
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
Describe the rest of the NER process (conserved in both types)
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
What is Xeroderma Pigmentosum (XP) and what are the symptoms?
- 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
What causes XP?
- Mutations in XPA-XPG genes
- Results in dysfunctional XPA-XPG proteins
What are XP complementation groups?
Different types of XP dependent on the XP protein/s that are affected
i.e Complementation group A = XPA affected
Which complementation groups gives less severe phenotypes?
- Severity of disease depends on complementation group
- Some give neurological problems
- XPC and XPE groups give milder phenotype and no neurological disorders
How is p53 involved in repair of CPDs?
- CPDs cause significantly less helix distortion
- 6-4PPs recognised by XPC and repaired faster than CPDs
- p53 regulates CPD repair by upregulation of XPE
Cisplatin is commonly used to treat which cancers? What is its mechanism of action?
Testicular and ovarian
Causes DNA adducts, primarily intrastrand crosslink adducts
How is NER adapted to cause cisplatin resistance?
- Resistant cells enhance ERCC1
- Enhanced ERCC1 leads to enhanced NER
- Greater reapir of cisplatin induced DNA adducts
